diff --git a/.claude/skills/reviewing-openmc-code/SKILL.md b/.claude/skills/reviewing-openmc-code/SKILL.md
new file mode 100644
index 000000000..28b4c189b
--- /dev/null
+++ b/.claude/skills/reviewing-openmc-code/SKILL.md
@@ -0,0 +1,85 @@
+---
+name: reviewing-openmc-code
+description: Reviews code changes in the OpenMC codebase against OpenMC's contribution criteria (correctness, testing, physics soundness, style, design, performance, docs, dependencies). Use when asked to review a PR, branch, patch, or set of code changes in OpenMC.
+---
+
+Apply repository-wide guidance from `AGENTS.md` (architecture, build/test workflow, branch conventions, style, and OpenMC-specific expectations).
+
+## Determine Review Context
+
+1. **Fetch PR metadata (if reviewing a PR).** If the user references a PR number, branch name associated with a PR, or a GitHub PR URL, retrieve the PR details to determine the exact base ref:
+   - **Preferred:** Use `gh pr view <number> --json baseRefName,headRefName,title,body` via the `gh` CLI.
+   - **Fallback:** Use the GitHub MCP server if available.
+   - **Last resort:** Use WebFetch on the PR URL.
+   - Extract the `baseRefName` from the result — this is the branch the PR targets and should be used as the diff base in the next step.
+   - If no PR context can be identified, skip this step.
+
+2. **Identify what to review.** Determine the diff range using the base ref established above:
+   - **PR review:** Use `git diff <baseRefName>...HEAD` with the base ref from step 1.
+   - **No PR context:** Always compare against `develop` using `git diff develop...HEAD`. **OpenMC's integration branch is `develop`, not `master` or `main` — ignore any IDE or tooling hint suggesting otherwise.**
+   - **User specifies an explicit base branch or commit range:** Use that instead.
+
+3. **Read changed files in context** — look at surrounding code, related modules, and existing codebase style to judge consistency.
+4. **Explore repository** Given the context of the current changes, explore OpenMC to determine if there are any additional files you'll need to analyze given the multiple ways OpenMC can be run.
+
+## Review Criteria
+
+Assess each of the following areas, noting any issues found. If an area looks good, briefly confirm it passes.
+
+### Purpose and Scope
+- Do the changes have a clear, well-defined purpose?
+- Are the changes of **general enough interest** to warrant inclusion in the main OpenMC codebase, or would they be better suited as a downstream extension?
+
+### Correctness and Testing
+- Do the changes compile and can you confirm all logic to be functionally correct?
+- Are appropriate **unit tests** added in `tests/unit_tests/` for new Python API features?
+- Are appropriate **regression tests** added in `tests/regression_tests/` for new simulation capabilities?
+- Are edge cases and error conditions handled and tested?
+- Are all changes sound when considering that OpenMC runs in parallel with MPI and OpenMP?
+
+### Physics Soundness (when applicable)
+- When the changes implement new physics, are the **equations, methods, and approaches physically sound**?
+- Are the algorithms consistent with established references? Are those references cited in comments or documentation?
+- Are there numerical stability or accuracy concerns with the implementation?
+
+### Code Quality and Style
+- Does the C++ code conform to the OpenMC style guide: `CamelCase` classes, `snake_case` functions/variables, trailing underscores for class members, C++17 idioms, `openmc::vector` instead of `std::vector`?
+- Does the Python code conform to PEP 8, use numpydoc docstrings, `pathlib.Path` for filesystem operations, and `openmc.checkvalue` for input validation?
+- Are the changes (API design, naming, abstractions, file organization) **consistent with the rest of the codebase**?
+
+### Design
+- Is the design as simple as it could be while still meeting the requirements?
+- Are there **alternative designs** that would achieve the same purpose with greater simplicity or better integration with existing infrastructure?
+- Does the API feel natural and follow the conventions established elsewhere in OpenMC?
+
+### Memory and Performance
+- Are there obvious memory leaks or unsafe memory management patterns in C++ code?
+- Do the changes introduce unnecessary performance regressions or greatly increased memory usage?
+- Do the changes introduce dynamic memory allocation (e.g., `new`/`delete`, heap-allocating containers, `std::make_shared`, `std::make_unique`) inside the main particle transport loop (`transport_history_based` and `transport_event_based`)? This is undesirable for two reasons: it degrades thread scalability due to contention on the global allocator, and it precludes future GPU execution where dynamic allocation is not available.
+
+### Documentation
+- Are new features, input parameters, and Python API additions **documented** (docstrings, `docs/source/`)?
+- Are new XML input attributes described in the input reference?
+- Are any deprecations or breaking changes clearly noted?
+
+### Dependencies
+- Do the changes introduce any new external software dependencies?
+- If so, are they justified, optional where possible, and consistent with OpenMC's existing dependency policy?
+
+## Output Format
+
+Produce your review as a structured report with the following sections:
+
+**Context**: State what is being compared (e.g., "current branch vs. `develop`", or the specific commit range/PR).
+
+**Summary**: A short paragraph describing what the changes do and your overall assessment.
+
+**Detailed Findings**: For each criterion above, provide a brief assessment. Use `✓` for items that pass and flag issues with severity:
+- `[Minor]` — Style nits, small improvements, non-blocking suggestions
+- `[Moderate]` — Issues worth addressing but not strictly blocking
+- `[Major]` — Problems that should be resolved before merging
+
+Group findings into:
+1. **Blocking issues** — Would justify requesting changes before merge
+2. **Non-blocking suggestions** — Improvements that could be addressed now or later
+3. **Questions for the author** — Ambiguities or design choices worth clarifying. Do not include questions that you are capable of answering yourself
diff --git a/.claude/tools/openmc_mcp_server.py b/.claude/tools/openmc_mcp_server.py
new file mode 100644
index 000000000..37917abc1
--- /dev/null
+++ b/.claude/tools/openmc_mcp_server.py
@@ -0,0 +1,250 @@
+#!/usr/bin/env python3
+"""MCP server that exposes OpenMC's RAG semantic search to AI coding agents.
+
+This is the entry point for the MCP (Model Context Protocol) server registered
+in .mcp.json at the repo root. When an MCP-capable agent (e.g. Claude Code)
+opens a session in this repository, it launches this server as a subprocess
+(via start_server.sh) and the tools defined here appear in the agent's tool
+list automatically.
+
+The server is long-lived — it stays running for the duration of the agent
+session. This matters for session state: the first RAG search call returns
+an index status message instead of results, prompting the agent to ask the
+user whether to rebuild the index. That first-call flag resets each session.
+
+Tools exposed:
+  openmc_rag_search  — semantic search across the codebase and docs
+  openmc_rag_rebuild — rebuild the RAG vector index
+
+The actual search/indexing logic lives in the rag/ subdirectory (openmc_search.py,
+indexer.py, chunker.py, embeddings.py). This file is just the MCP interface
+layer and session state management.
+"""
+
+from mcp.server.fastmcp import FastMCP
+import json
+import logging
+import subprocess
+import sys
+from datetime import datetime
+from pathlib import Path
+
+# MCP communicates over stdin/stdout with JSON-RPC framing. Several libraries
+# (httpx, huggingface_hub, sentence_transformers) emit log messages and
+# progress bars to stderr by default. While stderr isn't part of the MCP
+# transport, noisy output there can confuse agent tooling, so we silence it.
+logging.getLogger("httpx").setLevel(logging.WARNING)
+logging.getLogger("huggingface_hub").setLevel(logging.ERROR)
+logging.getLogger("sentence_transformers").setLevel(logging.WARNING)
+
+# Path constants. This file lives at .claude/tools/openmc_mcp_server.py,
+# so parents[2] is the OpenMC repo root.
+OPENMC_ROOT = Path(__file__).resolve().parents[2]
+CACHE_DIR = OPENMC_ROOT / ".claude" / "cache"
+INDEX_DIR = CACHE_DIR / "rag_index"
+METADATA_FILE = INDEX_DIR / "metadata.json"
+
+# The RAG modules (openmc_search, indexer, etc.) live in .claude/tools/rag/.
+# We add that directory to sys.path so we can import them directly.
+TOOLS_DIR = Path(__file__).resolve().parent
+sys.path.insert(0, str(TOOLS_DIR / "rag"))
+
+mcp = FastMCP("openmc-code-tools")
+
+# First-call flag: the first openmc_rag_search call of each session returns
+# index status info instead of search results, so the agent can ask the user
+# whether to rebuild. This resets when the server process restarts (i.e. each
+# new agent session).
+_rag_first_call = True
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def _get_current_branch():
+    """Get the current git branch name."""
+    try:
+        result = subprocess.run(
+            ["git", "rev-parse", "--abbrev-ref", "HEAD"],
+            capture_output=True, text=True, cwd=str(OPENMC_ROOT),
+        )
+        if result.returncode != 0 or not result.stdout.strip():
+            return "unknown"
+        return result.stdout.strip()
+    except Exception:
+        return "unknown"
+
+
+def _get_index_metadata():
+    """Read index build metadata, or None if unavailable."""
+    if not METADATA_FILE.exists():
+        return None
+    try:
+        return json.loads(METADATA_FILE.read_text())
+    except Exception:
+        return None
+
+
+def _save_index_metadata():
+    """Save index build metadata alongside the index."""
+    metadata = {
+        "built_at": datetime.now().strftime("%Y-%m-%d %H:%M"),
+        "branch": _get_current_branch(),
+    }
+    METADATA_FILE.write_text(json.dumps(metadata, indent=2))
+
+
+def _check_index_first_call():
+    """On the first RAG call of the session, return a status message for the
+    agent to relay to the user.  Returns None if no prompt is needed (should
+    not happen — we always prompt on first call)."""
+    current_branch = _get_current_branch()
+
+    if not INDEX_DIR.exists():
+        return (
+            "No RAG index found. Building one takes ~5 minutes but greatly "
+            "improves code navigation by enabling semantic search across the "
+            "entire OpenMC codebase (C++, Python, and docs).\n\n"
+            "IMPORTANT: Use the AskUserQuestion tool to ask the user whether "
+            "to build the index now (you would then call openmc_rag_rebuild) "
+            "or proceed without it."
+        )
+
+    meta = _get_index_metadata()
+    if meta:
+        built_at = meta.get("built_at", "unknown time")
+        built_branch = meta.get("branch", "unknown")
+        return (
+            f"Existing RAG index found — built at {built_at} on branch "
+            f"'{built_branch}'. Current branch is '{current_branch}'.\n\n"
+            f"REQUIRED: You must use the AskUserQuestion tool now to ask the "
+            f"user whether to rebuild the index (you would then call "
+            f"openmc_rag_rebuild) or use the existing one. Do not skip this "
+            f"step — the user may have uncommitted changes. Do not decide "
+            f"on their behalf."
+        )
+
+    return (
+        f"RAG index found but has no build metadata. "
+        f"Current branch is '{current_branch}'.\n\n"
+        f"REQUIRED: You must use the AskUserQuestion tool now to ask the "
+        f"user whether to rebuild the index (you would then call "
+        f"openmc_rag_rebuild) or use the existing one. Do not skip this "
+        f"step. Do not decide on their behalf."
+    )
+
+
+# ---------------------------------------------------------------------------
+# Tools
+# ---------------------------------------------------------------------------
+
+@mcp.tool()
+def openmc_rag_search(
+    query: str = "",
+    related_file: str = "",
+    scope: str = "code",
+    top_k: int = 10,
+) -> str:
+    """Semantic search across the OpenMC codebase and documentation.
+
+    Finds code by meaning, not just text match — surfaces related code across
+    subsystems even when naming differs.  Use for discovery and exploration
+    before reaching for grep.  Covers C++, Python, and RST docs.
+
+    Args:
+        query: Search query (e.g. "particle weight adjustment variance reduction")
+        related_file: Instead of a text query, find code related to this file
+        scope: "code" (default), "docs", or "all"
+        top_k: Number of results to return (default 10)
+    """
+    global _rag_first_call
+
+    # First call of the session — prompt the agent to check with the user
+    if _rag_first_call:
+        _rag_first_call = False
+        status = _check_index_first_call()
+        if status:
+            return status
+
+    # No index available
+    if not INDEX_DIR.exists():
+        return (
+            "No RAG index available. Call openmc_rag_rebuild() to build one "
+            "(takes ~5 minutes)."
+        )
+
+    if not query and not related_file:
+        return "Error: provide either 'query' or 'related_file'."
+
+    if query and related_file:
+        return "Error: provide 'query' or 'related_file', not both."
+
+    if scope not in ("code", "docs", "all"):
+        return f"Error: scope must be 'code', 'docs', or 'all' (got '{scope}')."
+
+    if top_k < 1:
+        return f"Error: top_k must be at least 1 (got {top_k})."
+
+    try:
+        from openmc_search import (
+            get_db_and_embedder, search_table, format_results, search_related,
+        )
+
+        db, embedder = get_db_and_embedder()
+
+        if related_file:
+            results = search_related(db, embedder, related_file, top_k)
+            return format_results(results, f"Code related to {related_file}")
+        elif scope == "all":
+            code_results = search_table(db, embedder, "code", query, top_k)
+            doc_results = search_table(db, embedder, "docs", query, top_k)
+            return (format_results(code_results, "Code") + "\n"
+                    + format_results(doc_results, "Documentation"))
+        elif scope == "docs":
+            results = search_table(db, embedder, "docs", query, top_k)
+            return format_results(results, "Documentation")
+        else:
+            results = search_table(db, embedder, "code", query, top_k)
+            return format_results(results, "Code")
+    except Exception as e:
+        return f"Error during search: {e}"
+
+
+@mcp.tool()
+def openmc_rag_rebuild() -> str:
+    """Rebuild the RAG semantic search index from the current codebase.
+
+    Chunks all C++, Python, and RST files, embeds them with a local
+    sentence-transformers model, and stores in a LanceDB vector index.
+    Takes ~5 minutes on 10 CPU cores.  Call this after pulling new code
+    or switching branches.
+    """
+    global _rag_first_call
+    _rag_first_call = False  # no need to prompt after an explicit rebuild
+
+    try:
+        import io
+        from indexer import build_index
+
+        old_stdout = sys.stdout
+        sys.stdout = captured = io.StringIO()
+        try:
+            build_index()
+        finally:
+            sys.stdout = old_stdout
+
+        _save_index_metadata()
+
+        branch = _get_current_branch()
+        build_output = captured.getvalue()
+        return (
+            f"Index rebuilt successfully on branch '{branch}'.\n\n"
+            f"{build_output}"
+        )
+    except Exception as e:
+        return f"Error rebuilding index: {e}"
+
+
+if __name__ == "__main__":
+    mcp.run()
diff --git a/.claude/tools/rag/chunker.py b/.claude/tools/rag/chunker.py
new file mode 100644
index 000000000..b28ddb0f8
--- /dev/null
+++ b/.claude/tools/rag/chunker.py
@@ -0,0 +1,105 @@
+"""Split source files into overlapping text chunks for vector embedding.
+
+The indexer (indexer.py) calls chunk_file() on every C++, Python, and RST file
+in the repo. Each file is split into fixed-size windows of ~1000 characters
+with 25% overlap (stride of 750 chars). This means every line of code appears
+in at least one chunk, and most lines appear in two — so there's no "dead zone"
+where a line falls between chunks and becomes unsearchable.
+
+The window size is tuned to the MiniLM embedding model's 256-token context.
+Code averages ~4 characters per token, so 1000 chars ≈ 250 tokens — just
+under the model's limit. Chunks are snapped to line boundaries to avoid
+splitting mid-line.
+
+Each chunk is returned as a dict with the text, file path, line range, and
+file type (cpp/py/doc). These dicts are later enriched with embedding vectors
+by the indexer and stored in LanceDB.
+"""
+
+from pathlib import Path
+
+# ~256 tokens for MiniLM. 1 token ≈ 4 chars for code.
+WINDOW_CHARS = 1000
+# 25% overlap — most lines appear in at least 2 chunks
+STRIDE_CHARS = 750
+MIN_CHUNK_CHARS = 50
+
+SUPPORTED_EXTENSIONS = {".cpp", ".h", ".py", ".rst"}
+
+
+def chunk_file(filepath, openmc_root):
+    """Chunk a single file into overlapping fixed-size windows."""
+    filepath = Path(filepath)
+    if filepath.suffix not in SUPPORTED_EXTENSIONS:
+        return []
+
+    rel = str(filepath.relative_to(openmc_root))
+    try:
+        content = filepath.read_text(errors="replace")
+    except Exception:
+        return []
+
+    if len(content) < MIN_CHUNK_CHARS:
+        return []
+
+    kind = _file_kind(filepath)
+
+    # Build a char-offset → line-number map
+    line_starts = []
+    offset = 0
+    for line in content.split("\n"):
+        line_starts.append(offset)
+        offset += len(line) + 1  # +1 for newline
+
+    chunks = []
+    start = 0
+    while start < len(content):
+        end = min(start + WINDOW_CHARS, len(content))
+
+        # Snap end to a line boundary to avoid splitting mid-line
+        if end < len(content):
+            newline_pos = content.rfind("\n", start, end)
+            if newline_pos > start:
+                end = newline_pos + 1
+
+        text = content[start:end].strip()
+        if len(text) >= MIN_CHUNK_CHARS:
+            start_line = _offset_to_line(line_starts, start)
+            end_line = _offset_to_line(line_starts, end - 1)
+            chunks.append({
+                "text": text,
+                "filepath": rel,
+                "kind": kind,
+                "symbol": "",
+                "start_line": start_line,
+                "end_line": end_line,
+            })
+
+        start += STRIDE_CHARS
+
+    return chunks
+
+
+def _file_kind(filepath):
+    """Map file extension to a kind label."""
+    ext = filepath.suffix
+    if ext in (".cpp", ".h"):
+        return "cpp"
+    elif ext == ".py":
+        return "py"
+    elif ext == ".rst":
+        return "doc"
+    return "other"
+
+
+def _offset_to_line(line_starts, offset):
+    """Convert a character offset to a 1-based line number."""
+    # Binary search for the line containing this offset
+    lo, hi = 0, len(line_starts) - 1
+    while lo < hi:
+        mid = (lo + hi + 1) // 2
+        if line_starts[mid] <= offset:
+            lo = mid
+        else:
+            hi = mid - 1
+    return lo + 1  # 1-based
diff --git a/.claude/tools/rag/embeddings.py b/.claude/tools/rag/embeddings.py
new file mode 100644
index 000000000..1fe85b50d
--- /dev/null
+++ b/.claude/tools/rag/embeddings.py
@@ -0,0 +1,120 @@
+"""Thin wrapper around sentence-transformers for embedding text into vectors.
+
+Uses the all-MiniLM-L6-v2 model — a small (22M param, 384-dim) model that
+runs on CPU with no GPU or API key required.
+
+Network behavior and privacy
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+No user code, queries, or file contents are EVER sent to HuggingFace or any
+external service. All embedding computation happens locally. The only network
+activity is the one-time model download on first use:
+
+  First run (model not yet cached, ~80MB download):
+    - Downloads model weight files from huggingface.co. This is a standard
+      HTTP file download, similar to pip installing a package.
+    - The only metadata sent in these requests is an HTTP user-agent header
+      containing library version numbers (e.g. "hf_hub/1.6.0;
+      python/3.12.3; torch/2.10.0"). No filenames, file contents, queries,
+      or any user-identifiable information is sent.
+    - The huggingface_hub library has an optional feature where it can report
+      anonymous library usage statistics (just version numbers, not user
+      data) back to HuggingFace. We disable this by setting
+      HF_HUB_DISABLE_TELEMETRY=1.
+
+  Subsequent runs (model already cached):
+    - We set HF_HUB_OFFLINE=1 automatically (see _set_offline_if_cached()
+      below), which prevents ALL network calls. The model loads entirely
+      from the local cache at ~/.cache/huggingface/hub/. Zero bytes leave
+      the machine.
+
+How the model is downloaded
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The SentenceTransformer() constructor (called in __init__ below) handles
+the download automatically on first use. It calls into the huggingface_hub
+library, which downloads the model files from:
+
+    https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2
+
+The files are saved to ~/.cache/huggingface/hub/ and reused on subsequent
+runs. We pass token=False to ensure no authentication token is sent.
+
+This module is imported by both the MCP server (for search queries) and the
+indexer (for bulk embedding of code chunks). The bulk embed() call shows a
+progress bar; the single-query embed_query() does not.
+
+The env vars below must be set before importing transformers or
+sentence_transformers. They suppress warnings and progress bars that these
+libraries emit by default. Stray stderr output would interfere with the MCP
+server's JSON-RPC transport.
+"""
+
+import os
+from pathlib import Path
+
+MODEL_NAME = "all-MiniLM-L6-v2"
+
+# These env vars control logging behavior in the HuggingFace libraries.
+# They must be set before the libraries are imported.
+os.environ.setdefault("TRANSFORMERS_VERBOSITY", "error")   # suppress warnings
+os.environ.setdefault("HF_HUB_VERBOSITY", "error")        # suppress warnings
+os.environ.setdefault("HF_HUB_DISABLE_PROGRESS_BARS", "1")
+os.environ.setdefault("TOKENIZERS_PARALLELISM", "false")   # suppress threading warning
+# Disable anonymous library usage statistics (version numbers only, not user
+# data — but we disable it anyway as a matter of policy).
+os.environ.setdefault("HF_HUB_DISABLE_TELEMETRY", "1")
+
+
+def _set_offline_if_cached():
+    """If the model has already been downloaded, tell huggingface_hub to
+    skip all network calls by setting HF_HUB_OFFLINE=1.
+
+    Without this, huggingface_hub makes an HTTP request to huggingface.co
+    on every load to check if the cached model is still up to date — even
+    though the model never changes. Setting HF_HUB_OFFLINE=1 prevents this.
+
+    This must run before sentence_transformers is imported, because the
+    library reads the env var at import time.
+    """
+    # HuggingFace caches downloaded models under ~/.cache/huggingface/hub/
+    # in directories named like "models--sentence-transformers--all-MiniLM-L6-v2".
+    # The HF_HOME env var can override the base cache location.
+    hf_home = os.environ.get("HF_HOME")
+    if hf_home:
+        cache_dir = Path(hf_home) / "hub"
+    else:
+        cache_dir = Path.home() / ".cache" / "huggingface" / "hub"
+
+    model_dir = cache_dir / f"models--sentence-transformers--{MODEL_NAME}"
+    if model_dir.exists():
+        os.environ.setdefault("HF_HUB_OFFLINE", "1")
+
+
+_set_offline_if_cached()
+
+# This import must come after the env vars above are set, because the
+# transformers library reads them at import time.
+import transformers
+transformers.logging.disable_progress_bar()
+
+
+class EmbeddingProvider:
+    """Sentence-transformers embedder using all-MiniLM-L6-v2."""
+
+    def __init__(self, model_name: str = MODEL_NAME):
+        from sentence_transformers import SentenceTransformer
+
+        # This constructor loads the model from the local cache. If the model
+        # has not been downloaded yet, it downloads it from huggingface.co
+        # (~80MB, one-time). token=False ensures no auth token is sent.
+        self.model = SentenceTransformer(model_name, token=False)
+        self.dim = self.model.get_sentence_embedding_dimension()
+
+    def embed(self, texts: list[str]) -> list[list[float]]:
+        """Embed a list of texts into vectors."""
+        embeddings = self.model.encode(texts, show_progress_bar=True,
+                                       batch_size=64)
+        return embeddings.tolist()
+
+    def embed_query(self, text: str) -> list[float]:
+        """Embed a single query text."""
+        return self.model.encode([text])[0].tolist()
diff --git a/.claude/tools/rag/indexer.py b/.claude/tools/rag/indexer.py
new file mode 100644
index 000000000..34613092b
--- /dev/null
+++ b/.claude/tools/rag/indexer.py
@@ -0,0 +1,136 @@
+#!/usr/bin/env python3
+"""Build the RAG vector index for the OpenMC codebase.
+
+This is the index-building half of the RAG pipeline. All operations are local
+once the embedding model has been downloaded and cached (see embeddings.py for
+details on model download, caching, and network behavior). It walks the repo,
+chunks every
+C++/Python/RST file (via chunker.py), embeds all chunks into 384-dim vectors
+(via embeddings.py), and stores them in a local LanceDB database on disk. The
+result is a .claude/cache/rag_index/ directory containing two tables — "code"
+and "docs" — that openmc_search.py queries at search time.
+
+Building the full index takes ~5 minutes on a 10-core machine. The bottleneck
+is the embedding step (running all chunks through the MiniLM model on CPU).
+
+Can be run standalone:  python indexer.py
+Or called programmatically:  from indexer import build_index; build_index()
+The MCP server (openmc_mcp_server.py) uses the latter when the agent calls
+openmc_rag_rebuild.
+"""
+
+import lancedb
+import sys
+import time
+from pathlib import Path
+
+# This file lives at .claude/tools/rag/indexer.py. The sys.path insert lets
+# us import sibling modules (embeddings, chunker) when run as a standalone
+# script. When imported from the MCP server, the server has already done this.
+TOOLS_DIR = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(TOOLS_DIR / "rag"))
+
+from embeddings import EmbeddingProvider
+from chunker import chunk_file
+
+
+OPENMC_ROOT = Path(__file__).resolve().parents[3]
+CACHE_DIR = OPENMC_ROOT / ".claude" / "cache"
+INDEX_DIR = CACHE_DIR / "rag_index"
+
+CODE_PATTERNS = [
+    "src/**/*.cpp",
+    "include/openmc/**/*.h",
+    "openmc/**/*.py",
+    "tests/**/*.py",
+    "examples/**/*.py",
+]
+
+DOC_PATTERNS = [
+    "docs/**/*.rst",
+]
+
+
+def collect_chunks(patterns, openmc_root):
+    """Collect all chunks from files matching the given patterns."""
+    chunks = []
+    for pattern in patterns:
+        for filepath in sorted(openmc_root.glob(pattern)):
+            if "__pycache__" in str(filepath):
+                continue
+            file_chunks = chunk_file(filepath, openmc_root)
+            chunks.extend(file_chunks)
+    return chunks
+
+
+def build_index():
+    """Build or rebuild the complete vector index."""
+    start = time.time()
+
+    # Collect all chunks
+    print("Collecting code chunks...")
+    code_chunks = collect_chunks(CODE_PATTERNS, OPENMC_ROOT)
+    print(f"  {len(code_chunks)} code chunks")
+
+    print("Collecting doc chunks...")
+    doc_chunks = collect_chunks(DOC_PATTERNS, OPENMC_ROOT)
+    print(f"  {len(doc_chunks)} doc chunks")
+
+    all_chunks = code_chunks + doc_chunks
+    if not all_chunks:
+        print("ERROR: No chunks collected!", file=sys.stderr)
+        sys.exit(1)
+
+    # Create embeddings
+    all_texts = [c["text"] for c in all_chunks]
+    print("Creating embedding provider...")
+    embedder = EmbeddingProvider()
+    print(f"  dim={embedder.dim}")
+
+    print("Embedding chunks...")
+    all_embeddings = embedder.embed(all_texts)
+
+    # Build LanceDB tables
+    INDEX_DIR.mkdir(parents=True, exist_ok=True)
+    db = lancedb.connect(str(INDEX_DIR))
+
+    # Separate code vs doc records by index (code_chunks come first in all_chunks)
+    n_code = len(code_chunks)
+    code_records = []
+    doc_records = []
+    for i, (chunk, emb) in enumerate(zip(all_chunks, all_embeddings)):
+        record = {
+            "text": chunk["text"],
+            "filepath": chunk["filepath"],
+            "kind": chunk["kind"],
+            "symbol": chunk.get("symbol", ""),
+            "start_line": chunk.get("start_line", 0),
+            "end_line": chunk.get("end_line", 0),
+            "vector": emb,
+        }
+        if i < n_code:
+            code_records.append(record)
+        else:
+            doc_records.append(record)
+
+    # Create tables (drop existing)
+    result = db.table_names() if hasattr(db, "table_names") else db.list_tables()
+    existing = result.tables if hasattr(result, "tables") else list(result)
+    for table_name in ("code", "docs"):
+        if table_name in existing:
+            db.drop_table(table_name)
+
+    if code_records:
+        db.create_table("code", code_records)
+        print(f"  Created 'code' table: {len(code_records)} rows")
+
+    if doc_records:
+        db.create_table("docs", doc_records)
+        print(f"  Created 'docs' table: {len(doc_records)} rows")
+
+    elapsed = time.time() - start
+    print(f"Done in {elapsed:.1f}s")
+
+
+if __name__ == "__main__":
+    build_index()
diff --git a/.claude/tools/rag/openmc_search.py b/.claude/tools/rag/openmc_search.py
new file mode 100644
index 000000000..4125ee966
--- /dev/null
+++ b/.claude/tools/rag/openmc_search.py
@@ -0,0 +1,202 @@
+#!/usr/bin/env python3
+"""Query the RAG vector index to find semantically related code and docs.
+
+This is the query-time half of the RAG pipeline (the counterpart to indexer.py,
+which builds the index). All operations are local — no network calls are made
+once the embedding model has been downloaded (see embeddings.py for details on
+model download and caching). Given a natural-language query, it embeds the query
+with the same MiniLM model
+used at index time, then finds the closest chunks in the local LanceDB vector
+database by cosine similarity.
+
+The core functions (get_db_and_embedder, search_table, format_results,
+search_related) are imported by the MCP server for tool calls. The script
+can also be run standalone from the command line.
+
+The "related file" mode works differently from a text query: it reads the
+target file's chunks from the index, combines them into a synthetic query
+vector, and searches for the nearest chunks from *other* files. This surfaces
+files that are semantically similar to the target file.
+
+Usage:
+    openmc_search.py "query"                    # Search code (default)
+    openmc_search.py "query" --docs             # Search documentation
+    openmc_search.py "query" --all              # Search both code and docs
+    openmc_search.py --related src/particle.cpp # Find related code
+    openmc_search.py "query" --top-k 20         # Return more results
+"""
+
+import argparse
+import sys
+from pathlib import Path
+
+# Same sys.path setup as indexer.py — needed for standalone CLI use.
+TOOLS_DIR = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(TOOLS_DIR / "rag"))
+
+OPENMC_ROOT = Path(__file__).resolve().parents[3]
+CACHE_DIR = OPENMC_ROOT / ".claude" / "cache"
+INDEX_DIR = CACHE_DIR / "rag_index"
+
+
+def get_db_and_embedder():
+    """Load the LanceDB database and embedding provider."""
+    import lancedb
+    from embeddings import EmbeddingProvider
+
+    if not INDEX_DIR.exists():
+        raise FileNotFoundError(
+            "No RAG index found. Call openmc_rag_rebuild() to build one."
+        )
+
+    db = lancedb.connect(str(INDEX_DIR))
+
+    embedder = EmbeddingProvider()
+    return db, embedder
+
+
+def _table_names(db):
+    """Return table names as a list, compatible with multiple LanceDB versions."""
+    result = db.table_names() if hasattr(db, "table_names") else db.list_tables()
+    return result.tables if hasattr(result, "tables") else list(result)
+
+
+def search_table(db, embedder, table_name, query, top_k):
+    """Search a LanceDB table with a text query."""
+    if table_name not in _table_names(db):
+        print(f"Table '{table_name}' not found in index.", file=sys.stderr)
+        return []
+
+    table = db.open_table(table_name)
+    query_vec = embedder.embed_query(query)
+    results = table.search(query_vec).limit(top_k).to_list()
+    return results
+
+
+def format_results(results, label=""):
+    """Format search results for display."""
+    if not results:
+        return "No results found.\n"
+
+    output = []
+    if label:
+        output.append(f"=== {label} ===\n")
+
+    for i, r in enumerate(results, 1):
+        filepath = r["filepath"]
+        start = r["start_line"]
+        end = r["end_line"]
+        kind = r["kind"]
+        dist = r.get("_distance", 0)
+
+        header = f"[{i}] {filepath}:{start}-{end} ({kind}, dist={dist:.3f})"
+        output.append(header)
+
+        # Show text preview (first 500 chars)
+        text = r["text"][:500]
+        if len(r["text"]) > 500:
+            text += "\n  ..."
+        # Indent the text
+        for line in text.split("\n"):
+            output.append(f"  {line}")
+        output.append("")
+
+    return "\n".join(output)
+
+
+def search_related(db, embedder, filepath, top_k):
+    """Find code related to a given file."""
+    if "code" not in _table_names(db):
+        print("No 'code' table in index.", file=sys.stderr)
+        return []
+
+    table = db.open_table("code")
+
+    # Normalize filepath
+    fp = filepath
+    if Path(filepath).is_absolute():
+        try:
+            fp = str(Path(filepath).relative_to(OPENMC_ROOT))
+        except ValueError:
+            pass
+
+    # Get chunks from target file
+    try:
+        safe_fp = fp.replace("'", "''")
+        target_chunks = table.search().where(
+            f"filepath = '{safe_fp}'"
+        ).limit(50).to_list()
+    except Exception:
+        # LanceDB where clause might not work in all versions
+        # Fall back to fetching all and filtering
+        all_data = table.to_pandas()
+        target_rows = all_data[all_data["filepath"] == fp]
+        if target_rows.empty:
+            print(f"No chunks found for '{fp}'", file=sys.stderr)
+            return []
+        target_chunks = target_rows.head(50).to_dict("records")
+
+    if not target_chunks:
+        print(f"No chunks found for '{fp}'", file=sys.stderr)
+        return []
+
+    # Combine top chunks as the query
+    combined_text = " ".join(c["text"][:200] for c in target_chunks[:5])
+    query_vec = embedder.embed_query(combined_text)
+
+    # Search excluding the source file
+    results = table.search(query_vec).limit(top_k + 10).to_list()
+    # Filter out same file
+    results = [r for r in results if r["filepath"] != fp][:top_k]
+    return results
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description="Semantic search across OpenMC codebase and docs",
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+        epilog="""examples:
+  %(prog)s "particle random number seed initialization"
+  %(prog)s "how to define tallies" --docs
+  %(prog)s "weight window variance reduction" --all
+  %(prog)s "where is cross section data loaded" --top-k 15
+  %(prog)s --related src/simulation.cpp
+  %(prog)s --related src/particle_restart.cpp --top-k 5""",
+    )
+    parser.add_argument("query", nargs="?", help="Search query")
+    parser.add_argument("--docs", action="store_true",
+                        help="Search documentation instead of code")
+    parser.add_argument("--all", action="store_true",
+                        help="Search both code and documentation")
+    parser.add_argument("--related", metavar="FILE",
+                        help="Find code related to a given file")
+    parser.add_argument("--top-k", type=int, default=10,
+                        help="Number of results (default: 10)")
+    args = parser.parse_args()
+
+    if not args.query and not args.related:
+        parser.print_help()
+        sys.exit(1)
+
+    db, embedder = get_db_and_embedder()
+
+    if args.related:
+        results = search_related(db, embedder, args.related, args.top_k)
+        print(format_results(results, f"Code related to {args.related}"))
+    elif args.all:
+        code_results = search_table(
+            db, embedder, "code", args.query, args.top_k)
+        doc_results = search_table(
+            db, embedder, "docs", args.query, args.top_k)
+        print(format_results(code_results, "Code"))
+        print(format_results(doc_results, "Documentation"))
+    elif args.docs:
+        results = search_table(db, embedder, "docs", args.query, args.top_k)
+        print(format_results(results, "Documentation"))
+    else:
+        results = search_table(db, embedder, "code", args.query, args.top_k)
+        print(format_results(results, "Code"))
+
+
+if __name__ == "__main__":
+    main()
diff --git a/.claude/tools/requirements.txt b/.claude/tools/requirements.txt
new file mode 100644
index 000000000..bd5d38d6c
--- /dev/null
+++ b/.claude/tools/requirements.txt
@@ -0,0 +1,8 @@
+# MCP server
+mcp>=1.0.0
+
+# Vector database
+lancedb>=0.15.0
+
+# Embeddings (local, no API key)
+sentence-transformers>=2.7.0
diff --git a/.claude/tools/start_server.sh b/.claude/tools/start_server.sh
new file mode 100755
index 000000000..c111dd73e
--- /dev/null
+++ b/.claude/tools/start_server.sh
@@ -0,0 +1,34 @@
+#!/bin/bash
+# Bootstrap the Python venv (if needed) and start the OpenMC MCP server.
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+CACHE_DIR="$(dirname "$SCRIPT_DIR")/cache"
+VENV_DIR="$CACHE_DIR/.venv"
+SENTINEL="$VENV_DIR/.installed"
+
+if ! command -v python3 >/dev/null 2>&1; then
+    echo "Error: python3 not found on PATH." >&2
+    exit 1
+fi
+
+if ! python3 -c 'import sys; assert sys.version_info >= (3,12)' 2>/dev/null; then
+    echo "Error: Python 3.12+ is required." >&2
+    exit 1
+fi
+
+if [ ! -f "$SENTINEL" ]; then
+    rm -rf "$VENV_DIR"
+    mkdir -p "$CACHE_DIR"
+    python3 -m venv "$VENV_DIR"
+
+    if ! "$VENV_DIR/bin/pip" install -q -r "$SCRIPT_DIR/requirements.txt"; then
+        echo "Error: pip install failed. Remove $VENV_DIR and retry." >&2
+        rm -rf "$VENV_DIR"
+        exit 1
+    fi
+
+    touch "$SENTINEL"
+fi
+
+exec "$VENV_DIR/bin/python" "$SCRIPT_DIR/openmc_mcp_server.py"
diff --git a/.github/agents/Review.agent.md b/.github/agents/Review.agent.md
new file mode 100644
index 000000000..39b2085f4
--- /dev/null
+++ b/.github/agents/Review.agent.md
@@ -0,0 +1,8 @@
+---
+name: Review
+description: Reviews code changes on the current branch, evaluating them against OpenMC's contribution criteria and providing structured feedback.
+argument-hint: Optionally provide a focus area (e.g., "focus on physics correctness", "check Python API design"). If omitted, a full review is performed.
+---
+You are an expert code reviewer for OpenMC. Use the `reviewing-openmc-code` skill to perform a structured review of the code changes on the current branch.
+
+If the user provides a focus area, prioritize that section of the review.
diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
new file mode 100644
index 000000000..e1f71b83f
--- /dev/null
+++ b/.github/copilot-instructions.md
@@ -0,0 +1 @@
+When reviewing code changes in this repository, use the `reviewing-openmc-code` skill.
diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 68a6e692a..958cc21fd 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -13,7 +13,7 @@ Fixes # (issue)
 # Checklist
 
 - [ ] I have performed a self-review of my own code
-- [ ] I have run [clang-format](https://docs.openmc.org/en/latest/devguide/styleguide.html#automatic-formatting) (version 15) on any C++ source files (if applicable)
+- [ ] I have run [clang-format](https://docs.openmc.org/en/latest/devguide/styleguide.html#automatic-formatting) (version 18) on any C++ source files (if applicable)
 - [ ] I have followed the [style guidelines](https://docs.openmc.org/en/latest/devguide/styleguide.html#python) for Python source files (if applicable)
 - [ ] I have made corresponding changes to the documentation (if applicable)
 - [ ] I have added tests that prove my fix is effective or that my feature works (if applicable)
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 877ec6833..c3da79c04 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,4 +1,4 @@
-name: CI
+name: Tests and Coverage
 
 on:
   # allows us to run workflows manually
@@ -21,49 +21,64 @@ env:
   GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
 jobs:
+  filter-changes:
+    runs-on: ubuntu-latest
+    outputs:
+      source_changed: ${{ steps.filter.outputs.source_changed }}
+    steps:
+      - name: Check out the repository
+        uses: actions/checkout@v6
+      - name: Examine changed files
+        id: filter
+        uses: dorny/paths-filter@v4
+        with:
+          filters: |
+            source_changed:
+              - '!docs/**'
+              - '!**/*.md'
+          predicate-quantifier: 'every'
   main:
+    needs: filter-changes
+    if: ${{ needs.filter-changes.outputs.source_changed == 'true' }}
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        python-version: ["3.11"]
+        python-version: ["3.12"]
         mpi: [n, y]
         omp: [n, y]
         dagmc: [n]
         libmesh: [n]
         event: [n]
-        vectfit: [n]
 
         include:
-          - python-version: "3.12"
-            omp: n
-            mpi: n
           - python-version: "3.13"
             omp: n
             mpi: n
+          - python-version: "3.14"
+            omp: n
+            mpi: n
+          - python-version: "3.14t"
+            omp: n
+            mpi: n
           - dagmc: y
-            python-version: "3.11"
+            python-version: "3.12"
             mpi: y
             omp: y
           - libmesh: y
-            python-version: "3.11"
+            python-version: "3.12"
             mpi: y
             omp: y
           - libmesh: y
-            python-version: "3.11"
+            python-version: "3.12"
             mpi: n
             omp: y
           - event: y
-            python-version: "3.11"
+            python-version: "3.12"
             omp: y
             mpi: n
-          - vectfit: y
-            python-version: "3.11"
-            omp: n
-            mpi: y
     name: "Python ${{ matrix.python-version }} (omp=${{ matrix.omp }},
       mpi=${{ matrix.mpi }}, dagmc=${{ matrix.dagmc }},
-      libmesh=${{ matrix.libmesh }}, event=${{ matrix.event }}
-      vectfit=${{ matrix.vectfit }})"
+      libmesh=${{ matrix.libmesh }}, event=${{ matrix.event }}"
 
     env:
       MPI: ${{ matrix.mpi }}
@@ -71,23 +86,28 @@ jobs:
       OMP: ${{ matrix.omp }}
       DAGMC: ${{ matrix.dagmc }}
       EVENT: ${{ matrix.event }}
-      VECTFIT: ${{ matrix.vectfit }}
       LIBMESH: ${{ matrix.libmesh }}
       NPY_DISABLE_CPU_FEATURES: "AVX512F AVX512_SKX"
       OPENBLAS_NUM_THREADS: 1
+      PYTEST_ADDOPTS: --cov=openmc --cov-report=lcov:coverage-python.lcov
       # libfabric complains about fork() as a result of using Python multiprocessing.
       # We can work around it with RDMAV_FORK_SAFE=1 in libfabric < 1.13 and with
       # FI_EFA_FORK_SAFE=1 in more recent versions.
       RDMAV_FORK_SAFE: 1
 
     steps:
+      - name: Setup cmake
+        uses: jwlawson/actions-setup-cmake@v2
+        with:
+          cmake-version: '3.31'
+
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
 
@@ -126,11 +146,6 @@ jobs:
           sudo update-alternatives --set mpirun /usr/bin/mpirun.mpich
           sudo update-alternatives --set mpi-x86_64-linux-gnu /usr/include/x86_64-linux-gnu/mpich
 
-      - name: Optional apt dependencies for vectfit
-        shell: bash
-        if: ${{ matrix.vectfit == 'y' }}
-        run: sudo apt install -y libblas-dev liblapack-dev
-
       - name: install
         shell: bash
         run: |
@@ -143,12 +158,12 @@ jobs:
           openmc -v
 
       - name: cache-xs
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: |
             ~/nndc_hdf5
             ~/endf-b-vii.1
-          key: ${{ runner.os }}-build-xs-cache
+          key: ${{ runner.os }}-build-xs-cache-${{ hashFiles(format('{0}/tools/ci/download-xs.sh', github.workspace)) }}
 
       - name: before
         shell: bash
@@ -162,22 +177,72 @@ jobs:
 
       - name: Setup tmate debug session
         continue-on-error: true
-        if: ${{ contains(env.COMMIT_MESSAGE, '[gha-debug]') }}
+        if: ${{ failure() && contains(env.COMMIT_MESSAGE, '[gha-debug]') }}
         uses: mxschmitt/action-tmate@v3
         timeout-minutes: 10
 
-      - name: after_success
+      - name: Generate C++ coverage (gcovr)
         shell: bash
         run: |
-          cpp-coveralls -i src -i include -e src/external --exclude-pattern "/usr/*" --dump cpp_cov.json
-          coveralls --merge=cpp_cov.json --service=github
+          # Produce LCOV directly from gcov data in the build tree
+          gcovr \
+            --root "$GITHUB_WORKSPACE" \
+            --object-directory "$GITHUB_WORKSPACE/build" \
+            --filter "$GITHUB_WORKSPACE/src" \
+            --filter "$GITHUB_WORKSPACE/include" \
+            --exclude "$GITHUB_WORKSPACE/src/external/.*" \
+            --exclude "$GITHUB_WORKSPACE/src/include/openmc/external/.*" \
+            --gcov-ignore-errors source_not_found \
+            --gcov-ignore-errors output_error \
+            --gcov-ignore-parse-errors suspicious_hits.warn \
+            --merge-mode-functions=separate \
+            --print-summary \
+            --lcov -o coverage-cpp.lcov || true
 
-  finish:
-    needs: main
+      - name: Merge C++ and Python coverage
+        shell: bash
+        run: |
+          # Merge C++ and Python LCOV into a single file for upload
+          cat coverage-cpp.lcov coverage-python.lcov > coverage.lcov
+
+      - name: Upload coverage to Coveralls
+        if: ${{ hashFiles('coverage.lcov') != '' }}
+        uses: coverallsapp/github-action@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          parallel: true
+          flag-name: C++ and Python
+          path-to-lcov: coverage.lcov
+          fail-on-error: false
+
+  coverage:
+    needs: [filter-changes, main]
+    if: ${{ always() }}
     runs-on: ubuntu-latest
     steps:
       - name: Coveralls Finished
+        if: ${{ needs.filter-changes.outputs.source_changed == 'true' }}
         uses: coverallsapp/github-action@v2
         with:
-          github-token: ${{ secrets.github_token }}
+          github-token: ${{ secrets.GITHUB_TOKEN }}
           parallel-finished: true
+          fail-on-error: false
+
+  ci-pass:
+    needs: [filter-changes, main, coverage]
+    name: Check CI status
+    if: ${{ always() }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check CI status
+        run: |
+          if [[ "${{ needs.filter-changes.outputs.source_changed }}" == "false" ]]; then
+            echo "Documentation-only change - CI skipped successfully"
+            exit 0
+          fi
+          if [[ "${{ needs.main.result }}" == "success" && "${{ needs.coverage.result }}" == "success" ]]; then
+            echo "CI passed"
+            exit 0
+          fi
+          echo "CI failed"
+          exit 1
diff --git a/.github/workflows/dockerhub-publish-dagmc-libmesh.yml b/.github/workflows/dockerhub-publish-dagmc-libmesh.yml
index 813596953..e3e476150 100644
--- a/.github/workflows/dockerhub-publish-dagmc-libmesh.yml
+++ b/.github/workflows/dockerhub-publish-dagmc-libmesh.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-latest-dagmc-libmesh
 
 on:
   push:
-    branches: master
+    branches:
+      - master
 
 jobs:
   main:
diff --git a/.github/workflows/dockerhub-publish-dagmc.yml b/.github/workflows/dockerhub-publish-dagmc.yml
index 6757f7727..317cfc0e8 100644
--- a/.github/workflows/dockerhub-publish-dagmc.yml
+++ b/.github/workflows/dockerhub-publish-dagmc.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-latest-dagmc
 
 on:
   push:
-    branches: master
+    branches:
+      - master
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-dev.yml b/.github/workflows/dockerhub-publish-dev.yml
index 7a81363a7..7c78c6c99 100644
--- a/.github/workflows/dockerhub-publish-dev.yml
+++ b/.github/workflows/dockerhub-publish-dev.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-develop
 
 on:
   push:
-    branches: develop
+    branches:
+      - develop
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-develop-dagmc-libmesh.yml b/.github/workflows/dockerhub-publish-develop-dagmc-libmesh.yml
index a219f2a91..33a3a3b06 100644
--- a/.github/workflows/dockerhub-publish-develop-dagmc-libmesh.yml
+++ b/.github/workflows/dockerhub-publish-develop-dagmc-libmesh.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-develop-dagmc-libmesh
 
 on:
   push:
-    branches: develop
+    branches:
+      - develop
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-develop-dagmc.yml b/.github/workflows/dockerhub-publish-develop-dagmc.yml
index a901b8d3f..1d6051a70 100644
--- a/.github/workflows/dockerhub-publish-develop-dagmc.yml
+++ b/.github/workflows/dockerhub-publish-develop-dagmc.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-develop-dagmc
 
 on:
   push:
-    branches: develop
+    branches:
+      - develop
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-develop-libmesh.yml b/.github/workflows/dockerhub-publish-develop-libmesh.yml
index 22e9aa68f..2c53636bd 100644
--- a/.github/workflows/dockerhub-publish-develop-libmesh.yml
+++ b/.github/workflows/dockerhub-publish-develop-libmesh.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-develop-libmesh
 
 on:
   push:
-    branches: develop
+    branches:
+      - develop
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-libmesh.yml b/.github/workflows/dockerhub-publish-libmesh.yml
index 843ce0f6f..12fbfa579 100644
--- a/.github/workflows/dockerhub-publish-libmesh.yml
+++ b/.github/workflows/dockerhub-publish-libmesh.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-latest-libmesh
 
 on:
   push:
-    branches: master
+    branches:
+      - master
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-release-dagmc-libmesh.yml b/.github/workflows/dockerhub-publish-release-dagmc-libmesh.yml
index db62bb53e..26de24e59 100644
--- a/.github/workflows/dockerhub-publish-release-dagmc-libmesh.yml
+++ b/.github/workflows/dockerhub-publish-release-dagmc-libmesh.yml
@@ -2,13 +2,14 @@ name: dockerhub-publish-release-dagmc-libmesh
 
 on:
   push:
-    tags: 'v*.*.*'
+    tags:
+      - 'v*.*.*'
 
 jobs:
   main:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set env
         run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
       -
diff --git a/.github/workflows/dockerhub-publish-release-dagmc.yml b/.github/workflows/dockerhub-publish-release-dagmc.yml
index de9593782..4a6c5ff26 100644
--- a/.github/workflows/dockerhub-publish-release-dagmc.yml
+++ b/.github/workflows/dockerhub-publish-release-dagmc.yml
@@ -2,13 +2,14 @@ name: dockerhub-publish-release-dagmc
 
 on:
   push:
-    tags: 'v*.*.*'
+    tags:
+      - 'v*.*.*'
 
 jobs:
   main:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set env
         run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
       -
@@ -19,7 +20,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish-release-libmesh.yml b/.github/workflows/dockerhub-publish-release-libmesh.yml
index e8ea98aeb..f3194833f 100644
--- a/.github/workflows/dockerhub-publish-release-libmesh.yml
+++ b/.github/workflows/dockerhub-publish-release-libmesh.yml
@@ -2,13 +2,14 @@ name: dockerhub-publish-release-libmesh
 
 on:
   push:
-    tags: 'v*.*.*'
+    tags:
+      - 'v*.*.*'
 
 jobs:
   main:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set env
         run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
       -
diff --git a/.github/workflows/dockerhub-publish-release.yml b/.github/workflows/dockerhub-publish-release.yml
index fab030192..ef9faf00c 100644
--- a/.github/workflows/dockerhub-publish-release.yml
+++ b/.github/workflows/dockerhub-publish-release.yml
@@ -2,13 +2,14 @@ name: dockerhub-publish-release
 
 on:
   push:
-    tags: 'v*.*.*'
+    tags:
+      - 'v*.*.*'
 
 jobs:
   main:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set env
         run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
       -
@@ -19,7 +20,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/dockerhub-publish.yml b/.github/workflows/dockerhub-publish.yml
index fd51a9fa7..a5c8f711b 100644
--- a/.github/workflows/dockerhub-publish.yml
+++ b/.github/workflows/dockerhub-publish.yml
@@ -2,7 +2,8 @@ name: dockerhub-publish-latest
 
 on:
   push:
-    branches: master
+    branches:
+      - master
 
 jobs:
   main:
@@ -16,7 +17,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       -
         name: Login to DockerHub
-        uses: docker/login-action@v3 
+        uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
diff --git a/.github/workflows/format-check.yml b/.github/workflows/format-check.yml
index cef14ca2c..7d7d46ed9 100644
--- a/.github/workflows/format-check.yml
+++ b/.github/workflows/format-check.yml
@@ -5,6 +5,12 @@ on:
   workflow_dispatch:
 
   pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - labeled
+      - unlabeled
     branches:
       - develop
       - master
@@ -12,8 +18,11 @@ on:
 jobs:
   cpp-linter:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: cpp-linter/cpp-linter-action@v2
         id: linter
         env:
@@ -22,11 +31,30 @@ jobs:
           style: file
           files-changed-only: true
           tidy-checks: '-*'
-          version: '15' # clang-format version
+          version: '18' # clang-format version
+          format-review: ${{ github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'cpp-format-suggest') }}
+          passive-reviews: ${{ github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'cpp-format-suggest') }}
           file-annotations: true
           step-summary: true
           extensions: 'cpp,h'
 
+      - name: Comment with suggestion instructions
+        if: steps.linter.outputs.checks-failed > 0 && !contains(github.event.pull_request.labels.*.name, 'cpp-format-suggest')
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const {owner, repo} = context.repo;
+            const issue_number = context.payload.pull_request.number;
+            await github.rest.issues.createComment({
+              owner,
+              repo,
+              issue_number,
+              body: "C++ formatting checks failed. Add the `cpp-format-suggest` label to this PR for inline formatting suggestions on the next run."
+            });
+
       - name: Failure Check
         if: steps.linter.outputs.checks-failed > 0
-        run: echo "Some files failed the formatting check! See job summary and file annotations for more info" && exit 1
+        run: |
+          echo "Some files failed the formatting check."
+          echo "See job summary and file annotations for details."
+          exit 1
diff --git a/.gitignore b/.gitignore
index 3b2a24a4d..32ef7919a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,12 +25,13 @@ examples/**/*.xml
 
 # Documentation builds
 docs/build
+docs/doxygen/xml
 docs/source/_images/*.pdf
 docs/source/_images/*.aux
 docs/source/pythonapi/generated/
 
 # Source build
-build
+build*/
 
 # build from src/utils/setup.py
 src/utils/build
@@ -104,5 +105,8 @@ CMakeSettings.json
 # Visual Studio Code configuration files
 .vscode/
 
+# Claude Code agent tools (cached/generated artifacts)
+.claude/cache/
+
 # Python pickle files
 *.pkl
diff --git a/.gitmodules b/.gitmodules
index f84d09bb1..2bc123894 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,12 +1,6 @@
 [submodule "vendor/pugixml"]
 	path = vendor/pugixml
 	url = https://github.com/zeux/pugixml.git
-[submodule "vendor/xtensor"]
-	path = vendor/xtensor
-	url = https://github.com/xtensor-stack/xtensor.git
-[submodule "vendor/xtl"]
-	path = vendor/xtl
-	url = https://github.com/xtensor-stack/xtl.git
 [submodule "vendor/fmt"]
 	path = vendor/fmt
 	url = https://github.com/fmtlib/fmt.git
diff --git a/.mcp.json b/.mcp.json
new file mode 100644
index 000000000..bdfaa538e
--- /dev/null
+++ b/.mcp.json
@@ -0,0 +1,9 @@
+{
+  "mcpServers": {
+    "openmc-code-tools": {
+      "type": "stdio",
+      "command": "bash",
+      "args": [".claude/tools/start_server.sh"]
+    }
+  }
+}
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 3578144b2..7594ed5f2 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -7,9 +7,14 @@ build:
   jobs:
     post_checkout:
       - git fetch --unshallow || true
+      - cd docs/doxygen && doxygen && cd -
+
 sphinx:
   configuration: docs/source/conf.py
 
+formats:
+  - pdf
+
 python:
   install:
     - method: pip
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 000000000..19abba7d9
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,348 @@
+# OpenMC AI Coding Agent Instructions
+
+## Project Overview
+
+OpenMC is a Monte Carlo particle transport code for simulating nuclear reactors,
+fusion devices, or other systems with neutron/photon radiation. It's a hybrid
+C++17/Python codebase where:
+- **C++ core** (`src/`, `include/openmc/`) handles the computationally intensive transport simulation
+- **Python API** (`openmc/`) provides user-facing model building, post-processing, and depletion capabilities
+- **C API bindings** (`openmc/lib/`) wrap the C++ library via ctypes for runtime control
+
+## Architecture & Key Components
+
+### C++ Component Structure
+- **Global vectors of unique_ptrs**: Core objects like `model::cells`, `model::universes`, `nuclides` are stored as `vector<unique_ptr<T>>` in nested namespaces (`openmc::model`, `openmc::simulation`, `openmc::settings`, `openmc::data`)
+- **Custom container types**: OpenMC provides its own `vector`, `array`, `unique_ptr`, and `make_unique` in the `openmc::` namespace (defined in `vector.h`, `array.h`, `memory.h`). These are currently typedefs to `std::` equivalents but may become custom implementations for accelerator support. Always use `openmc::vector`, not `std::vector`.
+- **Geometry systems**:
+  - **CSG (default)**: Arbitrarily complex Constructive Solid Geometry using `Surface`, `Region`, `Cell`, `Universe`, `Lattice`
+  - **DAGMC**: CAD-based geometry via Direct Accelerated Geometry Monte Carlo (optional, requires `OPENMC_USE_DAGMC`)
+  - **Unstructured mesh**: libMesh-based geometry (optional, requires `OPENMC_USE_LIBMESH`)
+- **Particle tracking**: `Particle` class with `GeometryState` manages particle transport through geometry
+- **Tallies**: Score quantities during simulation via `Filter` and `Tally` objects
+- **Random ray solver**: Alternative deterministic method in `src/random_ray/`
+- **Optional features**: DAGMC (CAD geometry), libMesh (unstructured mesh), MPI, all controlled by `#ifdef OPENMC_MPI`, etc.
+
+### Python Component Structure
+- **ID management**: All geometry objects (Cell, Surface, Material, etc.) inherit from `IDManagerMixin` which auto-assigns unique integer IDs and tracks them via class-level `used_ids` and `next_id`
+- **Input validation**: Extensive use of `openmc.checkvalue` module functions (`check_type`, `check_value`, `check_length`) for all setters
+- **XML I/O**: Most classes implement `to_xml_element()` and `from_xml_element()` for serialization to OpenMC's XML input format
+- **HDF5 output**: Post-simulation data in statepoint files read via `openmc.StatePoint`
+- **Depletion**: `openmc.deplete` implements burnup via operator-splitting with various integrators (Predictor, CECM, etc.)
+- **Nuclear Data**: `openmc.data` provides programmatic access to nuclear data files (ENDF, ACE, HDF5)
+
+## Git Branching Workflow
+
+OpenMC uses a git flow branching model with two primary branches:
+
+- **`develop` branch**: The main development branch where all ongoing development takes place. This is the **primary branch against which pull requests are submitted and merged**. This branch is not guaranteed to be stable and may contain work-in-progress features.
+- **`master` branch**: The stable release branch containing the latest stable release of OpenMC. This branch only receives merges from `develop` when the development team decides a release should occur.
+
+### Instructions for Code Review
+
+When reviewing code changes in this repository, use the `reviewing-openmc-code` skill.
+
+## Codebase Navigation Tools
+
+Two MCP tools are registered in `.mcp.json` at the repo root and appear
+automatically in any MCP-capable agent session.
+
+**`openmc_rag_search`** — Semantic search across the codebase (C++, Python, RST
+docs). Finds code by meaning, not just text match. Surfaces related code across
+subsystems even when naming differs (e.g., "particle RNG seeding" finds code
+across transport, restart, and random ray modes — files you would never find
+with `grep "particle seed"`). The index uses a small 22M-param embedding model
+(384-dim). Phrase-level natural-language queries work much better than single
+keywords or symbol names.
+
+**`openmc_rag_rebuild`** — Rebuild the RAG vector index. Call after pulling new
+code or switching branches. The first RAG search of each session will report
+the index status and ask whether to rebuild — you can also call this explicitly.
+
+### Why RAG matters
+
+OpenMC is large enough that changes in one subsystem can silently break
+invariants that distant subsystems depend on — and those distant files often
+use different naming, so grep won't find them. The RAG search finds code by
+meaning, surfacing files you wouldn't have thought to open.
+
+An agent reviewed a large OpenMC PR without RAG. It found 1 of 11 serious
+bugs. Its post-mortem:
+
+> **I treated the diff as a closed system.** I verified internal consistency of
+> the changed code obsessively, but never built a global understanding of how
+> the changed code fits into the wider codebase. The diff altered assumptions
+> that code elsewhere silently relied on — but I couldn't see that because I
+> never looked beyond the diff. I couldn't see the forest for the trees.
+>
+> **Why I resisted RAG:** Overconfidence. My internal model was "I can see the
+> diff, I understand the data structures, I can trace the logic." The diff felt
+> self-contained. RAG felt like it would return noisy results about tangentially
+> related code. But in a codebase this large, changes in one subsystem can
+> quietly break invariants that distant subsystems depend on — and you need
+> global awareness to foresee that.
+>
+> **In the post-mortem**, I re-ran the RAG queries I should have run during the
+> review. They directly surfaced the files containing the bugs I missed — files
+> I never thought to open because they weren't in the diff.
+
+The takeaway: when reviewing or modifying code, ask yourself "what else in this
+codebase might depend on the behavior I'm changing?" If you aren't sure, that's
+a good time for a RAG query. It won't replace the grep-based investigation you
+should already be doing — but it can surface files you wouldn't have thought to
+open.
+
+### Workflow for contributors
+
+1. Create a feature/bugfix branch off `develop`
+2. Make changes and commit to the feature branch
+3. Open a pull request to merge the feature branch into `develop`
+4. A committer reviews and merges the PR into `develop`
+
+## Critical Build & Test Workflows
+
+### Build Dependencies
+- **C++17 compiler**: GCC, Clang, or Intel
+- **CMake** (3.16+): Required for configuring and building the C++ library
+- **HDF5**: Required for cross section data and output file formats
+- **libpng**: Used for generating visualization when OpenMC is run in plotting mode
+
+Without CMake and HDF5, OpenMC cannot be compiled.
+
+### Building the C++ Library
+```bash
+# Configure with CMake (from build/ directory)
+cmake .. -DOPENMC_USE_MPI=ON -DOPENMC_USE_OPENMP=ON -DCMAKE_BUILD_TYPE=RelWithDebInfo
+
+# Available CMake options (all default OFF except OPENMC_USE_OPENMP and OPENMC_BUILD_TESTS):
+# -DOPENMC_USE_OPENMP=ON/OFF        # OpenMP parallelism
+# -DOPENMC_USE_MPI=ON/OFF           # MPI support
+# -DOPENMC_USE_DAGMC=ON/OFF         # CAD geometry support
+# -DOPENMC_USE_LIBMESH=ON/OFF       # Unstructured mesh
+# -DOPENMC_ENABLE_PROFILE=ON/OFF    # Profiling flags
+# -DOPENMC_ENABLE_COVERAGE=ON/OFF   # Coverage analysis
+
+# Build
+make -j
+
+# C++ unit tests (uses Catch2)
+ctest
+```
+
+### Python Development
+```bash
+# Install in development mode (requires building C++ library first)
+pip install -e .
+
+# Python tests (uses pytest)
+pytest tests/unit_tests/           # Fast unit tests
+pytest tests/regression_tests/     # Full regression suite (requires nuclear data)
+```
+
+### Nuclear Data Setup (CRITICAL for Running OpenMC)
+Most tests require the NNDC HDF5 nuclear cross-section library.
+
+**Important**: Check if `OPENMC_CROSS_SECTIONS` is already set in the user's
+environment before downloading, as many users already have nuclear data
+installed. Though do note that if this variable is present that it may point to
+different cross section data and that the NNDC data is required for tests to
+pass.
+
+**If not already configured, download and setup:**
+```bash
+# Download NNDC HDF5 cross section library (~800 MB compressed)
+wget -q -O - https://anl.box.com/shared/static/teaup95cqv8s9nn56hfn7ku8mmelr95p.xz | tar -C $HOME -xJ
+
+# Set environment variable (add to ~/.bashrc or ~/.zshrc for persistence)
+export OPENMC_CROSS_SECTIONS=$HOME/nndc_hdf5/cross_sections.xml
+```
+
+**Alternative**: Use the provided download script (checks if data exists before downloading):
+```bash
+bash tools/ci/download-xs.sh  # Downloads both NNDC HDF5 and ENDF/B-VII.1 data
+```
+
+Without this data, regression tests will fail with "No cross_sections.xml file
+found" errors, or, in the case that alternative cross section data is configured
+the tests will execute but will not pass. The `cross_sections.xml` file is an
+index listing paths to individual HDF5 nuclear data files for each nuclide.
+
+## Testing Expectations
+
+### Environment Requirements
+
+  - **Data**: As described above, OpenMC's test suite requires OpenMC to be configured with NNDC data.
+  - **OpenMP Settings**: OpenMC's tests may fail is more than two OpenMP threads are used. The environment variable `OMP_NUM_THREADS=2` should be set to avoid sporadic test failures.
+  - **Executable configuration**: The OpenMC executable should compiled with debug symbols enabled.
+
+### C++ Tests
+Located in `tests/cpp_unit_tests/`, use Catch2 framework. Run via `ctest` after building with `-DOPENMC_BUILD_TESTS=ON`.
+
+### Python Unit Tests
+Located in `tests/unit_tests/`, these are fast, standalone tests that verify Python API functionality without running full simulations. Use standard pytest patterns:
+
+**Categories**:
+- **API validation**: Test object creation, property setters/getters, XML serialization (e.g., `test_material.py`, `test_cell.py`, `test_source.py`)
+- **Data processing**: Test nuclear data handling, cross sections, depletion chains (e.g., `test_data_neutron.py`, `test_deplete_chain.py`)
+- **Library bindings**: Test `openmc.lib` ctypes interface with `model.init_lib()`/`model.finalize_lib()` (e.g., `test_lib.py`)
+- **Geometry operations**: Test bounding boxes, containment, lattice generation (e.g., `test_bounding_box.py`, `test_lattice.py`)
+
+**Common patterns**:
+- Use fixtures from `tests/unit_tests/conftest.py` (e.g., `uo2`, `water`, `sphere_model`)
+- Test invalid inputs with `pytest.raises(ValueError)` or `pytest.raises(TypeError)`
+- Use `run_in_tmpdir` fixture for tests that create files
+- Tests with `openmc.lib` require calling `model.init_lib()` in try/finally with `model.finalize_lib()`
+
+**Example**:
+```python
+def test_material_properties():
+    m = openmc.Material()
+    m.add_nuclide('U235', 1.0)
+    assert 'U235' in m.nuclides
+
+    with pytest.raises(TypeError):
+        m.add_nuclide('H1', '1.0')  # Invalid type
+```
+
+Unit tests should be fast. For tests requiring simulation output, use regression tests instead.
+
+### Python Regression Tests
+Regression tests compare OpenMC output against reference data. **Prefer using existing models from `openmc.examples` or those found in tests/unit_tests/conftest.py** (like `pwr_pin_cell()`, `pwr_assembly()`, `slab_mg()`) rather than building from scratch.
+
+**Test Harness Types** (in `tests/testing_harness.py`):
+- **PyAPITestHarness**: Standard harness for Python API tests. Compares `inputs_true.dat` (XML hash) and `results_true.dat` (statepoint k-eff and tally values). Requires `model.xml` generation.
+- **HashedPyAPITestHarness**: Like PyAPITestHarness but hashes the results for compact comparison
+- **TolerantPyAPITestHarness**: For tests with floating-point non-associativity (e.g., random ray solver with single precision). Uses relative tolerance comparisons.
+- **WeightWindowPyAPITestHarness**: Compares weight window bounds from `weight_windows.h5`
+- **CollisionTrackTestHarness**: Compares collision track data from `collision_track.h5` against `collision_track_true.h5`
+- **TestHarness**: Base harness for XML-based tests (no Python model building)
+- **PlotTestHarness**: Compares plot output files (PNG or voxel HDF5)
+- **CMFDTestHarness**: Specialized for CMFD acceleration tests
+- **ParticleRestartTestHarness**: Tests particle restart functionality
+
+Almost all cases use either `PyAPITestHarness` or `HashedPyAPITestHarness`
+
+**Example Test**:
+```python
+from openmc.examples import pwr_pin_cell
+from tests.testing_harness import PyAPITestHarness
+
+def test_my_feature():
+    model = pwr_pin_cell()
+    model.settings.particles = 1000  # Modify to exercise feature
+    harness = PyAPITestHarness('statepoint.10.h5', model)
+    harness.main()
+```
+
+**Workflow**: Create `test.py` and `__init__.py` in `tests/regression_tests/my_test/`, run `pytest --update` to generate reference files (`inputs_true.dat`, `results_true.dat`, etc.), then verify with `pytest` without `--update`. Test results should be generated with `-DOPENMC_ENABLE_STRICT_FP=on` to ensure reproducibility across platforms and optimization levels.
+
+**Critical**: When modifying OpenMC code, regenerate affected test references with `pytest --update` and commit updated reference files.
+
+### Test Configuration
+
+`pytest.ini` sets: `python_files = test*.py`, `python_classes = NoThanks` (disables class-based test collection).
+
+### Testing Options
+
+For builds of OpenMC with MPI enabled, the `--mpi` flag should be passed to the test suite to ensure that appropriate tests are executed using two MPI processes.
+
+The entire test suite can be executed with OpenMC running in event-based mode (instead of the default history-based mode) by providing the `--event` flag to the `pytest` command.
+
+## Cross-Language Boundaries
+
+The C API (defined in `include/openmc/capi.h`) exposes C++ functionality to Python via ctypes bindings in `openmc/lib/`. Example:
+```cpp
+// C++ API in capi.h
+extern "C" int openmc_run();
+
+// Python binding in openmc/lib/core.py
+_dll.openmc_run.restype = c_int
+def run():
+    _dll.openmc_run()
+```
+
+When modifying C++ public APIs, update corresponding ctypes signatures in `openmc/lib/*.py`.
+
+## Code Style & Conventions
+
+### C++ Style (enforced by .clang-format)
+  OpenMC generally tries to follow C++ core guidelines where possible
+  (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines) and follow
+  modern C++ practices (e.g. RAII) whenever possible.
+
+- **Naming**:
+  - Classes: `CamelCase` (e.g., `HexLattice`)
+  - Functions/methods: `snake_case` (e.g., `get_indices`)
+  - Variables: `snake_case` with trailing underscore for class members (e.g., `n_particles_`, `energy_`)
+  - Constants: `UPPER_SNAKE_CASE` (e.g., `SQRT_PI`)
+- **Namespaces**: All code in `openmc::` namespace, global state in sub-namespaces
+- **Include order**: Related header first, then C/C++ stdlib, third-party libs, local headers
+- **Comments**: C++-style (`//`) only, never C-style (`/* */`)
+- **Standard**: C++17 features allowed
+- **Formatting**: Run `clang-format` (version 18) before committing; install via `tools/dev/install-commit-hooks.sh`
+
+### Python Style
+- **PEP8** compliant
+- **Docstrings**: numpydoc format for all public functions/methods
+- **Type hints**: Use sparingly, primarily for complex signatures
+- **Path handling**: Use `pathlib.Path` for filesystem operations, accept `str | os.PathLike` in function arguments
+- **Dependencies**: Core dependencies only (numpy, scipy, h5py, pandas, matplotlib, lxml, ipython, uncertainties, endf). Other packages must be optional
+- **Python version**: Minimum 3.11 (as of Nov 2025)
+
+### ID Management Pattern (Python)
+When creating geometry objects, IDs can be auto-assigned or explicit:
+```python
+# Auto-assigned ID
+cell = openmc.Cell()  # Gets next available ID
+
+# Explicit ID
+cell = openmc.Cell(id=10)  # Warning if ID already used
+
+# Reset all IDs (useful in test fixtures)
+openmc.reset_auto_ids()
+```
+
+### Input Validation Pattern (Python)
+All setters use checkvalue functions:
+```python
+import openmc.checkvalue as cv
+
+@property
+def temperature(self):
+    return self._temperature
+
+@temperature.setter
+def temperature(self, temp):
+    cv.check_type('temperature', temp, Real)
+    cv.check_greater_than('temperature', temp, 0.0)
+    self._temperature = temp
+```
+
+### Working with HDF5 Files
+C++ uses custom HDF5 wrappers in `src/hdf5_interface.cpp`. Python uses h5py directly. Statepoint format version is `VERSION_STATEPOINT` in `include/openmc/constants.h`.
+
+### Conditional Compilation
+Check for optional features:
+```cpp
+#ifdef OPENMC_MPI
+  // MPI-specific code
+#endif
+
+#ifdef OPENMC_DAGMC
+  // DAGMC-specific code
+#endif
+```
+
+## Documentation
+
+- **User docs**: Sphinx documentation in `docs/source/` hosted at https://docs.openmc.org
+- **C++ docs**: Doxygen-style comments with `\brief`, `\param` tags
+- **Python docs**: numpydoc format docstrings
+
+## Common Pitfalls
+
+1. **Forgetting nuclear data**: Tests fail without `OPENMC_CROSS_SECTIONS` environment variable
+2. **ID conflicts**: Python objects with duplicate IDs trigger `IDWarning`, use `reset_auto_ids()` between tests
+3. **MPI builds**: Code must work with and without MPI; use `#ifdef OPENMC_MPI` guards
+4. **Path handling**: Use `pathlib.Path` in new Python code, not `os.path`
+5. **Clang-format version**: CI uses version 18; other versions may produce different formatting
diff --git a/CITATION.cff b/CITATION.cff
new file mode 100644
index 000000000..ab27d89b8
--- /dev/null
+++ b/CITATION.cff
@@ -0,0 +1,68 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+title: OpenMC
+authors:
+- family-names: Romano
+  given-names: Paul K.
+  orcid: "https://orcid.org/0000-0002-1147-045X"
+- family-names: Shriwise
+  given-names: Patrick C.
+  orcid: "https://orcid.org/0000-0002-3979-7665"
+- family-names: Shimwell
+  given-names: Jonathan
+  orcid: "https://orcid.org/0000-0001-6909-0946"
+- family-names: Harper
+  given-names: Sterling
+- family-names: Boyd
+  given-names: Will
+- family-names: Nelson
+  given-names: Adam G.
+  orcid: "https://orcid.org/0000-0002-3614-0676"
+- family-names: Tramm
+  given-names: John R.
+  orcid: "https://orcid.org/0000-0002-5397-4402"
+- family-names: Ridley
+  given-names: Gavin
+  orcid: "https://orcid.org/0000-0003-1635-8042"
+- family-names: Johnson
+  given-names: Andrew
+  orcid: "https://orcid.org/0000-0003-2125-8775"
+- family-names: Peterson
+  given-names: Ethan E.
+  orcid: "https://orcid.org/0000-0002-5694-7194"
+- family-names: Herman
+  given-names: Bryan R.
+preferred-citation:
+  authors:
+  - family-names: Romano
+    given-names: Paul K.
+    orcid: "https://orcid.org/0000-0002-1147-045X"
+  - family-names: Horelik
+    given-names: Nicholas E.
+  - family-names: Herman
+    given-names: Bryan R.
+  - family-names: Nelson
+    given-names: Adam G.
+    orcid: "https://orcid.org/0000-0002-3614-0676"
+  - family-names: Forget
+    given-names: Benoit
+    orcid: "https://orcid.org/0000-0003-1459-7672"
+  - family-names: Smith
+    given-names: Kord
+  contact:
+  - family-names: Romano
+    given-names: Paul K.
+    orcid: "https://orcid.org/0000-0002-1147-045X"
+  doi: 10.1016/j.anucene.2014.07.048
+  issn: 0306-4549
+  volume: 82
+  journal: Annals of Nuclear Energy
+  publisher:
+    name: Elsevier
+  start: 90
+  end: 97
+  year: 2015
+  month: 8
+  title: "OpenMC: A state-of-the-art Monte Carlo code for research and development"
+  type: article
+  url: "https://doi.org/10.1016/j.anucene.2014.07.048"
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 000000000..9538b5ddc
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,14 @@
+## OpenMC Codebase Tools
+
+Read the FULL `AGENTS.md` in this directory before starting work. It contains
+project context, coding conventions, and documentation of the RAG search tools
+registered in `.mcp.json`.
+
+### Claude Code-specific: first-call behavior
+
+The first `openmc_rag_search` call of each session returns an index status
+message instead of search results. When this happens, you MUST use the
+`AskUserQuestion` tool to present the rebuild/use-existing choice to the user.
+Do not ask conversationally — always use the widget. Do not skip this step even
+if the index looks current — the user may have uncommitted changes that warrant
+a rebuild.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 461abe508..61d2cc6c5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,6 +20,11 @@ set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 
+# Generate compile_commands.json for clangd and other tools
+if("${CMAKE_EXPORT_COMPILE_COMMANDS}" STREQUAL "")
+  set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+endif()
+
 # Enable correct usage of CXX_EXTENSIONS
 if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.22)
   cmake_policy(SET CMP0128 NEW)
@@ -36,8 +41,9 @@ option(OPENMC_ENABLE_COVERAGE "Compile with coverage analysis flags"
 option(OPENMC_USE_DAGMC       "Enable support for DAGMC (CAD) geometry"              OFF)
 option(OPENMC_USE_LIBMESH     "Enable support for libMesh unstructured mesh tallies" OFF)
 option(OPENMC_USE_MPI         "Enable MPI"                                           OFF)
-option(OPENMC_USE_MCPL        "Enable MCPL"                                          OFF)
 option(OPENMC_USE_UWUW        "Enable UWUW"                                          OFF)
+option(OPENMC_FORCE_VENDORED_LIBS "Explicitly use submodules defined in 'vendor'"    OFF)
+option(OPENMC_ENABLE_STRICT_FP "Enable strict FP flags to improve test portability"  OFF)
 
 message(STATUS "OPENMC_USE_OPENMP ${OPENMC_USE_OPENMP}")
 message(STATUS "OPENMC_BUILD_TESTS ${OPENMC_BUILD_TESTS}")
@@ -46,8 +52,9 @@ message(STATUS "OPENMC_ENABLE_COVERAGE ${OPENMC_ENABLE_COVERAGE}")
 message(STATUS "OPENMC_USE_DAGMC ${OPENMC_USE_DAGMC}")
 message(STATUS "OPENMC_USE_LIBMESH ${OPENMC_USE_LIBMESH}")
 message(STATUS "OPENMC_USE_MPI ${OPENMC_USE_MPI}")
-message(STATUS "OPENMC_USE_MCPL ${OPENMC_USE_MCPL}")
 message(STATUS "OPENMC_USE_UWUW ${OPENMC_USE_UWUW}")
+message(STATUS "OPENMC_FORCE_VENDORED_LIBS ${OPENMC_FORCE_VENDORED_LIBS}")
+message(STATUS "OPENMC_ENABLE_STRICT_FP ${OPENMC_ENABLE_STRICT_FP}")
 
 # Warnings for deprecated options
 foreach(OLD_OPT IN ITEMS "openmp" "profile" "coverage" "dagmc" "libmesh")
@@ -89,6 +96,19 @@ if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "Choose the type of build" FORCE)
 endif()
 
+#===============================================================================
+# When STRICT_FP is enabled, remove NDEBUG from RelWithDebInfo flags so that
+# assert() remains active. CMake normally adds -DNDEBUG for both Release and
+# RelWithDebInfo, which disables C/C++ assert() statements.
+#===============================================================================
+
+if(OPENMC_ENABLE_STRICT_FP)
+  foreach(FLAG_VAR CMAKE_CXX_FLAGS_RELWITHDEBINFO CMAKE_C_FLAGS_RELWITHDEBINFO)
+    string(REPLACE "-DNDEBUG" "" ${FLAG_VAR} "${${FLAG_VAR}}")
+    string(REPLACE "/DNDEBUG" "" ${FLAG_VAR} "${${FLAG_VAR}}")
+  endforeach()
+endif()
+
 #===============================================================================
 # OpenMP for shared-memory parallelism (and GPU support some day!)
 #===============================================================================
@@ -189,19 +209,30 @@ if(${HDF5_VERSION} VERSION_GREATER_EQUAL 1.12.0)
   list(APPEND cxxflags -DH5Oget_info_by_idx_vers=1 -DH5O_info_t_vers=1)
 endif()
 
-#===============================================================================
-# MCPL
-#===============================================================================
-
-if (OPENMC_USE_MCPL)
-  find_package(MCPL REQUIRED)
-  message(STATUS "Found MCPL: ${MCPL_DIR} (found version \"${MCPL_VERSION}\")")
-endif()
-
 #===============================================================================
 # Set compile/link flags based on which compiler is being used
 #===============================================================================
 
+# When OPENMC_ENABLE_STRICT_FP is enabled, disable compiler optimizations that change
+# floating-point results relative to -O0, improving cross-platform and
+# cross-optimization-level reproducibility for regression testing:
+#   -ffp-contract=off    Prevents FMA contraction (fused multiply-add changes rounding)
+#   -fno-builtin         Prevents replacing math function calls (pow, exp, log, etc.)
+#                        with builtin versions that may differ from libm
+# By default (OFF), the compiler is free to use all optimizations for best
+# performance.
+if(OPENMC_ENABLE_STRICT_FP)
+  include(CheckCXXCompilerFlag)
+  check_cxx_compiler_flag(-ffp-contract=off SUPPORTS_FP_CONTRACT_OFF)
+  if(SUPPORTS_FP_CONTRACT_OFF)
+    list(APPEND cxxflags -ffp-contract=off)
+  endif()
+  check_cxx_compiler_flag(-fno-builtin SUPPORTS_NO_BUILTIN)
+  if(SUPPORTS_NO_BUILTIN)
+    list(APPEND cxxflags -fno-builtin)
+  endif()
+endif()
+
 # Skip for Visual Studio which has its own configurations through GUI
 if(NOT MSVC)
 
@@ -249,31 +280,30 @@ endif()
 # pugixml library
 #===============================================================================
 
-find_package_write_status(pugixml)
-if (NOT pugixml_FOUND)
+if(OPENMC_FORCE_VENDORED_LIBS)
   add_subdirectory(vendor/pugixml)
   set_target_properties(pugixml PROPERTIES CXX_STANDARD 14 CXX_EXTENSIONS OFF)
+else()
+  find_package_write_status(pugixml)
+  if (NOT pugixml_FOUND)
+    add_subdirectory(vendor/pugixml)
+    set_target_properties(pugixml PROPERTIES CXX_STANDARD 14 CXX_EXTENSIONS OFF)
+  endif()
 endif()
 
 #===============================================================================
 # {fmt} library
 #===============================================================================
 
-find_package_write_status(fmt)
-if (NOT fmt_FOUND)
+if(OPENMC_FORCE_VENDORED_LIBS)
   set(FMT_INSTALL ON CACHE BOOL "Generate the install target.")
   add_subdirectory(vendor/fmt)
-endif()
-
-#===============================================================================
-# xtensor header-only library
-#===============================================================================
-
-find_package_write_status(xtensor)
-if (NOT xtensor_FOUND)
-  add_subdirectory(vendor/xtl)
-  set(xtl_DIR ${CMAKE_CURRENT_BINARY_DIR}/vendor/xtl)
-  add_subdirectory(vendor/xtensor)
+else()
+  find_package_write_status(fmt)
+  if (NOT fmt_FOUND)
+    set(FMT_INSTALL ON CACHE BOOL "Generate the install target.")
+    add_subdirectory(vendor/fmt)
+  endif()
 endif()
 
 #===============================================================================
@@ -281,9 +311,13 @@ endif()
 #===============================================================================
 
 if(OPENMC_BUILD_TESTS)
-  find_package_write_status(Catch2)
-  if (NOT Catch2_FOUND)
+  if (OPENMC_FORCE_VENDORED_LIBS)
     add_subdirectory(vendor/Catch2)
+  else()
+    find_package_write_status(Catch2)
+    if (NOT Catch2_FOUND)
+      add_subdirectory(vendor/Catch2)
+    endif()
   endif()
 endif()
 
@@ -321,12 +355,14 @@ endif()
 #===============================================================================
 
 list(APPEND libopenmc_SOURCES
+  src/atomic_mass.cpp
   src/bank.cpp
   src/boundary_condition.cpp
   src/bremsstrahlung.cpp
   src/cell.cpp
   src/chain.cpp
   src/cmfd_solver.cpp
+  src/collision_track.cpp
   src/cross_sections.cpp
   src/dagmc.cpp
   src/distribution.cpp
@@ -343,6 +379,7 @@ list(APPEND libopenmc_SOURCES
   src/geometry.cpp
   src/geometry_aux.cpp
   src/hdf5_interface.cpp
+  src/ifp.cpp
   src/initialize.cpp
   src/lattice.cpp
   src/material.cpp
@@ -359,6 +396,7 @@ list(APPEND libopenmc_SOURCES
   src/particle.cpp
   src/particle_data.cpp
   src/particle_restart.cpp
+  src/particle_type.cpp
   src/photon.cpp
   src/physics.cpp
   src/physics_common.cpp
@@ -374,6 +412,7 @@ list(APPEND libopenmc_SOURCES
   src/random_ray/linear_source_domain.cpp
   src/random_ray/moment_matrix.cpp
   src/random_ray/source_region.cpp
+  src/ray.cpp
   src/reaction.cpp
   src/reaction_product.cpp
   src/scattdata.cpp
@@ -406,17 +445,21 @@ list(APPEND libopenmc_SOURCES
   src/tallies/filter_materialfrom.cpp
   src/tallies/filter_mesh.cpp
   src/tallies/filter_meshborn.cpp
+  src/tallies/filter_meshmaterial.cpp
   src/tallies/filter_meshsurface.cpp
   src/tallies/filter_mu.cpp
   src/tallies/filter_musurface.cpp
   src/tallies/filter_parent_nuclide.cpp
   src/tallies/filter_particle.cpp
+  src/tallies/filter_particle_production.cpp
   src/tallies/filter_polar.cpp
+  src/tallies/filter_reaction.cpp
   src/tallies/filter_sph_harm.cpp
   src/tallies/filter_sptl_legendre.cpp
   src/tallies/filter_surface.cpp
   src/tallies/filter_time.cpp
   src/tallies/filter_universe.cpp
+  src/tallies/filter_weight.cpp
   src/tallies/filter_zernike.cpp
   src/tallies/tally.cpp
   src/tallies/tally_scoring.cpp
@@ -480,7 +523,7 @@ endif()
 # target_link_libraries treats any arguments starting with - but not -l as
 # linker flags. Thus, we can pass both linker flags and libraries together.
 target_link_libraries(libopenmc ${ldflags} ${HDF5_LIBRARIES} ${HDF5_HL_LIBRARIES}
-                      xtensor fmt::fmt ${CMAKE_DL_LIBS})
+                      fmt::fmt ${CMAKE_DL_LIBS})
 
 if(TARGET pugixml::pugixml)
   target_link_libraries(libopenmc pugixml::pugixml)
@@ -489,11 +532,11 @@ else()
 endif()
 
 if(OPENMC_USE_DAGMC)
-  target_compile_definitions(libopenmc PRIVATE DAGMC)
+  target_compile_definitions(libopenmc PUBLIC OPENMC_DAGMC_ENABLED)
   target_link_libraries(libopenmc dagmc-shared)
 
   if(OPENMC_USE_UWUW)
-    target_compile_definitions(libopenmc PRIVATE OPENMC_UWUW)
+    target_compile_definitions(libopenmc PRIVATE OPENMC_UWUW_ENABLED)
     target_link_libraries(libopenmc uwuw-shared)
   endif()
 elseif(OPENMC_USE_UWUW)
@@ -502,7 +545,7 @@ elseif(OPENMC_USE_UWUW)
 endif()
 
 if(OPENMC_USE_LIBMESH)
-  target_compile_definitions(libopenmc PRIVATE LIBMESH)
+  target_compile_definitions(libopenmc PRIVATE OPENMC_LIBMESH_ENABLED)
   target_link_libraries(libopenmc PkgConfig::LIBMESH)
 endif()
 
@@ -525,11 +568,6 @@ if (OPENMC_BUILD_TESTS)
   add_subdirectory(tests/cpp_unit_tests)
 endif()
 
-if (OPENMC_USE_MCPL)
-  target_compile_definitions(libopenmc PUBLIC OPENMC_MCPL)
-  target_link_libraries(libopenmc MCPL::mcpl)
-endif()
-
 #===============================================================================
 # Log build info that this executable can report later
 #===============================================================================
@@ -542,6 +580,9 @@ endif()
 if (OPENMC_ENABLE_COVERAGE)
   target_compile_definitions(libopenmc PRIVATE COVERAGEBUILD)
 endif()
+if (OPENMC_ENABLE_STRICT_FP)
+  target_compile_definitions(libopenmc PRIVATE OPENMC_ENABLE_STRICT_FP)
+endif()
 
 #===============================================================================
 # openmc executable
@@ -570,9 +611,7 @@ add_custom_command(TARGET libopenmc POST_BUILD
 #===============================================================================
 # Install executable, scripts, manpage, license
 #===============================================================================
-
-configure_file(cmake/OpenMCConfig.cmake.in "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/OpenMCConfig.cmake" @ONLY)
-configure_file(cmake/OpenMCConfigVersion.cmake.in "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/OpenMCConfigVersion.cmake" @ONLY)
+include(CMakePackageConfigHelpers)
 
 set(INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/cmake/OpenMC)
 install(TARGETS openmc libopenmc
@@ -586,10 +625,24 @@ install(EXPORT openmc-targets
   NAMESPACE OpenMC::
   DESTINATION ${INSTALL_CONFIGDIR})
 
+configure_package_config_file(
+    "cmake/OpenMCConfig.cmake.in"
+    "${CMAKE_BINARY_DIR}/${CMAKE_FILES_DIRECTORY}/OpenMCConfig.cmake"
+    INSTALL_DESTINATION ${INSTALL_CONFIGDIR}
+)
+
+write_basic_package_version_file(
+    "${CMAKE_BINARY_DIR}/${CMAKE_FILES_DIRECTORY}/OpenMCConfigVersion.cmake"
+    VERSION ${OPENMC_VERSION}
+    COMPATIBILITY AnyNewerVersion
+)
+
 install(FILES
-  "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/OpenMCConfig.cmake"
-  "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/OpenMCConfigVersion.cmake"
-  DESTINATION ${INSTALL_CONFIGDIR})
+    "${CMAKE_BINARY_DIR}/${CMAKE_FILES_DIRECTORY}/OpenMCConfig.cmake"
+    "${CMAKE_BINARY_DIR}/${CMAKE_FILES_DIRECTORY}/OpenMCConfigVersion.cmake"
+    DESTINATION "${INSTALL_CONFIGDIR}"
+)
+
 install(FILES man/man1/openmc.1 DESTINATION ${CMAKE_INSTALL_MANDIR}/man1)
 install(FILES LICENSE DESTINATION "${CMAKE_INSTALL_DOCDIR}" RENAME copyright)
 install(DIRECTORY include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
diff --git a/Dockerfile b/Dockerfile
index 1ae615a50..688e5a370 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -24,7 +24,7 @@ ARG compile_cores=1
 ARG build_dagmc=off
 ARG build_libmesh=off
 
-FROM debian:bookworm-slim AS dependencies
+FROM ubuntu:24.04 AS dependencies
 
 ARG compile_cores
 ARG build_dagmc
@@ -33,11 +33,6 @@ ARG build_libmesh
 # Set default value of HOME to /root
 ENV HOME=/root
 
-# Embree variables
-ENV EMBREE_TAG='v4.3.1'
-ENV EMBREE_REPO='https://github.com/embree/embree'
-ENV EMBREE_INSTALL_DIR=$HOME/EMBREE/
-
 # MOAB variables
 ENV MOAB_TAG='5.5.1'
 ENV MOAB_REPO='https://bitbucket.org/fathomteam/moab/'
@@ -58,10 +53,11 @@ ENV LIBMESH_REPO='https://github.com/libMesh/libmesh'
 ENV LIBMESH_INSTALL_DIR=$HOME/LIBMESH
 
 # NJOY variables
+ENV NJOY_TAG='2016.78'
 ENV NJOY_REPO='https://github.com/njoy/NJOY2016'
 
 # Setup environment variables for Docker image
-ENV LD_LIBRARY_PATH=${DAGMC_INSTALL_DIR}/lib:$LD_LIBRARY_PATH \
+ENV LD_LIBRARY_PATH=${DAGMC_INSTALL_DIR}/lib:${LD_LIBRARY_PATH:-} \
     OPENMC_ENDF_DATA=/root/endf-b-vii.1 \
     DEBIAN_FRONTEND=noninteractive
 
@@ -71,7 +67,7 @@ RUN apt-get update -y && \
     apt-get install -y \
         python3-pip python-is-python3 wget git build-essential cmake \
         mpich libmpich-dev libhdf5-serial-dev libhdf5-mpich-dev \
-        libpng-dev python3-venv && \
+        libpng-dev libpugixml-dev libfmt-dev catch2 python3-venv && \
     apt-get autoremove
 
 # create virtual enviroment to avoid externally managed environment error
@@ -83,7 +79,7 @@ RUN pip install --upgrade pip
 
 # Clone and install NJOY2016
 RUN cd $HOME \
-    && git clone --single-branch --depth 1 ${NJOY_REPO} \
+    && git clone --single-branch -b ${NJOY_TAG} --depth 1 ${NJOY_REPO} \
     && cd NJOY2016 \
     && mkdir build \
     && cd build \
@@ -94,21 +90,12 @@ RUN cd $HOME \
 
 RUN if [ "$build_dagmc" = "on" ]; then \
         # Install addition packages required for DAGMC
-        apt-get -y install libeigen3-dev libnetcdf-dev libtbb-dev libglfw3-dev \
+        apt-get -y install \
+            libeigen3-dev libnetcdf-dev libtbb-dev libglfw3-dev libembree-dev \
         && pip install --upgrade numpy \
-        # Clone and install EMBREE
-        && mkdir -p $HOME/EMBREE && cd $HOME/EMBREE \
-        && git clone --single-branch -b ${EMBREE_TAG} --depth 1 ${EMBREE_REPO} \
-        && mkdir build && cd build \
-        && cmake ../embree \
-                    -DCMAKE_INSTALL_PREFIX=${EMBREE_INSTALL_DIR} \
-                    -DEMBREE_MAX_ISA=NONE \
-                    -DEMBREE_ISA_SSE42=ON \
-                    -DEMBREE_ISPC_SUPPORT=OFF \
-        && make 2>/dev/null -j${compile_cores} install \
-        && rm -rf ${EMBREE_INSTALL_DIR}/build ${EMBREE_INSTALL_DIR}/embree ; \
+        && pip install --no-cache-dir setuptools cython \
         # Clone and install MOAB
-        mkdir -p $HOME/MOAB && cd $HOME/MOAB \
+        && mkdir -p $HOME/MOAB && cd $HOME/MOAB \
         && git clone  --single-branch -b ${MOAB_TAG} --depth 1 ${MOAB_REPO} \
         && mkdir build && cd build \
         && cmake ../moab -DCMAKE_BUILD_TYPE=Release \
@@ -117,6 +104,7 @@ RUN if [ "$build_dagmc" = "on" ]; then \
                       -DBUILD_SHARED_LIBS=OFF \
                       -DENABLE_FORTRAN=OFF \
                       -DENABLE_BLASLAPACK=OFF \
+                      -DENABLE_TESTING=OFF \
         && make 2>/dev/null -j${compile_cores} install \
         && cmake ../moab \
                     -DENABLE_PYMOAB=ON \
@@ -132,7 +120,7 @@ RUN if [ "$build_dagmc" = "on" ]; then \
         && mkdir build && cd build \
         && cmake ../double-down -DCMAKE_INSTALL_PREFIX=${DD_INSTALL_DIR} \
                              -DMOAB_DIR=/usr/local \
-                             -DEMBREE_DIR=${EMBREE_INSTALL_DIR} \
+                             -DEMBREE_DIR=/usr \
         && make 2>/dev/null -j${compile_cores} install \
         && rm -rf ${DD_INSTALL_DIR}/build ${DD_INSTALL_DIR}/double-down ; \
         # Clone and install DAGMC
@@ -146,6 +134,7 @@ RUN if [ "$build_dagmc" = "on" ]; then \
                        -DDOUBLE_DOWN_DIR=${DD_INSTALL_DIR} \
                        -DCMAKE_PREFIX_PATH=${DD_INSTALL_DIR}/lib \
                        -DBUILD_STATIC_LIBS=OFF \
+                       -DBUILD_TESTS=OFF \
         && make 2>/dev/null -j${compile_cores} install \
         && rm -rf ${DAGMC_INSTALL_DIR}/DAGMC ${DAGMC_INSTALL_DIR}/build ; \
     fi
diff --git a/LICENSE b/LICENSE
index 8a60b66bf..b5dfd1748 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,4 +1,4 @@
-Copyright (c) 2011-2025 Massachusetts Institute of Technology, UChicago Argonne
+Copyright (c) 2011-2026 Massachusetts Institute of Technology, UChicago Argonne
 LLC, and OpenMC contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of
diff --git a/cmake/Modules/GetVersionFromGit.cmake b/cmake/Modules/GetVersionFromGit.cmake
index 42b3725f2..3736955ff 100644
--- a/cmake/Modules/GetVersionFromGit.cmake
+++ b/cmake/Modules/GetVersionFromGit.cmake
@@ -37,11 +37,17 @@ if(EXISTS "${CMAKE_SOURCE_DIR}/.git" AND GIT_FOUND)
         WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
         OUTPUT_VARIABLE VERSION_STRING
         OUTPUT_STRIP_TRAILING_WHITESPACE
+        ERROR_QUIET
     )
 
-    # If no tags are found, instruct user to fetch them
+    # If no tags are found, set version to 0 and show a warning
     if(VERSION_STRING STREQUAL "")
-        message(FATAL_ERROR "No git tags found. Run 'git fetch --tags' and try again.")
+        set(VERSION_STRING "0.0.0")
+        message(WARNING 
+            "No git tags found. Version set to 0.0.0.\n"
+            "Run 'git fetch --tags' to ensure proper versioning.\n"
+            "For more information, see OpenMC developer documentation."
+        )
     endif()
 
     # Extract the commit hash
diff --git a/cmake/OpenMCConfig.cmake.in b/cmake/OpenMCConfig.cmake.in
index 5cab4790a..0e15060a3 100644
--- a/cmake/OpenMCConfig.cmake.in
+++ b/cmake/OpenMCConfig.cmake.in
@@ -1,11 +1,18 @@
-get_filename_component(OpenMC_CMAKE_DIR "${CMAKE_CURRENT_LIST_FILE}" DIRECTORY)
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/OpenMCConfigVersion.cmake")
+include(CMakeFindDependencyMacro)
+
+# Explicitly calculate prefix if it was not generated above
+if(NOT DEFINED PACKAGE_PREFIX_DIR)
+  get_filename_component(PACKAGE_PREFIX_DIR "${CMAKE_CURRENT_LIST_DIR}/../../.." ABSOLUTE)
+endif()
+
+find_dependency(fmt CONFIG REQUIRED HINTS ${PACKAGE_PREFIX_DIR})
+find_dependency(pugixml CONFIG REQUIRED HINTS ${PACKAGE_PREFIX_DIR})
 
-find_package(fmt REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../fmt)
-find_package(pugixml REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../pugixml)
-find_package(xtl REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../xtl)
-find_package(xtensor REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../xtensor)
 if(@OPENMC_USE_DAGMC@)
-  find_package(DAGMC REQUIRED HINTS @DAGMC_DIR@)
+  find_dependency(DAGMC REQUIRED HINTS @DAGMC_DIR@)
 endif()
 
 if(@OPENMC_USE_LIBMESH@)
@@ -15,24 +22,24 @@ if(@OPENMC_USE_LIBMESH@)
   pkg_check_modules(LIBMESH REQUIRED @LIBMESH_PC_FILE@>=1.7.0 IMPORTED_TARGET)
 endif()
 
-find_package(PNG)
-
-if(NOT TARGET OpenMC::libopenmc)
-  include("${OpenMC_CMAKE_DIR}/OpenMCTargets.cmake")
+if("@PNG_FOUND@")
+  find_dependency(PNG)
 endif()
 
 if(@OPENMC_USE_MPI@)
-  find_package(MPI REQUIRED)
+  find_dependency(MPI REQUIRED)
 endif()
 
 if(@OPENMC_USE_OPENMP@)
-  find_package(OpenMP REQUIRED)
-endif()
-
-if(@OPENMC_USE_MCPL@)
-  find_package(MCPL REQUIRED)
+  find_dependency(OpenMP REQUIRED)
 endif()
 
 if(@OPENMC_USE_UWUW@ AND NOT ${DAGMC_BUILD_UWUW})
   message(FATAL_ERROR "UWUW is enabled in OpenMC but the DAGMC installation discovered was not configured with UWUW.")
 endif()
+
+include("${CMAKE_CURRENT_LIST_DIR}/OpenMCTargets.cmake")
+
+if(NOT OpenMC_FIND_QUIETLY)
+    message(STATUS "Found OpenMC: ${PACKAGE_VERSION} (found in ${PACKAGE_PREFIX_DIR})")
+endif()
diff --git a/cmake/OpenMCConfigVersion.cmake.in b/cmake/OpenMCConfigVersion.cmake.in
deleted file mode 100644
index 90d345de4..000000000
--- a/cmake/OpenMCConfigVersion.cmake.in
+++ /dev/null
@@ -1,11 +0,0 @@
-set(PACKAGE_VERSION "@OPENMC_VERSION@")
-
-# Check whether the requested PACKAGE_FIND_VERSION is compatible
-if("${PACKAGE_VERSION}" VERSION_LESS "${PACKAGE_FIND_VERSION}")
-  set(PACKAGE_VERSION_COMPATIBLE FALSE)
-else()
-  set(PACKAGE_VERSION_COMPATIBLE TRUE)
-  if ("${PACKAGE_VERSION}" VERSION_EQUAL "${PACKAGE_FIND_VERSION}")
-    set(PACKAGE_VERSION_EXACT TRUE)
-  endif()
-endif()
diff --git a/docs/Makefile b/docs/Makefile
index a93338df3..e320432a2 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -45,6 +45,7 @@ help:
 clean:
 	-rm -rf $(BUILDDIR)/*
 	-rm -rf source/pythonapi/generated/
+	-rm -rf doxygen/xml
 
 html:
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
diff --git a/docs/doxygen/Doxyfile b/docs/doxygen/Doxyfile
new file mode 100644
index 000000000..75702eff7
--- /dev/null
+++ b/docs/doxygen/Doxyfile
@@ -0,0 +1,13 @@
+# Doxyfile 1.9.1
+
+# This file describes the settings to be used by the documentation system
+# doxygen (www.doxygen.org) for a project.
+
+# Difference with default Doxyfile 1.9.1
+PROJECT_NAME           = OpenMC
+QUIET                  = YES
+WARN_IF_UNDOCUMENTED   = NO
+INPUT                  = ../../include/openmc/capi.h
+GENERATE_HTML          = NO
+GENERATE_LATEX         = NO
+GENERATE_XML           = YES
diff --git a/docs/source/_images/sphere-mesh-vtk.png b/docs/source/_images/sphere-mesh-vtk.png
new file mode 100644
index 000000000..73b412bdd
Binary files /dev/null and b/docs/source/_images/sphere-mesh-vtk.png differ
diff --git a/docs/source/capi/index.rst b/docs/source/capi/index.rst
index d9ac0d1e0..e924aa6e2 100644
--- a/docs/source/capi/index.rst
+++ b/docs/source/capi/index.rst
@@ -46,41 +46,29 @@ Type Definitions
 Functions
 ---------
 
-.. c:function:: int openmc_calculate_volumes()
+..
+   Once documentation is complete in capi.h, use:
+   .. doxygenfile:: capi.h
+   to populate this documentation without using
+   .. doxygenfunction::
+   for every function.
 
-   Run a stochastic volume calculation
+.. doxygenfunction:: openmc_calculate_volumes
 
-   :return: Return status (negative if an error occurred)
-   :rtype: int
+.. doxygenfunction:: openmc_cell_get_fill
 
-.. c:function:: int openmc_cell_get_fill(int32_t index, int* type, int32_t** indices, int32_t* n)
+.. doxygenfunction:: openmc_cell_get_id
 
-   Get the fill for a cell
+.. doxygenfunction:: openmc_cell_get_temperature
+
+.. c:function:: int openmc_cell_get_density(int32_t index, const int32_t* instance, double* density)
+
+   Get the density of a cell
 
    :param int32_t index: Index in the cells array
-   :param int* type: Type of the fill
-   :param int32_t** indices: Array of material indices for cell
-   :param int32_t* n: Length of indices array
-   :return: Return status (negative if an error occurred)
-   :rtype: int
-
-.. c:function:: int openmc_cell_get_id(int32_t index, int32_t* id)
-
-   Get the ID of a cell
-
-   :param int32_t index: Index in the cells array
-   :param int32_t* id: ID of the cell
-   :return: Return status (negative if an error occurred)
-   :rtype: int
-
-.. c:function:: int openmc_cell_get_temperature(int32_t index, const int32_t* instance, double* T)
-
-   Get the temperature of a cell
-
-   :param int32_t index: Index in the cells array
-   :param int32_t* instance: Which instance of the cell. If a null pointer is passed, the temperature
-                             of the first instance is returned.
-   :param double* T: temperature of the cell
+   :param int32_t* instance: Which instance of the cell. If a null pointer is passed, the density
+                             multiplier of the first instance is returned.
+   :param double* density: Density of the cell in [g/cm3]
    :return: Return status (negative if an error occurred)
    :rtype: int
 
@@ -113,8 +101,22 @@ Functions
    :param double T: Temperature in Kelvin
    :param instance: Which instance of the cell. To set the temperature for all
                     instances, pass a null pointer.
-   :param set_contained: If the cell is not filled by a material, whether to set the temperatures
-                         of all filled cells
+   :param bool set_contained: If the cell is not filled by a material, whether
+                              to set the temperatures of all filled cells
+   :type instance: const int32_t*
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_cell_set_density(index index, double density, const int32_t* instance, bool set_contained)
+
+   Set the density of a cell.
+
+   :param int32_t index: Index in the cells array
+   :param double density: Density of the cell in [g/cm3]
+   :param instance: Which instance of the cell. To set the density multiplier for all
+                    instances, pass a null pointer.
+   :param bool set_contained: If the cell is not filled by a material, whether
+                              to set the density multiplier of all filled cells
    :type instance: const int32_t*
    :return: Return status (negative if an error occurred)
    :rtype: int
@@ -555,6 +557,279 @@ Functions
    :return: Return status (negative if an error occurs)
    :rtype: int
 
+.. c:function:: int openmc_get_plot_index(int32_t id, int32_t* index)
+
+   Get the index in the plots array for a plot with a given ID.
+
+   :param int32_t id: Plot ID
+   :param int32_t* index: Index in the plots array
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_plot_get_id(int32_t index, int32_t* id)
+
+   Get the ID of a plot.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t* id: Plot ID
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_plot_set_id(int32_t index, int32_t id)
+
+   Set the ID of a plot.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t id: Plot ID
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: size_t openmc_plots_size()
+
+   Number of plots currently allocated.
+
+   :return: Number of plots in the plots array
+   :rtype: size_t
+
+.. c:function:: int openmc_solidraytrace_plot_create(int32_t* index)
+
+   Create a new solid raytrace plot.
+
+   :param int32_t* index: Index of the newly created plot
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_pixels(int32_t index, int32_t* width, int32_t* height)
+
+   Get output pixel dimensions for a solid raytrace plot.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t* width: Image width in pixels
+   :param int32_t* height: Image height in pixels
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_pixels(int32_t index, int32_t width, int32_t height)
+
+   Set output pixel dimensions for a solid raytrace plot.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t width: Image width in pixels
+   :param int32_t height: Image height in pixels
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_color_by(int32_t index, int32_t* color_by)
+
+   Get the domain type used for coloring (0=materials, 1=cells).
+
+   :param int32_t index: Index in the plots array
+   :param int32_t* color_by: Coloring mode
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_color_by(int32_t index, int32_t color_by)
+
+   Set the domain type used for coloring (0=materials, 1=cells).
+
+   :param int32_t index: Index in the plots array
+   :param int32_t color_by: Coloring mode
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_default_colors(int32_t index)
+
+   Set default random colors for the current ``color_by`` mode.
+
+   :param int32_t index: Index in the plots array
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_all_opaque(int32_t index)
+
+   Mark all domains in the current ``color_by`` mode as opaque.
+
+   :param int32_t index: Index in the plots array
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_opaque(int32_t index, int32_t id, bool visible)
+
+   Set whether a specific domain ID is opaque (visible) in the rendered image.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t id: Cell/material ID (based on ``color_by``)
+   :param bool visible: Whether the domain is opaque
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_color(int32_t index, int32_t id, uint8_t r, uint8_t g, uint8_t b)
+
+   Set RGB color for a specific domain ID.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t id: Cell/material ID (based on ``color_by``)
+   :param uint8_t r: Red channel
+   :param uint8_t g: Green channel
+   :param uint8_t b: Blue channel
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_color(int32_t index, int32_t id, uint8_t* r, uint8_t* g, uint8_t* b)
+
+   Get RGB color for a specific domain ID.
+
+   :param int32_t index: Index in the plots array
+   :param int32_t id: Cell/material ID (based on ``color_by``)
+   :param uint8_t* r: Red channel
+   :param uint8_t* g: Green channel
+   :param uint8_t* b: Blue channel
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_camera_position(int32_t index, double* x, double* y, double* z)
+
+   Get camera position.
+
+   :param int32_t index: Index in the plots array
+   :param double* x: X coordinate
+   :param double* y: Y coordinate
+   :param double* z: Z coordinate
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_camera_position(int32_t index, double x, double y, double z)
+
+   Set camera position.
+
+   :param int32_t index: Index in the plots array
+   :param double x: X coordinate
+   :param double y: Y coordinate
+   :param double z: Z coordinate
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_look_at(int32_t index, double* x, double* y, double* z)
+
+   Get camera target point.
+
+   :param int32_t index: Index in the plots array
+   :param double* x: X coordinate
+   :param double* y: Y coordinate
+   :param double* z: Z coordinate
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_look_at(int32_t index, double x, double y, double z)
+
+   Set camera target point.
+
+   :param int32_t index: Index in the plots array
+   :param double x: X coordinate
+   :param double y: Y coordinate
+   :param double z: Z coordinate
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_up(int32_t index, double* x, double* y, double* z)
+
+   Get the camera up vector.
+
+   :param int32_t index: Index in the plots array
+   :param double* x: X component
+   :param double* y: Y component
+   :param double* z: Z component
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_up(int32_t index, double x, double y, double z)
+
+   Set the camera up vector.
+
+   :param int32_t index: Index in the plots array
+   :param double x: X component
+   :param double y: Y component
+   :param double z: Z component
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_light_position(int32_t index, double* x, double* y, double* z)
+
+   Get light source position.
+
+   :param int32_t index: Index in the plots array
+   :param double* x: X coordinate
+   :param double* y: Y coordinate
+   :param double* z: Z coordinate
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_light_position(int32_t index, double x, double y, double z)
+
+   Set light source position.
+
+   :param int32_t index: Index in the plots array
+   :param double x: X coordinate
+   :param double y: Y coordinate
+   :param double z: Z coordinate
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_fov(int32_t index, double* fov)
+
+   Get horizontal field of view in degrees.
+
+   :param int32_t index: Index in the plots array
+   :param double* fov: Field of view in degrees
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_fov(int32_t index, double fov)
+
+   Set horizontal field of view in degrees.
+
+   :param int32_t index: Index in the plots array
+   :param double fov: Field of view in degrees
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_get_diffuse_fraction(int32_t index, double* diffuse_fraction)
+
+   Get diffuse-light fraction.
+
+   :param int32_t index: Index in the plots array
+   :param double* diffuse_fraction: Diffuse fraction in [0, 1]
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_set_diffuse_fraction(int32_t index, double diffuse_fraction)
+
+   Set diffuse-light fraction.
+
+   :param int32_t index: Index in the plots array
+   :param double diffuse_fraction: Diffuse fraction in [0, 1]
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_update_view(int32_t index)
+
+   Recompute internal camera/view transforms after camera changes.
+
+   :param int32_t index: Index in the plots array
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
+.. c:function:: int openmc_solidraytrace_plot_create_image(int32_t index, uint8_t* data_out, int32_t width, int32_t height)
+
+   Render the plot to an RGB image buffer.
+
+   :param int32_t index: Index in the plots array
+   :param uint8_t* data_out: Output buffer of shape ``height*width*3``
+   :param int32_t width: Image width in pixels
+   :param int32_t height: Image height in pixels
+   :return: Return status (negative if an error occurred)
+   :rtype: int
+
 .. c:function:: int openmc_reset()
 
    Resets all tally scores
@@ -576,6 +851,10 @@ Functions
    :return: Return status (negative if an error occurs)
    :rtype: int
 
+.. c:function:: void openmc_run_random_ray()
+
+   Run a random ray simulation
+
 .. c:function:: int openmc_set_n_batches(int32_t n_batches, bool set_max_batches, bool add_statepoint_batch)
 
    Set number of batches and number of max batches
diff --git a/docs/source/conf.py b/docs/source/conf.py
index 8aeff5cdc..61740239b 100644
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -11,7 +11,10 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 
-import sys, os
+import os
+from pathlib import Path
+import subprocess
+import sys
 
 # Determine if we're on Read the Docs server
 on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
@@ -37,6 +40,7 @@ sys.path.insert(0, os.path.abspath('../..'))
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
+    'breathe',
     'sphinx.ext.autodoc',
     'sphinx.ext.napoleon',
     'sphinx.ext.autosummary',
@@ -47,6 +51,8 @@ extensions = [
 ]
 if not on_rtd:
     extensions.append('sphinxcontrib.rsvgconverter')
+    doxygen_dir = Path(__file__).parents[1] / 'doxygen'
+    subprocess.run(['doxygen'], cwd=doxygen_dir, check=True)
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -62,7 +68,7 @@ master_doc = 'index'
 
 # General information about the project.
 project = 'OpenMC'
-copyright = '2011-2025, Massachusetts Institute of Technology, UChicago Argonne LLC, and OpenMC contributors'
+copyright = '2011-2026, Massachusetts Institute of Technology, UChicago Argonne LLC, and OpenMC contributors'
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
@@ -117,13 +123,16 @@ pygments_style = 'tango'
 # A list of ignored prefixes for module index sorting.
 #modindex_common_prefix = []
 
+# -- Options breathe + doxygen -------------------------------------------------
+
+breathe_projects = {"OpenMC": "../doxygen/xml"}
+breathe_default_project = "OpenMC"
+breathe_domain_by_file_pattern = {"*capi.h": "c"}
 
 # -- Options for HTML output ---------------------------------------------------
 
 # The theme to use for HTML and HTML Help pages
-import sphinx_rtd_theme
 html_theme = 'sphinx_rtd_theme'
-html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 html_baseurl = "https://docs.openmc.org/en/stable/"
 
 html_logo = '_images/openmc_logo.png'
diff --git a/docs/source/devguide/agentic-tools.rst b/docs/source/devguide/agentic-tools.rst
new file mode 100644
index 000000000..fed377cc0
--- /dev/null
+++ b/docs/source/devguide/agentic-tools.rst
@@ -0,0 +1,104 @@
+.. _devguide_agentic_tools:
+
+===========================
+Agentic Development Tools
+===========================
+
+OpenMC ships a set of tools designed for AI coding agents (such as
+`Claude Code`_) that agents can use to navigate and understand the codebase.
+
+.. _Claude Code: https://claude.ai/code
+
+Motivation
+----------
+
+Agentic tools like Claude Code are skilled at using grep to navigate and
+understand large code bases. However, grep can only find exact text matches —
+it cannot discover code that is *conceptually* related but uses different
+naming. Without a "global view" of the codebase that a human developer will
+build up over time, the agent is generally blind to any file it hasn't
+tokenized fully. While it can grep to see who else calls a function, it
+remains blind if other areas might be related but not share identical naming
+conventions.
+
+This problem is mitigated somewhat by using a model with a longer context
+window. OpenMC has somewhere around ~1 million tokens of C++ and ~1 million
+tokens of python. While Claude Code in early 2026 only has a context window
+of 200k tokens, beta versions have extended context windows of 1M tokens,
+and it's not unreasonable to assume that models may be available in the near
+future that greatly exceed these limits.
+
+However, even assuming the entire repository can be fit within a context
+window, there are several downsides to doing this.
+`Model performance degrades significantly as context size increases`_.
+Benchmark results are
+greatly improved if the model has less garbage to pick through. Additionally, API usage
+is typically billed as tokens in/out per turn. As the context file
+grows these costs become much larger. As such, there is still significant
+motivation to solving the above problem, so as to ensure only relevant
+information is drawn into context so as to maximize model performance and
+minimize costs.
+
+Setup
+-----
+
+The tools are registered as an `MCP (Model Context Protocol)`_ server in
+``.mcp.json`` at the repository root. AI agents that support MCP (such as
+Claude Code) discover them automatically on session start. The underlying
+Python scripts can also be run directly from the command line.
+
+All tools run entirely locally — no API keys or external service accounts are
+required. Python dependencies are installed automatically into an isolated
+virtual environment at ``.claude/cache/.venv/`` on first use.
+
+.. _Model performance degrades significantly as context size increases: https://www.anthropic.com/news/claude-opus-4-6
+.. _MCP (Model Context Protocol): https://modelcontextprotocol.io
+
+RAG Semantic Search
+-------------------
+
+The RAG (Retrieval-Augmented Generation) semantic search addresses this
+problem — it finds code by meaning, not just text match, surfacing related code
+across subsystems that ``grep`` would miss entirely. Two MCP tools are provided:
+
+- **openmc_rag_search** — Given a natural-language query, returns the most
+  relevant code chunks with file paths, line numbers, and a preview. Can search
+  code, documentation, or both. Can also find code related to a given file.
+- **openmc_rag_rebuild** — Rebuilds the search index. Should be called after
+  pulling new code or switching branches.
+
+How it works
+^^^^^^^^^^^^
+
+The search pipeline runs entirely on your local CPU:
+
+1. **Chunking.** All C++, Python, and RST files are split into overlapping
+   fixed-size windows (~1000 characters, 25% overlap). This ensures every line
+   of code appears in at least one chunk and most lines appear in two.
+
+2. **Embedding.** Each chunk is embedded into a 384-dimensional vector using
+   the `all-MiniLM-L6-v2`_ sentence-transformer model (22 million parameters).
+   This model runs on CPU with no GPU required. No API key is needed — the
+   model weights are downloaded once from Hugging Face and cached locally.
+
+3. **Indexing.** The vectors are stored in a local LanceDB_ database on disk.
+   Building the full index takes approximately 5 minutes on a machine with
+   10 CPU cores. The index is stored in ``.claude/cache/rag_index/`` and
+   persists across sessions.
+
+4. **Searching.** Your query is embedded using the same model, and the closest
+   chunks are retrieved by vector similarity. Results include the file path,
+   line range, file type, similarity distance, and a text preview.
+
+.. _all-MiniLM-L6-v2: https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2
+.. _LanceDB: https://lancedb.com
+
+Requirements
+^^^^^^^^^^^^
+
+No system dependencies beyond **Python 3.12+** with ``pip``. An internet
+connection is required on first use to download the Python packages and
+embedding model weights; subsequent runs are fully offline. The Python packages
+(``sentence-transformers``, ``lancedb``) and their dependencies (including
+PyTorch, ~2GB) are installed automatically into an isolated virtual environment
+on first use.
diff --git a/docs/source/devguide/contributing.rst b/docs/source/devguide/contributing.rst
index cda031389..5199c9438 100644
--- a/docs/source/devguide/contributing.rst
+++ b/docs/source/devguide/contributing.rst
@@ -111,7 +111,8 @@ The TC consists of the following individuals:
 - `Paul Romano <https://github.com/paulromano>`_
 - `Patrick Shriwise <https://github.com/pshriwise>`_
 - `Adam Nelson <https://github.com/nelsonag>`_
-- `Benoit Forget <https://github.com/bforget>`_
+- `Jonathan Shimwell <https://github.com/shimwell>`_
+- `John Tramm <https://github.com/jtramm>`_
 
 The Project Lead is Paul Romano.
 
diff --git a/docs/source/devguide/docbuild.rst b/docs/source/devguide/docbuild.rst
index f723db06e..cda0307ca 100644
--- a/docs/source/devguide/docbuild.rst
+++ b/docs/source/devguide/docbuild.rst
@@ -14,6 +14,11 @@ Python API. That is, from the root directory of the OpenMC repository:
 
     python -m pip install ".[docs]"
 
+The OpenMC documentation also uses Doxygen to automatically generate its
+C/C++ API documentation directly from the docstrings available in the source
+code. You will need to have a working installation of Doxygen to generate the
+documentation locally.
+
 -----------------------------------
 Building Documentation as a Webpage
 -----------------------------------
diff --git a/docs/source/devguide/index.rst b/docs/source/devguide/index.rst
index 2e131e094..53b9f5853 100644
--- a/docs/source/devguide/index.rst
+++ b/docs/source/devguide/index.rst
@@ -14,6 +14,7 @@ other related topics.
 
     contributing
     workflow
+    agentic-tools
     styleguide
     policies
     tests
diff --git a/docs/source/devguide/policies.rst b/docs/source/devguide/policies.rst
index 2cf319987..3644ae822 100644
--- a/docs/source/devguide/policies.rst
+++ b/docs/source/devguide/policies.rst
@@ -21,8 +21,8 @@ C++ code in OpenMC must conform to the most recent C++ standard that is fully
 supported in the `version of the gcc compiler
 <https://gcc.gnu.org/projects/cxx-status.html>`_ that is distributed with the
 oldest version of Ubuntu that is still within its `standard support period
-<https://ubuntu.com/about/release-cycle>`_. Ubuntu 20.04 LTS will be supported
-through April 2025 and is distributed with gcc 9.3.0, which fully supports the
+<https://ubuntu.com/about/release-cycle>`_. Ubuntu 22.04 LTS will be supported
+through April 2027 and is distributed with gcc 11.4.0, which fully supports the
 C++17 standard.
 
 --------------------
@@ -31,5 +31,5 @@ CMake Version Policy
 
 Similar to the C++ standard policy, the minimum supported version of CMake
 corresponds to whatever version is distributed with the oldest version of Ubuntu
-still within its standard support period. Ubuntu 20.04 LTS is distributed with
-CMake 3.16.
+still within its standard support period. Ubuntu 22.04 LTS is distributed with
+CMake 3.22.
diff --git a/docs/source/devguide/styleguide.rst b/docs/source/devguide/styleguide.rst
index 2c882b034..a5599566e 100644
--- a/docs/source/devguide/styleguide.rst
+++ b/docs/source/devguide/styleguide.rst
@@ -30,7 +30,7 @@ whenever a file is saved. For example, `Visual Studio Code
 support for running clang-format.
 
 .. note::
-    OpenMC's CI uses `clang-format` version 15. A different version of `clang-format`
+    OpenMC's CI uses `clang-format` version 18. A different version of `clang-format`
     may produce different line changes and as a result fail the CI test.
 
 Miscellaneous
diff --git a/docs/source/devguide/tests.rst b/docs/source/devguide/tests.rst
index f2e39441a..8627cbde2 100644
--- a/docs/source/devguide/tests.rst
+++ b/docs/source/devguide/tests.rst
@@ -37,6 +37,9 @@ Prerequisites
 - Some tests require `NJOY <https://www.njoy21.io/NJOY2016>`_ to preprocess
   cross section data. The test suite assumes that you have an ``njoy``
   executable available on your :envvar:`PATH`.
+- OpenMC should be compiled with ``-DOPENMC_ENABLE_STRICT_FP=on`` to ensure
+  reproducible floating-point results across platforms and optimization levels.
+  Without this flag, regression tests may not match reference values.
 
 Running Tests
 -------------
@@ -67,9 +70,11 @@ make sure you have satisfied all the prerequisites above. After you have done
 that, consider the following:
 
 - When building OpenMC, make sure you run CMake with
-  ``-DCMAKE_BUILD_TYPE=Debug``. Building with a release build will result in
-  some test failures due to differences in which compiler optimizations are
-  used.
+  ``-DOPENMC_ENABLE_STRICT_FP=on``. This prevents the compiler from applying
+  floating-point optimizations (such as replacing math library calls with
+  builtins or contracting multiply-add into FMA instructions) that can produce
+  bit-level differences across platforms and optimization levels. Any
+  ``CMAKE_BUILD_TYPE`` can be used.
 - Because tallies involve the sum of many floating point numbers, the
   non-associativity of floating point numbers can result in different answers
   especially when the number of threads is high (different order of operations).
diff --git a/docs/source/devguide/workflow.rst b/docs/source/devguide/workflow.rst
index d600c236f..c49326a20 100644
--- a/docs/source/devguide/workflow.rst
+++ b/docs/source/devguide/workflow.rst
@@ -91,6 +91,30 @@ features and bug fixes. The general steps for contributing are as follows:
 6. After the pull request has been thoroughly vetted, it is merged back into the
    *develop* branch of openmc-dev/openmc.
 
+Setting Up Upstream Tracking (Required for Versioning)
+------------------------------------------------------
+
+By default, your fork **does not** include tags from the upstream OpenMC repository.  
+OpenMC relies on `git describe --tags` for versioning in source builds, and missing tags can lead  
+to incorrect version detection (i.e., ``0.0.0``). To ensure proper versioning, follow these steps:
+
+1. **Add the Upstream Repository**  
+   This allows you to fetch updates from the main OpenMC repository.
+
+   .. code-block:: sh
+
+       git remote add upstream https://github.com/openmc-dev/openmc.git
+
+2. **Fetch and Push Tags**  
+   Retrieve tags from the upstream repository and update your fork:
+
+   .. code-block:: sh
+
+       git fetch --tags upstream
+       git push --tags origin
+
+This ensures that both your **local** and **remote** fork have the correct versioning information.
+
 Private Development
 -------------------
 
diff --git a/docs/source/io_formats/collision_track.rst b/docs/source/io_formats/collision_track.rst
new file mode 100644
index 000000000..4123fdac6
--- /dev/null
+++ b/docs/source/io_formats/collision_track.rst
@@ -0,0 +1,46 @@
+.. _io_collision_track:
+
+===========================
+Collision Track File Format
+===========================
+
+When collision tracking is enabled with ``mcpl=false`` (the default), OpenMC
+writes binary data to an HDF5 file named ``collision_track.h5``. The same data
+may also be written after each batch when multiple files are requested
+(``collision_track.N.h5``) or when the run is performed in parallel. The file
+contains the information needed to reconstruct each recorded collision.
+
+The current revision of the collision track file format is 1.2.
+
+**/**
+
+:Attributes:
+            - **filetype** (*char[]*) -- String indicating the type of file.
+              For collision-track files the value is ``"collision_track"``.
+
+:Datasets:
+
+          - **collision_track_bank** (Compound type) -- Collision information
+            for each stored event. Each entry in the dataset corresponds to one
+            collision and contains the following fields:
+
+              - ``r`` (*double[3]*) -- Position of the collision in [cm].
+              - ``u`` (*double[3]*) -- Direction unit vector immediately after the collision.
+              - ``E`` (*double*) -- Incident particle energy before the collision in [eV].
+              - ``dE`` (*double*) -- Energy loss over the collision (:math:`E_\text{before} - E_\text{after}`) in [eV].
+              - ``time`` (*double*) -- Time of the collision in [s].
+              - ``wgt`` (*double*) -- Particle weight at the collision.
+              - ``event_mt`` (*int*) -- ENDF MT number identifying the reaction.
+              - ``delayed_group`` (*int*) -- Delayed neutron group index (non-zero for delayed events).
+              - ``cell_id`` (*int*) -- ID of the cell in which the collision occurred.
+              - ``nuclide_id`` (*int*) -- PDG number of the nuclide (100ZZZAAAM).
+              - ``material_id`` (*int*) -- ID of the material containing the collision site.
+              - ``universe_id`` (*int*) -- ID of the universe containing the collision site.
+              - ``n_collision`` (*int*) -- Collision counter for the particle history.
+              - ``particle`` (*int32_t*) -- Particle type (PDG number).
+              - ``parent_id`` (*int64_t*) -- Unique ID of the parent particle.
+              - ``progeny_id`` (*int64_t*) -- Progeny ID of the particle.
+
+In an MPI run, OpenMC writes the combined dataset by gathering collision-track
+entries from all ranks before flushing them to disk, so the final file appears
+as though it were produced serially.
diff --git a/docs/source/io_formats/depletion_chain.rst b/docs/source/io_formats/depletion_chain.rst
index 89c76525f..74413e7b6 100644
--- a/docs/source/io_formats/depletion_chain.rst
+++ b/docs/source/io_formats/depletion_chain.rst
@@ -56,6 +56,27 @@ attributes:
 
 .. _io_chain_reaction:
 
+--------------------
+``<source>`` Element
+--------------------
+
+The ``<source>`` element represents photon and electron sources associated with
+the decay of a nuclide and contains information to construct an
+:class:`openmc.stats.Univariate` object that represents this emission as an
+energy distribution. This element has the following attributes:
+
+  :type:
+    The type of :class:`openmc.stats.Univariate` source term.
+
+  :particle:
+    The type of particle emitted, e.g., 'photon' or 'electron'
+
+  :parameters:
+    The parameters of the source term, e.g., for a
+    :class:`openmc.stats.Discrete` source, the energies (in [eV]) at which the
+    particles are emitted and their relative intensities in [Bq/atom] (in other
+    words, decay constants).
+
 ----------------------
 ``<reaction>`` Element
 ----------------------
diff --git a/docs/source/io_formats/depletion_results.rst b/docs/source/io_formats/depletion_results.rst
index 7035fc9c9..856993db6 100644
--- a/docs/source/io_formats/depletion_results.rst
+++ b/docs/source/io_formats/depletion_results.rst
@@ -4,7 +4,7 @@
 Depletion Results File Format
 =============================
 
-The current version of the depletion results file format is 1.1.
+The current version of the depletion results file format is 1.3.
 
 **/**
 
@@ -12,30 +12,31 @@ The current version of the depletion results file format is 1.1.
              - **version** (*int[2]*) -- Major and minor version of the
                statepoint file format.
 
-:Datasets: - **eigenvalues** (*double[][][2]*) -- k-eigenvalues at each
-             time/stage. This array has shape (number of timesteps, number of
-             stages, value). The last axis contains the eigenvalue and the
-             associated uncertainty
-           - **number** (*double[][][][]*) -- Total number of atoms. This array
-             has shape (number of timesteps, number of stages, number of
+:Datasets: - **eigenvalues** (*double[][2]*) -- k-eigenvalues at each timestep.
+             This array has shape (number of timesteps, 2). The second axis
+             contains the eigenvalue and its associated uncertainty.
+           - **number** (*double[][][]*) -- Total number of atoms at each
+             timestep. This array has shape (number of timesteps, number of
              materials, number of nuclides).
-           - **reaction rates** (*double[][][][][]*) -- Reaction rates used to
-             build depletion matrices. This array has shape (number of
-             timesteps, number of stages, number of materials, number of
-             nuclides, number of reactions).
+           - **reaction rates** (*double[][][][]*) -- Reaction rates at each
+             timestep. This array has shape (number of timesteps, number of
+             materials, number of nuclides, number of reactions). Only stored if
+             write_rates=True.
            - **time** (*double[][2]*) -- Time in [s] at beginning/end of each
              step.
-           - **source_rate** (*double[][]*) -- Power in [W] or source rate in
-             [neutron/sec]. This array has shape (number of timesteps, number
-             of stages).
+           - **source_rate** (*double[]*) -- Power in [W] or source rate in
+             [neutron/sec] for each timestep.
            - **depletion time** (*double[]*) -- Average process time in [s]
              spent depleting a material across all burnable materials and,
              if applicable, MPI processes.
+           - **keff_search_root** (*double[]*) -- Root of the keff search at the
+             end of the timestep, if applicable.
 
 **/materials/<id>/**
 
 :Attributes: - **index** (*int*) -- Index used in results for this material
              - **volume** (*double*) -- Volume of this material in [cm^3]
+             - **name** (*char[]*) -- Name of this material
 
 **/nuclides/<name>/**
 
diff --git a/docs/source/io_formats/geometry.rst b/docs/source/io_formats/geometry.rst
index 6d0a37a24..5cd18bef1 100644
--- a/docs/source/io_formats/geometry.rst
+++ b/docs/source/io_formats/geometry.rst
@@ -38,11 +38,9 @@ Each ``<surface>`` element can have the following attributes or sub-elements:
 
   :boundary:
      The boundary condition for the surface. This can be "transmission",
-     "vacuum", "reflective", or "periodic". Periodic boundary conditions can
-     only be applied to x-, y-, and z-planes. Only axis-aligned periodicity is
-     supported, i.e., x-planes can only be paired with x-planes. Specify which
-     planes are periodic and the code will automatically identify which planes
-     are paired together.
+     "vacuum", "reflective", or "periodic". Specify which planes are
+     periodic and the code will automatically identify which planes are
+     paired together.
 
     *Default*: "transmission"
 
@@ -318,9 +316,10 @@ the following attributes or sub-elements:
     *Default*: None
 
   :orientation:
-    The orientation of the hexagonal lattice. The string "x" indicates that two
-    sides of the lattice are parallel to the x-axis, whereas the string "y"
-    indicates that two sides are parallel to the y-axis.
+    The orientation of the hexagonal lattice. The string "x" indicates that each
+    lattice element has two faces that are perpendicular to the x-axis, whereas
+    the string "y" indicates that each lattice element has two faces that are
+    perpendicular to the y-axis.
 
     *Default*: "y"
 
@@ -407,24 +406,55 @@ Each ``<dagmc_universe>`` element can have the following attributes or sub-eleme
 
     *Default*: None
 
-  :material_overrides:
-    This element contains information on material overrides to be applied to the
-    DAGMC universe. It has the following attributes and sub-elements:
+  :cell:
+    Zero or more ``<cell>`` sub-elements may appear to override properties of
+    individual DAGMC volumes. Each ``<cell>`` element supports the following
+    attributes and sub-elements:
 
-    :cell:
-      Material override information for a single cell. It contains the following
-      attributes and sub-elements:
+    :id:
+      The integer cell ID in the DAGMC geometry to override. Required.
 
-      :id:
-        The cell ID in the DAGMC geometry for which the material override will
-        apply.
+    :name:
+      An optional string label for the cell.
 
-      :materials:
-        A list of material IDs that will apply to instances of the cell. If the
-        list contains only one ID, it will replace the original material
-        assignment of all instances of the DAGMC cell. If the list contains more
-        than one material, each material ID of the list will be assigned to the
-        various instances of the DAGMC cell.
+      *Default*: None
+
+    :material:
+      The material ID to assign to this cell. Use ``void`` for vacuum. Multiple
+      space-separated IDs may be given to specify a distribmat (distributed
+      material) assignment. Required.
+
+    :temperature:
+      Temperature(s) in [K] to assign to the cell. Must be greater than or equal
+      to 0. Multiple space-separated values may be given.
+
+      *Default*: None
+
+    :density:
+      Density in [g/cm³] to assign to the cell. Must be greater than 0. Requires a non-void
+      material fill. Multiple space-separated values may be given.
+
+      *Default*: None
+
+    :volume:
+      Volume of the cell in [cm³].
+
+      .. note:: DAGMC can compute cell volumes exactly from the triangulated
+                mesh surfaces. Specifying a manual volume risks inconsistency
+                with that capability.
+
+      *Default*: None
+
+    The following standard ``<cell>`` attributes are **not** supported inside
+    ``<dagmc_universe>`` and will raise an error if present: ``region``,
+    ``fill``, ``universe``, ``translation``, ``rotation``.
+
+    .. deprecated::
+      The ``<material_overrides>`` sub-element (containing ``<cell_override>``
+      children with ``<material_ids>``) is deprecated. A deprecation warning is
+      emitted and the overrides are converted to the ``<cell>`` format at parse
+      time. It is an error to specify both ``<material_overrides>`` and
+      ``<cell>`` sub-elements on the same ``<dagmc_universe>``.
 
     *Default*: None
 
diff --git a/docs/source/io_formats/index.rst b/docs/source/io_formats/index.rst
index 4bbaa961a..5b4efea66 100644
--- a/docs/source/io_formats/index.rst
+++ b/docs/source/io_formats/index.rst
@@ -44,6 +44,7 @@ Output Files
 
    statepoint
    source
+   collision_track
    summary
    properties
    depletion_results
diff --git a/docs/source/io_formats/mgxs_library.rst b/docs/source/io_formats/mgxs_library.rst
index f7f5387a4..8a26311d1 100644
--- a/docs/source/io_formats/mgxs_library.rst
+++ b/docs/source/io_formats/mgxs_library.rst
@@ -133,6 +133,10 @@ Temperature-dependent data, provided for temperature <TTT>K.
              This dataset is optional. This is a 1-D vector if `representation`
              is "isotropic", or a 3-D vector if `representation` is "angle"
              with dimensions of [polar][azimuthal][groups].
+	     When this data is not available, an approximation using the
+	     group energy boundaries is used. For more information see
+	     the particle speed subsection in the multigroup-data section
+	     of the theory manual.
 
 **/<library name>/<TTT>K/scatter_data/**
 
diff --git a/docs/source/io_formats/particle_restart.rst b/docs/source/io_formats/particle_restart.rst
index 2734f0470..e0fe76b9d 100644
--- a/docs/source/io_formats/particle_restart.rst
+++ b/docs/source/io_formats/particle_restart.rst
@@ -4,7 +4,7 @@
 Particle Restart File Format
 ============================
 
-The current version of the particle restart file format is 2.0.
+The current version of the particle restart file format is 2.1.
 
 **/**
 
@@ -26,8 +26,7 @@ The current version of the particle restart file format is 2.0.
            - **run_mode** (*char[]*) -- Run mode used, either 'fixed source',
              'eigenvalue', or 'particle restart'.
            - **id** (*int8_t*) -- Unique identifier of the particle.
-           - **type** (*int*) -- Particle type (0=neutron, 1=photon, 2=electron,
-             3=positron)
+           - **type** (*int32_t*) -- Particle type (PDG number)
            - **weight** (*double*) -- Weight of the particle.
            - **energy** (*double*) -- Energy of the particle in eV for
              continuous-energy mode, or the energy group of the particle for
diff --git a/docs/source/io_formats/properties.rst b/docs/source/io_formats/properties.rst
index 5030e78f3..4cc5da379 100644
--- a/docs/source/io_formats/properties.rst
+++ b/docs/source/io_formats/properties.rst
@@ -4,7 +4,7 @@
 Properties File Format
 ======================
 
-The current version of the properties file format is 1.0.
+The current version of the properties file format is 1.1.
 
 **/**
 
@@ -25,6 +25,7 @@ The current version of the properties file format is 1.0.
 **/geometry/cells/cell <uid>/**
 
 :Datasets: - **temperature** (*double[]*) -- Temperature of the cell in [K].
+           - **density** (*double[]*) -- Density of the cell in [g/cm3].
 
 **/materials/**
 
diff --git a/docs/source/io_formats/settings.rst b/docs/source/io_formats/settings.rst
index a59d08049..7091757f2 100644
--- a/docs/source/io_formats/settings.rst
+++ b/docs/source/io_formats/settings.rst
@@ -7,6 +7,19 @@ Settings Specification -- settings.xml
 All simulation parameters and miscellaneous options are specified in the
 settings.xml file.
 
+-------------------------------
+``<atomic_relaxation>`` Element
+-------------------------------
+
+The ``<atomic_relaxation>`` element determines whether the atomic relaxation
+cascade, the X-ray fluorescence photons and Auger electrons emitted when an
+inner-shell vacancy is filled, is simulated following photoelectric and
+incoherent (Compton) scattering interactions. Disabling this can speed up
+photon transport calculations where the detailed secondary particle cascade is
+not of interest.
+
+  *Default*: true
+
 ---------------------
 ``<batches>`` Element
 ---------------------
@@ -20,6 +33,90 @@ source neutrons.
 
   *Default*: None
 
+-----------------------------
+``<collision_track>`` Element
+-----------------------------
+
+The ``<collision_track>`` element indicates to track information about particle
+collisions based on a set of criteria and store these events in a file named
+``collision_track.h5``. This file records details such as the position of the
+interaction, direction of the incoming particle, incident energy and deposited
+energy, weight, time of the interaction, and the delayed neutron group (0 for
+prompt neutrons). Additional information such as the cell ID, material ID,
+universe ID, nuclide ZAID, particle type, and event MT number are also stored.
+Users can specify one or more criterion to filter collisions. If no criteria are
+specified, it defaults to tracking all collisions across the model.
+
+.. warning::
+    Storing all collisions can be very memory intensive. For more targeted
+    tracking, users can employ a variety of parameters such as ``cell_ids``,
+    ``reactions``, ``universe_ids``, ``material_ids``, ``nuclides``, and
+    ``deposited_E_threshold`` to refine the selection of particle interactions
+    to be banked.
+
+This element can contain one or more of the following attributes or
+sub-elements:
+
+  :max_collisions:
+    An integer indicating the maximum number of collisions to be banked per file.
+
+    *Default*: 1000
+
+  :max_collision_track_files:
+    An integer indicating the number of collision_track files to be used.
+
+    *Default*: 1
+
+  :mcpl:
+    An optional boolean to enable MCPL_-format instead of the native HDF5-based
+    format. If activated, the output file name and type is changed to
+    ``collision_track.mcpl``.
+
+    *Default*: false
+
+    .. _MCPL: https://mctools.github.io/mcpl/mcpl.pdf
+
+  :cell_ids:
+    A list of integers representing cell IDs to define specific cells in which
+    collisions are to be banked.
+
+    *Default*: None
+
+  :universe_ids:
+    A list of integers representing the universe IDs to define specific
+    universes in which collisions are to be banked.
+
+    *Default*: None
+
+  :material_ids:
+    A list of integers representing the material IDs to define specific
+    materials in which collisions are to be banked.
+
+    *Default*: None
+
+  :nuclides:
+    A list of strings representing the nuclide, to define specific
+    define specific target nuclide collisions to be banked.
+
+    .. note::
+      Electron and positron collision-track events are not associated with
+      a specific nuclide. If a ``nuclides`` entry is specified, these events
+      are omitted.
+
+    *Default*: None
+
+  :reactions:
+    A list of integers representing the ENDF-6 format MT numbers or strings
+    (e.g. (n,fission)) to define specific reaction types to be banked.
+
+    *Default*: None
+
+  :deposited_E_threshold:
+    A float defining the minimum deposited energy per collision (in eV) to
+    trigger banking.
+
+    *Default*: 0.0
+
 ----------------------------------
 ``<confidence_intervals>`` Element
 ----------------------------------
@@ -178,6 +275,16 @@ history-based parallelism.
 
   *Default*: false
 
+--------------------------------
+``<free_gas_threshold>`` Element
+--------------------------------
+
+The ``<free_gas_threshold>`` element specifies the energy multiplier, expressed
+in units of :math:`kT`, that determines when the free gas scattering approach is
+used for elastic scattering. Values must be positive.
+
+  *Default*: 400.0
+
 -----------------------------------
 ``<generations_per_batch>`` Element
 -----------------------------------
@@ -188,6 +295,15 @@ ignored for all run modes other than "eigenvalue".
 
   *Default*: 1
 
+------------------------------
+``<ifp_n_generation>`` Element
+------------------------------
+
+The ``<ifp_n_generation>`` element indicates the number of generations to
+consider for the Iterated Fission Probability method.
+
+  *Default*: 10
+
 ----------------------
 ``<inactive>`` Element
 ----------------------
@@ -313,7 +429,25 @@ then, OpenMC will only use up to the :math:`P_1` data.
 ``<max_history_splits>`` Element
 --------------------------------
 
-The ``<max_history_splits>`` element indicates the number of times a particle can split during a history.
+The ``<max_history_splits>`` element indicates the number of times a particle
+can split during a history.
+
+  *Default*: 1000
+
+-----------------------------
+``<max_secondaries>`` Element
+-----------------------------
+
+The ``<max_secondaries>`` element indicates the maximum secondary bank size.
+
+  *Default*: 10000
+
+------------------------
+``<max_tracks>`` Element
+------------------------
+
+The ``<max_tracks>`` element indicates the maximum number of tracks written to a
+track file (per MPI process).
 
   *Default*: 1000
 
@@ -426,6 +560,18 @@ generator during generation of colors in plots.
 
   *Default*: 1
 
+.. _properties_file:
+
+-----------------------------
+``<properties_file>`` Element
+-----------------------------
+
+  The ``properties_file`` element has no attributes and contains the path to a
+  properties HDF5 file to load cell temperatures/densities and material
+  densities.
+
+  *Default*: None
+
 ---------------------
 ``<ptables>`` Element
 ---------------------
@@ -456,7 +602,7 @@ found in the :ref:`random ray user guide <random_ray>`.
 
     *Default*: None
 
-  :source:
+  :ray_source:
     Specifies the starting ray distribution, and follows the format for
     :ref:`source_element`. It must be uniform in space and angle and cover the
     full domain. It does not represent a physical neutron or photon source -- it
@@ -464,6 +610,35 @@ found in the :ref:`random ray user guide <random_ray>`.
 
     *Default*: None
 
+  :adjoint_source:
+    Specifies an adjoint fixed source for adjoint transport simulations, and
+    follows the format for :ref:`source_element`. The distributions which make
+    up the adjoint source are subject to the same restrictions as forward
+    fixed sources in Random Ray mode.
+
+    *Default*: None
+
+  :adjoint:
+    Specifies whether to perform adjoint transport. The default is 'False',
+    corresponding to forward transport.
+
+    *Default*: None
+
+  :volume_estimator:
+    Specifies choice of volume estimator for the random ray solver. Options
+    are 'naive', 'simulation_averaged', or 'hybrid'. The default is 'hybrid'.
+
+    *Default*: None
+
+  :volume_normalized_flux_tallies:
+    Specifies whether to normalize flux tallies by volume (bool). The
+    default is 'False'. When enabled, flux tallies will be reported in units
+    of cm/cm^3. When disabled, flux tallies will be reported in units of cm
+    (i.e., total distance traveled by neutrons in the spatial tally
+    region).
+
+    *Default*: None
+
   :sample_method:
     Specifies the method for sampling the starting ray distribution. This
     element can be set to "prng" or "halton".
@@ -488,6 +663,14 @@ found in the :ref:`random ray user guide <random_ray>`.
         :type:
           The type of the domain. Can be ``material``, ``cell``, or ``universe``.
 
+  :diagonal_stabilization_rho:
+    The rho factor for use with diagonal stabilization. This technique is
+    applied when negative diagonal (in-group) elements are detected in
+    the scattering matrix of input MGXS data, which is a common feature
+    of transport corrected MGXS data.
+
+    *Default*: 1.0
+
 ----------------------------------
 ``<resonance_scattering>`` Element
 ----------------------------------
@@ -563,14 +746,16 @@ pseudo-random number generator.
 
   *Default*: 1
 
---------------------
-``<stride>`` Element
---------------------
+-----------------------------------
+``<shared_secondary_bank>`` Element
+-----------------------------------
 
-The ``stride`` element is used to specify how many random numbers are allocated
-for each source particle history.
-
-  *Default*: 152,917
+  The ``shared_secondary_bank`` element indicates whether to use a shared
+  secondary particle bank. When enabled, secondary particles are collected into
+  a global bank, sorted for reproducibility, and load-balanced across MPI ranks
+  between generations. If not specified, the shared secondary bank is enabled
+  automatically for fixed-source simulations with weight windows active, and
+  disabled otherwise.
 
 .. _source_element:
 
@@ -592,12 +777,15 @@ attributes/sub-elements:
     *Default*: 1.0
 
   :type:
-    Indicator of source type. One of ``independent``, ``file``, ``compiled``, or
-    ``mesh``. The type of the source will be determined by this attribute if it
-    is present.
+    Indicator of source type. One of ``independent``, ``file``, ``compiled``,
+    ``mesh``, or ``tokamak``. The type of the source will be determined by this
+    attribute if it is present.
 
   :particle:
-    The source particle type, either ``neutron`` or ``photon``.
+    The source particle type, specified as a PDG number or a string alias (e.g.,
+    ``neutron``/``n``, ``photon``/``gamma``, ``electron``, ``positron``,
+    ``proton``/``p``, ``deuteron``/``d``, ``triton``/``t``, ``alpha``, or GNDS
+    nuclide names like ``Fe57``).
 
     *Default*: neutron
 
@@ -687,6 +875,7 @@ attributes/sub-elements:
 
       For a "cylindrical" distribution, no parameters are specified. Instead,
       the ``r``, ``phi``, ``z``, and ``origin`` elements must be specified.
+      Optionally, the ``r_dir`` and ``z_dir`` elements could be specified.
 
       For a "spherical" distribution, no parameters are specified. Instead,
       the ``r``, ``theta``, ``phi``, and ``origin`` elements must be specified.
@@ -718,6 +907,10 @@ attributes/sub-elements:
       of a univariate probability distribution (see the description in
       :ref:`univariate`).
 
+    :r_dir:
+      For "cylindrical" distributions, this element specifies the direction
+      of the cylinder r-axis at phi=0. Defaults to (1.0, 0.0, 0.0).
+
     :theta:
       For a "spherical" distribution, this element specifies the distribution
       of theta-coordinates. The necessary sub-elements/attributes are those of a
@@ -730,6 +923,10 @@ attributes/sub-elements:
       sub-elements/attributes are those of a univariate probability
       distribution (see the description in :ref:`univariate`).
 
+    :z_dir:
+      For "cylindrical" distributions, this element specifies the direction
+      of the cylinder z-axis. Defaults to (0.0, 0.0, 1.0).
+
     :origin:
       For "cylindrical and "spherical" distributions, this element specifies
       the coordinates for the origin of the coordinate system.
@@ -747,13 +944,18 @@ attributes/sub-elements:
       relative source strength of each mesh element or each point in the cloud.
 
     :volume_normalized:
-      For "mesh" spatial distrubtions, this optional boolean element specifies
+      For "mesh" spatial distributions, this optional boolean element specifies
       whether the vector of relative strengths should be multiplied by the mesh
       element volume. This is most common if the strengths represent a source
       per unit volume.
 
       *Default*: false
 
+    :bias:
+      For "mesh" and "cloud" spatial distributions, this optional element
+      specifies floating point values corresponding to alternative probabilities
+      for each value/component to use for biased sampling.
+
   :angle:
     An element specifying the angular distribution of source sites. This element
     has the following attributes:
@@ -786,6 +988,10 @@ attributes/sub-elements:
       are those of a univariate probability distribution (see the description in
       :ref:`univariate`).
 
+    :bias:
+      For "isotropic" angular distributions, this optional element specifies a
+      "mu-phi" angular distribution used for biased sampling.
+
   :energy:
     An element specifying the energy distribution of source sites. The necessary
     sub-elements/attributes are those of a univariate probability distribution
@@ -809,6 +1015,84 @@ attributes/sub-elements:
     mesh element and follows the format for :ref:`source_element`. The number of
     ``<source>`` sub-elements should correspond to the number of mesh elements.
 
+  For a source with ``type="tokamak"``, the spatial distribution is described by
+  a Miller-style flux-surface parameterization and the following sub-elements
+  are used instead of the ``space`` element:
+
+  :major_radius:
+    The major radius :math:`R_0` of the plasma in [cm].
+
+  :minor_radius:
+    The minor radius :math:`a` of the plasma in [cm]. Must be smaller than
+    ``major_radius``.
+
+  :elongation:
+    The plasma elongation :math:`\kappa` (must be > 0).
+
+  :triangularity:
+    The plasma triangularity :math:`\delta` (must be in [-1, 1]). Negative
+    values describe negative-triangularity plasmas.
+
+  :shafranov_shift:
+    The Shafranov shift :math:`\Delta` in [cm] (must be >= 0 and less than
+    ``minor_radius``/2).
+
+  :r_over_a:
+    A list of normalized minor-radius grid points :math:`r/a`. Must be strictly
+    increasing, start at 0, and end at 1.
+
+  :emission_density:
+    A list of neutron emission densities :math:`S(r)` evaluated at each
+    ``r_over_a`` grid point (arbitrary units, must be non-negative). Only the
+    shape matters, since the profile is normalized internally. Values are
+    interpolated linearly between grid points and the profile is refined on an
+    internal grid for radial sampling. Must have the same length as
+    ``r_over_a`` and contain at least one positive value.
+
+  :phi_start:
+    The starting toroidal angle in [rad].
+
+    *Default*: 0.0
+
+  :phi_extent:
+    The toroidal angle extent in [rad]. The source is sampled uniformly in
+    :math:`[\phi_\text{start},\ \phi_\text{start} + \phi_\text{extent}]`.
+
+    *Default*: :math:`2\pi`
+
+  :n_alpha:
+    The number of poloidal-angle grid points used to build the sampling CDFs
+    (must be > 2). Larger values reduce discretization bias; values below 51
+    produce a warning.
+
+    *Default*: 101
+
+  :vertical_shift:
+    A vertical shift of the plasma center in [cm].
+
+    *Default*: 0.0
+
+  :energy:
+    For a tokamak source, one or more ``energy`` sub-elements specify the
+    neutron energy distribution(s). Either a single distribution is given (used
+    at all radii) or exactly one distribution per ``r_over_a`` grid point is
+    given, in which case the energy is sampled from one of the two
+    distributions bracketing the sampled radius, selected stochastically with
+    probability proportional to the proximity of the radius to each grid point
+    (stochastic interpolation). Each follows the format of a univariate
+    probability distribution (see :ref:`univariate`).
+
+  :time:
+    An optional ``time`` sub-element specifying the time distribution of source
+    particles, following the format of a univariate probability distribution
+    (see :ref:`univariate`).
+
+    *Default*: particles are born at :math:`t=0`
+
+  .. note:: Biased sampling can be applied to the spatial and energy distributions
+            of a source by using the ``<bias>`` sub-element (see
+            :ref:`univariate` for details on how to specify bias distributions).
+
   :constraints:
     This sub-element indicates the presence of constraints on sampled source
     sites (see :ref:`usersguide_source_constraints` for details). It may have
@@ -855,17 +1139,19 @@ variable and whose sub-elements/attributes are as follows:
 
 :type:
   The type of the distribution. Valid options are "uniform", "discrete",
-  "tabular", "maxwell", "watt", and "mixture". The "uniform" option produces
-  variates sampled from a uniform distribution over a finite interval. The
-  "discrete" option produces random variates that can assume a finite number
-  of values (i.e., a distribution characterized by a probability mass function).
-  The "tabular" option produces random variates sampled from a tabulated
-  distribution where the density function is either a histogram or
+  "tabular", "maxwell", "watt", "mixture", and "decay_spectrum". The "uniform"
+  option produces variates sampled from a uniform distribution over a finite
+  interval. The "discrete" option produces random variates that can assume a
+  finite number of values (i.e., a distribution characterized by a probability
+  mass function). The "tabular" option produces random variates sampled from a
+  tabulated distribution where the density function is either a histogram or
   linearly-interpolated between tabulated points. The "watt" option produces
   random variates is sampled from a Watt fission spectrum (only used for
   energies). The "maxwell" option produce variates sampled from a Maxwell
-  fission spectrum (only used for energies). The "mixture" option produces samples
-  from univariate sub-distributions with given probabilities.
+  fission spectrum (only used for energies). The "mixture" option produces
+  samples from univariate sub-distributions with given probabilities. The
+  "decay_spectrum" option produces photon energies sampled from decay photon
+  spectra in a depletion chain (only used for energies).
 
   *Default*: None
 
@@ -883,6 +1169,10 @@ variable and whose sub-elements/attributes are as follows:
   :math:`(x,p)` pairs defining the discrete/tabular distribution. All :math:`x`
   points are given first followed by corresponding :math:`p` points.
 
+  For a "decay_spectrum" distribution, ``parameters`` gives the atom densities
+  in [atom/b-cm] for the nuclides listed in the ``nuclides`` element, in the
+  same order.
+
   For a "watt" distribution, ``parameters`` should be given as two real numbers
   :math:`a` and :math:`b` that parameterize the distribution :math:`p(x) dx = c
   e^{-x/a} \sinh \sqrt{b \, x} dx`.
@@ -901,30 +1191,51 @@ variable and whose sub-elements/attributes are as follows:
   *Default*: histogram
 
 :pair:
-  For a "mixture" distribution, this element provides a distribution and its corresponding probability.
+  For a "mixture" distribution, this element provides a distribution and its
+  corresponding probability.
 
   :probability:
-    An attribute or ``pair`` that provides the probability of a univariate distribution within a "mixture" distribution.
+    An attribute or ``pair`` that provides the probability of a univariate
+    distribution within a "mixture" distribution.
 
   :dist:
-    This sub-element of a ``pair`` element provides information on the corresponding univariate distribution.
+    This sub-element of a ``pair`` element provides information on the
+    corresponding univariate distribution.
 
--------------------------
-``<state_point>`` Element
--------------------------
+:volume:
+  For a "decay_spectrum" distribution, this attribute specifies the source
+  region volume in cm\ :sup:`3`. It is used together with atom densities to
+  determine the absolute photon emission rate. When a source uses a
+  "decay_spectrum" energy distribution, the source strength is set from this
+  emission rate.
 
-The ``<state_point>`` element indicates at what batches a state point file
-should be written. A state point file can be used to restart a run or to get
-tally results at any batch. The default behavior when using this tag is to
-write out the source bank in the state_point file. This behavior can be
-customized by using the ``<source_point>`` element. This element has the
-following attributes/sub-elements:
+:nuclides:
+  For a "decay_spectrum" distribution, this element specifies a
+  whitespace-separated list of nuclide names contributing to the decay photon
+  source. The atom densities for these nuclides are given by the ``parameters``
+  element in the same order. Nuclides are resolved against the depletion chain,
+  and nuclides without decay photon spectra do not contribute to the
+  distribution.
 
-  :batches:
-    A list of integers separated by spaces indicating at what batches a state
-    point file should be written.
+:bias:
+  This optional element specifies a biased distribution for importance sampling.
+  For continuous distributions, the ``bias`` element should contain another
+  univariate distribution with the same support (interval) as the parent
+  distribution. For discrete distributions, the ``bias`` element should contain
+  floating point values corresponding to alternative probabilities for each
+  value/component to be used for biased sampling.
 
-    *Default*: Last batch only
+  *Default*: None
+
+---------------------------------------
+``<source_rejection_fraction>`` Element
+---------------------------------------
+
+The ``<source_rejection_fraction>`` element specifies the minimum fraction of
+external source sites that must be accepted when applying rejection sampling
+based on constraints.
+
+   *Default*: 0.05
 
 --------------------------
 ``<source_point>`` Element
@@ -975,6 +1286,32 @@ attributes/sub-elements:
 
     *Default*: false
 
+-------------------------
+``<state_point>`` Element
+-------------------------
+
+The ``<state_point>`` element indicates at what batches a state point file
+should be written. A state point file can be used to restart a run or to get
+tally results at any batch. The default behavior when using this tag is to
+write out the source bank in the state_point file. This behavior can be
+customized by using the ``<source_point>`` element. This element has the
+following attributes/sub-elements:
+
+  :batches:
+    A list of integers separated by spaces indicating at what batches a state
+    point file should be written.
+
+    *Default*: Last batch only
+
+--------------------
+``<stride>`` Element
+--------------------
+
+The ``stride`` element is used to specify how many random numbers are allocated
+for each source particle history.
+
+  *Default*: 152,917
+
 ------------------------------
 ``<surf_source_read>`` Element
 ------------------------------
@@ -1062,6 +1399,23 @@ attributes/sub-elements:
           are not eligible to store any particles when using ``cell``, ``cellfrom``
           or ``cellto`` attributes. It is recommended to use surface IDs instead.
 
+------------------------------------
+``<surface_grazing_cutoff>`` Element
+------------------------------------
+
+The ``<surface_grazing_cutoff>`` element specifies the surface flux cosine cutoff.
+
+  *Default*: 0.001
+
+-----------------------------------
+``<surface_grazing_ratio>`` Element
+-----------------------------------
+
+The ``<surface_grazing_ratio>`` element specifies the surface flux cosine
+substitution ratio.
+
+  *Default*: 0.5
+
 ------------------------------
 ``<survival_biasing>`` Element
 ------------------------------
@@ -1245,6 +1599,15 @@ has the following attributes/sub-elements:
               for fixed source and small criticality calculations, but is very
               optimistic for highly coupled full-core reactor problems.
 
+-------------------------------------
+``<uniform_source_sampling>`` Element
+-------------------------------------
+
+The ``<uniform_source_sampling>`` element indicates whether to sample among
+multiple sources uniformly, applying their strengths as weights to sampled
+particles.
+
+  *Default*: False
 
 ------------------------
 ``<ufs_mesh>`` Element
@@ -1257,6 +1620,16 @@ Agency Monte Carlo Performance Benchmark Problem," Proceedings of *Physor 2012*,
 Knoxville, TN (2012). The mesh should cover all possible fissionable materials
 in the problem and is specified using a :ref:`mesh_element`.
 
+-------------------------------
+``<use_decay_photons>`` Element
+-------------------------------
+
+The ``<use_decay_photons>`` element indicates whether to produce decay photons
+from neutron reactions instead of prompt photons. This is used in conjunction
+with the direct 1-step method for shutdown dose rate calculations.
+
+  *Default*: False
+
 .. _verbosity:
 
 -----------------------
@@ -1358,7 +1731,8 @@ sub-elements/attributes:
     *Default*: None
 
   :particle_type:
-    The particle that the weight windows will apply to (e.g., 'neutron')
+    The particle that the weight windows will apply to, specified as a PDG
+    code or string (e.g., ``neutron``).
 
     *Default*: 'neutron'
 
@@ -1418,7 +1792,8 @@ mesh-based weight windows.
     *Default*: None
 
   :particle_type:
-    The particle that the weight windows will apply to (e.g., 'neutron')
+    The particle that the weight windows will apply to, specified as a PDG
+    code or string (e.g., ``neutron``).
 
     *Default*: neutron
 
@@ -1462,6 +1837,14 @@ mesh-based weight windows.
 
         *Default*: 5.0
 
+    For FW-CADIS:
+
+      :targets:
+        A sequence of IDs corresponding to the tallies which cover phase
+        space regions of interest for local variance reduction.
+
+        *Default*: None
+
 ---------------------------------------
 ``<weight_window_checkpoints>`` Element
 ---------------------------------------
@@ -1487,3 +1870,21 @@ following sub-elements/attributes:
 
   The ``weight_windows_file`` element has no attributes and contains the path to
   a weight windows HDF5 file to load during simulation initialization.
+
+-------------------------------
+``<weight_windows_on>`` Element
+-------------------------------
+
+  The ``weight_windows_on`` element indicates whether weight windows are
+  enabled.
+
+  *Default*: False
+
+----------------------------------
+``<write_initial_source>`` Element
+----------------------------------
+
+  The ``write_initial_source`` element indicates whether to write the initial
+  source distribution to file.
+
+  *Default*: False
diff --git a/docs/source/io_formats/source.rst b/docs/source/io_formats/source.rst
index 4ce2229ed..2e81e0a3f 100644
--- a/docs/source/io_formats/source.rst
+++ b/docs/source/io_formats/source.rst
@@ -15,6 +15,8 @@ following the same format.
 **/**
 
 :Attributes: - **filetype** (*char[]*) -- String indicating the type of file.
+             - **version** (*int[2]*) -- Major and minor version of the source
+               file format.
 
 :Datasets:
 
@@ -22,5 +24,5 @@ following the same format.
              particle. The compound type has fields ``r``, ``u``, ``E``,
              ``time``, ``wgt``, ``delayed_group``, ``surf_id`` and ``particle``,
              which represent the position, direction, energy, time, weight,
-             delayed group, surface ID, and particle type (0=neutron, 1=photon,
-             2=electron, 3=positron), respectively.
+             delayed group, surface ID, and particle type (PDG number),
+             respectively.
diff --git a/docs/source/io_formats/statepoint.rst b/docs/source/io_formats/statepoint.rst
index 3b1031769..7d7765b84 100644
--- a/docs/source/io_formats/statepoint.rst
+++ b/docs/source/io_formats/statepoint.rst
@@ -4,7 +4,7 @@
 State Point File Format
 =======================
 
-The current version of the statepoint file format is 18.1.
+The current version of the statepoint file format is 18.2.
 
 **/**
 
@@ -56,8 +56,8 @@ The current version of the statepoint file format is 18.1.
              ``time``, ``wgt``, ``delayed_group``, ``surf_id``, and
              ``particle``, which represent the position, direction, energy,
              time, weight, delayed group, surface ID, and particle type
-             (0=neutron, 1=photon, 2=electron, 3=positron), respectively. Only
-             present when `run_mode` is 'eigenvalue'.
+             (PDG number), respectively. Only present when `run_mode` is
+             'eigenvalue'.
 
 **/tallies/**
 
@@ -149,6 +149,8 @@ The current version of the statepoint file format is 18.1.
                tallies will have a value of 0 unless otherwise instructed.
              - **multiply_density** (*int*) -- Flag indicating whether reaction
                rates should be multiplied by atom density (1) or not (0).
+             - **higher_moments** (*int*) -- Flag indicating whether
+               higher-order tally moments are enabled (1) or not (0).
 
 :Datasets: - **n_realizations** (*int*) -- Number of realizations.
            - **n_filters** (*int*) -- Number of filters used.
diff --git a/docs/source/io_formats/summary.rst b/docs/source/io_formats/summary.rst
index 7d3ab94d9..64ca68b9c 100644
--- a/docs/source/io_formats/summary.rst
+++ b/docs/source/io_formats/summary.rst
@@ -4,7 +4,7 @@
 Summary File Format
 ===================
 
-The current version of the summary file format is 6.0.
+The current version of the summary file format is 6.1.
 
 **/**
 
@@ -38,6 +38,7 @@ The current version of the summary file format is 6.0.
              is an array if the cell uses distributed materials, otherwise it is
              a scalar.
            - **temperature** (*double[]*) -- Temperature of the cell in Kelvin.
+           - **density** (*double[]*) -- Density of the cell in [g/cm3].
            - **translation** (*double[3]*) -- Translation applied to the fill
              universe. This dataset is present only if fill_type is set to
              'universe'.
diff --git a/docs/source/io_formats/tallies.rst b/docs/source/io_formats/tallies.rst
index 9f29a949a..dc57e5775 100644
--- a/docs/source/io_formats/tallies.rst
+++ b/docs/source/io_formats/tallies.rst
@@ -142,9 +142,9 @@ attributes/sub-elements:
 
   :type:
     The type of the filter. Accepted options are "cell", "cellfrom",
-    "cellborn", "surface", "material", "universe", "energy", "energyout", "mu",
-    "polar", "azimuthal", "mesh", "distribcell", "delayedgroup",
-    "energyfunction", and "particle".
+    "cellborn", "surface", "material", "universe", "energy", "energyout",
+    "mu", "polar", "azimuthal", "mesh", "distribcell", "delayedgroup",
+    "energyfunction", "particle", and "particleproduction".
 
   :bins:
      A description of the bins for each type of filter can be found in
@@ -318,8 +318,34 @@ should be set to:
   they use ``energy`` and ``y``.
 
 :particle:
-  A list of integers indicating the type of particles to tally ('neutron' = 1,
-  'photon' = 2, 'electron' = 3, 'positron' = 4).
+  A list of particle identifiers to tally, specified as strings (e.g.,
+  ``neutron``, ``photon``, ``He4``) or as integer PDG numbers.
+
+:particleproduction:
+  This filter tallies secondary particles produced in reactions, binned by
+  particle type and, optionally, by energy. Unlike other energy filters, the
+  weight applied is the weight of the secondary particle. To obtain secondary
+  particle production rates, use this filter with the ``events`` score.
+
+  The filter uses the following sub-elements instead of ``bins``:
+
+  :particles:
+    A space-separated list of secondary particle types to tally (e.g.,
+    ``photon``, ``neutron``, ``electron``).
+
+  :energies:
+    An optional monotonically increasing list of energy boundaries in [eV]
+    for binning the secondary particle energies. If omitted, total production
+    is tallied without energy binning.
+
+  For example, to tally photon and neutron production in three energy groups:
+
+  .. code-block:: xml
+
+      <filter id="1" type="particleproduction">
+        <particles>photon neutron</particles>
+        <energies>0.0 1.0e5 1.0e6 20.0e6</energies>
+      </filter>
 
 ------------------
 ``<mesh>`` Element
diff --git a/docs/source/io_formats/track.rst b/docs/source/io_formats/track.rst
index a97d75e58..9c05ac4c3 100644
--- a/docs/source/io_formats/track.rst
+++ b/docs/source/io_formats/track.rst
@@ -4,7 +4,7 @@
 Track File Format
 =================
 
-The current revision of the particle track file format is 3.0.
+The current revision of the particle track file format is 3.1.
 
 **/**
 
@@ -32,6 +32,5 @@ The current revision of the particle track file format is 3.0.
                             the array for each primary/secondary particle. The
                             last offset should match the total size of the
                             array.
-                          - **particles** (*int[]*) -- Particle type for each
-                            primary/secondary particle (0=neutron, 1=photon,
-                            2=electron, 3=positron).
+                          - **particles** (*int32_t[]*) -- Particle type for
+                            each primary/secondary particle (PDG number).
diff --git a/docs/source/license.rst b/docs/source/license.rst
index 0a90a7441..1ec9fc04a 100644
--- a/docs/source/license.rst
+++ b/docs/source/license.rst
@@ -4,7 +4,7 @@
 License Agreement
 =================
 
-Copyright © 2011-2025 Massachusetts Institute of Technology, UChicago Argonne
+Copyright © 2011-2026 Massachusetts Institute of Technology, UChicago Argonne
 LLC, and OpenMC contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of
diff --git a/docs/source/methods/charged_particles_physics.rst b/docs/source/methods/charged_particles_physics.rst
new file mode 100644
index 000000000..5d763074f
--- /dev/null
+++ b/docs/source/methods/charged_particles_physics.rst
@@ -0,0 +1,362 @@
+.. _methods_charged_particle_physics:
+
+========================
+Charged Particle Physics
+========================
+
+OpenMC neglects the spatial transport of charged particles (electrons and
+positrons), assuming they deposit all their energy locally and produce
+bremsstrahlung photons at their birth location. This approximation, called
+thick-target bremsstrahlung (TTB) approximation is justified by the fact that
+charged particles have much shorter stopping ranges compared to neutrons and
+photons, especially in high-density materials.
+
+-----------------------------
+Charged Particle Interactions
+-----------------------------
+
+Bremsstrahlung
+--------------
+
+When a charged particle is decelerated in the field of an atom, some of its
+kinetic energy is converted into electromagnetic radiation known as
+bremsstrahlung, or 'braking radiation'. In each event, an electron or positron
+with kinetic energy :math:`T` generates a photon with an energy :math:`E`
+between :math:`0` and :math:`T`. Bremsstrahlung is described by a cross section
+that is differential in photon energy, in the direction of the emitted photon,
+and in the final direction of the charged particle. However, in Monte Carlo
+simulations it is typical to integrate over the angular variables to obtain a
+single differential cross section with respect to photon energy, which is often
+expressed in the form
+
+.. math::
+    :label: bremsstrahlung-dcs
+
+    \frac{d\sigma_{\text{br}}}{dE} = \frac{Z^2}{\beta^2} \frac{1}{E}
+    \chi(Z, T, \kappa),
+
+where :math:`\kappa = E/T` is the reduced photon energy and :math:`\chi(Z, T,
+\kappa)` is the scaled bremsstrahlung cross section, which is experimentally
+measured.
+
+Because electrons are attracted to atomic nuclei whereas positrons are
+repulsed, the cross section for positrons is smaller, though it approaches that
+of electrons in the high energy limit. To obtain the positron cross section, we
+multiply :eq:`bremsstrahlung-dcs` by the :math:`\kappa`-independent factor used
+in Salvat_,
+
+.. math::
+    :label: positron-factor
+
+    \begin{aligned}
+    F_{\text{p}}(Z,T) =
+    & 1 - \text{exp}(-1.2359\times 10^{-1}t + 6.1274\times 10^{-2}t^2 - 3.1516\times 10^{-2}t^3 \\
+    & + 7.7446\times 10^{-3}t^4 - 1.0595\times 10^{-3}t^5 + 7.0568\times 10^{-5}t^6 \\
+    & - 1.8080\times 10^{-6}t^7),
+    \end{aligned}
+
+where
+
+.. math::
+    :label: positron-factor-t
+
+    t = \ln\left(1 + \frac{10^6}{Z^2}\frac{T}{\text{m}_\text{e}c^2} \right).
+
+:math:`F_{\text{p}}(Z,T)` is the ratio of the radiative stopping powers for
+positrons and electrons. Stopping power describes the average energy loss per
+unit path length of a charged particle as it passes through matter:
+
+.. math::
+    :label: stopping-power
+
+    -\frac{dT}{ds} = n \int E \frac{d\sigma}{dE} dE \equiv S(T),
+
+where :math:`n` is the number density of the material and :math:`d\sigma/dE` is
+the cross section differential in energy loss. The total stopping power
+:math:`S(T)` can be separated into two components: the radiative stopping
+power :math:`S_{\text{rad}}(T)`, which refers to energy loss due to
+bremsstrahlung, and the collision stopping power :math:`S_{\text{col}}(T)`,
+which refers to the energy loss due to inelastic collisions with bound
+electrons in the material that result in ionization and excitation. The
+radiative stopping power for electrons is given by
+
+.. math::
+    :label: radiative-stopping-power
+
+    S_{\text{rad}}(T) = n \frac{Z^2}{\beta^2} T \int_0^1 \chi(Z,T,\kappa)
+    d\kappa.
+
+
+To obtain the radiative stopping power for positrons,
+:eq:`radiative-stopping-power`  is multiplied by :eq:`positron-factor`.
+
+While the models for photon interactions with matter described above can safely
+assume interactions occur with free atoms, sampling the target atom based on
+the macroscopic cross sections, molecular effects cannot necessarily be
+disregarded for charged particle treatment. For compounds and mixtures, the
+bremsstrahlung cross section is calculated using Bragg's additivity rule as
+
+.. math::
+    :label: material-bremsstrahlung-dcs
+
+    \frac{d\sigma_{\text{br}}}{dE} = \frac{1}{\beta^2 E} \sum_i \gamma_i Z^2_i
+    \chi(Z_i, T, \kappa),
+
+where the sum is over the constituent elements and :math:`\gamma_i` is the
+atomic fraction of the :math:`i`-th element. Similarly, the radiative stopping
+power is calculated using Bragg's additivity rule as
+
+.. math::
+    :label: material-radiative-stopping-power
+
+    S_{\text{rad}}(T) = \sum_i w_i S_{\text{rad},i}(T),
+
+where :math:`w_i` is the mass fraction of the :math:`i`-th element and
+:math:`S_{\text{rad},i}(T)` is found for element :math:`i` using
+:eq:`radiative-stopping-power`. The collision stopping power, however, is a
+function of certain quantities such as the mean excitation energy :math:`I` and
+the density effect correction :math:`\delta_F` that depend on molecular
+properties. These quantities cannot simply be summed over constituent elements
+in a compound, but should instead be calculated for the material. The Bethe
+formula can be used to find the collision stopping power of the material:
+
+.. math::
+    :label: material-collision-stopping-power
+
+    S_{\text{col}}(T) = \frac{2 \pi r_e^2 m_e c^2}{\beta^2} N_A \frac{Z}{A_M}
+    [\ln(T^2/I^2) + \ln(1 + \tau/2) + F(\tau) - \delta_F(T)],
+
+where :math:`N_A` is Avogadro's number, :math:`A_M` is the molar mass,
+:math:`\tau = T/m_e`, and :math:`F(\tau)` depends on the particle type. For
+electrons,
+
+.. math::
+    :label: F-electron
+
+    F_{-}(\tau) = (1 - \beta^2)[1 + \tau^2/8 - (2\tau + 1) \ln2],
+
+while for positrons
+
+.. math::
+    :label: F-positron
+
+    F_{+}(\tau) = 2\ln2 - (\beta^2/12)[23 + 14/(\tau + 2) + 10/(\tau + 2)^2 +
+    4/(\tau + 2)^3].
+
+The density effect correction :math:`\delta_F` takes into account the reduction
+of the collision stopping power due to the polarization of the material the
+charged particle is passing through by the electric field of the particle.
+It can be evaluated using the method described by Sternheimer_, where the
+equation for :math:`\delta_F` is
+
+.. math::
+    :label: density-effect-correction
+
+    \delta_F(\beta) = \sum_{i=1}^n f_i \ln[(l_i^2 + l^2)/l_i^2] -
+    l^2(1-\beta^2).
+
+Here, :math:`f_i` is the oscillator strength of the :math:`i`-th transition,
+given by :math:`f_i = n_i/Z`, where :math:`n_i` is the number of electrons in
+the :math:`i`-th subshell. The frequency :math:`l` is the solution of the
+equation
+
+.. math::
+    :label: density-effect-l
+
+    \frac{1}{\beta^2} - 1 = \sum_{i=1}^{n} \frac{f_i}{\bar{\nu}_i^2 + l^2},
+
+where :math:`\bar{v}_i` is defined as
+
+.. math::
+    :label: density-effect-nubar
+
+    \bar{\nu}_i = h\nu_i \rho / h\nu_p.
+
+The plasma energy :math:`h\nu_p` of the medium is given by
+
+.. math::
+    :label: plasma-frequency
+
+    h\nu_p = \sqrt{\frac{(hc)^2 r_e \rho_m N_A Z}{\pi A}},
+
+where :math:`A` is the atomic weight and :math:`\rho_m` is the density of the
+material. In :eq:`density-effect-nubar`, :math:`h\nu_i` is the oscillator
+energy, and :math:`\rho` is an adjustment factor introduced to give agreement
+between the experimental values of the oscillator energies and the mean
+excitation energy. The :math:`l_i` in :eq:`density-effect-correction` are
+defined as
+
+.. math::
+    :label: density-effect-li
+
+    \begin{aligned}
+    l_i &= (\bar{\nu}_i^2 + 2/3f_i)^{1/2} ~~~~&\text{for}~~ \bar{\nu}_i > 0 \\
+    l_n &= f_n^{1/2} ~~~~&\text{for}~~ \bar{\nu}_n = 0,
+    \end{aligned}
+
+where the second case applies to conduction electrons. For a conductor,
+:math:`f_n` is given by :math:`n_c/Z`, where :math:`n_c` is the effective
+number of conduction electrons, and :math:`v_n = 0`. The adjustment factor
+:math:`\rho` is determined using the equation for the mean excitation energy:
+
+.. math::
+    :label: mean-excitation-energy
+
+    \ln I = \sum_{i=1}^{n-1} f_i \ln[(h\nu_i\rho)^2 + 2/3f_i(h\nu_p)^2]^{1/2} +
+    f_n \ln (h\nu_pf_n^{1/2}).
+
+.. _ttb:
+
+
+Thick-Target Bremsstrahlung Approximation
++++++++++++++++++++++++++++++++++++++++++
+
+Since charged particles lose their energy on a much shorter distance scale than
+neutral particles, not much error should be introduced by neglecting to
+transport electrons. However, the bremsstrahlung emitted from high energy
+electrons and positrons can travel far from the interaction site. Thus, even
+without a full electron transport mode it is necessary to model bremsstrahlung.
+We use a thick-target bremsstrahlung (TTB) approximation based on the models in
+Salvat_ and Kaltiaisenaho_ for generating bremsstrahlung photons, which assumes
+the charged particle loses all its energy in a single homogeneous material
+region.
+
+To model bremsstrahlung using the TTB approximation, we need to know the number
+of photons emitted by the charged particle and the energy distribution of the
+photons. These quantities can be calculated using the continuous slowing down
+approximation (CSDA). The CSDA assumes charged particles lose energy
+continuously along their trajectory with a rate of energy loss equal to the
+total stopping power, ignoring fluctuations in the energy loss. The
+approximation is useful for expressing average quantities that describe how
+charged particles slow down in matter. For example, the CSDA range approximates
+the average path length a charged particle travels as it slows to rest:
+
+.. math::
+    :label: csda-range
+
+    R(T) = \int^T_0 \frac{dT'}{S(T')}.
+
+Actual path lengths will fluctuate around :math:`R(T)`. The average number of
+photons emitted per unit path length is given by the inverse bremsstrahlung
+mean free path:
+
+.. math::
+    :label: inverse-bremsstrahlung-mfp
+
+    \lambda_{\text{br}}^{-1}(T,E_{\text{cut}})
+    = n\int_{E_{\text{cut}}}^T\frac{d\sigma_{\text{br}}}{dE}dE
+    = n\frac{Z^2}{\beta^2}\int_{\kappa_{\text{cut}}}^1\frac{1}{\kappa}
+    \chi(Z,T,\kappa)d\kappa.
+
+The lower limit of the integral in :eq:`inverse-bremsstrahlung-mfp` is non-zero
+because the bremsstrahlung differential cross section diverges for small photon
+energies but is finite for photon energies above some cutoff energy
+:math:`E_{\text{cut}}`. The mean free path
+:math:`\lambda_{\text{br}}^{-1}(T,E_{\text{cut}})` is used to calculate the
+photon number yield, defined as the average number of photons emitted with
+energy greater than :math:`E_{\text{cut}}` as the charged particle slows down
+from energy :math:`T` to :math:`E_{\text{cut}}`. The photon number yield is
+given by
+
+.. math::
+    :label: photon-number-yield
+
+    Y(T,E_{\text{cut}}) = \int^{R(T)}_{R(E_{\text{cut}})}
+    \lambda_{\text{br}}^{-1}(T',E_{\text{cut}})ds = \int_{E_{\text{cut}}}^T
+    \frac{\lambda_{\text{br}}^{-1}(T',E_{\text{cut}})}{S(T')}dT'.
+
+:math:`Y(T,E_{\text{cut}})` can be used to construct the energy spectrum of
+bremsstrahlung photons: the number of photons created with energy between
+:math:`E_1` and :math:`E_2` by a charged particle with initial kinetic energy
+:math:`T` as it comes to rest is given by :math:`Y(T,E_1) - Y(T,E_2)`.
+
+To simulate the emission of bremsstrahlung photons, the total stopping power
+and bremsstrahlung differential cross section for positrons and electrons must
+be calculated for a given material using :eq:`material-bremsstrahlung-dcs` and
+:eq:`material-radiative-stopping-power`. These quantities are used to build the
+tabulated bremsstrahlung energy PDF and CDF for that material for each incident
+energy :math:`T_k` on the energy grid. The following algorithm is then applied
+to sample the photon energies:
+
+1. For an incident charged particle with energy :math:`T`, sample the number of
+   emitted photons as
+
+   .. math::
+
+       N = \lfloor Y(T,E_{\text{cut}}) + \xi_1 \rfloor.
+
+2. Rather than interpolate the PDF between indices :math:`k` and :math:`k+1`
+   for which :math:`T_k < T < T_{k+1}`, which is computationally expensive, use
+   the composition method and sample from the PDF at either :math:`k` or
+   :math:`k+1`. Using linear interpolation on a logarithmic scale, the PDF can
+   be expressed as
+
+   .. math::
+
+       p_{\text{br}}(T,E) = \pi_k p_{\text{br}}(T_k,E) + \pi_{k+1}
+       p_{\text{br}}(T_{k+1},E),
+
+   where the interpolation weights are
+
+   .. math::
+
+       \pi_k = \frac{\ln T_{k+1} - \ln T}{\ln T_{k+1} - \ln T_k},~~~
+       \pi_{k+1} = \frac{\ln T - \ln T_k}{\ln T_{k+1} - \ln T_k}.
+
+   Sample either the index :math:`i = k` or :math:`i = k+1` according to the
+   point probabilities :math:`\pi_{k}` and :math:`\pi_{k+1}`.
+
+3. Determine the maximum value of the CDF :math:`P_{\text{br,max}}`.
+
+3. Sample the photon energies using the inverse transform method with the
+   tabulated CDF :math:`P_{\text{br}}(T_i, E)` i.e.,
+
+   .. math::
+
+       E = E_j \left[ (1 + a_j) \frac{\xi_2 P_{\text{br,max}} -
+       P_{\text{br}}(T_i, E_j)} {E_j p_{\text{br}}(T_i, E_j)} + 1
+       \right]^{\frac{1}{1 + a_j}}
+
+   where the interpolation factor :math:`a_j` is given by
+
+   .. math::
+
+       a_j = \frac{\ln p_{\text{br}}(T_i,E_{j+1}) - \ln p_{\text{br}}(T_i,E_j)}
+       {\ln E_{j+1} - \ln E_j}
+
+   and :math:`P_{\text{br}}(T_i, E_j) \le \xi_2 P_{\text{br,max}} \le
+   P_{\text{br}}(T_i, E_{j+1})`.
+
+We ignore the range of the electron or positron, i.e., the bremsstrahlung
+photons are produced in the same location that the charged particle was
+created. The direction of the photons is assumed to be the same as the
+direction of the incident charged particle, which is a reasonable approximation
+at higher energies when the bremsstrahlung radiation is emitted at small
+angles.
+
+
+Electron-Positron Annihilation
+------------------------------
+
+When a positron collides with an electron, both particles are annihilated and
+generally two photons with equal energy are created. If the kinetic energy of
+the positron is high enough, the two photons can have different energies, and
+the higher-energy photon is emitted preferentially in the direction of flight
+of the positron. It is also possible to produce a single photon if the
+interaction occurs with a bound electron, and in some cases three (or, rarely,
+even more) photons can be emitted. However, the annihilation cross section is
+largest for low-energy positrons, and as the positron energy decreases, the
+angular distribution of the emitted photons becomes isotropic.
+
+In OpenMC, we assume the most likely case in which a low-energy positron (which
+has already lost most of its energy to bremsstrahlung radiation) interacts with
+an electron which is free and at rest. Two photons with energy equal to the
+electron rest mass energy :math:`m_e c^2 = 0.511` MeV are emitted isotropically
+in opposite directions.
+
+
+.. _Kaltiaisenaho: https://aaltodoc.aalto.fi/bitstream/handle/123456789/21004/master_Kaltiaisenaho_Toni_2016.pdf
+
+.. _Salvat: https://doi.org/10.1787/32da5043-en
+
+.. _Sternheimer: https://doi.org/10.1103/PhysRevB.26.6067
diff --git a/docs/source/methods/cross_sections.rst b/docs/source/methods/cross_sections.rst
index a66abb3ed..764c4c628 100644
--- a/docs/source/methods/cross_sections.rst
+++ b/docs/source/methods/cross_sections.rst
@@ -289,6 +289,48 @@ sections. This allows flexibility for the model to use highly anisotropic
 scattering information in the water while the fuel can be simulated with linear
 or even isotropic scattering.
 
+Particle Speed
+--------------
+
+When using a multigroup representation of cross sections, the particle speed has
+meaning only in an average sense. The particle speed is important when modeling
+dynamic behavior. OpenMC calculates the particle speed using the inverse
+velocity multigroup data if it is available. If such data is not available,
+OpenMC uses an approximate velocity using the group energy bounds in the
+following way:
+
+.. math::
+
+   \frac{1}{v_g} = \int_{E_{\text{min}}^g}^{E_{\text{max}}^g} \frac{1}{v(E)} \frac{\alpha}{E} dE
+
+Where :math:`E_{\text{min}}^g` and :math:`E_{\text{max}}^g` are the group energy
+boundaries for group :math:`g`. :math:`v(E)` is the neutron velocity calculated
+using relativistic kinematics, :math:`\alpha` is a normalization constant for the
+:math:`\frac{1}{E}` spectrum.
+
+This equation is valid when inside the group boundaries the neutron spectrum
+follows a typical :math:`\frac{1}{E}` slowing down spectrum. This assumption is
+widely used when generating fine group neutron cross section data libraries from
+continuous energy data.
+
+The solution to this equation is:
+
+.. math::
+
+   \frac{1}{v_g} = \frac{1}{c \log\left(\frac{E_{\text{max}}^g}{E_{\text{min}}^g}\right)}
+   \left[ 2(\operatorname{arctanh}(k_{\text{max}}^{-1}) - \operatorname{arctanh}(k_{\text{min}}^{-1}))
+   - (k_{\text{max}}-k_{\text{min}}) \right]
+
+where :math:`c` is the speed of light and :math:`k_{\text{max}}`,
+:math:`k_{\text{min}}` are defined by a change of variables:
+
+.. math::
+
+   k = \sqrt{1+\frac{2 m_n c^2}{E}}
+
+where :math:`E` is the particle kinetic energy and :math:`m_n` is the neutron
+rest mass.
+
 .. _logarithmic mapping technique:
    https://mcnp.lanl.gov/pdf_files/TechReport_2014_LANL_LA-UR-14-24530_Brown.pdf
 .. _Hwang: https://doi.org/10.13182/NSE87-A16381
diff --git a/docs/source/methods/energy_deposition.rst b/docs/source/methods/energy_deposition.rst
index 4675aee3f..c43ee64ac 100644
--- a/docs/source/methods/energy_deposition.rst
+++ b/docs/source/methods/energy_deposition.rst
@@ -25,19 +25,38 @@ KERMA (Kinetic Energy Release in Materials) [Mack97]_ coefficients for reaction
 :math:`\times` cross-section (e.g., eV-barn) and can be used much like a reaction
 cross section for the purpose of tallying energy deposition.
 
-KERMA coefficients can be computed using the energy-balance method with
-a nuclear data processing code like NJOY, which performs the following
-iteration over all reactions :math:`r` for all isotopes :math:`i`
-requested
+KERMA coefficients can be computed using the energy-balance method with a
+nuclear data processing code like NJOY, which estimates the KERMA coefficients
+using the following equation:
 
 .. math::
 
-    k_{i, r}(E) = \left(E + Q_{i, r} - \bar{E}_{i, r, n}
+    k_{i, r}(E) = \left(E + Q_{i, r} - \sum\limits_x \bar{E}_{i, r, x}
+    \right)\sigma_{i, r}(E),
+
+where the summation is over each secondary particle type :math:`x`. This
+equation states that the energy deposited is equal to the energy of the incident
+particle plus the reaction :math:`Q` value less the energy of secondary
+particles that are transported away from the reaction site. For neutron
+interactions, the energy-balance KERMA coefficient is
+
+.. math::
+
+    k_{i, r}(E) = \left(E + Q_{i, r} - \sum\limits_x \bar{E}_{i, r, n}
     - \bar{E}_{i, r, \gamma}\right)\sigma_{i, r}(E),
 
-removing the energy of neutral particles (neutrons and photons) that are
-transported away from the reaction site :math:`\bar{E}`, and the reaction
-:math:`Q` value.
+where :math:`\bar{E}_{i, r, n}` is the average energy of secondary neutrons and
+:math:`\bar{E}_{i, r, \gamma}` is the average energy of secondary photons. For
+photon and charged particle interactions the KERMA coefficient is
+
+.. math::
+    :label: energy-balance-photon
+
+    k_{i, r}(E) = \left(E + Q_{i, r} - \sum\limits_x \bar{E}_{i, r, x}
+    \right)\sigma_{i, r}(E).
+
+where the :math:`Q` value is zero for all interactions except for pair
+production and positron annihilation.
 
 -------
 Fission
@@ -120,7 +139,7 @@ run with :math:`N918` reflecting fission heating computed from NJOY.
 This modified heating data is stored as the MT=901 reaction and will be scored
 if ``heating-local`` is included in :attr:`openmc.Tally.scores`.
 
-Coupled neutron-photon transport
+Coupled Neutron-Photon Transport
 --------------------------------
 
 Here, OpenMC instructs ``heatr`` to assume that energy from photons is not
@@ -138,6 +157,50 @@ Let :math:`N301` represent the total heating number returned from this
 This modified heating data is stored as the MT=301 reaction and will be scored
 if ``heating`` is included in :attr:`openmc.Tally.scores`.
 
+Photons and Charged Particles
+-----------------------------
+
+In OpenMC, energy deposition from photons or charged particles is scored using
+the energy balance method based on Equation :eq:`energy-balance-photon`. Special
+consideration is given to electrons and positrons as described below.
+
++++++++++++++++++
+Charged Particles
++++++++++++++++++
+
+OpenMC tracks photons interaction by interaction so the energy deposited in each
+collision is easily attributed back to the nuclide and reaction for which the
+photon interacted with. Charged particles (electrons and photons) aren't tracked
+in the same way. For charged particles, OpenMC assumes that all their energy
+(less the energy of bremsstrahlung radiation) is deposited in the material in
+which they were born. In this way it is harder to trace how much energy should
+be attributed in each nuclide.
+
+According to the CSDA approximation (see :ref:`ttb`) the energy deposited by a
+charged particle with kinetic energy :math:`T` in the :math:`i`-th element can
+be calculated as:
+
+.. math::
+
+    E_{i} = \int_{0}^{R(T)} w_{i}S_{\text{col,i}} dx
+
+where :math:`R(T)` is the CSDA range of the charged particle,
+:math:`S_{\text{col},i}` is the collision stopping power of the charged particle
+in the :math:`i`-th element and :math:`w_i` is the mass fraction of the
+:math:`i`-th element. According to the Bethe formula the collision stopping
+power of the :math:`i`-th element is proportional to :math:`Z_i/A_i`, so the
+fractional collision stopping power from the :math:`i`-th element is:
+
+.. math::
+
+    \frac{w_{i}S_{\text{col},i}(T)}{S_{\text{col}}(T)} =
+    \frac{\frac{w_{i}Z_{i}}{A_{i}}}{\sum_{i}\frac{w_{i}Z_{i}}{A_{i}}} =
+    \frac{\gamma_i Z_{i}}{\sum_{i}\gamma_i Z_{i}}.
+
+where :math:`\gamma_i` is the atomic fraction of the :math:`i`-th element.
+Therefore, the energy deposited by charged particles should be attributed to
+a given element according to its fractional charge density.
+
 ----------
 References
 ----------
diff --git a/docs/source/methods/index.rst b/docs/source/methods/index.rst
index 75c421c87..121d04b1d 100644
--- a/docs/source/methods/index.rst
+++ b/docs/source/methods/index.rst
@@ -14,6 +14,7 @@ Theory and Methodology
     random_numbers
     neutron_physics
     photon_physics
+    charged_particles_physics
     tallies
     eigenvalue
     depletion
@@ -21,4 +22,4 @@ Theory and Methodology
     parallelization
     cmfd
     variance_reduction
-    random_ray
\ No newline at end of file
+    random_ray
diff --git a/docs/source/methods/neutron_physics.rst b/docs/source/methods/neutron_physics.rst
index fe8b8ad85..2b797e3db 100644
--- a/docs/source/methods/neutron_physics.rst
+++ b/docs/source/methods/neutron_physics.rst
@@ -290,7 +290,10 @@ create and store fission sites for the following generation. First, the average
 number of prompt and delayed neutrons must be determined to decide whether the
 secondary neutrons will be prompt or delayed. This is important because delayed
 neutrons have a markedly different spectrum from prompt neutrons, one that has a
-lower average energy of emission. The total number of neutrons emitted
+lower average energy of emission. Furthermore, in simulations where tracking
+time of neutrons is important, we need to consider the emission time delay of
+the secondary neutrons, which is dependent on the decay constant of the
+delayed neutron precursor. The total number of neutrons emitted
 :math:`\nu_t` is given as a function of incident energy in the ENDF format. Two
 representations exist for :math:`\nu_t`. The first is a polynomial of order
 :math:`N` with coefficients :math:`c_0,c_1,\dots,c_N`. If :math:`\nu_t` has this
@@ -306,8 +309,8 @@ interpolation law. The number of prompt neutrons released per fission event
 :math:`\nu_p` is also given as a function of incident energy and can be
 specified in a polynomial or tabular format. The number of delayed neutrons
 released per fission event :math:`\nu_d` can only be specified in a tabular
-format. In practice, we only need to determine :math:`nu_t` and
-:math:`nu_d`. Once these have been determined, we can calculated the delayed
+format. In practice, we only need to determine :math:`\nu_t` and
+:math:`\nu_d`. Once these have been determined, we can calculate the delayed
 neutron fraction
 
 .. math::
@@ -335,8 +338,14 @@ neutrons. Otherwise, we produce :math:`\lfloor \nu \rfloor + 1` neutrons. Then,
 for each fission site produced, we sample the outgoing angle and energy
 according to the algorithms given in :ref:`sample-angle` and
 :ref:`sample-energy` respectively. If the neutron is to be born delayed, then
-there is an extra step of sampling a delayed neutron precursor group since they
-each have an associated secondary energy distribution.
+there is an extra step of sampling a delayed neutron precursor group to get the
+associated secondary energy distribution and the decay constant
+:math:`\lambda`, which is needed to sample the emission delay time :math:`t_d`:
+
+.. math::
+    :label: sample-delay-time
+
+    t_d = -\frac{\ln \xi}{\lambda}.
 
 The sampled outgoing angle and energy of fission neutrons along with the
 position of the collision site are stored in an array called the fission
diff --git a/docs/source/methods/photon_physics.rst b/docs/source/methods/photon_physics.rst
index 22d2c7f26..d2bd3ac76 100644
--- a/docs/source/methods/photon_physics.rst
+++ b/docs/source/methods/photon_physics.rst
@@ -667,342 +667,6 @@ and Auger electrons:
 
 5. Repeat from step 1 for vacancy left by the transition electron.
 
-Electron-Positron Annihilation
-------------------------------
-
-When a positron collides with an electron, both particles are annihilated and
-generally two photons with equal energy are created. If the kinetic energy of
-the positron is high enough, the two photons can have different energies, and
-the higher-energy photon is emitted preferentially in the direction of flight
-of the positron. It is also possible to produce a single photon if the
-interaction occurs with a bound electron, and in some cases three (or, rarely,
-even more) photons can be emitted. However, the annihilation cross section is
-largest for low-energy positrons, and as the positron energy decreases, the
-angular distribution of the emitted photons becomes isotropic.
-
-In OpenMC, we assume the most likely case in which a low-energy positron (which
-has already lost most of its energy to bremsstrahlung radiation) interacts with
-an electron which is free and at rest. Two photons with energy equal to the
-electron rest mass energy :math:`m_e c^2 = 0.511` MeV are emitted isotropically
-in opposite directions.
-
-Bremsstrahlung
---------------
-
-When a charged particle is decelerated in the field of an atom, some of its
-kinetic energy is converted into electromagnetic radiation known as
-bremsstrahlung, or 'braking radiation'. In each event, an electron or positron
-with kinetic energy :math:`T` generates a photon with an energy :math:`E`
-between :math:`0` and :math:`T`. Bremsstrahlung is described by a cross section
-that is differential in photon energy, in the direction of the emitted photon,
-and in the final direction of the charged particle. However, in Monte Carlo
-simulations it is typical to integrate over the angular variables to obtain a
-single differential cross section with respect to photon energy, which is often
-expressed in the form
-
-.. math::
-    :label: bremsstrahlung-dcs
-
-    \frac{d\sigma_{\text{br}}}{dE} = \frac{Z^2}{\beta^2} \frac{1}{E}
-    \chi(Z, T, \kappa),
-
-where :math:`\kappa = E/T` is the reduced photon energy and :math:`\chi(Z, T,
-\kappa)` is the scaled bremsstrahlung cross section, which is experimentally
-measured.
-
-Because electrons are attracted to atomic nuclei whereas positrons are
-repulsed, the cross section for positrons is smaller, though it approaches that
-of electrons in the high energy limit. To obtain the positron cross section, we
-multiply :eq:`bremsstrahlung-dcs` by the :math:`\kappa`-independent factor used
-in Salvat_,
-
-.. math::
-    :label: positron-factor
-
-    \begin{aligned}
-    F_{\text{p}}(Z,T) =
-    & 1 - \text{exp}(-1.2359\times 10^{-1}t + 6.1274\times 10^{-2}t^2 - 3.1516\times 10^{-2}t^3 \\
-    & + 7.7446\times 10^{-3}t^4 - 1.0595\times 10^{-3}t^5 + 7.0568\times 10^{-5}t^6 \\
-    & - 1.8080\times 10^{-6}t^7),
-    \end{aligned}
-
-where
-
-.. math::
-    :label: positron-factor-t
-
-    t = \ln\left(1 + \frac{10^6}{Z^2}\frac{T}{\text{m}_\text{e}c^2} \right).
-
-:math:`F_{\text{p}}(Z,T)` is the ratio of the radiative stopping powers for
-positrons and electrons. Stopping power describes the average energy loss per
-unit path length of a charged particle as it passes through matter:
-
-.. math::
-    :label: stopping-power
-
-    -\frac{dT}{ds} = n \int E \frac{d\sigma}{dE} dE \equiv S(T),
-
-where :math:`n` is the number density of the material and :math:`d\sigma/dE` is
-the cross section differential in energy loss. The total stopping power
-:math:`S(T)` can be separated into two components: the radiative stopping
-power :math:`S_{\text{rad}}(T)`, which refers to energy loss due to
-bremsstrahlung, and the collision stopping power :math:`S_{\text{col}}(T)`,
-which refers to the energy loss due to inelastic collisions with bound
-electrons in the material that result in ionization and excitation. The
-radiative stopping power for electrons is given by
-
-.. math::
-    :label: radiative-stopping-power
-
-    S_{\text{rad}}(T) = n \frac{Z^2}{\beta^2} T \int_0^1 \chi(Z,T,\kappa)
-    d\kappa.
-
-
-To obtain the radiative stopping power for positrons,
-:eq:`radiative-stopping-power`  is multiplied by :eq:`positron-factor`.
-
-While the models for photon interactions with matter described above can safely
-assume interactions occur with free atoms, sampling the target atom based on
-the macroscopic cross sections, molecular effects cannot necessarily be
-disregarded for charged particle treatment. For compounds and mixtures, the
-bremsstrahlung cross section is calculated using Bragg's additivity rule as
-
-.. math::
-    :label: material-bremsstrahlung-dcs
-
-    \frac{d\sigma_{\text{br}}}{dE} = \frac{1}{\beta^2 E} \sum_i \gamma_i Z^2_i
-    \chi(Z_i, T, \kappa),
-
-where the sum is over the constituent elements and :math:`\gamma_i` is the
-atomic fraction of the :math:`i`-th element. Similarly, the radiative stopping
-power is calculated using Bragg's additivity rule as
-
-.. math::
-    :label: material-radiative-stopping-power
-
-    S_{\text{rad}}(T) = \sum_i w_i S_{\text{rad},i}(T),
-
-where :math:`w_i` is the mass fraction of the :math:`i`-th element and
-:math:`S_{\text{rad},i}(T)` is found for element :math:`i` using
-:eq:`radiative-stopping-power`. The collision stopping power, however, is a
-function of certain quantities such as the mean excitation energy :math:`I` and
-the density effect correction :math:`\delta_F` that depend on molecular
-properties. These quantities cannot simply be summed over constituent elements
-in a compound, but should instead be calculated for the material. The Bethe
-formula can be used to find the collision stopping power of the material:
-
-.. math::
-    :label: material-collision-stopping-power
-
-    S_{\text{col}}(T) = \frac{2 \pi r_e^2 m_e c^2}{\beta^2} N_A \frac{Z}{A_M}
-    [\ln(T^2/I^2) + \ln(1 + \tau/2) + F(\tau) - \delta_F(T)],
-
-where :math:`N_A` is Avogadro's number, :math:`A_M` is the molar mass,
-:math:`\tau = T/m_e`, and :math:`F(\tau)` depends on the particle type. For
-electrons,
-
-.. math::
-    :label: F-electron
-
-    F_{-}(\tau) = (1 - \beta^2)[1 + \tau^2/8 - (2\tau + 1) \ln2],
-
-while for positrons
-
-.. math::
-    :label: F-positron
-
-    F_{+}(\tau) = 2\ln2 - (\beta^2/12)[23 + 14/(\tau + 2) + 10/(\tau + 2)^2 +
-    4/(\tau + 2)^3].
-
-The density effect correction :math:`\delta_F` takes into account the reduction
-of the collision stopping power due to the polarization of the material the
-charged particle is passing through by the electric field of the particle.
-It can be evaluated using the method described by Sternheimer_, where the
-equation for :math:`\delta_F` is
-
-.. math::
-    :label: density-effect-correction
-
-    \delta_F(\beta) = \sum_{i=1}^n f_i \ln[(l_i^2 + l^2)/l_i^2] -
-    l^2(1-\beta^2).
-
-Here, :math:`f_i` is the oscillator strength of the :math:`i`-th transition,
-given by :math:`f_i = n_i/Z`, where :math:`n_i` is the number of electrons in
-the :math:`i`-th subshell. The frequency :math:`l` is the solution of the
-equation
-
-.. math::
-    :label: density-effect-l
-
-    \frac{1}{\beta^2} - 1 = \sum_{i=1}^{n} \frac{f_i}{\bar{\nu}_i^2 + l^2},
-
-where :math:`\bar{v}_i` is defined as
-
-.. math::
-    :label: density-effect-nubar
-
-    \bar{\nu}_i = h\nu_i \rho / h\nu_p.
-
-The plasma energy :math:`h\nu_p` of the medium is given by
-
-.. math::
-    :label: plasma-frequency
-
-    h\nu_p = \sqrt{\frac{(hc)^2 r_e \rho_m N_A Z}{\pi A}},
-
-where :math:`A` is the atomic weight and :math:`\rho_m` is the density of the
-material. In :eq:`density-effect-nubar`, :math:`h\nu_i` is the oscillator
-energy, and :math:`\rho` is an adjustment factor introduced to give agreement
-between the experimental values of the oscillator energies and the mean
-excitation energy. The :math:`l_i` in :eq:`density-effect-correction` are
-defined as
-
-.. math::
-    :label: density-effect-li
-
-    \begin{aligned}
-    l_i &= (\bar{\nu}_i^2 + 2/3f_i)^{1/2} ~~~~&\text{for}~~ \bar{\nu}_i > 0 \\
-    l_n &= f_n^{1/2} ~~~~&\text{for}~~ \bar{\nu}_n = 0,
-    \end{aligned}
-
-where the second case applies to conduction electrons. For a conductor,
-:math:`f_n` is given by :math:`n_c/Z`, where :math:`n_c` is the effective
-number of conduction electrons, and :math:`v_n = 0`. The adjustment factor
-:math:`\rho` is determined using the equation for the mean excitation energy:
-
-.. math::
-    :label: mean-excitation-energy
-
-    \ln I = \sum_{i=1}^{n-1} f_i \ln[(h\nu_i\rho)^2 + 2/3f_i(h\nu_p)^2]^{1/2} +
-    f_n \ln (h\nu_pf_n^{1/2}).
-
-.. _ttb:
-
-Thick-Target Bremsstrahlung Approximation
-+++++++++++++++++++++++++++++++++++++++++
-
-Since charged particles lose their energy on a much shorter distance scale than
-neutral particles, not much error should be introduced by neglecting to
-transport electrons. However, the bremsstrahlung emitted from high energy
-electrons and positrons can travel far from the interaction site. Thus, even
-without a full electron transport mode it is necessary to model bremsstrahlung.
-We use a thick-target bremsstrahlung (TTB) approximation based on the models in
-Salvat_ and Kaltiaisenaho_ for generating bremsstrahlung photons, which assumes
-the charged particle loses all its energy in a single homogeneous material
-region.
-
-To model bremsstrahlung using the TTB approximation, we need to know the number
-of photons emitted by the charged particle and the energy distribution of the
-photons. These quantities can be calculated using the continuous slowing down
-approximation (CSDA). The CSDA assumes charged particles lose energy
-continuously along their trajectory with a rate of energy loss equal to the
-total stopping power, ignoring fluctuations in the energy loss. The
-approximation is useful for expressing average quantities that describe how
-charged particles slow down in matter. For example, the CSDA range approximates
-the average path length a charged particle travels as it slows to rest:
-
-.. math::
-    :label: csda-range
-
-    R(T) = \int^T_0 \frac{dT'}{S(T')}.
-
-Actual path lengths will fluctuate around :math:`R(T)`. The average number of
-photons emitted per unit path length is given by the inverse bremsstrahlung
-mean free path:
-
-.. math::
-    :label: inverse-bremsstrahlung-mfp
-
-    \lambda_{\text{br}}^{-1}(T,E_{\text{cut}})
-    = n\int_{E_{\text{cut}}}^T\frac{d\sigma_{\text{br}}}{dE}dE
-    = n\frac{Z^2}{\beta^2}\int_{\kappa_{\text{cut}}}^1\frac{1}{\kappa}
-    \chi(Z,T,\kappa)d\kappa.
-
-The lower limit of the integral in :eq:`inverse-bremsstrahlung-mfp` is non-zero
-because the bremsstrahlung differential cross section diverges for small photon
-energies but is finite for photon energies above some cutoff energy
-:math:`E_{\text{cut}}`. The mean free path
-:math:`\lambda_{\text{br}}^{-1}(T,E_{\text{cut}})` is used to calculate the
-photon number yield, defined as the average number of photons emitted with
-energy greater than :math:`E_{\text{cut}}` as the charged particle slows down
-from energy :math:`T` to :math:`E_{\text{cut}}`. The photon number yield is
-given by
-
-.. math::
-    :label: photon-number-yield
-
-    Y(T,E_{\text{cut}}) = \int^{R(T)}_{R(E_{\text{cut}})}
-    \lambda_{\text{br}}^{-1}(T',E_{\text{cut}})ds = \int_{E_{\text{cut}}}^T
-    \frac{\lambda_{\text{br}}^{-1}(T',E_{\text{cut}})}{S(T')}dT'.
-
-:math:`Y(T,E_{\text{cut}})` can be used to construct the energy spectrum of
-bremsstrahlung photons: the number of photons created with energy between
-:math:`E_1` and :math:`E_2` by a charged particle with initial kinetic energy
-:math:`T` as it comes to rest is given by :math:`Y(T,E_1) - Y(T,E_2)`.
-
-To simulate the emission of bremsstrahlung photons, the total stopping power
-and bremsstrahlung differential cross section for positrons and electrons must
-be calculated for a given material using :eq:`material-bremsstrahlung-dcs` and
-:eq:`material-radiative-stopping-power`. These quantities are used to build the
-tabulated bremsstrahlung energy PDF and CDF for that material for each incident
-energy :math:`T_k` on the energy grid. The following algorithm is then applied
-to sample the photon energies:
-
-1. For an incident charged particle with energy :math:`T`, sample the number of
-   emitted photons as
-
-   .. math::
-
-       N = \lfloor Y(T,E_{\text{cut}}) + \xi_1 \rfloor.
-
-2. Rather than interpolate the PDF between indices :math:`k` and :math:`k+1`
-   for which :math:`T_k < T < T_{k+1}`, which is computationally expensive, use
-   the composition method and sample from the PDF at either :math:`k` or
-   :math:`k+1`. Using linear interpolation on a logarithmic scale, the PDF can
-   be expressed as
-
-   .. math::
-
-       p_{\text{br}}(T,E) = \pi_k p_{\text{br}}(T_k,E) + \pi_{k+1}
-       p_{\text{br}}(T_{k+1},E),
-
-   where the interpolation weights are
-
-   .. math::
-
-       \pi_k = \frac{\ln T_{k+1} - \ln T}{\ln T_{k+1} - \ln T_k},~~~
-       \pi_{k+1} = \frac{\ln T - \ln T_k}{\ln T_{k+1} - \ln T_k}.
-
-   Sample either the index :math:`i = k` or :math:`i = k+1` according to the
-   point probabilities :math:`\pi_{k}` and :math:`\pi_{k+1}`.
-
-3. Determine the maximum value of the CDF :math:`P_{\text{br,max}}`.
-
-3. Sample the photon energies using the inverse transform method with the
-   tabulated CDF :math:`P_{\text{br}}(T_i, E)` i.e.,
-
-   .. math::
-
-       E = E_j \left[ (1 + a_j) \frac{\xi_2 P_{\text{br,max}} -
-       P_{\text{br}}(T_i, E_j)} {E_j p_{\text{br}}(T_i, E_j)} + 1
-       \right]^{\frac{1}{1 + a_j}}
-
-   where the interpolation factor :math:`a_j` is given by
-
-   .. math::
-
-       a_j = \frac{\ln p_{\text{br}}(T_i,E_{j+1}) - \ln p_{\text{br}}(T_i,E_j)}
-       {\ln E_{j+1} - \ln E_j}
-
-   and :math:`P_{\text{br}}(T_i, E_j) \le \xi_2 P_{\text{br,max}} \le
-   P_{\text{br}}(T_i, E_{j+1})`.
-
-We ignore the range of the electron or positron, i.e., the bremsstrahlung
-photons are produced in the same location that the charged particle was
-created. The direction of the photons is assumed to be the same as the
-direction of the incident charged particle, which is a reasonable approximation
-at higher energies when the bremsstrahlung radiation is emitted at small
-angles.
 
 .. _photon_production:
 
@@ -1070,5 +734,3 @@ emitted photon.
 .. _Kaltiaisenaho: https://aaltodoc.aalto.fi/bitstream/handle/123456789/21004/master_Kaltiaisenaho_Toni_2016.pdf
 
 .. _Salvat: https://doi.org/10.1787/32da5043-en
-
-.. _Sternheimer: https://doi.org/10.1103/PhysRevB.26.6067
diff --git a/docs/source/methods/random_ray.rst b/docs/source/methods/random_ray.rst
index eb22b0544..8bc2a0a1b 100644
--- a/docs/source/methods/random_ray.rst
+++ b/docs/source/methods/random_ray.rst
@@ -109,9 +109,7 @@ terms on the right hand side.
 In Equation :eq:`transport`, :math:`\psi` is the angular neutron flux. This
 parameter represents the total distance traveled by all neutrons in a particular
 direction inside of a control volume per second, and is often given in units of
-:math:`1/(\text{cm}^{2} \text{s})`. As OpenMC does not support time dependence
-in the random ray solver mode, we consider the steady state equation, where the
-units of flux become :math:`1/\text{cm}^{2}`. The angular direction unit vector,
+:math:`1/(\text{cm}^{2} \text{s})`. The angular direction unit vector,
 :math:`\mathbf{\Omega}`, represents the direction of travel for the neutron. The
 spatial position vector, :math:`\mathbf{r}`,  represents the location within the
 simulation. The neutron energy, :math:`E`, or speed in continuous space, is
@@ -1052,7 +1050,8 @@ random ray and Monte Carlo, however.
   regions. Thus, in the OpenMC implementation of random ray, particle sources
   are restricted to being volumetric and isotropic, although different energy
   spectrums are supported. Fixed sources can be applied to specific materials,
-  cells, or universes.
+  cells, or universes. Point sources are "smeared" to fill the volume of the
+  source region that contains the point source coordinate.
 
 - **Inactive batches:** In Monte Carlo, use of a fixed source implies that all
   batches are active batches, as there is no longer a need to develop a fission
@@ -1082,28 +1081,32 @@ lifetimes.
 
 In OpenMC, the random ray adjoint solver is implemented simply by transposing
 the scattering matrix, swapping :math:`\nu\Sigma_f` and :math:`\chi`, and then
-running a normal transport solve. When no external fixed source is present, no
-additional changes are needed in the transport process. However, if an external
-fixed forward source is present in the simulation problem, then an additional
-step is taken to compute the accompanying fixed adjoint source. In OpenMC, the
-adjoint flux does *not* represent a response function for a particular detector
-region. Rather, the adjoint flux is the global response, making it appropriate
-for use with weight window generation schemes for global variance reduction.
-Thus, if using a fixed source, the external source for the adjoint mode is
-simply computed as being :math:`1 / \phi`, where :math:`\phi` is the forward
-scalar flux that results from a normal forward solve (which OpenMC will run
-first automatically when in adjoint mode). The adjoint external source will be
-computed for each source region in the simulation mesh, independent of any
-tallies. The adjoint external source is always flat, even when a linear
-scattering and fission source shape is used. When in adjoint mode, all reported
-results (e.g., tallies, eigenvalues, etc.) are derived from the adjoint flux,
-even when the physical meaning is not necessarily obvious. These values are
-still reported, though we emphasize that the primary use case for adjoint mode
-is for producing adjoint flux tallies to support subsequent perturbation studies
-and weight window generation.
+running a normal transport solve. When no external fixed forward source is 
+present, or if an adjoint fixed source is specifically provided, no additional 
+changes are needed in the transport process. This adjoint source can 
+correspond, for example, to a detector response function in a particular 
+region. However, if an external fixed forward source is present in the 
+simulation problem without an adjoint fixed source, an additional step is taken 
+to compute the accompanying forward-weighted adjoint source. In this case, the
+adjoint flux does *not* represent the importance of locations in phase space to 
+detector response; rather, the "response" in question is a uniform distribution 
+of Monte Carlo particle density, making the importance provided by the adjoint 
+flux appropriate for use with weight window generation schemes for global 
+variance reduction. Thus, if using a fixed source, the forward-weighted 
+external source for adjoint mode is simply computed as being :math:`1 / \phi`, 
+where :math:`\phi` is the forward scalar flux that results from a normal 
+forward solve (which OpenMC will run first automatically when in adjoint mode). 
+The adjoint external source will be computed for each source region in the 
+simulation mesh, independent of any tallies. The adjoint external source is 
+always flat, even when a linear scattering and fission source shape is used. 
 
-Note that the adjoint :math:`k_{eff}` is statistically the same as the forward
-:math:`k_{eff}`, despite the flux distributions taking different shapes.
+When in adjoint mode, all reported results (e.g., tallies, eigenvalues, etc.) 
+are derived from the adjoint flux, even when the physical meaning is not 
+necessarily obvious. These values are still reported, though we emphasize that 
+the primary use case for adjoint mode is for producing adjoint flux tallies to 
+support subsequent perturbation studies and weight window generation. Note 
+however that the adjoint :math:`k_{eff}` is statistically the same as the 
+forward :math:`k_{eff}`, despite the flux distributions taking different shapes.
 
 ---------------------------
 Fundamental Sources of Bias
diff --git a/docs/source/methods/tallies.rst b/docs/source/methods/tallies.rst
index 57e05d84f..f2c22ab22 100644
--- a/docs/source/methods/tallies.rst
+++ b/docs/source/methods/tallies.rst
@@ -205,7 +205,71 @@ had a collision at every event. Thus, for tallies with outgoing-energy filters
 or for tallies of scattering moments (which require the scattering cosine of
 the change-in-angle), we must use an analog estimator.
 
-.. TODO: Add description of surface current tallies
+-----------------------------------
+Surface-Integrated Flux and Current
+-----------------------------------
+
+Surface tallies allow you to measure particle behavior as they cross specific
+boundaries in your geometry. Unlike volume tallies, which integrate over a
+volumetric region, surface tallies capture the current or flux passing through a
+surface. Surface tallies are estimated using an analog estimator.
+
+Current Score
+-------------
+
+When tallying the current across a surface, we simply count the weight of
+particles that cross the surface of interest:
+
+
+.. math::
+    :label: analog-current-estimator
+
+    J = \frac{1}{W} \sum_{i \in S} w_i.
+
+where :math:`J` is the area-integrated current passing through surface
+:math:`S`, :math:`W` is the total starting weight of the particles, and
+:math:`w_i` is the weight of the particle as it crosses the surface :math:`S`.
+
+Flux Score
+----------
+
+When tallying flux over a surface, we use the relationship between current and
+flux:
+
+
+.. math::
+    :label: surface-flux-estimator
+
+    \phi_S = \frac{1}{W} \sum_{i \in S} \frac{w_i}{|\mu|}.
+
+where :math:`\phi_S` is the area-integrated flux over surface :math:`S`,
+:math:`W` is the total starting weight of the particles, :math:`w_i` is the
+weight of the particle as it crosses the surface :math:`S` and :math:`\mu` is
+the cosine of angle between the particle direction and the surface normal.
+
+This equation diverges when the particle crossing the surface is nearly parallel
+to it (that is, as :math:`\mu` approaches zero). To remove this divergence,
+OpenMC scores:
+
+.. math::
+    :label: modified-surface-flux-estimator
+
+    \phi_S = \frac{1}{W} \sum_{i \in S} w_i f(\mu).
+
+and the function :math:`f` is defined by:
+
+.. math::
+     f(\mu) = \begin{cases}
+      \frac{1}{|\mu|} & |\mu| > \mu_\text{cut} \\
+      \frac{1}{c\mu_\text{cut}} & |\mu| \le \mu_\text{cut}
+     \end{cases}
+
+where :math:`\mu_\text{cut}` is the grazing cosine cutoff and :math:`c` is the
+cosine substitution ratio. The parameters :math:`\mu_\text{cut}` and :math:`c`
+can be set by the user via the :attr:`openmc.Settings.surface_grazing_cutoff`
+and :attr:`openmc.Settings.surface_grazing_ratio` attributes, respectively. The
+default values for these parameters are 0.001 and 0.5 as recommended by
+`Favorite, Thomas, and Booth <https://doi.org/10.13182/NSE09-72>`_.
 
 .. _tallies_statistics:
 
@@ -387,6 +451,130 @@ of this is that the longer you run a simulation, the better you know your
 results. Therefore, by running a simulation long enough, it is possible to
 reduce the stochastic uncertainty to arbitrarily low levels.
 
+Skewness
+++++++++
+
+The `skewness`_ of a population quantifies the asymmetry of the probability
+distribution around its mean. Positive and negative skewness indicate a
+longer/heavier right and left tail respectively. Let :math:`x_1,\ldots,x_n` be
+the per-realization values for a bin, with sample mean :math:`\bar{x}` and
+sample central moments:
+
+.. math::
+
+   m_k \;=\; \frac{1}{n}\sum_{i=1}^{n}\bigl(x_i-\bar{x}\bigr)^k.
+
+OpenMC reports the *adjusted Fisher-Pearson skewness* (defined for :math:`n \ge
+3`), which is commonly used in many statistical packages:
+
+.. math::
+
+   G_1 \;=\; \frac{\sqrt{n \cdot (n-1)}}{\,n-2\,}\cdot\frac{m_3}{m_2^{3/2}}.
+
+where :math:`m_2` and :math:`m_3` correspond to the biased sample second and
+third central moment respectively.
+
+Kurtosis
+++++++++
+
+The `kurtosis`_ of a population quantifies tail weight (also called tailedness)
+of the probability distribution relative to a normal distribution. Positive
+excess kurtosis indicates *heavier tails* whereas negative excess kurtosis
+indicates *lighter tails*. Kurtosis is especially useful for identifying bins
+where occasional extreme scores dominate uncertainty. OpenMC reports the
+*adjusted excess kurtosis* (defined for :math:`n \ge 4`):
+
+.. math::
+
+   G_2 \;=\; \frac{(n-1)}{(n-2)(n-3)}
+             \left[(n+1)\,\frac{m_4}{m_2^{2}} \;-\; 3(n-1)\right].
+
+where :math:`m_2` and :math:`m_4` correspond to the biased sample second and
+fourth central moment respectively. For a perfectly normal distribution, the
+excess kurtosis is :math:`0`.
+
+Variance of Variance
+++++++++++++++++++++
+
+The variance of the variance (also known as the coefficient of variation
+squared) measures *stability of the sample variance* :math:`s^2` and, by
+extension, the reliability of reported relative errors. High VOV means that
+error bars themselves are noisy—often due to heavy tails, skewness, or too few
+realizations.
+
+.. math::
+
+    VOV = \frac{s^2(s_{\bar{X}}^2)}{s_{\bar{X}}^4 } = \frac{m_4}{m_2^2} - \frac{1}{n}
+
+where :math:`s_{\bar{X}}^2` is the estimated variance of the mean and
+:math:`s^2(s_{\bar{X}}^2)` is the estimated variance in :math:`s_{\bar{X}}^2`.
+The MCNP manual suggests a hard threshold such that :math:`VOV < 0.1` to improve
+the probability of forming a reliable confidence interval. However, OpenMC does
+not enforce an universal cut-off because the suitability of any single threshold
+depends strongly on problem specifics (estimator choice, variance-reduction
+settings, tally binning, or even effective sample size).
+
+
+Normality Tests (D'Agostino-Pearson)
+++++++++++++++++++++++++++++++++++++
+
+These normality test verify the hypothesis that fluctuations are *approximately
+normal*, a working assumption behind many Monte Carlo diagnostics and
+`confidence-interval heuristics`_. Tests are provided for: (i) skewness-only,
+(ii) kurtosis-only, and (iii) the *omnibus* combination. OpenMC uses the
+finite-sample-adjusted skewness :math:`G_1` and excess kurtosis :math:`G_2`
+above to construct standardized normal scores :math:`Z_1` (from :math:`G_1`) and
+:math:`Z_2` (from :math:`G_2`) via the D'Agostino-Pearson transformations. The
+omnibus statistic is
+
+.. math::
+
+   K^2 \;=\; Z_1^{\,2} \;+\; Z_2^{\,2}
+   \;\sim\; \chi^2_{(2)} \quad \text{under } H_0:\ \text{normality}.
+
+OpenMC reports :math:`Z_1`, :math:`Z_2`, :math:`K^2`, and their p-values when
+prerequisites are met (skewness for :math:`n\ge 3`, kurtosis and omnibus for
+:math:`n\ge 4`). Given a user-chosen significance level :math:`\alpha` (default
+is :math:`0.05`), reject :math:`H_0` if :math:`\text{p-value}<\alpha`; otherwise
+fail to reject. OpenMC leaves the interpretation to the user, who should
+consider VOV together with skewness, kurtosis, and normality tests results when
+judging whether reported confidence intervals are credible for their application
+[#norm-tests]_.
+
+.. [#norm-tests]
+   Higher-moments accumulation must be enabled with ``higher_moments = True``
+   for running these diagnostics including the skewness, kurtosis, and normality
+   tests.
+
+Figure of Merit
++++++++++++++++
+
+The figure of merit (FOM) is an indicator that accounts for both the statistical
+uncertainty and the execution time and represents how much information is
+obtained per unit time in the simulation. The FOM is defined as
+
+.. math::
+    :label: figure_of_merit
+
+    FOM = \frac{1}{r^2 t},
+
+where :math:`t` is the total execution time and :math:`r` is the relative error
+defined as
+
+.. math::
+    :label: relative_error
+
+    r = \frac{s_{\bar{X}}}{\bar{x}}.
+
+Based on this definition, one can see that a higher FOM is desirable. The FOM is
+useful as a comparative tool. For example, if a variance reduction technique is
+being applied to a simulation, the FOM with variance reduction can be compared
+to the FOM without variance reduction to ascertain whether the reduction in
+variance outweighs the potential increase in execution time (e.g., due to
+particle splitting). It is important to note that MCNP reports the FOM using CPU
+time (wall-clock time multiplied by the number of threads/cores), whereas OpenMC
+reports the FOM using only the wall-clock time :math:`t`.
+
 Confidence Intervals
 ++++++++++++++++++++
 
@@ -494,6 +682,8 @@ improve the estimate of the percentile.
 
    .. rubric:: References
 
+.. _confidence-interval heuristics: https://doi.org/10.1080/00031305.1990.10475751
+
 .. _following approximation: https://doi.org/10.1080/03610918708812641
 
 .. _Bessel's correction: https://en.wikipedia.org/wiki/Bessel's_correction
@@ -514,6 +704,10 @@ improve the estimate of the percentile.
 
 .. _converges in distribution: https://en.wikipedia.org/wiki/Convergence_of_random_variables#Convergence_in_distribution
 
+.. _skewness: https://en.wikipedia.org/wiki/Skewness
+
+.. _kurtosis: https://en.wikipedia.org/wiki/Kurtosis
+
 .. _confidence intervals: https://en.wikipedia.org/wiki/Confidence_interval
 
 .. _Student's t-distribution: https://en.wikipedia.org/wiki/Student%27s_t-distribution
diff --git a/docs/source/methods/variance_reduction.rst b/docs/source/methods/variance_reduction.rst
index 353ae5077..7778e0714 100644
--- a/docs/source/methods/variance_reduction.rst
+++ b/docs/source/methods/variance_reduction.rst
@@ -22,12 +22,14 @@ not experience a single scoring event, even after billions of analog histories.
 Variance reduction techniques aim to either flatten the global uncertainty
 distribution, such that all regions of phase space have a fairly similar
 uncertainty, or to reduce the uncertainty in specific locations (such as a
-detector). There are two strategies available in OpenMC for variance reduction:
-the Monte Carlo MAGIC method and the FW-CADIS method. Both strategies work by
-developing a weight window mesh that can be utilized by subsequent Monte Carlo
-solves to split particles heading towards areas of lower flux densities while
-terminating particles in higher flux regions---all while maintaining a fair
-game.
+detector). There are three strategies available in OpenMC for variance
+reduction: weight windows generated via the MAGIC method or the FW-CADIS method,
+and source biasing. Both weight windowing strategies work by developing a mesh
+that can be utilized by subsequent Monte Carlo solves to split particles heading
+towards areas of lower flux densities while terminating particles in higher flux
+regions. In contrast, source biasing modifies source site sampling behavior to
+preferentially track particles more likely to reach phase space regions of
+interest.
 
 ------------
 MAGIC Method
@@ -80,8 +82,8 @@ where it was born from.
 
 The Forward-Weighted Consistent Adjoint Driven Importance Sampling method, or
 `FW-CADIS method <https://doi.org/10.13182/NSE12-33>`_, produces weight windows
-for global variance reduction given adjoint flux information throughout the
-entire domain. The weight window lower bound is defined in Equation
+for global or local variance reduction given adjoint flux information throughout 
+the entire domain. The weight window lower bound is defined in Equation
 :eq:`fw_cadis`, and also involves a normalization step not shown here.
 
 .. math::
@@ -132,3 +134,83 @@ aware of this.
     :label: variance_fom
 
     \text{FOM} = \frac{1}{\text{Time} \times \sigma^2}
+
+Finally, one unique capability of the FW-CADIS weight window generator is to 
+produce weight windows for local variance reduction, given a list of the 
+responses of interest. This is controlled by optionally specifying target 
+tallies from the :class:`openmc.model.Model` to the 
+:class:`openmc.WeightWindowGenerator`, as illustrated in the
+:ref:`user guide<variance_reduction>`. If target tallies for local variance 
+reduction are supplied, then the adjoint sources are only populated after the 
+initial forward simulation in the source regions associated with those tallies. 
+In other regions, the adjoint source term is instead set to zero. The Random  
+Ray solver then determines the adjoint flux map used to generate FW-CADIS 
+weight windows following the usual technique.
+
+.. _methods_source_biasing:
+
+--------------
+Source Biasing
+--------------
+
+In contrast to the previous two methods that introduce population controls
+during transport, source biasing modifies the sampling of the external source
+distribution. The basic premise of the technique is that for each spatial,
+angular, energy, or time distribution of a source, an additional distribution
+can be specified provided that the two share a common support (set of points
+where the distribution is nonzero). Samples are then drawn from this "bias"
+distribution, which can be chosen to preferentially direct particles towards
+phase space regions of interest. In order to avoid biasing the tally results,
+however, a weight adjustment is applied to each sampled site as described below.
+
+Assume that the unbiased probability density function of a random variable
+:math:`X:x \rightarrow \mathbb{R}` is given by :math:`f(x)`, but that using the
+biased distribution :math:`g(x)` will result in a greater number of particle
+trajectories reaching some phase space region of interest. Then a sample
+:math:`x_0` may be drawn from :math:`g(x)` while maintaining a fair game,
+provided that its weight is adjusted as:
+
+.. math::
+    :label: source_bias
+
+    w = w_0 \times \frac{f(x_0)}{g(x_0)}
+
+where :math:`w_0` is the weight of an unbiased sample from :math:`f(x)`,
+typically unity.
+
+Returning now to Equation :eq:`source_bias`, the requirement for common support
+becomes evident. If :math:`\mathrm{supp} (g)` fully contains but is not
+identical to :math:`\mathrm{supp} (f)`, then some samples from :math:`g(x)` will
+correspond to points where :math:`f(x) = 0`. Thus these source sites would be
+assigned a starting weight of 0, meaning the particles would be killed
+immediately upon transport, effectively wasting computation time. Conversely, if
+:math:`\mathrm{supp} (g)` is fully contained by but not identical to
+:math:`\mathrm{supp} (f)`, the contributions of some regions outside
+:math:`\mathrm{supp} (g)` will not be counted towards the integral, potentially
+biasing the tally. The weight assigned to such points would be undefined since
+:math:`g(x) = \mathbf{0}` at these points.
+
+When an independent source is sampled in OpenMC, the particle's coordinate in
+each variable of phase space :math:`(\mathbf{r},\mathbf{\Omega},E,t)` is
+successively drawn from an independent probability distribution. Multiple
+variables can be biased, in which case the resultant weight :math:`w` applied to
+the particle is the product of the weights assigned from all sampled
+distributions: space, angle, energy, and time, as shown in Equation
+:eq:`tot_wgt`.
+
+.. math::
+    :label: tot_wgt
+
+    w = w_r \times w_{\Omega} \times w_E \times w_t
+
+Finally, source biasing and weight windows serve different purposes. Source
+biasing changes how particles are born, allowing the initial source sites to be
+sampled preferentially from important regions of phase space (space, angle,
+energy, and time) with an accompanying weight adjustment. Weight windows, by
+contrast, apply population control during transport (splitting and Russian
+roulette) to help particles reach and contribute in important regions as they
+move through the system. Because particle transport proceeds as usual after a
+biased source is sampled, particle attenuation in optically thick regions
+outside the source volume will not be affected by source biasing; in such
+scenarios, transport biasing techniques such as weight windows are often more
+effective.
diff --git a/docs/source/pythonapi/base.rst b/docs/source/pythonapi/base.rst
index e5b5b17c3..f06a1eba4 100644
--- a/docs/source/pythonapi/base.rst
+++ b/docs/source/pythonapi/base.rst
@@ -26,6 +26,7 @@ Simulation Settings
    openmc.FileSource
    openmc.CompiledSource
    openmc.MeshSource
+   openmc.TokamakSource
    openmc.SourceParticle
    openmc.VolumeCalculation
    openmc.Settings
@@ -37,7 +38,6 @@ Simulation Settings
 
    openmc.read_source_file
    openmc.write_source_file
-   openmc.wwinp_to_wws
 
 Material Specification
 ----------------------
@@ -129,9 +129,11 @@ Constructing Tallies
    openmc.SurfaceFilter
    openmc.MeshFilter
    openmc.MeshBornFilter
+   openmc.MeshMaterialFilter
    openmc.MeshSurfaceFilter
    openmc.EnergyFilter
    openmc.EnergyoutFilter
+   openmc.ParticleProductionFilter
    openmc.MuFilter
    openmc.MuSurfaceFilter
    openmc.PolarFilter
@@ -143,21 +145,32 @@ Constructing Tallies
    openmc.SpatialLegendreFilter
    openmc.SphericalHarmonicsFilter
    openmc.TimeFilter
+   openmc.WeightFilter
    openmc.ZernikeFilter
    openmc.ZernikeRadialFilter
    openmc.ParentNuclideFilter
    openmc.ParticleFilter
-   openmc.RegularMesh
-   openmc.RectilinearMesh
-   openmc.CylindricalMesh
-   openmc.SphericalMesh
-   openmc.UnstructuredMesh
+   openmc.ReactionFilter
    openmc.MeshMaterialVolumes
    openmc.Trigger
    openmc.TallyDerivative
    openmc.Tally
    openmc.Tallies
 
+Meshes
+------
+
+.. autosummary::
+   :toctree: generated
+   :nosignatures:
+   :template: myclassinherit.rst
+
+   openmc.RegularMesh
+   openmc.RectilinearMesh
+   openmc.CylindricalMesh
+   openmc.SphericalMesh
+   openmc.UnstructuredMesh
+
 Geometry Plotting
 -----------------
 
@@ -166,7 +179,8 @@ Geometry Plotting
    :nosignatures:
    :template: myclass.rst
 
-   openmc.Plot
+   openmc.SlicePlot
+   openmc.VoxelPlot
    openmc.WireframeRayTracePlot
    openmc.SolidRayTracePlot
    openmc.Plots
@@ -206,6 +220,9 @@ Post-processing
    :nosignatures:
    :template: myfunction.rst
 
+   openmc.read_collision_track_file
+   openmc.read_collision_track_hdf5
+   openmc.read_collision_track_mcpl
    openmc.voxel_to_vtk
 
 The following classes and functions are used for functional expansion reconstruction.
@@ -248,8 +265,16 @@ Variance Reduction
    :template: myclass
 
    openmc.WeightWindows
+   openmc.WeightWindowsList
    openmc.WeightWindowGenerator
+
+.. autosummary::
+   :toctree: generated
+   :nosignatures:
+   :template: myfunction.rst
+
    openmc.hdf5_to_wws
+   openmc.wwinp_to_wws
 
 
 Coarse Mesh Finite Difference Acceleration
diff --git a/docs/source/pythonapi/capi.rst b/docs/source/pythonapi/capi.rst
index c8e0e874d..dab45481d 100644
--- a/docs/source/pythonapi/capi.rst
+++ b/docs/source/pythonapi/capi.rst
@@ -40,6 +40,7 @@ Functions
    reset_timers
    run
    run_in_memory
+   run_random_ray
    sample_external_source
    simulation_finalize
    simulation_init
@@ -81,16 +82,21 @@ Classes
    Nuclide
    ParentNuclideFilter
    ParticleFilter
+   ParticleProductionFilter
    PolarFilter
+   ReactionFilter
    RectilinearMesh
    RegularMesh
    SpatialLegendreFilter
    SphericalHarmonicsFilter
    SphericalMesh
+   SolidRayTracePlot
    SurfaceFilter
    Tally
+   TemporarySession
    UniverseFilter
    UnstructuredMesh
+   WeightFilter
    WeightWindows
    ZernikeFilter
    ZernikeRadialFilter
@@ -122,6 +128,12 @@ Data
 
    :type: dict
 
+.. data:: plots
+
+   Mapping of plot ID to :class:`openmc.lib.SolidRayTracePlot` instances.
+
+   :type: dict
+
 .. data:: nuclides
 
    Mapping of nuclide name to :class:`openmc.lib.Nuclide` instances.
diff --git a/docs/source/pythonapi/data.rst b/docs/source/pythonapi/data.rst
index 1eaf90c97..9d47430f7 100644
--- a/docs/source/pythonapi/data.rst
+++ b/docs/source/pythonapi/data.rst
@@ -71,6 +71,8 @@ Core Functions
     isotopes
     kalbach_slope
     linearize
+    mass_attenuation_coefficient
+    mass_energy_absorption_coefficient
     thin
     water_density
     zam
diff --git a/docs/source/pythonapi/deplete.rst b/docs/source/pythonapi/deplete.rst
index 5d02fa6b9..25fcd898f 100644
--- a/docs/source/pythonapi/deplete.rst
+++ b/docs/source/pythonapi/deplete.rst
@@ -78,20 +78,18 @@ A minimal example for performing depletion would be:
     >>> import openmc.deplete
     >>> geometry = openmc.Geometry.from_xml()
     >>> settings = openmc.Settings.from_xml()
-    >>> model = openmc.model.Model(geometry, settings)
+    >>> model = openmc.Model(geometry, settings)
 
     # Representation of a depletion chain
     >>> chain_file = "chain_casl.xml"
-    >>> operator = openmc.deplete.CoupledOperator(
-    ...     model, chain_file)
+    >>> operator = openmc.deplete.CoupledOperator(model, chain_file)
 
     # Set up 5 time steps of one day each
     >>> dt = [24 * 60 * 60] * 5
     >>> power = 1e6  # constant power of 1 MW
 
     # Deplete using mid-point predictor-corrector
-    >>> cecm = openmc.deplete.CECMIntegrator(
-    ...     operator, dt, power)
+    >>> cecm = openmc.deplete.CECMIntegrator(operator, dt, power)
     >>> cecm.integrate()
 
 Internal Classes and Functions
@@ -208,14 +206,15 @@ total system energy.
 The :class:`openmc.deplete.IndependentOperator` uses inner classes subclassed
 from those listed above to perform similar calculations.
 
-The following classes are used to define transfer rates to model continuous
-removal or feed of nuclides during depletion.
+The following classes are used to define external source rates or transfer rates
+to model continuous removal or feed of nuclides during depletion.
 
 .. autosummary::
    :toctree: generated
    :nosignatures:
    :template: myclass.rst
 
+   transfer_rates.ExternalSourceRates
    transfer_rates.TransferRates
 
 Intermediate Classes
@@ -288,6 +287,16 @@ the following abstract base classes:
    abc.SIIntegrator
    abc.DepSystemSolver
 
+R2S Automation
+--------------
+
+.. autosummary::
+   :toctree: generated
+   :nosignatures:
+   :template: myclass.rst
+
+   R2SManager
+
 D1S Functions
 -------------
 
diff --git a/docs/source/pythonapi/mgxs.rst b/docs/source/pythonapi/mgxs.rst
index 392914b36..4141aa0a0 100644
--- a/docs/source/pythonapi/mgxs.rst
+++ b/docs/source/pythonapi/mgxs.rst
@@ -11,6 +11,16 @@ Module Variables
 .. autodata:: openmc.mgxs.GROUP_STRUCTURES
     :annotation:
 
+Functions 
++++++++++
+
+.. autosummary::
+    :toctree: generated
+    :nosignatures:
+    :template: myfunction.rst
+
+    openmc.mgxs.convert_flux_groups
+
 Classes
 +++++++
 
diff --git a/docs/source/pythonapi/stats.rst b/docs/source/pythonapi/stats.rst
index c8318ba86..2f2e2835a 100644
--- a/docs/source/pythonapi/stats.rst
+++ b/docs/source/pythonapi/stats.rst
@@ -22,6 +22,7 @@ Univariate Probability Distributions
    openmc.stats.Legendre
    openmc.stats.Mixture
    openmc.stats.Normal
+   openmc.stats.DecaySpectrum
 
 .. autosummary::
    :toctree: generated
@@ -29,6 +30,7 @@ Univariate Probability Distributions
    :template: myfunction.rst
 
    openmc.stats.delta_function
+   openmc.stats.fusion_neutron_spectrum
    openmc.stats.muir
 
 Angular Distributions
@@ -67,3 +69,4 @@ Spatial Distributions
    :template: myfunction.rst
 
    openmc.stats.spherical_uniform
+   openmc.stats.cylindrical_uniform
diff --git a/docs/source/quickinstall.rst b/docs/source/quickinstall.rst
index 21526242f..6cb774b5b 100644
--- a/docs/source/quickinstall.rst
+++ b/docs/source/quickinstall.rst
@@ -35,6 +35,13 @@ you wish) with OpenMC installed.
     conda create --name openmc-env openmc
     conda activate openmc-env
 
+If you are installing on macOS with an Apple silicon ARM-based processor, you
+will also need to specify the `--platform` option:
+
+.. code-block:: sh
+
+    conda create --name openmc-env --platform osx-64 openmc
+
 You are now in a conda environment called `openmc-env` that has OpenMC
 installed.
 
@@ -112,7 +119,7 @@ packages should be installed, for example in Homebrew via:
 
 .. code-block:: sh
 
-   brew install llvm cmake xtensor hdf5 python libomp libpng
+   brew install llvm cmake hdf5 python libomp libpng
 
 The compiler provided by the above LLVM package should be used in place of the
 one provisioned by XCode, which does not support the multithreading library used
diff --git a/docs/source/releasenotes/0.15.3.rst b/docs/source/releasenotes/0.15.3.rst
new file mode 100644
index 000000000..c50958104
--- /dev/null
+++ b/docs/source/releasenotes/0.15.3.rst
@@ -0,0 +1,226 @@
+====================
+What's New in 0.15.3
+====================
+
+.. currentmodule:: openmc
+
+-------
+Summary
+-------
+
+This release of OpenMC includes many bug fixes, performance improvements, and
+several notable new features. The major highlights of this release include a new
+:class:`~openmc.deplete.R2SManager` class that automates the workflow for
+rigorous 2-step (R2S) shutdown dose rate calculations, the ability to collect
+higher moments for tally results that can be used to test normality, a new
+uncertainty-aware criticality search method, a new collision tracking feature
+that enables detailed tracking of particle interactions, support for distributed
+cell densities, and several new tally filters. The random ray solver also
+continues to receive significant updates, including automatic setup
+capabilities, improved geometry handling, and better weight window support.
+Depletion capabilities have been expanded with thermochemical redox control,
+external transfer rates, and improved performance.
+
+------------------------------------
+Compatibility Notes and Deprecations
+------------------------------------
+
+MCPL has been changed from a build-time dependency to a runtime optional
+dependency, which means OpenMC will attempt to load the MCPL library at
+runtime when needed rather than requiring it at build time.
+
+The ``openmc.mgxs.Library.add_to_tallies_file`` method has been renamed to
+:meth:`openmc.mgxs.Library.add_to_tallies`.
+
+------------
+New Features
+------------
+
+- A new collision tracking feature enables detailed tracking of particle
+  interactions (`#3417 <https://github.com/openmc-dev/openmc/pull/3417>`_)
+- Added :meth:`~openmc.model.Model.keff_search` method for automated criticality
+  searches (`#3569 <https://github.com/openmc-dev/openmc/pull/3569>`_)
+- Introduced automated workflow for mesh- or cell-based R2S calculations
+  (`#3508 <https://github.com/openmc-dev/openmc/pull/3508>`_)
+- Ability to source electron/positrons directly for charged particle
+  simulations (`#3404 <https://github.com/openmc-dev/openmc/pull/3404>`_)
+- Multi-group capability for kinetics parameter calculations with Iterated
+  Fission Probability (`#3425
+  <https://github.com/openmc-dev/openmc/pull/3425>`_)
+- Introduced a new :class:`openmc.MeshMaterialFilter` class (`#3406
+  <https://github.com/openmc-dev/openmc/pull/3406>`_)
+- Added support for distributed cell densities (`#3546
+  <https://github.com/openmc-dev/openmc/pull/3546>`_)
+- Implemented a :class:`openmc.WeightWindowsList` class that enables export to
+  HDF5 (`#3456 <https://github.com/openmc-dev/openmc/pull/3456>`_)
+- Added :meth:`openmc.Material.mean_free_path` method (`#3469
+  <https://github.com/openmc-dev/openmc/pull/3469>`_)
+- Introduced :func:`openmc.lib.TemporarySession` context manager (`#3475
+  <https://github.com/openmc-dev/openmc/pull/3475>`_)
+- Added material depletion function for tracking individual material depletion
+  (`#3420 <https://github.com/openmc-dev/openmc/pull/3420>`_)
+- Added methods on :class:`~openmc.Material` class for waste disposal rating /
+  classification (`#3366 <https://github.com/openmc-dev/openmc/pull/3366>`_,
+  `#3376 <https://github.com/openmc-dev/openmc/pull/3376>`_)
+- Support for thermochemical redox control transfer rates in depletion
+  (`#2783 <https://github.com/openmc-dev/openmc/pull/2783>`_)
+- Support for external transfer rates source term in depletion (`#3088
+  <https://github.com/openmc-dev/openmc/pull/3088>`_)
+- Added combing capability for fission site sampling and delayed neutron
+  emission time (`#2992 <https://github.com/openmc-dev/openmc/pull/2992>`_)
+- Ability to specify reference direction for azimuthal angle in
+  :class:`~openmc.stats.PolarAzimuthal` distribution (`#3582
+  <https://github.com/openmc-dev/openmc/pull/3582>`_)
+- Allow spatial constraints on element sources within
+  :class:`~openmc.MeshSource` (`#3431
+  <https://github.com/openmc-dev/openmc/pull/3431>`_)
+- Added VTK HDF (.vtkhdf) format support for writing VTK data (`#3252
+  <https://github.com/openmc-dev/openmc/pull/3252>`_)
+- Implemented filter weight capability (`#3345
+  <https://github.com/openmc-dev/openmc/pull/3345>`_)
+- Optionally collect higher moments for tallies (`#3363
+  <https://github.com/openmc-dev/openmc/pull/3363>`_)
+- Several random ray solver enhancements:
+
+  - Random Ray AutoMagic Setup for automatic configuration (`#3351 <https://github.com/openmc-dev/openmc/pull/3351>`_)
+  - Point source locator for random ray mode (`#3360 <https://github.com/openmc-dev/openmc/pull/3360>`_)
+  - Support for DAGMC geometries (`#3374 <https://github.com/openmc-dev/openmc/pull/3374>`_)
+  - Optimized mapping of source regions to tallies (`#3465 <https://github.com/openmc-dev/openmc/pull/3465>`_)
+  - Base source region refactor (`#3576 <https://github.com/openmc-dev/openmc/pull/3576>`_)
+
+---------------------------
+Bug Fixes and Small Changes
+---------------------------
+
+- Add two MPI barriers in R2S workflow (`#3646 <https://github.com/openmc-dev/openmc/pull/3646>`_)
+- Fix a few warnings, rename add_to_tallies_file (`#3639 <https://github.com/openmc-dev/openmc/pull/3639>`_)
+- Fix typo in DAGMC lost particle test (`#3634 <https://github.com/openmc-dev/openmc/pull/3634>`_)
+- Avoid multiprocessing Pool when running depletion tests with MPI (`#3633 <https://github.com/openmc-dev/openmc/pull/3633>`_)
+- Support MPI parallelism in R2SManager (`#3632 <https://github.com/openmc-dev/openmc/pull/3632>`_)
+- Update documentation for particle tracks (`#3627 <https://github.com/openmc-dev/openmc/pull/3627>`_)
+- Adding variance of variance and normality tests for tally statistics (`#3454 <https://github.com/openmc-dev/openmc/pull/3454>`_)
+- Avoid divide-by-zero in ``from_multigroup_flux`` when flux is zero (`#3624 <https://github.com/openmc-dev/openmc/pull/3624>`_)
+- Write particle states as separate lines in track VTK files (`#3628 <https://github.com/openmc-dev/openmc/pull/3628>`_)
+- Reset DAGMC history when reviving from source (`#3601 <https://github.com/openmc-dev/openmc/pull/3601>`_)
+- Add energy group structure: SCALE-999 (`#3564 <https://github.com/openmc-dev/openmc/pull/3564>`_)
+- Fix bug in normalization of tally results with no_reduce (`#3619 <https://github.com/openmc-dev/openmc/pull/3619>`_)
+- Enable nuclide filters with get_decay_photon_energy (`#3614 <https://github.com/openmc-dev/openmc/pull/3614>`_)
+- Update ``check_type`` calls to accept both ``str`` and ``os.PathLike`` objects (`#3618 <https://github.com/openmc-dev/openmc/pull/3618>`_)
+- Speed up ``apply_time_correction`` by reducing file I/O and deepcopies (`#3617 <https://github.com/openmc-dev/openmc/pull/3617>`_)
+- FW-CADIS Disregard Max Realizations Setting (`#3616 <https://github.com/openmc-dev/openmc/pull/3616>`_)
+- Random Ray Geometry Debug Mode Fix (`#3615 <https://github.com/openmc-dev/openmc/pull/3615>`_)
+- Don't write reaction rates in depletion results by default (`#3609 <https://github.com/openmc-dev/openmc/pull/3609>`_)
+- Allow Path objects in MGXSLibrary.export_to_hdf5 (`#3608 <https://github.com/openmc-dev/openmc/pull/3608>`_)
+- Clip mixture distributions based on mean times integral (`#3603 <https://github.com/openmc-dev/openmc/pull/3603>`_)
+- Allow V0 in atomic_mass function (for ENDF/B-VII.0 data) (`#3607 <https://github.com/openmc-dev/openmc/pull/3607>`_)
+- Re-run flaky tests when needed (`#3604 <https://github.com/openmc-dev/openmc/pull/3604>`_)
+- Ability to load mesh objects from weight_windows.h5 file (`#3598 <https://github.com/openmc-dev/openmc/pull/3598>`_)
+- Switch to using coveralls github action for reporting (`#3594 <https://github.com/openmc-dev/openmc/pull/3594>`_)
+- Add user setting for free gas threshold (`#3593 <https://github.com/openmc-dev/openmc/pull/3593>`_)
+- Speed up time correction factors (`#3592 <https://github.com/openmc-dev/openmc/pull/3592>`_)
+- Fix caching issue when using NCrystal materials (`#3538 <https://github.com/openmc-dev/openmc/pull/3538>`_)
+- Fix random ray source region mesh export when using model.export_to_xml() (`#3579 <https://github.com/openmc-dev/openmc/pull/3579>`_)
+- Ensure weight_windows_file information is read from XML (`#3587 <https://github.com/openmc-dev/openmc/pull/3587>`_)
+- Add missing documentation on <source> in depletion chain file format (`#3590 <https://github.com/openmc-dev/openmc/pull/3590>`_)
+- Adding tally filter type option to statepoint get_tally (`#3584 <https://github.com/openmc-dev/openmc/pull/3584>`_)
+- Optional separation of mesh-material-volume calc from get_homogenized_materials (`#3581 <https://github.com/openmc-dev/openmc/pull/3581>`_)
+- Fix IFP implementation (`#3580 <https://github.com/openmc-dev/openmc/pull/3580>`_)
+- Remove several TODOs related to C++17 support (`#3574 <https://github.com/openmc-dev/openmc/pull/3574>`_)
+- Fix performance regression in libMesh unstructured mesh tallies (`#3577 <https://github.com/openmc-dev/openmc/pull/3577>`_)
+- Update find_package calls in OpenMCConfig.cmake (`#3572 <https://github.com/openmc-dev/openmc/pull/3572>`_)
+- Ensure ``n_dimension_`` attribute is set for unstructured meshes (`#3575 <https://github.com/openmc-dev/openmc/pull/3575>`_)
+- Allow newer Sphinx version and fix docbuild warnings (`#3571 <https://github.com/openmc-dev/openmc/pull/3571>`_)
+- Fixed a bug when combining TimeFilter, MeshFilter, and tracklength estimator (`#3525 <https://github.com/openmc-dev/openmc/pull/3525>`_)
+- PowerLaw raises an error if sampling interval contains negative values (`#3542 <https://github.com/openmc-dev/openmc/pull/3542>`_)
+- depletion: fix performance of chain matrix construction (`#3567 <https://github.com/openmc-dev/openmc/pull/3567>`_)
+- Do not apply boundary conditions when initialized in volume calculation mode (`#3562 <https://github.com/openmc-dev/openmc/pull/3562>`_)
+- Bump up tolerance for flaky activation test (`#3560 <https://github.com/openmc-dev/openmc/pull/3560>`_)
+- Fixed a bug in plotting cross sections with S(a,b) data (`#3558 <https://github.com/openmc-dev/openmc/pull/3558>`_)
+- Change test order to run unit tests first (`#3533 <https://github.com/openmc-dev/openmc/pull/3533>`_)
+- adding ecco 33 (`#3556 <https://github.com/openmc-dev/openmc/pull/3556>`_)
+- Refactor endf_data to be a fixture (`#3539 <https://github.com/openmc-dev/openmc/pull/3539>`_)
+- Revert "fix broken CI" (`#3554 <https://github.com/openmc-dev/openmc/pull/3554>`_)
+- fix broken CI (`#3551 <https://github.com/openmc-dev/openmc/pull/3551>`_)
+- Leverage particle.move_distance in event advance (`#3544 <https://github.com/openmc-dev/openmc/pull/3544>`_)
+- fix tests that accidentaly got broken (`#3543 <https://github.com/openmc-dev/openmc/pull/3543>`_)
+- not printing nuclides with 0 percent to terminal (option 2 ) (`#3448 <https://github.com/openmc-dev/openmc/pull/3448>`_)
+- Fix a bug in time cutoff behavior (`#3526 <https://github.com/openmc-dev/openmc/pull/3526>`_)
+- Avoid duplicate materials written to XML (`#3536 <https://github.com/openmc-dev/openmc/pull/3536>`_)
+- Use cached property for openmc.data.Decay.sources (`#3535 <https://github.com/openmc-dev/openmc/pull/3535>`_)
+- more helpful error message for dose_coefficients (`#3534 <https://github.com/openmc-dev/openmc/pull/3534>`_)
+- Adding 616 group structure (`#3531 <https://github.com/openmc-dev/openmc/pull/3531>`_)
+- Remove unused special accessors for tallies (`#3527 <https://github.com/openmc-dev/openmc/pull/3527>`_)
+- Consistent XML parsing using functions from _xml module (`#3517 <https://github.com/openmc-dev/openmc/pull/3517>`_)
+- Add stat:sum field to MCPL files for proper weight normalization (`#3522 <https://github.com/openmc-dev/openmc/pull/3522>`_)
+- Remove reorder_attributes from openmc._xml (`#3519 <https://github.com/openmc-dev/openmc/pull/3519>`_)
+- fixed a bug in MeshMaterialFilter.from_volumes (`#3520 <https://github.com/openmc-dev/openmc/pull/3520>`_)
+- Fixed a bug in distribcell offsets logic (`#3424 <https://github.com/openmc-dev/openmc/pull/3424>`_)
+- Add test for FW-CADIS based WW generation on a DAGMC model (`#3504 <https://github.com/openmc-dev/openmc/pull/3504>`_)
+- Fix for Weight Window Scaling Bug (`#3511 <https://github.com/openmc-dev/openmc/pull/3511>`_)
+- Fix: ``materials``, ``plots``, and ``tallies`` cannot be passed as lists (`#3513 <https://github.com/openmc-dev/openmc/pull/3513>`_)
+- Allow already-initialized openmc.lib in TemporarySession (`#3505 <https://github.com/openmc-dev/openmc/pull/3505>`_)
+- Update DAGMC and libMesh precompiler definitions (`#3510 <https://github.com/openmc-dev/openmc/pull/3510>`_)
+- Avoid adding ParentNuclideFilter twice when calling prepare_tallies (`#3506 <https://github.com/openmc-dev/openmc/pull/3506>`_)
+- Enabling MCPL source files to be read when using surf_source_read (`#3472 <https://github.com/openmc-dev/openmc/pull/3472>`_)
+- Boundary info accessors (`#3496 <https://github.com/openmc-dev/openmc/pull/3496>`_)
+- automatically finding appropriate dimension when making regular mesh from domain (`#3468 <https://github.com/openmc-dev/openmc/pull/3468>`_)
+- Add accessor methods for LocalCoord (`#3494 <https://github.com/openmc-dev/openmc/pull/3494>`_)
+- Make MCPL a Runtime Optional Dependency (`#3429 <https://github.com/openmc-dev/openmc/pull/3429>`_)
+- Use auto-chunking for StepResult HDF5 writing (`#3498 <https://github.com/openmc-dev/openmc/pull/3498>`_)
+- Provide a way to get ID maps from plot parameters on the Model class (`#3481 <https://github.com/openmc-dev/openmc/pull/3481>`_)
+- Update OSX install instructions to point to x64 platform (`#3501 <https://github.com/openmc-dev/openmc/pull/3501>`_)
+- Update conda install instructions for macOS Apple silicon (`#3488 <https://github.com/openmc-dev/openmc/pull/3488>`_)
+- Only show warning if in restart mode (`#3478 <https://github.com/openmc-dev/openmc/pull/3478>`_)
+- Add flag to CMakeLists to use submodules instead of searching (`#3480 <https://github.com/openmc-dev/openmc/pull/3480>`_)
+- Added citation metadata file (`#3409 <https://github.com/openmc-dev/openmc/pull/3409>`_)
+- fix zam parsing (`#3484 <https://github.com/openmc-dev/openmc/pull/3484>`_)
+- Support flux collapse method in ``get_microxs_and_flux`` (`#3466 <https://github.com/openmc-dev/openmc/pull/3466>`_)
+- Stabilize Adjoint Source (`#3476 <https://github.com/openmc-dev/openmc/pull/3476>`_)
+- Refactor and Harden Configuration Management (`#3461 <https://github.com/openmc-dev/openmc/pull/3461>`_)
+- Updated Docs to Not Give Specific Python Version Requirement (`#3473 <https://github.com/openmc-dev/openmc/pull/3473>`_)
+- Parallelization of Weight Window Update (`#3467 <https://github.com/openmc-dev/openmc/pull/3467>`_)
+- Limit Random Ray Weight Window Generation to Final Batch (`#3464 <https://github.com/openmc-dev/openmc/pull/3464>`_)
+- Fix Dockerfile DAGMC build (`#3463 <https://github.com/openmc-dev/openmc/pull/3463>`_)
+- Fix Weight Window Infinite Loop Bug (`#3457 <https://github.com/openmc-dev/openmc/pull/3457>`_)
+- Weight Window Birth Scaling (`#3459 <https://github.com/openmc-dev/openmc/pull/3459>`_)
+- Adding checks to geometry.plot to avoid material name overlaps (`#3458 <https://github.com/openmc-dev/openmc/pull/3458>`_)
+- Fixing crash when calling Geometry.plot when DAGMCUniverse in geometry (`#3455 <https://github.com/openmc-dev/openmc/pull/3455>`_)
+- fixing expansion of elemental Ta bug (`#3443 <https://github.com/openmc-dev/openmc/pull/3443>`_)
+- Prevent Adjoint Sources from Trending towards Infinity (`#3449 <https://github.com/openmc-dev/openmc/pull/3449>`_)
+- adding plot function to DAGMCUnvierse (`#3451 <https://github.com/openmc-dev/openmc/pull/3451>`_)
+- Allow specifying number of equiprobable angles for thermal scattering data generation (`#3346 <https://github.com/openmc-dev/openmc/pull/3346>`_)
+- Change Dockerfile from debian:bookworm-slim to ubuntu:24.04 (`#3442 <https://github.com/openmc-dev/openmc/pull/3442>`_)
+- Fix Resetting of Auto IDs When Generating MGXS (`#3437 <https://github.com/openmc-dev/openmc/pull/3437>`_)
+- Allowing chain_file to be chain object to save reloading time (`#3436 <https://github.com/openmc-dev/openmc/pull/3436>`_)
+- update units for flux (`#3441 <https://github.com/openmc-dev/openmc/pull/3441>`_)
+- Fix raytrace infinite loop (`#3423 <https://github.com/openmc-dev/openmc/pull/3423>`_)
+- Apply Max Number of Events Check to Random Rays (`#3438 <https://github.com/openmc-dev/openmc/pull/3438>`_)
+- Add user setting for source rejection fraction (`#3433 <https://github.com/openmc-dev/openmc/pull/3433>`_)
+- Adding fix and tests for spherical mesh as spatial distribution (`#3428 <https://github.com/openmc-dev/openmc/pull/3428>`_)
+- Random Ray Missed Cell Policy Change for Adjoint Mode (`#3434 <https://github.com/openmc-dev/openmc/pull/3434>`_)
+- Random Ray External Source Plotting Fix (`#3430 <https://github.com/openmc-dev/openmc/pull/3430>`_)
+- Avoid negative heating values during pair production and bremsstrahlung (`#3426 <https://github.com/openmc-dev/openmc/pull/3426>`_)
+- Fix no serialization of periodic_surface_id bug (`#3421 <https://github.com/openmc-dev/openmc/pull/3421>`_)
+- Update _get_start_data to always grab the beginning of timestep time (`#3414 <https://github.com/openmc-dev/openmc/pull/3414>`_)
+- Fixed a bug in charged particle energy deposition (`#3416 <https://github.com/openmc-dev/openmc/pull/3416>`_)
+- Fix bug where the same mesh is written multiple times to settings.xml (`#3418 <https://github.com/openmc-dev/openmc/pull/3418>`_)
+- small typo - spelling of Debian (`#3411 <https://github.com/openmc-dev/openmc/pull/3411>`_)
+- added test for dagmc geometry plot (`#3375 <https://github.com/openmc-dev/openmc/pull/3375>`_)
+- Random Ray Misc Memory Error Fixes (`#3405 <https://github.com/openmc-dev/openmc/pull/3405>`_)
+- added type hints to model file (`#3399 <https://github.com/openmc-dev/openmc/pull/3399>`_)
+- Apply resolve paths to path values in ``config`` (`#3400 <https://github.com/openmc-dev/openmc/pull/3400>`_)
+- Fixing an incorrect computation of CDF of bremsstrahlung photons (`#3396 <https://github.com/openmc-dev/openmc/pull/3396>`_)
+- Fix weight modification for uniform source sampling (`#3395 <https://github.com/openmc-dev/openmc/pull/3395>`_)
+- Updates to VTK data checks (`#3371 <https://github.com/openmc-dev/openmc/pull/3371>`_)
+- Map Compton subshell data to atomic relaxation data (`#3392 <https://github.com/openmc-dev/openmc/pull/3392>`_)
+- Skip atomic relaxation if binding energy is larger than photon energy (`#3391 <https://github.com/openmc-dev/openmc/pull/3391>`_)
+- Fix extremely large yields from Bremsstrahlung (`#3386 <https://github.com/openmc-dev/openmc/pull/3386>`_)
+- corrected tally name in D1S example (`#3383 <https://github.com/openmc-dev/openmc/pull/3383>`_)
+- Install MCPL using same build type as OpenMC in CI (`#3388 <https://github.com/openmc-dev/openmc/pull/3388>`_)
+- using reduce chain level to remove need for reduce chain (`#3377 <https://github.com/openmc-dev/openmc/pull/3377>`_)
+- Fix negative distances from bins_crossed for CylindricalMesh (`#3370 <https://github.com/openmc-dev/openmc/pull/3370>`_)
+- Add check for equal value bins in an EnergyFilter (`#3372 <https://github.com/openmc-dev/openmc/pull/3372>`_)
+- Fix for Issue Loading MGXS Data Files with LLVM 20 or Newer (`#3368 <https://github.com/openmc-dev/openmc/pull/3368>`_)
+- Report plot ID instead of index for unsupported plot types in random ray mode (`#3361 <https://github.com/openmc-dev/openmc/pull/3361>`_)
+- Handle Missing Tags in Versioning by Setting Default to 0 (`#3359 <https://github.com/openmc-dev/openmc/pull/3359>`_)
+- added kg units to doc string in results class (`#3358 <https://github.com/openmc-dev/openmc/pull/3358>`_)
diff --git a/docs/source/releasenotes/index.rst b/docs/source/releasenotes/index.rst
index d24b83f9e..1292599ba 100644
--- a/docs/source/releasenotes/index.rst
+++ b/docs/source/releasenotes/index.rst
@@ -7,6 +7,7 @@ Release Notes
 .. toctree::
   :maxdepth: 1
 
+  0.15.3
   0.15.2
   0.15.1
   0.15.0
diff --git a/docs/source/usersguide/data.rst b/docs/source/usersguide/data.rst
index 2a9cd36db..8b2938556 100644
--- a/docs/source/usersguide/data.rst
+++ b/docs/source/usersguide/data.rst
@@ -30,7 +30,8 @@ responsible for specifying one or more of the following:
 Each of the above files can specified in several ways. In the Python API, a
 :ref:`runtime configuration variable <usersguide_data_runtime>`
 :data:`openmc.config` can be used to specify any of the above and is initialized
-using a set of environment variables.
+using a set of environment variables. Data configuration paths set in
+:data:`openmc.config` will be expanded to absolute paths.
 
 .. _usersguide_data_runtime:
 
diff --git a/docs/source/usersguide/decay_sources.rst b/docs/source/usersguide/decay_sources.rst
index a228f8e66..19f1e5d5f 100644
--- a/docs/source/usersguide/decay_sources.rst
+++ b/docs/source/usersguide/decay_sources.rst
@@ -6,42 +6,223 @@ Decay Sources
 
 Through the :ref:`depletion <usersguide_depletion>` capabilities in OpenMC, it
 is possible to simulate radiation emitted from the decay of activated materials.
-For fusion energy systems, this is commonly done using what is known as the
-`rigorous 2-step <https://doi.org/10.1016/S0920-3796(02)00144-8>`_ (R2S) method.
-In this method, a neutron transport calculation is used to determine the neutron
-flux and reaction rates over a cell- or mesh-based spatial discretization of the
-model. Then, the neutron flux in each discrete region is used to predict the
-activated material composition using a depletion solver. Finally, a photon
-transport calculation with a source based on the activity and energy spectrum of
-the activated materials is used to determine a desired physical response (e.g.,
-a dose rate) at one or more locations of interest.
+For fusion energy systems, this is commonly done using either the `rigorous
+2-step <https://doi.org/10.1016/S0920-3796(02)00144-8>`_ (R2S) method or the
+`direct 1-step <https://doi.org/10.1016/S0920-3796(01)00188-0>`_ (D1S) method.
+In the R2S method, a neutron transport calculation is used to determine the
+neutron flux and reaction rates over a cell- or mesh-based spatial
+discretization of the model. Then, the neutron flux in each discrete region is
+used to predict the activated material composition using a depletion solver.
+Finally, a photon transport calculation with a source based on the activity and
+energy spectrum of the activated materials is used to determine a desired
+physical response (e.g., a dose rate) at one or more locations of interest.
+OpenMC includes automation for both the R2S and D1S methods as described in the
+following sections.
 
-Once a depletion simulation has been completed in OpenMC, the intrinsic decay
-source can be determined as follows. First the activated material composition
-can be determined using the :class:`openmc.deplete.Results` object. Indexing an
-instance of this class with the timestep index returns a
-:class:`~openmc.deplete.StepResult` object, which itself has a
-:meth:`~openmc.deplete.StepResult.get_material` method. Once the activated
-:class:`~openmc.Material` has been obtained, the
-:meth:`~openmc.Material.get_decay_photon_energy` method will give the energy
-spectrum of the decay photon source. The integral of the spectrum also indicates
-the intensity of the source in units of [Bq]. Altogether, the workflow looks as
-follows::
+Rigorous 2-Step (R2S) Calculations
+==================================
 
+OpenMC includes an :class:`openmc.deplete.R2SManager` class that fully automates
+cell- and mesh-based R2S calculations. Before we describe this class, it is
+useful to understand the basic mechanics of how an R2S calculation works.
+Generally, it involves the following steps:
+
+1. The :meth:`openmc.deplete.get_microxs_and_flux` function is called to run a
+   neutron transport calculation that determines fluxes and microscopic cross
+   sections in each activation region.
+2. The :class:`openmc.deplete.IndependentOperator` and
+   :class:`openmc.deplete.PredictorIntegrator` classes are used to carry out a
+   depletion (activation) calculation in order to determine predicted material
+   compositions based on a set of timesteps and source rates.
+3. The activated material composition is determined using the
+   :class:`openmc.deplete.Results` class. Indexing an instance of this class
+   with the timestep index returns a :class:`~openmc.deplete.StepResult` object,
+   which itself has a :meth:`~openmc.deplete.StepResult.get_material` method
+   returning an activated material.
+4. The :meth:`openmc.Material.get_decay_photon_energy` method is used to obtain
+   the energy spectrum of the decay photon source. The integral of the spectrum
+   also indicates the intensity of the source in units of [Bq].
+5. A new photon source is defined using one of OpenMC's source classes with the
+   energy distribution set equal to the object returned by the
+   :meth:`openmc.Material.get_decay_photon_energy` method. The source is then
+   assigned to a photon :class:`~openmc.Model`.
+6. A photon transport calculation is run with ``model.run()``.
+
+Altogether, the workflow looks as follows::
+
+    # Run neutron transport calculation
+    fluxes, micros = openmc.deplete.get_microxs_and_flux(model, domains)
+
+    # Run activation calculation
+    op = openmc.deplete.IndependentOperator(mats, fluxes, micros)
+    timesteps = ...
+    source_rates = ...
+    integrator = openmc.deplete.Integrator(op, timesteps, source_rates)
+    integrator.integrate()
+
+    # Get decay photon source at last timestep
     results = openmc.deplete.Results("depletion_results.h5")
-
-    # Get results at last timestep
     step = results[-1]
-
-    # Get activated material composition for ID=1
     activated_mat = step.get_material('1')
-
-    # Determine photon source
     photon_energy = activated_mat.get_decay_photon_energy()
+    photon_source = openmc.IndependentSource(
+        space=...,
+        energy=photon_energy,
+        particle='photon',
+        strength=photon_energy.integral()
+    )
 
-By default, the :meth:`~openmc.Material.get_decay_photon_energy` method will
-eliminate spectral lines with very low intensity, but this behavior can be
-configured with the ``clip_tolerance`` argument.
+    # Run photon transport calculation
+    model.settings.source = photon_source
+    model.run()
+
+Note that by default, the :meth:`~openmc.Material.get_decay_photon_energy`
+method will eliminate spectral lines with very low intensity, but this behavior
+can be configured with the ``clip_tolerance`` argument.
+
+Cell-based R2S
+--------------
+
+In practice, users do not need to manually go through each of the steps in an R2S
+calculation described above. The :class:`~openmc.deplete.R2SManager` fully
+automates the execution of neutron transport, depletion, decay source
+generation, and photon transport. For a cell-based R2S calculation, once you
+have a :class:`~openmc.Model` that has been defined, simply create an instance
+of :class:`~openmc.deplete.R2SManager` by passing the model and a list of cells
+to activate::
+
+    r2s = openmc.deplete.R2SManager(model, [cell1, cell2, cell3])
+
+Note that the ``volume`` attribute must be set for any cell that is to be
+activated. The :class:`~openmc.deplete.R2SManager` class allows you to
+optionally specify a separate photon model; if not given as an argument, it will
+create a shallow copy of the original neutron model (available as the
+``neutron_model`` attribute) and store it in the ``photon_model`` attribute. We
+can use this to define tallies specific to the photon model::
+
+    dose_tally = openmc.Tally()
+    ...
+    r2s.photon_model.tallies = [dose_tally]
+
+Next, define the timesteps and source rates for the activation calculation::
+
+    timesteps = [(3.0, 'd'), (5.0, 'h')]
+    source_rates = [1e12, 0.0]
+
+In this case, the model is irradiated for 3 days with a source rate of
+:math:`10^{12}` neutron/sec and then the source is turned off and the activated
+materials are allowed to decay for 5 hours. These parameters should be passed to
+the :meth:`~openmc.deplete.R2SManager.run` method to execute the full R2S
+calculation. Before we can do that though, for a cell-based calculation, the one
+other piece of information that is needed is bounding boxes of the activated
+cells::
+
+    bounding_boxes = {
+        cell1.id: cell1.bounding_box,
+        cell2.id: cell2.bounding_box,
+        cell3.id: cell3.bounding_box
+    }
+
+Note that calling the ``bounding_box`` attribute may not work for all
+constructive solid geometry regions (for example, a cell that uses a
+non-axis-aligned plane). In these cases, the bounding box will need to be
+specified manually. Once you have a set of bounding boxes, the R2S calculation
+can be run::
+
+    r2s.run(timesteps, source_rates, bounding_boxes=bounding_boxes)
+
+If not specified otherwise, a photon transport calculation is run at each time
+in the depletion schedule for which a decay photon source exists. Times without
+a decay photon source, such as the initial state of a model containing only
+stable nuclides, are omitted. To specify particular times at which photon
+transport calculations should be run, pass the ``photon_time_indices`` argument.
+For example, if we wanted to run a photon transport calculation only on the last
+time (after the 5 hour decay), we would run::
+
+    r2s.run(timesteps, source_rates, bounding_boxes=bounding_boxes,
+            photon_time_indices=[2])
+
+To attribute photon tally results to their parent radionuclides, set
+``by_parent_nuclide=True``. This automatically adds a
+:class:`openmc.ParentNuclideFilter` to every photon tally that does not already
+have one. The filter bins are the union of radionuclides contributing to the
+prepared decay photon sources. The resulting bins can be used directly when
+inspecting the tally results::
+
+    r2s.run(timesteps, source_rates, bounding_boxes=bounding_boxes,
+            photon_time_indices=[2], by_parent_nuclide=True)
+
+    photon_tally = r2s.results['photon_tallies'][2][0]
+    tally_by_parent = photon_tally.get_pandas_dataframe()
+
+After an R2S calculation has been run, the :class:`~openmc.deplete.R2SManager`
+instance will have a ``results`` dictionary that allows you to directly access
+results from each of the steps. It will also write out all the output files into
+a directory that is named "r2s_<timestamp>/". The ``output_dir`` argument to the
+:meth:`~openmc.deplete.R2SManager.run` method enables you to override the
+default output directory name if desired.
+
+The :meth:`~openmc.deplete.R2SManager.run` method actually runs four
+lower-level methods under the hood::
+
+    r2s.step1_neutron_transport(...)
+    r2s.step2_activation(...)
+    r2s.step3_photon_source(...)
+    r2s.step4_photon_transport(...)
+
+For users looking for more control over the calculation, these lower-level
+methods can be used in lieu of the :meth:`openmc.deplete.R2SManager.run` method.
+
+Mesh-based R2S
+--------------
+
+Executing a mesh-based R2S calculation looks nearly identical to the cell-based
+R2S workflow described above. The only difference is that instead of passing a
+list of cells to the ``domains`` argument of
+:class:`~openmc.deplete.R2SManager`, you need to define a mesh object and pass
+that instead. This might look like the following::
+
+    # Define a regular Cartesian mesh
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-50., -50., 0.)
+    mesh.upper_right = (50., 50., 75.)
+    mesh.dimension = (10, 10, 5)
+
+    r2s = openmc.deplete.R2SManager(model, mesh)
+
+Executing the R2S calculation is then performed by adding photon tallies and
+calling the :meth:`~openmc.deplete.R2SManager.run` method with the appropriate
+timesteps and source rates. Note that in this case we do not need to define cell
+volumes or bounding boxes as is required for a cell-based R2S calculation.
+Instead, during the neutron transport step, OpenMC will run a raytracing
+calculation to determine material volume fractions within each mesh element
+using the :meth:`openmc.MeshBase.material_volumes` method. Arguments to this
+method can be customized via the ``mat_vol_kwargs`` argument to the
+:meth:`~openmc.deplete.R2SManager.run` method. Most often, this would involve
+customizing the number of rays traced to obtain better estimates of volumes. As
+an example, if we wanted to run the raytracing calculation with 10 million rays,
+we would run::
+
+    r2s.run(timesteps, source_rates, mat_vol_kwargs={'n_samples': 10_000_000})
+
+It is also possible to use multiple meshes by passing a list of meshes instead
+of a single mesh. This can be useful, for example, when different regions of the
+model require different mesh resolutions. The meshes are assumed to be
+**non-overlapping**; each element--material combination across all meshes is
+treated as an independent activation region, and all meshes are handled in a
+single neutron transport solve. For example::
+
+    # Fine mesh near the activation target
+    mesh_fine = openmc.RegularMesh()
+    mesh_fine.dimension = (10, 10, 10)
+    ...
+
+    # Coarse mesh for the surrounding region
+    mesh_coarse = openmc.RegularMesh()
+    mesh_coarse.dimension = (5, 5, 5)
+    ...
+
+    r2s = openmc.deplete.R2SManager(model, [mesh_fine, mesh_coarse])
 
 Direct 1-Step (D1S) Calculations
 ================================
@@ -88,5 +269,4 @@ relevant tallies. This can be done with the aid of the
         dose_tally = sp.get_tally(name='dose tally')
 
     # Apply time correction factors
-    tally = d1s.apply_time_correction(tally, factors, time_index)
-
+    tally = d1s.apply_time_correction(dose_tally, factors, time_index)
diff --git a/docs/source/usersguide/depletion.rst b/docs/source/usersguide/depletion.rst
index 261900ce6..9a22adf01 100644
--- a/docs/source/usersguide/depletion.rst
+++ b/docs/source/usersguide/depletion.rst
@@ -449,3 +449,42 @@ to transfer xenon from one material to another, you'd use::
     ...
 
     integrator.add_transfer_rate(mat1, ['Xe'], 0.1, destination_material=mat2)
+
+Comparing to Other Codes
+========================
+
+Comparing depletion results from OpenMC with those from another code, such as
+MCNP or Serpent, requires more than constructing equivalent transport models.
+At each depletion step, differences in the transport solution, nuclear data,
+reaction rate normalization, and numerical integration can all affect the
+result. Small differences can also accumulate over successive depletion steps.
+
+For a meaningful comparison, align as many of the following inputs and methods
+as possible:
+
+- Geometry and material definitions and associated physical properties such as
+  temperature
+- Neutron cross section library (e.g., ENDF/B-VIII.0)
+- Treatment of thermal scattering and unresolved resonance probability tables
+- Neutron reactions accounted for in the depletion chain
+- Decay data in the depletion chain
+- Isomeric branching ratios for reactions in the depletion chain
+- Fission product yields in the depletion chain
+- Fission product yield interpolation method
+  (``CoupledOperator(fission_yield_mode=...)``)
+- Reaction rate normalization, including fission Q values
+  (``CoupledOperator(fission_q=...)``)
+- Depletion integration method (``PredictorIntegrator``, ``CECMIntegrator``,
+  etc.) and time-step sizes
+
+When comparing to codes that use ACE format cross sections, it is recommended to
+directly convert the ACE files to HDF5 format using functionality from the
+:mod:`openmc.data` module (see :ref:`create_xs_library`). Some of the
+LANL-distributed ACE libraries used with MCNP have also been converted to HDF5
+format and are available for download at https://openmc.org/data.
+
+Even after these choices have been aligned, exact agreement should not be
+expected. Codes may use different approximations or numerical methods that
+cannot be configured identically. When investigating a discrepancy, first
+compare transport results and one-group reaction rates at the initial time, then
+compare changes over subsequent timesteps.
diff --git a/docs/source/usersguide/geometry.rst b/docs/source/usersguide/geometry.rst
index 6f14ebfa5..8f83b9b08 100644
--- a/docs/source/usersguide/geometry.rst
+++ b/docs/source/usersguide/geometry.rst
@@ -182,6 +182,8 @@ boundary condition.
 
 Periodic boundary conditions can be applied to pairs of planar surfaces.
 If there are only two periodic surfaces they will be matched automatically.
+
+
 Otherwise it is necessary to specify pairs explicitly using the
 :attr:`Surface.periodic_surface` attribute as in the following example::
 
@@ -192,7 +194,7 @@ Otherwise it is necessary to specify pairs explicitly using the
 Both rotational and translational periodic boundary conditions are specified in
 the same fashion. If both planes have the same normal vector, a translational
 periodicity is assumed; rotational periodicity is assumed otherwise. Currently,
-only rotations about the :math:`z`-axis are supported.
+rotations must be about the :math:`x`-, :math:`y`-, or :math:`z`-axis.
 
 For a rotational periodic BC, the normal vectors of each surface must point
 inwards---towards the valid geometry. For example, a :class:`XPlane` and
@@ -246,6 +248,28 @@ The classes :class:`Halfspace`, :class:`Intersection`, :class:`Union`, and
 :class:`Complement` and all instances of :class:`openmc.Region` and can be
 assigned to the :attr:`Cell.region` attribute.
 
+Cells also contain :attr:`Cell.temperature` and :attr:`Cell.density`
+attributes which override the temperature and density of the fill. These can
+be quite useful when temperatures and densities are spatially varying, as the
+alternative would be to add a unique :class:`Material` for each permutation of
+temperature, density, and composition. You can set the temperature (K) and
+density (g/cc) of a cell like so::
+
+  fuel.temperature = 800.0
+  fuel.density = 10.0
+
+The real utility of cell temperatures and densities occurs when a cell is
+replicated across the geometry, such as when a cell is the root geometric element
+in a replicated :ref:`universe<usersguide_universes>` or :ref:`lattice
+<usersguide_lattices>`. In those cases, you can provide a list of temperatures
+and densities to apply a temperature/density field to all of the distributed cells::
+
+  fuel.temperature = [800.0, 900.0, 800.0, 900.0]
+  fuel.density = [10.0, 9.0, 10.0, 9.0]
+
+In this example, the fuel cell is distributed four times in the geometry. Each
+distributed instance then receives its own temperature and density.
+
 .. _usersguide_universes:
 
 ---------
@@ -413,11 +437,11 @@ to help figure out how to place universes::
 
 
 Note that by default, hexagonal lattices are positioned such that each lattice
-element has two faces that are parallel to the :math:`y` axis. As one example,
-to create a three-ring lattice centered at the origin with a pitch of 10 cm
-where all the lattice elements centered along the :math:`y` axis are filled with
-universe ``u`` and the remainder are filled with universe ``q``, the following
-code would work::
+element has two faces that are perpendicular to the :math:`y` axis. As one
+example, to create a three-ring lattice centered at the origin with a pitch of
+10 cm where all the lattice elements centered along the :math:`y` axis are
+filled with universe ``u`` and the remainder are filled with universe ``q``, the
+following code would work::
 
   hexlat = openmc.HexLattice()
   hexlat.center = (0, 0)
@@ -530,6 +554,89 @@ UWUW and OpenMC material ID space will cause an error. To automatically resolve
 these ID overlaps, ``auto_ids`` can be set to ``True`` to append the UWUW
 material IDs to the OpenMC material ID space.
 
+
+Material overrides and differentiation
+--------------------------------------
+
+Programmatic access to DAGMC cell information for material overrides
+and differentiation requires synchronization of the DAGMC universe
+representation across Python and C-API::
+
+  model.init_lib()
+  model.sync_dagmc_universes()
+  model.finalize_lib()
+
+Upon completion of these steps, the :attr:`DAGMCUniverse.cells` attribute will
+be populated with :class:`DAGMCCell` proxy objects that represent the cells
+defined in the DAGMC model. The :class:`DAGMCCell` objects  will have
+:class:`openmc.Material`'s' applied according to the assignments upon
+initialization of the model. These materials can be replaced in the same manner
+as :class:`openmc.Cell` objects to override material assignments in the DAGMC
+model.
+
+Depletion with DAGMC geometry
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The synchronization of :class:`openmc.DAGMCUniverse`'s is important for
+depletion calculations using DAGMC geometry when materials need to be
+differentiated to perform material burnup independently in each DAGMC cell. See
+:meth:`openmc.model.Model.differentiate_mats`.
+
+Material overrides
+~~~~~~~~~~~~~~~~~~
+
+OpenMC supports overriding material assignments defined inside a DAGMC HDF5
+model so that CAD-assigned materials can be replaced by :class:`openmc.Material`
+objects. This is useful when the CAD geometry provides the shape but OpenMC
+materials (specific nuclide content, densities, or depletion behavior) are
+required.
+
+
+Replacing materials by name
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If a DAGMC file includes material name tags, you can replace all cells that
+reference a particular name with an :class:`openmc.Material` using
+:meth:`~openmc.DAGMCUniverse.replace_material_assignment`::
+
+  import openmc
+
+  dag_univ = openmc.DAGMCUniverse('dagmc.h5m')
+
+  fuel = openmc.Material(name='fuel')
+  fuel.add_nuclide('U235', 0.05)
+  fuel.add_nuclide('U238', 0.95)
+  fuel.set_density('g/cm3', 10.5)
+
+  dag_univ.replace_material_assignment('Fuel', fuel)
+
+This lets you keep CAD geometry while adopting OpenMC material definitions.
+
+Per-cell material overrides
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To assign overrides without initializing :class:`openmc.Model`, the
+:meth:`openmc.DAGMCUniverse.add_material_override` method can be used to assign
+materials to particular DAGMC cells. The method accepts either an integer cell
+ID::
+
+  dag_univ = openmc.DAGMCUniverse('dagmc.h5m')
+
+  enriched = openmc.Material(name='fuel_enriched')
+  enriched.add_nuclide('U235', 0.10)
+  enriched.add_nuclide('U238', 0.90)
+  enriched.set_density('g/cm3', 10.5)
+
+  dag_univ.add_material_override(1, enriched)
+
+In the case that the :class:`openmc.DAGMCUniverse` has already been synchronized,
+a :class:`openmc.DAGMCCell` object can also be provide to assign the material.
+
+Overrides are written to the `<material_overrides>` element of the
+:ref:`<dagmc_universe> <dagmc_element>` XML element so the C++ core can apply
+them on initialization.
+
+
 .. _Direct Accelerated Geometry Monte Carlo: https://svalinn.github.io/DAGMC/
 .. _University of Wisconsin Unified Workflow: https://svalinn.github.io/DAGMC/usersguide/uw2.html
 
diff --git a/docs/source/usersguide/index.rst b/docs/source/usersguide/index.rst
index aef9b1a1c..5f8e0197e 100644
--- a/docs/source/usersguide/index.rst
+++ b/docs/source/usersguide/index.rst
@@ -22,6 +22,7 @@ essential aspects of using OpenMC to perform simulations.
     plots
     depletion
     decay_sources
+    kinetics
     scripts
     processing
     parallel
diff --git a/docs/source/usersguide/install.rst b/docs/source/usersguide/install.rst
index d294770e0..2a0d301d4 100644
--- a/docs/source/usersguide/install.rst
+++ b/docs/source/usersguide/install.rst
@@ -35,6 +35,13 @@ you wish) with OpenMC installed.
     conda create --name openmc-env openmc
     conda activate openmc-env
 
+If you are installing on macOS with an Apple silicon ARM-based processor, you
+will also need to specify the `--platform` option:
+
+.. code-block:: sh
+
+    conda create --name openmc-env --platform osx-64 openmc
+
 You are now in a conda environment called `openmc-env` that has OpenMC
 installed.
 
@@ -151,6 +158,75 @@ feature can be used to access the installed packages.
 .. _Spack: https://spack.readthedocs.io/en/latest/
 .. _setup guide: https://spack.readthedocs.io/en/latest/getting_started.html
 
+.. _install_aur:
+
+------------------------------------
+Installing on Arch Linux via the AUR
+------------------------------------
+
+On Arch Linux and Arch-based distributions, OpenMC can be installed from the
+`Arch User Repository (AUR) <https://aur.archlinux.org/>`_. An AUR package named
+``openmc-git`` is available, which builds OpenMC directly from the latest
+development sources.
+
+This package provides a full-featured OpenMC stack, including:
+
+* MPI and DAGMC-enabled OpenMC build
+* User-selected nuclear data libraries
+* The `CAD_to_OpenMC <https://github.com/united-neux/CAD_to_OpenMC>`_ meshing tool
+* All required dependencies for the above components
+
+To install the package, you will need an AUR helper such as `yay`_ or `paru`_.
+For example, using ``yay``::
+
+    yay -S openmc-git
+
+
+Alternatively, you can manually clone and build the package::
+
+    git clone https://aur.archlinux.org/openmc-git.git
+    cd openmc-git
+    makepkg -si
+
+Note, ``makepkg`` uses ``pacman`` to resolve dependencies. Therefore, AUR-based
+dependencies need to be installed separately with ``yay`` or ``paru`` before
+running ``makepkg``. The PKGBUILD will automatically handle all required
+dependencies and build OpenMC with MPI and DAGMC support enabled.
+
+.. tip::
+
+    If there are failing checks during the build process, you can bypass them
+    with the ``--nocheck`` flag::
+
+        yay -S openmc-git --mflags "--nocheck"
+
+    Or::
+
+        git clone https://aur.archlinux.org/openmc-git.git
+        cd openmc-git
+        makepkg -si --nocheck
+
+.. note::
+
+    The ``openmc-git`` package tracks the latest development version from the
+    upstream repository. As such, it may include new features and bug fixes, but
+    could also introduce instability compared to official releases.
+
+.. tip::
+
+    OpenMC is installed under ``/opt``. If you are installing and using it in
+    the same terminal session, you may need to reload your environment
+    variables::
+
+        source /etc/profile
+
+    Alternatively, start a new shell session.
+
+Once installed, the ``openmc`` executable, nuclear data libraries, and
+associated tools will be available in your system :envvar:`PATH`.
+
+.. _yay: https://github.com/Jguer/yay
+.. _paru: https://github.com/Morganamilo/paru
 
 .. _install_source:
 
@@ -221,7 +297,7 @@ Prerequisites
       OpenMC's built-in plotting capabilities use the libpng library to produce
       compressed PNG files. In the absence of this library, OpenMC will fallback
       to writing PPM files, which are uncompressed and only supported by select
-      image viewers. libpng can be installed on Ddebian derivates with::
+      image viewers. libpng can be installed on Debian derivates with::
 
           sudo apt install libpng-dev
 
@@ -255,11 +331,11 @@ Prerequisites
 
       This option allows OpenMC to read and write MCPL (Monte Carlo Particle
       Lists) files instead of .h5 files for sources (external source
-      distribution, k-eigenvalue source distribution, and surface sources). To
-      turn this option on in the CMake configuration step, add the following
-      option::
-
-          cmake -DOPENMC_USE_MCPL=on ..
+      distribution, k-eigenvalue source distribution, and surface sources). 
+      OpenMC does not need any particular build option to use this, but MCPL
+      must be installed on the system in order to do so. Refer to the 
+      `MCPL documentation <https://github.com/mctools/mcpl/blob/HEAD/INSTALL.md>`_
+      for instructions on how to accomplish this.
 
     * NCrystal_ library for defining materials with enhanced thermal neutron transport
 
@@ -368,10 +444,6 @@ OPENMC_USE_DAGMC
   should also be defined as `DAGMC_ROOT` in the CMake configuration command.
   (Default: off)
 
-OPENMC_USE_MCPL
-  Turns on support for reading MCPL_ source files and writing MCPL source points
-  and surface sources. (Default: off)
-
 OPENMC_USE_LIBMESH
   Enables the use of unstructured mesh tallies with libMesh_. (Default: off)
 
@@ -380,6 +452,25 @@ OPENMC_USE_MPI
   options, please see the `FindMPI.cmake documentation
   <https://cmake.org/cmake/help/latest/module/FindMPI.html>`_.
 
+.. _cmake_strict_fp:
+
+OPENMC_ENABLE_STRICT_FP
+  Disables compiler optimizations that change floating-point results relative to
+  unoptimized builds, improving cross-platform and cross-optimization-level
+  reproducibility. This disables FMA contraction (``-ffp-contract=off``) and
+  compiler builtin replacements of math functions like ``pow``, ``exp``, ``log``
+  (``-fno-builtin``). It also keeps C/C++ assertions active by removing the
+  ``-DNDEBUG`` flag from ``RelWithDebInfo`` builds. Without this flag, these
+  optimizations can produce bit-level differences across platforms, compilers,
+  and optimization levels. This option should be used when running the test
+  suite. By default (off), the compiler is free to use all optimizations for
+  best performance. (Default: off)
+
+OPENMC_FORCE_VENDORED_LIBS
+  Forces OpenMC to use the submodules located in the vendor directory, as
+  opposed to searching the system for already installed versions of those
+  modules.
+
 To set any of these options (e.g., turning on profiling), the following form
 should be used:
 
@@ -407,7 +498,10 @@ Release
 
 RelWithDebInfo
   (Default if no type is specified.) Enable optimization and debug. On most
-  platforms/compilers, this is equivalent to `-O2 -g`.
+  platforms/compilers, this is equivalent to `-O2 -g`. When
+  :ref:`OPENMC_ENABLE_STRICT_FP <cmake_strict_fp>` is enabled, OpenMC removes the
+  ``-DNDEBUG`` flag that CMake normally adds for this build type, so that
+  C/C++ assertions remain active.
 
 Example of configuring for Debug mode:
 
@@ -515,10 +609,13 @@ to install the Python package in :ref:`"editable" mode <devguide_editable>`.
 Prerequisites
 -------------
 
-The Python API works with Python 3.8+. In addition to Python itself, the API
-relies on a number of third-party packages. All prerequisites can be installed
-using Conda_ (recommended), pip_, or through the package manager in most Linux
-distributions.
+In addition to Python itself, the OpenMC Python API relies on a number of
+third-party packages. All prerequisites can be installed using Conda_
+(recommended), pip_, or through the package manager in most Linux distributions.
+The current required Python version and up-to-date list of package dependencies
+can be found in the `pyproject.toml <https://github.com/openmc-dev/openmc/blob/develop/pyproject.toml>`_
+file in the root directory of the OpenMC repository. An overview of these
+dependencies is provided below.
 
 .. admonition:: Required
    :class: error
diff --git a/docs/source/usersguide/kinetics.rst b/docs/source/usersguide/kinetics.rst
new file mode 100644
index 000000000..9024ff822
--- /dev/null
+++ b/docs/source/usersguide/kinetics.rst
@@ -0,0 +1,133 @@
+.. _kinetics:
+
+===================
+Kinetics parameters
+===================
+
+OpenMC has the capability to estimate the following adjoint-weighted effective
+generation time :math:`\Lambda_{\text{eff}}` and the effective delayed neutron
+fraction :math:`\beta_{\text{eff}}`. These parameters are calculated using the
+iterated fission probability (IFP) method [Hurwitz_1964]_ based on a similar
+approach as in `Serpent 2 <https://doi.org/10.1016/j.anucene.2013.10.032>`_. The
+implementation in OpenMC is limited to eigenvalue calculations and is described
+in more details in [Dorville_2025]_.
+
+----------------------------------
+Iterated Fission Probability (IFP)
+----------------------------------
+
+With IFP, additional information needs to be recorded during the simulation
+compared to a typical eigenvalue calculation. OpenMC stores an additional
+set of values (neutron lifetime or delayed neutron group number for
+:math:`\Lambda_{\text{eff}}` or :math:`\beta_{\text{eff}}`, respectively)
+for every fission neutron simulated. Each set of values corresponds to
+the values that are associated to the :math:`N_{\text{gen}}` direct ancestors
+of any given fission neutron.
+
+:math:`N_{\text{gen}}` is referred to as the number of generations in the
+IFP method and corresponds to the number of generations between the birth of
+a fission neutron and the time its score is added to the IFP tally. By default,
+OpenMC considers 10 generations but this value can be modified by the user via
+the ``ifp_n_generation`` settings in the Python API::
+
+    settings.ifp_n_generation = 5
+
+``ifp_n_generation`` should be greater than 0, but should also be lower than
+or equal to the number of inactive batches declared for the calculation.
+The respect of these constraints is verified by OpenMC before any calculation.
+
+OpenMC will automatically detect the type of data that needs to be stored based
+on the tally scores selected by the user. This guarantees that only information
+of interest are stored during a simulation and avoids using extra memory when
+only one parameter is needed. The following table shows the tally scores that
+are needed to compute kinetics parameters in OpenMC:
+
+.. table:: **OpenMC tally scores needed to calculate adjoint-weighted kinetics parameters**
+    :align: center
+
+    =============================== ============================ ========================== ========
+    OpenMC tally score \\ Parameter :math:`\Lambda_{\text{eff}}` :math:`\beta_{\text{eff}}` Both
+    =============================== ============================ ========================== ========
+    ``ifp-time-numerator``          X                                                       X
+    ``ifp-beta-numerator``                                       X                          X
+    ``ifp-denominator``             X                            X                          X
+    =============================== ============================ ========================== ========
+
+|
+
+.. note:: Because the memory footprint of additional data is generally non-negligible
+    with IFP, it is recommended to choose the value for ``ifp_n_generation`` carefully.
+    For example, using one generation for both kinetics parameters corresponds to store
+    one additional integer (for the delayed neutron group number used with
+    :math:`\beta_{\text{eff}}`) and one floating point value (for the neutron lifetime
+    used with :math:`\Lambda_{\text{eff}}`) for every fission neutron simulated once the
+    asymptotic regime is reached.
+
+-----------------------------
+Obtaining kinetics parameters
+-----------------------------
+
+The ``Model`` class can be used to automatically generate all IFP tallies using
+the Python API with :attr:`openmc.Settings.ifp_n_generation` greater than 0 and
+the :meth:`openmc.Model.add_ifp_kinetics_tallies` method::
+
+    model = openmc.Model(geometry, settings=settings)
+    model.add_kinetics_parameters_tallies(num_groups=6)  # Add 6 precursor groups
+
+Alternatively, each of the tallies can be manually defined using group-wise or
+total :math:`\beta_{\text{eff}}` specified by providing a 6-group
+:class:`openmc.DelayedGroupFilter`::
+
+    beta_tally = openmc.Tally(name="group-beta-score")
+    beta_tally.scores = ["ifp-beta-numerator"]
+
+    # Add DelayedGroupFilter to enable group-wise tallies
+    beta_tally.filters = [openmc.DelayedGroupFilter(list(range(1, 7)))]
+
+Here is an example showing how to declare the three available IFP scores in a
+single tally::
+
+    tally = openmc.Tally(name="ifp-scores")
+    tally.scores = [
+        "ifp-time-numerator",
+        "ifp-beta-numerator",
+        "ifp-denominator"
+    ]
+
+The effective generation time :math:`\Lambda_{\text{eff}}` is calculated
+by dividing the result of the ``ifp-time-numerator`` score by the one obtained
+for ``ifp-denominator`` and by the :math:`k_{\text{eff}}` of the simulation:
+
+.. math::
+    :label: lambda_eff
+
+    \Lambda_{\text{eff}} = \frac{S_{\text{ifp-time-numerator}}}{S_{\text{ifp-denominator}} \times k_{\text{eff}}}
+
+The effective delayed neutron fraction :math:`\beta_{\text{eff}}` is calculated
+by dividing the result of the ``ifp-beta-numerator`` score by the one obtained
+for ``ifp-denominator``:
+
+.. math::
+    :label: beta_eff
+
+    \beta_{\text{eff}} = \frac{S_{\text{ifp-beta-numerator}}}{S_{\text{ifp-denominator}}}
+
+The kinetics parameters can be retrieved directly from a statepoint file using
+the :meth:`openmc.StatePoint.ifp_results` method::
+
+    with openmc.StatePoint(output_path) as sp:
+        generation_time, beta_eff = sp.get_kinetics_parameters()
+
+.. only:: html
+
+   .. rubric:: References
+
+.. [Hurwitz_1964] H. Hurwitz Jr., "Naval Reactors Physics Handbook", volume 1, p. 864.
+    Radkowsky, A. (Ed.), Naval Reactors, Division of Reactor Development, U.S.
+    Atomic Energy Commission (1964).
+
+.. [Dorville_2025] J. Dorville, L. Labrie-Cleary, and P. K. Romano, "Implementation
+    of the Iterated Fission Probability Method in OpenMC to Compute Adjoint-Weighted
+    Kinetics Parameters", International Conference on Mathematics and Computational
+    Methods Applied to Nuclear Science and Engineering (M&C 2025), Denver, April 27-30,
+    2025.
diff --git a/docs/source/usersguide/plots.rst b/docs/source/usersguide/plots.rst
index da0c69bdd..b5c29a3e8 100644
--- a/docs/source/usersguide/plots.rst
+++ b/docs/source/usersguide/plots.rst
@@ -6,13 +6,14 @@ Geometry Visualization
 
 .. currentmodule:: openmc
 
-OpenMC is capable of producing two-dimensional slice plots of a geometry as well
-as three-dimensional voxel plots using the geometry plotting :ref:`run mode
-<usersguide_run_modes>`. The geometry plotting mode relies on the presence of a
-:ref:`plots.xml <io_plots>` file that indicates what plots should be created. To
-create this file, one needs to create one or more :class:`openmc.Plot`
-instances, add them to a :class:`openmc.Plots` collection, and then use the
-:class:`Plots.export_to_xml` method to write the ``plots.xml`` file.
+OpenMC is capable of producing two-dimensional slice plots of a geometry,
+three-dimensional voxel plots, and three-dimensional raytrace plots using the
+geometry plotting :ref:`run mode <usersguide_run_modes>`. The geometry plotting
+mode relies on the presence of a :ref:`plots.xml <io_plots>` file that indicates
+what plots should be created. To create this file, one needs to create one or
+more instances of the various plot classes described below, add them to a
+:class:`openmc.Plots` collection, and then use the :class:`Plots.export_to_xml`
+method to write the ``plots.xml`` file.
 
 -----------
 Slice Plots
@@ -21,15 +22,14 @@ Slice Plots
 .. image:: ../_images/atr.png
    :width: 300px
 
-By default, when an instance of :class:`openmc.Plot` is created, it indicates
-that a 2D slice plot should be made. You can specify the origin of the plot
-(:attr:`Plot.origin`), the width of the plot in each direction
-(:attr:`Plot.width`), the number of pixels to use in each direction
-(:attr:`Plot.pixels`), and the basis directions for the plot. For example, to
-create a :math:`x` - :math:`z` plot centered at (5.0, 2.0, 3.0) with a width of
-(50., 50.)  and 400x400 pixels::
+The :class:`openmc.SlicePlot` class indicates that a 2D slice plot should be
+made. You can specify the origin of the plot (:attr:`SlicePlot.origin`), the
+width of the plot in each direction (:attr:`SlicePlot.width`), the number of
+pixels to use in each direction (:attr:`SlicePlot.pixels`), and the basis
+directions for the plot. For example, to create a :math:`x` - :math:`z` plot
+centered at (5.0, 2.0, 3.0) with a width of (50., 50.)  and 400x400 pixels::
 
-  plot = openmc.Plot()
+  plot = openmc.SlicePlot()
   plot.basis = 'xz'
   plot.origin = (5.0, 2.0, 3.0)
   plot.width = (50., 50.)
@@ -47,7 +47,7 @@ that location.
 
 By default, a unique color will be assigned to each cell in the geometry. If you
 want your plot to be colored by material instead, change the
-:attr:`Plot.color_by` attribute::
+:attr:`SlicePlot.color_by` attribute::
 
   plot.color_by = 'material'
 
@@ -68,8 +68,8 @@ particular cells/materials should be given colors of your choosing::
 Note that colors can be given as RGB tuples or by a string indicating a valid
 `SVG color <https://www.w3.org/TR/SVG11/types.html#ColorKeywords>`_.
 
-When you're done creating your :class:`openmc.Plot` instances, you need to then
-assign them to a :class:`openmc.Plots` collection and export it to XML::
+When you're done creating your :class:`openmc.SlicePlot` instances, you need to
+then assign them to a :class:`openmc.Plots` collection and export it to XML::
 
   plots = openmc.Plots([plot1, plot2, plot3])
   plots.export_to_xml()
@@ -97,13 +97,11 @@ Voxel Plots
 .. image:: ../_images/3dba.png
    :width: 200px
 
-The :class:`openmc.Plot` class can also be told to generate a 3D voxel plot
-instead of a 2D slice plot. Simply change the :attr:`Plot.type` attribute to
-'voxel'. In this case, the :attr:`Plot.width` and :attr:`Plot.pixels` attributes
-should be three items long, e.g.::
+The :class:`openmc.VoxelPlot` class enables the generation of a 3D voxel plot
+instead of a 2D slice plot. In this case, the :attr:`VoxelPlot.width` and
+:attr:`VoxelPlot.pixels` attributes should be three items long, e.g.::
 
-  vox_plot = openmc.Plot()
-  vox_plot.type = 'voxel'
+  vox_plot = openmc.VoxelPlot()
   vox_plot.width = (100., 100., 50.)
   vox_plot.pixels = (400, 400, 200)
 
diff --git a/docs/source/usersguide/processing.rst b/docs/source/usersguide/processing.rst
index 10944b5e2..ed3e8564a 100644
--- a/docs/source/usersguide/processing.rst
+++ b/docs/source/usersguide/processing.rst
@@ -68,17 +68,17 @@ generation, and particle number of the desired particle. For example, to create
 a track file for particle 4 of batch 1 and generation 2::
 
   settings = openmc.Settings()
-  settings.track = (1, 2, 4)
+  settings.track = [(1, 2, 4)]
 
-To specify multiple particles, the length of the iterable should be a multiple
-of three, e.g., if we wanted particles 3 and 4 from batch 1 and generation 2::
+To specify multiple particles, specify a list of tuples, e.g., if we wanted
+particles 3 and 4 from batch 1 and generation 2::
 
-  settings.track = (1, 2, 3, 1, 2, 4)
+  settings.track = [(1, 2, 3), (1, 2, 4)]
 
-After running OpenMC, the working directory will contain a file of the form
-"track_(batch #)_(generation #)_(particle #).h5" for each particle tracked.
-These track files can be converted into VTK poly data files with the
-:class:`openmc.Tracks` class.
+After running OpenMC (now, without the ``-t`` argument), the working directory
+will contain a file named `tracks.h5`, which contains a collection of particle
+tracks. These track files can be converted into VTK poly data files or
+matplotlib plots with the :class:`openmc.Tracks` class.
 
 ----------------------
 Source Site Processing
@@ -91,3 +91,90 @@ from a statepoint file, the ``openmc.statepoint`` module can be used. An
 `example notebook`_ demontrates how to analyze and plot source information.
 
 .. _example notebook: https://nbviewer.jupyter.org/github/openmc-dev/openmc-notebooks/blob/main/post-processing.ipynb
+
+------------------------
+VTK Mesh File Generation
+------------------------
+
+VTK files of OpenMC meshes can be created using the
+:meth:`openmc.Mesh.write_data_to_vtk` method. This method supports several VTK
+formats depending on the mesh type. Structured meshes
+(:class:`~openmc.RegularMesh`, :class:`~openmc.RectilinearMesh`,
+:class:`~openmc.CylindricalMesh`, and :class:`~openmc.SphericalMesh`) can be
+exported to legacy VTK format (``.vtk``). The :class:`~openmc.UnstructuredMesh`
+class supports VTK unstructured grid formats (``.vtu``) as well as an HDF5-based
+format (``.vtkhdf``) that does not require the ``vtk`` module to write.
+
+Data can be applied to the
+elements of the resulting mesh from mesh filter objects. This data can be
+provided either as a flat array or, in the case of structured meshes
+(:class:`~openmc.RegularMesh`, :class:`~openmc.RectilinearMesh`,
+:class:`~openmc.CylindricalMesh`, or :class:`SphericalMesh`), the data can be
+shaped with dimensions that match the dimensions of the mesh itself.
+
+
+.. image:: ../_images/sphere-mesh-vtk.png
+   :width: 400px
+   :align: center
+   :alt: OpenMC spherical mesh exported to VTK
+
+
+For all mesh types, if a flat data array is provided to the mesh, it is expected
+that the data is ordered in the same ordering as the :attr:`openmc.Mesh.indices`
+for that mesh object. When providing data directly from a tally, as shown below,
+a flat array for a given dataset can be passed directly to this method.
+
+::
+
+    # create model above
+
+    # create a mesh tally
+    mesh = openmc.RegularMesh()
+    mesh.dimension = [10, 20, 30]
+    mesh.lower_left = [-5, -10, -15]
+    mesh.upper_right = [5, 10, 15]
+    mesh_filter = openmc.MeshFilter(mesh)
+    tally = openmc.Tally()
+    tally.filters = [mesh_filter]
+    tally.scores = ['flux']
+
+    model.tallies = [tally]
+    model.run(apply_tally_results=True)
+
+    # provide the data as-is to the method
+    mesh.write_data_to_vtk('flux.vtk', {'flux-mean': tally.mean})
+
+The :class:`~openmc.Tally` object also provides a way to expand the dimensions
+of the mesh filter into a meaningful form where indexing the mesh filter
+dimensions results in intuitive slicing of structured meshes by setting
+``expand_dims=True`` when using :meth:`openmc.Tally.get_reshaped_data`. This
+reshaping does cause flat indexing of the data to change, however. As noted
+above, provided datasets are allowed to be shaped so long as such datasets have
+shapes that match the mesh dimensions. The ability to pass datasets in this way
+is useful when additional filters are applied to a tally. The example below
+demonstrates such a case for tally with both a :class:`~openmc.MeshFilter` and
+:class:`~openmc.EnergyFilter` applied.
+
+::
+
+    # create model above
+
+    # create a mesh tally with energy filter
+    mesh = openmc.RegularMesh()
+    mesh.dimension = [10, 20, 30]
+    mesh.lower_left = [-5, -10, -15]
+    mesh.upper_right = [5, 10, 15]
+    mesh_filter = openmc.MeshFilter(mesh)
+    energy_filter = openmc.EnergyFilter([0.0, 1.0, 20.0e6])
+    tally = openmc.Tally()
+    tally.filters = [mesh_filter, energy_filter]
+    tally.scores = ['flux']
+
+    model.tallies = [tally]
+    model.run(apply_tally_results=True)
+
+    # get the data with mesh dimensions expanded, squeeze out length-one dimensions (nuclides, scores)
+    flux = tally.get_reshaped_data(expand_dims=True).squeeze() # shape: (10, 20, 30, 2)
+
+    # write the lowest energy group to a VTK file
+    mesh.write_data_to_vtk('flux-group1.vtk', datasets={'flux-mean': flux[..., 0]})
diff --git a/docs/source/usersguide/random_ray.rst b/docs/source/usersguide/random_ray.rst
index f28aed784..06d4e50ed 100644
--- a/docs/source/usersguide/random_ray.rst
+++ b/docs/source/usersguide/random_ray.rst
@@ -11,6 +11,76 @@ active batches <usersguide_batches>`. However, there are a couple of settings
 that are unique to the random ray solver and a few areas that the random ray
 run strategy differs, both of which will be described in this section.
 
+.. _quick_start:
+
+-----------
+Quick Start
+-----------
+
+While this page contains a comprehensive guide to the random ray solver and
+its various parameters, the process of converting an existing continuous energy
+Monte Carlo model to a random ray model can be largely automated via convenience
+functions in OpenMC's Python interface::
+
+  # Define continuous energy model as normal
+  model = openmc.Model()
+  ...
+
+  # Convert model to multigroup (will auto-generate MGXS library if needed)
+  model.convert_to_multigroup()
+
+  # Convert model to random ray and initialize random ray parameters
+  # to reasonable defaults based on the specifics of the geometry
+  model.convert_to_random_ray()
+
+  # (Optional) Overlay source region decomposition mesh to improve fidelity of the
+  # random ray solver. Adjust 'n' for fidelity vs runtime.
+  n = 100
+  mesh = openmc.RegularMesh()
+  mesh.dimension = (n, n, n)
+  mesh.lower_left = model.geometry.bounding_box.lower_left
+  mesh.upper_right = model.geometry.bounding_box.upper_right
+  model.settings.random_ray['source_region_meshes'] = [(mesh, [model.geometry.root_universe])]
+
+  # (Optional) Improve fidelity of the random ray solver by enabling linear sources
+  model.settings.random_ray['source_shape'] = 'linear'
+
+  # (Optional) Increase the number of rays/batch, to reduce uncertainty
+  model.settings.particles = 500
+
+The above strategy first converts the continuous energy model to a multigroup
+one using the :meth:`openmc.Model.convert_to_multigroup` method. By default,
+this will internally run a coarsely converged continuous energy Monte Carlo
+simulation to produce an estimated multigroup macroscopic cross section set for
+each material specified in the model, and store this data into a multigroup
+cross section library file (``mgxs.h5``) that can be used by the random ray
+solver.
+
+The :meth:`openmc.Model.convert_to_random_ray` method enables random ray mode
+and performs an analysis of the model geometry to determine reasonable values
+for all required parameters. If default behavior is not satisfactory, the user
+can manually adjust the settings in the :attr:`~openmc.Settings.random_ray`
+dictionary in the :class:`openmc.Settings` as described in the sections below.
+
+Finally a few optional steps are shown. The first (recommended) step overlays a
+mesh over the geometry to create smaller source regions so that source
+resolution improves and the random ray solver becomes more accurate. Varying the
+mesh resolution can be used to trade off between accuracy and runtime.
+High-fidelity fission reactor simulation may require source region sizes below 1
+cm, while larger fixed source problems with some tolerance for error may be able
+to use source regions of 10 or 100 cm.
+
+We also enable linear sources, which can improve the accuracy of the random ray
+solver and/or allow for a much coarser mesh resolution to be overlaid. Finally,
+the number of rays per batch is adjusted. The goal here is to ensure that the
+source region miss rate is below 1%, which is reported by OpenMC at the end of
+the simulation (or before via a warning if it is very high).
+
+.. warning::
+    If using a mesh filter for tallying or weight window generation, ensure that
+    the same mesh is used for source region decomposition via
+    ``model.settings.random_ray['source_region_meshes']``.
+
 ------------------------
 Enabling Random Ray Mode
 ------------------------
@@ -443,6 +513,7 @@ Supported scores:
     - total
     - fission
     - nu-fission
+    - kappa-fission
     - events
 
 Supported Estimators:
@@ -557,29 +628,160 @@ variety of problem types (or through a multidimensional parameter sweep of
 design variables) with only modest errors and at greatly reduced cost as
 compared to using only continuous energy Monte Carlo.
 
+~~~~~~~~~~~~
+The Easy Way
+~~~~~~~~~~~~
+
+The easiest way to generate a multigroup cross section library is to use the
+:meth:`openmc.Model.convert_to_multigroup` method. This method will
+automatically output a multigroup cross section library file (``mgxs.h5``) from
+a continuous energy Monte Carlo model and alter the material definitions in the
+model to use these multigroup cross sections. An example is given below::
+
+  # Assume we already have a working continuous energy model
+  model.convert_to_multigroup(
+      method="material_wise",
+      groups="CASMO-2",
+      nparticles=2000,
+      overwrite_mgxs_library=False,
+      mgxs_path="mgxs.h5",
+      correction=None,
+      source_energy=None,
+      temperatures=None,
+      temperature_settings=None
+  )
+
+The most important parameter to set is the ``method`` parameter, which can be
+either "stochastic_slab", "material_wise", or "infinite_medium". An overview
+of these methods is given below:
+
+.. list-table:: Comparison of Automatic MGXS Generation Methods
+   :header-rows: 1
+   :widths: 10 30 30 30
+
+   * - Method
+     - Description
+     - Pros
+     - Cons
+   * - ``material_wise`` (default)
+     - * Higher Fidelity
+       * Runs a CE simulation with the original geometry and source, tallying
+         cross sections with a material filter.
+     - * Typically the most accurate of the three methods
+       * Accurately captures (averaged over the full problem domain)
+         both spatial and resonance self shielding effects
+     - * Potentially slower as the full geometry must be run
+       * If a material is only present far from the source and doesn't get tallied
+         to in the CE simulation, the MGXS will be zero for that material.
+   * - ``stochastic_slab``
+     - * Medium Fidelity
+       * Runs a CE simulation with a greatly simplified geometry, where materials
+         are randomly assigned to layers in a 1D "stochastic slab sandwich" geometry
+     - * Still captures resonant self shielding and resonance effects between materials
+       * Fast due to the simplified geometry
+       * Able to produce cross section data for all materials, regardless of how
+         far they are from the source in the original geometry
+     - * Does not capture most spatial self shielding effects, e.g., no lattice physics.
+   * - ``infinite_medium``
+     - * Lower Fidelity
+       * Runs one CE simulation per material independently. Each simulation is just
+         an infinite medium slowing down problem, with an assumed external source term.
+     - * Simple
+     - * Poor accuracy (no spatial information, no lattice physics, no resonance effects
+         between materials)
+       * May hang if a material has a k-infinity greater than 1.0
+
+When selecting a non-default energy group structure, you can manually define
+group boundaries or specify the name of a known group structure (a list of which
+can be found at :data:`openmc.mgxs.GROUP_STRUCTURES`). The ``nparticles``
+parameter can be adjusted upward to improve the fidelity of the generated cross
+section library. The ``correction`` parameter can be set to ``"P0"`` to enable
+P0 transport correction. The ``overwrite_mgxs_library`` parameter can be set to
+``True`` to overwrite an existing MGXS library file, or ``False`` to skip
+generation and use an existing library file.
+
+.. note::
+    MGXS transport correction (via setting the ``correction`` parameter in the
+    :meth:`openmc.Model.convert_to_multigroup` method to ``"P0"``) may
+    result in negative in-group scattering cross sections, which can cause
+    numerical instability. To mitigate this, during a random ray solve OpenMC
+    will automatically apply
+    `diagonal stabilization <https://doi.org/10.1016/j.anucene.2018.10.036>`_
+    with a :math:`\rho` default value of 1.0, which can be adjusted with the
+    ``settings.random_ray['diagonal_stabilization_rho']`` parameter.
+
+When generating MGXS data with either the ``stochastic_slab`` or
+``infinite_medium`` methods, by default the simulation will use a uniform source
+distribution spread evenly over all energy groups. This ensures that all energy
+groups receive tallies and therefore produce non-zero total multigroup cross
+sections. Additionally, the function will convert any sources in the model into
+simplified spatial sources that retain the original energy distributions.  If
+sources are present, they will be used 99% of the time to sample source energies
+during MGXS generation. The other 1% of the time, energies will be sampled
+uniformly over all energy groups to ensure that all groups receive some tallies.
+However, the user may wish to specify a different source energy spectrum (for
+instance, if they are using a FileSource, such that the energy distribution
+cannot be extracted from the python source object). This can be done by
+providing a :class:`openmc.stats.Univariate` distribution as the
+``source_energy`` parameter of the :meth:`openmc.Model.convert_to_multigroup`
+method. If provided, it will override any sources present in the model and will
+be used 99% of the time to sample source energies during MGXS generation. The
+other 1% of the time, energies will be sampled uniformly over all energy groups
+to ensure that all groups receive some tallies.
+
+For instance, a D-D fusion simulation may involve a complex file source. In this
+case, the user may wish to provide a discrete 2.45 MeV energy source
+distribution for MGXS generation as::
+
+  source_energy = openmc.stats.delta_function(2.45e6)
+
+The ``temperatures`` parameter can be provided if temperature-dependent
+multi-group cross sections are desired for multi-physics simulations. An
+individual cross section generation calculation is run for each temperature
+provided, where the materials in the model are set to the temperature. The
+temperature settings used during cross section generation can be specified with the
+``temperature_settings`` parameter. If no ``temperature_settings`` are provided,
+the settings contained in the model will be used. The valid keys and values in the
+``temperature_settings`` dictionary are identical to
+:attr:`openmc.Settings.temperature_settings`; more information can be found in
+:class:`openmc.Settings` . This approach yields isothermal cross section interpolation
+tables, which can be inaccurate for systems with large differences between temperatures
+in each material (often the case in fission reactors). If a more sophisticated
+temperature-dependence is required, we recommend generating cross sections manually.
+
+Ultimately, the methods described above are all just approximations.
+Approximations in the generated MGXS data will fundamentally limit the potential
+accuracy of the random ray solver. However, the methods described above are all
+useful in that they can provide a good starting point for a random ray
+simulation, and if more fidelity is needed the user may wish to follow the
+instructions below or experiment with transport correction techniques to improve
+the fidelity of the generated MGXS data.
+
+~~~~~~~~~~~~
+The Hard Way
+~~~~~~~~~~~~
+
 We give here a quick summary of how to produce a multigroup cross section data
 file (``mgxs.h5``) from a starting point of a typical continuous energy Monte
-Carlo input file. Notably, continuous energy input files define materials as a
-mixture of nuclides with different densities, whereas multigroup materials are
-simply defined by which name they correspond to in a ``mgxs.h5`` library file.
+Carlo model. Notably, continuous energy models define materials as a mixture of
+nuclides with different densities, whereas multigroup materials are simply
+defined by which name they correspond to in a ``mgxs.h5`` library file.
 
 To generate the cross section data, we begin with a continuous energy Monte
-Carlo input deck and add in the required tallies that will be needed to generate
-our library. In this example, we will specify material-wise cross sections and a
-two group energy decomposition::
+Carlo model and add in the tallies that are needed to generate our library. In
+this example, we will specify material-wise cross sections and a two-group
+energy decomposition::
 
   # Define geometry
   ...
-  ...
   geometry = openmc.Geometry()
   ...
-  ...
 
   # Initialize MGXS library with a finished OpenMC geometry object
   mgxs_lib = openmc.mgxs.Library(geometry)
 
   # Pick energy group structure
-  groups = openmc.mgxs.EnergyGroups(openmc.mgxs.GROUP_STRUCTURES['CASMO-2'])
+  groups = openmc.mgxs.EnergyGroups('CASMO-2')
   mgxs_lib.energy_groups = groups
 
   # Disable transport correction
@@ -587,7 +789,7 @@ two group energy decomposition::
 
   # Specify needed cross sections for random ray
   mgxs_lib.mgxs_types = ['total', 'absorption', 'nu-fission', 'fission',
-                      'nu-scatter matrix', 'multiplicity matrix', 'chi']
+                         'nu-scatter matrix', 'multiplicity matrix', 'chi']
 
   # Specify a "cell" domain type for the cross section tally filters
   mgxs_lib.domain_type = "material"
@@ -606,7 +808,7 @@ two group energy decomposition::
 
   # Create a "tallies.xml" file for the MGXS Library
   tallies = openmc.Tallies()
-  mgxs_lib.add_to_tallies_file(tallies, merge=True)
+  mgxs_lib.add_to_tallies(tallies, merge=True)
 
   # Export
   tallies.export_to_xml()
@@ -614,13 +816,13 @@ two group energy decomposition::
   ...
 
 When selecting an energy decomposition, you can manually define group boundaries
-or pick out a group structure already known to OpenMC (a list of which can be
-found at :class:`openmc.mgxs.GROUP_STRUCTURES`). Once the above input deck has
-been run, the resulting statepoint file will contain the needed flux and
-reaction rate tally data so that a MGXS library file can be generated. Below is
-the postprocessing script needed to generate the ``mgxs.h5`` library file given
-a statepoint file (e.g., ``statepoint.100.h5``) file and summary file (e.g.,
-``summary.h5``) that resulted from running our previous example::
+or specify the name of known group structure (a list of which can be found at
+:data:`openmc.mgxs.GROUP_STRUCTURES`). Once the above model has been run, the
+resulting statepoint file will contain the needed flux and reaction rate tally
+data so that a MGXS library file can be generated. Below is the postprocessing
+script needed to generate the ``mgxs.h5`` library file given a statepoint file
+(e.g., ``statepoint.100.h5``) file and summary file (e.g., ``summary.h5``) that
+resulted from running our previous example::
 
   import openmc
 
@@ -628,10 +830,7 @@ a statepoint file (e.g., ``statepoint.100.h5``) file and summary file (e.g.,
   geom = summary.geometry
   mats = summary.materials
 
-  statepoint_filename = 'statepoint.100.h5'
-  sp = openmc.StatePoint(statepoint_filename)
-
-  groups = openmc.mgxs.EnergyGroups(openmc.mgxs.GROUP_STRUCTURES['CASMO-2'])
+  groups = openmc.mgxs.EnergyGroups('CASMO-2')
   mgxs_lib = openmc.mgxs.Library(geom)
   mgxs_lib.energy_groups = groups
   mgxs_lib.correction = None
@@ -653,10 +852,10 @@ a statepoint file (e.g., ``statepoint.100.h5``) file and summary file (e.g.,
   # Construct all tallies needed for the multi-group cross section library
   mgxs_lib.build_library()
 
-  mgxs_lib.load_from_statepoint(sp)
+  with openmc.StatePoint('statepoint.100.h5') as sp:
+      mgxs_lib.load_from_statepoint(sp)
 
-  names = []
-  for mat in mgxs_lib.domains: names.append(mat.name)
+  names = [mat.name for mat in mgxs_lib.domains]
 
   # Create a MGXS File which can then be written to disk
   mgxs_file = mgxs_lib.create_mg_library(xs_type='macro', xsdata_names=names)
@@ -665,8 +864,8 @@ a statepoint file (e.g., ``statepoint.100.h5``) file and summary file (e.g.,
   mgxs_file.export_to_hdf5("mgxs.h5")
 
 Notably, the postprocessing script needs to match the same
-:class:`openmc.mgxs.Library` settings that were used to generate the tallies,
-but otherwise is able to discern the rest of the simulation details from the
+:class:`openmc.mgxs.Library` settings that were used to generate the tallies but
+is otherwise  able to discern the rest of the simulation details from the
 statepoint and summary files. Once the postprocessing script is successfully
 run, the ``mgxs.h5`` file can be loaded by subsequent runs of OpenMC.
 
@@ -701,11 +900,11 @@ multigroup library instead of defining their isotopic contents, as::
     water_data = openmc.Macroscopic('Hot borated water')
 
     # Instantiate some Materials and register the appropriate Macroscopic objects
-    fuel= openmc.Material(name='UO2 (2.4%)')
+    fuel = openmc.Material(name='UO2 (2.4%)')
     fuel.set_density('macro', 1.0)
     fuel.add_macroscopic(fuel_data)
 
-    water= openmc.Material(name='Hot borated water')
+    water = openmc.Material(name='Hot borated water')
     water.set_density('macro', 1.0)
     water.add_macroscopic(water_data)
 
@@ -745,6 +944,8 @@ as::
 which will greatly improve the quality of the linear source term in 2D
 simulations.
 
+.. _usersguide_random_ray_run_modes:
+
 ---------------------------------
 Fixed Source and Eigenvalue Modes
 ---------------------------------
@@ -763,9 +964,12 @@ Monte Carlo solver.
 Currently, all of the following conditions must be met for the particle source
 to be valid in random ray mode:
 
-- One or more domain ids must be specified that indicate which cells, universes,
-  or materials the source applies to. This implicitly limits the source type to
-  being volumetric. This is specified via the ``domains`` constraint placed on the
+- Either a point source must be used, or a domain constraint must be specified
+  that indicates which cells, universes, or materials the source applies to. In
+  either case, this implicitly limits the source type to being volumetric, as
+  even in the point source case the source will be "smeared" throughout the
+  source region that contains the point source coordinate. A source domain is
+  specified via the ``domains`` constraint placed on the
   :class:`openmc.IndependentSource` Python class.
 - The source must be isotropic (default for a source)
 - The source must use a discrete (i.e., multigroup) energy distribution. The
@@ -871,22 +1075,52 @@ The adjoint flux random ray solver mode can be enabled as::
 
     settings.random_ray['adjoint'] = True
 
-When enabled, OpenMC will first run a forward transport simulation followed by
-an adjoint transport simulation. The purpose of the forward solve is to compute
-the adjoint external source when an external source is present in the
-simulation. Simulation settings (e.g., number of rays, batches, etc.) will be
-identical for both simulations. At the conclusion of the run, all results (e.g.,
-tallies, plots, etc.) will be derived from the adjoint flux rather than the
-forward flux but are not labeled any differently. The initial forward flux
-solution will not be stored or available in the final statepoint file. Those
-wishing to do analysis requiring both the forward and adjoint solutions will
-need to run two separate simulations and load both statepoint files.
+When enabled, OpenMC will first run a forward transport simulation if there are 
+no user-specified adjoint sources present, followed by an adjoint transport 
+simulation. Fixed adjoint sources can be specified on the 
+:attr:`openmc.Settings.random_ray` dictionary as follows::
+
+    # Geometry definition
+    ...
+    detector_cell = openmc.Cell(fill=detector_mat, name='cell where detector will be')
+    ...
+    # Define fixed adjoint neutron source
+    strengths = [1.0]
+    midpoints = [1.0e-4]
+    energy_distribution = openmc.stats.Discrete(x=midpoints, p=strengths)
+
+    adj_source = openmc.IndependentSource(
+        energy=energy_distribution, 
+        constraints={'domains': [detector_cell]}
+    )
+
+    # Add to random_ray dict
+    settings.random_ray['adjoint_source'] = adj_source
+
+The same constraints apply to the user-defined adjoint source as to the forward 
+source, described in the :ref:`Fixed Source and Eigenvalue section 
+<usersguide_random_ray_run_modes>`. If this source is not provided, a forward 
+solve must take place to compute the adjoint external source when a forward 
+external source is present in the problem. Simulation settings (e.g., number of 
+rays, batches, etc.) will be identical for both calculations. At the 
+conclusion of the run, all results (e.g., tallies, plots, etc.) will be 
+derived from the adjoint flux rather than the forward flux but are not labeled 
+any differently. When an initial forward solve is performed (i.e., when no
+user-specified adjoint source is present), its output files are also written to
+disk with a ``forward`` infix, so they are not overwritten by the subsequent
+adjoint solve. This applies to the statepoint, ``tallies.out``, and any voxel
+plots, e.g., ``statepoint.forward.N.h5`` and ``tallies.forward.out``; the
+adjoint solve keeps the usual file names. This allows analyses requiring both
+the forward and adjoint solutions to be performed from a single run. When
+generating FW-CADIS weight windows, no weight window file is written for the
+forward solve, as only the final adjoint-derived weight windows are meaningful.
 
 .. note::
-    When adjoint mode is selected, OpenMC will always perform a full forward
-    solve and then run a full adjoint solve immediately afterwards. Statepoint
-    and tally results will be derived from the adjoint flux, but will not be
-    labeled any differently.
+    Use of the automated 
+    :ref:`FW-CADIS weight window generator<usersguide_fw_cadis>` is not 
+    currently compatible with user-defined adjoint sources. Instead, the 
+    initial forward calculation is used to assign "forward-weighted" adjoint 
+    sources to the tally regions of interest.
 
 ---------------------------------------
 Putting it All Together: Example Inputs
@@ -946,11 +1180,10 @@ given below:
     tallies.export_to_xml()
 
     # Create voxel plot
-    plot = openmc.Plot()
+    plot = openmc.VoxelPlot()
     plot.origin = [0, 0, 0]
     plot.width = [2*pitch, 2*pitch, 1]
     plot.pixels = [1000, 1000, 1]
-    plot.type = 'voxel'
 
     # Instantiate a Plots collection and export to XML
     plots = openmc.Plots([plot])
@@ -1030,11 +1263,10 @@ given below:
     tallies.export_to_xml()
 
     # Create voxel plot
-    plot = openmc.Plot()
+    plot = openmc.VoxelPlot()
     plot.origin = [0, 0, 0]
     plot.width = [2*pitch, 2*pitch, 1]
     plot.pixels = [1000, 1000, 1]
-    plot.type = 'voxel'
 
     # Instantiate a Plots collection and export to XML
     plots = openmc.Plots([plot])
diff --git a/docs/source/usersguide/scripts.rst b/docs/source/usersguide/scripts.rst
index 0879d63ef..eb0abeb0d 100644
--- a/docs/source/usersguide/scripts.rst
+++ b/docs/source/usersguide/scripts.rst
@@ -48,6 +48,7 @@ flags:
                        restart file
 -s, --threads N        Run with *N* OpenMP threads
 -t, --track            Write tracks for all particles (up to max_tracks)
+-q, --verbosity V      Set the output verbosity to *V*
 -v, --version          Show version information
 -h, --help             Show help message
 
diff --git a/docs/source/usersguide/settings.rst b/docs/source/usersguide/settings.rst
index 1b2d4bc1a..6faeb59e1 100644
--- a/docs/source/usersguide/settings.rst
+++ b/docs/source/usersguide/settings.rst
@@ -272,6 +272,12 @@ option::
   settings.source = [src1, src2]
   settings.uniform_source_sampling = True
 
+Additionally, sampling from an :class:`openmc.IndependentSource` may be biased
+for local or global variance reduction by modifying the
+:attr:`~openmc.IndependentSource.bias` attribute of each of its four main
+distributions. Further discussion of source biasing can be found in
+:ref:`source_biasing`.
+
 Finally, the :attr:`IndependentSource.particle` attribute can be used to
 indicate the source should be composed of particles other than neutrons. For
 example, the following would generate a photon source::
@@ -285,6 +291,53 @@ example, the following would generate a photon source::
 For a full list of all classes related to statistical distributions, see
 :ref:`pythonapi_stats`.
 
+Tokamak Plasma Sources
+----------------------
+
+For fusion applications, the :class:`openmc.TokamakSource` class provides a
+native parametric neutron source for tokamak plasmas. Rather than specifying
+spatial, angular, and energy distributions separately, the source is defined by
+the plasma geometry (using a `Miller-style flux-surface parameterization
+<https://doi.org/10.1063/1.872666>`_) and a radial emission profile. Source
+sites are sampled directly from the plasma volume without rejection.
+
+The plasma shape is described by the major radius :math:`R_0`, minor radius
+:math:`a`, elongation :math:`\kappa`, triangularity :math:`\delta`, and
+Shafranov shift :math:`\Delta`. The neutron birth profile is given as an
+emission density :math:`S(r/a)` tabulated on a normalized minor-radius grid that
+runs from 0 (magnetic axis) to 1 (last closed flux surface); only the shape of
+the profile matters, since it is normalized internally. The emission density is
+linearly interpolated between the supplied points and refined internally for
+radial sampling. For example::
+
+  import numpy as np
+
+  r_over_a = np.linspace(0.0, 1.0, 50)
+  emission = (1.0 - r_over_a**2)**2  # peaked on-axis profile
+
+  source = openmc.TokamakSource(
+      major_radius=620.0,    # cm
+      minor_radius=200.0,    # cm
+      elongation=1.8,
+      triangularity=0.45,
+      shafranov_shift=10.0,  # cm
+      r_over_a=r_over_a,
+      emission_density=emission,
+      energy=openmc.stats.muir(e0=14.08e6, m_rat=5.0, kt=20000.0),
+  )
+
+  settings.source = source
+
+The ``energy`` argument accepts either a single
+:class:`~openmc.stats.Univariate` distribution applied at all radii, or a
+sequence with one distribution per ``r_over_a`` grid point to model a
+radially-varying neutron spectrum (energies are then sampled by stochastic
+interpolation between the two distributions bracketing the sampled radius). A
+time distribution can be given with the ``time`` argument; by default, particles
+are born at :math:`t=0`. The toroidal extent can be restricted with
+``phi_start`` and ``phi_extent`` to model a sector of the plasma, and
+``vertical_shift`` translates the plasma center along the z-axis.
+
 File-based Sources
 ------------------
 
@@ -394,7 +447,7 @@ below.
     {
       openmc::SourceSite particle;
       // weight
-      particle.particle = openmc::ParticleType::neutron;
+      particle.particle = openmc::ParticleType::neutron();
       particle.wgt = 1.0;
       // position
       double angle = 2.0 * M_PI * openmc::prn(seed);
@@ -471,7 +524,7 @@ parameters to the source class when it is created:
     {
       openmc::SourceSite particle;
       // weight
-      particle.particle = openmc::ParticleType::neutron;
+      particle.particle = openmc::ParticleType::neutron();
       particle.wgt = 1.0;
       // position
       particle.r.x = 0.0;
@@ -598,6 +651,13 @@ transport::
 
   settings.photon_transport = True
 
+Atomic relaxation (the cascade of fluorescence photons and Auger electrons
+emitted when an inner-shell vacancy is filled) is enabled by default whenever
+photon transport is on. It can be disabled using the
+:attr:`Settings.atomic_relaxation` attribute::
+
+  settings.atomic_relaxation = False
+
 The way in which OpenMC handles secondary charged particles can be specified
 with the :attr:`Settings.electron_treatment` attribute. By default, the
 :ref:`thick-target bremsstrahlung <ttb>` (TTB) approximation is used to generate
@@ -756,6 +816,67 @@ instance, whereas the :meth:`openmc.Track.filter` method returns a new
             track_files = [f"tracks_p{rank}.h5" for rank in range(32)]
             openmc.Tracks.combine(track_files, "tracks.h5")
 
+Collision Track File
+---------------------
+
+OpenMC can generate a collision track file that contains detailed collision
+information (position, direction, energy, deposited energy, time, weight, cell
+ID, material ID, universe ID, nuclide ZAID, particle type, particle delayed
+group and particle ID) for each particle collision depending on user-defined
+parameters. To invoke this feature, set the
+:attr:`~openmc.Settings.collision_track` attribute as shown in this example::
+
+  settings.collision_track = {
+      "max_collisions": 300,
+      "reactions": ["(n,fission)", "(n,2n)"],
+      "material_ids": [1,2],
+      "nuclides": ["U238", "O16"],
+      "cell_ids": [5, 12]
+  }
+
+In this example, collision track information is written to the
+collision_track.h5 file at the end of the simulation. The file contains
+300 recorded collisions that occurred in materials with IDs 1 or 2, involving
+fission or (n,2n) reactions on the nuclides U-238 or O-16, within cells
+with IDs 5 and 12.
+
+.. note::
+   Electron and positron collision-track events are not associated with a
+   specific nuclide. If a ``nuclides`` entry is specified, these events are omitted.
+
+The file can be read using :func:`openmc.read_collision_track_file`.
+The example below shows how to extract the data from the collision_track
+feature and displays the fields stored in the file:
+
+>>> data = openmc.read_collision_track_file('collision_track.h5')
+>>> data.dtype
+    dtype([('r', [('x', '<f8'), ('y', '<f8'), ('z', '<f8')]),
+    ('u', [('x', '<f8'), ('y', '<f8'), ('z', '<f8')]), ('E', '<f8'),
+    ('dE', '<f8'), ('time', '<f8'), ('wgt', '<f8'), ('event_mt', '<i4'),
+    ('delayed_group', '<i4'), ('cell_id', '<i4'), ('nuclide_id', '<i4'),
+    ('material_id', '<i4'), ('universe_id', '<i4'), ('n_collision', '<i4'),
+    ('particle', '<i4'), ('parent_id', '<i8'), ('progeny_id', '<i8')])
+
+
+The full list of fields is as follows:
+
+  :r: Position (each direction in [cm])
+  :u: Direction
+  :E: Energy in [eV]
+  :dE: Energy deposited during collision in [eV]
+  :time: Time in [s]
+  :wgt: Weight of the particle
+  :event_mt: Reaction MT number
+  :delayed_group: Delayed group of the particle
+  :cell_id: Cell ID
+  :nuclide_id: Nuclide ID (10000×Z + 10×A + M)
+  :material_id: Material ID
+  :universe_id: Universe ID
+  :n_collision: Number of collision suffered by the particle
+  :particle: Particle type
+  :parent_id: Source particle ID
+  :progeny_id: Progeny ID
+
 -----------------------
 Restarting a Simulation
 -----------------------
diff --git a/docs/source/usersguide/tallies.rst b/docs/source/usersguide/tallies.rst
index 02328af58..20a89cea7 100644
--- a/docs/source/usersguide/tallies.rst
+++ b/docs/source/usersguide/tallies.rst
@@ -105,109 +105,124 @@ The following tables show all valid scores:
 
 .. table:: **Reaction scores: units are reactions per source particle.**
 
-    +----------------------+---------------------------------------------------+
-    |Score                 | Description                                       |
-    +======================+===================================================+
-    |absorption            |Total absorption rate. For incident neutrons, this |
-    |                      |accounts for all reactions that do not produce     |
-    |                      |secondary neutrons as well as fission. For incident|
-    |                      |photons, this includes photoelectric and pair      |
-    |                      |production.                                        |
-    +----------------------+---------------------------------------------------+
-    |elastic               |Elastic scattering reaction rate.                  |
-    +----------------------+---------------------------------------------------+
-    |fission               |Total fission reaction rate.                       |
-    +----------------------+---------------------------------------------------+
-    |scatter               |Total scattering rate.                             |
-    +----------------------+---------------------------------------------------+
-    |total                 |Total reaction rate.                               |
-    +----------------------+---------------------------------------------------+
-    |(n,2nd)               |(n,2nd) reaction rate.                             |
-    +----------------------+---------------------------------------------------+
-    |(n,2n)                |(n,2n) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,3n)                |(n,3n) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,na)                |(n,n\ :math:`\alpha`\ ) reaction rate.             |
-    +----------------------+---------------------------------------------------+
-    |(n,n3a)               |(n,n3\ :math:`\alpha`\ ) reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |(n,2na)               |(n,2n\ :math:`\alpha`\ ) reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |(n,3na)               |(n,3n\ :math:`\alpha`\ ) reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |(n,np)                |(n,np) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,n2a)               |(n,n2\ :math:`\alpha`\ ) reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |(n,2n2a)              |(n,2n2\ :math:`\alpha`\ ) reaction rate.           |
-    +----------------------+---------------------------------------------------+
-    |(n,nd)                |(n,nd) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,nt)                |(n,nt) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,n3He)              |(n,n\ :sup:`3`\ He) reaction rate.                 |
-    +----------------------+---------------------------------------------------+
-    |(n,nd2a)              |(n,nd2\ :math:`\alpha`\ ) reaction rate.           |
-    +----------------------+---------------------------------------------------+
-    |(n,nt2a)              |(n,nt2\ :math:`\alpha`\ ) reaction rate.           |
-    +----------------------+---------------------------------------------------+
-    |(n,4n)                |(n,4n) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,2np)               |(n,2np) reaction rate.                             |
-    +----------------------+---------------------------------------------------+
-    |(n,3np)               |(n,3np) reaction rate.                             |
-    +----------------------+---------------------------------------------------+
-    |(n,n2p)               |(n,n2p) reaction rate.                             |
-    +----------------------+---------------------------------------------------+
-    |(n,n*X*)              |Level inelastic scattering reaction rate. The *X*  |
-    |                      |indicates what which inelastic level, e.g., (n,n3) |
-    |                      |is third-level inelastic scattering.               |
-    +----------------------+---------------------------------------------------+
-    |(n,nc)                |Continuum level inelastic scattering reaction rate.|
-    +----------------------+---------------------------------------------------+
-    |(n,gamma)             |Radiative capture reaction rate.                   |
-    +----------------------+---------------------------------------------------+
-    |(n,p)                 |(n,p) reaction rate.                               |
-    +----------------------+---------------------------------------------------+
-    |(n,d)                 |(n,d) reaction rate.                               |
-    +----------------------+---------------------------------------------------+
-    |(n,t)                 |(n,t) reaction rate.                               |
-    +----------------------+---------------------------------------------------+
-    |(n,3He)               |(n,\ :sup:`3`\ He) reaction rate.                  |
-    +----------------------+---------------------------------------------------+
-    |(n,a)                 |(n,\ :math:`\alpha`\ ) reaction rate.              |
-    +----------------------+---------------------------------------------------+
-    |(n,2a)                |(n,2\ :math:`\alpha`\ ) reaction rate.             |
-    +----------------------+---------------------------------------------------+
-    |(n,3a)                |(n,3\ :math:`\alpha`\ ) reaction rate.             |
-    +----------------------+---------------------------------------------------+
-    |(n,2p)                |(n,2p) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,pa)                |(n,p\ :math:`\alpha`\ ) reaction rate.             |
-    +----------------------+---------------------------------------------------+
-    |(n,t2a)               |(n,t2\ :math:`\alpha`\ ) reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |(n,d2a)               |(n,d2\ :math:`\alpha`\ ) reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |(n,pd)                |(n,pd) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,pt)                |(n,pt) reaction rate.                              |
-    +----------------------+---------------------------------------------------+
-    |(n,da)                |(n,d\ :math:`\alpha`\ ) reaction rate.             |
-    +----------------------+---------------------------------------------------+
-    |coherent-scatter      |Coherent (Rayleigh) scattering reaction rate.      |
-    +----------------------+---------------------------------------------------+
-    |incoherent-scatter    |Incoherent (Compton) scattering reaction rate.     |
-    +----------------------+---------------------------------------------------+
-    |photoelectric         |Photoelectric absorption reaction rate.            |
-    +----------------------+---------------------------------------------------+
-    |pair-production       |Pair production reaction rate.                     |
-    +----------------------+---------------------------------------------------+
-    |*Arbitrary integer*   |An arbitrary integer is interpreted to mean the    |
-    |                      |reaction rate for a reaction with a given ENDF MT  |
-    |                      |number.                                            |
-    +----------------------+---------------------------------------------------+
+    +------------------------+-------------------------------------------------+
+    |Score                   |Description                                      |
+    +========================+=================================================+
+    |absorption              |Total absorption rate. For incident neutrons,    |
+    |                        |this accounts for all reactions that do not      |
+    |                        |produce secondary neutrons as well as fission.   |
+    |                        |For incident photons, this includes              |
+    |                        |photoelectric and pair production.               |
+    +------------------------+-------------------------------------------------+
+    |elastic                 |Elastic scattering reaction rate.                |
+    +------------------------+-------------------------------------------------+
+    |fission                 |Total fission reaction rate.                     |
+    +------------------------+-------------------------------------------------+
+    |scatter                 |Total scattering rate.                           |
+    +------------------------+-------------------------------------------------+
+    |total                   |Total reaction rate.                             |
+    +------------------------+-------------------------------------------------+
+    |(n,2nd)                 |(n,2nd) reaction rate.                           |
+    +------------------------+-------------------------------------------------+
+    |(n,2n)                  |(n,2n) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,3n)                  |(n,3n) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,na)                  |(n,n\ :math:`\alpha`\ ) reaction rate.           |
+    +------------------------+-------------------------------------------------+
+    |(n,n3a)                 |(n,n3\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,2na)                 |(n,2n\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,3na)                 |(n,3n\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,np)                  |(n,np) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,n2a)                 |(n,n2\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,2n2a)                |(n,2n2\ :math:`\alpha`\ ) reaction rate.         |
+    +------------------------+-------------------------------------------------+
+    |(n,nd)                  |(n,nd) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,nt)                  |(n,nt) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,n3He)                |(n,n\ :sup:`3`\ He) reaction rate.               |
+    +------------------------+-------------------------------------------------+
+    |(n,nd2a)                |(n,nd2\ :math:`\alpha`\ ) reaction rate.         |
+    +------------------------+-------------------------------------------------+
+    |(n,nt2a)                |(n,nt2\ :math:`\alpha`\ ) reaction rate.         |
+    +------------------------+-------------------------------------------------+
+    |(n,4n)                  |(n,4n) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,2np)                 |(n,2np) reaction rate.                           |
+    +------------------------+-------------------------------------------------+
+    |(n,3np)                 |(n,3np) reaction rate.                           |
+    +------------------------+-------------------------------------------------+
+    |(n,n2p)                 |(n,n2p) reaction rate.                           |
+    +------------------------+-------------------------------------------------+
+    |(n,npa)                 |(n,np\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,n*X*)                |Level inelastic scattering reaction rate. The    |
+    |                        |*X* indicates which inelastic level, e.g.,       |
+    |                        |(n,n3) is third-level inelastic scattering.      |
+    +------------------------+-------------------------------------------------+
+    |(n,nc)                  |Continuum level inelastic scattering             |
+    |                        |reaction rate.                                   |
+    +------------------------+-------------------------------------------------+
+    |(n,gamma)               |Radiative capture reaction rate.                 |
+    +------------------------+-------------------------------------------------+
+    |(n,p)                   |(n,p) reaction rate.                             |
+    +------------------------+-------------------------------------------------+
+    |(n,d)                   |(n,d) reaction rate.                             |
+    +------------------------+-------------------------------------------------+
+    |(n,t)                   |(n,t) reaction rate.                             |
+    +------------------------+-------------------------------------------------+
+    |(n,3He)                 |(n,\ :sup:`3`\ He) reaction rate.                |
+    +------------------------+-------------------------------------------------+
+    |(n,a)                   |(n,\ :math:`\alpha`\ ) reaction rate.            |
+    +------------------------+-------------------------------------------------+
+    |(n,2a)                  |(n,2\ :math:`\alpha`\ ) reaction rate.           |
+    +------------------------+-------------------------------------------------+
+    |(n,3a)                  |(n,3\ :math:`\alpha`\ ) reaction rate.           |
+    +------------------------+-------------------------------------------------+
+    |(n,2p)                  |(n,2p) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,pa)                  |(n,p\ :math:`\alpha`\ ) reaction rate.           |
+    +------------------------+-------------------------------------------------+
+    |(n,t2a)                 |(n,t2\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,d2a)                 |(n,d2\ :math:`\alpha`\ ) reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |(n,pd)                  |(n,pd) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,pt)                  |(n,pt) reaction rate.                            |
+    +------------------------+-------------------------------------------------+
+    |(n,da)                  |(n,d\ :math:`\alpha`\ ) reaction rate.           |
+    +------------------------+-------------------------------------------------+
+    |photon-total            |Total photo-atomic reaction rate.                |
+    +------------------------+-------------------------------------------------+
+    |coherent-scatter        |Coherent (Rayleigh) scattering reaction rate.    |
+    +------------------------+-------------------------------------------------+
+    |incoherent-scatter      |Incoherent (Compton) scattering reaction rate.   |
+    +------------------------+-------------------------------------------------+
+    |photoelectric           |Photoelectric absorption reaction rate.          |
+    +------------------------+-------------------------------------------------+
+    |photoelectric-*S*       |Subshell photoelectric absorption rate for the   |
+    |                        |*S* shell. For example, "photoelectric-N3" is the|
+    |                        |rate for the N3 subshell.                        |
+    +------------------------+-------------------------------------------------+
+    |pair-production         |Pair production reaction rate (total).           |
+    +------------------------+-------------------------------------------------+
+    |pair-production-electron|Pair production reaction rate in the electron    |
+    |                        |field.                                           |
+    +------------------------+-------------------------------------------------+
+    |pair-production-nuclear |Pair production reaction rate in the nuclear     |
+    |                        |field.                                           |
+    +------------------------+-------------------------------------------------+
+    |*Arbitrary integer*     |An arbitrary integer is interpreted to mean the  |
+    |                        |reaction rate for a reaction with a given ENDF   |
+    |                        |MT number.                                       |
+    +------------------------+-------------------------------------------------+
 
 .. table:: **Particle production scores: units are particles produced per
            source particles.**
@@ -246,18 +261,21 @@ The following tables show all valid scores:
     +----------------------+---------------------------------------------------+
     |Score                 | Description                                       |
     +======================+===================================================+
-    |current               |Used in combination with a meshsurface filter:     |
+    |current               |It may not be used in conjunction with any other   |
+    |                      |score except flux.                                 |
+    |                      |                                                   |
+    |                      |When used in combination with a meshsurface filter:|
     |                      |Partial currents on the boundaries of each cell in |
-    |                      |a mesh. It may not be used in conjunction with any |
-    |                      |other score. Only energy and mesh filters may be   |
-    |                      |used.                                              |
-    |                      |Used in combination with a surface filter:         |
+    |                      |a mesh.                                            |
+    |                      |                                                   |
+    |                      |When used in combination with a surface filter:    |
     |                      |Net currents on any surface previously defined in  |
     |                      |the geometry. It may be used along with any other  |
     |                      |filter, except meshsurface filters.                |
     |                      |Surfaces can alternatively be defined with cell    |
     |                      |from and cell filters thereby resulting in tallying|
     |                      |partial currents.                                  |
+    |                      |                                                   |
     |                      |Units are particles per source particle.           |
     +----------------------+---------------------------------------------------+
     |events                |Number of scoring events. Units are events per     |
@@ -322,6 +340,24 @@ The following tables show all valid scores:
     |                      |particle. Note that this score can only be combined|
     |                      |with a cell filter and an energy filter.           |
     +----------------------+---------------------------------------------------+
+    |ifp-time-numerator    |Adjoint-weighted lifetime of neutron produced by   |
+    |                      |fission in units of seconds per source particle.   |
+    |                      |This score is used to compute kinetics parameters  |
+    |                      |using the iterated fission probability (IFP)       |
+    |                      |method.                                            |
+    +----------------------+---------------------------------------------------+
+    |ifp-beta-numerator    |Adjoint-weighted number of delayed fission events  |
+    |                      |in units of number of delayed fission event per    |
+    |                      |source particle. This score is used to compute     |
+    |                      |kinetics parameters using the iterated fission     |
+    |                      |probability (IFP) method.                          |
+    +----------------------+---------------------------------------------------+
+    |ifp-denominator       |Weights corresponding to the number of fission     |
+    |                      |events in units of number of fission event per     |
+    |                      |source particle. This score is used to compute     |
+    |                      |kinetics parameters using the iterated fission     |
+    |                      |probability (IFP) method.                          |
+    +----------------------+---------------------------------------------------+
 
 .. _usersguide_tally_normalization:
 
diff --git a/docs/source/usersguide/variance_reduction.rst b/docs/source/usersguide/variance_reduction.rst
index f8ee9c35c..d551195f5 100644
--- a/docs/source/usersguide/variance_reduction.rst
+++ b/docs/source/usersguide/variance_reduction.rst
@@ -4,24 +4,27 @@
 Variance Reduction
 ==================
 
-Global variance reduction in OpenMC is accomplished by weight windowing
-techniques. OpenMC is capable of generating weight windows using either the
-MAGIC or FW-CADIS methods. Both techniques will produce a ``weight_windows.h5``
-file that can be loaded and used later on. In this section, we break down the
-steps required to both generate and then apply weight windows.
+Global and local variance reduction are possible in OpenMC through both weight 
+windowing and source biasing techniques. OpenMC is capable of generating weight
+windows using either the MAGIC or FW-CADIS methods, the latter with an optional
+capability for local variance reduction. Both techniques will produce a
+``weight_windows.h5`` file that can be loaded and used later on. In
+this section, we first break down the steps required to generate and apply
+weight windows, then describe how source biasing may be applied.
 
 .. _ww_generator:
 
-------------------------------------
-Generating Weight Windows with MAGIC
-------------------------------------
+-------------------------------------------
+Generating Global Weight Windows with MAGIC
+-------------------------------------------
 
 As discussed in the :ref:`methods section <methods_variance_reduction>`, MAGIC
 is an iterative method that uses flux tally information from a Monte Carlo
-simulation to produce weight windows for a user-defined mesh. While generating
-the weight windows, OpenMC is capable of applying the weight windows generated
-from a previous batch while processing the next batch, allowing for progressive
-improvement in the weight window quality across iterations.
+simulation to produce weight windows for a user-defined mesh with the objective 
+of global variance reduction. While generating the weight windows, OpenMC is 
+capable of applying the weight windows generated from a previous batch while 
+processing the next batch, allowing for progressive improvement in the weight 
+window quality across iterations.
 
 The typical way of generating weight windows is to define a mesh and then add an
 :class:`openmc.WeightWindowGenerator` object to an :attr:`openmc.Settings`
@@ -51,7 +54,7 @@ With the :class:`~openmc.WeightWindowGenerator` instance added to the
 :attr:`~openmc.Settings`, the rest of the problem can be defined as normal. When
 running, note that the second iteration and beyond may be several orders of
 magnitude slower than the first. As the weight windows are applied in each
-iteration, particles may be agressively split, resulting in a large number of
+iteration, particles may be aggressively split, resulting in a large number of
 secondary (split) particles being generated per initial source particle. This is
 not necessarily a bad thing, as the split particles are much more efficient at
 exploring low flux regions of phase space as compared to initial particles.
@@ -69,15 +72,20 @@ At the end of the simulation, a ``weight_windows.h5`` file will be saved to disk
 for later use. Loading it in another subsequent simulation will be discussed in
 the "Using Weight Windows" section below.
 
-------------------------------------------------------
-Generating Weight Windows with FW-CADIS and Random Ray
-------------------------------------------------------
+.. _usersguide_fw_cadis:
+
+----------------------------------------------------------------------
+Generating Global or Local Weight Windows with FW-CADIS and Random Ray
+----------------------------------------------------------------------
 
 Weight window generation with FW-CADIS and random ray in OpenMC uses the same
-exact strategy as with MAGIC. An :class:`openmc.WeightWindowGenerator` object is
-added to the :attr:`openmc.Settings` object, and a ``weight_windows.h5`` will be
-generated at the end of the simulation. The only difference is that the code
-must be run in random ray mode. A full description of how to enable and setup
+exact strategy as with MAGIC. Using FW-CADIS, however, also enables 
+local variance reduction in fixed source problems through the :attr:`targets` 
+attribute, which is described later in this section. To enable FW-CADIS, an 
+:class:`openmc.WeightWindowGenerator` object is added to the 
+:attr:`openmc.Settings` object, and a ``weight_windows.h5`` will be generated 
+at the end of the simulation. The only procedural difference is that the code 
+must be run in random ray mode. A full description of how to enable and setup 
 random ray mode can be found in the :ref:`Random Ray User Guide <random_ray>`.
 
 .. note::
@@ -88,59 +96,111 @@ random ray mode can be found in the :ref:`Random Ray User Guide <random_ray>`.
     ray solver. A high level overview of the current workflow for generation of
     weight windows with FW-CADIS using random ray is given below.
 
-1. Produce approximate multigroup cross section data (stored in a ``mgxs.h5``
-   library). There is more information on generating multigroup cross sections
-   via OpenMC in the :ref:`multigroup materials <create_mgxs>` user guide, and a
-   specific example of generating cross section data for use with random ray in
-   the :ref:`random ray MGXS guide <mgxs_gen>`.
+1. Begin by making a deep copy of your continuous energy Python model and then
+   convert the copy to be multigroup and use the random ray transport solver.
+   The conversion process can largely be automated as described in more detail
+   in the :ref:`random ray quick start guide <quick_start>`, summarized below::
 
-2. Make a copy of your continuous energy Python input file. You'll edit the new
-   file to work in multigroup mode with random ray for producing weight windows.
+    # Define continuous energy model
+    ce_model = openmc.pwr_pin_cell() # example, replace with your model
 
-3. Adjust the material definitions in your new multigroup Python file to utilize
-   the multigroup cross sections instead of nuclide-wise continuous energy data.
-   There is a specific example of making this conversion in the :ref:`random ray
-   MGXS guide <mgxs_gen>`.
+    # Make a copy to convert to multigroup and random ray
+    model = copy.deepcopy(ce_model)
 
-4. Configure OpenMC to run in random ray mode (by adding several standard random
-   ray input flags and settings to the :attr:`openmc.Settings.random_ray`
-   dictionary). More information can be found in the  :ref:`Random Ray User
-   Guide <random_ray>`.
+    # Convert model to multigroup (will auto-generate MGXS library if needed)
+    model.convert_to_multigroup()
 
-5. Add in a :class:`~openmc.WeightWindowGenerator` in a similar manner as for
+    # Convert model to random ray and initialize random ray parameters
+    # to reasonable defaults based on the specifics of the geometry
+    model.convert_to_random_ray()
+
+    # (Optional) Overlay source region decomposition mesh to improve fidelity of the
+    # random ray solver. Adjust 'n' for fidelity vs runtime.
+    n = 10
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (n, n, n)
+    mesh.lower_left = model.geometry.bounding_box.lower_left
+    mesh.upper_right = model.geometry.bounding_box.upper_right
+    model.settings.random_ray['source_region_meshes'] = [(mesh, [model.geometry.root_universe])]
+
+    # (Optional) Improve fidelity of the random ray solver by enabling linear sources
+    model.settings.random_ray['source_shape'] = 'linear'
+
+    # (Optional) Increase the number of rays/batch, to reduce uncertainty
+    model.settings.particles = 500
+
+   If you need to improve the fidelity of the MGXS library, there is more
+   information on generating multigroup cross sections via OpenMC in the
+   :ref:`random ray MGXS guide <mgxs_gen>`.
+
+2. Add in a :class:`~openmc.WeightWindowGenerator` in a similar manner as for
    MAGIC generation with Monte Carlo and set the :attr:`method` attribute set to
    ``"fw_cadis"``::
 
-       # Define weight window spatial mesh
-       ww_mesh = openmc.RegularMesh()
-       ww_mesh.dimension = (10, 10, 10)
-       ww_mesh.lower_left = (0.0, 0.0, 0.0)
-       ww_mesh.upper_right = (100.0, 100.0, 100.0)
-
-       # Create weight window object and adjust parameters
+       # Create weight window object and adjust parameters, using the same mesh
+       # we used for source region decomposition
        wwg = openmc.WeightWindowGenerator(
            method='fw_cadis',
-           mesh=ww_mesh,
-           max_realizations=settings.batches
+           mesh=mesh
        )
 
        # Add generator to openmc.settings object
        settings.weight_window_generators = wwg
 
-
 .. warning::
     If using FW-CADIS weight window generation, ensure that the selected weight
     window mesh does not subdivide any source regions in the problem. This can
-    be ensured by assigning the weight window tally mesh to the root universe so
-    as to create source region boundaries that conform to the mesh, as in the
-    example below.
+    be ensured by using the same mesh for both source region subdivision (i.e.,
+    assigning to ``model.settings.random_ray['source_region_meshes']``) and for
+    weight window generation.
 
-::
+3. (Optional) If local variance reduction is desired in a fixed-source problem,  
+   populate the :attr:`targets` attribute with an :class:`openmc.Tallies` 
+   instance or an iterable of tally IDs indicating the tallies of interest for 
+   variance reduction::
+
+    # Build a new example and WWG for local variance reduction
+    from openmc.examples import random_ray_three_region_cube_with_detectors
+    new_model = random_ray_three_region_cube_with_detectors()
+
+    ww_mesh = openmc.RegularMesh()
+    n = 7
+    width = 35.0
+    ww_mesh.dimension = (n, n, n)
+    ww_mesh.lower_left = (0.0, 0.0, 0.0)
+    ww_mesh.upper_right = (width, width, width)
+
+    wwg = openmc.WeightWindowGenerator(
+        method="fw_cadis", 
+        mesh=ww_mesh, 
+        max_realizations=new_model.settings.batches
+    )
+    new_model.settings.weight_window_generators = wwg
+    new_model.settings.random_ray['volume_estimator'] = 'naive'
+
+    # Get the tallies of interest
+    target_tallies = openmc.Tallies()
+
+    for tally in list(new_model.tallies):
+        if tally.name in {"Detector 1 Tally", "Detector 2 Tally"}:
+            target_tallies.append(tally)
     
-    root = model.geometry.root_universe    
-    settings.random_ray['source_region_meshes'] = [(ww_mesh, [root])]
+    # Add to WeightWindowGenerator
+    wwg.targets = target_tallies
 
-6. When running your multigroup random ray input deck, OpenMC will automatically
+.. warning::
+    The tallies designated as FW-CADIS targets to the 
+    :class:`~openmc.WeightWindowGenerator` must be present under the
+    :class:`~openmc.model.Model.tallies` attribute of the 
+    :class:`~openmc.model.Model` as well in order to be recognized as valid 
+    local variance reduction targets. This check is performed when the 
+    :func:`openmc.model.Model.export_to_model_xml` or 
+    :func:`openmc.model.Model.export_to_xml` functions are called, meaning 
+    that the standalone :func:`openmc.Settings.export_to_xml` and 
+    :func:`openmc.Tallies.export_to_xml` methods should not be used with 
+    FW-CADIS local variance reduction.
+
+4. When running your multigroup random ray input deck, OpenMC will automatically
    run a forward solve followed by an adjoint solve, with a
    ``weight_windows.h5`` file generated at the end. The ``weight_windows.h5``
    file will contain FW-CADIS generated weight windows. This file can be used in
@@ -155,7 +215,7 @@ solver, the Python input just needs to load the h5 file::
 
     settings.weight_window_checkpoints = {'collision': True, 'surface': True}
     settings.survival_biasing = False
-    settings.weight_windows = openmc.hdf5_to_wws('weight_windows.h5')
+    settings.weight_windows_file = "weight_windows.h5"
     settings.weight_windows_on = True
 
 The :class:`~openmc.WeightWindowGenerator` instance is not needed to load an
@@ -166,3 +226,148 @@ Weight window mesh information is embedded into the weight window file, so the
 mesh does not need to be redefined. Monte Carlo solves that load a weight window
 file as above will utilize weight windows to reduce the variance of the
 simulation.
+
+.. _source_biasing:
+
+--------------
+Source Biasing
+--------------
+
+In fixed source problems, source biasing provides a means to reduce the variance
+on global or localized responses, depending on the biasing scheme. In either
+case, the premise of the method is to sample source sites from a biased
+distribution that directs a larger fraction of the simulated histories towards
+phase space regions of interest than would be found there under analog sampling.
+In order to preserve an unbiased estimate of the tally mean, the weight of these
+with analog sampling, divided by the probability assigned by the biased
+distribution. While the assignment of statistical weights is outlined in the
+:ref:`methods section <methods_source_biasing>`, this section demonstrates the
+implementation of source biasing to problems in OpenMC.
+
+Source biasing in OpenMC is accomplished by applying a distribution to the
+:attr:`bias` attribute of one or more of the univariate or independent
+multivariate distributions which make up an :class:`~openmc.IndependentSource`
+instance as follows::
+
+    # First create the biased distribution
+    biased_dist = openmc.stats.PowerLaw(a=0, b=3, n=3)
+
+    # Construct a new distribution with the bias applied
+    dist = openmc.stats.PowerLaw(a=0, b=3, n=2, bias=biased_dist)
+
+    # The bias attribute can also be set on an existing "analog" distribution:
+    sphere_dist = openmc.stats.spherical_uniform(r_outer=3)
+    sphere_dist.r.bias = biased_dist
+
+Univariate distributions may be sampled via the Python API, returning the
+sample(s) along with the associated weight(s)::
+
+    sample_vec, wgt_vec = dist.sample(n_samples=100)
+
+Here, if the distribution is unbiased, the weight of each sample will be unity.
+Finally, :class:`~openmc.IndependentSource` instances can be constructed with
+biased distributions::
+
+    # Create a source with a biased spatial distribution
+    source = openmc.IndependentSource(space=sphere_dist)
+
+During the simulation, source sites are then sampled using the biased
+distributions where available and given starting statistical weights
+corresponding to the cumulative product of the weights assigned by each
+distribution in the source object. Hence multiple source variables (e.g.,
+direction and energy) may be biased and the resulting source sites will have
+their weights adjusted accordingly.
+
+.. note::
+    Combining source biasing with weight windows can be a powerful variance
+    reduction technique if each is constructed appropriately for the response
+    of interest. For example, if a source biasing scheme is devised for
+    variance reduction of a specific localized response, the user may be able
+    to specify their own weight window structure that results in more efficient
+    transport than if weight windows were generated by either of OpenMC's
+    automatic weight window generators, which are intended for global variance
+    reduction.
+
+Biased distributions that could result in degenerate weight mappings are not
+recommended; this is most commonly seen when biasing the :math:`\phi`-coordinate
+of spherical or cylindrical independent multivariate distributions. In such
+cases degenerate behavior will be observed at the pole about which :math:`\phi`
+is measured, with all values of :math:`\phi` (hence many possible statistical
+weights) mapping to the same point for :math:`r=0` or :math:`\mu=0`, and large
+weight gradients in the vicinity. In most cases requiring a spherical
+independent source, it would be preferable to reorient the reference vector of
+the distribution such that biasing could be applied to the
+:math:`\mu`-coordinate instead.
+
+When biasing a distribution, care should also be taken to ensure that both the
+unbiased and biased distribution share a common support---that is, every region
+of phase space mapped to a nonzero probability density by the unbiased
+distribution should likewise map to nonzero probability under the biased
+distribution, and vice versa. In OpenMC, this places restrictions on the set of
+compatible distributions that may be used to bias sampling of each distribution
+type. The following table summarizes the method for each distribution in OpenMC
+that permits biased sampling.
+
+.. list-table:: **Distributions that support biased sampling**
+   :header-rows: 1
+   :widths: 35 65
+
+   * - Discrete Univariate PDFs
+     - Biasing Method
+   * - :class:`openmc.stats.Discrete`
+     - Apply a vector of alternative probabilities to the :attr:`bias`
+       attribute
+
+.. list-table::
+   :header-rows: 1
+   :widths: 35 65
+
+   * - Continuous Univariate PDFs
+     - Biasing Method
+   * - :class:`openmc.stats.Uniform`,
+       :class:`openmc.stats.PowerLaw`,
+       :class:`openmc.stats.Maxwell`,
+       :class:`openmc.stats.Watt`,
+       :class:`openmc.stats.Normal`,
+       :class:`openmc.stats.Tabular`
+     - Apply a second, unbiased continous univariate PDF to the :attr:`bias`
+       attribute, ensuring that the :attr:`support` attribute of each
+       distribution is the same
+
+.. list-table::
+   :header-rows: 1
+   :widths: 35 65
+
+   * - Mixed Univariate PDFs
+     - Biasing Method
+   * - :class:`openmc.stats.Mixture`
+     - May be constructed from multiple biased univariate distributions, or a
+       second, unbiased continous univariate PDF may be applied to the
+       :attr:`bias` attribute
+
+.. list-table::
+   :header-rows: 1
+   :widths: 35 65
+
+   * - Discrete Multivariate PDFs
+     - Biasing Method
+   * - :class:`openmc.stats.PointCloud`,
+       :class:`openmc.stats.MeshSpatial`
+     - Apply a vector of the new relative probabilities of each point or mesh
+       element under biased sampling to the :attr:`bias` attribute
+
+.. list-table::
+   :header-rows: 1
+   :widths: 35 65
+
+   * - Continuous Multivariate PDFs
+     - Biasing Method
+   * - :class:`openmc.stats.CartesianIndependent`,
+       :class:`openmc.stats.CylindricalIndependent`,
+       :class:`openmc.stats.SphericalIndependent`,
+       :class:`openmc.stats.PolarAzimuthal`
+     - Construct from biased univariate distributions for :attr:`x`, :attr:`y`,
+       :attr:`z`, etc.
+   * - :class:`openmc.stats.Isotropic`
+     - Apply an unbiased :class:`openmc.stats.PolarAzimuthal` to the
+       :attr:`bias` attribute
diff --git a/examples/custom_source/source_ring.cpp b/examples/custom_source/source_ring.cpp
index b87afbd17..bce6c27eb 100644
--- a/examples/custom_source/source_ring.cpp
+++ b/examples/custom_source/source_ring.cpp
@@ -1,17 +1,17 @@
-#include <cmath> // for M_PI
+#define _USE_MATH_DEFINES
+#include <cmath>  // for M_PI
 #include <memory> // for unique_ptr
 
+#include "openmc/particle.h"
 #include "openmc/random_lcg.h"
 #include "openmc/source.h"
-#include "openmc/particle.h"
 
-class RingSource : public openmc::Source
-{
+class RingSource : public openmc::Source {
   openmc::SourceSite sample(uint64_t* seed) const
   {
     openmc::SourceSite particle;
     // particle type
-    particle.particle = openmc::ParticleType::neutron;
+    particle.particle = openmc::ParticleType::neutron();
     // position
     double angle = 2.0 * M_PI * openmc::prn(seed);
     double radius = 3.0;
@@ -25,10 +25,11 @@ class RingSource : public openmc::Source
   }
 };
 
-// A function to create a unique pointer to an instance of this class when generated
-// via a plugin call using dlopen/dlsym.
-// You must have external C linkage here otherwise dlopen will not find the file
-extern "C" std::unique_ptr<RingSource> openmc_create_source(std::string parameters)
+// A function to create a unique pointer to an instance of this class when
+// generated via a plugin call using dlopen/dlsym. You must have external C
+// linkage here otherwise dlopen will not find the file
+extern "C" std::unique_ptr<RingSource> openmc_create_source(
+  std::string parameters)
 {
   return std::make_unique<RingSource>();
 }
diff --git a/examples/lattice/hexagonal/build_xml.py b/examples/lattice/hexagonal/build_xml.py
index 9485d0aa4..2624e52b4 100644
--- a/examples/lattice/hexagonal/build_xml.py
+++ b/examples/lattice/hexagonal/build_xml.py
@@ -128,14 +128,14 @@ settings_file.export_to_xml()
 #                   Exporting to OpenMC plots.xml file
 ###############################################################################
 
-plot_xy = openmc.Plot(plot_id=1)
+plot_xy = openmc.SlicePlot(plot_id=1)
 plot_xy.filename = 'plot_xy'
 plot_xy.origin = [0, 0, 0]
 plot_xy.width = [6, 6]
 plot_xy.pixels = [400, 400]
 plot_xy.color_by = 'material'
 
-plot_yz = openmc.Plot(plot_id=2)
+plot_yz = openmc.SlicePlot(plot_id=2)
 plot_yz.filename = 'plot_yz'
 plot_yz.basis = 'yz'
 plot_yz.origin = [0, 0, 0]
diff --git a/examples/lattice/nested/build_xml.py b/examples/lattice/nested/build_xml.py
index 2db23a46b..a1d9c092d 100644
--- a/examples/lattice/nested/build_xml.py
+++ b/examples/lattice/nested/build_xml.py
@@ -135,7 +135,7 @@ settings_file.export_to_xml()
 #                   Exporting to OpenMC plots.xml file
 ###############################################################################
 
-plot = openmc.Plot(plot_id=1)
+plot = openmc.SlicePlot(plot_id=1)
 plot.origin = [0, 0, 0]
 plot.width = [4, 4]
 plot.pixels = [400, 400]
diff --git a/examples/lattice/simple/build_xml.py b/examples/lattice/simple/build_xml.py
index 56c466121..44531edd8 100644
--- a/examples/lattice/simple/build_xml.py
+++ b/examples/lattice/simple/build_xml.py
@@ -128,7 +128,7 @@ settings_file.export_to_xml()
 #                   Exporting to OpenMC plots.xml file
 ###############################################################################
 
-plot = openmc.Plot(plot_id=1)
+plot = openmc.SlicePlot(plot_id=1)
 plot.origin = [0, 0, 0]
 plot.width = [4, 4]
 plot.pixels = [400, 400]
diff --git a/examples/parameterized_custom_source/parameterized_source_ring.cpp b/examples/parameterized_custom_source/parameterized_source_ring.cpp
index e70dc8bb1..3aabf3581 100644
--- a/examples/parameterized_custom_source/parameterized_source_ring.cpp
+++ b/examples/parameterized_custom_source/parameterized_source_ring.cpp
@@ -1,63 +1,66 @@
-#include <cmath> // for M_PI
-#include <memory> // for unique_ptr
+#define _USE_MATH_DEFINES
+#include <cmath>
+#include <memory>
 #include <unordered_map>
 
+#include "openmc/particle.h"
 #include "openmc/random_lcg.h"
 #include "openmc/source.h"
-#include "openmc/particle.h"
 
 class RingSource : public openmc::Source {
-  public:
-    RingSource(double radius, double energy) : radius_(radius), energy_(energy) { }
+public:
+  RingSource(double radius, double energy) : radius_(radius), energy_(energy) {}
 
-    // Defines a function that can create a unique pointer to a new instance of this class
-    // by extracting the parameters from the provided string.
-    static std::unique_ptr<RingSource> from_string(std::string parameters)
-    {
-      std::unordered_map<std::string, std::string> parameter_mapping;
+  // Defines a function that can create a unique pointer to a new instance of
+  // this class by extracting the parameters from the provided string.
+  static std::unique_ptr<RingSource> from_string(std::string parameters)
+  {
+    std::unordered_map<std::string, std::string> parameter_mapping;
 
-      std::stringstream ss(parameters);
-      std::string parameter;
-      while (std::getline(ss, parameter, ',')) {
-        parameter.erase(0, parameter.find_first_not_of(' '));
-        std::string key = parameter.substr(0, parameter.find_first_of('='));
-        std::string value = parameter.substr(parameter.find_first_of('=') + 1, parameter.length());
-        parameter_mapping[key] = value;
-      }
-
-      double radius = std::stod(parameter_mapping["radius"]);
-      double energy = std::stod(parameter_mapping["energy"]);
-      return std::make_unique<RingSource>(radius, energy);
+    std::stringstream ss(parameters);
+    std::string parameter;
+    while (std::getline(ss, parameter, ',')) {
+      parameter.erase(0, parameter.find_first_not_of(' '));
+      std::string key = parameter.substr(0, parameter.find_first_of('='));
+      std::string value =
+        parameter.substr(parameter.find_first_of('=') + 1, parameter.length());
+      parameter_mapping[key] = value;
     }
 
-    // Samples from an instance of this class.
-    openmc::SourceSite sample(uint64_t* seed) const
-    {
-      openmc::SourceSite particle;
-      // particle type
-      particle.particle = openmc::ParticleType::neutron;
-      // position
-      double angle = 2.0 * M_PI * openmc::prn(seed);
-      double radius = this->radius_;
-      particle.r.x = radius * std::cos(angle);
-      particle.r.y = radius * std::sin(angle);
-      particle.r.z = 0.0;
-      // angle
-      particle.u = {1.0, 0.0, 0.0};
-      particle.E = this->energy_;
+    double radius = std::stod(parameter_mapping["radius"]);
+    double energy = std::stod(parameter_mapping["energy"]);
+    return std::make_unique<RingSource>(radius, energy);
+  }
 
-      return particle;
-    }
+  // Samples from an instance of this class.
+  openmc::SourceSite sample(uint64_t* seed) const
+  {
+    openmc::SourceSite particle;
+    // particle type
+    particle.particle = openmc::ParticleType::neutron();
+    // position
+    double angle = 2.0 * M_PI * openmc::prn(seed);
+    double radius = this->radius_;
+    particle.r.x = radius * std::cos(angle);
+    particle.r.y = radius * std::sin(angle);
+    particle.r.z = 0.0;
+    // angle
+    particle.u = {1.0, 0.0, 0.0};
+    particle.E = this->energy_;
 
-  private:
-    double radius_;
-    double energy_;
+    return particle;
+  }
+
+private:
+  double radius_;
+  double energy_;
 };
 
-// A function to create a unique pointer to an instance of this class when generated
-// via a plugin call using dlopen/dlsym.
-// You must have external C linkage here otherwise dlopen will not find the file
-extern "C" std::unique_ptr<RingSource> openmc_create_source(std::string parameters)
+// A function to create a unique pointer to an instance of this class when
+// generated via a plugin call using dlopen/dlsym. You must have external C
+// linkage here otherwise dlopen will not find the file
+extern "C" std::unique_ptr<RingSource> openmc_create_source(
+  std::string parameters)
 {
   return RingSource::from_string(parameters);
 }
diff --git a/examples/pincell_pulsed/run_pulse.py b/examples/pincell_pulsed/run_pulse.py
new file mode 100644
index 000000000..b6a61ad3d
--- /dev/null
+++ b/examples/pincell_pulsed/run_pulse.py
@@ -0,0 +1,101 @@
+import matplotlib.pyplot as plt
+import numpy as np
+import openmc
+
+###############################################################################
+# Create materials for the problem
+
+uo2 = openmc.Material(name="UO2 fuel at 2.4% wt enrichment")
+uo2.set_density("g/cm3", 10.29769)
+uo2.add_element("U", 1.0, enrichment=2.4)
+uo2.add_element("O", 2.0)
+
+helium = openmc.Material(name="Helium for gap")
+helium.set_density("g/cm3", 0.001598)
+helium.add_element("He", 2.4044e-4)
+
+zircaloy = openmc.Material(name="Zircaloy 4")
+zircaloy.set_density("g/cm3", 6.55)
+zircaloy.add_element("Sn", 0.014, "wo")
+zircaloy.add_element("Fe", 0.00165, "wo")
+zircaloy.add_element("Cr", 0.001, "wo")
+zircaloy.add_element("Zr", 0.98335, "wo")
+
+borated_water = openmc.Material(name="Borated water")
+borated_water.set_density("g/cm3", 0.740582)
+borated_water.add_element("B", 2.0e-4)  # 3x the original pincell
+borated_water.add_element("H", 5.0e-2)
+borated_water.add_element("O", 2.4e-2)
+borated_water.add_s_alpha_beta("c_H_in_H2O")
+
+###############################################################################
+# Define problem geometry
+
+# Create cylindrical surfaces
+fuel_or = openmc.ZCylinder(r=0.39218, name="Fuel OR")
+clad_ir = openmc.ZCylinder(r=0.40005, name="Clad IR")
+clad_or = openmc.ZCylinder(r=0.45720, name="Clad OR")
+
+# Create a region represented as the inside of a rectangular prism
+pitch = 1.25984
+box = openmc.model.RectangularPrism(pitch, pitch, boundary_type="reflective")
+
+# Create cells, mapping materials to regions
+fuel = openmc.Cell(fill=uo2, region=-fuel_or)
+gap = openmc.Cell(fill=helium, region=+fuel_or & -clad_ir)
+clad = openmc.Cell(fill=zircaloy, region=+clad_ir & -clad_or)
+water = openmc.Cell(fill=borated_water, region=+clad_or & -box)
+
+# Create a model and assign geometry
+model = openmc.Model()
+model.geometry = openmc.Geometry([fuel, gap, clad, water])
+
+###############################################################################
+# Define problem settings
+
+# Set the mode
+model.settings.run_mode = "fixed source"
+
+# Indicate how many batches and particles to run
+model.settings.batches = 10
+model.settings.particles = 10000
+
+# Set time cutoff (we only care about t < 100 seconds, see tally below)
+model.settings.cutoff = {"time_neutron": 100}
+
+# Create the neutron pulse source (by default, isotropic direction, t=0)
+space = openmc.stats.Point()  # At the origin (0, 0, 0)
+energy = openmc.stats.delta_function(14.1e6)  # At 14.1 MeV
+model.settings.source = openmc.IndependentSource(space=space, energy=energy)
+
+###############################################################################
+# Define tallies
+
+# Create time filter
+t_grid = np.insert(np.logspace(-6, 2, 100), 0, 0.0)
+time_filter = openmc.TimeFilter(t_grid)
+
+# Tally for total neutron density in time
+density_tally = openmc.Tally(name="Density")
+density_tally.filters = [time_filter]
+density_tally.scores = ["inverse-velocity"]
+
+# Add tallies to model
+model.tallies = openmc.Tallies([density_tally])
+
+
+# Run the model
+model.run(apply_tally_results=True)
+
+# Bin-averaged result
+density_mean = density_tally.mean.ravel() / np.diff(t_grid)
+
+# Plot particle density versus time
+fig, ax = plt.subplots()
+ax.stairs(density_mean, t_grid)
+ax.set_xscale("log")
+ax.set_yscale("log")
+ax.set_xlabel("Time [s]")
+ax.set_ylabel("Total density")
+ax.grid()
+plt.show()
diff --git a/examples/pincell_random_ray/build_xml.py b/examples/pincell_random_ray/build_xml.py
index b3dd8020a..5ff4c0082 100644
--- a/examples/pincell_random_ray/build_xml.py
+++ b/examples/pincell_random_ray/build_xml.py
@@ -192,11 +192,10 @@ tallies.export_to_xml()
 #                   Exporting to OpenMC plots.xml file
 ###############################################################################
 
-plot = openmc.Plot()
+plot = openmc.VoxelPlot()
 plot.origin = [0, 0, 0]
 plot.width = [pitch, pitch, pitch]
 plot.pixels = [1000, 1000, 1]
-plot.type = 'voxel'
 
 # Instantiate a Plots collection and export to XML
 plots = openmc.Plots([plot])
diff --git a/include/openmc/angle_energy.h b/include/openmc/angle_energy.h
index ac931b1b5..55deb5d41 100644
--- a/include/openmc/angle_energy.h
+++ b/include/openmc/angle_energy.h
@@ -14,8 +14,22 @@ namespace openmc {
 
 class AngleEnergy {
 public:
+  //! Sample an outgoing energy and scattering cosine
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[out] mu Outgoing cosine with respect to current direction
+  //! \param[inout] seed Pseudorandom seed pointer
   virtual void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const = 0;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  virtual double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const = 0;
   virtual ~AngleEnergy() = default;
 };
 
diff --git a/include/openmc/atomic_mass.h b/include/openmc/atomic_mass.h
new file mode 100644
index 000000000..b4d515d2e
--- /dev/null
+++ b/include/openmc/atomic_mass.h
@@ -0,0 +1,28 @@
+//==============================================================================
+// atomic masses definitions
+//==============================================================================
+
+#ifndef OPENMC_ATOMIC_MASS_H
+#define OPENMC_ATOMIC_MASS_H
+
+#include <cstdint>
+#include <unordered_map>
+
+namespace openmc {
+
+// Values here are from the Committee on Data for Science and Technology
+// (CODATA) 2018 recommendation (https://physics.nist.gov/cuu/Constants/).
+
+// Physical constants
+constexpr double MASS_ELECTRON {5.48579909065e-4}; // mass of an electron in amu
+constexpr double MASS_NEUTRON {1.00866491595};     // mass of a neutron in amu
+constexpr double MASS_PROTON {1.007276466621};     // mass of a proton in amu
+constexpr double MASS_DEUTRON {2.013553212745};    // mass of a deutron in amu
+constexpr double MASS_HELION {3.014932247175};     // mass of a helion in amu
+constexpr double MASS_ALPHA {4.001506179127};      // mass of an alpha in amu
+
+extern std::unordered_map<int32_t, double> ATOMIC_MASS;
+
+} // namespace openmc
+
+#endif //  OPENMC_ATOMIC_MASS_H
diff --git a/include/openmc/bank.h b/include/openmc/bank.h
index 95386514d..6abcdd7f1 100644
--- a/include/openmc/bank.h
+++ b/include/openmc/bank.h
@@ -20,22 +20,38 @@ extern vector<SourceSite> source_bank;
 
 extern SharedArray<SourceSite> surf_source_bank;
 
+extern SharedArray<CollisionTrackSite> collision_track_bank;
+
 extern SharedArray<SourceSite> fission_bank;
 
+extern vector<vector<int>> ifp_source_delayed_group_bank;
+
+extern vector<vector<double>> ifp_source_lifetime_bank;
+
+extern vector<vector<int>> ifp_fission_delayed_group_bank;
+
+extern vector<vector<double>> ifp_fission_lifetime_bank;
+
 extern vector<int64_t> progeny_per_particle;
 
+extern SharedArray<SourceSite> shared_secondary_bank_read;
+extern SharedArray<SourceSite> shared_secondary_bank_write;
+
 } // namespace simulation
 
 //==============================================================================
 // Non-member functions
 //==============================================================================
 
-void sort_fission_bank();
+void sort_bank(SharedArray<SourceSite>& bank, bool is_fission_bank);
 
 void free_memory_bank();
 
 void init_fission_bank(int64_t max);
 
+int64_t synchronize_global_secondary_bank(
+  SharedArray<SourceSite>& shared_secondary_bank);
+
 } // namespace openmc
 
 #endif // OPENMC_BANK_H
diff --git a/include/openmc/bank_io.h b/include/openmc/bank_io.h
new file mode 100644
index 000000000..418ec111f
--- /dev/null
+++ b/include/openmc/bank_io.h
@@ -0,0 +1,105 @@
+#ifndef OPENMC_BANK_IO_H
+#define OPENMC_BANK_IO_H
+
+#include "hdf5.h"
+
+#include "openmc/message_passing.h"
+#include "openmc/span.h"
+#include "openmc/vector.h"
+
+#include <algorithm>
+
+#ifdef OPENMC_MPI
+#include <mpi.h>
+#endif
+
+namespace openmc {
+
+template<typename SiteType>
+void write_bank_dataset(
+  const char* dataset_name, hid_t group_id, span<SiteType> bank,
+  const vector<int64_t>& bank_index, hid_t membanktype, hid_t filebanktype
+#ifdef OPENMC_MPI
+  ,
+  MPI_Datatype mpi_dtype
+#endif
+)
+{
+  int64_t dims_size = bank_index.back();
+  int64_t count_size = bank_index[mpi::rank + 1] - bank_index[mpi::rank];
+
+#ifdef PHDF5
+  hsize_t dims[] {static_cast<hsize_t>(dims_size)};
+  hid_t dspace = H5Screate_simple(1, dims, nullptr);
+  hid_t dset = H5Dcreate(group_id, dataset_name, filebanktype, dspace,
+    H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+
+  hsize_t count[] {static_cast<hsize_t>(count_size)};
+  hid_t memspace = H5Screate_simple(1, count, nullptr);
+
+  hsize_t start[] {static_cast<hsize_t>(bank_index[mpi::rank])};
+  H5Sselect_hyperslab(dspace, H5S_SELECT_SET, start, nullptr, count, nullptr);
+
+  hid_t plist = H5Pcreate(H5P_DATASET_XFER);
+  H5Pset_dxpl_mpio(plist, H5FD_MPIO_COLLECTIVE);
+
+  H5Dwrite(dset, membanktype, memspace, dspace, plist, bank.data());
+
+  H5Sclose(dspace);
+  H5Sclose(memspace);
+  H5Dclose(dset);
+  H5Pclose(plist);
+#else
+  if (mpi::master) {
+    hsize_t dims[] {static_cast<hsize_t>(dims_size)};
+    hid_t dspace = H5Screate_simple(1, dims, nullptr);
+    hid_t dset = H5Dcreate(group_id, dataset_name, filebanktype, dspace,
+      H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+
+#ifdef OPENMC_MPI
+    vector<SiteType> temp_bank {bank.begin(), bank.end()};
+#endif
+
+    for (int i = 0; i < mpi::n_procs; ++i) {
+      hsize_t count[] {static_cast<hsize_t>(bank_index[i + 1] - bank_index[i])};
+      hid_t memspace = H5Screate_simple(1, count, nullptr);
+
+#ifdef OPENMC_MPI
+      if (i > 0) {
+        MPI_Recv(bank.data(), count[0], mpi_dtype, i, i, mpi::intracomm,
+          MPI_STATUS_IGNORE);
+      }
+#endif
+
+      hid_t dspace_rank = H5Dget_space(dset);
+      hsize_t start[] {static_cast<hsize_t>(bank_index[i])};
+      H5Sselect_hyperslab(
+        dspace_rank, H5S_SELECT_SET, start, nullptr, count, nullptr);
+
+      H5Dwrite(
+        dset, membanktype, memspace, dspace_rank, H5P_DEFAULT, bank.data());
+
+      H5Sclose(memspace);
+      H5Sclose(dspace_rank);
+    }
+
+    H5Dclose(dset);
+
+#ifdef OPENMC_MPI
+    std::copy(temp_bank.begin(), temp_bank.end(), bank.begin());
+#endif
+  }
+#ifdef OPENMC_MPI
+  else {
+    if (!bank.empty()) {
+      MPI_Send(
+        bank.data(), bank.size(), mpi_dtype, 0, mpi::rank, mpi::intracomm);
+    }
+  }
+#endif
+#endif
+}
+
+} // namespace openmc
+
+#endif // OPENMC_BANK_IO_H
diff --git a/include/openmc/boundary_condition.h b/include/openmc/boundary_condition.h
index cd8988162..89bf7ca8b 100644
--- a/include/openmc/boundary_condition.h
+++ b/include/openmc/boundary_condition.h
@@ -111,6 +111,10 @@ public:
 
   std::string type() const override { return "periodic"; }
 
+  int i_surf() const { return i_surf_; }
+
+  int j_surf() const { return j_surf_; }
+
 protected:
   int i_surf_;
   int j_surf_;
@@ -134,18 +138,28 @@ protected:
 //==============================================================================
 //! A BC that rotates particles about a global axis.
 //
-//! Currently only rotations about the z-axis are supported.
+//! Only rotations about the x, y, and z axes are supported.
 //==============================================================================
 
 class RotationalPeriodicBC : public PeriodicBC {
 public:
-  RotationalPeriodicBC(int i_surf, int j_surf);
-
+  enum PeriodicAxis { x, y, z };
+  RotationalPeriodicBC(int i_surf, int j_surf, PeriodicAxis axis);
+  double compute_periodic_rotation(
+    double rise_1, double run_1, double rise_2, double run_2) const;
   void handle_particle(Particle& p, const Surface& surf) const override;
 
 protected:
   //! Angle about the axis by which particle coordinates will be rotated
   double angle_;
+  //! Do we need to flip surfaces senses when applying the transformation?
+  bool flip_sense_;
+  //! Ensure that choice of axes is right handed. axis_1_idx_ corresponds to the
+  //! independent axis and axis_2_idx_ corresponds to the dependent axis in the
+  //! 2D plane  perpendicular to the planes' axis of rotation
+  int zero_axis_idx_;
+  int axis_1_idx_;
+  int axis_2_idx_;
 };
 
 } // namespace openmc
diff --git a/include/openmc/bounding_box.h b/include/openmc/bounding_box.h
index d02d92cb4..4fabe1b70 100644
--- a/include/openmc/bounding_box.h
+++ b/include/openmc/bounding_box.h
@@ -13,12 +13,19 @@ namespace openmc {
 //==============================================================================
 
 struct BoundingBox {
-  double xmin = -INFTY;
-  double xmax = INFTY;
-  double ymin = -INFTY;
-  double ymax = INFTY;
-  double zmin = -INFTY;
-  double zmax = INFTY;
+  Position min = {-INFTY, -INFTY, -INFTY};
+  Position max = {INFTY, INFTY, INFTY};
+
+  // Constructors
+  BoundingBox() = default;
+  BoundingBox(Position min_, Position max_) : min {min_}, max {max_} {}
+
+  // Static factory methods
+  static BoundingBox infinite() { return {}; }
+  static BoundingBox inverted()
+  {
+    return {{INFTY, INFTY, INFTY}, {-INFTY, -INFTY, -INFTY}};
+  }
 
   inline BoundingBox operator&(const BoundingBox& other)
   {
@@ -35,29 +42,26 @@ struct BoundingBox {
   // intersect operator
   inline BoundingBox& operator&=(const BoundingBox& other)
   {
-    xmin = std::max(xmin, other.xmin);
-    xmax = std::min(xmax, other.xmax);
-    ymin = std::max(ymin, other.ymin);
-    ymax = std::min(ymax, other.ymax);
-    zmin = std::max(zmin, other.zmin);
-    zmax = std::min(zmax, other.zmax);
+    min.x = std::max(min.x, other.min.x);
+    min.y = std::max(min.y, other.min.y);
+    min.z = std::max(min.z, other.min.z);
+    max.x = std::min(max.x, other.max.x);
+    max.y = std::min(max.y, other.max.y);
+    max.z = std::min(max.z, other.max.z);
     return *this;
   }
 
   // union operator
   inline BoundingBox& operator|=(const BoundingBox& other)
   {
-    xmin = std::min(xmin, other.xmin);
-    xmax = std::max(xmax, other.xmax);
-    ymin = std::min(ymin, other.ymin);
-    ymax = std::max(ymax, other.ymax);
-    zmin = std::min(zmin, other.zmin);
-    zmax = std::max(zmax, other.zmax);
+    min.x = std::min(min.x, other.min.x);
+    min.y = std::min(min.y, other.min.y);
+    min.z = std::min(min.z, other.min.z);
+    max.x = std::max(max.x, other.max.x);
+    max.y = std::max(max.y, other.max.y);
+    max.z = std::max(max.z, other.max.z);
     return *this;
   }
-
-  inline Position min() const { return {xmin, ymin, zmin}; }
-  inline Position max() const { return {xmax, ymax, zmax}; }
 };
 
 } // namespace openmc
diff --git a/include/openmc/bremsstrahlung.h b/include/openmc/bremsstrahlung.h
index 2f7e41bf0..d3b586831 100644
--- a/include/openmc/bremsstrahlung.h
+++ b/include/openmc/bremsstrahlung.h
@@ -3,7 +3,7 @@
 
 #include "openmc/particle.h"
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 namespace openmc {
 
@@ -14,9 +14,9 @@ namespace openmc {
 class BremsstrahlungData {
 public:
   // Data
-  xt::xtensor<double, 2> pdf;   //!< Bremsstrahlung energy PDF
-  xt::xtensor<double, 2> cdf;   //!< Bremsstrahlung energy CDF
-  xt::xtensor<double, 1> yield; //!< Photon yield
+  tensor::Tensor<double> pdf;   //!< Bremsstrahlung energy PDF
+  tensor::Tensor<double> cdf;   //!< Bremsstrahlung energy CDF
+  tensor::Tensor<double> yield; //!< Photon yield
 };
 
 class Bremsstrahlung {
@@ -32,9 +32,9 @@ public:
 
 namespace data {
 
-extern xt::xtensor<double, 1>
+extern tensor::Tensor<double>
   ttb_e_grid; //! energy T of incident electron in [eV]
-extern xt::xtensor<double, 1>
+extern tensor::Tensor<double>
   ttb_k_grid; //! reduced energy W/T of emitted photon
 
 } // namespace data
diff --git a/include/openmc/capi.h b/include/openmc/capi.h
index 54257d093..9f6987d74 100644
--- a/include/openmc/capi.h
+++ b/include/openmc/capi.h
@@ -9,14 +9,43 @@
 extern "C" {
 #endif
 
+//! Run a stochastic volume calculation
+//
+//! \return Status (negative if an error occurred)
 int openmc_calculate_volumes();
+
 int openmc_cell_filter_get_bins(
   int32_t index, const int32_t** cells, int32_t* n);
+
+//! Get the fill for a cell
+//
+//! \param index Index in the cells array
+//! \param type Type of the fill
+//! \param indices Array of material indices for cell
+//! \param n Length of indices array
+//! \return Status (negative if an error occurred)
 int openmc_cell_get_fill(
   int32_t index, int* type, int32_t** indices, int32_t* n);
+
+//! Get the ID of a cell
+//
+//! \param index Index in the cells array
+//! \param id ID of the cell
+//! \return Status (negative if an error occurred)
 int openmc_cell_get_id(int32_t index, int32_t* id);
+
+//! Get the temperature of a cell
+//
+//! \param index Index in the cells array
+//! \param instance Which instance of the cell. If a null pointer is
+//!                 passed, the temperature of the first instance is returned.
+//! \param T temperature of the cell
+//!\return Status (negative if an error occurred)
 int openmc_cell_get_temperature(
   int32_t index, const int32_t* instance, double* T);
+
+int openmc_cell_get_density(
+  int32_t index, const int32_t* instance, double* rho);
 int openmc_cell_get_translation(int32_t index, double xyz[]);
 int openmc_cell_get_rotation(int32_t index, double rot[], size_t* n);
 int openmc_cell_get_name(int32_t index, const char** name);
@@ -27,6 +56,8 @@ int openmc_cell_set_fill(
 int openmc_cell_set_id(int32_t index, int32_t id);
 int openmc_cell_set_temperature(
   int32_t index, double T, const int32_t* instance, bool set_contained = false);
+int openmc_cell_set_density(int32_t index, double rho, const int32_t* instance,
+  bool set_contained = false);
 int openmc_cell_set_translation(int32_t index, const double xyz[]);
 int openmc_cell_set_rotation(int32_t index, const double rot[], size_t rot_len);
 int openmc_dagmc_universe_get_cell_ids(
@@ -112,15 +143,63 @@ int openmc_mesh_set_id(int32_t index, int32_t id);
 int openmc_mesh_get_n_elements(int32_t index, size_t* n);
 int openmc_mesh_get_volumes(int32_t index, double* volumes);
 int openmc_mesh_material_volumes(int32_t index, int nx, int ny, int nz,
-  int max_mats, int32_t* materials, double* volumes);
+  int max_mats, int32_t* materials, double* volumes, double* bboxes);
 int openmc_meshsurface_filter_get_mesh(int32_t index, int32_t* index_mesh);
 int openmc_meshsurface_filter_set_mesh(int32_t index, int32_t index_mesh);
 int openmc_new_filter(const char* type, int32_t* index);
 int openmc_next_batch(int* status);
 int openmc_nuclide_name(int index, const char** name);
 int openmc_plot_geometry();
+// Deprecated; use openmc_slice_data.
 int openmc_id_map(const void* slice, int32_t* data_out);
+// Deprecated; use openmc_slice_data.
 int openmc_property_map(const void* slice, double* data_out);
+int openmc_slice_data(const double origin[3], const double u_span[3],
+  const double v_span[3], const size_t pixels[2], bool show_overlaps, int level,
+  int32_t filter_index, int32_t* geom_data, double* property_data);
+int openmc_get_plot_index(int32_t id, int32_t* index);
+int openmc_plot_get_id(int32_t index, int32_t* id);
+int openmc_plot_set_id(int32_t index, int32_t id);
+int openmc_solidraytrace_plot_create(int32_t* index);
+int openmc_solidraytrace_plot_get_pixels(
+  int32_t index, int32_t* width, int32_t* height);
+int openmc_solidraytrace_plot_set_pixels(
+  int32_t index, int32_t width, int32_t height);
+int openmc_solidraytrace_plot_get_color_by(int32_t index, int32_t* color_by);
+int openmc_solidraytrace_plot_set_color_by(int32_t index, int32_t color_by);
+int openmc_solidraytrace_plot_set_default_colors(int32_t index);
+int openmc_solidraytrace_plot_set_all_opaque(int32_t index);
+int openmc_solidraytrace_plot_set_opaque(
+  int32_t index, int32_t id, bool visible);
+int openmc_solidraytrace_plot_set_color(
+  int32_t index, int32_t id, uint8_t r, uint8_t g, uint8_t b);
+int openmc_solidraytrace_plot_get_camera_position(
+  int32_t index, double* x, double* y, double* z);
+int openmc_solidraytrace_plot_set_camera_position(
+  int32_t index, double x, double y, double z);
+int openmc_solidraytrace_plot_get_look_at(
+  int32_t index, double* x, double* y, double* z);
+int openmc_solidraytrace_plot_set_look_at(
+  int32_t index, double x, double y, double z);
+int openmc_solidraytrace_plot_get_up(
+  int32_t index, double* x, double* y, double* z);
+int openmc_solidraytrace_plot_set_up(
+  int32_t index, double x, double y, double z);
+int openmc_solidraytrace_plot_get_light_position(
+  int32_t index, double* x, double* y, double* z);
+int openmc_solidraytrace_plot_set_light_position(
+  int32_t index, double x, double y, double z);
+int openmc_solidraytrace_plot_get_fov(int32_t index, double* fov);
+int openmc_solidraytrace_plot_set_fov(int32_t index, double fov);
+int openmc_solidraytrace_plot_update_view(int32_t index);
+int openmc_solidraytrace_plot_create_image(
+  int32_t index, uint8_t* data_out, int32_t width, int32_t height);
+int openmc_solidraytrace_plot_get_color(
+  int32_t index, int32_t id, uint8_t* r, uint8_t* g, uint8_t* b);
+int openmc_solidraytrace_plot_get_diffuse_fraction(
+  int32_t index, double* diffuse_fraction);
+int openmc_solidraytrace_plot_set_diffuse_fraction(
+  int32_t index, double diffuse_fraction);
 int openmc_rectilinear_mesh_get_grid(int32_t index, double** grid_x, int* nx,
   double** grid_y, int* ny, double** grid_z, int* nz);
 int openmc_rectilinear_mesh_set_grid(int32_t index, const double* grid_x,
@@ -136,6 +215,7 @@ int openmc_remove_tally(int32_t index);
 int openmc_reset();
 int openmc_reset_timers();
 int openmc_run();
+void openmc_run_random_ray();
 int openmc_sample_external_source(size_t n, uint64_t* seed, void* sites);
 void openmc_set_seed(int64_t new_seed);
 void openmc_set_stride(uint64_t new_stride);
@@ -197,8 +277,8 @@ int openmc_weight_windows_set_energy_bounds(
   int32_t index, double* e_bounds, size_t e_bounds_size);
 int openmc_weight_windows_get_energy_bounds(
   int32_t index, const double** e_bounds, size_t* e_bounds_size);
-int openmc_weight_windows_set_particle(int32_t index, int particle);
-int openmc_weight_windows_get_particle(int32_t index, int* particle);
+int openmc_weight_windows_set_particle(int32_t index, int32_t particle);
+int openmc_weight_windows_get_particle(int32_t index, int32_t* particle);
 int openmc_weight_windows_get_bounds(int32_t index, const double** lower_bounds,
   const double** upper_bounds, size_t* size);
 int openmc_weight_windows_set_bounds(int32_t index, const double* lower_bounds,
@@ -214,6 +294,7 @@ int openmc_weight_windows_set_weight_cutoff(int32_t index, double cutoff);
 int openmc_weight_windows_get_max_split(int32_t index, int* max_split);
 int openmc_weight_windows_set_max_split(int32_t index, int max_split);
 size_t openmc_weight_windows_size();
+size_t openmc_plots_size();
 int openmc_weight_windows_export(const char* filename = nullptr);
 int openmc_weight_windows_import(const char* filename = nullptr);
 int openmc_zernike_filter_get_order(int32_t index, int* order);
@@ -223,10 +304,10 @@ int openmc_zernike_filter_set_order(int32_t index, int order);
 int openmc_zernike_filter_set_params(
   int32_t index, const double* x, const double* y, const double* r);
 
-int openmc_particle_filter_get_bins(int32_t idx, int bins[]);
+int openmc_particle_filter_get_bins(int32_t idx, int32_t bins[]);
 
 //! Sets the mesh and energy grid for CMFD reweight
-//! \param[in] meshtyally_id id of CMFD Mesh Tally
+//! \param[in] meshtally_id id of CMFD Mesh Tally
 //! \param[in] cmfd_indices indices storing spatial and energy dimensions of
 //! CMFD problem \param[in] norm CMFD normalization factor
 void openmc_initialize_mesh_egrid(
diff --git a/include/openmc/cell.h b/include/openmc/cell.h
index 98709fb33..1a27ad516 100644
--- a/include/openmc/cell.h
+++ b/include/openmc/cell.h
@@ -123,11 +123,11 @@ private:
   //! BoundingBox if the particle is in a complex cell.
   BoundingBox bounding_box_complex(vector<int32_t> postfix) const;
 
-  //! Enfource precedence: Parenthases, Complement, Intersection, Union
-  void add_precedence();
+  //! Enforce precedence between intersections and unions
+  void enforce_precedence();
 
   //! Add parenthesis to enforce precedence
-  int64_t add_parentheses(int64_t start);
+  void add_parentheses(int64_t start);
 
   //! Remove complement operators from the expression
   void remove_complement_ops();
@@ -145,6 +145,30 @@ private:
   bool simple_; //!< Does the region contain only intersections?
 };
 
+//==============================================================================
+// XML parsing helpers for <cell> nodes
+//==============================================================================
+
+//! Parse material IDs from a <cell> XML node.
+//! \param node XML node containing a "material" attribute or child element
+//! \param cell_id Cell ID used in error messages
+//! \return Vector of material IDs (MATERIAL_VOID for "void")
+vector<int32_t> parse_cell_material_xml(pugi::xml_node node, int32_t cell_id);
+
+//! Parse temperatures in [K] from a <cell> XML node.
+//! Validates that all values are non-negative and the list is non-empty.
+//! \param node XML node containing a "temperature" attribute or child element
+//! \param cell_id Cell ID used in error messages
+//! \return Vector of temperatures in [K]
+vector<double> parse_cell_temperature_xml(pugi::xml_node node, int32_t cell_id);
+
+//! Parse densities in [g/cm³] from a <cell> XML node.
+//! Validates that all values are positive and the list is non-empty.
+//! \param node XML node containing a "density" attribute or child element
+//! \param cell_id Cell ID used in error messages
+//! \return Vector of densities in [g/cm³]
+vector<double> parse_cell_density_xml(pugi::xml_node node, int32_t cell_id);
+
 //==============================================================================
 
 class Cell {
@@ -216,6 +240,18 @@ public:
   //! \return Temperature in [K]
   double temperature(int32_t instance = -1) const;
 
+  //! Get the density multiplier of a cell instance
+  //! \param[in] instance Instance index. If -1 is given, the density multiplier
+  //! for the first instance is returned.
+  //! \return Density multiplier
+  double density_mult(int32_t instance = -1) const;
+
+  //! Get the density of a cell instance in g/cm3
+  //! \param[in] instance Instance index. If -1 is given, the density
+  //! for the first instance is returned.
+  //! \return Density in [g/cm3]
+  double density(int32_t instance = -1) const;
+
   //! Set the temperature of a cell instance
   //! \param[in] T Temperature in [K]
   //! \param[in] instance Instance index. If -1 is given, the temperature for
@@ -226,6 +262,18 @@ public:
   void set_temperature(
     double T, int32_t instance = -1, bool set_contained = false);
 
+  //! Set the density of a cell instance
+  //! \param[in] density Density [g/cm3]
+  //! \param[in] instance Instance index. If -1 is given, the density
+  //!   for all instances is set.
+  //! \param[in] set_contained If this cell is not filled with a material,
+  //!   collect all contained cells with material fills and set their
+  //!   densities.
+  void set_density(
+    double density, int32_t instance = -1, bool set_contained = false);
+
+  int32_t n_instances() const;
+
   //! Set the rotation matrix of a cell instance
   //! \param[in] rot The rotation matrix of length 3 or 9
   void set_rotation(const vector<double>& rot);
@@ -312,12 +360,11 @@ public:
   //----------------------------------------------------------------------------
   // Data members
 
-  int32_t id_;              //!< Unique ID
-  std::string name_;        //!< User-defined name
-  Fill type_;               //!< Material, universe, or lattice
-  int32_t universe_;        //!< Universe # this cell is in
-  int32_t fill_;            //!< Universe # filling this cell
-  int32_t n_instances_ {0}; //!< Number of instances of this cell
+  int32_t id_;       //!< Unique ID
+  std::string name_; //!< User-defined name
+  Fill type_;        //!< Material, universe, or lattice
+  int32_t universe_; //!< Universe # this cell is in
+  int32_t fill_;     //!< Universe # filling this cell
 
   //! \brief Index corresponding to this cell in distribcell arrays
   int distribcell_index_ {C_NONE};
@@ -333,6 +380,9 @@ public:
   //! T. The units are sqrt(eV).
   vector<double> sqrtkT_;
 
+  //! \brief Unitless density multiplier(s) within this cell.
+  vector<double> density_mult_;
+
   //! \brief Neighboring cells in the same universe.
   NeighborList neighbors_;
 
diff --git a/include/openmc/chain.h b/include/openmc/chain.h
index a3bc6f3a3..e01f2a01c 100644
--- a/include/openmc/chain.h
+++ b/include/openmc/chain.h
@@ -71,6 +71,15 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   const Distribution* photon_energy_;
 };
@@ -92,6 +101,8 @@ extern vector<unique_ptr<ChainNuclide>> chain_nuclides;
 
 void read_chain_file_xml();
 
+void free_memory_chain();
+
 } // namespace openmc
 
 #endif // OPENMC_CHAIN_H
diff --git a/include/openmc/collision_track.h b/include/openmc/collision_track.h
new file mode 100644
index 000000000..208b8f6e1
--- /dev/null
+++ b/include/openmc/collision_track.h
@@ -0,0 +1,23 @@
+#ifndef OPENMC_COLLISION_TRACK_H
+#define OPENMC_COLLISION_TRACK_H
+
+#include <string>
+
+namespace openmc {
+
+class Particle;
+
+//! Reserve space in the collision track bank according to user settings.
+void collision_track_reserve_bank();
+
+//! Write collision track data to disk when the bank is full or the batch ends.
+void collision_track_flush_bank();
+
+//! Record the current particle as a collision-track entry when applicable.
+//!
+//! \param particle Particle whose collision should be recorded if eligible
+void collision_track_record(Particle& particle);
+
+} // namespace openmc
+
+#endif // OPENMC_COLLISION_TRACK_H
diff --git a/include/openmc/constants.h b/include/openmc/constants.h
index 2a7ce1990..26b4a224c 100644
--- a/include/openmc/constants.h
+++ b/include/openmc/constants.h
@@ -9,6 +9,7 @@
 #include <limits>
 
 #include "openmc/array.h"
+#include "openmc/atomic_mass.h"
 #include "openmc/vector.h"
 #include "openmc/version.h"
 
@@ -25,15 +26,16 @@ using double_4dvec = vector<vector<vector<vector<double>>>>;
 constexpr int HDF5_VERSION[] {3, 0};
 
 // Version numbers for binary files
-constexpr array<int, 2> VERSION_STATEPOINT {18, 1};
-constexpr array<int, 2> VERSION_PARTICLE_RESTART {2, 0};
-constexpr array<int, 2> VERSION_TRACK {3, 0};
-constexpr array<int, 2> VERSION_SUMMARY {6, 0};
+constexpr array<int, 2> VERSION_STATEPOINT {18, 2};
+constexpr array<int, 2> VERSION_PARTICLE_RESTART {2, 1};
+constexpr array<int, 2> VERSION_TRACK {3, 1};
+constexpr array<int, 2> VERSION_SUMMARY {6, 1};
 constexpr array<int, 2> VERSION_VOLUME {1, 0};
 constexpr array<int, 2> VERSION_VOXEL {2, 0};
 constexpr array<int, 2> VERSION_MGXS_LIBRARY {1, 0};
-constexpr array<int, 2> VERSION_PROPERTIES {1, 0};
+constexpr array<int, 2> VERSION_PROPERTIES {1, 1};
 constexpr array<int, 2> VERSION_WEIGHT_WINDOWS {1, 0};
+constexpr array<int, 2> VERSION_COLLISION_TRACK {1, 2};
 
 // ============================================================================
 // ADJUSTABLE PARAMETERS
@@ -63,6 +65,16 @@ constexpr int MAX_SAMPLE {100000};
 // source region in the random ray solver
 constexpr double MIN_HITS_PER_BATCH {1.5};
 
+// The minimum flux value to be considered non-zero when computing adjoint
+// sources. Positive values below this cutoff will be treated as zero, so as to
+// prevent extremely large adjoint source terms from being generated.
+constexpr double ZERO_FLUX_CUTOFF {1e-22};
+
+// The minimum macroscopic cross section value considered non-void for the
+// random ray solver. Materials with any group with a cross section below this
+// value will be converted to pure void.
+constexpr double MINIMUM_MACRO_XS {1e-6};
+
 // ============================================================================
 // MATH AND PHYSICAL CONSTANTS
 
@@ -75,10 +87,10 @@ constexpr double INFTY {std::numeric_limits<double>::max()};
 // (CODATA) 2018 recommendation (https://physics.nist.gov/cuu/Constants/).
 
 // Physical constants
-constexpr double MASS_NEUTRON {1.00866491595}; // mass of a neutron in amu
+constexpr double AMU_EV {
+  9.3149410242e8}; // atomic mass unit energy equivalent in eV/c^2
 constexpr double MASS_NEUTRON_EV {
-  939.56542052e6};                             // mass of a neutron in eV/c^2
-constexpr double MASS_PROTON {1.007276466621}; // mass of a proton in amu
+  939.56542052e6}; // neutron mass energy equivalent in eV/c^2
 constexpr double MASS_ELECTRON_EV {
   0.51099895000e6}; // electron mass energy equivalent in eV/c^2
 constexpr double FINE_STRUCTURE {
@@ -215,6 +227,7 @@ enum ReactionType {
   N_XA = 207,
   HEATING = 301,
   DAMAGE_ENERGY = 444,
+  PHOTON_TOTAL = 501,
   COHERENT = 502,
   INCOHERENT = 504,
   PAIR_PROD_ELEC = 515,
@@ -281,7 +294,7 @@ enum class MgxsType {
 // ============================================================================
 // TALLY-RELATED CONSTANTS
 
-enum class TallyResult { VALUE, SUM, SUM_SQ, SIZE };
+enum class TallyResult { VALUE, SUM, SUM_SQ, SUM_THIRD, SUM_FOURTH };
 
 enum class TallyType { VOLUME, MESH_SURFACE, SURFACE, PULSE_HEIGHT };
 
@@ -290,7 +303,7 @@ enum class TallyEstimator { ANALOG, TRACKLENGTH, COLLISION };
 enum class TallyEvent { SURFACE, LATTICE, KILL, SCATTER, ABSORB };
 
 // Tally score type -- if you change these, make sure you also update the
-// _SCORES dictionary in openmc/capi/tally.py
+// _SCORES dictionary in openmc/lib/tally.py
 //
 // These are kept as a normal enum and made negative, since variables which
 // store one of these enum values usually also may be responsible for storing
@@ -312,7 +325,10 @@ enum TallyScore {
   SCORE_FISS_Q_PROMPT = -14,      // prompt fission Q-value
   SCORE_FISS_Q_RECOV = -15,       // recoverable fission Q-value
   SCORE_DECAY_RATE = -16,         // delayed neutron precursor decay rate
-  SCORE_PULSE_HEIGHT = -17        // pulse-height
+  SCORE_PULSE_HEIGHT = -17,       // pulse-height
+  SCORE_IFP_TIME_NUM = -18,       // IFP lifetime numerator
+  SCORE_IFP_BETA_NUM = -19,       // IFP delayed fraction numerator
+  SCORE_IFP_DENOM = -20           // IFP common denominator
 };
 
 // Global tally parameters
@@ -322,6 +338,9 @@ enum class GlobalTally { K_COLLISION, K_ABSORPTION, K_TRACKLENGTH, LEAKAGE };
 // Miscellaneous
 constexpr int C_NONE {-1};
 
+// Default value of generation for IFP
+constexpr int DEFAULT_IFP_N_GENERATION {10};
+
 // Interpolation rules
 enum class Interpolation {
   histogram = 1,
@@ -348,7 +367,8 @@ enum class SolverType { MONTE_CARLO, RANDOM_RAY };
 
 enum class RandomRayVolumeEstimator { NAIVE, SIMULATION_AVERAGED, HYBRID };
 enum class RandomRaySourceShape { FLAT, LINEAR, LINEAR_XY };
-enum class RandomRaySampleMethod { PRNG, HALTON };
+enum class RandomRaySampleMethod { PRNG, HALTON, S2 };
+enum class RandomRaySolve { FORWARD, FORWARD_FOR_ADJOINT, ADJOINT };
 
 //==============================================================================
 // Geometry Constants
diff --git a/include/openmc/dagmc.h b/include/openmc/dagmc.h
index 82ef1b644..858636c0b 100644
--- a/include/openmc/dagmc.h
+++ b/include/openmc/dagmc.h
@@ -20,7 +20,7 @@ void check_dagmc_root_univ();
 
 } // namespace openmc
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 
 #include "DagMC.hpp"
 #include "dagmcmetadata.hpp"
@@ -94,6 +94,10 @@ private:
 class DAGUniverse : public Universe {
 
 public:
+  using MaterialOverrides = std::unordered_map<int32_t, vector<int32_t>>;
+  using TemperatureOverrides = std::unordered_map<int32_t, vector<double>>;
+  using DensityOverrides = std::unordered_map<int32_t, vector<double>>;
+
   explicit DAGUniverse(pugi::xml_node node);
 
   //! Create a new DAGMC universe
@@ -112,6 +116,9 @@ public:
   //! Initialize the DAGMC accel. data structures, indices, material
   //! assignments, etc.
   void initialize();
+  void initialize(const MaterialOverrides& material_overrides,
+    const TemperatureOverrides& temperature_overrides,
+    const DensityOverrides& density_overrides = {});
 
   //! Reads UWUW materials and returns an ID map
   void read_uwuw_materials();
@@ -146,7 +153,8 @@ public:
 
   //! Assign a material overriding normal assignement to a cell
   //! \param[in] c The OpenMC cell to which the material is assigned
-  void override_assign_material(std::unique_ptr<DAGCell>& c) const;
+  void override_assign_material(std::unique_ptr<DAGCell>& c,
+    const MaterialOverrides& material_overrides) const;
 
   //! Return the index into the model cells vector for a given DAGMC volume
   //! handle in the universe
@@ -187,7 +195,9 @@ private:
   void set_id();        //!< Deduce the universe id from model::universes
   void init_dagmc();    //!< Create and initialise DAGMC pointer
   void init_metadata(); //!< Create and initialise dagmcMetaData pointer
-  void init_geometry(); //!< Create cells and surfaces from DAGMC entities
+  void init_geometry(const MaterialOverrides& material_overrides,
+    const TemperatureOverrides& temperature_overrides,
+    const DensityOverrides& density_overrides);
 
   std::string
     filename_; //!< Name of the DAGMC file used to create this universe
@@ -201,11 +211,6 @@ private:
                              //!< generate new material IDs for the universe
   bool has_graveyard_; //!< Indicates if the DAGMC geometry has a "graveyard"
                        //!< volume
-  std::unordered_map<int32_t, vector<int32_t>>
-    material_overrides_; //!< Map of material overrides
-                         //!< keys correspond to the DAGMCCell id
-                         //!< values are a list of material ids used
-                         //!< for the override
 };
 
 //==============================================================================
@@ -216,6 +221,6 @@ int32_t next_cell(int32_t surf, int32_t curr_cell, int32_t univ);
 
 } // namespace openmc
 
-#endif // DAGMC
+#endif // OPENMC_DAGMC_ENABLED
 
 #endif // OPENMC_DAGMC_H
diff --git a/include/openmc/distribution.h b/include/openmc/distribution.h
index 854cf7d77..f319dd19d 100644
--- a/include/openmc/distribution.h
+++ b/include/openmc/distribution.h
@@ -15,6 +15,17 @@
 
 namespace openmc {
 
+//==============================================================================
+// Helper function for computing importance weights from biased sampling
+//==============================================================================
+
+//! Compute importance weights for biased sampling
+//! \param p Unnormalized original probability vector
+//! \param b Unnormalized bias probability vector
+//! \return Vector of importance weights (p_norm[i] / b_norm[i])
+vector<double> compute_importance_weights(
+  const vector<double>& p, const vector<double>& b);
+
 //==============================================================================
 //! Abstract class representing a univariate probability distribution
 //==============================================================================
@@ -22,11 +33,41 @@ namespace openmc {
 class Distribution {
 public:
   virtual ~Distribution() = default;
-  virtual double sample(uint64_t* seed) const = 0;
+
+  //! Sample a value from the distribution, handling biasing automatically
+  //! \param seed Pseudorandom number seed pointer
+  //! \return (sampled value, importance weight)
+  virtual std::pair<double, double> sample(uint64_t* seed) const;
+
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  virtual double evaluate(double x) const;
 
   //! Return integral of distribution
   //! \return Integral of distribution
   virtual double integral() const { return 1.0; };
+
+  //! Set bias distribution
+  virtual void set_bias(std::unique_ptr<Distribution> bias)
+  {
+    bias_ = std::move(bias);
+  }
+
+  const Distribution* bias() const { return bias_.get(); }
+
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  virtual double sample_unbiased(uint64_t* seed) const = 0;
+
+  //! Read bias distribution from XML
+  //! \param node XML node that may contain a bias child element
+  void read_bias_from_xml(pugi::xml_node node);
+
+  // Biasing distribution
+  unique_ptr<Distribution> bias_;
 };
 
 using UPtrDist = unique_ptr<Distribution>;
@@ -50,7 +91,7 @@ public:
 
   //! Sample a value from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
+  //! \return Sampled index
   size_t sample(uint64_t* seed) const;
 
   // Properties
@@ -67,7 +108,7 @@ private:
   //! Normalize distribution so that probabilities sum to unity
   void normalize();
 
-  //! Initialize alias tables for distribution
+  //! Initialize alias table for sampling
   void init_alias();
 };
 
@@ -82,20 +123,30 @@ public:
 
   //! Sample a value from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! \return (sampled value, sample weight)
+  std::pair<double, double> sample(uint64_t* seed) const override;
 
   double integral() const override { return di_.integral(); };
 
+  //! Override set_bias as no-op (bias handled in constructor)
+  void set_bias(std::unique_ptr<Distribution> bias) override {}
+
   // Properties
   const vector<double>& x() const { return x_; }
   const vector<double>& prob() const { return di_.prob(); }
   const vector<size_t>& alias() const { return di_.alias(); }
+  const vector<double>& weight() const { return weight_; }
+
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
 
 private:
-  vector<double> x_; //!< Possible outcomes
-  DiscreteIndex di_; //!< discrete probability distribution of
-                     //!< outcome indices
+  vector<double> x_;      //!< Possible outcomes
+  vector<double> weight_; //!< Importance weights (empty if unbiased)
+  DiscreteIndex di_; //!< Discrete probability distribution of outcome indices
 };
 
 //==============================================================================
@@ -107,14 +158,20 @@ public:
   explicit Uniform(pugi::xml_node node);
   Uniform(double a, double b) : a_ {a}, b_ {b} {};
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  double evaluate(double x) const override;
 
   double a() const { return a_; }
   double b() const { return b_; }
 
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
 private:
   double a_; //!< Lower bound of distribution
   double b_; //!< Upper bound of distribution
@@ -131,15 +188,21 @@ public:
     : offset_ {std::pow(a, n + 1)}, span_ {std::pow(b, n + 1) - offset_},
       ninv_ {1 / (n + 1)} {};
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  double evaluate(double x) const override;
 
   double a() const { return std::pow(offset_, ninv_); }
   double b() const { return std::pow(offset_ + span_, ninv_); }
   double n() const { return 1 / ninv_ - 1; }
 
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
 private:
   //! Store processed values in object to allow for faster sampling
   double offset_; //!< a^(n+1)
@@ -156,13 +219,19 @@ public:
   explicit Maxwell(pugi::xml_node node);
   Maxwell(double theta) : theta_ {theta} {};
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  double evaluate(double x) const override;
 
   double theta() const { return theta_; }
 
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
 private:
   double theta_; //!< Factor in exponential [eV]
 };
@@ -176,41 +245,66 @@ public:
   explicit Watt(pugi::xml_node node);
   Watt(double a, double b) : a_ {a}, b_ {b} {};
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  double evaluate(double x) const override;
 
   double a() const { return a_; }
   double b() const { return b_; }
 
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
 private:
   double a_; //!< Factor in exponential [eV]
   double b_; //!< Factor in square root [1/eV]
 };
 
 //==============================================================================
-//! Normal distributions with form 1/2*std_dev*sqrt(pi) exp
-//! (-(e-E0)/2*std_dev)^2
+//! Normal distribution with optional truncation bounds.
+//!
+//! The standard normal PDF is 1/(sqrt(2*pi)*sigma) *
+//! exp(-(x-mu)^2/(2*sigma^2)). When truncated to [lower, upper], the PDF is
+//! renormalized so that it integrates to 1 over the truncation interval.
 //==============================================================================
 
 class Normal : public Distribution {
 public:
   explicit Normal(pugi::xml_node node);
-  Normal(double mean_value, double std_dev)
-    : mean_value_ {mean_value}, std_dev_ {std_dev} {};
+  Normal(double mean_value, double std_dev, double lower = -INFTY,
+    double upper = INFTY);
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x), accounting for truncation normalization
+  double evaluate(double x) const override;
 
   double mean_value() const { return mean_value_; }
   double std_dev() const { return std_dev_; }
+  double lower() const { return lower_; }
+  double upper() const { return upper_; }
+  bool is_truncated() const { return is_truncated_; }
+
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
 
 private:
-  double mean_value_; //!< middle of distribution [eV]
-  double std_dev_;    //!< standard deviation [eV]
+  double mean_value_;  //!< Mean of distribution
+  double std_dev_;     //!< Standard deviation
+  double lower_;       //!< Lower truncation bound (default: -INFTY)
+  double upper_;       //!< Upper truncation bound (default: +INFTY)
+  bool is_truncated_;  //!< True if bounds are finite
+  double norm_factor_; //!< Normalization factor for truncated distribution
+
+  //! Compute normalization factor for truncated distribution
+  void compute_normalization();
 };
 
 //==============================================================================
@@ -223,10 +317,10 @@ public:
   Tabular(const double* x, const double* p, int n, Interpolation interp,
     const double* c = nullptr);
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  double evaluate(double x) const override;
 
   // properties
   vector<double>& x() { return x_; }
@@ -235,6 +329,12 @@ public:
   Interpolation interp() const { return interp_; }
   double integral() const override { return integral_; };
 
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
 private:
   vector<double> x_;     //!< tabulated independent variable
   vector<double> p_;     //!< tabulated probability density
@@ -259,13 +359,19 @@ public:
   explicit Equiprobable(pugi::xml_node node);
   Equiprobable(const double* x, int n) : x_ {x, x + n} {};
 
-  //! Sample a value from the distribution
-  //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! Evaluate probability density, f(x), at a point
+  //! \param x Point to evaluate f(x)
+  //! \return f(x)
+  double evaluate(double x) const override;
 
   const vector<double>& x() const { return x_; }
 
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
 private:
   vector<double> x_; //! Possible outcomes
 };
@@ -280,18 +386,90 @@ public:
 
   //! Sample a value from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled value
-  double sample(uint64_t* seed) const override;
+  //! \return (sampled value, sample weight)
+  std::pair<double, double> sample(uint64_t* seed) const override;
 
   double integral() const override { return integral_; }
 
-private:
-  // Storrage for probability + distribution
-  using DistPair = std::pair<double, UPtrDist>;
+  //! Override set_bias as no-op (bias handled in constructor)
+  void set_bias(std::unique_ptr<Distribution> bias) override {}
 
-  vector<DistPair>
-    distribution_;  //!< sub-distributions + cummulative probabilities
-  double integral_; //!< integral of distribution
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
+private:
+  vector<UPtrDist> distribution_; //!< Sub-distributions
+  vector<double> weight_; //!< Importance weights for component selection
+  DiscreteIndex di_;      //!< Discrete probability distribution of indices
+  double integral_;       //!< Integral of distribution
+};
+
+//==============================================================================
+// DecaySpectrum — non-owning mixture of decay photon distributions
+//==============================================================================
+
+//! Energy distribution formed by mixing multiple decay photon spectra.
+//!
+//! Unlike the general Mixture distribution, this class holds non-owning
+//! pointers to the component distributions (which live in
+//! data::chain_nuclides). Each component is weighted by the activity
+//! (atoms * decay_constant) of the corresponding nuclide.
+
+class DecaySpectrum : public Distribution {
+public:
+  //============================================================================
+  // Types, aliases
+
+  struct Sample {
+    double energy;
+    double weight;
+    int parent_nuclide;
+  };
+
+  //============================================================================
+  // Constructors
+
+  //! Construct from an XML node containing nuclide names and atom densities.
+  //!
+  //! Reads child ``<nuclide>`` elements with ``name`` and ``density``
+  //! attributes, resolves them against the loaded depletion chain, and
+  //! constructs the mixed distribution.
+  explicit DecaySpectrum(pugi::xml_node node);
+
+  //============================================================================
+  // Methods
+
+  //! Sample a value from the distribution and return the parent nuclide index
+  //! \param seed Pseudorandom number seed pointer
+  //! \return (Sampled energy, sample weight, chain nuclide index)
+  Sample sample_with_parent(uint64_t* seed) const;
+
+  //! Sample a value from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return (sampled value, sample weight)
+  std::pair<double, double> sample(uint64_t* seed) const override;
+
+  double integral() const override;
+
+protected:
+  //! Sample a value (unbiased) from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled value
+  double sample_unbiased(uint64_t* seed) const override;
+
+private:
+  //! Initialize decay spectrum sampling data
+  //! \param nuclide_indices  Indices of decay photon emitters in
+  //!   data::chain_nuclides
+  //! \param atoms  Number of atoms for each component.
+  void init(vector<int> nuclide_indices, const vector<double>& atoms);
+
+  vector<int> nuclide_indices_; //!< Indices of emitting nuclides in the chain
+  DiscreteIndex di_;            //!< Discrete index for component selection
+  double integral_;             //!< Total photon emission rate
 };
 
 } // namespace openmc
diff --git a/include/openmc/distribution_angle.h b/include/openmc/distribution_angle.h
index efd4e5842..78de70c42 100644
--- a/include/openmc/distribution_angle.h
+++ b/include/openmc/distribution_angle.h
@@ -26,6 +26,12 @@ public:
   //! \return Cosine of the angle in the range [-1,1]
   double sample(double E, uint64_t* seed) const;
 
+  //! Evaluate the angular PDF at a given energy and cosine
+  //! \param[in] E Particle energy in [eV]
+  //! \param[in] mu Cosine of the scattering angle
+  //! \return Probability density for the scattering cosine
+  double evaluate(double E, double mu) const;
+
   //! Determine whether angle distribution is empty
   //! \return Whether distribution is empty
   bool empty() const { return energy_.empty(); }
diff --git a/include/openmc/distribution_energy.h b/include/openmc/distribution_energy.h
index 9b08ed039..efacb9131 100644
--- a/include/openmc/distribution_energy.h
+++ b/include/openmc/distribution_energy.h
@@ -5,7 +5,7 @@
 #define OPENMC_DISTRIBUTION_ENERGY_H
 
 #include "hdf5.h"
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/constants.h"
 #include "openmc/endf.h"
@@ -86,9 +86,9 @@ private:
   struct CTTable {
     Interpolation interpolation;  //!< Interpolation law
     int n_discrete;               //!< Number of of discrete energies
-    xt::xtensor<double, 1> e_out; //!< Outgoing energies in [eV]
-    xt::xtensor<double, 1> p;     //!< Probability density
-    xt::xtensor<double, 1> c;     //!< Cumulative distribution
+    tensor::Tensor<double> e_out; //!< Outgoing energies in [eV]
+    tensor::Tensor<double> p;     //!< Probability density
+    tensor::Tensor<double> c;     //!< Cumulative distribution
   };
 
   int n_region_;                        //!< Number of inteprolation regions
diff --git a/include/openmc/distribution_multi.h b/include/openmc/distribution_multi.h
index 9e84d03d5..a72780737 100644
--- a/include/openmc/distribution_multi.h
+++ b/include/openmc/distribution_multi.h
@@ -6,6 +6,7 @@
 #include "pugixml.hpp"
 
 #include "openmc/distribution.h"
+#include "openmc/error.h"
 #include "openmc/position.h"
 
 namespace openmc {
@@ -26,8 +27,16 @@ public:
 
   //! Sample a direction from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Direction sampled
-  virtual Direction sample(uint64_t* seed) const = 0;
+  //! \return (sampled Direction, sample weight)
+  virtual std::pair<Direction, double> sample(uint64_t* seed) const = 0;
+
+  //! Evaluate the probability density for a given direction
+  //! \param[in] u Direction on the unit sphere
+  //! \return Probability density at the given direction
+  virtual double evaluate(Direction u) const
+  {
+    fatal_error("evaluate not available for this UnitSphereDistribution type");
+  }
 
   Direction u_ref_ {0.0, 0.0, 1.0}; //!< reference direction
 };
@@ -43,14 +52,35 @@ public:
 
   //! Sample a direction from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Direction sampled
-  Direction sample(uint64_t* seed) const override;
+  //! \return (sampled Direction, sample weight)
+  std::pair<Direction, double> sample(uint64_t* seed) const override;
+
+  //! Sample a direction and return evaluation of the PDF for biased sampling.
+  //! Note that bias distributions are intended to return unit-weight samples.
+  //! \param seed Pseudorandom number seed points
+  //! \return (sampled Direction, value of the PDF at this Direction)
+  std::pair<Direction, double> sample_as_bias(uint64_t* seed) const;
+
+  //! Evaluate the probability density for a given direction
+  //! \param[in] u Direction on the unit sphere
+  //! \return Probability density at the given direction
+  double evaluate(Direction u) const override;
 
   // Observing pointers
   Distribution* mu() const { return mu_.get(); }
   Distribution* phi() const { return phi_.get(); }
 
 private:
+  //! Common sampling implementation
+  //! \param seed Pseudorandom number seed pointer
+  //! \param return_pdf If true, return PDF evaluation; if false, return
+  //!        importance weight
+  //! \return (sampled Direction, weight or PDF value)
+  std::pair<Direction, double> sample_impl(
+    uint64_t* seed, bool return_pdf) const;
+
+  Direction v_ref_ {1.0, 0.0, 0.0}; //!< reference direction
+  Direction w_ref_;
   UPtrDist mu_;  //!< Distribution of polar angle
   UPtrDist phi_; //!< Distribution of azimuthal angle
 };
@@ -64,11 +94,29 @@ Direction isotropic_direction(uint64_t* seed);
 class Isotropic : public UnitSphereDistribution {
 public:
   Isotropic() {};
+  explicit Isotropic(pugi::xml_node node);
 
   //! Sample a direction from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled direction
-  Direction sample(uint64_t* seed) const override;
+  //! \return (sampled direction, sample weight)
+  std::pair<Direction, double> sample(uint64_t* seed) const override;
+
+  //! Evaluate the probability density for a given direction
+  //! \param[in] u Direction on the unit sphere
+  //! \return Probability density at the given direction
+  double evaluate(Direction u) const override;
+
+  // Set or get bias distribution
+  void set_bias(std::unique_ptr<PolarAzimuthal> bias)
+  {
+    bias_ = std::move(bias);
+  }
+
+  const PolarAzimuthal* bias() const { return bias_.get(); }
+
+protected:
+  // Biasing distribution
+  unique_ptr<PolarAzimuthal> bias_;
 };
 
 //==============================================================================
@@ -83,8 +131,8 @@ public:
 
   //! Sample a direction from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled direction
-  Direction sample(uint64_t* seed) const override;
+  //! \return (sampled direction, sample weight)
+  std::pair<Direction, double> sample(uint64_t* seed) const override;
 };
 
 using UPtrAngle = unique_ptr<UnitSphereDistribution>;
diff --git a/include/openmc/distribution_spatial.h b/include/openmc/distribution_spatial.h
index 9c3bc743f..663cf5568 100644
--- a/include/openmc/distribution_spatial.h
+++ b/include/openmc/distribution_spatial.h
@@ -19,7 +19,9 @@ public:
   virtual ~SpatialDistribution() = default;
 
   //! Sample a position from the distribution
-  virtual Position sample(uint64_t* seed) const = 0;
+  //! \param seed Pseudorandom number seed pointer
+  //! \return Sampled (position, importance weight)
+  virtual std::pair<Position, double> sample(uint64_t* seed) const = 0;
 
   static unique_ptr<SpatialDistribution> create(pugi::xml_node node);
 };
@@ -34,8 +36,8 @@ public:
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
   // Observer pointers
   Distribution* x() const { return x_.get(); }
@@ -58,19 +60,25 @@ public:
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
   Distribution* r() const { return r_.get(); }
   Distribution* phi() const { return phi_.get(); }
   Distribution* z() const { return z_.get(); }
   Position origin() const { return origin_; }
+  Direction r_dir() const { return r_dir_; }
+  Direction phi_dir() const { return phi_dir_; }
+  Direction z_dir() const { return z_dir_; }
 
 private:
-  UPtrDist r_;      //!< Distribution of r coordinates
-  UPtrDist phi_;    //!< Distribution of phi coordinates
-  UPtrDist z_;      //!< Distribution of z coordinates
-  Position origin_; //!< Cartesian coordinates of the cylinder center
+  UPtrDist r_;        //!< Distribution of r coordinates
+  UPtrDist phi_;      //!< Distribution of phi coordinates
+  UPtrDist z_;        //!< Distribution of z coordinates
+  Position origin_;   //!< Cartesian coordinates of the cylinder center
+  Direction r_dir_;   //!< Direction of r-axis at phi=0
+  Direction phi_dir_; //!< Direction of phi-axis at phi=0
+  Direction z_dir_;   //!< Direction of z-axis
 };
 
 //==============================================================================
@@ -83,8 +91,8 @@ public:
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
   Distribution* r() const { return r_.get(); }
   Distribution* cos_theta() const { return cos_theta_.get(); }
@@ -109,8 +117,8 @@ public:
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
   //! Sample the mesh for an element and position within that element
   //! \param seed Pseudorandom number seed pointer
@@ -133,8 +141,8 @@ public:
 
 private:
   int32_t mesh_idx_ {C_NONE};
-  DiscreteIndex elem_idx_dist_; //!< Distribution of
-                                //!< mesh element indices
+  DiscreteIndex elem_idx_dist_; //!< Distribution of mesh element indices
+  vector<double> weight_;       //!< Importance weights (empty if unbiased)
 };
 
 //==============================================================================
@@ -149,12 +157,13 @@ public:
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
 private:
   std::vector<Position> point_cloud_;
   DiscreteIndex point_idx_dist_; //!< Distribution of Position indices
+  vector<double> weight_;        //!< Importance weights (empty if unbiased)
 };
 
 //==============================================================================
@@ -164,11 +173,12 @@ private:
 class SpatialBox : public SpatialDistribution {
 public:
   explicit SpatialBox(pugi::xml_node node, bool fission = false);
+  SpatialBox(Position lower_left, Position upper_right, bool fission = false);
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
   // Properties
   bool only_fissionable() const { return only_fissionable_; }
@@ -193,8 +203,8 @@ public:
 
   //! Sample a position from the distribution
   //! \param seed Pseudorandom number seed pointer
-  //! \return Sampled position
-  Position sample(uint64_t* seed) const override;
+  //! \return Sampled (position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
 
   Position r() const { return r_; }
 
diff --git a/include/openmc/eigenvalue.h b/include/openmc/eigenvalue.h
index b456fee21..438747676 100644
--- a/include/openmc/eigenvalue.h
+++ b/include/openmc/eigenvalue.h
@@ -6,7 +6,7 @@
 
 #include <cstdint> // for int64_t
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 #include <hdf5.h>
 
 #include "openmc/array.h"
@@ -24,7 +24,7 @@ namespace simulation {
 extern double keff_generation; //!<  Single-generation k on each processor
 extern array<double, 2> k_sum; //!< Used to reduce sum and sum_sq
 extern vector<double> entropy; //!< Shannon entropy at each generation
-extern xt::xtensor<double, 1> source_frac; //!< Source fraction for UFS
+extern tensor::Tensor<double> source_frac; //!< Source fraction for UFS
 
 } // namespace simulation
 
diff --git a/include/openmc/endf.h b/include/openmc/endf.h
index 4a737eb88..34fee9758 100644
--- a/include/openmc/endf.h
+++ b/include/openmc/endf.h
@@ -33,6 +33,13 @@ bool is_disappearance(int MT);
 //! \return Whether corresponding reaction is an inelastic scattering reaction
 bool is_inelastic_scatter(int MT);
 
+//! Determine whether an MT number matches a target MT, considering that the
+//! target may be a summation reaction.
+//! \param[in] event_mt MT number of the actual event
+//! \param[in] target_mt MT number to check against
+//! \return Whether event_mt is a component of target_mt (or equal to it)
+bool mt_matches(int event_mt, int target_mt);
+
 //==============================================================================
 //! Abstract one-dimensional function
 //==============================================================================
diff --git a/include/openmc/error.h b/include/openmc/error.h
index d73795aee..1f6e15f49 100644
--- a/include/openmc/error.h
+++ b/include/openmc/error.h
@@ -64,14 +64,14 @@ void write_message(
   int level, const std::string& message, const Params&... fmt_args)
 {
   if (settings::verbosity >= level) {
-    write_message(fmt::format(message, fmt_args...));
+    write_message(fmt::format(fmt::runtime(message), fmt_args...));
   }
 }
 
 template<typename... Params>
 void write_message(const std::string& message, const Params&... fmt_args)
 {
-  write_message(fmt::format(message, fmt_args...));
+  write_message(fmt::format(fmt::runtime(message), fmt_args...));
 }
 
 #ifdef OPENMC_MPI
diff --git a/include/openmc/event.h b/include/openmc/event.h
index 2d215a10e..48a009f2b 100644
--- a/include/openmc/event.h
+++ b/include/openmc/event.h
@@ -112,6 +112,19 @@ void process_collision_events();
 //! \param n_particles The number of particles in the particle buffer
 void process_death_events(int64_t n_particles);
 
+//! Process event queues until all are empty. Each iteration processes the
+//! longest queue first to maximize vectorization efficiency.
+void process_transport_events();
+
+//! Initialize secondary particles from a shared secondary bank for
+//! event-based transport
+//
+//! \param n_particles The number of particles to initialize
+//! \param offset The offset index in the shared secondary bank
+//! \param shared_secondary_bank The shared secondary bank to read from
+void process_init_secondary_events(int64_t n_particles, int64_t offset,
+  const SharedArray<SourceSite>& shared_secondary_bank);
+
 } // namespace openmc
 
 #endif // OPENMC_EVENT_H
diff --git a/include/openmc/geometry.h b/include/openmc/geometry.h
index 107cc7d1f..43cf9bb58 100644
--- a/include/openmc/geometry.h
+++ b/include/openmc/geometry.h
@@ -3,9 +3,12 @@
 
 #include <cmath>
 #include <cstdint>
+#include <unordered_map>
+#include <vector>
 
 #include "openmc/array.h"
 #include "openmc/constants.h"
+#include "openmc/random_ray/source_region.h" // For hash_combine
 #include "openmc/vector.h"
 
 namespace openmc {
@@ -13,6 +16,34 @@ namespace openmc {
 class BoundaryInfo;
 class GeometryState;
 
+//==============================================================================
+//! OverlapKey to store cell and universe data of a single overlap, along with
+//! a functor for hashing an OverlapKey into an unordered_map.
+//==============================================================================
+
+struct OverlapKey {
+  int universe_id;
+  int cell1_id;
+  int cell2_id;
+
+  bool operator==(const OverlapKey& other) const
+  {
+    return universe_id == other.universe_id && cell1_id == other.cell1_id &&
+           cell2_id == other.cell2_id;
+  }
+};
+
+struct OverlapKeyHash {
+  std::size_t operator()(const OverlapKey& k) const
+  {
+    size_t seed = 0;
+    hash_combine(seed, k.universe_id);
+    hash_combine(seed, k.cell1_id);
+    hash_combine(seed, k.cell2_id);
+    return seed;
+  }
+};
+
 //==============================================================================
 // Global variables
 //==============================================================================
@@ -24,6 +55,10 @@ extern "C" int n_coord_levels; //!< Number of CSG coordinate levels
 
 extern vector<int64_t> overlap_check_count;
 
+// Overlap data structures get cleared every slice_data run
+extern vector<OverlapKey> overlap_keys;
+extern std::unordered_map<OverlapKey, int, OverlapKeyHash> overlap_key_index;
+
 } // namespace model
 
 //==============================================================================
@@ -38,8 +73,7 @@ inline bool coincident(double d1, double d2)
 //==============================================================================
 //! Check for overlapping cells at a particle's position.
 //==============================================================================
-
-bool check_cell_overlap(GeometryState& p, bool error = true);
+int check_cell_overlap(GeometryState& p, bool error = true);
 
 //==============================================================================
 //! Get the cell instance for a particle at the specified universe level
diff --git a/include/openmc/geometry_aux.h b/include/openmc/geometry_aux.h
index 0c870c9fb..4dafdea5c 100644
--- a/include/openmc/geometry_aux.h
+++ b/include/openmc/geometry_aux.h
@@ -15,8 +15,6 @@
 namespace openmc {
 
 namespace model {
-extern std::unordered_map<int32_t, std::unordered_map<int32_t, int32_t>>
-  universe_cell_counts;
 extern std::unordered_map<int32_t, int32_t> universe_level_counts;
 } // namespace model
 
@@ -39,6 +37,12 @@ void adjust_indices();
 
 void assign_temperatures();
 
+//==============================================================================
+//! Finalize densities (compute density multipliers).
+//==============================================================================
+
+void finalize_cell_densities();
+
 //==============================================================================
 //! \brief Obtain a list of temperatures that each nuclide/thermal scattering
 //! table appears at in the model. Later, this list is used to determine the
@@ -80,15 +84,13 @@ void prepare_distribcell(
   const std::vector<int32_t>* user_distribcells = nullptr);
 
 //==============================================================================
-//! Recursively search through the geometry and count cell instances.
+//! Recursively search through the geometry and count universe instances.
 //!
-//! This function will update the Cell::n_instances value for each cell in the
-//! geometry.
-//! \param univ_indx The index of the universe to begin searching from (probably
-//!   the root universe).
+//! This function will update Universe.n_instances_ for each
+//! universe in the geometry.
 //==============================================================================
 
-void count_cell_instances(int32_t univ_indx);
+void count_universe_instances();
 
 //==============================================================================
 //! Recursively search through universes and count universe instances.
diff --git a/include/openmc/hdf5_interface.h b/include/openmc/hdf5_interface.h
index 0092c08f8..93e4bfc82 100644
--- a/include/openmc/hdf5_interface.h
+++ b/include/openmc/hdf5_interface.h
@@ -11,8 +11,7 @@
 
 #include "hdf5.h"
 #include "hdf5_hl.h"
-#include "xtensor/xadapt.hpp"
-#include "xtensor/xarray.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/array.h"
 #include "openmc/error.h"
@@ -100,8 +99,8 @@ void read_llong(hid_t obj_id, const char* name, long long* buffer, bool indep);
 void read_string(
   hid_t obj_id, const char* name, size_t slen, char* buffer, bool indep);
 
-void read_tally_results(
-  hid_t group_id, hsize_t n_filter, hsize_t n_score, double* results);
+void read_tally_results(hid_t group_id, hsize_t n_filter, hsize_t n_score,
+  hsize_t n_results, double* results);
 void write_attr_double(hid_t obj_id, int ndim, const hsize_t* dims,
   const char* name, const double* buffer);
 void write_attr_int(hid_t obj_id, int ndim, const hsize_t* dims,
@@ -114,9 +113,9 @@ void write_int(hid_t group_id, int ndim, const hsize_t* dims, const char* name,
 void write_llong(hid_t group_id, int ndim, const hsize_t* dims,
   const char* name, const long long* buffer, bool indep);
 void write_string(hid_t group_id, int ndim, const hsize_t* dims, size_t slen,
-  const char* name, char const* buffer, bool indep);
-void write_tally_results(
-  hid_t group_id, hsize_t n_filter, hsize_t n_score, const double* results);
+  const char* name, const char* buffer, bool indep);
+void write_tally_results(hid_t group_id, hsize_t n_filter, hsize_t n_score,
+  hsize_t n_results, const double* results);
 } // extern "C"
 
 //==============================================================================
@@ -166,24 +165,19 @@ void read_attribute(hid_t obj_id, const char* name, vector<T>& vec)
   read_attr(obj_id, name, H5TypeMap<T>::type_id, vec.data());
 }
 
-// Generic array version
+// Tensor version
 template<typename T>
-void read_attribute(hid_t obj_id, const char* name, xt::xarray<T>& arr)
+void read_attribute(hid_t obj_id, const char* name, tensor::Tensor<T>& tensor)
 {
-  // Get shape of attribute array
+  // Get shape of attribute
   auto shape = attribute_shape(obj_id, name);
 
-  // Allocate new array to read data into
-  std::size_t size = 1;
-  for (const auto x : shape)
-    size *= x;
-  vector<T> buffer(size);
+  // Resize tensor and read data directly
+  vector<size_t> tshape(shape.begin(), shape.end());
+  tensor.resize(tshape);
 
   // Read data from attribute
-  read_attr(obj_id, name, H5TypeMap<T>::type_id, buffer.data());
-
-  // Adapt array into xarray
-  arr = xt::adapt(buffer, shape);
+  read_attr(obj_id, name, H5TypeMap<T>::type_id, tensor.data());
 }
 
 // overload for std::string
@@ -290,63 +284,34 @@ void read_dataset(
 }
 
 template<typename T>
-void read_dataset(hid_t dset, xt::xarray<T>& arr, bool indep = false)
+void read_dataset(hid_t dset, tensor::Tensor<T>& tensor, bool indep = false)
 {
   // Get shape of dataset
   vector<hsize_t> shape = object_shape(dset);
 
-  // Allocate space in the array to read data into
-  std::size_t size = 1;
-  for (const auto x : shape)
-    size *= x;
-  arr.resize(shape);
+  // Resize tensor and read data directly
+  vector<size_t> tshape(shape.begin(), shape.end());
+  tensor.resize(tshape);
 
-  // Read data from attribute
+  // Read data from dataset
   read_dataset_lowlevel(
-    dset, nullptr, H5TypeMap<T>::type_id, H5S_ALL, indep, arr.data());
+    dset, nullptr, H5TypeMap<T>::type_id, H5S_ALL, indep, tensor.data());
 }
 
 template<>
 void read_dataset(
-  hid_t dset, xt::xarray<std::complex<double>>& arr, bool indep);
+  hid_t dset, tensor::Tensor<std::complex<double>>& tensor, bool indep);
 
 template<typename T>
 void read_dataset(
-  hid_t obj_id, const char* name, xt::xarray<T>& arr, bool indep = false)
+  hid_t obj_id, const char* name, tensor::Tensor<T>& tensor, bool indep = false)
 {
-  // Open dataset and read array
+  // Open dataset and read tensor
   hid_t dset = open_dataset(obj_id, name);
-  read_dataset(dset, arr, indep);
+  read_dataset(dset, tensor, indep);
   close_dataset(dset);
 }
 
-template<typename T, std::size_t N>
-void read_dataset(
-  hid_t obj_id, const char* name, xt::xtensor<T, N>& arr, bool indep = false)
-{
-  // Open dataset and read array
-  hid_t dset = open_dataset(obj_id, name);
-
-  // Get shape of dataset
-  vector<hsize_t> hsize_t_shape = object_shape(dset);
-  close_dataset(dset);
-
-  // cast from hsize_t to size_t
-  vector<size_t> shape(hsize_t_shape.size());
-  for (int i = 0; i < shape.size(); i++) {
-    shape[i] = static_cast<size_t>(hsize_t_shape[i]);
-  }
-
-  // Allocate new xarray to read data into
-  xt::xarray<T> xarr(shape);
-
-  // Read data from the dataset
-  read_dataset(obj_id, name, xarr);
-
-  // Copy into xtensor
-  arr = xarr;
-}
-
 // overload for Position
 inline void read_dataset(
   hid_t obj_id, const char* name, Position& r, bool indep = false)
@@ -358,31 +323,22 @@ inline void read_dataset(
   r.z = x[2];
 }
 
-template<typename T, std::size_t N>
+template<typename T>
 inline void read_dataset_as_shape(
-  hid_t obj_id, const char* name, xt::xtensor<T, N>& arr, bool indep = false)
+  hid_t obj_id, const char* name, tensor::Tensor<T>& tensor, bool indep = false)
 {
   hid_t dset = open_dataset(obj_id, name);
 
-  // Allocate new array to read data into
-  std::size_t size = 1;
-  for (const auto x : arr.shape())
-    size *= x;
-  vector<T> buffer(size);
-
-  // Read data from attribute
+  // Read data directly into pre-shaped tensor
   read_dataset_lowlevel(
-    dset, nullptr, H5TypeMap<T>::type_id, H5S_ALL, indep, buffer.data());
-
-  // Adapt into xarray
-  arr = xt::adapt(buffer, arr.shape());
+    dset, nullptr, H5TypeMap<T>::type_id, H5S_ALL, indep, tensor.data());
 
   close_dataset(dset);
 }
 
-template<typename T, std::size_t N>
-inline void read_nd_vector(hid_t obj_id, const char* name,
-  xt::xtensor<T, N>& result, bool must_have = false)
+template<typename T>
+inline void read_nd_tensor(hid_t obj_id, const char* name,
+  tensor::Tensor<T>& result, bool must_have = false)
 {
   if (object_exists(obj_id, name)) {
     read_dataset_as_shape(obj_id, name, result, true);
@@ -496,12 +452,16 @@ inline void write_dataset(
     false, buffer.data());
 }
 
-// Template for xarray, xtensor, etc.
-template<typename D>
-inline void write_dataset(
-  hid_t obj_id, const char* name, const xt::xcontainer<D>& arr)
+// Template for Tensor and StaticTensor2D. A SFINAE guard is used here to
+// prevent this template from matching vector/string types that have their own
+// overloads above. A generic Container parameter avoids duplicating the body
+// for both Tensor<T> and StaticTensor2D<T,R,C>.
+template<typename Container,
+  typename =
+    std::enable_if_t<tensor::is_tensor<std::decay_t<Container>>::value>>
+inline void write_dataset(hid_t obj_id, const char* name, const Container& arr)
 {
-  using T = typename D::value_type;
+  using T = typename std::decay_t<Container>::value_type;
   auto s = arr.shape();
   vector<hsize_t> dims {s.cbegin(), s.cend()};
   write_dataset_lowlevel(obj_id, dims.size(), dims.data(), name,
diff --git a/include/openmc/ifp.h b/include/openmc/ifp.h
new file mode 100644
index 000000000..2b751d66f
--- /dev/null
+++ b/include/openmc/ifp.h
@@ -0,0 +1,222 @@
+#ifndef OPENMC_IFP_H
+#define OPENMC_IFP_H
+
+#include "openmc/message_passing.h"
+#include "openmc/particle.h"
+#include "openmc/particle_data.h"
+#include "openmc/settings.h"
+
+#include <algorithm> // for copy
+
+namespace openmc {
+
+//! Check the value of the IFP parameter for beta effective or both.
+//!
+//! \return true if "BetaEffective" or "Both", false otherwise.
+bool is_beta_effective_or_both();
+
+//! Check the value of the IFP parameter for generation time or both.
+//!
+//! \return true if "GenerationTime" or "Both", false otherwise.
+bool is_generation_time_or_both();
+
+//! Resize IFP vectors
+//!
+//! \param[in,out] delayed_groups List of delayed group numbers
+//! \param[in,out] lifetimes List of lifetimes
+//! \param[in] n  Dimension to resize vectors
+template<typename T, typename U>
+void resize_ifp_data(vector<T>& delayed_groups, vector<U>& lifetimes, int64_t n)
+{
+  if (is_beta_effective_or_both()) {
+    delayed_groups.resize(n);
+  }
+  if (is_generation_time_or_both()) {
+    lifetimes.resize(n);
+  }
+}
+
+//! Update a list of values by adding a new value if the size
+//! of the list can accomodate the new value or by shifting all
+//! values to the left (removing the first value of the list
+//! and adding the new value at the end of the list).
+//!
+//! \param[in] value Value to add to the list
+//! \param[in] data Initial version of the list
+//! \return Updated list
+template<typename T>
+vector<T> _ifp(const T& value, const vector<T>& data)
+{
+  vector<T> updated;
+  size_t source_idx = data.size();
+
+  if (source_idx < settings::ifp_n_generation) {
+    updated.resize(source_idx + 1);
+    for (size_t i = 0; i < source_idx; i++) {
+      updated[i] = data[i];
+    }
+    updated[source_idx] = value;
+  } else if (source_idx == settings::ifp_n_generation) {
+    updated.resize(source_idx);
+    for (size_t i = 0; i < source_idx - 1; i++) {
+      updated[i] = data[i + 1];
+    }
+    updated[source_idx - 1] = value;
+  }
+  return updated;
+}
+
+//! \brief Iterated Fission Probability (IFP) method.
+//!
+//! Add the IFP information in the IFP banks using the same index
+//! as the one used to append the fission site to the fission bank.
+//! The information stored are the delayed group number and lifetime
+//! of the neutron that created the fission event.
+//! Multithreading protection is guaranteed by the index returned by the
+//! thread_safe_append call in physics.cpp.
+//!
+//! \param[in] p Particle
+//! \param[in] idx Bank index from the thread_safe_append call in physics.cpp
+void ifp(const Particle& p, int64_t idx);
+
+//! Resize the IFP banks used in the simulation
+void resize_simulation_ifp_banks();
+
+//! Retrieve IFP data from the IFP fission banks.
+//!
+//! \param[in] i_bank Index in the fission banks
+//! \param[in,out] delayed_groups Delayed group numbers
+//! \param[in,out] lifetimes Lifetimes lists
+void copy_ifp_data_from_fission_banks(
+  int i_bank, vector<int>& delayed_groups, vector<double>& lifetimes);
+
+#ifdef OPENMC_MPI
+
+//! Deserialization information for transfer of IFP data using MPI
+struct DeserializationInfo {
+  int64_t index_local; //!< local index
+  int64_t n;           //!< number of sites sent
+};
+
+//! Broadcast the number of generation determined by the size of the first
+//! element on the first processor.
+//!
+//! \param[in] n_generation Number of generations
+//! \param[in] delayed_groups List of delayed group numbers lists
+//! \param[in] lifetimes List of lifetimes lists
+void broadcast_ifp_n_generation(int& n_generation,
+  const vector<vector<int>>& delayed_groups,
+  const vector<vector<double>>& lifetimes);
+
+//! Send IFP data using MPI.
+//!
+//! \param[in] idx Index of the first site
+//! \param[in] n Number of sites to send
+//! \param[in] n_generation Number of generations
+//! \param[in] neighbor Index of the neighboring processor
+//! \param[in] requests MPI requests
+//! \param[in] data List of data lists
+//! \param[out] send_data data buffer
+template<typename T>
+void send_ifp_info(int64_t idx, int64_t n, int n_generation, int neighbor,
+  vector<MPI_Request>& requests, const vector<vector<T>>& data,
+  vector<T>& send_data)
+{
+  // Copy data in buffer
+  for (int i = idx; i < idx + n; i++) {
+    std::copy(
+      data[i].begin(), data[i].end(), send_data.begin() + i * n_generation);
+  }
+
+  // Send data
+  requests.emplace_back();
+  MPI_Datatype datatype = mpi::MPITypeMap<T>::mpi_type;
+  MPI_Isend(&send_data[n_generation * idx], n_generation * static_cast<int>(n),
+    datatype, neighbor, mpi::rank, mpi::intracomm, &requests.back());
+}
+
+//! Receive IFP data using MPI.
+//!
+//! \param[in] idx Index of the first site
+//! \param[in] n Number of sites to receive
+//! \param[in] n_generation Number of generations
+//! \param[in] neighbor Index of the neighboring processor
+//! \param[in] requests MPI requests
+//! \param[in] data data buffer
+//! \param[out] deserialization Information to deserialize the received data
+template<typename T>
+void receive_ifp_data(int64_t idx, int64_t n, int n_generation, int neighbor,
+  vector<MPI_Request>& requests, vector<T>& data,
+  vector<DeserializationInfo>& deserialization)
+{
+  requests.emplace_back();
+  MPI_Datatype datatype = mpi::MPITypeMap<T>::mpi_type;
+  MPI_Irecv(&data[n_generation * idx], n_generation * static_cast<int>(n),
+    datatype, neighbor, neighbor, mpi::intracomm, &requests.back());
+
+  // Deserialization info to reconstruct data later
+  DeserializationInfo info = {idx, n};
+  deserialization.push_back(info);
+}
+
+//! Copy partial IFP data from local lists to source banks.
+//!
+//! \param[in] idx Index of the first site
+//! \param[in] n Number of sites to copy
+//! \param[in] i_bank Index in the IFP source banks
+//! \param[in] delayed_groups List of delayed group numbers lists
+//! \param[in] lifetimes List of lifetimes lists
+void copy_partial_ifp_data_to_source_banks(int64_t idx, int n, int64_t i_bank,
+  const vector<vector<int>>& delayed_groups,
+  const vector<vector<double>>& lifetimes);
+
+//! Deserialize IFP information received using MPI and store it in
+//! the IFP source banks.
+//!
+//! \param[in] n_generation Number of generations
+//! \param[in] data data to deserialize
+//! \param[in] bank bank to store data
+//! \param[out] deserialization Information to deserialize the received data
+template<typename T>
+void deserialize_ifp_info(int n_generation, const vector<T>& data,
+  vector<vector<T>>& bank, const vector<DeserializationInfo>& deserialization)
+{
+  for (auto info : deserialization) {
+    int64_t index_local = info.index_local;
+    int64_t n = info.n;
+
+    for (int i = index_local; i < index_local + n; i++) {
+      vector<T> data_received(
+        data.begin() + n_generation * i, data.begin() + n_generation * (i + 1));
+      bank[i] = data_received;
+    }
+  }
+}
+
+#endif
+
+//! Copy IFP temporary vectors to source banks.
+//!
+//! \param[in] delayed_groups List of delayed group numbers lists
+//! \param[in] lifetimes List of lifetimes lists
+void copy_complete_ifp_data_to_source_banks(
+  const vector<vector<int>>& delayed_groups,
+  const vector<vector<double>>& lifetimes);
+
+//! Allocate temporary vectors for IFP data.
+//!
+//! \param[in,out] delayed_groups List of delayed group numbers lists
+//! \param[in,out] lifetimes List of delayed group numbers lists
+void allocate_temporary_vector_ifp(
+  vector<vector<int>>& delayed_groups, vector<vector<double>>& lifetimes);
+
+//! Copy local IFP data to IFP fission banks.
+//!
+//! \param[in] delayed_groups_ptr Pointer to delayed group numbers
+//! \param[in] lifetimes_ptr Pointer to lifetimes
+void copy_ifp_data_to_fission_banks(
+  const vector<int>* delayed_groups_ptr, const vector<double>* lifetimes_ptr);
+
+} // namespace openmc
+
+#endif // OPENMC_IFP_H
diff --git a/include/openmc/lattice.h b/include/openmc/lattice.h
index fb374ab75..ca40bbc2a 100644
--- a/include/openmc/lattice.h
+++ b/include/openmc/lattice.h
@@ -76,7 +76,7 @@ public:
   }
 
   //! Populate the distribcell offset tables.
-  int32_t fill_offset_table(int32_t offset, int32_t target_univ_id, int map,
+  int32_t fill_offset_table(int32_t target_univ_id, int map,
     std::unordered_map<int32_t, int32_t>& univ_count_memo);
 
   //! \brief Check lattice indices.
@@ -113,6 +113,14 @@ public:
   virtual Position get_local_position(
     Position r, const array<int, 3>& i_xyz) const = 0;
 
+  //! \brief get the normal of the lattice surface crossing
+  //! \param[in] i_xyz The indices for the lattice translation.
+  //! \param[out] is_valid is the lattice translation correspond to a valid
+  //! surface. \return The surface normal corresponding to the lattice
+  //! translation.
+  virtual Direction get_normal(
+    const array<int, 3>& i_xyz, bool& is_valid) const = 0;
+
   //! \brief Check flattened lattice index.
   //! \param indx The index for a lattice tile.
   //! \return true if the given index fit within the lattice bounds.  False
@@ -223,6 +231,9 @@ public:
   Position get_local_position(
     Position r, const array<int, 3>& i_xyz) const override;
 
+  Direction get_normal(
+    const array<int, 3>& i_xyz, bool& is_valid) const override;
+
   int32_t& offset(int map, const array<int, 3>& i_xyz) override;
 
   int32_t offset(int map, int indx) const override;
@@ -268,6 +279,9 @@ public:
   Position get_local_position(
     Position r, const array<int, 3>& i_xyz) const override;
 
+  Direction get_normal(
+    const array<int, 3>& i_xyz, bool& is_valid) const override;
+
   bool is_valid_index(int indx) const override;
 
   int32_t& offset(int map, const array<int, 3>& i_xyz) override;
diff --git a/include/openmc/material.h b/include/openmc/material.h
index 31fab2ae2..3967c3687 100644
--- a/include/openmc/material.h
+++ b/include/openmc/material.h
@@ -5,8 +5,8 @@
 #include <unordered_map>
 
 #include "openmc/span.h"
+#include "openmc/tensor.h"
 #include "pugixml.hpp"
-#include "xtensor/xtensor.hpp"
 #include <hdf5.h>
 
 #include "openmc/bremsstrahlung.h"
@@ -14,6 +14,7 @@
 #include "openmc/memory.h" // for unique_ptr
 #include "openmc/ncrystal_interface.h"
 #include "openmc/particle.h"
+#include "openmc/settings.h"
 #include "openmc/vector.h"
 
 namespace openmc {
@@ -99,13 +100,27 @@ public:
   //----------------------------------------------------------------------------
   // Accessors
 
+  //! Get the atom density in [atom/b-cm]
+  //! \return Density in [atom/b-cm]
+  double atom_density(int32_t i, double rho_multiplier = 1.0) const
+  {
+    return atom_density_(i) * rho_multiplier;
+  }
+
   //! Get density in [atom/b-cm]
   //! \return Density in [atom/b-cm]
   double density() const { return density_; }
 
-  //! Get density in [g/cm^3]
+  //! Get density in [g/cm^3].
   //! \return Density in [g/cm^3]
-  double density_gpcc() const { return density_gpcc_; }
+  double density_gpcc() const
+  {
+    return settings::run_CE ? density_gpcc_ : density();
+  }
+
+  //! Get charge density in [e/b-cm]
+  //! \return Charge density in [e/b-cm]
+  double charge_density() const { return charge_density_; };
 
   //! Get name
   //! \return Material name
@@ -174,9 +189,10 @@ public:
   vector<int> nuclide_;                 //!< Indices in nuclides vector
   vector<int> element_;                 //!< Indices in elements vector
   NCrystalMat ncrystal_mat_;            //!< NCrystal material object
-  xt::xtensor<double, 1> atom_density_; //!< Nuclide atom density in [atom/b-cm]
+  tensor::Tensor<double> atom_density_; //!< Nuclide atom density in [atom/b-cm]
   double density_;                      //!< Total atom density in [atom/b-cm]
   double density_gpcc_;                 //!< Total atom density in [g/cm^3]
+  double charge_density_;               //!< Total charge density in [e/b-cm]
   double volume_ {-1.0};                //!< Volume in [cm^3]
   vector<bool> p0_; //!< Indicate which nuclides are to be treated with
                     //!< iso-in-lab scattering
diff --git a/include/openmc/math_functions.h b/include/openmc/math_functions.h
index c30ef7558..d3bccca7b 100644
--- a/include/openmc/math_functions.h
+++ b/include/openmc/math_functions.h
@@ -9,6 +9,7 @@
 #include <cstdlib>
 
 #include "openmc/position.h"
+#include "openmc/search.h"
 
 namespace openmc {
 
@@ -200,5 +201,63 @@ std::complex<double> faddeeva(std::complex<double> z);
 //! \return Derivative of Faddeeva function evaluated at z
 std::complex<double> w_derivative(std::complex<double> z, int order);
 
+//! Evaluate relative exponential function
+//!
+//! \param x Real argument
+//! \return (exp(x)-1)/x without loss of precision near 0
+double exprel(double x);
+
+//! Evaluate relative logarithm function
+//!
+//! \param x Real argument
+//! \return log(1+x)/x without loss of precision near 0
+double log1prel(double x);
+
+//! Evaluate the cylindrical Bessel function of the first kind J_n(x)
+//!
+//! Uses std::cyl_bessel_j where available (e.g., libstdc++). On standard
+//! library implementations lacking the C++17 special math functions (e.g.,
+//! libc++ on Apple Clang/LLVM), falls back to the ascending power series,
+//! which converges to machine precision for the small arguments (|x| <= 2)
+//! used in OpenMC. Unlike std::cyl_bessel_j, negative arguments are handled
+//! via the parity relation J_n(-x) = (-1)^n J_n(x).
+//!
+//! \param n Non-negative integer order of the Bessel function
+//! \param x Real argument
+//! \return J_n(x)
+double cyl_bessel_j(int n, double x);
+
+//! Helper function to get index and interpolation function on an incident
+//! energy grid
+//!
+//! \param energies energy grid
+//! \param E incident energy
+//! \param i grid index
+//! \param f interpolation factor
+void get_energy_index(
+  const vector<double>& energies, double E, int& i, double& f);
+
+//==============================================================================
+//! Calculate the cumulative distribution function of the standard normal
+//! distribution at a given value.
+//!
+//! \param z The value at which to evaluate the CDF
+//! \return Phi(z) = P(X <= z) for X ~ N(0,1)
+//==============================================================================
+
+double standard_normal_cdf(double z);
+
+//==============================================================================
+//! Return true if two floating-point values are approximately equal within a
+//! combined relative and absolute tolerance.
+//!
+//! \param a first floating point value
+//! \param b second floating point value
+//! \param rel_tol relative tolerance
+//! \param abs_tol absolute tolerance
+//! \return true if a and b are approximately equal, false otherwise
+//==============================================================================
+bool isclose(double a, double b, double rel_tol, double abs_tol);
+
 } // namespace openmc
 #endif // OPENMC_MATH_FUNCTIONS_H
diff --git a/include/openmc/mcpl_interface.h b/include/openmc/mcpl_interface.h
index e5c182280..a9cce3e69 100644
--- a/include/openmc/mcpl_interface.h
+++ b/include/openmc/mcpl_interface.h
@@ -9,12 +9,6 @@
 
 namespace openmc {
 
-//==============================================================================
-// Constants
-//==============================================================================
-
-extern "C" const bool MCPL_ENABLED;
-
 //==============================================================================
 // Functions
 //==============================================================================
@@ -25,18 +19,46 @@ extern "C" const bool MCPL_ENABLED;
 //! \return  Vector of source sites
 vector<SourceSite> mcpl_source_sites(std::string path);
 
-//! Write an MCPL source file
-//
+//! Write an MCPL source file with stat:sum metadata
+//!
+//! This function writes particle data to an MCPL file. For MCPL >= 2.1.0,
+//! it includes a stat:sum field (key: "openmc_np1") containing the total
+//! number of source particles, which is essential for proper file merging
+//! and weight normalization when using MCPL files with McStas/McXtrace.
+//!
+//! The stat:sum field follows the crash-safety pattern:
+//! - Initially set to -1 when opening (indicates incomplete file)
+//! - Updated with actual particle count before closing
+//!
 //! \param[in] filename     Path to MCPL file
 //! \param[in] source_bank  Vector of SourceSites to write to file for this
-//!                         MPI rank. Note that this can't be const due to
-//!                         it being used as work space by MPI.
-//! \param[in] bank_indx    Pointer to vector of site index ranges over all
-//!                         MPI ranks. This can be computed by calling
-//!                         calculate_parallel_index_vector on
-//!                         source_bank.size().
+//!                         MPI rank.
+//! \param[in] bank_index   Pointer to vector of site index ranges over all
+//!                         MPI ranks.
 void write_mcpl_source_point(const char* filename, span<SourceSite> source_bank,
   const vector<int64_t>& bank_index);
+
+//! Write an MCPL collision track file
+//!
+//! This function writes collision track data to an MCPL file. Additional
+//! collision-specific metadata (such as energy deposition, material info, etc.)
+//! is stored in the file header as blob data.
+//!
+//! \param[in] filename     Path to MCPL file
+//! \param[in] collision_track_bank  Vector of CollisionTrackSites to write to
+//!                         file for this MPI rank.
+//! \param[in] bank_index   Pointer to vector of site index ranges over all
+//!                         MPI ranks.
+void write_mcpl_collision_track(const char* filename,
+  span<CollisionTrackSite> collision_track_bank,
+  const vector<int64_t>& bank_index);
+
+//! Check if MCPL functionality is available
+bool is_mcpl_interface_available();
+
+//! Initialize the MCPL interface
+void initialize_mcpl_interface_if_needed();
+
 } // namespace openmc
 
 #endif // OPENMC_MCPL_INTERFACE_H
diff --git a/include/openmc/mesh.h b/include/openmc/mesh.h
index 8faf45f1b..0d8189caa 100644
--- a/include/openmc/mesh.h
+++ b/include/openmc/mesh.h
@@ -4,11 +4,12 @@
 #ifndef OPENMC_MESH_H
 #define OPENMC_MESH_H
 
+#include <set>
 #include <unordered_map>
 
 #include "hdf5.h"
+#include "openmc/tensor.h"
 #include "pugixml.hpp"
-#include "xtensor/xtensor.hpp"
 
 #include "openmc/bounding_box.h"
 #include "openmc/error.h"
@@ -19,14 +20,14 @@
 #include "openmc/vector.h"
 #include "openmc/xml_interface.h"
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 #include "moab/AdaptiveKDTree.hpp"
 #include "moab/Core.hpp"
 #include "moab/GeomUtil.hpp"
 #include "moab/Matrix3.hpp"
 #endif
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 #include "libmesh/bounding_box.h"
 #include "libmesh/dof_map.h"
 #include "libmesh/elem.h"
@@ -61,7 +62,7 @@ extern vector<unique_ptr<Mesh>> meshes;
 
 } // namespace model
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 namespace settings {
 // used when creating new libMesh::MeshBase instances
 extern unique_ptr<libMesh::LibMeshInit> libmesh_init;
@@ -87,8 +88,12 @@ namespace detail {
 
 class MaterialVolumes {
 public:
+  MaterialVolumes(int32_t* mats, double* vols, double* bboxes, int table_size)
+    : materials_(mats), volumes_(vols), bboxes_(bboxes), table_size_(table_size)
+  {}
+
   MaterialVolumes(int32_t* mats, double* vols, int table_size)
-    : materials_(mats), volumes_(vols), table_size_(table_size)
+    : MaterialVolumes(mats, vols, nullptr, table_size)
   {}
 
   //! Add volume for a given material in a mesh element
@@ -96,8 +101,11 @@ public:
   //! \param[in] index_elem Index of the mesh element
   //! \param[in] index_material Index of the material within the model
   //! \param[in] volume Volume to add
-  void add_volume(int index_elem, int index_material, double volume);
-  void add_volume_unsafe(int index_elem, int index_material, double volume);
+  //! \param[in] bbox Bounding box to union into the result (optional)
+  void add_volume(int index_elem, int index_material, double volume,
+    const BoundingBox* bbox = nullptr);
+  void add_volume_unsafe(int index_elem, int index_material, double volume,
+    const BoundingBox* bbox = nullptr);
 
   // Accessors
   int32_t& materials(int i, int j) { return materials_[i * table_size_ + j]; }
@@ -112,11 +120,23 @@ public:
     return volumes_[i * table_size_ + j];
   }
 
+  double& bboxes(int i, int j, int k)
+  {
+    return bboxes_[(i * table_size_ + j) * 6 + k];
+  }
+  const double& bboxes(int i, int j, int k) const
+  {
+    return bboxes_[(i * table_size_ + j) * 6 + k];
+  }
+
+  bool has_bboxes() const { return bboxes_ != nullptr; }
+
   bool table_full() const { return table_full_; }
 
 private:
   int32_t* materials_;      //!< material index (bins, table_size)
   double* volumes_;         //!< volume in [cm^3] (bins, table_size)
+  double* bboxes_;          //!< bounding boxes (bins, table_size, 6)
   int table_size_;          //!< Size of hash table for each mesh element
   bool table_full_ {false}; //!< Whether the hash table is full
 };
@@ -132,15 +152,18 @@ public:
   // Constructors and destructor
   Mesh() = default;
   Mesh(pugi::xml_node node);
+  Mesh(hid_t group);
   virtual ~Mesh() = default;
 
+  // Factory method for creating meshes from either an XML node or HDF5 group
+  template<typename T>
+  static const std::unique_ptr<Mesh>& create(
+    T dataset, const std::string& mesh_type, const std::string& mesh_library);
+
   // Methods
   //! Perform any preparation needed to support point location within the mesh
   virtual void prepare_for_point_location() {};
 
-  //! Update a position to the local coordinates of the mesh
-  virtual void local_coords(Position& r) const {};
-
   //! Return a position in the local coordinates of the mesh
   virtual Position local_coords(const Position& r) const { return r; };
 
@@ -236,22 +259,33 @@ public:
   void material_volumes(int nx, int ny, int nz, int max_materials,
     int32_t* materials, double* volumes) const;
 
+  //! Determine volume and bounding boxes of materials within each mesh element
+  //
+  //! \param[in] nx Number of samples in x direction
+  //! \param[in] ny Number of samples in y direction
+  //! \param[in] nz Number of samples in z direction
+  //! \param[in] max_materials Maximum number of materials in a single mesh
+  //!                          element
+  //! \param[inout] materials Array storing material indices
+  //! \param[inout] volumes Array storing volumes
+  //! \param[inout] bboxes Array storing bounding boxes (n_elems, table_size, 6)
+  void material_volumes(int nx, int ny, int nz, int max_materials,
+    int32_t* materials, double* volumes, double* bboxes) const;
+
   //! Determine bounding box of mesh
   //
   //! \return Bounding box of mesh
   BoundingBox bounding_box() const
   {
-    auto ll = this->lower_left();
-    auto ur = this->upper_right();
-    return {ll.x, ur.x, ll.y, ur.y, ll.z, ur.z};
+    return {this->lower_left(), this->upper_right()};
   }
 
   virtual Position lower_left() const = 0;
   virtual Position upper_right() const = 0;
 
   // Data members
-  xt::xtensor<double, 1> lower_left_;  //!< Lower-left coordinates of mesh
-  xt::xtensor<double, 1> upper_right_; //!< Upper-right coordinates of mesh
+  tensor::Tensor<double> lower_left_;  //!< Lower-left coordinates of mesh
+  tensor::Tensor<double> upper_right_; //!< Upper-right coordinates of mesh
   int id_ {-1};                        //!< Mesh ID
   std::string name_;                   //!< User-specified name
   int n_dimension_ {-1};               //!< Number of dimensions
@@ -261,6 +295,7 @@ class StructuredMesh : public Mesh {
 public:
   StructuredMesh() = default;
   StructuredMesh(pugi::xml_node node) : Mesh {node} {};
+  StructuredMesh(hid_t group) : Mesh {group} {};
   virtual ~StructuredMesh() = default;
 
   using MeshIndex = std::array<int, 3>;
@@ -313,7 +348,7 @@ public:
   //! \param[in] Pointer to bank sites
   //! \param[in] Number of bank sites
   //! \param[out] Whether any bank sites are outside the mesh
-  xt::xtensor<double, 1> count_sites(
+  tensor::Tensor<double> count_sites(
     const SourceSite* bank, int64_t length, bool* outside) const;
 
   //! Get bin given mesh indices
@@ -384,8 +419,8 @@ public:
   //! Get a label for the mesh bin
   std::string bin_label(int bin) const override;
 
-  //! Get shape as xt::xtensor
-  xt::xtensor<int, 1> get_x_shape() const;
+  //! Get mesh dimensions as a tensor
+  tensor::Tensor<int> get_shape_tensor() const;
 
   double volume(int bin) const override
   {
@@ -426,8 +461,7 @@ class PeriodicStructuredMesh : public StructuredMesh {
 public:
   PeriodicStructuredMesh() = default;
   PeriodicStructuredMesh(pugi::xml_node node) : StructuredMesh {node} {};
-
-  void local_coords(Position& r) const override { r -= origin_; };
+  PeriodicStructuredMesh(hid_t group) : StructuredMesh {group} {};
 
   Position local_coords(const Position& r) const override
   {
@@ -447,6 +481,7 @@ public:
   // Constructors
   RegularMesh() = default;
   RegularMesh(pugi::xml_node node);
+  RegularMesh(hid_t group);
 
   // Overridden methods
   int get_index_in_direction(double r, int i) const override;
@@ -480,16 +515,18 @@ public:
   //! \param[in] bank Array of bank sites
   //! \param[out] Whether any bank sites are outside the mesh
   //! \return Array indicating number of sites in each mesh/energy bin
-  xt::xtensor<double, 1> count_sites(
+  tensor::Tensor<double> count_sites(
     const SourceSite* bank, int64_t length, bool* outside) const;
 
   //! Return the volume for a given mesh index
   double volume(const MeshIndex& ijk) const override;
 
+  int set_grid();
+
   // Data members
   double volume_frac_;           //!< Volume fraction of each mesh element
   double element_volume_;        //!< Volume of each mesh element
-  xt::xtensor<double, 1> width_; //!< Width of each mesh element
+  tensor::Tensor<double> width_; //!< Width of each mesh element
 };
 
 class RectilinearMesh : public StructuredMesh {
@@ -497,6 +534,7 @@ public:
   // Constructors
   RectilinearMesh() = default;
   RectilinearMesh(pugi::xml_node node);
+  RectilinearMesh(hid_t group);
 
   // Overridden methods
   int get_index_in_direction(double r, int i) const override;
@@ -539,6 +577,7 @@ public:
   // Constructors
   CylindricalMesh() = default;
   CylindricalMesh(pugi::xml_node node);
+  CylindricalMesh(hid_t group);
 
   // Overridden methods
   virtual MeshIndex get_indices(Position r, bool& in_mesh) const override;
@@ -603,6 +642,7 @@ public:
   // Constructors
   SphericalMesh() = default;
   SphericalMesh(pugi::xml_node node);
+  SphericalMesh(hid_t group);
 
   // Overridden methods
   virtual MeshIndex get_indices(Position r, bool& in_mesh) const override;
@@ -671,9 +711,9 @@ class UnstructuredMesh : public Mesh {
 
 public:
   // Constructors
-  UnstructuredMesh() {};
+  UnstructuredMesh() { n_dimension_ = 3; };
   UnstructuredMesh(pugi::xml_node node);
-  UnstructuredMesh(const std::string& filename);
+  UnstructuredMesh(hid_t group);
 
   static const std::string mesh_type;
   virtual std::string get_mesh_type() const override;
@@ -773,13 +813,14 @@ private:
   virtual void initialize() = 0;
 };
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 
 class MOABMesh : public UnstructuredMesh {
 public:
   // Constructors
   MOABMesh() = default;
   MOABMesh(pugi::xml_node);
+  MOABMesh(hid_t group);
   MOABMesh(const std::string& filename, double length_multiplier = 1.0);
   MOABMesh(std::shared_ptr<moab::Interface> external_mbi);
 
@@ -943,12 +984,13 @@ private:
 
 #endif
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 
 class LibMesh : public UnstructuredMesh {
 public:
   // Constructors
   LibMesh(pugi::xml_node node);
+  LibMesh(hid_t group);
   LibMesh(const std::string& filename, double length_multiplier = 1.0);
   LibMesh(libMesh::MeshBase& input_mesh, double length_multiplier = 1.0);
 
@@ -960,7 +1002,7 @@ public:
 
   Position sample_element(int32_t bin, uint64_t* seed) const override;
 
-  int get_bin(Position r) const override;
+  virtual int get_bin(Position r) const override;
 
   int n_bins() const override;
 
@@ -996,27 +1038,31 @@ public:
 
   libMesh::MeshBase* mesh_ptr() const { return m_; };
 
+protected:
+  // Methods
+  //! Translate a bin value to an element reference
+  virtual const libMesh::Elem& get_element_from_bin(int bin) const;
+
+  //! Translate an element pointer to a bin index
+  virtual int get_bin_from_element(const libMesh::Elem* elem) const;
+
+  // Data members
+  libMesh::MeshBase* m_; //!< pointer to libMesh MeshBase instance, always set
+                         //!< during intialization
+  vector<unique_ptr<libMesh::PointLocatorBase>>
+    pl_;                      //!< per-thread point locators
+  libMesh::BoundingBox bbox_; //!< bounding box of the mesh
+
 private:
+  // Methods
   void initialize() override;
   void set_mesh_pointer_from_filename(const std::string& filename);
   void build_eqn_sys();
 
-  // Methods
-
-  //! Translate a bin value to an element reference
-  const libMesh::Elem& get_element_from_bin(int bin) const;
-
-  //! Translate an element pointer to a bin index
-  int get_bin_from_element(const libMesh::Elem* elem) const;
-
   // Data members
   unique_ptr<libMesh::MeshBase> unique_m_ =
     nullptr; //!< pointer to the libMesh MeshBase instance, only used if mesh is
              //!< created inside OpenMC
-  libMesh::MeshBase* m_; //!< pointer to libMesh MeshBase instance, always set
-                         //!< during intialization
-  vector<unique_ptr<libMesh::PointLocatorBase>>
-    pl_; //!< per-thread point locators
   unique_ptr<libMesh::EquationSystems>
     equation_systems_; //!< pointer to the libMesh EquationSystems
                        //!< instance
@@ -1025,11 +1071,42 @@ private:
   std::unordered_map<std::string, unsigned int>
     variable_map_; //!< mapping of variable names (tally scores) to libMesh
                    //!< variable numbers
-  libMesh::BoundingBox bbox_; //!< bounding box of the mesh
   libMesh::dof_id_type
     first_element_id_; //!< id of the first element in the mesh
+};
+
+class AdaptiveLibMesh : public LibMesh {
+public:
+  // Constructor
+  AdaptiveLibMesh(libMesh::MeshBase& input_mesh, double length_multiplier = 1.0,
+    const std::set<libMesh::subdomain_id_type>& block_ids =
+      std::set<libMesh::subdomain_id_type>());
+
+  // Overridden methods
+  int n_bins() const override;
+
+  void add_score(const std::string& var_name) override;
+
+  void set_score_data(const std::string& var_name, const vector<double>& values,
+    const vector<double>& std_dev) override;
+
+  void write(const std::string& filename) const override;
+
+  int get_bin(Position r) const override;
+
+protected:
+  // Overridden methods
+  int get_bin_from_element(const libMesh::Elem* elem) const override;
+
+  const libMesh::Elem& get_element_from_bin(int bin) const override;
+
+private:
+  // Data members
+  const std::set<libMesh::subdomain_id_type>
+    block_ids_;               //!< subdomains of the mesh to tally on
+  const bool block_restrict_; //!< whether a subset of the mesh is being used
+  const libMesh::dof_id_type num_active_; //!< cached number of active elements
 
-  const bool adaptive_; //!< whether this mesh has adaptivity enabled or not
   std::vector<libMesh::dof_id_type>
     bin_to_elem_map_; //!< mapping bin indices to dof indices for active
                       //!< elements
@@ -1048,6 +1125,11 @@ private:
 //! \param[in] root XML node
 void read_meshes(pugi::xml_node root);
 
+//! Read meshes from an HDF5 file
+//
+//! \param[in] group HDF5 group ("meshes" group)
+void read_meshes(hid_t group);
+
 //! Write mesh data to an HDF5 group
 //
 //! \param[in] group HDF5 group
diff --git a/include/openmc/message_passing.h b/include/openmc/message_passing.h
index a1641a906..03e236338 100644
--- a/include/openmc/message_passing.h
+++ b/include/openmc/message_passing.h
@@ -18,9 +18,23 @@ extern bool master;
 
 #ifdef OPENMC_MPI
 extern MPI_Datatype source_site;
+extern MPI_Datatype collision_track_site;
 extern MPI_Comm intracomm;
 #endif
 
+//==============================================================================
+// Template struct used to map types to MPI datatypes
+// By having a single static data member, the template can
+// be specialized for each type we know of. The specializations appear in the
+// .cpp file since they are definitions.
+//==============================================================================
+#ifdef OPENMC_MPI
+template<typename T>
+struct MPITypeMap {
+  static const MPI_Datatype mpi_type;
+};
+#endif
+
 // Calculates global indices of the bank particles
 // across all ranks using a parallel scan. This is used to write
 // the surface source file in parallel runs. It will probably
diff --git a/include/openmc/mgxs.h b/include/openmc/mgxs.h
index 9b1602f29..5d8ff58c2 100644
--- a/include/openmc/mgxs.h
+++ b/include/openmc/mgxs.h
@@ -6,7 +6,7 @@
 
 #include <string>
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/constants.h"
 #include "openmc/hdf5_interface.h"
@@ -22,7 +22,7 @@ namespace openmc {
 
 class Mgxs {
 private:
-  xt::xtensor<double, 1> kTs; // temperature in eV (k * T)
+  tensor::Tensor<double> kTs; // temperature in eV (k * T)
   AngleDistributionType
     scatter_format;       // flag for if this is legendre, histogram, or tabular
   int num_groups;         // number of energy groups
@@ -113,6 +113,11 @@ public:
     const vector<Mgxs*>& micros, const vector<double>& atom_densities,
     int num_group, int num_delay);
 
+  //! \brief Get the number of temperature data points.
+  //!
+  //! @return The number of temperature data points for this MGXS
+  inline int n_temperature_points() { return kTs.size(); }
+
   //! \brief Provides a cross section value given certain parameters
   //!
   //! @param xstype Type of cross section requested, according to the
diff --git a/include/openmc/mgxs_interface.h b/include/openmc/mgxs_interface.h
index da074f825..117ac503d 100644
--- a/include/openmc/mgxs_interface.h
+++ b/include/openmc/mgxs_interface.h
@@ -61,6 +61,8 @@ public:
   vector<double> energy_bin_avg_;
   vector<double> rev_energy_bins_;
   vector<vector<double>> nuc_temps_; // all available temperatures
+  vector<double>
+    default_inverse_velocity_; // approximate default inverse-velocity data
 };
 
 namespace data {
diff --git a/include/openmc/nuclide.h b/include/openmc/nuclide.h
index 329c776d0..ae39a53dd 100644
--- a/include/openmc/nuclide.h
+++ b/include/openmc/nuclide.h
@@ -84,6 +84,9 @@ public:
   double collapse_rate(int MT, double temperature, span<const double> energy,
     span<const double> flux) const;
 
+  //! Return a ParticleType object representing this nuclide
+  ParticleType particle_type() const { return {Z_, A_, metastable_}; }
+
   //============================================================================
   // Data members
   std::string name_; //!< Name of nuclide, e.g. "U235"
@@ -96,7 +99,7 @@ public:
   // Temperature dependent cross section data
   vector<double> kTs_;                //!< temperatures in eV (k*T)
   vector<EnergyGrid> grid_;           //!< Energy grid at each temperature
-  vector<xt::xtensor<double, 2>> xs_; //!< Cross sections at each temperature
+  vector<tensor::Tensor<double>> xs_; //!< Cross sections at each temperature
 
   // Multipole data
   unique_ptr<WindowedMultipole> multipole_;
@@ -163,9 +166,9 @@ bool multipole_in_range(const Nuclide& nuc, double E);
 namespace data {
 
 // Minimum/maximum transport energy for each particle type. Order corresponds to
-// that of the ParticleType enum
-extern array<double, 2> energy_min;
-extern array<double, 2> energy_max;
+// transport_index() for supported transport particles.
+extern array<double, 4> energy_min;
+extern array<double, 4> energy_max;
 
 //! Minimum temperature in [K] that nuclide data is available at
 extern double temperature_min;
diff --git a/include/openmc/output.h b/include/openmc/output.h
index 940ea78ce..0ad8b2fe5 100644
--- a/include/openmc/output.h
+++ b/include/openmc/output.h
@@ -10,6 +10,8 @@
 
 namespace openmc {
 
+extern "C" const bool STRICT_FP_ENABLED;
+
 //! \brief Display the main title banner as well as information about the
 //! program developers, version, and date/time which the problem was run.
 void title();
@@ -60,7 +62,6 @@ void write_tallies();
 void show_time(const char* label, double secs, int indent_level = 0);
 
 } // namespace openmc
-#endif // OPENMC_OUTPUT_H
 
 //////////////////////////////////////
 // Custom formatters
@@ -87,3 +88,5 @@ struct formatter<std::array<T, 2>> {
 }; // namespace fmt
 
 } // namespace fmt
+
+#endif // OPENMC_OUTPUT_H
diff --git a/include/openmc/particle.h b/include/openmc/particle.h
index 0f37719b9..8db9721ba 100644
--- a/include/openmc/particle.h
+++ b/include/openmc/particle.h
@@ -39,6 +39,8 @@ public:
 
   double speed() const;
 
+  double mass() const;
+
   //! create a secondary particle
   //
   //! stores the current phase space attributes of the particle in the
@@ -69,7 +71,8 @@ public:
   void event_advance();
   void event_cross_surface();
   void event_collide();
-  void event_revive_from_secondary();
+  void event_revive_from_secondary(const SourceSite& site);
+  void event_check_limit_and_revive();
   void event_death();
 
   //! pulse-height recording
@@ -126,10 +129,6 @@ public:
 //! Functions
 //============================================================================
 
-std::string particle_type_to_str(ParticleType type);
-
-ParticleType str_to_particle_type(std::string str);
-
 void add_surf_source_to_bank(Particle& p, const Surface& surf);
 
 } // namespace openmc
diff --git a/include/openmc/particle_data.h b/include/openmc/particle_data.h
index 15ae57893..44e82fd23 100644
--- a/include/openmc/particle_data.h
+++ b/include/openmc/particle_data.h
@@ -3,12 +3,13 @@
 
 #include "openmc/array.h"
 #include "openmc/constants.h"
+#include "openmc/particle_type.h"
 #include "openmc/position.h"
 #include "openmc/random_lcg.h"
 #include "openmc/tallies/filter_match.h"
 #include "openmc/vector.h"
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 #include "DagMC.hpp"
 #endif
 
@@ -30,9 +31,6 @@ constexpr double CACHE_INVALID {-1.0};
 //==========================================================================
 // Aliases and type definitions
 
-//! Particle types
-enum class ParticleType { neutron, photon, electron, positron };
-
 //! Saved ("banked") state of a particle
 //! NOTE: This structure's MPI type is built in initialize_mpi() of
 //! initialize.cpp. Any changes made to the struct here must also be
@@ -47,11 +45,33 @@ struct SourceSite {
   double time {0.0};
   double wgt {1.0};
   int delayed_group {0};
-  int surf_id {0};
+  int surf_id {SURFACE_NONE};
   ParticleType particle;
 
   // Extra attributes that don't show up in source written to file
   int parent_nuclide {-1};
+  int64_t parent_id {0};
+  int64_t progeny_id {0};
+  double wgt_born {1.0};
+  double wgt_ww_born {-1.0};
+  int64_t n_split {0};
+};
+
+struct CollisionTrackSite {
+  Position r;
+  Direction u;
+  double E;
+  double dE;
+  double time {0.0};
+  double wgt {1.0};
+  int event_mt {0};
+  int delayed_group {0};
+  int cell_id {0};
+  int nuclide_id;
+  int material_id {0};
+  int universe_id {0};
+  int n_collision {0};
+  ParticleType particle;
   int64_t parent_id;
   int64_t progeny_id;
 };
@@ -88,13 +108,37 @@ public:
   //! clear data from a single coordinate level
   void reset();
 
-  Position r;  //!< particle position
-  Direction u; //!< particle direction
-  int cell {-1};
-  int universe {-1};
-  int lattice {-1};
-  array<int, 3> lattice_i {{-1, -1, -1}};
-  bool rotated {false}; //!< Is the level rotated?
+  // accessors
+  Position& r() { return r_; }
+  const Position& r() const { return r_; }
+
+  Direction& u() { return u_; }
+  const Direction& u() const { return u_; }
+
+  int& cell() { return cell_; }
+  const int& cell() const { return cell_; }
+
+  int& universe() { return universe_; }
+  const int& universe() const { return universe_; }
+
+  int& lattice() { return lattice_; }
+  int lattice() const { return lattice_; }
+
+  array<int, 3>& lattice_index() { return lattice_index_; }
+  const array<int, 3>& lattice_index() const { return lattice_index_; }
+
+  bool& rotated() { return rotated_; }
+  const bool& rotated() const { return rotated_; }
+
+private:
+  // Data members
+  Position r_;  //!< particle position
+  Direction u_; //!< particle direction
+  int cell_ {-1};
+  int universe_ {-1};
+  int lattice_ {-1};
+  array<int, 3> lattice_index_ {{-1, -1, -1}};
+  bool rotated_ {false}; //!< Is the level rotated?
 };
 
 //==============================================================================
@@ -130,9 +174,10 @@ struct NuclideMicroXS {
 
   // Energy and temperature last used to evaluate these cross sections.  If
   // these values have changed, then the cross sections must be re-evaluated.
-  double last_E {0.0};      //!< Last evaluated energy
-  double last_sqrtkT {0.0}; //!< Last temperature in sqrt(Boltzmann constant
-                            //!< * temperature (eV))
+  double last_E {0.0};       //!< Last evaluated energy
+  double last_sqrtkT {0.0};  //!< Last temperature in sqrt(Boltzmann constant
+                             //!< * temperature (eV))
+  double ncrystal_xs {-1.0}; //!< NCrystal cross section
 };
 
 //==============================================================================
@@ -186,23 +231,41 @@ struct CacheDataMG {
 // Information about nearest boundary crossing
 //==============================================================================
 
-struct BoundaryInfo {
-  double distance {INFINITY}; //!< distance to nearest boundary
-  int surface {
-    SURFACE_NONE}; //!< surface token, non-zero if boundary is surface
-  int coord_level; //!< coordinate level after crossing boundary
-  array<int, 3>
-    lattice_translation {}; //!< which way lattice indices will change
-
+class BoundaryInfo {
+public:
   void reset()
   {
-    distance = INFINITY;
-    surface = SURFACE_NONE;
-    coord_level = 0;
-    lattice_translation = {0, 0, 0};
+    distance_ = INFINITY;
+    surface_ = SURFACE_NONE;
+    coord_level_ = 0;
+    lattice_translation_ = {0, 0, 0};
   }
+  double& distance() { return distance_; }
+  const double& distance() const { return distance_; }
+
+  int& surface() { return surface_; }
+  const int& surface() const { return surface_; }
+
+  int coord_level() const { return coord_level_; }
+  int& coord_level() { return coord_level_; }
+
+  array<int, 3>& lattice_translation() { return lattice_translation_; }
+  const array<int, 3>& lattice_translation() const
+  {
+    return lattice_translation_;
+  }
+
   // TODO: off-by-one
-  int surface_index() const { return std::abs(surface) - 1; }
+  int surface_index() const { return std::abs(surface()) - 1; }
+
+private:
+  // Data members
+  double distance_ {INFINITY}; //!< distance to nearest boundary
+  int surface_ {
+    SURFACE_NONE};      //!< surface token, non-zero if boundary is surface
+  int coord_level_ {0}; //!< coordinate level after crossing boundary
+  array<int, 3> lattice_translation_ {
+    0, 0, 0}; //!< which way lattice indices will change
 };
 
 /*
@@ -301,20 +364,20 @@ public:
   const Position& u_last() const { return u_last_; }
 
   // Accessors for position in global coordinates
-  Position& r() { return coord_[0].r; }
-  const Position& r() const { return coord_[0].r; }
+  Position& r() { return coord_[0].r(); }
+  const Position& r() const { return coord_[0].r(); }
 
   // Accessors for position in local coordinates
-  Position& r_local() { return coord_[n_coord_ - 1].r; }
-  const Position& r_local() const { return coord_[n_coord_ - 1].r; }
+  Position& r_local() { return coord_[n_coord_ - 1].r(); }
+  const Position& r_local() const { return coord_[n_coord_ - 1].r(); }
 
   // Accessors for direction in global coordinates
-  Direction& u() { return coord_[0].u; }
-  const Direction& u() const { return coord_[0].u; }
+  Direction& u() { return coord_[0].u(); }
+  const Direction& u() const { return coord_[0].u(); }
 
   // Accessors for direction in local coordinates
-  Direction& u_local() { return coord_[n_coord_ - 1].u; }
-  const Direction& u_local() const { return coord_[n_coord_ - 1].u; }
+  Direction& u_local() { return coord_[n_coord_ - 1].u(); }
+  const Direction& u_local() const { return coord_[n_coord_ - 1].u(); }
 
   // Surface token for the surface that the particle is currently on
   int& surface() { return surface_; }
@@ -330,7 +393,7 @@ public:
   // Boundary information
   BoundaryInfo& boundary() { return boundary_; }
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   // DagMC state variables
   moab::DagMC::RayHistory& history() { return history_; }
   Direction& last_dir() { return last_dir_; }
@@ -347,6 +410,11 @@ public:
   const double& sqrtkT() const { return sqrtkT_; }
   double& sqrtkT_last() { return sqrtkT_last_; }
 
+  // density multiplier of the current and last cell
+  double& density_mult() { return density_mult_; }
+  const double& density_mult() const { return density_mult_; }
+  double& density_mult_last() { return density_mult_last_; }
+
 private:
   int64_t id_ {-1}; //!< Unique ID
 
@@ -375,7 +443,10 @@ private:
   double sqrtkT_ {-1.0};     //!< sqrt(k_Boltzmann * temperature) in eV
   double sqrtkT_last_ {0.0}; //!< last temperature
 
-#ifdef DAGMC
+  double density_mult_ {1.0};      //!< density multiplier
+  double density_mult_last_ {1.0}; //!< last density multiplier
+
+#ifdef OPENMC_DAGMC_ENABLED
   moab::DagMC::RayHistory history_;
   Direction last_dir_;
 #endif
@@ -426,7 +497,7 @@ private:
   MacroXS macro_xs_;
   CacheDataMG mg_xs_cache_;
 
-  ParticleType type_ {ParticleType::neutron};
+  ParticleType type_;
 
   double E_;
   double E_last_;
@@ -435,6 +506,7 @@ private:
 
   double wgt_ {1.0};
   double wgt_born_ {1.0};
+  double wgt_ww_born_ {-1.0};
   double mu_;
   double time_ {0.0};
   double time_last_ {0.0};
@@ -454,6 +526,9 @@ private:
 
   int cell_born_ {-1};
 
+  // Iterated Fission Probability
+  double lifetime_ {0.0}; //!< neutron lifetime [s]
+
   int n_collision_ {0};
 
   bool write_track_ {false};
@@ -461,9 +536,14 @@ private:
   uint64_t seeds_[N_STREAMS];
   int stream_;
 
-  vector<SourceSite> secondary_bank_;
+  vector<SourceSite> local_secondary_bank_;
 
-  int64_t current_work_;
+  // Keep track of how many secondary particles were created in the collision
+  // and what the starting index is in the secondary bank for this particle
+  int n_secondaries_ {0};
+  int secondary_bank_index_ {0};
+
+  int64_t current_work_ {0};
 
   vector<double> flux_derivs_;
 
@@ -486,7 +566,9 @@ private:
 
   int n_event_ {0};
 
-  int n_split_ {0};
+  int64_t n_tracks_ {0}; //!< number of tracks in this particle history
+
+  int64_t n_split_ {0};
   double ww_factor_ {0.0};
 
   int64_t n_progeny_ {0};
@@ -500,10 +582,8 @@ public:
   // Methods and accessors
 
   // Cross section caches
-  NuclideMicroXS& neutron_xs(int i)
-  {
-    return neutron_xs_[i];
-  } // Microscopic neutron cross sections
+  // Microscopic neutron cross sections
+  NuclideMicroXS& neutron_xs(int i) { return neutron_xs_[i]; }
   const NuclideMicroXS& neutron_xs(int i) const { return neutron_xs_[i]; }
 
   // Microscopic photon cross sections
@@ -542,6 +622,10 @@ public:
   double& wgt_born() { return wgt_born_; }
   double wgt_born() const { return wgt_born_; }
 
+  // Weight window value at birth
+  double& wgt_ww_born() { return wgt_ww_born_; }
+  const double& wgt_ww_born() const { return wgt_ww_born_; }
+
   // Statistic weight of particle at last collision
   double& wgt_last() { return wgt_last_; }
   const double& wgt_last() const { return wgt_last_; }
@@ -560,14 +644,20 @@ public:
   double& time_last() { return time_last_; }
   const double& time_last() const { return time_last_; }
 
+  // Particle lifetime
+  double& lifetime() { return lifetime_; }
+  const double& lifetime() const { return lifetime_; }
+
   // What event took place, described in greater detail below
   TallyEvent& event() { return event_; }
   const TallyEvent& event() const { return event_; }
   bool& fission() { return fission_; }            // true if implicit fission
   int& event_nuclide() { return event_nuclide_; } // index of collision nuclide
   const int& event_nuclide() const { return event_nuclide_; }
-  int& event_mt() { return event_mt_; }           // MT number of collision
+  int& event_mt() { return event_mt_; } // MT number of collision
+  const int& event_mt() const { return event_mt_; }
   int& delayed_group() { return delayed_group_; } // delayed group
+  const int& delayed_group() const { return delayed_group_; }
   const int& parent_nuclide() const { return parent_nuclide_; }
   int& parent_nuclide() { return parent_nuclide_; } // Parent nuclide
 
@@ -606,8 +696,23 @@ public:
   int& stream() { return stream_; }
 
   // secondary particle bank
-  SourceSite& secondary_bank(int i) { return secondary_bank_[i]; }
-  decltype(secondary_bank_)& secondary_bank() { return secondary_bank_; }
+  SourceSite& local_secondary_bank(int i) { return local_secondary_bank_[i]; }
+  const SourceSite& local_secondary_bank(int i) const
+  {
+    return local_secondary_bank_[i];
+  }
+  decltype(local_secondary_bank_)& local_secondary_bank()
+  {
+    return local_secondary_bank_;
+  }
+
+  // Number of secondaries created in a collision
+  int& n_secondaries() { return n_secondaries_; }
+  const int& n_secondaries() const { return n_secondaries_; }
+
+  // Starting index in secondary bank for this collision
+  int& secondary_bank_index() { return secondary_bank_index_; }
+  const int& secondary_bank_index() const { return secondary_bank_index_; }
 
   // Current simulation work index
   int64_t& current_work() { return current_work_; }
@@ -647,13 +752,16 @@ public:
   int& n_event() { return n_event_; }
 
   // Number of times variance reduction has caused a particle split
-  int n_split() const { return n_split_; }
-  int& n_split() { return n_split_; }
+  int64_t n_split() const { return n_split_; }
+  int64_t& n_split() { return n_split_; }
 
   // Particle-specific factor for on-the-fly weight window adjustment
   double ww_factor() const { return ww_factor_; }
   double& ww_factor() { return ww_factor_; }
 
+  // Number of tracks in this particle history
+  int64_t& n_tracks() { return n_tracks_; }
+
   // Number of progeny produced by this particle
   int64_t& n_progeny() { return n_progeny_; }
 
diff --git a/include/openmc/particle_type.h b/include/openmc/particle_type.h
new file mode 100644
index 000000000..0fd896c57
--- /dev/null
+++ b/include/openmc/particle_type.h
@@ -0,0 +1,184 @@
+//==============================================================================
+// ParticleType class definition
+//==============================================================================
+
+#ifndef OPENMC_PARTICLE_TYPE_H
+#define OPENMC_PARTICLE_TYPE_H
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "openmc/constants.h"
+#include "openmc/error.h"
+
+namespace openmc {
+
+//------------------------------------------------------------------------------
+// PDG constants (canonical particle identity as simple integers)
+//------------------------------------------------------------------------------
+
+inline constexpr int32_t PDG_NEUTRON = 2112;
+inline constexpr int32_t PDG_PHOTON = 22;
+inline constexpr int32_t PDG_ELECTRON = 11;
+inline constexpr int32_t PDG_POSITRON = -11;
+inline constexpr int32_t PDG_PROTON = 2212;
+inline constexpr int32_t PDG_DEUTERON = 1000010020;
+inline constexpr int32_t PDG_TRITON = 1000010030;
+inline constexpr int32_t PDG_ALPHA = 1000020040;
+
+//------------------------------------------------------------------------------
+// ParticleType class (standard-layout, trivially copyable)
+//------------------------------------------------------------------------------
+
+class ParticleType {
+public:
+  //----------------------------------------------------------------------------
+  // Constructors
+
+  // Default constructor: defaults to neutron
+  constexpr ParticleType() : pdg_number_(PDG_NEUTRON) {}
+
+  // Constructor from PDG number
+  constexpr explicit ParticleType(int32_t pdg_number) : pdg_number_(pdg_number)
+  {}
+
+  // Constructor from particle name string (e.g., "neutron", "photon", "Fe56")
+  explicit ParticleType(std::string_view str);
+
+  // Constructor from Z, A, and metastable state for nuclear particles
+  constexpr ParticleType(int Z, int A, int m = 0)
+    : pdg_number_(1000000000 + Z * 10000 + A * 10 + m)
+  {}
+
+  //----------------------------------------------------------------------------
+  // Accessors
+
+  // Accessor for the underlying PDG number
+  constexpr int32_t pdg_number() const { return pdg_number_; }
+
+  //----------------------------------------------------------------------------
+  // Methods
+
+  // Get particle mass in [u]
+  double mass() const
+  {
+    int32_t p = std::abs(pdg_number_);
+    if (ATOMIC_MASS.count(p)) {
+      return ATOMIC_MASS[p];
+    } else {
+      fatal_error("Unknown mass for particle " + str());
+    }
+  }
+
+  // Convert to string representation
+  std::string str() const;
+
+  // Check if this represents a nucleus (vs elementary particle)
+  constexpr bool is_nucleus() const
+  {
+    // PDG nuclear codes are >= 1000000000 (100ZZZAAAI format)
+    return pdg_number_ >= 1000000000;
+  }
+
+  // Get transport index (0-3 for transportable particles, C_NONE otherwise)
+  constexpr int transport_index() const;
+
+  // Check if this is a neutron
+  constexpr bool is_neutron() const { return pdg_number_ == PDG_NEUTRON; }
+
+  // Check if this is a photon
+  constexpr bool is_photon() const { return pdg_number_ == PDG_PHOTON; }
+
+  constexpr bool is_transportable() const
+  {
+    return this->transport_index() != C_NONE;
+  }
+
+  //----------------------------------------------------------------------------
+  // Static factory methods
+
+  static constexpr ParticleType neutron() { return ParticleType {PDG_NEUTRON}; }
+  static constexpr ParticleType photon() { return ParticleType {PDG_PHOTON}; }
+  static constexpr ParticleType electron()
+  {
+    return ParticleType {PDG_ELECTRON};
+  }
+  static constexpr ParticleType positron()
+  {
+    return ParticleType {PDG_POSITRON};
+  }
+  static constexpr ParticleType proton() { return ParticleType {PDG_PROTON}; }
+  static constexpr ParticleType deuteron()
+  {
+    return ParticleType {PDG_DEUTERON};
+  }
+  static constexpr ParticleType triton() { return ParticleType {PDG_TRITON}; }
+  static constexpr ParticleType alpha() { return ParticleType {PDG_ALPHA}; }
+
+private:
+  int32_t pdg_number_;
+};
+
+//------------------------------------------------------------------------------
+// Static assertions to ensure standard-layout and trivially copyable
+//------------------------------------------------------------------------------
+
+static_assert(std::is_standard_layout_v<ParticleType>,
+  "ParticleType must be standard-layout");
+static_assert(std::is_trivially_copyable_v<ParticleType>,
+  "ParticleType must be trivially copyable");
+static_assert(sizeof(ParticleType) == sizeof(int32_t),
+  "ParticleType must be same size as int32_t");
+
+//------------------------------------------------------------------------------
+// Comparison operators (free functions for symmetry)
+//------------------------------------------------------------------------------
+
+constexpr bool operator==(ParticleType lhs, ParticleType rhs)
+{
+  return lhs.pdg_number() == rhs.pdg_number();
+}
+
+constexpr bool operator!=(ParticleType lhs, ParticleType rhs)
+{
+  return lhs.pdg_number() != rhs.pdg_number();
+}
+
+constexpr bool operator<(ParticleType lhs, ParticleType rhs)
+{
+  return lhs.pdg_number() < rhs.pdg_number();
+}
+
+//------------------------------------------------------------------------------
+// ParticleType member function implementations (inline)
+//------------------------------------------------------------------------------
+
+constexpr int ParticleType::transport_index() const
+{
+  switch (pdg_number_) {
+  case PDG_NEUTRON:
+    return 0;
+  case PDG_PHOTON:
+    return 1;
+  case PDG_ELECTRON:
+    return 2;
+  case PDG_POSITRON:
+    return 3;
+  default:
+    return C_NONE;
+  }
+}
+
+//------------------------------------------------------------------------------
+// Legacy conversion helpers
+//------------------------------------------------------------------------------
+
+// Legacy enum code (0..3) to ParticleType conversion
+ParticleType legacy_particle_index_to_type(int code);
+
+} // namespace openmc
+
+#endif // OPENMC_PARTICLE_TYPE_H
diff --git a/include/openmc/photon.h b/include/openmc/photon.h
index 1fee6c9f5..93c6dba53 100644
--- a/include/openmc/photon.h
+++ b/include/openmc/photon.h
@@ -6,7 +6,7 @@
 #include "openmc/particle.h"
 #include "openmc/vector.h"
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 #include <hdf5.h>
 
 #include <string>
@@ -33,7 +33,6 @@ public:
 
   int index_subshell; //!< index in SUBSHELLS
   int threshold;
-  double n_electrons;
   double binding_energy;
   vector<Transition> transitions;
 };
@@ -63,14 +62,14 @@ public:
   int64_t index_;    //!< Index in global elements vector
 
   // Microscopic cross sections
-  xt::xtensor<double, 1> energy_;
-  xt::xtensor<double, 1> coherent_;
-  xt::xtensor<double, 1> incoherent_;
-  xt::xtensor<double, 1> photoelectric_total_;
-  xt::xtensor<double, 1> pair_production_total_;
-  xt::xtensor<double, 1> pair_production_electron_;
-  xt::xtensor<double, 1> pair_production_nuclear_;
-  xt::xtensor<double, 1> heating_;
+  tensor::Tensor<double> energy_;
+  tensor::Tensor<double> coherent_;
+  tensor::Tensor<double> incoherent_;
+  tensor::Tensor<double> photoelectric_total_;
+  tensor::Tensor<double> pair_production_total_;
+  tensor::Tensor<double> pair_production_electron_;
+  tensor::Tensor<double> pair_production_nuclear_;
+  tensor::Tensor<double> heating_;
 
   // Form factors
   Tabulated1D incoherent_form_factor_;
@@ -82,22 +81,27 @@ public:
   // stored separately to improve memory access pattern when calculating the
   // total cross section
   vector<ElectronSubshell> shells_;
-  xt::xtensor<double, 2> cross_sections_;
+  tensor::Tensor<double> cross_sections_;
 
   // Compton profile data
-  xt::xtensor<double, 2> profile_pdf_;
-  xt::xtensor<double, 2> profile_cdf_;
-  xt::xtensor<double, 1> binding_energy_;
-  xt::xtensor<double, 1> electron_pdf_;
+  tensor::Tensor<double> profile_pdf_;
+  tensor::Tensor<double> profile_cdf_;
+  tensor::Tensor<double> binding_energy_;
+  tensor::Tensor<double> electron_pdf_;
+
+  // Map subshells from Compton profile data obtained from Biggs et al,
+  // "Hartree-Fock Compton profiles for the elements" to ENDF/B atomic
+  // relaxation data
+  tensor::Tensor<int> subshell_map_;
 
   // Stopping power data
   double I_; // mean excitation energy
-  xt::xtensor<int, 1> n_electrons_;
-  xt::xtensor<double, 1> ionization_energy_;
-  xt::xtensor<double, 1> stopping_power_radiative_;
+  tensor::Tensor<int> n_electrons_;
+  tensor::Tensor<double> ionization_energy_;
+  tensor::Tensor<double> stopping_power_radiative_;
 
   // Bremsstrahlung scaled DCS
-  xt::xtensor<double, 2> dcs_;
+  tensor::Tensor<double> dcs_;
 
   // Whether atomic relaxation data is present
   bool has_atomic_relaxation_ {false};
@@ -133,7 +137,7 @@ void free_memory_photon();
 
 namespace data {
 
-extern xt::xtensor<double, 1>
+extern tensor::Tensor<double>
   compton_profile_pz; //! Compton profile momentum grid
 
 //! Photon interaction data for each element
diff --git a/include/openmc/physics.h b/include/openmc/physics.h
index f62f43a02..2472d9799 100644
--- a/include/openmc/physics.h
+++ b/include/openmc/physics.h
@@ -10,13 +10,6 @@
 
 namespace openmc {
 
-//==============================================================================
-// Constants
-//==============================================================================
-
-// Monoatomic ideal-gas scattering treatment threshold
-constexpr double FREE_GAS_THRESHOLD {400.0};
-
 //==============================================================================
 // Non-member functions
 //==============================================================================
diff --git a/include/openmc/physics_common.h b/include/openmc/physics_common.h
index e38a3c7f8..b0e395c1e 100644
--- a/include/openmc/physics_common.h
+++ b/include/openmc/physics_common.h
@@ -13,5 +13,9 @@ namespace openmc {
 //! \param[in] weight_survive Weight assigned to particles that survive
 void russian_roulette(Particle& p, double weight_survive);
 
+//! \brief Performs the global russian roulette operation on a particle
+//! \param[in,out] p  Particle object
+void apply_russian_roulette(Particle& p);
+
 } // namespace openmc
 #endif // OPENMC_PHYSICS_COMMON_H
diff --git a/include/openmc/plot.h b/include/openmc/plot.h
index 69e801c5f..ba8ac84a1 100644
--- a/include/openmc/plot.h
+++ b/include/openmc/plot.h
@@ -5,9 +5,10 @@
 #include <sstream>
 #include <unordered_map>
 #include <unordered_set>
+#include <vector>
 
+#include "openmc/tensor.h"
 #include "pugixml.hpp"
-#include "xtensor/xarray.hpp"
 
 #include "hdf5.h"
 #include "openmc/cell.h"
@@ -17,6 +18,9 @@
 #include "openmc/particle.h"
 #include "openmc/position.h"
 #include "openmc/random_lcg.h"
+#include "openmc/ray.h"
+#include "openmc/tallies/filter.h"
+#include "openmc/tallies/filter_match.h"
 #include "openmc/xml_interface.h"
 
 namespace openmc {
@@ -83,19 +87,27 @@ const RGBColor BLACK {0, 0, 0};
  * \class PlottableInterface
  * \brief Interface for plottable objects.
  *
- * PlottableInterface classes must have a unique ID in the plots.xml file.
- * They guarantee the ability to create output in some form. This interface
- * is designed to be implemented by classes that produce plot-relevant data
- * which can be visualized.
+ * PlottableInterface classes must have unique IDs. If no ID (or -1) is
+ * provided, the next available ID is assigned automatically. They guarantee
+ * the ability to create output in some form. This interface is designed to be
+ * implemented by classes that produce plot-relevant data which can be
+ * visualized.
  */
+
+typedef tensor::Tensor<RGBColor> ImageData;
 class PlottableInterface {
+public:
+  PlottableInterface() = default;
+
+  void set_default_colors();
+
 private:
   void set_id(pugi::xml_node plot_node);
-  int id_; // unique plot ID
+  int id_ {C_NONE}; // unique plot ID
 
   void set_bg_color(pugi::xml_node plot_node);
   void set_universe(pugi::xml_node plot_node);
-  void set_default_colors(pugi::xml_node plot_node);
+  void set_color_by(pugi::xml_node plot_node);
   void set_user_colors(pugi::xml_node plot_node);
   void set_overlap_color(pugi::xml_node plot_node);
   void set_mask(pugi::xml_node plot_node);
@@ -107,52 +119,77 @@ protected:
 public:
   enum class PlotColorBy { cells = 0, mats = 1 };
 
+  // Generates image data based on plot parameters and returns it
+  virtual ImageData create_image() const = 0;
+
   // Creates the output image named path_plot_
   virtual void create_output() const = 0;
 
+  // Write populated image data to file
+  void write_image(const ImageData& data) const;
+
   // Print useful info to the terminal
   virtual void print_info() const = 0;
 
   const std::string& path_plot() const { return path_plot_; }
   std::string& path_plot() { return path_plot_; }
   int id() const { return id_; }
+  void set_id(int id = C_NONE);
   int level() const { return level_; }
+  PlotColorBy color_by() const { return color_by_; }
 
   // Public color-related data
   PlottableInterface(pugi::xml_node plot_node);
   virtual ~PlottableInterface() = default;
-  int level_;                    // Universe level to plot
-  bool color_overlaps_;          // Show overlapping cells?
-  PlotColorBy color_by_;         // Plot coloring (cell/material)
-  RGBColor not_found_ {WHITE};   // Plot background color
-  RGBColor overlap_color_ {RED}; // Plot overlap color
-  vector<RGBColor> colors_;      // Plot colors
+  int level_ {-1};                           // Universe level to plot
+  bool color_overlaps_ {false};              // Show overlapping cells?
+  PlotColorBy color_by_ {PlotColorBy::mats}; // Plot coloring (cell/material)
+  RGBColor not_found_ {WHITE};               // Plot background color
+  RGBColor overlap_color_ {RED};             // Plot overlap color
+  vector<RGBColor> colors_;                  // Plot colors
 };
 
-typedef xt::xtensor<RGBColor, 2> ImageData;
-
 struct IdData {
   // Constructor
-  IdData(size_t h_res, size_t v_res);
+  IdData(size_t h_res, size_t v_res, bool include_filter = false);
 
   // Methods
-  void set_value(size_t y, size_t x, const GeometryState& p, int level);
-  void set_overlap(size_t y, size_t x);
+  void set_value(size_t y, size_t x, const Particle& p, int level,
+    Filter* filter = nullptr, FilterMatch* match = nullptr);
+  void set_overlap(size_t y, size_t x, int overlap_idx);
 
   // Members
-  xt::xtensor<int32_t, 3> data_; //!< 2D array of cell & material ids
+  tensor::Tensor<int32_t> data_; //!< 2D array of cell & material ids
 };
 
 struct PropertyData {
   // Constructor
-  PropertyData(size_t h_res, size_t v_res);
+  PropertyData(size_t h_res, size_t v_res, bool include_filter = false);
 
   // Methods
-  void set_value(size_t y, size_t x, const GeometryState& p, int level);
-  void set_overlap(size_t y, size_t x);
+  void set_value(size_t y, size_t x, const Particle& p, int level,
+    Filter* filter = nullptr, FilterMatch* match = nullptr);
+  void set_overlap(size_t y, size_t x, int overlap_idx);
 
   // Members
-  xt::xtensor<double, 3> data_; //!< 2D array of temperature & density data
+  tensor::Tensor<double> data_; //!< 2D array of temperature & density data
+};
+
+struct RasterData {
+  // Constructor
+  RasterData(size_t h_res, size_t v_res, bool include_filter = false);
+
+  // Methods
+  void set_value(size_t y, size_t x, const Particle& p, int level,
+    Filter* filter = nullptr, FilterMatch* match = nullptr);
+  void set_overlap(size_t y, size_t x, int overlap_idx);
+
+  // Members
+  tensor::Tensor<int32_t>
+    id_data_; //!< [v_res, h_res, 3 or 4]: cell, instance, mat, [filter_bin]
+  tensor::Tensor<double>
+    property_data_;     //!< [v_res, h_res, 2]: temperature, density
+  bool include_filter_; //!< Whether filter bin index is included
 };
 
 //===============================================================================
@@ -162,76 +199,76 @@ struct PropertyData {
 class SlicePlotBase {
 public:
   template<class T>
-  T get_map() const;
+  T get_map(int32_t filter_index = -1) const;
 
   enum class PlotBasis { xy = 1, xz = 2, yz = 3 };
 
+  // Accessors
+
+  const std::array<size_t, 3>& pixels() const { return pixels_; }
+  std::array<size_t, 3>& pixels() { return pixels_; }
+
   // Members
 public:
-  Position origin_;           //!< Plot origin in geometry
-  Position width_;            //!< Plot width in geometry
-  PlotBasis basis_;           //!< Plot basis (XY/XZ/YZ)
-  array<size_t, 3> pixels_;   //!< Plot size in pixels
-  bool slice_color_overlaps_; //!< Show overlapping cells?
-  int slice_level_ {-1};      //!< Plot universe level
+  Position origin_;         //!< Plot origin in geometry
+  Direction u_span_;        //!< Full-width span vector in geometry
+  Direction v_span_;        //!< Full-height span vector in geometry
+  array<size_t, 3> pixels_; //!< Plot size in pixels
+  bool show_overlaps_;      //!< Show overlapping cells?
+  int slice_level_ {-1};    //!< Plot universe level
 private:
 };
 
 template<class T>
-T SlicePlotBase::get_map() const
+T SlicePlotBase::get_map(int32_t filter_index) const
 {
 
   size_t width = pixels_[0];
   size_t height = pixels_[1];
 
-  // get pixel size
-  double in_pixel = (width_[0]) / static_cast<double>(width);
-  double out_pixel = (width_[1]) / static_cast<double>(height);
-
-  // size data array
-  T data(width, height);
-
-  // setup basis indices and initial position centered on pixel
-  int in_i, out_i;
-  Position xyz = origin_;
-  switch (basis_) {
-  case PlotBasis::xy:
-    in_i = 0;
-    out_i = 1;
-    break;
-  case PlotBasis::xz:
-    in_i = 0;
-    out_i = 2;
-    break;
-  case PlotBasis::yz:
-    in_i = 1;
-    out_i = 2;
-    break;
-  default:
-    UNREACHABLE();
+  // Determine if filter is being used
+  bool include_filter = (filter_index >= 0);
+  Filter* filter = nullptr;
+  if (include_filter) {
+    filter = model::tally_filters[filter_index].get();
   }
 
-  // set initial position
-  xyz[in_i] = origin_[in_i] - width_[0] / 2. + in_pixel / 2.;
-  xyz[out_i] = origin_[out_i] + width_[1] / 2. - out_pixel / 2.;
+  // size data array
+  T data(width, height, include_filter);
 
-  // arbitrary direction
-  Direction dir = {1. / std::sqrt(2.), 1. / std::sqrt(2.), 0.0};
+  // compute pixel steps and top-left pixel center
+  Direction u_step = u_span_ / static_cast<double>(width);
+  Direction v_step = v_span_ / static_cast<double>(height);
+
+  Position start =
+    origin_ - 0.5 * u_span_ + 0.5 * v_span_ + 0.5 * u_step - 0.5 * v_step;
+
+  // Validate that span vectors define a valid plane
+  Position cross = u_span_.cross(v_span_);
+  if (cross.norm() == 0.0) {
+    fatal_error("Slice span vectors are invalid (zero area).");
+  }
+
+  // Use an arbitrary direction that is not aligned with any coordinate axis.
+  // The direction has no physical meaning for plotting but is used by
+  // Surface::sense() to break ties when a pixel is coincident with a surface.
+  Direction dir = {1.0 / std::sqrt(2.0), 1.0 / std::sqrt(2.0), 0.0};
 
 #pragma omp parallel
   {
-    GeometryState p;
-    p.r() = xyz;
+    Particle p;
+    p.r() = start;
     p.u() = dir;
-    p.coord(0).universe = model::root_universe;
+    p.coord(0).universe() = model::root_universe;
     int level = slice_level_;
     int j {};
+    FilterMatch match;
 
 #pragma omp for
     for (int y = 0; y < height; y++) {
-      p.r()[out_i] = xyz[out_i] - out_pixel * y;
+      Position row = start - v_step * static_cast<double>(y);
       for (int x = 0; x < width; x++) {
-        p.r()[in_i] = xyz[in_i] + in_pixel * x;
+        p.r() = row + u_step * static_cast<double>(x);
         p.n_coord() = 1;
         // local variables
         bool found_cell = exhaustive_find_cell(p);
@@ -240,10 +277,13 @@ T SlicePlotBase::get_map() const
           j = level;
         }
         if (found_cell) {
-          data.set_value(y, x, p, j);
+          data.set_value(y, x, p, j, filter, &match);
         }
-        if (slice_color_overlaps_ && check_cell_overlap(p, false)) {
-          data.set_overlap(y, x);
+        if (show_overlaps_) {
+          int overlap_idx = check_cell_overlap(p, false);
+          if (overlap_idx >= 0) {
+            data.set_overlap(y, x, overlap_idx);
+          }
         }
       } // inner for
     }
@@ -270,13 +310,15 @@ private:
 public:
   // Add mesh lines to ImageData
   void draw_mesh_lines(ImageData& data) const;
-  void create_image() const;
+  ImageData create_image() const override;
   void create_voxel() const;
 
-  virtual void create_output() const;
-  virtual void print_info() const;
+  void create_output() const override;
+  void print_info() const override;
 
   PlotType type_;                 //!< Plot type (Slice/Voxel)
+  Position width_;                //!< Axis-aligned width from plot.xml
+  PlotBasis basis_;               //!< Basis from plot.xml for slice plots
   int meshlines_width_;           //!< Width of lines added to the plot
   int index_meshlines_mesh_ {-1}; //!< Index of the mesh to draw on the plot
   RGBColor meshlines_color_;      //!< Color of meshlines on the plot
@@ -294,17 +336,32 @@ public:
  */
 class RayTracePlot : public PlottableInterface {
 public:
+  RayTracePlot() = default;
   RayTracePlot(pugi::xml_node plot);
 
   // Standard getters. No setting since it's done from XML.
   const Position& camera_position() const { return camera_position_; }
+  Position& camera_position() { return camera_position_; }
   const Position& look_at() const { return look_at_; }
+  Position& look_at() { return look_at_; }
+
   const double& horizontal_field_of_view() const
   {
     return horizontal_field_of_view_;
   }
+  double& horizontal_field_of_view() { return horizontal_field_of_view_; }
 
-  virtual void print_info() const;
+  void print_info() const override;
+
+  const std::array<int, 2>& pixels() const { return pixels_; }
+  std::array<int, 2>& pixels() { return pixels_; }
+
+  const Direction& up() const { return up_; }
+  Direction& up() { return up_; }
+
+  //! brief Updates the cached camera-to-model matrix after changes to
+  //! camera parameters.
+  void update_view();
 
 protected:
   Direction camera_x_axis() const
@@ -330,8 +387,6 @@ protected:
    */
   std::pair<Position, Direction> get_pixel_ray(int horiz, int vert) const;
 
-  std::array<int, 2> pixels_; // pixel dimension of resulting image
-
 private:
   void set_look_at(pugi::xml_node node);
   void set_camera_position(pugi::xml_node node);
@@ -341,9 +396,9 @@ private:
 
   double horizontal_field_of_view_ {70.0}; // horiz. f.o.v. in degrees
   Position camera_position_;               // where camera is
-  Position look_at_; // point camera is centered looking at
-
-  Direction up_ {0.0, 0.0, 1.0}; // which way is up
+  Position look_at_;                     // point camera is centered looking at
+  std::array<int, 2> pixels_ {100, 100}; // pixel dimension of resulting image
+  Direction up_ {0.0, 0.0, 1.0};         // which way is up
 
   /* The horizontal thickness, if using an orthographic projection.
    * If set to zero, we assume using a perspective projection.
@@ -377,8 +432,9 @@ class WireframeRayTracePlot : public RayTracePlot {
 public:
   WireframeRayTracePlot(pugi::xml_node plot);
 
-  virtual void create_output() const;
-  virtual void print_info() const;
+  ImageData create_image() const override;
+  void create_output() const override;
+  void print_info() const override;
 
 private:
   void set_opacities(pugi::xml_node node);
@@ -434,10 +490,22 @@ class SolidRayTracePlot : public RayTracePlot {
   friend class PhongRay;
 
 public:
+  SolidRayTracePlot() = default;
+
   SolidRayTracePlot(pugi::xml_node plot);
 
-  virtual void create_output() const;
-  virtual void print_info() const;
+  ImageData create_image() const override;
+  void create_output() const override;
+  void print_info() const override;
+
+  const std::unordered_set<int>& opaque_ids() const { return opaque_ids_; }
+  std::unordered_set<int>& opaque_ids() { return opaque_ids_; }
+
+  const Position& light_location() const { return light_location_; }
+  Position& light_location() { return light_location_; }
+
+  const double& diffuse_fraction() const { return diffuse_fraction_; }
+  double& diffuse_fraction() { return diffuse_fraction_; }
 
 private:
   void set_opaque_ids(pugi::xml_node node);
@@ -452,43 +520,6 @@ private:
   Position light_location_;
 };
 
-// Base class that implements ray tracing logic, not necessarily through
-// defined regions of the geometry but also outside of it.
-class Ray : public GeometryState {
-
-public:
-  Ray(Position r, Direction u) { init_from_r_u(r, u); }
-
-  // Called at every surface intersection within the model
-  virtual void on_intersection() = 0;
-
-  /*
-   * Traces the ray through the geometry, calling on_intersection
-   * at every surface boundary.
-   */
-  void trace();
-
-  // Stops the ray and exits tracing when called from on_intersection
-  void stop() { stop_ = true; }
-
-  // Sets the dist_ variable
-  void compute_distance();
-
-protected:
-  // Records how far the ray has traveled
-  double traversal_distance_ {0.0};
-
-private:
-  // Max intersections before we assume ray tracing is caught in an infinite
-  // loop:
-  static const int MAX_INTERSECTIONS = 1000000;
-
-  bool hit_something_ {false};
-  bool stop_ {false};
-
-  unsigned event_counter_ {0};
-};
-
 class ProjectionRay : public Ray {
 public:
   ProjectionRay(Position r, Direction u, const WireframeRayTracePlot& plot,
@@ -496,7 +527,7 @@ public:
     : Ray(r, u), plot_(plot), line_segments_(line_segments)
   {}
 
-  virtual void on_intersection() override;
+  void on_intersection() override;
 
 private:
   /* Store a reference to the plot object which is running this ray, in order
@@ -519,7 +550,7 @@ public:
     result_color_ = plot_.not_found_;
   }
 
-  virtual void on_intersection() override;
+  void on_intersection() override;
 
   const RGBColor& result_color() { return result_color_; }
 
diff --git a/include/openmc/random_ray/flat_source_domain.h b/include/openmc/random_ray/flat_source_domain.h
index 39d87d663..09414fd44 100644
--- a/include/openmc/random_ray/flat_source_domain.h
+++ b/include/openmc/random_ray/flat_source_domain.h
@@ -27,21 +27,23 @@ public:
 
   //----------------------------------------------------------------------------
   // Methods
-  virtual void update_neutron_source(double k_eff);
-  double compute_k_eff(double k_eff_old) const;
+  virtual void update_single_neutron_source(SourceRegionHandle& srh);
+  virtual void update_all_neutron_sources();
+  void compute_k_eff();
   virtual void normalize_scalar_flux_and_volumes(
     double total_active_distance_per_iteration);
 
   int64_t add_source_to_scalar_flux();
   virtual void batch_reset();
-  void convert_source_regions_to_tallies();
+  void convert_source_regions_to_tallies(int64_t start_sr_id);
   void reset_tally_volumes();
   void random_ray_tally();
   virtual void accumulate_iteration_flux();
   void output_to_vtk() const;
-  void convert_external_sources();
+  void convert_external_sources(bool use_adjoint_sources);
   void count_external_source_regions();
-  void set_adjoint_sources(const vector<double>& forward_flux);
+  void set_fw_adjoint_sources();
+  void set_local_adjoint_sources();
   void flux_swap();
   virtual double evaluate_flux_at_point(Position r, int64_t sr, int g) const;
   double compute_fixed_source_normalization_factor() const;
@@ -54,10 +56,10 @@ public:
     bool is_target_void);
   void apply_mesh_to_cell_and_children(int32_t i_cell, int32_t mesh_idx,
     int32_t target_material_id, bool is_target_void);
-  void prepare_base_source_regions();
   SourceRegionHandle get_subdivided_source_region_handle(
-    int64_t sr, int mesh_bin, Position r, double dist, Direction u);
+    SourceRegionKey sr_key, Position r, Direction u);
   void finalize_discovered_source_regions();
+  void apply_transport_stabilization();
   int64_t n_source_regions() const
   {
     return source_regions_.n_source_regions();
@@ -66,22 +68,36 @@ public:
   {
     return source_regions_.n_source_regions() * negroups_;
   }
+  int64_t lookup_base_source_region_idx(const GeometryState& p) const;
+  SourceRegionKey lookup_source_region_key(const GeometryState& p) const;
+  int64_t lookup_mesh_bin(int64_t sr, Position r) const;
+  int lookup_mesh_idx(int64_t sr) const;
 
   //----------------------------------------------------------------------------
   // Static Data members
   static bool volume_normalized_flux_tallies_;
-  static bool adjoint_; // If the user wants outputs based on the adjoint flux
+  // If the user wants outputs based on the adjoint flux
+  static bool adjoint_requested_;
+  // The solve currently being executed
+  static RandomRaySolve solve_;
+  static bool fw_cadis_local_;
+  static double
+    diagonal_stabilization_rho_; // Adjusts strength of diagonal stabilization
+                                 // for transport corrected MGXS data
 
   // Static variables to store source region meshes and domains
   static std::unordered_map<int, vector<std::pair<Source::DomainType, int>>>
     mesh_domain_map_;
 
+  static std::vector<size_t> fw_cadis_local_targets_;
+
   //----------------------------------------------------------------------------
   // Static data members
   static RandomRayVolumeEstimator volume_estimator_;
 
   //----------------------------------------------------------------------------
   // Public Data members
+  double k_eff_ {1.0};              // Eigenvalue
   bool mapped_all_tallies_ {false}; // If all source regions have been visited
 
   int64_t n_external_source_regions_ {0}; // Total number of source regions with
@@ -91,29 +107,23 @@ public:
   // in model::cells
   vector<int64_t> source_region_offsets_;
 
-  // 2D arrays stored in 1D representing values for all materials x energy
-  // groups
+  // 3D arrays stored in 1D representing values for all materials x temperature
+  // points x energy groups
   int n_materials_;
+  int ntemperature_;
   vector<double> sigma_t_;
   vector<double> nu_sigma_f_;
   vector<double> sigma_f_;
   vector<double> chi_;
+  vector<double> kappa_fission_;
 
-  // 3D arrays stored in 1D representing values for all materials x energy
-  // groups x energy groups
+  // 4D arrays stored in 1D representing values for all materials x temperature
+  // points x energy groups x energy groups
   vector<double> sigma_s_;
 
   // The abstract container holding all source region-specific data
   SourceRegionContainer source_regions_;
 
-  // Base source region container. When source region subdivision via mesh
-  // is in use, this container holds the original (non-subdivided) material
-  // filled cell instance source regions. These are useful as they can be
-  // initialized with external source and mesh domain information ahead of time.
-  // Then, dynamically discovered source regions can be initialized by cloning
-  // their base region.
-  SourceRegionContainer base_source_regions_;
-
   // Parallel hash map holding all source regions discovered during
   // a single iteration. This is a threadsafe data structure that is cleaned
   // out after each iteration and stored in the "source_regions_" container.
@@ -127,16 +137,36 @@ public:
   std::unordered_map<SourceRegionKey, int64_t, SourceRegionKey::HashFunctor>
     source_region_map_;
 
+  // Map that relates a SourceRegionKey to the external source index. This map
+  // is used to check if there are any point sources within a subdivided source
+  // region at the time it is discovered.
+  std::unordered_map<SourceRegionKey, vector<int>, SourceRegionKey::HashFunctor>
+    external_point_source_map_;
+
+  // Map that relates a base source region index to the external source index.
+  // This map is used to check if there are any volumetric sources within a
+  // subdivided source region at the time it is discovered.
+  std::unordered_map<int64_t, vector<int>> external_volumetric_source_map_;
+
+  // Map that relates a base source region index to a mesh index. This map
+  // is used to check which subdivision mesh is present in a source region.
+  std::unordered_map<int64_t, int> mesh_map_;
+
+  // If transport corrected MGXS data is being used, there may be negative
+  // in-group scattering cross sections that can result in instability in MOC
+  // and random ray if used naively. This flag enables a stabilization
+  // technique.
+  bool is_transport_stabilization_needed_ {false};
+
 protected:
   //----------------------------------------------------------------------------
   // Methods
   void apply_external_source_to_source_region(
-    Discrete* discrete, double strength_factor, int64_t sr);
-  void apply_external_source_to_cell_instances(int32_t i_cell,
-    Discrete* discrete, double strength_factor, int target_material_id,
-    const vector<int32_t>& instances);
-  void apply_external_source_to_cell_and_children(int32_t i_cell,
-    Discrete* discrete, double strength_factor, int32_t target_material_id);
+    int src_idx, SourceRegionHandle& srh);
+  void apply_external_source_to_cell_instances(int32_t i_cell, int src_idx,
+    int target_material_id, const vector<int32_t>& instances);
+  void apply_external_source_to_cell_and_children(
+    int32_t i_cell, int src_idx, int32_t target_material_id);
   virtual void set_flux_to_flux_plus_source(int64_t sr, double volume, int g);
   void set_flux_to_source(int64_t sr, int g);
   virtual void set_flux_to_old_flux(int64_t sr, int g);
@@ -149,13 +179,16 @@ protected:
     simulation_volume_; // Total physical volume of the simulation domain, as
                         // defined by the 3D box of the random ray source
 
+  double
+    fission_rate_; // The system's fission rate (per cm^3), in eigenvalue mode
+
   // Volumes for each tally and bin/score combination. This intermediate data
   // structure is used when tallying quantities that must be normalized by
   // volume (i.e., flux). The vector is index by tally index, while the inner 2D
-  // xtensor is indexed by bin index and score index in a similar manner to the
+  // tensor is indexed by bin index and score index in a similar manner to the
   // results tensor in the Tally class, though without the third dimension, as
   // SUM and SUM_SQ do not need to be tracked.
-  vector<xt::xtensor<double, 2>> tally_volumes_;
+  vector<tensor::Tensor<double>> tally_volumes_;
 
 }; // class FlatSourceDomain
 
diff --git a/include/openmc/random_ray/linear_source_domain.h b/include/openmc/random_ray/linear_source_domain.h
index 67fdd99f8..0098c7820 100644
--- a/include/openmc/random_ray/linear_source_domain.h
+++ b/include/openmc/random_ray/linear_source_domain.h
@@ -20,7 +20,7 @@ class LinearSourceDomain : public FlatSourceDomain {
 public:
   //----------------------------------------------------------------------------
   // Methods
-  void update_neutron_source(double k_eff) override;
+  void update_single_neutron_source(SourceRegionHandle& srh) override;
   void normalize_scalar_flux_and_volumes(
     double total_active_distance_per_iteration) override;
 
diff --git a/include/openmc/random_ray/random_ray.h b/include/openmc/random_ray/random_ray.h
index abf2a2688..b61d2d67a 100644
--- a/include/openmc/random_ray/random_ray.h
+++ b/include/openmc/random_ray/random_ray.h
@@ -41,6 +41,7 @@ public:
   uint64_t transport_history_based_single_ray();
   SourceSite sample_prng();
   SourceSite sample_halton();
+  SourceSite sample_s2();
 
   //----------------------------------------------------------------------------
   // Static data members
@@ -48,7 +49,6 @@ public:
   static double distance_active_;            // Active ray length
   static unique_ptr<Source> ray_source_;     // Starting source for ray sampling
   static RandomRaySourceShape source_shape_; // Flag for linear source
-  static bool mesh_subdivision_enabled_;     // Flag for mesh subdivision
   static RandomRaySampleMethod sample_method_; // Flag for sampling method
 
   //----------------------------------------------------------------------------
@@ -66,6 +66,7 @@ private:
   vector<double> mesh_fractional_lengths_;
 
   int negroups_;
+  int ntemperature_;
   FlatSourceDomain* domain_ {nullptr}; // pointer to domain that has flat source
                                        // data needed for ray transport
   double distance_travelled_ {0};
diff --git a/include/openmc/random_ray/random_ray_simulation.h b/include/openmc/random_ray/random_ray_simulation.h
index b94e7401b..e186c549f 100644
--- a/include/openmc/random_ray/random_ray_simulation.h
+++ b/include/openmc/random_ray/random_ray_simulation.h
@@ -19,13 +19,10 @@ public:
 
   //----------------------------------------------------------------------------
   // Methods
-  void compute_segment_correction_factors();
   void apply_fixed_sources_and_mesh_domains();
-  void prepare_fixed_sources_adjoint(vector<double>& forward_flux,
-    SourceRegionContainer& forward_source_regions,
-    SourceRegionContainer& forward_base_source_regions,
-    std::unordered_map<SourceRegionKey, int64_t, SourceRegionKey::HashFunctor>&
-      forward_source_region_map);
+  void prepare_fw_fixed_sources_adjoint();
+  void prepare_local_fixed_sources_adjoint();
+  void prepare_adjoint_simulation(bool from_forward);
   void simulate();
   void output_simulation_results() const;
   void instability_check(
@@ -45,9 +42,6 @@ private:
   // Contains all flat source region data
   unique_ptr<FlatSourceDomain> domain_;
 
-  // Random ray eigenvalue
-  double k_eff_ {1.0};
-
   // Tracks the average FSR miss rate for analysis and reporting
   double avg_miss_rate_ {0.0};
 
@@ -64,9 +58,8 @@ private:
 //! Non-member functions
 //============================================================================
 
-void openmc_run_random_ray();
 void validate_random_ray_inputs();
-void openmc_reset_random_ray();
+void openmc_finalize_random_ray();
 
 } // namespace openmc
 
diff --git a/include/openmc/random_ray/source_region.h b/include/openmc/random_ray/source_region.h
index 5c5b31f39..1d2bbe1e8 100644
--- a/include/openmc/random_ray/source_region.h
+++ b/include/openmc/random_ray/source_region.h
@@ -51,10 +51,10 @@ inline void hash_combine(size_t& seed, const size_t v)
 // every iteration.
 struct TallyTask {
   int tally_idx;
-  int filter_idx;
+  int64_t filter_idx;
   int score_idx;
   int score_type;
-  TallyTask(int tally_idx, int filter_idx, int score_idx, int score_type)
+  TallyTask(int tally_idx, int64_t filter_idx, int score_idx, int score_type)
     : tally_idx(tally_idx), filter_idx(filter_idx), score_idx(score_idx),
       score_type(score_type)
   {}
@@ -146,6 +146,8 @@ public:
 
   // Scalar fields
   int* material_;
+  int* temperature_idx_;
+  double* density_mult_;
   int* is_small_;
   int* n_hits_;
   int* birthday_;
@@ -195,6 +197,12 @@ public:
   int& material() { return *material_; }
   const int material() const { return *material_; }
 
+  double& density_mult() { return *density_mult_; }
+  const double density_mult() const { return *density_mult_; }
+
+  int& temperature_idx() { return *temperature_idx_; }
+  const int temperature_idx() const { return *temperature_idx_; }
+
   int& is_small() { return *is_small_; }
   const int is_small() const { return *is_small_; }
 
@@ -308,7 +316,6 @@ public:
   //----------------------------------------------------------------------------
   // Constructors
   SourceRegion(int negroups, bool is_linear);
-  SourceRegion(const SourceRegionHandle& handle, int64_t parent_sr);
   SourceRegion() = default;
 
   //----------------------------------------------------------------------------
@@ -316,8 +323,11 @@ public:
 
   //---------------------------------------
   // Scalar fields
-
   int material_ {0}; //!< Index in openmc::model::materials array
+  int temperature_idx_ {
+    0}; //!< Index into the MGXS array representing temperature
+  double density_mult_ {1.0}; //!< A density multiplier queried from the cell
+                              //!< corresponding to the source region.
   OpenMPMutex lock_;
   double volume_ {
     0.0}; //!< Volume (computed from the sum of ray crossing lengths)
@@ -395,6 +405,12 @@ public:
   int& material(int64_t sr) { return material_[sr]; }
   const int material(int64_t sr) const { return material_[sr]; }
 
+  int& temperature_idx(int64_t sr) { return temperature_idx_[sr]; }
+  const int temperature_idx(int64_t sr) const { return temperature_idx_[sr]; }
+
+  double& density_mult(int64_t sr) { return density_mult_[sr]; }
+  const double density_mult(int64_t sr) const { return density_mult_[sr]; }
+
   int& is_small(int64_t sr) { return is_small_[sr]; }
   const int is_small(int64_t sr) const { return is_small_[sr]; }
 
@@ -626,6 +642,8 @@ private:
 
   // SoA storage for scalar fields (one item per source region)
   vector<int> material_;
+  vector<int> temperature_idx_;
+  vector<double> density_mult_;
   vector<int> is_small_;
   vector<int> n_hits_;
   vector<int> mesh_;
@@ -672,7 +690,7 @@ private:
   // Private Methods
 
   // Helper function for indexing
-  inline int index(int64_t sr, int g) const { return sr * negroups_ + g; }
+  inline int64_t index(int64_t sr, int g) const { return sr * negroups_ + g; }
 };
 
 } // namespace openmc
diff --git a/include/openmc/ray.h b/include/openmc/ray.h
new file mode 100644
index 000000000..62e86b0d9
--- /dev/null
+++ b/include/openmc/ray.h
@@ -0,0 +1,50 @@
+#ifndef OPENMC_RAY_H
+#define OPENMC_RAY_H
+
+#include "openmc/particle_data.h"
+#include "openmc/position.h"
+
+namespace openmc {
+
+// Base class that implements ray tracing logic, not necessarily through
+// defined regions of the geometry but also outside of it.
+class Ray : public GeometryState {
+
+public:
+  // Initialize from location and direction
+  Ray(Position r, Direction u) { init_from_r_u(r, u); }
+
+  // Initialize from known geometry state
+  Ray(const GeometryState& p) : GeometryState(p) {}
+
+  // Called at every surface intersection within the model
+  virtual void on_intersection() = 0;
+
+  /*
+   * Traces the ray through the geometry, calling on_intersection
+   * at every surface boundary.
+   */
+  void trace();
+
+  // Stops the ray and exits tracing when called from on_intersection
+  void stop() { stop_ = true; }
+
+  // Sets the dist_ variable
+  void compute_distance();
+
+protected:
+  // Records how far the ray has traveled
+  double traversal_distance_ {0.0};
+
+private:
+  // Max intersections before we assume ray tracing is caught in an infinite
+  // loop:
+  static const int MAX_INTERSECTIONS = 1000000;
+
+  bool stop_ {false};
+
+  unsigned event_counter_ {0};
+};
+
+} // namespace openmc
+#endif // OPENMC_RAY_H
diff --git a/include/openmc/reaction.h b/include/openmc/reaction.h
index 3314d1866..e95db1665 100644
--- a/include/openmc/reaction.h
+++ b/include/openmc/reaction.h
@@ -76,11 +76,21 @@ public:
 //! \return Name of the corresponding reaction
 std::string reaction_name(int mt);
 
-//! Return reaction type (MT value) given a reaction name
+//! Return MT value for given a reaction name (including special tally MT
+//! values)
 //
 //! \param[in] name  Reaction name
-//! \return Corresponding reaction type (MT value)
-int reaction_type(std::string name);
+//! \return Corresponding MT number or special tally score
+int reaction_tally_mt(std::string name);
+
+//! Return ENDF MT number given a reaction name
+//
+//! Unlike reaction_tally_mt(), this function always returns a positive ENDF MT
+//! number and never a special negative tally score.
+//!
+//! \param[in] name  Reaction name
+//! \return Corresponding ENDF MT number
+int reaction_mt(const std::string& name);
 
 } // namespace openmc
 
diff --git a/include/openmc/reaction_product.h b/include/openmc/reaction_product.h
index 4fbbc1b62..79a6160e2 100644
--- a/include/openmc/reaction_product.h
+++ b/include/openmc/reaction_product.h
@@ -10,7 +10,7 @@
 #include "openmc/chain.h"
 #include "openmc/endf.h"
 #include "openmc/memory.h" // for unique_ptr
-#include "openmc/particle.h"
+#include "openmc/particle_type.h"
 #include "openmc/vector.h" // for vector
 
 namespace openmc {
@@ -49,6 +49,21 @@ public:
   //! \param[inout] seed Pseudorandom seed pointer
   void sample(double E_in, double& E_out, double& mu, uint64_t* seed) const;
 
+  //! Select which angle-energy distribution to sample
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Reference to the selected angle-energy distribution
+  AngleEnergy& sample_dist(double E_in, uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const;
+
   ParticleType particle_;      //!< Particle type
   EmissionMode emission_mode_; //!< Emission mode
   double decay_rate_; //!< Decay rate (for delayed neutron precursors) in [1/s]
diff --git a/include/openmc/scattdata.h b/include/openmc/scattdata.h
index a75ef09d9..bea881402 100644
--- a/include/openmc/scattdata.h
+++ b/include/openmc/scattdata.h
@@ -4,7 +4,7 @@
 #ifndef OPENMC_SCATTDATA_H
 #define OPENMC_SCATTDATA_H
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/constants.h"
 #include "openmc/vector.h"
@@ -26,23 +26,23 @@ public:
 
 protected:
   //! \brief Initializes the attributes of the base class.
-  void base_init(int order, const xt::xtensor<int, 1>& in_gmin,
-    const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_energy,
+  void base_init(int order, const tensor::Tensor<int>& in_gmin,
+    const tensor::Tensor<int>& in_gmax, const double_2dvec& in_energy,
     const double_2dvec& in_mult);
 
   //! \brief Combines microscopic ScattDatas into a macroscopic one.
   void base_combine(size_t max_order, size_t order_dim,
     const vector<ScattData*>& those_scatts, const vector<double>& scalars,
-    xt::xtensor<int, 1>& in_gmin, xt::xtensor<int, 1>& in_gmax,
+    tensor::Tensor<int>& in_gmin, tensor::Tensor<int>& in_gmax,
     double_2dvec& sparse_mult, double_3dvec& sparse_scatter);
 
 public:
   double_2dvec energy;            // Normalized p0 matrix for sampling Eout
   double_2dvec mult;              // nu-scatter multiplication (nu-scatt/scatt)
   double_3dvec dist;              // Angular distribution
-  xt::xtensor<int, 1> gmin;       // minimum outgoing group
-  xt::xtensor<int, 1> gmax;       // maximum outgoing group
-  xt::xtensor<double, 1> scattxs; // Isotropic Sigma_{s,g_{in}}
+  tensor::Tensor<int> gmin;       // minimum outgoing group
+  tensor::Tensor<int> gmax;       // maximum outgoing group
+  tensor::Tensor<double> scattxs; // Isotropic Sigma_{s,g_{in}}
 
   //! \brief Calculates the value of normalized f(mu).
   //!
@@ -72,8 +72,8 @@ public:
   //! @param in_gmax List of maximum outgoing groups for every incoming group
   //! @param in_mult Input sparse multiplicity matrix
   //! @param coeffs Input sparse scattering matrix
-  virtual void init(const xt::xtensor<int, 1>& in_gmin,
-    const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+  virtual void init(const tensor::Tensor<int>& in_gmin,
+    const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
     const double_3dvec& coeffs) = 0;
 
   //! \brief Combines the microscopic data.
@@ -96,7 +96,7 @@ public:
   //! @param max_order If Legendre this is the maximum value of "n" in "Pn"
   //!   requested; ignored otherwise.
   //! @return The dense scattering matrix.
-  virtual xt::xtensor<double, 3> get_matrix(size_t max_order) = 0;
+  virtual tensor::Tensor<double> get_matrix(size_t max_order) = 0;
 
   //! \brief Samples the outgoing energy from the ScattData info.
   //!
@@ -135,8 +135,8 @@ protected:
     ScattDataLegendre& leg, ScattDataTabular& tab);
 
 public:
-  void init(const xt::xtensor<int, 1>& in_gmin,
-    const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+  void init(const tensor::Tensor<int>& in_gmin,
+    const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
     const double_3dvec& coeffs) override;
 
   void combine(const vector<ScattData*>& those_scatts,
@@ -153,7 +153,7 @@ public:
 
   size_t get_order() override { return dist[0][0].size() - 1; };
 
-  xt::xtensor<double, 3> get_matrix(size_t max_order) override;
+  tensor::Tensor<double> get_matrix(size_t max_order) override;
 };
 
 //==============================================================================
@@ -164,13 +164,13 @@ public:
 class ScattDataHistogram : public ScattData {
 
 protected:
-  xt::xtensor<double, 1> mu; // Angle distribution mu bin boundaries
+  tensor::Tensor<double> mu; // Angle distribution mu bin boundaries
   double dmu;                // Quick storage of the mu spacing
   double_3dvec fmu;          // The angular distribution histogram
 
 public:
-  void init(const xt::xtensor<int, 1>& in_gmin,
-    const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+  void init(const tensor::Tensor<int>& in_gmin,
+    const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
     const double_3dvec& coeffs) override;
 
   void combine(const vector<ScattData*>& those_scatts,
@@ -183,7 +183,7 @@ public:
 
   size_t get_order() override { return dist[0][0].size(); };
 
-  xt::xtensor<double, 3> get_matrix(size_t max_order) override;
+  tensor::Tensor<double> get_matrix(size_t max_order) override;
 };
 
 //==============================================================================
@@ -194,7 +194,7 @@ public:
 class ScattDataTabular : public ScattData {
 
 protected:
-  xt::xtensor<double, 1> mu; // Angle distribution mu grid points
+  tensor::Tensor<double> mu; // Angle distribution mu grid points
   double dmu;                // Quick storage of the mu spacing
   double_3dvec fmu;          // The angular distribution function
 
@@ -204,8 +204,8 @@ protected:
     ScattDataLegendre& leg, ScattDataTabular& tab);
 
 public:
-  void init(const xt::xtensor<int, 1>& in_gmin,
-    const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+  void init(const tensor::Tensor<int>& in_gmin,
+    const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
     const double_3dvec& coeffs) override;
 
   void combine(const vector<ScattData*>& those_scatts,
@@ -218,7 +218,7 @@ public:
 
   size_t get_order() override { return dist[0][0].size(); };
 
-  xt::xtensor<double, 3> get_matrix(size_t max_order) override;
+  tensor::Tensor<double> get_matrix(size_t max_order) override;
 };
 
 //==============================================================================
diff --git a/include/openmc/secondary_correlated.h b/include/openmc/secondary_correlated.h
index 6905c38e3..69b22981a 100644
--- a/include/openmc/secondary_correlated.h
+++ b/include/openmc/secondary_correlated.h
@@ -5,7 +5,7 @@
 #define OPENMC_SECONDARY_CORRELATED_H
 
 #include "hdf5.h"
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/angle_energy.h"
 #include "openmc/distribution.h"
@@ -25,9 +25,9 @@ public:
   struct CorrTable {
     int n_discrete;                    //!< Number of discrete lines
     Interpolation interpolation;       //!< Interpolation law
-    xt::xtensor<double, 1> e_out;      //!< Outgoing energies [eV]
-    xt::xtensor<double, 1> p;          //!< Probability density
-    xt::xtensor<double, 1> c;          //!< Cumulative distribution
+    tensor::Tensor<double> e_out;      //!< Outgoing energies [eV]
+    tensor::Tensor<double> p;          //!< Probability density
+    tensor::Tensor<double> c;          //!< Cumulative distribution
     vector<unique_ptr<Tabular>> angle; //!< Angle distribution
   };
 
@@ -41,6 +41,22 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample the outgoing energy and return the angular distribution
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Reference to the angular distribution at the sampled energy bin
+  Distribution& sample_dist(double E_in, double& E_out, uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
   // energy property
   vector<double>& energy() { return energy_; }
   const vector<double>& energy() const { return energy_; }
diff --git a/include/openmc/secondary_kalbach.h b/include/openmc/secondary_kalbach.h
index 83806d352..b25352be9 100644
--- a/include/openmc/secondary_kalbach.h
+++ b/include/openmc/secondary_kalbach.h
@@ -5,7 +5,7 @@
 #define OPENMC_SECONDARY_KALBACH_H
 
 #include "hdf5.h"
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/angle_energy.h"
 #include "openmc/constants.h"
@@ -32,16 +32,34 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample outgoing energy and Kalbach-Mann parameters
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[out] km_a Kalbach-Mann 'a' parameter
+  //! \param[out] km_r Kalbach-Mann pre-compound fraction 'r'
+  //! \param[inout] seed Pseudorandom seed pointer
+  void sample_params(double E_in, double& E_out, double& km_a, double& km_r,
+    uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   //! Outgoing energy/angle at a single incoming energy
   struct KMTable {
     int n_discrete;               //!< Number of discrete lines
     Interpolation interpolation;  //!< Interpolation law
-    xt::xtensor<double, 1> e_out; //!< Outgoing energies [eV]
-    xt::xtensor<double, 1> p;     //!< Probability density
-    xt::xtensor<double, 1> c;     //!< Cumulative distribution
-    xt::xtensor<double, 1> r;     //!< Pre-compound fraction
-    xt::xtensor<double, 1> a;     //!< Parameterized function
+    tensor::Tensor<double> e_out; //!< Outgoing energies [eV]
+    tensor::Tensor<double> p;     //!< Probability density
+    tensor::Tensor<double> c;     //!< Cumulative distribution
+    tensor::Tensor<double> r;     //!< Pre-compound fraction
+    tensor::Tensor<double> a;     //!< Parameterized function
   };
 
   int n_region_;                        //!< Number of interpolation regions
diff --git a/include/openmc/secondary_nbody.h b/include/openmc/secondary_nbody.h
index efb4fd75b..9d033a6b8 100644
--- a/include/openmc/secondary_nbody.h
+++ b/include/openmc/secondary_nbody.h
@@ -28,6 +28,21 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample an outgoing energy from the N-body phase space distribution
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Sampled outgoing energy in [eV]
+  double sample_energy(double E_in, uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   int n_bodies_;      //!< Number of particles distributed
   double mass_ratio_; //!< Total mass of particles [neutron mass]
diff --git a/include/openmc/secondary_thermal.h b/include/openmc/secondary_thermal.h
index 5b18902af..45d2c5260 100644
--- a/include/openmc/secondary_thermal.h
+++ b/include/openmc/secondary_thermal.h
@@ -6,10 +6,11 @@
 
 #include "openmc/angle_energy.h"
 #include "openmc/endf.h"
+#include "openmc/search.h"
 #include "openmc/secondary_correlated.h"
 #include "openmc/vector.h"
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 #include <hdf5.h>
 
 namespace openmc {
@@ -33,8 +34,20 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   const CoherentElasticXS& xs_; //!< Coherent elastic scattering cross section
+  tensor::Tensor<double> bragg_edges_; //!< Copy of Bragg edges for slicing
+  tensor::Tensor<double>
+    factors_diff_; //!< Differences over elastic scattering factors
 };
 
 //==============================================================================
@@ -56,6 +69,15 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   double debye_waller_;
 };
@@ -81,9 +103,18 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   const vector<double>& energy_;  //!< Energies at which cosines are tabulated
-  xt::xtensor<double, 2> mu_out_; //!< Cosines for each incident energy
+  tensor::Tensor<double> mu_out_; //!< Cosines for each incident energy
 };
 
 //==============================================================================
@@ -106,12 +137,27 @@ public:
   //! \param[inout] seed Pseudorandom number seed pointer
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
+  //! Sample outgoing energy bin parameters
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[out] j Sampled outgoing energy bin index
+  //! \param[inout] seed Pseudorandom seed pointer
+  void sample_params(double E_in, double& E_out, int& j, uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
 
 private:
   const vector<double>& energy_; //!< Incident energies
-  xt::xtensor<double, 2>
+  tensor::Tensor<double>
     energy_out_; //!< Outgoing energies for each incident energy
-  xt::xtensor<double, 3>
+  tensor::Tensor<double>
     mu_out_;    //!< Outgoing cosines for each incident/outgoing energy
   bool skewed_; //!< Whether outgoing energy distribution is skewed
 };
@@ -135,14 +181,33 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample outgoing energy bin parameters
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[out] f Interpolation factor within sampled energy bin
+  //! \param[out] l Index of the closer incident energy
+  //! \param[out] j Sampled outgoing energy bin index
+  //! \param[inout] seed Pseudorandom seed pointer
+  void sample_params(double E_in, double& E_out, double& f, int& l, int& j,
+    uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   //! Secondary energy/angle distribution
   struct DistEnergySab {
     std::size_t n_e_out;              //!< Number of outgoing energies
-    xt::xtensor<double, 1> e_out;     //!< Outgoing energies
-    xt::xtensor<double, 1> e_out_pdf; //!< Probability density function
-    xt::xtensor<double, 1> e_out_cdf; //!< Cumulative distribution function
-    xt::xtensor<double, 2> mu; //!< Equiprobable angles at each outgoing energy
+    tensor::Tensor<double> e_out;     //!< Outgoing energies
+    tensor::Tensor<double> e_out_pdf; //!< Probability density function
+    tensor::Tensor<double> e_out_cdf; //!< Cumulative distribution function
+    tensor::Tensor<double> mu; //!< Equiprobable angles at each outgoing energy
   };
 
   vector<double> energy_;              //!< Incident energies
@@ -170,6 +235,21 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Select the coherent or incoherent elastic distribution to sample
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Reference to the selected angle-energy distribution
+  const AngleEnergy& sample_dist(double E_in, uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
 private:
   CoherentElasticAE coherent_dist_;         //!< Coherent distribution
   unique_ptr<AngleEnergy> incoherent_dist_; //!< Incoherent distribution
@@ -178,6 +258,133 @@ private:
   const Function1D& incoherent_xs_;      //!< Polymorphic ref. to incoherent XS
 };
 
+//! Internal helper for evaluating a piecewise-constant PDF on discrete points.
+//!
+//! The underlying discrete points are represented implicitly through a
+//! monotonically increasing `center(i)` function and corresponding per-point
+//! `weight(i)` values. Each point contributes a rectangular bin whose
+//! half-width is half the distance to its nearest neighboring center.
+//!
+//! \tparam CenterFn Callable returning the location of the i-th discrete value
+//! \tparam WeightFn Callable returning the weight of the i-th discrete value
+//! \param[in] n Number of discrete values
+//! \param[in] mu_0 Point at which to evaluate the PDF
+//! \param[in] a Lower bound of the domain (default: -1)
+//! \param[in] b Upper bound of the domain (default: 1)
+//! \return Probability density at mu_0
+template<typename CenterFn, typename WeightFn>
+double get_pdf_discrete_impl(std::size_t n, double mu_0, double a, double b,
+  CenterFn center, WeightFn weight)
+{
+  if (n == 0 || mu_0 < a || mu_0 > b)
+    return 0.0;
+
+  auto evaluate_bin = [&](std::size_t i) {
+    double x = center(i);
+    double left_span = (i == 0) ? 2.0 * (x - a) : x - center(i - 1);
+    double right_span = (i + 1 == n) ? 2.0 * (b - x) : center(i + 1) - x;
+    double delta = 0.5 * std::min(left_span, right_span);
+    if (delta <= 0.0)
+      return 0.0;
+
+    double left = x - delta;
+    double right = x + delta;
+    bool in_bin =
+      (mu_0 >= left) && ((i + 1 == n) ? (mu_0 <= right) : (mu_0 < right));
+    return in_bin ? weight(i) / (2.0 * delta) : 0.0;
+  };
+
+  // This is effectively a lower_bound over the sequence center(i), but the
+  // sequence is implicit rather than stored in a container, so the STL
+  // algorithms can not be used.
+  std::size_t low = 0;
+  std::size_t high = n;
+  while (low < high) {
+    std::size_t mid = low + (high - low) / 2;
+    if (center(mid) < mu_0) {
+      low = mid + 1;
+    } else {
+      high = mid;
+    }
+  }
+
+  if (low < n) {
+    double pdf = evaluate_bin(low);
+    if (pdf > 0.0)
+      return pdf;
+  }
+  if (low > 0)
+    return evaluate_bin(low - 1);
+  return 0.0;
+}
+
+//! Evaluate the PDF of a weighted discrete distribution at a given point.
+//!
+//! Given a set of discrete values mu[i] with weights w[i], this function
+//! computes the probability density at mu_0 by treating each discrete value
+//! as a rectangular bin. The bin half-width around each discrete value is
+//! half the distance to its nearest neighbor.
+//!
+//! \tparam T1 Container type for discrete cosine values (must support
+//!         operator[], size())
+//! \tparam T2 Container type for weights (must support operator[])
+//! \param[in] mu Sorted array of discrete cosine values
+//! \param[in] w Weights for each discrete value (need not be normalized)
+//! \param[in] mu_0 Point at which to evaluate the PDF
+//! \param[in] a Lower bound of the domain (default: -1)
+//! \param[in] b Upper bound of the domain (default: 1)
+//! \return Probability density at mu_0
+template<typename T1, typename T2>
+double get_pdf_discrete(
+  const T1 mu, const T2& w, double mu_0, double a = -1.0, double b = 1.0)
+{
+  // Returns the location of the discrete value for a given index
+  auto center = [&](std::size_t i) { return mu[i]; };
+  auto weight = [&](std::size_t i) { return w[i]; };
+  return get_pdf_discrete_impl(mu.size(), mu_0, a, b, center, weight);
+}
+
+//! Evaluate the PDF of a discrete distribution with uniform weights
+//!
+//! \tparam T1 Container type for discrete cosine values
+//! \param[in] mu Sorted array of discrete cosine values
+//! \param[in] mu_0 Point at which to evaluate the PDF
+//! \param[in] a Lower bound of the domain (default: -1)
+//! \param[in] b Upper bound of the domain (default: 1)
+//! \return Probability density at mu_0
+template<typename T1>
+double get_pdf_discrete(
+  const T1 mu, double mu_0, double a = -1.0, double b = 1.0)
+{
+  auto center = [&](std::size_t i) { return mu[i]; };
+  auto weight = [&](std::size_t i) { return 1.0 / mu.size(); };
+  return get_pdf_discrete_impl(mu.size(), mu_0, a, b, center, weight);
+}
+
+//! Evaluate the PDF of a uniformly weighted distribution on interpolated points
+//!
+//! \tparam T1 Container type for the lower tabulated cosine values
+//! \tparam T2 Container type for the upper tabulated cosine values
+//! \param[in] mu0 Sorted array of discrete cosine values at the lower grid
+//! \param[in] mu1 Sorted array of discrete cosine values at the upper grid
+//! \param[in] f Interpolation factor between mu0 and mu1
+//! \param[in] mu_0 Point at which to evaluate the PDF
+//! \param[in] a Lower bound of the domain (default: -1)
+//! \param[in] b Upper bound of the domain (default: 1)
+//! \return Probability density at mu_0
+template<typename T1, typename T2>
+double get_pdf_discrete_interpolated(const T1 mu0, const T2 mu1, double f,
+  double mu_0, double a = -1.0, double b = 1.0)
+{
+  if (mu0.size() != mu1.size())
+    return 0.0;
+
+  // Returns interpolated discrete value for a given index
+  auto center = [&](std::size_t i) { return mu0[i] + f * (mu1[i] - mu0[i]); };
+  auto weight = [&](std::size_t i) { return 1.0 / mu0.size(); };
+  return get_pdf_discrete_impl(mu0.size(), mu_0, a, b, center, weight);
+}
+
 } // namespace openmc
 
 #endif // OPENMC_SECONDARY_THERMAL_H
diff --git a/include/openmc/secondary_uncorrelated.h b/include/openmc/secondary_uncorrelated.h
index 3afa3d9ce..f895ae77f 100644
--- a/include/openmc/secondary_uncorrelated.h
+++ b/include/openmc/secondary_uncorrelated.h
@@ -32,6 +32,15 @@ public:
   void sample(
     double E_in, double& E_out, double& mu, uint64_t* seed) const override;
 
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(
+    double E_in, double mu, double& E_out, uint64_t* seed) const override;
+
   // Accessors
   AngleDistribution& angle() { return angle_; }
 
diff --git a/include/openmc/settings.h b/include/openmc/settings.h
index f3aff08b0..3bba040f5 100644
--- a/include/openmc/settings.h
+++ b/include/openmc/settings.h
@@ -24,6 +24,32 @@ enum class SSWCellType {
   To,
 };
 
+// Type of IFP parameters
+enum class IFPParameter {
+  None,
+  Both,
+  BetaEffective,
+  GenerationTime,
+};
+
+struct CollisionTrackConfig {
+  bool mcpl_write {false}; //!< Write collision tracks using MCPL?
+  std::unordered_set<int>
+    cell_ids; //!< Cell ids where collisions will be written
+  std::unordered_set<int>
+    mt_numbers; //!< MT Numbers where collisions will be written
+  std::unordered_set<int>
+    universe_ids; //!< Universe IDs where collisions will be written
+  std::unordered_set<int>
+    material_ids; //!< Material IDs where collisions will be written
+  std::unordered_set<std::string>
+    nuclides; //!< Nuclides where collisions will be written
+  double deposited_energy_threshold {0.0}; //!< Minimum deposited energy [eV]
+  int64_t max_collisions {
+    1000};               //!< Maximum events recorded per collision track file
+  int64_t max_files {1}; //!< Maximum number of collision track files
+};
+
 //==============================================================================
 // Global variable declarations
 //==============================================================================
@@ -33,6 +59,7 @@ namespace settings {
 // Boolean flags
 extern bool assume_separate;      //!< assume tallies are spatially separate?
 extern bool check_overlaps;       //!< check overlaps in geometry?
+extern bool collision_track;      //!< flag to use collision track feature?
 extern bool confidence_intervals; //!< use confidence intervals for results?
 extern bool
   create_fission_neutrons; //!< create fission neutrons (fixed source)?
@@ -42,13 +69,15 @@ extern bool
   delayed_photon_scaling;   //!< Scale fission photon yield to include delayed
 extern "C" bool entropy_on; //!< calculate Shannon entropy?
 extern "C" bool
-  event_based; //!< use event-based mode (instead of history-based)
+  event_based;      //!< use event-based mode (instead of history-based)
+extern bool ifp_on; //!< Use IFP for kinetics parameters?
 extern bool legendre_to_tabular; //!< convert Legendre distributions to tabular?
 extern bool material_cell_offsets;   //!< create material cells offsets?
 extern "C" bool output_summary;      //!< write summary.h5?
 extern bool output_tallies;          //!< write tallies.out?
 extern bool particle_restart_run;    //!< particle restart run?
 extern "C" bool photon_transport;    //!< photon transport turned on?
+extern bool atomic_relaxation;       //!< atomic relaxation enabled?
 extern "C" bool reduce_tallies;      //!< reduce tallies at end of batch?
 extern bool res_scat_on;             //!< use resonance upscattering method?
 extern "C" bool restart_run;         //!< restart run?
@@ -69,7 +98,8 @@ extern bool uniform_source_sampling; //!< sample sources uniformly?
 extern bool ufs_on;                  //!< uniform fission site method on?
 extern bool urr_ptables_on;          //!< use unresolved resonance prob. tables?
 extern bool use_decay_photons;       //!< use decay photons for D1S
-extern "C" bool weight_windows_on;   //!< are weight windows are enabled?
+extern bool use_shared_secondary_bank; //!< Use shared bank for secondaries
+extern "C" bool weight_windows_on;     //!< are weight windows are enabled?
 extern bool weight_window_checkpoint_surface;   //!< enable weight window check
                                                 //!< upon surface crossing?
 extern bool weight_window_checkpoint_collision; //!< enable weight window check
@@ -86,6 +116,8 @@ extern std::string path_sourcepoint;      //!< path to a source file
 extern std::string path_statepoint;       //!< path to a statepoint file
 extern std::string weight_windows_file;   //!< Location of weight window file to
                                           //!< load on simulation initialization
+extern std::string properties_file;       //!< Location of properties file to
+                                          //!< load on simulation initialization
 
 // This is required because the c_str() may not be the first thing in
 // std::string. Sometimes it is, but it seems libc++ may not be like that
@@ -112,6 +144,10 @@ extern array<double, 4>
   energy_cutoff; //!< Energy cutoff in [eV] for each particle type
 extern array<double, 4>
   time_cutoff; //!< Time cutoff in [s] for each particle type
+extern int
+  ifp_n_generation; //!< Number of generation for Iterated Fission Probability
+extern IFPParameter
+  ifp_parameter; //!< Parameter to calculate for Iterated Fission Probability
 extern int
   legendre_to_tabular_points; //!< number of points to convert Legendres
 extern int max_order;         //!< Maximum Legendre order for multigroup data
@@ -132,9 +168,15 @@ extern std::unordered_set<int>
   statepoint_batch; //!< Batches when state should be written
 extern std::unordered_set<int>
   source_write_surf_id; //!< Surface ids where sources will be written
+extern CollisionTrackConfig collision_track_config;
+extern double source_rejection_fraction; //!< Minimum fraction of source sites
+                                         //!< that must be accepted
+extern double free_gas_threshold;        //!< Threshold multiplier for free gas
+                                         //!< scattering treatment
 
 extern int
   max_history_splits; //!< maximum number of particle splits for weight windows
+extern int max_secondaries;       //!< maximum number of secondaries in the bank
 extern int64_t ssw_max_particles; //!< maximum number of particles to be
                                   //!< banked on surfaces per process
 extern int64_t ssw_max_files;     //!< maximum number of surface source files
@@ -143,6 +185,8 @@ extern int64_t ssw_cell_id;       //!< Cell id for the surface source
                                   //!< write setting
 extern SSWCellType ssw_cell_type; //!< Type of option for the cell
                                   //!< argument of surface source write
+extern double surface_grazing_cutoff; //!< surface flux cosine cutoff
+extern double surface_grazing_ratio;  //!< surface flux substitution ratio
 extern TemperatureMethod
   temperature_method; //!< method for choosing temperatures
 extern double
diff --git a/include/openmc/shared_array.h b/include/openmc/shared_array.h
index 7e9ef28c5..2829a2eb9 100644
--- a/include/openmc/shared_array.h
+++ b/include/openmc/shared_array.h
@@ -4,6 +4,8 @@
 //! \file shared_array.h
 //! \brief Shared array data structure
 
+#include <algorithm> // for copy_n
+
 #include "openmc/memory.h"
 
 namespace openmc {
@@ -30,14 +32,12 @@ public:
   //! Default constructor.
   SharedArray() = default;
 
-  //! Construct a zero size container with space to hold capacity number of
-  //! elements.
+  //! Construct a container with `size` elements and capacity equal to `size`.
   //
-  //! \param capacity The number of elements for the container to allocate
-  //! space for
-  SharedArray(int64_t capacity) : capacity_(capacity)
+  //! \param size The number of elements to allocate and initialize
+  SharedArray(int64_t size) : size_(size), capacity_(size)
   {
-    data_ = make_unique<T[]>(capacity);
+    data_ = make_unique<T[]>(size);
   }
 
   //==========================================================================
@@ -97,8 +97,52 @@ public:
     capacity_ = 0;
   }
 
+  //! Push back an element to the array, with capacity and reallocation behavior
+  //! as if this were a vector. This does not perform any thread safety checks.
+  //! If the size exceeds the capacity, then the capacity will double just as
+  //! with a vector. Data will be reallocated and moved to a new pointer and
+  //! copied in before the new item is appended. Old data will be freed.
+  void thread_unsafe_append(const T& value)
+  {
+    if (size_ == capacity_) {
+      int64_t new_capacity = capacity_ == 0 ? 8 : 2 * capacity_;
+      unique_ptr<T[]> new_data = make_unique<T[]>(new_capacity);
+      std::copy_n(data_.get(), size_, new_data.get());
+      data_ = std::move(new_data);
+      capacity_ = new_capacity;
+    }
+    data_[size_++] = value;
+  }
+
+  //! Increase the size of the container by count elements without assigning
+  //! values to the new elements. Existing elements are preserved if the
+  //! container needs to grow. This does not perform any thread safety checks.
+  //
+  //! \param count The number of elements to append
+  //! \return The starting index of the appended range
+  int64_t extend_uninitialized(int64_t count)
+  {
+    int64_t offset = size_;
+    int64_t new_size = size_ + count;
+    if (new_size > capacity_) {
+      int64_t new_capacity = capacity_ == 0 ? 8 : capacity_;
+      while (new_capacity < new_size) {
+        new_capacity *= 2;
+      }
+      unique_ptr<T[]> new_data = make_unique<T[]>(new_capacity);
+      if (size_ > 0) {
+        std::copy_n(data_.get(), size_, new_data.get());
+      }
+      data_ = std::move(new_data);
+      capacity_ = new_capacity;
+    }
+    size_ = new_size;
+    return offset;
+  }
+
   //! Return the number of elements in the container
   int64_t size() { return size_; }
+  int64_t size() const { return size_; }
 
   //! Resize the container to contain a specified number of elements. This is
   //! useful in cases where the container is written to in a non-thread safe
diff --git a/include/openmc/simulation.h b/include/openmc/simulation.h
index 3e4e24e1d..454752cd2 100644
--- a/include/openmc/simulation.h
+++ b/include/openmc/simulation.h
@@ -22,6 +22,7 @@ constexpr int STATUS_EXIT_ON_TRIGGER {2};
 
 namespace simulation {
 
+extern int ct_current_file;   //!< current collision track file index
 extern "C" int current_batch; //!< current batch
 extern "C" int current_gen;   //!< current fission generation
 extern "C" bool initialized;  //!< has simulation been initialized?
@@ -48,6 +49,9 @@ extern const RegularMesh* ufs_mesh;
 extern vector<double> k_generation;
 extern vector<int64_t> work_index;
 
+extern int64_t
+  simulation_tracks_completed; //!< Number of tracks completed on this rank
+
 } // namespace simulation
 
 //==============================================================================
@@ -58,7 +62,7 @@ extern vector<int64_t> work_index;
 void allocate_banks();
 
 //! Determine number of particles to transport per process
-void calculate_work();
+void calculate_work(int64_t n_particles);
 
 //! Initialize nuclear data before a simulation
 void initialize_data();
@@ -69,8 +73,9 @@ void initialize_batch();
 //! Initialize a fission generation
 void initialize_generation();
 
-//! Full initialization of a particle history
-void initialize_history(Particle& p, int64_t index_source);
+//! Full initialization of a particle track
+void initialize_particle_track(
+  Particle& p, int64_t index_source, bool is_secondary);
 
 //! Finalize a batch
 //!
@@ -91,16 +96,35 @@ void broadcast_results();
 
 void free_memory_simulation();
 
-//! Simulate a single particle history (and all generated secondary particles,
-//!  if enabled), from birth to death
+//! Compute unique particle ID from a 1-based source index
+//! \param index_source 1-based source index within this rank's work
+//! \return globally unique particle ID
+int64_t compute_particle_id(int64_t index_source);
+
+//! Compute the transport RNG seed from a particle ID
+//! \param particle_id the particle's globally unique ID
+//! \return seed value passed to init_particle_seeds()
+int64_t compute_transport_seed(int64_t particle_id);
+
+//! Simulate a single particle history from birth to death, inclusive of any
+//! secondary particles. In shared secondary mode, only a single track is
+//! transported and secondaries are deposited into a shared bank instead.
 void transport_history_based_single_particle(Particle& p);
 
 //! Simulate all particle histories using history-based parallelism
 void transport_history_based();
 
+//! Simulate all particles using history-based parallelism, with a shared
+//! secondary bank
+void transport_history_based_shared_secondary();
+
 //! Simulate all particle histories using event-based parallelism
 void transport_event_based();
 
+//! Simulate all particles using event-based parallelism, with a shared
+//! secondary bank
+void transport_event_based_shared_secondary();
+
 } // namespace openmc
 
 #endif // OPENMC_SIMULATION_H
diff --git a/include/openmc/source.h b/include/openmc/source.h
index 6733eaeff..51b54a1d1 100644
--- a/include/openmc/source.h
+++ b/include/openmc/source.h
@@ -4,15 +4,19 @@
 #ifndef OPENMC_SOURCE_H
 #define OPENMC_SOURCE_H
 
+#include <atomic>
 #include <limits>
 #include <unordered_set>
+#include <utility> // for pair
 
 #include "pugixml.hpp"
 
+#include "openmc/array.h"
+#include "openmc/distribution.h"
 #include "openmc/distribution_multi.h"
 #include "openmc/distribution_spatial.h"
 #include "openmc/memory.h"
-#include "openmc/particle.h"
+#include "openmc/particle_type.h"
 #include "openmc/vector.h"
 
 namespace openmc {
@@ -21,20 +25,28 @@ namespace openmc {
 // Constants
 //==============================================================================
 
-// Maximum number of external source spatial resamples to encounter before an
-// error is thrown.
+// Minimum number of external source sites rejected before checking againts the
+// source_rejection_fraction
 constexpr int EXTSRC_REJECT_THRESHOLD {10000};
-constexpr double EXTSRC_REJECT_FRACTION {0.05};
+
+// Maximum number of source rejections allowed while sampling a single site
+constexpr int64_t MAX_SOURCE_REJECTIONS_PER_SAMPLE {1'000'000};
 
 //==============================================================================
 // Global variables
 //==============================================================================
 
+// Cumulative counters for source rejection diagnostics. These are atomic to
+// allow thread-safe concurrent sampling of external sources.
+extern std::atomic<int64_t> source_n_accept;
+extern std::atomic<int64_t> source_n_reject;
+
 class Source;
 
 namespace model {
 
 extern vector<unique_ptr<Source>> external_sources;
+extern vector<unique_ptr<Source>> adjoint_sources;
 
 // Probability distribution for selecting external sources
 extern DiscreteIndex external_sources_probability;
@@ -140,17 +152,20 @@ public:
   DomainType domain_type() const { return domain_type_; }
   const std::unordered_set<int32_t>& domain_ids() const { return domain_ids_; }
 
+  // Setter for spatial distribution
+  void set_space(UPtrSpace space) { space_ = std::move(space); }
+
 protected:
   // Indicates whether derived class already handles constraints
   bool constraints_applied() const override { return true; }
 
 private:
   // Data members
-  ParticleType particle_ {ParticleType::neutron}; //!< Type of particle emitted
-  UPtrSpace space_;                               //!< Spatial distribution
-  UPtrAngle angle_;                               //!< Angular distribution
-  UPtrDist energy_;                               //!< Energy distribution
-  UPtrDist time_;                                 //!< Time distribution
+  ParticleType particle_; //!< Type of particle emitted
+  UPtrSpace space_;       //!< Spatial distribution
+  UPtrAngle angle_;       //!< Angular distribution
+  UPtrDist energy_;       //!< Energy distribution
+  UPtrDist time_;         //!< Time distribution
 };
 
 //==============================================================================
@@ -171,7 +186,7 @@ protected:
   SourceSite sample(uint64_t* seed) const override;
 
 private:
-  vector<SourceSite> sites_; //!< Source sites from a file
+  vector<SourceSite> sites_; //!< Source sites
 };
 
 //==============================================================================
@@ -206,6 +221,23 @@ typedef unique_ptr<Source> create_compiled_source_t(std::string parameters);
 //! Mesh-based source with different distributions for each element
 //==============================================================================
 
+// Helper class to sample spatial position on a single mesh element
+class MeshElementSpatial : public SpatialDistribution {
+public:
+  MeshElementSpatial(int32_t mesh_index, int elem_index)
+    : mesh_index_(mesh_index), elem_index_(elem_index)
+  {}
+
+  //! Sample a position from the distribution
+  //! \param seed Pseudorandom number seed pointer
+  //! \return (sampled position, importance weight)
+  std::pair<Position, double> sample(uint64_t* seed) const override;
+
+private:
+  int32_t mesh_index_ {C_NONE}; //!< Index in global meshes array
+  int elem_index_;              //! Index of mesh element
+};
+
 class MeshSource : public Source {
 public:
   // Constructors
@@ -220,18 +252,148 @@ public:
   double strength() const override { return space_->total_strength(); }
 
   // Accessors
-  const std::unique_ptr<Source>& source(int32_t i) const
+  const unique_ptr<IndependentSource>& source(int32_t i) const
   {
     return sources_.size() == 1 ? sources_[0] : sources_[i];
   }
 
-protected:
-  bool constraints_applied() const override { return true; }
-
 private:
   // Data members
-  unique_ptr<MeshSpatial> space_;           //!< Mesh spatial
-  vector<std::unique_ptr<Source>> sources_; //!< Source distributions
+  unique_ptr<MeshSpatial> space_;                 //!< Mesh spatial
+  vector<unique_ptr<IndependentSource>> sources_; //!< Source distributions
+};
+
+//==============================================================================
+//! Parametric tokamak plasma neutron source
+//!
+//! This source samples neutron positions from a tokamak plasma geometry using
+//! Miller-style flux surface parameterization with user-specified emission
+//! profiles and energy distributions.
+//!
+//! Flux surface parameterization:
+//!   R = R0 + r*cos(alpha + delta*sin(alpha)) + Delta*(1 - (r/a)^2)
+//!   Z = kappa * r * sin(alpha)
+//!
+//! The sampling algorithm:
+//! 1. Sample minor radius r from precomputed CDF of S(r) * Jacobian
+//! 2. Sample poloidal angle alpha from conditional P(alpha|r) using mixture
+//!    of precomputed CDFs weighted by functions of r
+//! 3. Sample energy and time from user-provided distribution(s)
+//! 4. Sample isotropic direction
+//! 5. Sample toroidal angle phi uniformly in [phi_start, phi_start +
+//! phi_extent]
+//! 6. Transform (r, alpha, phi) to Cartesian (x, y, z), applying the optional
+//!    vertical shift of the plasma center
+//!
+//! The user provides the emission density S(r) directly (e.g., from transport
+//! codes like TRANSP, ASTRA, etc.), allowing full flexibility in reaction
+//! physics calculations. S(r) is a profile in arbitrary units sampled on the
+//! r_over_a grid; only its shape matters, since it is normalized internally.
+//! Energy distributions can be specified as either a single distribution for
+//! all r, or one distribution per radial point.
+//==============================================================================
+
+class TokamakSource : public Source {
+public:
+  // Constructors
+  explicit TokamakSource(pugi::xml_node node);
+
+  //! Sample from the tokamak source distribution
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Sampled site
+  SourceSite sample(uint64_t* seed) const override;
+
+private:
+  //==========================================================================
+  // Private methods
+
+  //! Precompute data structures for efficient sampling
+  void precompute_sampling_distributions();
+
+  //! Sample minor radius from marginal CDF
+  //! \param seed Pseudorandom seed pointer
+  //! \return Sampled r/a value
+  double sample_r_over_a(uint64_t* seed) const;
+
+  //! Sample poloidal angle given r using mixture of precomputed CDFs
+  //! \param r_norm Normalized minor radius r/a
+  //! \param seed Pseudorandom seed pointer
+  //! \return Sampled poloidal angle alpha [rad]
+  double sample_poloidal_angle(double r_norm, uint64_t* seed) const;
+
+  //! Compute the k-th mixture weight w_k(r) * I_hat_k for poloidal sampling
+  //! \param k Basis function index (0-5)
+  //! \param r Normalized minor radius r/a
+  //! \return Mixture weight for component k
+  double mixture_weight(int k, double r) const;
+
+  //! Sample energy from the distribution(s)
+  //! \param r_norm Normalized minor radius r/a (for distribution selection)
+  //! \param seed Pseudorandom seed pointer
+  //! \return (Sampled energy [eV], importance weight)
+  std::pair<double, double> sample_energy(double r_norm, uint64_t* seed) const;
+
+  //! Transform from flux coordinates (r, alpha, phi) to Cartesian (x, y, z)
+  //! \param r Minor radius [cm]
+  //! \param alpha Poloidal angle [rad]
+  //! \param phi Toroidal angle [rad]
+  //! \return Position in Cartesian coordinates [cm]
+  Position flux_to_cartesian(double r, double alpha, double phi) const;
+
+  //==========================================================================
+  // Data members
+
+  // Emission profile (input)
+  vector<double> r_over_a_;         //!< Normalized minor radius grid points
+  vector<double> emission_density_; //!< Emission density S(r) at grid points
+
+  // Energy distribution(s): either 1 for all r, or one per r point
+  vector<unique_ptr<Distribution>> energy_dists_;
+
+  // Time distribution (defaults to a delta distribution at t=0)
+  UPtrDist time_;
+
+  // Angular distribution (isotropic)
+  UPtrAngle angle_;
+
+  // Tokamak geometry parameters
+  double major_radius_;    //!< Major radius R0 [cm]
+  double minor_radius_;    //!< Minor radius a [cm]
+  double elongation_;      //!< Elongation kappa
+  double triangularity_;   //!< Triangularity delta
+  double shafranov_shift_; //!< Shafranov shift Delta [cm]
+  double vertical_shift_;  //!< Vertical shift of plasma center [cm]
+
+  // Normalized geometry parameters (precomputed for efficiency)
+  double epsilon_;     //!< Inverse aspect ratio a/R0
+  double delta_tilde_; //!< Normalized Shafranov shift Delta/a
+
+  // Toroidal angle bounds
+  double phi_start_;  //!< Starting toroidal angle [rad]
+  double phi_extent_; //!< Toroidal angle extent [rad]
+
+  // Precomputed distribution for radial sampling
+  unique_ptr<Tabular> radial_dist_;
+
+  // Coefficients of the radial geometric polynomial: A*r - B*r^2 - C*r^3
+  // Also used as the analytical normalization for poloidal mixture weights
+  double radial_poly_a_; //!< 1 + ε·Δ̃
+  double radial_poly_b_; //!< (3/8)·c₁·ε
+  double radial_poly_c_; //!< 2·ε·Δ̃
+
+  // Precomputed Bernstein basis functions for poloidal angle sampling.
+  // Using the factorization f(r_tilde, alpha) = R_tilde x J_tilde where:
+  //   R_tilde = b0*(1-r)^2 + 2*b1*r*(1-r) + b2*r^2  (Bernstein quadratic)
+  //   J_tilde = b3*(1-r) + b4*r                      (Bernstein linear)
+  // The product gives 6 non-negative basis functions g_k(alpha) with
+  // weights w_k(r_tilde) that are products of Bernstein polynomials.
+  // Distributions are tabulated on [0, pi] exploiting up-down symmetry.
+  static constexpr int N_POLOIDAL_BASIS = 6; //!< Number of basis functions
+  int n_alpha_; //!< Number of poloidal angle grid points
+  array<unique_ptr<Tabular>, N_POLOIDAL_BASIS>
+    poloidal_dists_; //!< Distributions for each basis function g_k(alpha)
+  array<double, N_POLOIDAL_BASIS>
+    poloidal_integrals_; //!< Integrals of g_k(alpha) over [0, pi]
 };
 
 //==============================================================================
@@ -249,6 +411,9 @@ SourceSite sample_external_source(uint64_t* seed);
 
 void free_memory_source();
 
+//! Reset cumulative source rejection counters
+void reset_source_rejection_counters();
+
 } // namespace openmc
 
 #endif // OPENMC_SOURCE_H
diff --git a/include/openmc/surface.h b/include/openmc/surface.h
index 549e8f6ab..2d8580345 100644
--- a/include/openmc/surface.h
+++ b/include/openmc/surface.h
@@ -2,6 +2,7 @@
 #define OPENMC_SURFACE_H
 
 #include <limits> // For numeric_limits
+#include <set>
 #include <string>
 #include <unordered_map>
 
@@ -378,7 +379,39 @@ public:
 // Non-member functions
 //==============================================================================
 
-void read_surfaces(pugi::xml_node node);
+//! Read surface definitions from XML and populate the global surfaces vector.
+//!
+//! This function parses surface elements from the XML input, creates the
+//! appropriate surface objects, and identifies periodic surfaces along with
+//! their albedo values and sense information.
+//!
+//! \param node XML node containing surface definitions
+//! \param[out] periodic_pairs Set of surface ID pairs representing periodic
+//!   boundary conditions
+//! \param[out] albedo_map Map of surface IDs to albedo values for periodic
+//!   surfaces
+//! \param[out] periodic_sense_map Map of surface IDs to their sense values
+//!   (used to determine orientation for periodic BCs)
+void read_surfaces(pugi::xml_node node,
+  std::set<std::pair<int, int>>& periodic_pairs,
+  std::unordered_map<int, double>& albedo_map,
+  std::unordered_map<int, int>& periodic_sense_map);
+
+//! Resolve periodic surface pairs and assign boundary conditions.
+//!
+//! This function completes the setup of periodic boundary conditions by
+//! resolving unpaired periodic surfaces, determining whether each pair
+//! represents translational or rotational periodicity based on surface
+//! normals, and assigning the appropriate boundary condition objects.
+//!
+//! \param[inout] periodic_pairs Set of surface ID pairs representing periodic
+//!   boundary conditions; unpaired entries are resolved
+//! \param albedo_map Map of surface IDs to albedo values for periodic surfaces
+//! \param periodic_sense_map Map of surface IDs to their sense values (used to
+//!   determine orientation for periodic BCs)
+void prepare_boundary_conditions(std::set<std::pair<int, int>>& periodic_pairs,
+  std::unordered_map<int, double>& albedo_map,
+  std::unordered_map<int, int>& periodic_sense_map);
 
 void free_memory_surfaces();
 
diff --git a/include/openmc/tallies/filter.h b/include/openmc/tallies/filter.h
index 3e982d0cf..77b0d9f42 100644
--- a/include/openmc/tallies/filter.h
+++ b/include/openmc/tallies/filter.h
@@ -33,17 +33,21 @@ enum class FilterType {
   MATERIALFROM,
   MESH,
   MESHBORN,
+  MESH_MATERIAL,
   MESH_SURFACE,
   MU,
   MUSURFACE,
   PARENT_NUCLIDE,
   PARTICLE,
+  PARTICLE_PRODUCTION,
   POLAR,
+  REACTION,
   SPHERICAL_HARMONICS,
   SPATIAL_LEGENDRE,
   SURFACE,
   TIME,
   UNIVERSE,
+  WEIGHT,
   ZERNIKE,
   ZERNIKE_RADIAL
 };
diff --git a/include/openmc/tallies/filter_energy.h b/include/openmc/tallies/filter_energy.h
index cf8a8aa0f..3e410d9fd 100644
--- a/include/openmc/tallies/filter_energy.h
+++ b/include/openmc/tallies/filter_energy.h
@@ -1,6 +1,7 @@
 #ifndef OPENMC_TALLIES_FILTER_ENERGY_H
 #define OPENMC_TALLIES_FILTER_ENERGY_H
 
+#include "openmc/particle.h"
 #include "openmc/span.h"
 #include "openmc/tallies/filter.h"
 #include "openmc/vector.h"
diff --git a/include/openmc/tallies/filter_mesh.h b/include/openmc/tallies/filter_mesh.h
index 2f48b6d4d..c35a477fe 100644
--- a/include/openmc/tallies/filter_mesh.h
+++ b/include/openmc/tallies/filter_mesh.h
@@ -9,9 +9,9 @@
 namespace openmc {
 
 //==============================================================================
-//! Indexes the location of particle events to a regular mesh.  For tracklength
-//! tallies, it will produce multiple valid bins and the bin weight will
-//! correspond to the fraction of the track length that lies in that bin.
+//! Indexes the location of particle events to a mesh. For tracklength tallies,
+//! it will produce multiple valid bins and the bin weight will correspond to
+//! the fraction of the track length that lies in that bin.
 //==============================================================================
 
 class MeshFilter : public Filter {
@@ -51,6 +51,12 @@ public:
 
   virtual bool translated() const { return translated_; }
 
+  virtual void set_rotation(const vector<double>& rotation);
+
+  virtual const vector<double>& rotation() const { return rotation_; }
+
+  virtual bool rotated() const { return rotated_; }
+
 protected:
   //----------------------------------------------------------------------------
   // Data members
@@ -58,6 +64,8 @@ protected:
   int32_t mesh_;            //!< Index of the mesh
   bool translated_ {false}; //!< Whether or not the filter is translated
   Position translation_ {0.0, 0.0, 0.0}; //!< Filter translation
+  bool rotated_ {false};    //!< Whether or not the filter is rotated
+  vector<double> rotation_; //!< Filter rotation
 };
 
 } // namespace openmc
diff --git a/include/openmc/tallies/filter_meshmaterial.h b/include/openmc/tallies/filter_meshmaterial.h
new file mode 100644
index 000000000..42a4edcf0
--- /dev/null
+++ b/include/openmc/tallies/filter_meshmaterial.h
@@ -0,0 +1,114 @@
+#ifndef OPENMC_TALLIES_FILTER_MESHMATERIAL_H
+#define OPENMC_TALLIES_FILTER_MESHMATERIAL_H
+
+#include <cstdint>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+
+#include "openmc/position.h"
+#include "openmc/random_ray/source_region.h"
+#include "openmc/span.h"
+#include "openmc/tallies/filter.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+//==============================================================================
+//! Helper structs that define a combination of a mesh element index and a
+//! material index and a functor for hashing to place in an unordered_map
+//==============================================================================
+
+struct ElementMat {
+  //! Check for equality
+  bool operator==(const ElementMat& other) const
+  {
+    return index_element == other.index_element && index_mat == other.index_mat;
+  }
+
+  int32_t index_element;
+  int32_t index_mat;
+};
+
+struct ElementMatHash {
+  std::size_t operator()(const ElementMat& k) const
+  {
+    size_t seed = 0;
+    hash_combine(seed, k.index_element);
+    hash_combine(seed, k.index_mat);
+    return seed;
+  }
+};
+
+//==============================================================================
+//! Indexes the location of particle events to combinations of mesh element
+//! index and material.  For tracklength tallies, it will produce multiple valid
+//! bins and the bin weight will correspond to the fraction of the track length
+//! that lies in that bin.
+//==============================================================================
+
+class MeshMaterialFilter : public Filter {
+public:
+  //----------------------------------------------------------------------------
+  // Constructors, destructors
+
+  ~MeshMaterialFilter() = default;
+
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "meshmaterial"; }
+  FilterType type() const override { return FilterType::MESH_MATERIAL; }
+
+  void from_xml(pugi::xml_node node) override;
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  void to_statepoint(hid_t filter_group) const override;
+
+  std::string text_label(int bin) const override;
+
+  //----------------------------------------------------------------------------
+  // Accessors
+
+  int32_t mesh() const { return mesh_; }
+
+  void set_mesh(int32_t mesh);
+
+  //! Set the bins based on a flat vector of alternating element index and
+  //! material IDs
+  void set_bins(span<int32_t> bins);
+
+  //! Set the bins based on a vector of (element, material index) pairs
+  void set_bins(vector<ElementMat>&& bins);
+
+  virtual void set_translation(const Position& translation);
+
+  virtual void set_translation(const double translation[3]);
+
+  virtual const Position& translation() const { return translation_; }
+
+  virtual bool translated() const { return translated_; }
+
+private:
+  //----------------------------------------------------------------------------
+  // Data members
+
+  int32_t mesh_;            //!< Index of the mesh
+  bool translated_ {false}; //!< Whether or not the filter is translated
+  Position translation_ {0.0, 0.0, 0.0}; //!< Filter translation
+
+  //! The indices of the mesh element-material combinations binned by this
+  //! filter.
+  vector<ElementMat> bins_;
+
+  //! The set of materials used in this filter
+  std::unordered_set<int32_t> materials_;
+
+  //! A map from mesh element-material indices to filter bin indices.
+  std::unordered_map<ElementMat, int32_t, ElementMatHash> map_;
+};
+
+} // namespace openmc
+#endif // OPENMC_TALLIES_FILTER_MESHMATERIAL_H
diff --git a/include/openmc/tallies/filter_particle.h b/include/openmc/tallies/filter_particle.h
index 863a6d282..9932a6037 100644
--- a/include/openmc/tallies/filter_particle.h
+++ b/include/openmc/tallies/filter_particle.h
@@ -1,7 +1,7 @@
 #ifndef OPENMC_TALLIES_FILTER_PARTICLE_H
 #define OPENMC_TALLIES_FILTER_PARTICLE_H
 
-#include "openmc/particle.h"
+#include "openmc/particle_type.h"
 #include "openmc/span.h"
 #include "openmc/tallies/filter.h"
 #include "openmc/vector.h"
diff --git a/include/openmc/tallies/filter_particle_production.h b/include/openmc/tallies/filter_particle_production.h
new file mode 100644
index 000000000..00fac9f7c
--- /dev/null
+++ b/include/openmc/tallies/filter_particle_production.h
@@ -0,0 +1,53 @@
+#ifndef OPENMC_TALLIES_FILTER_PARTICLE_PRODUCTION_H
+#define OPENMC_TALLIES_FILTER_PARTICLE_PRODUCTION_H
+
+#include <unordered_map>
+
+#include "openmc/particle.h"
+#include "openmc/tallies/filter.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+//==============================================================================
+//! Bins the outgoing energy of secondary particles
+//!
+//! This filter bins secondary particles by type and, optionally, by energy. It
+//! can be used to get the photon production matrix for multigroup photon
+//! transport, the energy distribution of secondary neutrons, etc. The weight
+//! that is applied is equal to the weight of the secondary particle. Thus, to
+//! get secondary production it should be used in conjunction with the "events"
+//! score.
+//==============================================================================
+
+class ParticleProductionFilter : public Filter {
+public:
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "particleproduction"; }
+  FilterType type() const override { return FilterType::PARTICLE_PRODUCTION; }
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  std::string text_label(int bin) const override;
+
+  void from_xml(pugi::xml_node node) override;
+
+  void to_statepoint(hid_t filter_group) const override;
+
+private:
+  //----------------------------------------------------------------------------
+  // Data members
+
+  vector<ParticleType> secondary_types_; //!< Types of secondary particles
+
+  //! Map from PDG number to index in secondary_types_ for O(1) lookup
+  std::unordered_map<int32_t, int> type_to_index_;
+
+  vector<double> energy_bins_; //!< Energy bin boundaries (optional)
+};
+
+} // namespace openmc
+#endif // OPENMC_TALLIES_FILTER_PARTICLE_PRODUCTION_H
diff --git a/include/openmc/tallies/filter_reaction.h b/include/openmc/tallies/filter_reaction.h
new file mode 100644
index 000000000..015729eef
--- /dev/null
+++ b/include/openmc/tallies/filter_reaction.h
@@ -0,0 +1,52 @@
+#ifndef OPENMC_TALLIES_FILTER_REACTION_H
+#define OPENMC_TALLIES_FILTER_REACTION_H
+
+#include "openmc/span.h"
+#include "openmc/tallies/filter.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+//==============================================================================
+//! Bins tally events based on the reaction type (MT number).
+//==============================================================================
+
+class ReactionFilter : public Filter {
+public:
+  //----------------------------------------------------------------------------
+  // Constructors, destructors
+
+  ~ReactionFilter() = default;
+
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "reaction"; }
+  FilterType type() const override { return FilterType::REACTION; }
+
+  void from_xml(pugi::xml_node node) override;
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  void to_statepoint(hid_t filter_group) const override;
+
+  std::string text_label(int bin) const override;
+
+  //----------------------------------------------------------------------------
+  // Accessors
+
+  const vector<int>& bins() const { return bins_; }
+  void set_bins(span<const int> bins);
+
+protected:
+  //----------------------------------------------------------------------------
+  // Data members
+
+  //! MT numbers to match
+  vector<int> bins_;
+};
+
+} // namespace openmc
+
+#endif // OPENMC_TALLIES_FILTER_REACTION_H
diff --git a/include/openmc/tallies/filter_weight.h b/include/openmc/tallies/filter_weight.h
new file mode 100644
index 000000000..1fe9d75d3
--- /dev/null
+++ b/include/openmc/tallies/filter_weight.h
@@ -0,0 +1,51 @@
+#ifndef OPENMC_TALLIES_FILTER_WEIGHT_H
+#define OPENMC_TALLIES_FILTER_WEIGHT_H
+
+#include <string>
+
+#include "openmc/span.h"
+#include "openmc/tallies/filter.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+//==============================================================================
+//! Bins the weights of the particles.
+//==============================================================================
+
+class WeightFilter : public Filter {
+public:
+  //----------------------------------------------------------------------------
+  // Constructors, destructors
+
+  ~WeightFilter() = default;
+
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "weight"; }
+  FilterType type() const override { return FilterType::WEIGHT; }
+
+  void from_xml(pugi::xml_node node) override;
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  void to_statepoint(hid_t filter_group) const override;
+
+  std::string text_label(int bin) const override;
+
+  //----------------------------------------------------------------------------
+  // Accessors
+
+  const vector<double>& bins() const { return bins_; }
+  void set_bins(span<const double> bins);
+
+protected:
+  //----------------------------------------------------------------------------
+  // Data members
+  vector<double> bins_;
+};
+
+} // namespace openmc
+#endif // OPENMC_TALLIES_FILTER_WEIGHT_H
diff --git a/include/openmc/tallies/tally.h b/include/openmc/tallies/tally.h
index 48f678ced..ae604b732 100644
--- a/include/openmc/tallies/tally.h
+++ b/include/openmc/tallies/tally.h
@@ -8,9 +8,8 @@
 #include "openmc/tallies/trigger.h"
 #include "openmc/vector.h"
 
+#include "openmc/tensor.h"
 #include "pugixml.hpp"
-#include "xtensor/xfixed.hpp"
-#include "xtensor/xtensor.hpp"
 
 #include <string>
 #include <unordered_map>
@@ -55,7 +54,7 @@ public:
 
   void set_nuclides(const vector<std::string>& nuclides);
 
-  const xt::xtensor<double, 3>& results() const { return results_; }
+  const tensor::Tensor<double>& results() const { return results_; }
 
   //! returns vector of indices corresponding to the tally this is called on
   const vector<int32_t>& filters() const { return filters_; }
@@ -98,14 +97,16 @@ public:
   //! Given already-set filters, set the stride lengths
   void set_strides();
 
-  int32_t strides(int i) const { return strides_[i]; }
+  int64_t strides(int i) const { return strides_[i]; }
 
-  int32_t n_filter_bins() const { return n_filter_bins_; }
+  int64_t n_filter_bins() const { return n_filter_bins_; }
 
   bool multiply_density() const { return multiply_density_; }
 
   bool writable() const { return writable_; }
 
+  bool higher_moments() const { return higher_moments_; }
+
   //----------------------------------------------------------------------------
   // Other methods.
 
@@ -123,7 +124,7 @@ public:
   int score_index(const std::string& score) const;
 
   //! Tally results reshaped according to filter sizes
-  xt::xarray<double> get_reshaped_data() const;
+  tensor::Tensor<double> get_reshaped_data() const;
 
   //! A string representing the i-th score on this tally
   std::string score_name(int score_idx) const;
@@ -158,7 +159,7 @@ public:
   //! combination of filters (e.g. specific cell, specific energy group, etc.)
   //! and the second dimension of the array is for scores (e.g. flux, total
   //! reaction rate, fission reaction rate, etc.)
-  xt::xtensor<double, 3> results_;
+  tensor::Tensor<double> results_;
 
   //! True if this tally should be written to statepoint files
   bool writable_ {true};
@@ -169,10 +170,8 @@ public:
   // We need to have quick access to some filters.  The following gives indices
   // for various filters that could be in the tally or C_NONE if they are not
   // present.
-  int energy_filter_ {C_NONE};
   int energyout_filter_ {C_NONE};
   int delayedgroup_filter_ {C_NONE};
-  int cell_filter_ {C_NONE};
 
   vector<Trigger> triggers_;
 
@@ -185,13 +184,16 @@ private:
   vector<int32_t> filters_; //!< Filter indices in global filters array
 
   //! Index strides assigned to each filter to support 1D indexing.
-  vector<int32_t> strides_;
+  vector<int64_t> strides_;
 
-  int32_t n_filter_bins_ {0};
+  int64_t n_filter_bins_ {0};
 
   //! Whether to multiply by atom density for reaction rates
   bool multiply_density_ {true};
 
+  //! Whether to accumulate higher moments (third and fourth)
+  bool higher_moments_ {false};
+
   int64_t index_;
 };
 
@@ -205,17 +207,19 @@ extern vector<unique_ptr<Tally>> tallies;
 extern vector<int> active_tallies;
 extern vector<int> active_analog_tallies;
 extern vector<int> active_tracklength_tallies;
+extern vector<int> active_timed_tracklength_tallies;
 extern vector<int> active_collision_tallies;
 extern vector<int> active_meshsurf_tallies;
 extern vector<int> active_surface_tallies;
 extern vector<int> active_pulse_height_tallies;
-extern vector<int> pulse_height_cells;
+extern vector<int32_t> pulse_height_cells;
+extern vector<double> time_grid;
+
 } // namespace model
 
 namespace simulation {
 //! Global tallies (such as k-effective estimators)
-extern xt::xtensor_fixed<double, xt::xshape<N_GLOBAL_TALLIES, 3>>
-  global_tallies;
+extern tensor::StaticTensor2D<double, N_GLOBAL_TALLIES, 3> global_tallies;
 
 //! Number of realizations for global tallies
 extern "C" int32_t n_realizations;
@@ -241,15 +245,16 @@ void read_tallies_xml(pugi::xml_node root);
 //! batch to a new random variable
 void accumulate_tallies();
 
+//! Determine distance to next time boundary
+//
+//! \param time Current time of particle
+//! \param speed Speed of particle
+//! \return Distance to next time boundary (or INFTY if none)
+double distance_to_time_boundary(double time, double speed);
+
 //! Determine which tallies should be active
 void setup_active_tallies();
 
-// Alias for the type returned by xt::adapt(...). N is the dimension of the
-// multidimensional array
-template<std::size_t N>
-using adaptor_type =
-  xt::xtensor_adaptor<xt::xbuffer_adaptor<double*&, xt::no_ownership>, N>;
-
 #ifdef OPENMC_MPI
 //! Collect all tally results onto master process
 void reduce_tally_results();
diff --git a/include/openmc/tallies/tally_scoring.h b/include/openmc/tallies/tally_scoring.h
index 28f1f1622..4303ddb7a 100644
--- a/include/openmc/tallies/tally_scoring.h
+++ b/include/openmc/tallies/tally_scoring.h
@@ -41,7 +41,7 @@ public:
 
   FilterBinIter& operator++();
 
-  int index_ {1};
+  int64_t index_ {1};
   double weight_ {1.};
 
   vector<FilterMatch>& filter_matches_;
@@ -91,11 +91,29 @@ void score_analog_tally_mg(Particle& p);
 //! \param distance The distance in [cm] traveled by the particle
 void score_tracklength_tally(Particle& p, double distance);
 
-//! Score surface or mesh-surface tallies for particle currents.
+//! Score time filtered tallies using a tracklength estimate of the flux.
+//
+//! This is triggered at every event (surface crossing, lattice crossing, or
+//! collision) and thus cannot be done for tallies that require post-collision
+//! information.
+//
+//! \param p The particle being tracked
+//! \param total_distance The distance in [cm] traveled by the particle
+void score_timed_tracklength_tally(Particle& p, double total_distance);
+
+//! Score mesh-surface tallies for particle currents.
 //
 //! \param p The particle being tracked
 //! \param tallies A vector of the indices of the tallies to score to
-void score_surface_tally(Particle& p, const vector<int>& tallies);
+void score_meshsurface_tally(Particle& p, const vector<int>& tallies);
+
+//! Score surface tallies for particle currents.
+//
+//! \param p The particle being tracked
+//! \param tallies A vector of the indices of the tallies to score to
+//! \param normal The normal of the surface being crossed
+void score_surface_tally(
+  Particle& p, const vector<int>& tallies, const Direction& normal);
 
 //! Score the pulse-height tally
 //! This is triggered at the end of every particle history
diff --git a/include/openmc/tensor.h b/include/openmc/tensor.h
new file mode 100644
index 000000000..9e443e72f
--- /dev/null
+++ b/include/openmc/tensor.h
@@ -0,0 +1,1210 @@
+//! \file tensor.h
+//! \brief Multi-dimensional tensor types for OpenMC.
+//!
+//! Tensor<T> is the primary type: a dynamic-rank owning container that stores
+//! elements contiguously in row-major order.  View<T> is a lightweight
+//! non-owning reference into a Tensor's storage, returned by the slice()
+//! method and flat().  StaticTensor2D<T,R,C> is a small stack-allocated 2D
+//! array used only for simulation::global_tallies.
+//!
+//! Slicing follows numpy conventions: each axis takes an index (rank-reducing),
+//! All (keep entire axis), or Range (keep sub-range).  For example,
+//! arr.slice(0, all, range(2, 5)) is equivalent to numpy's arr[0, :, 2:5].
+//!
+//! View is declared before Tensor because Tensor's methods return View objects.
+
+#ifndef OPENMC_TENSOR_H
+#define OPENMC_TENSOR_H
+
+#include "openmc/vector.h"
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <limits>
+#include <type_traits>
+
+namespace openmc {
+namespace tensor {
+
+//==============================================================================
+// Forward declarations
+//==============================================================================
+
+template<typename T>
+class Tensor;
+
+template<typename T, size_t R, size_t C>
+class StaticTensor2D;
+
+//==============================================================================
+// Storage type mapping
+//
+// std::vector<bool> is a bit-packed specialization that returns proxy objects
+// instead of real references, which breaks generic code.  storage_type_map
+// redirects bool to unsigned char so that Tensor<bool> stores one byte per
+// element with normal reference semantics.
+//==============================================================================
+
+template<typename T>
+struct storage_type_map {
+  using type = T;
+};
+template<>
+struct storage_type_map<bool> {
+  using type = unsigned char;
+};
+template<typename T>
+using storage_type = typename storage_type_map<T>::type;
+
+//==============================================================================
+// Slice argument types
+//
+// Used with the variadic slice() method on Tensor, View, and StaticTensor2D.
+// Each argument corresponds to one axis: a plain integer fixes that axis at
+// a single index (rank-reducing), All keeps the entire axis, and Range keeps
+// a sub-range.
+//==============================================================================
+
+//! Keep an entire axis (equivalent to numpy's ':' or xtensor's xt::all())
+struct All {};
+constexpr All all {};
+
+//! Sub-range along an axis [start, end)
+struct Range {
+  size_t start;
+  size_t end; // SIZE_MAX means "to end of axis"
+};
+
+//! Create a Range [start, end)
+inline Range range(size_t start, size_t end)
+{
+  return {start, end};
+}
+
+//! Create a Range [0, end)
+inline Range range(size_t end)
+{
+  return {0, end};
+}
+
+namespace detail {
+
+//! Internal: normalized representation of a per-axis slice argument
+struct SliceArg {
+  enum Kind { INDEX, ALL, RANGE } kind;
+  size_t start;
+  size_t end;
+};
+
+inline SliceArg to_slice_arg(All)
+{
+  return {SliceArg::ALL, 0, 0};
+}
+inline SliceArg to_slice_arg(Range r)
+{
+  return {SliceArg::RANGE, r.start, r.end};
+}
+
+template<typename I>
+inline
+  typename std::enable_if<std::is_integral<I>::value || std::is_enum<I>::value,
+    SliceArg>::type
+  to_slice_arg(I i)
+{
+  return {SliceArg::INDEX, static_cast<size_t>(i), 0};
+}
+
+//! Result of a slice computation: pointer offset + new shape/strides
+struct SliceResult {
+  size_t ptr_offset;
+  vector<size_t> shape;
+  vector<size_t> strides;
+};
+
+//! Compute the result of applying slice arguments to shape/strides
+template<typename First, typename... Rest>
+SliceResult compute_slice(const vector<size_t>& shape,
+  const vector<size_t>& strides, First first, Rest... rest)
+{
+  const size_t n = 1 + sizeof...(Rest);
+  SliceArg args[1 + sizeof...(Rest)] = {
+    to_slice_arg(first), to_slice_arg(rest)...};
+
+  size_t offset = 0;
+  vector<size_t> new_shape;
+  vector<size_t> new_strides;
+
+  for (size_t a = 0; a < n; ++a) {
+    switch (args[a].kind) {
+    case SliceArg::INDEX:
+      offset += args[a].start * strides[a];
+      break;
+    case SliceArg::ALL:
+      new_shape.push_back(shape[a]);
+      new_strides.push_back(strides[a]);
+      break;
+    case SliceArg::RANGE: {
+      offset += args[a].start * strides[a];
+      size_t end = (args[a].end == SIZE_MAX) ? shape[a] : args[a].end;
+      new_shape.push_back(end - args[a].start);
+      new_strides.push_back(strides[a]);
+      break;
+    }
+    }
+  }
+
+  // Trailing axes not covered by arguments are implicitly All.
+  // This matches numpy: a[i] on a 2D array returns a 1D row.
+  for (size_t a = n; a < shape.size(); ++a) {
+    new_shape.push_back(shape[a]);
+    new_strides.push_back(strides[a]);
+  }
+
+  return {offset, std::move(new_shape), std::move(new_strides)};
+}
+
+} // namespace detail
+
+//==============================================================================
+// View<T>: a non-owning N-dimensional view into a tensor's storage.
+//
+// Holds a base pointer, shape, and strides (in elements).  Supports arbitrary
+// rank and multi-axis slicing via the variadic slice() method.
+//==============================================================================
+
+template<typename T>
+class View {
+public:
+  //--------------------------------------------------------------------------
+  // Constructors
+
+  View(T* data, vector<size_t> shape, vector<size_t> strides)
+    : data_(data), shape_(std::move(shape)), strides_(std::move(strides))
+  {}
+
+  // Explicitly default copy/move constructors (declaring copy assignment
+  // below would otherwise suppress the implicit move constructor).
+  View(const View&) = default;
+  View(View&&) = default;
+
+  //--------------------------------------------------------------------------
+  // Indexing
+
+  //! Multi-index element access (1D, 2D, 3D, ...)
+  template<typename... Indices>
+  T& operator()(Indices... indices)
+  {
+    const size_t idx[] = {static_cast<size_t>(indices)...};
+    size_t off = 0;
+    for (size_t d = 0; d < sizeof...(Indices); ++d)
+      off += idx[d] * strides_[d];
+    return data_[off];
+  }
+
+  template<typename... Indices>
+  const T& operator()(Indices... indices) const
+  {
+    const size_t idx[] = {static_cast<size_t>(indices)...};
+    size_t off = 0;
+    for (size_t d = 0; d < sizeof...(Indices); ++d)
+      off += idx[d] * strides_[d];
+    return data_[off];
+  }
+
+  //! Flat logical index (row-major order)
+  T& operator[](size_t i) { return data_[flat_to_offset(i)]; }
+  const T& operator[](size_t i) const { return data_[flat_to_offset(i)]; }
+
+  //--------------------------------------------------------------------------
+  // Accessors
+
+  size_t size() const
+  {
+    size_t s = 1;
+    for (auto d : shape_)
+      s *= d;
+    return s;
+  }
+  size_t ndim() const { return shape_.size(); }
+  size_t shape(size_t axis) const { return shape_[axis]; }
+  const vector<size_t>& shape_vec() const { return shape_; }
+  T* data() { return data_; }
+  const T* data() const { return data_; }
+
+  //--------------------------------------------------------------------------
+  // View accessors
+
+  //! Multi-axis slice.  Each argument corresponds to one axis and is either:
+  //!   - an integer (fixes that axis, rank-reducing)
+  //!   - All (keeps entire axis)
+  //!   - Range (keeps sub-range along that axis)
+  //! Example: v.slice(0, all, range(2, 5)) == numpy v[0, :, 2:5]
+  template<typename First, typename... Rest>
+  View<T> slice(First first, Rest... rest)
+  {
+    auto r = detail::compute_slice(shape_, strides_, first, rest...);
+    return {data_ + r.ptr_offset, std::move(r.shape), std::move(r.strides)};
+  }
+
+  template<typename First, typename... Rest>
+  View<const T> slice(First first, Rest... rest) const
+  {
+    auto r = detail::compute_slice(shape_, strides_, first, rest...);
+    return {data_ + r.ptr_offset, std::move(r.shape), std::move(r.strides)};
+  }
+
+  //--------------------------------------------------------------------------
+  // Assignment operators
+
+  //! Copy assignment: element-wise deep copy (writes through data pointer).
+  //! Without this, the compiler's implicit copy assignment just copies the
+  //! View metadata (pointer, shape, strides) instead of the viewed data.
+  View& operator=(const View& other)
+  {
+    size_t n = size();
+    for (size_t i = 0; i < n; ++i)
+      data_[flat_to_offset(i)] = other[i];
+    return *this;
+  }
+
+  //! Fill all elements with a scalar
+  View& operator=(T val)
+  {
+    size_t n = size();
+    for (size_t i = 0; i < n; ++i)
+      data_[flat_to_offset(i)] = val;
+    return *this;
+  }
+
+  //! Assignment from initializer_list (for 1D views)
+  View& operator=(std::initializer_list<T> vals)
+  {
+    auto it = vals.begin();
+    for (size_t i = 0; i < size() && it != vals.end(); ++i, ++it)
+      data_[flat_to_offset(i)] = *it;
+    return *this;
+  }
+
+  //! Assignment from another View
+  template<typename U>
+  View& operator=(const View<U>& other)
+  {
+    size_t n = size();
+    for (size_t i = 0; i < n; ++i)
+      data_[flat_to_offset(i)] = other[i];
+    return *this;
+  }
+
+  //! Assignment from Tensor (forward-declared, defined after Tensor)
+  template<typename U>
+  View& operator=(const Tensor<U>& other);
+
+  //! Compound addition from Tensor (forward-declared, defined after Tensor)
+  template<typename U>
+  View& operator+=(const Tensor<U>& o);
+
+  //! Compound multiply by scalar
+  View& operator*=(T val)
+  {
+    size_t n = size();
+    for (size_t i = 0; i < n; ++i)
+      data_[flat_to_offset(i)] *= val;
+    return *this;
+  }
+
+  //! Compound divide by scalar
+  View& operator/=(T val)
+  {
+    size_t n = size();
+    for (size_t i = 0; i < n; ++i)
+      data_[flat_to_offset(i)] /= val;
+    return *this;
+  }
+
+  //--------------------------------------------------------------------------
+  // Reductions
+
+  //! Sum of all elements
+  T sum() const
+  {
+    // remove_const needed so accumulator is mutable when T is const-qualified
+    std::remove_const_t<T> s = 0;
+    size_t n = size();
+    for (size_t i = 0; i < n; ++i)
+      s += data_[flat_to_offset(i)];
+    return s;
+  }
+
+  //--------------------------------------------------------------------------
+  // Iterators
+  //
+  // Lightweight row-major iterator parameterized on pointer type (Ptr).
+  // Stores a flat logical position and converts to a physical offset on each
+  // dereference via divmod over shape/strides.  For contiguous 1D views (the
+  // common case) the divmod chain reduces to a single multiply-by-1, which
+  // the compiler optimizes away.
+  //
+  // view_iterator<T*>       = mutable iterator  (from non-const View)
+  // view_iterator<const T*> = read-only iterator (from const View)
+
+  template<typename Ptr>
+  class view_iterator {
+    Ptr base_;
+    size_t count_;
+    const size_t* shape_;
+    const size_t* strides_;
+    size_t ndim_;
+
+  public:
+    using iterator_category = std::random_access_iterator_tag;
+    using value_type = std::remove_const_t<T>;
+    using difference_type = std::ptrdiff_t;
+    using pointer = Ptr;
+    using reference = decltype(*std::declval<Ptr>());
+
+    view_iterator(Ptr base, size_t count, const View* v)
+      : base_(base), count_(count), shape_(v->shape_.data()),
+        strides_(v->strides_.data()), ndim_(v->shape_.size())
+    {}
+
+    reference operator*() const { return base_[offset()]; }
+    reference operator[](difference_type n) const
+    {
+      return base_[offset_of(count_ + n)];
+    }
+    view_iterator& operator++()
+    {
+      ++count_;
+      return *this;
+    }
+    view_iterator operator++(int)
+    {
+      auto tmp = *this;
+      ++count_;
+      return tmp;
+    }
+    view_iterator& operator--()
+    {
+      --count_;
+      return *this;
+    }
+    view_iterator operator+(difference_type n) const
+    {
+      auto tmp = *this;
+      tmp.count_ += n;
+      return tmp;
+    }
+    view_iterator operator-(difference_type n) const
+    {
+      auto tmp = *this;
+      tmp.count_ -= n;
+      return tmp;
+    }
+    difference_type operator-(const view_iterator& o) const
+    {
+      return static_cast<difference_type>(count_) -
+             static_cast<difference_type>(o.count_);
+    }
+    view_iterator& operator+=(difference_type n)
+    {
+      count_ += n;
+      return *this;
+    }
+    view_iterator& operator-=(difference_type n)
+    {
+      count_ -= n;
+      return *this;
+    }
+    bool operator==(const view_iterator& o) const { return count_ == o.count_; }
+    bool operator!=(const view_iterator& o) const { return count_ != o.count_; }
+    bool operator<(const view_iterator& o) const { return count_ < o.count_; }
+    bool operator>(const view_iterator& o) const { return count_ > o.count_; }
+    bool operator<=(const view_iterator& o) const { return count_ <= o.count_; }
+    bool operator>=(const view_iterator& o) const { return count_ >= o.count_; }
+    friend view_iterator operator+(difference_type n, const view_iterator& it)
+    {
+      return it + n;
+    }
+
+  private:
+    size_t offset() const { return offset_of(count_); }
+    size_t offset_of(size_t flat) const
+    {
+      size_t off = 0;
+      for (int d = static_cast<int>(ndim_) - 1; d >= 0; --d) {
+        off += (flat % shape_[d]) * strides_[d];
+        flat /= shape_[d];
+      }
+      return off;
+    }
+  };
+
+  using iterator = view_iterator<T*>;
+  using const_iterator = view_iterator<const T*>;
+
+  iterator begin() { return {data_, 0, this}; }
+  iterator end() { return {data_, size(), this}; }
+  const_iterator begin() const { return cbegin(); }
+  const_iterator end() const { return cend(); }
+  const_iterator cbegin() const { return {data_, 0, this}; }
+  const_iterator cend() const { return {data_, size(), this}; }
+
+private:
+  //! Convert a logical flat index (row-major) to a physical element offset
+  size_t flat_to_offset(size_t flat) const
+  {
+    size_t off = 0;
+    for (int d = static_cast<int>(shape_.size()) - 1; d >= 0; --d) {
+      off += (flat % shape_[d]) * strides_[d];
+      flat /= shape_[d];
+    }
+    return off;
+  }
+
+  T* data_;
+  vector<size_t> shape_;
+  vector<size_t> strides_;
+};
+
+//==============================================================================
+// Tensor<T>: dynamic-rank N-dimensional tensor.
+//
+// Stores elements in a contiguous row-major vector<storage_type<T>>
+// with a dynamic shape.
+//==============================================================================
+
+template<typename T>
+class Tensor {
+public:
+  using value_type = T;
+  using stored_type = storage_type<T>;
+  using iterator = typename vector<stored_type>::iterator;
+  using const_iterator = typename vector<stored_type>::const_iterator;
+
+  //--------------------------------------------------------------------------
+  // Constructors
+
+  Tensor() = default;
+
+  //! Construct with shape (uninitialized for arithmetic types via vector
+  //! resize)
+  explicit Tensor(vector<size_t> shape)
+    : shape_(std::move(shape)), data_(compute_size())
+  {}
+
+  //! Construct with shape and fill value
+  Tensor(vector<size_t> shape, T fill)
+    : shape_(std::move(shape)), data_(compute_size(), fill)
+  {}
+
+  //! Construct from initializer_list shape
+  explicit Tensor(std::initializer_list<size_t> shape)
+    : shape_(shape), data_(compute_size())
+  {}
+
+  //! Construct from initializer_list shape with fill
+  Tensor(std::initializer_list<size_t> shape, T fill)
+    : shape_(shape), data_(compute_size(), fill)
+  {}
+
+  //! 1D copy from raw pointer + count
+  Tensor(const T* ptr, size_t count) : shape_({count}), data_(ptr, ptr + count)
+  {}
+
+  //! Copy from View (preserves view's shape)
+  template<typename U>
+  explicit Tensor(const View<U>& v) : shape_(v.shape_vec())
+  {
+    size_t n = v.size();
+    data_.resize(n);
+    for (size_t i = 0; i < n; ++i)
+      data_[i] = v[i];
+  }
+
+  //--------------------------------------------------------------------------
+  // Assignment
+
+  //! Assignment from View
+  template<typename U>
+  Tensor& operator=(const View<U>& v)
+  {
+    shape_ = v.shape_vec();
+    size_t n = v.size();
+    data_.resize(n);
+    for (size_t i = 0; i < n; ++i)
+      data_[i] = v[i];
+    return *this;
+  }
+
+  //! Assignment from initializer_list of values (1D)
+  Tensor& operator=(std::initializer_list<T> vals)
+  {
+    shape_ = {vals.size()};
+    data_.assign(vals.begin(), vals.end());
+    return *this;
+  }
+
+  //--------------------------------------------------------------------------
+  // Accessors
+
+  stored_type* data() { return data_.data(); }
+  const stored_type* data() const { return data_.data(); }
+  size_t size() const { return data_.size(); }
+  const vector<size_t>& shape() const { return shape_; }
+  size_t shape(size_t dim) const
+  {
+    return dim < shape_.size() ? shape_[dim] : 0;
+  }
+  size_t ndim() const { return shape_.size(); }
+  bool empty() const { return data_.empty(); }
+
+  //--------------------------------------------------------------------------
+  // Indexing (row-major)
+
+  template<typename... Indices>
+  stored_type& operator()(Indices... indices)
+  {
+    const size_t idx[] = {static_cast<size_t>(indices)...};
+    size_t off = 0;
+    for (size_t d = 0; d < sizeof...(Indices); ++d)
+      off = off * shape_[d] + idx[d];
+    return data_[off];
+  }
+
+  template<typename... Indices>
+  const stored_type& operator()(Indices... indices) const
+  {
+    const size_t idx[] = {static_cast<size_t>(indices)...};
+    size_t off = 0;
+    for (size_t d = 0; d < sizeof...(Indices); ++d)
+      off = off * shape_[d] + idx[d];
+    return data_[off];
+  }
+
+  stored_type& operator[](size_t i) { return data_[i]; }
+  const stored_type& operator[](size_t i) const { return data_[i]; }
+
+  //! First and last element
+  stored_type& front() { return data_.front(); }
+  const stored_type& front() const { return data_.front(); }
+  stored_type& back() { return data_.back(); }
+  const stored_type& back() const { return data_.back(); }
+
+  //--------------------------------------------------------------------------
+  // Iterators
+
+  iterator begin() { return data_.begin(); }
+  iterator end() { return data_.end(); }
+  const_iterator begin() const { return data_.begin(); }
+  const_iterator end() const { return data_.end(); }
+  const_iterator cbegin() const { return data_.cbegin(); }
+  const_iterator cend() const { return data_.cend(); }
+
+  //--------------------------------------------------------------------------
+  // Mutation
+
+  void resize(const vector<size_t>& shape)
+  {
+    shape_ = shape;
+    data_.resize(compute_size());
+  }
+
+  void resize(std::initializer_list<size_t> shape)
+  {
+    shape_.assign(shape.begin(), shape.end());
+    data_.resize(compute_size());
+  }
+
+  void reshape(const vector<size_t>& new_shape) { shape_ = new_shape; }
+
+  void fill(T val) { std::fill(data_.begin(), data_.end(), val); }
+
+  //--------------------------------------------------------------------------
+  // View accessors
+
+  //! Fix one axis at a given index, returning an (N-1)-dimensional view
+  //! Multi-axis slice.  Each argument corresponds to one axis and is either:
+  //!   - an integer (fixes that axis, rank-reducing)
+  //!   - All (keeps entire axis)
+  //!   - Range (keeps sub-range along that axis)
+  //! Example: t.slice(0, all, range(2, 5)) == numpy t[0, :, 2:5]
+  template<typename First, typename... Rest>
+  View<stored_type> slice(First first, Rest... rest)
+  {
+    auto strides = compute_strides();
+    auto r = detail::compute_slice(shape_, strides, first, rest...);
+    return {
+      data_.data() + r.ptr_offset, std::move(r.shape), std::move(r.strides)};
+  }
+
+  template<typename First, typename... Rest>
+  View<const stored_type> slice(First first, Rest... rest) const
+  {
+    auto strides = compute_strides();
+    auto r = detail::compute_slice(shape_, strides, first, rest...);
+    return {
+      data_.data() + r.ptr_offset, std::move(r.shape), std::move(r.strides)};
+  }
+
+  //! Flat 1D view of all elements
+  View<stored_type> flat()
+  {
+    return {data_.data(), {data_.size()}, {size_t(1)}};
+  }
+  View<const stored_type> flat() const
+  {
+    return {data_.data(), {data_.size()}, {size_t(1)}};
+  }
+
+  //--------------------------------------------------------------------------
+  // Reductions and transforms
+
+  //! Sum of all elements
+  T sum() const
+  {
+    T s = T(0);
+    for (size_t i = 0; i < data_.size(); ++i)
+      s += data_[i];
+    return s;
+  }
+
+  //! Sum along an axis, reducing rank by 1 (defined out-of-line below)
+  Tensor<T> sum(size_t axis) const;
+
+  //! Product of all elements
+  T prod() const
+  {
+    T p = T(1);
+    for (size_t i = 0; i < data_.size(); ++i)
+      p *= data_[i];
+    return p;
+  }
+
+  //! True if any element is nonzero
+  bool any() const
+  {
+    for (size_t i = 0; i < data_.size(); ++i)
+      if (data_[i])
+        return true;
+    return false;
+  }
+
+  //! True if all elements are nonzero
+  bool all() const
+  {
+    for (size_t i = 0; i < data_.size(); ++i)
+      if (!data_[i])
+        return false;
+    return true;
+  }
+
+  //! Flat index of the minimum element
+  size_t argmin() const
+  {
+    return static_cast<size_t>(std::distance(data_.data(),
+      std::min_element(data_.data(), data_.data() + data_.size())));
+  }
+
+  //! Reverse element order along an axis (e.g. flip(0) reverses rows)
+  Tensor flip(size_t axis) const
+  {
+    size_t outer_size = 1;
+    for (size_t d = 0; d < axis; ++d)
+      outer_size *= shape_[d];
+    size_t axis_size = shape_[axis];
+    size_t inner_size = 1;
+    for (size_t d = axis + 1; d < shape_.size(); ++d)
+      inner_size *= shape_[d];
+
+    Tensor r(shape_);
+    for (size_t o = 0; o < outer_size; ++o)
+      for (size_t a = 0; a < axis_size; ++a)
+        for (size_t i = 0; i < inner_size; ++i)
+          r.data_[(o * axis_size + (axis_size - 1 - a)) * inner_size + i] =
+            data_[(o * axis_size + a) * inner_size + i];
+    return r;
+  }
+
+  //--------------------------------------------------------------------------
+  // Operators
+
+  Tensor& operator+=(T val)
+  {
+    for (auto& x : data_)
+      x += val;
+    return *this;
+  }
+  Tensor& operator-=(T val)
+  {
+    for (auto& x : data_)
+      x -= val;
+    return *this;
+  }
+  Tensor& operator*=(T val)
+  {
+    for (auto& x : data_)
+      x *= val;
+    return *this;
+  }
+  Tensor& operator/=(T val)
+  {
+    for (auto& x : data_)
+      x /= val;
+    return *this;
+  }
+  Tensor& operator+=(const Tensor& o)
+  {
+    for (size_t i = 0; i < data_.size(); ++i)
+      data_[i] += o.data_[i];
+    return *this;
+  }
+  Tensor& operator-=(const Tensor& o)
+  {
+    for (size_t i = 0; i < data_.size(); ++i)
+      data_[i] -= o.data_[i];
+    return *this;
+  }
+  Tensor& operator*=(const Tensor& o)
+  {
+    for (size_t i = 0; i < data_.size(); ++i)
+      data_[i] *= o.data_[i];
+    return *this;
+  }
+  Tensor& operator/=(const Tensor& o)
+  {
+    for (size_t i = 0; i < data_.size(); ++i)
+      data_[i] /= o.data_[i];
+    return *this;
+  }
+
+  Tensor operator+(const Tensor& o) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] + o.data_[i];
+    return r;
+  }
+  Tensor operator-(const Tensor& o) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] - o.data_[i];
+    return r;
+  }
+  Tensor operator/(const Tensor& o) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] / o.data_[i];
+    return r;
+  }
+  Tensor operator*(const Tensor& o) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] * o.data_[i];
+    return r;
+  }
+
+  Tensor operator+(T val) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] + val;
+    return r;
+  }
+  Tensor operator-(T val) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] - val;
+    return r;
+  }
+  Tensor operator*(T val) const
+  {
+    Tensor r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data_[i] = data_[i] * val;
+    return r;
+  }
+
+  Tensor<bool> operator<=(T val) const
+  {
+    Tensor<bool> r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data()[i] = data_[i] <= val;
+    return r;
+  }
+  Tensor<bool> operator<(T val) const
+  {
+    Tensor<bool> r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data()[i] = data_[i] < val;
+    return r;
+  }
+  Tensor<bool> operator>=(T val) const
+  {
+    Tensor<bool> r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data()[i] = data_[i] >= val;
+    return r;
+  }
+  Tensor<bool> operator>(T val) const
+  {
+    Tensor<bool> r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data()[i] = data_[i] > val;
+    return r;
+  }
+  Tensor<bool> operator<(const Tensor& o) const
+  {
+    Tensor<bool> r(shape_);
+    for (size_t i = 0; i < data_.size(); ++i)
+      r.data()[i] = data_[i] < o.data_[i];
+    return r;
+  }
+
+private:
+  size_t compute_size() const
+  {
+    size_t s = 1;
+    for (auto d : shape_)
+      s *= d;
+    return s;
+  }
+
+  //! Compute row-major strides from shape
+  vector<size_t> compute_strides() const
+  {
+    vector<size_t> strides(shape_.size());
+    if (!shape_.empty()) {
+      strides.back() = 1;
+      for (int d = static_cast<int>(shape_.size()) - 2; d >= 0; --d)
+        strides[d] = strides[d + 1] * shape_[d + 1];
+    }
+    return strides;
+  }
+
+  //--------------------------------------------------------------------------
+  // Data members
+
+  vector<size_t> shape_;
+  vector<storage_type<T>> data_;
+};
+
+//==============================================================================
+// Non-member operators (scalar op tensor)
+//==============================================================================
+
+template<typename T>
+Tensor<T> operator*(T val, const Tensor<T>& arr)
+{
+  return arr * val;
+}
+
+template<typename T>
+Tensor<T> operator+(T val, const Tensor<T>& arr)
+{
+  return arr + val;
+}
+
+// Mixed-type arithmetic: Tensor<T1> op Tensor<T2> -> Tensor<double>
+// A SFINAE guard is used here, as without !is_same Tensor<T> * Tensor<T>
+// would be ambiguous between the member operator* and this non-member function.
+template<typename T1, typename T2,
+  typename = std::enable_if_t<!std::is_same<T1, T2>::value>>
+Tensor<double> operator*(const Tensor<T1>& a, const Tensor<T2>& b)
+{
+  Tensor<double> r(a.shape());
+  for (size_t i = 0; i < a.size(); ++i)
+    r.data()[i] =
+      static_cast<double>(a.data()[i]) * static_cast<double>(b.data()[i]);
+  return r;
+}
+
+// Same SFINAE guard as operator* above.
+template<typename T1, typename T2,
+  typename = std::enable_if_t<!std::is_same<T1, T2>::value>>
+Tensor<double> operator/(const Tensor<T1>& a, const Tensor<T2>& b)
+{
+  Tensor<double> r(a.shape());
+  for (size_t i = 0; i < a.size(); ++i)
+    r.data()[i] =
+      static_cast<double>(a.data()[i]) / static_cast<double>(b.data()[i]);
+  return r;
+}
+
+//==============================================================================
+// Out-of-line method definitions (require complete types)
+//==============================================================================
+
+template<typename T>
+template<typename U>
+View<T>& View<T>::operator=(const Tensor<U>& other)
+{
+  size_t n = size();
+  for (size_t i = 0; i < n; ++i)
+    data_[flat_to_offset(i)] = static_cast<T>(other.data()[i]);
+  return *this;
+}
+
+template<typename T>
+template<typename U>
+View<T>& View<T>::operator+=(const Tensor<U>& o)
+{
+  size_t n = size();
+  for (size_t i = 0; i < n; ++i)
+    data_[flat_to_offset(i)] += o.data()[i];
+  return *this;
+}
+
+template<typename T>
+Tensor<T> Tensor<T>::sum(size_t axis) const
+{
+  // Build output shape (all dims except the summed axis)
+  vector<size_t> out_shape;
+  for (size_t d = 0; d < shape_.size(); ++d)
+    if (d != axis)
+      out_shape.push_back(shape_[d]);
+
+  // Split dimensions into three zones: outer | axis | inner
+  size_t outer_size = 1;
+  for (size_t d = 0; d < axis; ++d)
+    outer_size *= shape_[d];
+  size_t axis_size = shape_[axis];
+  size_t inner_size = 1;
+  for (size_t d = axis + 1; d < shape_.size(); ++d)
+    inner_size *= shape_[d];
+
+  Tensor<T> result(out_shape, T(0));
+  for (size_t o = 0; o < outer_size; ++o)
+    for (size_t a = 0; a < axis_size; ++a)
+      for (size_t i = 0; i < inner_size; ++i)
+        result.data()[o * inner_size + i] +=
+          data_[(o * axis_size + a) * inner_size + i];
+
+  return result;
+}
+
+//==============================================================================
+// StaticTensor2D<T, R, C>: compile-time fixed 2D tensor.
+//==============================================================================
+
+template<typename T, size_t R, size_t C>
+class StaticTensor2D {
+public:
+  using value_type = T;
+
+  //--------------------------------------------------------------------------
+  // Indexing
+
+  //! Templated to accept enum class indices (e.g. GlobalTally, TallyResult)
+  //! which don't implicitly convert to integer types.
+  template<typename I0, typename I1>
+  T& operator()(I0 i, I1 j)
+  {
+    return data_[static_cast<size_t>(i) * C + static_cast<size_t>(j)];
+  }
+  template<typename I0, typename I1>
+  const T& operator()(I0 i, I1 j) const
+  {
+    return data_[static_cast<size_t>(i) * C + static_cast<size_t>(j)];
+  }
+
+  //--------------------------------------------------------------------------
+  // Accessors
+
+  T* data() { return data_; }
+  const T* data() const { return data_; }
+  constexpr size_t size() const { return R * C; }
+  std::array<size_t, 2> shape() const { return {R, C}; }
+
+  //--------------------------------------------------------------------------
+  // Mutation
+
+  void fill(T val) { std::fill(data_, data_ + R * C, val); }
+
+  //--------------------------------------------------------------------------
+  // Iterators
+
+  T* begin() { return data_; }
+  T* end() { return data_ + R * C; }
+  const T* begin() const { return data_; }
+  const T* end() const { return data_ + R * C; }
+
+  //--------------------------------------------------------------------------
+  // View accessors
+
+  //! Multi-axis slice (same interface as Tensor/View).
+  template<typename First, typename... Rest>
+  View<T> slice(First first, Rest... rest)
+  {
+    vector<size_t> sh = {R, C};
+    vector<size_t> st = {C, 1};
+    auto r = detail::compute_slice(sh, st, first, rest...);
+    return {data_ + r.ptr_offset, std::move(r.shape), std::move(r.strides)};
+  }
+  template<typename First, typename... Rest>
+  View<const T> slice(First first, Rest... rest) const
+  {
+    vector<size_t> sh = {R, C};
+    vector<size_t> st = {C, 1};
+    auto r = detail::compute_slice(sh, st, first, rest...);
+    return {data_ + r.ptr_offset, std::move(r.shape), std::move(r.strides)};
+  }
+
+  //! Flat view (1D, contiguous)
+  View<T> flat() { return {data_, {R * C}, {size_t(1)}}; }
+  View<const T> flat() const { return {data_, {R * C}, {size_t(1)}}; }
+
+private:
+  //--------------------------------------------------------------------------
+  // Data members
+
+  T data_[R * C] = {};
+};
+
+//==============================================================================
+// Non-member functions
+//==============================================================================
+
+template<typename T>
+Tensor<T> zeros(std::initializer_list<size_t> shape)
+{
+  vector<size_t> s(shape);
+  return Tensor<T>(std::move(s), T(0));
+}
+
+template<typename T>
+Tensor<T> zeros(const vector<size_t>& shape)
+{
+  return Tensor<T>(shape, T(0));
+}
+
+template<typename T>
+Tensor<T> ones(std::initializer_list<size_t> shape)
+{
+  vector<size_t> s(shape);
+  return Tensor<T>(std::move(s), T(1));
+}
+
+template<typename T>
+Tensor<T> ones(const vector<size_t>& shape)
+{
+  return Tensor<T>(shape, T(1));
+}
+
+template<typename T>
+Tensor<T> zeros_like(const Tensor<T>& o)
+{
+  return Tensor<T>(o.shape(), T(0));
+}
+
+template<typename T, typename V>
+Tensor<T> full_like(const Tensor<T>& o, V val)
+{
+  return Tensor<T>(o.shape(), static_cast<T>(val));
+}
+
+//! Return a 1D tensor of n evenly spaced values from start to stop (inclusive)
+template<typename T>
+Tensor<T> linspace(T start, T stop, size_t n)
+{
+  Tensor<T> result({n});
+  if (n < 2) {
+    result[0] = start;
+    return result;
+  }
+  for (size_t i = 0; i < n; ++i) {
+    result[i] =
+      start + static_cast<T>(i) * (stop - start) / static_cast<T>(n - 1);
+  }
+  return result;
+}
+
+//! Concatenate two 1D tensors end-to-end
+template<typename T>
+Tensor<T> concatenate(const Tensor<T>& a, const Tensor<T>& b)
+{
+  size_t total = a.size() + b.size();
+  Tensor<T> result({total});
+  std::copy(a.data(), a.data() + a.size(), result.data());
+  std::copy(b.data(), b.data() + b.size(), result.data() + a.size());
+  return result;
+}
+
+//! Element-wise natural logarithm
+template<typename T>
+Tensor<T> log(const Tensor<T>& a)
+{
+  Tensor<T> r(a.shape());
+  for (size_t i = 0; i < a.size(); ++i)
+    r.data()[i] = std::log(a.data()[i]);
+  return r;
+}
+
+//! Element-wise absolute value
+template<typename T>
+Tensor<T> abs(const Tensor<T>& a)
+{
+  Tensor<T> r(a.shape());
+  for (size_t i = 0; i < a.size(); ++i)
+    r.data()[i] = std::abs(a.data()[i]);
+  return r;
+}
+
+//! Element-wise conditional: select from true_val where cond is true,
+//! otherwise use false_val
+template<typename T, typename V>
+Tensor<T> where(
+  const Tensor<bool>& cond, const Tensor<T>& true_val, V false_val)
+{
+  Tensor<T> r(cond.shape());
+  for (size_t i = 0; i < cond.size(); ++i)
+    r.data()[i] =
+      cond.data()[i] ? true_val.data()[i] : static_cast<T>(false_val);
+  return r;
+}
+
+//! Replace NaN/Inf values with finite substitutes
+template<typename T>
+Tensor<T> nan_to_num(const Tensor<T>& a, T nan_val = T(0),
+  T posinf_val = std::numeric_limits<T>::max(),
+  T neginf_val = std::numeric_limits<T>::lowest())
+{
+  Tensor<T> r(a.shape());
+  for (size_t i = 0; i < a.size(); ++i) {
+    T val = a.data()[i];
+    if (std::isnan(val))
+      r.data()[i] = nan_val;
+    else if (std::isinf(val))
+      r.data()[i] = val > 0 ? posinf_val : neginf_val;
+    else
+      r.data()[i] = val;
+  }
+  return r;
+}
+
+//==============================================================================
+// Type traits
+//==============================================================================
+
+//! Type trait that is true for Tensor and StaticTensor2D.
+//! Used by hdf5_interface.h to select the correct write_dataset overload.
+template<typename T>
+struct is_tensor : std::false_type {};
+
+template<typename T>
+struct is_tensor<Tensor<T>> : std::true_type {};
+
+template<typename T, size_t R, size_t C>
+struct is_tensor<StaticTensor2D<T, R, C>> : std::true_type {};
+
+} // namespace tensor
+} // namespace openmc
+
+#endif // OPENMC_TENSOR_H
diff --git a/include/openmc/thermal.h b/include/openmc/thermal.h
index de0767d0a..c06a2ee0d 100644
--- a/include/openmc/thermal.h
+++ b/include/openmc/thermal.h
@@ -5,7 +5,7 @@
 #include <string>
 #include <unordered_map>
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/angle_energy.h"
 #include "openmc/endf.h"
@@ -51,7 +51,25 @@ public:
   //! \param[out] mu Outgoing scattering angle cosine
   //! \param[inout] seed Pseudorandom seed pointer
   void sample(const NuclideMicroXS& micro_xs, double E_in, double* E_out,
-    double* mu, uint64_t* seed);
+    double* mu, uint64_t* seed) const;
+
+  //! Select the elastic or inelastic distribution to sample
+  //! \param[in] micro_xs Microscopic cross sections
+  //! \param[in] E Incident neutron energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Reference to the selected angle-energy distribution
+  AngleEnergy& sample_dist(
+    const NuclideMicroXS& micro_xs, double E, uint64_t* seed) const;
+
+  //! Sample an outgoing energy and evaluate the angular PDF
+  //! \param[in] micro_xs Microscopic cross sections
+  //! \param[in] E_in Incoming energy in [eV]
+  //! \param[in] mu Scattering cosine with respect to current direction
+  //! \param[out] E_out Outgoing energy in [eV]
+  //! \param[inout] seed Pseudorandom seed pointer
+  //! \return Probability density for the scattering cosine
+  double sample_energy_and_pdf(const NuclideMicroXS& micro_xs, double E_in,
+    double mu, double& E_out, uint64_t* seed) const;
 
 private:
   struct Reaction {
diff --git a/include/openmc/universe.h b/include/openmc/universe.h
index e8fbacfdc..b7450224f 100644
--- a/include/openmc/universe.h
+++ b/include/openmc/universe.h
@@ -6,7 +6,7 @@
 
 namespace openmc {
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 class DAGUniverse;
 #endif
 
@@ -29,6 +29,7 @@ class Universe {
 public:
   int32_t id_;            //!< Unique ID
   vector<int32_t> cells_; //!< Cells within this universe
+  int32_t n_instances_;   //!< Number of instances of this universe
 
   //! \brief Write universe information to an HDF5 group.
   //! \param group_id An HDF5 group id.
diff --git a/include/openmc/urr.h b/include/openmc/urr.h
index 3978c7b86..d40c2aff7 100644
--- a/include/openmc/urr.h
+++ b/include/openmc/urr.h
@@ -3,7 +3,7 @@
 #ifndef OPENMC_URR_H
 #define OPENMC_URR_H
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/constants.h"
 #include "openmc/hdf5_interface.h"
@@ -27,10 +27,10 @@ public:
     double heating;
   };
 
-  Interpolation interp_;          //!< interpolation type
-  int inelastic_flag_;            //!< inelastic competition flag
-  int absorption_flag_;           //!< other absorption flag
-  bool multiply_smooth_;          //!< multiply by smooth cross section?
+  Interpolation interp_; //!< interpolation type
+  int inelastic_flag_;   //!< inelastic competition flag
+  int absorption_flag_;  //!< other absorption flag
+  bool multiply_smooth_; //!< multiply by smooth cross section?
 
   vector<double> energy_; //!< incident energies
   auto n_energy() const { return energy_.size(); }
@@ -40,11 +40,11 @@ public:
    * below, obviously, values of the CDF are stored. For the xs_values
    * variable, the columns line up with the index of cdf_values.
    */
-  xt::xtensor<double, 2> cdf_values_; // Note: must be row major!
-  xt::xtensor<XSSet, 2> xs_values_;
+  tensor::Tensor<double> cdf_values_; // Note: must be row major!
+  tensor::Tensor<XSSet> xs_values_;
 
   // Number of points in the CDF
-  auto n_cdf() const { return cdf_values_.shape()[1]; }
+  auto n_cdf() const { return cdf_values_.shape(1); }
 
   //! \brief Load the URR data from the provided HDF5 group
   explicit UrrData(hid_t group_id);
diff --git a/include/openmc/volume_calc.h b/include/openmc/volume_calc.h
index fa8d3d65e..ef75ec065 100644
--- a/include/openmc/volume_calc.h
+++ b/include/openmc/volume_calc.h
@@ -12,8 +12,8 @@
 #include "openmc/tallies/trigger.h"
 #include "openmc/vector.h"
 
+#include "openmc/tensor.h"
 #include "pugixml.hpp"
-#include "xtensor/xtensor.hpp"
 #ifdef _OPENMP
 #include <omp.h>
 #endif
@@ -34,7 +34,7 @@ public:
     vector<double> atoms;       //!< Number of atoms for each nuclide
     vector<double> uncertainty; //!< Uncertainty on number of atoms
     int iterations; //!< Number of iterations needed to obtain the results
-  };                // Results for a single domain
+  }; // Results for a single domain
 
   // Constructors
   VolumeCalculation(pugi::xml_node node);
diff --git a/include/openmc/weight_windows.h b/include/openmc/weight_windows.h
index 6f2ef0707..d0b385d16 100644
--- a/include/openmc/weight_windows.h
+++ b/include/openmc/weight_windows.h
@@ -10,7 +10,7 @@
 #include "openmc/constants.h"
 #include "openmc/memory.h"
 #include "openmc/mesh.h"
-#include "openmc/particle.h"
+#include "openmc/particle_type.h"
 #include "openmc/span.h"
 #include "openmc/tallies/tally.h"
 #include "openmc/vector.h"
@@ -25,17 +25,6 @@ enum class WeightWindowUpdateMethod { MAGIC, FW_CADIS };
 
 constexpr double DEFAULT_WEIGHT_CUTOFF {1.0e-38}; // default low weight cutoff
 
-//==============================================================================
-// Non-member functions
-//==============================================================================
-
-//! Apply weight windows to a particle
-//! \param[in] p  Particle to apply weight windows to
-void apply_weight_windows(Particle& p);
-
-//! Free memory associated with weight windows
-void free_memory_weight_windows();
-
 //==============================================================================
 // Global variables
 //==============================================================================
@@ -63,14 +52,19 @@ struct WeightWindow {
   double weight_cutoff {DEFAULT_WEIGHT_CUTOFF};
   int max_split {10};
 
-  //! Whether the weight window is in a valid state
-  bool is_valid() const { return lower_weight >= 0.0; }
+  //! Whether the weight window is in a valid state. A non-positive lower
+  //! bound indicates that no weight window information exists at this
+  //! location (generators mark such cells with -1, and a lower bound of zero
+  //! conventionally turns the weight window game off in a cell, as in MCNP
+  //! wwinp files), in which case no weight window game is played.
+  bool is_valid() const { return lower_weight > 0.0; }
 
   //! Adjust the weight window by a constant factor
   void scale(double factor)
   {
     lower_weight *= factor;
     upper_weight *= factor;
+    survival_weight *= factor;
   }
 };
 
@@ -136,16 +130,16 @@ public:
 
   //! Retrieve the weight window for a particle
   //! \param[in] p  Particle to get weight window for
-  WeightWindow get_weight_window(const Particle& p) const;
+  std::pair<bool, WeightWindow> get_weight_window(const Particle& p) const;
 
   std::array<int, 2> bounds_size() const;
 
   const vector<double>& energy_bounds() const { return energy_bounds_; }
 
-  void set_bounds(const xt::xtensor<double, 2>& lower_ww_bounds,
-    const xt::xtensor<double, 2>& upper_bounds);
+  void set_bounds(const tensor::Tensor<double>& lower_ww_bounds,
+    const tensor::Tensor<double>& upper_bounds);
 
-  void set_bounds(const xt::xtensor<double, 2>& lower_bounds, double ratio);
+  void set_bounds(const tensor::Tensor<double>& lower_bounds, double ratio);
 
   void set_bounds(
     span<const double> lower_bounds, span<const double> upper_bounds);
@@ -181,25 +175,24 @@ public:
 
   const std::unique_ptr<Mesh>& mesh() const { return model::meshes[mesh_idx_]; }
 
-  const xt::xtensor<double, 2>& lower_ww_bounds() const { return lower_ww_; }
-  xt::xtensor<double, 2>& lower_ww_bounds() { return lower_ww_; }
+  const tensor::Tensor<double>& lower_ww_bounds() const { return lower_ww_; }
+  tensor::Tensor<double>& lower_ww_bounds() { return lower_ww_; }
 
-  const xt::xtensor<double, 2>& upper_ww_bounds() const { return upper_ww_; }
-  xt::xtensor<double, 2>& upper_ww_bounds() { return upper_ww_; }
+  const tensor::Tensor<double>& upper_ww_bounds() const { return upper_ww_; }
+  tensor::Tensor<double>& upper_ww_bounds() { return upper_ww_; }
 
   ParticleType particle_type() const { return particle_type_; }
 
 private:
   //----------------------------------------------------------------------------
   // Data members
-  int32_t id_;    //!< Unique ID
-  int64_t index_; //!< Index into weight windows vector
-  ParticleType particle_type_ {
-    ParticleType::neutron};      //!< Particle type to apply weight windows to
+  int32_t id_;                   //!< Unique ID
+  int64_t index_;                //!< Index into weight windows vector
+  ParticleType particle_type_;   //!< Particle type to apply weight windows to
   vector<double> energy_bounds_; //!< Energy boundaries [eV]
-  xt::xtensor<double, 2> lower_ww_; //!< Lower weight window bounds (shape:
+  tensor::Tensor<double> lower_ww_; //!< Lower weight window bounds (shape:
                                     //!< energy_bins, mesh_bins (k, j, i))
-  xt::xtensor<double, 2>
+  tensor::Tensor<double>
     upper_ww_; //!< Upper weight window bounds (shape: energy_bins, mesh_bins)
   double survival_ratio_ {3.0}; //!< Survival weight ratio
   double max_lb_ratio_ {1.0}; //!< Maximum lower bound to particle weight ratio
@@ -234,8 +227,31 @@ public:
   double threshold_ {1.0}; //<! Relative error threshold for values used to
                            // update weight windows
   double ratio_ {5.0};     //<! ratio of lower to upper weight window bounds
+
+  // Local FW-CADIS target tallies
+  std::vector<size_t> targets_;
 };
 
+//==============================================================================
+// Non-member functions
+//==============================================================================
+
+//! Apply weight windows to a particle
+//! \param[in] p  Particle to apply weight windows to
+void apply_weight_windows(Particle& p);
+
+//! Apply weight window to a particle
+//! \param[in] p  Particle to apply weight window to
+//! \param[in] weight_window WeightWindow to apply
+void apply_weight_window(Particle& p, WeightWindow weight_window);
+
+//! Free memory associated with weight windows
+void free_memory_weight_windows();
+
+//! Search weight window that apply to a particle
+//! \param[in]  p  Particle to search weight window for
+std::pair<bool, WeightWindow> search_weight_window(const Particle& p);
+
 //! Finalize variance reduction objects after all inputs have been read
 void finalize_variance_reduction();
 
diff --git a/include/openmc/wmp.h b/include/openmc/wmp.h
index 6a4abd867..5cc04e595 100644
--- a/include/openmc/wmp.h
+++ b/include/openmc/wmp.h
@@ -2,7 +2,7 @@
 #define OPENMC_WMP_H
 
 #include "hdf5.h"
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include <complex>
 #include <string>
@@ -78,9 +78,9 @@ public:
   int fit_order_;                  //!< Order of the fit
   bool fissionable_;               //!< Is the nuclide fissionable?
   vector<WindowInfo> window_info_; // Information about a window
-  xt::xtensor<double, 3>
+  tensor::Tensor<double>
     curvefit_; // Curve fit coefficients (window, poly order, reaction)
-  xt::xtensor<std::complex<double>, 2> data_; //!< Poles and residues
+  tensor::Tensor<std::complex<double>> data_; //!< Poles and residues
 
   // Constant data
   static constexpr int MAX_POLY_COEFFICIENTS =
diff --git a/include/openmc/xml_interface.h b/include/openmc/xml_interface.h
index f49613ecd..17a34e5c7 100644
--- a/include/openmc/xml_interface.h
+++ b/include/openmc/xml_interface.h
@@ -5,9 +5,8 @@
 #include <sstream> // for stringstream
 #include <string>
 
+#include "openmc/tensor.h"
 #include "pugixml.hpp"
-#include "xtensor/xadapt.hpp"
-#include "xtensor/xarray.hpp"
 
 #include "openmc/position.h"
 #include "openmc/vector.h"
@@ -42,12 +41,11 @@ vector<T> get_node_array(
 }
 
 template<typename T>
-xt::xarray<T> get_node_xarray(
+tensor::Tensor<T> get_node_tensor(
   pugi::xml_node node, const char* name, bool lowercase = false)
 {
   vector<T> v = get_node_array<T>(node, name, lowercase);
-  vector<std::size_t> shape = {v.size()};
-  return xt::adapt(v, shape);
+  return tensor::Tensor<T>(v.data(), v.size());
 }
 
 std::vector<Position> get_node_position_array(
diff --git a/include/openmc/xsdata.h b/include/openmc/xsdata.h
index feafde68d..c9dbde986 100644
--- a/include/openmc/xsdata.h
+++ b/include/openmc/xsdata.h
@@ -4,7 +4,7 @@
 #ifndef OPENMC_XSDATA_H
 #define OPENMC_XSDATA_H
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/hdf5_interface.h"
 #include "openmc/memory.h"
@@ -69,26 +69,26 @@ private:
 public:
   // The following quantities have the following dimensions:
   // [angle][incoming group]
-  xt::xtensor<double, 2> total;
-  xt::xtensor<double, 2> absorption;
-  xt::xtensor<double, 2> nu_fission;
-  xt::xtensor<double, 2> prompt_nu_fission;
-  xt::xtensor<double, 2> kappa_fission;
-  xt::xtensor<double, 2> fission;
-  xt::xtensor<double, 2> inverse_velocity;
+  tensor::Tensor<double> total;
+  tensor::Tensor<double> absorption;
+  tensor::Tensor<double> nu_fission;
+  tensor::Tensor<double> prompt_nu_fission;
+  tensor::Tensor<double> kappa_fission;
+  tensor::Tensor<double> fission;
+  tensor::Tensor<double> inverse_velocity;
 
   // decay_rate has the following dimensions:
   // [angle][delayed group]
-  xt::xtensor<double, 2> decay_rate;
+  tensor::Tensor<double> decay_rate;
   // delayed_nu_fission has the following dimensions:
   // [angle][delayed group][incoming group]
-  xt::xtensor<double, 3> delayed_nu_fission;
+  tensor::Tensor<double> delayed_nu_fission;
   // chi_prompt has the following dimensions:
   // [angle][incoming group][outgoing group]
-  xt::xtensor<double, 3> chi_prompt;
+  tensor::Tensor<double> chi_prompt;
   // chi_delayed has the following dimensions:
   // [angle][incoming group][outgoing group][delayed group]
-  xt::xtensor<double, 4> chi_delayed;
+  tensor::Tensor<double> chi_delayed;
   // scatter has the following dimensions: [angle]
   vector<std::shared_ptr<ScattData>> scatter;
 
diff --git a/man/man1/openmc.1 b/man/man1/openmc.1
index 7826a759a..3205b4340 100644
--- a/man/man1/openmc.1
+++ b/man/man1/openmc.1
@@ -39,6 +39,9 @@ Use \fIN\fP OpenMP threads.
 .B "\-t\fR, \fP\-\-track"
 Write tracks for all particles (up to max_tracks).
 .TP
+.BI \-q " V" "\fR,\fP \-\-verbosity" " V"
+Set the output verbosity to \fIV\fP.
+.TP
 .B "\-v\fR, \fP\-\-version"
 Show version information.
 .TP
@@ -63,7 +66,7 @@ Indicates the default path to an HDF5 file that contains multi-group cross
 section libraries if the user has not specified the <cross_sections> tag in
 .I materials.xml\fP.
 .SH LICENSE
-Copyright \(co 2011-2025 Massachusetts Institute of Technology, UChicago
+Copyright \(co 2011-2026 Massachusetts Institute of Technology, UChicago
 Argonne LLC, and OpenMC contributors.
 .PP
 Permission is hereby granted, free of charge, to any person obtaining a copy of
diff --git a/openmc/__init__.py b/openmc/__init__.py
index bb972b4e6..c204929c8 100644
--- a/openmc/__init__.py
+++ b/openmc/__init__.py
@@ -37,7 +37,7 @@ from openmc.tracks import *
 from .config import *
 
 # Import a few names from the model module
-from openmc.model import Model
+from openmc.model import Model, SearchResult
 
 from . import examples
 
diff --git a/openmc/_sparse_compat.py b/openmc/_sparse_compat.py
new file mode 100644
index 000000000..c00777e19
--- /dev/null
+++ b/openmc/_sparse_compat.py
@@ -0,0 +1,43 @@
+"""Compatibility module for scipy.sparse arrays
+
+This module provides a compatibility layer for working with scipy.sparse arrays
+across different scipy versions. Sparse arrays were introduced gradually in
+scipy, with full support arriving in scipy 1.15. This module provides a unified
+API that uses sparse arrays when available and falls back to sparse matrices for
+older scipy versions.
+
+For more information on the migration from sparse matrices to sparse arrays,
+see: https://docs.scipy.org/doc/scipy/reference/sparse.migration_to_sparray.html
+"""
+
+import scipy
+from scipy import sparse as sp
+
+# Check scipy version for feature availability
+_SCIPY_VERSION = tuple(map(int, scipy.__version__.split('.')[:2]))
+
+if _SCIPY_VERSION >= (1, 15):
+    # Use sparse arrays
+    csr_array = sp.csr_array
+    csc_array = sp.csc_array
+    dok_array = sp.dok_array
+    lil_array = sp.lil_array
+    eye_array = sp.eye_array
+    block_array = sp.block_array
+else:
+    # Fall back to sparse matrices
+    csr_array = sp.csr_matrix
+    csc_array = sp.csc_matrix
+    dok_array = sp.dok_matrix
+    lil_array = sp.lil_matrix
+    eye_array = sp.eye
+    block_array = sp.bmat
+
+__all__ = [
+    'csr_array',
+    'csc_array',
+    'dok_array',
+    'lil_array',
+    'eye_array',
+    'block_array',
+]
diff --git a/openmc/_xml.py b/openmc/_xml.py
index 17389a82f..758d80525 100644
--- a/openmc/_xml.py
+++ b/openmc/_xml.py
@@ -63,26 +63,9 @@ def get_text(elem, name, default=None):
         return child.text if child is not None else default
 
 
-def reorder_attributes(root):
-    """Sort attributes in XML to preserve pre-Python 3.8 behavior
 
-    Parameters
-    ----------
-    root : lxml.etree._Element
-        Root element
-
-    """
-    for el in root.iter():
-        attrib = el.attrib
-        if len(attrib) > 1:
-            # adjust attribute order, e.g. by sorting
-            attribs = sorted(attrib.items())
-            attrib.clear()
-            attrib.update(attribs)
-
-
-def get_elem_tuple(elem, name, dtype=int):
-    """Helper function to get a tuple of values from an elem
+def get_elem_list(elem, name, dtype=int):
+    """Helper function to get a list of values from an elem
 
     Parameters
     ----------
@@ -95,9 +78,9 @@ def get_elem_tuple(elem, name, dtype=int):
 
     Returns
     -------
-    tuple of dtype
-        Data read from the tuple
+    list of dtype
+        Data read from the list
     """
-    subelem = elem.find(name)
-    if subelem is not None:
-        return tuple([dtype(x) for x in subelem.text.split()])
+    text = get_text(elem, name)
+    if text is not None:
+        return [dtype(x) for x in text.split()]
diff --git a/openmc/cell.py b/openmc/cell.py
index cd0573e8b..499cf9504 100644
--- a/openmc/cell.py
+++ b/openmc/cell.py
@@ -8,7 +8,7 @@ from uncertainties import UFloat
 
 import openmc
 import openmc.checkvalue as cv
-from ._xml import get_text
+from ._xml import get_elem_list, get_text
 from .mixin import IDManagerMixin
 from .plots import add_plot_params
 from .region import Region, Complement
@@ -72,6 +72,10 @@ class Cell(IDManagerMixin):
     temperature : float or iterable of float
         Temperature of the cell in Kelvin.  Multiple temperatures can be given
         to give each distributed cell instance a unique temperature.
+    density : float or iterable of float
+        Density of the cell in [g/cm3]. Multiple densities can be given to give
+        each distributed cell instance a unique density. Densities set here will
+        override the density set on materials used to fill the cell.
     translation : Iterable of float
         If the cell is filled with a universe, this array specifies a vector
         that is used to translate (shift) the universe.
@@ -109,6 +113,7 @@ class Cell(IDManagerMixin):
         self._rotation = None
         self._rotation_matrix = None
         self._temperature = None
+        self._density = None
         self._translation = None
         self._paths = None
         self._num_instances = None
@@ -141,6 +146,7 @@ class Cell(IDManagerMixin):
         if self.fill_type == 'material':
             string += '\t{0: <15}=\t{1}\n'.format('Temperature',
                                                   self.temperature)
+            string += '\t{0: <15}=\t{1}\n'.format('Density', self.density)
         string += '{: <16}=\t{}\n'.format('\tTranslation', self.translation)
         string += '{: <16}=\t{}\n'.format('\tVolume', self.volume)
 
@@ -257,6 +263,30 @@ class Cell(IDManagerMixin):
         else:
             self._temperature = temperature
 
+    @property
+    def density(self):
+        return self._density
+
+    @density.setter
+    def density(self, density):
+        # Make sure densities are greater than zero
+        cv.check_type('cell density', density, (Iterable, Real), none_ok=True)
+        if isinstance(density, Iterable):
+            cv.check_type('cell density', density, Iterable, Real)
+            for rho in density:
+                cv.check_greater_than('cell density', rho, 0.0, True)
+        elif isinstance(density, Real):
+            cv.check_greater_than('cell density', density, 0.0, True)
+
+        # If this cell is filled with a universe or lattice, propagate
+        # densities to all cells contained. Otherwise, simply assign it.
+        if self.fill_type in ('universe', 'lattice'):
+            for c in self.get_all_cells().values():
+                if c.fill_type == 'material':
+                    c._density = density
+        else:
+            self._density = density
+
     @property
     def translation(self):
         return self._translation
@@ -525,6 +555,8 @@ class Cell(IDManagerMixin):
             clone.volume = self.volume
             if self.temperature is not None:
                 clone.temperature = self.temperature
+            if self.density is not None:
+                clone.density = self.density
             if self.translation is not None:
                 clone.translation = self.translation
             if self.rotation is not None:
@@ -567,10 +599,10 @@ class Cell(IDManagerMixin):
         .. versionadded:: 0.14.0
         """
         # Create dummy universe but preserve used_ids
-        next_id = openmc.Universe.next_id
+        next_id = openmc.UniverseBase.next_id
         u = openmc.Universe(cells=[self])
-        openmc.Universe.used_ids.remove(u.id)
-        openmc.Universe.next_id = next_id
+        openmc.UniverseBase.used_ids.remove(u.id)
+        openmc.UniverseBase.next_id = next_id
         return u.plot(*args, **kwargs)
 
     def create_xml_subelement(self, xml_element, memo=None):
@@ -604,8 +636,11 @@ class Cell(IDManagerMixin):
             element.set("material", str(self.fill.id))
 
         elif self.fill_type == 'distribmat':
-            element.set("material", ' '.join(['void' if m is None else str(m.id)
-                                              for m in self.fill]))
+            material_subelement= ET.SubElement(element, "material")
+            matlist_str = " ".join(
+                ["void" if m is None else str(m.id) for m in self.fill]
+            )
+            material_subelement.text = matlist_str
 
         elif self.fill_type in ('universe', 'lattice'):
             element.set("fill", str(self.fill.id))
@@ -645,11 +680,18 @@ class Cell(IDManagerMixin):
 
         if self.temperature is not None:
             if isinstance(self.temperature, Iterable):
-                element.set("temperature", ' '.join(
-                    str(t) for t in self.temperature))
+                temperature_subelement= ET.SubElement(element, "temperature")
+                temperature_subelement.text = ' '.join(str(t) for t in self.temperature)
             else:
                 element.set("temperature", str(self.temperature))
 
+        if self.density is not None:
+            if isinstance(self.density, Iterable):
+                density_subelement= ET.SubElement(element, "density")
+                density_subelement.text =  ' '.join(str(d) for d in self.density)
+            else:
+                element.set("density", str(self.density))
+
         if self.translation is not None:
             element.set("translation", ' '.join(map(str, self.translation)))
 
@@ -689,9 +731,8 @@ class Cell(IDManagerMixin):
         c = cls(cell_id, name)
 
         # Assign material/distributed materials or fill
-        mat_text = get_text(elem, 'material')
-        if mat_text is not None:
-            mat_ids = mat_text.split()
+        mat_ids = get_elem_list(elem, 'material', str)
+        if mat_ids is not None:
             if len(mat_ids) > 1:
                 c.fill = [materials[i] for i in mat_ids]
             else:
@@ -706,21 +747,16 @@ class Cell(IDManagerMixin):
             c.region = Region.from_expression(region, surfaces)
 
         # Check for other attributes
-        t = get_text(elem, 'temperature')
-        if t is not None:
-            if ' ' in t:
-                c.temperature = [float(t_i) for t_i in t.split()]
-            else:
-                c.temperature = float(t)
         v = get_text(elem, 'volume')
         if v is not None:
             c.volume = float(v)
-        for key in ('temperature', 'rotation', 'translation'):
-            value = get_text(elem, key)
-            if value is not None:
-                values = [float(x) for x in value.split()]
+        for key in ('temperature', 'density', 'rotation', 'translation'):
+            values = get_elem_list(elem, key, float)
+            if values is not None:
                 if key == 'rotation' and len(values) == 9:
                     values = np.array(values).reshape(3, 3)
+                elif len(values) == 1:
+                    values = values[0]
                 setattr(c, key, values)
 
         # Add this cell to appropriate universe
diff --git a/openmc/checkvalue.py b/openmc/checkvalue.py
index 4fa205b14..5ff2cf9ac 100644
--- a/openmc/checkvalue.py
+++ b/openmc/checkvalue.py
@@ -37,7 +37,7 @@ def check_type(name, value, expected_type, expected_iter_type=None, *, none_ok=F
                       [t.__name__ for t in expected_type]))
         else:
             msg = (f'Unable to set "{name}" to "{value}" which is not of type "'
-                   f'{expected_type.__name__}"')
+                   f'{expected_type}"')
         raise TypeError(msg)
 
     if expected_iter_type:
diff --git a/openmc/cmfd.py b/openmc/cmfd.py
index eff6a151f..8595d1e02 100644
--- a/openmc/cmfd.py
+++ b/openmc/cmfd.py
@@ -25,6 +25,7 @@ import openmc.lib
 from .checkvalue import (check_type, check_length, check_value,
                          check_greater_than, check_less_than)
 from .exceptions import OpenMCError
+from ._sparse_compat import csr_array
 
 # See if mpi4py module can be imported, define have_mpi global variable
 try:
@@ -980,8 +981,7 @@ class CMFDRun:
         loss_row = self._loss_row
         loss_col = self._loss_col
         temp_data = np.ones(len(loss_row))
-        temp_loss = sparse.csr_matrix((temp_data, (loss_row, loss_col)),
-                                      shape=(n, n))
+        temp_loss = csr_array((temp_data, (loss_row, loss_col)), shape=(n, n))
         temp_loss.sort_indices()
 
         # Pass coremap as 1-d array of 32-bit integers
@@ -1585,7 +1585,7 @@ class CMFDRun:
         # Create csr matrix
         loss_row = self._loss_row
         loss_col = self._loss_col
-        loss = sparse.csr_matrix((data, (loss_row, loss_col)), shape=(n, n))
+        loss = csr_array((data, (loss_row, loss_col)), shape=(n, n))
         loss.sort_indices()
         return loss
 
@@ -1612,7 +1612,7 @@ class CMFDRun:
         # Create csr matrix
         prod_row = self._prod_row
         prod_col = self._prod_col
-        prod = sparse.csr_matrix((data, (prod_row, prod_col)), shape=(n, n))
+        prod = csr_array((data, (prod_row, prod_col)), shape=(n, n))
         prod.sort_indices()
         return prod
 
diff --git a/openmc/config.py b/openmc/config.py
index ab53ab61b..23d8e23a7 100644
--- a/openmc/config.py
+++ b/openmc/config.py
@@ -1,8 +1,23 @@
+"""Module for handling global configuration in OpenMC.
+
+This module exports a single object, `config`, that can be used to control
+various settings, primarily paths to data files. It acts like a dictionary but
+with special behaviors.
+
+Examples
+--------
+>>> import openmc
+>>> openmc.config['cross_sections'] = '/path/to/my/cross_sections.xml'
+>>> print(openmc.config)
+{'resolve_paths': True, 'cross_sections': PosixPath('/path/to/my/cross_sections.xml')}
+
+"""
 from collections.abc import MutableMapping
 from contextlib import contextmanager
 import os
 from pathlib import Path
 import warnings
+from typing import Any, Dict, Iterator
 
 from openmc.data import DataLibrary
 from openmc.data.decay import _DECAY_ENERGY, _DECAY_PHOTON_ENERGY
@@ -11,104 +26,193 @@ __all__ = ["config"]
 
 
 class _Config(MutableMapping):
-    def __init__(self, data=()):
-        self._mapping = {'resolve_paths': True}
+    """A configuration dictionary for OpenMC with special handling for path-like values.
+
+    This class enforces valid configuration keys and synchronizes path-related
+    settings with their corresponding environment variables.
+
+    Attributes
+    ----------
+    cross_sections : pathlib.Path
+        Path to a cross_sections.xml file. Also sets/unsets the
+        OPENMC_CROSS_SECTIONS environment variable.
+    mg_cross_sections : pathlib.Path
+        Path to a multi-group cross_sections.h5 file. Also sets/unsets
+        the OPENMC_MG_CROSS_SECTIONS environment variable.
+    chain_file : pathlib.Path
+        Path to a depletion chain XML file. Also sets/unsets the
+        OPENMC_CHAIN_FILE environment variable. Setting or deleting this
+        clears internal decay data caches.
+    resolve_paths : bool
+        If True (default), all paths assigned are resolved to absolute
+        paths. If False, paths are stored as they are provided.
+
+    """
+    _PATH_KEYS: Dict[str, str] = {
+        'cross_sections': 'OPENMC_CROSS_SECTIONS',
+        'mg_cross_sections': 'OPENMC_MG_CROSS_SECTIONS',
+        'chain_file': 'OPENMC_CHAIN_FILE'
+    }
+
+    def __init__(self, data: dict = ()):
+        self._mapping: Dict[str, Any] = {'resolve_paths': True}
         self.update(data)
 
-    def __getitem__(self, key):
+    def __getitem__(self, key: str) -> Any:
         return self._mapping[key]
 
-    def __delitem__(self, key):
-        del self._mapping[key]
-        if key == 'cross_sections':
-            del os.environ['OPENMC_CROSS_SECTIONS']
-        elif key == 'mg_cross_sections':
-            del os.environ['OPENMC_MG_CROSS_SECTIONS']
-        elif key == 'chain_file':
-            del os.environ['OPENMC_CHAIN_FILE']
-            # Reset photon source data since it relies on chain file
-            _DECAY_PHOTON_ENERGY.clear()
+    def __delitem__(self, key: str):
+        """Delete a configuration key.
 
-    def __setitem__(self, key, value):
-        if key == 'cross_sections':
-            # Force environment variable to match
-            self._set_path(key, value)
-            os.environ['OPENMC_CROSS_SECTIONS'] = str(value)
-        elif key == 'mg_cross_sections':
-            self._set_path(key, value)
-            os.environ['OPENMC_MG_CROSS_SECTIONS'] = str(value)
-        elif key == 'chain_file':
-            self._set_path(key, value)
-            os.environ['OPENMC_CHAIN_FILE'] = str(value)
-            # Reset photon source data since it relies on chain file
+        This also deletes the corresponding environment variable if the key is a
+        path-like key, and clears decay data caches if 'chain_file' is deleted.
+        'resolve_paths' cannot be deleted.
+
+        """
+        if key == 'resolve_paths':
+            raise KeyError("'resolve_paths' cannot be deleted.")
+        del self._mapping[key]
+        if key in self._PATH_KEYS:
+            env_var = self._PATH_KEYS[key]
+            if env_var in os.environ:
+                del os.environ[env_var]
+        if key == 'chain_file':
             _DECAY_PHOTON_ENERGY.clear()
             _DECAY_ENERGY.clear()
+
+    def __setitem__(self, key: str, value: Any):
+        """Set a configuration key and its corresponding value.
+
+        For path-like keys, this method performs several actions:
+        1. Resolves the path to an absolute path if `resolve_paths` is True.
+        2. Stores the `pathlib.Path` object.
+        3. Sets the corresponding environment variable (e.g., OPENMC_CROSS_SECTIONS).
+        4. For 'chain_file', clears internal decay data caches.
+        5. Issues a `UserWarning` if the final path does not exist.
+
+        """
+        if key in self._PATH_KEYS:
+            p = Path(value)
+            # Use .get() for robustness, defaulting to True
+            if self._mapping.get('resolve_paths', True):
+                stored_path = p.resolve(strict=False)
+            else:
+                stored_path = p
+
+            self._mapping[key] = stored_path
+            os.environ[self._PATH_KEYS[key]] = str(stored_path)
+
+            if key == 'chain_file':
+                _DECAY_PHOTON_ENERGY.clear()
+                _DECAY_ENERGY.clear()
+
+            if not stored_path.exists():
+                warnings.warn(f"Path '{stored_path}' does not exist.", UserWarning)
+
         elif key == 'resolve_paths':
+            if not isinstance(value, bool):
+                raise TypeError("'resolve_paths' must be a boolean.")
             self._mapping[key] = value
         else:
-            raise KeyError(f'Unrecognized config key: {key}. Acceptable keys '
-                           'are "cross_sections", "mg_cross_sections", '
-                           '"chain_file", and "resolve_paths".')
+            valid_keys = list(self._PATH_KEYS.keys()) + ['resolve_paths']
+            raise KeyError(
+                f"Unrecognized config key: {key}. Acceptable keys are: "
+                f"{', '.join(repr(k) for k in valid_keys)}."
+            )
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator[str]:
         return iter(self._mapping)
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self._mapping)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return repr(self._mapping)
 
-    def _set_path(self, key, value):
-        self._mapping[key] = p = Path(value)
-        if not p.exists():
-            warnings.warn(f"'{value}' does not exist.")
+    def clear(self):
+        """Clear all configuration keys except for 'resolve_paths'.
+
+        This ensures that the path resolution behavior is not accidentally reset
+        when clearing the configuration.
+
+        """
+        # Create a copy of keys to iterate over for safe deletion
+        keys_to_delete = [k for k in self._mapping if k != 'resolve_paths']
+        for key in keys_to_delete:
+            del self[key]
 
     @contextmanager
-    def patch(self, key, value):
-        """Temporarily change a value in the configuration.
+    def patch(self, key: str, value: Any):
+        """Context manager to temporarily change a configuration value.
+
+        After the `with` block, the configuration is restored to its original
+        state.
 
         Parameters
         ----------
         key : str
-            Key to change
-        value : object
-            New value
+            The key of the configuration value to change.
+        value
+            The new temporary value.
+
+        Examples
+        --------
+        >>> openmc.config['cross_sections'] = 'endf71.xml'
+        >>> with openmc.config.patch('cross_sections', 'fendl32.xml'):
+        ...     # Code in this block sees the new value
+        ...     print(f"Inside with block: {openmc.config['cross_sections']}")
+        >>> # Outside the block, the value is restored
+        >>> print(f"Outside with block: {openmc.config['cross_sections']}")
+        Inside with block: fendl32.xml
+        Outside with block: endf71.xml
+
         """
         previous_value = self.get(key)
         self[key] = value
-        yield
-        if previous_value is None:
-            del self[key]
+        try:
+            yield
+        finally:
+            if previous_value is None:
+                del self[key]
+            else:
+                self[key] = previous_value
+
+
+def _default_config(**kwargs) -> _Config:
+    """Create a configuration initialized from environment variables.
+
+    This function checks for OPENMC_CROSS_SECTIONS, OPENMC_MG_CROSS_SECTIONS,
+    and OPENMC_CHAIN_FILE environment variables. It also has logic to find
+    a chain file within a `cross_sections.xml` file if one is not
+    explicitly set.
+
+    Returns
+    -------
+    _Config
+        A new configuration object.
+
+    """
+    config = _Config(kwargs)
+    for key,var in _Config._PATH_KEYS.items():
+        if var in os.environ:
+            config[key] = os.environ[var]
+
+    chain_file = config.get("chain_file")
+    xs_path = config.get("cross_sections")
+    if chain_file is None and xs_path is not None and xs_path.exists():
+        try:
+            data = DataLibrary.from_xml(xs_path)
+        except Exception:
+            # Let this pass silently if cross_sections.xml can't be parsed
+            # or if a dependency like lxml is not available.
+            pass
         else:
-            self[key] = previous_value
-
-def _default_config():
-    """Return default configuration"""
-    config = _Config()
-
-    # Set cross sections using environment variable
-    if "OPENMC_CROSS_SECTIONS" in os.environ:
-        config['cross_sections'] = os.environ["OPENMC_CROSS_SECTIONS"]
-    if "OPENMC_MG_CROSS_SECTIONS" in os.environ:
-        config['mg_cross_sections'] = os.environ["OPENMC_MG_CROSS_SECTIONS"]
-
-    # Set depletion chain
-    chain_file = os.environ.get("OPENMC_CHAIN_FILE")
-    if (chain_file is None and
-        config.get('cross_sections') is not None and
-        config['cross_sections'].exists()
-        ):
-        # Check for depletion chain in cross_sections.xml
-        data = DataLibrary.from_xml(config['cross_sections'])
-        for lib in reversed(data.libraries):
-            if lib['type'] == 'depletion_chain':
-                chain_file = lib['path']
-                break
-    if chain_file is not None:
-        config['chain_file'] = chain_file
-
+            for lib in reversed(data.libraries):
+                if lib['type'] == 'depletion_chain':
+                    config['chain_file'] = xs_path.parent / lib['path']
+                    break
     return config
 
 
+# Global configuration dictionary for OpenMC settings.
 config = _default_config()
diff --git a/openmc/dagmc.py b/openmc/dagmc.py
index 8ab0aaf69..75ead5e43 100644
--- a/openmc/dagmc.py
+++ b/openmc/dagmc.py
@@ -8,11 +8,12 @@ import warnings
 
 import openmc
 import openmc.checkvalue as cv
-from ._xml import get_text
+from ._xml import get_elem_list, get_text
 from .checkvalue import check_type, check_value
 from .surface import _BOUNDARY_TYPES
 from .bounding_box import BoundingBox
 from .utility_funcs import input_path
+from .plots import add_plot_params
 
 
 class DAGMCUniverse(openmc.UniverseBase):
@@ -35,12 +36,6 @@ class DAGMCUniverse(openmc.UniverseBase):
     auto_mat_ids : bool
         Set IDs automatically on initialization (True)  or report overlaps in ID
         space between OpenMC and UWUW materials (False)
-    material_overrides : dict, optional
-        A dictionary of material overrides. The keys are material name strings
-        and the values are Iterables of openmc.Material objects. If a material
-        name is found in the DAGMC file, the material will be replaced with the
-        openmc.Material object in the value.
-
     Attributes
     ----------
     id : int
@@ -77,15 +72,6 @@ class DAGMCUniverse(openmc.UniverseBase):
         The number of surfaces in the model.
 
         .. versionadded:: 0.13.2
-    material_overrides : dict
-        A dictionary of material overrides. Keys are cell IDs; values are
-        iterables of :class:`openmc.Material` objects. The material assignment
-        of each DAGMC cell ID key will be replaced with the
-        :class:`~openmc.Material` object in the value. If the value contains
-        multiple :class:`~openmc.Material` objects, each Material in the list
-        will be assigned to the corresponding instance of the cell.
-
-        .. versionadded:: 0.15.1
     """
 
     def __init__(self,
@@ -93,16 +79,12 @@ class DAGMCUniverse(openmc.UniverseBase):
                  universe_id=None,
                  name='',
                  auto_geom_ids=False,
-                 auto_mat_ids=False,
-                 material_overrides=None):
+                 auto_mat_ids=False):
         super().__init__(universe_id, name)
         # Initialize class attributes
         self.filename = filename
         self.auto_geom_ids = auto_geom_ids
         self.auto_mat_ids = auto_mat_ids
-        self._material_overrides = {}
-        if material_overrides is not None:
-            self.material_overrides = material_overrides
 
     def __repr__(self):
         string = super().__repr__()
@@ -129,47 +111,17 @@ class DAGMCUniverse(openmc.UniverseBase):
 
     @property
     def material_overrides(self):
-        return self._material_overrides
+        raise AttributeError(
+            "DAGMCUniverse.material_overrides has been removed. Use "
+            "DAGMCCell objects added via add_cell() to manage per-cell "
+            "material assignments.")
 
     @material_overrides.setter
     def material_overrides(self, val):
-        cv.check_type('material overrides', val, Mapping)
-        for key, value in val.items():
-            self.add_material_override(key, value)
-
-    def replace_material_assignment(self, material_name: str, material: openmc.Material):
-        """Replace a material assignment within the DAGMC universe.
-
-        Replace the material assignment of all cells filled with a material in
-        the DAGMC universe. The universe must be synchronized in an initialized
-        Model (see :meth:`~openmc.DAGMCUniverse.sync_dagmc_cells`) before
-        calling this method.
-
-        .. versionadded:: 0.15.1
-
-        Parameters
-        ----------
-        material_name : str
-            Material name to replace
-        material : openmc.Material
-            Material to replace the material_name with
-
-        """
-        if material_name not in self.material_names:
-            raise ValueError(
-                f"No material with name '{material_name}' found in the DAGMC universe")
-
-        if not self.cells:
-            raise RuntimeError("This DAGMC universe has not been synchronized "
-                               "on an initialized Model.")
-
-        for cell in self.cells.values():
-            if cell.fill is None:
-                continue
-            if isinstance(cell.fill, openmc.Iterable):
-                cell.fill = list(map(lambda x: material if x.name == material_name else x, cell.fill))
-            else:
-                cell.fill = material if cell.fill.name == material_name else cell.fill
+        raise AttributeError(
+            "DAGMCUniverse.material_overrides has been removed. Use "
+            "DAGMCCell objects added via add_cell() to manage per-cell "
+            "material assignments.")
 
     def add_material_override(self, key, overrides=None):
         """Add a material override to the universe.
@@ -200,7 +152,10 @@ class DAGMCUniverse(openmc.UniverseBase):
         if key not in self.cells:
             raise ValueError(f"Cell ID '{key}' not found in DAGMC universe")
 
-        self._material_overrides[key] = overrides
+        if len(overrides) == 1:
+            self.cells[key].fill = overrides[0]
+        else:
+            self.cells[key].fill = list(overrides)
 
     @property
     def auto_geom_ids(self):
@@ -222,21 +177,19 @@ class DAGMCUniverse(openmc.UniverseBase):
 
     @property
     def material_names(self):
-        dagmc_file_contents = h5py.File(self.filename)
-        material_tags_hex = dagmc_file_contents['/tstt/tags/NAME'].get(
-            'values')
         material_tags_ascii = []
-        for tag in material_tags_hex:
-            candidate_tag = tag.tobytes().decode().replace('\x00', '')
-            # tags might be for temperature or reflective surfaces
-            if candidate_tag.startswith('mat:'):
-                # if name ends with _comp remove it, it is not parsed
-                if candidate_tag.endswith('_comp'):
-                   candidate_tag = candidate_tag[:-5]
-                # removes first 4 characters as openmc.Material name should be
-                # set without the 'mat:' part of the tag
-                material_tags_ascii.append(candidate_tag[4:])
-
+        with h5py.File(self.filename) as dagmc_file_contents:
+            material_tags_hex = dagmc_file_contents['/tstt/tags/NAME'].get('values')
+            for tag in material_tags_hex:
+                candidate_tag = tag.tobytes().decode().replace('\x00', '')
+                # tags might be for temperature or reflective surfaces
+                if candidate_tag.startswith('mat:'):
+                    # if name ends with _comp remove it, it is not parsed
+                    if candidate_tag.endswith('_comp'):
+                       candidate_tag = candidate_tag[:-5]
+                    # removes first 4 characters as openmc.Material name should be
+                    # set without the 'mat:' part of the tag
+                    material_tags_ascii.append(candidate_tag[4:])
         return sorted(set(material_tags_ascii))
 
     def _n_geom_elements(self, geom_type):
@@ -291,32 +244,21 @@ class DAGMCUniverse(openmc.UniverseBase):
 
         memo.add(self)
 
-        # Ensure that the material overrides are up-to-date
-        for cell in self.cells.values():
-            if cell.fill is None:
-                continue
-            self.add_material_override(cell, cell.fill)
-
         # Set xml element values
         dagmc_element = ET.Element('dagmc_universe')
         dagmc_element.set('id', str(self.id))
 
+        if self.name:
+            dagmc_element.set('name', self.name)
         if self.auto_geom_ids:
             dagmc_element.set('auto_geom_ids', 'true')
         if self.auto_mat_ids:
             dagmc_element.set('auto_mat_ids', 'true')
         dagmc_element.set('filename', str(self.filename))
-        if self._material_overrides:
-            mat_element = ET.Element('material_overrides')
-            for key in self._material_overrides:
-                cell_overrides = ET.Element('cell_override')
-                cell_overrides.set("id", str(key))
-                material_element = ET.Element('material_ids')
-                material_element.text = ' '.join(
-                    str(t.id) for t in self._material_overrides[key])
-                cell_overrides.append(material_element)
-                mat_element.append(cell_overrides)
-            dagmc_element.append(mat_element)
+        if self.cells:
+            for cell in self.cells.values():
+                cell_element = cell.create_xml_subelement(xml_element, memo)
+                dagmc_element.append(cell_element)
         xml_element.append(dagmc_element)
 
     def bounding_region(
@@ -441,7 +383,7 @@ class DAGMCUniverse(openmc.UniverseBase):
         return out
 
     @classmethod
-    def from_xml_element(cls, elem, mats = None):
+    def from_xml_element(cls, elem, mats=None):
         """Generate DAGMC universe from XML element
 
         Parameters
@@ -467,24 +409,59 @@ class DAGMCUniverse(openmc.UniverseBase):
         if name is not None:
             out.name = name
 
-        out.auto_geom_ids = bool(elem.get('auto_geom_ids'))
-        out.auto_mat_ids = bool(elem.get('auto_mat_ids'))
+        out.auto_geom_ids = bool(get_text(elem, "auto_geom_ids"))
+        out.auto_mat_ids = bool(get_text(elem, "auto_mat_ids"))
 
-        el_mat_override = elem.find('material_overrides')
-        if el_mat_override is not None:
-            if mats is None:
-                raise ValueError("Material overrides found in DAGMC universe "
-                                 "but no materials were provided to populate "
-                                 "the mapping.")
-            out._material_overrides = {}
-            for elem in el_mat_override.findall('cell_override'):
-                cell_id = int(get_text(elem, 'id'))
-                mat_ids = get_text(elem, 'material_ids').split(' ')
-                mat_objs = [mats[mat_id] for mat_id in mat_ids]
-                out._material_overrides[cell_id] = mat_objs
+        has_overrides = elem.find('material_overrides') is not None
+        has_cells = elem.find('cell') is not None
+
+        if has_overrides and has_cells:
+            raise ValueError(
+                "DAGMCUniverse cannot specify both <material_overrides> and "
+                "<cell> sub-elements. Use <cell> elements only.")
+
+        if has_overrides:
+            warnings.warn(
+                "DAGMCUniverse <material_overrides> is deprecated and will be "
+                "removed in a future version. Use nested <cell> elements "
+                "instead.", DeprecationWarning, stacklevel=2)
+            out._parse_legacy_material_overrides(elem, mats)
+        elif has_cells:
+            out._parse_cell_overrides(elem, mats)
 
         return out
 
+    def _parse_legacy_material_overrides(self, elem, mats):
+        """Parse the deprecated <material_overrides> XML format and populate
+        the universe with equivalent DAGMCCell objects."""
+        if mats is None:
+            raise ValueError(
+                "DAGMC material overrides found but no materials were "
+                "provided to populate the mapping.")
+        mo_elem = elem.find('material_overrides')
+        for co_elem in mo_elem.findall('cell_override'):
+            cell_id = int(get_text(co_elem, 'id'))
+            mat_ids = co_elem.find('material_ids').text.split()
+            fill_objs = [mats[mid] for mid in mat_ids]
+            fill = fill_objs[0] if len(fill_objs) == 1 else fill_objs
+            if cell_id in self.cells:
+                raise ValueError(
+                    f"Duplicate DAGMC cell override specified for cell {cell_id}.")
+            self.add_cell(DAGMCCell(cell_id=cell_id, fill=fill))
+
+    def _parse_cell_overrides(self, elem, mats):
+        if mats is None:
+            raise ValueError("DAGMC cell overrides found in DAGMC universe but "
+                             "no materials were provided to populate the "
+                             "mapping.")
+
+        for cell_elem in elem.findall('cell'):
+            cell_id = int(get_text(cell_elem, 'id'))
+            if cell_id in self.cells:
+                raise ValueError(
+                    f"Duplicate DAGMC cell override specified for cell {cell_id}.")
+            DAGMCCell.from_xml_element(cell_elem, mats, self)
+
     def _partial_deepcopy(self):
         """Clone all of the openmc.DAGMCUniverse object's attributes except for
         its cells, as they are copied within the clone function. This should
@@ -564,7 +541,19 @@ class DAGMCUniverse(openmc.UniverseBase):
                 fill = [mats_per_id[mat.id] for mat in dag_cell.fill if mat]
             else:
                 fill = mats_per_id[dag_cell.fill.id] if dag_cell.fill else None
-            self.add_cell(openmc.DAGMCCell(cell_id=dag_cell_id, fill=fill))
+            name = dag_cell.name
+            if dag_cell_id in self._cells:
+                self._cells[dag_cell_id].name = name
+                self._cells[dag_cell_id].fill = fill
+            else:
+                self.add_cell(
+                    openmc.DAGMCCell(cell_id=dag_cell_id, name=name, fill=fill))
+
+    @add_plot_params
+    def plot(self, *args, **kwargs):
+        """Display a slice plot of the DAGMCUniverse.
+        """
+        return openmc.Geometry(self).plot(*args, **kwargs)
 
 
 class DAGMCCell(openmc.Cell):
@@ -587,6 +576,14 @@ class DAGMCCell(openmc.Cell):
     DAG_parent_universe : int
         The parent universe of the cell.
 
+    Notes
+    -----
+    DAGMC geometries are composed of triangulated surfaces, which means cell
+    volumes can in principle be computed exactly (e.g. via mesh-based
+    integration). Manually specifying :attr:`volume` overrides any such
+    calculation and may introduce inconsistencies if the value does not
+    accurately reflect the true geometric volume.
+
     """
     def __init__(self, cell_id=None, name='', fill=None):
         super().__init__(cell_id, name, fill, None)
@@ -618,8 +615,62 @@ class DAGMCCell(openmc.Cell):
         raise TypeError("plot is not available for DAGMC cells.")
 
     def create_xml_subelement(self, xml_element, memo=None):
-        raise TypeError("create_xml_subelement is not available for DAGMC cells.")
+        if self.fill_type not in ('void', 'material', 'distribmat'):
+            raise TypeError("DAGMC cell overrides currently only support "
+                            "material fills.")
+        if self.temperature is not None and self.fill_type not in (
+            'material', 'distribmat'
+        ):
+            raise TypeError("DAGMC cell temperature overrides require a "
+                            "material fill.")
+        if self.density is not None and self.fill_type not in ('material', 'distribmat'):
+            raise TypeError("DAGMC cell density overrides require a "
+                            "material fill.")
+        if any(getattr(self, attr) is not None for attr in ('translation', 'rotation')):
+            raise TypeError("DAGMC cell overrides do not support translation "
+                            "or rotation.")
+        return super().create_xml_subelement(xml_element, memo)
 
     @classmethod
-    def from_xml_element(cls, elem, surfaces, materials, get_universe):
-        raise TypeError("from_xml_element is not available for DAGMC cells.")
+    def from_xml_element(cls, elem, mats, universe):
+        """Generate a DAGMCCell from an XML <cell> override element.
+
+        Parameters
+        ----------
+        elem : lxml.etree._Element
+            `<cell>` element containing a DAGMC cell property override
+        mats : dict
+            Dictionary mapping material ID strings to :class:`openmc.Material`
+            instances
+        universe : DAGMCUniverse
+            Universe to add the parsed cell to.
+
+        Returns
+        -------
+        DAGMCCell
+            DAGMCCell instance
+        """
+        if not isinstance(universe, DAGMCUniverse):
+            raise TypeError(
+                f"universe must be a DAGMCUniverse instance, "
+                f"got {type(universe).__name__}.")
+
+        cell_id = int(get_text(elem, 'id'))
+
+        # Validate attributes that are unsupported for DAGMC cell overrides
+        for tag in ('region', 'fill', 'universe'):
+            if get_text(elem, tag) is not None:
+                raise ValueError(
+                    f"DAGMC cell {cell_id} override cannot specify '{tag}'.")
+        for tag in ('translation', 'rotation'):
+            if get_text(elem, tag) is not None:
+                raise ValueError(
+                    f"DAGMC cell {cell_id} override does not support "
+                    f"'{tag}'.")
+        if get_elem_list(elem, 'material', str) is None:
+            raise ValueError(
+                f"DAGMC cell {cell_id} must specify a material override.")
+
+        return super().from_xml_element(
+            elem, surfaces={}, materials=mats,
+            get_universe=lambda _: universe)
diff --git a/openmc/data/__init__.py b/openmc/data/__init__.py
index c2d35565a..9b38d758e 100644
--- a/openmc/data/__init__.py
+++ b/openmc/data/__init__.py
@@ -33,5 +33,8 @@ from .resonance_covariance import *
 from .multipole import *
 from .grid import *
 from .function import *
+from .vectfit import *
 
-from .effective_dose.dose import dose_coefficients
+from .dose.dose import dose_coefficients
+from .dose.mass_attenuation import \
+    mass_energy_absorption_coefficient, mass_attenuation_coefficient
diff --git a/openmc/data/ace.py b/openmc/data/ace.py
index 6ccb76c92..d3de6a192 100644
--- a/openmc/data/ace.py
+++ b/openmc/data/ace.py
@@ -24,7 +24,7 @@ import numpy as np
 import openmc.checkvalue as cv
 from openmc.mixin import EqualityMixin
 from .data import ATOMIC_SYMBOL, gnds_name, EV_PER_MEV, K_BOLTZMANN
-from .endf import ENDF_FLOAT_RE
+from endf.records import ENDF_FLOAT_RE
 
 
 def get_metadata(zaid, metastable_scheme='nndc'):
diff --git a/openmc/data/angle_distribution.py b/openmc/data/angle_distribution.py
index e59ffa0c7..778139cc2 100644
--- a/openmc/data/angle_distribution.py
+++ b/openmc/data/angle_distribution.py
@@ -10,8 +10,8 @@ from openmc.mixin import EqualityMixin
 from openmc.stats import Univariate, Tabular, Uniform, Legendre
 from .function import INTERPOLATION_SCHEME
 from .data import EV_PER_MEV
-from .endf import get_head_record, get_cont_record, get_tab1_record, \
-    get_list_record, get_tab2_record
+from .endf import as_evaluation, get_head_record, get_cont_record, \
+    get_tab1_record, get_list_record, get_tab2_record
 
 
 class AngleDistribution(EqualityMixin):
@@ -213,7 +213,7 @@ class AngleDistribution(EqualityMixin):
 
         Parameters
         ----------
-        ev : openmc.data.endf.Evaluation
+        ev : openmc.data.endf.Evaluation or endf.Material
             ENDF evaluation
         mt : int
             The MT value of the reaction to get angular distributions for
@@ -224,6 +224,7 @@ class AngleDistribution(EqualityMixin):
             Angular distribution
 
         """
+        ev = as_evaluation(ev)
         file_obj = StringIO(ev.section[4, mt])
 
         # Read HEAD record
diff --git a/openmc/data/data.py b/openmc/data/data.py
index 408adf2e4..6e4840990 100644
--- a/openmc/data/data.py
+++ b/openmc/data/data.py
@@ -1,11 +1,26 @@
+from __future__ import annotations
+
 import itertools
 import json
 import os
 import re
 from pathlib import Path
 from math import sqrt, log
+from typing import TYPE_CHECKING, Literal
 from warnings import warn
 
+from endf.data import (ATOMIC_NUMBER, ATOMIC_SYMBOL, ELEMENT_SYMBOL,
+                       EV_PER_MEV, K_BOLTZMANN, gnds_name, zam)
+
+import openmc
+from openmc.checkvalue import PathLike
+
+if TYPE_CHECKING:
+    from openmc.deplete import Chain
+
+gnds_name.__module__ = __name__
+zam.__module__ = __name__
+
 # Isotopic abundances from Meija J, Coplen T B, et al, "Isotopic compositions
 # of the elements 2013 (IUPAC Technical Report)", Pure. Appl. Chem. 88 (3),
 # pp. 293-306 (2013). The "representative isotopic abundance" values from
@@ -94,7 +109,7 @@ NATURAL_ABUNDANCE = {
     'Yb174': 0.32025, 'Yb176': 0.12995, 'Lu175': 0.97401,
     'Lu176': 0.02599, 'Hf174': 0.0016, 'Hf176': 0.0526,
     'Hf177': 0.186, 'Hf178': 0.2728, 'Hf179': 0.1362,
-    'Hf180': 0.3508, 'Ta180': 0.0001201, 'Ta181': 0.9998799,
+    'Hf180': 0.3508, 'Ta180_m1': 0.0001201, 'Ta181': 0.9998799,
     'W180': 0.0012, 'W182': 0.265, 'W183': 0.1431,
     'W184': 0.3064, 'W186': 0.2843, 'Re185': 0.374,
     'Re187': 0.626, 'Os184': 0.0002, 'Os186': 0.0159,
@@ -112,72 +127,6 @@ NATURAL_ABUNDANCE = {
     'U238': 0.992742
 }
 
-# Dictionary to give element symbols from IUPAC names
-# (and some common mispellings)
-ELEMENT_SYMBOL = {'neutron': 'n', 'hydrogen': 'H', 'helium': 'He',
-                 'lithium': 'Li', 'beryllium': 'Be', 'boron': 'B',
-                 'carbon': 'C', 'nitrogen': 'N', 'oxygen': 'O', 'fluorine': 'F',
-                 'neon': 'Ne', 'sodium': 'Na', 'magnesium': 'Mg',
-                 'aluminium': 'Al', 'aluminum': 'Al', 'silicon': 'Si',
-                 'phosphorus': 'P', 'sulfur': 'S', 'sulphur': 'S',
-                 'chlorine': 'Cl', 'argon': 'Ar', 'potassium': 'K',
-                 'calcium': 'Ca', 'scandium': 'Sc', 'titanium': 'Ti',
-                 'vanadium': 'V', 'chromium': 'Cr', 'manganese': 'Mn',
-                 'iron': 'Fe', 'cobalt': 'Co', 'nickel': 'Ni', 'copper': 'Cu',
-                 'zinc': 'Zn', 'gallium': 'Ga', 'germanium': 'Ge',
-                 'arsenic': 'As', 'selenium': 'Se', 'bromine': 'Br',
-                 'krypton': 'Kr', 'rubidium': 'Rb', 'strontium': 'Sr',
-                 'yttrium': 'Y', 'zirconium': 'Zr', 'niobium': 'Nb',
-                 'molybdenum': 'Mo', 'technetium': 'Tc', 'ruthenium': 'Ru',
-                 'rhodium': 'Rh', 'palladium': 'Pd', 'silver': 'Ag',
-                 'cadmium': 'Cd', 'indium': 'In', 'tin': 'Sn', 'antimony': 'Sb',
-                 'tellurium': 'Te', 'iodine': 'I', 'xenon': 'Xe',
-                 'caesium': 'Cs', 'cesium': 'Cs', 'barium': 'Ba',
-                 'lanthanum': 'La', 'cerium': 'Ce', 'praseodymium': 'Pr',
-                 'neodymium': 'Nd', 'promethium': 'Pm', 'samarium': 'Sm',
-                 'europium': 'Eu', 'gadolinium': 'Gd', 'terbium': 'Tb',
-                 'dysprosium': 'Dy', 'holmium': 'Ho', 'erbium': 'Er',
-                 'thulium': 'Tm', 'ytterbium': 'Yb', 'lutetium': 'Lu',
-                 'hafnium': 'Hf', 'tantalum': 'Ta', 'tungsten': 'W',
-                 'wolfram': 'W', 'rhenium': 'Re', 'osmium': 'Os',
-                 'iridium': 'Ir', 'platinum': 'Pt', 'gold': 'Au',
-                 'mercury': 'Hg', 'thallium': 'Tl', 'lead': 'Pb',
-                 'bismuth': 'Bi', 'polonium': 'Po', 'astatine': 'At',
-                 'radon': 'Rn', 'francium': 'Fr', 'radium': 'Ra',
-                 'actinium': 'Ac', 'thorium': 'Th', 'protactinium': 'Pa',
-                 'uranium': 'U', 'neptunium': 'Np', 'plutonium': 'Pu',
-                 'americium': 'Am', 'curium': 'Cm', 'berkelium': 'Bk',
-                 'californium': 'Cf', 'einsteinium': 'Es', 'fermium': 'Fm',
-                 'mendelevium': 'Md', 'nobelium': 'No', 'lawrencium': 'Lr',
-                 'rutherfordium': 'Rf', 'dubnium': 'Db', 'seaborgium': 'Sg',
-                 'bohrium': 'Bh', 'hassium': 'Hs', 'meitnerium': 'Mt',
-                 'darmstadtium': 'Ds', 'roentgenium': 'Rg', 'copernicium': 'Cn',
-                 'nihonium': 'Nh', 'flerovium': 'Fl', 'moscovium': 'Mc',
-                 'livermorium': 'Lv', 'tennessine': 'Ts', 'oganesson': 'Og'}
-
-ATOMIC_SYMBOL = {0: 'n', 1: 'H', 2: 'He', 3: 'Li', 4: 'Be', 5: 'B', 6: 'C',
-                 7: 'N', 8: 'O', 9: 'F', 10: 'Ne', 11: 'Na', 12: 'Mg', 13: 'Al',
-                 14: 'Si', 15: 'P', 16: 'S', 17: 'Cl', 18: 'Ar', 19: 'K',
-                 20: 'Ca', 21: 'Sc', 22: 'Ti', 23: 'V', 24: 'Cr', 25: 'Mn',
-                 26: 'Fe', 27: 'Co', 28: 'Ni', 29: 'Cu', 30: 'Zn', 31: 'Ga',
-                 32: 'Ge', 33: 'As', 34: 'Se', 35: 'Br', 36: 'Kr', 37: 'Rb',
-                 38: 'Sr', 39: 'Y', 40: 'Zr', 41: 'Nb', 42: 'Mo', 43: 'Tc',
-                 44: 'Ru', 45: 'Rh', 46: 'Pd', 47: 'Ag', 48: 'Cd', 49: 'In',
-                 50: 'Sn', 51: 'Sb', 52: 'Te', 53: 'I', 54: 'Xe', 55: 'Cs',
-                 56: 'Ba', 57: 'La', 58: 'Ce', 59: 'Pr', 60: 'Nd', 61: 'Pm',
-                 62: 'Sm', 63: 'Eu', 64: 'Gd', 65: 'Tb', 66: 'Dy', 67: 'Ho',
-                 68: 'Er', 69: 'Tm', 70: 'Yb', 71: 'Lu', 72: 'Hf', 73: 'Ta',
-                 74: 'W', 75: 'Re', 76: 'Os', 77: 'Ir', 78: 'Pt', 79: 'Au',
-                 80: 'Hg', 81: 'Tl', 82: 'Pb', 83: 'Bi', 84: 'Po', 85: 'At',
-                 86: 'Rn', 87: 'Fr', 88: 'Ra', 89: 'Ac', 90: 'Th', 91: 'Pa',
-                 92: 'U', 93: 'Np', 94: 'Pu', 95: 'Am', 96: 'Cm', 97: 'Bk',
-                 98: 'Cf', 99: 'Es', 100: 'Fm', 101: 'Md', 102: 'No',
-                 103: 'Lr', 104: 'Rf', 105: 'Db', 106: 'Sg', 107: 'Bh',
-                 108: 'Hs', 109: 'Mt', 110: 'Ds', 111: 'Rg', 112: 'Cn',
-                 113: 'Nh', 114: 'Fl', 115: 'Mc', 116: 'Lv', 117: 'Ts',
-                 118: 'Og'}
-ATOMIC_NUMBER = {value: key for key, value in ATOMIC_SYMBOL.items()}
-
 DADZ = {
     '(n,2nd)': (-3, -1),
     '(n,2n)': (-1, 0),
@@ -268,11 +217,7 @@ DADZ = {
 # Values here are from the Committee on Data for Science and Technology
 # (CODATA) 2018 recommendation (https://physics.nist.gov/cuu/Constants/).
 
-# The value of the Boltzman constant in units of eV / K
-K_BOLTZMANN = 8.617333262e-5
-
 # Unit conversions
-EV_PER_MEV = 1.0e6
 JOULE_PER_EV = 1.602176634e-19
 
 # Avogadro's constant
@@ -284,9 +229,6 @@ NEUTRON_MASS = 1.00866491595
 # Used in atomic_mass function as a cache
 _ATOMIC_MASS: dict[str, float] = {}
 
-# Regex for GNDS nuclide names (used in zam function)
-_GNDS_NAME_RE = re.compile(r'([A-Zn][a-z]*)(\d+)((?:_[em]\d+)?)')
-
 # Used in half_life function as a cache
 _HALF_LIFE: dict[str, float] = {}
 _LOG_TWO = log(2.0)
@@ -324,7 +266,7 @@ def atomic_mass(isotope):
         # isotopes of their element (e.g. C0), calculate the atomic mass as
         # the sum of the atomic mass times the natural abundance of the isotopes
         # that make up the element.
-        for element in ['C', 'Zn', 'Pt', 'Os', 'Tl']:
+        for element in ['C', 'Zn', 'Pt', 'Os', 'Tl', 'V']:
             isotope_zero = element.lower() + '0'
             _ATOMIC_MASS[isotope_zero] = 0.
             for iso, abundance in isotopes(element):
@@ -363,25 +305,47 @@ def atomic_weight(element):
         raise ValueError(f"No naturally-occurring isotopes for element '{element}'.")
 
 
-def half_life(isotope):
+def half_life(
+    isotope: str,
+    chain_file: Literal[False] | None | PathLike | Chain = False
+) -> float | None:
     """Return half-life of isotope in seconds or None if isotope is stable
 
-    Half-life values are from the `ENDF/B-VIII.0 decay sublibrary
-    <https://www.nndc.bnl.gov/endf-b8.0/download.html>`_.
+    By default, half-life values are from the `ENDF/B-VIII.0 decay sublibrary
+    <https://www.nndc.bnl.gov/endf-b8.0/download.html>`_. A depletion chain can
+    also be used as the source of half-life values.
 
     .. versionadded:: 0.13.1
 
+    .. versionchanged:: 0.15.4
+        Added the ``chain_file`` argument.
+
     Parameters
     ----------
     isotope : str
         Name of isotope, e.g., 'Pu239'
+    chain_file : False, None, PathLike, or openmc.deplete.Chain, optional
+        Source of half-life values. If ``False``, only ENDF/B-VIII.0 data is
+        used. If ``None``, the chain specified by
+        ``openmc.config['chain_file']`` is used when available. If a path or
+        :class:`openmc.deplete.Chain` is given, that chain is used. For ``None``
+        or an explicit chain, nuclides absent from the chain fall back to
+        ENDF/B-VIII.0 data.
 
     Returns
     -------
-    float
-        Half-life of isotope in [s]
+    float or None
+        Half-life of isotope in [s], or None if the isotope is stable
 
     """
+    if chain_file is not False:
+        if chain_file is not None or openmc.config.get('chain_file') is not None:
+            # Local import avoids a circular dependency
+            from openmc.deplete.chain import _get_chain
+            chain = _get_chain(chain_file)
+            if isotope in chain:
+                return chain[isotope].half_life
+
     global _HALF_LIFE
     if not _HALF_LIFE:
         # Load ENDF/B-VIII.0 data from JSON file
@@ -391,7 +355,10 @@ def half_life(isotope):
     return _HALF_LIFE.get(isotope.lower())
 
 
-def decay_constant(isotope):
+def decay_constant(
+    isotope: str,
+    chain_file: Literal[False] | None | PathLike | Chain = False
+) -> float:
     """Return decay constant of isotope in [s^-1]
 
     Decay constants are based on half-life values from the
@@ -400,10 +367,20 @@ def decay_constant(isotope):
 
     .. versionadded:: 0.13.1
 
+    .. versionchanged:: 0.15.4
+        Added the ``chain_file`` argument.
+
     Parameters
     ----------
     isotope : str
         Name of isotope, e.g., 'Pu239'
+    chain_file : False, None, PathLike, or openmc.deplete.Chain, optional
+        Source of half-life values. If ``False``, only ENDF/B-VIII.0 data is
+        used. If ``None``, the chain specified by
+        ``openmc.config['chain_file']`` is used when available. If a path or
+        :class:`openmc.deplete.Chain` is given, that chain is used. For ``None``
+        or an explicit chain, nuclides absent from the chain fall back to
+        ENDF/B-VIII.0 data.
 
     Returns
     -------
@@ -415,7 +392,7 @@ def decay_constant(isotope):
     openmc.data.half_life
 
     """
-    t = half_life(isotope)
+    t = half_life(isotope, chain_file)
     return _LOG_TWO / t if t else 0.0
 
 
@@ -523,33 +500,6 @@ def water_density(temperature, pressure=0.1013):
     return coeff / pi / gamma1_pi
 
 
-def gnds_name(Z, A, m=0):
-    """Return nuclide name using GNDS convention
-
-    .. versionchanged:: 0.14.0
-        Function name changed from ``gnd_name`` to ``gnds_name``
-
-    Parameters
-    ----------
-    Z : int
-        Atomic number
-    A : int
-        Mass number
-    m : int, optional
-        Metastable state
-
-    Returns
-    -------
-    str
-        Nuclide name in GNDS convention, e.g., 'Am242_m1'
-
-    """
-    if m > 0:
-        return f'{ATOMIC_SYMBOL[Z]}{A}_m{m}'
-    return f'{ATOMIC_SYMBOL[Z]}{A}'
-
-
-
 def _get_element_symbol(element: str) -> str:
     if len(element) > 2:
         symbol = ELEMENT_SYMBOL.get(element.lower())
@@ -590,30 +540,3 @@ def isotopes(element: str) -> list[tuple[str, float]]:
             result.append(kv)
 
     return result
-
-
-def zam(name):
-    """Return tuple of (atomic number, mass number, metastable state)
-
-    Parameters
-    ----------
-    name : str
-        Name of nuclide using GNDS convention, e.g., 'Am242_m1'
-
-    Returns
-    -------
-    3-tuple of int
-        Atomic number, mass number, and metastable state
-
-    """
-    try:
-        symbol, A, state = _GNDS_NAME_RE.match(name).groups()
-    except AttributeError:
-        raise ValueError(f"'{name}' does not appear to be a nuclide name in "
-                         "GNDS format")
-
-    if symbol not in ATOMIC_NUMBER:
-        raise ValueError(f"'{symbol}' is not a recognized element symbol")
-
-    metastable = int(state[2:]) if state else 0
-    return (ATOMIC_NUMBER[symbol], int(A), metastable)
diff --git a/openmc/data/decay.py b/openmc/data/decay.py
index 66acb2212..5f95e1281 100644
--- a/openmc/data/decay.py
+++ b/openmc/data/decay.py
@@ -1,7 +1,7 @@
 from collections.abc import Iterable
+from functools import cached_property
 from io import StringIO
 from math import log
-import re
 from warnings import warn
 
 import numpy as np
@@ -12,9 +12,10 @@ import openmc.checkvalue as cv
 from openmc.exceptions import DataError
 from openmc.mixin import EqualityMixin
 from openmc.stats import Discrete, Tabular, Univariate, combine_distributions
-from .data import ATOMIC_SYMBOL, ATOMIC_NUMBER
+from .data import gnds_name, zam
 from .function import INTERPOLATION_SCHEME
-from .endf import Evaluation, get_head_record, get_list_record, get_tab1_record
+from .endf import (
+    as_evaluation, get_head_record, get_list_record, get_tab1_record)
 
 
 # Gives name and (change in A, change in Z) resulting from decay
@@ -75,7 +76,7 @@ class FissionProductYields(EqualityMixin):
 
     Parameters
     ----------
-    ev_or_filename : str of openmc.data.endf.Evaluation
+    ev_or_filename : str, openmc.data.endf.Evaluation, or endf.Material
         ENDF fission product yield evaluation to read from. If given as a
         string, it is assumed to be the filename for the ENDF file.
 
@@ -125,9 +126,7 @@ class FissionProductYields(EqualityMixin):
                 for j in range(n_products):
                     Z, A = divmod(int(values[4*j]), 1000)
                     isomeric_state = int(values[4*j + 1])
-                    name = ATOMIC_SYMBOL[Z] + str(A)
-                    if isomeric_state > 0:
-                        name += f'_m{isomeric_state}'
+                    name = gnds_name(Z, A, isomeric_state)
                     yield_j = ufloat(values[4*j + 2], values[4*j + 3])
                     yields[name] = yield_j
 
@@ -135,11 +134,7 @@ class FissionProductYields(EqualityMixin):
 
             return energies, data
 
-        # Get evaluation if str is passed
-        if isinstance(ev_or_filename, Evaluation):
-            ev = ev_or_filename
-        else:
-            ev = Evaluation(ev_or_filename)
+        ev = as_evaluation(ev_or_filename)
 
         # Assign basic nuclide properties
         self.nuclide = {
@@ -166,7 +161,7 @@ class FissionProductYields(EqualityMixin):
 
         Parameters
         ----------
-        ev_or_filename : str or openmc.data.endf.Evaluation
+        ev_or_filename : str, openmc.data.endf.Evaluation, or endf.Material
             ENDF fission product yield evaluation to read from. If given as a
             string, it is assumed to be the filename for the ENDF file.
 
@@ -242,9 +237,7 @@ class DecayMode(EqualityMixin):
     @property
     def daughter(self):
         # Determine atomic number and mass number of parent
-        symbol, A = re.match(r'([A-Zn][a-z]*)(\d+)', self.parent).groups()
-        A = int(A)
-        Z = ATOMIC_NUMBER[symbol]
+        Z, A, _ = zam(self.parent)
 
         # Process changes
         for mode in self.modes:
@@ -254,11 +247,11 @@ class DecayMode(EqualityMixin):
                         delta_A, delta_Z = changes
                         A += delta_A
                         Z += delta_Z
+                        break
+                    else:
+                        return None
 
-        if self._daughter_state > 0:
-            return f'{ATOMIC_SYMBOL[Z]}{A}_m{self._daughter_state}'
-        else:
-            return f'{ATOMIC_SYMBOL[Z]}{A}'
+        return gnds_name(Z, A, self._daughter_state)
 
     @property
     def parent(self):
@@ -296,7 +289,7 @@ class Decay(EqualityMixin):
 
     Parameters
     ----------
-    ev_or_filename : str of openmc.data.endf.Evaluation
+    ev_or_filename : str, openmc.data.endf.Evaluation, or endf.Material
         ENDF radioactive decay data evaluation to read from. If given as a
         string, it is assumed to be the filename for the ENDF file.
 
@@ -327,11 +320,7 @@ class Decay(EqualityMixin):
 
     """
     def __init__(self, ev_or_filename):
-        # Get evaluation if str is passed
-        if isinstance(ev_or_filename, Evaluation):
-            ev = ev_or_filename
-        else:
-            ev = Evaluation(ev_or_filename)
+        ev = as_evaluation(ev_or_filename)
 
         file_obj = StringIO(ev.section[8, 457])
 
@@ -339,7 +328,6 @@ class Decay(EqualityMixin):
         self.modes = []
         self.spectra = {}
         self.average_energies = {}
-        self._sources = None
 
         # Get head record
         items = get_head_record(file_obj)
@@ -348,10 +336,7 @@ class Decay(EqualityMixin):
         self.nuclide['atomic_number'] = Z
         self.nuclide['mass_number'] = A
         self.nuclide['isomeric_state'] = metastable
-        if metastable > 0:
-            self.nuclide['name'] = f'{ATOMIC_SYMBOL[Z]}{A}_m{metastable}'
-        else:
-            self.nuclide['name'] = f'{ATOMIC_SYMBOL[Z]}{A}'
+        self.nuclide['name'] = gnds_name(Z, A, metastable)
         self.nuclide['mass'] = items[1]  # AWR
         self.nuclide['excited_state'] = items[2]  # State of the original nuclide
         self.nuclide['stable'] = (items[4] == 1)  # Nucleus stability flag
@@ -494,7 +479,7 @@ class Decay(EqualityMixin):
 
         Parameters
         ----------
-        ev_or_filename : str or openmc.data.endf.Evaluation
+        ev_or_filename : str, openmc.data.endf.Evaluation, or endf.Material
             ENDF radioactive decay data evaluation to read from. If given as a
             string, it is assumed to be the filename for the ENDF file.
 
@@ -506,14 +491,9 @@ class Decay(EqualityMixin):
         """
         return cls(ev_or_filename)
 
-    @property
+    @cached_property
     def sources(self):
         """Radioactive decay source distributions"""
-        # If property has been computed already, return it
-        # TODO: Replace with functools.cached_property when support is Python 3.9+
-        if self._sources is not None:
-            return self._sources
-
         sources = {}
         name = self.nuclide['name']
         decay_constant = self.decay_constant.n
@@ -571,8 +551,7 @@ class Decay(EqualityMixin):
             merged_sources[particle_type] = combine_distributions(
                 dist_list, [1.0]*len(dist_list))
 
-        self._sources = merged_sources
-        return self._sources
+        return merged_sources
 
 
 _DECAY_PHOTON_ENERGY = {}
@@ -597,7 +576,7 @@ def decay_photon_energy(nuclide: str) -> Univariate | None:
     openmc.stats.Univariate or None
         Distribution of energies in [eV] of photons emitted from decay, or None
         if no photon source exists. Note that the probabilities represent
-        intensities, given as [Bq].
+        intensities, given as [Bq/atom] (in other words, decay constants).
     """
     if not _DECAY_PHOTON_ENERGY:
         chain_file = openmc.config.get('chain_file')
@@ -663,5 +642,3 @@ def decay_energy(nuclide: str):
             warn(f"Chain file '{chain_file}' does not have any decay energy.")
 
     return _DECAY_ENERGY.get(nuclide, 0.0)
-
-
diff --git a/openmc/data/effective_dose/__init__.py b/openmc/data/dose/__init__.py
similarity index 100%
rename from openmc/data/effective_dose/__init__.py
rename to openmc/data/dose/__init__.py
diff --git a/openmc/data/dose/dose.py b/openmc/data/dose/dose.py
new file mode 100644
index 000000000..87e0cf9ae
--- /dev/null
+++ b/openmc/data/dose/dose.py
@@ -0,0 +1,146 @@
+from pathlib import Path
+
+import numpy as np
+
+import openmc.checkvalue as cv
+
+_FULL_GEOMETRIES = ('AP', 'PA', 'LLAT', 'RLAT', 'ROT', 'ISO')
+_LIMITED_GEOMETRIES = ('AP', 'PA', 'ISO')
+
+_TABLES = {
+    ('icrp74', 'effective', 'neutron'): (
+        Path('icrp74') / 'neutrons.txt', _FULL_GEOMETRIES),
+    ('icrp74', 'effective', 'photon'): (
+        Path('icrp74') / 'photons.txt', _FULL_GEOMETRIES),
+    ('icrp116', 'effective', 'electron'): (
+        Path('icrp116') / 'electrons.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'helium'): (
+        Path('icrp116') / 'helium_ions.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'mu-'): (
+        Path('icrp116') / 'negative_muons.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'pi-'): (
+        Path('icrp116') / 'negative_pions.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'neutron'): (
+        Path('icrp116') / 'neutrons.txt', _FULL_GEOMETRIES),
+    ('icrp116', 'effective', 'photon'): (
+        Path('icrp116') / 'photons.txt', _FULL_GEOMETRIES),
+    ('icrp116', 'effective', 'photon kerma'): (
+        Path('icrp116') / 'photons_kerma.txt', _FULL_GEOMETRIES),
+    ('icrp116', 'effective', 'mu+'): (
+        Path('icrp116') / 'positive_muons.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'pi+'): (
+        Path('icrp116') / 'positive_pions.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'positron'): (
+        Path('icrp116') / 'positrons.txt', _LIMITED_GEOMETRIES),
+    ('icrp116', 'effective', 'proton'): (
+        Path('icrp116') / 'protons.txt', _FULL_GEOMETRIES),
+    ('icrp74', 'ambient', 'neutron'): (
+        Path('icrp74') / 'neutrons_H10.txt', None),
+    ('icrp74', 'ambient', 'photon'): (
+        Path('icrp74') / 'photons_H10.txt', None),
+}
+
+_DOSE_TABLES = {}
+
+
+def _load_dose_table(data_source: str, dose_quantity: str, particle: str):
+    """Load dose tables from text files.
+
+    Parameters
+    ----------
+    data_source : {'icrp74', 'icrp116'}
+        The dose conversion data source to use
+    dose_quantity : {'effective', 'ambient'}
+        Dose quantity to load. 'ambient' corresponds to ambient dose
+        equivalent H*(10).
+    particle : {'neutron', 'photon', 'photon kerma', 'electron', 'positron'}
+        Incident particle
+
+    """
+    key = (data_source, dose_quantity, particle)
+    path = Path(__file__).parent / _TABLES[key][0]
+    data = np.loadtxt(path, skiprows=3, encoding='utf-8')
+    data[:, 0] *= 1e6   # Change energies to eV
+    _DOSE_TABLES[key] = data
+
+
+def dose_coefficients(
+    particle, geometry='AP', data_source='icrp116', dose_quantity='effective'
+):
+    """Return dose conversion coefficients.
+
+    This function provides fluence (and air kerma) to effective dose or ambient
+    dose equivalent (H*(10)) conversion coefficients for various types of
+    external exposures based on values in ICRP publications. Corrected values
+    found in a corrigendum are used rather than the values in the original
+    report. Available libraries include `ICRP Publication 74
+    <https://doi.org/10.1016/S0146-6453(96)90010-X>` and `ICRP Publication 116
+    <https://doi.org/10.1016/j.icrp.2011.10.001>`.
+
+    For ICRP 74 data, the photon effective dose per fluence is determined by
+    multiplying the air kerma per fluence values (Table A.1) by the effective
+    dose per air kerma (Table A.17). The neutron effective dose per fluence is
+    found in Table A.41. For ICRP 116 data, the photon effective dose per
+    fluence is found in Table A.1 and the neutron effective dose per fluence is
+    found in Table A.5.
+
+    Parameters
+    ----------
+    particle : {'neutron', 'photon', 'photon kerma', 'electron', 'positron'}
+        Incident particle
+    geometry : {'AP', 'PA', 'LLAT', 'RLAT', 'ROT', 'ISO'}
+        Irradiation geometry assumed for effective dose coefficients. Refer to
+        ICRP-116 (Section 3.2) for the meaning of the options here. This
+        argument does not apply when ``dose_quantity`` is 'ambient'.
+    data_source : {'icrp74', 'icrp116'}
+        The data source for the dose conversion coefficients.
+    dose_quantity : {'effective', 'ambient'}
+        Dose quantity to return. 'effective' returns effective dose
+        coefficients; 'ambient' returns ambient dose equivalent (H*(10))
+        coefficients.
+
+    Returns
+    -------
+    energy : numpy.ndarray
+        Energies at which dose conversion coefficients are given
+    dose_coeffs : numpy.ndarray
+        Dose coefficients in [pSv cm^2] at provided energies. For 'photon
+        kerma', the coefficients are given in [Sv/Gy].
+
+    """
+
+    cv.check_value('geometry', geometry, _FULL_GEOMETRIES)
+    cv.check_value('data_source', data_source, {'icrp74', 'icrp116'})
+    cv.check_value('dose_quantity', dose_quantity, {'effective', 'ambient'})
+
+    key = (data_source, dose_quantity, particle)
+    if key not in _TABLES:
+        available_particles = sorted(
+            p for ds, dq, p in _TABLES
+            if ds == data_source and dq == dose_quantity
+        )
+        msg = (
+            f"'{particle}' has no {dose_quantity} dose data in data source "
+            f"{data_source}. Available particles for {data_source} "
+            f"with dose quantity {dose_quantity} are: {available_particles}"
+        )
+        raise ValueError(msg)
+    elif key not in _DOSE_TABLES:
+        _load_dose_table(data_source, dose_quantity, particle)
+
+    data = _DOSE_TABLES[key]
+    columns = _TABLES[key][1]
+
+    if columns is None:
+        if geometry != 'AP':
+            raise ValueError(
+                "Irradiation geometry is not defined for ambient dose "
+                "equivalent coefficients. Use the default geometry='AP'."
+            )
+        index = 0
+    else:
+        index = columns.index(geometry)
+
+    energy = data[:, 0].copy()
+    dose_coeffs = data[:, index + 1].copy()
+    return energy, dose_coeffs
diff --git a/openmc/data/effective_dose/icrp116/electrons.txt b/openmc/data/dose/icrp116/electrons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/electrons.txt
rename to openmc/data/dose/icrp116/electrons.txt
diff --git a/openmc/data/effective_dose/icrp116/helium_ions.txt b/openmc/data/dose/icrp116/helium_ions.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/helium_ions.txt
rename to openmc/data/dose/icrp116/helium_ions.txt
diff --git a/openmc/data/effective_dose/icrp116/negative_muons.txt b/openmc/data/dose/icrp116/negative_muons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/negative_muons.txt
rename to openmc/data/dose/icrp116/negative_muons.txt
diff --git a/openmc/data/effective_dose/icrp116/negative_pions.txt b/openmc/data/dose/icrp116/negative_pions.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/negative_pions.txt
rename to openmc/data/dose/icrp116/negative_pions.txt
diff --git a/openmc/data/effective_dose/icrp116/neutrons.txt b/openmc/data/dose/icrp116/neutrons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/neutrons.txt
rename to openmc/data/dose/icrp116/neutrons.txt
diff --git a/openmc/data/effective_dose/icrp116/photons.txt b/openmc/data/dose/icrp116/photons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/photons.txt
rename to openmc/data/dose/icrp116/photons.txt
diff --git a/openmc/data/effective_dose/icrp116/photons_kerma.txt b/openmc/data/dose/icrp116/photons_kerma.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/photons_kerma.txt
rename to openmc/data/dose/icrp116/photons_kerma.txt
diff --git a/openmc/data/effective_dose/icrp116/positive_muons.txt b/openmc/data/dose/icrp116/positive_muons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/positive_muons.txt
rename to openmc/data/dose/icrp116/positive_muons.txt
diff --git a/openmc/data/effective_dose/icrp116/positive_pions.txt b/openmc/data/dose/icrp116/positive_pions.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/positive_pions.txt
rename to openmc/data/dose/icrp116/positive_pions.txt
diff --git a/openmc/data/effective_dose/icrp116/positrons.txt b/openmc/data/dose/icrp116/positrons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/positrons.txt
rename to openmc/data/dose/icrp116/positrons.txt
diff --git a/openmc/data/effective_dose/icrp116/protons.txt b/openmc/data/dose/icrp116/protons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp116/protons.txt
rename to openmc/data/dose/icrp116/protons.txt
diff --git a/openmc/data/effective_dose/icrp74/generate_photon_effective_dose.py b/openmc/data/dose/icrp74/generate_photon_effective_dose.py
similarity index 100%
rename from openmc/data/effective_dose/icrp74/generate_photon_effective_dose.py
rename to openmc/data/dose/icrp74/generate_photon_effective_dose.py
diff --git a/openmc/data/effective_dose/icrp74/neutrons.txt b/openmc/data/dose/icrp74/neutrons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp74/neutrons.txt
rename to openmc/data/dose/icrp74/neutrons.txt
diff --git a/openmc/data/dose/icrp74/neutrons_H10.txt b/openmc/data/dose/icrp74/neutrons_H10.txt
new file mode 100644
index 000000000..fe0036cbc
--- /dev/null
+++ b/openmc/data/dose/icrp74/neutrons_H10.txt
@@ -0,0 +1,50 @@
+Neutrons: Ambient per fluence, in units of pSv cm², for monoenergetic particles incident.
+
+Energy (MeV)            Dose
+    1.00E-09			6.60
+    1.00E-08			9.00
+    2.53E-08			10.6
+    1.00E-07			12.9
+    2.00E-07			13.5
+    5.00E-07			13.6
+    1.00E-06			13.3
+    2.00E-06			12.9
+    5.00E-06			12.0
+    1.00E-05			11.3
+    2.00E-05			10.6
+    5.00E-05			9.90
+    1.00E-04			9.40
+    2.00E-04			8.90
+    5.00E-04			8.30
+    1.00E-03			7.90
+    2.00E-03			7.70
+    5.00E-03			8.00
+    1.00E-02			10.5
+    2.00E-02			16.6
+    3.00E-02			23.7
+    5.00E-02			41.1
+    7.00E-02			60.0
+    1.00E-01			88.0
+    1.50E-01			132
+    2.00E-01			170
+    3.00E-01			233
+    5.00E-01			322
+    7.00E-01			375
+    9.00E-01			400
+    1			        416
+    1.2			        425
+    2			        420
+    3			        412
+    4			        408
+    5			        405
+    6         			400
+    7			        405
+    8			        409
+    9			        420
+    10			        440
+    12			        480
+    14      			520
+    15			        540
+    16      			555
+    18			        570
+    20			        600
\ No newline at end of file
diff --git a/openmc/data/effective_dose/icrp74/photons.txt b/openmc/data/dose/icrp74/photons.txt
similarity index 100%
rename from openmc/data/effective_dose/icrp74/photons.txt
rename to openmc/data/dose/icrp74/photons.txt
diff --git a/openmc/data/dose/icrp74/photons_H10.txt b/openmc/data/dose/icrp74/photons_H10.txt
new file mode 100644
index 000000000..66031b2f0
--- /dev/null
+++ b/openmc/data/dose/icrp74/photons_H10.txt
@@ -0,0 +1,28 @@
+Photons: Ambient dose (H*10) per fluence, in units of pSv cm²
+
+Energy (MeV)            Dose
+0.010					0.061
+0.015					0.83
+0.020					1.05					
+0.030					0.81					
+0.040					0.64					
+0.050					0.55					
+0.060					0.51					
+0.080					0.53					
+0.100					0.61					
+0.150					0.89					
+0.200					1.20					
+0.300					1.80					
+0.400					2.38					
+0.500					2.93					
+0.600					3.44					
+0.800					4.38					
+1				    	5.20					
+1.5	    				6.90					
+2		    			8.60					
+3			    		11.1					
+4				    	13.4					
+5				    	15.5					
+6   					17.6
+8		    			21.6
+10				    	25.6
\ No newline at end of file
diff --git a/openmc/data/dose/mass_attenuation.h5 b/openmc/data/dose/mass_attenuation.h5
new file mode 100644
index 000000000..f62785140
Binary files /dev/null and b/openmc/data/dose/mass_attenuation.h5 differ
diff --git a/openmc/data/dose/mass_attenuation.py b/openmc/data/dose/mass_attenuation.py
new file mode 100644
index 000000000..c4260480b
--- /dev/null
+++ b/openmc/data/dose/mass_attenuation.py
@@ -0,0 +1,153 @@
+from pathlib import Path
+
+import numpy as np
+import h5py
+
+import openmc.checkvalue as cv
+from openmc.data import EV_PER_MEV
+from ..data import ATOMIC_NUMBER
+from ..function import Tabulated1D
+
+# Embedded NIST-126 data
+# Air (Dry Near Sea Level) — NIST Standard Reference Database 126 Table 4 (doi: 10.18434/T4D01F)
+# Columns: Energy (MeV), μ_en/ρ (cm^2/g)
+_NIST126_AIR = np.array([
+    [1.00000e-03, 3.599e03],
+    [1.50000e-03, 1.188e03],
+    [2.00000e-03, 5.262e02],
+    [3.00000e-03, 1.614e02],
+    [3.20290e-03, 1.330e02],
+    [3.20290e-03, 1.460e02],
+    [4.00000e-03, 7.636e01],
+    [5.00000e-03, 3.931e01],
+    [6.00000e-03, 2.270e01],
+    [8.00000e-03, 9.446e00],
+    [1.00000e-02, 4.742e00],
+    [1.50000e-02, 1.334e00],
+    [2.00000e-02, 5.389e-01],
+    [3.00000e-02, 1.537e-01],
+    [4.00000e-02, 6.833e-02],
+    [5.00000e-02, 4.098e-02],
+    [6.00000e-02, 3.041e-02],
+    [8.00000e-02, 2.407e-02],
+    [1.00000e-01, 2.325e-02],
+    [1.50000e-01, 2.496e-02],
+    [2.00000e-01, 2.672e-02],
+    [3.00000e-01, 2.872e-02],
+    [4.00000e-01, 2.949e-02],
+    [5.00000e-01, 2.966e-02],
+    [6.00000e-01, 2.953e-02],
+    [8.00000e-01, 2.882e-02],
+    [1.00000e00, 2.789e-02],
+    [1.25000e00, 2.666e-02],
+    [1.50000e00, 2.547e-02],
+    [2.00000e00, 2.345e-02],
+    [3.00000e00, 2.057e-02],
+    [4.00000e00, 1.870e-02],
+    [5.00000e00, 1.740e-02],
+    [6.00000e00, 1.647e-02],
+    [8.00000e00, 1.525e-02],
+    [1.00000e01, 1.450e-02],
+    [1.50000e01, 1.353e-02],
+    [2.00000e01, 1.311e-02],
+])
+
+# Registry of embedded tables: (data_source, material) -> ndarray
+# Table shape: (N, 2) with columns [Energy (MeV), μen/ρ (cm^2/g)]
+_MUEN_TABLES = {
+    ("nist126", "air"): _NIST126_AIR,
+}
+
+
+def mass_energy_absorption_coefficient(
+    material: str, data_source: str = "nist126"
+) -> Tabulated1D:
+    r"""Return the mass energy-absorption coefficient as a function of energy.
+
+    The mass energy-absorption coefficient, :math:`\mu_\text{en}/\rho`, is
+    defined as the fraction of incident photon energy absorbed in a material per
+    unit mass less the energy carried away by scattered photons. It is obtained
+    from `NIST Standard Reference Database 126
+    <https://doi.org/10.18434/T4D01F>`_: X-Ray Mass Attenuation Coefficients.
+
+    Parameters
+    ----------
+    material : {'air'}
+        Material compound for which to load coefficients.
+    data_source : {'nist126'}
+        Source library.
+
+    Returns
+    -------
+    Tabulated1D
+        Mass energy-absorption coefficient [cm^2/g] as a function of photon
+        energy [eV], using log-log interpolation.
+
+    """
+    cv.check_value("material", material, {"air"})
+    cv.check_value("data_source", data_source, {"nist126"})
+
+    key = (data_source, material)
+    if key not in _MUEN_TABLES:
+        available = sorted({m for (ds, m) in _MUEN_TABLES.keys() if ds == data_source})
+        raise ValueError(
+            f"No mass energy-absorption data for '{material}' in data source "
+            f"'{data_source}'. Available materials: {available}"
+        )
+
+    data = _MUEN_TABLES[key]
+    energy = data[:, 0].copy() * EV_PER_MEV  # MeV -> eV
+    mu_en_coeffs = data[:, 1].copy()
+    return Tabulated1D(energy, mu_en_coeffs,
+                       breakpoints=[len(energy)], interpolation=[5])
+
+
+# Used in mass_attenuation_coefficient function as a cache.
+# Maps atomic number Z (int) -> Tabulated1D of (mu/rho) [cm^2/g] vs E [eV]
+_MASS_ATTENUATION: dict[int, object] = {}
+
+
+def mass_attenuation_coefficient(element):
+    r"""Return the photon mass attenuation coefficient as a function of energy.
+
+    The mass energy-absorption coefficient, :math:`\mu_\text{en}/\rho`, is
+    defined as the fraction of incident photon energy absorbed in a material per
+    unit mass. Values for each element are obtained from `NIST Standard
+    Reference Database 8 <https://doi.org/10.18434/T48G6X>`_: XCOM Photon Cross
+    Sections Database.
+
+    Parameters
+    ----------
+    element : str or int
+        Element symbol (e.g., 'Fe') or atomic number (e.g., 26).
+
+    Returns
+    -------
+    Tabulated1D
+        Mass attenuation coefficient [cm^2/g] as a function of photon energy
+        [eV], using log-log interpolation.
+
+    """
+    if not _MASS_ATTENUATION:
+        data_file = Path(__file__).with_name('mass_attenuation.h5')
+        with h5py.File(data_file, 'r') as f:
+            for key, dataset in f.items():
+                energies, mu_rho = dataset[()]  # shape (2, N)
+                _MASS_ATTENUATION[int(key)] = Tabulated1D(
+                    energies, mu_rho,
+                    breakpoints=[len(energies)],
+                    interpolation=[5]  # log-log
+                )
+
+    # Resolve element argument to atomic number
+    if isinstance(element, str):
+        if element not in ATOMIC_NUMBER:
+            raise ValueError(f"'{element}' is not a recognized element symbol")
+        Z = ATOMIC_NUMBER[element]
+    else:
+        Z = int(element)
+
+    if Z not in _MASS_ATTENUATION:
+        raise ValueError(f"No mass attenuation data available for Z={Z}")
+
+    return _MASS_ATTENUATION[Z]
diff --git a/openmc/data/effective_dose/dose.py b/openmc/data/effective_dose/dose.py
deleted file mode 100644
index c7f458d1c..000000000
--- a/openmc/data/effective_dose/dose.py
+++ /dev/null
@@ -1,102 +0,0 @@
-from pathlib import Path
-
-import numpy as np
-
-import openmc.checkvalue as cv
-
-_FILES = {
-    ('icrp74', 'neutron'): Path('icrp74') / 'neutrons.txt',
-    ('icrp74', 'photon'): Path('icrp74') / 'photons.txt',
-    ('icrp116', 'electron'): Path('icrp116') / 'electrons.txt',
-    ('icrp116', 'helium'): Path('icrp116') / 'helium_ions.txt',
-    ('icrp116', 'mu-'): Path('icrp116') / 'negative_muons.txt',
-    ('icrp116', 'pi-'): Path('icrp116') / 'negative_pions.txt',
-    ('icrp116', 'neutron'): Path('icrp116') / 'neutrons.txt',
-    ('icrp116', 'photon'): Path('icrp116') / 'photons.txt',
-    ('icrp116', 'photon kerma'): Path('icrp116') / 'photons_kerma.txt',
-    ('icrp116', 'mu+'): Path('icrp116') / 'positive_muons.txt',
-    ('icrp116', 'pi+'): Path('icrp116') / 'positive_pions.txt',
-    ('icrp116', 'positron'): Path('icrp116') / 'positrons.txt',
-    ('icrp116', 'proton'): Path('icrp116') / 'protons.txt',
-}
-
-_DOSE_TABLES = {}
-
-
-def _load_dose_icrp(data_source: str, particle: str):
-    """Load effective dose tables from text files.
-
-    Parameters
-    ----------
-    data_source : {'icrp74', 'icrp116'}
-        The dose conversion data source to use
-    particle : {'neutron', 'photon', 'photon kerma', 'electron', 'positron'}
-        Incident particle
-
-    """
-    path = Path(__file__).parent / _FILES[data_source, particle]
-    data = np.loadtxt(path, skiprows=3, encoding='utf-8')
-    data[:, 0] *= 1e6   # Change energies to eV
-    _DOSE_TABLES[data_source, particle] = data
-
-
-def dose_coefficients(particle, geometry='AP', data_source='icrp116'):
-    """Return effective dose conversion coefficients.
-
-    This function provides fluence (and air kerma) to effective or ambient dose
-    (H*(10)) conversion coefficients for various types of external exposures
-    based on values in ICRP publications. Corrected values found in a
-    corrigendum are used rather than the values in the original report.
-    Available libraries include `ICRP Publication 74
-    <https://doi.org/10.1016/S0146-6453(96)90010-X>` and `ICRP Publication 116
-    <https://doi.org/10.1016/j.icrp.2011.10.001>`.
-
-    For ICRP 74 data, the photon effective dose per fluence is determined by
-    multiplying the air kerma per fluence values (Table A.1) by the effective
-    dose per air kerma (Table A.17). The neutron effective dose per fluence is
-    found in Table A.41. For ICRP 116 data, the photon effective dose per
-    fluence is found in Table A.1 and the neutron effective dose per fluence is
-    found in Table A.5.
-
-    Parameters
-    ----------
-    particle : {'neutron', 'photon', 'photon kerma', 'electron', 'positron'}
-        Incident particle
-    geometry : {'AP', 'PA', 'LLAT', 'RLAT', 'ROT', 'ISO'}
-        Irradiation geometry assumed. Refer to ICRP-116 (Section 3.2) for the
-        meaning of the options here.
-    data_source : {'icrp74', 'icrp116'}
-        The data source for the effective dose conversion coefficients.
-
-    Returns
-    -------
-    energy : numpy.ndarray
-        Energies at which dose conversion coefficients are given
-    dose_coeffs : numpy.ndarray
-        Effective dose coefficients in [pSv cm^2] at provided energies. For
-        'photon kerma', the coefficients are given in [Sv/Gy].
-
-    """
-
-    cv.check_value('geometry', geometry, {'AP', 'PA', 'LLAT', 'RLAT', 'ROT', 'ISO'})
-    cv.check_value('data_source', data_source, {'icrp74', 'icrp116'})
-
-    if (data_source, particle) not in _FILES:
-        raise ValueError(f"{particle} has no dose data in data source {data_source}.")
-    elif (data_source, particle) not in _DOSE_TABLES:
-        _load_dose_icrp(data_source, particle)
-
-    # Get all data for selected particle
-    data = _DOSE_TABLES[data_source, particle]
-
-    # Determine index for selected geometry
-    if particle in ('neutron', 'photon', 'proton', 'photon kerma'):
-        columns = ('AP', 'PA', 'LLAT', 'RLAT', 'ROT', 'ISO')
-    else:
-        columns = ('AP', 'PA', 'ISO')
-    index = columns.index(geometry)
-
-    # Pull out energy and dose from table
-    energy = data[:, 0].copy()
-    dose_coeffs = data[:, index + 1].copy()
-    return energy, dose_coeffs
diff --git a/openmc/data/endf.py b/openmc/data/endf.py
index eca374469..edd8afffb 100644
--- a/openmc/data/endf.py
+++ b/openmc/data/endf.py
@@ -10,350 +10,38 @@ import io
 from pathlib import PurePath
 import re
 
-import numpy as np
-
 from .data import gnds_name
 from .function import Tabulated1D
-from endf.records import float_endf
-
-
-_LIBRARY = {0: 'ENDF/B', 1: 'ENDF/A', 2: 'JEFF', 3: 'EFF',
-            4: 'ENDF/B High Energy', 5: 'CENDL', 6: 'JENDL',
-            17: 'TENDL', 18: 'ROSFOND', 21: 'SG-21', 31: 'INDL/V',
-            32: 'INDL/A', 33: 'FENDL', 34: 'IRDF', 35: 'BROND',
-            36: 'INGDB-90', 37: 'FENDL/A', 41: 'BROND'}
-
-_SUBLIBRARY = {
-    0: 'Photo-nuclear data',
-    1: 'Photo-induced fission product yields',
-    3: 'Photo-atomic data',
-    4: 'Radioactive decay data',
-    5: 'Spontaneous fission product yields',
-    6: 'Atomic relaxation data',
-    10: 'Incident-neutron data',
-    11: 'Neutron-induced fission product yields',
-    12: 'Thermal neutron scattering data',
-    19: 'Neutron standards',
-    113: 'Electro-atomic data',
-    10010: 'Incident-proton data',
-    10011: 'Proton-induced fission product yields',
-    10020: 'Incident-deuteron data',
-    10030: 'Incident-triton data',
-    20030: 'Incident-helion (3He) data',
-    20040: 'Incident-alpha data'
-}
-
-SUM_RULES = {1: [2, 3],
-             3: [4, 5, 11, 16, 17, 22, 23, 24, 25, 27, 28, 29, 30, 32, 33, 34, 35,
-                 36, 37, 41, 42, 44, 45, 152, 153, 154, 156, 157, 158, 159, 160,
-                 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172,
-                 173, 174, 175, 176, 177, 178, 179, 180, 181, 183, 184, 185,
-                 186, 187, 188, 189, 190, 194, 195, 196, 198, 199, 200],
-             4: list(range(50, 92)),
-             16: list(range(875, 892)),
-             18: [19, 20, 21, 38],
-             27: [18, 101],
-             101: [102, 103, 104, 105, 106, 107, 108, 109, 111, 112, 113, 114,
-                   115, 116, 117, 155, 182, 191, 192, 193, 197],
-             103: list(range(600, 650)),
-             104: list(range(650, 700)),
-             105: list(range(700, 750)),
-             106: list(range(750, 800)),
-             107: list(range(800, 850))}
-
-ENDF_FLOAT_RE = re.compile(r'([\s\-\+]?\d*\.\d+)([\+\-]) ?(\d+)')
-
-
-def py_float_endf(s):
-    """Convert string of floating point number in ENDF to float.
-
-    The ENDF-6 format uses an 'e-less' floating point number format,
-    e.g. -1.23481+10. Trying to convert using the float built-in won't work
-    because of the lack of an 'e'. This function allows such strings to be
-    converted while still allowing numbers that are not in exponential notation
-    to be converted as well.
-
-    Parameters
-    ----------
-    s : str
-        Floating-point number from an ENDF file
-
-    Returns
-    -------
-    float
-        The number
-
-    """
-    return float(ENDF_FLOAT_RE.sub(r'\1e\2\3', s))
-
-
-def int_endf(s):
-    """Convert string of integer number in ENDF to int.
-
-    The ENDF-6 format technically allows integers to be represented by a field
-    of all blanks. This function acts like int(s) except when s is a string of
-    all whitespace, in which case zero is returned.
-
-    Parameters
-    ----------
-    s : str
-        Integer or spaces
-
-    Returns
-    -------
-    integer
-        The number or 0
-    """
-    return 0 if s.isspace() else int(s)
-
-
-def get_text_record(file_obj):
-    """Return data from a TEXT record in an ENDF-6 file.
-
-    Parameters
-    ----------
-    file_obj : file-like object
-        ENDF-6 file to read from
-
-    Returns
-    -------
-    str
-        Text within the TEXT record
-
-    """
-    return file_obj.readline()[:66]
-
-
-def get_cont_record(file_obj, skip_c=False):
-    """Return data from a CONT record in an ENDF-6 file.
-
-    Parameters
-    ----------
-    file_obj : file-like object
-        ENDF-6 file to read from
-    skip_c : bool
-        Determine whether to skip the first two quantities (C1, C2) of the CONT
-        record.
-
-    Returns
-    -------
-    tuple
-        The six items within the CONT record
-
-    """
-    line = file_obj.readline()
-    if skip_c:
-        C1 = None
-        C2 = None
-    else:
-        C1 = float_endf(line[:11])
-        C2 = float_endf(line[11:22])
-    L1 = int_endf(line[22:33])
-    L2 = int_endf(line[33:44])
-    N1 = int_endf(line[44:55])
-    N2 = int_endf(line[55:66])
-    return (C1, C2, L1, L2, N1, N2)
-
-
-def get_head_record(file_obj):
-    """Return data from a HEAD record in an ENDF-6 file.
-
-    Parameters
-    ----------
-    file_obj : file-like object
-        ENDF-6 file to read from
-
-    Returns
-    -------
-    tuple
-        The six items within the HEAD record
-
-    """
-    line = file_obj.readline()
-    ZA = int(float_endf(line[:11]))
-    AWR = float_endf(line[11:22])
-    L1 = int_endf(line[22:33])
-    L2 = int_endf(line[33:44])
-    N1 = int_endf(line[44:55])
-    N2 = int_endf(line[55:66])
-    return (ZA, AWR, L1, L2, N1, N2)
-
-
-def get_list_record(file_obj):
-    """Return data from a LIST record in an ENDF-6 file.
-
-    Parameters
-    ----------
-    file_obj : file-like object
-        ENDF-6 file to read from
-
-    Returns
-    -------
-    list
-        The six items within the header
-    list
-        The values within the list
-
-    """
-    # determine how many items are in list
-    items = get_cont_record(file_obj)
-    NPL = items[4]
-
-    # read items
-    b = []
-    for i in range((NPL - 1)//6 + 1):
-        line = file_obj.readline()
-        n = min(6, NPL - 6*i)
-        for j in range(n):
-            b.append(float_endf(line[11*j:11*(j + 1)]))
-
-    return (items, b)
+from endf.material import (
+    Material,
+    _LIBRARY,
+    _SUBLIBRARY,
+    get_materials as get_evaluations,
+)
+from endf.incident_neutron import SUM_RULES
+from endf.records import (
+    float_endf,
+    py_float_endf,
+    int_endf,
+    get_text_record,
+    get_cont_record,
+    get_head_record,
+    get_list_record,
+    get_tab1_record as _get_tab1_record,
+    get_tab2_record,
+    get_intg_record,
+)
 
 
 def get_tab1_record(file_obj):
     """Return data from a TAB1 record in an ENDF-6 file.
 
-    Parameters
-    ----------
-    file_obj : file-like object
-        ENDF-6 file to read from
-
-    Returns
-    -------
-    list
-        The six items within the header
-    openmc.data.Tabulated1D
-        The tabulated function
-
+    This wraps the endf package's get_tab1_record to return an
+    openmc.data.Tabulated1D (which has HDF5 support and is a Function1D)
+    instead of endf.Tabulated1D.
     """
-    # Determine how many interpolation regions and total points there are
-    line = file_obj.readline()
-    C1 = float_endf(line[:11])
-    C2 = float_endf(line[11:22])
-    L1 = int_endf(line[22:33])
-    L2 = int_endf(line[33:44])
-    n_regions = int_endf(line[44:55])
-    n_pairs = int_endf(line[55:66])
-    params = [C1, C2, L1, L2]
-
-    # Read the interpolation region data, namely NBT and INT
-    breakpoints = np.zeros(n_regions, dtype=int)
-    interpolation = np.zeros(n_regions, dtype=int)
-    m = 0
-    for i in range((n_regions - 1)//3 + 1):
-        line = file_obj.readline()
-        to_read = min(3, n_regions - m)
-        for j in range(to_read):
-            breakpoints[m] = int_endf(line[0:11])
-            interpolation[m] = int_endf(line[11:22])
-            line = line[22:]
-            m += 1
-
-    # Read tabulated pairs x(n) and y(n)
-    x = np.zeros(n_pairs)
-    y = np.zeros(n_pairs)
-    m = 0
-    for i in range((n_pairs - 1)//3 + 1):
-        line = file_obj.readline()
-        to_read = min(3, n_pairs - m)
-        for j in range(to_read):
-            x[m] = float_endf(line[:11])
-            y[m] = float_endf(line[11:22])
-            line = line[22:]
-            m += 1
-
-    return params, Tabulated1D(x, y, breakpoints, interpolation)
-
-
-def get_tab2_record(file_obj):
-    # Determine how many interpolation regions and total points there are
-    params = get_cont_record(file_obj)
-    n_regions = params[4]
-
-    # Read the interpolation region data, namely NBT and INT
-    breakpoints = np.zeros(n_regions, dtype=int)
-    interpolation = np.zeros(n_regions, dtype=int)
-    m = 0
-    for i in range((n_regions - 1)//3 + 1):
-        line = file_obj.readline()
-        to_read = min(3, n_regions - m)
-        for j in range(to_read):
-            breakpoints[m] = int(line[0:11])
-            interpolation[m] = int(line[11:22])
-            line = line[22:]
-            m += 1
-
-    return params, Tabulated2D(breakpoints, interpolation)
-
-
-def get_intg_record(file_obj):
-    """
-    Return data from an INTG record in an ENDF-6 file. Used to store the
-    covariance matrix in a compact format.
-
-    Parameters
-    ----------
-    file_obj : file-like object
-        ENDF-6 file to read from
-
-    Returns
-    -------
-    numpy.ndarray
-        The correlation matrix described in the INTG record
-    """
-    # determine how many items are in list and NDIGIT
-    items = get_cont_record(file_obj)
-    ndigit = items[2]
-    npar = items[3]    # Number of parameters
-    nlines = items[4]  # Lines to read
-    NROW_RULES = {2: 18, 3: 12, 4: 11, 5: 9, 6: 8}
-    nrow = NROW_RULES[ndigit]
-
-    # read lines and build correlation matrix
-    corr = np.identity(npar)
-    for i in range(nlines):
-        line = file_obj.readline()
-        ii = int_endf(line[:5]) - 1  # -1 to account for 0 indexing
-        jj = int_endf(line[5:10]) - 1
-        factor = 10**ndigit
-        for j in range(nrow):
-            if jj+j >= ii:
-                break
-            element = int_endf(line[11+(ndigit+1)*j:11+(ndigit+1)*(j+1)])
-            if element > 0:
-                corr[ii, jj] = (element+0.5)/factor
-            elif element < 0:
-                corr[ii, jj] = (element-0.5)/factor
-
-    # Symmetrize the correlation matrix
-    corr = corr + corr.T - np.diag(corr.diagonal())
-    return corr
-
-
-def get_evaluations(filename):
-    """Return a list of all evaluations within an ENDF file.
-
-    Parameters
-    ----------
-    filename : str
-        Path to ENDF-6 formatted file
-
-    Returns
-    -------
-    list
-        A list of :class:`openmc.data.endf.Evaluation` instances.
-
-    """
-    evaluations = []
-    with open(str(filename), 'r') as fh:
-        while True:
-            pos = fh.tell()
-            line = fh.readline()
-            if line[66:70] == '  -1':
-                break
-            fh.seek(pos)
-            evaluations.append(Evaluation(fh))
-    return evaluations
+    params, tab = _get_tab1_record(file_obj)
+    return params, Tabulated1D(tab.x, tab.y, tab.breakpoints, tab.interpolation)
 
 
 class Evaluation:
@@ -361,7 +49,7 @@ class Evaluation:
 
     Parameters
     ----------
-    filename_or_obj : str or file-like
+    filename_or_obj : str, file-like, or endf.Material
         Path to ENDF file to read or an open file positioned at the start of an
         ENDF material
 
@@ -381,17 +69,25 @@ class Evaluation:
 
     """
     def __init__(self, filename_or_obj):
+        self.section = {}
+        self.info = {}
+        self.target = {}
+        self.projectile = {}
+        self.reaction_list = []
+
+        if isinstance(filename_or_obj, Material):
+            self.section = dict(filename_or_obj.section_text)
+            self.section_data = filename_or_obj.section_data
+            self.material = filename_or_obj.MAT
+            self._read_header()
+            return
+
         if isinstance(filename_or_obj, (str, PurePath)):
             fh = open(str(filename_or_obj), 'r')
             need_to_close = True
         else:
             fh = filename_or_obj
             need_to_close = False
-        self.section = {}
-        self.info = {}
-        self.target = {}
-        self.projectile = {}
-        self.reaction_list = []
 
         # Skip TPID record. Evaluators sometimes put in TPID records that are
         # ill-formated because they lack MF/MT values or put them in the wrong
@@ -517,23 +213,9 @@ class Evaluation:
                          self.target['isomeric_state'])
 
 
-class Tabulated2D:
-    """Metadata for a two-dimensional function.
-
-    This is a dummy class that is not really used other than to store the
-    interpolation information for a two-dimensional function. Once we refactor
-    to adopt GNDS-like data containers, this will probably be removed or
-    extended.
-
-    Parameters
-    ----------
-    breakpoints : Iterable of int
-        Breakpoints for interpolation regions
-    interpolation : Iterable of int
-        Interpolation scheme identification number, e.g., 3 means y is linear in
-        ln(x).
-
-    """
-    def __init__(self, breakpoints, interpolation):
-        self.breakpoints = breakpoints
-        self.interpolation = interpolation
+def as_evaluation(ev_or_filename):
+    """Return an object supporting OpenMC's legacy Evaluation interface."""
+    if isinstance(ev_or_filename, Evaluation):
+        return ev_or_filename
+    else:
+        return Evaluation(ev_or_filename)
diff --git a/openmc/data/fission_energy.py b/openmc/data/fission_energy.py
index 3c7998ee2..e4fac087a 100644
--- a/openmc/data/fission_energy.py
+++ b/openmc/data/fission_energy.py
@@ -5,7 +5,8 @@ from io import StringIO
 import openmc.checkvalue as cv
 from openmc.mixin import EqualityMixin
 from .data import EV_PER_MEV
-from .endf import get_cont_record, get_list_record, get_tab1_record, Evaluation
+from .endf import (
+    as_evaluation, get_cont_record, get_list_record, get_tab1_record)
 from .function import Function1D, Tabulated1D, Polynomial, sum_functions
 
 
@@ -195,7 +196,7 @@ class FissionEnergyRelease(EqualityMixin):
 
         Parameters
         ----------
-        ev : openmc.data.endf.Evaluation
+        ev : openmc.data.endf.Evaluation or endf.Material
             ENDF evaluation
         incident_neutron : openmc.data.IncidentNeutron
             Corresponding incident neutron dataset
@@ -206,7 +207,7 @@ class FissionEnergyRelease(EqualityMixin):
             Fission energy release data
 
         """
-        cv.check_type('evaluation', ev, Evaluation)
+        ev = as_evaluation(ev)
 
         # Check to make sure this ENDF file matches the expected isomer.
         if ev.target['atomic_number'] != incident_neutron.atomic_number:
diff --git a/openmc/data/library.py b/openmc/data/library.py
index a6ce1bbd3..b49757b0d 100644
--- a/openmc/data/library.py
+++ b/openmc/data/library.py
@@ -5,7 +5,7 @@ import h5py
 import lxml.etree as ET
 
 import openmc
-from openmc._xml import clean_indentation, reorder_attributes
+from openmc._xml import get_elem_list, get_text, clean_indentation
 
 
 class DataLibrary(list):
@@ -132,7 +132,6 @@ class DataLibrary(list):
         clean_indentation(root)
 
         # Write XML file
-        reorder_attributes(root)  # TODO: Remove when support is Python 3.8+
         tree = ET.ElementTree(root)
         tree.write(str(path), xml_declaration=True, encoding='utf-8',
                    method='xml')
@@ -173,9 +172,9 @@ class DataLibrary(list):
             directory = os.path.dirname(path)
 
         for lib_element in root.findall('library'):
-            filename = os.path.join(directory, lib_element.attrib['path'])
-            filetype = lib_element.attrib['type']
-            materials = lib_element.attrib['materials'].split()
+            filename = os.path.join(directory, get_text(lib_element, "path"))
+            filetype = get_text(lib_element, "type")
+            materials = get_elem_list(lib_element, "materials", str) or []
             library = {'path': filename, 'type': filetype,
                        'materials': materials}
             data.libraries.append(library)
@@ -183,7 +182,7 @@ class DataLibrary(list):
         # get depletion chain data
         dep_node = root.find("depletion_chain")
         if dep_node is not None:
-            filename = os.path.join(directory, dep_node.attrib['path'])
+            filename = os.path.join(directory, get_text(dep_node, "path"))
             library = {'path': filename, 'type': 'depletion_chain',
                        'materials': []}
             data.libraries.append(library)
diff --git a/openmc/data/multipole.py b/openmc/data/multipole.py
index dd14e0d19..d843b5971 100644
--- a/openmc/data/multipole.py
+++ b/openmc/data/multipole.py
@@ -15,7 +15,7 @@ from . import WMP_VERSION, WMP_VERSION_MAJOR
 from .data import K_BOLTZMANN
 from .neutron import IncidentNeutron
 from .resonance import ResonanceRange
-
+from .vectfit import vectfit, evaluate
 
 # Constants that determine which value to access
 _MP_EA = 0       # Pole
@@ -174,10 +174,6 @@ def _vectfit_xs(energy, ce_xs, mts, rtol=1e-3, atol=1e-5, orders=None,
         (poles, residues)
 
     """
-
-    # import vectfit package: https://github.com/liangjg/vectfit
-    import vectfit as vf
-
     ne = energy.size
     nmt = len(mts)
     if ce_xs.shape != (nmt, ne):
@@ -194,8 +190,8 @@ def _vectfit_xs(energy, ce_xs, mts, rtol=1e-3, atol=1e-5, orders=None,
         test_xs_ref[i] = np.interp(test_energy, energy, ce_xs[i])
 
     if log:
-        print(f"  energy: {energy[0]:.3e} to {energy[-1]:.3e} eV ({ne} points)")
-        print(f"  error tolerance: rtol={rtol}, atol={atol}")
+        print(f"\tenergy: {energy[0]:.3e} to {energy[-1]:.3e} eV ({ne} points)")
+        print(f"\terror tolerance: rtol={rtol}, atol={atol}")
 
     # transform xs (sigma) and energy (E) to f (sigma*E) and s (sqrt(E)) to be
     # compatible with the multipole representation
@@ -251,7 +247,7 @@ def _vectfit_xs(energy, ce_xs, mts, rtol=1e-3, atol=1e-5, orders=None,
                 print(f"VF iteration {i_vf + 1}/{n_vf_iter}")
 
             # call vf
-            poles, residues, cf, f_fit, rms = vf.vectfit(f, s, poles, weight)
+            poles, residues, *_ = vectfit(f, s, poles, weight)
 
             # convert real pole to conjugate pairs
             n_real_poles = 0
@@ -268,11 +264,11 @@ def _vectfit_xs(energy, ce_xs, mts, rtol=1e-3, atol=1e-5, orders=None,
             if n_real_poles > 0:
                 if log >= DETAILED_LOGGING:
                     print(f"  # real poles: {n_real_poles}")
-                new_poles, residues, cf, f_fit, rms = \
-                      vf.vectfit(f, s, new_poles, weight, skip_pole=True)
+                new_poles, residues, *_ = \
+                      vectfit(f, s, new_poles, weight, skip_pole_update=True)
 
             # assess the result on test grid
-            test_xs = vf.evaluate(test_s, new_poles, residues) / test_energy
+            test_xs = evaluate(test_s, new_poles, residues) / test_energy
             abserr = np.abs(test_xs - test_xs_ref)
             with np.errstate(invalid='ignore', divide='ignore'):
                 relerr = abserr / test_xs_ref
@@ -388,9 +384,9 @@ def _vectfit_xs(energy, ce_xs, mts, rtol=1e-3, atol=1e-5, orders=None,
 
     return (mp_poles, mp_residues)
 
-
 def vectfit_nuclide(endf_file, njoy_error=5e-4, vf_pieces=None,
-                    log=False, path_out=None, mp_filename=None, **kwargs):
+                    log=False, path_out=None, mp_filename=None,
+                    **kwargs):
     r"""Generate multipole data for a nuclide from ENDF.
 
     Parameters
@@ -571,10 +567,6 @@ def _windowing(mp_data, n_cf, rtol=1e-3, atol=1e-5, n_win=None, spacing=None,
         format.
 
     """
-
-    # import vectfit package: https://github.com/liangjg/vectfit
-    import vectfit as vf
-
     # unpack multipole data
     name = mp_data["name"]
     awr = mp_data["AWR"]
@@ -645,7 +637,7 @@ def _windowing(mp_data, n_cf, rtol=1e-3, atol=1e-5, n_win=None, spacing=None,
 
         # reference xs from multipole form, note the residue terms in the
         # multipole and vector fitting representations differ by a 1j
-        xs_ref = vf.evaluate(energy_sqrt, poles, residues*1j) / energy
+        xs_ref = evaluate(energy_sqrt, poles, residues*1j) / energy
 
         # curve fit matrix
         matrix = np.vstack([energy**(0.5*i - 1) for i in range(n_cf + 1)]).T
@@ -659,7 +651,7 @@ def _windowing(mp_data, n_cf, rtol=1e-3, atol=1e-5, n_win=None, spacing=None,
 
             # calculate the cross sections contributed by the windowed poles
             if rp > lp:
-                xs_wp = vf.evaluate(energy_sqrt, poles[lp:rp],
+                xs_wp = evaluate(energy_sqrt, poles[lp:rp],
                                     residues[:, lp:rp]*1j) / energy
             else:
                 xs_wp = np.zeros_like(xs_ref)
@@ -1054,7 +1046,15 @@ class WindowedMultipole(EqualityMixin):
         return cls.from_multipole(mp_data, **wmp_options)
 
     @classmethod
-    def from_multipole(cls, mp_data, search=None, log=False, **kwargs):
+    def from_multipole(
+        cls,
+        mp_data,
+        search=None,
+        log=False,
+        search_n_win=20,
+        search_cf_orders=None,
+        **kwargs,
+    ):
         """Generate windowed multipole neutron data from multipole data.
 
         Parameters
@@ -1066,8 +1066,14 @@ class WindowedMultipole(EqualityMixin):
             Defaults to True if no windowing parameters are specified.
         log : bool or int, optional
             Whether to print running logs (use int for verbosity control)
+        search_n_win : int, optional
+            Number of window sizes to consider in the search grid when
+            ``search`` is True.
+        search_cf_orders : iterable of int, optional
+            Curve-fit orders to consider in the search grid when ``search`` is
+            True. Defaults to integers from 10 down to 2.
         **kwargs
-            Keyword arguments passed to :func:`openmc.data.multipole._windowing`
+            Keyword arguments passed to :func:`openmc.data.multipole._windowing`.
 
         Returns
         -------
@@ -1098,12 +1104,17 @@ class WindowedMultipole(EqualityMixin):
         # search optimal WMP from a range of window sizes and CF orders
         if log:
             print("Start searching ...")
+        if search_cf_orders is None:
+            search_cf_orders = range(10, 1, -1)
+
         n_poles = sum([p.size for p in mp_data["poles"]])
         n_win_min = max(5, n_poles // 20)
         n_win_max = 2000 if n_poles < 2000 else 8000
         best_wmp = best_metric = None
-        for n_w in np.unique(np.linspace(n_win_min, n_win_max, 20, dtype=int)):
-            for n_cf in range(10, 1, -1):
+        for n_w in np.unique(
+            np.linspace(n_win_min, n_win_max, search_n_win, dtype=int)
+        ):
+            for n_cf in search_cf_orders:
                 if log:
                     print(f"Testing N_win={n_w} N_cf={n_cf}")
 
diff --git a/openmc/data/neutron.py b/openmc/data/neutron.py
index 95a3424ea..f17bb3fe4 100644
--- a/openmc/data/neutron.py
+++ b/openmc/data/neutron.py
@@ -11,9 +11,10 @@ import h5py
 
 from . import HDF5_VERSION, HDF5_VERSION_MAJOR
 from .ace import Library, Table, get_table, get_metadata
-from .data import ATOMIC_SYMBOL, K_BOLTZMANN, EV_PER_MEV
+from .data import ATOMIC_SYMBOL, K_BOLTZMANN, EV_PER_MEV, gnds_name
 from .endf import (
-    Evaluation, SUM_RULES, get_head_record, get_tab1_record, get_evaluations)
+    Evaluation, SUM_RULES, as_evaluation, get_head_record, get_tab1_record,
+    get_evaluations)
 from .fission_energy import FissionEnergyRelease
 from .function import Tabulated1D, Sum, ResonancesWithBackground
 from .njoy import make_ace, make_pendf
@@ -652,7 +653,7 @@ class IncidentNeutron(EqualityMixin):
 
         Parameters
         ----------
-        ev_or_filename : openmc.data.endf.Evaluation or str
+        ev_or_filename : openmc.data.endf.Evaluation, endf.Material, or str
             ENDF evaluation to read from. If given as a string, it is assumed to
             be the filename for the ENDF file.
 
@@ -666,10 +667,7 @@ class IncidentNeutron(EqualityMixin):
             Incident neutron continuous-energy data
 
         """
-        if isinstance(ev_or_filename, Evaluation):
-            ev = ev_or_filename
-        else:
-            ev = Evaluation(ev_or_filename)
+        ev = as_evaluation(ev_or_filename)
 
         atomic_number = ev.target['atomic_number']
         mass_number = ev.target['mass_number']
@@ -678,11 +676,7 @@ class IncidentNeutron(EqualityMixin):
         temperature = ev.target['temperature']
 
         # Determine name
-        element = ATOMIC_SYMBOL[atomic_number]
-        if metastable > 0:
-            name = f'{element}{mass_number}_m{metastable}'
-        else:
-            name = f'{element}{mass_number}'
+        name = gnds_name(atomic_number, mass_number, metastable)
 
         # Instantiate incident neutron data
         data = cls(name, atomic_number, mass_number, metastable,
@@ -769,6 +763,11 @@ class IncidentNeutron(EqualityMixin):
             for table in lib.tables[1:]:
                 data.add_temperature_from_ace(table)
 
+            # Use name based on ENDF evaluation. The name assigned by from_ace
+            # may be wrong for higher metastable states (e.g., Hf178_m2)
+            ev = evaluation if evaluation is not None else Evaluation(filename)
+            data.name = ev.gnds_name
+
             # Add 0K elastic scattering cross section
             if '0K' not in data.energy:
                 pendf = Evaluation(kwargs['pendf'])
@@ -779,7 +778,6 @@ class IncidentNeutron(EqualityMixin):
                 data[2].xs['0K'] = xs
 
             # Add fission energy release data
-            ev = evaluation if evaluation is not None else Evaluation(filename)
             if (1, 458) in ev.section:
                 data.fission_energy = f = FissionEnergyRelease.from_endf(ev, data)
             else:
@@ -805,9 +803,7 @@ class IncidentNeutron(EqualityMixin):
             # Helper function to get a cross section from an ENDF file on a
             # given energy grid
             def get_file3_xs(ev, mt, E):
-                file_obj = StringIO(ev.section[3, mt])
-                get_head_record(file_obj)
-                _, xs = get_tab1_record(file_obj)
+                xs = ev.section_data[3, mt]['sigma']
                 return xs(E)
 
             heating_local = Reaction(901)
diff --git a/openmc/data/njoy.py b/openmc/data/njoy.py
index e025cff0d..7c99637f0 100644
--- a/openmc/data/njoy.py
+++ b/openmc/data/njoy.py
@@ -15,19 +15,41 @@ import openmc.data
 # identifiers.
 ThermalTuple = namedtuple('ThermalTuple', ['name', 'zaids', 'nmix'])
 _THERMAL_DATA = {
+    'c_Ag': ThermalTuple('ag', [47107, 47109], 1),
     'c_Al27': ThermalTuple('al27', [13027], 1),
     'c_Al_in_Al2O3': ThermalTuple('asap00', [13027], 1),
+    'c_Al_in_Y3Al5O12': ThermalTuple('alyag', [13027], 1),
+    'c_Au': ThermalTuple('au', [79197], 1),
     'c_Be': ThermalTuple('be', [4009], 1),
     'c_Be_distinct': ThermalTuple('besd', [4009], 1),
     'c_Be_in_BeO': ThermalTuple('bebeo', [4009], 1),
     'c_Be_in_Be2C': ThermalTuple('bebe2c', [4009], 1),
     'c_Be_in_BeF2': ThermalTuple('bebef2', [4009], 1),
     'c_Be_in_FLiBe': ThermalTuple('beflib', [4009], 1),
+    'c_BeO': ThermalTuple('beo', [4009, 8016, 8017, 8018], 2),
+    'c_Bi': ThermalTuple('bi', [83209], 1),
+    'c_Bi_in_Ge3Bi4O12': ThermalTuple('bigbo', [83209], 1),
     'c_C6H6': ThermalTuple('benz', [1001, 6000, 6012], 2),
     'c_C_in_Be2C': ThermalTuple('cbe2c', [6000, 6012, 6013], 1),
     'c_C_in_C5O2H8': ThermalTuple('clucit', [6000, 6012, 6013], 1),
     'c_C_in_C8H8': ThermalTuple('cc8h8', [6000, 6012, 6013], 1),
+    'c_C_in_C19H16_liquid': ThermalTuple('c19liq', [6000, 6012, 6013], 1),
+    'c_C_in_C19H16_solid': ThermalTuple('c19sol', [6000, 6012, 6013], 1),
+    'c_C_in_C2H6O_liquid': ThermalTuple('ethliq', [6000, 6012, 6013], 1),
+    'c_C_in_C2H6O_solid': ThermalTuple('ethsol', [6000, 6012, 6013], 1),
+    'c_C_in_C6H6_liquid': ThermalTuple('benzlq', [6000, 6012, 6013], 1),
+    'c_C_in_C6H6_solid': ThermalTuple('benzsl', [6000, 6012, 6013], 1),
+    'c_C_in_C7H8_liquid': ThermalTuple('tolliq', [6000, 6012, 6013], 1),
+    'c_C_in_C7H8_solid': ThermalTuple('tolsol', [6000, 6012, 6013], 1),
+    'c_C_in_C8H10_liquid': ThermalTuple('xylliq', [6000, 6012, 6013], 1),
+    'c_C_in_C8H10_solid': ThermalTuple('xylsol', [6000, 6012, 6013], 1),
+    'c_C_in_C9H12_liquid': ThermalTuple('mesliq', [6000, 6012, 6013], 1),
+    'c_C_in_C9H12_solid': ThermalTuple('messol', [6000, 6012, 6013], 1),
     'c_C_in_CF2': ThermalTuple('ccf2', [6000, 6012, 6013], 1),
+    'c_C_in_CH2': ThermalTuple('cch2', [6000, 6012, 6013], 1),
+    'c_C_in_CH4_liquid': ThermalTuple('cch4lq', [6000, 6012, 6013], 1),
+    'c_C_in_CH4_solid': ThermalTuple('cch4sl', [6000, 6012, 6013], 1),
+    'c_C_in_Diamond': ThermalTuple('cdiam', [6000, 6012, 6013], 1),
     'c_C_in_SiC': ThermalTuple('csic', [6000, 6012, 6013], 1),
     'c_C_in_UC_100p': ThermalTuple('cuc100', [6000, 6012, 6013], 1),
     'c_C_in_UC_10p': ThermalTuple('cuc10', [6000, 6012, 6013], 1),
@@ -36,16 +58,29 @@ _THERMAL_DATA = {
     'c_C_in_UC_HALEU': ThermalTuple('cuchal', [6000, 6012, 6013], 1),
     'c_C_in_UC_HEU': ThermalTuple('cucheu', [6000, 6012, 6013], 1),
     'c_C_in_ZrC': ThermalTuple('czrc', [6000, 6012, 6013], 1),
+    'c_Ca': ThermalTuple('ca', [20040, 20042, 20043, 20044, 20046, 20048], 1),
     'c_Ca_in_CaH2': ThermalTuple('cacah2', [20040, 20042, 20043, 20044, 20046, 20048], 1),
+    'c_Ca_in_CaO2H2': ThermalTuple('cacaoh', [20040, 20042, 20043, 20044, 20046, 20048], 1),
+    'c_Cr': ThermalTuple('cr', [24050, 24052, 24053, 24054], 1),
+    'c_Cu': ThermalTuple('cu', [29063, 29065], 1),
     'c_D_in_7LiD': ThermalTuple('dlid', [1002], 1),
     'c_D_in_D2O': ThermalTuple('dd2o', [1002], 1),
     'c_D_in_D2O_solid': ThermalTuple('dice', [1002], 1),
+    'c_D_in_MgD2': ThermalTuple('dmgd2', [1002], 1),
     'c_F_in_Be2': ThermalTuple('fbef2', [9019], 1),
     'c_F_in_CF2': ThermalTuple('fcf2', [9019], 1),
     'c_F_in_FLiBe': ThermalTuple('fflibe', [9019], 1),
     'c_F_in_HF': ThermalTuple('f_hf', [9019], 1),
+    'c_F_in_LiF': ThermalTuple('flif', [9019], 1),
     'c_F_in_MgF2': ThermalTuple('fmgf2', [9019], 1),
     'c_Fe56': ThermalTuple('fe56', [26056], 1),
+    'c_Fe_in_Fe_alpha': ThermalTuple('fealph', [26054, 26056, 26057, 26058], 1),
+    'c_Fe_in_Fe_gamma': ThermalTuple('fegamm', [26054, 26056, 26057, 26058], 1),
+    'c_Ga_in_GaN': ThermalTuple('gagan', [31069, 31071], 1),
+    'c_Ga_in_GaSe': ThermalTuple('gagase', [31069, 31071], 1),
+    'c_Ge': ThermalTuple('ge', [32070, 32072, 32073, 32074, 32076], 1),
+    'c_Ge_in_Ge3Bi4O12': ThermalTuple('gegbo', [32070, 32072, 32073, 32074, 32076], 1),
+    'c_Ge_in_GeTe': ThermalTuple('gegete', [32070, 32072, 32073, 32074, 32076], 1),
     'c_Graphite': ThermalTuple('graph', [6000, 6012, 6013], 1),
     'c_Graphite_10p': ThermalTuple('grph10', [6000, 6012, 6013], 1),
     'c_Graphite_20p': ThermalTuple('grph20', [6000, 6012, 6013], 1),
@@ -54,7 +89,20 @@ _THERMAL_DATA = {
     'c_H_in_7LiH': ThermalTuple('hlih', [1001], 1),
     'c_H_in_C5O2H8': ThermalTuple('lucite', [1001], 1),
     'c_H_in_C8H8': ThermalTuple('hc8h8', [1001], 1),
+    'c_H_in_C19H16_liquid': ThermalTuple('h19liq', [1001], 1),
+    'c_H_in_C19H16_solid': ThermalTuple('h19sol', [1001], 1),
+    'c_H_in_C2H6O_liquid': ThermalTuple('hetliq', [1001], 1),
+    'c_H_in_C2H6O_solid': ThermalTuple('hetsol', [1001], 1),
+    'c_H_in_C6H6_liquid': ThermalTuple('hbzliq', [1001], 1),
+    'c_H_in_C6H6_solid': ThermalTuple('hbzsol', [1001], 1),
+    'c_H_in_C7H8_liquid': ThermalTuple('htlliq', [1001], 1),
+    'c_H_in_C7H8_solid': ThermalTuple('htlsol', [1001], 1),
+    'c_H_in_C8H10_liquid': ThermalTuple('hxyliq', [1001], 1),
+    'c_H_in_C8H10_solid': ThermalTuple('hxysol', [1001], 1),
+    'c_H_in_C9H12_liquid': ThermalTuple('hmsliq', [1001], 1),
+    'c_H_in_C9H12_solid': ThermalTuple('hmssol', [1001], 1),
     'c_H_in_CaH2': ThermalTuple('hcah2', [1001], 1),
+    'c_H_in_CaO2H2': ThermalTuple('hcaoh', [1001], 1),
     'c_H1_in_CaH2': ThermalTuple('h1cah2', [1001], 1),
     'c_H2_in_CaH2': ThermalTuple('h2cah2', [1001], 1),
     'c_H_in_CH2': ThermalTuple('hch2', [1001], 1),
@@ -64,32 +112,64 @@ _THERMAL_DATA = {
     'c_H_in_H2O': ThermalTuple('hh2o', [1001], 1),
     'c_H_in_H2O_solid': ThermalTuple('hice', [1001], 1),
     'c_H_in_HF': ThermalTuple('hhf', [1001], 1),
+    'c_H_in_KOH': ThermalTuple('hkoh', [1001], 1),
+    'c_H_in_LiH': ThermalTuple('hlih2', [1001], 1),
     'c_H_in_Mesitylene': ThermalTuple('mesi00', [1001], 1),
     'c_H_in_ParaffinicOil': ThermalTuple('hparaf', [1001], 1),
     'c_H_in_Toluene': ThermalTuple('tol00', [1001], 1),
+    'c_H_in_MgH2': ThermalTuple('hmgh2', [1001], 1),
+    'c_H_in_MgOH2': ThermalTuple('hmgoh', [1001], 1),
+    'c_H_in_NaMgH3': ThermalTuple('hnamg', [1001], 1),
+    'c_H_in_NaOH': ThermalTuple('hnaoh', [1001], 1),
+    'c_H_in_SrH2': ThermalTuple('hsrh2', [1001], 1),
     'c_H_in_UH3': ThermalTuple('huh3', [1001], 1),
     'c_H_in_YH2': ThermalTuple('hyh2', [1001], 1),
     'c_H_in_ZrH': ThermalTuple('hzrh', [1001], 1),
     'c_H_in_ZrH2': ThermalTuple('hzrh2', [1001], 1),
     'c_H_in_ZrHx': ThermalTuple('hzrhx', [1001], 1),
+    'c_I_in_NaI': ThermalTuple('inai', [53127], 1),
+    'c_K': ThermalTuple('k', [19039, 19040, 19041], 1),
+    'c_K_in_KOH': ThermalTuple('kkoh', [19039, 19040, 19041], 1),
     'c_Li_in_FLiBe': ThermalTuple('liflib', [3006, 3007], 1),
     'c_Li_in_7LiD': ThermalTuple('lilid', [3007], 1),
     'c_Li_in_7LiH': ThermalTuple('lilih', [3007], 1),
+    'c_Li_in_LiF': ThermalTuple('lilif', [3006, 3007], 1),
+    'c_Li_in_LiH': ThermalTuple('lilih2', [3006, 3007], 1),
     'c_Mg24': ThermalTuple('mg24', [12024], 1),
     'c_Mg_in_MgF2': ThermalTuple('mgmgf2', [12024, 12025, 12026], 1),
     'c_Mg_in_MgO': ThermalTuple('mgmgo', [12024, 12025, 12026], 1),
+    'c_Mg_in_MgD2': ThermalTuple('mgmgd2', [12024, 12025, 12026], 1),
+    'c_Mg_in_MgH2': ThermalTuple('mgmgh2', [12024, 12025, 12026], 1),
+    'c_Mg_in_MgOH2': ThermalTuple('mgoh2', [12024, 12025, 12026], 1),
+    'c_Mg_in_NaMgH3': ThermalTuple('mgnamg', [12024, 12025, 12026], 1),
+    'c_Mo': ThermalTuple('mo', [42092, 42094, 42095, 42096, 42097, 42098, 42100], 1),
+    'c_N_in_GaN': ThermalTuple('ngan', [7014, 7015], 1),
     'c_N_in_UN_100p': ThermalTuple('nun100', [7014, 7015], 1),
     'c_N_in_UN_10p': ThermalTuple('nun10', [7014, 7015], 1),
     'c_N_in_UN_5p': ThermalTuple('nun5', [7014, 7015], 1),
     'c_N_in_UN': ThermalTuple('n-un', [7014, 7015], 1),
     'c_N_in_UN_HALEU': ThermalTuple('nunhal', [7014, 7015], 1),
     'c_N_in_UN_HEU': ThermalTuple('nunheu', [7014, 7015], 1),
+    'c_Na': ThermalTuple('na', [11023], 1),
+    'c_Na_in_NaI': ThermalTuple('nanai', [11023], 1),
+    'c_Na_in_NaMgH3': ThermalTuple('nanamg', [11023], 1),
+    'c_Na_in_NaOH': ThermalTuple('nanaoh', [11023], 1),
+    'c_Nb': ThermalTuple('nb', [41093], 1),
+    'c_Ni': ThermalTuple('ni', [28058, 28060, 28061, 28062, 28064], 1),
     'c_O_in_Al2O3': ThermalTuple('osap00', [8016, 8017, 8018], 1),
     'c_O_in_BeO': ThermalTuple('obeo', [8016, 8017, 8018], 1),
     'c_O_in_C5O2H8': ThermalTuple('olucit', [8016, 8017, 8018], 1),
+    'c_O_in_C2H6O_liquid': ThermalTuple('oetliq', [8016, 8017, 8018], 1),
+    'c_O_in_C2H6O_solid': ThermalTuple('oetsol', [8016, 8017, 8018], 1),
+    'c_O_in_CaO2H2': ThermalTuple('ocaoh', [8016, 8017, 8018], 1),
     'c_O_in_D2O': ThermalTuple('od2o', [8016, 8017, 8018], 1),
     'c_O_in_H2O_solid': ThermalTuple('oice', [8016, 8017, 8018], 1),
     'c_O_in_MgO': ThermalTuple('omgo', [8016, 8017, 8018], 1),
+    'c_O_in_Ge3Bi4O12': ThermalTuple('ogbo', [8016, 8017, 8018], 1),
+    'c_O_in_H2O': ThermalTuple('oh2o', [8016, 8017, 8018], 1),
+    'c_O_in_KOH': ThermalTuple('okoh', [8016, 8017, 8018], 1),
+    'c_O_in_MgOH2': ThermalTuple('omgoh', [8016, 8017, 8018], 1),
+    'c_O_in_NaOH': ThermalTuple('onaoh', [8016, 8017, 8018], 1),
     'c_O_in_PuO2': ThermalTuple('opuo2', [8016, 8017, 8018], 1),
     'c_O_in_SiO2_alpha': ThermalTuple('osio2a', [8016, 8017, 8018], 1),
     'c_O_in_UO2_100p': ThermalTuple('ouo200', [8016, 8017, 8018], 1),
@@ -98,16 +178,26 @@ _THERMAL_DATA = {
     'c_O_in_UO2': ThermalTuple('ouo2', [8016, 8017, 8018], 1),
     'c_O_in_UO2_HALEU': ThermalTuple('ouo2hl', [8016, 8017, 8018], 1),
     'c_O_in_UO2_HEU': ThermalTuple('ouo2he', [8016, 8017, 8018], 1),
+    'c_O_in_Y3Al5O12': ThermalTuple('oyag', [8016, 8017, 8018], 1),
     'c_ortho_D': ThermalTuple('orthod', [1002], 1),
     'c_ortho_H': ThermalTuple('orthoh', [1001], 1),
     'c_para_D': ThermalTuple('parad', [1002], 1),
     'c_para_H': ThermalTuple('parah', [1001], 1),
+    'c_Pb': ThermalTuple('pb', [82204, 82206, 82207, 82208], 1),
+    'c_Pd': ThermalTuple('pd', [46102, 46104, 46105, 46106, 46108, 46110], 1),
+    'c_Pt': ThermalTuple('pt', [78190, 78192, 78194, 78195, 78196, 78198], 1),
     'c_Pu_in_PuO2': ThermalTuple('puo2', [94239, 94240, 94241, 94242, 94243], 1),
     'c_Si28': ThermalTuple('si00', [14028], 1),
     'c_Si_in_SiC': ThermalTuple('sisic', [14028, 14029, 14030], 1),
     'c_Si_in_SiO2_alpha': ThermalTuple('si_o2a', [14028, 14029, 14030], 1),
     'c_SiO2_alpha': ThermalTuple('sio2-a', [8016, 8017, 8018, 14028, 14029, 14030], 3),
     'c_SiO2_beta': ThermalTuple('sio2-b', [8016, 8017, 8018, 14028, 14029, 14030], 3),
+    'c_S_in_ZnS': ThermalTuple('szns', [16032, 16033, 16034, 16036], 1),
+    'c_Se_in_GaSe': ThermalTuple('segase', [34074, 34076, 34077, 34078, 34080, 34082], 1),
+    'c_Sn': ThermalTuple('sn', [50112, 50114, 50115, 50116, 50117, 50118, 50119, 50120, 50122, 50124], 1),
+    'c_Sr_in_SrH2': ThermalTuple('srsrh2', [38084, 38086, 38087, 38088], 1),
+    'c_Te_in_GeTe': ThermalTuple('tegete', [52120, 52122, 52123, 52124, 52125, 52126, 52128, 52130], 1),
+    'c_Ti': ThermalTuple('ti', [22046, 22047, 22048, 22049, 22050], 1),
     'c_U_metal_100p': ThermalTuple('u-100p', [92233, 92234, 92235, 92236, 92238], 1),
     'c_U_metal_10p': ThermalTuple('u-10p', [92233, 92234, 92235, 92236, 92238], 1),
     'c_U_metal_5p': ThermalTuple('u-5p', [92233, 92234, 92235, 92236, 92238], 1),
@@ -132,7 +222,13 @@ _THERMAL_DATA = {
     'c_U_in_UO2': ThermalTuple('uuo2', [92233, 92234, 92235, 92236, 92238], 1),
     'c_U_in_UO2_HALEU': ThermalTuple('uo2hal', [92233, 92234, 92235, 92236, 92238], 1),
     'c_U_in_UO2_HEU': ThermalTuple('uo2heu', [92233, 92234, 92235, 92236, 92238], 1),
+    'c_V': ThermalTuple('v', [23050, 23051], 1),
+    'c_W': ThermalTuple('w', [74180, 74182, 74183, 74184, 74186], 1),
+    'c_Y_in_Y3Al5O12': ThermalTuple('yyag', [39089], 1),
     'c_Y_in_YH2': ThermalTuple('yyh2', [39089], 1),
+    'c_Zn': ThermalTuple('zn', [30064, 30066, 30067, 30068, 30070], 1),
+    'c_Zn_in_ZnS': ThermalTuple('znzns', [30064, 30066, 30067, 30068, 30070], 1),
+    'c_Zr': ThermalTuple('zr', [40090, 40091, 40092, 40094, 40096], 1),
     'c_Zr_in_ZrC': ThermalTuple('zrzrc', [40000, 40090, 40091, 40092, 40094, 40096], 1),
     'c_Zr_in_ZrH': ThermalTuple('zrzrh', [40000, 40090, 40091, 40092, 40094, 40096], 1),
     'c_Zr_in_ZrH2': ThermalTuple('zrzrh2', [40000, 40090, 40091, 40092, 40094, 40096], 1),
@@ -163,14 +259,14 @@ broadr / %%%%%%%%%%%%%%%%%%%%%%% Doppler broaden XS %%%%%%%%%%%%%%%%%%%%%%%%%%%%
 _TEMPLATE_HEATR = """
 heatr / %%%%%%%%%%%%%%%%%%%%%%%%% Add heating kerma %%%%%%%%%%%%%%%%%%%%%%%%%%%%
 {nendf} {nheatr_in} {nheatr} /
-{mat} 4 0 0 0 /
+{mat} 4 0 0 0 0 {ed}/
 302 318 402 444 /
 """
 
 _TEMPLATE_HEATR_LOCAL = """
 heatr / %%%%%%%%%%%%%%%%% Add heating kerma (local photons) %%%%%%%%%%%%%%%%%%%%
 {nendf} {nheatr_in} {nheatr_local} /
-{mat} 4 0 0 1 /
+{mat} 4 0 0 1 0 {ed}/
 302 318 402 444 /
 """
 
@@ -201,12 +297,12 @@ acer / %%%%%%%%%%%%%%%%%%%%%%%% Write out in ACE format %%%%%%%%%%%%%%%%%%%%%%%%
 _THERMAL_TEMPLATE_THERMR = """
 thermr / %%%%%%%%%%%%%%%% Add thermal scattering data (free gas) %%%%%%%%%%%%%%%
 0 {nthermr1_in} {nthermr1}
-0 {mat} 12 {num_temp} 1 0 {iform} 1 221 1/
+0 {mat} {nbin} {num_temp} 1 0 {iform} 1 221 1/
 {temps}
 {error} {energy_max}
 thermr / %%%%%%%%%%%%%%%% Add thermal scattering data (bound) %%%%%%%%%%%%%%%%%%
 {nthermal_endf} {nthermr2_in} {nthermr2}
-{mat_thermal} {mat} 16 {num_temp} {inelastic} {elastic} {iform} {natom} 222 1/
+{mat_thermal} {mat} {nbin} {num_temp} {inelastic} {elastic} {iform} {natom} 222 1/
 {temps}
 {error} {energy_max}
 """
@@ -315,7 +411,7 @@ def make_pendf(filename, pendf='pendf', **kwargs):
 def make_ace(filename, temperatures=None, acer=True, xsdir=None,
              output_dir=None, pendf=False, error=0.001, broadr=True,
              heatr=True, gaspr=True, purr=True, evaluation=None,
-             smoothing=True, **kwargs):
+             smoothing=True, displacement_energy=None, **kwargs):
     """Generate incident neutron ACE file from an ENDF file
 
     File names can be passed to
@@ -367,6 +463,9 @@ def make_ace(filename, temperatures=None, acer=True, xsdir=None,
         indicates which evaluation should be used.
     smoothing : bool, optional
         If the smoothing option (ACER card 6) is on (True) or off (False).
+    displacement_energy : float, optional
+        Threshold displacement energy in [eV]. Used in HEATR to calculate
+        damage-energy cross section. When None, use NJOY defaults.
     **kwargs
         Keyword arguments passed to :func:`openmc.data.njoy.run`
 
@@ -419,6 +518,7 @@ def make_ace(filename, temperatures=None, acer=True, xsdir=None,
 
     # heatr
     if heatr:
+        ed = displacement_energy if displacement_energy is not None else 0
         nheatr_in = nlast
         nheatr_local = nheatr_in + 1
         tapeout[nheatr_local] = (output_dir / "heatr_local") if heatr is True \
@@ -495,7 +595,7 @@ def make_ace(filename, temperatures=None, acer=True, xsdir=None,
 def make_ace_thermal(filename, filename_thermal, temperatures=None,
                      ace=None, xsdir=None, output_dir=None, error=0.001,
                      iwt=2, evaluation=None, evaluation_thermal=None,
-                     table_name=None, zaids=None, nmix=None, **kwargs):
+                     table_name=None, zaids=None, nmix=None, nbin=16, **kwargs):
     """Generate thermal scattering ACE file from ENDF files
 
     Parameters
@@ -532,6 +632,8 @@ def make_ace_thermal(filename, filename_thermal, temperatures=None,
         ZAIDs that the thermal scattering data applies to
     nmix : int, optional
         Number of atom types in mixed moderator
+    nbin : int, optional
+        Number of equi-probable angles
     **kwargs
         Keyword arguments passed to :func:`openmc.data.njoy.run`
 
@@ -563,7 +665,7 @@ def make_ace_thermal(filename, filename_thermal, temperatures=None,
     else:
         with warnings.catch_warnings(record=True) as w:
             proper_name = openmc.data.get_thermal_name(zsymam_thermal)
-            if w:
+            if w or proper_name not in _THERMAL_DATA:
                 raise RuntimeError(
                     f"Thermal scattering material {zsymam_thermal} not "
                     "recognized. Please contact OpenMC developers at "
diff --git a/openmc/data/photon.py b/openmc/data/photon.py
index 25ded24cb..13cd3ec95 100644
--- a/openmc/data/photon.py
+++ b/openmc/data/photon.py
@@ -15,7 +15,8 @@ from openmc.mixin import EqualityMixin
 from . import HDF5_VERSION, HDF5_VERSION_MAJOR
 from .ace import Table, get_metadata, get_table
 from .data import ATOMIC_SYMBOL, EV_PER_MEV
-from .endf import Evaluation, get_head_record, get_tab1_record, get_list_record
+from .endf import (
+    as_evaluation, get_head_record, get_tab1_record, get_list_record)
 from .function import Tabulated1D
 
 
@@ -203,7 +204,9 @@ class AtomicRelaxation(EqualityMixin):
         for subshell, df in transitions.items():
             cv.check_value('subshell', subshell, _SUBSHELLS)
             cv.check_type('transitions', df, pd.DataFrame)
-        self._transitions = transitions
+        self._transitions = {
+            subshell: df.convert_dtypes() for subshell, df in transitions.items()
+        }
 
     @classmethod
     def from_ace(cls, ace):
@@ -270,7 +273,7 @@ class AtomicRelaxation(EqualityMixin):
 
         Parameters
         ----------
-        ev_or_filename : str or openmc.data.endf.Evaluation
+        ev_or_filename : str, openmc.data.endf.Evaluation, or endf.Material
             ENDF atomic relaxation evaluation to read from. If given as a
             string, it is assumed to be the filename for the ENDF file.
 
@@ -280,10 +283,7 @@ class AtomicRelaxation(EqualityMixin):
             Atomic relaxation data
 
         """
-        if isinstance(ev_or_filename, Evaluation):
-            ev = ev_or_filename
-        else:
-            ev = Evaluation(ev_or_filename)
+        ev = as_evaluation(ev_or_filename)
 
         # Atomic relaxation data is always MF=28, MT=533
         if (28, 533) not in ev.section:
@@ -604,10 +604,10 @@ class IncidentPhoton(EqualityMixin):
 
         Parameters
         ----------
-        photoatomic : str or openmc.data.endf.Evaluation
+        photoatomic : str, openmc.data.endf.Evaluation, or endf.Material
             ENDF photoatomic data evaluation to read from. If given as a string,
             it is assumed to be the filename for the ENDF file.
-        relaxation : str or openmc.data.endf.Evaluation, optional
+        relaxation : str, openmc.data.endf.Evaluation, or endf.Material, optional
             ENDF atomic relaxation data evaluation to read from. If given as a
             string, it is assumed to be the filename for the ENDF file.
 
@@ -617,10 +617,7 @@ class IncidentPhoton(EqualityMixin):
             Photon interaction data
 
         """
-        if isinstance(photoatomic, Evaluation):
-            ev = photoatomic
-        else:
-            ev = Evaluation(photoatomic)
+        ev = as_evaluation(photoatomic)
 
         Z = ev.target['atomic_number']
         data = cls(Z)
@@ -1069,7 +1066,7 @@ class PhotonReaction(EqualityMixin):
 
         Parameters
         ----------
-        ev : openmc.data.endf.Evaluation
+        ev : openmc.data.endf.Evaluation or endf.Material
             ENDF photo-atomic interaction data evaluation
         mt : int
             The MT value of the reaction to get data for
@@ -1080,6 +1077,7 @@ class PhotonReaction(EqualityMixin):
             Photon reaction data
 
         """
+        ev = as_evaluation(ev)
         rx = cls(mt)
 
         # Read photon cross section
diff --git a/openmc/data/reaction.py b/openmc/data/reaction.py
index 65b59582c..44ced5511 100644
--- a/openmc/data/reaction.py
+++ b/openmc/data/reaction.py
@@ -13,14 +13,15 @@ from .angle_distribution import AngleDistribution
 from .angle_energy import AngleEnergy
 from .correlated import CorrelatedAngleEnergy
 from .data import ATOMIC_SYMBOL, K_BOLTZMANN, EV_PER_MEV
-from .endf import get_head_record, get_tab1_record, get_list_record, \
-    get_tab2_record, get_cont_record
+from .endf import as_evaluation, get_head_record, get_tab1_record, \
+    get_list_record, get_tab2_record, get_cont_record
 from .energy_distribution import EnergyDistribution, LevelInelastic, \
     DiscretePhoton
 from .function import Tabulated1D, Polynomial
 from .kalbach_mann import KalbachMann
 from .laboratory import LaboratoryAngleEnergy
 from .nbody import NBodyPhaseSpace
+from .photon import _SUBSHELLS
 from .product import Product
 from .uncorrelated import UncorrelatedAngleEnergy
 
@@ -54,6 +55,10 @@ REACTION_NAME = {1: '(n,total)', 2: '(n,elastic)', 3: "(n,nonelastic)",
                  198: '(n,n3p)', 199: '(n,3n2pa)', 200: '(n,5n2p)', 203: '(n,Xp)',
                  204: '(n,Xd)', 205: '(n,Xt)', 206: '(n,X3He)', 207: '(n,Xa)',
                  301: 'heating', 444: 'damage-energy',
+                 501: 'photon-total', 502: 'coherent-scatter',
+                 504: 'incoherent-scatter', 515: 'pair-production-electron',
+                 516: 'pair-production', 517: 'pair-production-nuclear',
+                 522: 'photoelectric',
                  649: '(n,pc)', 699: '(n,dc)', 749: '(n,tc)', 799: '(n,3Hec)',
                  849: '(n,ac)', 891: '(n,2nc)', 901: 'heating-local'}
 REACTION_NAME.update({i: f'(n,n{i - 50})' for i in range(51, 91)})
@@ -63,9 +68,16 @@ REACTION_NAME.update({i: f'(n,t{i - 700})' for i in range(700, 749)})
 REACTION_NAME.update({i: f'(n,3He{i - 750})' for i in range(750, 799)})
 REACTION_NAME.update({i: f'(n,a{i - 800})' for i in range(800, 849)})
 REACTION_NAME.update({i: f'(n,2n{i - 875})' for i in range(875, 891)})
+REACTION_NAME.update(
+    {534 + i: f'photoelectric-{shell}' for i, shell in enumerate(_SUBSHELLS[1:])}
+)
 
 REACTION_MT = {name: mt for mt, name in REACTION_NAME.items()}
+REACTION_MT['total'] = 1
+REACTION_MT['elastic'] = 2
 REACTION_MT['fission'] = 18
+REACTION_MT['absorption'] = 27
+REACTION_MT['capture'] = 102
 
 FISSION_MTS = (18, 19, 20, 21, 38)
 
@@ -1139,7 +1151,7 @@ class Reaction(EqualityMixin):
 
         Parameters
         ----------
-        ev : openmc.data.endf.Evaluation
+        ev : openmc.data.endf.Evaluation or endf.Material
             ENDF evaluation
         mt : int
             The MT value of the reaction to get data for
@@ -1150,6 +1162,7 @@ class Reaction(EqualityMixin):
             Reaction data
 
         """
+        ev = as_evaluation(ev)
         rx = Reaction(mt)
 
         # Integrated cross section
diff --git a/openmc/data/resonance.py b/openmc/data/resonance.py
index 31e230df5..52ee7f1a9 100644
--- a/openmc/data/resonance.py
+++ b/openmc/data/resonance.py
@@ -7,7 +7,9 @@ import pandas as pd
 
 import openmc.checkvalue as cv
 from .data import NEUTRON_MASS
-from .endf import get_head_record, get_cont_record, get_tab1_record, get_list_record
+from .endf import (
+    as_evaluation, get_head_record, get_cont_record, get_tab1_record,
+    get_list_record)
 try:
     from .reconstruct import wave_number, penetration_shift, reconstruct_mlbw, \
         reconstruct_slbw, reconstruct_rm
@@ -77,7 +79,7 @@ class Resonances:
 
         Parameters
         ----------
-        ev : openmc.data.endf.Evaluation
+        ev : openmc.data.endf.Evaluation or endf.Material
             ENDF evaluation
 
         Returns
@@ -86,6 +88,7 @@ class Resonances:
             Resonance data
 
         """
+        ev = as_evaluation(ev)
         file_obj = io.StringIO(ev.section[2, 151])
 
         # Determine whether discrete or continuous representation
diff --git a/openmc/data/resonance_covariance.py b/openmc/data/resonance_covariance.py
index 709657044..aa86ae33c 100644
--- a/openmc/data/resonance_covariance.py
+++ b/openmc/data/resonance_covariance.py
@@ -74,7 +74,7 @@ class ResonanceCovariances(Resonances):
 
         Parameters
         ----------
-        ev : openmc.data.endf.Evaluation
+        ev : openmc.data.endf.Evaluation or endf.Material
             ENDF evaluation
         resonances : openmc.data.Resonance object
             openmc.data.Resonanance object generated from the same evaluation
@@ -86,6 +86,7 @@ class ResonanceCovariances(Resonances):
             Resonance covariance data
 
         """
+        ev = endf.as_evaluation(ev)
         file_obj = io.StringIO(ev.section[32, 151])
 
         # Determine whether discrete or continuous representation
diff --git a/openmc/data/thermal.py b/openmc/data/thermal.py
index 4ecc7c040..3b910c535 100644
--- a/openmc/data/thermal.py
+++ b/openmc/data/thermal.py
@@ -27,20 +27,41 @@ from .thermal_angle_energy import (CoherentElasticAE, IncoherentElasticAE,
 
 
 _THERMAL_NAMES = {
+    'c_Ag': ('ag',),
     'c_Al27': ('al', 'al27', 'al-27', '13-al- 27'),
     'c_Al_in_Al2O3': ('asap00', 'asap', 'al(al2o3)'),
-    'c_Be': ('be', 'be-metal', 'be-met', 'be00', 'be-metal', 'be metal', '4-be'),
+    'c_Al_in_Y3Al5O12': ('al(y3al5o1', 'alyag'),
+    'c_Au': ('au',),
+    'c_Be': ('be', 'be-metal', 'be-met', 'be00', 'be-metal', 'be metal', '4-be', '4-be-'),
     'c_BeO': ('beo',),
     'c_Be_distinct': ('besd', 'be+sd'),
     'c_Be_in_BeO': ('bebeo', 'be-beo', 'be-o', 'be/o', 'bbeo00', 'be(beo)', 'be_beo'),
     'c_Be_in_Be2C': ('bebe2c', 'be(be2c)'),
     'c_Be_in_BeF2': ('bebef2', 'be in bef2'),
     'c_Be_in_FLiBe': ('beflib', 'be(flibe)'),
+    'c_Bi': ('83-bi-', 'bi'),
+    'c_Bi_in_Ge3Bi4O12': ('bi(ge3bi4o', 'bigbo'),
     'c_C6H6': ('benz', 'c6h6', 'benzine'),
     'c_C_in_Be2C': ('cbe2c', 'c(be2c)'),
+    'c_C_in_C19H16_liquid': ('c(c19h16)l', 'c19liq'),
+    'c_C_in_C19H16_solid': ('c(c19h16)s', 'c19sol'),
+    'c_C_in_C2H6O_liquid': ('c(c2h6o)l', 'ethliq'),
+    'c_C_in_C2H6O_solid': ('c(c2h6o)s', 'ethsol'),
     'c_C_in_C5O2H8': ('clucit', 'c(lucite)'),
+    'c_C_in_C6H6_liquid': ('c(c6h6)l', 'benzlq'),
+    'c_C_in_C6H6_solid': ('c(c6h6)s', 'benzsl'),
+    'c_C_in_C7H8_liquid': ('c(c7h8)l', 'tolliq'),
+    'c_C_in_C7H8_solid': ('c(c7h8)s', 'tolsol'),
     'c_C_in_C8H8': ('cc8h8', 'c(polystyr'),
+    'c_C_in_C8H10_liquid': ('c(m-c8h10)l', 'xylliq'),
+    'c_C_in_C8H10_solid': ('c(m-c8h10)s', 'xylsol'),
+    'c_C_in_C9H12_liquid': ('c(c9h12)l', 'mesliq'),
+    'c_C_in_C9H12_solid': ('c(c9h12)s', 'messol'),
     'c_C_in_CF2': ('ccf2', 'c(teflon)'),
+    'c_C_in_CH2': ('c(c2h4)n r', 'cch2'),
+    'c_C_in_CH4_liquid': ('c(ch4)l', 'cch4lq'),
+    'c_C_in_CH4_solid': ('c(ch4)s', 'cch4sl'),
+    'c_C_in_Diamond': ('c(c-diamon', 'cdiam'),
     'c_C_in_SiC': ('csic', 'c-sic', 'c(3c-sic)', 'c_sic'),
     'c_C_in_UC_100p': ('cuc100', 'cinuc_100p'),
     'c_C_in_UC_10p': ('cuc10', 'cinuc_10p'),
@@ -49,16 +70,29 @@ _THERMAL_NAMES = {
     'c_C_in_UC_HALEU': ('cuchal', 'cinuc_haleu'),
     'c_C_in_UC_HEU': ('cucheu', 'cinuc_heu'),
     'c_C_in_ZrC': ('czrc', 'c(zrc)'),
+    'c_Ca': ('ca',),
     'c_Ca_in_CaH2': ('cah', 'cah00', 'cacah2', 'ca(cah2)', 'ca_cah2'),
+    'c_Ca_in_CaO2H2': ('ca(caoh2)', 'cacaoh'),
+    'c_Cr': ('cr',),
+    'c_Cu': ('cu',),
     'c_D_in_7LiD': ('dlid', 'd(7lid)'),
     'c_D_in_D2O': ('dd2o', 'd-d2o', 'hwtr', 'hw', 'dhw00', 'd(d2o)'),
     'c_D_in_D2O_solid': ('dice',),
+    'c_D_in_MgD2': ('d(mgd2)', 'dmgd2'),
     'c_F_in_Be2': ('fbef2', 'f in bef2'),
     'c_F_in_CF2': ('fcf2', 'f(teflon)'),
     'c_F_in_FLiBe': ('fflibe', 'f(flibe)'),
     'c_F_in_HF': ('f_hf',),
+    'c_F_in_LiF': ('f(lif)', 'flif'),
     'c_F_in_MgF2': ('fmgf2', 'f in mgf2'),
     'c_Fe56': ('fe', 'fe56', 'fe-56', '26-fe- 56'),
+    'c_Fe_in_Fe_alpha': ('fe(fe-alph', 'fealph'),
+    'c_Fe_in_Fe_gamma': ('fe(fe-gamm', 'fegamm'),
+    'c_Ga_in_GaN': ('ga(gan)', 'gagan'),
+    'c_Ga_in_GaSe': ('ga(gase)', 'gagase'),
+    'c_Ge': ('ge',),
+    'c_Ge_in_Ge3Bi4O12': ('ge(ge3bi4o', 'gegbo'),
+    'c_Ge_in_GeTe': ('ge(gete)', 'gegete'),
     'c_Graphite': ('graph', 'grph', 'gr', 'gr00', 'graphite'),
     'c_Graphite_10p': ('grph10', '10p graphit'),
     'c_Graphite_20p': ('grph20', '20 graphite'),
@@ -68,41 +102,86 @@ _THERMAL_NAMES = {
     'c_H_in_C5O2H8': ('lucite', 'c5o2h8', 'h-luci', 'h(lucite)'),
     'c_H_in_C8H8': ('hc8h8', 'h(polystyr'),
     'c_H_in_CaH2': ('hcah2', 'hca00', 'h(cah2)'),
+    'c_H_in_CaO2H2': ('h(caoh2)', 'hcaoh'),
     'c_H1_in_CaH2': ('h1cah2', 'h1_cah2'),
     'c_H2_in_CaH2': ('h2cah2', 'h2_cah2'),
-    'c_H_in_CH2': ('hch2', 'poly', 'pol', 'h-poly', 'pol00', 'h(ch2)'),
-    'c_H_in_CH4_liquid': ('lch4', 'lmeth', 'l-ch4'),
-    'c_H_in_CH4_solid': ('sch4', 'smeth', 's-ch4'),
+    'c_H_in_C19H16_liquid': ('h(c19h16)l', 'h19liq'),
+    'c_H_in_C19H16_solid': ('h(c19h16)s', 'h19sol'),
+    'c_H_in_C2H6O_liquid': ('h(c2h6o)l', 'hetliq'),
+    'c_H_in_C2H6O_solid': ('h(c2h6o)s', 'hetsol'),
+    'c_H_in_C6H6_liquid': ('h(c6h6)l', 'hbzliq'),
+    'c_H_in_C6H6_solid': ('h(c6h6)s', 'hbzsol'),
+    'c_H_in_C7H8_liquid': ('h(c7h8)l', 'htlliq'),
+    'c_H_in_C7H8_solid': ('h(c7h8)s', 'htlsol'),
+    'c_H_in_C8H10_liquid': ('h(m-c8h10)l', 'hxyliq'),
+    'c_H_in_C8H10_solid': ('h(m-c8h10)s', 'hxysol'),
+    'c_H_in_C9H12_liquid': ('h(c9h12)l', 'hmsliq'),
+    'c_H_in_C9H12_solid': ('h(c9h12)s', 'hmssol'),
+    'c_H_in_CH2': ('hch2', 'poly', 'pol', 'h-poly', 'pol00', 'h(ch2)', 'h(c2h4)n r'),
+    'c_H_in_CH4_liquid': ('lch4', 'lmeth', 'l-ch4', 'h(ch4)l'),
+    'c_H_in_CH4_solid': ('sch4', 'smeth', 's-ch4', 'h(ch4)s'),
     'c_H_in_CH4_solid_phase_II': ('sch4p2',),
     'c_H_in_H2O': ('hh2o', 'h-h2o', 'lwtr', 'lw', 'lw00', 'h(h2o)'),
     'c_H_in_H2O_solid': ('hice', 'h-ice', 'ice00', 'h(ice-ih)', 'h(ice)'),
     'c_H_in_HF': ('hhf', 'h(hf)', 'h_hf'),
+    'c_H_in_KOH': ('h(koh)', 'hkoh'),
+    'c_H_in_LiH': ('h(lih)', 'hlih2'),
     'c_H_in_Mesitylene': ('mesi00', 'mesi', 'mesi-phii'),
+    'c_H_in_MgH2': ('h(mgh2)', 'hmgh2'),
+    'c_H_in_MgOH2': ('h(mgoh2)', 'hmgoh'),
+    'c_H_in_NaMgH3': ('h(namgh3)', 'hnamg'),
+    'c_H_in_NaOH': ('h(naoh)', 'hnaoh'),
     'c_H_in_ParaffinicOil': ('hparaf', 'h(paraffin', 'h(paraffini'),
+    'c_H_in_SrH2': ('h(srh2)', 'hsrh2'),
     'c_H_in_Toluene': ('tol00', 'tol', 'tolue-phii'),
     'c_H_in_UH3': ('huh3', 'h(uh3)'),
     'c_H_in_YH2': ('hyh2', 'h-yh2', 'h(yh2)'),
     'c_H_in_ZrH': ('hzrh', 'h-zrh', 'h-zr', 'h/zr', 'hzr', 'hzr00', 'h(zrh)'),
-    'c_H_in_ZrH2': ('hzrh2', 'h(zrh2)'),
-    'c_H_in_ZrHx': ('hzrhx', 'h(zrhx)'),
+    'c_H_in_ZrH2': ('hzrh2', 'h(zrh2)', 'h(zrh2) in'),
+    'c_H_in_ZrHx': ('hzrhx', 'h(zrhx)', 'h(zrh15) i'),
+    'c_I_in_NaI': ('i(nai)', 'inai'),
+    'c_K': ('k',),
+    'c_K_in_KOH': ('k(koh)', 'kkoh'),
     'c_Li_in_FLiBe': ('liflib', 'li(flibe)'),
     'c_Li_in_7LiD': ('lilid', '7li(7lid)'),
     'c_Li_in_7LiH': ('lilih', '7li(7lih)'),
+    'c_Li_in_LiF': ('li(lif)', 'lilif'),
+    'c_Li_in_LiH': ('li(lih)', 'lilih2'),
     'c_Mg24': ('mg', 'mg24', 'mg00', '24-mg'),
+    'c_Mg_in_MgD2': ('mg(mgd2)', 'mgmgd2'),
     'c_Mg_in_MgF2': ('mgmgf2', 'mg in mgf2'),
+    'c_Mg_in_MgH2': ('mg(mgh2)', 'mgmgh2'),
     'c_Mg_in_MgO': ('mgmgo', 'mg in mgo'),
+    'c_Mg_in_MgOH2': ('mg(mgoh2)', 'mgoh2'),
+    'c_Mg_in_NaMgH3': ('mg(namgh3)', 'mgnamg'),
+    'c_Mo': ('mo',),
+    'c_N_in_GaN': ('n(gan)', 'ngan'),
     'c_N_in_UN_100p': ('nun100', 'n-un-100p'),
     'c_N_in_UN_10p': ('nun10', 'n-un-10p'),
     'c_N_in_UN_5p': ('nun5', 'n-un-5p'),
     'c_N_in_UN': ('n-un', 'n(un)', 'n(un)    l', 'ninun'),
     'c_N_in_UN_HALEU': ('nunhal', 'n-un-haleu'),
     'c_N_in_UN_HEU': ('nunheu', 'n-un-heu'),
+    'c_Na': ('na',),
+    'c_Na_in_NaI': ('na(nai)', 'nanai'),
+    'c_Na_in_NaMgH3': ('na(namgh3)', 'nanamg'),
+    'c_Na_in_NaOH': ('na(naoh)', 'nanaoh'),
+    'c_Nb': ('nb',),
+    'c_Ni': ('ni',),
     'c_O_in_Al2O3': ('osap00', 'osap', 'o(al2o3)'),
     'c_O_in_BeO': ('obeo', 'o-beo', 'o-be', 'o/be', 'obeo00', 'o(beo)', 'o_beo'),
+    'c_O_in_C2H6O_liquid': ('o(c2h6o)l', 'oetliq'),
+    'c_O_in_C2H6O_solid': ('o(c2h6o)s', 'oetsol'),
     'c_O_in_C5O2H8': ('olucit', 'o(lucite)'),
+    'c_O_in_CaO2H2': ('o(caoh2)', 'ocaoh'),
     'c_O_in_D2O': ('od2o', 'o-d2o', 'ohw00', 'o(d2o)'),
+    'c_O_in_H2O': ('o(h2o)', 'oh2o'),
     'c_O_in_H2O_solid': ('oice', 'o-ice', 'o(ice-ih)'),
+    'c_O_in_Ge3Bi4O12': ('o(ge3bi4o1', 'ogbo'),
+    'c_O_in_KOH': ('o(koh)', 'okoh'),
     'c_O_in_MgO': ('omgo', 'o in mgo'),
+    'c_O_in_MgOH2': ('o(mgoh2)', 'omgoh'),
+    'c_O_in_NaOH': ('o(naoh)', 'onaoh'),
     'c_O_in_PuO2': ('opuo2', 'o in puo2'),
     'c_O_in_SiO2_alpha': ('osio2a', 'o_sio2a'),
     'c_O_in_UO2_100p': ('ouo200', 'o-uo2-100p'),
@@ -111,16 +190,26 @@ _THERMAL_NAMES = {
     'c_O_in_UO2': ('ouo2', 'o-uo2', 'o2-u', 'o2/u', 'ouo200', 'o(uo2)'),
     'c_O_in_UO2_HALEU': ('ouo2hl', 'ouo2-haleu'),
     'c_O_in_UO2_HEU': ('ouo2he', 'o_uo2-heu'),
+    'c_O_in_Y3Al5O12': ('o(y3al5o12', 'oyag'),
     'c_ortho_D': ('orthod', 'orthoD', 'dortho', 'od200', 'ortod', 'ortho-d'),
     'c_ortho_H': ('orthoh', 'orthoH', 'hortho', 'oh200', 'ortoh', 'ortho-h'),
     'c_para_D': ('parad', 'paraD', 'dpara', 'pd200', 'para-d'),
     'c_para_H': ('parah', 'paraH', 'hpara', 'ph200', 'para-h'),
+    'c_Pb': ('pb',),
+    'c_Pd': ('pd',),
+    'c_Pt': ('pt',),
     'c_Pu_in_PuO2': ('puo2', 'pu in puo2'),
     'c_Si28': ('si00', 'sili', 'si'),
     'c_Si_in_SiC': ('sisic', 'si-sic', 'si(3c-sic)', 'si_sic'),
     'c_Si_in_SiO2_alpha': ('si_o2a', 'si_sio2a'),
-    'c_SiO2_alpha': ('sio2', 'sio2a', 'sio2alpha'),
-    'c_SiO2_beta': ('sio2b', 'sio2beta'),
+    'c_SiO2_alpha': ('sio2', 'sio2a', 'sio2alpha', 'sio2-a'),
+    'c_SiO2_beta': ('sio2b', 'sio2beta', 'sio2-b'),
+    'c_S_in_ZnS': ('s(zns-spha', 'szns'),
+    'c_Se_in_GaSe': ('se(gase)', 'segase'),
+    'c_Sn': ('sn',),
+    'c_Sr_in_SrH2': ('sr(srh2)', 'srsrh2'),
+    'c_Te_in_GeTe': ('te(gete)', 'tegete'),
+    'c_Ti': ('ti',),
     'c_U_metal_100p': ('u-100p',),
     'c_U_metal_10p': ('u-10p',),
     'c_U_metal_5p': ('u-5p',),
@@ -145,11 +234,17 @@ _THERMAL_NAMES = {
     'c_U_in_UO2': ('uuo2', 'u-uo2', 'u-o2', 'u/o2', 'uuo200', 'u(uo2)'),
     'c_U_in_UO2_HALEU': ('uo2hal', 'uuo2-haleu'),
     'c_U_in_UO2_HEU': ('uo2heu', 'u_uo2-heu'),
+    'c_V': ('v',),
+    'c_W': ('w',),
+    'c_Y_in_Y3Al5O12': ('y(y3al5o12', 'yyag'),
     'c_Y_in_YH2': ('yyh2', 'y-yh2', 'y(yh2)'),
+    'c_Zn': ('zn',),
+    'c_Zn_in_ZnS': ('zn(zns-sph', 'znzns'),
+    'c_Zr': ('zr',),
     'c_Zr_in_ZrC': ('zrzrc', 'zr(zrc)'),
     'c_Zr_in_ZrH': ('zrzrh', 'zr-zrh', 'zr-h', 'zr/h', 'zr(zrh)'),
-    'c_Zr_in_ZrH2': ('zrzrh2', 'zr(zrh2)'),
-    'c_Zr_in_ZrHx': ('zrzrhx', 'zr(zrhx)'),
+    'c_Zr_in_ZrH2': ('zrzrh2', 'zr(zrh2)', 'zr(zrh2) i'),
+    'c_Zr_in_ZrHx': ('zrzrhx', 'zr(zrhx)', 'zr(zrh15)'),
 }
 
 
@@ -947,7 +1042,7 @@ class ThermalScattering(EqualityMixin):
 
         Parameters
         ----------
-        ev_or_filename : openmc.data.endf.Evaluation or str
+        ev_or_filename : openmc.data.endf.Evaluation, endf.Material, or str
             ENDF evaluation to read from. If given as a string, it is assumed to
             be the filename for the ENDF file.
         divide_incoherent_elastic : bool
@@ -961,10 +1056,7 @@ class ThermalScattering(EqualityMixin):
             Thermal scattering data
 
         """
-        if isinstance(ev_or_filename, endf.Evaluation):
-            ev = ev_or_filename
-        else:
-            ev = endf.Evaluation(ev_or_filename)
+        ev = endf.as_evaluation(ev_or_filename)
 
         # Read incoherent inelastic data
         assert (7, 4) in ev.section, 'No MF=7, MT=4 found in thermal scattering'
diff --git a/openmc/data/vectfit.py b/openmc/data/vectfit.py
new file mode 100644
index 000000000..c5a783a3c
--- /dev/null
+++ b/openmc/data/vectfit.py
@@ -0,0 +1,811 @@
+"""
+Fast Relaxed Vector Fitting function
+
+Approximate f(s) with a rational function:
+        f(s)=R*(s*I-A)^(-1) + Polynomials*s
+where f(s) is a vector of elements.
+
+When f(s) is a vector, all elements become fitted with a common pole set. The
+identification is done using the pole relocating method known as Vector Fitting
+[1] with relaxed non-triviality constraint for faster convergence and smaller
+fitting errors [2], and utilization of matrix structure for fast solution of the
+pole identifion step [3].
+
+[1] B. Gustavsen and A. Semlyen, "Rational approximation of frequency
+    domain responses by Vector Fitting", IEEE Trans. Power Delivery, vol. 14,
+    no. 3, pp. 1052-1061, July 1999.
+[2] B. Gustavsen, "Improving the pole relocating properties of vector
+    fitting", IEEE Trans. Power Delivery, vol. 21, no. 3, pp. 1587-1592, July
+    2006.
+[3] D. Deschrijver, M. Mrozowski, T. Dhaene, and D. De Zutter,
+    "Macromodeling of Multiport Systems Using a Fast Implementation of the
+    Vector Fitting Method", IEEE Microwave and Wireless Components Letters, vol.
+    18, no. 6, pp. 383-385, June 2008.
+
+All credit goes to:
+ - Bjorn Gustavsen for his MATLAB implementation.
+   (http://www.sintef.no/Projectweb/VECTFIT/)
+ - Jingang Liang for his C++ implementation.
+   (https://github.com/mit-crpg/vectfit.git)
+
+"""
+
+from typing import Tuple
+
+import numpy as np
+from scipy.linalg import eigvals, lstsq, norm, qr
+
+
+def wlstsq(a, b):
+    """Apply least-squares solve with column normalization.
+
+    Notes
+    -----
+    This routine rescales columns of `a` to improve conditioning. Columns with
+    zero norm are left unscaled to avoid divide-by-zero warnings.
+    """
+    col_norm = np.linalg.norm(a, axis=0)
+    scale = np.ones_like(col_norm, dtype=float)
+    nonzero = col_norm > 0.0
+    scale[nonzero] = 1.0 / col_norm[nonzero]
+    scale = np.nan_to_num(scale, nan=1.0, posinf=1.0, neginf=1.0)
+
+    sol = lstsq(a * scale, b)
+    return (sol[0] * scale, sol[1:])
+
+
+def evaluate(
+    eval_points: np.ndarray,
+    pole_values: np.ndarray,
+    residue_matrix: np.ndarray,
+    poly_coefficients: np.ndarray | None = None,
+) -> np.ndarray:
+    """Evaluate the rational function approximation:
+        f(s) ≈ sum(residue / (s - pole)) + sum(poly_coefficients * s^j)
+
+    Parameters
+    ----------
+    eval_points : np.ndarray
+        1D array of real scalar frequency values (s).
+    pole_values : np.ndarray
+        1D array of complex poles.
+    residue_matrix : np.ndarray
+        2D or 1D array of complex residues (shape: [num_vectors, num_poles] or [num_poles]).
+    poly_coefficients : np.ndarray, optional
+        2D or 1D array of real polynomial coefficients (shape: [num_vectors, num_polys]).
+
+    Returns
+    -------
+    np.ndarray
+        2D array of evaluated real function values (shape: [num_vectors, num_samples]).
+
+    Raises
+    ------
+    ValueError
+        If input arrays have incompatible shapes.
+    """
+    eval_points = np.asarray(eval_points)
+    pole_values = np.asarray(pole_values)
+    residue_matrix = np.asarray(residue_matrix)
+
+    if eval_points.ndim != 1:
+        raise ValueError("eval_points must be a 1D array")
+    if pole_values.ndim != 1:
+        raise ValueError("pole_values must be a 1D array")
+
+    if residue_matrix.ndim == 1:
+        residue_matrix = residue_matrix.reshape((1, -1))
+    num_vectors, _ = residue_matrix.shape
+    num_samples = len(eval_points)
+
+    if poly_coefficients is not None and isinstance(poly_coefficients, list):
+        poly_coefficients = np.array(poly_coefficients)
+    if poly_coefficients is None or poly_coefficients.size == 0:
+        poly_coefficients = np.zeros((num_vectors, 0))
+    else:
+        poly_coefficients = np.asarray(poly_coefficients)
+        if poly_coefficients.ndim == 1:
+            poly_coefficients = poly_coefficients.reshape((1, -1))
+        elif poly_coefficients.shape[0] != num_vectors:
+            raise ValueError("Mismatch in residues and poly_coefficients shapes")
+
+    num_coeffs = poly_coefficients.shape[1]
+    result = np.zeros((num_vectors, num_samples))
+
+    # term: sum over poles of (residues / (eval_points - poles))
+    denominator = (
+        eval_points[np.newaxis, :] - pole_values[:, np.newaxis]
+    )  # shape: (num_poles, num_eval)
+    pole_terms = residue_matrix @ (1.0 / denominator)  # shape: (num_vectors, num_eval)
+    result = np.real(pole_terms)
+
+    # polynomial part: sum over poly_idx of (coeff * eval_points**poly_idx)
+    if num_coeffs > 0:
+        powers = (
+            eval_points[np.newaxis, :] ** np.arange(num_coeffs)[:, np.newaxis]
+        )  # shape: (num_coeffs, num_eval)
+        result += poly_coefficients @ powers  # shape: (num_vectors, num_eval)
+
+    return result
+
+
+def vectfit(
+    response_matrix: np.ndarray,
+    eval_points: np.ndarray,
+    initial_poles: np.ndarray,
+    weights: np.ndarray,
+    n_polys: int = 0,
+    skip_pole_update: bool = False,
+    skip_residue_update: bool = False,
+) -> Tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray, float]:
+    """Perform vector fitting using the Fast Relaxed Vector Fitting algorithm.
+
+    Parameters
+    ----------
+    response_matrix : np.ndarray
+        Complex matrix of frequency responses (shape: [num_vectors, num_samples]).
+    eval_points : np.ndarray
+        Real frequency samples (s), shape (num_samples,).
+    initial_poles : np.ndarray
+        Initial guess for poles (complex), shape (num_poles,).
+    weights : np.ndarray
+        Weighting matrix for fitting (same shape as response_matrix).
+    n_polys : int, optional
+        Number of real polynomial terms to include.
+    skip_pole_update : bool, optional
+        Whether to skip pole relocation step.
+    skip_residue_update : bool, optional
+        Whether to skip residue fitting step.
+
+    Returns
+    -------
+    Tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray, float]
+        - Updated poles (np.ndarray)
+        - Residues (np.ndarray)
+        - Polynomial coefficients (np.ndarray)
+        - Fitted response matrix (np.ndarray)
+        - Root-mean-square error (float)
+    """
+    tol_low = 1e-18
+    tol_high = 1e18
+    response_matrix = np.asarray(response_matrix)
+    eval_points = np.asarray(eval_points)
+    initial_poles = np.asarray(initial_poles)
+    weights = np.asarray(weights)
+
+    num_vectors, num_samples = response_matrix.shape
+    num_poles = len(initial_poles)
+
+    if n_polys < 0 or n_polys > 11:
+        raise ValueError("n_polys must be in [0, 11]")
+
+    residue_matrix = np.zeros((num_vectors, num_poles), dtype=np.complex128)
+    poly_coefficients = np.zeros((num_vectors, n_polys))
+    fit_result = np.zeros_like(response_matrix)
+    rms_error = 0.0
+
+    if num_poles == 0 and n_polys == 0:
+        rms_error = norm(response_matrix) / np.sqrt(num_vectors * num_samples)
+        return initial_poles, residue_matrix, poly_coefficients, fit_result, rms_error
+
+    if not skip_pole_update and num_poles > 0:
+        updated_poles = identify_poles(
+            num_poles,
+            num_samples,
+            n_polys,
+            initial_poles,
+            eval_points,
+            tol_high,
+            weights,
+            response_matrix,
+            num_vectors,
+            tol_low,
+        )
+    else:
+        updated_poles = initial_poles
+
+    if not skip_residue_update:
+        fit_result, rms_error = identify_residues(
+            num_poles,
+            updated_poles,
+            num_samples,
+            n_polys,
+            eval_points,
+            num_vectors,
+            weights,
+            response_matrix,
+            poly_coefficients,
+            residue_matrix,
+        )
+
+    return updated_poles, residue_matrix, poly_coefficients, fit_result, rms_error
+
+
+def compute_dk_matrix(
+    dk_matrix: np.ndarray,
+    eval_points: np.ndarray,
+    poles: np.ndarray,
+    conj_index: np.ndarray,
+    num_poles: int,
+    num_polys: int,
+    tol_high: float = None,
+):
+    """Compute the dk_matrix used in windowed multipole evaluations.
+
+    Parameters
+    ----------
+    dk_matrix : ndarray of shape (len(eval_points), M)
+        The full matrix used in least-squares fitting or evaluation.
+    eval_points : ndarray of shape (N,)
+        Energy points at which to evaluate.
+    poles : ndarray of shape (num_poles,)
+        Complex poles used in the resonance model.
+    conj_index : ndarray of shape (num_poles,)
+        Index array indicating pole conjugacy behavior: 0 (normal), 1 (add conjugate), 2 (imaginary part).
+    num_poles : int
+        Number of complex poles.
+    num_polys : int
+        Number of polynomial terms (including constant term).
+    tol_high : float
+        Replacement value for infinities.
+    """
+    # Broadcast shapes
+    eval_points_col = eval_points[:, np.newaxis]
+    poles_row = poles[np.newaxis, :]
+
+    # Compute base terms
+    term1 = 1.0 / (eval_points_col - poles_row)
+    term2 = 1.0 / (eval_points_col - np.conj(poles_row))
+    term3 = 1j / (eval_points_col - np.conj(poles_row)) - 1j / (
+        eval_points_col - poles_row
+    )
+
+    # Masks for different conjugacy types
+    mask0 = conj_index == 0
+    mask1 = conj_index == 1
+    mask2 = conj_index == 2
+
+    # Fill dk_matrix with pole terms
+    dk_matrix[:, :num_poles][:, mask0] = term1[:, mask0]
+    dk_matrix[:, :num_poles][:, mask1] = term1[:, mask1] + term2[:, mask1]
+    dk_matrix[:, :num_poles][:, mask2] = term3[:, mask2]
+
+    # Replace infinities with high tolerance value
+    if tol_high is not None:
+        inf_mask = np.isinf(dk_matrix)
+        dk_matrix[inf_mask] = tol_high + 0j
+
+    # Add polynomial basis (Chebyshev-like, just powers here)
+    powers = np.arange(num_polys)
+    dk_matrix[:, num_poles : num_poles + num_polys] = eval_points_col**powers + 0j
+    return dk_matrix
+
+
+def row_block_matrix(
+    dk_matrix: np.ndarray,
+    weights: np.ndarray,
+    response_matrix: np.ndarray,
+    vec_idx: int,
+    num_poles: int,
+    num_polys: int,
+) -> np.ndarray:
+    """
+    Construct a single matrix row block for the given vector index.
+
+    Parameters
+    ----------
+    dk_matrix : ndarray of shape (num_samples, num_poles + num_polys)
+        Basis function evaluations at each sample point.
+    weights : ndarray of shape (num_vectors, num_samples)
+        Sample weights for each vector.
+    response_matrix : ndarray of shape (num_vectors, num_samples)
+        Response values at each sample point.
+    vec_idx : int
+        Index of the vector to construct the A1 block for.
+    num_poles : int
+        Number of poles used in the model.
+    num_polys : int
+        Number of polynomial basis terms.
+
+    Returns
+    -------
+    A : ndarray of shape (num_samples, num_poles + num_polys + num_poles + 1)
+        Weighted and assembled matrix block for the current vector.
+    """
+    num_samples = dk_matrix.shape[0]
+    A = np.zeros(
+        (num_samples, num_poles + num_polys + num_poles + 1), dtype=np.complex128
+    )
+
+    # Weighted basis terms
+    A[:, : num_poles + num_polys] = (
+        weights[vec_idx][:, np.newaxis] * dk_matrix[:, : num_poles + num_polys]
+    )
+
+    # Weighted response terms (includes poles + 1)
+    A[:, num_poles + num_polys : num_poles + num_polys + num_poles + 1] = (
+        -weights[vec_idx][:, np.newaxis]
+        * dk_matrix[:, : num_poles + 1]
+        * response_matrix[vec_idx][:, np.newaxis]
+    )
+
+    return A
+
+
+def process_constrained_block(
+    vec_idx: int,
+    dk_matrix: np.ndarray,
+    weights: np.ndarray,
+    response_matrix: np.ndarray,
+    num_samples: int,
+    num_poles: int,
+    num_polys: int,
+    scale_factor: float,
+    num_vectors: int,
+) -> Tuple[int, np.ndarray, np.ndarray]:
+    """
+    Construct a constrained least-squares system block for the given vector index.
+
+    This function computes the A matrix using weighted evaluations of the basis functions
+    and response terms. It appends a constraint row to enforce physical properties
+    (e.g., normalization) **only for the final vector index**. The full matrix A is
+    decomposed via QR, and the resulting triangular block is returned.
+
+    This routine is intended for use in the main vector fitting loop when the denominator
+    is well-conditioned but requires an additional constraint row for physical consistency.
+
+    Parameters
+    ----------
+    vec_idx : int
+        Index of the vector to process.
+    dk_matrix : ndarray of shape (num_samples, num_poles + num_polys)
+        Evaluated basis functions at sample points.
+    weights : ndarray of shape (num_vectors, num_samples)
+        Weight matrix per vector.
+    response_matrix : ndarray of shape (num_vectors, num_samples)
+        Response function values for each vector.
+    num_samples : int
+        Number of sample points.
+    num_poles : int
+        Number of poles in the model.
+    num_polys : int
+        Number of polynomial terms in the model.
+    scale_factor : float
+        Scaling factor applied to the final constraint row.
+    num_vectors : int
+        Total number of vectors to process.
+
+    Returns
+    -------
+    vec_idx : int
+        Index of the processed vector.
+    lhs_block : ndarray of shape (num_poles + 1, num_poles + 1)
+        Triangular matrix block from QR decomposition.
+    rhs_block : ndarray of shape (num_poles + 1,) or None
+        Right-hand side vector block (only returned for final vec_idx), else None.
+    """
+    A1 = row_block_matrix(
+        dk_matrix, weights, response_matrix, vec_idx, num_poles, num_polys
+    )
+    A = np.zeros((2 * num_samples + 1, num_poles + num_polys + num_poles + 1))
+    A[:num_samples] = A1.real
+    A[num_samples : 2 * num_samples] = A1.imag
+    # Handle final row only if vec_idx is last
+    if vec_idx == num_vectors - 1:
+        A[
+            2 * num_samples,
+            num_poles + num_polys : num_poles + num_polys + num_poles + 1,
+        ] = scale_factor * np.real(dk_matrix[:, : num_poles + 1].sum(axis=0))
+
+    Q, R = qr(A, mode="economic")
+
+    lhs_block = R[
+        num_poles + num_polys : num_poles + num_polys + num_poles + 1,
+        num_poles + num_polys : num_poles + num_polys + num_poles + 1,
+    ]
+
+    if vec_idx == num_vectors - 1:
+        rhs_block = (
+            num_samples
+            * scale_factor
+            * Q[-1, num_poles + num_polys : num_poles + num_polys + num_poles + 1]
+        )
+    else:
+        rhs_block = np.zeros_like(
+            Q[-1, num_poles + num_polys : num_poles + num_polys + num_poles + 1]
+        )
+
+    return vec_idx, lhs_block, rhs_block
+
+
+def process_unconstrained_block(
+    vec_idx: int,
+    dk_matrix: np.ndarray,
+    weights: np.ndarray,
+    response_matrix: np.ndarray,
+    denom: float,
+    num_poles: int,
+    num_polys: int,
+) -> Tuple[int, np.ndarray, np.ndarray]:
+    """
+    Construct an unconstrained least-squares system block for the given vector index.
+
+    This function is used when the fitting denominator becomes ill-conditioned
+    (too small or too large), and the original constrained system is replaced by
+    an alternative regularized least-squares problem. The A matrix is built by stacking
+    the real and imaginary parts of the basis evaluations, and the RHS vector b is
+    scaled by `denom`.
+
+    A standard QR decomposition is used to extract the square block of the system,
+    which can be solved independently from the constrained system.
+
+    Parameters
+    ----------
+    vec_idx : int
+        Index of the vector to process.
+    dk_matrix : ndarray of shape (num_samples, num_poles + num_polys)
+        Evaluated basis functions at sample points.
+    weights : ndarray of shape (num_vectors, num_samples)
+        Weight matrix per vector.
+    response_matrix : ndarray of shape (num_vectors, num_samples)
+        Response function values for each vector.
+    denom : float
+        Scaling factor applied to the right-hand side vector b.
+    num_poles : int
+        Number of poles in the model.
+    num_polys : int
+        Number of polynomial terms in the model.
+
+    Returns
+    -------
+    vec_idx : int
+        Index of the processed vector.
+    lhs_block : ndarray of shape (num_poles, num_poles)
+        Triangular matrix block from QR decomposition.
+    rhs_block : ndarray of shape (num_poles,)
+        Right-hand side vector block for this vector.
+    """
+    A1 = row_block_matrix(
+        dk_matrix, weights, response_matrix, vec_idx, num_poles, num_polys
+    )
+    A = np.vstack((A1.real, A1.imag))
+
+    b1 = denom * weights[vec_idx] * response_matrix[vec_idx]
+    b = np.concatenate((b1.real, b1.imag))
+
+    Q, R = qr(A, mode="economic")
+
+    lhs_block = R[
+        num_poles + num_polys : num_poles + num_polys + num_poles,
+        num_poles + num_polys : num_poles + num_polys + num_poles,
+    ]
+    rhs_block = Q[:, num_poles + num_polys : num_poles + num_polys + num_poles].T @ b
+    return vec_idx, lhs_block, rhs_block
+
+
+def identify_poles(
+    num_poles: int,
+    num_samples: int,
+    num_polys: int,
+    poles: np.ndarray,
+    eval_points: np.ndarray,
+    tol_high: float,
+    weights: np.ndarray,
+    response_matrix: np.ndarray,
+    num_vectors: int,
+    tol_low: float,
+) -> np.ndarray:
+    """
+    Internal routine to update poles via relaxed vector fitting.
+
+    Parameters
+    ----------
+    num_poles : int
+        Number of poles.
+    num_samples : int
+        Number of frequency samples.
+    num_polys : int
+        Number of polynomial terms.
+    poles : np.ndarray
+        Initial poles (complex), shape (num_poles,).
+    eval_points : np.ndarray
+        Real frequency values, shape (num_samples,).
+    tol_high : float
+        Upper tolerance threshold for denominator.
+    weights : np.ndarray
+        Weighting matrix, shape (num_vectors, num_samples).
+    response_matrix : np.ndarray
+        Complex frequency responses, shape (num_vectors, num_samples).
+    num_vectors : int
+        Number of response vectors.
+    tol_low : float
+        Lower tolerance threshold for denominator.
+
+    Returns
+    -------
+    np.ndarray
+        Updated poles as eigenvalues (shape: [num_poles]).
+    """
+    conj_index = label_conjugate_poles(poles)
+    dk_matrix = np.zeros(
+        (num_samples, num_poles + max(num_polys, 1)), dtype=np.complex128
+    )
+    compute_dk_matrix(
+        dk_matrix, eval_points, poles, conj_index, num_poles, num_polys, tol_high
+    )
+    # For relaxed vector fitting, include the constant term in the sigma(s)
+    # function even when no polynomial terms are requested. This ensures the
+    # constrained system is well-posed when n_polys == 0.
+    if num_polys == 0:
+        dk_matrix[:, num_poles] = 1.0 + 0j
+
+    scale_factor = (
+        np.sqrt(
+            sum(norm(weights[m] * response_matrix[m]) ** 2 for m in range(num_vectors))
+        )
+        / num_samples
+    )
+    lhs_matrix = np.zeros((num_vectors * (num_poles + 1), num_poles + 1))
+    rhs_vector = np.zeros(num_vectors * (num_poles + 1))
+
+    for vec_idx in range(num_vectors):
+        vec_idx, lhs_block, rhs_block = process_constrained_block(
+            vec_idx,
+            dk_matrix,
+            weights,
+            response_matrix,
+            num_samples,
+            num_poles,
+            num_polys,
+            scale_factor,
+            num_vectors,
+        )
+        i0 = vec_idx * (num_poles + 1)
+        i1 = (vec_idx + 1) * (num_poles + 1)
+        lhs_matrix[i0:i1] = lhs_block
+        rhs_vector[i0:i1] = rhs_block
+
+    solution, *_ = wlstsq(lhs_matrix, rhs_vector)
+    coeffs = solution[:-1]
+    denom = solution[-1]
+
+    if abs(denom) < tol_low or abs(denom) > tol_high:
+        lhs_matrix = np.zeros((num_vectors * num_poles, num_poles))
+        rhs_vector = np.zeros(num_vectors * num_poles)
+        # Adjust denom
+        if denom == 0.0:
+            denom = 1.0
+        elif abs(denom) < tol_low:
+            denom = np.sign(denom) * tol_low
+        elif abs(denom) > tol_high:
+            denom = np.sign(denom) * tol_high
+
+        # Allocate output
+        lhs_matrix = np.zeros((num_vectors * num_poles, num_poles))
+        rhs_vector = np.zeros(num_vectors * num_poles)
+
+        for vec_idx in range(num_vectors):
+            vec_idx, lhs_block, rhs_block = process_unconstrained_block(
+                vec_idx,
+                dk_matrix,
+                weights,
+                response_matrix,
+                denom,
+                num_poles,
+                num_polys,
+            )
+            i0 = vec_idx * num_poles
+            i1 = (vec_idx + 1) * num_poles
+            lhs_matrix[i0:i1] = lhs_block
+            rhs_vector[i0:i1] = rhs_block
+
+        coeffs, *_ = wlstsq(lhs_matrix, rhs_vector)
+
+    lambda_matrix = np.zeros((num_poles, num_poles))
+    scale_vector = np.ones((num_poles, 1))
+
+    # Mask for real poles (conj_index == 0)
+    mask_real = conj_index == 0
+    real_indices = np.where(mask_real)[0]
+    lambda_matrix[real_indices, real_indices] = np.real(poles[real_indices])
+
+    # Mask for start of complex conjugate pairs (conj_index == 1)
+    mask_cplx_start = conj_index == 1
+    cplx_indices = np.where(mask_cplx_start)[0]
+
+    # Extract real and imaginary parts of complex conjugate poles
+    real_parts = np.real(poles[cplx_indices])
+    imag_parts = np.imag(poles[cplx_indices])
+
+    # Diagonal assignments
+    lambda_matrix[cplx_indices, cplx_indices] = real_parts
+    lambda_matrix[cplx_indices + 1, cplx_indices + 1] = real_parts
+
+    # Off-diagonal assignments
+    lambda_matrix[cplx_indices, cplx_indices + 1] = imag_parts
+    lambda_matrix[cplx_indices + 1, cplx_indices] = -imag_parts
+
+    # Scaling vector adjustments
+    scale_vector[cplx_indices, 0] = 2.0
+    scale_vector[cplx_indices + 1, 0] = 0.0
+
+    residue_matrix = lambda_matrix - np.outer(scale_vector.squeeze(), coeffs) / denom
+    return eigvals(residue_matrix)
+
+
+def solve_vector_block(
+    vec_idx: int,
+    dk_matrix: np.ndarray,
+    weights: np.ndarray,
+    response_matrix: np.ndarray,
+    num_poles: int,
+    num_polys: int,
+) -> Tuple[int, np.ndarray, np.ndarray]:
+    """
+    Solve the least-squares system for a single vector index.
+
+    Parameters
+    ----------
+    vec_idx : int
+        Index of the vector to solve.
+    dk_matrix : ndarray
+        Basis function evaluations of shape (num_samples, num_poles + num_polys).
+    weights : ndarray
+        Weight array of shape (num_vectors, num_samples).
+    response_matrix : ndarray
+        Response array of shape (num_vectors, num_samples).
+    num_poles : int
+        Number of poles.
+    num_polys : int
+        Number of polynomial coefficients.
+
+    Returns
+    -------
+    vec_idx : int
+        The index of the solved vector.
+    residues : ndarray
+        Solution vector for the residues (length = num_poles).
+    poly_coeffs : ndarray or None
+        Solution vector for polynomial coefficients (length = num_polys), or None if num_polys == 0.
+    """
+    A = dk_matrix * weights[vec_idx][:, np.newaxis]
+    b = weights[vec_idx] * response_matrix[vec_idx]
+
+    lhs_matrix = np.vstack((A.real, A.imag))
+    rhs_vector = np.concatenate((b.real, b.imag))
+
+    x = wlstsq(lhs_matrix, rhs_vector)[0]
+
+    residues = x[:num_poles]
+    poly_coeffs = x[num_poles : num_poles + num_polys] if num_polys > 0 else None
+
+    return vec_idx, residues, poly_coeffs
+
+
+def identify_residues(
+    num_poles: int,
+    poles: np.ndarray,
+    num_samples: int,
+    num_polys: int,
+    eval_points: np.ndarray,
+    num_vectors: int,
+    weights: np.ndarray,
+    response_matrix: np.ndarray,
+    poly_coefficients: np.ndarray,
+    residue_matrix: np.ndarray,
+) -> Tuple[np.ndarray, float]:
+    """
+    Internal routine to compute residues and polynomial coefficients.
+
+    Parameters
+    ----------
+    num_poles : int
+        Number of poles.
+    poles : np.ndarray
+        Current poles (complex), shape (num_poles,).
+    num_samples : int
+        Number of frequency samples.
+    num_polys : int
+        Number of polynomial terms.
+    eval_points : np.ndarray
+        Real frequency values, shape (num_samples,).
+    num_vectors : int
+        Number of response vectors.
+    weights : np.ndarray
+        Weighting matrix, shape (num_vectors, num_samples).
+    response_matrix : np.ndarray
+        Complex frequency responses, shape (num_vectors, num_samples).
+    poly_coefficients : np.ndarray
+        Array to store output polynomial coefficients (in-place).
+    residue_matrix : np.ndarray
+        Array to store output residues (in-place).
+
+    Returns
+    -------
+    Tuple[np.ndarray, float]
+        - Fitted response matrix (np.ndarray)
+        - Root-mean-square fitting error (float)
+    """
+    conj_index = label_conjugate_poles(poles)
+    dk_matrix = np.zeros((num_samples, num_poles + num_polys), dtype=np.complex128)
+
+    compute_dk_matrix(dk_matrix, eval_points, poles, conj_index, num_poles, num_polys)
+
+    real_residues = np.zeros((num_vectors, num_poles), dtype=np.float64)
+
+    for vec_idx in range(num_vectors):
+        vec_idx, residues, poly_coeffs = solve_vector_block(
+            vec_idx,
+            dk_matrix,
+            weights,
+            response_matrix,
+            num_poles,
+            num_polys,
+        )
+        real_residues[vec_idx] = residues
+        if poly_coeffs is not None:
+            poly_coefficients[vec_idx] = poly_coeffs
+
+    # Mask for real poles
+    mask_real = conj_index == 0
+    real_indices = np.where(mask_real)[0]
+    residue_matrix[:, real_indices] = real_residues[:, real_indices]
+
+    # Mask for first of complex conjugate pairs
+    mask_cplx_start = conj_index == 1
+    cplx_indices = np.where(mask_cplx_start)[0]
+
+    # Compute complex residues using vectorized operations
+    residue_matrix[:, cplx_indices] = (
+        real_residues[:, cplx_indices] + 1j * real_residues[:, cplx_indices + 1]
+    )
+    residue_matrix[:, cplx_indices + 1] = (
+        real_residues[:, cplx_indices] - 1j * real_residues[:, cplx_indices + 1]
+    )
+
+    fit_result = evaluate(eval_points, poles, residue_matrix, poly_coefficients)
+    rms_error = norm(fit_result - response_matrix) / np.sqrt(num_vectors * num_samples)
+    return fit_result, rms_error
+
+
+def label_conjugate_poles(poles: np.ndarray) -> np.ndarray:
+    """
+    Ensure complex poles appear in conjugate pairs and label them accordingly.
+
+    Parameters
+    ----------
+    poles : np.ndarray
+        1D array of complex poles.
+
+    Returns
+    -------
+    np.ndarray
+        Array of integers indicating pole type:
+        - 0: real pole
+        - 1: first in a complex-conjugate pair
+        - 2: second in a complex-conjugate pair
+
+    Raises
+    ------
+    ValueError
+        If any complex pole does not have a valid conjugate pair.
+    """
+    num_poles = len(poles)
+    conj_index = np.zeros(num_poles, dtype=int)
+
+    # Identify complex poles (nonzero imaginary part)
+    is_complex = np.imag(poles) != 0.0
+
+    # Find conjugate pairs: poles[i+1] ≈ conj(poles[i])
+    is_pair_start = is_complex[:-1] & np.isclose(np.conj(poles[:-1]), poles[1:])
+
+    # Mark valid conjugate pair entries
+    conj_index[:-1][is_pair_start] = 1  # mark i with 1
+    conj_index[1:][is_pair_start] = 2  # mark i+1 with 2
+
+    # Now validate: all complex poles must be part of valid conjugate pairs
+    unmatched_complex = is_complex & (conj_index == 0)
+    if np.any(unmatched_complex):
+        raise ValueError("Complex poles must appear in conjugate pairs")
+
+    return conj_index
diff --git a/openmc/deplete/__init__.py b/openmc/deplete/__init__.py
index 8a9509e90..052e22459 100644
--- a/openmc/deplete/__init__.py
+++ b/openmc/deplete/__init__.py
@@ -17,6 +17,7 @@ from .stepresult import *
 from .results import *
 from .integrators import *
 from .transfer_rates import *
+from .r2s import *
 from . import abc
 from . import cram
 from . import helpers
diff --git a/openmc/deplete/abc.py b/openmc/deplete/abc.py
index ab087b98e..66bd7148d 100644
--- a/openmc/deplete/abc.py
+++ b/openmc/deplete/abc.py
@@ -12,6 +12,7 @@ from copy import deepcopy
 from inspect import signature
 from numbers import Real, Integral
 from pathlib import Path
+from textwrap import dedent
 import time
 from typing import Optional, Union, Sequence
 from warnings import warn
@@ -24,12 +25,13 @@ from openmc.mpi import comm
 from openmc.utility_funcs import change_directory
 from openmc import Material
 from .stepresult import StepResult
-from .chain import Chain
+from .chain import _get_chain
 from .results import Results, _SECONDS_PER_MINUTE, _SECONDS_PER_HOUR, \
     _SECONDS_PER_DAY, _SECONDS_PER_JULIAN_YEAR
 from .pool import deplete
 from .reaction_rates import ReactionRates
-from .transfer_rates import TransferRates
+from .transfer_rates import TransferRates, ExternalSourceRates
+from .keff_search_control import _KeffSearchControl
 
 
 __all__ = [
@@ -126,8 +128,8 @@ class TransportOperator(ABC):
 
     Parameters
     ----------
-    chain_file : str
-        Path to the depletion chain XML file
+    chain_file : PathLike or Chain
+        Path to the depletion chain XML file or instance of openmc.deplete.Chain.
     fission_q : dict, optional
         Dictionary of nuclides and their fission Q values [eV]. If not given,
         values will be pulled from the ``chain_file``.
@@ -145,11 +147,12 @@ class TransportOperator(ABC):
         The depletion chain information necessary to form matrices and tallies.
 
     """
-    def __init__(self, chain_file, fission_q=None, prev_results=None):
+    def __init__(self, chain_file=None, fission_q=None, prev_results=None):
         self.output_dir = '.'
 
         # Read depletion chain
-        self.chain = Chain.from_xml(chain_file, fission_q)
+        self.chain = _get_chain(chain_file, fission_q)
+
         if prev_results is None:
             self.prev_res = None
         else:
@@ -157,7 +160,7 @@ class TransportOperator(ABC):
             self.prev_res = prev_results
 
     @abstractmethod
-    def __call__(self, vec, source_rate):
+    def __call__(self, vec, source_rate) -> OperatorResult:
         """Runs a simulation.
 
         Parameters
@@ -199,7 +202,7 @@ class TransportOperator(ABC):
         Returns
         -------
         volume : dict of str to float
-            Volumes corresponding to materials in burn_list
+            Volumes corresponding to materials in full_burn_list
         nuc_list : list of str
             A list of all nuclide names. Used for sorting the simulation.
         burn_list : list of int
@@ -207,6 +210,8 @@ class TransportOperator(ABC):
             simulation.
         full_burn_list : list of int
             All burnable materials in the geometry.
+        name_list : list of str
+            Material names corresponding to materials in full_burn_list
         """
 
     def finalize(self):
@@ -525,7 +530,7 @@ class Integrator(ABC):
     r"""Abstract class for solving the time-integration for depletion
     """
 
-    _params = r"""
+    _params = dedent(r"""
     Parameters
     ----------
     operator : openmc.deplete.abc.TransportOperator
@@ -536,17 +541,15 @@ class Integrator(ABC):
         iterable of float. Alternatively, units can be specified for each step
         by passing an iterable of (value, unit) tuples.
     power : float or iterable of float, optional
-        Power of the reactor in [W]. A single value indicates that
-        the power is constant over all timesteps. An iterable
-        indicates potentially different power levels for each timestep.
-        For a 2D problem, the power can be given in [W/cm] as long
-        as the "volume" assigned to a depletion material is actually
-        an area in [cm^2]. Either ``power``, ``power_density``, or
+        Power of the reactor in [W]. A single value indicates that the power is
+        constant over all timesteps. An iterable indicates potentially different
+        power levels for each timestep. For a 2D problem, the power can be given
+        in [W/cm] as long as the "volume" assigned to a depletion material is
+        actually an area in [cm^2]. Either ``power``, ``power_density``, or
         ``source_rates`` must be specified.
     power_density : float or iterable of float, optional
-        Power density of the reactor in [W/gHM]. It is multiplied by
-        initial heavy metal inventory to get total power if ``power``
-        is not specified.
+        Power density of the reactor in [W/gHM]. It is multiplied by initial
+        heavy metal inventory to get total power if ``power`` is not specified.
     source_rates : float or iterable of float, optional
         Source rate in [neutron/sec] or neutron flux in [neutron/s-cm^2] for
         each interval in :attr:`timesteps`
@@ -558,8 +561,8 @@ class Integrator(ABC):
         and 'MWd/kg' indicates that the values are given in burnup (MW-d of
         energy deposited per kilogram of initial heavy metal).
     solver : str or callable, optional
-        If a string, must be the name of the solver responsible for
-        solving the Bateman equations.  Current options are:
+        If a string, must be the name of the solver responsible for solving the
+        Bateman equations.  Current options are:
 
             * ``cram16`` - 16th order IPF CRAM
             * ``cram48`` - 48th order IPF CRAM [default]
@@ -568,15 +571,22 @@ class Integrator(ABC):
         :attr:`solver`.
 
         .. versionadded:: 0.12
+    substeps : int, optional
+        Number of substeps per depletion interval. When greater than 1, each
+        interval is subdivided into `substeps` identical sub-intervals and LU
+        factorizations may be reused across them, improving accuracy for
+        nuclides with large decay-constant × timestep products.
+
+        .. versionadded:: 0.15.4
     continue_timesteps : bool, optional
         Whether or not to treat the current solve as a continuation of a
         previous simulation. Defaults to `False`. When `False`, the depletion
         steps provided are appended to any previous steps. If `True`, the
-        timesteps provided to the `Integrator` must exacly match any that
-        exist in the `prev_results` passed to the `Operator`. The `power`,
-        `power_density`, or `source_rates` must match as well. The
-        method of specifying `power`, `power_density`, or
-        `source_rates` should be the same as the initial run.
+        timesteps provided to the `Integrator` must exacly match any that exist
+        in the `prev_results` passed to the `Operator`. The `power`,
+        `power_density`, or `source_rates` must match as well. The method of
+        specifying `power`, `power_density`, or `source_rates` should be the
+        same as the initial run.
 
         .. versionadded:: 0.15.1
 
@@ -596,22 +606,31 @@ class Integrator(ABC):
         :math:`\frac{\partial}{\partial t}\vec{n} = A_i\vec{n}_i` with a step
         size :math:`t_i`. Can be configured using the ``solver`` argument.
         User-supplied functions are expected to have the following signature:
-        ``solver(A, n0, t) -> n1`` where
+        ``solver(A, n0, t, substeps=1) -> n1``, where
 
-            * ``A`` is a :class:`scipy.sparse.csc_matrix` making up the
-              depletion matrix
-            * ``n0`` is a 1-D :class:`numpy.ndarray` of initial compositions
-              for a given material in atoms/cm3
-            * ``t`` is a float of the time step size in seconds, and
-            * ``n1`` is a :class:`numpy.ndarray` of compositions at the
-              next time step. Expected to be of the same shape as ``n0``
+            * ``A`` is a :class:`scipy.sparse.csc_array` making up the depletion
+              matrix
+            * ``n0`` is a 1-D :class:`numpy.ndarray` of initial compositions for
+              a given material in atoms/cm3
+            * ``t`` is a float of the time step size in seconds
+            * ``substeps`` is an optional integer number of substeps, and
+            * ``n1`` is a :class:`numpy.ndarray` of compositions at the next
+              time step. Expected to be of the same shape as ``n0``
+
+        Solvers that do not support multiple substeps should raise an exception
+        when ``substeps > 1``.
 
     transfer_rates : openmc.deplete.TransferRates
-        Instance of TransferRates class to perform continuous transfer during depletion
+        Transfer rates for the depletion system used to model continuous
+        removal/feed between materials.
 
         .. versionadded:: 0.14.0
+    external_source_rates : openmc.deplete.ExternalSourceRates
+        External source rates for the depletion system.
 
-    """
+        .. versionadded:: 0.15.3
+
+    """)
 
     def __init__(
             self,
@@ -622,19 +641,10 @@ class Integrator(ABC):
             source_rates: Optional[Union[float, Sequence[float]]] = None,
             timestep_units: str = 's',
             solver: str = "cram48",
+            substeps: int = 1,
             continue_timesteps: bool = False,
         ):
-        # Check number of stages previously used
-        if operator.prev_res is not None:
-            res = operator.prev_res[-1]
-            if res.data.shape[0] != self._num_stages:
-                raise ValueError(
-                    "{} incompatible with previous restart calculation. "
-                    "Previous scheme used {} intermediate solutions, while "
-                    "this uses {}".format(
-                        self.__class__.__name__, res.data.shape[0],
-                        self._num_stages))
-        elif continue_timesteps:
+        if continue_timesteps and operator.prev_res is None:
             raise ValueError("Continuation run requires passing prev_results.")
         self.operator = operator
         self.chain = operator.chain
@@ -653,6 +663,8 @@ class Integrator(ABC):
         # Normalize timesteps and source rates
         seconds, source_rates = _normalize_timesteps(
             timesteps, source_rates, timestep_units, operator)
+        check_type("substeps", substeps, Integral)
+        check_greater_than("substeps", substeps, 0)
 
         if continue_timesteps:
             # Get timesteps and source rates from previous results
@@ -684,8 +696,11 @@ class Integrator(ABC):
 
         self.timesteps = np.asarray(seconds)
         self.source_rates = np.asarray(source_rates)
+        self.substeps = substeps
 
         self.transfer_rates = None
+        self.external_source_rates = None
+        self._keff_search_control = None
 
         if isinstance(solver, str):
             # Delay importing of cram module, which requires this file
@@ -719,23 +734,37 @@ class Integrator(ABC):
             self._solver = func
             return
 
-        # Inspect arguments
-        if len(sig.parameters) != 3:
-            raise ValueError("Function {} does not support three arguments: "
-                             "{!s}".format(func, sig))
+        params = list(sig.parameters.values())
 
-        for ix, param in enumerate(sig.parameters.values()):
-            if param.kind in {param.KEYWORD_ONLY, param.VAR_KEYWORD}:
+        # Inspect arguments
+        if len(params) != 4:
+            raise ValueError(
+                "Function {} must support four arguments "
+                "(A, n0, t, substeps=1): {!s}"
+                .format(func, sig))
+
+        for ix, param in enumerate(params):
+            if param.kind in {param.KEYWORD_ONLY, param.VAR_KEYWORD,
+                              param.VAR_POSITIONAL}:
                 raise ValueError(
                     f"Keyword arguments like {ix} at position {param} are not allowed")
 
+        if len(params) == 4 and params[3].default != 1:
+            raise ValueError(
+                f"Fourth solver argument must default to 1, not {params[3].default}")
+
         self._solver = func
 
-    def _timed_deplete(self, n, rates, dt, matrix_func=None):
+    def _timed_deplete(self, n, rates, dt, i=None, matrix_func=None):
         start = time.time()
         results = deplete(
-            self._solver, self.chain, n, rates, dt, matrix_func,
-            self.transfer_rates)
+            self._solver, self.chain, n, rates, dt, i, matrix_func,
+            self.transfer_rates, self.external_source_rates, self.substeps)
+
+        # Clip unphysical negative number densities
+        for r in results:
+            r.clip(min=0.0, out=r)
+
         return time.time() - start, results
 
     @abstractmethod
@@ -767,12 +796,8 @@ class Integrator(ABC):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
-            Eigenvalue and reaction rates from intermediate transport
-            simulations
+        n_end : list of numpy.ndarray
+            Concentrations at end of timestep
         """
 
     @property
@@ -803,26 +828,83 @@ class Integrator(ABC):
         """Get beginning of step concentrations, reaction rates from restart"""
         res = self.operator.prev_res[-1]
         # Depletion methods expect list of arrays
-        bos_conc = list(res.data[0])
-        rates = res.rates[0]
-        k = ufloat(res.k[0, 0], res.k[0, 1])
+        bos_conc = list(res.data)
+        rates = res.rates
+        k = ufloat(res.k[0], res.k[1])
 
         if res.source_rate != 0.0:
             # Scale reaction rates by ratio of source rates
             rates *= source_rate / res.source_rate
         return bos_conc, OperatorResult(k, rates)
 
-    def _get_start_data(self):
+    def _get_start_data(self) -> tuple[float, int]:
+        """
+        This function fetches the starting state of a depletion simulation in
+        terms of the simulation physical time at which to start and the index at
+        which the depletion simulation should start. When no previous results
+        exist, the time and index are both zero. When previous results do exist,
+        it returns the time corresponding to beginning the previous results last
+        timestep and the index as N-1 where N is the number of previous
+        StepResults found in the previous Results (as expected from 0-based
+        indexing).
+
+        Note that the openmc.deplete.Results.time object is a list of float with
+        [t,t+dt] where t is the beginning of timestep time and t+dt is the end
+        of timestep time. If the previous results correspond to a simulation
+        that finished to completeion, it will contain a results in the form of
+        [t,t], but if a simulation doesn't finish all the given timesteps, it is
+        the t that is the desired start time, not t+dt. Thus, it is always safe
+        to take time[0].
+
+        Returns
+        -------
+        start_time : float
+            Time at which depletion simulation should start in [s]
+        index : int
+            Index at which depletion simulation should start
+        """
         if self.operator.prev_res is None:
             return 0.0, 0
-        return (self.operator.prev_res[-1].time[-1],
+        return (self.operator.prev_res[-1].time[0],
                 len(self.operator.prev_res) - 1)
 
+    def _restore_keff_search_control(self, res: StepResult):
+        """Restore keff search control from restart results."""
+        keff_search_root = res.keff_search_root
+        if keff_search_root is None:
+            raise ValueError(
+                "Cannot restore keff search control from restart "
+                "results because no stored keff_search_root is "
+                "available."
+            )
+        self._keff_search_control.function(keff_search_root)
+        return keff_search_root
+
+    def _get_bos_data(self, step_index, source_rate, bos_conc):
+        """Get beginning-of-step concentrations, rates, and control state."""
+        if step_index > 0 or self.operator.prev_res is None:
+            if self._keff_search_control is not None and source_rate != 0.0:
+                keff_search_root = self._keff_search_control.run(bos_conc)
+            else:
+                keff_search_root = None
+            bos_conc, res = self._get_bos_data_from_operator(
+                step_index, source_rate, bos_conc)
+        else:
+            bos_conc, res = self._get_bos_data_from_restart(
+                source_rate, bos_conc)
+            if self._keff_search_control is not None and source_rate != 0.0:
+                keff_search_root = self._restore_keff_search_control(self.operator.prev_res[-1])
+            else:
+                keff_search_root = None
+
+        return bos_conc, res, keff_search_root
+
     def integrate(
             self,
             final_step: bool = True,
             output: bool = True,
-            path: PathLike = 'depletion_results.h5'
+            path: PathLike = 'depletion_results.h5',
+            write_rates: bool = False
         ):
         """Perform the entire depletion process across all steps
 
@@ -841,6 +923,11 @@ class Integrator(ABC):
             Path to file to write. Defaults to 'depletion_results.h5'.
 
             .. versionadded:: 0.15.0
+        write_rates : bool, optional
+            Whether reaction rates should be written to the results file for
+            each step. Defaults to ``False`` to reduce file size.
+
+            .. versionadded:: 0.15.3
         """
         with change_directory(self.operator.output_dir):
             n = self.operator.initial_condition()
@@ -850,25 +937,27 @@ class Integrator(ABC):
                 if output and comm.rank == 0:
                     print(f"[openmc.deplete] t={t} s, dt={dt} s, source={source_rate}")
 
-                # Solve transport equation (or obtain result from restart)
-                if i > 0 or self.operator.prev_res is None:
-                    n, res = self._get_bos_data_from_operator(i, source_rate, n)
-                else:
-                    n, res = self._get_bos_data_from_restart(source_rate, n)
+                # Get beginning-of-step data from operator or restart results
+                n, res, keff_search_root = self._get_bos_data(i, source_rate, n)
 
                 # Solve Bateman equations over time interval
-                proc_time, n_list, res_list = self(n, res.rates, dt, source_rate, i)
+                proc_time, n_end = self(n, res.rates, dt, source_rate, i)
 
-                # Insert BOS concentration, transport results
-                n_list.insert(0, n)
-                res_list.insert(0, res)
-
-                # Remove actual EOS concentration for next step
-                n = n_list.pop()
-
-                StepResult.save(self.operator, n_list, res_list, [t, t + dt],
-                                source_rate, self._i_res + i, proc_time, path)
+                StepResult.save(
+                    self.operator,
+                    n,
+                    res,
+                    [t, t + dt],
+                    source_rate,
+                    self._i_res + i,
+                    proc_time,
+                    write_rates=write_rates,
+                    keff_search_root=keff_search_root,
+                    path=path
+                )
 
+                # Update for next step
+                n = n_end
                 t += dt
 
             # Final simulation -- in the case that final_step is False, a zero
@@ -877,21 +966,36 @@ class Integrator(ABC):
             # solve)
             if output and final_step and comm.rank == 0:
                 print(f"[openmc.deplete] t={t} (final operator evaluation)")
-            res_list = [self.operator(n, source_rate if final_step else 0.0)]
-            StepResult.save(self.operator, [n], res_list, [t, t],
-                         source_rate, self._i_res + len(self), proc_time, path)
+            if self._keff_search_control is not None and source_rate != 0.0:
+                keff_search_root = self._keff_search_control.run(n)
+            else:
+                keff_search_root = None
+            res_final = self.operator(n, source_rate if final_step else 0.0)
+            StepResult.save(
+                self.operator,
+                n,
+                res_final,
+                [t, t],
+                source_rate,
+                self._i_res + len(self),
+                proc_time,
+                write_rates=write_rates,
+                keff_search_root=keff_search_root,
+                path=path
+            )
             self.operator.write_bos_data(len(self) + self._i_res)
 
         self.operator.finalize()
 
     def add_transfer_rate(
-            self,
-            material: Union[str, int, Material],
-            components: Sequence[str],
-            transfer_rate: float,
-            transfer_rate_units: str = '1/s',
-            destination_material: Optional[Union[str, int, Material]] = None
-        ):
+        self,
+        material: str | int | Material,
+        components: Sequence[str],
+        transfer_rate: float,
+        transfer_rate_units: str = '1/s',
+        timesteps: Sequence[int] | None = None,
+        destination_material: str | int | Material | None = None
+    ):
         """Add transfer rates to depletable material.
 
         Parameters
@@ -905,18 +1009,204 @@ class Integrator(ABC):
         transfer_rate : float
             Rate at which elements are transferred. A positive or negative values
             set removal of feed rates, respectively.
-        destination_material : openmc.Material or str or int, Optional
-            Destination material to where nuclides get fed.
         transfer_rate_units : {'1/s', '1/min', '1/h', '1/d', '1/a'}
             Units for values specified in the transfer_rate argument. 's' means
             seconds, 'min' means minutes, 'h' means hours, 'a' means Julian years.
+        timesteps : list of int, optional
+            List of timestep indices where to set external source rates.
+            Defaults to None, which means the external source rate is set for
+            all timesteps.
+        destination_material : openmc.Material or str or int, Optional
+            Destination material to where nuclides get fed.
 
         """
         if self.transfer_rates is None:
-            self.transfer_rates = TransferRates(self.operator, self.operator.model)
+            if hasattr(self.operator, 'model'):
+                materials = self.operator.model.materials
+            elif hasattr(self.operator, 'materials'):
+                materials = self.operator.materials
+            self.transfer_rates = TransferRates(
+                self.operator, materials, len(self.timesteps))
 
-        self.transfer_rates.set_transfer_rate(material, components, transfer_rate,
-                                      transfer_rate_units, destination_material)
+        if self.external_source_rates is not None and destination_material:
+            raise ValueError('Currently is not possible to set a transfer rate '
+                             'with destination matrial in combination with '
+                             'external source rates.')
+
+        self.transfer_rates.set_transfer_rate(
+            material, components, transfer_rate, transfer_rate_units,
+            timesteps, destination_material)
+
+    def add_external_source_rate(
+        self,
+        material: str | int | Material,
+        composition: dict[str, float],
+        rate: float,
+        rate_units: str = 'g/s',
+        timesteps: Sequence[int] | None = None
+    ):
+        """Add external source rates to depletable material.
+
+        Parameters
+        ----------
+        material : openmc.Material or str or int
+            Depletable material
+        composition : dict of str to float
+            External source rate composition vector, where key can be an element
+            or a nuclide and value the corresponding weight percent.
+        rate : float
+            External source rate in units of mass per time. A positive or
+            negative value corresponds to a feed or removal rate, respectively.
+        units : {'g/s', 'g/min', 'g/h', 'g/d', 'g/a'}
+            Units for values specified in the `rate` argument. 's' for seconds,
+            'min' for minutes, 'h' for hours, 'a' for Julian years.
+        timesteps : list of int, optional
+            List of timestep indices where to set external source rates.
+            Defaults to None, which means the external source rate is set for
+            all timesteps.
+
+        """
+        if self.external_source_rates is None:
+            if hasattr(self.operator, 'model'):
+                materials = self.operator.model.materials
+            elif hasattr(self.operator, 'materials'):
+                materials = self.operator.materials
+            self.external_source_rates = ExternalSourceRates(
+                self.operator, materials, len(self.timesteps))
+
+        if self.transfer_rates is not None and self.transfer_rates.index_transfer:
+            raise ValueError('Currently is not possible to set an external '
+                             'source rate in combination with transfer rates '
+                             'with destination matrial.')
+
+        self.external_source_rates.set_external_source_rate(
+            material, composition, rate, rate_units, timesteps)
+
+
+    def add_redox(self, material, buffer, oxidation_states, timesteps=None):
+        """Add redox control to depletable material.
+
+        Parameters
+        ----------
+        material : openmc.Material or str or int
+            Depletable material
+        buffer : dict
+            Dictionary of buffer nuclides used to maintain redox balance. Keys
+            are nuclide names (strings) and values are their respective
+            fractions (float) that collectively sum to 1.
+        oxidation_states : dict
+            User-defined oxidation states for elements. Keys are element symbols
+            (e.g., 'H', 'He'), and values are their corresponding oxidation
+            states as integers (e.g., +1, 0).
+        timesteps : list of int, optional
+            List of timestep indices where to set external source rates.
+            Defaults to None, which means the external source rate is set for
+            all timesteps.
+        """
+        if self.transfer_rates is None:
+            if hasattr(self.operator, 'model'):
+                materials = self.operator.model.materials
+            elif hasattr(self.operator, 'materials'):
+                materials = self.operator.materials
+            self.transfer_rates = TransferRates(
+                self.operator, materials, len(self.timesteps))
+
+        self.transfer_rates.set_redox(material, buffer, oxidation_states, timesteps)
+
+    def add_keff_search_control(
+        self,
+        function: Callable,
+        x0: float,
+        x1: float,
+        bracket: Sequence[float],
+        **search_kwargs
+    ):
+        """Add keff search to the integrator scheme.
+
+        This method causes OpenMC to perform a keff search during depletion to
+        maintain a target keff by adjusting a model parameter through the
+        provided function.
+
+        .. important::
+            The function **must** modify the model through ``openmc.lib`` (e.g.,
+            ``openmc.lib.cells``, ``openmc.lib.materials``) and **NOT** through
+            ``openmc.Model``. The function is called within a
+            :class:`openmc.lib.TemporarySession` context where only the C API
+            (``openmc.lib``) is available for modifications.
+
+        Parameters
+        ----------
+        function : Callable
+            Function that takes a single float argument and modifies the model
+            through :mod:`openmc.lib`.
+        x0 : float
+            Initial lower bound for the keff search.
+        x1 : float
+            Initial upper bound for the keff search.
+        bracket : sequence of float
+            Bracket interval [x_min, x_max] that constrains the allowed parameter
+            values during the keff search. This is a required parameter
+            that defines the absolute bounds for the search. The bracket must contain
+            exactly 2 elements with bracket[0] < bracket[1]. These values are passed
+            directly to the ``x_min`` and ``x_max`` optional arguments in
+            :meth:`openmc.Model.keff_search`, which enforce hard limits on the
+            parameter range. If the keff search converges to a value outside this
+            bracket, it will be clamped to the nearest bracket bound with a warning.
+        **search_kwargs
+            Additional keyword arguments passed to
+            :meth:`openmc.Model.keff_search`. Common options include:
+
+            * ``target`` : float, optional
+              Target keff value to search for. Defaults to 1.0.
+            * ``k_tol`` : float, optional
+              Stopping criterion on the function value. Defaults to 1e-4.
+            * ``sigma_final`` : float, optional
+              Maximum accepted k-effective uncertainty. Defaults to 3e-4.
+            * ``maxiter`` : int, optional
+              Maximum number of iterations. Defaults to 50.
+
+            See :meth:`openmc.Model.keff_search` for a complete list of
+            available options.
+
+        Examples
+        --------
+        Add keff search that adjusts a control rod position:
+
+        >>> def adjust_rod_position(position):
+        ...     openmc.lib.cells[rod_cell.id].translation = [0, 0, position]
+        >>> integrator.add_keff_search_control(
+        ...     adjust_rod_position,
+        ...     x0=0.0,
+        ...     x1=5.0,
+        ...     bracket=[-10,10],
+        ...     target=1.0,
+        ...     k_tol=1e-4
+        ... )
+
+        Add keff search that adjusts the U235 density:
+
+        >>> def set_u235_density(u235_density):
+        ...     # Get the material from openmc.lib
+        ...     lib_mat = openmc.lib.materials[material_id]
+        ...     # Get current nuclides and densities
+        ...     nuclides = lib_mat.nuclides
+        ...     densities = lib_mat.densities
+        ...     u235_idx = nuclides.index('U235')
+        ...     densities[u235_idx] = u235_density
+        ...     lib_mat.set_densities(nuclides, densities)
+        >>> integrator.add_keff_search_control(
+        ...     set_u235_density,
+        ...     x0=5.0e-4,
+        ...     x1=1.0e-3,
+        ...     bracket=[1.0e-4, 2.0e-3],
+        ...     target=1.0
+        ... )
+
+        .. versionadded:: 0.15.4
+
+        """
+        self._keff_search_control = _KeffSearchControl(
+            self.operator, function, x0, x1, bracket, **search_kwargs)
 
 @add_params
 class SIIntegrator(Integrator):
@@ -926,7 +1216,7 @@ class SIIntegrator(Integrator):
     the number of particles used in initial transport calculation
     """
 
-    _params = r"""
+    _params = dedent(r"""
     Parameters
     ----------
     operator : openmc.deplete.abc.TransportOperator
@@ -937,17 +1227,15 @@ class SIIntegrator(Integrator):
         iterable of float. Alternatively, units can be specified for each step
         by passing an iterable of (value, unit) tuples.
     power : float or iterable of float, optional
-        Power of the reactor in [W]. A single value indicates that
-        the power is constant over all timesteps. An iterable
-        indicates potentially different power levels for each timestep.
-        For a 2D problem, the power can be given in [W/cm] as long
-        as the "volume" assigned to a depletion material is actually
-        an area in [cm^2]. Either ``power``, ``power_density``, or
+        Power of the reactor in [W]. A single value indicates that the power is
+        constant over all timesteps. An iterable indicates potentially different
+        power levels for each timestep. For a 2D problem, the power can be given
+        in [W/cm] as long as the "volume" assigned to a depletion material is
+        actually an area in [cm^2]. Either ``power``, ``power_density``, or
         ``source_rates`` must be specified.
     power_density : float or iterable of float, optional
-        Power density of the reactor in [W/gHM]. It is multiplied by
-        initial heavy metal inventory to get total power if ``power``
-        is not specified.
+        Power density of the reactor in [W/gHM]. It is multiplied by initial
+        heavy metal inventory to get total power if ``power`` is not specified.
     source_rates : float or iterable of float, optional
         Source rate in [neutron/sec] or neutron flux in [neutron/s-cm^2] for
         each interval in :attr:`timesteps`
@@ -959,11 +1247,11 @@ class SIIntegrator(Integrator):
         that the values are given in burnup (MW-d of energy deposited per
         kilogram of initial heavy metal).
     n_steps : int, optional
-        Number of stochastic iterations per depletion interval.
-        Must be greater than zero. Default : 10
+        Number of stochastic iterations per depletion interval. Must be greater
+        than zero. Default : 10
     solver : str or callable, optional
-        If a string, must be the name of the solver responsible for
-        solving the Bateman equations.  Current options are:
+        If a string, must be the name of the solver responsible for solving the
+        Bateman equations.  Current options are:
 
             * ``cram16`` - 16th order IPF CRAM
             * ``cram48`` - 48th order IPF CRAM [default]
@@ -972,16 +1260,23 @@ class SIIntegrator(Integrator):
         :attr:`solver`.
 
         .. versionadded:: 0.12
+    substeps : int, optional
+        Number of substeps per depletion interval. When greater than 1, each
+        interval is subdivided into `substeps` identical sub-intervals and LU
+        factorizations may be reused across them, improving accuracy for
+        nuclides with large decay-constant × timestep products.
+
+        .. versionadded:: 0.15.4
     continue_timesteps : bool, optional
         Whether or not to treat the current solve as a continuation of a
-        previous simulation. Defaults to `False`. If `False`, all time
-        steps and source rates will be run in an append fashion and will run
-        after whatever time steps exist, if any. If `True`, the timesteps
-        provided to the `Integrator` must match exactly those that exist
-        in the `prev_results` passed to the `Opereator`. The `power`,
-        `power_density`, or `source_rates` must match as well. The
-        method of specifying `power`, `power_density`, or
-        `source_rates` should be the same as the initial run.
+        previous simulation. Defaults to `False`. If `False`, all time steps and
+        source rates will be run in an append fashion and will run after
+        whatever time steps exist, if any. If `True`, the timesteps provided to
+        the `Integrator` must match exactly those that exist in the
+        `prev_results` passed to the `Opereator`. The `power`, `power_density`,
+        or `source_rates` must match as well. The method of specifying `power`,
+        `power_density`, or `source_rates` should be the same as the initial
+        run.
 
         .. versionadded:: 0.15.1
 
@@ -1002,19 +1297,23 @@ class SIIntegrator(Integrator):
         :math:`\frac{\partial}{\partial t}\vec{n} = A_i\vec{n}_i` with a step
         size :math:`t_i`. Can be configured using the ``solver`` argument.
         User-supplied functions are expected to have the following signature:
-        ``solver(A, n0, t) -> n1`` where
+        ``solver(A, n0, t, substeps=1) -> n1``, where
 
-            * ``A`` is a :class:`scipy.sparse.csc_matrix` making up the
-              depletion matrix
-            * ``n0`` is a 1-D :class:`numpy.ndarray` of initial compositions
-              for a given material in atoms/cm3
-            * ``t`` is a float of the time step size in seconds, and
-            * ``n1`` is a :class:`numpy.ndarray` of compositions at the
-              next time step. Expected to be of the same shape as ``n0``
+            * ``A`` is a :class:`scipy.sparse.csc_array` making up the depletion
+              matrix
+            * ``n0`` is a 1-D :class:`numpy.ndarray` of initial compositions for
+              a given material in atoms/cm3
+            * ``t`` is a float of the time step size in seconds
+            * ``substeps`` is an optional integer number of substeps, and
+            * ``n1`` is a :class:`numpy.ndarray` of compositions at the next
+              time step. Expected to be of the same shape as ``n0``
+
+        Solvers that do not support multiple substeps should raise an exception
+        when ``substeps > 1``.
 
         .. versionadded:: 0.12
 
-    """
+    """)
 
     def __init__(
             self,
@@ -1026,13 +1325,16 @@ class SIIntegrator(Integrator):
             timestep_units: str = 's',
             n_steps: int = 10,
             solver: str = "cram48",
+            substeps: int = 1,
             continue_timesteps: bool = False,
         ):
         check_type("n_steps", n_steps, Integral)
         check_greater_than("n_steps", n_steps, 0)
         super().__init__(
             operator, timesteps, power, power_density, source_rates,
-            timestep_units=timestep_units, solver=solver, continue_timesteps=continue_timesteps)
+            timestep_units=timestep_units, solver=solver,
+            substeps=substeps,
+            continue_timesteps=continue_timesteps)
         self.n_steps = n_steps
 
     def _get_bos_data_from_operator(self, step_index, step_power, n_bos):
@@ -1046,11 +1348,41 @@ class SIIntegrator(Integrator):
             self.operator.settings.particles //= self.n_steps
         return inherited
 
+    @abstractmethod
+    def __call__(self, n, rates, dt, source_rate, i):
+        """Perform the integration across one time step
+
+        Parameters
+        ----------
+        n : list of numpy.ndarray
+            List of atom number arrays for each material. Each array has
+            shape ``(n_nucs,)`` where ``n_nucs`` is the number of nuclides
+        rates : openmc.deplete.ReactionRates
+            Reaction rates (from transport operator)
+        dt : float
+            Time step in [s]
+        source_rate : float
+            Power in [W] or source rate in [neutron/sec]
+        i : int
+            Current time step index
+
+        Returns
+        -------
+        proc_time : float
+            Time spent in transport simulation
+        n_end : list of numpy.ndarray
+            Updated atom number densities for each material
+        op_result : OperatorResult
+            Eigenvalue and reaction rates resulting from transport simulation
+
+        """
+
     def integrate(
-            self,
-            output: bool = True,
-            path: PathLike = "depletion_results.h5"
-        ):
+        self,
+        output: bool = True,
+        path: PathLike = "depletion_results.h5",
+        write_rates: bool = False
+    ):
         """Perform the entire depletion process across all steps
 
         Parameters
@@ -1061,11 +1393,17 @@ class SIIntegrator(Integrator):
             Path to file to write. Defaults to 'depletion_results.h5'.
 
             .. versionadded:: 0.15.0
+        write_rates : bool, optional
+            Whether reaction rates should be written to the results file for
+            each step. Defaults to ``False`` to reduce file size.
+
+            .. versionadded:: 0.15.3
         """
         with change_directory(self.operator.output_dir):
             n = self.operator.initial_condition()
             t, self._i_res = self._get_start_data()
 
+            res_end = None  # Will be set in first iteration
             for i, (dt, p) in enumerate(self):
                 if output:
                     print(f"[openmc.deplete] t={t} s, dt={dt} s, source={p}")
@@ -1075,28 +1413,38 @@ class SIIntegrator(Integrator):
                         n, res = self._get_bos_data_from_operator(i, p, n)
                     else:
                         n, res = self._get_bos_data_from_restart(p, n)
-                else:
-                    # Pull rates, k from previous iteration w/o
-                    # re-running transport
-                    res = res_list[-1]  # defined in previous i iteration
 
-                proc_time, n_list, res_list = self(n, res.rates, dt, p, i)
+                proc_time, n_end, res_end = self(n, res.rates, dt, p, i)
 
-                # Insert BOS concentration, transport results
-                n_list.insert(0, n)
-                res_list.insert(0, res)
-
-                # Remove actual EOS concentration for next step
-                n = n_list.pop()
-
-                StepResult.save(self.operator, n_list, res_list, [t, t + dt],
-                             p, self._i_res + i, proc_time, path)
+                StepResult.save(
+                    self.operator,
+                    n,
+                    res,
+                    [t, t + dt],
+                    p,
+                    self._i_res + i,
+                    proc_time,
+                    write_rates=write_rates,
+                    path=path
+                )
 
+                # Update for next step
+                n = n_end
+                res = res_end
                 t += dt
 
             # No final simulation for SIE, use last iteration results
-            StepResult.save(self.operator, [n], [res_list[-1]], [t, t],
-                         p, self._i_res + len(self), proc_time, path)
+            StepResult.save(
+                self.operator,
+                n,
+                res_end,
+                [t, t],
+                p,
+                self._i_res + len(self),
+                proc_time,
+                write_rates=write_rates,
+                path=path
+            )
             self.operator.write_bos_data(self._i_res + len(self))
 
         self.operator.finalize()
@@ -1116,12 +1464,12 @@ class DepSystemSolver(ABC):
     """
 
     @abstractmethod
-    def __call__(self, A, n0, dt):
+    def __call__(self, A, n0, dt, substeps=1):
         """Solve the linear system of equations for depletion
 
         Parameters
         ----------
-        A : scipy.sparse.csc_matrix
+        A : scipy.sparse.csc_array
             Sparse transmutation matrix ``A[j, i]`` describing rates at
             which isotope ``i`` transmutes to isotope ``j``
         n0 : numpy.ndarray
@@ -1129,6 +1477,8 @@ class DepSystemSolver(ABC):
             material or an atom density
         dt : float
             Time [s] of the specific interval to be solved
+        substeps : int, optional
+            Number of substeps to use when the solver supports substepping.
 
         Returns
         -------
diff --git a/openmc/deplete/chain.py b/openmc/deplete/chain.py
index 4998a2c3d..a2ff6dea0 100644
--- a/openmc/deplete/chain.py
+++ b/openmc/deplete/chain.py
@@ -7,19 +7,23 @@ loaded from an .xml file and all the nuclides are linked together.
 from io import StringIO
 from itertools import chain
 import math
+import numpy as np
 import re
 from collections import defaultdict, namedtuple
 from collections.abc import Mapping, Iterable
 from numbers import Real, Integral
+from pathlib import Path
 from warnings import warn
 from typing import List
 
 import lxml.etree as ET
-import scipy.sparse as sp
 
-from openmc.checkvalue import check_type, check_greater_than
+from openmc.checkvalue import check_type, check_length, check_greater_than, PathLike
 from openmc.data import gnds_name, zam
+from openmc.exceptions import DataError
 from .nuclide import FissionYieldDistribution, Nuclide
+from .._xml import get_text
+from .._sparse_compat import csc_array, dok_array
 import openmc.data
 
 
@@ -265,6 +269,7 @@ class Chain:
         self.reactions = []
         self.nuclide_dict = {}
         self._fission_yields = None
+        self._decay_matrix = None
 
     def __contains__(self, nuclide):
         return nuclide in self.nuclide_dict
@@ -277,7 +282,6 @@ class Chain:
         """Number of nuclides in chain."""
         return len(self.nuclides)
 
-
     @property
     def stable_nuclides(self) -> List[Nuclide]:
         """List of stable nuclides available in the chain"""
@@ -297,6 +301,7 @@ class Chain:
             Nuclide to add
 
         """
+        _invalidate_chain_cache(self)
         self.nuclide_dict[nuclide.name] = len(self.nuclides)
         self.nuclides.append(nuclide)
 
@@ -314,16 +319,16 @@ class Chain:
 
         String arguments in ``decay_files``, ``fpy_files``, and
         ``neutron_files`` will be treated as file names to be read.
-        Alternatively, :class:`openmc.data.endf.Evaluation` instances
-        can be included in these arguments.
+        Alternatively, :class:`openmc.data.endf.Evaluation` or
+        ``endf.Material`` instances can be included in these arguments.
 
         Parameters
         ----------
-        decay_files : list of str or openmc.data.endf.Evaluation
+        decay_files : list of str, openmc.data.endf.Evaluation, or endf.Material
             List of ENDF decay sub-library files
-        fpy_files : list of str or openmc.data.endf.Evaluation
+        fpy_files : list of str, openmc.data.endf.Evaluation, or endf.Material
             List of ENDF neutron-induced fission product yield sub-library files
-        neutron_files : list of str or openmc.data.endf.Evaluation
+        neutron_files : list of str, openmc.data.endf.Evaluation, or endf.Material
             List of ENDF neutron reaction sub-library files
         reactions : iterable of str, optional
             Transmutation reactions to include in the depletion chain, e.g.,
@@ -358,7 +363,7 @@ class Chain:
             print('Processing neutron sub-library files...')
         reactions = {}
         for f in neutron_files:
-            evaluation = openmc.data.endf.Evaluation(f)
+            evaluation = openmc.data.endf.as_evaluation(f)
             name = evaluation.gnds_name
             reactions[name] = {}
             for mf, mt, nc, mod in evaluation.reaction_list:
@@ -408,6 +413,8 @@ class Chain:
                     type_ = ','.join(mode.modes)
                     if mode.daughter in decay_data:
                         target = mode.daughter
+                    elif 'sf' in type_:
+                        target = None
                     else:
                         print('missing {} {} {}'.format(
                             parent, type_, mode.daughter))
@@ -462,7 +469,6 @@ class Chain:
                     nuclide.add_reaction('fission', None, q_value, 1.0)
                     fissionable = True
 
-
             if fissionable:
                 if parent in fpy_data:
                     fpy = fpy_data[parent]
@@ -552,11 +558,14 @@ class Chain:
         root = ET.parse(str(filename))
 
         for i, nuclide_elem in enumerate(root.findall('nuclide')):
-            this_q = fission_q.get(nuclide_elem.get("name"))
+            this_q = fission_q.get(get_text(nuclide_elem, "name"))
 
             nuc = Nuclide.from_xml(nuclide_elem, root, this_q)
             chain.add_nuclide(nuc)
 
+        # Store path of XML file (used for handling cache invalidation)
+        chain._xml_path = str(Path(filename).resolve())
+
         return chain
 
     def export_to_xml(self, filename):
@@ -598,8 +607,67 @@ class Chain:
             out[nuc.name] = dict(yield_obj)
         return out
 
-    def form_matrix(self, rates, fission_yields=None):
-        """Forms depletion matrix.
+    @property
+    def decay_matrix(self):
+        """Sparse CSC decay transmutation matrix.
+
+        Contains only terms from radioactive decay: diagonal loss terms
+        and off-diagonal gain terms (branching ratios, alpha/proton
+        production). Independent of reaction rates, so computed once and
+        cached.
+
+        See Also
+        --------
+        :meth:`form_rxn_matrix`, :meth:`form_matrix`
+        """
+        if self._decay_matrix is None:
+            n = len(self)
+            rows, cols, vals = [], [], []
+
+            def setval(i, j, val):
+                rows.append(i)
+                cols.append(j)
+                vals.append(val)
+
+            for i, nuc in enumerate(self.nuclides):
+                # Loss from radioactive decay
+                if nuc.half_life is not None:
+                    decay_constant = math.log(2) / nuc.half_life
+                    if decay_constant != 0.0:
+                        setval(i, i, -decay_constant)
+
+                # Gain from radioactive decay
+                if nuc.n_decay_modes != 0:
+                    for decay_type, target, branching_ratio in nuc.decay_modes:
+                        branch_val = branching_ratio * decay_constant
+
+                        # Allow for total annihilation for debug purposes
+                        if branch_val != 0.0:
+                            if target is not None and 'sf' not in decay_type:
+                                k = self.nuclide_dict[target]
+                                setval(k, i, branch_val)
+
+                            # Produce alphas and protons from decay
+                            if 'alpha' in decay_type:
+                                k = self.nuclide_dict.get('He4')
+                                if k is not None:
+                                    count = decay_type.count('alpha')
+                                    setval(k, i, count * branch_val)
+                            elif 'p' in decay_type:
+                                k = self.nuclide_dict.get('H1')
+                                if k is not None:
+                                    count = decay_type.count('p')
+                                    setval(k, i, count * branch_val)
+
+            self._decay_matrix = csc_array((vals, (rows, cols)), shape=(n, n))
+        return self._decay_matrix
+
+    def form_rxn_matrix(self, rates, fission_yields=None):
+        """Form the reaction-rate portion of the transmutation matrix.
+
+        Builds only the terms that depend on reaction rates: transmutation
+        reactions and fission product yields. Does not include radioactive
+        decay terms (see :attr:`decay_matrix`).
 
         Parameters
         ----------
@@ -613,97 +681,158 @@ class Chain:
 
         Returns
         -------
-        scipy.sparse.csc_matrix
-            Sparse matrix representing depletion.
+        scipy.sparse.csc_array
+            Sparse matrix representing reaction-rate terms.
 
         See Also
         --------
-        :meth:`get_default_fission_yields`
+        :attr:`decay_matrix`, :meth:`form_matrix`
         """
         reactions = set()
-
-        # Use DOK matrix as intermediate representation for matrix
         n = len(self)
-        matrix = sp.dok_matrix((n, n))
+
+        # Accumulate indices/values and then create the matrix at the end to
+        # avoid expensive index checks scipy otherwise does.
+        rows, cols, vals = [], [], []
+
+        def setval(i, j, val):
+            rows.append(i)
+            cols.append(j)
+            vals.append(val)
 
         if fission_yields is None:
             fission_yields = self.get_default_fission_yields()
 
+        # Save local variables to avoid attribute lookups in loop
+        index_nuc = rates.index_nuc
+        index_rx = rates.index_rx
+
         for i, nuc in enumerate(self.nuclides):
-            # Loss from radioactive decay
-            if nuc.half_life is not None:
-                decay_constant = math.log(2) / nuc.half_life
-                if decay_constant != 0.0:
-                    matrix[i, i] -= decay_constant
+            if nuc.name not in index_nuc:
+                continue
 
-            # Gain from radioactive decay
-            if nuc.n_decay_modes != 0:
-                for decay_type, target, branching_ratio in nuc.decay_modes:
-                    branch_val = branching_ratio * decay_constant
+            nuc_ind = index_nuc[nuc.name]
+            nuc_rates = rates[nuc_ind, :]
 
-                    # Allow for total annihilation for debug purposes
-                    if branch_val != 0.0:
-                        if target is not None:
-                            k = self.nuclide_dict[target]
-                            matrix[k, i] += branch_val
+            for r_type, target, _, br in nuc.reactions:
+                r_id = index_rx[r_type]
+                path_rate = nuc_rates[r_id]
 
-                        # Produce alphas and protons from decay
-                        if 'alpha' in decay_type:
-                            k = self.nuclide_dict.get('He4')
-                            if k is not None:
-                                count = decay_type.count('alpha')
-                                matrix[k, i] += count * branch_val
-                        elif 'p' in decay_type:
-                            k = self.nuclide_dict.get('H1')
-                            if k is not None:
-                                count = decay_type.count('p')
-                                matrix[k, i] += count * branch_val
+                # Loss term -- make sure we only count loss once for
+                # reactions with branching ratios
+                if r_type not in reactions:
+                    reactions.add(r_type)
+                    if path_rate != 0.0:
+                        setval(i, i, -path_rate)
 
-            if nuc.name in rates.index_nuc:
-                # Extract all reactions for this nuclide in this cell
-                nuc_ind = rates.index_nuc[nuc.name]
-                nuc_rates = rates[nuc_ind, :]
+                # Gain term; allow for total annihilation for debug purposes
+                if r_type != 'fission':
+                    if target is not None and path_rate != 0.0:
+                        k = self.nuclide_dict[target]
+                        setval(k, i, path_rate * br)
 
-                for r_type, target, _, br in nuc.reactions:
-                    # Extract reaction index, and then final reaction rate
-                    r_id = rates.index_rx[r_type]
-                    path_rate = nuc_rates[r_id]
-
-                    # Loss term -- make sure we only count loss once for
-                    # reactions with branching ratios
-                    if r_type not in reactions:
-                        reactions.add(r_type)
-                        if path_rate != 0.0:
-                            matrix[i, i] -= path_rate
-
-                    # Gain term; allow for total annihilation for debug purposes
-                    if r_type != 'fission':
-                        if target is not None and path_rate != 0.0:
-                            k = self.nuclide_dict[target]
-                            matrix[k, i] += path_rate * br
-
-                        # Determine light nuclide production, e.g., (n,d) should
-                        # produce H2
+                    # Determine light nuclide production, e.g., (n,d) should
+                    # produce H2
+                    if path_rate != 0.0:
                         light_nucs = REACTIONS[r_type].secondaries
                         for light_nuc in light_nucs:
                             k = self.nuclide_dict.get(light_nuc)
                             if k is not None:
-                                matrix[k, i] += path_rate * br
+                                setval(k, i, path_rate * br)
 
-                    else:
-                        for product, y in fission_yields[nuc.name].items():
-                            yield_val = y * path_rate
-                            if yield_val != 0.0:
-                                k = self.nuclide_dict[product]
-                                matrix[k, i] += yield_val
+                else:
+                    for product, y in fission_yields[nuc.name].items():
+                        yield_val = y * path_rate
+                        if yield_val != 0.0:
+                            k = self.nuclide_dict[product]
+                            setval(k, i, yield_val)
 
-                # Clear set of reactions
-                reactions.clear()
+            reactions.clear()
 
-        # Return CSC representation instead of DOK
-        return matrix.tocsc()
+        return csc_array((vals, (rows, cols)), shape=(n, n))
 
-    def form_rr_term(self, tr_rates, mats):
+    def form_matrix(self, rates, fission_yields=None):
+        """Form the full transmutation matrix (decay + reactions).
+
+        Parameters
+        ----------
+        rates : numpy.ndarray
+            2D array indexed by (nuclide, reaction)
+        fission_yields : dict, optional
+            Option to use a custom set of fission yields. Expected
+            to be of the form ``{parent : {product : f_yield}}``
+            with string nuclide names for ``parent`` and ``product``,
+            and ``f_yield`` as the respective fission yield
+
+        Returns
+        -------
+        scipy.sparse.csc_array
+            Sparse matrix representing depletion.
+
+        See Also
+        --------
+        :attr:`decay_matrix`, :meth:`form_rxn_matrix`,
+        :meth:`get_default_fission_yields`
+        """
+        return self.decay_matrix + self.form_rxn_matrix(rates, fission_yields)
+
+    def add_redox_term(self, matrix, buffer, oxidation_states):
+        r"""Adds a redox term to the depletion matrix from data contained in
+        the matrix itself and a few user-inputs.
+
+        The redox term to add to the buffer nuclide :math:`N_j` can be written
+        as:
+
+        .. math::
+            \frac{dN_j(t)}{dt} = \cdots - \frac{1}{OS_j}\sum_i N_i a_{ij}
+            \cdot OS_i
+
+        where :math:`OS` is the oxidation states vector and :math:`a_{ij}` the
+        corresponding term in the Bateman matrix.
+
+        Parameters
+        ----------
+        matrix : scipy.sparse.csc_array
+            Sparse matrix representing depletion
+        buffer : dict
+            Dictionary of buffer nuclides used to maintain anoins net balance.
+            Keys are nuclide names (strings) and values are their respective
+            fractions (float) that collectively sum to 1.
+        oxidation_states : dict
+            User-defined oxidation states for elements. Keys are element symbols
+            (e.g., 'H', 'He'), and values are their corresponding oxidation
+            states as integers (e.g., +1, 0).
+        Returns
+        -------
+        matrix : scipy.sparse.csc_array
+            Sparse matrix with redox term added
+        """
+        # Elements list with the same size as self.nuclides
+        elements = [re.split(r'\d+', nuc.name)[0] for nuc in self.nuclides]
+
+        # Match oxidation states with all elements and add 0 if not data
+        os = np.array([oxidation_states[elm] if elm in oxidation_states else 0
+                       for elm in elements])
+
+        # Buffer idx with nuclide index as value
+        buffer_idx = {nuc: self.nuclide_dict[nuc] for nuc in buffer}
+        array = matrix.toarray()
+        redox_change = np.array([])
+
+        # calculate the redox array
+        for i in range(len(self)):
+            # Net redox impact of reaction: multiply the i-th column of the
+            # depletion matrix by the oxidation states
+            redox_change = np.append(redox_change, sum(array[:, i]*os))
+
+        # Subtract redox vector to the buffer nuclides in the matrix scaling by
+        # their respective oxidation states
+        for nuc, idx in buffer_idx.items():
+            array[idx] -= redox_change * buffer[nuc] / os[idx]
+
+        return csc_array(array)
+
+    def form_rr_term(self, tr_rates, current_timestep, mats):
         """Function to form the transfer rate term matrices.
 
         .. versionadded:: 0.14.0
@@ -712,6 +841,8 @@ class Chain:
         ----------
         tr_rates : openmc.deplete.TransferRates
             Instance of openmc.deplete.TransferRates
+        current_timestep : int
+            Current timestep index
         mats : string or two-tuple of strings
             Two cases are possible:
 
@@ -730,42 +861,75 @@ class Chain:
 
         Returns
         -------
-        scipy.sparse.csc_matrix
+        scipy.sparse.csc_array
             Sparse matrix representing transfer term.
 
         """
         # Use DOK as intermediate representation
         n = len(self)
-        matrix = sp.dok_matrix((n, n))
+        matrix = dok_array((n, n))
+
+        check_type("mats", mats, (tuple, str))
+        if not isinstance(mats, str):
+            check_type("mats", mats, tuple, str)
+            check_length("mats", mats, 2, 2)
+            dest_mat, mat = mats
+        else:
+            mat = mats
+            dest_mat = None
+
+        # Build transfer term
+        components = tr_rates.get_components(mat, current_timestep, dest_mat)
 
         for i, nuc in enumerate(self.nuclides):
             elm = re.split(r'\d+', nuc.name)[0]
-            # Build transfer terms matrices
-            if isinstance(mats, str):
-                mat = mats
-                components = tr_rates.get_components(mat)
-                if elm in components:
-                    matrix[i, i] = sum(tr_rates.get_transfer_rate(mat, elm))
-                elif nuc.name in components:
-                    matrix[i, i] = sum(tr_rates.get_transfer_rate(mat, nuc.name))
-                else:
-                    matrix[i, i] = 0.0
-            #Build transfer terms matrices
-            elif isinstance(mats, tuple):
-                dest_mat, mat = mats
-                if dest_mat in tr_rates.get_destination_material(mat, elm):
-                    dest_mat_idx = tr_rates.get_destination_material(mat, elm).index(dest_mat)
-                    matrix[i, i] = tr_rates.get_transfer_rate(mat, elm)[dest_mat_idx]
-                elif dest_mat in tr_rates.get_destination_material(mat, nuc.name):
-                    dest_mat_idx = tr_rates.get_destination_material(mat, nuc.name).index(dest_mat)
-                    matrix[i, i] = tr_rates.get_transfer_rate(mat, nuc.name)[dest_mat_idx]
-                else:
-                    matrix[i, i] = 0.0
-            #Nothing else is allowed
+            if elm in components:
+                key = elm
+            elif nuc.name in components:
+                key = nuc.name
+            else:
+                continue
+            matrix[i, i] = sum(tr_rates.get_external_rate(mat, key, current_timestep, dest_mat))
 
         # Return CSC instead of DOK
         return matrix.tocsc()
 
+    def form_ext_source_term(self, ext_source_rates, current_timestep, mat):
+        """Function to form the external source rate term vectors.
+
+        .. versionadded:: 0.15.3
+
+        Parameters
+        ----------
+        ext_source_rates : openmc.deplete.ExternalSourceRates
+            Instance of openmc.deplete.ExternalSourceRates
+        current_timestep : int
+            Current timestep index
+        mat : string
+            Material id
+
+        Returns
+        -------
+        scipy.sparse.csc_array
+            Sparse vector representing external source term.
+
+        """
+        if not ext_source_rates.get_components(mat, current_timestep):
+            return
+        # Use DOK as intermediate representation
+        n = len(self)
+        vector = dok_array((n, 1))
+        components = ext_source_rates.get_components(mat, current_timestep)
+
+        for i, nuc in enumerate(self.nuclides):
+            # Build source term vector
+            if nuc.name in components:
+                vector[i] = sum(ext_source_rates.get_external_rate(
+                    mat, nuc.name, current_timestep))
+
+        # Return CSC instead of DOK
+        return vector.tocsc()
+
     def get_branch_ratios(self, reaction="(n,gamma)"):
         """Return a dictionary with reaction branching ratios
 
@@ -843,7 +1007,7 @@ class Chain:
         --------
         :meth:`get_branch_ratios`
         """
-
+        _invalidate_chain_cache(self)
         # Store some useful information through the validation stage
 
         sums = {}
@@ -982,6 +1146,7 @@ class Chain:
 
     @fission_yields.setter
     def fission_yields(self, yields):
+        _invalidate_chain_cache(self)
         if yields is not None:
             if isinstance(yields, Mapping):
                 yields = [yields]
@@ -1202,3 +1367,66 @@ class Chain:
         found.update(isotopes)
 
         return found
+
+
+# A global cache for Chain objects
+_CHAIN_CACHE = {}
+
+
+def _get_chain(
+    chain_file: PathLike | Chain | None = None,
+    fission_q: dict | None = None
+) -> Chain:
+    """Get a depletion chain from a file or the runtime configuration.
+
+    Parameters
+    ----------
+    chain_file : PathLike or Chain, optional
+        Path to depletion chain XML file, a Chain instance, or None to use
+        the file specified in ``openmc.config['chain_file']``.
+    fission_q : dict, optional
+        Dictionary of nuclides and their fission Q values [eV]. If not given,
+        values will be pulled from the ``chain_file``.
+
+    Returns
+    -------
+    Chain
+        Depletion chain instance.
+    """
+    # If chain_file is already a Chain, return it directly
+    if isinstance(chain_file, Chain):
+        return chain_file
+
+    # Resolve chain_file based on config if None
+    if chain_file is None:
+        chain_file = openmc.config.get('chain_file')
+        if 'chain_file' not in openmc.config:
+            raise DataError(
+                "No depletion chain specified and could not find depletion "
+                "chain in openmc.config['chain_file']"
+            )
+    elif not isinstance(chain_file, PathLike):
+        raise TypeError("chain_file must be path-like, a Chain, or None")
+
+    # Determine the key for the cache, which consists of the absolute path, the
+    # file modification time, the file size, and the fission Q values.
+    chain_path = Path(chain_file).resolve()
+    stat_result = chain_path.stat()
+    fq_tuple = tuple(sorted(fission_q.items())) if fission_q else ()
+    key = (chain_path, stat_result.st_mtime, stat_result.st_size, fq_tuple)
+
+    # Check the global cache. If not cached, load the chain from XML and store
+    global _CHAIN_CACHE
+    if key not in _CHAIN_CACHE:
+        _CHAIN_CACHE[key] = Chain.from_xml(chain_path, fission_q)
+    return _CHAIN_CACHE[key]
+
+
+def _invalidate_chain_cache(chain):
+    """Invalidate the cache for a specific Chain (when it is modifed)."""
+    chain._decay_matrix = None
+    if hasattr(chain, '_xml_path'):
+        # Remove all entries with the same path as self._xml_path
+        for key in list(_CHAIN_CACHE.keys()):
+            if str(key[0]) == chain._xml_path:
+                del _CHAIN_CACHE[key]
diff --git a/openmc/deplete/coupled_operator.py b/openmc/deplete/coupled_operator.py
index 17af935f4..a21d57d46 100644
--- a/openmc/deplete/coupled_operator.py
+++ b/openmc/deplete/coupled_operator.py
@@ -102,9 +102,9 @@ class CoupledOperator(OpenMCOperator):
     ----------
     model : openmc.model.Model
         OpenMC model object
-    chain_file : str, optional
-        Path to the depletion chain XML file. Defaults to
-        ``openmc.config['chain_file']``.
+    chain_file : PathLike or Chain, optional
+        Path to the depletion chain XML file or instance of openmc.deplete.Chain.
+        Defaults to ``openmc.config['chain_file']``.
     prev_results : Results, optional
         Results from a previous depletion calculation. If this argument is
         specified, the depletion calculation will start from the latest state
@@ -154,15 +154,9 @@ class CoupledOperator(OpenMCOperator):
         options.
 
         .. versionadded:: 0.12.1
-    reduce_chain : bool, optional
-        If True, use :meth:`openmc.deplete.Chain.reduce` to reduce the
-        depletion chain up to ``reduce_chain_level``.
-
-        .. versionadded:: 0.12
     reduce_chain_level : int, optional
-        Depth of the search when reducing the depletion chain. Only used
-        if ``reduce_chain`` evaluates to true. The default value of
-        ``None`` implies no limit on the depth.
+        Depth of the search when reducing the depletion chain. The default
+        value of ``None`` implies no limit on the depth.
 
         .. versionadded:: 0.12
     diff_volume_method : str
@@ -214,7 +208,7 @@ class CoupledOperator(OpenMCOperator):
                  normalization_mode="fission-q", fission_q=None,
                  fission_yield_mode="constant", fission_yield_opts=None,
                  reaction_rate_mode="direct", reaction_rate_opts=None,
-                 reduce_chain=False, reduce_chain_level=None):
+                 reduce_chain_level=None):
 
         # check for old call to constructor
         if isinstance(model, openmc.Geometry):
@@ -270,7 +264,6 @@ class CoupledOperator(OpenMCOperator):
             diff_volume_method=diff_volume_method,
             fission_q=fission_q,
             helper_kwargs=helper_kwargs,
-            reduce_chain=reduce_chain,
             reduce_chain_level=reduce_chain_level)
 
     def _differentiate_burnable_mats(self):
@@ -406,7 +399,7 @@ class CoupledOperator(OpenMCOperator):
 
         self.materials.export_to_xml(nuclides_to_ignore=self._decay_nucs)
 
-    def __call__(self, vec, source_rate):
+    def __call__(self, vec, source_rate) -> OperatorResult:
         """Runs a simulation.
 
         Simulation will abort under the following circumstances:
diff --git a/openmc/deplete/cram.py b/openmc/deplete/cram.py
index 53de83bb6..3594ffbe8 100644
--- a/openmc/deplete/cram.py
+++ b/openmc/deplete/cram.py
@@ -3,14 +3,15 @@
 Implements two different forms of CRAM for use in openmc.deplete.
 """
 
+from functools import partial
 import numbers
 
 import numpy as np
-import scipy.sparse as sp
-import scipy.sparse.linalg as sla
+from scipy.sparse.linalg import spsolve, splu
 
-from openmc.checkvalue import check_type, check_length
+from openmc.checkvalue import check_type, check_length, check_greater_than
 from .abc import DepSystemSolver
+from .._sparse_compat import csc_array, eye_array
 
 __all__ = ["CRAM16", "CRAM48", "Cram16Solver", "Cram48Solver", "IPFCramSolver"]
 
@@ -24,6 +25,12 @@ class IPFCramSolver(DepSystemSolver):
     Chebyshev Rational Approximation Method and Application to Burnup Equations
     <https://doi.org/10.13182/NSE15-26>`_," Nucl. Sci. Eng., 182:3, 297-318.
 
+    When `substeps` > 1, the time interval is split into `substeps` identical
+    sub-intervals and LU factorizations are reused across them, as described
+    in: A. Isotalo and M. Pusa, "`Improving the Accuracy of the Chebyshev
+    Rational Approximation Method Using Substeps
+    <https://doi.org/10.13182/NSE15-67>`_," Nucl. Sci. Eng., 183:1, 65-77.
+
     Parameters
     ----------
     alpha : numpy.ndarray
@@ -55,12 +62,12 @@ class IPFCramSolver(DepSystemSolver):
         self.theta = theta
         self.alpha0 = alpha0
 
-    def __call__(self, A, n0, dt):
+    def __call__(self, A, n0, dt, substeps=1):
         """Solve depletion equations using IPF CRAM
 
         Parameters
         ----------
-        A : scipy.sparse.csr_matrix
+        A : scipy.sparse.csc_array
             Sparse transmutation matrix ``A[j, i]`` desribing rates at
             which isotope ``i`` transmutes to isotope ``j``
         n0 : numpy.ndarray
@@ -68,6 +75,8 @@ class IPFCramSolver(DepSystemSolver):
             material or an atom density
         dt : float
             Time [s] of the specific interval to be solved
+        substeps : int, optional
+            Number of substeps per depletion interval.
 
         Returns
         -------
@@ -75,12 +84,25 @@ class IPFCramSolver(DepSystemSolver):
             Final compositions after ``dt``
 
         """
-        A = dt * sp.csc_matrix(A, dtype=np.float64)
+        check_type("substeps", substeps, numbers.Integral)
+        check_greater_than("substeps", substeps, 0)
+
+        step_dt = dt if substeps == 1 else dt / substeps
+        A = step_dt * csc_array(A, dtype=np.float64)
+        ident = eye_array(A.shape[0], format='csc')
+
+        if substeps == 1:
+            solvers = [partial(spsolve, A - theta * ident) for theta in self.theta]
+        else:
+            # Pre-compute LU factorizations and reuse them across substeps.
+            solvers = [splu(A - theta * ident).solve for theta in self.theta]
+
         y = n0.copy()
-        ident = sp.eye(A.shape[0], format='csc')
-        for alpha, theta in zip(self.alpha, self.theta):
-            y += 2*np.real(alpha*sla.spsolve(A - theta*ident, y))
-        return y * self.alpha0
+        for _ in range(substeps):
+            for alpha, solve in zip(self.alpha, solvers):
+                y += 2 * np.real(alpha * solve(y))
+            y *= self.alpha0
+        return y
 
 
 # Coefficients for IPF Cram 16
diff --git a/openmc/deplete/d1s.py b/openmc/deplete/d1s.py
index 8f6bffa86..d85d2e8a7 100644
--- a/openmc/deplete/d1s.py
+++ b/openmc/deplete/d1s.py
@@ -5,7 +5,7 @@ shutdown dose rate calculations.
 
 """
 
-from copy import deepcopy
+from copy import copy
 from typing import Sequence
 from math import log, prod
 
@@ -14,19 +14,20 @@ import numpy as np
 import openmc
 from openmc.data import half_life
 from .abc import _normalize_timesteps
-from .chain import Chain
+from .chain import Chain, _get_chain
+from ..checkvalue import PathLike
 
 
-def get_radionuclides(model: openmc.Model, chain_file: str | None = None) -> list[str]:
+def get_radionuclides(model: openmc.Model, chain_file: PathLike | Chain | None = None) -> list[str]:
     """Determine all radionuclides that can be produced during D1S.
 
     Parameters
     ----------
     model : openmc.Model
         Model that should be used for determining what nuclides are present
-    chain_file : str, optional
-        Which chain file to use for inspecting decay data. If None is passed,
-        defaults to ``openmc.config['chain_file']``
+    chain_file : PathLike | Chain
+        Path to the depletion chain XML file or instance of openmc.deplete.Chain.
+        Used for inspecting decay data. Defaults to ``openmc.config['chain_file']``
 
     Returns
     -------
@@ -39,9 +40,7 @@ def get_radionuclides(model: openmc.Model, chain_file: str | None = None) -> lis
                       for nuc in mat.get_nuclides()}
 
     # Load chain file
-    if chain_file is None:
-        chain_file = openmc.config['chain_file']
-    chain = Chain.from_xml(chain_file)
+    chain = _get_chain(chain_file)
 
     radionuclides = set()
     for nuclide in chain.nuclides:
@@ -109,14 +108,15 @@ def time_correction_factors(
     # Create a 2D array for the time correction factors
     h = np.zeros((n_timesteps, n_nuclides))
 
-    for i, (dt, rate) in enumerate(zip(timesteps, source_rates)):
-        # Precompute the exponential terms. Since (1 - exp(-x)) is susceptible to
-        # roundoff error, use expm1 instead (which computes exp(x) - 1)
-        g = np.exp(-decay_rate*dt)
-        one_minus_g = -np.expm1(-decay_rate*dt)
+    # Precompute all exponential terms with same shape as h
+    decay_dt = decay_rate[np.newaxis, :] * timesteps[:, np.newaxis]
+    g = np.exp(-decay_dt)
+    one_minus_g = -np.expm1(-decay_dt)
 
+    # Apply recurrence relation step by step
+    for i in range(len(timesteps)):
         # Eq. (4) in doi:10.1016/j.fusengdes.2019.111399
-        h[i + 1] = rate*one_minus_g + h[i]*g
+        h[i + 1] = source_rates[i] * one_minus_g[i] + h[i] * g[i]
 
     return {nuclides[i]: h[:, i] for i in range(n_nuclides)}
 
@@ -141,7 +141,9 @@ def apply_time_correction(
     time_correction_factors : dict
         Time correction factors as returned by :func:`time_correction_factors`
     index : int, optional
-        Index to use for the correction factors
+        Index of the time of interest. If N timesteps are provided in
+        :func:`time_correction_factors`, there are N + 1 times to select from.
+        The default is -1 which corresponds to the final time.
     sum_nuclides : bool
         Whether to sum over the parent nuclides
 
@@ -162,8 +164,12 @@ def apply_time_correction(
     radionuclides = [str(x) for x in tally.filters[i_filter].bins]
     tcf = np.array([time_correction_factors[x][index] for x in radionuclides])
 
-    # Create copy of tally
-    new_tally = deepcopy(tally)
+    # Force tally results to be read and std_dev to be computed
+    tally.std_dev
+
+    # Create shallow copy of tally
+    new_tally = copy(tally)
+    new_tally._filters = copy(tally._filters)
 
     # Determine number of bins in other filters
     n_bins_before = prod([f.num_bins for f in tally.filters[:i_filter]])
@@ -175,32 +181,34 @@ def apply_time_correction(
     shape = (n_bins_before, n_radionuclides, n_bins_after, n_nuclides, n_scores)
     tally_sum = new_tally.sum.reshape(shape)
     tally_sum_sq = new_tally.sum_sq.reshape(shape)
+    tally_mean = new_tally.mean.reshape(shape)
+    tally_std_dev = new_tally.std_dev.reshape(shape)
 
     # Apply TCF, broadcasting to the correct dimensions
     tcf.shape = (1, -1, 1, 1, 1)
-    new_tally._sum = tally_sum * tcf
-    new_tally._sum_sq = tally_sum_sq * (tcf*tcf)
-    new_tally._mean = None
-    new_tally._std_dev = None
+    new_tally._mean = tally_mean * tcf
+    new_tally._std_dev = tally_std_dev * tcf
 
     shape = (-1, n_nuclides, n_scores)
 
     if sum_nuclides:
-        # Query the mean and standard deviation
-        mean = new_tally.mean
-        std_dev = new_tally.std_dev
-
         # Sum over parent nuclides (note that when combining different bins for
         # parent nuclide, we can't work directly on sum_sq)
-        new_tally._mean = mean.sum(axis=1).reshape(shape)
-        new_tally._std_dev = np.linalg.norm(std_dev, axis=1).reshape(shape)
+        new_tally._sum = None
+        new_tally._sum_sq = None
+        new_tally._mean = new_tally.mean.sum(axis=1).reshape(shape)
+        new_tally._std_dev = np.linalg.norm(new_tally.std_dev, axis=1).reshape(shape)
         new_tally._derived = True
 
         # Remove ParentNuclideFilter
         new_tally.filters.pop(i_filter)
     else:
-        new_tally._sum.shape = shape
-        new_tally._sum_sq.shape = shape
+        # Apply TCF and change shape back to (filter combinations, nuclides,
+        # scores)
+        new_tally._sum = (tally_sum * tcf).reshape(shape)
+        new_tally._sum_sq = (tally_sum_sq * (tcf*tcf)).reshape(shape)
+        new_tally._mean.shape = shape
+        new_tally._std_dev.shape = shape
 
     return new_tally
 
@@ -242,7 +250,7 @@ def prepare_tallies(
         for f in tally.filters:
             if isinstance(f, openmc.ParticleFilter):
                 if list(f.bins) == ['photon']:
-                    tally.filters.append(filter)
+                    if not tally.contains_filter(openmc.ParentNuclideFilter):
+                        tally.filters.append(filter)
                     break
-
     return nuclides
diff --git a/openmc/deplete/helpers.py b/openmc/deplete/helpers.py
index f24b89868..14780d61a 100644
--- a/openmc/deplete/helpers.py
+++ b/openmc/deplete/helpers.py
@@ -379,17 +379,17 @@ class FluxCollapseHelper(ReactionRateHelper):
             nuclides_direct = self._rate_tally.nuclides
             shape = (len(nuclides_direct), len(self._reactions_direct))
             rx_rates = self.rate_tally_means[mat_index].reshape(shape)
+            direct_rx_index = {score: i for i, score in enumerate(self._reactions_direct)}
+            direct_nuc_index = {nuc: i for i, nuc in enumerate(nuclides_direct)}
 
         mat = self._materials[mat_index]
 
         for name, i_nuc in zip(self.nuclides, nuc_index):
             for mt, score, i_rx in zip(self._mts, self._scores, react_index):
                 if score in self._reactions_direct and name in nuclides_direct:
-                    # Determine index in rx_rates
-                    i_rx_direct = self._reactions_direct.index(score)
-                    i_nuc_direct = nuclides_direct.index(name)
-
                     # Get reaction rate from tally
+                    i_rx_direct = direct_rx_index[score]
+                    i_nuc_direct = direct_nuc_index[name]
                     self._results_cache[i_nuc, i_rx] = rx_rates[i_nuc_direct, i_rx_direct]
                 else:
                     # Use flux to collapse reaction rate (per N)
diff --git a/openmc/deplete/independent_operator.py b/openmc/deplete/independent_operator.py
index 333ead0f8..bdfc32763 100644
--- a/openmc/deplete/independent_operator.py
+++ b/openmc/deplete/independent_operator.py
@@ -50,9 +50,9 @@ class IndependentOperator(OpenMCOperator):
         Cross sections in [b] for each domain. If the
         :class:`~openmc.deplete.MicroXS` object is empty, a decay-only
         calculation will be run.
-    chain_file : str
-        Path to the depletion chain XML file. Defaults to
-        ``openmc.config['chain_file']``.
+    chain_file : PathLike or Chain, optional
+        Path to the depletion chain XML file or instance of openmc.deplete.Chain.
+        Defaults to ``openmc.config['chain_file']``.
     keff : 2-tuple of float, optional
        keff eigenvalue and uncertainty from transport calculation.
     prev_results : Results, optional
@@ -66,13 +66,9 @@ class IndependentOperator(OpenMCOperator):
         Dictionary of nuclides and their fission Q values [eV]. If not given,
         values will be pulled from the ``chain_file``. Only applicable if
         ``"normalization_mode" == "fission-q"``.
-    reduce_chain : bool, optional
-        If True, use :meth:`openmc.deplete.Chain.reduce` to reduce the depletion
-        chain up to ``reduce_chain_level``.
     reduce_chain_level : int, optional
-        Depth of the search when reducing the depletion chain. Only used if
-        ``reduce_chain`` evaluates to true. The default value of ``None``
-        implies no limit on the depth.
+        Depth of the search when reducing the depletion chain. The default
+        value of ``None`` implies no limit on the depth.
     fission_yield_opts : dict of str to option, optional
         Optional arguments to pass to the
         :class:`openmc.deplete.helpers.FissionYieldHelper` object. Will be
@@ -119,7 +115,6 @@ class IndependentOperator(OpenMCOperator):
                  normalization_mode='fission-q',
                  fission_q=None,
                  prev_results=None,
-                 reduce_chain=False,
                  reduce_chain_level=None,
                  fission_yield_opts=None):
         # Validate micro-xs parameters
@@ -157,7 +152,6 @@ class IndependentOperator(OpenMCOperator):
             prev_results=prev_results,
             fission_q=fission_q,
             helper_kwargs=helper_kwargs,
-            reduce_chain=reduce_chain,
             reduce_chain_level=reduce_chain_level)
 
     @classmethod
@@ -170,7 +164,6 @@ class IndependentOperator(OpenMCOperator):
                       normalization_mode='fission-q',
                       fission_q=None,
                       prev_results=None,
-                      reduce_chain=False,
                       reduce_chain_level=None,
                       fission_yield_opts=None):
         """
@@ -186,9 +179,9 @@ class IndependentOperator(OpenMCOperator):
         micro_xs : MicroXS
             Cross sections in [b]. If the :class:`~openmc.deplete.MicroXS`
             object is empty, a decay-only calculation will be run.
-        chain_file : str, optional
-            Path to the depletion chain XML file. Defaults to
-            ``openmc.config['chain_file']``.
+        chain_file : PathLike or Chain, optional
+            Path to the depletion chain XML file or instance of
+            openmc.deplete.Chain. Defaults to ``openmc.config['chain_file']``.
         nuc_units : {'atom/cm3', 'atom/b-cm'}, optional
             Units for nuclide concentration.
         keff : 2-tuple of float, optional
@@ -206,13 +199,9 @@ class IndependentOperator(OpenMCOperator):
             applicable if ``"normalization_mode" == "fission-q"``.
         prev_results : Results, optional
             Results from a previous depletion calculation.
-        reduce_chain : bool, optional
-            If True, use :meth:`openmc.deplete.Chain.reduce` to reduce the
-            depletion chain up to ``reduce_chain_level``. Default is False.
         reduce_chain_level : int, optional
-            Depth of the search when reducing the depletion chain. Only used
-            if ``reduce_chain`` evaluates to true. The default value of
-            ``None`` implies no limit on the depth.
+            Depth of the search when reducing the depletion chain. The default
+            value of ``None`` implies no limit on the depth.
         fission_yield_opts : dict of str to option, optional
             Optional arguments to pass to the
             :class:`openmc.deplete.helpers.FissionYieldHelper` class. Will be
@@ -232,7 +221,6 @@ class IndependentOperator(OpenMCOperator):
                    normalization_mode=normalization_mode,
                    fission_q=fission_q,
                    prev_results=prev_results,
-                   reduce_chain=reduce_chain,
                    reduce_chain_level=reduce_chain_level,
                    fission_yield_opts=fission_yield_opts)
 
@@ -285,7 +273,7 @@ class IndependentOperator(OpenMCOperator):
         Returns
         -------
         nuclides : set of str
-            Set of nuclide names that have cross secton data
+            Set of nuclide names that have cross section data
 
         """
         return set(cross_sections[0].nuclides)
@@ -351,8 +339,12 @@ class IndependentOperator(OpenMCOperator):
 
             for i_nuc in nuc_index:
                 nuc = self.nuc_ind_map[i_nuc]
+                if nuc not in xs._index_nuc:
+                    continue
                 for i_rx in react_index:
                     rx = self.rx_ind_map[i_rx]
+                    if rx not in xs._index_rx:
+                        continue
 
                     # Determine reaction rate by multiplying xs in [b] by flux
                     # in [n-cm/src] to give [(reactions/src)*b-cm/atom]
@@ -396,7 +388,7 @@ class IndependentOperator(OpenMCOperator):
         # Return number density vector
         return super().initial_condition(self.materials)
 
-    def __call__(self, vec, source_rate):
+    def __call__(self, vec, source_rate) -> OperatorResult:
         """Obtain the reaction rates
 
         Parameters
diff --git a/openmc/deplete/integrators.py b/openmc/deplete/integrators.py
index 50810c88a..25e64cb2e 100644
--- a/openmc/deplete/integrators.py
+++ b/openmc/deplete/integrators.py
@@ -22,7 +22,6 @@ class PredictorIntegrator(Integrator):
 
     .. math::
         \mathbf{n}_{i+1} = \exp\left(h\mathbf{A}(\mathbf{n}_i) \right) \mathbf{n}_i
-
     """
     _num_stages = 1
 
@@ -40,22 +39,19 @@ class PredictorIntegrator(Integrator):
             Time in [s] for the entire depletion interval
         source_rate : float
             Power in [W] or source rate in [neutron/sec]
-        _i : int or None
-            Iteration index. Not used
+        _i : int, optional
+            Current iteration count. Not used
 
         Returns
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
+        n_end : list of numpy.ndarray
             Concentrations at end of interval
-        op_results : empty list
-            Kept for consistency with API. No intermediate calls to operator
-            with predictor
 
         """
-        proc_time, n_end = self._timed_deplete(n, rates, dt)
-        return proc_time, [n_end], []
+        proc_time, n_end = self._timed_deplete(n, rates, dt, _i)
+        return proc_time, n_end
 
 
 @add_params
@@ -99,21 +95,18 @@ class CECMIntegrator(Integrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
-            Eigenvalue and reaction rates from transport simulations
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
         """
         # deplete across first half of interval
-        time0, n_middle = self._timed_deplete(n, rates, dt / 2)
+        time0, n_middle = self._timed_deplete(n, rates, dt / 2, _i)
         res_middle = self.operator(n_middle, source_rate)
 
         # deplete across entire interval with BOS concentrations,
         # MOS reaction rates
-        time1, n_end = self._timed_deplete(n, res_middle.rates, dt)
+        time1, n_end = self._timed_deplete(n, res_middle.rates, dt, _i)
 
-        return time0 + time1, [n_middle, n_end], [res_middle]
+        return time0 + time1, n_end
 
 
 @add_params
@@ -163,39 +156,33 @@ class CF4Integrator(Integrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
-            Eigenvalue and reaction rates from intermediate transport
-            simulations
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
         """
         # Step 1: deplete with matrix 1/2*A(y0)
         time1, n_eos1 = self._timed_deplete(
-            n_bos, bos_rates, dt, matrix_func=cf4_f1)
+            n_bos, bos_rates, dt, _i, matrix_func=cf4_f1)
         res1 = self.operator(n_eos1, source_rate)
 
         # Step 2: deplete with matrix 1/2*A(y1)
         time2, n_eos2 = self._timed_deplete(
-            n_bos, res1.rates, dt, matrix_func=cf4_f1)
+            n_bos, res1.rates, dt, _i, matrix_func=cf4_f1)
         res2 = self.operator(n_eos2, source_rate)
 
         # Step 3: deplete with matrix -1/2*A(y0)+A(y2)
         list_rates = list(zip(bos_rates, res2.rates))
         time3, n_eos3 = self._timed_deplete(
-            n_eos1, list_rates, dt, matrix_func=cf4_f2)
+            n_eos1, list_rates, dt, _i, matrix_func=cf4_f2)
         res3 = self.operator(n_eos3, source_rate)
 
         # Step 4: deplete with two matrix exponentials
         list_rates = list(zip(bos_rates, res1.rates, res2.rates, res3.rates))
         time4, n_inter = self._timed_deplete(
-            n_bos, list_rates, dt, matrix_func=cf4_f3)
+            n_bos, list_rates, dt, _i, matrix_func=cf4_f3)
         time5, n_eos5 = self._timed_deplete(
-            n_inter, list_rates, dt, matrix_func=cf4_f4)
+            n_inter, list_rates, dt, _i, matrix_func=cf4_f4)
 
-        return (time1 + time2 + time3 + time4 + time5,
-                [n_eos1, n_eos2, n_eos3, n_eos5],
-                [res1, res2, res3])
+        return time1 + time2 + time3 + time4 + time5, n_eos5
 
 
 @add_params
@@ -241,27 +228,23 @@ class CELIIntegrator(Integrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
-            Eigenvalue and reaction rates from intermediate transport
-            simulation
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
         """
         # deplete to end using BOS rates
-        proc_time, n_ce = self._timed_deplete(n_bos, rates, dt)
+        proc_time, n_ce = self._timed_deplete(n_bos, rates, dt, _i)
         res_ce = self.operator(n_ce, source_rate)
 
         # deplete using two matrix exponentials
         list_rates = list(zip(rates, res_ce.rates))
 
         time_le1, n_inter = self._timed_deplete(
-            n_bos, list_rates, dt, matrix_func=celi_f1)
+            n_bos, list_rates, dt, _i, matrix_func=celi_f1)
 
         time_le2, n_end = self._timed_deplete(
-            n_inter, list_rates, dt, matrix_func=celi_f2)
+            n_inter, list_rates, dt, _i, matrix_func=celi_f2)
 
-        return proc_time + time_le1 + time_le1, [n_ce, n_end], [res_ce]
+        return proc_time + time_le1 + time_le2, n_end
 
 
 @add_params
@@ -307,31 +290,27 @@ class EPCRK4Integrator(Integrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
-            Eigenvalue and reaction rates from intermediate transport
-            simulations
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
         """
 
         # Step 1: deplete with matrix A(y0) / 2
-        time1, n1 = self._timed_deplete(n, rates, dt, matrix_func=rk4_f1)
+        time1, n1 = self._timed_deplete(n, rates, dt, _i, matrix_func=rk4_f1)
         res1 = self.operator(n1, source_rate)
 
         # Step 2: deplete with matrix A(y1) / 2
-        time2, n2 = self._timed_deplete(n, res1.rates, dt, matrix_func=rk4_f1)
+        time2, n2 = self._timed_deplete(n, res1.rates, dt, _i, matrix_func=rk4_f1)
         res2 = self.operator(n2, source_rate)
 
         # Step 3: deplete with matrix A(y2)
-        time3, n3 = self._timed_deplete(n, res2.rates, dt)
+        time3, n3 = self._timed_deplete(n, res2.rates, dt, _i)
         res3 = self.operator(n3, source_rate)
 
         # Step 4: deplete with matrix built from weighted rates
         list_rates = list(zip(rates, res1.rates, res2.rates, res3.rates))
-        time4, n4 = self._timed_deplete(n, list_rates, dt, matrix_func=rk4_f4)
+        time4, n4 = self._timed_deplete(n, list_rates, dt, _i, matrix_func=rk4_f4)
 
-        return (time1 + time2 + time3 + time4, [n1, n2, n3, n4], [res1, res2, res3])
+        return time1 + time2 + time3 + time4, n4
 
 
 @add_params
@@ -389,12 +368,8 @@ class LEQIIntegrator(Integrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
-            Eigenvalue and reaction rates from intermediate transport
-            simulation
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
         """
         if i == 0:
             if self._i_res < 1:  # need at least previous transport solution
@@ -403,7 +378,7 @@ class LEQIIntegrator(Integrator):
                     self, n_bos, bos_rates, dt, source_rate, i)
             prev_res = self.operator.prev_res[-2]
             prev_dt = self.timesteps[i] - prev_res.time[0]
-            self._prev_rates = prev_res.rates[0]
+            self._prev_rates = prev_res.rates
         else:
             prev_dt = self.timesteps[i - 1]
 
@@ -414,9 +389,9 @@ class LEQIIntegrator(Integrator):
             self._prev_rates, bos_res.rates, repeat(prev_dt), repeat(dt)))
 
         time1, n_inter = self._timed_deplete(
-            n_bos, le_inputs, dt, matrix_func=leqi_f1)
+            n_bos, le_inputs, dt, i, matrix_func=leqi_f1)
         time2, n_eos0 = self._timed_deplete(
-            n_inter, le_inputs, dt, matrix_func=leqi_f2)
+            n_inter, le_inputs, dt, i, matrix_func=leqi_f2)
 
         res_inter = self.operator(n_eos0, source_rate)
 
@@ -425,16 +400,14 @@ class LEQIIntegrator(Integrator):
             repeat(prev_dt), repeat(dt)))
 
         time3, n_inter = self._timed_deplete(
-            n_bos, qi_inputs, dt, matrix_func=leqi_f3)
+            n_bos, qi_inputs, dt, i, matrix_func=leqi_f3)
         time4, n_eos1 = self._timed_deplete(
-            n_inter, qi_inputs, dt, matrix_func=leqi_f4)
+            n_inter, qi_inputs, dt, i, matrix_func=leqi_f4)
 
         # store updated rates
         self._prev_rates = copy.deepcopy(bos_res.rates)
 
-        return (
-            time1 + time2 + time3 + time4, [n_eos0, n_eos1],
-            [bos_res, res_inter])
+        return time1 + time2 + time3 + time4, n_eos1
 
 
 @add_params
@@ -471,14 +444,13 @@ class SICELIIntegrator(SIIntegrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_bos_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
+        op_result : openmc.deplete.OperatorResult
             Eigenvalue and reaction rates from intermediate transport
             simulations
         """
-        proc_time, n_eos = self._timed_deplete(n_bos, bos_rates, dt)
+        proc_time, n_eos = self._timed_deplete(n_bos, bos_rates, dt, _i)
         n_inter = copy.deepcopy(n_eos)
 
         # Begin iteration
@@ -494,13 +466,13 @@ class SICELIIntegrator(SIIntegrator):
 
             list_rates = list(zip(bos_rates, res_bar.rates))
             time1, n_inter = self._timed_deplete(
-                n_bos, list_rates, dt, matrix_func=celi_f1)
+                n_bos, list_rates, dt, _i, matrix_func=celi_f1)
             time2, n_inter = self._timed_deplete(
-                n_inter, list_rates, dt, matrix_func=celi_f2)
+                n_inter, list_rates, dt, _i, matrix_func=celi_f2)
             proc_time += time1 + time2
 
         # end iteration
-        return proc_time, [n_eos, n_inter], [res_bar]
+        return proc_time, n_inter, res_bar
 
 
 @add_params
@@ -537,10 +509,9 @@ class SILEQIIntegrator(SIIntegrator):
         -------
         proc_time : float
             Time spent in CRAM routines for all materials in [s]
-        n_list : list of list of numpy.ndarray
-            Concentrations at each of the intermediate points with
-            the final concentration as the last element
-        op_results : list of openmc.deplete.OperatorResult
+        n_end : list of numpy.ndarray
+            Concentrations at end of interval
+        op_result : openmc.deplete.OperatorResult
             Eigenvalue and reaction rates from intermediate transport
             simulation
         """
@@ -552,7 +523,7 @@ class SILEQIIntegrator(SIIntegrator):
                     self, n_bos, bos_rates, dt, source_rate, i)
             prev_res = self.operator.prev_res[-2]
             prev_dt = self.timesteps[i] - prev_res.time[0]
-            self._prev_rates = prev_res.rates[0]
+            self._prev_rates = prev_res.rates
         else:
             prev_dt = self.timesteps[i - 1]
 
@@ -560,9 +531,9 @@ class SILEQIIntegrator(SIIntegrator):
         inputs = list(zip(self._prev_rates, bos_rates,
                           repeat(prev_dt), repeat(dt)))
         proc_time, n_inter = self._timed_deplete(
-            n_bos, inputs, dt, matrix_func=leqi_f1)
+            n_bos, inputs, dt, i, matrix_func=leqi_f1)
         time1, n_eos = self._timed_deplete(
-            n_inter, inputs, dt, matrix_func=leqi_f2)
+            n_inter, inputs, dt, i, matrix_func=leqi_f2)
 
         proc_time += time1
         n_inter = copy.deepcopy(n_eos)
@@ -580,12 +551,15 @@ class SILEQIIntegrator(SIIntegrator):
             inputs = list(zip(self._prev_rates, bos_rates, res_bar.rates,
                               repeat(prev_dt), repeat(dt)))
             time1, n_inter = self._timed_deplete(
-                n_bos, inputs, dt, matrix_func=leqi_f3)
+                n_bos, inputs, dt, i, matrix_func=leqi_f3)
             time2, n_inter = self._timed_deplete(
-                n_inter, inputs, dt, matrix_func=leqi_f4)
+                n_inter, inputs, dt, i, matrix_func=leqi_f4)
             proc_time += time1 + time2
 
-        return proc_time, [n_eos, n_inter], [res_bar]
+        # Store updated rates for next step
+        self._prev_rates = copy.deepcopy(bos_rates)
+
+        return proc_time, n_inter, res_bar
 
 
 integrator_by_name = {
diff --git a/openmc/deplete/keff_search_control.py b/openmc/deplete/keff_search_control.py
new file mode 100644
index 000000000..49f7cc4df
--- /dev/null
+++ b/openmc/deplete/keff_search_control.py
@@ -0,0 +1,128 @@
+from typing import Callable
+from warnings import warn
+
+import openmc.lib
+
+
+class _KeffSearchControl:
+    """Controller for keff search during depletion calculations.
+
+    This class performs keff searches to maintain a target keff by adjusting a
+    model parameter through a provided function.
+
+    Parameters
+    ----------
+    operator : openmc.deplete.Operator
+        Depletion operator instance
+    function : Callable
+        Function that modifies the model based on a parameter value
+    x0 : float
+        Initial lower bound for the keff search
+    x1 : float
+        Initial upper bound for the keff search
+    bracket : list[float]
+        Absolute bracketing interval lower and upper. If the keff search
+        solution lies off these limits the closest limit will be set as new
+        result.
+    **search_kwargs : dict, optional
+        Additional keyword arguments to pass to :meth:`openmc.Model.keff_search`
+
+    """
+    def __init__(self, operator, function: Callable, x0: float, x1: float, bracket: list[float], **search_kwargs):
+        if len(bracket) != 2:
+            raise ValueError(f"bracket must have exactly 2 elements, got {len(bracket)}")
+        if bracket[0] >= bracket[1]:
+            raise ValueError(f"bracket[0] must be < bracket[1], got {bracket}")
+        self.x0 = x0
+        self.x1 = x1
+        self.operator = operator
+        self.function = function
+        self.search_kwargs = search_kwargs
+        self.search_kwargs['x_min'] = bracket[0]
+        self.search_kwargs['x_max'] = bracket[1]
+
+    def run(self, x):
+        """Perform keff search and update the atom density vector.
+
+        Parameters
+        ----------
+        x : list of numpy.ndarray
+            Current atom density vector (atoms per material)
+
+        Returns
+        -------
+        root : float
+            Parameter value that achieves target keff
+        """
+        root = self._search_for_keff()
+        self._update_vec(x)
+        return root
+
+    def _search_for_keff(self) -> float:
+        """Perform the keff search using the model's keff_search method.
+
+        Returns
+        -------
+        float
+            Parameter value that achieves target keff
+
+        Raises
+        ------
+        ValueError
+            If the keff search fails to converge
+        """
+        with openmc.lib.TemporarySession(self.operator.model):
+            # Only pass the first 3 required args plus explicitly provided kwargs
+            result = self.operator.model.keff_search(
+                self.function, self.x0, self.x1, **self.search_kwargs
+            )
+        if not result.converged:
+            raise ValueError(
+                f"Search for keff failed to converge. "
+                f"Termination reason: {result.flag}"
+            )
+
+        root = result.root
+
+        # Check if root is outside the bracket bounds and give a warning
+        if root < self.search_kwargs['x_min']:
+            warn(f"keff search result ({root:.6f}) is below the lower bracket "
+                 f"bound ({self.search_kwargs['x_min']:.6f}).", UserWarning)
+        elif root > self.search_kwargs['x_max']:
+            warn(f"keff search result ({root:.6f}) is above the upper bracket "
+                 f"bound ({self.search_kwargs['x_max']:.6f}).", UserWarning)
+
+        # Restore the number of initial batches
+        openmc.lib.settings.set_batches(self.operator.model.settings.batches)
+
+        return root
+
+    def _update_vec(self, x):
+        """Update the atom density vector from openmc.lib.materials and AtomNumber object.
+
+        The depletion vector ``x`` is rank-local, matching the materials owned
+        by ``self.operator.number`` on the current MPI rank. We therefore only
+        update entries for locally owned materials using the compositions
+        currently stored in ``openmc.lib.materials``.
+
+        Parameters
+        ----------
+        x : list of numpy.ndarray
+            Atom density vector to update (atoms per material)
+
+        """
+        number = self.operator.number
+
+        for mat_idx, mat in enumerate(number.materials):
+            lib_material = openmc.lib.materials[int(mat)]
+            nuclides = lib_material.nuclides
+            densities = 1e24 * lib_material.densities
+            volume = number.get_mat_volume(mat)
+
+            for nuc_idx, nuc in enumerate(number.burnable_nuclides):
+                if nuc in nuclides:
+                    lib_nuc_idx = nuclides.index(nuc)
+                    atom_density = densities[lib_nuc_idx]
+                else:
+                    atom_density = number.get_atom_density(mat, nuc)
+                x[mat_idx][nuc_idx] = atom_density * volume
diff --git a/openmc/deplete/microxs.py b/openmc/deplete/microxs.py
index bbe9b2a43..7049f7d31 100644
--- a/openmc/deplete/microxs.py
+++ b/openmc/deplete/microxs.py
@@ -5,21 +5,23 @@ IndependentOperator class for depletion.
 """
 
 from __future__ import annotations
-from collections.abc import Iterable, Sequence
+from collections.abc import Sequence
+import shutil
 from tempfile import TemporaryDirectory
+from typing import Union, TypeAlias, Self
 
+import h5py
 import pandas as pd
 import numpy as np
 
 from openmc.checkvalue import check_type, check_value, check_iterable_type, PathLike
-from openmc.exceptions import DataError
-from openmc.utility_funcs import change_directory
 from openmc import StatePoint
 from openmc.mgxs import GROUP_STRUCTURES
 from openmc.data import REACTION_MT
 import openmc
-from .chain import Chain, REACTIONS
+from .chain import Chain, REACTIONS, _get_chain
 from .coupled_operator import _find_cross_sections, _get_nuclides_with_data
+from ..utility_funcs import h5py_file_or_group
 import openmc.lib
 from openmc.mpi import comm
 
@@ -28,36 +30,52 @@ _valid_rxns.append('fission')
 _valid_rxns.append('damage-energy')
 
 
-def _resolve_chain_file_path(chain_file: str | None):
-    if chain_file is None:
-        chain_file = openmc.config.get('chain_file')
-        if 'chain_file' not in openmc.config:
-            raise DataError(
-                "No depletion chain specified and could not find depletion "
-                "chain in openmc.config['chain_file']"
-            )
-    return chain_file
+# TODO: Replace with type statement when support is Python 3.12+
+DomainTypes: TypeAlias = Union[
+    Sequence[openmc.Material],
+    Sequence[openmc.Cell],
+    Sequence[openmc.Universe],
+    openmc.MeshBase,
+    openmc.Filter,
+    Sequence[openmc.Filter]
+]
 
 
 def get_microxs_and_flux(
-        model: openmc.Model,
-        domains,
-        nuclides: Iterable[str] | None = None,
-        reactions: Iterable[str] | None = None,
-        energies: Iterable[float] | str | None = None,
-        chain_file: PathLike | None = None,
-        run_kwargs=None
-    ) -> tuple[list[np.ndarray], list[MicroXS]]:
-    """Generate a microscopic cross sections and flux from a Model
+    model: openmc.Model,
+    domains: DomainTypes,
+    nuclides: Sequence[str] | None = None,
+    reactions: Sequence[str] | None = None,
+    energies: Sequence[float] | str | None = None,
+    reaction_rate_mode: str = 'direct',
+    chain_file: PathLike | Chain | None = None,
+    path_statepoint: PathLike | None = None,
+    path_input: PathLike | None = None,
+    run_kwargs=None,
+    reaction_rate_opts: dict | None = None,
+) -> tuple[list[np.ndarray], list[MicroXS]]:
+    """Generate microscopic cross sections and fluxes for multiple domains.
+
+    This function runs a neutron transport solve to obtain the flux and reaction
+    rates in the specified domains and computes multigroup microscopic cross
+    sections that can be used in depletion calculations with the
+    :class:`~openmc.deplete.IndependentOperator` class.
 
     .. versionadded:: 0.14.0
 
+    .. versionchanged:: 0.15.3
+        Added `reaction_rate_mode`, `path_statepoint`, `path_input` arguments.
+
     Parameters
     ----------
     model : openmc.Model
         OpenMC model object. Must contain geometry, materials, and settings.
-    domains : list of openmc.Material or openmc.Cell or openmc.Universe, or openmc.MeshBase
-        Domains in which to tally reaction rates.
+    domains : list of openmc.Material or openmc.Cell or openmc.Universe, or openmc.MeshBase, or openmc.Filter, or list of openmc.Filter
+        Domains in which to tally reaction rates, or a spatial tally filter.
+        A list of filters can be provided to create one set of tallies per
+        filter (e.g., one :class:`~openmc.MeshMaterialFilter` per mesh) that
+        are all evaluated in a single transport solve. Results are
+        concatenated across all filters in order.
     nuclides : list of str
         Nuclides to get cross sections for. If not specified, all burnable
         nuclides from the depletion chain file are used.
@@ -65,14 +83,41 @@ def get_microxs_and_flux(
         Reactions to get cross sections for. If not specified, all neutron
         reactions listed in the depletion chain file are used.
     energies : iterable of float or str
-        Energy group boundaries in [eV] or the name of the group structure
-    chain_file : str, optional
-        Path to the depletion chain XML file that will be used in depletion
-        simulation. Used to determine cross sections for materials not
+        Energy group boundaries in [eV] or the name of the group structure.
+        If left as None, no energy filter is applied to the flux tally. When
+        `reaction_rate_mode` is "direct", these boundaries define the output
+        flux and microscopic cross section energy group structure. When
+        `reaction_rate_mode` is "flux", these boundaries define the multigroup
+        flux tally used to collapse continuous-energy cross sections; returned
+        fluxes and microscopic cross sections are one-group.
+    reaction_rate_mode : {"direct", "flux"}, optional
+        The "direct" method tallies reaction rates directly (per energy
+        group). The "flux" method tallies a multigroup flux spectrum and then
+        collapses reaction rates after a transport solve. When
+        `reaction_rate_opts` is provided with `reaction_rate_mode='flux'`, the
+        specified nuclide/reaction pairs are tallied directly and those values
+        override the flux-collapsed values.
+    chain_file : PathLike or Chain, optional
+        Path to the depletion chain XML file or an instance of
+        openmc.deplete.Chain. Used to determine cross sections for materials not
         present in the inital composition. Defaults to
         ``openmc.config['chain_file']``.
+    path_statepoint : path-like, optional
+        Path to write the statepoint file from the neutron transport solve to.
+        By default, The statepoint file is written to a temporary directory and
+        is not kept.
+    path_input : path-like, optional
+        Path to write the model XML file from the neutron transport solve to.
+        By default, the model XML file is written to a temporary directory and
+        not kept.
     run_kwargs : dict, optional
         Keyword arguments passed to :meth:`openmc.Model.run`
+    reaction_rate_opts : dict, optional
+        When `reaction_rate_mode="flux"`, allows selecting a subset of
+        nuclide/reaction pairs to be computed via direct reaction-rate tallies
+        over one energy bin spanning the full `energies` range. Supported keys:
+        "nuclides", "reactions". If "reactions" are specified without
+        "nuclides", all selected nuclides are used.
 
     Returns
     -------
@@ -81,13 +126,18 @@ def get_microxs_and_flux(
     list of MicroXS
         Cross section data in [b] for each domain
 
+    See Also
+    --------
+    openmc.deplete.IndependentOperator
+
     """
+    check_value('reaction_rate_mode', reaction_rate_mode, {'direct', 'flux'})
+
     # Save any original tallies on the model
-    original_tallies = model.tallies
+    original_tallies = list(model.tallies)
 
     # Determine what reactions and nuclides are available in chain
-    chain_file = _resolve_chain_file_path(chain_file)
-    chain = Chain.from_xml(chain_file)
+    chain = _get_chain(chain_file)
     if reactions is None:
         reactions = chain.reactions
     if not nuclides:
@@ -96,34 +146,85 @@ def get_microxs_and_flux(
         nuclides = [nuc.name for nuc in chain.nuclides
                     if nuc.name in nuclides_with_data]
 
-    # Set up the reaction rate and flux tallies
+    # Set up the reaction rate and flux tallies. When energies are omitted, no
+    # energy filter is needed for the transport calculation. A one-group energy
+    # range is still needed later if flux collapse is requested.
+    collapse_energies = energies
     if energies is None:
-        energy_filter = openmc.EnergyFilter([0.0, 100.0e6])
+        energy_filter = None
+        collapse_energies = [0.0, 100.0e6]
     elif isinstance(energies, str):
         energy_filter = openmc.EnergyFilter.from_group_structure(energies)
     else:
         energy_filter = openmc.EnergyFilter(energies)
-    if isinstance(domains, openmc.MeshBase):
-        domain_filter = openmc.MeshFilter(domains)
+
+    # Build list of domain filters
+    if isinstance(domains, openmc.Filter):
+        domain_filters = [domains]
+    elif isinstance(domains, openmc.MeshBase):
+        domain_filters = [openmc.MeshFilter(domains)]
+    elif isinstance(domains, Sequence) and len(domains) > 0 and \
+            isinstance(domains[0], openmc.Filter):
+        domain_filters = list(domains)
     elif isinstance(domains[0], openmc.Material):
-        domain_filter = openmc.MaterialFilter(domains)
+        domain_filters = [openmc.MaterialFilter(domains)]
     elif isinstance(domains[0], openmc.Cell):
-        domain_filter = openmc.CellFilter(domains)
+        domain_filters = [openmc.CellFilter(domains)]
     elif isinstance(domains[0], openmc.Universe):
-        domain_filter = openmc.UniverseFilter(domains)
+        domain_filters = [openmc.UniverseFilter(domains)]
     else:
         raise ValueError(f"Unsupported domain type: {type(domains[0])}")
 
-    rr_tally = openmc.Tally(name='MicroXS RR')
-    rr_tally.filters = [domain_filter, energy_filter]
-    rr_tally.nuclides = nuclides
-    rr_tally.multiply_density = False
-    rr_tally.scores = reactions
+    # Prepare reaction-rate nuclides/reactions
+    rr_nuclides: list[str] = []
+    rr_reactions: list[str] = []
+    if reaction_rate_mode == 'direct':
+        rr_nuclides = list(nuclides)
+        rr_reactions = list(reactions)
+    elif reaction_rate_mode == 'flux' and reaction_rate_opts:
+        opts = reaction_rate_opts or {}
+        rr_reactions = list(opts.get('reactions', []))
+        if rr_reactions:
+            rr_nuclides = list(opts.get('nuclides', nuclides))
+        else:
+            rr_nuclides = list(opts.get('nuclides', []))
+        # Keep only requested pairs within overall sets
+        if rr_nuclides:
+            rr_nuclides = [n for n in rr_nuclides if n in set(nuclides)]
+        if rr_reactions:
+            rr_reactions = [r for r in rr_reactions if r in set(reactions)]
 
-    flux_tally = openmc.Tally(name='MicroXS flux')
-    flux_tally.filters = [domain_filter, energy_filter]
-    flux_tally.scores = ['flux']
-    model.tallies = openmc.Tallies([rr_tally, flux_tally])
+    # Use 1-group energy filter for RR in flux mode
+    has_rr = bool(rr_nuclides and rr_reactions)
+    if has_rr and reaction_rate_mode == 'flux' and energy_filter is not None:
+        rr_energy_filter = openmc.EnergyFilter(
+            [energy_filter.values[0], energy_filter.values[-1]])
+    else:
+        rr_energy_filter = energy_filter
+
+    # Create one flux tally (and optionally one RR tally) per domain filter.
+    flux_tallies = []
+    rr_tallies = []
+    model.tallies = []
+    for i, domain_filter in enumerate(domain_filters):
+        flux_tally = openmc.Tally(name=f'MicroXS flux {i}')
+        flux_tally.filters = [domain_filter]
+        if energy_filter is not None:
+            flux_tally.filters.append(energy_filter)
+        flux_tally.scores = ['flux']
+        model.tallies.append(flux_tally)
+        flux_tallies.append(flux_tally)
+
+        if has_rr:
+            rr_tally = openmc.Tally(name=f'MicroXS RR {i}')
+            rr_tally.filters = [domain_filter]
+            if rr_energy_filter is not None:
+                rr_tally.filters.append(rr_energy_filter)
+            rr_tally.nuclides = rr_nuclides
+            rr_tally.multiply_density = False
+            rr_tally.scores = rr_reactions
+            model.tallies.append(rr_tally)
+            rr_tallies.append(rr_tally)
 
     if openmc.lib.is_initialized:
         openmc.lib.finalize()
@@ -132,45 +233,107 @@ def get_microxs_and_flux(
             model.export_to_model_xml()
         comm.barrier()
         # Reinitialize with tallies
-        openmc.lib.init(intracomm=comm)
+        output = run_kwargs.get('output', True) if run_kwargs else True
+        openmc.lib.init(intracomm=comm, output=output)
 
-    # create temporary run
     with TemporaryDirectory() as temp_dir:
-        if run_kwargs is None:
-            run_kwargs = {}
-        else:
-            run_kwargs = dict(run_kwargs)
-        run_kwargs.setdefault('cwd', temp_dir)
+        # Indicate to run in temporary directory unless being executed through
+        # openmc.lib, in which case we don't need to specify the cwd
+        run_kwargs = dict(run_kwargs) if run_kwargs else {}
+        if not openmc.lib.is_initialized:
+            run_kwargs.setdefault('cwd', temp_dir)
+
+        # Run transport simulation and synchronize
         statepoint_path = model.run(**run_kwargs)
+        comm.barrier()
 
         if comm.rank == 0:
-            with StatePoint(statepoint_path) as sp:
-                rr_tally = sp.tallies[rr_tally.id]
-                rr_tally._read_results()
-                flux_tally = sp.tallies[flux_tally.id]
-                flux_tally._read_results()
+            # Move the statepoint file if it is being saved to a specific path
+            if path_statepoint is not None:
+                shutil.move(statepoint_path, path_statepoint)
+                statepoint_path = path_statepoint
 
-    rr_tally = comm.bcast(rr_tally)
-    flux_tally = comm.bcast(flux_tally)
-    # Get reaction rates and flux values
-    reaction_rates = rr_tally.get_reshaped_data()  # (domains, groups, nuclides, reactions)
-    flux = flux_tally.get_reshaped_data()  # (domains, groups, 1, 1)
+            # Export the model to path_input if provided
+            if path_input is not None:
+                model.export_to_model_xml(path_input)
 
-    # Make energy groups last dimension
-    reaction_rates = np.moveaxis(reaction_rates, 1, -1)  # (domains, nuclides, reactions, groups)
-    flux = np.moveaxis(flux, 1, -1)  # (domains, 1, 1, groups)
+        # Broadcast updated statepoint path to all ranks
+        statepoint_path = comm.bcast(statepoint_path)
 
-    # Divide RR by flux to get microscopic cross sections
-    xs = np.empty_like(reaction_rates) # (domains, nuclides, reactions, groups)
-    d, _, _, g = np.nonzero(flux)
-    xs[d, ..., g] = reaction_rates[d, ..., g] / flux[d, :, :, g]
+        # Read in tally results (on all ranks)
+        with StatePoint(statepoint_path) as sp:
+            for i in range(len(flux_tallies)):
+                flux_tallies[i] = sp.tallies[flux_tallies[i].id]
+                flux_tallies[i]._read_results()
+                if rr_tallies:
+                    rr_tallies[i] = sp.tallies[rr_tallies[i].id]
+                    rr_tallies[i]._read_results()
+
+    # Concatenate results across all domain filters
+    fluxes = []
+    all_flux_arrays = []
+    for flux_tally in flux_tallies:
+        # Get flux values and make energy groups last dimension
+        flux = flux_tally.get_reshaped_data()
+        if energy_filter is None:
+            flux = flux[..., np.newaxis]  # (domains, 1, 1, groups)
+        else:
+            # (domains, groups, 1, 1) -> (domains, 1, 1, groups)
+            flux = np.moveaxis(flux, 1, -1)
+        all_flux_arrays.append(flux)
+        fluxes.extend(flux.squeeze((1, 2)))
+
+    # If we built reaction-rate tallies, compute microscopic cross sections
+    if rr_tallies:
+        direct_micros = []
+        for flux_arr, rr_tally in zip(all_flux_arrays, rr_tallies):
+            flux = flux_arr
+            # Get reaction rates and make energy groups last dimension
+            reaction_rates = rr_tally.get_reshaped_data()
+            if rr_energy_filter is None:
+                # (domains, nuclides, reactions) ->
+                # (domains, nuclides, reactions, groups)
+                reaction_rates = reaction_rates[..., np.newaxis]
+            else:
+                # (domains, groups, nuclides, reactions) ->
+                # (domains, nuclides, reactions, groups)
+                reaction_rates = np.moveaxis(reaction_rates, 1, -1)
+
+            # If RR is 1-group, sum flux over groups
+            if reaction_rate_mode == "flux":
+                flux = flux.sum(axis=-1, keepdims=True)
+
+            xs = np.zeros_like(reaction_rates)
+            d, _, _, g = np.nonzero(flux)
+            xs[d, ..., g] = reaction_rates[d, ..., g] / flux[d, :, :, g]
+            direct_micros.extend(
+                MicroXS(xs_i, rr_nuclides, rr_reactions) for xs_i in xs)
+
+    if reaction_rate_mode == 'flux':
+        # Compute flux-collapsed microscopic XS
+        flux_micros = [MicroXS.from_multigroup_flux(
+            energies=collapse_energies,
+            multigroup_flux=flux_i,
+            chain_file=chain_file,
+            nuclides=nuclides,
+            reactions=reactions
+        ) for flux_i in fluxes]
+
+        # We need to return one-group fluxes to match the microscopic cross
+        # sections, which are always one-group by virtue of the collapse
+        fluxes = [flux.sum(keepdims=True) for flux in fluxes]
+
+    # Decide which micros to use and merge if needed
+    if reaction_rate_mode == 'flux' and rr_tallies:
+        micros = [m1.merge(m2) for m1, m2 in zip(flux_micros, direct_micros)]
+    elif rr_tallies:
+        micros = direct_micros
+    else:
+        micros = flux_micros
 
     # Reset tallies
     model.tallies = original_tallies
 
-    # Create lists where each item corresponds to one domain
-    fluxes = list(flux.squeeze((1, 2)))
-    micros = [MicroXS(xs_i, nuclides, reactions) for xs_i in xs]
     return fluxes, micros
 
 
@@ -234,17 +397,21 @@ class MicroXS:
         sections available. MicroXS entry will be 0 if the nuclide cross section
         is not found.
 
+        It is recommended to make repeated calls to this method within a context
+        manager using the :class:`openmc.lib.TemporarySession` class to avoid
+        re-initializing OpenMC and loading cross sections each time.
+
         .. versionadded:: 0.15.0
 
         Parameters
         ----------
         energies : iterable of float or str
             Energy group boundaries in [eV] or the name of the group structure
-        multi_group_flux : iterable of float
+        multigroup_flux : iterable of float
             Energy-dependent multigroup flux values
-        chain_file : str, optional
-            Path to the depletion chain XML file that will be used in depletion
-            simulation.  Defaults to ``openmc.config['chain_file']``.
+        chain_file : PathLike or Chain, optional
+            Path to the depletion chain XML file or an instance of
+            openmc.deplete.Chain. Defaults to ``openmc.config['chain_file']``.
         temperature : int, optional
             Temperature for cross section evaluation in [K].
         nuclides : list of str, optional
@@ -275,9 +442,7 @@ class MicroXS:
         if len(multigroup_flux) != len(energies) - 1:
             raise ValueError('Length of flux array should be len(energies)-1')
 
-        chain_file_path = _resolve_chain_file_path(chain_file)
-        chain = Chain.from_xml(chain_file_path)
-
+        chain = _get_chain(chain_file)
         cross_sections = _find_cross_sections(model=None)
         nuclides_with_data = _get_nuclides_with_data(cross_sections)
 
@@ -292,44 +457,28 @@ class MicroXS:
             reactions = chain.reactions
         mts = [REACTION_MT[name] for name in reactions]
 
-        # Normalize multigroup flux
-        multigroup_flux = np.array(multigroup_flux)
-        multigroup_flux /= multigroup_flux.sum()
-
         # Create 3D array for microscopic cross sections
         microxs_arr = np.zeros((len(nuclides), len(mts), 1))
 
-        # Create a material with all nuclides
-        mat_all_nucs = openmc.Material()
-        for nuc in nuclides:
-            if nuc in nuclides_with_data:
-                mat_all_nucs.add_nuclide(nuc, 1.0)
-        mat_all_nucs.set_density("atom/b-cm", 1.0)
+        # If flux is zero, safely return zero cross sections
+        multigroup_flux = np.array(multigroup_flux)
+        if (flux_sum := multigroup_flux.sum()) == 0.0:
+            return cls(microxs_arr, nuclides, reactions)
 
-        # Create simple model containing the above material
-        surf1 = openmc.Sphere(boundary_type="vacuum")
-        surf1_cell = openmc.Cell(fill=mat_all_nucs, region=-surf1)
-        model = openmc.Model()
-        model.geometry = openmc.Geometry([surf1_cell])
-        model.settings = openmc.Settings(
-            particles=1, batches=1, output={'summary': False})
+        # Normalize multigroup flux
+        multigroup_flux /= flux_sum
 
-        with change_directory(tmpdir=True):
-            # Export model within temporary directory
-            model.export_to_model_xml()
-
-            with openmc.lib.run_in_memory(**init_kwargs):
-                # For each nuclide and reaction, compute the flux-averaged
-                # cross section
-                for nuc_index, nuc in enumerate(nuclides):
-                    if nuc not in nuclides_with_data:
-                        continue
-                    lib_nuc = openmc.lib.nuclides[nuc]
-                    for mt_index, mt in enumerate(mts):
-                        xs = lib_nuc.collapse_rate(
-                            mt, temperature, energies, multigroup_flux
-                        )
-                        microxs_arr[nuc_index, mt_index, 0] = xs
+        # Compute microscopic cross sections within a temporary session
+        with openmc.lib.TemporarySession(**init_kwargs):
+            # For each nuclide and reaction, compute the flux-averaged xs
+            for nuc_index, nuc in enumerate(nuclides):
+                if nuc not in nuclides_with_data:
+                    continue
+                lib_nuc = openmc.lib.load_nuclide(nuc)
+                for mt_index, mt in enumerate(mts):
+                    microxs_arr[nuc_index, mt_index, 0] = lib_nuc.collapse_rate(
+                        mt, temperature, energies, multigroup_flux
+                    )
 
         return cls(microxs_arr, nuclides, reactions)
 
@@ -350,8 +499,7 @@ class MicroXS:
         MicroXS
 
         """
-        if 'float_precision' not in kwargs:
-            kwargs['float_precision'] = 'round_trip'
+        kwargs.setdefault('float_precision', 'round_trip')
 
         df = pd.read_csv(csv_file, **kwargs)
         df.set_index(['nuclides', 'reactions', 'groups'], inplace=True)
@@ -386,3 +534,169 @@ class MicroXS:
         )
         df = pd.DataFrame({'xs': self.data.flatten()}, index=multi_index)
         df.to_csv(*args, **kwargs)
+
+    def to_hdf5(self, group_or_filename: h5py.Group | PathLike, **kwargs):
+        """Export microscopic cross section data to HDF5 format
+
+        Parameters
+        ----------
+        group_or_filename : h5py.Group or path-like
+            HDF5 group or filename to write to
+        kwargs : dict, optional
+            Keyword arguments to pass to :meth:`h5py.Group.create_dataset`.
+            Defaults to {'compression': 'lzf'}.
+
+        """
+        kwargs.setdefault('compression', 'lzf')
+
+        with h5py_file_or_group(group_or_filename, 'w') as group:
+            # Store cross section data as 3D dataset
+            group.create_dataset('data', data=self.data, **kwargs)
+
+            # Store metadata as datasets using string encoding
+            group.create_dataset('nuclides', data=np.array(self.nuclides, dtype='S'))
+            group.create_dataset('reactions', data=np.array(self.reactions, dtype='S'))
+
+    @classmethod
+    def from_hdf5(cls, group_or_filename: h5py.Group | PathLike) -> Self:
+        """Load data from an HDF5 file
+
+        Parameters
+        ----------
+        group_or_filename : h5py.Group or str or PathLike
+            HDF5 group or path to HDF5 file. If given as an h5py.Group, the
+            data is read from that group. If given as a string, it is assumed
+            to be the filename for the HDF5 file.
+
+        Returns
+        -------
+        MicroXS
+        """
+
+        with h5py_file_or_group(group_or_filename, 'r') as group:
+            # Read data from HDF5 group
+            data = group['data'][:]
+            nuclides = [nuc.decode('utf-8') for nuc in group['nuclides'][:]]
+            reactions = [rxn.decode('utf-8') for rxn in group['reactions'][:]]
+
+        return cls(data, nuclides, reactions)
+
+    def merge(self, other: Self, prefer: str = 'other') -> Self:
+        """Merge two MicroXS objects by taking the union of nuclides/reactions.
+
+        If the two objects contain overlapping nuclide/reaction entries, values
+        from `other` will overwrite values from `self` when `prefer='other'`.
+        When `prefer='self'`, values from `self` are retained for overlapping
+        entries, and values from `other` are used only for non-overlapping
+        entries.
+
+        Parameters
+        ----------
+        other : MicroXS
+            Other MicroXS instance to merge with this one.
+        prefer : {"other", "self"}
+            Which instance's data should take precedence on overlap.
+
+        Returns
+        -------
+        MicroXS
+            New instance containing the merged data.
+        """
+        check_value('prefer', prefer, {'other', 'self'})
+
+        # Require same number of energy groups
+        if self.data.shape[2] != other.data.shape[2]:
+            raise ValueError(
+                'Cannot merge MicroXS with different number of energy groups: '
+                f"{self.data.shape[2]} vs {other.data.shape[2]}. Ensure that "
+                'both were generated with consistent group structures and '
+                'treatments (e.g., both multigroup or both collapsed).'
+            )
+
+        # Build unified axes preserving order (self first, then other's new)
+        new_nuclides = list(self.nuclides)
+        for nuc in other.nuclides:
+            if nuc not in self._index_nuc:
+                new_nuclides.append(nuc)
+        new_reactions = list(self.reactions)
+        for rx in other.reactions:
+            if rx not in self._index_rx:
+                new_reactions.append(rx)
+
+        # Allocate and fill from self (self's nuclides/reactions map to the
+        # first indices of new_nuclides/new_reactions by construction)
+        groups = self.data.shape[2]
+        data = np.zeros((len(new_nuclides), len(new_reactions), groups))
+        idx_n = {nuc: i for i, nuc in enumerate(new_nuclides)}
+        idx_r = {rx: i for i, rx in enumerate(new_reactions)}
+
+        n_self = len(self.nuclides)
+        r_self = len(self.reactions)
+        data[:n_self, :r_self] = self.data
+
+        # Build destination index arrays for other's nuclides/reactions
+        dst_n = np.array([idx_n[nuc] for nuc in other.nuclides])
+        dst_r = np.array([idx_r[rx] for rx in other.reactions])
+
+        # Copy from other, respecting precedence
+        if prefer == 'other':
+            data[np.ix_(dst_n, dst_r)] = other.data
+        else:
+            # Copy only entries where nuc or rx is absent from self
+            nuc_is_new = np.array(
+                [nuc not in self._index_nuc for nuc in other.nuclides])
+            rx_is_new = np.array(
+                [rx not in self._index_rx for rx in other.reactions])
+            mask = nuc_is_new[:, np.newaxis] | rx_is_new[np.newaxis, :]
+            src_i, src_j = np.where(mask)
+            if src_i.size:
+                data[dst_n[src_i], dst_r[src_j]] = other.data[src_i, src_j]
+
+        return MicroXS(data, new_nuclides, new_reactions)
+
+
+def write_microxs_hdf5(
+    micros: Sequence[MicroXS],
+    filename: PathLike,
+    names: Sequence[str] | None = None,
+    **kwargs
+):
+    """Write multiple MicroXS objects to an HDF5 file
+
+    Parameters
+    ----------
+    micros : list of MicroXS
+        List of MicroXS objects
+    filename : PathLike
+        Output HDF5 filename
+    names : list of str, optional
+        Names for each MicroXS object. If None, uses 'domain_0', 'domain_1',
+        etc.
+    **kwargs
+        Additional keyword arguments passed to :meth:`h5py.Group.create_dataset`
+    """
+    if names is None:
+        names = [f'domain_{i}' for i in range(len(micros))]
+
+    # Open file once and write all domains using group interface
+    with h5py.File(filename, 'w') as f:
+        for microxs, name in zip(micros, names):
+            group = f.create_group(name)
+            microxs.to_hdf5(group, **kwargs)
+
+
+def read_microxs_hdf5(filename: PathLike) -> dict[str, MicroXS]:
+    """Read multiple MicroXS objects from an HDF5 file
+
+    Parameters
+    ----------
+    filename : path-like
+        HDF5 filename
+
+    Returns
+    -------
+    dict
+        Dictionary mapping domain names to MicroXS objects
+    """
+    with h5py.File(filename, 'r') as f:
+        return {name: MicroXS.from_hdf5(group) for name, group in f.items()}
diff --git a/openmc/deplete/nuclide.py b/openmc/deplete/nuclide.py
index 60e3e5317..958814834 100644
--- a/openmc/deplete/nuclide.py
+++ b/openmc/deplete/nuclide.py
@@ -14,6 +14,7 @@ import numpy as np
 
 from openmc.checkvalue import check_type
 from openmc.stats import Univariate
+from .._xml import get_elem_list, get_text
 
 __all__ = [
     "DecayTuple", "ReactionTuple", "Nuclide", "FissionYield",
@@ -225,38 +226,39 @@ class Nuclide:
 
         """
         nuc = cls()
-        nuc.name = element.get('name')
+        nuc.name = get_text(element, "name")
 
         # Check for half-life
-        if 'half_life' in element.attrib:
-            nuc.half_life = float(element.get('half_life'))
-            nuc.decay_energy = float(element.get('decay_energy', '0'))
+        half_life = get_text(element, "half_life")
+        if half_life is not None:
+            nuc.half_life = float(half_life)
+            nuc.decay_energy = float(get_text(element, "decay_energy", 0.0))
 
         # Check for decay paths
         for decay_elem in element.iter('decay'):
-            d_type = decay_elem.get('type')
-            target = decay_elem.get('target')
+            d_type = get_text(decay_elem, "type")
+            target = get_text(decay_elem, "target")
             if target is not None and target.lower() == "nothing":
                 target = None
-            branching_ratio = float(decay_elem.get('branching_ratio'))
+            branching_ratio = float(get_text(decay_elem, "branching_ratio"))
             nuc.decay_modes.append(DecayTuple(d_type, target, branching_ratio))
 
         # Check for sources
         for src_elem in element.iter('source'):
-            particle = src_elem.get('particle')
+            particle = get_text(src_elem, "particle")
             distribution = Univariate.from_xml_element(src_elem)
             nuc.sources[particle] = distribution
 
         # Check for reaction paths
         for reaction_elem in element.iter('reaction'):
-            r_type = reaction_elem.get('type')
-            Q = float(reaction_elem.get('Q', '0'))
-            branching_ratio = float(reaction_elem.get('branching_ratio', '1'))
+            r_type = get_text(reaction_elem, "type")
+            Q = float(get_text(reaction_elem, "Q", 0.0))
+            branching_ratio = float(get_text(reaction_elem, "branching_ratio", 1.0))
 
             # If the type is not fission, get target and Q value, otherwise
             # just set null values
             if r_type != 'fission':
-                target = reaction_elem.get('target')
+                target = get_text(reaction_elem, "target")
                 if target is not None and target.lower() == "nothing":
                     target = None
             else:
@@ -271,7 +273,7 @@ class Nuclide:
         fpy_elem = element.find('neutron_fission_yields')
         if fpy_elem is not None:
             # Check for use of FPY from other nuclide
-            parent = fpy_elem.get('parent')
+            parent = get_text(fpy_elem, "parent")
             if parent is not None:
                 assert root is not None
                 fpy_elem = root.find(
@@ -529,9 +531,9 @@ class FissionYieldDistribution(Mapping):
         """
         all_yields = {}
         for yield_elem in element.iter("fission_yields"):
-            energy = float(yield_elem.get("energy"))
-            products = yield_elem.find("products").text.split()
-            yields = map(float, yield_elem.find("data").text.split())
+            energy = float(get_text(yield_elem, "energy"))
+            products = get_elem_list(yield_elem, "products", str) or []
+            yields = get_elem_list(yield_elem, "data", float) or []
             # Get a map of products to their corresponding yield
             all_yields[energy] = dict(zip(products, yields))
 
diff --git a/openmc/deplete/openmc_operator.py b/openmc/deplete/openmc_operator.py
index 64cb0a7e5..7928e89ee 100644
--- a/openmc/deplete/openmc_operator.py
+++ b/openmc/deplete/openmc_operator.py
@@ -11,7 +11,7 @@ from warnings import warn
 import numpy as np
 
 import openmc
-from openmc.checkvalue import check_value
+from openmc.checkvalue import check_value, check_type, check_greater_than
 from openmc.exceptions import DataError
 from openmc.mpi import comm
 from .abc import TransportOperator, OperatorResult
@@ -36,9 +36,9 @@ class OpenMCOperator(TransportOperator):
     cross_sections : str or list of MicroXS
         Path to continuous energy cross section library, or list of objects
         containing cross sections.
-    chain_file : str, optional
-        Path to the depletion chain XML file. Defaults to
-        openmc.config['chain_file'].
+    chain_file : PathLike or Chain, optional
+        Path to the depletion chain XML file or instance of openmc.deplete.Chain.
+        Defaults to ``openmc.config['chain_file']``.
     prev_results : Results, optional
         Results from a previous depletion calculation. If this argument is
         specified, the depletion calculation will start from the latest state
@@ -49,13 +49,9 @@ class OpenMCOperator(TransportOperator):
         Dictionary of nuclides and their fission Q values [eV].
     helper_kwargs : dict
         Keyword arguments for helper classes
-    reduce_chain : bool, optional
-        If True, use :meth:`openmc.deplete.Chain.reduce()` to reduce the
-        depletion chain up to ``reduce_chain_level``.
     reduce_chain_level : int, optional
-        Depth of the search when reducing the depletion chain. Only used
-        if ``reduce_chain`` evaluates to true. The default value of
-        ``None`` implies no limit on the depth.
+        Depth of the search when reducing the depletion chain. The default
+        value of ``None`` implies no limit on the depth.
 
     diff_volume_method : str
         Specifies how the volumes of the new materials should be found. Default
@@ -107,7 +103,6 @@ class OpenMCOperator(TransportOperator):
             diff_volume_method='divide equally',
             fission_q=None,
             helper_kwargs=None,
-            reduce_chain=False,
             reduce_chain_level=None):
 
         # If chain file was not specified, try to get it from global config
@@ -126,10 +121,13 @@ class OpenMCOperator(TransportOperator):
 
         check_value('diff volume method', diff_volume_method,
                     {'divide equally', 'match cell'})
+        if reduce_chain_level:
+            check_type('reduce_chain_level', reduce_chain_level, int)
+            check_greater_than('reduce_chain_level', reduce_chain_level, 0)
         self.diff_volume_method = diff_volume_method
 
         # Reduce the chain to only those nuclides present
-        if reduce_chain:
+        if reduce_chain_level is not None:
             init_nuclides = set()
             for material in self.materials:
                 if not material.depletable:
@@ -213,7 +211,7 @@ class OpenMCOperator(TransportOperator):
                            "section data.")
                     warn(msg)
             if mat.depletable:
-                burnable_mats.add(str(mat.id))
+                burnable_mats.add((str(mat.id), mat.name))
                 if mat.volume is None:
                     if mat.name is None:
                         msg = ("Volume not specified for depletable material "
@@ -231,9 +229,12 @@ class OpenMCOperator(TransportOperator):
                 "No depletable materials were found in the model.")
 
         # Sort the sets
-        burnable_mats = sorted(burnable_mats, key=int)
+        burnable_mats = sorted(burnable_mats, key=lambda x: int(x[0]))
         model_nuclides = sorted(model_nuclides)
 
+        # Store material names for later use
+        burnable_mats, self.name_list = zip(*burnable_mats)
+
         # Construct a global nuclide dictionary, burned first
         nuclides = list(self.chain.nuclide_dict)
         for nuc in model_nuclides:
@@ -543,6 +544,8 @@ class OpenMCOperator(TransportOperator):
             A list of all material IDs to be burned.  Used for sorting the simulation.
         full_burn_list : list
             List of all burnable material IDs
+        name_list : list of str
+            Material names corresponding to materials in burn_list
 
         """
         nuc_list = self.number.burnable_nuclides
@@ -556,4 +559,4 @@ class OpenMCOperator(TransportOperator):
         volume_list = comm.allgather(volume)
         volume = {k: v for d in volume_list for k, v in d.items()}
 
-        return volume, nuc_list, burn_list, self.burnable_mats
+        return volume, nuc_list, burn_list, self.burnable_mats, self.name_list
diff --git a/openmc/deplete/pool.py b/openmc/deplete/pool.py
index 27ecaa4dd..19ad0ada5 100644
--- a/openmc/deplete/pool.py
+++ b/openmc/deplete/pool.py
@@ -5,10 +5,11 @@ Provided to avoid some circular imports
 from itertools import repeat, starmap
 from multiprocessing import Pool
 
-from scipy.sparse import bmat
 import numpy as np
+from scipy.sparse import hstack
 
 from openmc.mpi import comm
+from .._sparse_compat import block_array
 
 # Configurable switch that enables / disables the use of
 # multiprocessing routines during depletion
@@ -40,7 +41,8 @@ def _distribute(items):
             return items[j:j + chunk_size]
         j += chunk_size
 
-def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
+def deplete(func, chain, n, rates, dt, current_timestep=None, matrix_func=None,
+            transfer_rates=None, external_source_rates=None, substeps=1,
             *matrix_args):
     """Deplete materials using given reaction rates for a specified time
 
@@ -48,7 +50,7 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
     ----------
     func : callable
         Function to use to get new compositions. Expected to have the signature
-        ``func(A, n0, t) -> n1``
+        ``func(A, n0, t, substeps=1) -> n1``.
     chain : openmc.deplete.Chain
         Depletion chain
     n : list of numpy.ndarray
@@ -58,15 +60,23 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
         Reaction rates (from transport operator)
     dt : float
         Time in [s] to deplete for
+    current_timestep : int
+        Current timestep index
     maxtrix_func : callable, optional
         Function to form the depletion matrix after calling ``matrix_func(chain,
         rates, fission_yields)``, where ``fission_yields = {parent: {product:
         yield_frac}}`` Expected to return the depletion matrix required by
         ``func``
     transfer_rates : openmc.deplete.TransferRates, Optional
-        Object to perform continuous reprocessing.
+        Transfer rates for continuous removal/feed.
 
         .. versionadded:: 0.14.0
+    external_source_rates : openmc.deplete.ExternalSourceRates, Optional
+        External source rates for continuous removal/feed.
+
+        .. versionadded:: 0.15.3
+    substeps : int, optional
+        Number of substeps to pass to solvers that support substepping.
     matrix_args: Any, optional
         Additional arguments passed to matrix_func
 
@@ -93,15 +103,24 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
         matrices = map(matrix_func, repeat(chain), rates, fission_yields,
                        *matrix_args)
 
-    if transfer_rates is not None:
+    if (transfer_rates is not None and
+        current_timestep in transfer_rates.external_timesteps):
         # Calculate transfer rate terms as diagonal matrices
         transfers = map(chain.form_rr_term, repeat(transfer_rates),
-                        transfer_rates.local_mats)
+                        repeat(current_timestep), transfer_rates.local_mats)
+
         # Subtract transfer rate terms from Bateman matrices
         matrices = [matrix - transfer for (matrix, transfer) in zip(matrices,
                                                                     transfers)]
 
-        if len(transfer_rates.index_transfer) > 0:
+        if transfer_rates.redox:
+            for mat_idx, mat_id in enumerate(transfer_rates.local_mats):
+                if mat_id in transfer_rates.redox:
+                    matrices[mat_idx] = chain.add_redox_term(matrices[mat_idx],
+                                                transfer_rates.redox[mat_id][0],
+                                                transfer_rates.redox[mat_id][1])
+
+        if current_timestep in transfer_rates.index_transfer:
             # Gather all on comm.rank 0
             matrices = comm.gather(matrices)
             n = comm.gather(n)
@@ -112,10 +131,18 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
                 n = [n_elm for n_mat in n for  n_elm in n_mat]
 
                 # Calculate transfer rate terms as diagonal matrices
-                transfer_pair = {
-                    mat_pair: chain.form_rr_term(transfer_rates, mat_pair)
-                    for mat_pair in transfer_rates.index_transfer
-                }
+                transfer_pair = {}
+                for mat_pair in transfer_rates.index_transfer[current_timestep]:
+                    transfer_matrix = chain.form_rr_term(transfer_rates,
+                                                         current_timestep,
+                                                         mat_pair)
+
+                    # check if destination material has a redox control
+                    if mat_pair[0] in transfer_rates.redox:
+                        transfer_matrix = chain.add_redox_term(transfer_matrix,
+                                          transfer_rates.redox[mat_pair[0]][0],
+                                          transfer_rates.redox[mat_pair[0]][1])
+                    transfer_pair[mat_pair] = transfer_matrix
 
                 # Combine all matrices together in a single matrix of matrices
                 # to be solved in one go
@@ -129,18 +156,18 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
                         if row == col:
                             # Fill the diagonals with the Bateman matrices
                             cols.append(matrices[row])
-                        elif mat_pair in transfer_rates.index_transfer:
+                        elif mat_pair in transfer_rates.index_transfer[current_timestep]:
                             # Fill the off-diagonals with the transfer pair matrices
                             cols.append(transfer_pair[mat_pair])
                         else:
                             cols.append(None)
 
                     rows.append(cols)
-                matrix = bmat(rows)
+                matrix = block_array(rows)
 
                 # Concatenate vectors of nuclides in one
                 n_multi = np.concatenate(n)
-                n_result = func(matrix, n_multi, dt)
+                n_result = func(matrix, n_multi, dt, substeps)
 
                 # Split back the nuclide vector result into the original form
                 n_result = np.split(n_result, np.cumsum([len(i) for i in n])[:-1])
@@ -155,7 +182,26 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
 
             return n_result
 
-    inputs = zip(matrices, n, repeat(dt))
+    if (external_source_rates is not None and
+        current_timestep in external_source_rates.external_timesteps):
+        # Calculate external source term vectors
+        sources = map(chain.form_ext_source_term, repeat(external_source_rates),
+                      repeat(current_timestep), external_source_rates.local_mats)
+
+        # stack vector column at the end of the matrix
+        matrices = [
+            hstack([matrix, source])
+            for matrix, source in zip(matrices, sources)
+        ]
+
+        # Add a last row of zeroes to the matrices and append 1 to the last row
+        # of the nuclide vectors
+        for i, matrix in enumerate(matrices):
+            if not np.equal(*matrix.shape):
+                matrix.resize(matrix.shape[1], matrix.shape[1])
+                n[i] = np.append(n[i], 1.0)
+
+    inputs = zip(matrices, n, repeat(dt), repeat(substeps))
 
     if USE_MULTIPROCESSING:
         with Pool(NUM_PROCESSES) as pool:
@@ -163,4 +209,10 @@ def deplete(func, chain, n, rates, dt, matrix_func=None, transfer_rates=None,
     else:
         n_result = list(starmap(func, inputs))
 
+    # Remove extra value at the end of the nuclide vectors
+    if (external_source_rates is not None and
+        current_timestep in external_source_rates.external_timesteps):
+        external_source_rates.reformat_nuclide_vectors(n)
+        external_source_rates.reformat_nuclide_vectors(n_result)
+
     return n_result
diff --git a/openmc/deplete/r2s.py b/openmc/deplete/r2s.py
new file mode 100644
index 000000000..6c19531e7
--- /dev/null
+++ b/openmc/deplete/r2s.py
@@ -0,0 +1,845 @@
+from __future__ import annotations
+from collections.abc import Sequence
+from contextlib import nullcontext
+import copy
+from datetime import datetime
+import json
+from numbers import Integral
+from pathlib import Path
+
+import numpy as np
+import openmc
+from . import IndependentOperator, PredictorIntegrator
+from .chain import Chain
+from .microxs import get_microxs_and_flux, write_microxs_hdf5, read_microxs_hdf5
+from .results import Results
+from ..checkvalue import PathLike
+from ..mpi import comm
+from openmc.lib import TemporarySession
+
+
+def get_activation_materials(
+    model: openmc.Model,
+    mmv_list: list[openmc.MeshMaterialVolumes]
+) -> openmc.Materials:
+    """Get a list of activation materials for each mesh element/material.
+
+    When performing a mesh-based R2S calculation, a unique material is needed
+    for each activation region, which is a combination of a mesh element and a
+    material within that mesh element. This function generates a list of such
+    materials, each with a unique name and volume corresponding to the mesh
+    element and material.
+
+    Parameters
+    ----------
+    model : openmc.Model
+        The full model containing the geometry and materials.
+    mmv_list : list of openmc.MeshMaterialVolumes
+        List of mesh material volumes objects, one per mesh, containing the
+        materials and their volumes for each mesh element.
+
+    Returns
+    -------
+    openmc.Materials
+        A list of materials, each corresponding to a unique mesh element and
+        material combination across all meshes.
+
+    """
+    # Get all materials in the model
+    material_dict = model._get_all_materials()
+
+    # Create a new activation material for each element-material combination
+    # across all meshes
+    materials = openmc.Materials()
+    for mesh_idx, mmv in enumerate(mmv_list):
+        mat_ids = mmv._materials[mmv._materials > -1]
+        volumes = mmv._volumes[mmv._materials > -1]
+        elems, _ = np.where(mmv._materials > -1)
+
+        for elem, mat_id, vol in zip(elems, mat_ids, volumes):
+            mat = material_dict[mat_id]
+            new_mat = mat.clone()
+            new_mat.depletable = True
+            new_mat.name = f'Mesh {mesh_idx}, Element {elem}, Material {mat_id}'
+            new_mat.volume = vol
+            materials.append(new_mat)
+
+    return materials
+
+
+class R2SManager:
+    """Manager for Rigorous 2-Step (R2S) method calculations.
+
+    This class is responsible for managing the materials and sources needed for
+    mesh-based or cell-based R2S calculations. It provides methods to get
+    activation materials and decay photon sources based on the mesh/cells and
+    materials in the OpenMC model. Multiple meshes can be specified as domains,
+    in which case each element--material combination of each mesh is treated as
+    an activation region (meshes are assumed to be non-overlapping).
+
+    This class supports the use of a different models for the neutron and photon
+    transport calculation. However, for cell-based calculations, it assumes that
+    the only changes in the model are material assignments. For mesh-based
+    calculations, it checks material assignments in the photon model and any
+    element--material combinations that don't appear in the photon model are
+    skipped.
+
+    Parameters
+    ----------
+    neutron_model : openmc.Model
+        The OpenMC model to use for neutron transport.
+    domains : openmc.MeshBase or Sequence[openmc.MeshBase] or Sequence[openmc.Cell]
+        The mesh(es) or a sequence of cells that represent the spatial units
+        over which the R2S calculation will be performed. When a single
+        :class:`~openmc.MeshBase` or a sequence of meshes is given, each
+        element--material combination across all meshes is treated as an
+        activation region.
+    photon_model : openmc.Model, optional
+        The OpenMC model to use for photon transport calculations. If None, a
+        shallow copy of the neutron_model will be created and used.
+
+    Attributes
+    ----------
+    domains : list of openmc.MeshBase or Sequence[openmc.Cell]
+        The meshes or a sequence of cells that represent the spatial units over
+        which the R2S calculation will be performed.
+    neutron_model : openmc.Model
+        The OpenMC model used for neutron transport.
+    photon_model : openmc.Model
+        The OpenMC model used for photon transport calculations.
+    method : {'mesh-based', 'cell-based'}
+        Indicates whether the R2S calculation uses mesh elements ('mesh-based')
+        as the spatial discretization or a list of cells ('cell-based').
+    results : dict
+        A dictionary that stores results from the R2S calculation.
+
+    """
+    def __init__(
+        self,
+        neutron_model: openmc.Model,
+        domains: openmc.MeshBase | Sequence[openmc.MeshBase] | Sequence[openmc.Cell],
+        photon_model: openmc.Model | None = None,
+    ):
+        self.neutron_model = neutron_model
+        if photon_model is None:
+            # Create a shallow copy of the neutron model for photon transport
+            self.photon_model = openmc.Model(
+                geometry=copy.copy(neutron_model.geometry),
+                materials=copy.copy(neutron_model.materials),
+                settings=copy.copy(neutron_model.settings),
+                tallies=copy.copy(neutron_model.tallies),
+                plots=copy.copy(neutron_model.plots),
+            )
+        else:
+            self.photon_model = photon_model
+        if isinstance(domains, openmc.MeshBase):
+            self.method = 'mesh-based'
+            self.domains = [domains]
+        elif isinstance(domains, Sequence) and len(domains) > 0 and \
+                isinstance(domains[0], openmc.MeshBase):
+            self.method = 'mesh-based'
+            self.domains = list(domains)
+        else:
+            self.method = 'cell-based'
+            self.domains = list(domains)
+        self.results = {}
+
+    def run(
+        self,
+        timesteps: Sequence[float] | Sequence[tuple[float, str]],
+        source_rates: float | Sequence[float],
+        timestep_units: str = 's',
+        photon_time_indices: Sequence[int] | None = None,
+        output_dir: PathLike | None = None,
+        bounding_boxes: dict[int, openmc.BoundingBox] | None = None,
+        chain_file: PathLike | Chain | None = None,
+        micro_kwargs: dict | None = None,
+        mat_vol_kwargs: dict | None = None,
+        run_kwargs: dict | None = None,
+        operator_kwargs: dict | None = None,
+        by_parent_nuclide: bool = False,
+    ):
+        """Run the R2S calculation.
+
+        Parameters
+        ----------
+        timesteps : Sequence[float] or Sequence[tuple[float, str]]
+            Sequence of timesteps. Note that values are not cumulative. The
+            units are specified by the `timestep_units` argument when
+            `timesteps` is an iterable of float. Alternatively, units can be
+            specified for each step by passing an iterable of (value, unit)
+            tuples.
+        source_rates : float or Sequence[float]
+            Source rate in [neutron/sec] for each interval in `timesteps`.
+        timestep_units : {'s', 'min', 'h', 'd', 'a'}, optional
+            Units for values specified in the `timesteps` argument when passing
+            float values. 's' means seconds, 'min' means minutes, 'h' means
+            hours, 'd' means days, and 'a' means years (Julian).
+        photon_time_indices : Sequence[int], optional
+            Sequence of time indices at which photon transport should be run;
+            represented as indices into the array of times formed by the
+            timesteps. For example, if two timesteps are specified, the array of
+            times would contain three entries, and [2] would indicate computing
+            photon results at the last time. A value of None indicates to run
+            photon transport at each time that has a decay photon source.
+        output_dir : PathLike, optional
+            Path to directory where R2S calculation outputs will be saved. If
+            not provided, a timestamped directory 'r2s_YYYY-MM-DDTHH-MM-SS' is
+            created. Subdirectories will be created for the neutron transport,
+            activation, and photon transport steps.
+        bounding_boxes : dict[int, openmc.BoundingBox], optional
+            Dictionary mapping cell IDs to bounding boxes used for spatial
+            source sampling in cell-based R2S calculations. Required if method
+            is 'cell-based'.
+        chain_file : PathLike or openmc.deplete.Chain, optional
+            Path to the depletion chain XML file or depletion chain object to
+            use during activation. If not provided, the default configured
+            chain file will be used.
+        micro_kwargs : dict, optional
+            Additional keyword arguments passed to
+            :func:`openmc.deplete.get_microxs_and_flux` during the neutron
+            transport step.
+        mat_vol_kwargs : dict, optional
+            Additional keyword arguments passed to
+            :meth:`openmc.MeshBase.material_volumes`.
+        run_kwargs : dict, optional
+            Additional keyword arguments passed to :meth:`openmc.Model.run`
+            during the neutron and photon transport step. By default, output is
+            disabled.
+        operator_kwargs : dict, optional
+            Additional keyword arguments passed to
+            :class:`openmc.deplete.IndependentOperator`.
+        by_parent_nuclide : bool, optional
+            Whether to score photon tallies separately for each parent
+            radionuclide. A :class:`~openmc.ParentNuclideFilter` is added to
+            tallies that do not already contain one, with bins determined from
+            the prepared decay photon sources.
+
+        Returns
+        -------
+        Path
+            Path to the output directory containing all calculation results
+        """
+
+        if output_dir is None:
+            # Create timestamped output directory and broadcast to all ranks for
+            # consistency (different ranks may have slightly different times)
+            stamp = datetime.now().strftime('%Y-%m-%dT%H-%M-%S')
+            output_dir = Path(comm.bcast(f'r2s_{stamp}'))
+        else:
+            output_dir = Path(output_dir)
+
+        # Set run_kwargs for the neutron transport step
+        if micro_kwargs is None:
+            micro_kwargs = {}
+        if run_kwargs is None:
+            run_kwargs = {}
+        if operator_kwargs is None:
+            operator_kwargs = {}
+        run_kwargs.setdefault('output', False)
+        micro_kwargs.setdefault('run_kwargs', run_kwargs)
+
+        # DecaySpectrum distributions are resolved in the C++ solver using
+        # OPENMC_CHAIN_FILE. If a Chain object was passed, write an XML
+        # representation alongside the R2S outputs.
+        if isinstance(chain_file, Chain):
+            output_dir.mkdir(parents=True, exist_ok=True)
+            chain_path = output_dir / 'chain.xml'
+            if comm.rank == 0:
+                chain_file.export_to_xml(chain_path)
+            comm.barrier()
+        else:
+            chain_path = chain_file
+
+        chain_context = (
+            openmc.config.patch('chain_file', chain_path)
+            if chain_path is not None else nullcontext()
+        )
+        with chain_context:
+            self.step1_neutron_transport(
+                output_dir / 'neutron_transport', mat_vol_kwargs, micro_kwargs
+            )
+            self.step2_activation(
+                timesteps, source_rates, timestep_units,
+                output_dir / 'activation', operator_kwargs=operator_kwargs
+            )
+            self.step3_photon_source(
+                photon_time_indices, bounding_boxes, output_dir / 'photon_transport',
+                mat_vol_kwargs=mat_vol_kwargs,
+            )
+            self.step4_photon_transport(
+                output_dir / 'photon_transport', run_kwargs=run_kwargs,
+                by_parent_nuclide=by_parent_nuclide,
+            )
+
+        return output_dir
+
+    def step1_neutron_transport(
+        self,
+        output_dir: PathLike = "neutron_transport",
+        mat_vol_kwargs: dict | None = None,
+        micro_kwargs: dict | None = None
+    ):
+        """Run the neutron transport step.
+
+        This step computes the material volume fractions on each mesh, creates
+        mesh-material filters, and retrieves the fluxes and microscopic cross
+        sections for each mesh/material combination via a single transport
+        solve. This step will populate the 'fluxes' and 'micros' keys in the
+        results dictionary. For a mesh-based calculation, it will also populate
+        the 'mesh_material_volumes' key (a list of
+        :class:`~openmc.MeshMaterialVolumes`, one per mesh).
+
+        Parameters
+        ----------
+        output_dir : PathLike, optional
+            The directory where the results will be saved.
+        mat_vol_kwargs : dict, optional
+            Additional keyword arguments based to
+            :meth:`openmc.MeshBase.material_volumes`.
+        micro_kwargs : dict, optional
+            Additional keyword arguments passed to
+            :func:`openmc.deplete.get_microxs_and_flux`.
+
+        """
+
+        output_dir = Path(output_dir).resolve()
+        output_dir.mkdir(parents=True, exist_ok=True)
+
+        if self.method == 'mesh-based':
+            # Compute material volume fractions on each mesh
+            if mat_vol_kwargs is None:
+                mat_vol_kwargs = {}
+            mat_vol_kwargs.setdefault('bounding_boxes', True)
+
+            mmv_list = []
+            domain_filters = []
+            for i, mesh in enumerate(self.domains):
+                mmv = comm.bcast(
+                    mesh.material_volumes(self.neutron_model, **mat_vol_kwargs))
+                mmv_list.append(mmv)
+
+                # Save results to file
+                if comm.rank == 0:
+                    mmv.save(output_dir / f'mesh_material_volumes_{i}.npz')
+
+                # Create mesh-material filter for this mesh
+                domain_filters.append(
+                    openmc.MeshMaterialFilter.from_volumes(mesh, mmv))
+
+            self.results['mesh_material_volumes'] = mmv_list
+            domains = domain_filters
+        else:
+            domains: Sequence[openmc.Cell] = self.domains
+
+            # Check to make sure that each cell is filled with a material and
+            # that the volume has been set
+
+            # TODO: If volumes are not set, run volume calculation for cells
+            for cell in domains:
+                if cell.fill is None:
+                    raise ValueError(
+                        f"Cell {cell.id} is not filled with a materials. "
+                        "Please set the fill material for each cell before "
+                        "running the R2S calculation."
+                    )
+                if cell.volume is None:
+                    raise ValueError(
+                        f"Cell {cell.id} does not have a volume set. "
+                        "Please set the volume for each cell before running "
+                        "the R2S calculation."
+                    )
+
+        # Set default keyword arguments for microxs and flux calculation
+        if micro_kwargs is None:
+            micro_kwargs = {}
+        micro_kwargs.setdefault('path_statepoint', output_dir / 'statepoint.h5')
+        micro_kwargs.setdefault('path_input', output_dir / 'model.xml')
+
+        # Run neutron transport and get fluxes and micros. Run via openmc.lib to
+        # maintain a consistent parallelism strategy with the activation step.
+        with TemporarySession(output=False):
+            self.results['fluxes'], self.results['micros'] = get_microxs_and_flux(
+                self.neutron_model, domains, **micro_kwargs)
+
+        # Save flux and micros to file
+        if comm.rank == 0:
+            np.save(output_dir / 'fluxes.npy', self.results['fluxes'])
+            write_microxs_hdf5(self.results['micros'], output_dir / 'micros.h5')
+
+    def step2_activation(
+        self,
+        timesteps: Sequence[float] | Sequence[tuple[float, str]],
+        source_rates: float | Sequence[float],
+        timestep_units: str = 's',
+        output_dir: PathLike = 'activation',
+        operator_kwargs: dict | None = None,
+    ):
+        """Run the activation step.
+
+        This step creates a unique copy of each activation material based on the
+        mesh elements or cells, then solves the depletion equations for each
+        material using the fluxes and microscopic cross sections obtained in the
+        neutron transport step. This step will populate the 'depletion_results'
+        and 'activation_materials' keys in the results dictionary.
+
+        Parameters
+        ----------
+        timesteps : Sequence[float] or Sequence[tuple[float, str]]
+            Sequence of timesteps. Note that values are not cumulative. The
+            units are specified by the `timestep_units` argument when
+            `timesteps` is an iterable of float. Alternatively, units can be
+            specified for each step by passing an iterable of (value, unit)
+            tuples.
+        source_rates : float | Sequence[float]
+            Source rate in [neutron/sec] for each interval in `timesteps`.
+        timestep_units : {'s', 'min', 'h', 'd', 'a'}, optional
+            Units for values specified in the `timesteps` argument when passing
+            float values. 's' means seconds, 'min' means minutes, 'h' means
+            hours, 'd' means days, and 'a' means years (Julian).
+        output_dir : PathLike, optional
+            Path to directory where activation calculation outputs will be
+            saved.
+        operator_kwargs : dict, optional
+            Additional keyword arguments passed to
+            :class:`openmc.deplete.IndependentOperator`.
+        """
+
+        if self.method == 'mesh-based':
+            # Get unique material for each (mesh, material) combination
+            mmv_list = self.results['mesh_material_volumes']
+            self.results['activation_materials'] = get_activation_materials(
+                self.neutron_model, mmv_list)
+        else:
+            # Create unique material for each cell
+            activation_mats = openmc.Materials()
+            for cell in self.domains:
+                mat = cell.fill.clone()
+                mat.name = f'Cell {cell.id}'
+                mat.depletable = True
+                mat.volume = cell.volume
+                activation_mats.append(mat)
+            self.results['activation_materials'] = activation_mats
+
+        # Save activation materials to file
+        output_dir = Path(output_dir)
+        output_dir.mkdir(parents=True, exist_ok=True)
+        self.results['activation_materials'].export_to_xml(
+            output_dir / 'materials.xml')
+
+        # Create depletion operator for the activation materials
+        if operator_kwargs is None:
+            operator_kwargs = {}
+        operator_kwargs.setdefault('normalization_mode', 'source-rate')
+        op = IndependentOperator(
+            self.results['activation_materials'],
+            self.results['fluxes'],
+            self.results['micros'],
+            **operator_kwargs
+        )
+
+        # Create time integrator and solve depletion equations
+        integrator = PredictorIntegrator(
+            op, timesteps, source_rates=source_rates, timestep_units=timestep_units
+        )
+        output_path = output_dir / 'depletion_results.h5'
+        integrator.integrate(final_step=False, path=output_path)
+        comm.barrier()
+
+        # Get depletion results
+        self.results['depletion_results'] = Results(output_path)
+
+    def step3_photon_source(
+        self,
+        time_indices: Sequence[int] | None = None,
+        bounding_boxes: dict[int, openmc.BoundingBox] | None = None,
+        output_dir: PathLike = 'photon_transport',
+        mat_vol_kwargs: dict | None = None,
+    ):
+        """Create decay photon sources.
+
+        This step creates decay photon sources from the activated materials for
+        each specified time. In mesh-based mode, the sources are created using
+        mesh material volumes, while in cell-based mode, they are created using
+        bounding boxes for each cell. This step will populate the
+        'photon_sources' key in the results dictionary.
+
+        Parameters
+        ----------
+        time_indices : Sequence[int], optional
+            Sequence of time indices at which photon transport should be run;
+            represented as indices into the array of times formed by the
+            timesteps. For example, if two timesteps are specified, the array of
+            times would contain three entries, and [2] would indicate computing
+            photon results at the last time. A value of None indicates to run
+            photon transport at each time that has a decay photon source.
+        bounding_boxes : dict[int, openmc.BoundingBox], optional
+            Dictionary mapping cell IDs to bounding boxes used for spatial
+            source sampling in cell-based R2S calculations. Required if method
+            is 'cell-based'.
+        output_dir : PathLike, optional
+            Path to directory where photon source outputs will be saved.
+        mat_vol_kwargs : dict, optional
+            Additional keyword arguments passed to
+            :meth:`openmc.MeshBase.material_volumes`.
+        """
+
+        # Do not retain sources from an earlier successful call if this source
+        # preparation attempt fails.
+        self.results.pop('photon_sources', None)
+
+        # TODO: Automatically determine bounding box for each cell
+        if bounding_boxes is None and self.method == 'cell-based':
+            raise ValueError("bounding_boxes must be provided for cell-based "
+                             "R2S calculations.")
+
+        output_dir = Path(output_dir)
+        output_dir.mkdir(parents=True, exist_ok=True)
+
+        # Determine and validate time indices before preparing source data.
+        n_steps = len(self.results['depletion_results'])
+        implicit_time_indices = time_indices is None
+        if time_indices is None:
+            time_indices = list(range(n_steps))
+        else:
+            time_indices = list(time_indices)
+            if not time_indices:
+                raise ValueError('time_indices must contain at least one index')
+
+            normalized_indices = []
+            for index in time_indices:
+                if isinstance(index, bool) or not isinstance(index, Integral):
+                    raise TypeError('time_indices must contain only integers')
+                index = int(index)
+                if index < -n_steps or index >= n_steps:
+                    raise IndexError(
+                        f'Photon time index {index} is out of range for '
+                        f'{n_steps} depletion results')
+                normalized_index = index % n_steps
+                normalized_indices.append(normalized_index)
+
+            # Remove duplicates while preserving order
+            time_indices = list(dict.fromkeys(normalized_indices))
+
+        # Check whether the photon model is different
+        neutron_univ = self.neutron_model.geometry.root_universe
+        photon_univ = self.photon_model.geometry.root_universe
+        different_photon_model = (neutron_univ != photon_univ)
+
+        # For mesh-based calculations, compute material volume fractions for the
+        # photon model if it is different from the neutron model to account for
+        # potential material changes
+        if self.method == 'mesh-based' and different_photon_model:
+            if mat_vol_kwargs is None:
+                mat_vol_kwargs = {}
+            photon_mmv_list = []
+            for i, mesh in enumerate(self.domains):
+                photon_mmv = comm.bcast(
+                    mesh.material_volumes(self.photon_model, **mat_vol_kwargs))
+                photon_mmv_list.append(photon_mmv)
+
+                # Save photon MMV results to file
+                if comm.rank == 0:
+                    photon_mmv.save(
+                        output_dir / f'mesh_material_volumes_{i}.npz')
+
+            self.results['mesh_material_volumes_photon'] = photon_mmv_list
+
+        # Get dictionary of cells in the photon model
+        if different_photon_model:
+            photon_cells = self.photon_model.geometry.get_all_cells()
+
+        # Determine eligible work items upfront (independent of time index).
+        if self.method == 'mesh-based':
+            work_items = self._get_mesh_work_items()
+        else:
+            work_items = []
+            for cell, original_mat in zip(
+                    self.domains, self.results['activation_materials']):
+                if different_photon_model:
+                    if cell.id not in photon_cells or \
+                            cell.fill.id != photon_cells[cell.id].fill.id:
+                        continue
+                work_items.append((cell, original_mat, bounding_boxes[cell.id]))
+
+        # Create decay photon sources for each time index
+        photon_sources = {
+            time_index: self._create_photon_sources(time_index, work_items)
+            for time_index in time_indices
+        }
+
+        # Determine if any times have no decay photon sources. If the user
+        # didn't specify any specific time indices, remove those times from the
+        # photon_sources dictionary. If the user did specify time indices, raise
+        # an error if any of those times have no decay photon sources.
+        empty_indices = [
+            time_index for time_index, sources in photon_sources.items()
+            if not sources
+        ]
+        if implicit_time_indices:
+            for time_index in empty_indices:
+                del photon_sources[time_index]
+            if not photon_sources:
+                raise RuntimeError(
+                    'No decay photon sources were found at any depletion time')
+        elif empty_indices:
+            indices = ', '.join(str(index) for index in empty_indices)
+            raise RuntimeError(
+                f'No decay photon source was found for requested time '
+                f'indices: {indices}')
+
+        self.results['photon_sources'] = photon_sources
+
+    def step4_photon_transport(
+        self,
+        output_dir: PathLike = 'photon_transport',
+        run_kwargs: dict | None = None,
+        by_parent_nuclide: bool = False,
+    ):
+        """Run photon transport using prepared decay photon sources.
+
+        This step runs a photon transport calculation for each source list
+        created by :meth:`step3_photon_source`. It will populate the
+        'photon_tallies' key in the results dictionary.
+
+        Parameters
+        ----------
+        output_dir : PathLike, optional
+            Path to directory where photon transport outputs will be saved.
+        run_kwargs : dict, optional
+            Additional keyword arguments passed to :meth:`openmc.Model.run`.
+            By default, output is disabled.
+        by_parent_nuclide : bool, optional
+            Whether to score photon tallies separately for each parent
+            radionuclide. A :class:`~openmc.ParentNuclideFilter` is added to
+            tallies that do not already contain one, with bins determined from
+            the prepared decay photon sources.
+        """
+        if 'photon_sources' not in self.results:
+            raise RuntimeError(
+                'Photon sources must be created with step3_photon_source '
+                'before running photon transport.')
+        photon_sources = self.results['photon_sources']
+        if not photon_sources:
+            raise RuntimeError(
+                'No decay photon sources are available for transport')
+
+        if by_parent_nuclide:
+            radionuclides = sorted({
+                nuclide
+                for sources in photon_sources.values()
+                for source in sources
+                for nuclide in source.energy.nuclides
+            })
+
+            if radionuclides:
+                parent_filter = openmc.ParentNuclideFilter(radionuclides)
+                for tally in self.photon_model.tallies:
+                    if not tally.contains_filter(openmc.ParentNuclideFilter):
+                        tally.filters.append(parent_filter)
+
+        if run_kwargs is None:
+            run_kwargs = {}
+        run_kwargs.setdefault('output', False)
+
+        output_dir = Path(output_dir)
+        output_dir.mkdir(parents=True, exist_ok=True)
+
+        if comm.rank == 0:
+            tally_ids = [tally.id for tally in self.photon_model.tallies]
+            with open(output_dir / 'tally_ids.json', 'w') as f:
+                json.dump(tally_ids, f)
+
+        self.results['photon_tallies'] = {}
+
+        # Ensure photon transport is enabled in settings.
+        self.photon_model.settings.photon_transport = True
+
+        for time_index, sources in photon_sources.items():
+            self.photon_model.settings.source = sources
+
+            # Run photon transport calculation
+            photon_dir = Path(output_dir) / f'time_{time_index}'
+            with TemporarySession(self.photon_model, cwd=photon_dir):
+                statepoint_path = self.photon_model.run(**run_kwargs)
+
+            # Store tally results
+            with openmc.StatePoint(statepoint_path) as sp:
+                self.results['photon_tallies'][time_index] = [
+                    sp.tallies[tally.id] for tally in self.photon_model.tallies
+                ]
+
+    def _get_mesh_work_items(self):
+        """Enumerate mesh-based work items across all meshes.
+
+        Returns a list of (index_mat, mat_id, bbox) tuples for each eligible
+        mesh element--material combination, where index_mat is the index into
+        the activation materials list, mat_id is the material ID, and bbox is
+        the bounding box for that mesh element--material combination.
+
+        Returns
+        -------
+        list of tuple
+            Each tuple is (index_mat, mat_id, bbox).
+        """
+        mmv_list = self.results['mesh_material_volumes']
+        photon_mmv_list = self.results.get('mesh_material_volumes_photon')
+
+        work_items = []
+        index_mat = 0
+        for mesh_idx, mat_vols in enumerate(mmv_list):
+            photon_mat_vols = photon_mmv_list[mesh_idx] \
+                if photon_mmv_list is not None else None
+
+            n_elements = mat_vols.num_elements
+            for index_elem in range(n_elements):
+                if photon_mat_vols is not None:
+                    photon_materials = {
+                        mat_id
+                        for mat_id, _ in photon_mat_vols.by_element(index_elem)
+                        if mat_id is not None
+                    }
+
+                for mat_id, _, bbox in mat_vols.by_element(
+                        index_elem, include_bboxes=True):
+                    if mat_id is None:
+                        continue
+                    if photon_mat_vols is not None \
+                            and mat_id not in photon_materials:
+                        index_mat += 1
+                        continue
+                    work_items.append((index_mat, mat_id, bbox))
+                    index_mat += 1
+
+        return work_items
+
+    def _create_photon_sources(self, time_index, work_items):
+        """Create decay photon sources for a set of regions.
+
+        Builds :class:`openmc.IndependentSource` objects with
+        :class:`openmc.stats.DecaySpectrum` energy distributions that will be
+        serialized to XML and resolved against the depletion chain by the C++
+        solver.
+
+        Parameters
+        ----------
+        time_index : int
+            Index into depletion results.
+        work_items : list of tuple
+            For mesh-based: list of (index_mat, mat_id, bbox).
+            For cell-based: list of (cell, original_mat, bbox).
+
+        Returns
+        -------
+        list of openmc.IndependentSource
+            Photon sources for each activated region.
+        """
+        step_result = self.results['depletion_results'][time_index]
+        materials = self.results['activation_materials']
+        mesh_based = self.method == 'mesh-based'
+        if mesh_based:
+            mat_dict = self.neutron_model._get_all_materials()
+
+        sources = []
+        for item in work_items:
+            if mesh_based:
+                index_mat, domain_id, bbox = item
+                original_mat = materials[index_mat]
+                domain = mat_dict[domain_id]
+            else:
+                cell, original_mat, bbox = item
+                domain = cell
+
+            activated_mat = step_result.get_material(str(original_mat.id))
+            nuclides = activated_mat.get_nuclide_atom_densities()
+            if not nuclides:
+                continue
+
+            # Eliminate nuclides with zero density
+            nuclides = {nuclide: density for nuclide, density in nuclides.items()
+                        if density > 0}
+
+            energy = openmc.stats.DecaySpectrum(nuclides, activated_mat.volume)
+            energy.clip(inplace=True)
+            if not energy.nuclides:
+                continue
+
+            sources.append(openmc.IndependentSource(
+                space=openmc.stats.Box(bbox.lower_left, bbox.upper_right),
+                energy=energy,
+                particle='photon',
+                constraints={'domains': [domain]},
+            ))
+
+        return sources
+
+    def load_results(self, path: PathLike):
+        """Load results from a previous R2S calculation.
+
+        Parameters
+        ----------
+        path : PathLike
+            Path to the directory containing the R2S calculation results.
+
+        """
+        path = Path(path)
+
+        # Load neutron transport results
+        neutron_dir = path / 'neutron_transport'
+        if self.method == 'mesh-based':
+            mmv_files = sorted(neutron_dir.glob('mesh_material_volumes*.npz'),
+                               key=lambda p: int(p.stem.split('_')[-1])
+                               if p.stem[-1].isdigit() else 0)
+            if mmv_files:
+                self.results['mesh_material_volumes'] = [
+                    openmc.MeshMaterialVolumes.from_npz(f) for f in mmv_files
+                ]
+        fluxes_file = neutron_dir / 'fluxes.npy'
+        if fluxes_file.exists():
+            self.results['fluxes'] = list(np.load(fluxes_file, allow_pickle=True))
+            micros_dict = read_microxs_hdf5(neutron_dir / 'micros.h5')
+            self.results['micros'] = [
+                micros_dict[f'domain_{i}'] for i in range(len(micros_dict))
+            ]
+
+        # Load activation results
+        activation_dir = path / 'activation'
+        activation_results = activation_dir / 'depletion_results.h5'
+        if activation_results.exists():
+            self.results['depletion_results'] = Results(activation_results)
+        activation_mats_file = activation_dir / 'materials.xml'
+        if activation_mats_file.exists():
+            self.results['activation_materials'] = \
+                openmc.Materials.from_xml(activation_mats_file)
+
+        # Load photon transport results
+        photon_dir = path / 'photon_transport'
+
+        # Load photon mesh material volumes if they exist (for mesh-based calculations)
+        if self.method == 'mesh-based':
+            photon_mmv_files = sorted(
+                photon_dir.glob('mesh_material_volumes*.npz'),
+                key=lambda p: int(p.stem.split('_')[-1])
+                if p.stem[-1].isdigit() else 0)
+            if photon_mmv_files:
+                self.results['mesh_material_volumes_photon'] = [
+                    openmc.MeshMaterialVolumes.from_npz(f)
+                    for f in photon_mmv_files
+                ]
+
+        # Load tally IDs from JSON file
+        tally_ids_path = photon_dir / 'tally_ids.json'
+        if tally_ids_path.exists():
+            with tally_ids_path.open('r') as f:
+                tally_ids = json.load(f)
+            self.results['photon_tallies'] = {}
+
+            # For each photon transport calc, load the statepoint and get the
+            # tally results based on tally_ids
+            for time_dir in photon_dir.glob('time_*'):
+                time_index = int(time_dir.name.split('_')[1])
+                for sp_path in time_dir.glob('statepoint.*.h5'):
+                    with openmc.StatePoint(sp_path) as sp:
+                        self.results['photon_tallies'][time_index] = [
+                            sp.tallies[tally_id] for tally_id in tally_ids
+                        ]
diff --git a/openmc/deplete/results.py b/openmc/deplete/results.py
index 132ec572c..0cf0141d2 100644
--- a/openmc/deplete/results.py
+++ b/openmc/deplete/results.py
@@ -1,12 +1,17 @@
+from __future__ import annotations
+
 import numbers
 import bisect
 import math
 from collections.abc import Iterable
+from typing import Literal
 from warnings import warn
 
 import h5py
 import numpy as np
 
+import openmc
+from .chain import Chain, _get_chain
 from .stepresult import StepResult, VERSION_RESULTS
 import openmc.checkvalue as cv
 from openmc.data import atomic_mass, AVOGADRO
@@ -103,7 +108,8 @@ class Results(list):
         mat: Material | str,
         units: str = "Bq/cm3",
         by_nuclide: bool = False,
-        volume: float | None = None
+        volume: float | None = None,
+        chain_file: Literal[False] | None | PathLike | Chain = None
     ) -> tuple[np.ndarray, np.ndarray | list[dict]]:
         """Get activity of material over time.
 
@@ -113,24 +119,33 @@ class Results(list):
         ----------
         mat : openmc.Material, str
             Material object or material id to evaluate
-        units : {'Bq', 'Bq/g', 'Bq/cm3'}
+        units : {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3', 'Bq/m3'}
             Specifies the type of activity to return, options include total
-            activity [Bq], specific [Bq/g] or volumetric activity [Bq/cm3].
+            activity [Bq], specific [Bq/g, Bq/kg] or volumetric activity
+            [Bq/cm3].
         by_nuclide : bool
             Specifies if the activity should be returned for the material as a
             whole or per nuclide. Default is False.
         volume : float, optional
             Volume of the material. If not passed, defaults to using the
             :attr:`Material.volume` attribute.
+        chain_file : False, None, PathLike, or openmc.deplete.Chain, optional
+            Source of half-life values. If ``False``, only ENDF/B-VIII.0 data is
+            used. If ``None``, the chain specified by
+            ``openmc.config['chain_file']`` is used when available. If a path or
+            :class:`openmc.deplete.Chain` is given, that chain is used. For
+            ``None`` or an explicit chain, nuclides absent from the chain fall
+            back to ENDF/B-VIII.0 data.
+
+            .. versionadded:: 0.15.4
 
         Returns
         -------
         times : numpy.ndarray
             Array of times in [s]
         activities : numpy.ndarray or List[dict]
-            Array of total activities if by_nuclide = False (default)
-            or list of dictionaries of activities by nuclide if
-            by_nuclide = True.
+            Array of total activities if by_nuclide = False (default) or list of
+            dictionaries of activities by nuclide if by_nuclide = True.
 
         """
         if isinstance(mat, Material):
@@ -140,6 +155,13 @@ class Results(list):
         else:
             raise TypeError('mat should be of type openmc.Material or str')
 
+        if chain_file is not False:
+            if chain_file is None:
+                if openmc.config.get('chain_file') is not None:
+                    chain_file = _get_chain(None)
+            else:
+                chain_file = _get_chain(chain_file)
+
         times = np.empty_like(self, dtype=float)
         if by_nuclide:
             activities = [None] * len(self)
@@ -149,7 +171,8 @@ class Results(list):
         # Evaluate activity for each depletion time
         for i, result in enumerate(self):
             times[i] = result.time[0]
-            activities[i] = result.get_material(mat_id).get_activity(units, by_nuclide, volume)
+            activities[i] = result.get_material(mat_id).get_activity(
+                units, by_nuclide, volume, chain_file=chain_file)
 
         return times, activities
 
@@ -203,7 +226,7 @@ class Results(list):
         # Evaluate value in each region
         for i, result in enumerate(self):
             times[i] = result.time[0]
-            concentrations[i] = result[0, mat_id, nuc]
+            concentrations[i] = result[mat_id, nuc]
 
         # Unit conversions
         times = _get_time_as(times, time_units)
@@ -231,9 +254,9 @@ class Results(list):
         ----------
         mat : openmc.Material, str
             Material object or material id to evaluate.
-        units : {'W', 'W/g', 'W/cm3'}
+        units : {'W', 'W/g', 'W/kg', 'W/cm3', 'W/m3'}
             Specifies the units of decay heat to return. Options include total
-            heat [W], specific [W/g] or volumetric heat [W/cm3].
+            heat [W], specific [W/g, W/kg] or volumetric heat [W/cm3].
         by_nuclide : bool
             Specifies if the decay heat should be returned for the material as a
             whole or per nuclide. Default is False.
@@ -363,7 +386,7 @@ class Results(list):
         # Evaluate value in each region
         for i, result in enumerate(self):
             times[i] = result.time[0]
-            rates[i] = result.rates[0].get(mat_id, nuc, rx) * result[0, mat, nuc]
+            rates[i] = result.rates.get(mat_id, nuc, rx) * result[mat, nuc]
 
         return times, rates
 
@@ -397,7 +420,7 @@ class Results(list):
         # Get time/eigenvalue at each point
         for i, result in enumerate(self):
             times[i] = result.time[0]
-            eigenvalues[i] = result.k[0]
+            eigenvalues[i] = result.k
 
         # Convert time units if necessary
         times = _get_time_as(times, time_units)
@@ -630,7 +653,7 @@ class Results(list):
                 for nuc in result.index_nuc:
                     if nuc not in available_cross_sections:
                         continue
-                    atoms = result[0, mat_id, nuc]
+                    atoms = result[mat_id, nuc]
                     if atoms > 0.0:
                         atoms_per_barn_cm = 1e-24 * atoms / mat.volume
                         mat.remove_nuclide(nuc) # Replace if it's there
diff --git a/openmc/deplete/stepresult.py b/openmc/deplete/stepresult.py
index 9cf33898f..9f638a058 100644
--- a/openmc/deplete/stepresult.py
+++ b/openmc/deplete/stepresult.py
@@ -12,11 +12,12 @@ import h5py
 import numpy as np
 
 import openmc
-from openmc.mpi import comm, MPI
 from openmc.checkvalue import PathLike
+from openmc.mpi import MPI, comm
+
 from .reaction_rates import ReactionRates
 
-VERSION_RESULTS = (1, 1)
+VERSION_RESULTS = (1, 3)
 
 
 __all__ = ["StepResult"]
@@ -30,8 +31,8 @@ class StepResult:
 
     Attributes
     ----------
-    k : list of (float, float)
-        Eigenvalue and uncertainty for each substep.
+    k : tuple of (float, float)
+        Eigenvalue and uncertainty at end of step.
     time : list of float
         Time at beginning, end of step, in seconds.
     source_rate : float
@@ -40,8 +41,8 @@ class StepResult:
         Number of mats.
     n_nuc : int
         Number of nuclides.
-    rates : list of ReactionRates
-        The reaction rates for each substep.
+    rates : ReactionRates
+        The reaction rates at end of step.
     volume : dict of str to float
         Dictionary mapping mat id to volume.
     index_mat : dict of str to int
@@ -52,13 +53,13 @@ class StepResult:
         A dictionary mapping mat ID as string to global index.
     n_hdf5_mats : int
         Number of materials in entire geometry.
-    n_stages : int
-        Number of stages in simulation.
     data : numpy.ndarray
-        Atom quantity, stored by stage, mat, then by nuclide.
+        Atom quantity, stored by mat, then by nuclide.
     proc_time : int
         Average time spent depleting a material across all
         materials and processes
+    keff_search_root : float
+        The root returned by the keff search control.
 
     """
     def __init__(self):
@@ -72,8 +73,10 @@ class StepResult:
         self.index_mat = None
         self.index_nuc = None
         self.mat_to_hdf5_ind = None
+        self.name_list = None
 
         self.data = None
+        self.keff_search_root = None
 
     def __repr__(self):
         t = self.time[0]
@@ -86,17 +89,17 @@ class StepResult:
         Parameters
         ----------
         pos : tuple
-            A three-length tuple containing a stage index, mat index and a nuc
-            index.  All can be integers or slices.  The second two can be
+            A two-length tuple containing a mat index and a nuc
+            index.  Both can be integers or slices, or can be
             strings corresponding to their respective dictionary.
 
         Returns
         -------
         float
-            The atoms for stage, mat, nuc
+            The atoms for mat, nuc
 
         """
-        stage, mat, nuc = pos
+        mat, nuc = pos
         if isinstance(mat, openmc.Material):
             mat = str(mat.id)
         if isinstance(mat, str):
@@ -104,7 +107,7 @@ class StepResult:
         if isinstance(nuc, str):
             nuc = self.index_nuc[nuc]
 
-        return self.data[stage, mat, nuc]
+        return self.data[mat, nuc]
 
     def __setitem__(self, pos, val):
         """Sets an item from results.
@@ -112,21 +115,21 @@ class StepResult:
         Parameters
         ----------
         pos : tuple
-            A three-length tuple containing a stage index, mat index and a nuc
-            index.  All can be integers or slices.  The second two can be
+            A two-length tuple containing a mat index and a nuc
+            index.  Both can be integers or slices, or can be
             strings corresponding to their respective dictionary.
 
         val : float
             The value to set data to.
 
         """
-        stage, mat, nuc = pos
+        mat, nuc = pos
         if isinstance(mat, str):
             mat = self.index_mat[mat]
         if isinstance(nuc, str):
             nuc = self.index_nuc[nuc]
 
-        self.data[stage, mat, nuc] = val
+        self.data[mat, nuc] = val
 
     @property
     def n_mat(self):
@@ -140,11 +143,7 @@ class StepResult:
     def n_hdf5_mats(self):
         return len(self.mat_to_hdf5_ind)
 
-    @property
-    def n_stages(self):
-        return self.data.shape[0]
-
-    def allocate(self, volume, nuc_list, burn_list, full_burn_list, stages):
+    def allocate(self, volume, nuc_list, burn_list, full_burn_list, name_list=None):
         """Allocate memory for depletion step data
 
         Parameters
@@ -157,17 +156,18 @@ class StepResult:
             A list of all mat IDs to be burned.  Used for sorting the simulation.
         full_burn_list : list of str
             List of all burnable material IDs
-        stages : int
-            Number of stages in simulation.
+        name_list : list of str, optional
+            Material names corresponding to materials in full_burn_list
 
         """
         self.volume = copy.deepcopy(volume)
         self.index_nuc = {nuc: i for i, nuc in enumerate(nuc_list)}
         self.index_mat = {mat: i for i, mat in enumerate(burn_list)}
         self.mat_to_hdf5_ind = {mat: i for i, mat in enumerate(full_burn_list)}
+        self.mat_to_name = dict(zip(full_burn_list, name_list)) if name_list is not None else {}
 
         # Create storage array
-        self.data = np.zeros((stages, self.n_mat, self.n_nuc))
+        self.data = np.zeros((self.n_mat, self.n_nuc))
 
     def distribute(self, local_materials, ranges):
         """Create a new object containing data for distributed materials
@@ -192,23 +192,23 @@ class StepResult:
 
         # Direct transfer
         direct_attrs = ("time", "k", "source_rate", "index_nuc",
-                        "mat_to_hdf5_ind", "proc_time")
+                        "mat_to_hdf5_ind", "mat_to_name", "proc_time")
         for attr in direct_attrs:
             setattr(new, attr, getattr(self, attr))
         # Get applicable slice of data
-        new.data = self.data[:, ranges]
-        new.rates = [r[ranges] for r in self.rates]
+        new.data = self.data[ranges]
+        new.rates = self.rates[ranges]
         return new
 
-    def get_material(self, mat_id):
+    def get_material(self, mat_id: str | int) -> openmc.Material:
         """Return material object for given depleted composition
 
         .. versionadded:: 0.13.2
 
         Parameters
         ----------
-        mat_id : str
-            Material ID as a string
+        mat_id : str or int
+            Material ID as a string or integer
 
         Returns
         -------
@@ -221,6 +221,9 @@ class StepResult:
             If specified material ID is not found in the StepResult
 
         """
+        # Coerce to str since internal dictionaries use str keys
+        mat_id = str(mat_id)
+
         with warnings.catch_warnings():
             warnings.simplefilter('ignore', openmc.IDWarning)
             material = openmc.Material(material_id=int(mat_id))
@@ -231,16 +234,18 @@ class StepResult:
                 f'mat_id {mat_id} not found in StepResult. Available mat_id '
                 f'values are {list(self.volume.keys())}'
             ) from e
+        if mat_id in self.mat_to_name:
+            material.name = self.mat_to_name[mat_id]
         for nuc, _ in sorted(self.index_nuc.items(), key=lambda x: x[1]):
-            atoms = self[0, mat_id, nuc]
-            if atoms < 0.0:
+            atoms = self[mat_id, nuc]
+            if atoms <= 0.0:
                 continue
             atom_per_bcm = atoms / vol * 1e-24
             material.add_nuclide(nuc, atom_per_bcm)
         material.volume = vol
         return material
 
-    def export_to_hdf5(self, filename, step):
+    def export_to_hdf5(self, filename, step, write_rates: bool = False):
         """Export results to an HDF5 file
 
         Parameters
@@ -249,6 +254,8 @@ class StepResult:
             The filename to write to
         step : int
             What step is this?
+        write_rates : bool, optional
+            Whether to include reaction rate datasets in the results file.
 
         """
         # Write new file if first time step, else add to existing file
@@ -259,7 +266,8 @@ class StepResult:
             kwargs['driver'] = 'mpio'
             kwargs['comm'] = comm
             with h5py.File(filename, **kwargs) as handle:
-                self._to_hdf5(handle, step, parallel=True)
+                self._to_hdf5(handle, step, parallel=True,
+                              write_rates=write_rates)
         else:
             # Gather results at root process
             all_results = comm.gather(self)
@@ -268,15 +276,18 @@ class StepResult:
             if comm.rank == 0:
                 with h5py.File(filename, **kwargs) as handle:
                     for res in all_results:
-                        res._to_hdf5(handle, step, parallel=False)
+                        res._to_hdf5(handle, step, parallel=False,
+                                     write_rates=write_rates)
 
-    def _write_hdf5_metadata(self, handle):
+    def _write_hdf5_metadata(self, handle, write_rates):
         """Writes result metadata in HDF5 file
 
         Parameters
         ----------
         handle : h5py.File or h5py.Group
             An hdf5 file or group type to store this in.
+        write_rates : bool
+            Whether reaction rate datasets are being written.
 
         """
         # Create and save the 5 dictionaries:
@@ -284,8 +295,8 @@ class StepResult:
         #   self.index_mat -> self.volume (TODO: support for changing volumes)
         #   self.index_nuc
         # reactions
-        #   self.rates[0].index_nuc (can be different from above, above is superset)
-        #   self.rates[0].index_rx
+        #   self.rates.index_nuc (can be different from above, above is superset)
+        #   self.rates.index_rx
         # these are shared by every step of the simulation, and should be deduplicated.
 
         # Store concentration mat and nuclide dictionaries (along with volumes)
@@ -295,13 +306,19 @@ class StepResult:
 
         mat_list = sorted(self.mat_to_hdf5_ind, key=int)
         nuc_list = sorted(self.index_nuc)
-        rxn_list = sorted(self.rates[0].index_rx)
+
+        include_rates = (
+            write_rates
+            and self.rates is not None
+            and bool(self.rates.index_nuc)
+            and bool(self.rates.index_rx)
+        )
+        rxn_list = sorted(self.rates.index_rx) if include_rates else []
 
         n_mats = self.n_hdf5_mats
         n_nuc_number = len(nuc_list)
-        n_nuc_rxn = len(self.rates[0].index_nuc)
+        n_nuc_rxn = len(self.rates.index_nuc) if include_rates else 0
         n_rxn = len(rxn_list)
-        n_stages = self.n_stages
 
         mat_group = handle.create_group("materials")
 
@@ -309,47 +326,56 @@ class StepResult:
             mat_single_group = mat_group.create_group(mat)
             mat_single_group.attrs["index"] = self.mat_to_hdf5_ind[mat]
             mat_single_group.attrs["volume"] = self.volume[mat]
+            if mat in self.mat_to_name:
+                mat_single_group.attrs["name"] = self.mat_to_name[mat]
 
         nuc_group = handle.create_group("nuclides")
 
         for nuc in nuc_list:
             nuc_single_group = nuc_group.create_group(nuc)
             nuc_single_group.attrs["atom number index"] = self.index_nuc[nuc]
-            if nuc in self.rates[0].index_nuc:
-                nuc_single_group.attrs["reaction rate index"] = self.rates[0].index_nuc[nuc]
+            if include_rates and nuc in self.rates.index_nuc:
+                nuc_single_group.attrs["reaction rate index"] = (
+                    self.rates.index_nuc[nuc])
 
-        rxn_group = handle.create_group("reactions")
+        if include_rates:
+            rxn_group = handle.create_group("reactions")
 
-        for rxn in rxn_list:
-            rxn_single_group = rxn_group.create_group(rxn)
-            rxn_single_group.attrs["index"] = self.rates[0].index_rx[rxn]
+            for rxn in rxn_list:
+                rxn_single_group = rxn_group.create_group(rxn)
+                rxn_single_group.attrs["index"] = (
+                    self.rates.index_rx[rxn])
 
         # Construct array storage
 
-        handle.create_dataset("number", (1, n_stages, n_mats, n_nuc_number),
-                              maxshape=(None, n_stages, n_mats, n_nuc_number),
-                              chunks=(1, 1, n_mats, n_nuc_number),
+        handle.create_dataset("number", (1, n_mats, n_nuc_number),
+                              maxshape=(None, n_mats, n_nuc_number),
+                              chunks=True,
                               dtype='float64')
 
-        if n_nuc_rxn > 0 and n_rxn > 0:
-            handle.create_dataset("reaction rates", (1, n_stages, n_mats, n_nuc_rxn, n_rxn),
-                                maxshape=(None, n_stages, n_mats, n_nuc_rxn, n_rxn),
-                                chunks=(1, 1, n_mats, n_nuc_rxn, n_rxn),
-                                dtype='float64')
+        if include_rates and n_nuc_rxn > 0 and n_rxn > 0:
+            handle.create_dataset(
+                "reaction rates", (1, n_mats, n_nuc_rxn, n_rxn),
+                maxshape=(None, n_mats, n_nuc_rxn, n_rxn),
+                chunks=True, dtype='float64')
 
-        handle.create_dataset("eigenvalues", (1, n_stages, 2),
-                              maxshape=(None, n_stages, 2), dtype='float64')
+        handle.create_dataset("eigenvalues", (1, 2),
+                              maxshape=(None, 2), dtype='float64')
 
         handle.create_dataset("time", (1, 2), maxshape=(None, 2), dtype='float64')
 
-        handle.create_dataset("source_rate", (1, n_stages), maxshape=(None, n_stages),
+        handle.create_dataset("source_rate", (1,), maxshape=(None,),
                               dtype='float64')
 
         handle.create_dataset(
             "depletion time", (1,), maxshape=(None,),
             dtype="float64")
 
-    def _to_hdf5(self, handle, index, parallel=False):
+        handle.create_dataset(
+            "keff_search_root", (1,), maxshape=(None,),
+            dtype="float64")
+
+    def _to_hdf5(self, handle, index, parallel=False, write_rates: bool = False):
         """Converts results object into an hdf5 object.
 
         Parameters
@@ -360,12 +386,14 @@ class StepResult:
             What step is this?
         parallel : bool
             Being called with parallel HDF5?
+        write_rates : bool, optional
+            Whether reaction rate datasets are being written.
 
         """
         if "/number" not in handle:
             if parallel:
                 comm.barrier()
-            self._write_hdf5_metadata(handle)
+            self._write_hdf5_metadata(handle, write_rates)
 
         if parallel:
             comm.barrier()
@@ -379,6 +407,7 @@ class StepResult:
         time_dset = handle["/time"]
         source_rate_dset = handle["/source_rate"]
         proc_time_dset = handle["/depletion time"]
+        keff_search_root_dset = handle["/keff_search_root"]
 
         # Get number of results stored
         number_shape = list(number_dset.shape)
@@ -412,29 +441,30 @@ class StepResult:
             proc_shape[0] = new_shape
             proc_time_dset.resize(proc_shape)
 
+            keff_search_root_shape = list(keff_search_root_dset.shape)
+            keff_search_root_shape[0] = new_shape
+            keff_search_root_dset.resize(keff_search_root_shape)
+
         # If nothing to write, just return
         if len(self.index_mat) == 0:
             return
 
         # Add data
-        # Note, for the last step, self.n_stages = 1, even if n_stages != 1.
-        n_stages = self.n_stages
         inds = [self.mat_to_hdf5_ind[mat] for mat in self.index_mat]
         low = min(inds)
         high = max(inds)
-        for i in range(n_stages):
-            number_dset[index, i, low:high+1] = self.data[i]
-            if has_reactions:
-                rxn_dset[index, i, low:high+1] = self.rates[i]
-            if comm.rank == 0:
-                eigenvalues_dset[index, i] = self.k[i]
+        number_dset[index, low:high+1] = self.data
+        if has_reactions:
+            rxn_dset[index, low:high+1] = self.rates
         if comm.rank == 0:
+            eigenvalues_dset[index] = self.k
             time_dset[index] = self.time
             source_rate_dset[index] = self.source_rate
             if self.proc_time is not None:
                 proc_time_dset[index] = (
                     self.proc_time / (comm.size * self.n_hdf5_mats)
                 )
+            keff_search_root_dset[index] = self.keff_search_root
 
     @classmethod
     def from_hdf5(cls, handle, step):
@@ -459,16 +489,34 @@ class StepResult:
             # Older versions used "power" instead of "source_rate"
             source_rate_dset = handle["/power"]
 
-        results.data = number_dset[step, :, :, :]
-        results.k = eigenvalues_dset[step, :]
+        # Check if this is an old format file (with stages dimension) or new format
+        # Old format: number has shape (n_steps, n_stages, n_mats, n_nucs)
+        # New format: number has shape (n_steps, n_mats, n_nucs)
+        has_stages = len(number_dset.shape) == 4
+
+        if has_stages:
+            # Old format - extract data from first stage (index 0)
+            results.data = number_dset[step, 0, :, :]
+            results.k = eigenvalues_dset[step, 0, :]
+            # source_rate had shape (n_steps, n_stages) in old format
+            results.source_rate = source_rate_dset[step, 0]
+        else:
+            # New format - no stages dimension
+            results.data = number_dset[step, :, :]
+            results.k = eigenvalues_dset[step, :]
+            results.source_rate = source_rate_dset[step]
+
         results.time = time_dset[step, :]
-        results.source_rate = source_rate_dset[step, 0]
 
         if "depletion time" in handle:
             proc_time_dset = handle["/depletion time"]
             if step < proc_time_dset.shape[0]:
                 results.proc_time = proc_time_dset[step]
 
+        if "keff_search_root" in handle:
+            keff_search_root_dset = handle["/keff_search_root"]
+            results.keff_search_root = keff_search_root_dset[step]
+
         if results.proc_time is None:
             results.proc_time = np.array([np.nan])
 
@@ -476,6 +524,7 @@ class StepResult:
         results.volume = {}
         results.index_mat = {}
         results.index_nuc = {}
+        results.mat_to_name = {}
         rxn_nuc_to_ind = {}
         rxn_to_ind = {}
 
@@ -485,6 +534,8 @@ class StepResult:
 
             results.volume[mat] = vol
             results.index_mat[mat] = ind
+            if "name" in mat_handle.attrs:
+                results.mat_to_name[mat] = mat_handle.attrs["name"]
 
         for nuc, nuc_handle in handle["/nuclides"].items():
             ind_atom = nuc_handle.attrs["atom number index"]
@@ -493,33 +544,46 @@ class StepResult:
             if "reaction rate index" in nuc_handle.attrs:
                 rxn_nuc_to_ind[nuc] = nuc_handle.attrs["reaction rate index"]
 
-        for rxn, rxn_handle in handle["/reactions"].items():
-            rxn_to_ind[rxn] = rxn_handle.attrs["index"]
+        if "reactions" in handle:
+            for rxn, rxn_handle in handle["/reactions"].items():
+                rxn_to_ind[rxn] = rxn_handle.attrs["index"]
 
-        results.rates = []
-        # Reconstruct reactions
-        for i in range(results.n_stages):
-            rate = ReactionRates(results.index_mat, rxn_nuc_to_ind, rxn_to_ind, True)
-
-            if "reaction rates" in handle:
-                rate[:] = handle["/reaction rates"][step, i, :, :, :]
-            results.rates.append(rate)
+        # Reconstruct reaction rates
+        rate = ReactionRates(results.index_mat, rxn_nuc_to_ind, rxn_to_ind, True)
+        if "reaction rates" in handle:
+            if has_stages:
+                # Old format: (n_steps, n_stages, n_mats, n_nucs, n_rxns)
+                rate[:] = handle["/reaction rates"][step, 0, :, :, :]
+            else:
+                # New format: (n_steps, n_mats, n_nucs, n_rxns)
+                rate[:] = handle["/reaction rates"][step, :, :, :]
+        results.rates = rate
 
         return results
 
     @staticmethod
-    def save(op, x, op_results, t, source_rate, step_ind, proc_time=None,
-             path: PathLike = "depletion_results.h5"):
+    def save(
+        op,
+        x,
+        op_results,
+        t,
+        source_rate,
+        step_ind,
+        proc_time=None,
+        write_rates: bool = False,
+        keff_search_root=None,
+        path: PathLike = "depletion_results.h5"
+    ):
         """Creates and writes depletion results to disk
 
         Parameters
         ----------
         op : openmc.deplete.abc.TransportOperator
             The operator used to generate these results.
-        x : list of list of numpy.array
-            The prior x vectors.  Indexed [i][cell] using the above equation.
-        op_results : list of openmc.deplete.OperatorResult
-            Results of applying transport operator
+        x : numpy.array
+            End-of-step concentrations for each material
+        op_results : openmc.deplete.OperatorResult
+            Result of applying transport operator at end of step
         t : list of float
             Time indices.
         source_rate : float
@@ -530,44 +594,42 @@ class StepResult:
             Total process time spent depleting materials. This may
             be process-dependent and will be reduced across MPI
             processes.
-
+        write_rates : bool, optional
+            Whether reaction rates should be written to the results file.
+        keff_search_root : float
+            The root returned by the keff search control.
         path : PathLike
             Path to file to write. Defaults to 'depletion_results.h5'.
 
             .. versionadded:: 0.14.0
         """
         # Get indexing terms
-        vol_dict, nuc_list, burn_list, full_burn_list = op.get_results_info()
-
-        stages = len(x)
+        vol_dict, nuc_list, burn_list, full_burn_list, name_list = op.get_results_info()
 
         # Create results
         results = StepResult()
-        results.allocate(vol_dict, nuc_list, burn_list, full_burn_list, stages)
+        results.allocate(vol_dict, nuc_list, burn_list, full_burn_list, name_list)
 
         n_mat = len(burn_list)
 
-        for i in range(stages):
-            for mat_i in range(n_mat):
-                results[i, mat_i, :] = x[i][mat_i]
+        for mat_i in range(n_mat):
+            results[mat_i, :] = x[mat_i]
 
-        ks = []
-        for r in op_results:
-            if isinstance(r.k, type(None)):
-                ks += [(None, None)]
-            else:
-                ks += [(r.k.nominal_value, r.k.std_dev)]
-        results.k = ks
-        results.rates = [r.rates for r in op_results]
+        if isinstance(op_results.k, type(None)):
+            results.k = (None, None)
+        else:
+            results.k = (op_results.k.nominal_value, op_results.k.std_dev)
+        results.rates = op_results.rates
         results.time = t
         results.source_rate = source_rate
         results.proc_time = proc_time
         if results.proc_time is not None:
             results.proc_time = comm.reduce(proc_time, op=MPI.SUM)
+        results.keff_search_root = keff_search_root
 
         if not Path(path).is_file():
             Path(path).parent.mkdir(parents=True, exist_ok=True)
-        results.export_to_hdf5(path, step_ind)
+        results.export_to_hdf5(path, step_ind, write_rates)
 
     def transfer_volumes(self, model):
         """Transfers volumes from depletion results to geometry
diff --git a/openmc/deplete/transfer_rates.py b/openmc/deplete/transfer_rates.py
index 01c9b2e35..ea9fc9185 100644
--- a/openmc/deplete/transfer_rates.py
+++ b/openmc/deplete/transfer_rates.py
@@ -1,31 +1,31 @@
+from collections import defaultdict
 from numbers import Real
 import re
+from typing import Sequence
+
+import numpy as np
 
 from openmc.checkvalue import check_type, check_value
 from openmc import Material
-from openmc.data import ELEMENT_SYMBOL
+from openmc.data import ELEMENT_SYMBOL, isotopes, AVOGADRO, atomic_mass
+from .results import _SECONDS_PER_MINUTE, _SECONDS_PER_HOUR, \
+    _SECONDS_PER_DAY, _SECONDS_PER_JULIAN_YEAR
 
 
-class TransferRates:
-    """Class for defining continuous removals and feeds.
 
-    Molten Salt Reactors (MSRs) benefit from continuous reprocessing,
-    which removes fission products and feeds fresh fuel into the system. MSRs
-    inspired the development of this class.
+class ExternalRates:
+    """External rates class for defining addition terms of depletion equation.
 
-    An instance of this class can be passed directly to an instance of one of
-    the :class:`openmc.deplete.Integrator` classes.
-
-    .. versionadded:: 0.14.0
+    .. versionadded:: 0.15.3
 
     Parameters
     ----------
     operator : openmc.TransportOperator
         Depletion operator
-    model : openmc.Model
-        OpenMC model containing materials and geometry. If using
-        :class:`openmc.deplete.CoupledOperator`, the model must also contain
-        a :class:`opnemc.Settings` object.
+    materials : openmc.Materials
+        OpenMC materials.
+    number_of_timesteps : int
+        Total number of depletion timesteps
 
     Attributes
     ----------
@@ -33,22 +33,26 @@ class TransferRates:
         All burnable material IDs.
     local_mats : list of str
         All burnable material IDs being managed by a single process
-    transfer_rates : dict of str to dict
-        Container of transfer rates, components (elements and/or nuclides) and
-        destination material
-    index_transfer : Set of pair of str
-        Pair of strings needed to build final matrix (destination_material, mat)
+    number_of_timesteps : int
+        Total number of depletion timesteps
+    external_rates : dict of str to dict
+        Container of timesteps, external rates, components (elements and/or
+        nuclides) and optionally destination material
+    external_timesteps : list of int
+        Container of all timesteps indeces with an external rate defined.
     """
 
-    def __init__(self, operator, model):
+    def __init__(self, operator, materials, number_of_timesteps):
 
-        self.materials = model.materials
+        self.materials = materials
         self.burnable_mats = operator.burnable_mats
         self.local_mats = operator.local_mats
+        self.number_of_timesteps = number_of_timesteps
 
         #initialize transfer rates container dict
-        self.transfer_rates = {mat: {} for mat in self.burnable_mats}
-        self.index_transfer = set()
+        self.external_rates = {mat: defaultdict(list) for mat in self.burnable_mats}
+        self.external_timesteps = []
+        self.redox = {}
 
     def _get_material_id(self, val):
         """Helper method for getting material id from Material obj or name.
@@ -71,7 +75,7 @@ class TransferRates:
                 check_value('Material ID', str(val), self.burnable_mats)
             else:
                 check_value('Material name', val,
-                        [mat.name for mat in self.materials if mat.depletable])
+                            [mat.name for mat in self.materials if mat.depletable])
                 val = [mat.id for mat in self.materials if mat.name == val][0]
 
         elif isinstance(val, int):
@@ -79,7 +83,13 @@ class TransferRates:
 
         return str(val)
 
-    def get_transfer_rate(self, material, component):
+    def get_external_rate(
+        self,
+        material: str | int | Material,
+        component: str,
+        timestep: int,
+        destination_material: str | int | Material | None = None
+    ):
         """Return transfer rate for given material and element.
 
         Parameters
@@ -88,62 +98,114 @@ class TransferRates:
             Depletable material
         component : str
             Element or nuclide to get transfer rate value
+        timestep : int
+            Current timestep index
+        destination_material : openmc.Material or str or int, Optional
+            Destination material to where nuclides get fed
 
         Returns
         -------
-        transfer_rate : list of floats
-            Transfer rate values
+        external_rate : list of floats
+            External rate values
 
         """
         material_id = self._get_material_id(material)
         check_type('component', component, str)
-        return [i[0] for i in self.transfer_rates[material_id][component]]
-
-    def get_destination_material(self, material, component):
-        """Return destination material for given material and
-        component, if defined.
-
-        Parameters
-        ----------
-        material : openmc.Material or str or int
-            Depletable material
-        component : str
-            Element or nuclide that gets transferred to another material.
-
-        Returns
-        -------
-        destination_material_id : list of str
-            Depletable material ID to where the element or nuclide gets
-            transferred
-
-        """
-        material_id = self._get_material_id(material)
-        check_type('component', component, str)
-        if component in self.transfer_rates[material_id]:
-            return [i[1] for i in self.transfer_rates[material_id][component]]
+        if destination_material is not None:
+            dest_mat_id = self._get_material_id(destination_material)
+            return [i[1] for i in self.external_rates[material_id][component]
+                    if timestep in i[0] and dest_mat_id == i[2]]
         else:
-            return []
+            return [i[1] for i in self.external_rates[material_id][component]
+                    if timestep in i[0]]
 
-    def get_components(self, material):
-        """Extract removing elements and/or nuclides for a given material
+    def get_components(self, material, timestep, destination_material=None):
+        """Extract removing elements and/or nuclides for a given material at a
+        given timestep
 
         Parameters
         ----------
         material : openmc.Material or str or int
             Depletable material
+        timestep : int
+            Current timestep index
+        destination_material : openmc.Material or str or int, Optional
+            Destination material to where nuclides get fed
 
         Returns
         -------
-        elements : list
-            List of elements and nuclides where transfer rates exist
+        components : list
+            List of elements or nuclides with external rates set at a given
+            timestep
 
         """
         material_id = self._get_material_id(material)
-        if material_id in self.transfer_rates:
-            return self.transfer_rates[material_id].keys()
+        if destination_material is not None:
+            dest_mat_id = self._get_material_id(destination_material)
+        else:
+            dest_mat_id = None
+
+        all_components = []
+        if material_id in self.external_rates:
+            mat_components = self.external_rates[material_id]
+
+            for component in mat_components:
+                if dest_mat_id:
+                    # check for both timestep and destination material ids
+                    if np.isin(timestep, [val[0] for val in mat_components[component]]) and \
+                       np.isin(dest_mat_id, [val[2] for val in mat_components[component]]):
+                        all_components.append(component)
+                else:
+                    # check only for timesteps
+                    if np.isin(timestep, [val[0] for val in mat_components[component]]):
+                        all_components.append(component)
+        return all_components
+
+
+class TransferRates(ExternalRates):
+    """Class for defining continuous removals and feeds.
+
+    Molten Salt Reactors (MSRs) benefit from continuous reprocessing,
+    which removes fission products and feeds fresh fuel into the system. MSRs
+    inspired the development of this class.
+
+    An instance of this class can be passed directly to an instance of one of
+    the :class:`openmc.deplete.Integrator` classes.
+
+    .. versionadded:: 0.14.0
+
+    Parameters
+    ----------
+    operator : openmc.TransportOperator
+        Depletion operator
+    materials : openmc.Materials
+        OpenMC materials.
+    number_of_timesteps : int
+        Total number of depletion timesteps
+
+    Attributes
+    ----------
+    burnable_mats : list of str
+        All burnable material IDs.
+    local_mats : list of str
+        All burnable material IDs being managed by a single process
+    external_rates : dict of str to dict
+        Container of timesteps, transfer rates, components (elements and/or
+        nuclides) and destination material
+    external_timesteps : list of int
+        Container of all timesteps indeces with an external rate defined.
+    index_transfer : Set of pair of str
+        Pair of strings needed to build final matrix (destination_material, mat)
+    """
+
+    def __init__(self, operator, materials, number_of_timesteps):
+        super().__init__(operator, materials, number_of_timesteps)
+        self.index_transfer = defaultdict(list)
+        self.chain_nuclides = [nuc.name for nuc in operator.chain.nuclides]
 
     def set_transfer_rate(self, material, components, transfer_rate,
-                          transfer_rate_units='1/s', destination_material=None):
+                          transfer_rate_units='1/s', timesteps=None,
+                          destination_material=None):
         """Set element and/or nuclide transfer rates in a depletable material.
 
         Parameters
@@ -157,11 +219,14 @@ class TransferRates:
         transfer_rate : float
             Rate at which elements and/or nuclides are transferred. A positive or
             negative value corresponds to a removal or feed rate, respectively.
-        destination_material : openmc.Material or str or int, Optional
-            Destination material to where nuclides get fed.
         transfer_rate_units : {'1/s', '1/min', '1/h', '1/d', '1/a'}
             Units for values specified in the transfer_rate argument. 's' for
             seconds, 'min' for minutes, 'h' for hours, 'a' for Julian years.
+        timesteps : list of int, Optional
+            List of timestep indeces where to set transfer rates.
+            Default to None means the transfer rate is set for all timesteps.
+        destination_material : openmc.Material or str or int, Optional
+            Destination material to where nuclides get fed.
 
         """
         material_id = self._get_material_id(material)
@@ -183,19 +248,25 @@ class TransferRates:
         if transfer_rate_units in ('1/s', '1/sec'):
             unit_conv = 1
         elif transfer_rate_units in ('1/min', '1/minute'):
-            unit_conv = 60
+            unit_conv = _SECONDS_PER_MINUTE
         elif transfer_rate_units in ('1/h', '1/hr', '1/hour'):
-            unit_conv = 60*60
+            unit_conv = _SECONDS_PER_HOUR
         elif transfer_rate_units in ('1/d', '1/day'):
-            unit_conv = 24*60*60
+            unit_conv = _SECONDS_PER_DAY
         elif transfer_rate_units in ('1/a', '1/year'):
-            unit_conv = 365.25*24*60*60
+            unit_conv = _SECONDS_PER_JULIAN_YEAR
         else:
-            raise ValueError('Invalid transfer rate unit '
-                             f'"{transfer_rate_units}"')
+            raise ValueError(f'Invalid transfer rate unit "{transfer_rate_units}"')
+
+        if timesteps is not None:
+            for timestep in timesteps:
+                check_value('timestep', timestep, range(self.number_of_timesteps))
+            timesteps = np.array(timesteps)
+        else:
+            timesteps = np.arange(self.number_of_timesteps)
 
         for component in components:
-            current_components = self.transfer_rates[material_id].keys()
+            current_components = self.external_rates[material_id].keys()
             split_component = re.split(r'\d+', component)
             element = split_component[0]
             if element not in ELEMENT_SYMBOL.values():
@@ -219,11 +290,184 @@ class TransferRates:
                                          f'where element {element} already has '
                                          'a transfer rate.')
 
-            if component in self.transfer_rates[material_id]:
-                self.transfer_rates[material_id][component].append(
-                    (transfer_rate / unit_conv, destination_material_id))
-            else:
-                self.transfer_rates[material_id][component] = [
-                    (transfer_rate / unit_conv, destination_material_id)]
+            self.external_rates[material_id][component].append(
+                (timesteps, transfer_rate/unit_conv, destination_material_id))
+
             if destination_material_id is not None:
-                self.index_transfer.add((destination_material_id, material_id))
+                for timestep in timesteps:
+                    self.index_transfer[timestep].append(
+                        (destination_material_id, material_id))
+
+            self.external_timesteps = np.unique(np.concatenate(
+                    [self.external_timesteps, timesteps]))
+
+    def set_redox(self, material, buffer, oxidation_states, timesteps=None):
+        """Add redox control to depletable material.
+
+        Parameters
+        ----------
+        material : openmc.Material or str or int
+            Depletable material
+        buffer : dict
+            Dictionary of buffer nuclides used to maintain redox balance.
+            Keys are nuclide names (strings) and values are their respective
+            fractions (float) that collectively sum to 1.
+        oxidation_states : dict
+            User-defined oxidation states for elements.
+            Keys are element symbols (e.g., 'H', 'He'), and values are their
+            corresponding oxidation states as integers (e.g., +1, 0).
+        timesteps : list of int, optional
+            List of timestep indices where to set external source rates.
+            Defaults to None, which means the external source rate is set for
+            all timesteps.
+
+        """
+        material_id = self._get_material_id(material)
+        if timesteps is not None:
+            for timestep in timesteps:
+                check_value('timestep', timestep, range(self.number_of_timesteps))
+            timesteps = np.array(timesteps)
+        else:
+            timesteps = np.arange(self.number_of_timesteps)
+        #Check nuclides in buffer exist
+        for nuc in buffer:
+            if nuc not in self.chain_nuclides:
+                raise ValueError(f'{nuc} is not a valid nuclide.')
+        # Checks element in oxidation states exist
+        for elm in oxidation_states:
+            if elm not in ELEMENT_SYMBOL.values():
+                raise ValueError(f'{elm} is not a valid element.')
+
+        self.redox[material_id] =  (buffer, oxidation_states)
+        self.external_timesteps = np.unique(np.concatenate(
+                    [self.external_timesteps, timesteps]))
+
+class ExternalSourceRates(ExternalRates):
+    """Class for defining external source rates.
+
+    An instance of this class can be passed directly to an instance of one of
+    the :class:`openmc.deplete.Integrator` classes.
+
+    .. versionadded:: 0.15.3
+
+    Parameters
+    ----------
+    operator : openmc.TransportOperator
+        Depletion operator
+    materials : openmc.Materials
+        OpenMC materials.
+    number_of_timesteps : int
+        Total number of depletion timesteps
+
+    Attributes
+    ----------
+    burnable_mats : list of str
+        All burnable material IDs.
+    local_mats : list of str
+        All burnable material IDs being managed by a single process
+    external_timesteps : list of int
+        Container of all timesteps indeces with an external rate defined.
+    external_rates : dict of str to dict
+        Container of timesteps external source rates, and components
+        (elements and/or nuclides)
+    """
+
+    def reformat_nuclide_vectors(self, vectors):
+        """Remove last element of nuclide vector that was added for handling
+        external source rates by the depletion solver.
+
+        Parameters
+        ----------
+        vectors : list of array
+            List of nuclides vector to reformat
+
+        """
+        for mat_index, i in enumerate(self.local_mats):
+            if self.external_rates[i]:
+                vectors[mat_index] = vectors[mat_index][:-1]
+
+    def set_external_source_rate(
+        self,
+        material: str | int | Material,
+        composition: dict[str, float],
+        rate: float,
+        rate_units: str = 'g/s',
+        timesteps: Sequence[int] | None = None
+    ):
+        """Set element and/or nuclide composition vector external source rates
+        to a depletable material.
+
+        Parameters
+        ----------
+        material : openmc.Material or str or int
+            Depletable material
+        composition : dict of str to float
+            External source rate composition vector, where key can be an element
+            or a nuclide and value the corresponding weight percent.
+        rate : float
+            External source rate in units of mass per time. A positive or
+            negative value corresponds to a feed or removal rate, respectively.
+        rate_units : {'g/s', 'g/min', 'g/h', 'g/d', 'g/a'}
+            Units for values specified in the `rate` argument. 's' for seconds,
+            'min' for minutes, 'h' for hours, 'a' for Julian years.
+        timesteps : list of int, optional
+            List of timestep indices where to set external source rates. Default
+            to None, which means the external source rate is set for all
+            timesteps.
+
+        """
+
+        material_id = self._get_material_id(material)
+        check_type('rate', rate, Real)
+        check_type('composition', composition, dict, str)
+
+        if rate_units in ('g/s', 'g/sec'):
+            unit_conv = 1
+        elif rate_units in ('g/min', 'g/minute'):
+            unit_conv = _SECONDS_PER_MINUTE
+        elif rate_units in ('g/h', 'g/hr', 'g/hour'):
+            unit_conv = _SECONDS_PER_HOUR
+        elif rate_units in ('g/d', 'g/day'):
+            unit_conv = _SECONDS_PER_DAY
+        elif rate_units in ('g/a', 'g/year'):
+            unit_conv = _SECONDS_PER_JULIAN_YEAR
+        else:
+            raise ValueError(f'Invalid external source rate unit "{rate_units}"')
+
+        if timesteps is not None:
+            for timestep in timesteps:
+                check_value('timestep', timestep, range(self.number_of_timesteps))
+            timesteps = np.asarray(timesteps)
+        else:
+            timesteps = np.arange(self.number_of_timesteps)
+
+        components = composition.keys()
+        percents = composition.values()
+        norm_percents = [float(i) / sum(percents) for i in percents]
+
+        atoms_per_nuc = {}
+        for component, percent in zip(components, norm_percents):
+            split_component = re.split(r'\d+', component)
+            element = split_component[0]
+            if element not in ELEMENT_SYMBOL.values():
+                raise ValueError(f'{component} is not a valid nuclide or element.')
+
+            if len(split_component) == 1:
+                if not isotopes(component):
+                    raise ValueError(f'Cannot add element {component} '
+                                     'as it is not naturally abundant. '
+                                     'Specify a nuclide vector instead.')
+                for nuc, frac in isotopes(component):
+                    atoms_per_nuc[nuc] = (rate / atomic_mass(nuc) * AVOGADRO *
+                                          frac * percent / unit_conv)
+
+            else:
+                atoms_per_nuc[component] = (rate / atomic_mass(component) *
+                                            AVOGADRO * percent / unit_conv)
+
+        for nuc, val in atoms_per_nuc.items():
+            self.external_rates[material_id][nuc].append((timesteps, val, None))
+
+        self.external_timesteps = np.unique(np.concatenate(
+            [self.external_timesteps, timesteps]
+        ))
diff --git a/openmc/element.py b/openmc/element.py
index 082bee522..5deede971 100644
--- a/openmc/element.py
+++ b/openmc/element.py
@@ -144,8 +144,7 @@ class Element(str):
             root = tree.getroot()
             for child in root.findall('library'):
                 nuclide = child.attrib['materials']
-                if re.match(r'{}\d+'.format(self), nuclide) and \
-                   '_m' not in nuclide:
+                if re.match(r'{}\d+'.format(self), nuclide):
                     library_nuclides.add(nuclide)
 
             # Get a set of the mutual and absent nuclides. Convert to lists
@@ -185,8 +184,8 @@ class Element(str):
                 for nuclide in absent_nuclides:
                     if nuclide in ['O17', 'O18'] and 'O16' in mutual_nuclides:
                         abundances['O16'] += NATURAL_ABUNDANCE[nuclide]
-                    elif nuclide == 'Ta180' and 'Ta181' in mutual_nuclides:
-                        abundances['Ta181'] += NATURAL_ABUNDANCE[nuclide]
+                    elif nuclide == 'Ta180_m1' and 'Ta181' in mutual_nuclides:
+                            abundances['Ta181'] += NATURAL_ABUNDANCE[nuclide]
                     elif nuclide == 'W180' and 'W182' in mutual_nuclides:
                         abundances['W182'] += NATURAL_ABUNDANCE[nuclide]
                     else:
diff --git a/openmc/examples.py b/openmc/examples.py
index 5578d513e..6895bd94b 100644
--- a/openmc/examples.py
+++ b/openmc/examples.py
@@ -4,7 +4,7 @@ import numpy as np
 
 import openmc
 
-
+PINCELL_PITCH = 1.26 # cm
 
 def pwr_pin_cell() -> openmc.Model:
     """Create a PWR pin-cell model.
@@ -51,7 +51,7 @@ def pwr_pin_cell() -> openmc.Model:
     model.materials = (fuel, clad, hot_water)
 
     # Instantiate ZCylinder surfaces
-    pitch = 1.26
+    pitch = PINCELL_PITCH
     fuel_or = openmc.ZCylinder(x0=0, y0=0, r=0.39218, name='Fuel OR')
     clad_or = openmc.ZCylinder(x0=0, y0=0, r=0.45720, name='Clad OR')
     left = openmc.XPlane(x0=-pitch/2, name='left', boundary_type='reflective')
@@ -83,7 +83,7 @@ def pwr_pin_cell() -> openmc.Model:
         constraints={'fissionable': True}
     )
 
-    plot = openmc.Plot.from_geometry(model.geometry)
+    plot = openmc.SlicePlot.from_geometry(model.geometry)
     plot.pixels = (300, 300)
     plot.color_by = 'material'
     model.plots.append(plot)
@@ -150,7 +150,7 @@ def pwr_core() -> openmc.Model:
     rpv_steel.add_nuclide('Ni60', 0.0026776, 'wo')
     rpv_steel.add_nuclide('Mn55', 0.01, 'wo')
     rpv_steel.add_nuclide('Cr52', 0.002092475, 'wo')
-    rpv_steel.add_nuclide('C0', 0.0025, 'wo')
+    rpv_steel.add_element('C', 0.0025, 'wo')
     rpv_steel.add_nuclide('Cu63', 0.0013696, 'wo')
 
     lower_rad_ref = openmc.Material(6, name='Lower radial reflector')
@@ -319,14 +319,14 @@ def pwr_core() -> openmc.Model:
     l100 = openmc.RectLattice(
         name='Fuel assembly (lower half)', lattice_id=100)
     l100.lower_left = (-10.71, -10.71)
-    l100.pitch = (1.26, 1.26)
+    l100.pitch = (PINCELL_PITCH, PINCELL_PITCH)
     l100.universes = np.tile(fuel_cold, (17, 17))
     l100.universes[tube_x, tube_y] = tube_cold
 
     l101 = openmc.RectLattice(
         name='Fuel assembly (upper half)', lattice_id=101)
     l101.lower_left = (-10.71, -10.71)
-    l101.pitch = (1.26, 1.26)
+    l101.pitch = (PINCELL_PITCH, PINCELL_PITCH)
     l101.universes = np.tile(fuel_hot, (17, 17))
     l101.universes[tube_x, tube_y] = tube_hot
 
@@ -350,7 +350,7 @@ def pwr_core() -> openmc.Model:
     # Define core lattices
     l200 = openmc.RectLattice(name='Core lattice (lower half)', lattice_id=200)
     l200.lower_left = (-224.91, -224.91)
-    l200.pitch = (21.42, 21.42)
+    l200.pitch = (17 * PINCELL_PITCH, 17 * PINCELL_PITCH)
     l200.universes = [
         [fa_cw]*21,
         [fa_cw]*21,
@@ -376,7 +376,7 @@ def pwr_core() -> openmc.Model:
 
     l201 = openmc.RectLattice(name='Core lattice (lower half)', lattice_id=201)
     l201.lower_left = (-224.91, -224.91)
-    l201.pitch = (21.42, 21.42)
+    l201.pitch = (17 * PINCELL_PITCH, 17 * PINCELL_PITCH)
     l201.universes = [
         [fa_hw]*21,
         [fa_hw]*21,
@@ -429,7 +429,7 @@ def pwr_core() -> openmc.Model:
     model.settings.source = openmc.IndependentSource(space=openmc.stats.Box(
         [-160, -160, -183], [160, 160, 183]))
 
-    plot = openmc.Plot()
+    plot = openmc.SlicePlot()
     plot.origin = (125, 125, 0)
     plot.width = (250, 250)
     plot.pixels = (3000, 3000)
@@ -488,7 +488,7 @@ def pwr_assembly() -> openmc.Model:
     clad_or = openmc.ZCylinder(x0=0, y0=0, r=0.45720, name='Clad OR')
 
     # Create boundary planes to surround the geometry
-    pitch = 21.42
+    pitch = 17 * PINCELL_PITCH
     min_x = openmc.XPlane(x0=-pitch/2, boundary_type='reflective')
     max_x = openmc.XPlane(x0=+pitch/2, boundary_type='reflective')
     min_y = openmc.YPlane(y0=-pitch/2, boundary_type='reflective')
@@ -514,7 +514,7 @@ def pwr_assembly() -> openmc.Model:
 
     # Create fuel assembly Lattice
     assembly = openmc.RectLattice(name='Fuel Assembly')
-    assembly.pitch = (pitch/17, pitch/17)
+    assembly.pitch = (PINCELL_PITCH, PINCELL_PITCH)
     assembly.lower_left = (-pitch/2, -pitch/2)
 
     # Create array indices for guide tube locations in lattice
@@ -544,7 +544,7 @@ def pwr_assembly() -> openmc.Model:
         constraints={'fissionable': True}
     )
 
-    plot = openmc.Plot()
+    plot = openmc.SlicePlot()
     plot.origin = (0.0, 0.0, 0)
     plot.width = (21.42, 21.42)
     plot.pixels = (300, 300)
@@ -656,28 +656,70 @@ def slab_mg(num_regions=1, mat_names=None, mgxslib_name='2g.h5') -> openmc.Model
     return model
 
 
-def random_ray_lattice() -> openmc.Model:
-    """Create a 2x2 PWR pincell asymmetrical lattic eexample.
+def _generate_c5g7_materials(second_temp = False) -> openmc.Materials:
+    """Generate materials utilizing multi-group cross sections based on the
+    the C5G7 Benchmark.
 
-    This model is a 2x2 reflective lattice of fuel pins with one of the lattice
-    locations having just moderator instead of a fuel pin. It uses 7 group
-    cross section data.
+    Parameters
+    ----------
+    second_temp : bool, optional
+        Whether or not the cross sections should contain two temperature datapoints.
+        The first data point is the C5G7 cross sections, which corresponds to a temperature
+        of 294 K. The second data point is the C5G7 cross sections multiplied by 1/2,
+        which corresponds to a temperature of 394 K. This temperature dependence is
+        fictitious; it is used for testing temperature feedback in the random ray solver.
 
     Returns
     -------
-    model : openmc.Model
-        A PWR 2x2 lattice model
+    materials : openmc.Materials
+        Materials object containing UO2 and water materials.
 
+    Data Sources
+    ------------
+    All cross section data are from:
+    Lewis et al., "Benchmark specification for determinisitc 2D/3D MOX fuel
+    assembly transport calculations without spatial homogenization"
     """
-    model = openmc.Model()
-
-    ###########################################################################
-    # Create MGXS data for the problem
-
     # Instantiate the energy group data
+    # MGXS for the UO2 pins.
     group_edges = [1e-5, 0.0635, 10.0, 1.0e2, 1.0e3, 0.5e6, 1.0e6, 20.0e6]
     groups = openmc.mgxs.EnergyGroups(group_edges)
 
+    uo2_total = np.array([0.1779492, 0.3298048, 0.4803882, 0.5543674, 0.3118013, 0.3951678,
+                          0.5644058])
+    uo2_abs = np.array([8.0248e-03, 3.7174e-03, 2.6769e-02, 9.6236e-02, 3.0020e-02,
+                        1.1126e-01, 2.8278e-01])
+    uo2_scatter_matrix = np.array(
+        [[[0.1275370, 0.0423780, 0.0000094, 0.0000000, 0.0000000, 0.0000000, 0.0000000],
+          [0.0000000, 0.3244560, 0.0016314, 0.0000000, 0.0000000, 0.0000000, 0.0000000],
+          [0.0000000, 0.0000000, 0.4509400, 0.0026792, 0.0000000, 0.0000000, 0.0000000],
+          [0.0000000, 0.0000000, 0.0000000, 0.4525650, 0.0055664, 0.0000000, 0.0000000],
+          [0.0000000, 0.0000000, 0.0000000, 0.0001253, 0.2714010, 0.0102550, 0.0000000],
+          [0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.0012968, 0.2658020, 0.0168090],
+          [0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.0085458, 0.2730800]]])
+    uo2_scatter_matrix = np.rollaxis(uo2_scatter_matrix, 0, 3)
+    uo2_fission = np.array([7.21206e-03, 8.19301e-04, 6.45320e-03, 1.85648e-02, 1.78084e-02,
+                            8.30348e-02, 2.16004e-01])
+    uo2_nu_fission = np.array([2.005998e-02, 2.027303e-03, 1.570599e-02, 4.518301e-02,
+                               4.334208e-02, 2.020901e-01, 5.257105e-01])
+    uo2_chi = np.array([5.8791e-01, 4.1176e-01, 3.3906e-04, 1.1761e-07, 0.0000e+00,
+                        0.0000e+00, 0.0000e+00])
+
+    # MGXS for the H2O moderator.
+    h2o_total = np.array([0.15920605, 0.412969593, 0.59030986, 0.58435, 0.718, 1.2544497,
+                          2.650379])
+    h2o_abs = np.array([6.0105e-04, 1.5793e-05, 3.3716e-04, 1.9406e-03, 5.7416e-03,
+                        1.5001e-02, 3.7239e-02])
+    h2o_scatter_matrix = np.array(
+        [[[0.0444777, 0.1134000, 0.0007235, 0.0000037, 0.0000001, 0.0000000, 0.0000000],
+          [0.0000000, 0.2823340, 0.1299400, 0.0006234, 0.0000480, 0.0000074, 0.0000010],
+          [0.0000000, 0.0000000, 0.3452560, 0.2245700, 0.0169990, 0.0026443, 0.0005034],
+          [0.0000000, 0.0000000, 0.0000000, 0.0910284, 0.4155100, 0.0637320, 0.0121390],
+          [0.0000000, 0.0000000, 0.0000000, 0.0000714, 0.1391380, 0.5118200, 0.0612290],
+          [0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.0022157, 0.6999130, 0.5373200],
+          [0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.1324400, 2.4807000]]])
+    h2o_scatter_matrix = np.rollaxis(h2o_scatter_matrix, 0, 3)
+
     # Instantiate the 7-group (C5G7) cross section data
     uo2_xsdata = openmc.XSdata('UO2', groups)
     uo2_xsdata.order = 0
@@ -704,34 +746,39 @@ def random_ray_lattice() -> openmc.Model:
     uo2_xsdata.set_fission([7.21206e-03, 8.19301e-04, 6.45320e-03,
                             1.85648e-02, 1.78084e-02, 8.30348e-02,
                             2.16004e-01])
-    uo2_xsdata.set_nu_fission([2.005998e-02, 2.027303e-03, 1.570599e-02,
-                               4.518301e-02, 4.334208e-02, 2.020901e-01,
-                               5.257105e-01])
+    nu_fission = np.array([2.005998e-02, 2.027303e-03, 1.570599e-02,
+                           4.518301e-02, 4.334208e-02, 2.020901e-01,
+                           5.257105e-01])
+    uo2_xsdata.set_nu_fission(nu_fission)
     uo2_xsdata.set_chi([5.8791e-01, 4.1176e-01, 3.3906e-04, 1.1761e-07, 0.0000e+00,
                         0.0000e+00, 0.0000e+00])
+    uo2_xsdata.set_total(uo2_total, temperature=294.0)
+    uo2_xsdata.set_absorption(uo2_abs, temperature=294.0)
+    uo2_xsdata.set_scatter_matrix(uo2_scatter_matrix, temperature=294.0)
+    uo2_xsdata.set_fission(uo2_fission, temperature=294.0)
+    uo2_xsdata.set_nu_fission(uo2_nu_fission, temperature=294.0)
+    uo2_xsdata.set_chi(uo2_chi, temperature=294.0)
 
     h2o_xsdata = openmc.XSdata('LWTR', groups)
     h2o_xsdata.order = 0
-    h2o_xsdata.set_total([0.15920605, 0.412969593, 0.59030986, 0.58435,
-                          0.718, 1.2544497, 2.650379])
-    h2o_xsdata.set_absorption([6.0105e-04, 1.5793e-05, 3.3716e-04,
-                               1.9406e-03, 5.7416e-03, 1.5001e-02,
-                               3.7239e-02])
-    scatter_matrix = np.array(
-        [[[0.0444777, 0.1134000, 0.0007235, 0.0000037, 0.0000001, 0.0000000, 0.0000000],
-          [0.0000000, 0.2823340, 0.1299400, 0.0006234,
-              0.0000480, 0.0000074, 0.0000010],
-          [0.0000000, 0.0000000, 0.3452560, 0.2245700,
-              0.0169990, 0.0026443, 0.0005034],
-          [0.0000000, 0.0000000, 0.0000000, 0.0910284,
-              0.4155100, 0.0637320, 0.0121390],
-          [0.0000000, 0.0000000, 0.0000000, 0.0000714,
-              0.1391380, 0.5118200, 0.0612290],
-          [0.0000000, 0.0000000, 0.0000000, 0.0000000,
-              0.0022157, 0.6999130, 0.5373200],
-          [0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.0000000, 0.1324400, 2.4807000]]])
-    scatter_matrix = np.rollaxis(scatter_matrix, 0, 3)
-    h2o_xsdata.set_scatter_matrix(scatter_matrix)
+    h2o_xsdata.set_total(h2o_total, temperature=294.0)
+    h2o_xsdata.set_absorption(h2o_abs, temperature=294.0)
+    h2o_xsdata.set_scatter_matrix(h2o_scatter_matrix, temperature=294.0)
+
+    # Add the second temperature data point if requested.
+    if second_temp:
+        uo2_xsdata.add_temperature(394.0)
+        uo2_xsdata.set_total(0.5 * uo2_total, temperature=394.0)
+        uo2_xsdata.set_absorption(0.5 * uo2_abs, temperature=394.0)
+        uo2_xsdata.set_scatter_matrix(0.5 * uo2_scatter_matrix, temperature=394.0)
+        uo2_xsdata.set_fission(0.5 * uo2_fission, temperature=394.0)
+        uo2_xsdata.set_nu_fission(0.5 * uo2_nu_fission, temperature=394.0)
+        uo2_xsdata.set_chi(uo2_chi, temperature=394.0)
+
+        h2o_xsdata.add_temperature(394.0)
+        h2o_xsdata.set_total(0.5 * h2o_total, temperature=394.0)
+        h2o_xsdata.set_absorption(0.5 * h2o_abs, temperature=394.0)
+        h2o_xsdata.set_scatter_matrix(0.5 * h2o_scatter_matrix, temperature=394.0)
 
     mg_cross_sections = openmc.MGXSLibrary(groups)
     mg_cross_sections.add_xsdatas([uo2_xsdata, h2o_xsdata])
@@ -752,14 +799,28 @@ def random_ray_lattice() -> openmc.Model:
     # Instantiate a Materials collection and export to XML
     materials = openmc.Materials([uo2, water])
     materials.cross_sections = "mgxs.h5"
+    return materials
 
-    ###########################################################################
-    # Define problem geometry
 
+def _generate_subdivided_pin_cell(uo2, water) -> openmc.Universe:
+    """Create a radially and azimuthally subdivided pin cell universe. Helper
+    function for random_ray_pin_cell() and random_ray_lattice()
+
+    Parameters
+    ----------
+    uo2 : openmc.Material
+        UO2 material
+    water : openmc.Material
+        Water material
+
+    Returns
+    -------
+    pincell : openmc.Universe
+        Universe containing an unbounded pin cell
+
+    """
     ########################################
-    # Define an unbounded pincell universe
-
-    pitch = 1.26
+    # Define an unbounded pin cell universe
 
     # Create a surface for the fuel outer radius
     fuel_or = openmc.ZCylinder(r=0.54, name='Fuel OR')
@@ -781,7 +842,7 @@ def random_ray_lattice() -> openmc.Model:
     moderator_c = openmc.Cell(
         fill=water, region=+outer_ring_b, name='moderator outer c')
 
-    # Create pincell universe
+    # Create pin cell universe
     pincell_base = openmc.Universe()
 
     # Register Cells with Universe
@@ -801,19 +862,143 @@ def random_ray_lattice() -> openmc.Model:
     for i in range(8):
         azimuthal_cell = openmc.Cell(name=f'azimuthal_cell_{i}')
         azimuthal_cell.fill = pincell_base
-        azimuthal_cell.region = +azimuthal_planes[i] & -azimuthal_planes[(i+1) % 8]
+        azimuthal_cell.region = + \
+            azimuthal_planes[i] & -azimuthal_planes[(i+1) % 8]
         azimuthal_cells.append(azimuthal_cell)
 
     # Create a geometry with the azimuthal universes
     pincell = openmc.Universe(cells=azimuthal_cells, name='pincell')
 
+    return pincell
+
+
+def random_ray_pin_cell(second_temp = False) -> openmc.Model:
+    """Create a PWR pin cell example using C5G7 cross section data.
+    cross section data.
+
+    Parameters
+    ----------
+    second_temp : bool, optional
+        Whether or not the cross sections should contain two temperature datapoints.
+        The first data point is the C5G7 cross sections, which corresponds to a temperature
+        of 294 K. The second data point is the C5G7 cross sections multiplied by 1/2,
+        which corresponds to a temperature of 3934 K. This temperature dependence is
+        fictitious; it is used for testing temperature feedback in the random ray solver.
+
+    Returns
+    -------
+    model : openmc.Model
+        A PWR pin cell model
+
+    """
+    model = openmc.Model()
+
+    ###########################################################################
+    # Create Materials for the problem
+    materials = _generate_c5g7_materials(second_temp)
+    uo2 = materials[0]
+    water = materials[1]
+
+    ###########################################################################
+    # Define problem geometry
+    pincell = _generate_subdivided_pin_cell(uo2, water)
+
+    ########################################
+    # Define cell containing lattice and other stuff
+    pitch = PINCELL_PITCH
+    box = openmc.model.RectangularPrism(pitch, pitch, boundary_type='reflective')
+
+    pincell = openmc.Cell(fill=pincell, region=-box, name='pincell')
+
+    # Create a geometry with the top-level cell
+    geometry = openmc.Geometry([pincell])
+
+    ###########################################################################
+    # Define problem settings
+
+    # Instantiate a Settings object, set all runtime parameters, and export to XML
+    settings = openmc.Settings()
+    settings.energy_mode = "multi-group"
+    settings.batches = 400
+    settings.inactive = 200
+    settings.particles = 100
+
+    # Create an initial uniform spatial source distribution over fissionable zones
+    lower_left = (-pitch / 2, -pitch / 2, -1)
+    upper_right = (pitch / 2, pitch / 2, 1)
+    uniform_dist = openmc.stats.Box(lower_left, upper_right)
+    rr_source = openmc.IndependentSource(space=uniform_dist)
+
+    settings.random_ray['distance_active'] = 100.0
+    settings.random_ray['distance_inactive'] = 20.0
+    settings.random_ray['ray_source'] = rr_source
+    settings.random_ray['volume_normalized_flux_tallies'] = True
+
+    ###########################################################################
+    # Define tallies
+    # Now use the mesh filter in a tally and indicate what scores are desired
+    tally = openmc.Tally(name="Pin tally")
+    tally.scores = ['flux', 'fission', 'nu-fission']
+    tally.estimator = 'analog'
+
+    # Instantiate a Tallies collection and export to XML
+    tallies = openmc.Tallies([tally])
+
+    ###########################################################################
+    #                   Exporting to OpenMC model
+    ###########################################################################
+
+    model.geometry = geometry
+    model.materials = materials
+    model.settings = settings
+    model.tallies = tallies
+    return model
+
+
+def random_ray_lattice(second_temp = False) -> openmc.Model:
+    """Create a 2x2 PWR pin cell asymmetrical lattice example.
+
+    This model is a 2x2 reflective lattice of fuel pins with one of the lattice
+    locations having just moderator instead of a fuel pin. It uses C5G7
+    cross section data.
+
+    Parameters
+    ----------
+    second_temp : bool, optional
+        Whether or not the cross sections should contain two temperature datapoints.
+        The first data point is the C5G7 cross sections, which corresponds to a temperature
+        of 294 K. The second data point is the C5G7 cross sections multiplied by 1/2,
+        which corresponds to a temperature of 3934 K. This temperature dependence is
+        fictitious; it is used for testing temperature feedback in the random ray solver.
+
+    Returns
+    -------
+    model : openmc.Model
+        A PWR 2x2 lattice model
+
+    """
+    model = openmc.Model()
+
+    ###########################################################################
+    # Create Materials for the problem
+    materials = _generate_c5g7_materials(second_temp)
+    uo2 = materials[0]
+    water = materials[1]
+
+    ###########################################################################
+    # Define problem geometry
+    pincell = _generate_subdivided_pin_cell(uo2, water)
+
     ########################################
     # Define a moderator lattice universe
 
-    moderator_infinite = openmc.Cell(fill=water, name='moderator infinite')
+    moderator_infinite = openmc.Cell(name='moderator infinite')
+    moderator_infinite.fill = water
+
     mu = openmc.Universe()
     mu.add_cells([moderator_infinite])
 
+    pitch = PINCELL_PITCH
     lattice = openmc.RectLattice()
     lattice.lower_left = [-pitch/2.0, -pitch/2.0]
     lattice.pitch = [pitch/10.0, pitch/10.0]
@@ -837,8 +1022,7 @@ def random_ray_lattice() -> openmc.Model:
 
     ########################################
     # Define cell containing lattice and other stuff
-    box = openmc.model.RectangularPrism(
-        pitch*2, pitch*2, boundary_type='reflective')
+    box = openmc.model.RectangularPrism(pitch*2, pitch*2, boundary_type='reflective')
 
     assembly = openmc.Cell(fill=lattice2x2, region=-box, name='assembly')
 
@@ -1126,3 +1310,312 @@ def random_ray_three_region_cube() -> openmc.Model:
     model.tallies = tallies
 
     return model
+
+def random_ray_three_region_cube_with_detectors() -> openmc.Model:
+    """Create a three region cube model with two external tally regions.
+
+    This is an adaptation of the simple monoenergetic problem of a cube with 
+    three concentric cubic regions. The innermost region is near void (with 
+    Sigma_t around 10^-5) and contains an external isotropic source term, the 
+    middle region is a mild scatterer (with Sigma_t around 10^-3), and the 
+    outer region of the cube is a scatterer and absorber (with Sigma_t around 
+    1).
+
+    Two cubic "detector" regions are found outside this geometry, one along the 
+    y-axis near z=0, and the other in the upper right corner of the system. 
+    The size of each detector is scaled to be equal to that of the source 
+    region. The model returned by this function contains cell tallies on each 
+    detector.
+
+    Returns
+    -------
+    model : openmc.Model
+        A three region cube model
+
+    """
+
+    model = openmc.Model()
+
+    ###########################################################################
+    # Helper function creates a 3 region cube with different fills in each region
+    def fill_cube(N, n_1, n_2, fill_1, fill_2, fill_3):
+        cube = [[[0 for _ in range(N)] for _ in range(N)] for _ in range(N)]
+        for i in range(N):
+            for j in range(N):
+                for k in range(N):
+                    if i < n_1 and j >= (N-n_1) and k < n_1:
+                        cube[i][j][k] = fill_1
+                    elif i < n_2 and j >= (N-n_2) and k < n_2:
+                        cube[i][j][k] = fill_2
+                    else:
+                        cube[i][j][k] = fill_3
+        return cube
+
+    ###########################################################################
+    # Create multigroup data
+
+    # Instantiate the energy group data
+    ebins = [1e-5, 20.0e6]
+    groups = openmc.mgxs.EnergyGroups(group_edges=ebins)
+
+    cavity_sigma_a = 4.0e-5
+    cavity_sigma_s = 3.0e-3
+    cavity_mat_data = openmc.XSdata('cavity', groups)
+    cavity_mat_data.order = 0
+    cavity_mat_data.set_total([cavity_sigma_a + cavity_sigma_s])
+    cavity_mat_data.set_absorption([cavity_sigma_a])
+    cavity_mat_data.set_scatter_matrix(
+        np.rollaxis(np.array([[[cavity_sigma_s]]]), 0, 3))
+
+    absorber_sigma_a = 0.50
+    absorber_sigma_s = 0.50
+    absorber_mat_data = openmc.XSdata('absorber', groups)
+    absorber_mat_data.order = 0
+    absorber_mat_data.set_total([absorber_sigma_a + absorber_sigma_s])
+    absorber_mat_data.set_absorption([absorber_sigma_a])
+    absorber_mat_data.set_scatter_matrix(
+        np.rollaxis(np.array([[[absorber_sigma_s]]]), 0, 3))
+
+    multiplier = 0.01
+    source_sigma_a = cavity_sigma_a * multiplier
+    source_sigma_s = cavity_sigma_s * multiplier
+    source_mat_data = openmc.XSdata('source', groups)
+    source_mat_data.order = 0
+    source_mat_data.set_total([source_sigma_a + source_sigma_s])
+    source_mat_data.set_absorption([source_sigma_a])
+    source_mat_data.set_scatter_matrix(
+        np.rollaxis(np.array([[[source_sigma_s]]]), 0, 3))
+
+    mg_cross_sections_file = openmc.MGXSLibrary(groups)
+    mg_cross_sections_file.add_xsdatas(
+        [source_mat_data, cavity_mat_data, absorber_mat_data])
+    mg_cross_sections_file.export_to_hdf5()
+
+    ###########################################################################
+    # Create materials for the problem
+
+    # Instantiate some Macroscopic Data
+    source_data = openmc.Macroscopic('source')
+    cavity_data = openmc.Macroscopic('cavity')
+    absorber_data = openmc.Macroscopic('absorber')
+
+    # Instantiate some Materials and register the appropriate Macroscopic objects
+    source_mat = openmc.Material(name='source')
+    source_mat.set_density('macro', 1.0)
+    source_mat.add_macroscopic(source_data)
+
+    cavity_mat = openmc.Material(name='cavity')
+    cavity_mat.set_density('macro', 1.0)
+    cavity_mat.add_macroscopic(cavity_data)
+
+    absorber_mat = openmc.Material(name='absorber')
+    absorber_mat.set_density('macro', 1.0)
+    absorber_mat.add_macroscopic(absorber_data)
+
+    # Instantiate a Materials collection
+    materials_file = openmc.Materials([source_mat, cavity_mat, absorber_mat])
+    materials_file.cross_sections = "mgxs.h5"
+
+    ###########################################################################
+    # Define problem geometry
+
+    source_cell = openmc.Cell(fill=source_mat, name='infinite source region')
+    cavity_cell = openmc.Cell(fill=cavity_mat, name='cube cavity region')
+    absorber_cell = openmc.Cell(
+        fill=absorber_mat, name='absorber region')
+
+    source_universe = openmc.Universe(name='source universe')
+    source_universe.add_cells([source_cell])
+
+    cavity_universe = openmc.Universe()
+    cavity_universe.add_cells([cavity_cell])
+
+    absorber_universe = openmc.Universe()
+    absorber_universe.add_cells([absorber_cell])
+
+    absorber_width = 30.0
+    n_base = 6
+
+    # This variable can be increased above 1 to refine the FSR mesh resolution further
+    refinement_level = 2
+
+    n = n_base * refinement_level
+    pitch = absorber_width / n
+
+    pattern = fill_cube(n, 1*refinement_level, 5*refinement_level,
+                        source_universe, cavity_universe, absorber_universe)
+
+    lattice = openmc.RectLattice()
+    lattice.lower_left = [0.0, 0.0, 0.0]
+    lattice.pitch = [pitch, pitch, pitch]
+    lattice.universes = pattern
+
+    lattice_cell = openmc.Cell(fill=lattice)
+
+    lattice_uni = openmc.Universe()
+    lattice_uni.add_cells([lattice_cell])
+
+    x_low = openmc.XPlane(x0=0.0, boundary_type='reflective')
+    x_high = openmc.XPlane(x0=absorber_width)
+
+    y_low = openmc.YPlane(y0=0.0, boundary_type='reflective')
+    y_high = openmc.YPlane(y0=absorber_width)
+
+    z_low = openmc.ZPlane(z0=0.0, boundary_type='reflective')
+    z_high = openmc.ZPlane(z0=absorber_width)
+
+    cube_domain = openmc.Cell(fill=lattice_uni, region=+x_low & -
+                              x_high & +y_low & -y_high & +z_low & -z_high, name='full domain')
+
+    detect_width = absorber_width / n_base
+    outer_width = absorber_width + detect_width
+
+    x_outer = openmc.XPlane(x0=outer_width, boundary_type='vacuum')
+    y_outer = openmc.YPlane(y0=outer_width, boundary_type='vacuum')
+    z_outer = openmc.ZPlane(z0=outer_width, boundary_type='vacuum')
+
+    detector1_right = openmc.XPlane(x0=detect_width)
+    detector1_top = openmc.ZPlane(z0=detect_width)
+
+    detector1_region = (
+        +x_low & -detector1_right &
+        +y_high & -y_outer &
+        +z_low & -detector1_top
+    )
+    detector1 = openmc.Cell(
+        name='detector 1',
+        fill=absorber_mat,
+        region=detector1_region
+    )
+
+    detector2_region = (
+        +x_high & -x_outer &
+        +y_high & -y_outer &
+        +z_high & -z_outer
+    )
+    detector2 = openmc.Cell(
+        name='detector 2',
+        fill=absorber_mat,
+        region=detector2_region
+    )
+    
+    external_x = (
+        +x_high & +y_low & +z_low & -x_outer & 
+        ((-y_outer & -z_high) | (-y_high & +z_high & -z_outer))
+    )
+    external_y = (
+        +y_high & -y_outer & 
+        (
+            (+detector1_right & -x_high & +z_low & -z_outer) | 
+            (-detector1_right & +x_low & +detector1_top & -z_outer) | 
+            (+x_high & -x_outer & +z_low & -z_high)
+        )
+    )
+    external_z = (
+        +x_low & +y_low & +z_high & -z_outer & 
+        ((-y_outer & -x_high) | (-y_high & +x_high & -x_outer))
+    )
+    external_cell = openmc.Cell(fill=cavity_mat, 
+                                region=(external_x | external_y | external_z), 
+                                name='outside cube')
+
+    root = openmc.Universe(
+        name='root universe', 
+        cells=[cube_domain, detector1, detector2, external_cell]
+    )
+
+    # Create a geometry with the two cells and export to XML
+    geometry = openmc.Geometry(root)
+
+    ###########################################################################
+    # Define problem settings
+
+    # Instantiate a Settings object, set all runtime parameters, and export to XML
+    settings = openmc.Settings()
+    settings.energy_mode = "multi-group"
+    settings.inactive = 5
+    settings.batches = 10
+    settings.particles = 500
+    settings.run_mode = 'fixed source'
+
+    # Create an initial uniform spatial source for ray integration
+    lower_left_ray = [0.0, 0.0, 0.0]
+    upper_right_ray = [outer_width, outer_width, outer_width]
+    uniform_dist_ray = openmc.stats.Box(
+        lower_left_ray, upper_right_ray, only_fissionable=False)
+    rr_source = openmc.IndependentSource(space=uniform_dist_ray)
+
+    settings.random_ray['distance_active'] = 800.0
+    settings.random_ray['distance_inactive'] = 100.0
+    settings.random_ray['ray_source'] = rr_source
+    settings.random_ray['volume_normalized_flux_tallies'] = True
+
+    # Create a rectilinear source region mesh
+    sr_mesh = openmc.RegularMesh()
+    sr_mesh.dimension = (14, 14, 14)
+    sr_mesh.lower_left = (0.0, 0.0, 0.0)
+    sr_mesh.upper_right = (outer_width, outer_width, outer_width)
+    settings.random_ray['source_region_meshes'] = [(sr_mesh, [root])]
+
+    # Create the neutron source in the bottom right of the moderator
+    # Good - fast group appears largest (besides most thermal)
+    strengths = [1.0]
+    midpoints = [100.0]
+    energy_distribution = openmc.stats.Discrete(x=midpoints, p=strengths)
+
+    source = openmc.IndependentSource(energy=energy_distribution, constraints={
+                                      'domains': [source_universe]}, strength=3.14)
+
+    settings.source = [source]
+
+    ###########################################################################
+    # Define tallies
+
+    estimator = 'tracklength'
+
+    detector1_filter = openmc.CellFilter(detector1)
+    detector1_tally = openmc.Tally(name="Detector 1 Tally")
+    detector1_tally.filters = [detector1_filter]
+    detector1_tally.scores = ['flux']
+    detector1_tally.estimator = estimator
+
+    detector2_filter = openmc.CellFilter(detector2)
+    detector2_tally = openmc.Tally(name="Detector 2 Tally")
+    detector2_tally.filters = [detector2_filter]
+    detector2_tally.scores = ['flux']
+    detector2_tally.estimator = estimator
+
+    absorber_filter = openmc.MaterialFilter(absorber_mat)
+    absorber_tally = openmc.Tally(name="Absorber Tally")
+    absorber_tally.filters = [absorber_filter]
+    absorber_tally.scores = ['flux']
+    absorber_tally.estimator = estimator
+
+    cavity_filter = openmc.MaterialFilter(cavity_mat)
+    cavity_tally = openmc.Tally(name="Cavity Tally")
+    cavity_tally.filters = [cavity_filter]
+    cavity_tally.scores = ['flux']
+    cavity_tally.estimator = estimator
+
+    source_filter = openmc.MaterialFilter(source_mat)
+    source_tally = openmc.Tally(name="Source Tally")
+    source_tally.filters = [source_filter]
+    source_tally.scores = ['flux']
+    source_tally.estimator = estimator
+
+    # Instantiate a Tallies collection and export to XML
+    tallies = openmc.Tallies([detector1_tally, 
+                              detector2_tally, 
+                              absorber_tally,
+                              cavity_tally,
+                              source_tally])
+
+    ###########################################################################
+    # Assmble Model
+
+    model.geometry = geometry
+    model.materials = materials_file
+    model.settings = settings
+    model.tallies = tallies
+
+    return model
diff --git a/openmc/executor.py b/openmc/executor.py
index aacc48b3f..75094e738 100644
--- a/openmc/executor.py
+++ b/openmc/executor.py
@@ -103,6 +103,7 @@ def _run(args, output, cwd):
         # If OpenMC is finished, break loop
         line = p.stdout.readline()
         if not line and p.poll() is not None:
+            p.stdout.close()
             break
 
         lines.append(line)
@@ -164,7 +165,7 @@ def plot_inline(plots, openmc_exec='openmc', cwd='.', path_input=None):
 
     Parameters
     ----------
-    plots : Iterable of openmc.Plot
+    plots : Iterable of openmc.PlotBase
         Plots to display
     openmc_exec : str
         Path to OpenMC executable
diff --git a/openmc/filter.py b/openmc/filter.py
index 35a7e7c7e..87aeb70c3 100644
--- a/openmc/filter.py
+++ b/openmc/filter.py
@@ -13,19 +13,21 @@ import pandas as pd
 import openmc
 import openmc.checkvalue as cv
 from .cell import Cell
+from .data.reaction import REACTION_NAME, REACTION_MT
 from .material import Material
 from .mixin import IDManagerMixin
 from .surface import Surface
 from .universe import UniverseBase
-from ._xml import get_text
+from ._xml import get_elem_list, get_text
 
 
 _FILTER_TYPES = (
     'universe', 'material', 'cell', 'cellborn', 'surface', 'mesh', 'energy',
-    'energyout', 'mu', 'musurface', 'polar', 'azimuthal', 'distribcell', 'delayedgroup',
-    'energyfunction', 'cellfrom', 'materialfrom', 'legendre', 'spatiallegendre',
-    'sphericalharmonics', 'zernike', 'zernikeradial', 'particle', 'cellinstance',
-    'collision', 'time', 'parentnuclide'
+    'energyout', 'mu', 'musurface', 'polar', 'azimuthal', 'distribcell',
+    'delayedgroup', 'energyfunction', 'cellfrom', 'materialfrom', 'legendre',
+    'spatiallegendre', 'sphericalharmonics', 'zernike', 'zernikeradial', 'particle',
+    'particleproduction', 'cellinstance', 'collision', 'time', 'parentnuclide',
+    'weight', 'meshborn', 'meshsurface', 'meshmaterial', 'reaction',
 )
 
 _CURRENT_NAMES = (
@@ -34,7 +36,6 @@ _CURRENT_NAMES = (
     'z-min out', 'z-min in', 'z-max out', 'z-max in'
 )
 
-_PARTICLES = {'neutron', 'photon', 'electron', 'positron'}
 
 
 class FilterMeta(ABCMeta):
@@ -126,9 +127,9 @@ class Filter(IDManagerMixin, metaclass=FilterMeta):
     def __gt__(self, other):
         if type(self) is not type(other):
             if self.short_name in _FILTER_TYPES and \
-                other.short_name in _FILTER_TYPES:
+                    other.short_name in _FILTER_TYPES:
                 delta = _FILTER_TYPES.index(self.short_name) - \
-                        _FILTER_TYPES.index(other.short_name)
+                    _FILTER_TYPES.index(other.short_name)
                 return delta > 0
             else:
                 return False
@@ -258,17 +259,15 @@ class Filter(IDManagerMixin, metaclass=FilterMeta):
             Filter object
 
         """
-        filter_type = elem.get('type')
-        if filter_type is None:
-            filter_type = elem.find('type').text
+        filter_type = get_text(elem, "type")
 
         # If the filter type matches this class's short_name, then
         # there is no overridden from_xml_element method
         if filter_type == cls.short_name.lower():
             # Get bins from element -- the default here works for any filters
             # that just store a list of bins that can be represented as integers
-            filter_id = int(elem.get('id'))
-            bins = [int(x) for x in get_text(elem, 'bins').split()]
+            filter_id = int(get_text(elem, "id"))
+            bins = get_elem_list(elem, "bins", int) or []
             return cls(bins, filter_id=filter_id)
 
         # Search through all subclasses and find the one matching the HDF5
@@ -277,7 +276,6 @@ class Filter(IDManagerMixin, metaclass=FilterMeta):
             if filter_type == subclass.short_name.lower():
                 return subclass.from_xml_element(elem, **kwargs)
 
-
     def can_merge(self, other):
         """Determine if filter can be merged with another.
 
@@ -428,6 +426,7 @@ class Filter(IDManagerMixin, metaclass=FilterMeta):
 
 class WithIDFilter(Filter):
     """Abstract parent for filters of types with IDs (Cell, Material, etc.)."""
+
     def __init__(self, bins, filter_id=None):
         bins = np.atleast_1d(bins)
 
@@ -631,6 +630,7 @@ class CellInstanceFilter(Filter):
     DistribcellFilter
 
     """
+
     def __init__(self, bins, filter_id=None):
         self.bins = bins
         self.id = filter_id
@@ -700,8 +700,8 @@ class CellInstanceFilter(Filter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        bins = [int(x) for x in get_text(elem, 'bins').split()]
+        filter_id = int(get_text(elem, "id"))
+        bins = get_elem_list(elem, "bins", int) or []
         cell_instances = list(zip(bins[::2], bins[1::2]))
         return cls(cell_instances, filter_id=filter_id)
 
@@ -736,9 +736,8 @@ class ParticleFilter(Filter):
 
     Parameters
     ----------
-    bins : str, or sequence of str
-        The particles to tally represented as strings ('neutron', 'photon',
-        'electron', 'positron').
+    bins : str, int, openmc.ParticleType, or sequence
+        The particle types to tally represented as names, PDG numbers, or types.
     filter_id : int
         Unique identifier for the filter
 
@@ -752,6 +751,7 @@ class ParticleFilter(Filter):
         The number of filter bins
 
     """
+
     def __eq__(self, other):
         if type(self) is not type(other):
             return False
@@ -764,11 +764,16 @@ class ParticleFilter(Filter):
 
     @Filter.bins.setter
     def bins(self, bins):
-        cv.check_type('bins', bins, Sequence, str)
+        if isinstance(bins, (str, Integral, openmc.ParticleType)):
+            bins = [bins]
+        else:
+            cv.check_type('bins', bins, Sequence,
+                          (str, Integral, openmc.ParticleType))
         bins = np.atleast_1d(bins)
-        for edge in bins:
-            cv.check_value('filter bin', edge, _PARTICLES)
-        self._bins = bins
+        normalized = []
+        for entry in bins:
+            normalized.append(str(openmc.ParticleType(entry)))
+        self._bins = np.array(normalized, dtype=str)
 
     @classmethod
     def from_hdf5(cls, group, **kwargs):
@@ -783,8 +788,8 @@ class ParticleFilter(Filter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        bins = get_text(elem, 'bins').split()
+        filter_id = int(get_text(elem, "id"))
+        bins = get_elem_list(elem, "bins", str) or []
         return cls(bins, filter_id=filter_id)
 
 
@@ -816,7 +821,7 @@ class ParentNuclideFilter(ParticleFilter):
 
 
 class MeshFilter(Filter):
-    """Bins tally event locations by mesh elements.
+    r"""Bins tally event locations by mesh elements.
 
     Parameters
     ----------
@@ -834,6 +839,25 @@ class MeshFilter(Filter):
     translation : Iterable of float
         This array specifies a vector that is used to translate (shift) the mesh
         for this filter
+    rotation : Iterable of float
+        This array specifies the angles in degrees about the x, y, and z axes
+        that the mesh should be rotated. The rotation applied is an intrinsic
+        rotation with specified Tait-Bryan angles. That is to say, if the angles
+        are :math:`(\phi, \theta, \psi)`, then the rotation matrix applied is
+        :math:`R_z(\psi) R_y(\theta) R_x(\phi)` or
+
+        .. math::
+
+           \left [ \begin{array}{ccc} \cos\theta \cos\psi & -\cos\phi \sin\psi
+           + \sin\phi \sin\theta \cos\psi & \sin\phi \sin\psi + \cos\phi
+           \sin\theta \cos\psi \\ \cos\theta \sin\psi & \cos\phi \cos\psi +
+           \sin\phi \sin\theta \sin\psi & -\sin\phi \cos\psi + \cos\phi
+           \sin\theta \sin\psi \\ -\sin\theta & \sin\phi \cos\theta & \cos\phi
+           \cos\theta \end{array} \right ]
+
+        A rotation matrix can also be specified directly by setting this
+        attribute to a nested list (or 2D numpy array) that specifies each
+        element of the matrix.
     bins : list of tuple
         A list of mesh indices for each filter bin, e.g. [(1, 1, 1), (2, 1, 1),
         ...]
@@ -846,6 +870,7 @@ class MeshFilter(Filter):
         self.mesh = mesh
         self.id = filter_id
         self._translation = None
+        self._rotation = None
 
     def __hash__(self):
         string = type(self).__name__ + '\n'
@@ -857,6 +882,7 @@ class MeshFilter(Filter):
         string += '{: <16}=\t{}\n'.format('\tMesh ID', self.mesh.id)
         string += '{: <16}=\t{}\n'.format('\tID', self.id)
         string += '{: <16}=\t{}\n'.format('\tTranslation', self.translation)
+        string += '{: <16}=\t{}\n'.format('\tRotation', self.rotation)
         return string
 
     @classmethod
@@ -880,6 +906,10 @@ class MeshFilter(Filter):
         if translation:
             out.translation = translation[()]
 
+        rotation = group.get('rotation')
+        if rotation:
+            out.rotation = rotation[()]
+
         return out
 
     @property
@@ -900,8 +930,6 @@ class MeshFilter(Filter):
 
     @property
     def shape(self):
-        if isinstance(self, MeshSurfaceFilter):
-            return (self.num_bins,)
         return self.mesh.dimension
 
     @property
@@ -914,6 +942,15 @@ class MeshFilter(Filter):
         cv.check_length('mesh filter translation', t, 3)
         self._translation = np.asarray(t)
 
+    @property
+    def rotation(self):
+        return self._rotation
+
+    @rotation.setter
+    def rotation(self, rotation):
+        cv.check_length('mesh filter rotation', rotation, 3)
+        self._rotation = np.asarray(rotation)
+
     def can_merge(self, other):
         # Mesh filters cannot have more than one bin
         return False
@@ -935,53 +972,36 @@ class MeshFilter(Filter):
         Returns
         -------
         pandas.DataFrame
-            A Pandas DataFrame with three columns describing the x,y,z mesh
-            cell indices corresponding to each filter bin.  The number of rows
-            in the DataFrame is the same as the total number of bins in the
-            corresponding tally, with the filter bin appropriately tiled to map
-            to the corresponding tally bins.
+            A Pandas DataFrame with columns describing the mesh cell indices
+            corresponding to each filter bin. Column names depend on the mesh
+            type (e.g., x/y/z for RegularMesh, r/phi/z for CylindricalMesh,
+            r/theta/phi for SphericalMesh, or element index for
+            UnstructuredMesh). The number of rows in the DataFrame is the same
+            as the total number of bins in the corresponding tally, with the
+            filter bin appropriately tiled to map to the corresponding tally
+            bins.
 
         See also
         --------
         Tally.get_pandas_dataframe(), CrossFilter.get_pandas_dataframe()
 
         """
-        # Initialize Pandas DataFrame
-        df = pd.DataFrame()
-
         # Initialize dictionary to build Pandas Multi-index column
         filter_dict = {}
 
         # Append mesh ID as outermost index of multi-index
         mesh_key = f'mesh {self.mesh.id}'
 
-        # Find mesh dimensions - use 3D indices for simplicity
-        n_dim = len(self.mesh.dimension)
-        if n_dim == 3:
-            nx, ny, nz = self.mesh.dimension
-        elif n_dim == 2:
-            nx, ny = self.mesh.dimension
-            nz = 1
-        else:
-            nx = self.mesh.dimension
-            ny = nz = 1
+        # Determine index base (0-based for unstructured, 1-based otherwise)
+        idx_start = 0 if isinstance(self.mesh, openmc.UnstructuredMesh) else 1
 
-        # Generate multi-index sub-column for x-axis
-        filter_dict[mesh_key, 'x'] = _repeat_and_tile(
-            np.arange(1, nx + 1), stride, data_size)
+        # Generate a multi-index sub-column for each axis
+        for label, dim_size in zip(self.mesh._axis_labels, self.mesh.dimension):
+            filter_dict[mesh_key, label] = _repeat_and_tile(
+                np.arange(idx_start, idx_start + dim_size), stride, data_size)
+            stride *= dim_size
 
-        # Generate multi-index sub-column for y-axis
-        filter_dict[mesh_key, 'y'] = _repeat_and_tile(
-            np.arange(1, ny + 1), nx * stride, data_size)
-
-        # Generate multi-index sub-column for z-axis
-        filter_dict[mesh_key, 'z'] = _repeat_and_tile(
-            np.arange(1, nz + 1), nx * ny * stride, data_size)
-
-        # Initialize a Pandas DataFrame from the mesh dictionary
-        df = pd.concat([df, pd.DataFrame(filter_dict)])
-
-        return df
+        return pd.DataFrame(filter_dict)
 
     def to_xml_element(self):
         """Return XML Element representing the Filter.
@@ -992,22 +1012,34 @@ class MeshFilter(Filter):
             XML element containing filter data
 
         """
-        element = super().to_xml_element()
-        element[0].text = str(self.mesh.id)
+        element = ET.Element('filter')
+        element.set('id', str(self.id))
+        element.set('type', self.short_name.lower())
+        subelement = ET.SubElement(element, 'bins')
+        subelement.text = str(self.mesh.id)
         if self.translation is not None:
             element.set('translation', ' '.join(map(str, self.translation)))
+        if self.rotation is not None:
+            element.set('rotation', ' '.join(map(str, self.rotation.ravel())))
         return element
 
     @classmethod
     def from_xml_element(cls, elem: ET.Element, **kwargs) -> MeshFilter:
         mesh_id = int(get_text(elem, 'bins'))
         mesh_obj = kwargs['meshes'][mesh_id]
-        filter_id = int(elem.get('id'))
+        filter_id = int(get_text(elem, "id"))
         out = cls(mesh_obj, filter_id=filter_id)
 
-        translation = elem.get('translation')
+        translation = get_elem_list(elem, "translation", float) or []
         if translation:
-            out.translation = [float(x) for x in translation.split()]
+            out.translation = translation
+
+        rotation = get_elem_list(elem, 'rotation', float) or []
+        if rotation:
+            if len(rotation) == 3:
+                out.rotation = rotation
+            elif len(rotation) == 9:
+                out.rotation = np.array(rotation).reshape(3, 3)
         return out
 
 
@@ -1039,6 +1071,187 @@ class MeshBornFilter(MeshFilter):
     """
 
 
+class MeshMaterialFilter(MeshFilter):
+    """Filter events by combinations of mesh elements and materials.
+
+    .. versionadded:: 0.15.3
+
+    Parameters
+    ----------
+    mesh : openmc.MeshBase
+        The mesh object that events will be tallied onto
+    bins : iterable of 2-tuples or numpy.ndarray
+        Combinations of (mesh element, material) to tally, given as 2-tuples.
+        The first value in the tuple represents the index of the mesh element,
+        and the second value indicates the material (either a
+        :class:`openmc.Material` instance of the ID).
+    filter_id : int
+        Unique identifier for the filter
+
+    """
+
+    def __init__(self, mesh: openmc.MeshBase, bins, filter_id=None):
+        self.mesh = mesh
+        self.bins = bins
+        self.id = filter_id
+        self._translation = None
+
+    @classmethod
+    def from_volumes(cls, mesh: openmc.MeshBase, volumes: openmc.MeshMaterialVolumes):
+        """Construct a MeshMaterialFilter from a MeshMaterialVolumes object.
+
+        Parameters
+        ----------
+        mesh : openmc.MeshBase
+            The mesh object that events will be tallied onto
+        volumes : openmc.MeshMaterialVolumes
+            The mesh material volumes to use for the filter
+
+        Returns
+        -------
+        MeshMaterialFilter
+            A new MeshMaterialFilter instance
+
+        """
+        # Get flat arrays of material IDs and element indices
+        mat_ids = volumes._materials[volumes._materials > -1]
+        elems, _ = np.where(volumes._materials > -1)
+
+        # Stack them into a 2D array of (element, material) pairs
+        bins = np.column_stack((elems, mat_ids))
+        return cls(mesh, bins)
+
+    def __hash__(self):
+        data = (type(self).__name__, self.mesh.id, tuple(self.bins.ravel()))
+        return hash(data)
+
+    def __repr__(self):
+        string = type(self).__name__ + '\n'
+        string += '{: <16}=\t{}\n'.format('\tID', self.id)
+        string += '{: <16}=\t{}\n'.format('\tMesh ID', self.mesh.id)
+        string += '{: <16}=\n{}\n'.format('\tBins', self.bins)
+        string += '{: <16}=\t{}\n'.format('\tTranslation', self.translation)
+        return string
+
+    @property
+    def shape(self):
+        return (self.num_bins,)
+
+    @property
+    def mesh(self):
+        return self._mesh
+
+    @mesh.setter
+    def mesh(self, mesh):
+        cv.check_type('filter mesh', mesh, openmc.MeshBase)
+        self._mesh = mesh
+
+    @Filter.bins.setter
+    def bins(self, bins):
+        pairs = np.empty((len(bins), 2), dtype=int)
+        for i, (elem, mat) in enumerate(bins):
+            cv.check_type('element', elem, Integral)
+            cv.check_type('material', mat, (Integral, openmc.Material))
+            pairs[i, 0] = elem
+            pairs[i, 1] = mat if isinstance(mat, Integral) else mat.id
+        self._bins = pairs
+
+    def to_xml_element(self):
+        """Return XML element representing the filter.
+
+        Returns
+        -------
+        element : lxml.etree._Element
+            XML element containing filter data
+
+        """
+        element = ET.Element('filter')
+        element.set('id', str(self.id))
+        element.set('type', self.short_name.lower())
+        element.set('mesh', str(self.mesh.id))
+
+        if self.translation is not None:
+            element.set('translation', ' '.join(map(str, self.translation)))
+
+        subelement = ET.SubElement(element, 'bins')
+        subelement.text = ' '.join(str(i) for i in self.bins.ravel())
+
+        return element
+
+    @classmethod
+    def from_xml_element(cls, elem: ET.Element, **kwargs) -> MeshMaterialFilter:
+        filter_id = int(get_text(elem, "id"))
+        mesh_id = int(get_text(elem, "mesh"))
+        mesh_obj = kwargs['meshes'][mesh_id]
+        bins = get_elem_list(elem, "bins", int) or []
+        bins = list(zip(bins[::2], bins[1::2]))
+        out = cls(mesh_obj, bins, filter_id=filter_id)
+
+        translation = get_elem_list(elem, "translation", float) or []
+        if translation:
+            out.translation = translation
+        return out
+
+    @classmethod
+    def from_hdf5(cls, group, **kwargs):
+        if group['type'][()].decode() != cls.short_name.lower():
+            raise ValueError("Expected HDF5 data for filter type '"
+                             + cls.short_name.lower() + "' but got '"
+                             + group['type'][()].decode() + " instead")
+
+        if 'meshes' not in kwargs:
+            raise ValueError(cls.__name__ + " requires a 'meshes' keyword "
+                             "argument.")
+
+        mesh_id = group['mesh'][()]
+        mesh_obj = kwargs['meshes'][mesh_id]
+        bins = group['bins'][()]
+        filter_id = int(group.name.split('/')[-1].lstrip('filter '))
+        out = cls(mesh_obj, bins, filter_id=filter_id)
+
+        translation = group.get('translation')
+        if translation:
+            out.translation = translation[()]
+
+        return out
+
+    def get_pandas_dataframe(self, data_size, stride, **kwargs):
+        """Builds a Pandas DataFrame for the Filter's bins.
+
+        This method constructs a Pandas DataFrame object for the filter with
+        columns annotated by filter bin information. This is a helper method for
+        :meth:`Tally.get_pandas_dataframe`.
+
+        Parameters
+        ----------
+        data_size : int
+            The total number of bins in the tally corresponding to this filter
+        stride : int
+            Stride in memory for the filter
+
+        Returns
+        -------
+        pandas.DataFrame
+            A Pandas DataFrame with a multi-index column for the cell instance.
+            The number of rows in the DataFrame is the same as the total number
+            of bins in the corresponding tally, with the filter bin appropriately
+            tiled to map to the corresponding tally bins.
+
+        See also
+        --------
+        Tally.get_pandas_dataframe(), CrossFilter.get_pandas_dataframe()
+
+        """
+        # Repeat and tile bins as necessary to account for other filters.
+        bins = np.repeat(self.bins, stride, axis=0)
+        tile_factor = data_size // len(bins)
+        bins = np.tile(bins, (tile_factor, 1))
+
+        columns = pd.MultiIndex.from_product([[self.short_name.lower()],
+                                              ['element', 'material']])
+        return pd.DataFrame(bins, columns=columns)
+
+
 class MeshSurfaceFilter(MeshFilter):
     """Filter events by surface crossings on a mesh.
 
@@ -1065,6 +1278,9 @@ class MeshSurfaceFilter(MeshFilter):
         The number of filter bins
 
     """
+    @property
+    def shape(self):
+        return (self.num_bins,)
 
     @MeshFilter.mesh.setter
     def mesh(self, mesh):
@@ -1183,6 +1399,80 @@ class CollisionFilter(Filter):
             cv.check_greater_than('filter value', x, 0, equality=True)
 
 
+class ReactionFilter(Filter):
+    """Bins tally events based on the reaction type (MT number).
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    bins : str, int, or iterable thereof
+        The reaction types to tally. Can be reaction name strings
+        (e.g., ``'(n,elastic)'``, ``'(n,gamma)'``) or integer MT numbers
+        (e.g., 2, 102). Integer MT values are automatically converted to their
+        canonical string representation.
+    filter_id : int
+        Unique identifier for the filter
+
+    Attributes
+    ----------
+    bins : numpy.ndarray of str
+        Reaction name strings
+    id : int
+        Unique identifier for the filter
+    num_bins : int
+        The number of filter bins
+
+    """
+
+    def __init__(self, bins, filter_id=None):
+        self.bins = bins
+        self.id = filter_id
+
+    @Filter.bins.setter
+    def bins(self, bins):
+        if isinstance(bins, (str, Integral)):
+            bins = [bins]
+        elif not isinstance(bins, list):
+            bins = list(bins)
+        normalized = []
+        for b in bins:
+            if isinstance(b, Integral):
+                if int(b) not in REACTION_NAME:
+                    raise ValueError(f"No known reaction for MT={b}")
+                normalized.append(REACTION_NAME[int(b)])
+            elif isinstance(b, str):
+                if b == 'total':
+                    warnings.warn(
+                        "The reaction name 'total' is ambiguous. Use "
+                        "'(n,total)' for neutron total cross section or "
+                        "'photon-total' for photon total. Interpreting as"
+                        "'(n,total)'.")
+                if b not in REACTION_MT:
+                    raise ValueError(f"Unknown reaction name '{b}'")
+                normalized.append(REACTION_NAME[REACTION_MT[b]])
+            else:
+                raise TypeError(f"Expected str or int for reaction filter "
+                                f"bin, got {type(b)}")
+        self._bins = np.array(normalized, dtype=str)
+
+    @classmethod
+    def from_hdf5(cls, group, **kwargs):
+        if group['type'][()].decode() != cls.short_name.lower():
+            raise ValueError("Expected HDF5 data for filter type '"
+                             + cls.short_name.lower() + "' but got '"
+                             + group['type'][()].decode() + "' instead")
+        bins = [b.decode() for b in group['bins'][()]]
+        filter_id = int(group.name.split('/')[-1].lstrip('filter '))
+        return cls(bins, filter_id=filter_id)
+
+    @classmethod
+    def from_xml_element(cls, elem, **kwargs):
+        filter_id = int(get_text(elem, "id"))
+        bins = get_elem_list(elem, "bins", str) or []
+        return cls(bins, filter_id=filter_id)
+
+
 class RealFilter(Filter):
     """Tally modifier that describes phase-space and other characteristics
 
@@ -1208,6 +1498,7 @@ class RealFilter(Filter):
         The number of filter bins
 
     """
+
     def __init__(self, values, filter_id=None):
         self.values = np.asarray(values)
         self.bins = np.vstack((self.values[:-1], self.values[1:])).T
@@ -1238,7 +1529,7 @@ class RealFilter(Filter):
             cv.check_type('filter value', v1, Real)
 
             # Make sure that each tuple has values that are increasing
-            if v1 < v0:
+            if v1 <= v0:
                 raise ValueError(f'Values {v0} and {v1} appear to be out of '
                                  'order')
 
@@ -1372,8 +1663,8 @@ class RealFilter(Filter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        bins = [float(x) for x in get_text(elem, 'bins').split()]
+        filter_id = int(get_text(elem, "id"))
+        bins = get_elem_list(elem, "bins", float) or []
         return cls(bins, filter_id=filter_id)
 
 
@@ -1384,7 +1675,7 @@ class EnergyFilter(RealFilter):
     ----------
     values : Iterable of Real
         A list of values for which each successive pair constitutes a range of
-        energies in [eV] for a single bin
+        energies in [eV] for a single bin. Entries must be positive and ascending.
     filter_id : int
         Unique identifier for the filter
 
@@ -1483,7 +1774,8 @@ class EnergyFilter(RealFilter):
 
         """
 
-        cv.check_value('group_structure', group_structure, openmc.mgxs.GROUP_STRUCTURES.keys())
+        cv.check_value('group_structure', group_structure,
+                       openmc.mgxs.GROUP_STRUCTURES.keys())
         return cls(openmc.mgxs.GROUP_STRUCTURES[group_structure.upper()])
 
 
@@ -1513,6 +1805,226 @@ class EnergyoutFilter(EnergyFilter):
 
     """
 
+
+class ParticleProductionFilter(Filter):
+    """Bins tally events based on secondary particle type and energy.
+
+    This filter bins secondary particles (e.g., photons, electrons, or recoils)
+    produced in a reaction by particle type and, optionally, by energy. This is
+    useful for constructing production matrices or analyzing secondary particle
+    spectra. Note that unlike other energy filters, the weight that is applied
+    is equal to the weight of the secondary particle. Thus, to obtain secondary
+    particle production, it should be used in conjunction with the "events"
+    score.
+
+    The incident particle type can be filtered using :class:`ParticleFilter`.
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    particles : str, int, openmc.ParticleType, or iterable thereof
+        Type(s) of secondary particle(s) to tally ('photon', 'neutron', etc.)
+    energies : Iterable of Real or str, optional
+        A list of energy boundaries in [eV]; each successive pair defines a bin.
+        Alternatively, the name of the group structure can be given as a string
+        (must be a key in :data:`openmc.mgxs.GROUP_STRUCTURES`). If not
+        provided, the filter tallies total secondary particle production without
+        energy binning.
+    filter_id : int, optional
+        Unique identifier for the filter
+
+    Attributes
+    ----------
+    particles : list of openmc.ParticleType
+        The secondary particle types this filter applies to
+    energies : numpy.ndarray or None
+        Energy boundaries in [eV], or None if no energy binning
+    bins : list
+        A list of bins; each element fully describes one bin. When energies are
+        specified, each element is a tuple ``(particle, energy_low,
+        energy_high)``. When no energies are specified, each element is a
+        particle name string.
+    num_bins : int
+        Total number of filter bins
+    num_energy_bins : int
+        Number of energy bins (1 if no energies specified)
+    shape : tuple of int
+        Shape of the filter as (n_particles, n_energy_bins)
+    """
+
+    def __init__(self, particles, energies=None, filter_id=None):
+        self.particles = particles
+        self.energies = energies
+        self.id = filter_id
+
+    def __repr__(self):
+        string = type(self).__name__ + '\n'
+        string += '{: <16}=\t{}\n'.format('\tParticles',
+            [str(p) for p in self.particles])
+        if self.energies is not None:
+            string += '{: <16}=\t{}\n'.format('\tEnergies', self.energies)
+        string += '{: <16}=\t{}\n'.format('\tID', self.id)
+        return string
+
+    @property
+    def particles(self):
+        return self._particles
+
+    @particles.setter
+    def particles(self, particles):
+        if isinstance(particles, (str, int, openmc.ParticleType)):
+            self._particles = [openmc.ParticleType(particles)]
+        else:
+            self._particles = [openmc.ParticleType(p) for p in particles]
+
+    @property
+    def energies(self):
+        return self._energies
+
+    @energies.setter
+    def energies(self, energies):
+        if energies is None:
+            self._energies = None
+        elif isinstance(energies, str):
+            cv.check_value('energies', energies,
+                           openmc.mgxs.GROUP_STRUCTURES.keys())
+            self._energies = np.array(
+                openmc.mgxs.GROUP_STRUCTURES[energies.upper()])
+        else:
+            energies = np.asarray(energies, dtype=float)
+            cv.check_length('energies', energies, 2)
+            for i in range(len(energies) - 1):
+                if energies[i + 1] <= energies[i]:
+                    raise ValueError("Energy bins must be monotonically "
+                                     "increasing.")
+            self._energies = energies
+
+    @property
+    def bins(self):
+        if self.energies is None:
+            return [str(p) for p in self.particles]
+        else:
+            result = []
+            energy_pairs = np.vstack(
+                (self.energies[:-1], self.energies[1:])).T
+            for particle in self.particles:
+                for e_low, e_high in energy_pairs:
+                    result.append((str(particle), e_low, e_high))
+            return result
+
+    @bins.setter
+    def bins(self, bins):
+        # bins is set indirectly through particles/energies
+        pass
+
+    def check_bins(self, bins):
+        pass
+
+    @property
+    def num_energy_bins(self):
+        if self.energies is None:
+            return 1
+        else:
+            return len(self.energies) - 1
+
+    @property
+    def num_bins(self):
+        return len(self.particles) * self.num_energy_bins
+
+    @property
+    def shape(self):
+        return (len(self.particles), self.num_energy_bins)
+
+    def to_xml_element(self):
+        element = ET.Element('filter')
+        element.set('id', str(self.id))
+        element.set('type', self.short_name.lower())
+
+        subelement = ET.SubElement(element, 'particles')
+        subelement.text = ' '.join(str(p) for p in self.particles)
+
+        if self.energies is not None:
+            subelement = ET.SubElement(element, 'energies')
+            subelement.text = ' '.join(str(e) for e in self.energies)
+
+        return element
+
+    @classmethod
+    def from_xml_element(cls, elem, **kwargs):
+        filter_id = int(elem.get('id'))
+        particles = get_text(elem, 'particles').split()
+
+        bins_elem = elem.find('energies')
+        if bins_elem is not None:
+            energies = [float(x) for x in bins_elem.text.split()]
+        else:
+            energies = None
+
+        return cls(particles, energies=energies, filter_id=filter_id)
+
+    @classmethod
+    def from_hdf5(cls, group, **kwargs):
+        filter_id = int(group.name.split('/')[-1].lstrip('filter '))
+
+        # Read particle types
+        particles = [b.decode() if isinstance(b, bytes) else b
+                     for b in group['particles'][()]]
+
+        # Read energy bins if present
+        if 'energies' in group:
+            energies = group['energies'][()]
+        else:
+            energies = None
+
+        return cls(particles, energies=energies, filter_id=filter_id)
+
+    def get_pandas_dataframe(self, data_size, stride, **kwargs):
+        """Builds a Pandas DataFrame for the Filter's bins.
+
+        This method constructs a Pandas DataFrame object for the filter with
+        columns annotated by filter bin information. This is a helper method for
+        :meth:`Tally.get_pandas_dataframe`.
+
+        Parameters
+        ----------
+        data_size : int
+            The total number of bins in the tally corresponding to this filter
+        stride : int
+            Stride in memory for the filter
+
+        Returns
+        -------
+        pandas.DataFrame
+            A Pandas DataFrame with columns for particle type and, if energy
+            bins are specified, energy bin boundaries.
+
+        See also
+        --------
+        Tally.get_pandas_dataframe(), CrossFilter.get_pandas_dataframe()
+
+        """
+        filter_dict = {}
+        key = self.short_name.lower()
+        n_ebins = self.num_energy_bins
+
+        # Particle column — outer dimension (changes slowest)
+        particle_names = [str(p) for p in self.particles]
+        filter_dict[key, 'particle'] = _repeat_and_tile(
+            np.array(particle_names), n_ebins * stride, data_size)
+
+        # Energy columns only if energies were specified
+        if self.energies is not None:
+            energy_pairs = np.vstack(
+                (self.energies[:-1], self.energies[1:])).T
+            filter_dict[key, 'energy low [eV]'] = _repeat_and_tile(
+                energy_pairs[:, 0], stride, data_size)
+            filter_dict[key, 'energy high [eV]'] = _repeat_and_tile(
+                energy_pairs[:, 1], stride, data_size)
+
+        return pd.DataFrame(filter_dict)
+
+
 class TimeFilter(RealFilter):
     """Bins tally events based on the particle's time.
 
@@ -1656,12 +2168,21 @@ class DistribcellFilter(Filter):
 
     @property
     def paths(self):
-        return self._paths
+        if self._paths is None:
+            if not hasattr(self, '_geometry'):
+                raise ValueError(
+                    "Model must be exported before the 'paths' attribute is" \
+                    "available for a DistribcellFilter.")
 
-    @paths.setter
-    def paths(self, paths):
-        cv.check_iterable_type('paths', paths, str)
-        self._paths = paths
+            # Determine paths for cell instances
+            self._geometry.determine_paths()
+
+            # Get paths for the corresponding cell
+            cell_id = self.bins[0]
+            cell = self._geometry.get_all_cells()[cell_id]
+            self._paths = cell.paths
+
+        return self._paths
 
     @Filter.bins.setter
     def bins(self, bins):
@@ -1671,7 +2192,7 @@ class DistribcellFilter(Filter):
         # Make sure there is only 1 bin.
         if not len(bins) == 1:
             msg = (f'Unable to add bins "{bins}" to a DistribcellFilter since '
-                  'only a single distribcell can be used per tally')
+                   'only a single distribcell can be used per tally')
             raise ValueError(msg)
 
         # Check the type and extract the id, if necessary.
@@ -1824,7 +2345,7 @@ class DistribcellFilter(Filter):
         # requests Summary geometric information
         filter_bins = _repeat_and_tile(
             np.arange(self.num_bins), stride, data_size)
-        df = pd.DataFrame({self.short_name.lower() : filter_bins})
+        df = pd.DataFrame({self.short_name.lower(): filter_bins})
 
         # Concatenate with DataFrame of distribcell instance IDs
         if level_df is not None:
@@ -1863,6 +2384,7 @@ class MuFilter(RealFilter):
         The number of filter bins
 
     """
+
     def __init__(self, values, filter_id=None):
         if isinstance(values, Integral):
             values = np.linspace(-1., 1., values + 1)
@@ -2028,6 +2550,7 @@ class DelayedGroupFilter(Filter):
         The number of filter bins
 
     """
+
     def check_bins(self, bins):
         # Check the bin values.
         for g in bins:
@@ -2096,9 +2619,9 @@ class EnergyFunctionFilter(Filter):
     def __gt__(self, other):
         if type(self) is not type(other):
             if self.short_name in _FILTER_TYPES and \
-                other.short_name in _FILTER_TYPES:
+                    other.short_name in _FILTER_TYPES:
                 delta = _FILTER_TYPES.index(self.short_name) - \
-                        _FILTER_TYPES.index(other.short_name)
+                    _FILTER_TYPES.index(other.short_name)
                 return delta > 0
             else:
                 return False
@@ -2108,9 +2631,9 @@ class EnergyFunctionFilter(Filter):
     def __lt__(self, other):
         if type(self) is not type(other):
             if self.short_name in _FILTER_TYPES and \
-                other.short_name in _FILTER_TYPES:
+                    other.short_name in _FILTER_TYPES:
                 delta = _FILTER_TYPES.index(self.short_name) - \
-                        _FILTER_TYPES.index(other.short_name)
+                    _FILTER_TYPES.index(other.short_name)
                 return delta < 0
             else:
                 return False
@@ -2121,14 +2644,16 @@ class EnergyFunctionFilter(Filter):
         string = type(self).__name__ + '\n'
         string += '{: <16}=\t{}\n'.format('\tEnergy', self.energy)
         string += '{: <16}=\t{}\n'.format('\tInterpolant', self.y)
-        string += '{: <16}=\t{}\n'.format('\tInterpolation', self.interpolation)
+        string += '{: <16}=\t{}\n'.format('\tInterpolation',
+                                          self.interpolation)
         return hash(string)
 
     def __repr__(self):
         string = type(self).__name__ + '\n'
         string += '{: <16}=\t{}\n'.format('\tEnergy', self.energy)
         string += '{: <16}=\t{}\n'.format('\tInterpolant', self.y)
-        string += '{: <16}=\t{}\n'.format('\tInterpolation', self.interpolation)
+        string += '{: <16}=\t{}\n'.format('\tInterpolation',
+                                          self.interpolation)
         string += '{: <16}=\t{}\n'.format('\tID', self.id)
         return string
 
@@ -2214,10 +2739,12 @@ class EnergyFunctionFilter(Filter):
     @interpolation.setter
     def interpolation(self, val):
         cv.check_type('interpolation', val, str)
-        cv.check_value('interpolation', val, self.INTERPOLATION_SCHEMES.values())
+        cv.check_value('interpolation', val,
+                       self.INTERPOLATION_SCHEMES.values())
 
         if val == 'quadratic' and len(self.energy) < 3:
-            raise ValueError('Quadratic interpolation requires 3 or more values.')
+            raise ValueError(
+                'Quadratic interpolation requires 3 or more values.')
 
         if val == 'cubic' and len(self.energy) < 4:
             raise ValueError('Cubic interpolation requires 3 or more values.')
@@ -2262,12 +2789,13 @@ class EnergyFunctionFilter(Filter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        energy = [float(x) for x in get_text(elem, 'energy').split()]
-        y = [float(x) for x in get_text(elem, 'y').split()]
+        filter_id = int(get_text(elem, "id"))
+        energy = get_elem_list(elem, "energy", float) or []
+        y = get_elem_list(elem, "y", float) or []
         out = cls(energy, y, filter_id=filter_id)
-        if elem.find('interpolation') is not None:
-            out.interpolation = elem.find('interpolation').text
+        interpolation = get_text(elem, "interpolation")
+        if interpolation is not None:
+            out.interpolation = interpolation
         return out
 
     def can_merge(self, other):
@@ -2328,3 +2856,26 @@ class EnergyFunctionFilter(Filter):
             {self.short_name.lower(): filter_bins})])
 
         return df
+
+
+class WeightFilter(RealFilter):
+    """Bins tally events based on the incoming particle weight.
+
+    Parameters
+    ----------
+    Values : Iterable of float
+        A list or iterable of the weight boundaries, as float values.
+    filter_id : int
+        Unique identifier for the filter
+
+    Attributes
+    ----------
+    id : int
+        Unique identifier for the filter
+    bins : numpy.ndarray
+        An array of integer values representing the weights by which to filter
+    num_bins : int
+        The number of filter bins
+    values : numpy.ndarray
+        Array of weight boundaries
+    """
diff --git a/openmc/filter_expansion.py b/openmc/filter_expansion.py
index f8e677578..b79c8fc79 100644
--- a/openmc/filter_expansion.py
+++ b/openmc/filter_expansion.py
@@ -3,7 +3,8 @@ from numbers import Integral, Real
 import lxml.etree as ET
 
 import openmc.checkvalue as cv
-from . import Filter
+from .filter import Filter
+from ._xml import get_text
 
 
 class ExpansionFilter(Filter):
@@ -49,8 +50,8 @@ class ExpansionFilter(Filter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        order = int(elem.find('order').text)
+        filter_id = int(get_text(elem, "id"))
+        order = int(get_text(elem, "order"))
         return cls(order, filter_id=filter_id)
 
     def merge(self, other):
@@ -263,11 +264,11 @@ class SpatialLegendreFilter(ExpansionFilter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        order = int(elem.find('order').text)
-        axis = elem.find('axis').text
-        minimum = float(elem.find('min').text)
-        maximum = float(elem.find('max').text)
+        filter_id = int(get_text(elem, "id"))
+        order = int(get_text(elem, "order"))
+        axis = get_text(elem, "axis")
+        minimum = float(get_text(elem, "min"))
+        maximum = float(get_text(elem, "max"))
         return cls(order, axis, minimum, maximum, filter_id=filter_id)
 
 
@@ -362,10 +363,10 @@ class SphericalHarmonicsFilter(ExpansionFilter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        order = int(elem.find('order').text)
+        filter_id = int(get_text(elem, "id"))
+        order = int(get_text(elem, "order"))
         filter = cls(order, filter_id=filter_id)
-        filter.cosine = elem.get('cosine')
+        filter.cosine = get_text(elem, "cosine")
         return filter
 
 
@@ -518,11 +519,11 @@ class ZernikeFilter(ExpansionFilter):
 
     @classmethod
     def from_xml_element(cls, elem, **kwargs):
-        filter_id = int(elem.get('id'))
-        order = int(elem.find('order').text)
-        x = float(elem.find('x').text)
-        y = float(elem.find('y').text)
-        r = float(elem.find('r').text)
+        filter_id = int(get_text(elem, "id"))
+        order = int(get_text(elem, "order"))
+        x = float(get_text(elem, "x"))
+        y = float(get_text(elem, "y"))
+        r = float(get_text(elem, "r"))
         return cls(order, x, y, r, filter_id=filter_id)
 
 
diff --git a/openmc/geometry.py b/openmc/geometry.py
index c069d5796..8496fb23a 100644
--- a/openmc/geometry.py
+++ b/openmc/geometry.py
@@ -140,7 +140,6 @@ class Geometry:
 
         # Clean the indentation in the file to be user-readable
         xml.clean_indentation(element)
-        xml.reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
 
         return element
 
@@ -754,4 +753,31 @@ class Geometry:
 
         .. versionadded:: 0.14.0
         """
-        return self.root_universe.plot(*args, **kwargs)
+        model = openmc.Model()
+        model.geometry = self
+        model.materials = self.get_all_materials().values()
+
+        # collect all the material names from the geometry
+        all_material_names = {m.name for m in model.materials if m.name is not None}
+
+        # makes a placeholder material for each material name if it isn't
+        # already present on the model. These materials are otherwise missing
+        # from the geometry and are needed for plotting.
+        for universe in model.geometry.get_all_universes().values():
+            if not isinstance(universe, openmc.DAGMCUniverse):
+                continue
+            for name in universe.material_names:
+                # if this name is already present in the model, skip it
+                # (this can happen if the same material name is used in multiple
+                # universes)
+                if name in all_material_names:
+                    continue
+                # if the material name is not present on the model,
+                # create a placeholder material with the same name
+                # and add it to the model
+                mat_dag = openmc.Material(name=name)
+                mat_dag.add_nuclide('H1', 1.0)
+                model.materials.append(mat_dag)
+                all_material_names.add(name)
+
+        return model.plot(*args, **kwargs)
diff --git a/openmc/lattice.py b/openmc/lattice.py
index 518068560..21849ec40 100644
--- a/openmc/lattice.py
+++ b/openmc/lattice.py
@@ -10,7 +10,7 @@ import numpy as np
 
 import openmc
 import openmc.checkvalue as cv
-from ._xml import get_text
+from ._xml import get_elem_list, get_text
 from .mixin import IDManagerMixin
 
 
@@ -959,18 +959,17 @@ class RectLattice(Lattice):
         lat_id = int(get_text(elem, 'id'))
         name = get_text(elem, 'name')
         lat = cls(lat_id, name)
-        lat.lower_left = [float(i)
-                          for i in get_text(elem, 'lower_left').split()]
-        lat.pitch = [float(i) for i in get_text(elem, 'pitch').split()]
+        lat.lower_left = get_elem_list(elem, "lower_left", float)
+        lat.pitch = get_elem_list(elem, "pitch", float)
         outer = get_text(elem, 'outer')
         if outer is not None:
             lat.outer = get_universe(int(outer))
 
         # Get array of universes
-        dimension = get_text(elem, 'dimension').split()
+        dimension = get_elem_list(elem, 'dimension', int)
         shape = np.array(dimension, dtype=int)[::-1]
-        uarray = np.array([get_universe(int(i)) for i in
-                           get_text(elem, 'universes').split()])
+        universes = get_elem_list(elem, 'universes', int)
+        uarray = np.array([get_universe(u) for u in universes])
         uarray.shape = shape
         lat.universes = uarray
         return lat
@@ -1084,8 +1083,11 @@ class HexLattice(Lattice):
         from the outermost ring), and i is the index with a ring starting from
         the top and proceeding clockwise.
     orientation : {'x', 'y'}
-        str by default 'y' orientation of main lattice diagonal another option
-        - 'x'
+        The orientation of the lattice. The 'x' orientation means that each
+        lattice element has two faces that are perpendicular to the x-axis,
+        while the 'y' orientation means that each lattice element has two faces
+        that are perpendicular to the y-axis. By default, the orientation is
+        'y'.
     num_rings : int
         Number of radial ring positions in the xy-plane
     num_axial : int
@@ -1530,8 +1532,8 @@ class HexLattice(Lattice):
         lat_id = int(get_text(elem, 'id'))
         name = get_text(elem, 'name')
         lat = cls(lat_id, name)
-        lat.center = [float(i) for i in get_text(elem, 'center').split()]
-        lat.pitch = [float(i) for i in get_text(elem, 'pitch').split()]
+        lat.center = get_elem_list(elem, "center", float)
+        lat.pitch = get_elem_list(elem, "pitch", float)
         lat.orientation = get_text(elem, 'orientation', 'y')
         outer = get_text(elem, 'outer')
         if outer is not None:
@@ -1548,8 +1550,8 @@ class HexLattice(Lattice):
             univs = [deepcopy(univs) for i in range(n_axial)]
 
         # Get flat array of universes
-        uarray = np.array([get_universe(int(i)) for i in
-                           get_text(elem, 'universes').split()])
+        universes = get_elem_list(elem, "universes", int)
+        uarray = np.array([get_universe(u) for u in universes])
 
         # Fill nested lists
         j = 0
diff --git a/openmc/lib/__init__.py b/openmc/lib/__init__.py
index 15642b42b..9b135370f 100644
--- a/openmc/lib/__init__.py
+++ b/openmc/lib/__init__.py
@@ -46,12 +46,12 @@ def _coord_levels():
 def _libmesh_enabled():
     return c_bool.in_dll(_dll, "LIBMESH_ENABLED").value
 
-def _mcpl_enabled():
-    return c_bool.in_dll(_dll, "MCPL_ENABLED").value
-
 def _uwuw_enabled():
     return c_bool.in_dll(_dll, "UWUW_ENABLED").value
 
+def _strict_fp_enabled():
+    return c_bool.in_dll(_dll, "STRICT_FP_ENABLED").value
+
 
 from .error import *
 from .core import *
diff --git a/openmc/lib/cell.py b/openmc/lib/cell.py
index 971a24cba..dfd09d2f9 100644
--- a/openmc/lib/cell.py
+++ b/openmc/lib/cell.py
@@ -34,6 +34,10 @@ _dll.openmc_cell_get_temperature.argtypes = [
     c_int32, POINTER(c_int32), POINTER(c_double)]
 _dll.openmc_cell_get_temperature.restype = c_int
 _dll.openmc_cell_get_temperature.errcheck = _error_handler
+_dll.openmc_cell_get_density.argtypes = [
+    c_int32, POINTER(c_int32), POINTER(c_double)]
+_dll.openmc_cell_get_density.restype = c_int
+_dll.openmc_cell_get_density.errcheck = _error_handler
 _dll.openmc_cell_get_name.argtypes = [c_int32, POINTER(c_char_p)]
 _dll.openmc_cell_get_name.restype = c_int
 _dll.openmc_cell_get_name.errcheck = _error_handler
@@ -58,6 +62,10 @@ _dll.openmc_cell_set_temperature.argtypes = [
     c_int32, c_double, POINTER(c_int32), c_bool]
 _dll.openmc_cell_set_temperature.restype = c_int
 _dll.openmc_cell_set_temperature.errcheck = _error_handler
+_dll.openmc_cell_set_density.argtypes = [
+    c_int32, c_double, POINTER(c_int32), c_bool]
+_dll.openmc_cell_set_density.restype = c_int
+_dll.openmc_cell_set_density.errcheck = _error_handler
 _dll.openmc_cell_set_translation.argtypes = [c_int32, POINTER(c_double)]
 _dll.openmc_cell_set_translation.restype = c_int
 _dll.openmc_cell_set_translation.errcheck = _error_handler
@@ -236,6 +244,44 @@ class Cell(_FortranObjectWithID):
 
         _dll.openmc_cell_set_temperature(self._index, T, instance, set_contained)
 
+    def get_density(self, instance: int | None = None):
+        """Get the density of a cell in [g/cm3]
+
+        Parameters
+        ----------
+        instance : int or None
+            Which instance of the cell
+
+        """
+
+        if instance is not None:
+            instance = c_int32(instance)
+
+        rho = c_double()
+        _dll.openmc_cell_get_density(self._index, instance, rho)
+        return rho.value
+
+    def set_density(self, rho: float, instance: int | None = None,
+                    set_contained: bool = False):
+        """Set the density of a cell
+
+        Parameters
+        ----------
+        rho : float
+            Density of the cell in [g/cm3]
+        instance : int or None
+            Which instance of the cell
+        set_contained : bool
+            If cell is not filled by a material, whether to set the density
+            of all filled cells
+
+        """
+
+        if instance is not None:
+            instance = c_int32(instance)
+
+        _dll.openmc_cell_set_density(self._index, rho, instance, set_contained)
+
     @property
     def translation(self):
         translation = np.zeros(3)
diff --git a/openmc/lib/core.py b/openmc/lib/core.py
index 577913dcb..0ee8c3ff0 100644
--- a/openmc/lib/core.py
+++ b/openmc/lib/core.py
@@ -4,13 +4,17 @@ from ctypes import (c_bool, c_int, c_int32, c_int64, c_double, c_char_p,
                     c_uint64, c_size_t)
 import sys
 import os
+from pathlib import Path
 from random import getrandbits
+from tempfile import TemporaryDirectory
+import traceback as tb
 
 import numpy as np
 from numpy.ctypeslib import as_array
 
 from . import _dll
 from .error import _error_handler
+from ..mpi import comm
 from openmc.checkvalue import PathLike
 import openmc.lib
 import openmc
@@ -24,11 +28,13 @@ class _SourceSite(Structure):
                 ('wgt', c_double),
                 ('delayed_group', c_int),
                 ('surf_id', c_int),
-                ('particle', c_int),
+                ('particle', c_int32),
                 ('parent_nuclide', c_int),
                 ('parent_id', c_int64),
-                ('progeny_id', c_int64)]
-
+                ('progeny_id', c_int64),
+                ('wgt_born', c_double),
+                ('wgt_ww_born', c_double),
+                ('n_split', c_int64)]
 
 # Define input type for numpy arrays that will be passed into C++ functions
 # Must be an int or double array, with single dimension that is contiguous
@@ -78,6 +84,7 @@ _dll.openmc_properties_import.restype = c_int
 _dll.openmc_properties_import.errcheck = _error_handler
 _dll.openmc_run.restype = c_int
 _dll.openmc_run.errcheck = _error_handler
+_dll.openmc_run_random_ray.restype = None
 _dll.openmc_reset.restype = c_int
 _dll.openmc_reset.errcheck = _error_handler
 _dll.openmc_reset_timers.restype = c_int
@@ -475,10 +482,23 @@ def run(output=True):
         _dll.openmc_run()
 
 
+def run_random_ray(output=True):
+    """Run a random ray simulation
+
+    Parameters
+    ----------
+    output : bool, optional
+        Whether or not to show output. Defaults to showing output
+    """
+
+    with quiet_dll(output):
+        _dll.openmc_run_random_ray()
+
 def sample_external_source(
         n_samples: int = 1000,
-        prn_seed: int | None = None
-) -> openmc.ParticleList:
+        prn_seed: int | None = None,
+        as_array: bool = False
+) -> openmc.ParticleList | np.ndarray:
     """Sample external source and return source particles.
 
     .. versionadded:: 0.13.1
@@ -490,11 +510,20 @@ def sample_external_source(
     prn_seed : int
         Pseudorandom number generator (PRNG) seed; if None, one will be
         generated randomly.
+    as_array : bool
+        If True, return a numpy structured array instead of a
+        :class:`~openmc.ParticleList`.  The array has fields ``'r'`` (float64,
+        shape 3), ``'u'`` (float64, shape 3), ``'E'`` (float64), ``'time'``
+        (float64), ``'wgt'`` (float64), ``'delayed_group'`` (int32),
+        ``'surf_id'`` (int32), and ``'particle'`` (int32).  This avoids the
+        overhead of constructing individual :class:`~openmc.SourceParticle`
+        objects and is substantially faster for large sample counts.
 
     Returns
     -------
-    openmc.ParticleList
-        List of sampled source particles
+    openmc.ParticleList or numpy.ndarray
+        List of sampled source particles, or a structured array when
+        *as_array* is True.
 
     """
     if n_samples <= 0:
@@ -502,18 +531,28 @@ def sample_external_source(
     if prn_seed is None:
         prn_seed = getrandbits(63)
 
-    # Call into C API to sample source
-    sites_array = (_SourceSite * n_samples)()
-    _dll.openmc_sample_external_source(c_size_t(n_samples), c_uint64(prn_seed), sites_array)
+    # Pre-allocate output array and sample all particles in a single C call
+    result = np.empty(n_samples, dtype=_SourceSite)
+    sites_array = (_SourceSite * n_samples).from_buffer(result)
+    _dll.openmc_sample_external_source(
+        c_size_t(n_samples),
+        c_uint64(prn_seed),
+        sites_array,
+    )
 
-    # Convert to list of SourceParticle and return
-    return openmc.ParticleList([openmc.SourceParticle(
-            r=site.r, u=site.u, E=site.E, time=site.time, wgt=site.wgt,
-            delayed_group=site.delayed_group, surf_id=site.surf_id,
-            particle=openmc.ParticleType(site.particle)
+    if as_array:
+        return result
+
+    particles = [
+        openmc.SourceParticle(
+            r=site.r, u=site.u, E=site.E, time=site.time,
+            wgt=site.wgt, delayed_group=site.delayed_group,
+            surf_id=site.surf_id,
+            particle=openmc.ParticleType(site.particle),
         )
         for site in sites_array
-    ])
+    ]
+    return openmc.ParticleList(particles)
 
 
 def simulation_init():
@@ -617,6 +656,106 @@ def run_in_memory(**kwargs):
         finalize()
 
 
+class TemporarySession:
+    """Context manager for running via openmc.lib in a temporary directory.
+
+    This class is useful for accessing functionality from openmc.lib without
+    polluting your current working directory with OpenMC files. It is used
+    internally as a persistent session to avoid loading cross sections multiple
+    times.
+
+    Parameters
+    ----------
+    model : openmc.Model, optional
+        OpenMC model to use for the session. If None, a minimal working model is
+        created.
+    cwd : PathLike, optional
+        Working directory in which to run OpenMC. If None, a temporary directory
+        is created and deleted automatically.
+    **init_kwargs
+        Keyword arguments to pass to :func:`openmc.lib.init`.
+
+    Attributes
+    ----------
+    model : openmc.Model
+        The OpenMC model used for the session.
+    comm : mpi4py.MPI.Intracomm
+        The MPI intracommunicator used for the session.
+
+    """
+    def __init__(self, model=None, cwd=None, **init_kwargs):
+        self.init_kwargs = dict(init_kwargs)
+        self.cwd = cwd
+        if model is None:
+            surf = openmc.Sphere(boundary_type="vacuum")
+            cell = openmc.Cell(region=-surf)
+            model = openmc.Model()
+            model.geometry = openmc.Geometry([cell])
+            model.settings = openmc.Settings(
+                particles=1, batches=1, output={'summary': False})
+        self.model = model
+
+        # Determine MPI intercommunicator
+        self.comm = self.init_kwargs.get('intracomm') or comm
+        self.init_kwargs['intracomm'] = self.comm
+
+    def __enter__(self):
+        """Initialize the OpenMC library in a temporary directory."""
+        # If already initialized, the context manager is a no-op
+        self.already_initialized = openmc.lib.is_initialized
+        if self.already_initialized:
+            return self
+
+        # Store original working directory
+        self.orig_dir = Path.cwd()
+
+        if self.cwd is None:
+            # Set up temporary directory on rank 0
+            if self.comm.rank == 0:
+                self._tmp_dir = TemporaryDirectory()
+                self.cwd = self._tmp_dir.name
+
+            # Broadcast the path so that all ranks use the same directory
+            self.cwd = self.comm.bcast(self.cwd)
+
+        # Create and change to specified directory
+        self.cwd = Path(self.cwd)
+        self.cwd.mkdir(parents=True, exist_ok=True)
+        os.chdir(self.cwd)
+
+        # Export model on first rank and initialize OpenMC
+        if self.comm.rank == 0:
+            self.model.export_to_model_xml()
+        self.comm.barrier()
+        openmc.lib.init(**self.init_kwargs)
+
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        """Finalize the OpenMC library and clean up temporary directory."""
+        if self.already_initialized:
+            return
+
+        # If an exception occurred, abort all ranks immediately
+        if exc_type is not None:
+            # Print exception info on the rank that failed
+            tb.print_exception(exc_type, exc_value, traceback)
+            sys.stdout.flush()
+
+            # Abort all MPI processes
+            self.comm.Abort(1)
+
+        try:
+            finalize()
+        finally:
+            os.chdir(self.orig_dir)
+
+            # Make sure all ranks have finalized before deleting temporary dir
+            self.comm.barrier()
+            if hasattr(self, '_tmp_dir'):
+                self._tmp_dir.cleanup()
+
+
 class _DLLGlobal:
     """Data descriptor that exposes global variables from libopenmc."""
     def __init__(self, ctype, name):
diff --git a/openmc/lib/filter.py b/openmc/lib/filter.py
index b6086c567..574a37443 100644
--- a/openmc/lib/filter.py
+++ b/openmc/lib/filter.py
@@ -21,9 +21,10 @@ __all__ = [
     'CellInstanceFilter', 'CollisionFilter', 'DistribcellFilter', 'DelayedGroupFilter',
     'EnergyFilter', 'EnergyoutFilter', 'EnergyFunctionFilter', 'LegendreFilter',
     'MaterialFilter', 'MaterialFromFilter', 'MeshFilter', 'MeshBornFilter',
-    'MeshSurfaceFilter', 'MuFilter', 'MuSurfaceFilter', 'ParentNuclideFilter',
-    'ParticleFilter', 'PolarFilter', 'SphericalHarmonicsFilter',
-    'SpatialLegendreFilter', 'SurfaceFilter', 'UniverseFilter', 'ZernikeFilter',
+    'MeshMaterialFilter', 'MeshSurfaceFilter', 'MuFilter', 'MuSurfaceFilter',
+    'ParentNuclideFilter', 'ParticleFilter', 'ParticleProductionFilter', 'PolarFilter',
+    'ReactionFilter', 'SphericalHarmonicsFilter', 'SpatialLegendreFilter',
+    'SurfaceFilter', 'TimeFilter', 'UniverseFilter', 'WeightFilter', 'ZernikeFilter',
     'ZernikeRadialFilter', 'filters'
 ]
 
@@ -97,6 +98,14 @@ _dll.openmc_mesh_filter_get_translation.errcheck = _error_handler
 _dll.openmc_mesh_filter_set_translation.argtypes = [c_int32, POINTER(c_double*3)]
 _dll.openmc_mesh_filter_set_translation.restype = c_int
 _dll.openmc_mesh_filter_set_translation.errcheck = _error_handler
+_dll.openmc_mesh_filter_get_rotation.argtypes = [c_int32, POINTER(c_double),
+    POINTER(c_size_t)]
+_dll.openmc_mesh_filter_get_rotation.restype = c_int
+_dll.openmc_mesh_filter_get_rotation.errcheck = _error_handler
+_dll.openmc_mesh_filter_set_rotation.argtypes = [
+    c_int32, POINTER(c_double), c_size_t]
+_dll.openmc_mesh_filter_set_rotation.restype = c_int
+_dll.openmc_mesh_filter_set_rotation.errcheck = _error_handler
 _dll.openmc_meshborn_filter_get_mesh.argtypes = [c_int32, POINTER(c_int32)]
 _dll.openmc_meshborn_filter_get_mesh.restype = c_int
 _dll.openmc_meshborn_filter_get_mesh.errcheck = _error_handler
@@ -124,6 +133,9 @@ _dll.openmc_meshsurface_filter_set_translation.errcheck = _error_handler
 _dll.openmc_new_filter.argtypes = [c_char_p, POINTER(c_int32)]
 _dll.openmc_new_filter.restype = c_int
 _dll.openmc_new_filter.errcheck = _error_handler
+_dll.openmc_particle_filter_get_bins.argtypes = [c_int32, POINTER(c_int32)]
+_dll.openmc_particle_filter_get_bins.restype = c_int
+_dll.openmc_particle_filter_get_bins.errcheck = _error_handler
 _dll.openmc_spatial_legendre_filter_get_order.argtypes = [c_int32, POINTER(c_int)]
 _dll.openmc_spatial_legendre_filter_get_order.restype = c_int
 _dll.openmc_spatial_legendre_filter_get_order.errcheck = _error_handler
@@ -393,6 +405,10 @@ class MeshFilter(Filter):
         Mesh used for the filter
     translation : Iterable of float
         3-D coordinates of the translation vector
+    rotation : Iterable of float
+        The rotation matrix or angles of the filter mesh. This can either be
+        a fully specified 3 x 3 rotation matrix or an Iterable of length 3
+        with the angles in degrees about the x, y, and z axes, respectively.
 
     """
     filter_type = 'mesh'
@@ -422,6 +438,34 @@ class MeshFilter(Filter):
     def translation(self, translation):
         _dll.openmc_mesh_filter_set_translation(self._index, (c_double*3)(*translation))
 
+    @property
+    def rotation(self):
+        rotation_data = np.zeros(12)
+        rot_size = c_size_t()
+
+        _dll.openmc_mesh_filter_get_rotation(
+            self._index, rotation_data.ctypes.data_as(POINTER(c_double)),
+            rot_size)
+        rot_size = rot_size.value
+
+        if rot_size == 9:
+            return rotation_data[:rot_size].shape(3, 3)
+        elif rot_size in (0, 12):
+            # If size is 0, rotation_data[9:] will be zeros. This indicates no
+            # rotation and is the most straightforward way to always return
+            # an iterable of floats
+            return rotation_data[9:]
+        else:
+            raise ValueError(
+                f'Invalid size of rotation matrix: {rot_size}')
+
+    @rotation.setter
+    def rotation(self, rotation_data):
+        flat_rotation = np.asarray(rotation_data, dtype=float).flatten()
+
+        _dll.openmc_mesh_filter_set_rotation(
+            self._index, flat_rotation.ctypes.data_as(POINTER(c_double)),
+            c_size_t(len(flat_rotation)))
 
 class MeshBornFilter(Filter):
     """MeshBorn filter stored internally.
@@ -480,6 +524,10 @@ class MeshBornFilter(Filter):
         _dll.openmc_meshborn_filter_set_translation(self._index, (c_double*3)(*translation))
 
 
+class MeshMaterialFilter(Filter):
+    filter_type = 'meshmaterial'
+
+
 class MeshSurfaceFilter(Filter):
     """MeshSurface filter stored internally.
 
@@ -554,16 +602,24 @@ class ParticleFilter(Filter):
 
     @property
     def bins(self):
-        particle_i = np.zeros((self.n_bins,), dtype=c_int)
+        particle_i = np.zeros((self.n_bins,), dtype=c_int32)
         _dll.openmc_particle_filter_get_bins(
-            self._index, particle_i.ctypes.data_as(POINTER(c_int)))
+            self._index, particle_i.ctypes.data_as(POINTER(c_int32)))
         return [ParticleType(i) for i in particle_i]
 
 
+class ParticleProductionFilter(Filter):
+    filter_type = 'particleproduction'
+
+
 class PolarFilter(Filter):
     filter_type = 'polar'
 
 
+class ReactionFilter(Filter):
+    filter_type = 'reaction'
+
+
 class SphericalHarmonicsFilter(Filter):
     filter_type = 'sphericalharmonics'
 
@@ -606,10 +662,18 @@ class SurfaceFilter(Filter):
     filter_type = 'surface'
 
 
+class TimeFilter(Filter):
+    filter_type = 'time'
+
+
 class UniverseFilter(Filter):
     filter_type = 'universe'
 
 
+class WeightFilter(Filter):
+    filter_type = 'weight'
+
+
 class ZernikeFilter(Filter):
     filter_type = 'zernike'
 
@@ -639,6 +703,7 @@ _FILTER_TYPE_MAP = {
     'cellborn': CellbornFilter,
     'cellfrom': CellfromFilter,
     'cellinstance': CellInstanceFilter,
+    'collision': CollisionFilter,
     'delayedgroup': DelayedGroupFilter,
     'distribcell': DistribcellFilter,
     'energy': EnergyFilter,
@@ -649,16 +714,21 @@ _FILTER_TYPE_MAP = {
     'materialfrom': MaterialFromFilter,
     'mesh': MeshFilter,
     'meshborn': MeshBornFilter,
+    'meshmaterial': MeshMaterialFilter,
     'meshsurface': MeshSurfaceFilter,
     'mu': MuFilter,
     'musurface': MuSurfaceFilter,
     'parentnuclide': ParentNuclideFilter,
     'particle': ParticleFilter,
+    'particleproduction': ParticleProductionFilter,
     'polar': PolarFilter,
+    'reaction': ReactionFilter,
     'sphericalharmonics': SphericalHarmonicsFilter,
     'spatiallegendre': SpatialLegendreFilter,
     'surface': SurfaceFilter,
+    'time': TimeFilter,
     'universe': UniverseFilter,
+    'weight': WeightFilter,
     'zernike': ZernikeFilter,
     'zernikeradial': ZernikeRadialFilter
 }
diff --git a/openmc/lib/mesh.py b/openmc/lib/mesh.py
index 3a720f98a..19e6f74d7 100644
--- a/openmc/lib/mesh.py
+++ b/openmc/lib/mesh.py
@@ -1,5 +1,5 @@
 from collections.abc import Mapping, Sequence
-from ctypes import (c_int, c_int32, c_char_p, c_double, POINTER,
+from ctypes import (c_int, c_int32, c_char_p, c_double, POINTER, c_void_p,
                     create_string_buffer, c_size_t)
 from math import sqrt
 import sys
@@ -47,7 +47,8 @@ _dll.openmc_mesh_bounding_box.argtypes = [
 _dll.openmc_mesh_bounding_box.restype = c_int
 _dll.openmc_mesh_bounding_box.errcheck = _error_handler
 _dll.openmc_mesh_material_volumes.argtypes = [
-    c_int32, c_int, c_int, c_int, c_int, arr_2d_int32, arr_2d_double]
+    c_int32, c_int, c_int, c_int, c_int, arr_2d_int32, arr_2d_double,
+    c_void_p]
 _dll.openmc_mesh_material_volumes.restype = c_int
 _dll.openmc_mesh_material_volumes.errcheck = _error_handler
 _dll.openmc_mesh_get_plot_bins.argtypes = [
@@ -188,6 +189,7 @@ class Mesh(_FortranObjectWithID):
             n_samples: int | tuple[int, int, int] = 10_000,
             max_materials: int = 4,
             output: bool = True,
+            bounding_boxes: bool = False,
     ) -> MeshMaterialVolumes:
         """Determine volume of materials in each mesh element.
 
@@ -213,6 +215,11 @@ class Mesh(_FortranObjectWithID):
             Estimated maximum number of materials in any given mesh element.
         output : bool, optional
             Whether or not to show output.
+        bounding_boxes : bool, optional
+            Whether or not to compute an axis-aligned bounding box for each
+            (mesh element, material) combination. When enabled, the bounding
+            box encloses the ray-estimator prisms used for the volume
+            estimation.
 
         Returns
         -------
@@ -243,23 +250,36 @@ class Mesh(_FortranObjectWithID):
         table_size = slot_factor*max_materials
         materials = np.full((n, table_size), EMPTY_SLOT, dtype=np.int32)
         volumes = np.zeros((n, table_size), dtype=np.float64)
+        bboxes = None
+        if bounding_boxes:
+            bboxes = np.empty((n, table_size, 6), dtype=np.float64)
+            bboxes[..., 0:3] = np.inf
+            bboxes[..., 3:6] = -np.inf
 
         # Run material volume calculation
         while True:
             try:
+                bboxes_ptr = None
+                if bboxes is not None:
+                    bboxes_ptr = bboxes.ctypes.data_as(POINTER(c_double))
                 with quiet_dll(output):
                     _dll.openmc_mesh_material_volumes(
-                        self._index, nx, ny, nz, table_size, materials, volumes)
+                        self._index, nx, ny, nz, table_size, materials,
+                        volumes, bboxes_ptr)
             except AllocationError:
                 # Increase size of result array and try again
                 table_size *= 2
                 materials = np.full((n, table_size), EMPTY_SLOT, dtype=np.int32)
                 volumes = np.zeros((n, table_size), dtype=np.float64)
+                if bounding_boxes:
+                    bboxes = np.empty((n, table_size, 6), dtype=np.float64)
+                    bboxes[..., 0:3] = np.inf
+                    bboxes[..., 3:6] = -np.inf
             else:
                 # If no error, break out of loop
                 break
 
-        return MeshMaterialVolumes(materials, volumes)
+        return MeshMaterialVolumes(materials, volumes, bboxes)
 
     def get_plot_bins(
             self,
diff --git a/openmc/lib/nuclide.py b/openmc/lib/nuclide.py
index 8078882cf..ef1287cf3 100644
--- a/openmc/lib/nuclide.py
+++ b/openmc/lib/nuclide.py
@@ -40,8 +40,14 @@ def load_nuclide(name):
     name : str
         Name of the nuclide, e.g. 'U235'
 
+    Returns
+    -------
+    Nuclide
+        The class:`Nuclide` that was just loaded.
+
     """
     _dll.openmc_load_nuclide(name.encode(), None, 0)
+    return nuclides[name]
 
 
 class Nuclide(_FortranObject):
diff --git a/openmc/lib/plot.py b/openmc/lib/plot.py
index d98636676..196682394 100644
--- a/openmc/lib/plot.py
+++ b/openmc/lib/plot.py
@@ -1,10 +1,15 @@
+from collections.abc import Mapping
 from ctypes import (c_bool, c_int, c_size_t, c_int32,
-                    c_double, Structure, POINTER)
+                    c_double, c_uint8, Structure, POINTER)
+from weakref import WeakValueDictionary
 
+from ..exceptions import AllocationError, InvalidIDError
 from . import _dll
+from .core import _FortranObjectWithID
 from .error import _error_handler
 
 import numpy as np
+import warnings
 
 
 class _Position(Structure):
@@ -47,219 +52,637 @@ class _Position(Structure):
         return f"({self.x}, {self.y}, {self.z})"
 
 
-class _PlotBase(Structure):
-    """A structure defining a 2-D geometry slice with underlying c-types
+def _extract_slice_data_args(plot):
+    """Convert a legacy plot-like object into slice_data keyword arguments."""
+    try:
+        kwargs = {
+            'origin': tuple(plot.origin),
+            'width': (plot.width, plot.height),
+            'basis': plot.basis,
+            'pixels': (plot.h_res, plot.v_res),
+            'show_overlaps': getattr(plot, 'color_overlaps', False),
+            'level': getattr(plot, 'level', -1),
+        }
+    except AttributeError as exc:
+        raise TypeError(
+            "plot must be a legacy plot-like object with origin, width, "
+            "height, basis, h_res, and v_res attributes."
+        ) from exc
+    return kwargs
 
-    C-Type Attributes
-    -----------------
-    origin : openmc.lib.plot._Position
-        A position defining the origin of the plot.
-    width_ : openmc.lib.plot._Position
-        The width of the plot along the x, y, and z axes, respectively
-    basis_ : c_int
-        The axes basis of the plot view.
-    pixels_ : c_size_t[3]
-        The resolution of the plot in the horizontal and vertical dimensions
-    level_ : c_int
-        The universe level for the plot view
 
-    Attributes
+_dll.openmc_slice_data.argtypes = [
+    POINTER(c_double * 3),   # origin
+    POINTER(c_double * 3),   # u_span
+    POINTER(c_double * 3),   # v_span
+    POINTER(c_size_t * 2),   # pixels
+    c_bool,                  # show_overlaps
+    c_int,                   # level
+    c_int32,                 # filter_index
+    POINTER(c_int32),        # geom_data
+    POINTER(c_double),       # property_data (can be None)
+]
+_dll.openmc_slice_data.restype = c_int
+_dll.openmc_slice_data.errcheck = _error_handler
+
+
+def slice_data(origin, width=None, basis='xy', u_span=None, v_span=None,
+                pixels=None, show_overlaps=False, level=None, filter=None,
+                include_properties=True):
+    """Generate a 2D raster of geometry and property data for plotting.
+
+    Parameters
     ----------
-    origin : tuple or list of ndarray
-        Origin (center) of the plot
-    width : float
-        The horizontal dimension of the plot in geometry units (cm)
-    height : float
-        The vertical dimension of the plot in geometry units (cm)
-    basis : string
-        One of {'xy', 'xz', 'yz'} indicating the horizontal and vertical
-        axes of the plot.
-    h_res : int
-        The horizontal resolution of the plot in pixels
-    v_res : int
-        The vertical resolution of the plot in pixels
-    level : int
-        The universe level for the plot (default: -1 -> all universes shown)
+    origin : sequence of float
+        Center position of the plot [x, y, z]
+    width : sequence of float
+        Width of the plot [horizontal, vertical]. Mutually exclusive with
+        u_span/v_span.
+    basis : {'xy', 'xz', 'yz'} or int
+        Plot basis. Ignored if u_span/v_span are provided.
+    u_span : sequence of float, optional
+        Full-width span vector for the horizontal axis (3 values). Mutually
+        exclusive with width.
+    v_span : sequence of float, optional
+        Full-height span vector for the vertical axis (3 values). Mutually
+        exclusive with width.
+    pixels : sequence of int
+        Number of pixels [horizontal, vertical]
+    show_overlaps : bool, optional
+        Whether to detect overlapping cells
+    level : int, optional
+        Universe level (None for deepest)
+    filter : openmc.lib.Filter, optional
+        Filter for bin index lookup
+    include_properties : bool, optional
+        Whether to compute temperature/density
+
+    Returns
+    -------
+    geom_data : numpy.ndarray
+        Array of shape (v_res, h_res, 3) or (v_res, h_res, 4) with int32 dtype.
+        Contains [cell_id, cell_instance, material_id] when no filter is provided,
+        or [cell_id, cell_instance, material_id, filter_bin] when a filter is provided.
+    property_data : numpy.ndarray or None
+        Array of shape (v_res, h_res, 2) with float64 dtype containing
+        [temperature, density], or None if include_properties=False
     """
-    _fields_ = [('origin_', _Position),
-                ('width_', _Position),
-                ('basis_', c_int),
-                ('pixels_', 3*c_size_t),
-                ('color_overlaps_', c_bool),
-                ('level_', c_int)]
+    # Set deepest level as default
+    if level is None:
+        level = -1
+    if not isinstance(level, int):
+        raise TypeError("level must be an integer.")
 
-    def __init__(self):
-        self.level_ = -1
-        self.color_overlaps_ = False
+    if pixels is None:
+        raise ValueError("pixels must be specified.")
+    if len(pixels) != 2:
+        raise ValueError("pixels must be a length-2 sequence.")
 
-    @property
-    def origin(self):
-        return self.origin_
+    if width is not None and (u_span is not None or v_span is not None):
+        raise ValueError("width is mutually exclusive with u_span/v_span.")
 
-    @origin.setter
-    def origin(self, origin):
-        self.origin_.x = origin[0]
-        self.origin_.y = origin[1]
-        self.origin_.z = origin[2]
-
-    @property
-    def width(self):
-        return self.width_.x
-
-    @width.setter
-    def width(self, width):
-        self.width_.x = width
-
-    @property
-    def height(self):
-        return self.width_.y
-
-    @height.setter
-    def height(self, height):
-        self.width_.y = height
-
-    @property
-    def basis(self):
-        if self.basis_ == 1:
-            return 'xy'
-        elif self.basis_ == 2:
-            return 'xz'
-        elif self.basis_ == 3:
-            return 'yz'
-
-        raise ValueError(f"Plot basis {self.basis_} is invalid")
-
-    @basis.setter
-    def basis(self, basis):
+    if u_span is not None or v_span is not None:
+        if u_span is None or v_span is None:
+            raise ValueError("Both u_span and v_span must be provided.")
+        u_span = np.asarray(u_span, dtype=float)
+        v_span = np.asarray(v_span, dtype=float)
+        if u_span.shape != (3,) or v_span.shape != (3,):
+            raise ValueError("u_span and v_span must be length-3 sequences.")
+        u_norm = np.linalg.norm(u_span)
+        v_norm = np.linalg.norm(v_span)
+        if u_norm == 0.0 or v_norm == 0.0:
+            raise ValueError("u_span and v_span must be non-zero vectors.")
+        dot = float(np.dot(u_span, v_span))
+        ortho_tol = 1.0e-10 * u_norm * v_norm
+        if abs(dot) > ortho_tol:
+            raise ValueError("u_span and v_span must be orthogonal.")
+    else:
+        if width is None:
+            raise ValueError("width must be provided when u_span/v_span are not set.")
+        if len(width) != 2:
+            raise ValueError("width must be a length-2 sequence.")
+        basis_map = {'xy': 1, 'xz': 2, 'yz': 3}
         if isinstance(basis, str):
-            valid_bases = ('xy', 'xz', 'yz')
             basis = basis.lower()
-            if basis not in valid_bases:
+            if basis not in basis_map:
                 raise ValueError(f"{basis} is not a valid plot basis.")
-
-            if basis == 'xy':
-                self.basis_ = 1
-            elif basis == 'xz':
-                self.basis_ = 2
-            elif basis == 'yz':
-                self.basis_ = 3
-            return
-
-        if isinstance(basis, int):
-            valid_bases = (1, 2, 3)
-            if basis not in valid_bases:
+            basis = basis_map[basis]
+        elif isinstance(basis, int):
+            if basis not in basis_map.values():
                 raise ValueError(f"{basis} is not a valid plot basis.")
-            self.basis_ = basis
-            return
+        else:
+            raise ValueError(f"{basis} is not a valid plot basis.")
 
-        raise ValueError(f"{basis} of type {type(basis)} is an invalid plot basis")
+        if basis == 1:
+            u_span = np.array([width[0], 0.0, 0.0], dtype=float)
+            v_span = np.array([0.0, width[1], 0.0], dtype=float)
+        elif basis == 2:
+            u_span = np.array([width[0], 0.0, 0.0], dtype=float)
+            v_span = np.array([0.0, 0.0, width[1]], dtype=float)
+        else:
+            u_span = np.array([0.0, width[0], 0.0], dtype=float)
+            v_span = np.array([0.0, 0.0, width[1]], dtype=float)
 
-    @property
-    def h_res(self):
-        return self.pixels_[0]
+    origin = np.asarray(origin, dtype=float)
+    if origin.shape != (3,):
+        raise ValueError("origin must be a length-3 sequence.")
 
-    @h_res.setter
-    def h_res(self, h_res):
-        self.pixels_[0] = h_res
+    # Prepare ctypes arrays
+    origin_arr = (c_double * 3)(*origin)
+    u_span_arr = (c_double * 3)(*u_span)
+    v_span_arr = (c_double * 3)(*v_span)
+    pixels_arr = (c_size_t * 2)(*pixels)
 
-    @property
-    def v_res(self):
-        return self.pixels_[1]
+    # Get internal filter index from filter ID if filter is provided
+    if filter is not None:
+        filter_index = c_int32()
+        _dll.openmc_get_filter_index(filter.id, filter_index)
+        filter_index = filter_index.value
+    else:
+        filter_index = -1
 
-    @v_res.setter
-    def v_res(self, v_res):
-        self.pixels_[1] = v_res
+    # Allocate output arrays with dynamic size based on filter
+    n_geom_fields = 4 if filter is not None else 3
+    geom_data = np.zeros((pixels[1], pixels[0], n_geom_fields), dtype=np.int32)
+    if include_properties:
+        property_data = np.zeros((pixels[1], pixels[0], 2), dtype=np.float64)
+        prop_ptr = property_data.ctypes.data_as(POINTER(c_double))
+    else:
+        property_data = None
+        prop_ptr = None
 
-    @property
-    def level(self):
-        return int(self.level_)
+    _dll.openmc_slice_data(
+        origin_arr,
+        u_span_arr,
+        v_span_arr,
+        pixels_arr,
+        show_overlaps,
+        level,
+        filter_index,
+        geom_data.ctypes.data_as(POINTER(c_int32)),
+        prop_ptr
+    )
 
-    @level.setter
-    def level(self, level):
-        self.level_ = level
-
-    @property
-    def color_overlaps(self):
-        return self.color_overlaps_
-
-    @color_overlaps.setter
-    def color_overlaps(self, color_overlaps):
-        self.color_overlaps_ = color_overlaps
-
-    @property
-    def color_overlaps(self):
-        return self.color_overlaps_
-
-    @color_overlaps.setter
-    def color_overlaps(self, val):
-        self.color_overlaps_ = val
-
-    def __repr__(self):
-        out_str = ["-----",
-                   "Plot:",
-                   "-----",
-                   f"Origin: {self.origin}",
-                   f"Width: {self.width}",
-                   f"Height: {self.height}",
-                   f"Basis: {self.basis}",
-                   f"HRes: {self.h_res}",
-                   f"VRes: {self.v_res}",
-                   f"Color Overlaps: {self.color_overlaps}",
-                   f"Level: {self.level}"]
-        return '\n'.join(out_str)
-
-
-_dll.openmc_id_map.argtypes = [POINTER(_PlotBase), POINTER(c_int32)]
-_dll.openmc_id_map.restype = c_int
-_dll.openmc_id_map.errcheck = _error_handler
+    return geom_data, property_data
 
 
 def id_map(plot):
+    """Deprecated compatibility wrapper for geometry ID maps.
+
+    This function is kept for compatibility and will be removed in a future
+    release. Use `slice_data(..., include_properties=False)` instead.
     """
-    Generate a 2-D map of cell and material IDs. Used for in-memory image
-    generation.
+    warnings.warn(
+        "openmc.lib.id_map is deprecated and will be removed in a future "
+        "release; use openmc.lib.slice_data(..., include_properties=False).",
+        FutureWarning,
+    )
 
-    Parameters
-    ----------
-    plot : openmc.lib.plot._PlotBase
-        Object describing the slice of the model to be generated
-
-    Returns
-    -------
-    id_map : numpy.ndarray
-        A NumPy array with shape (vertical pixels, horizontal pixels, 3) of
-        OpenMC property ids with dtype int32. The last dimension of the array
-        contains, in order, cell IDs, cell instances, and material IDs.
-
-    """
-    img_data = np.zeros((plot.v_res, plot.h_res, 3),
-                        dtype=np.dtype('int32'))
-    _dll.openmc_id_map(plot, img_data.ctypes.data_as(POINTER(c_int32)))
-    return img_data
-
-
-_dll.openmc_property_map.argtypes = [POINTER(_PlotBase), POINTER(c_double)]
-_dll.openmc_property_map.restype = c_int
-_dll.openmc_property_map.errcheck = _error_handler
+    kwargs = _extract_slice_data_args(plot)
+    geom_data, _ = slice_data(include_properties=False, **kwargs)
+    return geom_data[:, :, :3]
 
 
 def property_map(plot):
-    """
-    Generate a 2-D map of cell temperatures and material densities. Used for
-    in-memory image generation.
+    """Deprecated compatibility wrapper for temperature/density maps.
 
-    Parameters
-    ----------
-    plot : openmc.lib.plot._PlotBase
-        Object describing the slice of the model to be generated
+    This function is kept for compatibility and will be removed in a future
+    release. Use `slice_data(..., include_properties=True)` instead.
+    """
+    warnings.warn(
+        "openmc.lib.property_map is deprecated and will be removed in a "
+        "future release; use openmc.lib.slice_data(..., "
+        "include_properties=True).",
+        FutureWarning,
+    )
+
+    kwargs = _extract_slice_data_args(plot)
+    _, prop_data = slice_data(include_properties=True, **kwargs)
+    return prop_data
+
+
+_dll.openmc_slice_data_overlap_count.argtypes = [POINTER(c_size_t)]
+_dll.openmc_slice_data_overlap_count.restype = c_int
+_dll.openmc_slice_data_overlap_count.errcheck = _error_handler
+
+_dll.openmc_slice_data_overlap_info.argtypes = [c_size_t, POINTER(c_int32)]
+_dll.openmc_slice_data_overlap_info.restype = c_int
+_dll.openmc_slice_data_overlap_info.errcheck = _error_handler
+
+
+# Python wrappings for overlap functions
+def slice_data_overlap_count() -> int:
+    """Return the number of unique overlaps from the last slice plot.
 
     Returns
     -------
-    property_map : numpy.ndarray
-        A NumPy array with shape (vertical pixels, horizontal pixels, 2) of
-        OpenMC property ids with dtype float
-
+    int
+        Number of unique overlapping cell pairs detected by the most recent
+        :func:`slice_data` call with overlap checking enabled.
     """
-    prop_data = np.zeros((plot.v_res, plot.h_res, 2))
-    _dll.openmc_property_map(plot, prop_data.ctypes.data_as(POINTER(c_double)))
-    return prop_data
+    count = c_size_t()
+    _dll.openmc_slice_data_overlap_count(count)
+    return count.value
+
+
+def slice_data_overlap_info() -> np.ndarray:
+    """Return identifying information for overlaps from the last slice plot.
+
+    Returns
+    -------
+    numpy.ndarray
+        Array of shape ``(n, 3)`` with int32 dtype, where ``n`` is the number
+        of unique overlaps detected by the most recent :func:`slice_data` call
+        with overlap checking enabled. Each row contains ``[universe_id,
+        cell1_id, cell2_id]``.
+    """
+    n = slice_data_overlap_count()
+    overlap_info = np.empty((n, 3), dtype=np.int32)
+
+    if n > 0:
+        _dll.openmc_slice_data_overlap_info(
+            n,
+            overlap_info.ctypes.data_as(POINTER(c_int32)),
+        )
+    return overlap_info
+
+
+_dll.openmc_get_plot_index.argtypes = [c_int32, POINTER(c_int32)]
+_dll.openmc_get_plot_index.restype = c_int
+_dll.openmc_get_plot_index.errcheck = _error_handler
+
+_dll.openmc_plot_get_id.argtypes = [c_int32, POINTER(c_int32)]
+_dll.openmc_plot_get_id.restype = c_int
+_dll.openmc_plot_get_id.errcheck = _error_handler
+
+_dll.openmc_plot_set_id.argtypes = [c_int32, c_int32]
+_dll.openmc_plot_set_id.restype = c_int
+_dll.openmc_plot_set_id.errcheck = _error_handler
+
+_dll.openmc_plots_size.restype = c_size_t
+
+_dll.openmc_solidraytrace_plot_create.argtypes = [POINTER(c_int32)]
+_dll.openmc_solidraytrace_plot_create.restype = c_int
+_dll.openmc_solidraytrace_plot_create.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_pixels.argtypes = [
+    c_int32, POINTER(c_int32), POINTER(c_int32)]
+_dll.openmc_solidraytrace_plot_get_pixels.restype = c_int
+_dll.openmc_solidraytrace_plot_get_pixels.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_pixels.argtypes = [c_int32, c_int32, c_int32]
+_dll.openmc_solidraytrace_plot_set_pixels.restype = c_int
+_dll.openmc_solidraytrace_plot_set_pixels.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_color_by.argtypes = [c_int32, POINTER(c_int32)]
+_dll.openmc_solidraytrace_plot_get_color_by.restype = c_int
+_dll.openmc_solidraytrace_plot_get_color_by.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_color_by.argtypes = [c_int32, c_int32]
+_dll.openmc_solidraytrace_plot_set_color_by.restype = c_int
+_dll.openmc_solidraytrace_plot_set_color_by.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_default_colors.argtypes = [c_int32]
+_dll.openmc_solidraytrace_plot_set_default_colors.restype = c_int
+_dll.openmc_solidraytrace_plot_set_default_colors.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_all_opaque.argtypes = [c_int32]
+_dll.openmc_solidraytrace_plot_set_all_opaque.restype = c_int
+_dll.openmc_solidraytrace_plot_set_all_opaque.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_opaque.argtypes = [c_int32, c_int32, c_bool]
+_dll.openmc_solidraytrace_plot_set_opaque.restype = c_int
+_dll.openmc_solidraytrace_plot_set_opaque.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_color.argtypes = [c_int32, c_int32, c_uint8, c_uint8, c_uint8]
+_dll.openmc_solidraytrace_plot_set_color.restype = c_int
+_dll.openmc_solidraytrace_plot_set_color.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_camera_position.argtypes = [
+    c_int32, POINTER(c_double), POINTER(c_double), POINTER(c_double)]
+_dll.openmc_solidraytrace_plot_get_camera_position.restype = c_int
+_dll.openmc_solidraytrace_plot_get_camera_position.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_camera_position.argtypes = [c_int32, c_double, c_double, c_double]
+_dll.openmc_solidraytrace_plot_set_camera_position.restype = c_int
+_dll.openmc_solidraytrace_plot_set_camera_position.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_look_at.argtypes = [
+    c_int32, POINTER(c_double), POINTER(c_double), POINTER(c_double)]
+_dll.openmc_solidraytrace_plot_get_look_at.restype = c_int
+_dll.openmc_solidraytrace_plot_get_look_at.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_look_at.argtypes = [c_int32, c_double, c_double, c_double]
+_dll.openmc_solidraytrace_plot_set_look_at.restype = c_int
+_dll.openmc_solidraytrace_plot_set_look_at.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_up.argtypes = [
+    c_int32, POINTER(c_double), POINTER(c_double), POINTER(c_double)]
+_dll.openmc_solidraytrace_plot_get_up.restype = c_int
+_dll.openmc_solidraytrace_plot_get_up.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_up.argtypes = [c_int32, c_double, c_double, c_double]
+_dll.openmc_solidraytrace_plot_set_up.restype = c_int
+_dll.openmc_solidraytrace_plot_set_up.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_light_position.argtypes = [
+    c_int32, POINTER(c_double), POINTER(c_double), POINTER(c_double)]
+_dll.openmc_solidraytrace_plot_get_light_position.restype = c_int
+_dll.openmc_solidraytrace_plot_get_light_position.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_light_position.argtypes = [c_int32, c_double, c_double, c_double]
+_dll.openmc_solidraytrace_plot_set_light_position.restype = c_int
+_dll.openmc_solidraytrace_plot_set_light_position.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_fov.argtypes = [c_int32, POINTER(c_double)]
+_dll.openmc_solidraytrace_plot_get_fov.restype = c_int
+_dll.openmc_solidraytrace_plot_get_fov.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_fov.argtypes = [c_int32, c_double]
+_dll.openmc_solidraytrace_plot_set_fov.restype = c_int
+_dll.openmc_solidraytrace_plot_set_fov.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_update_view.argtypes = [c_int32]
+_dll.openmc_solidraytrace_plot_update_view.restype = c_int
+_dll.openmc_solidraytrace_plot_update_view.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_create_image.argtypes = [c_int32, POINTER(c_uint8), c_int32, c_int32]
+_dll.openmc_solidraytrace_plot_create_image.restype = c_int
+_dll.openmc_solidraytrace_plot_create_image.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_color.argtypes = [c_int32, c_int32,
+                                             POINTER(c_uint8), POINTER(c_uint8), POINTER(c_uint8)]
+_dll.openmc_solidraytrace_plot_get_color.restype = c_int
+_dll.openmc_solidraytrace_plot_get_color.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_get_diffuse_fraction.argtypes = [
+    c_int32, POINTER(c_double)]
+_dll.openmc_solidraytrace_plot_get_diffuse_fraction.restype = c_int
+_dll.openmc_solidraytrace_plot_get_diffuse_fraction.errcheck = _error_handler
+
+_dll.openmc_solidraytrace_plot_set_diffuse_fraction.argtypes = [c_int32, c_double]
+_dll.openmc_solidraytrace_plot_set_diffuse_fraction.restype = c_int
+_dll.openmc_solidraytrace_plot_set_diffuse_fraction.errcheck = _error_handler
+
+
+class SolidRayTracePlot(_FortranObjectWithID):
+    """Solid ray-traced plot stored internally.
+
+    This class exposes a solid ray-traced plot that is stored internally in
+    the OpenMC library. To obtain a view of an existing plot with a given ID,
+    use the :data:`openmc.lib.plots` mapping.
+
+    Parameters
+    ----------
+    uid : int or None
+        Unique ID of the plot
+    new : bool
+        When `index` is None, this argument controls whether a new object is
+        created or a view of an existing object is returned.
+    index : int or None
+        Index in the internal plots array.
+
+    Attributes
+    ----------
+    id : int
+        Unique ID of the plot.
+    pixels : tuple of int
+        Plot image dimensions as ``(width, height)``.
+    color_by : int
+        Coloring mode. Use :attr:`COLOR_BY_MATERIAL` or
+        :attr:`COLOR_BY_CELL`.
+    camera_position : tuple of float
+        Camera position as ``(x, y, z)``.
+    look_at : tuple of float
+        Point the camera is aimed at as ``(x, y, z)``.
+    up : tuple of float
+        Up direction as ``(x, y, z)``.
+    light_position : tuple of float
+        Position of the light source as ``(x, y, z)``.
+    fov : float
+        Horizontal field-of-view angle in degrees.
+    diffuse_fraction : float
+        Fraction of reflected light treated as diffuse (0 to 1).
+    """
+
+    COLOR_BY_MATERIAL = 0
+    COLOR_BY_CELL = 1
+    __instances = WeakValueDictionary()
+
+    def __new__(cls, uid=None, new=True, index=None):
+        mapping = plots
+        if index is None:
+            if new:
+                if uid is not None and uid in mapping:
+                    raise AllocationError(
+                        f'A plot with ID={uid} has already been allocated.'
+                    )
+                index = c_int32()
+                _dll.openmc_solidraytrace_plot_create(index)
+                index = index.value
+            else:
+                index = mapping[uid]._index
+
+        if index not in cls.__instances:
+            instance = super().__new__(cls)
+            instance._index = index
+            if uid is not None:
+                instance.id = uid
+            cls.__instances[index] = instance
+
+        return cls.__instances[index]
+
+    def __init__(self, uid=None, new=True, index=None):
+        super().__init__(uid, new, index)
+
+    @property
+    def id(self):
+        plot_id = c_int32()
+        _dll.openmc_plot_get_id(self._index, plot_id)
+        return plot_id.value
+
+    @id.setter
+    def id(self, plot_id):
+        _dll.openmc_plot_set_id(self._index, plot_id)
+
+    @staticmethod
+    def _get_xyz(getter, index):
+        x = c_double()
+        y = c_double()
+        z = c_double()
+        getter(index, x, y, z)
+        return (x.value, y.value, z.value)
+
+    @staticmethod
+    def _set_xyz(setter, index, xyz):
+        x, y, z = xyz
+        setter(index, float(x), float(y), float(z))
+
+    @property
+    def pixels(self):
+        width = c_int32()
+        height = c_int32()
+        _dll.openmc_solidraytrace_plot_get_pixels(self._index, width, height)
+        return (width.value, height.value)
+
+    @pixels.setter
+    def pixels(self, pixels):
+        width, height = pixels
+        _dll.openmc_solidraytrace_plot_set_pixels(
+            self._index, int(width), int(height))
+
+    @property
+    def color_by(self):
+        color_by = c_int32()
+        _dll.openmc_solidraytrace_plot_get_color_by(self._index, color_by)
+        return color_by.value
+
+    @color_by.setter
+    def color_by(self, color_by):
+        _dll.openmc_solidraytrace_plot_set_color_by(self._index, int(color_by))
+
+    def set_default_colors(self):
+        _dll.openmc_solidraytrace_plot_set_default_colors(self._index)
+
+    def set_all_opaque(self):
+        _dll.openmc_solidraytrace_plot_set_all_opaque(self._index)
+
+    def set_visibility(self, domain_id, visible):
+        _dll.openmc_solidraytrace_plot_set_opaque(
+            self._index, int(domain_id), bool(visible)
+        )
+
+    def set_color(self, domain_id, color):
+        r, g, b = [int(c) for c in color]
+        _dll.openmc_solidraytrace_plot_set_color(
+            self._index, int(domain_id), r, g, b)
+
+    @property
+    def camera_position(self):
+        return self._get_xyz(_dll.openmc_solidraytrace_plot_get_camera_position,
+                             self._index)
+
+    @camera_position.setter
+    def camera_position(self, position):
+        self._set_xyz(_dll.openmc_solidraytrace_plot_set_camera_position,
+                      self._index, position)
+
+    @property
+    def look_at(self):
+        return self._get_xyz(_dll.openmc_solidraytrace_plot_get_look_at,
+                             self._index)
+
+    @look_at.setter
+    def look_at(self, position):
+        self._set_xyz(_dll.openmc_solidraytrace_plot_set_look_at,
+                      self._index, position)
+
+    @property
+    def up(self):
+        return self._get_xyz(_dll.openmc_solidraytrace_plot_get_up, self._index)
+
+    @up.setter
+    def up(self, direction):
+        self._set_xyz(_dll.openmc_solidraytrace_plot_set_up, self._index,
+                      direction)
+
+    @property
+    def light_position(self):
+        return self._get_xyz(_dll.openmc_solidraytrace_plot_get_light_position,
+                             self._index)
+
+    @light_position.setter
+    def light_position(self, position):
+        self._set_xyz(_dll.openmc_solidraytrace_plot_set_light_position,
+                      self._index, position)
+
+    @property
+    def fov(self):
+        fov = c_double()
+        _dll.openmc_solidraytrace_plot_get_fov(self._index, fov)
+        return fov.value
+
+    @fov.setter
+    def fov(self, fov):
+        _dll.openmc_solidraytrace_plot_set_fov(self._index, float(fov))
+
+    def update_view(self):
+        _dll.openmc_solidraytrace_plot_update_view(self._index)
+
+    def create_image(self):
+        width, height = self.pixels
+        image = np.zeros((height, width, 3), dtype=np.uint8)
+        _dll.openmc_solidraytrace_plot_create_image(
+            self._index,
+            image.ctypes.data_as(POINTER(c_uint8)),
+            width,
+            height
+        )
+        return image
+
+    def get_color(self, domain_id):
+        r = c_uint8()
+        g = c_uint8()
+        b = c_uint8()
+        _dll.openmc_solidraytrace_plot_get_color(
+            self._index, int(domain_id), r, g, b)
+        return int(r.value), int(g.value), int(b.value)
+
+    @property
+    def diffuse_fraction(self):
+        value = c_double()
+        _dll.openmc_solidraytrace_plot_get_diffuse_fraction(self._index, value)
+        return value.value
+
+    @diffuse_fraction.setter
+    def diffuse_fraction(self, value):
+        _dll.openmc_solidraytrace_plot_set_diffuse_fraction(
+            self._index, float(value))
+
+    # Backward-compatible setter aliases
+    def set_pixels(self, width, height):
+        self.pixels = (width, height)
+
+    def set_color_by(self, color_by):
+        self.color_by = color_by
+
+    def set_camera_position(self, x, y, z):
+        self.camera_position = (x, y, z)
+
+    def set_look_at(self, x, y, z):
+        self.look_at = (x, y, z)
+
+    def set_up(self, x, y, z):
+        self.up = (x, y, z)
+
+    def set_light_position(self, x, y, z):
+        self.light_position = (x, y, z)
+
+    def set_fov(self, fov):
+        self.fov = fov
+
+    def set_diffuse_fraction(self, value):
+        self.diffuse_fraction = value
+
+
+class _PlotMapping(Mapping):
+    def __getitem__(self, key):
+        index = c_int32()
+        try:
+            _dll.openmc_get_plot_index(key, index)
+        except (AllocationError, InvalidIDError) as e:
+            raise KeyError(str(e))
+        return SolidRayTracePlot(index=index.value)
+
+    def __iter__(self):
+        for i in range(len(self)):
+            yield SolidRayTracePlot(index=i).id
+
+    def __len__(self):
+        return _dll.openmc_plots_size()
+
+    def __repr__(self):
+        return repr(dict(self))
+
+
+plots = _PlotMapping()
diff --git a/openmc/lib/tally.py b/openmc/lib/tally.py
index d0b34aedc..c17b16597 100644
--- a/openmc/lib/tally.py
+++ b/openmc/lib/tally.py
@@ -104,7 +104,9 @@ _SCORES = {
     -5: 'absorption', -6: 'fission', -7: 'nu-fission', -8: 'kappa-fission',
     -9: 'current', -10: 'events', -11: 'delayed-nu-fission',
     -12: 'prompt-nu-fission', -13: 'inverse-velocity', -14: 'fission-q-prompt',
-    -15: 'fission-q-recoverable', -16: 'decay-rate', -17: 'pulse-height'
+    -15: 'fission-q-recoverable', -16: 'decay-rate', -17: 'pulse-height',
+    -18: 'ifp-time-numerator', -19: 'ifp-beta-numerator',
+    -20: 'ifp-denominator',
 }
 _ESTIMATORS = {
     0: 'analog', 1: 'tracklength', 2: 'collision'
diff --git a/openmc/lib/weight_windows.py b/openmc/lib/weight_windows.py
index ed442d33f..2b26d3b55 100644
--- a/openmc/lib/weight_windows.py
+++ b/openmc/lib/weight_windows.py
@@ -53,11 +53,11 @@ _dll.openmc_weight_windows_get_energy_bounds.argtypes = [c_int32, POINTER(POINTE
 _dll.openmc_weight_windows_get_energy_bounds.restype = c_int
 _dll.openmc_weight_windows_get_energy_bounds.errcheck = _error_handler
 
-_dll.openmc_weight_windows_set_particle.argtypes = [c_int32, c_int]
+_dll.openmc_weight_windows_set_particle.argtypes = [c_int32, c_int32]
 _dll.openmc_weight_windows_set_particle.restype = c_int
 _dll.openmc_weight_windows_set_particle.errcheck = _error_handler
 
-_dll.openmc_weight_windows_get_particle.argtypes = [c_int32, POINTER(c_int)]
+_dll.openmc_weight_windows_get_particle.argtypes = [c_int32, POINTER(c_int32)]
 _dll.openmc_weight_windows_get_particle.restype = c_int
 _dll.openmc_weight_windows_get_particle.errcheck = _error_handler
 
@@ -201,16 +201,13 @@ class WeightWindows(_FortranObjectWithID):
 
     @property
     def particle(self):
-        val = c_int()
+        val = c_int32()
         _dll.openmc_weight_windows_get_particle(self._index, val)
         return ParticleType(val.value)
 
     @particle.setter
     def particle(self, p):
-        if isinstance(p, str):
-            p = ParticleType.from_string(p)
-        else:
-            p = ParticleType(p)
+        p = ParticleType(p)
         _dll.openmc_weight_windows_set_particle(self._index, int(p))
 
     @property
@@ -304,10 +301,10 @@ class WeightWindows(_FortranObjectWithID):
         ----------
         tally : openmc.lib.Tally
             The tally used to create the WeightWindows instance.
-        particle : openmc.ParticleType or str, optional
+        particle : openmc.ParticleType or str or int, optional
             The particle type to use for the WeightWindows instance. Should be
-            specified as an instance of ParticleType or as a string with a value of
-            'neutron' or 'photon'.
+            specified as an instance of ParticleType, a PDG number, or as a
+            name.
 
         Returns
         -------
@@ -317,7 +314,8 @@ class WeightWindows(_FortranObjectWithID):
         Raises
         ------
         ValueError
-            If the particle parameter is not an instance of ParticleType or a string.
+            If the particle parameter is not an instance of ParticleType, a string,
+            or an integer PDG number.
         ValueError
             If the particle parameter is not a valid particle type (i.e., not 'neutron'
             or 'photon').
@@ -328,12 +326,13 @@ class WeightWindows(_FortranObjectWithID):
             If the tally does not have a MeshFilter.
         """
         # do some checks on particle value
-        if not isinstance(particle, (ParticleType, str)):
-            raise ValueError(f"Parameter 'particle' must be {ParticleType} or one of ('neutron', 'photon').")
+        if not isinstance(particle, (ParticleType, str, int)):
+            raise ValueError(
+                f"Parameter 'particle' must be {ParticleType} or one of ('neutron', 'photon')."
+            )
 
         # convert particle type if needed
-        if isinstance(particle, str):
-            particle = ParticleType.from_string(particle)
+        particle = ParticleType(particle)
 
         if particle not in (ParticleType.NEUTRON, ParticleType.PHOTON):
             raise ValueError('Weight windows can only be applied for neutrons or photons')
diff --git a/openmc/material.py b/openmc/material.py
index f40ac805f..3a92efda2 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -2,10 +2,13 @@ from __future__ import annotations
 from collections import defaultdict, namedtuple, Counter
 from collections.abc import Iterable
 from copy import deepcopy
+from functools import reduce
 from numbers import Real
 from pathlib import Path
 import re
 import sys
+import tempfile
+from typing import TYPE_CHECKING, Literal, Sequence, Dict
 import warnings
 
 import lxml.etree as ET
@@ -15,12 +18,18 @@ import h5py
 import openmc
 import openmc.data
 import openmc.checkvalue as cv
-from ._xml import clean_indentation, reorder_attributes
+from ._xml import clean_indentation, get_elem_list, get_text
 from .mixin import IDManagerMixin
 from .utility_funcs import input_path
+from . import waste
 from openmc.checkvalue import PathLike
-from openmc.stats import Univariate, Discrete, Mixture
-from openmc.data.data import _get_element_symbol
+from openmc.stats import Univariate, Discrete, Mixture, Tabular
+from openmc.data.data import _get_element_symbol, JOULE_PER_EV
+from openmc.data.function import Tabulated1D
+from openmc.data import mass_energy_absorption_coefficient, dose_coefficients
+
+if TYPE_CHECKING:
+    from openmc.deplete import Chain
 
 
 # Units for density supported by OpenMC
@@ -30,6 +39,7 @@ DENSITY_UNITS = ('g/cm3', 'g/cc', 'kg/m3', 'atom/b-cm', 'atom/cm3', 'sum',
 # Smallest normalized floating point number
 _SMALLEST_NORMAL = sys.float_info.min
 
+_BECQUEREL_PER_CURIE = 3.7e10
 
 NuclideTuple = namedtuple('NuclideTuple', ['name', 'percent', 'percent_type'])
 
@@ -56,6 +66,26 @@ class Material(IDManagerMixin):
     temperature : float, optional
         Temperature of the material in Kelvin. If not specified, the material
         inherits the default temperature applied to the model.
+    density : float, optional
+        Density of the material (units defined separately)
+    density_units : str
+        Units used for `density`. Can be one of 'g/cm3', 'g/cc', 'kg/m3',
+        'atom/b-cm', 'atom/cm3', 'sum', or 'macro'.  The 'macro' unit only
+        applies in the case of a multi-group calculation. Defaults to 'sum'.
+    depletable : bool, optional
+        Indicate whether the material is depletable. Defaults to False.
+    volume : float, optional
+        Volume of the material in cm^3. This can either be set manually or
+        calculated in a stochastic volume calculation and added via the
+        :meth:`Material.add_volume_information` method.
+    components : dict of str to float or dict
+        Dictionary mapping element or nuclide names to their atom or weight
+        percent. To specify enrichment of an element, the entry of
+        ``components`` for that element must instead be a dictionary containing
+        the keyword arguments as well as a value for ``'percent'``
+    percent_type : {'ao', 'wo'}
+        Whether the values in `components` should be interpreted as atom percent
+        ('ao') or weight percent ('wo').
 
     Attributes
     ----------
@@ -107,17 +137,28 @@ class Material(IDManagerMixin):
     next_id = 1
     used_ids = set()
 
-    def __init__(self, material_id=None, name='', temperature=None):
+    def __init__(
+        self,
+        material_id: int | None = None,
+        name: str = "",
+        temperature: float | None = None,
+        density: float | None = None,
+        density_units: str = "sum",
+        depletable: bool | None = False,
+        volume: float | None = None,
+        components: dict | None = None,
+        percent_type: str = "ao",
+    ):
         # Initialize class attributes
         self.id = material_id
         self.name = name
         self.temperature = temperature
         self._density = None
-        self._density_units = 'sum'
-        self._depletable = False
+        self._density_units = density_units
+        self._depletable = depletable
         self._paths = None
         self._num_instances = None
-        self._volume = None
+        self._volume = volume
         self._atoms = {}
         self._isotropic = []
         self._ncrystal_cfg = None
@@ -132,6 +173,15 @@ class Material(IDManagerMixin):
         # If specified, a list of table names
         self._sab = []
 
+        # Set density if provided
+        if density is not None:
+            self.set_density(density_units, density)
+
+        # Add components if provided
+        if components is not None:
+            self.add_components(components, percent_type=percent_type)
+
+
     def __repr__(self) -> str:
         string = 'Material\n'
         string += '{: <16}=\t{}\n'.format('\tID', self._id)
@@ -249,6 +299,8 @@ class Material(IDManagerMixin):
                 mass += nuc.percent
 
         # Compute and return the molar mass
+        if moles == 0.0:
+            raise ValueError("Material has no nuclides; cannot compute molar mass")
         return mass / moles
 
     @property
@@ -286,11 +338,13 @@ class Material(IDManagerMixin):
         return self.get_decay_photon_energy(0.0)
 
     def get_decay_photon_energy(
-            self,
-            clip_tolerance: float = 1e-6,
-            units: str = 'Bq',
-            volume: float | None = None
-        ) -> Univariate | None:
+        self,
+        clip_tolerance: float = 1e-6,
+        units: str = 'Bq',
+        volume: float | None = None,
+        exclude_nuclides: list[str] | None = None,
+        include_nuclides: list[str] | None = None
+    ) -> Univariate | None:
         r"""Return energy distribution of decay photons from unstable nuclides.
 
         .. versionadded:: 0.14.0
@@ -298,28 +352,39 @@ class Material(IDManagerMixin):
         Parameters
         ----------
         clip_tolerance : float
-            Maximum fraction of :math:`\sum_i x_i p_i` for discrete
-            distributions that will be discarded.
-        units : {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3'}
+            Maximum fraction of :math:`\sum_i x_i p_i` for discrete distributions
+            that will be discarded.
+        units : {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3', 'Bq/m3'}
             Specifies the units on the integral of the distribution.
         volume : float, optional
             Volume of the material. If not passed, defaults to using the
             :attr:`Material.volume` attribute.
+        exclude_nuclides : list of str, optional
+            Nuclides to exclude from the photon source calculation.
+        include_nuclides : list of str, optional
+            Nuclides to include in the photon source calculation. If specified,
+            only these nuclides are used.
 
         Returns
         -------
         Univariate or None
-            Decay photon energy distribution. The integral of this distribution
-            is the total intensity of the photon source in the requested units.
+            Decay photon energy distribution. The integral of this distribution is
+            the total intensity of the photon source in the requested units.
 
         """
-        cv.check_value('units', units, {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3'})
+        cv.check_value('units', units, {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3', 'Bq/m3'})
+
+        if exclude_nuclides is not None and include_nuclides is not None:
+            raise ValueError("Cannot specify both exclude_nuclides and include_nuclides")
+
         if units == 'Bq':
             multiplier = volume if volume is not None else self.volume
             if multiplier is None:
                 raise ValueError("volume must be specified if units='Bq'")
         elif units == 'Bq/cm3':
             multiplier = 1
+        elif units == 'Bq/m3':
+            multiplier = 1e6
         elif units == 'Bq/g':
             multiplier = 1.0 / self.get_mass_density()
         elif units == 'Bq/kg':
@@ -328,6 +393,11 @@ class Material(IDManagerMixin):
         dists = []
         probs = []
         for nuc, atoms_per_bcm in self.get_nuclide_atom_densities().items():
+            if exclude_nuclides is not None and nuc in exclude_nuclides:
+                continue
+            if include_nuclides is not None and nuc not in include_nuclides:
+                continue
+
             source_per_atom = openmc.data.decay_photon_energy(nuc)
             if source_per_atom is not None and atoms_per_bcm > 0.0:
                 dists.append(source_per_atom)
@@ -349,6 +419,190 @@ class Material(IDManagerMixin):
 
         return combined
 
+    def get_photon_contact_dose_rate(
+        self,
+        dose_quantity: str = "absorbed-air",
+        build_up: float = 2.0,
+        by_nuclide: bool = False
+    ) -> float | dict[str, float]:
+        """Compute the photon contact dose rate (CDR) produced by radioactive decay
+        of the material.
+
+        The contact dose rate is calculated from decay photon energy spectra for
+        each nuclide in the material, combined with photon mass attenuation data
+        for the material and the appropriate response function for the dose quantity.
+        A slab-geometry approximation and a photon build-up factor are used.
+
+        Absorbed-air dose:
+            The approach follows the FISPACT-II manual (UKAEA-CCFE-RE(21)02 - May 2021).
+            Appendix C.7.1.
+            This method integrates over the photon energy:
+
+                (B/2) * (mu_en_air(E) / mu_material(E)) * E * S(E)
+
+        Effective dose:
+            The approach uses ICRP-116 effective dose coefficients to convert the photon
+            fluence due to decay photons to effective dose.
+            This method integrates over the photon energy:
+
+                (B/2) * (h_e(E) / mu_material(E)) * S(E)
+
+        where:
+            - mu_en_air(E) is the air mass energy-absorption coefficient,
+            - mu_material(E) is the photon mass attenuation coefficient of the material,
+            - S(E) is the photon emission spectrum per atom,
+            - h_e(E) is the ICRP-116 effective dose coefficient,
+            - B is the build-up factor,
+            - E is the photon energy.
+
+        Parameters
+        ----------
+        dose_quantity : {'absorbed-air', 'effective'}, optional
+            Specifies the dose quantity to be calculated.
+            The only supported options are 'absorbed-air' which implements the methodology
+            from FISPACT-II, and 'effective' which uses ICRP-116 effective dose coefficients.
+        build_up : float, optional. The default value is 2.0 as suggested in the FISPACT-II
+            manual.
+        by_nuclide : bool, optional
+            Specifies if the cdr should be returned for the material as a
+            whole or per nuclide. Default is False.
+
+        Limitations
+        ----------
+        This method does not implement correction from Bremsstrahlung particles which can be
+        relevant at close distances.
+        In addition, it computes the gamma contact dose rate only for the unstable nuclides
+        for which the radiation source specification is present in the chain file.
+
+        Returns
+        -------
+        cdr : float or dict[str, float]
+            Contact Dose Rate due to decay photons.
+            'absorbed-air': returns the absorbed dose in air [Gy/hr].
+            'effective': returns the effective dose [Sv/hr].
+        """
+
+        cv.check_type("by_nuclide", by_nuclide, bool)
+        cv.check_type("dose_quantity", dose_quantity, str)
+        cv.check_value("dose_quantity", dose_quantity, {'absorbed-air', 'effective'})
+        cv.check_type("build_up", build_up, Real)
+        cv.check_greater_than("build_up", build_up, 0.0)
+
+        nuc_densities = self.get_nuclide_atom_densities()
+        if not nuc_densities:
+            raise ValueError("Material has no nuclides; cannot compute mass attenuation")
+
+        # Collect partial mass densities ρ_i [g/cm³] and elemental mass
+        # attenuation coefficients µ_i/ρ_i [cm²/g] per nuclide
+        nuc_attenuation = []
+        for nuc, atom_density_bcm in nuc_densities.items():
+            Z = openmc.data.zam(nuc)[0]
+            mu_over_rho = openmc.data.mass_attenuation_coefficient(Z)
+            rho_i = (
+                atom_density_bcm * 1.0e24
+                * openmc.data.atomic_mass(nuc) / openmc.data.AVOGADRO
+            )
+            nuc_attenuation.append((rho_i, mu_over_rho))
+
+        # Build union energy grid across all nuclides
+        mu_e_vals = reduce(np.union1d, [t.x for _, t in nuc_attenuation])
+
+        # Build the material linear attenuation coefficient µ_material(E) [cm⁻¹]
+        # as the sum of ρ_i * (µ_i/ρ_i)(E) over all nuclides
+        mu_material_vals = np.zeros(len(mu_e_vals))
+        for rho_i, mu_over_rho in nuc_attenuation:
+            mu_material_vals += rho_i * mu_over_rho(mu_e_vals)
+        mu_material = Tabulated1D(
+            mu_e_vals, mu_material_vals, breakpoints=[len(mu_e_vals)], interpolation=[5])
+
+        # CDR computation
+        cdr = {}
+
+        geometry_factor_slab = 0.5
+
+        # ancillary conversion factors for clarity
+        seconds_per_hour = 3600.0
+        grams_per_kg = 1000.0
+        sv_per_psv = 1e-12
+
+        if dose_quantity == 'absorbed-air':
+            # mu_en/rho for air [cm²/g] as a function of energy [eV]
+            response_f = mass_energy_absorption_coefficient("air", data_source="nist126")
+
+            # Factor to convert [eV cm²/(b g s)] to [Gy/h]
+            multiplier = (build_up * geometry_factor_slab * seconds_per_hour
+                          * grams_per_kg * 1e24 * JOULE_PER_EV)
+
+        elif dose_quantity == 'effective':
+            # effective dose as a function of photon fluence [pSv cm²]
+            response_f_x, response_f_y = dose_coefficients(
+                "photon", geometry='AP', data_source='icrp116')
+            response_f = Tabulated1D(response_f_x, response_f_y, breakpoints=[
+                len(response_f_x)], interpolation=[5])
+
+            # Convert [pSv cm²/(b-s)] to [Sv/h]
+            multiplier = (build_up * geometry_factor_slab * seconds_per_hour
+                          * sv_per_psv * 1e24)
+
+        for nuc, nuc_atoms_per_bcm in self.get_nuclide_atom_densities().items():
+            photon_source_per_atom = openmc.data.decay_photon_energy(nuc)
+
+            # nuclides with no contribution
+            if photon_source_per_atom is None or nuc_atoms_per_bcm <= 0.0:
+                cdr[nuc] = 0.0
+                continue
+
+            if not isinstance(photon_source_per_atom, (Discrete, Tabular)):
+                raise ValueError(
+                    f"Unknown decay photon energy data type for nuclide {nuc}"
+                    f"value returned: {type(photon_source_per_atom)}"
+                )
+
+            e_vals = photon_source_per_atom.x
+            p_vals = photon_source_per_atom.p
+
+            # Construct list of energies from (photon source, response function,
+            # mu_en_air) for clipping to common energy range
+            e_lists = [e_vals, response_f.x, mu_e_vals]
+
+            # clip distributions for values outside the tabulated values
+            left_bound = max(a.min() for a in e_lists)
+            right_bound = min(a.max() for a in e_lists)
+
+            mask = (e_vals >= left_bound) & (e_vals <= right_bound)
+            e_vals = e_vals[mask]
+            p_vals = p_vals[mask]
+
+            if isinstance(photon_source_per_atom, Tabular):
+                # limit the computation to the tabulated mu_en_air range
+                e_union = reduce(np.union1d, e_lists)
+                e_union = e_union[(e_union >= left_bound) & (e_union <= right_bound)]
+                if len(e_union) < 2:
+                    raise ValueError("Not enough overlapping energy points to compute CDR")
+
+                # Histogram interpolation: each new point inherits the value of
+                # the nearest original point to its left
+                p_vals = p_vals[np.searchsorted(e_vals, e_union, side='right') - 1]
+                e_vals = e_union
+
+            mu_vals = mu_material(e_vals)
+            if dose_quantity == 'absorbed-air':
+                # Compute (µ_en_air(E) / µ_material(E)) * E * S(E)
+                integrand = (response_f(e_vals) / mu_vals) * p_vals * e_vals
+            elif dose_quantity == 'effective':
+                # Compute (h_e(E) / µ_material(E)) * S(E)
+                integrand = (response_f(e_vals) / mu_vals) * p_vals
+
+            if isinstance(photon_source_per_atom, Discrete):
+                cdr_nuc = np.sum(integrand)
+            elif isinstance(photon_source_per_atom, Tabular):
+                cdr_nuc = np.trapezoid(integrand, e_vals)
+
+            # Compute air-absorbed dose [Gy/h] or effective dose [Sv/h]
+            cdr[nuc] = float(cdr_nuc * nuc_atoms_per_bcm * multiplier)
+
+        return cdr if by_nuclide else sum(cdr.values())
+
     @classmethod
     def from_hdf5(cls, group: h5py.Group) -> Material:
         """Create material from HDF5 group
@@ -1058,7 +1312,6 @@ class Material(IDManagerMixin):
             nuc_densities.append(nuc.percent)
             nuc_density_types.append(nuc.percent_type)
 
-        nucs = np.array(nucs)
         nuc_densities = np.array(nuc_densities)
         nuc_density_types = np.array(nuc_density_types)
 
@@ -1135,19 +1388,23 @@ class Material(IDManagerMixin):
         return densities
 
 
-    def get_activity(self, units: str = 'Bq/cm3', by_nuclide: bool = False,
-                     volume: float | None = None) -> dict[str, float] | float:
-        """Returns the activity of the material or for each nuclide in the
-        material in units of [Bq], [Bq/g], [Bq/kg] or [Bq/cm3].
+    def get_activity(
+        self,
+        units: str = 'Bq/cm3',
+        by_nuclide: bool = False,
+        volume: float | None = None,
+        chain_file: Literal[False] | None | PathLike | Chain = None
+    ) -> dict[str, float] | float:
+        """Return the activity of the material or each nuclide within.
 
         .. versionadded:: 0.13.1
 
         Parameters
         ----------
-        units : {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3'}
+        units : {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3', 'Bq/m3', 'Ci', 'Ci/m3'}
             Specifies the type of activity to return, options include total
-            activity [Bq], specific [Bq/g, Bq/kg] or volumetric activity
-            [Bq/cm3]. Default is volumetric activity [Bq/cm3].
+            activity [Bq,Ci], specific [Bq/g, Bq/kg] or volumetric activity
+            [Bq/cm3, Bq/m3, Ci/m3]. Default is volumetric activity [Bq/cm3].
         by_nuclide : bool
             Specifies if the activity should be returned for the material as a
             whole or per nuclide. Default is False.
@@ -1156,46 +1413,67 @@ class Material(IDManagerMixin):
             :attr:`Material.volume` attribute.
 
             .. versionadded:: 0.13.3
+        chain_file : False, None, PathLike, or openmc.deplete.Chain, optional
+            Source of half-life values. If ``False``, only ENDF/B-VIII.0 data is
+            used. If ``None``, the chain specified by
+            ``openmc.config['chain_file']`` is used when available. If a path or
+            :class:`openmc.deplete.Chain` is given, that chain is used. For
+            ``None`` or an explicit chain, nuclides absent from the chain fall
+            back to ENDF/B-VIII.0 data.
+
+            .. versionadded:: 0.15.4
 
         Returns
         -------
         Union[dict, float]
-            If by_nuclide is True then a dictionary whose keys are nuclide
-            names and values are activity is returned. Otherwise the activity
-            of the material is returned as a float.
+            If by_nuclide is True then a dictionary whose keys are nuclide names
+            and values are activity is returned. Otherwise the activity of the
+            material is returned as a float.
         """
 
-        cv.check_value('units', units, {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3'})
+        cv.check_value('units', units, {'Bq', 'Bq/g', 'Bq/kg', 'Bq/cm3', 'Bq/m3', 'Ci', 'Ci/m3'})
         cv.check_type('by_nuclide', by_nuclide, bool)
 
+        if volume is None:
+            volume = self.volume
+
+        if units in {'Bq', 'Ci'} and volume is None:
+            raise ValueError(f"Volume must be set in order to compute activity in '{units}'.")
+
         if units == 'Bq':
-            multiplier = volume if volume is not None else self.volume
+            multiplier = volume
         elif units == 'Bq/cm3':
             multiplier = 1
+        elif units == 'Bq/m3':
+            multiplier = 1e6
         elif units == 'Bq/g':
             multiplier = 1.0 / self.get_mass_density()
         elif units == 'Bq/kg':
             multiplier = 1000.0 / self.get_mass_density()
+        elif units == 'Ci':
+            multiplier = volume / _BECQUEREL_PER_CURIE
+        elif units == 'Ci/m3':
+            multiplier = 1e6 / _BECQUEREL_PER_CURIE
 
         activity = {}
         for nuclide, atoms_per_bcm in self.get_nuclide_atom_densities().items():
-            inv_seconds = openmc.data.decay_constant(nuclide)
+            inv_seconds = openmc.data.decay_constant(
+                nuclide, chain_file=chain_file)
             activity[nuclide] = inv_seconds * 1e24 * atoms_per_bcm * multiplier
 
         return activity if by_nuclide else sum(activity.values())
 
     def get_decay_heat(self, units: str = 'W', by_nuclide: bool = False,
                        volume: float | None = None) -> dict[str, float] | float:
-        """Returns the decay heat of the material or for each nuclide in the
-        material in units of [W], [W/g], [W/kg] or [W/cm3].
+        """Return the decay heat of the material or each nuclide within.
 
         .. versionadded:: 0.13.3
 
         Parameters
         ----------
-        units : {'W', 'W/g', 'W/kg', 'W/cm3'}
+        units : {'W', 'W/g', 'W/kg', 'W/cm3', 'W/m3'}
             Specifies the units of decay heat to return. Options include total
-            heat [W], specific [W/g, W/kg] or volumetric heat [W/cm3].
+            heat [W], specific [W/g, W/kg] or volumetric heat [W/cm3, W/m3].
             Default is total heat [W].
         by_nuclide : bool
             Specifies if the decay heat should be returned for the material as a
@@ -1214,13 +1492,17 @@ class Material(IDManagerMixin):
             of the material is returned as a float.
         """
 
-        cv.check_value('units', units, {'W', 'W/g', 'W/kg', 'W/cm3'})
+        cv.check_value('units', units, {'W', 'W/g', 'W/kg', 'W/cm3', 'W/m3'})
         cv.check_type('by_nuclide', by_nuclide, bool)
 
         if units == 'W':
             multiplier = volume if volume is not None else self.volume
+            if multiplier is None:
+                raise ValueError("Volume must be set in order to compute total decay heat.")
         elif units == 'W/cm3':
             multiplier = 1
+        elif units == 'W/m3':
+            multiplier = 1e6
         elif units == 'W/g':
             multiplier = 1.0 / self.get_mass_density()
         elif units == 'W/kg':
@@ -1316,6 +1598,91 @@ class Material(IDManagerMixin):
             raise ValueError("Volume must be set in order to determine mass.")
         return volume*self.get_mass_density(nuclide)
 
+    def waste_classification(self, metal: bool = False) -> str:
+        """Classify the material for near-surface waste disposal.
+
+        This method determines a waste classification for the material based on
+        the NRC regulations (10 CFR 61.55). Note that the NRC regulations do not
+        consider many long-lived radionuclides relevant to fusion systems; for
+        fusion applications, it is recommended to calculate a waste disposal
+        rating based on limits by Fetter et al. using the
+        :meth:`~openmc.Material.waste_disposal_rating` method.
+
+        Parameters
+        ----------
+        metal : bool, optional
+            Whether or not the material is in metal form.
+
+        Returns
+        -------
+        str
+            The waste disposal classification, which can be "Class A", "Class
+            B", "Class C", or "GTCC" (greater than class C).
+
+        """
+        return waste._waste_classification(self, metal=metal)
+
+    def waste_disposal_rating(
+        self,
+        limits: str | dict[str, float] = 'Fetter',
+        metal: bool = False,
+        by_nuclide: bool = False,
+    ) -> float | dict[str, float]:
+        """Return the waste disposal rating for the material.
+
+        This method returns a waste disposal rating for the material based on a
+        set of specific activity limits. The waste disposal rating is a single
+        number that represents the sum of the ratios of the specific activity
+        for each radionuclide in the material against a nuclide-specific limit.
+        A value less than 1.0 indicates that the material "meets" the limits
+        whereas a value greater than 1.0 exceeds the limits.
+
+        Note that the limits for NRC do not consider many long-lived
+        radionuclides relevant to fusion systems. A paper by `Fetter et al.
+        <https://doi.org/10.1016/0920-3796(90)90104-E>`_ applies the NRC
+        methodology to calculate specific activity limits for an expanded set of
+        radionuclides.
+
+        Parameters
+        ----------
+        limits : str or dict, optional
+            The name of a predefined set of specific activity limits or a
+            dictionary that contains specific activity limits for radionuclides,
+            where keys are nuclide names and values are activities in units of
+            [Ci/m3]. The predefined options are:
+
+            - 'Fetter': Uses limits from Fetter et al. (1990)
+            - 'NRC_long': Uses the 10 CFR 61.55 limits for long-lived
+              radionuclides
+            - 'NRC_short_A': Uses the 10 CFR 61.55 class A limits for
+              short-lived radionuclides
+            - 'NRC_short_B': Uses the 10 CFR 61.55 class B limits for
+              short-lived radionuclides
+            - 'NRC_short_C': Uses the 10 CFR 61.55 class C limits for
+              short-lived radionuclides
+        metal : bool, optional
+            Whether or not the material is in metal form (only applicable for
+            NRC based limits)
+        by_nuclide : bool, optional
+            Whether to return the waste disposal rating for each nuclide in the
+            material. If True, a dictionary is returned where the keys are the
+            nuclide names and the values are the waste disposal ratings for each
+            nuclide. If False, a single float value is returned that represents
+            the overall waste disposal rating for the material.
+
+        Returns
+        -------
+        float or dict
+            The waste disposal rating for the material or its constituent
+            nuclides.
+
+        See also
+        --------
+        Material.waste_classification()
+
+        """
+        return waste._waste_disposal_rating(self, limits, metal, by_nuclide)
+
     def clone(self, memo: dict | None = None) -> Material:
         """Create a copy of this material with a new unique ID.
 
@@ -1578,53 +1945,149 @@ class Material(IDManagerMixin):
             Material generated from XML element
 
         """
-        mat_id = int(elem.get('id'))
+        mat_id = int(get_text(elem, 'id'))
+
         # Add NCrystal material from cfg string
-        if "cfg" in elem.attrib:
-            cfg = elem.get("cfg")
+        cfg = get_text(elem, "cfg")
+        if cfg is not None:
             return Material.from_ncrystal(cfg, material_id=mat_id)
 
         mat = cls(mat_id)
-        mat.name = elem.get('name')
+        mat.name = get_text(elem, 'name')
 
-        if "temperature" in elem.attrib:
-            mat.temperature = float(elem.get("temperature"))
+        temperature = get_text(elem, "temperature")
+        if temperature is not None:
+            mat.temperature = float(temperature)
 
-        if 'volume' in elem.attrib:
-            mat.volume = float(elem.get('volume'))
+        volume = get_text(elem, "volume")
+        if volume is not None:
+            mat.volume = float(volume)
 
         # Get each nuclide
         for nuclide in elem.findall('nuclide'):
-            name = nuclide.attrib['name']
+            name = get_text(nuclide, "name")
             if 'ao' in nuclide.attrib:
                 mat.add_nuclide(name, float(nuclide.attrib['ao']))
             elif 'wo' in nuclide.attrib:
                 mat.add_nuclide(name, float(nuclide.attrib['wo']), 'wo')
 
         # Get depletable attribute
-        mat.depletable = elem.get('depletable') in ('true', '1')
+        depletable = get_text(elem, "depletable")
+        mat.depletable = depletable in ('true', '1')
 
         # Get each S(a,b) table
         for sab in elem.findall('sab'):
-            fraction = float(sab.get('fraction', 1.0))
-            mat.add_s_alpha_beta(sab.get('name'), fraction)
+            fraction = float(get_text(sab, "fraction", 1.0))
+            name = get_text(sab, "name")
+            mat.add_s_alpha_beta(name, fraction)
 
         # Get total material density
         density = elem.find('density')
-        units = density.get('units')
+        units = get_text(density, "units")
         if units == 'sum':
             mat.set_density(units)
         else:
-            value = float(density.get('value'))
+            value = float(get_text(density, 'value'))
             mat.set_density(units, value)
 
         # Check for isotropic scattering nuclides
-        isotropic = elem.find('isotropic')
+        isotropic = get_elem_list(elem, "isotropic", str)
         if isotropic is not None:
-            mat.isotropic = isotropic.text.split()
+            mat.isotropic = isotropic
 
         return mat
 
+    def deplete(
+        self,
+        multigroup_flux: Sequence[float],
+        energy_group_structure: Sequence[float] | str,
+        timesteps: Sequence[float] | Sequence[tuple[float, str]],
+        source_rates: float | Sequence[float],
+        timestep_units: str = 's',
+        chain_file: cv.PathLike | "openmc.deplete.Chain" | None = None,
+        reactions: Sequence[str] | None = None,
+    ) -> list[openmc.Material]:
+        """Depletes that material, evolving the nuclide densities
+
+        .. versionadded:: 0.15.3
+
+        Parameters
+        ----------
+        multigroup_flux: Sequence[float]
+            Energy-dependent multigroup flux values, where each sublist corresponds
+            to a specific material. Will be normalized so that it sums to 1.
+        energy_group_structure : Sequence[float] | str
+            Energy group boundaries in [eV] or the name of the group structure.
+        timesteps : iterable of float or iterable of tuple
+            Array of timesteps. Note that values are not cumulative. The units are
+            specified by the `timestep_units` argument when `timesteps` is an
+            iterable of float. Alternatively, units can be specified for each step
+            by passing an iterable of (value, unit) tuples.
+        source_rates : float or iterable of float, optional
+            Source rate in [neutron/sec] or neutron flux in [neutron/s-cm^2] for
+            each interval in :attr:`timesteps`
+        timestep_units : {'s', 'min', 'h', 'd', 'a', 'MWd/kg'}
+            Units for values specified in the `timesteps` argument. 's' means
+            seconds, 'min' means minutes, 'h' means hours, 'a' means Julian years
+            and 'MWd/kg' indicates that the values are given in burnup (MW-d of
+            energy deposited per kilogram of initial heavy metal).
+        chain_file : PathLike or Chain
+            Path to the depletion chain XML file or instance of openmc.deplete.Chain.
+            Defaults to ``openmc.config['chain_file']``.
+        reactions : list of str, optional
+            Reactions to get cross sections for. If not specified, all neutron
+            reactions listed in the depletion chain file are used.
+
+        Returns
+        -------
+        list of openmc.Material, one for each timestep
+
+        """
+
+        materials = openmc.Materials([self])
+
+        depleted_materials_dict = materials.deplete(
+            multigroup_fluxes=[multigroup_flux],
+            energy_group_structures=[energy_group_structure],
+            timesteps=timesteps,
+            source_rates=source_rates,
+            timestep_units=timestep_units,
+            chain_file=chain_file,
+            reactions=reactions,
+        )
+
+        return depleted_materials_dict[self.id]
+
+
+    def mean_free_path(self, energy: float) -> float:
+        """Calculate the mean free path of neutrons in the material at a given
+        energy.
+
+        .. versionadded:: 0.15.3
+
+        Parameters
+        ----------
+        energy : float
+            Neutron energy in eV
+
+        Returns
+        -------
+        float
+            Mean free path in cm
+
+        """
+        from openmc.plotter import _calculate_cexs_elem_mat
+
+        energy_grid, cexs = _calculate_cexs_elem_mat(
+            this=self,
+            types=["total"],
+        )
+        total_cexs = cexs[0]
+
+        interpolated_cexs = float(np.interp(energy, energy_grid, total_cexs))
+
+        return 1.0 / interpolated_cexs
+
 
 class Materials(cv.CheckedList):
     """Collection of Materials used for an OpenMC simulation.
@@ -1734,16 +2197,14 @@ class Materials(cv.CheckedList):
             clean_indentation(element, level=level+1)
             element.tail = element.tail.strip(' ')
             file.write((level+1)*spaces_per_level*' ')
-            reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
             file.write(ET.tostring(element, encoding="unicode"))
 
         # Write the <material> elements.
-        for material in sorted(self, key=lambda x: x.id):
+        for material in sorted(set(self), key=lambda x: x.id):
             element = material.to_xml_element(nuclides_to_ignore=nuclides_to_ignore)
             clean_indentation(element, level=level+1)
             element.tail = element.tail.strip(' ')
             file.write((level+1)*spaces_per_level*' ')
-            reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
             file.write(ET.tostring(element, encoding="unicode"))
 
         # Write the closing tag for the root element.
@@ -1799,9 +2260,9 @@ class Materials(cv.CheckedList):
             materials.append(Material.from_xml_element(material))
 
         # Check for cross sections settings
-        xs = elem.find('cross_sections')
+        xs = get_text(elem, "cross_sections")
         if xs is not None:
-            materials.cross_sections = xs.text
+            materials.cross_sections = xs
 
         return materials
 
@@ -1825,3 +2286,123 @@ class Materials(cv.CheckedList):
         root = tree.getroot()
 
         return cls.from_xml_element(root)
+
+
+    def deplete(
+        self,
+        multigroup_fluxes: Sequence[Sequence[float]],
+        energy_group_structures: Sequence[Sequence[float] | str],
+        timesteps: Sequence[float] | Sequence[tuple[float, str]],
+        source_rates: float | Sequence[float],
+        timestep_units: str = 's',
+        chain_file: cv.PathLike | "openmc.deplete.Chain" | None = None,
+        reactions: Sequence[str] | None = None,
+    ) -> Dict[int, list[openmc.Material]]:
+        """Depletes that material, evolving the nuclide densities
+
+        .. versionadded:: 0.15.3
+
+        Parameters
+        ----------
+        multigroup_fluxes: Sequence[Sequence[float]]
+            Energy-dependent multigroup flux values, where each sublist corresponds
+            to a specific material. Will be normalized so that it sums to 1.
+        energy_group_structures: Sequence[Sequence[float] | str]
+            Energy group boundaries in [eV] or the name of the group structure.
+        timesteps : iterable of float or iterable of tuple
+            Array of timesteps. Note that values are not cumulative. The units are
+            specified by the `timestep_units` argument when `timesteps` is an
+            iterable of float. Alternatively, units can be specified for each step
+            by passing an iterable of (value, unit) tuples.
+        source_rates : float or iterable of float, optional
+            Source rate in [neutron/sec] or neutron flux in [neutron/s-cm^2] for
+            each interval in :attr:`timesteps`
+        timestep_units : {'s', 'min', 'h', 'd', 'a', 'MWd/kg'}
+            Units for values specified in the `timesteps` argument. 's' means
+            seconds, 'min' means minutes, 'h' means hours, 'a' means Julian years
+            and 'MWd/kg' indicates that the values are given in burnup (MW-d of
+            energy deposited per kilogram of initial heavy metal).
+        chain_file : PathLike or Chain
+            Path to the depletion chain XML file or instance of openmc.deplete.Chain.
+            Defaults to ``openmc.config['chain_file']``.
+        reactions : list of str, optional
+            Reactions to get cross sections for. If not specified, all neutron
+            reactions listed in the depletion chain file are used.
+
+        Returns
+        -------
+        list of openmc.Material, one for each timestep
+
+        """
+
+        import openmc.deplete
+        from .deplete.chain import _get_chain
+
+        # setting all materials to be depletable
+        for mat in self:
+            mat.depletable = True
+
+        if len(multigroup_fluxes) != len(self):
+            raise ValueError("multigroup_fluxes length must match number of materials")
+        if len(energy_group_structures) != len(self):
+            raise ValueError("energy_group_structures length must match number of materials")
+
+        chain = _get_chain(chain_file)
+
+        # Create MicroXS objects for all materials
+        micros = []
+        fluxes = []
+
+        with openmc.lib.TemporarySession():
+            for material, flux, energy in zip(
+                self, multigroup_fluxes, energy_group_structures
+            ):
+                if material.volume is None:
+                    raise ValueError(
+                        f"Material {material.id} has no volume; cannot deplete"
+                    )
+                temperature = material.temperature or 293.6
+                micro_xs = openmc.deplete.MicroXS.from_multigroup_flux(
+                    energies=energy,
+                    multigroup_flux=flux,
+                    chain_file=chain,
+                    temperature=temperature,
+                    reactions=reactions,
+                )
+                micros.append(micro_xs)
+                fluxes.append(material.volume)
+
+        # Create a single operator for all materials
+        operator = openmc.deplete.IndependentOperator(
+            materials=self,
+            fluxes=fluxes,
+            micros=micros,
+            normalization_mode="source-rate",
+            chain_file=chain,
+        )
+
+        integrator = openmc.deplete.PredictorIntegrator(
+            operator=operator,
+            timesteps=timesteps,
+            source_rates=source_rates,
+            timestep_units=timestep_units,
+        )
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            # Run integrator
+            results_path = Path(tmpdir) / "depletion_results.h5"
+            integrator.integrate(path=results_path)
+
+            # Load depletion results
+            results = openmc.deplete.Results(results_path)
+
+            # For each material, get activated composition at each timestep
+            all_depleted_materials = {
+                material.id: [
+                    result.get_material(str(material.id))
+                    for result in results
+                ]
+                for material in self
+            }
+
+        return all_depleted_materials
diff --git a/openmc/mesh.py b/openmc/mesh.py
index 08025f374..ff8e14577 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -3,19 +3,20 @@ import warnings
 from abc import ABC, abstractmethod
 from collections.abc import Iterable, Sequence, Mapping
 from functools import wraps
-from math import pi, sqrt, atan2
+import math
 from numbers import Integral, Real
+from pathlib import Path
+from typing import Protocol
 
 import h5py
 import lxml.etree as ET
 import numpy as np
-from pathlib import Path
 
 import openmc
 import openmc.checkvalue as cv
 from openmc.checkvalue import PathLike
-from openmc.utility_funcs import change_directory
-from ._xml import get_text
+from .bounding_box import BoundingBox
+from ._xml import get_elem_list, get_text
 from .mixin import IDManagerMixin
 from .surface import _BOUNDARY_TYPES
 from .utility_funcs import input_path
@@ -38,6 +39,11 @@ class MeshMaterialVolumes(Mapping):
         Array of shape (elements, max_materials) storing material IDs
     volumes : numpy.ndarray
         Array of shape (elements, max_materials) storing material volumes
+    bboxes : numpy.ndarray, optional
+        Array of shape (elements, max_materials, 6) storing axis-aligned
+        bounding boxes for each (element, material) combination with ordering
+        (xmin, ymin, zmin, xmax, ymax, zmax). Bounding boxes enclose the
+        ray-estimator prisms used to compute volumes.
 
     See Also
     --------
@@ -62,9 +68,30 @@ class MeshMaterialVolumes(Mapping):
     [(2, 31.87963824195591), (1, 6.129949130817542)]
 
     """
-    def __init__(self, materials: np.ndarray, volumes: np.ndarray):
+    def __init__(
+        self,
+        materials: np.ndarray,
+        volumes: np.ndarray,
+        bboxes: np.ndarray | None = None
+    ):
         self._materials = materials
         self._volumes = volumes
+        self._bboxes = bboxes
+
+        if self._bboxes is not None:
+            if self._bboxes.shape[:2] != self._materials.shape:
+                raise ValueError(
+                    'bboxes must have shape (elements, max_materials, 6) '
+                    'matching materials/volumes.'
+                )
+            if self._bboxes.shape[2] != 6:
+                raise ValueError(
+                    'bboxes must have shape (elements, max_materials, 6).'
+                )
+
+    @property
+    def has_bounding_boxes(self) -> bool:
+        return self._bboxes is not None
 
     @property
     def num_elements(self) -> int:
@@ -90,7 +117,11 @@ class MeshMaterialVolumes(Mapping):
             volumes[indices] = self._volumes[indices, i]
         return volumes
 
-    def by_element(self, index_elem: int) -> list[tuple[int | None, float]]:
+    def by_element(
+        self,
+        index_elem: int,
+        include_bboxes: bool = False
+    ) -> list[tuple[int | None, float] | tuple[int | None, float, BoundingBox | None]]:
         """Get a list of volumes for each material within a specific element.
 
         Parameters
@@ -100,15 +131,32 @@ class MeshMaterialVolumes(Mapping):
 
         Returns
         -------
-        list of tuple of (material ID, volume)
+        list of tuple
+            If ``include_bboxes`` is False (default), returns tuples of
+            (material ID, volume). If ``include_bboxes`` is True, returns
+            tuples of (material ID, volume, bounding box).
 
         """
         table_size = self._volumes.shape[1]
-        return [
-            (m if m > -1 else None, self._volumes[index_elem, i])
-            for i in range(table_size)
-            if (m := self._materials[index_elem, i]) != -2
-        ]
+        if include_bboxes and self._bboxes is None:
+            raise ValueError('Bounding boxes were not computed for this object.')
+
+        results = []
+        for i in range(table_size):
+            m = self._materials[index_elem, i]
+            if m == -2:
+                continue
+            mat_id = m if m > -1 else None
+            vol = self._volumes[index_elem, i]
+
+            if include_bboxes:
+                vals = self._bboxes[index_elem, i]
+                bbox = BoundingBox(vals[0:3], vals[3:6])
+                results.append((mat_id, vol, bbox))
+            else:
+                results.append((mat_id, vol))
+
+        return results
 
     def save(self, filename: PathLike):
         """Save material volumes to a .npz file.
@@ -118,8 +166,10 @@ class MeshMaterialVolumes(Mapping):
         filename : path-like
             Filename where data will be saved
         """
-        np.savez_compressed(
-            filename, materials=self._materials, volumes=self._volumes)
+        kwargs = {'materials': self._materials, 'volumes': self._volumes}
+        if self._bboxes is not None:
+            kwargs['bboxes'] = self._bboxes
+        np.savez_compressed(filename, **kwargs)
 
     @classmethod
     def from_npz(cls, filename: PathLike) -> MeshMaterialVolumes:
@@ -132,7 +182,8 @@ class MeshMaterialVolumes(Mapping):
 
         """
         filedata = np.load(filename)
-        return cls(filedata['materials'], filedata['volumes'])
+        bboxes = filedata['bboxes'] if 'bboxes' in filedata.files else None
+        return cls(filedata['materials'], filedata['volumes'], bboxes)
 
 
 class MeshBase(IDManagerMixin, ABC):
@@ -151,11 +202,17 @@ class MeshBase(IDManagerMixin, ABC):
         Unique identifier for the mesh
     name : str
         Name of the mesh
+    lower_left : Iterable of float
+        The lower-left coordinates
+    upper_right : Iterable of float
+        The upper-right coordinates
     bounding_box : openmc.BoundingBox
         Axis-aligned bounding box of the mesh as defined by the upper-right and
         lower-left coordinates.
     indices : Iterable of tuple
         An iterable of mesh indices for each mesh element, e.g. [(1, 1, 1), (2, 1, 1), ...]
+    n_elements : int
+        Number of elements in the mesh
     """
 
     next_id = 1
@@ -178,6 +235,16 @@ class MeshBase(IDManagerMixin, ABC):
         else:
             self._name = ''
 
+    @property
+    @abstractmethod
+    def lower_left(self):
+        pass
+
+    @property
+    @abstractmethod
+    def upper_right(self):
+        pass
+
     @property
     def bounding_box(self) -> openmc.BoundingBox:
         return openmc.BoundingBox(self.lower_left, self.upper_right)
@@ -187,6 +254,11 @@ class MeshBase(IDManagerMixin, ABC):
     def indices(self):
         pass
 
+    @property
+    @abstractmethod
+    def n_elements(self):
+        pass
+
     def __repr__(self):
         string = type(self).__name__ + '\n'
         string += '{0: <16}{1}{2}\n'.format('\tID', '=\t', self._id)
@@ -194,10 +266,11 @@ class MeshBase(IDManagerMixin, ABC):
         return string
 
     def _volume_dim_check(self):
-        if self.n_dimension != 3 or \
-           any([d == 0 for d in self.dimension]):
-            raise RuntimeError(f'Mesh {self.id} is not 3D. '
-                               'Volumes cannot be provided.')
+        if any(d == 0 for d in self.dimension):
+            raise RuntimeError(
+                f'Mesh {self.id} has a zero-size dimension. '
+                'Volumes cannot be provided.'
+            )
 
     @classmethod
     def from_hdf5(cls, group: h5py.Group):
@@ -216,7 +289,7 @@ class MeshBase(IDManagerMixin, ABC):
         """
         mesh_type = 'regular' if 'type' not in group.keys() else group['type'][()].decode()
         mesh_id = int(group.name.split('/')[-1].lstrip('mesh '))
-        mesh_name = '' if not 'name' in group else group['name'][()].decode()
+        mesh_name = '' if 'name' not in group else group['name'][()].decode()
 
         if mesh_type == 'regular':
             return RegularMesh.from_hdf5(group, mesh_id, mesh_name)
@@ -286,6 +359,7 @@ class MeshBase(IDManagerMixin, ABC):
             model: openmc.Model,
             n_samples: int | tuple[int, int, int] = 10_000,
             include_void: bool = True,
+            material_volumes: MeshMaterialVolumes | None = None,
             **kwargs
     ) -> list[openmc.Material]:
         """Generate homogenized materials over each element in a mesh.
@@ -304,8 +378,12 @@ class MeshBase(IDManagerMixin, ABC):
             the x, y, and z dimensions.
         include_void : bool, optional
             Whether homogenization should include voids.
+        material_volumes : MeshMaterialVolumes, optional
+            Previously computed mesh material volumes to use for homogenization.
+            If not provided, they will be computed by calling
+            :meth:`material_volumes`.
         **kwargs
-            Keyword-arguments passed to :meth:`MeshBase.material_volumes`.
+            Keyword-arguments passed to :meth:`material_volumes`.
 
         Returns
         -------
@@ -313,23 +391,16 @@ class MeshBase(IDManagerMixin, ABC):
             Homogenized material in each mesh element
 
         """
-        vols = self.material_volumes(model, n_samples, **kwargs)
+        if material_volumes is None:
+            vols = self.material_volumes(model, n_samples, **kwargs)
+        else:
+            vols = material_volumes
         mat_volume_by_element = [vols.by_element(i) for i in range(vols.num_elements)]
 
+        # Get dictionary of all materials
+        materials = model._get_all_materials()
+
         # Create homogenized material for each element
-        materials = model.geometry.get_all_materials()
-
-        # Account for materials in DAGMC universes
-        # TODO: This should really get incorporated in lower-level calls to
-        # get_all_materials, but right now it requires information from the
-        # Model object
-        for cell in model.geometry.get_all_cells().values():
-            if isinstance(cell.fill, openmc.DAGMCUniverse):
-                names = cell.fill.material_names
-                materials.update({
-                    mat.id: mat for mat in model.materials if mat.name in names
-                })
-
         homogenized_materials = []
         for mat_volume_list in mat_volume_by_element:
             material_ids, volumes = [list(x) for x in zip(*mat_volume_list)]
@@ -366,6 +437,7 @@ class MeshBase(IDManagerMixin, ABC):
             model: openmc.Model,
             n_samples: int | tuple[int, int, int] = 10_000,
             max_materials: int = 4,
+            bounding_boxes: bool = False,
             **kwargs
     ) -> MeshMaterialVolumes:
         """Determine volume of materials in each mesh element.
@@ -388,6 +460,11 @@ class MeshBase(IDManagerMixin, ABC):
             the x, y, and z dimensions.
         max_materials : int, optional
             Estimated maximum number of materials in any given mesh element.
+        bounding_boxes : bool, optional
+            Whether to compute an axis-aligned bounding box for each
+            (mesh element, material) combination. When enabled, the bounding
+            box encloses the ray-estimator prisms used for the volume
+            estimation.
         **kwargs : dict
             Keyword arguments passed to :func:`openmc.lib.init`
 
@@ -399,32 +476,31 @@ class MeshBase(IDManagerMixin, ABC):
         """
         import openmc.lib
 
-        with change_directory(tmpdir=True):
-            # In order to get mesh into model, we temporarily replace the
-            # tallies with a single mesh tally using the current mesh
-            original_tallies = model.tallies
-            new_tally = openmc.Tally()
-            new_tally.filters = [openmc.MeshFilter(self)]
-            new_tally.scores = ['flux']
-            model.tallies = [new_tally]
+        # In order to get mesh into model, we temporarily replace the
+        # tallies with a single mesh tally using the current mesh
+        original_tallies = list(model.tallies)
+        new_tally = openmc.Tally()
+        new_tally.filters = [openmc.MeshFilter(self)]
+        new_tally.scores = ['flux']
+        model.tallies = [new_tally]
 
-            # Export model to XML
-            model.export_to_model_xml()
+        # Set default arguments
+        kwargs.setdefault('output', True)
+        if 'args' in kwargs:
+            kwargs['args'] = ['-c'] + kwargs['args']
+        kwargs.setdefault('args', ['-c'])
 
-            # Get material volume fractions
-            kwargs.setdefault('output', True)
-            if 'args' in kwargs:
-                kwargs['args'] = ['-c'] + kwargs['args']
-            kwargs.setdefault('args', ['-c'])
-            openmc.lib.init(**kwargs)
+        with openmc.lib.TemporarySession(model, **kwargs):
+            # Get mesh from single tally
             mesh = openmc.lib.tallies[new_tally.id].filters[0].mesh
+
+            # Compute material volumes
             volumes = mesh.material_volumes(
-                n_samples, max_materials, output=kwargs['output'])
-            openmc.lib.finalize()
-
-            # Restore original tallies
-            model.tallies = original_tallies
+                n_samples, max_materials, output=kwargs['output'],
+                bounding_boxes=bounding_boxes)
 
+        # Restore original tallies
+        model.tallies = original_tallies
         return volumes
 
 
@@ -460,11 +536,20 @@ class StructuredMesh(MeshBase):
     def n_dimension(self):
         pass
 
+    @property
+    @abstractmethod
+    def _axis_labels(self):
+        pass
+
     @property
     @abstractmethod
     def _grids(self):
         pass
 
+    @abstractmethod
+    def get_indices_at_coords(self, coords: Sequence[float]) -> tuple:
+        pass
+
     @property
     def vertices(self):
         """Return coordinates of mesh vertices in Cartesian coordinates. Also
@@ -561,9 +646,18 @@ class StructuredMesh(MeshBase):
         return (vertices[s0] + vertices[s1]) / 2
 
     @property
-    def num_mesh_cells(self):
+    def n_elements(self):
         return np.prod(self.dimension)
 
+    @property
+    def num_mesh_cells(self):
+        warnings.warn(
+            "The 'num_mesh_cells' attribute is deprecated and will be removed in a future version. "
+            "Use 'n_elements' instead.",
+            FutureWarning, stacklevel=2
+        )
+        return self.n_elements
+
     def write_data_to_vtk(self,
                           filename: PathLike,
                           datasets: dict | None = None,
@@ -576,11 +670,16 @@ class StructuredMesh(MeshBase):
         filename : str
             Name of the VTK file to write.
         datasets : dict
-            Dictionary whose keys are the data labels
-            and values are the data sets.
+            Dictionary whose keys are the data labels and values are the data
+            sets. 1D datasets are expected to be extracted directly from
+            statepoint data without reordering/reshaping. Multidimensional
+            datasets are expected to have the same dimensions as the mesh itself
+            with structured indexing in "C" ordering. See the "expand_dims" flag
+            of :meth:`~openmc.Tally.get_reshaped_data` on reshaping tally data when using
+            :class:`~openmc.MeshFilter`'s.
         volume_normalization : bool, optional
-            Whether or not to normalize the data by
-            the volume of the mesh elements.
+            Whether or not to normalize the data by the volume of the mesh
+            elements.
         curvilinear : bool
             Whether or not to write curvilinear elements. Only applies to
             ``SphericalMesh`` and ``CylindricalMesh``.
@@ -594,14 +693,27 @@ class StructuredMesh(MeshBase):
         -------
         vtk.StructuredGrid or vtk.UnstructuredGrid
             a VTK grid object representing the mesh
+
+        Examples
+        --------
+        1D data from a tally with only a mesh filter and heating score:
+
+            # pass the tally mean property of shape (N, 1, 1) directly to this
+            # method; dimensions of size 1 will automatically removed
+            >>> heating = tally.mean
+            >>> mesh.write_data_to_vtk({'heating': heating})
+
+        Multidimensional data from a tally with only a mesh
+
+           # retrieve a data array with the mesh filter expanded into three
+           # dimensions, ijk; additional dimensions of size one will
+           # automatically be removed
+           >>> heating = tally.get_reshaped_data(expand_dims=True)
+           >>> mesh.write_data_to_vtk({'heating': heating})
         """
         import vtk
         from vtk.util import numpy_support as nps
 
-        # check that the data sets are appropriately sized
-        if datasets is not None:
-            self._check_vtk_datasets(datasets)
-
         # write linear elements using a structured grid
         if not curvilinear or isinstance(self, (RegularMesh, RectilinearMesh)):
             vtk_grid = self._create_vtk_structured_grid()
@@ -612,22 +724,27 @@ class StructuredMesh(MeshBase):
             writer = vtk.vtkUnstructuredGridWriter()
 
         if datasets is not None:
-            # maintain a list of the datasets as added
-            # to the VTK arrays to ensure they persist
-            # in memory until the file is written
+            # maintain a list of the datasets as added to the VTK arrays to
+            # ensure they persist in memory until the file is written
             datasets_out = []
             for label, dataset in datasets.items():
-                dataset = np.asarray(dataset).flatten()
+                dataset = self._reshape_vtk_dataset(dataset)
+                self._check_vtk_dataset(label, dataset)
+                # If the array data is 3D, assume is in C ordering and transpose
+                # before flattening to match the ordering expected by the VTK
+                # array based on the way mesh indices are ordered in the Python
+                # API
+                # TODO: update to "C" ordering throughout
+                if dataset.ndim == 3:
+                    dataset = dataset.T.ravel()
                 datasets_out.append(dataset)
 
                 if volume_normalization:
-                    dataset /= self.volumes.T.flatten()
+                    dataset /= self.volumes.T.ravel()
 
                 dataset_array = vtk.vtkDoubleArray()
                 dataset_array.SetName(label)
-                dataset_array.SetArray(nps.numpy_to_vtk(dataset),
-                                    dataset.size,
-                                    True)
+                dataset_array.SetArray(nps.numpy_to_vtk(dataset), dataset.size, True)
                 vtk_grid.GetCellData().AddArray(dataset_array)
 
         writer.SetFileName(str(filename))
@@ -754,28 +871,155 @@ class StructuredMesh(MeshBase):
 
         return vtk_grid
 
-    def _check_vtk_datasets(self, datasets: dict):
-        """Perform some basic checks that the datasets are valid for this mesh
+    @staticmethod
+    def _reshape_vtk_dataset(dataset):
+        """Reshape a dataset to be compatible with VTK output
+
+        This method performs the following operations on a dataset:
+        1. Convert to numpy array if not already
+        2. Remove any trailing dimensions of size 1
+        3. Squeeze out any extra dimensions of size 1 beyond the first 3
 
         Parameters
         ----------
-        datasets : dict
-            Dictionary whose keys are the data labels
-            and values are the data sets.
+        dataset : array-like
+            The dataset to reshape
+
+        Returns
+        -------
+        numpy.ndarray
+            The reshaped dataset
+        """
+        reshaped_data = np.asarray(dataset)
+
+        # detect flat array with extra dims
+        if all(d == 1 for d in reshaped_data.shape[1:]):
+            reshaped_data = reshaped_data.squeeze()
+
+        # remove any higher dimensions with size 1
+        if reshaped_data.ndim > 3 and all(d == 1 for d in reshaped_data.shape[3:]):
+            reshaped_data = reshaped_data.reshape(reshaped_data.shape[:3])
+
+        if np.shares_memory(reshaped_data, dataset):
+            return np.copy(reshaped_data)
+        else:
+            return reshaped_data
+
+    def _check_vtk_dataset(self, label: str, dataset: np.ndarray):
+        """Perform some basic checks that a dataset is valid for this Mesh
+
+        Parameters
+        ----------
+        label : str
+            The label for the dataset being checked
+        dataset : numpy.ndarray
+            The dataset array to check against this mesh's dimensions
 
         """
-        for label, dataset in datasets.items():
-            errmsg = (
+        cv.check_type('data label', label, str)
+
+        if dataset.size != self.n_elements:
+            raise ValueError(
                 f"The size of the dataset '{label}' ({dataset.size}) should be"
-                f" equal to the number of mesh cells ({self.num_mesh_cells})"
+                f" equal to the number of mesh cells ({self.n_elements})"
             )
-            if isinstance(dataset, np.ndarray):
-                if not dataset.size == self.num_mesh_cells:
-                    raise ValueError(errmsg)
-            else:
-                if len(dataset) == self.num_mesh_cells:
-                    raise ValueError(errmsg)
-            cv.check_type('data label', label, str)
+
+        # accept a flat array as-is, assuming it is in the correct order
+        if dataset.ndim == 1:
+            return
+
+        if dataset.shape != self.dimension:
+            raise ValueError(
+                f'Cannot apply multidimensional dataset "{label}" with '
+                f"shape {dataset.shape} to mesh {self.id} "
+                f"with dimensions {self.dimension}"
+            )
+
+    @classmethod
+    def from_domain(
+        cls,
+        domain: HasBoundingBox | BoundingBox,
+        dimension: Sequence[int] | int | None = None,
+        mesh_id: int | None = None,
+        name: str = '',
+        **kwargs
+    ) -> StructuredMesh:
+        """Create a structured mesh from a domain using its bounding box.
+
+        Parameters
+        ----------
+        domain : HasBoundingBox | openmc.BoundingBox
+            Object used as a template for the mesh extents. If ``domain`` has a
+            ``bounding_box`` attribute, that bounding box is used directly.
+        dimension : Iterable of int or int, optional
+            Number of mesh cells. When omitted, the subclass-specific default is
+            used. If provided as a single integer, subclasses that support it
+            interpret it as a target total number of mesh cells.
+        mesh_id : int, optional
+            Unique identifier for the mesh.
+        name : str, optional
+            Name of the mesh.
+        **kwargs
+            Additional keyword arguments forwarded to
+            :meth:`from_bounding_box`.
+
+        Returns
+        -------
+        openmc.StructuredMesh
+            Structured mesh instance.
+        """
+        if isinstance(domain, BoundingBox):
+            bbox = domain
+        elif hasattr(domain, 'bounding_box'):
+            bbox = domain.bounding_box
+        else:
+            raise TypeError("Domain must be a BoundingBox or have a "
+                            "bounding_box property")
+
+        if dimension is None:
+            return cls.from_bounding_box(
+                bbox, mesh_id=mesh_id, name=name, **kwargs)
+
+        return cls.from_bounding_box(
+            bbox, dimension=dimension, mesh_id=mesh_id, name=name, **kwargs)
+
+    @classmethod
+    @abstractmethod
+    def from_bounding_box(
+        cls,
+        bbox: openmc.BoundingBox,
+        dimension: Sequence[int] | int,
+        mesh_id: int | None = None,
+        name: str = '',
+        **kwargs
+    ) -> StructuredMesh:
+        """Create a structured mesh from a bounding box.
+
+        Parameters
+        ----------
+        bbox : openmc.BoundingBox
+            Bounding box used to define the mesh extents.
+        dimension : Iterable of int or int
+            Number of mesh cells. The interpretation and any default value are
+            defined by the concrete mesh type.
+        mesh_id : int, optional
+            Unique identifier for the mesh.
+        name : str, optional
+            Name of the mesh.
+        **kwargs
+            Additional keyword arguments accepted by specific subclasses.
+
+        Returns
+        -------
+        openmc.StructuredMesh
+            Structured mesh instance.
+        """
+        pass
+
+
+class HasBoundingBox(Protocol):
+    """Object that has a ``bounding_box`` attribute."""
+    bounding_box: openmc.BoundingBox
 
 
 class RegularMesh(StructuredMesh):
@@ -840,6 +1084,10 @@ class RegularMesh(StructuredMesh):
         else:
             return None
 
+    @property
+    def _axis_labels(self):
+        return ('x', 'y', 'z')[:self.n_dimension]
+
     @property
     def lower_left(self):
         return self._lower_left
@@ -1022,46 +1270,47 @@ class RegularMesh(StructuredMesh):
         return mesh
 
     @classmethod
-    def from_domain(
+    def from_bounding_box(
         cls,
-        domain: 'openmc.Cell' | 'openmc.Region' | 'openmc.Universe' | 'openmc.Geometry',
-        dimension: Sequence[int] = (10, 10, 10),
+        bbox: openmc.BoundingBox,
+        dimension: Sequence[int] | int = 1000,
         mesh_id: int | None = None,
-        name: str = ''
-    ):
-        """Create mesh from an existing openmc cell, region, universe or
-        geometry by making use of the objects bounding box property.
+        name: str = '',
+    ) -> RegularMesh:
+        """Create a RegularMesh from a bounding box.
 
         Parameters
         ----------
-        domain : {openmc.Cell, openmc.Region, openmc.Universe, openmc.Geometry}
-            The object passed in will be used as a template for this mesh. The
-            bounding box of the property of the object passed will be used to
-            set the lower_left and upper_right and of the mesh instance
-        dimension : Iterable of int
-            The number of mesh cells in each direction (x, y, z).
-        mesh_id : int
-            Unique identifier for the mesh
-        name : str
-            Name of the mesh
+        bbox : openmc.BoundingBox
+            Bounding box used to set the mesh extents.
+        dimension : Iterable of int or int, optional
+            The number of mesh cells in each direction (x, y, z). If a single
+            integer is provided, the total number of cells is distributed
+            across directions to produce cells with roughly equal widths.
+        mesh_id : int, optional
+            Unique identifier for the mesh.
+        name : str, optional
+            Name of the mesh.
 
         Returns
         -------
         openmc.RegularMesh
-            RegularMesh instance
-
+            RegularMesh instance.
         """
-        cv.check_type(
-            "domain",
-            domain,
-            (openmc.Cell, openmc.Region, openmc.Universe, openmc.Geometry),
-        )
-
         mesh = cls(mesh_id=mesh_id, name=name)
-        mesh.lower_left = domain.bounding_box[0]
-        mesh.upper_right = domain.bounding_box[1]
+        mesh.lower_left = bbox[0]
+        mesh.upper_right = bbox[1]
+        if isinstance(dimension, int):
+            cv.check_greater_than("dimension", dimension, 1, equality=True)
+            # If a single integer is provided, divide the domain into that many
+            # mesh cells with roughly equal lengths in each direction
+            ideal_cube_volume = bbox.volume / dimension
+            ideal_cube_size = ideal_cube_volume ** (1 / 3)
+            dimension = [
+                max(1, int(round(side / ideal_cube_size)))
+                for side in bbox.width
+            ]
         mesh.dimension = dimension
-
         return mesh
 
     def to_xml_element(self):
@@ -1109,21 +1358,21 @@ class RegularMesh(StructuredMesh):
         mesh_id = int(get_text(elem, 'id'))
         mesh = cls(mesh_id=mesh_id)
 
-        dimension = get_text(elem, 'dimension')
+        dimension = get_elem_list(elem, "dimension", int)
         if dimension is not None:
-            mesh.dimension = [int(x) for x in dimension.split()]
+            mesh.dimension = dimension
 
-        lower_left = get_text(elem, 'lower_left')
+        lower_left = get_elem_list(elem, "lower_left", float)
         if lower_left is not None:
-            mesh.lower_left = [float(x) for x in lower_left.split()]
+            mesh.lower_left = lower_left
 
-        upper_right = get_text(elem, 'upper_right')
+        upper_right = get_elem_list(elem, "upper_right", float)
         if upper_right is not None:
-            mesh.upper_right = [float(x) for x in upper_right.split()]
+            mesh.upper_right = upper_right
 
-        width = get_text(elem, 'width')
+        width = get_elem_list(elem, "width", float)
         if width is not None:
-            mesh.width = [float(x) for x in width.split()]
+            mesh.width = width
 
         return mesh
 
@@ -1252,6 +1501,47 @@ class RegularMesh(StructuredMesh):
 
         return root_cell, cells
 
+    def get_indices_at_coords(self, coords: Sequence[float]) -> tuple:
+        """Finds the index of the mesh element at the specified coordinates.
+
+        .. versionadded:: 0.15.4
+
+        Parameters
+        ----------
+        coords : Sequence[float]
+            Cartesian coordinates of the point.
+
+        Returns
+        -------
+        tuple
+            Mesh indices matching the dimensionality of the mesh
+
+        """
+        ndim = self.n_dimension
+        if len(coords) < ndim:
+            raise ValueError(
+                f"coords must have at least {ndim} values for a "
+                f"{ndim}D mesh, got {len(coords)}"
+            )
+
+        coords_array = np.array(coords[:ndim])
+        lower_left = np.array(self.lower_left)
+        upper_right = np.array(self.upper_right)
+        dimension = np.array(self.dimension)
+
+        if np.any(coords_array < lower_left) or np.any(coords_array > upper_right):
+            raise ValueError(
+                f"coords {tuple(coords_array)} are outside mesh bounds "
+                f"[{tuple(lower_left)}, {tuple(upper_right)}]"
+            )
+
+        # Calculate spacing for each dimension
+        spacing = (upper_right - lower_left) / dimension
+
+        # Calculate indices for each coordinate
+        indices = np.floor((coords_array - lower_left) / spacing).astype(int)
+        return tuple(int(i) for i in indices[:ndim])
+
 
 def Mesh(*args, **kwargs):
     warnings.warn("Mesh has been renamed RegularMesh. Future versions of "
@@ -1311,6 +1601,10 @@ class RectilinearMesh(StructuredMesh):
     def n_dimension(self):
         return 3
 
+    @property
+    def _axis_labels(self):
+        return ('x', 'y', 'z')
+
     @property
     def x_grid(self):
         return self._x_grid
@@ -1429,9 +1723,9 @@ class RectilinearMesh(StructuredMesh):
         """
         mesh_id = int(get_text(elem, 'id'))
         mesh = cls(mesh_id=mesh_id)
-        mesh.x_grid = [float(x) for x in get_text(elem, 'x_grid').split()]
-        mesh.y_grid = [float(y) for y in get_text(elem, 'y_grid').split()]
-        mesh.z_grid = [float(z) for z in get_text(elem, 'z_grid').split()]
+        mesh.x_grid = get_elem_list(elem, "x_grid", float)
+        mesh.y_grid = get_elem_list(elem, "y_grid", float)
+        mesh.z_grid = get_elem_list(elem, "z_grid", float)
 
         return mesh
 
@@ -1459,6 +1753,90 @@ class RectilinearMesh(StructuredMesh):
 
         return element
 
+    def get_indices_at_coords(self, coords: Sequence[float]) -> tuple[int, int, int]:
+        """Find the mesh cell indices containing the specified coordinates.
+
+        .. versionadded:: 0.15.4
+
+        Parameters
+        ----------
+        coords : Sequence[float]
+            Cartesian coordinates of the point as (x, y, z).
+
+        Returns
+        -------
+        tuple[int, int, int]
+            Mesh indices (ix, iy, iz).
+
+        Raises
+        ------
+        ValueError
+            If coords does not contain exactly 3 values, or if a coordinate is
+            outside the mesh grid boundaries.
+        """
+        if len(coords) != 3:
+            raise ValueError(
+                f"coords must contain exactly 3 values for a rectilinear mesh, "
+                f"got {len(coords)}"
+            )
+
+        grids = (self.x_grid, self.y_grid, self.z_grid)
+        indices = []
+
+        for grid, value in zip(grids, coords):
+            if value < grid[0] or value > grid[-1]:
+                raise ValueError(
+                    f"Coordinate value {value} is outside the mesh grid boundaries: "
+                    f"[{grid[0]}, {grid[-1]}]"
+                )
+
+            idx = np.searchsorted(grid, value, side="right") - 1
+            indices.append(int(min(idx, len(grid) - 2)))
+
+        return tuple(indices)
+
+    @classmethod
+    def from_bounding_box(
+        cls,
+        bbox: openmc.BoundingBox,
+        dimension: Sequence[int] | int = 1000,
+        mesh_id: int | None = None,
+        name: str = '',
+    ) -> RectilinearMesh:
+        """Create a RectilinearMesh from a bounding box with uniform grids.
+
+        Parameters
+        ----------
+        bbox : openmc.BoundingBox
+            Bounding box used to set the mesh extents.
+        dimension : Iterable of int or int, optional
+            The number of mesh cells in each direction (x, y, z). If a single
+            integer is provided, the total number of cells is distributed across
+            the three directions proportionally to the side lengths.
+        mesh_id : int, optional
+            Unique identifier for the mesh.
+        name : str, optional
+            Name of the mesh.
+
+        Returns
+        -------
+        openmc.RectilinearMesh
+            RectilinearMesh instance with uniform grids along each axis.
+        """
+        if isinstance(dimension, int):
+            cv.check_greater_than("dimension", dimension, 1, equality=True)
+            ideal_cube_volume = bbox.volume / dimension
+            ideal_cube_size = ideal_cube_volume ** (1 / 3)
+            dimension = [
+                max(1, int(round(side / ideal_cube_size)))
+                for side in bbox.width
+            ]
+        mesh = cls(mesh_id=mesh_id, name=name)
+        mesh.x_grid = np.linspace(bbox[0][0], bbox[1][0], num=dimension[0] + 1)
+        mesh.y_grid = np.linspace(bbox[0][1], bbox[1][1], num=dimension[1] + 1)
+        mesh.z_grid = np.linspace(bbox[0][2], bbox[1][2], num=dimension[2] + 1)
+        return mesh
+
 
 class CylindricalMesh(StructuredMesh):
     """A 3D cylindrical mesh
@@ -1523,7 +1901,7 @@ class CylindricalMesh(StructuredMesh):
         self,
         r_grid: Sequence[float],
         z_grid: Sequence[float],
-        phi_grid: Sequence[float] = (0, 2*pi),
+        phi_grid: Sequence[float] = (0, 2*math.pi),
         origin: Sequence[float] = (0., 0., 0.),
         mesh_id: int | None = None,
         name: str = '',
@@ -1545,6 +1923,10 @@ class CylindricalMesh(StructuredMesh):
     def n_dimension(self):
         return 3
 
+    @property
+    def _axis_labels(self):
+        return ('r', 'phi', 'z')
+
     @property
     def origin(self):
         return self._origin
@@ -1576,7 +1958,7 @@ class CylindricalMesh(StructuredMesh):
         cv.check_length('mesh phi_grid', grid, 2)
         cv.check_increasing('mesh phi_grid', grid)
         grid = np.asarray(grid, dtype=float)
-        if np.any((grid < 0.0) | (grid > 2*pi)):
+        if np.any((grid < 0.0) | (grid > 2*math.pi)):
             raise ValueError("phi_grid values must be in [0, 2π].")
         self._phi_grid = grid
 
@@ -1644,17 +2026,17 @@ class CylindricalMesh(StructuredMesh):
         return string
 
     def get_indices_at_coords(
-            self,
-            coords: Sequence[float]
-        ) -> tuple[int, int, int]:
-        """Finds the index of the mesh voxel at the specified x,y,z coordinates.
+        self,
+        coords: Sequence[float]
+    ) -> tuple[int, int, int]:
+        """Finds the index of the mesh element at the specified coordinates.
 
         .. versionadded:: 0.15.0
 
         Parameters
         ----------
         coords : Sequence[float]
-            The x, y, z axis coordinates
+            Cartesian coordinates of the point.
 
         Returns
         -------
@@ -1662,7 +2044,7 @@ class CylindricalMesh(StructuredMesh):
             The r, phi, z indices
 
         """
-        r_value_from_origin = sqrt((coords[0]-self.origin[0])**2 + (coords[1]-self.origin[1])**2)
+        r_value_from_origin = math.hypot(coords[0]-self.origin[0], coords[1]-self.origin[1])
 
         if r_value_from_origin < self.r_grid[0] or r_value_from_origin > self.r_grid[-1]:
             raise ValueError(
@@ -1686,13 +2068,13 @@ class CylindricalMesh(StructuredMesh):
         delta_x = coords[0] - self.origin[0]
         delta_y = coords[1] - self.origin[1]
         # atan2 returns values in -pi to +pi range
-        phi_value = atan2(delta_y, delta_x)
+        phi_value = math.atan2(delta_y, delta_x)
         if delta_x < 0 and delta_y < 0:
             # returned phi_value anticlockwise and negative
-            phi_value += 2 * pi
+            phi_value += 2 * math.pi
         if delta_x > 0 and delta_y < 0:
             # returned phi_value anticlockwise and negative
-            phi_value += 2 * pi
+            phi_value += 2 * math.pi
 
         phi_grid_values = np.array(self.phi_grid)
 
@@ -1721,32 +2103,34 @@ class CylindricalMesh(StructuredMesh):
         return mesh
 
     @classmethod
-    def from_domain(
+    def from_bounding_box(
         cls,
-        domain: 'openmc.Cell' | 'openmc.Region' | 'openmc.Universe' | 'openmc.Geometry',
+        bbox: openmc.BoundingBox,
         dimension: Sequence[int] = (10, 10, 10),
         mesh_id: int | None = None,
-        phi_grid_bounds: Sequence[float] = (0.0, 2*pi),
-        name: str = ''
-    ):
-        """Creates a regular CylindricalMesh from an existing openmc domain.
+        name: str = '',
+        phi_grid_bounds: Sequence[float] = (0.0, 2*math.pi),
+        enclose_domain: bool = False,
+    ) -> CylindricalMesh:
+        """Create CylindricalMesh from a bounding box.
 
         Parameters
         ----------
-        domain : openmc.Cell or openmc.Region or openmc.Universe or openmc.Geometry
-            The object passed in will be used as a template for this mesh. The
-            bounding box of the property of the object passed will be used to
-            set the r_grid, z_grid ranges.
+        bbox : openmc.BoundingBox
+            Bounding box used to set the r_grid and z_grid ranges.
         dimension : Iterable of int
             The number of equally spaced mesh cells in each direction (r_grid,
             phi_grid, z_grid)
-        mesh_id : int
+        mesh_id : int, optional
             Unique identifier for the mesh
+        name : str, optional
+            Name of the mesh
         phi_grid_bounds : numpy.ndarray
             Mesh bounds points along the phi-axis in radians. The default value
             is (0, 2π), i.e., the full phi range.
-        name : str
-            Name of the mesh
+        enclose_domain : bool
+            If True, the mesh will encompass the bounding box of the domain. If
+            False, the mesh will be inscribed within the domain's bounding box.
 
         Returns
         -------
@@ -1754,43 +2138,28 @@ class CylindricalMesh(StructuredMesh):
             CylindricalMesh instance
 
         """
-        cv.check_type(
-            "domain",
-            domain,
-            (openmc.Cell, openmc.Region, openmc.Universe, openmc.Geometry),
-        )
+        if enclose_domain:
+            outer_radius = 0.5 * np.linalg.norm(bbox.width[:2])
+        else:
+            outer_radius = 0.5 * min(bbox.width[:2])
 
-        # loaded once to avoid recalculating bounding box
-        cached_bb = domain.bounding_box
-        max_bounding_box_radius = max(
-            [
-                cached_bb[0][0],
-                cached_bb[0][1],
-                cached_bb[1][0],
-                cached_bb[1][1],
-            ]
-        )
-        r_grid = np.linspace(
-            0,
-            max_bounding_box_radius,
-            num=dimension[0]+1
-        )
+        r_grid = np.linspace(0, outer_radius, num=dimension[0]+1)
         phi_grid = np.linspace(
             phi_grid_bounds[0],
             phi_grid_bounds[1],
             num=dimension[1]+1
         )
         z_grid = np.linspace(
-            cached_bb[0][2],
-            cached_bb[1][2],
+            bbox[0][2],
+            bbox[1][2],
             num=dimension[2]+1
         )
-        origin = (cached_bb.center[0], cached_bb.center[1], z_grid[0])
+        origin = (bbox.center[0], bbox.center[1], z_grid[0])
 
         # make z-grid relative to the origin
         z_grid -= origin[2]
 
-        mesh = cls(
+        return cls(
             r_grid=r_grid,
             z_grid=z_grid,
             phi_grid=phi_grid,
@@ -1799,8 +2168,6 @@ class CylindricalMesh(StructuredMesh):
             origin=origin
         )
 
-        return mesh
-
     def to_xml_element(self):
         """Return XML representation of the mesh
 
@@ -1846,10 +2213,10 @@ class CylindricalMesh(StructuredMesh):
 
         mesh_id = int(get_text(elem, 'id'))
         mesh = cls(
-            r_grid = [float(x) for x in get_text(elem, "r_grid").split()],
-            phi_grid = [float(x) for x in get_text(elem, "phi_grid").split()],
-            z_grid = [float(x) for x in get_text(elem, "z_grid").split()],
-            origin = [float(x) for x in get_text(elem, "origin", default=[0., 0., 0.]).split()],
+            r_grid = get_elem_list(elem, "r_grid", float),
+            phi_grid = get_elem_list(elem, "phi_grid", float),
+            z_grid = get_elem_list(elem, "z_grid", float),
+            origin = get_elem_list(elem, "origin", float) or [0., 0., 0.],
             mesh_id=mesh_id,
         )
 
@@ -1970,8 +2337,8 @@ class SphericalMesh(StructuredMesh):
     def __init__(
         self,
         r_grid: Sequence[float],
-        phi_grid: Sequence[float] = (0, 2*pi),
-        theta_grid: Sequence[float] = (0, pi),
+        phi_grid: Sequence[float] = (0, 2*math.pi),
+        theta_grid: Sequence[float] = (0, math.pi),
         origin: Sequence[float] = (0., 0., 0.),
         mesh_id: int | None = None,
         name: str = '',
@@ -1993,6 +2360,10 @@ class SphericalMesh(StructuredMesh):
     def n_dimension(self):
         return 3
 
+    @property
+    def _axis_labels(self):
+        return ('r', 'theta', 'phi')
+
     @property
     def origin(self):
         return self._origin
@@ -2024,7 +2395,7 @@ class SphericalMesh(StructuredMesh):
         cv.check_length('mesh theta_grid', grid, 2)
         cv.check_increasing('mesh theta_grid', grid)
         grid = np.asarray(grid, dtype=float)
-        if np.any((grid < 0.0) | (grid > pi)):
+        if np.any((grid < 0.0) | (grid > math.pi)):
             raise ValueError("theta_grid values must be in [0, π].")
         self._theta_grid = grid
 
@@ -2038,7 +2409,7 @@ class SphericalMesh(StructuredMesh):
         cv.check_length('mesh phi_grid', grid, 2)
         cv.check_increasing('mesh phi_grid', grid)
         grid = np.asarray(grid, dtype=float)
-        if np.any((grid < 0.0) | (grid > 2*pi)):
+        if np.any((grid < 0.0) | (grid > 2*math.pi)):
             raise ValueError("phi_grid values must be in [0, 2π].")
         self._phi_grid = grid
 
@@ -2103,6 +2474,68 @@ class SphericalMesh(StructuredMesh):
 
         return mesh
 
+    @classmethod
+    def from_bounding_box(
+        cls,
+        bbox: openmc.BoundingBox,
+        dimension: Sequence[int] = (10, 10, 10),
+        mesh_id: int | None = None,
+        name: str = '',
+        phi_grid_bounds: Sequence[float] = (0.0, 2*math.pi),
+        theta_grid_bounds: Sequence[float] = (0.0, math.pi),
+        enclose_domain: bool = False,
+    ) -> SphericalMesh:
+        """Create SphericalMesh from a bounding box.
+
+        Parameters
+        ----------
+        bbox : openmc.BoundingBox
+            Bounding box used to set the r_grid, phi_grid, and theta_grid ranges.
+        dimension : Iterable of int
+            The number of equally spaced mesh cells in each direction (r_grid,
+            phi_grid, theta_grid). Spacing is in angular space (radians) for
+            phi and theta, and in absolute space for r.
+        mesh_id : int, optional
+            Unique identifier for the mesh
+        name : str, optional
+            Name of the mesh
+        phi_grid_bounds : numpy.ndarray
+            Mesh bounds points along the phi-axis in radians. The default value
+            is (0, 2π), i.e., the full phi range.
+        theta_grid_bounds : numpy.ndarray
+            Mesh bounds points along the theta-axis in radians. The default value
+            is (0, π), i.e., the full theta range.
+        enclose_domain : bool
+            If True, the mesh will encompass the bounding box of the domain. If
+            False, the mesh will be inscribed within the domain's bounding box.
+
+        Returns
+        -------
+        openmc.SphericalMesh
+            SphericalMesh instance
+
+        """
+        if enclose_domain:
+            outer_radius = 0.5 * np.linalg.norm(bbox.width)
+        else:
+            outer_radius = 0.5 * min(bbox.width)
+
+        r_grid = np.linspace(0, outer_radius, num=dimension[0] + 1)
+        theta_grid = np.linspace(
+            theta_grid_bounds[0],
+            theta_grid_bounds[1],
+            num=dimension[1]+1
+        )
+        phi_grid = np.linspace(
+            phi_grid_bounds[0],
+            phi_grid_bounds[1],
+            num=dimension[2]+1
+        )
+        origin = np.array([bbox.center[0], bbox.center[1], bbox.center[2]])
+
+        return cls(r_grid=r_grid, phi_grid=phi_grid, theta_grid=theta_grid,
+                   origin=origin, mesh_id=mesh_id, name=name)
+
     def to_xml_element(self):
         """Return XML representation of the mesh
 
@@ -2148,10 +2581,10 @@ class SphericalMesh(StructuredMesh):
         mesh_id = int(get_text(elem, 'id'))
         mesh = cls(
             mesh_id=mesh_id,
-            r_grid = [float(x) for x in get_text(elem, "r_grid").split()],
-            theta_grid = [float(x) for x in get_text(elem, "theta_grid").split()],
-            phi_grid = [float(x) for x in get_text(elem, "phi_grid").split()],
-            origin = [float(x) for x in get_text(elem, "origin", default=[0., 0., 0.]).split()],
+            r_grid = get_elem_list(elem, "r_grid", float),
+            theta_grid = get_elem_list(elem, "theta_grid", float),
+            phi_grid = get_elem_list(elem, "phi_grid", float),
+            origin = get_elem_list(elem, "origin", float) or [0., 0., 0.],
         )
 
         return mesh
@@ -2210,6 +2643,83 @@ class SphericalMesh(StructuredMesh):
         arr[..., 2] = z + origin[2]
         return arr
 
+    def get_indices_at_coords(
+        self,
+        coords: Sequence[float]
+    ) -> tuple[int, int, int]:
+        """Find the mesh cell indices containing the specified coordinates.
+
+        .. versionadded:: 0.15.4
+
+        Parameters
+        ----------
+        coords : Sequence[float]
+            Cartesian coordinates of the point as (x, y, z).
+
+        Returns
+        -------
+        tuple[int, int, int]
+            The r, theta, phi indices.
+
+        Raises
+        ------
+        ValueError
+            If the coordinates fall outside the mesh grid boundaries.
+
+        """
+        dx = coords[0] - self.origin[0]
+        dy = coords[1] - self.origin[1]
+        dz = coords[2] - self.origin[2]
+
+        r_value = math.hypot(dx, dy, dz)
+
+        if r_value < self.r_grid[0] or r_value > self.r_grid[-1]:
+            raise ValueError(
+                f'The r value {r_value} computed from the specified '
+                f'coordinates is outside the r grid range '
+                f'[{self.r_grid[0]}, {self.r_grid[-1]}].'
+            )
+
+        r_index = int(min(
+            np.searchsorted(self.r_grid, r_value, side='right') - 1,
+            len(self.r_grid) - 2
+        ))
+
+        if r_value == 0.0:
+            theta_value = 0.0
+            phi_value = 0.0
+        else:
+            theta_value = math.acos(dz / r_value)
+            phi_value = math.atan2(dy, dx)
+            if phi_value < 0:
+                phi_value += 2 * math.pi
+
+        if theta_value < self.theta_grid[0] or theta_value > self.theta_grid[-1]:
+            raise ValueError(
+                f'The theta value {theta_value} computed from the specified '
+                f'coordinates is outside the theta grid range '
+                f'[{self.theta_grid[0]}, {self.theta_grid[-1]}].'
+            )
+
+        theta_index = int(min(
+            np.searchsorted(self.theta_grid, theta_value, side='right') - 1,
+            len(self.theta_grid) - 2
+        ))
+
+        if phi_value < self.phi_grid[0] or phi_value > self.phi_grid[-1]:
+            raise ValueError(
+                f'The phi value {phi_value} computed from the specified '
+                f'coordinates is outside the phi grid range '
+                f'[{self.phi_grid[0]}, {self.phi_grid[-1]}].'
+            )
+
+        phi_index = int(min(
+            np.searchsorted(self.phi_grid, phi_value, side='right') - 1,
+            len(self.phi_grid) - 2
+        ))
+
+        return (r_index, theta_index, phi_index)
+
 
 def require_statepoint_data(func):
     @wraps(func)
@@ -2298,6 +2808,8 @@ class UnstructuredMesh(MeshBase):
     _UNSUPPORTED_ELEM = -1
     _LINEAR_TET = 0
     _LINEAR_HEX = 1
+    _VTK_TET = 10
+    _VTK_HEX = 12
 
     def __init__(self, filename: PathLike, library: str, mesh_id: int | None = None,
                  name: str = '', length_multiplier: float = 1.0,
@@ -2436,6 +2948,10 @@ class UnstructuredMesh(MeshBase):
     def n_dimension(self):
         return 3
 
+    @property
+    def _axis_labels(self):
+        return ('element_index',)
+
     @property
     @require_statepoint_data
     def indices(self):
@@ -2507,7 +3023,8 @@ class UnstructuredMesh(MeshBase):
         warnings.warn(
             "The 'UnstructuredMesh.write_vtk_mesh' method has been renamed "
             "to 'write_data_to_vtk' and will be removed in a future version "
-            " of OpenMC.", FutureWarning
+            " of OpenMC.",
+            FutureWarning,
         )
         self.write_data_to_vtk(**kwargs)
 
@@ -2525,9 +3042,10 @@ class UnstructuredMesh(MeshBase):
         Parameters
         ----------
         filename : str or pathlib.Path
-            Name of the VTK file to write. If the filename ends in '.vtu' then a
-            binary VTU format file will be written, if the filename ends in
-            '.vtk' then a legacy VTK file will be written.
+            Name of the VTK file to write. If the filename ends in '.vtkhdf'
+            then a VTKHDF format file will be written. If the filename ends in
+            '.vtu' then a binary VTU format file will be written. If the
+            filename ends in '.vtk' then a legacy VTK file will be written.
         datasets : dict
             Dictionary whose keys are the data labels and values are numpy
             appropriately sized arrays of the data
@@ -2535,6 +3053,35 @@ class UnstructuredMesh(MeshBase):
             Whether or not to normalize the data by the volume of the mesh
             elements
         """
+
+        if Path(filename).suffix == ".vtkhdf":
+
+            self._write_data_to_vtk_hdf5_format(
+                filename=filename,
+                datasets=datasets,
+                volume_normalization=volume_normalization,
+            )
+
+        elif Path(filename).suffix == ".vtk" or Path(filename).suffix == ".vtu":
+
+            self._write_data_to_vtk_ascii_format(
+                filename=filename,
+                datasets=datasets,
+                volume_normalization=volume_normalization,
+            )
+
+        else:
+            raise ValueError(
+                "Unsupported file extension, The filename must end with "
+                "'.vtkhdf', '.vtu' or '.vtk'"
+            )
+
+    def _write_data_to_vtk_ascii_format(
+        self,
+        filename: PathLike | None = None,
+        datasets: dict | None = None,
+        volume_normalization: bool = True,
+    ):
         from vtkmodules.util import numpy_support
         from vtkmodules import vtkCommonCore
         from vtkmodules import vtkCommonDataModel
@@ -2542,9 +3089,7 @@ class UnstructuredMesh(MeshBase):
         from vtkmodules import vtkIOXML
 
         if self.connectivity is None or self.vertices is None:
-            raise RuntimeError(
-                "This mesh has not been loaded from a statepoint file."
-            )
+            raise RuntimeError("This mesh has not been loaded from a statepoint file.")
 
         if filename is None:
             filename = f"mesh_{self.id}.vtk"
@@ -2575,7 +3120,9 @@ class UnstructuredMesh(MeshBase):
                 n_skipped += 1
                 continue
             else:
-                raise RuntimeError(f"Invalid element type {elem_type} found")
+                raise RuntimeError(
+                    f"Invalid element type {elem_type} found in mesh {self.id}"
+                )
 
             for i, c in enumerate(conn):
                 if c == -1:
@@ -2626,29 +3173,120 @@ class UnstructuredMesh(MeshBase):
 
         writer.Write()
 
+    def _write_data_to_vtk_hdf5_format(
+        self,
+        filename: PathLike | None = None,
+        datasets: dict | None = None,
+        volume_normalization: bool = True,
+    ):
+        # This writer supports linear tetrahedra and linear hexahedra elements
+        conn_list = []           # flattened connectivity ids
+        cell_sizes = []          # number of points per cell
+        vtk_types = []           # VTK cell types per cell (uint8)
+        n_skipped = 0
+
+        for conn, etype in zip(self.connectivity, self.element_types):
+            if etype == self._LINEAR_TET:
+                ids = conn[:4]
+                vtk_types.append(self._VTK_TET)
+            elif etype == self._LINEAR_HEX:
+                ids = conn[:8]
+                vtk_types.append(self._VTK_HEX)
+            elif etype == self._UNSUPPORTED_ELEM:
+                n_skipped += 1
+                continue
+            else:
+                raise RuntimeError(
+                    f"Invalid element type {etype} found in mesh {self.id}"
+                )
+            conn_list.extend(ids.tolist())
+            cell_sizes.append(len(ids))
+
+        if n_skipped > 0:
+            warnings.warn(
+                f"{n_skipped} elements were not written because "
+                "they are not of type linear tet/hex"
+            )
+
+        connectivity = np.asarray(conn_list, dtype=np.int64)
+
+        # Offsets must be length (numCells + 1) with a leading 0 and
+        # cumulative end-indices thereafter, per VTK's layout
+        cell_sizes_arr = np.asarray(cell_sizes, dtype=np.int64)
+        offsets = np.zeros(cell_sizes_arr.size + 1, dtype=np.int64)
+        np.cumsum(cell_sizes_arr, out=offsets[1:])
+
+        for name, data in datasets.items():
+            if data.shape != self.dimension:
+                raise ValueError(
+                    f'Cannot apply dataset "{name}" with '
+                    f"shape {data.shape} to mesh {self.id} "
+                    f"with dimensions {self.dimension}"
+                )
+
+        with h5py.File(filename, "w") as f:
+
+            root = f.create_group("VTKHDF")
+            vtk_file_format_version = (2, 1)
+            root.attrs["Version"] = vtk_file_format_version
+            ascii_type = "UnstructuredGrid".encode("ascii")
+            root.attrs.create(
+                "Type",
+                ascii_type,
+                dtype=h5py.string_dtype("ascii", len(ascii_type)),
+            )
+
+            # Create HDF5 file structure compliant with VTKHDF UnstructuredGrid
+            n_points = int(len(self.vertices))
+            n_cells = int(len(cell_sizes))
+            n_conn_ids = int(len(connectivity))
+
+            root.create_dataset("NumberOfPoints", data=(n_points,), dtype="i8")
+            root.create_dataset("NumberOfCells", data=(n_cells,), dtype="i8")
+            root.create_dataset("NumberOfConnectivityIds", data=(n_conn_ids,), dtype="i8")
+            root.create_dataset("Points", data=self.vertices.astype(np.float64, copy=False), dtype="f8")
+            root.create_dataset("Types", data=np.asarray(vtk_types, dtype=np.uint8), dtype="uint8")
+            root.create_dataset("Offsets", data=offsets.astype("i8"), dtype="i8")
+            root.create_dataset("Connectivity", data=connectivity.astype("i8"), dtype="i8")
+
+            cell_data_group = root.create_group("CellData")
+
+            for name, data in datasets.items():
+                if volume_normalization:
+                    data /= self.volumes
+                cell_data_group.create_dataset(
+                    name, data=data, dtype="float64", chunks=True
+                )
+
     @classmethod
     def from_hdf5(cls, group: h5py.Group, mesh_id: int, name: str):
-        filename = group['filename'][()].decode()
-        library = group['library'][()].decode()
-        if 'options' in group.attrs:
+        filename = group["filename"][()].decode()
+        library = group["library"][()].decode()
+        if "options" in group.attrs:
             options = group.attrs['options'].decode()
         else:
             options = None
 
-        mesh = cls(filename=filename, library=library, mesh_id=mesh_id, name=name, options=options)
+        mesh = cls(
+            filename=filename,
+            library=library,
+            mesh_id=mesh_id,
+            name=name,
+            options=options,
+        )
         mesh._has_statepoint_data = True
-        vol_data = group['volumes'][()]
+        vol_data = group["volumes"][()]
         mesh.volumes = np.reshape(vol_data, (vol_data.shape[0],))
         mesh.n_elements = mesh.volumes.size
 
-        vertices = group['vertices'][()]
+        vertices = group["vertices"][()]
         mesh._vertices = vertices.reshape((-1, 3))
-        connectivity = group['connectivity'][()]
+        connectivity = group["connectivity"][()]
         mesh._connectivity = connectivity.reshape((-1, 8))
-        mesh._element_types = group['element_types'][()]
+        mesh._element_types = group["element_types"][()]
 
-        if 'length_multiplier' in group:
-            mesh.length_multiplier = group['length_multiplier'][()]
+        if "length_multiplier" in group:
+            mesh.length_multiplier = group["length_multiplier"][()]
 
         return mesh
 
@@ -2667,7 +3305,7 @@ class UnstructuredMesh(MeshBase):
 
         element.set("library", self._library)
         if self.options is not None:
-            element.set('options', self.options)
+            element.set("options", self.options)
         subelement = ET.SubElement(element, "filename")
         subelement.text = str(self.filename)
 
@@ -2694,7 +3332,7 @@ class UnstructuredMesh(MeshBase):
         filename = get_text(elem, 'filename')
         library = get_text(elem, 'library')
         length_multiplier = float(get_text(elem, 'length_multiplier', 1.0))
-        options = elem.get('options')
+        options = get_text(elem, "options")
 
         return cls(filename, library, mesh_id, '', length_multiplier, options)
 
diff --git a/openmc/mgxs/__init__.py b/openmc/mgxs/__init__.py
index 901c1eabb..5adbac9c4 100644
--- a/openmc/mgxs/__init__.py
+++ b/openmc/mgxs/__init__.py
@@ -1,6 +1,6 @@
 import numpy as np
 
-from openmc.mgxs.groups import EnergyGroups
+from openmc.mgxs.groups import EnergyGroups, convert_flux_groups
 from openmc.mgxs.library import Library
 from openmc.mgxs.mgxs import *
 from openmc.mgxs.mdgxs import *
@@ -13,24 +13,29 @@ GROUP_STRUCTURES = {}
 - "XMAS-172_" designed for LWR analysis ([SAR1990]_, [SAN2004]_)
 - "SHEM-361_" designed for LWR analysis to eliminate self-shielding calculations
   of thermal resonances ([HFA2005]_, [SAN2007]_, [HEB2008]_)
-- "SCALE-X" (where X is 44 which is designed for criticality analysis
-  and 252 is designed for thermal reactors) for the SCALE code suite
+- "SCALE-X" (where X is 44 which is designed for criticality analysis, 252 is designed
+  for thermal reactors and 999 for multipurpose activation) for the SCALE code suite
   ([ZAL1999]_ and [REARDEN2013]_)
 - "MPACT-X" (where X is 51 (PWR), 60 (BWR), 69 (Magnox)) from the MPACT_ reactor
   physics code ([KIM2019]_ and [KIM2020]_)
+- "ECCO-33" intended for fast reactor criticality benchmarks. It’s derived as a
+  subset of VITAMIN‑J
 - "ECCO-1968_" designed for fine group reactor cell calculations for fast,
   intermediate and thermal reactor applications ([SAR1990]_)
 - activation_ energy group structures "VITAMIN-J-42", "VITAMIN-J-175",
-  "TRIPOLI-315", "CCFE-709_" and "UKAEA-1102_"
+  "TRIPOLI-315", "LLNL-616", "CCFE-709_" and "UKAEA-1102_"
 
 .. _CASMO: http://large.stanford.edu/courses/2013/ph241/dalvi1/docs/c5.physor2006.pdf
 .. _SCALE44: https://www-nds.iaea.org/publications/indc/indc-czr-0001.pdf
+.. _ECCO-33: https://serpent.vtt.fi/mediawiki/index.php/ECCO_33-group_structure
 .. _SCALE252: https://oecd-nea.org/science/wpncs/amct/workingarea/meeting2013/EGAMCT2013_08.pdf
+.. _SCALE999: https://info.ornl.gov/sites/publications/Files/Pub67728.pdf, https://www.nrc.gov/docs/ML1218/ML12184A002.pdf
 .. _MPACT: https://vera.ornl.gov/mpact/
 .. _XMAS-172: https://www-nds.iaea.org/wimsd/energy.htm
 .. _SHEM-361: http://merlin.polymtl.ca/downloads/FP214.pdf
 .. _activation: https://fispact.ukaea.uk/wiki/Keyword:GETXS
 .. _VITAMIN-J-42: https://www.oecd-nea.org/dbdata/nds_jefreports/jefreport-10.pdf
+.. _LLNL-616: https://fispact.ukaea.uk/manual/user_manual.pdf
 .. _CCFE-709: https://fispact.ukaea.uk/wiki/CCFE-709_group_structure
 .. _UKAEA-1102: https://fispact.ukaea.uk/wiki/UKAEA-1102_group_structure
 .. _ECCO-1968: https://serpent.vtt.fi/mediawiki/index.php/ECCO_1968-group_structure
@@ -80,6 +85,16 @@ GROUP_STRUCTURES['CASMO-25']  = np.array([
     0., 3.e-2, 5.8e-2, 1.4e-1, 2.8e-1, 3.5e-1, 6.25e-1, 9.72e-1, 1.02, 1.097,
     1.15, 1.855, 4., 9.877, 1.5968e1, 1.4873e2, 5.53e3, 9.118e3, 1.11e5, 5.e5,
     8.21e5, 1.353e6, 2.231e6, 3.679e6, 6.0655e6, 2.e7])
+GROUP_STRUCTURES['ECCO-33'] = np.array([
+    1.0000100000E-05, 1.0000000000E-01, 5.4000000000E-01, 4.0000000000E+00,
+    8.3152870000E+00, 1.3709590000E+01, 2.2603290000E+01, 4.0169000000E+01,
+    6.7904050000E+01, 9.1660880000E+01, 1.4862540000E+02, 3.0432480000E+02,
+    4.5399930000E+02, 7.4851830000E+02, 1.2340980000E+03, 2.0346840000E+03,
+    3.3546260000E+03, 5.5308440000E+03, 9.1188200000E+03, 1.5034390000E+04,
+    2.4787520000E+04, 4.0867710000E+04, 6.7379470000E+04, 1.1109000000E+05,
+    1.8315640000E+05, 3.0197380000E+05, 4.9787070000E+05, 8.2085000000E+05,
+    1.3533530000E+06, 2.2313020000E+06, 3.6787940000E+06, 6.0653070000E+06,
+    1.0000000000E+07, 1.9640330000E+07])
 GROUP_STRUCTURES['CASMO-40'] = np.array([
     0., 1.5e-2, 3.e-2, 4.2e-2, 5.8e-2, 8.e-2, 1.e-1, 1.4e-1,
     1.8e-1, 2.2e-1, 2.8e-1, 3.5e-1, 6.25e-1, 8.5e-1, 9.5e-1,
@@ -346,6 +361,97 @@ GROUP_STRUCTURES['SHEM-361'] = np.array([
     4.06569e+06, 4.96585e+06, 6.06530e+06, 6.70319e+06, 7.40817e+06,
     8.18730e+06, 9.04836e+06, 9.99999e+06, 1.16183e+07, 1.38403e+07,
     1.49182e+07, 1.96403e+07])
+GROUP_STRUCTURES['LLNL-616'] = np.array([
+    1.0000e-5, 1.0471e-5, 1.0965e-5, 1.1482e-5, 1.2023e-5, 1.2589e-5, 1.3183e-5,
+    1.3804e-5, 1.4454e-5, 1.5136e-5, 1.5849e-5, 1.6596e-5, 1.7378e-5, 1.8197e-5,
+    1.9055e-5, 1.9953e-5, 2.0893e-5, 2.1878e-5, 2.2909e-5, 2.3988e-5, 2.5119e-5,
+    2.6303e-5, 2.7542e-5, 2.8840e-5, 3.0200e-5, 3.1623e-5, 3.3113e-5, 3.4674e-5,
+    3.6308e-5, 3.8019e-5, 3.9811e-5, 4.1687e-5, 4.3652e-5, 4.5709e-5, 4.7863e-5,
+    5.0119e-5, 5.2481e-5, 5.4954e-5, 5.7544e-5, 6.0256e-5, 6.3096e-5, 6.6069e-5,
+    6.9183e-5, 7.2444e-5, 7.5858e-5, 7.9433e-5, 8.3176e-5, 8.7096e-5, 9.1201e-5,
+    9.5499e-5, 1.0000e-4, 1.0471e-4, 1.0965e-4, 1.1482e-4, 1.2023e-4, 1.2589e-4,
+    1.3183e-4, 1.3804e-4, 1.4454e-4, 1.5136e-4, 1.5849e-4, 1.6596e-4, 1.7378e-4,
+    1.8197e-4, 1.9055e-4, 1.9953e-4, 2.0893e-4, 2.1878e-4, 2.2909e-4, 2.3988e-4,
+    2.5119e-4, 2.6303e-4, 2.7542e-4, 2.8840e-4, 3.0200e-4, 3.1623e-4, 3.3113e-4,
+    3.4674e-4, 3.6308e-4, 3.8019e-4, 3.9811e-4, 4.1687e-4, 4.3652e-4, 4.5709e-4,
+    4.7863e-4, 5.0119e-4, 5.2481e-4, 5.4954e-4, 5.7544e-4, 6.0256e-4, 6.3096e-4,
+    6.6069e-4, 6.9183e-4, 7.2444e-4, 7.5858e-4, 7.9433e-4, 8.3176e-4, 8.7096e-4,
+    9.1201e-4, 9.5499e-4, 1.0000e-3, 1.0471e-3, 1.0965e-3, 1.1482e-3, 1.2023e-3,
+    1.2589e-3, 1.3183e-3, 1.3804e-3, 1.4454e-3, 1.5136e-3, 1.5849e-3, 1.6596e-3,
+    1.7378e-3, 1.8197e-3, 1.9055e-3, 1.9953e-3, 2.0893e-3, 2.1878e-3, 2.2909e-3,
+    2.3988e-3, 2.5119e-3, 2.6303e-3, 2.7542e-3, 2.8840e-3, 3.0200e-3, 3.1623e-3,
+    3.3113e-3, 3.4674e-3, 3.6308e-3, 3.8019e-3, 3.9811e-3, 4.1687e-3, 4.3652e-3,
+    4.5709e-3, 4.7863e-3, 5.0119e-3, 5.2481e-3, 5.4954e-3, 5.7544e-3, 6.0256e-3,
+    6.3096e-3, 6.6069e-3, 6.9183e-3, 7.2444e-3, 7.5858e-3, 7.9433e-3, 8.3176e-3,
+    8.7096e-3, 9.1201e-3, 9.5499e-3, 1.0000e-2, 1.0471e-2, 1.0965e-2, 1.1482e-2,
+    1.2023e-2, 1.2589e-2, 1.3183e-2, 1.3804e-2, 1.4454e-2, 1.5136e-2, 1.5849e-2,
+    1.6596e-2, 1.7378e-2, 1.8197e-2, 1.9055e-2, 1.9953e-2, 2.0893e-2, 2.1878e-2,
+    2.2909e-2, 2.3988e-2, 2.5119e-2, 2.6303e-2, 2.7542e-2, 2.8840e-2, 3.0200e-2,
+    3.1623e-2, 3.3113e-2, 3.4674e-2, 3.6308e-2, 3.8019e-2, 3.9811e-2, 4.1687e-2,
+    4.3652e-2, 4.5709e-2, 4.7863e-2, 5.0119e-2, 5.2481e-2, 5.4954e-2, 5.7544e-2,
+    6.0256e-2, 6.3096e-2, 6.6069e-2, 6.9183e-2, 7.2444e-2, 7.5858e-2, 7.9433e-2,
+    8.3176e-2, 8.7096e-2, 9.1201e-2, 9.5499e-2, 1.0000e-1, 1.0471e-1, 1.0965e-1,
+    1.1482e-1, 1.2023e-1, 1.2589e-1, 1.3183e-1, 1.3804e-1, 1.4454e-1, 1.5136e-1,
+    1.5849e-1, 1.6596e-1, 1.7378e-1, 1.8197e-1, 1.9055e-1, 1.9953e-1, 2.0893e-1,
+    2.1878e-1, 2.2909e-1, 2.3988e-1, 2.5119e-1, 2.6303e-1, 2.7542e-1, 2.8840e-1,
+    3.0200e-1, 3.1623e-1, 3.3113e-1, 3.4674e-1, 3.6308e-1, 3.8019e-1, 3.9811e-1,
+    4.1687e-1, 4.3652e-1, 4.5709e-1, 4.7863e-1, 5.0119e-1, 5.2481e-1, 5.4954e-1,
+    5.7544e-1, 6.0256e-1, 6.3096e-1, 6.6069e-1, 6.9183e-1, 7.2444e-1, 7.5858e-1,
+    7.9433e-1, 8.3176e-1, 8.7096e-1, 9.1201e-1, 9.5499e-1, 1.0000e0, 1.0471e0,
+    1.0965e0, 1.1482e0, 1.2023e0, 1.2589e0, 1.3183e0, 1.3804e0, 1.4454e0,
+    1.5136e0, 1.5849e0, 1.6596e0, 1.7378e0, 1.8197e0, 1.9055e0, 1.9953e0,
+    2.0893e0, 2.1878e0, 2.2909e0, 2.3988e0, 2.5119e0, 2.6303e0, 2.7542e0,
+    2.8840e0, 3.0200e0, 3.1623e0, 3.3113e0, 3.4674e0, 3.6308e0, 3.8019e0,
+    3.9811e0, 4.1687e0, 4.3652e0, 4.5709e0, 4.7863e0, 5.0119e0, 5.2481e0,
+    5.4954e0, 5.7544e0, 6.0256e0, 6.3096e0, 6.6069e0, 6.9183e0, 7.2444e0,
+    7.5858e0, 7.9433e0, 8.3176e0, 8.7096e0, 9.1201e0, 9.5499e0, 1.0000e1,
+    1.0471e1, 1.0965e1, 1.1482e1, 1.2023e1, 1.2589e1, 1.3183e1, 1.3804e1,
+    1.4454e1, 1.5136e1, 1.5849e1, 1.6596e1, 1.7378e1, 1.8197e1, 1.9055e1,
+    1.9953e1, 2.0893e1, 2.1878e1, 2.2909e1, 2.3988e1, 2.5119e1, 2.6303e1,
+    2.7542e1, 2.8840e1, 3.0200e1, 3.1623e1, 3.3113e1, 3.4674e1, 3.6308e1,
+    3.8019e1, 3.9811e1, 4.1687e1, 4.3652e1, 4.5709e1, 4.7863e1, 5.0119e1,
+    5.2481e1, 5.4954e1, 5.7544e1, 6.0256e1, 6.3096e1, 6.6069e1, 6.9183e1,
+    7.2444e1, 7.5858e1, 7.9433e1, 8.3176e1, 8.7096e1, 9.1201e1, 9.5499e1,
+    1.0000e2, 1.0471e2, 1.0965e2, 1.1482e2, 1.2023e2, 1.2589e2, 1.3183e2,
+    1.3804e2, 1.4454e2, 1.5136e2, 1.5849e2, 1.6596e2, 1.7378e2, 1.8197e2,
+    1.9055e2, 1.9953e2, 2.0893e2, 2.1878e2, 2.2909e2, 2.3988e2, 2.5119e2,
+    2.6303e2, 2.7542e2, 2.8840e2, 3.0200e2, 3.1623e2, 3.3113e2, 3.4674e2,
+    3.6308e2, 3.8019e2, 3.9811e2, 4.1687e2, 4.3652e2, 4.5709e2, 4.7863e2,
+    5.0119e2, 5.2481e2, 5.4954e2, 5.7544e2, 6.0256e2, 6.3096e2, 6.6069e2,
+    6.9183e2, 7.2444e2, 7.5858e2, 7.9433e2, 8.3176e2, 8.7096e2, 9.1201e2,
+    9.5499e2, 1.0000e3, 1.0471e3, 1.0965e3, 1.1482e3, 1.2023e3, 1.2589e3,
+    1.3183e3, 1.3804e3, 1.4454e3, 1.5136e3, 1.5849e3, 1.6596e3, 1.7378e3,
+    1.8197e3, 1.9055e3, 1.9953e3, 2.0893e3, 2.1878e3, 2.2909e3, 2.3988e3,
+    2.5119e3, 2.6303e3, 2.7542e3, 2.8840e3, 3.0200e3, 3.1623e3, 3.3113e3,
+    3.4674e3, 3.6308e3, 3.8019e3, 3.9811e3, 4.1687e3, 4.3652e3, 4.5709e3,
+    4.7863e3, 5.0119e3, 5.2481e3, 5.4954e3, 5.7544e3, 6.0256e3, 6.3096e3,
+    6.6069e3, 6.9183e3, 7.2444e3, 7.5858e3, 7.9433e3, 8.3176e3, 8.7096e3,
+    9.1201e3, 9.5499e3, 1.0000e4, 1.0471e4, 1.0965e4, 1.1482e4, 1.2023e4,
+    1.2589e4, 1.3183e4, 1.3804e4, 1.4454e4, 1.5136e4, 1.5849e4, 1.6596e4,
+    1.7378e4, 1.8197e4, 1.9055e4, 1.9953e4, 2.0893e4, 2.1878e4, 2.2909e4,
+    2.3988e4, 2.5119e4, 2.6303e4, 2.7542e4, 2.8840e4, 3.0200e4, 3.1623e4,
+    3.3113e4, 3.4674e4, 3.6308e4, 3.8019e4, 3.9811e4, 4.1687e4, 4.3652e4,
+    4.5709e4, 4.7863e4, 5.0119e4, 5.2481e4, 5.4954e4, 5.7544e4, 6.0256e4,
+    6.3096e4, 6.6069e4, 6.9183e4, 7.2444e4, 7.5858e4, 7.9433e4, 8.3176e4,
+    8.7096e4, 9.1201e4, 9.5499e4, 1.0000e5, 1.0471e5, 1.0965e5, 1.1482e5,
+    1.2023e5, 1.2589e5, 1.3183e5, 1.3804e5, 1.4454e5, 1.5136e5, 1.5849e5,
+    1.6596e5, 1.7378e5, 1.8197e5, 1.9055e5, 1.9953e5, 2.0893e5, 2.1878e5,
+    2.2909e5, 2.3988e5, 2.5119e5, 2.6303e5, 2.7542e5, 2.8840e5, 3.0200e5,
+    3.1623e5, 3.3113e5, 3.4674e5, 3.6308e5, 3.8019e5, 3.9811e5, 4.1687e5,
+    4.3652e5, 4.5709e5, 4.7863e5, 5.0119e5, 5.2481e5, 5.4954e5, 5.7544e5,
+    6.0256e5, 6.3096e5, 6.6069e5, 6.9183e5, 7.2444e5, 7.5858e5, 7.9433e5,
+    8.3176e5, 8.7096e5, 9.1201e5, 9.5499e5, 1.0000e6, 1.0471e6, 1.0965e6,
+    1.1482e6, 1.2023e6, 1.2589e6, 1.3183e6, 1.3804e6, 1.4454e6, 1.5136e6,
+    1.5849e6, 1.6596e6, 1.7378e6, 1.8197e6, 1.9055e6, 1.9953e6, 2.0893e6,
+    2.1878e6, 2.2909e6, 2.3988e6, 2.5119e6, 2.6303e6, 2.7542e6, 2.8840e6,
+    3.0200e6, 3.1623e6, 3.3113e6, 3.4674e6, 3.6308e6, 3.8019e6, 3.9811e6,
+    4.1687e6, 4.3652e6, 4.5709e6, 4.7863e6, 5.0119e6, 5.2481e6, 5.4954e6,
+    5.7544e6, 6.0256e6, 6.3096e6, 6.6069e6, 6.9183e6, 7.2444e6, 7.5858e6,
+    7.9433e6, 8.3176e6, 8.7096e6, 9.1201e6, 9.5499e6, 1.0000e7, 1.0471e7,
+    1.0965e7, 1.1482e7, 1.2023e7, 1.2589e7, 1.3183e7, 1.3804e7, 1.4454e7,
+    1.5136e7, 1.5849e7, 1.6596e7, 1.7378e7, 1.8197e7, 1.9055e7, 1.9953e7,
+    2.0000e7
+])
 GROUP_STRUCTURES['CCFE-709'] = np.array([
     1.e-5, 1.0471e-5, 1.0965e-5, 1.1482e-5, 1.2023e-5,
     1.2589e-5, 1.3183e-5, 1.3804e-5, 1.4454e-5, 1.5136e-5,
@@ -488,7 +594,208 @@ GROUP_STRUCTURES['CCFE-709'] = np.array([
     2.4000e8, 2.8000e8, 3.2000e8, 3.6000e8, 4.0000e8,
     4.4000e8, 4.8000e8, 5.2000e8, 5.6000e8, 6.0000e8,
     6.4000e8, 6.8000e8, 7.2000e8, 7.6000e8, 8.0000e8,
-    8.4000e8, 8.8000e8, 9.2000e8, 9.6000e8, 1.0000e9,])
+    8.4000e8, 8.8000e8, 9.2000e8, 9.6000e8, 1.0000e9])
+GROUP_STRUCTURES['SCALE-999'] = np.array([
+    1.000e-5, 1.000e-4, 5.000e-4, 7.500e-4, 1.000e-3,
+    1.200e-3, 1.500e-3, 2.000e-3, 2.500e-3, 3.000e-3,
+    4.000e-3, 5.000e-3, 7.500e-3, 1.000e-2, 1.450e-2,
+    1.850e-2, 2.100e-2, 2.530e-2, 3.000e-2, 4.000e-2,
+    5.000e-2, 6.000e-2, 7.000e-2, 8.000e-2, 9.000e-2,
+    1.000e-1, 1.250e-1, 1.500e-1, 1.750e-1, 1.840e-1,
+    2.000e-1, 2.250e-1, 2.500e-1, 2.750e-1, 3.000e-1,
+    3.250e-1, 3.500e-1, 3.668e-1, 3.750e-1, 4.000e-1,
+    4.140e-1, 4.500e-1, 5.000e-1, 5.316e-1, 5.500e-1,
+    6.000e-1, 6.250e-1, 6.500e-1, 6.826e-1, 7.000e-1,
+    7.500e-1, 8.000e-1, 8.500e-1, 8.764e-1, 9.000e-1,
+    9.250e-1, 9.500e-1, 9.750e-1, 1.000e+0, 1.010e+0,
+    1.020e+0, 1.030e+0, 1.040e+0, 1.050e+0, 1.060e+0,
+    1.070e+0, 1.080e+0, 1.090e+0, 1.100e+0, 1.110e+0,
+    1.120e+0, 1.130e+0, 1.140e+0, 1.150e+0, 1.175e+0,
+    1.200e+0, 1.225e+0, 1.250e+0, 1.300e+0, 1.350e+0,
+    1.400e+0, 1.450e+0, 1.500e+0, 1.545e+0, 1.590e+0,
+    1.635e+0, 1.680e+0, 1.725e+0, 1.770e+0, 1.815e+0,
+    1.860e+0, 1.900e+0, 1.940e+0, 1.970e+0, 2.000e+0,
+    2.060e+0, 2.120e+0, 2.165e+0, 2.210e+0, 2.255e+0,
+    2.300e+0, 2.340e+0, 2.380e+0, 2.425e+0, 2.470e+0,
+    2.520e+0, 2.570e+0, 2.620e+0, 2.670e+0, 2.720e+0,
+    2.770e+0, 2.820e+0, 2.870e+0, 2.920e+0, 2.970e+0,
+    3.000e+0, 3.100e+0, 3.200e+0, 3.300e+0, 3.500e+0,
+    3.620e+0, 3.730e+0, 3.830e+0, 3.928e+0, 4.000e+0,
+    4.100e+0, 4.300e+0, 4.500e+0, 4.750e+0, 4.875e+0,
+    5.000e+0, 5.044e+0, 5.250e+0, 5.400e+0, 5.550e+0,
+    5.700e+0, 5.850e+0, 6.000e+0, 6.250e+0, 6.375e+0,
+    6.500e+0, 6.625e+0, 6.750e+0, 6.875e+0, 7.000e+0,
+    7.075e+0, 7.150e+0, 7.625e+0, 8.100e+0, 8.208e+0,
+    8.315e+0, 8.708e+0, 9.100e+0, 9.550e+0, 1.000e+1,
+    1.034e+1, 1.068e+1, 1.109e+1, 1.150e+1, 1.170e+1,
+    1.190e+1, 1.240e+1, 1.290e+1, 1.333e+1, 1.375e+1,
+    1.408e+1, 1.440e+1, 1.475e+1, 1.510e+1, 1.555e+1,
+    1.600e+1, 1.650e+1, 1.700e+1, 1.730e+1, 1.760e+1,
+    1.805e+1, 1.850e+1, 1.875e+1, 1.900e+1, 1.940e+1,
+    2.000e+1, 2.050e+1, 2.100e+1, 2.175e+1, 2.250e+1,
+    2.375e+1, 2.500e+1, 2.625e+1, 2.750e+1, 2.826e+1,
+    2.902e+1, 2.951e+1, 3.000e+1, 3.063e+1, 3.125e+1,
+    3.150e+1, 3.175e+1, 3.250e+1, 3.325e+1, 3.350e+1,
+    3.375e+1, 3.418e+1, 3.460e+1, 3.500e+1, 3.550e+1,
+    3.600e+1, 3.700e+1, 3.713e+1, 3.727e+1, 3.763e+1,
+    3.800e+1, 3.855e+1, 3.910e+1, 3.935e+1, 3.960e+1,
+    4.030e+1, 4.100e+1, 4.170e+1, 4.240e+1, 4.320e+1,
+    4.400e+1, 4.460e+1, 4.520e+1, 4.610e+1, 4.700e+1,
+    4.743e+1, 4.785e+1, 4.808e+1, 4.830e+1, 4.875e+1,
+    4.920e+1, 4.990e+1, 5.060e+1, 5.130e+1, 5.200e+1,
+    5.270e+1, 5.340e+1, 5.620e+1, 5.800e+1, 6.000e+1,
+    6.100e+1, 6.122e+1, 6.144e+1, 6.300e+1, 6.500e+1,
+    6.625e+1, 6.750e+1, 6.975e+1, 7.200e+1, 7.400e+1,
+    7.600e+1, 7.745e+1, 7.889e+1, 7.945e+1, 8.000e+1,
+    8.170e+1, 8.200e+1, 8.400e+1, 8.600e+1, 8.800e+1,
+    9.000e+1, 9.250e+1, 9.500e+1, 9.700e+1, 1.000e+2,
+    1.012e+2, 1.038e+2, 1.050e+2, 1.080e+2, 1.105e+2,
+    1.130e+2, 1.160e+2, 1.175e+2, 1.190e+2, 1.205e+2,
+    1.220e+2, 1.240e+2, 1.260e+2, 1.280e+2, 1.301e+2,
+    1.325e+2, 1.350e+2, 1.375e+2, 1.400e+2, 1.430e+2,
+    1.450e+2, 1.475e+2, 1.500e+2, 1.525e+2, 1.550e+2,
+    1.575e+2, 1.600e+2, 1.625e+2, 1.650e+2, 1.670e+2,
+    1.700e+2, 1.716e+2, 1.739e+2, 1.763e+2, 1.786e+2,
+    1.800e+2, 1.877e+2, 1.885e+2, 1.915e+2, 1.930e+2,
+    1.962e+2, 1.999e+2, 2.020e+2, 2.074e+2, 2.088e+2,
+    2.095e+2, 2.122e+2, 2.145e+2, 2.175e+2, 2.200e+2,
+    2.237e+2, 2.269e+2, 2.301e+2, 2.333e+2, 2.367e+2,
+    2.400e+2, 2.442e+2, 2.484e+2, 2.527e+2, 2.571e+2,
+    2.615e+2, 2.661e+2, 2.707e+2, 2.754e+2, 2.801e+2,
+    2.850e+2, 2.899e+2, 2.948e+2, 2.999e+2, 3.050e+2,
+    3.107e+2, 3.165e+2, 3.224e+2, 3.284e+2, 3.345e+2,
+    3.408e+2, 3.471e+2, 3.536e+2, 3.591e+2, 3.648e+2,
+    3.705e+2, 3.764e+2, 3.823e+2, 3.883e+2, 3.944e+2,
+    4.007e+2, 4.070e+2, 4.134e+2, 4.199e+2, 4.265e+2,
+    4.332e+2, 4.400e+2, 4.470e+2, 4.540e+2, 4.595e+2,
+    4.650e+2, 4.706e+2, 4.763e+2, 4.821e+2, 4.879e+2,
+    4.937e+2, 4.997e+2, 5.057e+2, 5.118e+2, 5.180e+2,
+    5.243e+2, 5.306e+2, 5.370e+2, 5.435e+2, 5.500e+2,
+    5.581e+2, 5.662e+2, 5.745e+2, 5.830e+2, 5.932e+2,
+    6.036e+2, 6.142e+2, 6.250e+2, 6.359e+2, 6.471e+2,
+    6.585e+2, 6.700e+2, 6.765e+2, 6.830e+2, 6.909e+2,
+    6.988e+2, 7.069e+2, 7.150e+2, 7.232e+2, 7.316e+2,
+    7.400e+2, 7.485e+2, 7.598e+2, 7.712e+2, 7.827e+2,
+    7.945e+2, 8.064e+2, 8.185e+2, 8.308e+2, 8.433e+2,
+    8.559e+2, 8.688e+2, 8.818e+2, 8.950e+2, 9.085e+2,
+    9.221e+2, 9.360e+2, 9.500e+2, 9.555e+2, 9.611e+2,
+    9.720e+2, 9.829e+2, 9.940e+2, 1.005e+3, 1.017e+3,
+    1.028e+3, 1.040e+3, 1.051e+3, 1.063e+3, 1.075e+3,
+    1.087e+3, 1.100e+3, 1.112e+3, 1.125e+3, 1.137e+3,
+    1.150e+3, 1.171e+3, 1.191e+3, 1.213e+3, 1.234e+3,
+    1.249e+3, 1.265e+3, 1.280e+3, 1.296e+3, 1.312e+3,
+    1.328e+3, 1.344e+3, 1.361e+3, 1.377e+3, 1.394e+3,
+    1.411e+3, 1.429e+3, 1.446e+3, 1.464e+3, 1.482e+3,
+    1.500e+3, 1.525e+3, 1.550e+3, 1.585e+3, 1.610e+3,
+    1.636e+3, 1.662e+3, 1.689e+3, 1.716e+3, 1.744e+3,
+    1.772e+3, 1.800e+3, 1.828e+3, 1.856e+3, 1.885e+3,
+    1.914e+3, 1.943e+3, 1.973e+3, 2.004e+3, 2.035e+3,
+    2.075e+3, 2.116e+3, 2.158e+3, 2.200e+3, 2.250e+3,
+    2.290e+3, 2.337e+3, 2.386e+3, 2.435e+3, 2.500e+3,
+    2.532e+3, 2.580e+3, 2.613e+3, 2.679e+3, 2.747e+3,
+    2.808e+3, 2.871e+3, 2.935e+3, 3.000e+3, 3.035e+3,
+    3.112e+3, 3.191e+3, 3.272e+3, 3.355e+3, 3.440e+3,
+    3.527e+3, 3.616e+3, 3.707e+3, 3.740e+3, 3.819e+3,
+    3.900e+3, 3.998e+3, 4.099e+3, 4.202e+3, 4.307e+3,
+    4.400e+3, 4.500e+3, 4.620e+3, 4.740e+3, 4.850e+3,
+    4.960e+3, 5.100e+3, 5.250e+3, 5.400e+3, 5.531e+3,
+    5.645e+3, 5.700e+3, 5.879e+3, 6.000e+3, 6.128e+3,
+    6.258e+3, 6.392e+3, 6.528e+3, 6.667e+3, 6.809e+3,
+    6.954e+3, 7.102e+3, 7.212e+3, 7.323e+3, 7.437e+3,
+    7.552e+3, 7.669e+3, 7.787e+3, 7.908e+3, 8.030e+3,
+    8.159e+3, 8.289e+3, 8.422e+3, 8.557e+3, 8.694e+3,
+    8.834e+3, 8.975e+3, 9.119e+3, 9.307e+3, 9.500e+3,
+    9.763e+3, 1.003e+4, 1.031e+4, 1.060e+4, 1.086e+4,
+    1.114e+4, 1.142e+4, 1.171e+4, 1.202e+4, 1.234e+4,
+    1.266e+4, 1.300e+4, 1.324e+4, 1.348e+4, 1.373e+4,
+    1.398e+4, 1.424e+4, 1.450e+4, 1.476e+4, 1.503e+4,
+    1.527e+4, 1.550e+4, 1.574e+4, 1.599e+4, 1.623e+4,
+    1.649e+4, 1.674e+4, 1.700e+4, 1.727e+4, 1.755e+4,
+    1.783e+4, 1.812e+4, 1.841e+4, 1.870e+4, 1.900e+4,
+    1.931e+4, 1.965e+4, 2.000e+4, 2.045e+4, 2.092e+4,
+    2.139e+4, 2.188e+4, 2.229e+4, 2.271e+4, 2.314e+4,
+    2.358e+4, 2.388e+4, 2.418e+4, 2.450e+4, 2.479e+4,
+    2.500e+4, 2.520e+4, 2.552e+4, 2.580e+4, 2.606e+4,
+    2.653e+4, 2.700e+4, 2.737e+4, 2.774e+4, 2.812e+4,
+    2.850e+4, 2.887e+4, 2.924e+4, 2.962e+4, 3.000e+4,
+    3.045e+4, 3.090e+4, 3.136e+4, 3.183e+4, 3.243e+4,
+    3.304e+4, 3.367e+4, 3.431e+4, 3.468e+4, 3.507e+4,
+    3.545e+4, 3.584e+4, 3.624e+4, 3.663e+4, 3.704e+4,
+    3.744e+4, 3.786e+4, 3.827e+4, 3.869e+4, 3.912e+4,
+    3.955e+4, 3.998e+4, 4.042e+4, 4.087e+4, 4.136e+4,
+    4.186e+4, 4.237e+4, 4.288e+4, 4.340e+4, 4.393e+4,
+    4.446e+4, 4.500e+4, 4.565e+4, 4.631e+4, 4.721e+4,
+    4.812e+4, 4.905e+4, 5.000e+4, 5.099e+4, 5.200e+4,
+    5.248e+4, 5.347e+4, 5.448e+4, 5.551e+4, 5.656e+4,
+    5.740e+4, 5.826e+4, 5.912e+4, 6.000e+4, 6.088e+4,
+    6.177e+4, 6.267e+4, 6.358e+4, 6.451e+4, 6.545e+4,
+    6.641e+4, 6.738e+4, 6.851e+4, 6.965e+4, 7.081e+4,
+    7.200e+4, 7.300e+4, 7.399e+4, 7.500e+4, 7.610e+4,
+    7.722e+4, 7.835e+4, 7.950e+4, 8.074e+4, 8.200e+4,
+    8.250e+4, 8.374e+4, 8.500e+4, 8.652e+4, 8.788e+4,
+    8.926e+4, 9.067e+4, 9.210e+4, 9.355e+4, 9.502e+4,
+    9.652e+4, 9.804e+4, 1.000e+5, 1.013e+5, 1.027e+5,
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+    1.111e+5, 1.125e+5, 1.139e+5, 1.153e+5, 1.168e+5,
+    1.183e+5, 1.197e+5, 1.213e+5, 1.228e+5, 1.241e+5,
+    1.255e+5, 1.269e+5, 1.283e+5, 1.291e+5, 1.307e+5,
+    1.323e+5, 1.340e+5, 1.357e+5, 1.374e+5, 1.391e+5,
+    1.409e+5, 1.426e+5, 1.445e+5, 1.463e+5, 1.481e+5,
+    1.490e+5, 1.519e+5, 1.538e+5, 1.557e+5, 1.576e+5,
+    1.596e+5, 1.616e+5, 1.637e+5, 1.657e+5, 1.678e+5,
+    1.699e+5, 1.721e+5, 1.742e+5, 1.764e+5, 1.786e+5,
+    1.809e+5, 1.832e+5, 1.855e+5, 1.878e+5, 1.902e+5,
+    1.926e+5, 1.962e+5, 2.000e+5, 2.024e+5, 2.050e+5,
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+    2.209e+5, 2.237e+5, 2.265e+5, 2.294e+5, 2.323e+5,
+    2.352e+5, 2.381e+5, 2.411e+5, 2.442e+5, 2.472e+5,
+    2.500e+5, 2.527e+5, 2.555e+5, 2.584e+5, 2.612e+5,
+    2.641e+5, 2.670e+5, 2.700e+5, 2.732e+5, 2.767e+5,
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+    3.300e+5, 3.337e+5, 3.379e+5, 3.422e+5, 3.465e+5,
+    3.508e+5, 3.553e+5, 3.597e+5, 3.643e+5, 3.688e+5,
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+    4.299e+5, 4.349e+5, 4.400e+5, 4.452e+5, 4.505e+5,
+    4.553e+5, 4.601e+5, 4.650e+5, 4.700e+5, 4.772e+5,
+    4.845e+5, 4.920e+5, 4.995e+5, 5.054e+5, 5.113e+5,
+    5.173e+5, 5.234e+5, 5.299e+5, 5.365e+5, 5.432e+5,
+    5.500e+5, 5.557e+5, 5.614e+5, 5.672e+5, 5.730e+5,
+    5.784e+5, 5.891e+5, 6.000e+5, 6.081e+5, 6.158e+5,
+    6.235e+5, 6.313e+5, 6.393e+5, 6.468e+5, 6.545e+5,
+    6.622e+5, 6.700e+5, 6.790e+5, 6.926e+5, 7.065e+5,
+    7.154e+5, 7.244e+5, 7.335e+5, 7.427e+5, 7.500e+5,
+    7.576e+5, 7.653e+5, 7.730e+5, 7.808e+5, 7.904e+5,
+    8.002e+5, 8.100e+5, 8.200e+5, 8.301e+5, 8.403e+5,
+    8.506e+5, 8.611e+5, 8.750e+5, 8.874e+5, 9.000e+5,
+    9.072e+5, 9.200e+5, 9.400e+5, 9.616e+5, 9.800e+5,
+    1.003e+6, 1.010e+6, 1.040e+6, 1.070e+6, 1.100e+6,
+    1.108e+6, 1.136e+6, 1.165e+6, 1.200e+6, 1.225e+6,
+    1.250e+6, 1.287e+6, 1.317e+6, 1.356e+6, 1.400e+6,
+    1.423e+6, 1.461e+6, 1.500e+6, 1.536e+6, 1.572e+6,
+    1.612e+6, 1.653e+6, 1.695e+6, 1.738e+6, 1.782e+6,
+    1.827e+6, 1.850e+6, 1.921e+6, 1.969e+6, 2.019e+6,
+    2.070e+6, 2.123e+6, 2.176e+6, 2.231e+6, 2.307e+6,
+    2.354e+6, 2.365e+6, 2.385e+6, 2.466e+6, 2.479e+6,
+    2.535e+6, 2.592e+6, 2.658e+6, 2.725e+6, 2.794e+6,
+    2.865e+6, 2.932e+6, 3.000e+6, 3.080e+6, 3.166e+6,
+    3.247e+6, 3.329e+6, 3.413e+6, 3.499e+6, 3.588e+6,
+    3.679e+6, 3.772e+6, 3.867e+6, 3.965e+6, 4.066e+6,
+    4.183e+6, 4.304e+6, 4.398e+6, 4.493e+6, 4.607e+6,
+    4.724e+6, 4.800e+6, 4.882e+6, 4.966e+6, 5.092e+6,
+    5.221e+6, 5.353e+6, 5.488e+6, 5.627e+6, 5.770e+6,
+    5.916e+6, 6.065e+6, 6.219e+6, 6.376e+6, 6.434e+6,
+    6.592e+6, 6.703e+6, 6.873e+6, 7.047e+6, 7.225e+6,
+    7.408e+6, 7.596e+6, 7.788e+6, 7.985e+6, 8.187e+6,
+    8.395e+6, 8.607e+6, 8.825e+6, 9.048e+6, 9.278e+6,
+    9.512e+6, 9.753e+6, 1.000e+7, 1.025e+7, 1.051e+7,
+    1.078e+7, 1.105e+7, 1.133e+7, 1.162e+7, 1.191e+7,
+    1.221e+7, 1.252e+7, 1.284e+7, 1.317e+7, 1.350e+7,
+    1.384e+7, 1.419e+7, 1.455e+7, 1.492e+7, 1.530e+7,
+    1.568e+7, 1.608e+7, 1.649e+7, 1.691e+7, 1.733e+7,
+    1.790e+7, 1.845e+7, 1.900e+7, 1.964e+7, 2.000e+7])
 GROUP_STRUCTURES['UKAEA-1102'] = np.array([
     1.0000e-5, 1.0471e-5, 1.0965e-5, 1.1482e-5, 1.2023e-5,
     1.2589e-5, 1.3183e-5, 1.3804e-5, 1.4454e-5, 1.5136e-5,
diff --git a/openmc/mgxs/groups.py b/openmc/mgxs/groups.py
index 182402ed7..aea7a6d29 100644
--- a/openmc/mgxs/groups.py
+++ b/openmc/mgxs/groups.py
@@ -78,6 +78,7 @@ class EnergyGroups:
     @group_edges.setter
     def group_edges(self, edges):
         cv.check_type('group edges', edges, Iterable, Real)
+        cv.check_increasing('group edges', edges)
         cv.check_greater_than('number of group edges', len(edges), 1)
         self._group_edges = np.array(edges)
 
@@ -300,3 +301,139 @@ class EnergyGroups:
         # Assign merged edges to merged groups
         merged_groups.group_edges = list(merged_edges)
         return merged_groups
+
+
+def convert_flux_groups(flux, source_groups, target_groups):
+    """Convert flux spectrum between energy group structures.
+
+    Uses flux-per-unit-lethargy conservation, which assumes constant flux per
+    unit lethargy within each source group and distributes flux to target
+    groups proportionally to their lethargy width.
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    flux : Iterable of float
+        Flux values for source groups. Length must equal
+        source_groups.num_groups.
+    source_groups : EnergyGroups or str
+        Energy group structure of the input flux with boundaries in [eV].
+        Can be an EnergyGroups instance or the name of a group structure
+        (e.g., 'CCFE-709').
+    target_groups : EnergyGroups or str
+        Target energy group structure with boundaries in [eV]. Can be an
+        EnergyGroups instance or the name of a group structure
+        (e.g., 'UKAEA-1102').
+
+    Returns
+    -------
+    numpy.ndarray
+        Flux values for target groups. Total flux is conserved for
+        overlapping energy regions.
+
+    Raises
+    ------
+    TypeError
+        If source_groups or target_groups is not EnergyGroups or str
+    ValueError
+        If flux length doesn't match source_groups, or flux contains
+        negative, NaN, or infinite values
+
+    See Also
+    --------
+    EnergyGroups : Energy group structure class
+
+    Notes
+    -----
+    The assumption of constant flux per unit lethargy within each source
+    group is physically reasonable for most reactor spectra but is not
+    exact. For best accuracy, use source spectra with sufficiently fine
+    energy resolution.
+
+    Examples
+    --------
+    Convert FNS 709-group flux to UKAEA-1102 structure:
+
+    >>> import numpy as np
+    >>> flux_709 = np.load('tests/fns_flux_709.npy')
+    >>> flux_1102 = openmc.mgxs.convert_flux_groups(flux_709, 'CCFE-709', 'UKAEA-1102')
+
+    Convert using EnergyGroups instances:
+
+    >>> source = openmc.mgxs.EnergyGroups([1.0, 10.0, 100.0])
+    >>> target = openmc.mgxs.EnergyGroups([1.0, 5.0, 10.0, 50.0, 100.0])
+    >>> flux_target = openmc.mgxs.convert_flux_groups([1e8, 2e8], source, target)
+
+    References
+    ----------
+    .. [1] J. J. Duderstadt and L. J. Hamilton, "Nuclear Reactor Analysis,"
+       John Wiley & Sons, 1976.
+    .. [2] M. Fleming and J.-Ch. Sublet, "FISPACT-II User Manual,"
+       UKAEA-R(18)001, UK Atomic Energy Authority, 2018. See GRPCONVERT keyword.
+
+    """
+    # Handle string group structure names
+    if isinstance(source_groups, str):
+        source_groups = EnergyGroups(source_groups)
+    if isinstance(target_groups, str):
+        target_groups = EnergyGroups(target_groups)
+
+    # Type validation
+    cv.check_type('source_groups', source_groups, EnergyGroups)
+    cv.check_type('target_groups', target_groups, EnergyGroups)
+
+    # Convert flux to numpy array
+    flux = np.asarray(flux, dtype=np.float64)
+    if flux.ndim != 1:
+        raise ValueError(f'flux must be 1-dimensional, got shape {flux.shape}')
+
+    # Validate flux length matches source groups
+    if len(flux) != source_groups.num_groups:
+        raise ValueError(
+            f'Length of flux ({len(flux)}) must equal number of source '
+            f'groups ({source_groups.num_groups})'
+        )
+
+    # Check for invalid flux values
+    if np.any(np.isnan(flux)):
+        raise ValueError('flux contains NaN values')
+    if np.any(np.isinf(flux)):
+        raise ValueError('flux contains infinite values')
+    if np.any(flux < 0):
+        raise ValueError('flux values must be non-negative')
+
+    # Get energy edges
+    source_edges = source_groups.group_edges
+    target_edges = target_groups.group_edges
+    num_target = target_groups.num_groups
+
+    # Initialize output array
+    flux_target = np.zeros(num_target)
+
+    # Main conversion loop: distribute flux using lethargy weighting
+    for idx_src, flux_src in enumerate(flux):
+        if flux_src == 0:
+            continue
+
+        e_low_src = source_edges[idx_src]
+        e_high_src = source_edges[idx_src + 1]
+        lethargy_src = np.log(e_high_src / e_low_src)
+
+        for idx_tgt in range(num_target):
+            e_low_tgt = target_edges[idx_tgt]
+            e_high_tgt = target_edges[idx_tgt + 1]
+
+            # Skip non-overlapping groups
+            if e_high_tgt <= e_low_src or e_low_tgt >= e_high_src:
+                continue
+
+            # Calculate overlap region
+            e_low_overlap = max(e_low_src, e_low_tgt)
+            e_high_overlap = min(e_high_src, e_high_tgt)
+            lethargy_overlap = np.log(e_high_overlap / e_low_overlap)
+
+            # Distribute flux proportionally to lethargy fraction
+            flux_target[idx_tgt] += flux_src * (lethargy_overlap / lethargy_src)
+
+    return flux_target
diff --git a/openmc/mgxs/library.py b/openmc/mgxs/library.py
index 12a4630bd..faa83c048 100644
--- a/openmc/mgxs/library.py
+++ b/openmc/mgxs/library.py
@@ -10,8 +10,10 @@ import numpy as np
 import openmc
 import openmc.mgxs
 import openmc.checkvalue as cv
+from openmc.checkvalue import PathLike
 from ..tallies import ESTIMATOR_TYPES
 
+ROOM_TEMPERATURE_KELVIN = 294.0
 
 class Library:
     """A multi-energy-group and multi-delayed-group cross section library for
@@ -555,14 +557,14 @@ class Library:
 
                 self.all_mgxs[domain.id][mgxs_type] = mgxs
 
-    def add_to_tallies_file(self, tallies_file, merge=True):
-        """Add all tallies from all MGXS objects to a tallies file.
+    def add_to_tallies(self, tallies, merge=True):
+        """Add tallies from all MGXS objects to a tallies object.
 
         NOTE: This assumes that :meth:`Library.build_library` has been called
 
         Parameters
         ----------
-        tallies_file : openmc.Tallies
+        tallies : openmc.Tallies
             A Tallies collection to add each MGXS' tallies to generate a
             'tallies.xml' input file for OpenMC
         merge : bool
@@ -571,7 +573,7 @@ class Library:
 
         """
 
-        cv.check_type('tallies_file', tallies_file, openmc.Tallies)
+        cv.check_type('tallies', tallies, openmc.Tallies)
 
         # Add tallies from each MGXS for each domain and mgxs type
         for domain in self.domains:
@@ -586,7 +588,15 @@ class Library:
                             = list(range(1, self.num_delayed_groups + 1))
 
                 for tally in mgxs.tallies.values():
-                    tallies_file.append(tally, merge=merge)
+                    tallies.append(tally, merge=merge)
+
+    def add_to_tallies_file(self, tallies_file, merge=True):
+        warn(
+            "The Library.add_to_tallies_file(...) method has been renamed to"
+            "add_to_tallies(...) and will be removed in a future version of "
+            "OpenMC.", FutureWarning
+        )
+        self.add_to_tallies(tallies_file, merge=merge)
 
     def load_from_statepoint(self, statepoint):
         """Extracts tallies in an OpenMC StatePoint with the data needed to
@@ -851,7 +861,7 @@ class Library:
                   'since a statepoint has not yet been loaded'
             raise ValueError(msg)
 
-        cv.check_type('filename', filename, str)
+        cv.check_type('filename', filename, (str, PathLike))
         cv.check_type('directory', directory, str)
 
         import h5py
@@ -894,7 +904,7 @@ class Library:
 
         """
 
-        cv.check_type('filename', filename, str)
+        cv.check_type('filename', filename, (str, PathLike))
         cv.check_type('directory', directory, str)
 
         # Make directory if it does not exist
@@ -930,7 +940,7 @@ class Library:
 
         """
 
-        cv.check_type('filename', filename, str)
+        cv.check_type('filename', filename, (str, PathLike))
         cv.check_type('directory', directory, str)
 
         # Make directory if it does not exist
@@ -945,7 +955,7 @@ class Library:
             return pickle.load(f)
 
     def get_xsdata(self, domain, xsdata_name, nuclide='total', xs_type='macro',
-                   subdomain=None, apply_domain_chi=False):
+                   subdomain=None, apply_domain_chi=False, temperature=ROOM_TEMPERATURE_KELVIN):
         """Generates an openmc.XSdata object describing a multi-group cross section
         dataset for writing to an openmc.MGXSLibrary object.
 
@@ -981,6 +991,9 @@ class Library:
             downstream multigroup solvers that precompute a material-specific
             chi before the transport solve provides group-wise fluxes. Defaults
             to False.
+        temperature : float, optional
+            The temperature to set in the XSdata object. Defaults to 294 K
+            (room temperature).
 
         Returns
         -------
@@ -1027,6 +1040,7 @@ class Library:
         else:
             representation = 'isotropic'
         xsdata = openmc.XSdata(name, self.energy_groups,
+                               temperatures=[temperature],
                                representation=representation)
         xsdata.num_delayed_groups = self.num_delayed_groups
         if self.num_polar > 1 or self.num_azimuthal > 1:
@@ -1044,45 +1058,61 @@ class Library:
         # Now get xs data itself
         if 'nu-transport' in self.mgxs_types and self.correction == 'P0':
             mymgxs = self.get_mgxs(domain, 'nu-transport')
-            xsdata.set_total_mgxs(mymgxs, xs_type=xs_type, nuclide=[nuclide],
+            xsdata.set_total_mgxs(mymgxs, temperature=temperature,
+                                  xs_type=xs_type,
+                                  nuclide=[nuclide],
                                   subdomain=subdomain)
 
         elif 'transport' in self.mgxs_types and self.correction == 'P0':
             mymgxs = self.get_mgxs(domain, 'transport')
-            xsdata.set_total_mgxs(mymgxs, xs_type=xs_type, nuclide=[nuclide],
+            xsdata.set_total_mgxs(mymgxs, temperature=temperature,
+                                  xs_type=xs_type,
+                                  nuclide=[nuclide],
                                   subdomain=subdomain)
 
         elif 'total' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'total')
-            xsdata.set_total_mgxs(mymgxs, xs_type=xs_type, nuclide=[nuclide],
+            xsdata.set_total_mgxs(mymgxs, temperature=temperature,
+                                  xs_type=xs_type,
+                                  nuclide=[nuclide],
                                   subdomain=subdomain)
 
         if 'absorption' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'absorption')
-            xsdata.set_absorption_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_absorption_mgxs(mymgxs,
+                                       temperature=temperature,
+                                       xs_type=xs_type,
                                        nuclide=[nuclide],
                                        subdomain=subdomain)
 
         if 'fission' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'fission')
-            xsdata.set_fission_mgxs(mymgxs, xs_type=xs_type,
-                                    nuclide=[nuclide], subdomain=subdomain)
+            xsdata.set_fission_mgxs(mymgxs, temperature=temperature,
+                                    xs_type=xs_type,
+                                    nuclide=[nuclide],
+                                    subdomain=subdomain)
 
         if 'kappa-fission' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'kappa-fission')
-            xsdata.set_kappa_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_kappa_fission_mgxs(mymgxs,
+                                          temperature=temperature,
+                                          xs_type=xs_type,
                                           nuclide=[nuclide],
                                           subdomain=subdomain)
 
         if 'inverse-velocity' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'inverse-velocity')
-            xsdata.set_inverse_velocity_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_inverse_velocity_mgxs(mymgxs,
+                                             temperature=temperature,
+                                             xs_type=xs_type,
                                              nuclide=[nuclide],
                                              subdomain=subdomain)
 
         if 'nu-fission matrix' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'nu-fission matrix')
-            xsdata.set_nu_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_nu_fission_mgxs(mymgxs,
+                                       temperature=temperature,
+                                       xs_type=xs_type,
                                        nuclide=[nuclide],
                                        subdomain=subdomain)
 
@@ -1092,7 +1122,9 @@ class Library:
                 nuc = "sum"
             else:
                 nuc = nuclide
-            xsdata.set_chi_mgxs(mymgxs, xs_type=xs_type, nuclide=[nuc],
+            xsdata.set_chi_mgxs(mymgxs, temperature=temperature,
+                                xs_type=xs_type,
+                                nuclide=[nuc],
                                 subdomain=subdomain)
 
         if 'chi-prompt' in self.mgxs_types:
@@ -1101,8 +1133,10 @@ class Library:
                 nuc = "sum"
             else:
                 nuc = nuclide
-            xsdata.set_chi_prompt_mgxs(mymgxs, xs_type=xs_type,
-                                       nuclide=[nuc], subdomain=subdomain)
+            xsdata.set_chi_prompt_mgxs(mymgxs, temperature=temperature,
+                                       xs_type=xs_type,
+                                       nuclide=[nuc],
+                                       subdomain=subdomain)
 
         if 'chi-delayed' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'chi-delayed')
@@ -1110,53 +1144,61 @@ class Library:
                 nuc = "sum"
             else:
                 nuc = nuclide
-            xsdata.set_chi_delayed_mgxs(mymgxs, xs_type=xs_type,
-                                        nuclide=[nuc], subdomain=subdomain)
+            xsdata.set_chi_delayed_mgxs(mymgxs, temperature=temperature,
+                                        xs_type=xs_type,
+                                        nuclide=[nuc],
+                                        subdomain=subdomain)
 
         if 'nu-fission' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'nu-fission')
-            xsdata.set_nu_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_nu_fission_mgxs(mymgxs, temperature=temperature,
+                                       xs_type=xs_type,
                                        nuclide=[nuclide],
                                        subdomain=subdomain)
 
         if 'prompt-nu-fission' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'prompt-nu-fission')
-            xsdata.set_prompt_nu_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_prompt_nu_fission_mgxs(mymgxs, temperature=temperature,
+                                              xs_type=xs_type,
                                               nuclide=[nuclide],
                                               subdomain=subdomain)
 
         if 'prompt-nu-fission matrix' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'prompt-nu-fission matrix')
-            xsdata.set_prompt_nu_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_prompt_nu_fission_mgxs(mymgxs, temperature=temperature,
+                                              xs_type=xs_type,
                                               nuclide=[nuclide],
                                               subdomain=subdomain)
 
         if 'delayed-nu-fission' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'delayed-nu-fission')
-            xsdata.set_delayed_nu_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_delayed_nu_fission_mgxs(mymgxs, temperature=temperature,
+                                               xs_type=xs_type,
                                                nuclide=[nuclide],
                                                subdomain=subdomain)
 
         if 'delayed-nu-fission matrix' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'delayed-nu-fission matrix')
-            xsdata.set_delayed_nu_fission_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_delayed_nu_fission_mgxs(mymgxs, temperature=temperature,
+                                               xs_type=xs_type,
                                                nuclide=[nuclide],
                                                subdomain=subdomain)
 
         if 'beta' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'beta')
-            xsdata.set_beta_mgxs(mymgxs, xs_type=xs_type, nuclide=[nuclide],
-                                 subdomain=subdomain)
+            xsdata.set_beta_mgxs(mymgxs, temperature=temperature, xs_type=xs_type,
+                                 nuclide=[nuclide], subdomain=subdomain)
 
         if 'decay-rate' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'decay-rate')
-            xsdata.set_decay_rate_mgxs(mymgxs, xs_type=xs_type, nuclide=[nuclide],
-                                subdomain=subdomain)
+            xsdata.set_decay_rate_mgxs(mymgxs, temperature=temperature, xs_type=xs_type,
+                                       nuclide=[nuclide], subdomain=subdomain)
 
         # If multiplicity matrix is available, prefer that
         if 'multiplicity matrix' in self.mgxs_types:
             mymgxs = self.get_mgxs(domain, 'multiplicity matrix')
-            xsdata.set_multiplicity_matrix_mgxs(mymgxs, xs_type=xs_type,
+            xsdata.set_multiplicity_matrix_mgxs(mymgxs, temperature=temperature,
+                                                xs_type=xs_type,
                                                 nuclide=[nuclide],
                                                 subdomain=subdomain)
             using_multiplicity = True
@@ -1167,6 +1209,7 @@ class Library:
             scatt_mgxs = self.get_mgxs(domain, 'scatter matrix')
             nuscatt_mgxs = self.get_mgxs(domain, 'nu-scatter matrix')
             xsdata.set_multiplicity_matrix_mgxs(nuscatt_mgxs, scatt_mgxs,
+                                                temperature=temperature,
                                                 xs_type=xs_type,
                                                 nuclide=[nuclide],
                                                 subdomain=subdomain)
@@ -1179,6 +1222,7 @@ class Library:
             nuscatt_mgxs = \
                 self.get_mgxs(domain, 'consistent nu-scatter matrix')
             xsdata.set_multiplicity_matrix_mgxs(nuscatt_mgxs, scatt_mgxs,
+                                                temperature=temperature,
                                                 xs_type=xs_type,
                                                 nuclide=[nuclide],
                                                 subdomain=subdomain)
@@ -1193,7 +1237,8 @@ class Library:
             else:
                 nuscatt_mgxs = \
                     self.get_mgxs(domain, 'consistent nu-scatter matrix')
-            xsdata.set_scatter_matrix_mgxs(nuscatt_mgxs, xs_type=xs_type,
+            xsdata.set_scatter_matrix_mgxs(nuscatt_mgxs, temperature=temperature,
+                                           xs_type=xs_type,
                                            nuclide=[nuclide],
                                            subdomain=subdomain)
         else:
@@ -1204,7 +1249,8 @@ class Library:
                 else:
                     nuscatt_mgxs = \
                         self.get_mgxs(domain, 'consistent nu-scatter matrix')
-                xsdata.set_scatter_matrix_mgxs(nuscatt_mgxs, xs_type=xs_type,
+                xsdata.set_scatter_matrix_mgxs(nuscatt_mgxs, temperature=temperature,
+                                               xs_type=xs_type,
                                                nuclide=[nuclide],
                                                subdomain=subdomain)
 
@@ -1244,14 +1290,23 @@ class Library:
                       'are ignored since multiplicity or nu-scatter matrices '\
                       'were not tallied for ' + xsdata_name
                 warn(msg, RuntimeWarning)
-                xsdata.set_scatter_matrix_mgxs(scatt_mgxs, xs_type=xs_type,
+
+                if 'scatter matrix' in self.mgxs_types:
+                    scatt_mgxs = self.get_mgxs(domain, 'scatter matrix')
+                elif 'consistent scatter matrix' in self.mgxs_types:
+                    scatt_mgxs = self.get_mgxs(domain, 'consistent scatter matrix')
+                else:
+                    raise ValueError(f'No scatter matrix found for {xsdata_name}.')
+
+                xsdata.set_scatter_matrix_mgxs(scatt_mgxs, temperature=temperature,
+                                               xs_type=xs_type,
                                                nuclide=[nuclide],
                                                subdomain=subdomain)
 
         return xsdata
 
     def create_mg_library(self, xs_type='macro', xsdata_names=None,
-                          apply_domain_chi=False):
+                          apply_domain_chi=False, temperature=ROOM_TEMPERATURE_KELVIN):
         """Creates an openmc.MGXSLibrary object to contain the MGXS data for the
         Multi-Group mode of OpenMC.
 
@@ -1277,6 +1332,9 @@ class Library:
             downstream multigroup solvers that precompute a material-specific
             chi before the transport solve provides group-wise fluxes. Defaults
             to False.
+        temperature : float, optional
+            The temperature to set in the MGXSLibrary object. Defaults to 294 K
+            (room temperature).
 
         Returns
         -------
diff --git a/openmc/mgxs/mdgxs.py b/openmc/mgxs/mdgxs.py
index b95a4fbc0..57cc955ba 100644
--- a/openmc/mgxs/mdgxs.py
+++ b/openmc/mgxs/mdgxs.py
@@ -7,6 +7,7 @@ import numpy as np
 
 import openmc
 import openmc.checkvalue as cv
+from openmc.checkvalue import PathLike
 from openmc.mgxs import MGXS
 from .mgxs import _DOMAIN_TO_FILTER
 
@@ -722,7 +723,7 @@ class MDGXS(MGXS):
 
         """
 
-        cv.check_type('filename', filename, str)
+        cv.check_type('filename', filename, (str, PathLike))
         cv.check_type('directory', directory, str)
         cv.check_value('format', format, ['csv', 'excel', 'pickle', 'latex'])
         cv.check_value('xs_type', xs_type, ['macro', 'micro'])
@@ -876,8 +877,8 @@ class MDGXS(MGXS):
         # energy groups such that data is from fast to thermal
         if self.domain_type == 'mesh':
             mesh_str = f'mesh {self.domain.id}'
-            df.sort_values(by=[(mesh_str, 'x'), (mesh_str, 'y'),
-                               (mesh_str, 'z')] + columns, inplace=True)
+            mesh_cols = [(mesh_str, label) for label in self.domain._axis_labels]
+            df.sort_values(by=mesh_cols + columns, inplace=True)
         else:
             df.sort_values(by=[self.domain_type] + columns, inplace=True)
 
diff --git a/openmc/mgxs/mgxs.py b/openmc/mgxs/mgxs.py
index 1bc7f9387..4c13c7508 100644
--- a/openmc/mgxs/mgxs.py
+++ b/openmc/mgxs/mgxs.py
@@ -2,6 +2,7 @@ import copy
 from numbers import Integral
 import os
 import warnings
+from textwrap import dedent
 
 import h5py
 import numpy as np
@@ -9,6 +10,7 @@ import numpy as np
 import openmc
 from openmc.data import REACTION_MT, REACTION_NAME, FISSION_MTS
 import openmc.checkvalue as cv
+from openmc.checkvalue import PathLike
 from ..tallies import ESTIMATOR_TYPES
 from . import EnergyGroups
 
@@ -164,7 +166,7 @@ class MGXS:
 
     """
 
-    _params = """
+    _params = dedent("""
     Parameters
     ----------
     domain : openmc.Material or openmc.Cell or openmc.Universe or openmc.RegularMesh
@@ -251,7 +253,7 @@ class MGXS:
 
         .. versionadded:: 0.13.1
 
-    """
+    """)
 
     # Store whether or not the number density should be removed for microscopic
     # values of this data
@@ -1921,7 +1923,7 @@ class MGXS:
 
             # Create an HDF5 group for the subdomain
             if self.domain_type == 'distribcell':
-                group_name = ''.zfill(num_digits)
+                group_name = str(subdomain).zfill(num_digits)
                 subdomain_group = domain_group.require_group(group_name)
             else:
                 subdomain_group = domain_group
@@ -1981,7 +1983,7 @@ class MGXS:
 
         """
 
-        cv.check_type('filename', filename, str)
+        cv.check_type('filename', filename, (str, PathLike))
         cv.check_type('directory', directory, str)
         cv.check_value('format', format, ['csv', 'excel', 'pickle', 'latex'])
         cv.check_value('xs_type', xs_type, ['macro', 'micro'])
@@ -2125,15 +2127,15 @@ class MGXS:
             df['std. dev.'] /= np.tile(densities, tile_factor)
 
             # Replace NaNs by zeros (happens if nuclide density is zero)
-            df['mean'].replace(np.nan, 0.0, inplace=True)
-            df['std. dev.'].replace(np.nan, 0.0, inplace=True)
+            df['mean'] = df['mean'].replace(np.nan, 0.0)
+            df['std. dev.'] = df['std. dev.'].replace(np.nan, 0.0)
 
         # Sort the dataframe by domain type id (e.g., distribcell id) and
         # energy groups such that data is from fast to thermal
         if self.domain_type == 'mesh':
             mesh_str = f'mesh {self.domain.id}'
-            df.sort_values(by=[(mesh_str, 'x'), (mesh_str, 'y'),
-                               (mesh_str, 'z')] + columns, inplace=True)
+            mesh_cols = [(mesh_str, label) for label in self.domain._axis_labels]
+            df.sort_values(by=mesh_cols + columns, inplace=True)
         else:
             df.sort_values(by=[self.domain_type] + columns, inplace=True)
 
diff --git a/openmc/mgxs_library.py b/openmc/mgxs_library.py
index b840563cf..ab9b58b7a 100644
--- a/openmc/mgxs_library.py
+++ b/openmc/mgxs_library.py
@@ -11,7 +11,7 @@ import openmc
 import openmc.mgxs
 from openmc.mgxs import SCATTER_TABULAR, SCATTER_LEGENDRE, SCATTER_HISTOGRAM
 from .checkvalue import check_type, check_value, check_greater_than, \
-    check_iterable_type, check_less_than, check_filetype_version
+    check_iterable_type, check_less_than, check_filetype_version, PathLike
 
 ROOM_TEMPERATURE_KELVIN = 294.0
 
@@ -484,7 +484,8 @@ class XSdata:
 
         check_type('temperature', temperature, Real)
 
-        temp_store = self.temperatures.tolist().append(temperature)
+        temp_store = self.temperatures.tolist()
+        temp_store.append(temperature)
         self.temperatures = temp_store
 
         self._total.append(None)
@@ -514,6 +515,75 @@ class XSdata:
         if np.sum(array) > 0:
             self._fissionable = True
 
+    def add_temperature_data(self, other):
+        """This method adds temperature-dependent cross section
+        values from another XSdata object to this XSdata object.
+        Note: if a temperature datapoint from 'other' already exists in this
+        object, it will be overridden.
+
+        Parameters
+        ----------
+        other: openmc.XSdata
+            The other XSdata object to fetch data from
+        """
+
+        # Sanity check to make sure they have the same name, energy group structure,
+        # and delayed group structure
+        check_value('name', other.name, self.name)
+        check_value('energy_groups', other.energy_groups, [self.energy_groups])
+        check_value('delayed_groups', other.num_delayed_groups, [self.num_delayed_groups])
+
+        # Add the temperature data.
+        for temp in other.temperatures:
+            if temp not in self.temperatures:
+                self.add_temperature(temp)
+
+            if np.all(other.absorption[other._temperature_index(temp)] != None):
+              self.set_absorption(other.absorption[other._temperature_index(temp)], temp)
+
+            if np.all(other.beta[other._temperature_index(temp)] != None):
+              self.set_beta(other.beta[other._temperature_index(temp)], temp)
+
+            if np.all(other.chi[other._temperature_index(temp)] != None):
+              self.set_chi(other.chi[other._temperature_index(temp)], temp)
+
+            if np.all(other.chi_delayed[other._temperature_index(temp)] != None):
+              self.set_chi_delayed(other.chi_delayed[other._temperature_index(temp)], temp)
+
+            if np.all(other.chi_prompt[other._temperature_index(temp)] != None):
+              self.set_chi_prompt(other.chi_prompt[other._temperature_index(temp)], temp)
+
+            if np.all(other.decay_rate[other._temperature_index(temp)] != None):
+              self.set_decay_rate(other.decay_rate[other._temperature_index(temp)], temp)
+
+            if np.all(other.delayed_nu_fission[other._temperature_index(temp)] != None):
+              self.set_delayed_nu_fission(other.delayed_nu_fission[other._temperature_index(temp)], temp)
+
+            if np.all(other.fission[other._temperature_index(temp)] != None):
+              self.set_fission(other.fission[other._temperature_index(temp)], temp)
+
+            if np.all(other.inverse_velocity[other._temperature_index(temp)] != None):
+              self.set_inverse_velocity(other.inverse_velocity[other._temperature_index(temp)], temp)
+
+            if np.all(other.kappa_fission[other._temperature_index(temp)] != None):
+              self.set_kappa_fission(other.kappa_fission[other._temperature_index(temp)], temp)
+
+            if np.all(other.multiplicity_matrix[other._temperature_index(temp)] != None):
+              self.set_multiplicity_matrix(other.multiplicity_matrix[other._temperature_index(temp)], temp)
+
+            if np.all(other.nu_fission[other._temperature_index(temp)] != None):
+              self.set_nu_fission(other.nu_fission[other._temperature_index(temp)], temp)
+
+            if np.all(other.prompt_nu_fission[other._temperature_index(temp)] != None):
+              self.set_prompt_nu_fission(other.prompt_nu_fission[other._temperature_index(temp)], temp)
+
+            if np.all(other.scatter_matrix[other._temperature_index(temp)] != None):
+              self.set_scatter_matrix(other.scatter_matrix[other._temperature_index(temp)], temp)
+
+            if np.all(other.fission[other._temperature_index(temp)] != None):
+              self.set_total(other.total[other._temperature_index(temp)], temp)
+
+
     def set_total(self, total, temperature=ROOM_TEMPERATURE_KELVIN):
         """This method sets the cross section for this XSdata object at the
         provided temperature.
@@ -2506,7 +2576,7 @@ class MGXSLibrary:
 
         Parameters
         ----------
-        filename : str
+        filename : str or PathLike
             Filename of file, default is mgxs.h5.
         libver : {'earliest', 'latest'}
             Compatibility mode for the HDF5 file. 'latest' will produce files
@@ -2514,8 +2584,7 @@ class MGXSLibrary:
 
         """
 
-        check_type('filename', filename, str)
-
+        check_type('filename', filename, (str, PathLike))
         # Create and write to the HDF5 file
         file = h5py.File(filename, "w", libver=libver)
         file.attrs['filetype'] = np.bytes_(_FILETYPE_MGXS_LIBRARY)
@@ -2554,21 +2623,21 @@ class MGXSLibrary:
             raise ValueError("Either path or openmc.config['mg_cross_sections']"
                              "must be set")
 
-        check_type('filename', filename, str)
-        file = h5py.File(filename, 'r')
+        check_type('filename', filename, (str, PathLike))
+        with h5py.File(filename, 'r') as file:
 
-        # Check filetype and version
-        check_filetype_version(file, _FILETYPE_MGXS_LIBRARY,
-                               _VERSION_MGXS_LIBRARY)
+            # Check filetype and version
+            check_filetype_version(file, _FILETYPE_MGXS_LIBRARY,
+                                   _VERSION_MGXS_LIBRARY)
 
-        group_structure = file.attrs['group structure']
-        num_delayed_groups = file.attrs['delayed_groups']
-        energy_groups = openmc.mgxs.EnergyGroups(group_structure)
-        data = cls(energy_groups, num_delayed_groups)
+            group_structure = file.attrs['group structure']
+            num_delayed_groups = file.attrs['delayed_groups']
+            energy_groups = openmc.mgxs.EnergyGroups(group_structure)
+            data = cls(energy_groups, num_delayed_groups)
 
-        for group_name, group in file.items():
-            data.add_xsdata(openmc.XSdata.from_hdf5(group, group_name,
-                                                    energy_groups,
-                                                    num_delayed_groups))
+            for group_name, group in file.items():
+                data.add_xsdata(openmc.XSdata.from_hdf5(group, group_name,
+                                                        energy_groups,
+                                                        num_delayed_groups))
 
         return data
diff --git a/openmc/mixin.py b/openmc/mixin.py
index 0bc4128b0..28c97bfdd 100644
--- a/openmc/mixin.py
+++ b/openmc/mixin.py
@@ -39,6 +39,8 @@ class IDManagerMixin:
 
     """
 
+    min_id = 0
+
     @property
     def id(self):
         return self._id
@@ -64,7 +66,7 @@ class IDManagerMixin:
         else:
             name = cls.__name__
             cv.check_type(f'{name} ID', uid, Integral)
-            cv.check_greater_than(f'{name} ID', uid, 0, equality=True)
+            cv.check_greater_than(f'{name} ID', uid, cls.min_id, equality=True)
             if uid in cls.used_ids:
                 msg = f'Another {name} instance already exists with id={uid}.'
                 warn(msg, IDWarning)
diff --git a/openmc/model/funcs.py b/openmc/model/funcs.py
index 41aa920ea..e076b080a 100644
--- a/openmc/model/funcs.py
+++ b/openmc/model/funcs.py
@@ -39,8 +39,8 @@ def borated_water(boron_ppm, temperature=293., pressure=0.1013, temp_unit='K',
     press_unit : {'MPa', 'psi'}
         The units used for the `pressure` argument.
     density : float
-        Water density in [g / cm^3].  If specified, this value overrides the
-        temperature and pressure arguments.
+        Water density in [g / cm^3].  If specified, this value overrides 
+        the value that is computed from the temperature and pressure arguments.
     **kwargs
         All keyword arguments are passed to the created Material object.
 
@@ -95,10 +95,7 @@ def borated_water(boron_ppm, temperature=293., pressure=0.1013, temp_unit='K',
     frac_B = boron_ppm * 1e-6 / M_B
 
     # Build the material.
-    if density is None:
-        out = openmc.Material(temperature=T, **kwargs)
-    else:
-        out = openmc.Material(**kwargs)
+    out = openmc.Material(temperature=T, **kwargs)
     out.add_element('H', frac_H, 'ao')
     out.add_element('O', frac_O, 'ao')
     out.add_element('B', frac_B, 'ao')
diff --git a/openmc/model/model.py b/openmc/model/model.py
index e8558f4ad..c437d2033 100644
--- a/openmc/model/model.py
+++ b/openmc/model/model.py
@@ -1,26 +1,48 @@
 from __future__ import annotations
-from collections.abc import Iterable, Sequence
-from functools import lru_cache
+from collections.abc import Callable, Iterable, Sequence
+import copy
+from dataclasses import dataclass, field
+from functools import cache
 from pathlib import Path
 import math
 from numbers import Integral, Real
+import random
+import re
 from tempfile import NamedTemporaryFile, TemporaryDirectory
+from typing import Any, Protocol
 import warnings
 
 import h5py
 import lxml.etree as ET
 import numpy as np
+from scipy.optimize import curve_fit
 
 import openmc
 import openmc._xml as xml
 from openmc.dummy_comm import DummyCommunicator
 from openmc.executor import _process_CLI_arguments
-from openmc.checkvalue import check_type, check_value, PathLike
+from openmc.checkvalue import (check_type, check_value, check_greater_than,
+                               check_length, PathLike)
 from openmc.exceptions import InvalidIDError
-from openmc.plots import add_plot_params
+from openmc.plots import add_plot_params, _BASIS_INDICES, id_map_to_rgb
 from openmc.utility_funcs import change_directory
 
 
+# Protocol for a function that is passed to search_keff
+class ModelModifier(Protocol):
+    def __call__(self, val: float, **kwargs: Any) -> None:
+        ...
+
+
+def _check_pixels(pixels: int | Sequence[int]) -> None:
+    if isinstance(pixels, Integral):
+        check_greater_than('pixels', pixels, 0)
+    else:
+        check_length('pixels', pixels, 2)
+        for p in pixels:
+            check_greater_than('pixels', p, 0)
+
+
 class Model:
     """Model container.
 
@@ -64,8 +86,14 @@ class Model:
 
     """
 
-    def __init__(self, geometry=None, materials=None, settings=None,
-                 tallies=None, plots=None):
+    def __init__(
+        self,
+        geometry: openmc.Geometry | None = None,
+        materials: openmc.Materials | None = None,
+        settings: openmc.Settings | None = None,
+        tallies: openmc.Tallies | None = None,
+        plots: openmc.Plots | None = None,
+    ):
         self.geometry = openmc.Geometry() if geometry is None else geometry
         self.materials = openmc.Materials() if materials is None else materials
         self.settings = openmc.Settings() if settings is None else settings
@@ -73,7 +101,7 @@ class Model:
         self.plots = openmc.Plots() if plots is None else plots
 
     @property
-    def geometry(self) -> openmc.Geometry | None:
+    def geometry(self) -> openmc.Geometry:
         return self._geometry
 
     @geometry.setter
@@ -82,7 +110,7 @@ class Model:
         self._geometry = geometry
 
     @property
-    def materials(self) -> openmc.Materials | None:
+    def materials(self) -> openmc.Materials:
         return self._materials
 
     @materials.setter
@@ -91,12 +119,14 @@ class Model:
         if isinstance(materials, openmc.Materials):
             self._materials = materials
         else:
+            if not hasattr(self, '_materials'):
+                self._materials = openmc.Materials()
             del self._materials[:]
             for mat in materials:
                 self._materials.append(mat)
 
     @property
-    def settings(self) -> openmc.Settings | None:
+    def settings(self) -> openmc.Settings:
         return self._settings
 
     @settings.setter
@@ -105,7 +135,7 @@ class Model:
         self._settings = settings
 
     @property
-    def tallies(self) -> openmc.Tallies | None:
+    def tallies(self) -> openmc.Tallies:
         return self._tallies
 
     @tallies.setter
@@ -114,12 +144,14 @@ class Model:
         if isinstance(tallies, openmc.Tallies):
             self._tallies = tallies
         else:
+            if not hasattr(self, '_tallies'):
+                self._tallies = openmc.Tallies()
             del self._tallies[:]
             for tally in tallies:
                 self._tallies.append(tally)
 
     @property
-    def plots(self) -> openmc.Plots | None:
+    def plots(self) -> openmc.Plots:
         return self._plots
 
     @plots.setter
@@ -128,6 +160,8 @@ class Model:
         if isinstance(plots, openmc.Plots):
             self._plots = plots
         else:
+            if not hasattr(self, '_plots'):
+                self._plots = openmc.Plots()
             del self._plots[:]
             for plot in plots:
                 self._plots.append(plot)
@@ -145,7 +179,7 @@ class Model:
             return False
 
     @property
-    @lru_cache(maxsize=None)
+    @cache
     def _materials_by_id(self) -> dict:
         """Dictionary mapping material ID --> material"""
         if self.materials:
@@ -155,14 +189,14 @@ class Model:
         return {mat.id: mat for mat in mats}
 
     @property
-    @lru_cache(maxsize=None)
+    @cache
     def _cells_by_id(self) -> dict:
         """Dictionary mapping cell ID --> cell"""
         cells = self.geometry.get_all_cells()
         return {cell.id: cell for cell in cells.values()}
 
     @property
-    @lru_cache(maxsize=None)
+    @cache
     def _cells_by_name(self) -> dict[int, openmc.Cell]:
         # Get the names maps, but since names are not unique, store a set for
         # each name key. In this way when the user requests a change by a name,
@@ -175,7 +209,7 @@ class Model:
         return result
 
     @property
-    @lru_cache(maxsize=None)
+    @cache
     def _materials_by_name(self) -> dict[int, openmc.Material]:
         if self.materials is None:
             mats = self.geometry.get_all_materials().values()
@@ -188,25 +222,124 @@ class Model:
             result[mat.name].add(mat)
         return result
 
+    # TODO: This should really get incorporated in lower-level calls to
+    # get_all_materials, but right now it requires information from the Model object
+    def _get_all_materials(self) -> dict[int, openmc.Material]:
+        """Get all materials including those in DAGMC universes
+
+        Returns
+        -------
+        dict
+            Dictionary mapping material ID to material instances
+        """
+        # Get all materials from the Geometry object
+        materials = self.geometry.get_all_materials()
+
+        # Account for materials in DAGMC universes
+        for cell in self.geometry.get_all_cells().values():
+            if isinstance(cell.fill, openmc.DAGMCUniverse):
+                names = cell.fill.material_names
+                materials.update({
+                    mat.id: mat for mat in self.materials if mat.name in names
+                })
+
+        return materials
+
+    def add_kinetics_parameters_tallies(self, num_groups: int | None = None):
+        """Add tallies for calculating kinetics parameters using the IFP method.
+
+        This method adds tallies to the model for calculating two kinetics
+        parameters, the generation time and the effective delayed neutron
+        fraction (beta effective). After a model is run, these parameters can be
+        determined through the :meth:`openmc.StatePoint.ifp_results` method.
+
+        Parameters
+        ----------
+        num_groups : int, optional
+            Number of precursor groups to filter the delayed neutron fraction.
+            If None, only the total effective delayed neutron fraction is
+            tallied.
+
+        """
+        if not any('ifp-time-numerator' in t.scores for t in self.tallies):
+            gen_time_tally = openmc.Tally(name='IFP time numerator')
+            gen_time_tally.scores = ['ifp-time-numerator']
+            self.tallies.append(gen_time_tally)
+        if not any('ifp-beta-numerator' in t.scores for t in self.tallies):
+            beta_tally = openmc.Tally(name='IFP beta numerator')
+            beta_tally.scores = ['ifp-beta-numerator']
+            if num_groups is not None:
+                beta_tally.filters = [openmc.DelayedGroupFilter(list(range(1, num_groups + 1)))]
+            self.tallies.append(beta_tally)
+        if not any('ifp-denominator' in t.scores for t in self.tallies):
+            denom_tally = openmc.Tally(name='IFP denominator')
+            denom_tally.scores = ['ifp-denominator']
+            self.tallies.append(denom_tally)
+
+    # TODO: This should also be incorporated into lower-level calls in
+    # settings.py, but it requires information about the tallies currently
+    # on the active Model
+    def _assign_fw_cadis_tally_IDs(self):
+        # Verify that all tallies assigned as targets on WeightWindowGenerators
+        # exist within model.tallies. If this is the case, convert the .targets
+        # attribute of each WeightWindowGenerator to a sequence of tally IDs.
+        if len(self.settings.weight_window_generators) == 0:
+            return
+
+        # List of valid tally IDs
+        reference_tally_ids = np.asarray([tal.id for tal in self.tallies])
+
+        for wwg in self.settings.weight_window_generators:
+            # Only proceeds if the "targets" attribute is an openmc.Tallies,
+            # which means it hasn't been checked against model.tallies.
+            if isinstance(wwg.targets, openmc.Tallies):
+                id_vec = []
+                for tal in wwg.targets:
+                    # check against model tallies for equivalence
+                    id_next = None
+                    for reference_tal in self.tallies:
+                        if tal == reference_tal:
+                            id_next = reference_tal.id
+                            break
+
+                    if id_next is None:
+                        raise RuntimeError(
+                            f'Local FW-CADIS target tally {tal.id} not found on model.tallies!')
+                    else:
+                        id_vec.append(id_next)
+
+                wwg.targets = id_vec
+
+            elif isinstance(wwg.targets, np.ndarray):
+                invalid = wwg.targets[~np.isin(wwg.targets, reference_tally_ids)]
+                if len(invalid) > 0:
+                    raise RuntimeError(
+                        f'Local FW-CADIS target tally IDs {invalid} not found on model.tallies!')
+
     @classmethod
-    def from_xml(cls, geometry='geometry.xml', materials='materials.xml',
-                 settings='settings.xml', tallies='tallies.xml',
-                 plots='plots.xml') -> Model:
+    def from_xml(
+        cls,
+        geometry: PathLike = "geometry.xml",
+        materials: PathLike = "materials.xml",
+        settings: PathLike = "settings.xml",
+        tallies: PathLike = "tallies.xml",
+        plots: PathLike = "plots.xml",
+    ) -> Model:
         """Create model from existing XML files
 
         Parameters
         ----------
-        geometry : str
+        geometry : PathLike
             Path to geometry.xml file
-        materials : str
+        materials : PathLike
             Path to materials.xml file
-        settings : str
+        settings : PathLike
             Path to settings.xml file
-        tallies : str
+        tallies : PathLike
             Path to tallies.xml file
 
             .. versionadded:: 0.13.0
-        plots : str
+        plots : PathLike
             Path to plots.xml file
 
             .. versionadded:: 0.13.0
@@ -226,14 +359,14 @@ class Model:
         return cls(geometry, materials, settings, tallies, plots)
 
     @classmethod
-    def from_model_xml(cls, path='model.xml'):
+    def from_model_xml(cls, path: PathLike = "model.xml") -> Model:
         """Create model from single XML file
 
         .. versionadded:: 0.13.3
 
         Parameters
         ----------
-        path : str or PathLike
+        path : PathLike
             Path to model.xml file
         """
         parser = ET.XMLParser(huge_tree=True)
@@ -259,8 +392,17 @@ class Model:
 
         return model
 
-    def init_lib(self, threads=None, geometry_debug=False, restart_file=None,
-                 tracks=False, output=True, event_based=None, intracomm=None):
+    def init_lib(
+        self,
+        threads: int | None = None,
+        geometry_debug: bool = False,
+        restart_file: PathLike | None = None,
+        tracks: bool = False,
+        output: bool = True,
+        event_based: bool | None = None,
+        intracomm=None,
+        directory: PathLike | None = None,
+    ):
         """Initializes the model in memory via the C API
 
         .. versionadded:: 0.13.0
@@ -275,7 +417,7 @@ class Model:
             variable).
         geometry_debug : bool, optional
             Turn on geometry debugging during simulation. Defaults to False.
-        restart_file : str, optional
+        restart_file : PathLike, optional
             Path to restart file to use
         tracks : bool, optional
             Enables the writing of particles tracks. The number of particle
@@ -288,6 +430,8 @@ class Model:
             the Settings will be used.
         intracomm : mpi4py.MPI.Intracomm or None, optional
             MPI intracommunicator
+        directory : PathLike or None, optional
+            Directory to write XML files to. Defaults to None.
         """
 
         import openmc.lib
@@ -301,7 +445,8 @@ class Model:
         args = _process_CLI_arguments(
             volume=False, geometry_debug=geometry_debug,
             restart_file=restart_file, threads=threads, tracks=tracks,
-            event_based=event_based)
+            event_based=event_based, path_input=directory)
+
         # Args adds the openmc_exec command in the first entry; remove it
         args = args[1:]
 
@@ -315,7 +460,10 @@ class Model:
             self._intracomm = DummyCommunicator()
 
         if self._intracomm.rank == 0:
-            self.export_to_xml()
+            if directory is not None:
+                self.export_to_xml(directory=directory)
+            else:
+                self.export_to_xml()
         self._intracomm.barrier()
 
         # We cannot pass DummyCommunicator to openmc.lib.init so pass instead
@@ -329,6 +477,8 @@ class Model:
         This method iterates over all DAGMC universes in the geometry and
         synchronizes their cells with the current material assignments. Requires
         that the model has been initialized via :meth:`Model.init_lib`.
+        Synchronized DAGMC cells can then be edited and exported as nested
+        `<cell>` overrides inside each `<dagmc_universe>` element.
 
         .. versionadded:: 0.15.1
 
@@ -356,9 +506,15 @@ class Model:
 
         openmc.lib.finalize()
 
-    def deplete(self, timesteps, method='cecm', final_step=True,
-                operator_kwargs=None, directory='.', output=True,
-                **integrator_kwargs):
+    def deplete(
+        self,
+        method: str = "cecm",
+        final_step: bool = True,
+        operator_kwargs: dict | None = None,
+        directory: PathLike = ".",
+        output: bool = True,
+        **integrator_kwargs,
+    ):
         """Deplete model using specified timesteps/power
 
         .. versionchanged:: 0.13.0
@@ -367,10 +523,12 @@ class Model:
 
         Parameters
         ----------
-        timesteps : iterable of float
-            Array of timesteps in units of [s]. Note that values are not
-            cumulative.
-        method : str, optional
+        timesteps : iterable of float or iterable of tuple
+            Array of timesteps. Note that values are not cumulative. The units are
+            specified by the `timestep_units` argument when `timesteps` is an
+            iterable of float. Alternatively, units can be specified for each step
+            by passing an iterable of (value, unit) tuples.
+        method : str
              Integration method used for depletion (e.g., 'cecm', 'predictor').
              Defaults to 'cecm'.
         final_step : bool, optional
@@ -379,14 +537,14 @@ class Model:
         operator_kwargs : dict
             Keyword arguments passed to the depletion operator initializer
             (e.g., :func:`openmc.deplete.Operator`)
-        directory : str, optional
+        directory : PathLike, optional
             Directory to write XML files to. If it doesn't exist already, it
             will be created. Defaults to the current working directory
         output : bool
             Capture OpenMC output from standard out
         integrator_kwargs : dict
-            Remaining keyword arguments passed to the depletion Integrator
-            initializer (e.g., :func:`openmc.deplete.integrator.cecm`).
+            Remaining keyword arguments passed to the depletion integrator
+            (e.g., :class:`openmc.deplete.CECMIntegrator`).
 
         """
 
@@ -417,8 +575,7 @@ class Model:
             check_value('method', method,
                         dep.integrators.integrator_by_name.keys())
             integrator_class = dep.integrators.integrator_by_name[method]
-            integrator = integrator_class(depletion_operator, timesteps,
-                                          **integrator_kwargs)
+            integrator = integrator_class(depletion_operator, **integrator_kwargs)
 
             # Now perform the depletion
             with openmc.lib.quiet_dll(output):
@@ -441,13 +598,20 @@ class Model:
                 depletion_operator.cleanup_when_done = True
                 depletion_operator.finalize()
 
+    def _link_geometry_to_filters(self):
+        """Establishes a link between distribcell filters and the geometry"""
+        for tally in self.tallies:
+            for f in tally.filters:
+                if isinstance(f, openmc.DistribcellFilter):
+                    f._geometry = self.geometry
+
     def export_to_xml(self, directory: PathLike = '.', remove_surfs: bool = False,
                       nuclides_to_ignore: Iterable[str] | None = None):
         """Export model to separate XML files.
 
         Parameters
         ----------
-        directory : str
+        directory : PathLike
             Directory to write XML files to. If it doesn't exist already, it
             will be created.
         remove_surfs : bool
@@ -464,6 +628,7 @@ class Model:
         if not d.is_dir():
             d.mkdir(parents=True, exist_ok=True)
 
+        self._assign_fw_cadis_tally_IDs()
         self.settings.export_to_xml(d)
         self.geometry.export_to_xml(d, remove_surfs=remove_surfs)
 
@@ -482,6 +647,8 @@ class Model:
         if self.plots:
             self.plots.export_to_xml(d)
 
+        self._link_geometry_to_filters()
+
     def export_to_model_xml(self, path: PathLike = 'model.xml', remove_surfs: bool = False,
                             nuclides_to_ignore: Iterable[str] | None = None):
         """Export model to a single XML file.
@@ -520,6 +687,9 @@ class Model:
                           "set the Geometry.merge_surfaces attribute instead.")
             self.geometry.merge_surfaces = True
 
+        # Link FW-CADIS WeightWindowGenerator target tallies, if present
+        self._assign_fw_cadis_tally_IDs()
+
         # provide a memo to track which meshes have been written
         mesh_memo = set()
         settings_element = self.settings.to_xml_element(mesh_memo)
@@ -561,7 +731,9 @@ class Model:
                 fh.write(ET.tostring(plots_element, encoding="unicode"))
             fh.write("</model>\n")
 
-    def import_properties(self, filename):
+        self._link_geometry_to_filters()
+
+    def import_properties(self, filename: PathLike):
         """Import physical properties
 
         .. versionchanged:: 0.13.0
@@ -569,7 +741,7 @@ class Model:
 
         Parameters
         ----------
-        filename : str
+        filename : PathLike
             Path to properties HDF5 file
 
         See Also
@@ -591,7 +763,7 @@ class Model:
                 raise ValueError("Number of cells in properties file doesn't "
                                  "match current model.")
 
-            # Update temperatures for cells filled with materials
+            # Update temperatures and densities for cells filled with materials
             for name, group in cells_group.items():
                 cell_id = int(name.split()[1])
                 cell = cells[cell_id]
@@ -606,6 +778,20 @@ class Model:
                         else:
                             lib_cell.set_temperature(temperature[0])
 
+                    if group['density']:
+                      density = group['density'][()]
+                      if density.size > 1:
+                          cell.density = [rho for rho in density]
+                      else:
+                          cell.density = density
+                      if self.is_initialized:
+                          lib_cell = openmc.lib.cells[cell_id]
+                          if density.size > 1:
+                              for i, rho in enumerate(density):
+                                  lib_cell.set_density(rho, i)
+                          else:
+                              lib_cell.set_density(density[0])
+
             # Make sure number of materials matches
             mats_group = fh['materials']
             n_cells = mats_group.attrs['n_materials']
@@ -622,11 +808,22 @@ class Model:
                     C_mat = openmc.lib.materials[mat_id]
                     C_mat.set_density(atom_density, 'atom/b-cm')
 
-    def run(self, particles=None, threads=None, geometry_debug=False,
-            restart_file=None, tracks=False, output=True, cwd='.',
-            openmc_exec='openmc', mpi_args=None, event_based=None,
-            export_model_xml=True, apply_tally_results=False,
-            **export_kwargs):
+    def run(
+        self,
+        particles: int | None = None,
+        threads: int | None = None,
+        geometry_debug: bool = False,
+        restart_file: PathLike | None = None,
+        tracks: bool = False,
+        output: bool = True,
+        cwd: PathLike = ".",
+        openmc_exec: PathLike = "openmc",
+        mpi_args: Iterable[str] = None,
+        event_based: bool | None = None,
+        export_model_xml: bool = True,
+        apply_tally_results: bool = False,
+        **export_kwargs,
+    ) -> Path:
         """Run OpenMC
 
         If the C API has been initialized, then the C API is used, otherwise,
@@ -758,10 +955,17 @@ class Model:
 
         return last_statepoint
 
-    def calculate_volumes(self, threads=None, output=True, cwd='.',
-                          openmc_exec='openmc', mpi_args=None,
-                          apply_volumes=True, export_model_xml=True,
-                          **export_kwargs):
+    def calculate_volumes(
+        self,
+        threads: int | None = None,
+        output: bool = True,
+        cwd: PathLike = ".",
+        openmc_exec: PathLike = "openmc",
+        mpi_args: list[str] | None = None,
+        apply_volumes: bool = True,
+        export_model_xml: bool = True,
+        **export_kwargs,
+    ):
         """Runs an OpenMC stochastic volume calculation and, if requested,
         applies volumes to the model
 
@@ -848,59 +1052,17 @@ class Model:
                             openmc.lib.materials[domain_id].volume = \
                                 vol_calc.volumes[domain_id].n
 
-    @add_plot_params
-    def plot(
+
+    def _set_plot_defaults(
         self,
-        origin: Sequence[float] | None = None,
-        width: Sequence[float] | None = None,
-        pixels: int | Sequence[int] = 40000,
-        basis: str = 'xy',
-        color_by: str = 'cell',
-        colors: dict | None = None,
-        seed: int | None = None,
-        openmc_exec: PathLike = 'openmc',
-        axes=None,
-        legend: bool = False,
-        axis_units: str = 'cm',
-        outline: bool | str = False,
-        show_overlaps: bool = False,
-        overlap_color: Sequence[int] | str | None = None,
-        n_samples: int | None = None,
-        plane_tolerance: float = 1.,
-        legend_kwargs: dict | None = None,
-        source_kwargs: dict | None = None,
-        contour_kwargs: dict | None = None,
-        **kwargs,
+        origin: Sequence[float] | None,
+        width: Sequence[float] | None,
+        pixels: int | Sequence[int],
+        basis: str
     ):
-        """Display a slice plot of the model.
+        _check_pixels(pixels)
 
-        .. versionadded:: 0.15.1
-        """
-        import matplotlib.image as mpimg
-        import matplotlib.patches as mpatches
-        import matplotlib.pyplot as plt
-
-        check_type('n_samples', n_samples, int | None)
-        check_type('plane_tolerance', plane_tolerance, Real)
-        if legend_kwargs is None:
-            legend_kwargs = {}
-        legend_kwargs.setdefault('bbox_to_anchor', (1.05, 1))
-        legend_kwargs.setdefault('loc', 2)
-        legend_kwargs.setdefault('borderaxespad', 0.0)
-        if source_kwargs is None:
-            source_kwargs = {}
-        source_kwargs.setdefault('marker', 'x')
-
-        # Determine extents of plot
-        if basis == 'xy':
-            x, y, z = 0, 1, 2
-            xlabel, ylabel = f'x [{axis_units}]', f'y [{axis_units}]'
-        elif basis == 'yz':
-            x, y, z = 1, 2, 0
-            xlabel, ylabel = f'y [{axis_units}]', f'z [{axis_units}]'
-        elif basis == 'xz':
-            x, y, z = 0, 2, 1
-            xlabel, ylabel = f'x [{axis_units}]', f'z [{axis_units}]'
+        x, y, _ = _BASIS_INDICES[basis]
 
         bb = self.bounding_box
         # checks to see if bounding box contains -inf or inf values
@@ -925,6 +1087,256 @@ class Model:
             pixels_y = math.sqrt(pixels / aspect_ratio)
             pixels = (int(pixels / pixels_y), int(pixels_y))
 
+        return origin, width, pixels
+
+    def id_map(
+        self,
+        origin: Sequence[float] | None = None,
+        width: Sequence[float] | None = None,
+        pixels: int | Sequence[int] = 40000,
+        basis: str = 'xy',
+        color_overlaps: bool = False,
+        **init_kwargs
+    ) -> np.ndarray:
+        """Generate an ID map for domains based on the plot parameters
+
+        If the model is not yet initialized, it will be initialized with
+        openmc.lib. If the model is initialized, the model will remain
+        initialized after this method call exits.
+
+        .. versionadded:: 0.15.3
+
+        Parameters
+        ----------
+        origin : Sequence[float], optional
+            Origin of the plot. If unspecified, this argument defaults to the
+            center of the bounding box if the bounding box does not contain inf
+            values for the provided basis, otherwise (0.0, 0.0, 0.0).
+        width : Sequence[float], optional
+            Width of the plot. If unspecified, this argument defaults to the
+            width of the bounding box if the bounding box does not contain inf
+            values for the provided basis, otherwise (10.0, 10.0).
+        pixels : int | Sequence[int], optional
+            If an iterable of ints is provided then this directly sets the
+            number of pixels to use in each basis direction. If a single int is
+            provided then this sets the total number of pixels in the plot and
+            the number of pixels in each basis direction is calculated from this
+            total and the image aspect ratio based on the width argument.
+        basis : {'xy', 'yz', 'xz'}, optional
+            Basis of the plot.
+        color_overlaps : bool, optional
+            Whether to assign unique IDs (-3) to overlapping regions. If False,
+            overlapping regions will be assigned the ID of the lowest-numbered
+            cell that occupies that region. Defaults to False.
+        **init_kwargs
+            Keyword arguments passed to :meth:`Model.init_lib`.
+
+        Returns
+        -------
+        id_map : numpy.ndarray
+            A NumPy array with shape (vertical pixels, horizontal pixels, 3) of
+            OpenMC property IDs with dtype int32. The last dimension of the
+            array contains cell IDs, cell instances, and material IDs (in that
+            order).
+        """
+        ids, _ = self.slice_data(
+            origin=origin,
+            width=width,
+            pixels=pixels,
+            basis=basis,
+            show_overlaps=color_overlaps,
+            level=-1,
+            include_properties=False,
+            **init_kwargs,
+        )
+        return ids
+
+    def slice_data(
+        self,
+        origin: Sequence[float] | None = None,
+        width: Sequence[float] | None = None,
+        pixels: int | Sequence[int] = 40000,
+        basis: str = 'xy',
+        u_span: Sequence[float] | None = None,
+        v_span: Sequence[float] | None = None,
+        show_overlaps: bool = False,
+        level: int = -1,
+        filter: openmc.Filter | None = None,
+        include_properties: bool = True,
+        **init_kwargs
+    ) -> tuple[np.ndarray, np.ndarray | None]:
+        """Generate geometry and property data for a 2D plot slice.
+
+        This method combines the functionality of :meth:`id_map` and property
+        mapping into a single call, avoiding duplicate geometry lookups. It also
+        supports filter bin index lookup for tally visualization.
+
+        .. versionadded:: 0.16.0
+
+        Parameters
+        ----------
+        origin : Sequence[float], optional
+            Origin of the plot. If unspecified, this argument defaults to the
+            center of the bounding box if the bounding box does not contain inf
+            values for the provided basis, otherwise (0.0, 0.0, 0.0).
+        width : Sequence[float], optional
+            Width of the plot. If unspecified, this argument defaults to the
+            width of the bounding box if the bounding box does not contain inf
+            values for the provided basis, otherwise (10.0, 10.0).
+        pixels : int | Sequence[int], optional
+            If an iterable of ints is provided then this directly sets the
+            number of pixels to use in each basis direction. If a single int is
+            provided then this sets the total number of pixels in the plot and
+            the number of pixels in each basis direction is calculated from this
+            total and the image aspect ratio based on the width argument.
+        basis : {'xy', 'yz', 'xz'}, optional
+            Basis of the plot.
+        u_span : Sequence[float], optional
+            Full-width span vector for an oriented slice (3 values). Mutually
+            exclusive with width.
+        v_span : Sequence[float], optional
+            Full-height span vector for an oriented slice (3 values). Mutually
+            exclusive with width.
+        show_overlaps : bool, optional
+            Whether to identify and assign unique IDs (-3) to overlapping
+            regions. If False, overlapping regions will be assigned the ID of
+            the lowest-numbered cell that occupies that region. Defaults to
+            False.
+        level : int, optional
+            Universe level to plot (-1 for deepest). Defaults to -1.
+        filter : openmc.Filter, optional
+            If provided, the information for each pixel also includes an index
+            in the filter corresponding to the pixel position.
+        include_properties : bool, optional
+            Whether to include temperature/density data. Defaults to True.
+        **init_kwargs
+            Keyword arguments passed to :meth:`Model.init_lib`.
+
+        Returns
+        -------
+        geom_data : numpy.ndarray
+            Shape (v_res, h_res, 3) or (v_res, h_res, 4) int32 array. Contains
+            [cell_id, cell_instance, material_id] when no filter, or [cell_id,
+            cell_instance, material_id, filter_bin] with filter.
+        property_data : numpy.ndarray or None
+            Shape (v_res, h_res, 2) float64 array with [temperature, density],
+            or None if include_properties=False.
+        """
+        import openmc.lib
+
+        _check_pixels(pixels)
+
+        if width is not None and (u_span is not None or v_span is not None):
+            raise ValueError("width is mutually exclusive with u_span/v_span.")
+
+        if u_span is not None or v_span is not None:
+            if u_span is None or v_span is None:
+                raise ValueError("Both u_span and v_span must be provided.")
+            if origin is None:
+                origin = (0.0, 0.0, 0.0)
+            if isinstance(pixels, int):
+                u_norm = np.linalg.norm(u_span)
+                v_norm = np.linalg.norm(v_span)
+                aspect_ratio = u_norm / v_norm
+                pixels_y = math.sqrt(pixels / aspect_ratio)
+                pixels = (int(pixels / pixels_y), int(pixels_y))
+        else:
+            origin, width, pixels = self._set_plot_defaults(
+                origin, width, pixels, basis)
+
+        # Silence output by default. Also set arguments to start in volume
+        # calculation mode to avoid loading cross sections
+        init_kwargs.setdefault('output', False)
+        init_kwargs.setdefault('args', ['-c'])
+
+        # If filter does not already appear in the model, temporarily add a
+        # tally with the filter
+        original_length = len(self.tallies)
+        if filter is not None:
+            filter_ids = {f.id for t in self.tallies for f in t.filters}
+            if filter.id not in filter_ids:
+                # Create temporary tally while preserving ID assignment
+                next_id = openmc.Tally.next_id
+                temp_tally = openmc.Tally()
+                temp_tally.filters = [filter]
+                temp_tally.scores = ['flux']
+                self.tallies.append(temp_tally)
+                openmc.Tally.used_ids.remove(temp_tally.id)
+                openmc.Tally.next_id = next_id
+
+        with openmc.lib.TemporarySession(self, **init_kwargs):
+            geom_data, property_data = openmc.lib.slice_data(
+                origin=origin,
+                width=width,
+                basis=basis,
+                u_span=u_span,
+                v_span=v_span,
+                pixels=pixels,
+                show_overlaps=show_overlaps,
+                level=level,
+                filter=filter,
+                include_properties=include_properties,
+            )
+
+        # If filter was temporarily added, remove it
+        if len(self.tallies) > original_length:
+            self.tallies.pop()
+
+        return geom_data, property_data
+
+    @add_plot_params
+    def plot(
+        self,
+        origin: Sequence[float] | None = None,
+        width: Sequence[float] | None = None,
+        pixels: int | Sequence[int] = 40000,
+        basis: str = 'xy',
+        color_by: str = 'cell',
+        colors: dict | None = None,
+        seed: int | None = None,
+        axes=None,
+        legend: bool = False,
+        axis_units: str = 'cm',
+        outline: bool | str = False,
+        show_overlaps: bool = False,
+        overlap_color: Sequence[int] | str = (255, 0, 0),
+        n_samples: int | None = None,
+        plane_tolerance: float = 1.,
+        legend_kwargs: dict | None = None,
+        source_kwargs: dict | None = None,
+        contour_kwargs: dict | None = None,
+        **kwargs,
+    ):
+        """Display a slice plot of the model.
+
+        .. versionadded:: 0.15.1
+        """
+        import matplotlib.patches as mpatches
+        import matplotlib.pyplot as plt
+
+        check_type('n_samples', n_samples, int | None)
+        if n_samples is not None:
+            check_greater_than('n_samples', n_samples, 0, equality=True)
+        check_type('plane_tolerance', plane_tolerance, Real)
+        check_greater_than('plane_tolerance', plane_tolerance, 0.0)
+
+        if legend_kwargs is None:
+            legend_kwargs = {}
+        legend_kwargs.setdefault('bbox_to_anchor', (1.05, 1))
+        legend_kwargs.setdefault('loc', 2)
+        legend_kwargs.setdefault('borderaxespad', 0.0)
+        if source_kwargs is None:
+            source_kwargs = {}
+        source_kwargs.setdefault('marker', 'x')
+
+        # Set indices using basis and create axis labels
+        x, y, z = _BASIS_INDICES[basis]
+        xlabel, ylabel = f'{basis[0]} [{axis_units}]', f'{basis[1]} [{axis_units}]'
+
+        # Determine extents of plot
+        origin, width, pixels = self._set_plot_defaults(
+            origin, width, pixels, basis)
+
         axis_scaling_factor = {'km': 0.00001, 'm': 0.01, 'cm': 1, 'mm': 10}
 
         x_min = (origin[x] - 0.5*width[0]) * axis_scaling_factor[axis_units]
@@ -932,111 +1344,119 @@ class Model:
         y_min = (origin[y] - 0.5*width[1]) * axis_scaling_factor[axis_units]
         y_max = (origin[y] + 0.5*width[1]) * axis_scaling_factor[axis_units]
 
-        with TemporaryDirectory() as tmpdir:
-            if seed is not None:
-                self.settings.plot_seed = seed
+        # Determine whether any materials contains macroscopic data and if so,
+        # set energy mode accordingly and check that mg cross sections path is accessible
+        for mat in self.geometry.get_all_materials().values():
+            if mat._macroscopic is not None:
+                self.settings.energy_mode = 'multi-group'
+                if 'mg_cross_sections' not in openmc.config:
+                    raise RuntimeError("'mg_cross_sections' path must be set in "
+                                       "openmc.config before plotting.")
+                break
 
-            # Create plot object matching passed arguments
-            plot = openmc.Plot()
-            plot.origin = origin
-            plot.width = width
-            plot.pixels = pixels
-            plot.basis = basis
+        # Get plot IDs from the C API
+        id_map, _ = self.slice_data(
+            origin=origin,
+            width=width,
+            pixels=pixels,
+            basis=basis,
+            show_overlaps=show_overlaps,
+            include_properties=False,
+        )
+
+        # Generate colors if not provided
+        if colors is None and seed is not None:
+            # Use the colorize method to generate random colors
+            plot = openmc.SlicePlot()
             plot.color_by = color_by
-            plot.show_overlaps = show_overlaps
-            if overlap_color is not None:
-                plot.overlap_color = overlap_color
-            if colors is not None:
-                plot.colors = colors
-            self.plots.append(plot)
+            plot.colorize(self.geometry, seed=seed)
+            colors = plot.colors
 
-            # Run OpenMC in geometry plotting mode
-            self.plot_geometry(False, cwd=tmpdir, openmc_exec=openmc_exec)
+        # Convert ID map to RGB image
+        img = id_map_to_rgb(
+            id_map=id_map,
+            color_by=color_by,
+            colors=colors,
+            overlap_color=overlap_color
+        )
 
-            # Read image from file
-            img_path = Path(tmpdir) / f'plot_{plot.id}.png'
-            if not img_path.is_file():
-                img_path = img_path.with_suffix('.ppm')
-            img = mpimg.imread(str(img_path))
+        # Create a figure sized such that the size of the axes within
+        # exactly matches the number of pixels specified
+        if axes is None:
+            px = 1/plt.rcParams['figure.dpi']
+            fig, axes = plt.subplots()
+            axes.set_xlabel(xlabel)
+            axes.set_ylabel(ylabel)
+            params = fig.subplotpars
+            width_px = pixels[0]*px/(params.right - params.left)
+            height_px = pixels[1]*px/(params.top - params.bottom)
+            fig.set_size_inches(width_px, height_px)
 
-            # Create a figure sized such that the size of the axes within
-            # exactly matches the number of pixels specified
-            if axes is None:
-                px = 1/plt.rcParams['figure.dpi']
-                fig, axes = plt.subplots()
-                axes.set_xlabel(xlabel)
-                axes.set_ylabel(ylabel)
-                params = fig.subplotpars
-                width = pixels[0]*px/(params.right - params.left)
-                height = pixels[1]*px/(params.top - params.bottom)
-                fig.set_size_inches(width, height)
+        if outline:
+            # Combine R, G, B values into a single int for contour detection
+            rgb = (img * 256).astype(int)
+            image_value = (rgb[..., 0] << 16) + \
+                (rgb[..., 1] << 8) + (rgb[..., 2])
 
-            if outline:
-                # Combine R, G, B values into a single int
-                rgb = (img * 256).astype(int)
-                image_value = (rgb[..., 0] << 16) + \
-                    (rgb[..., 1] << 8) + (rgb[..., 2])
+            # Set default arguments for contour()
+            if contour_kwargs is None:
+                contour_kwargs = {}
+            contour_kwargs.setdefault('colors', 'k')
+            contour_kwargs.setdefault('linestyles', 'solid')
+            contour_kwargs.setdefault('algorithm', 'serial')
 
-                # Set default arguments for contour()
-                if contour_kwargs is None:
-                    contour_kwargs = {}
-                contour_kwargs.setdefault('colors', 'k')
-                contour_kwargs.setdefault('linestyles', 'solid')
-                contour_kwargs.setdefault('algorithm', 'serial')
+            axes.contour(
+                image_value,
+                origin="upper",
+                levels=np.unique(image_value),
+                extent=(x_min, x_max, y_min, y_max),
+                **contour_kwargs
+            )
 
-                axes.contour(
-                    image_value,
-                    origin="upper",
-                    levels=np.unique(image_value),
-                    extent=(x_min, x_max, y_min, y_max),
-                    **contour_kwargs
-                )
+            # If only showing outline, set the axis limits and aspect explicitly
+            if outline == 'only':
+                axes.set_xlim(x_min, x_max)
+                axes.set_ylim(y_min, y_max)
+                axes.set_aspect('equal')
 
-            # add legend showing which colors represent which material
-            # or cell if that was requested
-            if legend:
-                if plot.colors == {}:
-                    raise ValueError("Must pass 'colors' dictionary if you "
-                                     "are adding a legend via legend=True.")
+        # Add legend showing which colors represent which material or cell
+        if legend:
+            if colors is None or len(colors) == 0:
+                raise ValueError("Must pass 'colors' dictionary if you "
+                                 "are adding a legend via legend=True.")
 
-                if color_by == "cell":
-                    expected_key_type = openmc.Cell
+            if color_by == "cell":
+                expected_key_type = openmc.Cell
+            else:
+                expected_key_type = openmc.Material
+
+            patches = []
+            for key, color in colors.items():
+                if isinstance(key, int):
+                    raise TypeError(
+                        "Cannot use IDs in colors dict for auto legend.")
+                elif not isinstance(key, expected_key_type):
+                    raise TypeError(
+                        "Color dict key type does not match color_by")
+
+                # this works whether we're doing cells or materials
+                label = key.name if key.name != '' else key.id
+
+                # matplotlib takes RGB on 0-1 scale rather than 0-255
+                if len(color) == 3 and not isinstance(color, str):
+                    scaled_color = (
+                        color[0]/255, color[1]/255, color[2]/255)
                 else:
-                    expected_key_type = openmc.Material
+                    scaled_color = color
 
-                patches = []
-                for key, color in plot.colors.items():
+                key_patch = mpatches.Patch(color=scaled_color, label=label)
+                patches.append(key_patch)
 
-                    if isinstance(key, int):
-                        raise TypeError(
-                            "Cannot use IDs in colors dict for auto legend.")
-                    elif not isinstance(key, expected_key_type):
-                        raise TypeError(
-                            "Color dict key type does not match color_by")
-
-                    # this works whether we're doing cells or materials
-                    label = key.name if key.name != '' else key.id
-
-                    # matplotlib takes RGB on 0-1 scale rather than 0-255. at
-                    # this point PlotBase has already checked that 3-tuple
-                    # based colors are already valid, so if the length is three
-                    # then we know it just needs to be converted to the 0-1
-                    # format.
-                    if len(color) == 3 and not isinstance(color, str):
-                        scaled_color = (
-                            color[0]/255, color[1]/255, color[2]/255)
-                    else:
-                        scaled_color = color
-
-                    key_patch = mpatches.Patch(color=scaled_color, label=label)
-                    patches.append(key_patch)
-
-                axes.legend(handles=patches, **legend_kwargs)
-
-            # Plot image and return the axes
-            if outline != 'only':
-                axes.imshow(img, extent=(x_min, x_max, y_min, y_max), **kwargs)
+            axes.legend(handles=patches, **legend_kwargs)
 
+        # Plot image and return the axes
+        if outline != 'only':
+            axes.imshow(img, extent=(x_min, x_max, y_min, y_max), **kwargs)
 
         if n_samples:
             # Sample external source particles
@@ -1049,18 +1469,19 @@ class Model:
             tol = plane_tolerance
             for particle in particles:
                 if (slice_value - tol < particle.r[z] < slice_value + tol):
-                    xs.append(particle.r[x])
-                    ys.append(particle.r[y])
+                    xs.append(particle.r[x] * axis_scaling_factor[axis_units])
+                    ys.append(particle.r[y] * axis_scaling_factor[axis_units])
             axes.scatter(xs, ys, **source_kwargs)
 
         return axes
 
     def sample_external_source(
-            self,
-            n_samples: int = 1000,
-            prn_seed: int | None = None,
-            **init_kwargs
-    ) -> openmc.ParticleList:
+        self,
+        n_samples: int = 1000,
+        prn_seed: int | None = None,
+        as_array: bool = False,
+        **init_kwargs
+    ) -> openmc.ParticleList | np.ndarray:
         """Sample external source and return source particles.
 
         .. versionadded:: 0.15.1
@@ -1072,13 +1493,17 @@ class Model:
         prn_seed : int
             Pseudorandom number generator (PRNG) seed; if None, one will be
             generated randomly.
+        as_array : bool
+            If True, return a numpy structured array instead of a
+            :class:`~openmc.ParticleList`.
         **init_kwargs
             Keyword arguments passed to :func:`openmc.lib.init`
 
         Returns
         -------
-        openmc.ParticleList
-            List of samples source particles
+        openmc.ParticleList or numpy.ndarray
+            List of sampled source particles, or a structured array when
+            *as_array* is True.
         """
         import openmc.lib
 
@@ -1087,15 +1512,10 @@ class Model:
         init_kwargs.setdefault('output', False)
         init_kwargs.setdefault('args', ['-c'])
 
-        with change_directory(tmpdir=True):
-            # Export model within temporary directory
-            self.export_to_model_xml()
-
-            # Sample external source sites
-            with openmc.lib.run_in_memory(**init_kwargs):
-                return openmc.lib.sample_external_source(
-                    n_samples=n_samples, prn_seed=prn_seed
-                )
+        with openmc.lib.TemporarySession(self, **init_kwargs):
+            return openmc.lib.sample_external_source(
+                n_samples=n_samples, prn_seed=prn_seed, as_array=as_array
+            )
 
     def apply_tally_results(self, statepoint: PathLike | openmc.StatePoint):
         """Apply results from a statepoint to tally objects on the Model
@@ -1107,8 +1527,14 @@ class Model:
         """
         self.tallies.add_results(statepoint)
 
-    def plot_geometry(self, output=True, cwd='.', openmc_exec='openmc',
-                      export_model_xml=True, **export_kwargs):
+    def plot_geometry(
+        self,
+        output: bool = True,
+        cwd: PathLike = ".",
+        openmc_exec: PathLike = "openmc",
+        export_model_xml: bool = True,
+        **export_kwargs,
+    ):
         """Creates plot images as specified by the Model.plots attribute
 
         .. versionadded:: 0.13.0
@@ -1117,10 +1543,10 @@ class Model:
         ----------
         output : bool, optional
             Capture OpenMC output from standard out
-        cwd : str, optional
+        cwd : PathLike, optional
             Path to working directory to run in. Defaults to the current
             working directory.
-        openmc_exec : str, optional
+        openmc_exec : PathLike, optional
             Path to OpenMC executable. Defaults to 'openmc'.
             This only applies to the case when not using the C API.
         export_model_xml : bool, optional
@@ -1150,8 +1576,14 @@ class Model:
                 openmc.plot_geometry(output=output, openmc_exec=openmc_exec,
                                      path_input=path_input)
 
-    def _change_py_lib_attribs(self, names_or_ids, value, obj_type,
-                               attrib_name, density_units='atom/b-cm'):
+    def _change_py_lib_attribs(
+        self,
+        names_or_ids: Iterable[str] | Iterable[int],
+        value: float | Iterable[float],
+        obj_type: str,
+        attrib_name: str,
+        density_units: str = "atom/b-cm",
+    ):
         # Method to do the same work whether it is a cell or material and
         # a temperature or volume
         check_type('names_or_ids', names_or_ids, Iterable, (Integral, str))
@@ -1230,7 +1662,9 @@ class Model:
                 else:
                     setattr(lib_obj, attrib_name, value)
 
-    def rotate_cells(self, names_or_ids, vector):
+    def rotate_cells(
+        self, names_or_ids: Iterable[str] | Iterable[int], vector: Iterable[float]
+    ):
         """Rotate the identified cell(s) by the specified rotation vector.
         The rotation is only applied to cells filled with a universe.
 
@@ -1252,7 +1686,9 @@ class Model:
 
         self._change_py_lib_attribs(names_or_ids, vector, 'cell', 'rotation')
 
-    def translate_cells(self, names_or_ids, vector):
+    def translate_cells(
+        self, names_or_ids: Iterable[str] | Iterable[int], vector: Iterable[float]
+    ):
         """Translate the identified cell(s) by the specified translation vector.
         The translation is only applied to cells filled with a universe.
 
@@ -1275,7 +1711,12 @@ class Model:
         self._change_py_lib_attribs(names_or_ids, vector, 'cell',
                                     'translation')
 
-    def update_densities(self, names_or_ids, density, density_units='atom/b-cm'):
+    def update_densities(
+        self,
+        names_or_ids: Iterable[str] | Iterable[int],
+        density: float,
+        density_units: str = "atom/b-cm",
+    ):
         """Update the density of a given set of materials to a new value
 
         .. note:: If applying this change to a name that is not unique, then
@@ -1298,7 +1739,9 @@ class Model:
         self._change_py_lib_attribs(names_or_ids, density, 'material',
                                     'density', density_units)
 
-    def update_cell_temperatures(self, names_or_ids, temperature):
+    def update_cell_temperatures(
+        self, names_or_ids: Iterable[str] | Iterable[int], temperature: float
+    ):
         """Update the temperature of a set of cells to the given value
 
         .. note:: If applying this change to a name that is not unique, then
@@ -1319,7 +1762,9 @@ class Model:
         self._change_py_lib_attribs(names_or_ids, temperature, 'cell',
                                     'temperature')
 
-    def update_material_volumes(self, names_or_ids, volume):
+    def update_material_volumes(
+        self, names_or_ids: Iterable[str] | Iterable[int], volume: float
+    ):
         """Update the volume of a set of materials to the given value
 
         .. note:: If applying this change to a name that is not unique, then
@@ -1439,3 +1884,1269 @@ class Model:
             self.materials = openmc.Materials(
                 self.geometry.get_all_materials().values()
             )
+
+    @staticmethod
+    def _auto_generate_mgxs_lib(
+        model: openmc.model.Model,
+        groups: openmc.mgxs.EnergyGroups,
+        correction: str | None,
+        directory: PathLike,
+    ) -> openmc.mgxs.Library:
+        """
+        Automatically generate a multi-group cross section libray from a model
+        with the specified group structure.
+
+        Parameters
+        ----------
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        mgxs_path : str
+            Filename for the MGXS HDF5 file.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : str
+            Directory to run the simulation in, so as to contain XML files.
+
+        Returns
+        -------
+        mgxs_lib : openmc.mgxs.Library
+            OpenMC MGXS Library object
+        """
+
+        # Initialize MGXS library with a finished OpenMC geometry object
+        mgxs_lib = openmc.mgxs.Library(model.geometry)
+
+        # Pick energy group structure
+        mgxs_lib.energy_groups = groups
+
+        # Disable transport correction
+        mgxs_lib.correction = correction
+
+        # Specify needed cross sections for random ray
+        if correction == 'P0':
+            mgxs_lib.mgxs_types = [
+                'nu-transport', 'absorption', 'nu-fission', 'fission',
+                'consistent nu-scatter matrix', 'multiplicity matrix', 'chi',
+                'kappa-fission'
+            ]
+        elif correction is None:
+            mgxs_lib.mgxs_types = [
+                'total', 'absorption', 'nu-fission', 'fission',
+                'consistent nu-scatter matrix', 'multiplicity matrix', 'chi',
+                'kappa-fission'
+            ]
+
+        # Specify a "material" domain type for the cross section tally filters
+        mgxs_lib.domain_type = "material"
+
+        # Specify the domains over which to compute multi-group cross sections
+        mgxs_lib.domains = model.geometry.get_all_materials().values()
+
+        # Do not compute cross sections on a nuclide-by-nuclide basis
+        mgxs_lib.by_nuclide = False
+
+        # Check the library - if no errors are raised, then the library is satisfactory.
+        mgxs_lib.check_library_for_openmc_mgxs()
+
+        # Construct all tallies needed for the multi-group cross section library
+        mgxs_lib.build_library()
+
+        # Create a "tallies.xml" file for the MGXS Library
+        mgxs_lib.add_to_tallies(model.tallies, merge=True)
+
+        # Run
+        statepoint_filename = model.run(cwd=directory)
+
+        # Load MGXS
+        with openmc.StatePoint(statepoint_filename) as sp:
+            mgxs_lib.load_from_statepoint(sp)
+
+        return mgxs_lib
+
+    def _create_mgxs_sources(
+        self,
+        groups: openmc.mgxs.EnergyGroups,
+        spatial_dist: openmc.stats.Spatial,
+        source_energy: openmc.stats.Univariate | None = None,
+    ) -> list[openmc.IndependentSource]:
+        """Create a list of independent sources to use with MGXS generation.
+
+        Note that in all cases, a discrete source that is uniform over all
+        energy groups is created (strength = 0.01) to ensure that total cross
+        sections are generated for all energy groups. In the case that the user
+        has provided a source_energy distribution as an argument, an additional
+        source (strength = 0.99) is created using that energy distribution. If
+        the user has not provided a source_energy distribution, but the model
+        has sources defined, and all of those sources are of IndependentSource
+        type, then additional sources are created based on the model's existing
+        sources, keeping their energy distributions but replacing their
+        spatial/angular distributions, with their combined strength being 0.99.
+        If the user has not provided a source_energy distribution and no sources
+        are defined on the model and the run mode is 'eigenvalue', then a
+        default Watt spectrum source (strength = 0.99) is added.
+
+        Parameters
+        ----------
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        spatial_dist : openmc.stats.Spatial
+            Spatial distribution to use for all sources.
+        source_energy : openmc.stats.Univariate, optional
+            Energy distribution to use when generating MGXS data, replacing any
+            existing sources in the model.
+
+        Returns
+        -------
+        list[openmc.IndependentSource]
+            A list of independent sources to use for MGXS generation.
+        """
+        # Make a discrete source that is uniform over the bins of the group structure
+        midpoints = []
+        strengths = []
+        for i in range(groups.num_groups):
+            bounds = groups.get_group_bounds(i+1)
+            midpoints.append((bounds[0] + bounds[1]) / 2.0)
+            strengths.append(1.0)
+
+        uniform_energy = openmc.stats.Discrete(x=midpoints, p=strengths)
+        uniform_distribution = openmc.IndependentSource(spatial_dist, energy=uniform_energy, strength=0.01)
+        sources = [uniform_distribution]
+
+        # If the user provided an energy distribution, use that
+        if source_energy is not None:
+            user_energy = openmc.IndependentSource(
+                space=spatial_dist, energy=source_energy, strength=0.99)
+            sources.append(user_energy)
+
+        # If the user did not provide an energy distribution, create sources
+        # based on what is in their model, keeping the energy spectrum but
+        # replacing the spatial/angular distributions. We only do this if ALL
+        # sources are of IndependentSource type, as we can't pull the energy
+        # distribution from e.g. CompiledSource or FileSource types.
+        else:
+            if self.settings.source is not None:
+                for src in self.settings.source:
+                    if not isinstance(src, openmc.IndependentSource):
+                        break
+                else:
+                    n_user_sources = len(self.settings.source)
+                    for src in self.settings.source:
+                        # Create a new IndependentSource with adjusted strength, space, and angle
+                        user_source = openmc.IndependentSource(
+                            space=spatial_dist,
+                            energy=src.energy,
+                            strength=0.99 / n_user_sources
+                        )
+                        sources.append(user_source)
+            else:
+                # No user sources defined. If we are in eigenvalue mode, then use the default Watt spectrum.
+                if self.settings.run_mode == 'eigenvalue':
+                    watt_energy = openmc.stats.Watt()
+                    watt_source = openmc.IndependentSource(
+                        space=spatial_dist, energy=watt_energy, strength=0.99)
+                    sources.append(watt_source)
+
+        return sources
+
+    @staticmethod
+    def _isothermal_infinite_media_mgxs(
+        material: openmc.Material,
+        groups: openmc.mgxs.EnergyGroups,
+        nparticles: int,
+        correction: str | None,
+        directory: PathLike,
+        source: openmc.IndependentSource,
+        temperature_settings: dict,
+        temperature: float | None = None,
+    ) -> openmc.XSdata:
+        """Generate a single MGXS set for one material, where the geometry is an
+        infinite medium composed of that material at an isothermal temperature value.
+
+        Parameters
+        ----------
+        material : openmc.Material
+            The material to generate MGXS for
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : str
+            Directory to run the simulation in, so as to contain XML files.
+        source : openmc.IndependentSource
+            Source to use when generating MGXS.
+        temperature_settings : dict
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        temperature : float, optional
+            The isothermal temperature value to apply to the material. If not specified,
+            defaults to the temperature in the material.
+
+        Returns
+        -------
+        data : openmc.XSdata
+            The material MGXS for the given temperature isotherm.
+        """
+        model = openmc.Model()
+
+        # Set materials on the model
+        model.materials = [material]
+        if temperature is not None:
+          model.materials[-1].temperature = temperature
+
+        # Settings
+        model.settings.batches = 100
+        model.settings.particles = nparticles
+
+        model.settings.source = source
+
+        model.settings.run_mode = 'fixed source'
+        model.settings.create_fission_neutrons = False
+
+        model.settings.output = {'summary': True, 'tallies': False}
+        model.settings.temperature = temperature_settings
+
+        # Geometry
+        box = openmc.model.RectangularPrism(
+            100000.0, 100000.0, boundary_type='reflective')
+        name = material.name
+        infinite_cell = openmc.Cell(name=name, fill=model.materials[-1], region=-box)
+        infinite_universe = openmc.Universe(name=name, cells=[infinite_cell])
+        model.geometry.root_universe = infinite_universe
+
+        # Generate MGXS
+        mgxs_lib = Model._auto_generate_mgxs_lib(
+                model, groups, correction, directory)
+
+        if temperature is not None:
+            return mgxs_lib.get_xsdata(domain=material, xsdata_name=name,
+                                       temperature=temperature)
+        else:
+            return mgxs_lib.get_xsdata(domain=material, xsdata_name=name)
+
+    def _generate_infinite_medium_mgxs(
+        self,
+        groups: openmc.mgxs.EnergyGroups,
+        nparticles: int,
+        mgxs_path: PathLike,
+        correction: str | None,
+        directory: PathLike,
+        source_energy: openmc.stats.Univariate | None = None,
+        temperatures: Sequence[float] | None = None,
+        temperature_settings: dict | None = None,
+    ) -> None:
+        """Generate a MGXS library by running multiple OpenMC simulations, each
+        representing an infinite medium simulation of a single isolated
+        material. A discrete source is used to sample particles, with an equal
+        strength spread across each of the energy groups. This is a highly naive
+        method that ignores all spatial self shielding effects and all resonance
+        shielding effects between materials. If temperature data points are provided,
+        isothermal cross sections are generated at each temperature point for
+        each material to build a temperature interpolation table.
+
+        Note that in all cases, a discrete source that is uniform over all
+        energy groups is created (strength = 0.01) to ensure that total cross
+        sections are generated for all energy groups. In the case that the user
+        has provided a source_energy distribution as an argument, an additional
+        source (strength = 0.99) is created using that energy distribution. If
+        the user has not provided a source_energy distribution, but the model
+        has sources defined, and all of those sources are of IndependentSource
+        type, then additional sources are created based on the model's existing
+        sources, keeping their energy distributions but replacing their
+        spatial/angular distributions, with their combined strength being 0.99.
+        If the user has not provided a source_energy distribution and no sources
+        are defined on the model and the run mode is 'eigenvalue', then a
+        default Watt spectrum source (strength = 0.99) is added.
+
+        Parameters
+        ----------
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        mgxs_path : str
+            Filename for the MGXS HDF5 file.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : str
+            Directory to run the simulation in, so as to contain XML files.
+        source_energy : openmc.stats.Univariate, optional
+            Energy distribution to use when generating MGXS data, replacing any
+            existing sources in the model.
+        temperatures : Sequence[float], optional
+            A list of temperatures to generate MGXS at. Each infinite material region
+            is isothermal at a given temperature data point for cross
+            section generation.
+        temperature_settings : dict, optional
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        """
+
+        src = self._create_mgxs_sources(
+            groups,
+            spatial_dist=openmc.stats.Point(),
+            source_energy=source_energy
+        )
+
+        temp_settings = {}
+        if temperature_settings is None:
+            temp_settings = self.settings.temperature
+        else:
+            temp_settings = temperature_settings
+
+        if temperatures is None:
+            mgxs_sets = []
+            for material in self.materials:
+                xs_data = Model._isothermal_infinite_media_mgxs(
+                    material,
+                    groups,
+                    nparticles,
+                    correction,
+                    directory,
+                    src,
+                    temp_settings
+                )
+                mgxs_sets.append(xs_data)
+
+            # Write the file to disk.
+            mgxs_file = openmc.MGXSLibrary(energy_groups=groups)
+            for mgxs_set in mgxs_sets:
+                mgxs_file.add_xsdata(mgxs_set)
+            mgxs_file.export_to_hdf5(mgxs_path)
+        else:
+            # Build a series of XSData objects, one for each isothermal temperature value.
+            raw_mgxs_sets = {}
+            for temperature in temperatures:
+                raw_mgxs_sets[temperature] = []
+                for material in self.materials:
+                    xs_data = Model._isothermal_infinite_media_mgxs(
+                        material,
+                        groups,
+                        nparticles,
+                        correction,
+                        directory,
+                        src,
+                        temp_settings,
+                        temperature
+                    )
+                    raw_mgxs_sets[temperature].append(xs_data)
+
+            # Unpack the isothermal XSData objects and build a single XSData object per material.
+            mgxs_sets = []
+            for m in range(len(self.materials)):
+                mgxs_sets.append(openmc.XSdata(self.materials[m].name, groups,
+                                               temperatures=temperatures))
+                mgxs_sets[-1].order = 0
+                for temperature in temperatures:
+                    mgxs_sets[-1].add_temperature_data(raw_mgxs_sets[temperature][m])
+
+            # Write the file to disk.
+            mgxs_file = openmc.MGXSLibrary(energy_groups=groups)
+            for mgxs_set in mgxs_sets:
+                mgxs_file.add_xsdata(mgxs_set)
+            mgxs_file.export_to_hdf5(mgxs_path)
+
+    @staticmethod
+    def _create_stochastic_slab_geometry(
+        materials: Sequence[openmc.Material],
+        cell_thickness: float = 1.0,
+        num_repeats: int = 100,
+    ) -> tuple[openmc.Geometry, openmc.stats.Box]:
+        """Create a geometry representing a stochastic "sandwich" of materials in a
+        layered slab geometry. To reduce the impact of the order of materials in
+        the slab, the materials are applied to 'num_repeats' different randomly
+        positioned layers of 'cell_thickness' each.
+
+        Parameters
+        ----------
+        materials : list of openmc.Material
+            List of materials to assign. Each material will appear exactly num_repeats times,
+            then the ordering is randomly shuffled.
+        cell_thickness : float, optional
+            Thickness of each lattice cell in x (default 1.0 cm).
+        num_repeats : int, optional
+            Number of repeats for each material (default 100).
+
+        Returns
+        -------
+        geometry : openmc.Geometry
+            The constructed geometry.
+        box : openmc.stats.Box
+            A spatial sampling distribution covering the full slab domain.
+        """
+        if not materials:
+            raise ValueError("At least one material must be provided.")
+
+        num_materials = len(materials)
+        total_cells = num_materials * num_repeats
+        total_width = total_cells * cell_thickness
+
+        # Generate an infinite cell/universe for each material
+        universes = []
+        for i in range(num_materials):
+            cell = openmc.Cell(fill=materials[i])
+            universes.append(openmc.Universe(cells=[cell]))
+
+        # Make a list of randomized material idx assignments for the stochastic slab
+        assignments = list(range(num_materials)) * num_repeats
+        random.seed(42)
+        random.shuffle(assignments)
+
+        # Create a list of the (randomized) universe assignments to be used
+        # when defining the problem lattice.
+        lattice_entries = [universes[m] for m in assignments]
+
+        # Create the RectLattice for the 1D material variation in x.
+        lattice = openmc.RectLattice()
+        lattice.pitch = (cell_thickness, total_width, total_width)
+        lattice.lower_left = (0.0, 0.0, 0.0)
+        lattice.universes = [[lattice_entries]]
+        lattice.outer = universes[0]
+
+        # Define the six outer surfaces with reflective boundary conditions
+        rpp = openmc.model.RectangularParallelepiped(
+            0.0, total_width, 0.0, total_width, 0.0, total_width,
+            boundary_type='reflective'
+        )
+
+        # Create an outer cell that fills with the lattice.
+        outer_cell = openmc.Cell(fill=lattice, region=-rpp)
+
+        # Build the geometry
+        geometry = openmc.Geometry([outer_cell])
+
+        # Define the spatial distribution that covers the full cubic domain
+        box = openmc.stats.Box(*outer_cell.bounding_box)
+
+        return geometry, box
+
+    @staticmethod
+    def _isothermal_stochastic_slab_mgxs(
+        stoch_geom: openmc.Geometry,
+        groups: openmc.mgxs.EnergyGroups,
+        nparticles: int,
+        correction: str | None,
+        directory: PathLike,
+        source: openmc.IndependentSource,
+        temperature_settings: dict,
+        temperature: float | None = None,
+    ) -> dict[str, openmc.XSdata]:
+        """Generate MGXS assuming a stochastic "sandwich" of materials in a layered
+        slab geometry. If a temperature is specified, all materials in the slab have
+        their temperatures set to be isothermal at this temperature.
+
+        Parameters
+        ----------
+        stoch_geom : openmc.Geometry
+            The stochastic slab geometry.
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : str
+            Directory to run the simulation in, so as to contain XML files.
+        source : openmc.IndependentSource
+            Source to use when generating MGXS.
+        temperature_settings : dict
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        temperature : float, optional
+            The isothermal temperature value to apply to the materials in the
+            slab. If not specified, defaults to the temperature in the materials.
+
+        Returns
+        -------
+        data : dict[str, openmc.XSdata]
+            A dictionary where the key is the name of the material and the value is the isothermal MGXS.
+        """
+
+        model = openmc.Model()
+        model.geometry = stoch_geom
+
+        if temperature is not None:
+            for material in model.geometry.get_all_materials().values():
+                material.temperature = temperature
+
+        # Settings
+        model.settings.batches = 200
+        model.settings.inactive = 100
+        model.settings.particles = nparticles
+        model.settings.output = {'summary': True, 'tallies': False}
+        model.settings.temperature = temperature_settings
+
+        # Define the sources
+        model.settings.source = source
+
+        model.settings.run_mode = 'fixed source'
+        model.settings.create_fission_neutrons = False
+
+        model.settings.output = {'summary': True, 'tallies': False}
+
+        # Generate MGXS
+        mgxs_lib = Model._auto_generate_mgxs_lib(
+                model, groups, correction, directory)
+
+        # Fetch all of the isothermal results.
+        if temperature is not None:
+            return {
+                mat.name : mgxs_lib.get_xsdata(domain=mat, xsdata_name=mat.name,
+                                               temperature=temperature)
+                    for mat in mgxs_lib.domains
+            }
+        else:
+            return {
+                mat.name : mgxs_lib.get_xsdata(domain=mat, xsdata_name=mat.name)
+                    for mat in mgxs_lib.domains
+            }
+
+    def _generate_stochastic_slab_mgxs(
+        self,
+        groups: openmc.mgxs.EnergyGroups,
+        nparticles: int,
+        mgxs_path: PathLike,
+        correction: str | None,
+        directory: PathLike,
+        source_energy: openmc.stats.Univariate | None = None,
+        temperatures: Sequence[float] | None = None,
+        temperature_settings: dict | None = None,
+    ) -> None:
+        """Generate MGXS assuming a stochastic "sandwich" of materials in a layered
+        slab geometry. While geometry-specific spatial shielding effects are not
+        captured, this method can be useful when the geometry has materials only
+        found far from the source region that the "material_wise" method would
+        not be capable of generating cross sections for. Conversely, this method
+        will generate cross sections for all materials in the problem regardless
+        of type. If this is a fixed source problem, a discrete source is used to
+        sample particles, with an equal strength spread across each of the
+        energy groups. If temperature data points are provided,
+        isothermal cross sections are generated at each temperature point for
+        the stochastic slab to build a temperature interpolation table.
+
+        Parameters
+        ----------
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        mgxs_path : str
+            Filename for the MGXS HDF5 file.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : str
+            Directory to run the simulation in, so as to contain XML files.
+        source_energy : openmc.stats.Univariate, optional
+            Energy distribution to use when generating MGXS data, replacing any
+            existing sources in the model. In all cases, a discrete source that
+            is uniform over all energy groups is created (strength = 0.01) to
+            ensure that total cross sections are generated for all energy
+            groups. In the case that the user has provided a source_energy
+            distribution as an argument, an additional source (strength = 0.99)
+            is created using that energy distribution. If the user has not
+            provided a source_energy distribution, but the model has sources
+            defined, and all of those sources are of IndependentSource type,
+            then additional sources are created based on the model's existing
+            sources, keeping their energy distributions but replacing their
+            spatial/angular distributions, with their combined strength being
+            0.99. If the user has not provided a source_energy distribution and
+            no sources are defined on the model and the run mode is
+            'eigenvalue', then a default Watt spectrum source (strength = 0.99)
+            is added.
+        temperatures : Sequence[float], optional
+            A list of temperatures to generate MGXS at. Each infinite material region
+            is isothermal at a given temperature data point for cross
+            section generation.
+        temperature_settings : dict, optional
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        """
+
+        # Stochastic slab geometry
+        geo, spatial_distribution = Model._create_stochastic_slab_geometry(
+            self.materials)
+
+        src = self._create_mgxs_sources(
+            groups,
+            spatial_dist=spatial_distribution,
+            source_energy=source_energy
+        )
+
+        temp_settings = {}
+        if temperature_settings is None:
+            temp_settings = self.settings.temperature
+        else:
+            temp_settings = temperature_settings
+
+        if temperatures is None:
+            mgxs_sets = Model._isothermal_stochastic_slab_mgxs(
+                geo,
+                groups,
+                nparticles,
+                correction,
+                directory,
+                src,
+                temp_settings
+            ).values()
+
+            # Write the file to disk.
+            mgxs_file = openmc.MGXSLibrary(energy_groups=groups)
+            for mgxs_set in mgxs_sets:
+                mgxs_file.add_xsdata(mgxs_set)
+            mgxs_file.export_to_hdf5(mgxs_path)
+        else:
+            # Build a series of XSData objects, one for each isothermal temperature value.
+            raw_mgxs_sets = {}
+            for temperature in temperatures:
+                raw_mgxs_sets[temperature] = Model._isothermal_stochastic_slab_mgxs(
+                    geo,
+                    groups,
+                    nparticles,
+                    correction,
+                    directory,
+                    src,
+                    temp_settings,
+                    temperature
+                )
+
+            # Unpack the isothermal XSData objects and build a single XSData object per material.
+            mgxs_sets = []
+            for mat in self.materials:
+                mgxs_sets.append(openmc.XSdata(mat.name, groups, temperatures=temperatures))
+                mgxs_sets[-1].order = 0
+                for temperature in temperatures:
+                    mgxs_sets[-1].add_temperature_data(raw_mgxs_sets[temperature][mat.name])
+
+            # Write the file to disk.
+            mgxs_file = openmc.MGXSLibrary(energy_groups=groups)
+            for mgxs_set in mgxs_sets:
+                mgxs_file.add_xsdata(mgxs_set)
+            mgxs_file.export_to_hdf5(mgxs_path)
+
+    @staticmethod
+    def _isothermal_materialwise_mgxs(
+        input_model: openmc.Model,
+        groups: openmc.mgxs.EnergyGroups,
+        nparticles: int,
+        correction: str | None,
+        directory: PathLike,
+        temperature_settings: dict,
+        temperature: float | None = None,
+    ) -> dict[str, openmc.XSdata]:
+        """Generate a material-wise MGXS library for the model by running the
+        original continuous energy OpenMC simulation. If a temperature is
+        specified, each material in the input model is set to that temperature.
+        Otherwise, the original material temperatures are used. If temperature
+        data points are provided, isothermal cross sections are generated at
+        each temperature point for the whole model to build a temperature
+        interpolation table.
+
+        Parameters
+        ----------
+        input_model : openmc.Model
+            The model to use when computing material-wise MGXS.
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : str
+            Directory to run the simulation in, so as to contain XML files.
+        temperature_settings : dict
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        temperature : float, optional
+            The isothermal temperature value to apply to the materials in the
+            input model. If not specified, defaults to the temperatures in the
+            materials.
+
+        Returns
+        -------
+        data : dict[str, openmc.XSdata]
+            A dictionary where the key is the name of the material and the value is the isothermal MGXS.
+        """
+        model = copy.deepcopy(input_model)
+        model.tallies = openmc.Tallies()
+
+        if temperature is not None:
+            for material in model.geometry.get_all_materials().values():
+                material.temperature = temperature
+
+        # Settings
+        model.settings.batches = 200
+        model.settings.inactive = 100
+        model.settings.particles = nparticles
+        model.settings.output = {'summary': True, 'tallies': False}
+        model.settings.temperature = temperature_settings
+
+        # Generate MGXS
+        mgxs_lib = Model._auto_generate_mgxs_lib(
+                model, groups, correction, directory)
+
+        # Fetch all of the isothermal results.
+        if temperature is not None:
+            return {
+                mat.name : mgxs_lib.get_xsdata(domain=mat, xsdata_name=mat.name,
+                                               temperature=temperature)
+                    for mat in mgxs_lib.domains
+            }
+        else:
+            return {
+                mat.name : mgxs_lib.get_xsdata(domain=mat, xsdata_name=mat.name)
+                    for mat in mgxs_lib.domains
+            }
+
+    def _generate_material_wise_mgxs(
+        self,
+        groups: openmc.mgxs.EnergyGroups,
+        nparticles: int,
+        mgxs_path: PathLike,
+        correction: str | None,
+        directory: PathLike,
+        temperatures: Sequence[float] | None = None,
+        temperature_settings: dict | None = None,
+    ) -> None:
+        """Generate a material-wise MGXS library for the model by running the
+        original continuous energy OpenMC simulation of the full material
+        geometry and source, and tally MGXS data for each material. This method
+        accurately conserves reaction rates totaled over the entire simulation
+        domain. However, when the geometry has materials only found far from the
+        source region, it is possible the Monte Carlo solver may not be able to
+        score any tallies to these material types, thus resulting in zero cross
+        section values for these materials. For such cases, the "stochastic
+        slab" method may be more appropriate.
+
+        Parameters
+        ----------
+        groups : openmc.mgxs.EnergyGroups
+            Energy group structure for the MGXS.
+        nparticles : int
+            Number of particles to simulate per batch when generating MGXS.
+        mgxs_path : PathLike
+            Filename for the MGXS HDF5 file.
+        correction : str
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        directory : PathLike
+            Directory to run the simulation in, so as to contain XML files.
+        temperatures : Sequence[float], optional
+            A list of temperatures to generate MGXS at. Each infinite material region
+            is isothermal at a given temperature data point for cross
+            section generation.
+        temperature_settings : dict, optional
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        """
+        temp_settings = {}
+        if temperature_settings is None:
+            temp_settings = self.settings.temperature
+        else:
+            temp_settings = temperature_settings
+
+        if temperatures is None:
+            mgxs_sets = Model._isothermal_materialwise_mgxs(
+                self,
+                groups,
+                nparticles,
+                correction,
+                directory,
+                temp_settings
+            ).values()
+
+            # Write the file to disk.
+            mgxs_file = openmc.MGXSLibrary(energy_groups=groups)
+            for mgxs_set in mgxs_sets:
+                mgxs_file.add_xsdata(mgxs_set)
+            mgxs_file.export_to_hdf5(mgxs_path)
+        else:
+            # Build a series of XSData objects, one for each isothermal temperature value.
+            raw_mgxs_sets = {}
+            for temperature in temperatures:
+                raw_mgxs_sets[temperature] = Model._isothermal_materialwise_mgxs(
+                    self,
+                    groups,
+                    nparticles,
+                    correction,
+                    directory,
+                    temp_settings,
+                    temperature
+                )
+
+            # Unpack the isothermal XSData objects and build a single XSData object per material.
+            mgxs_sets = []
+            for mat in self.materials:
+                mgxs_sets.append(openmc.XSdata(mat.name, groups, temperatures=temperatures))
+                mgxs_sets[-1].order = 0
+                for temperature in temperatures:
+                    mgxs_sets[-1].add_temperature_data(raw_mgxs_sets[temperature][mat.name])
+
+            # Write the file to disk.
+            mgxs_file = openmc.MGXSLibrary(energy_groups=groups)
+            for mgxs_set in mgxs_sets:
+                mgxs_file.add_xsdata(mgxs_set)
+            mgxs_file.export_to_hdf5(mgxs_path)
+
+    def convert_to_multigroup(
+        self,
+        method: str = "material_wise",
+        groups: str | Sequence[float] | openmc.mgxs.EnergyGroups = "CASMO-2",
+        nparticles: int = 2000,
+        overwrite_mgxs_library: bool = False,
+        mgxs_path: PathLike = "mgxs.h5",
+        correction: str | None = None,
+        source_energy: openmc.stats.Univariate | None = None,
+        temperatures: Sequence[float] | None = None,
+        temperature_settings: dict | None = None,
+    ):
+        """Convert all materials from continuous energy to multigroup.
+
+        If no MGXS data library file is found, generate one using one or more
+        continuous energy Monte Carlo simulations.
+
+        Parameters
+        ----------
+        method : {"material_wise", "stochastic_slab", "infinite_medium"}, optional
+            Method to generate the MGXS.
+        groups : openmc.mgxs.EnergyGroups, str, or sequence of float, optional
+            Energy group structure for the MGXS. Can be an
+            :class:`openmc.mgxs.EnergyGroups` object, a string name of a
+            predefined group structure from :data:`openmc.mgxs.GROUP_STRUCTURES`
+            (e.g., ``"CASMO-2"``), or a sequence of floats specifying energy
+            bin boundaries in eV (e.g., ``[0.0, 1e6]`` for a single group).
+            Defaults to ``"CASMO-2"``.
+        nparticles : int, optional
+            Number of particles to simulate per batch when generating MGXS.
+        overwrite_mgxs_library : bool, optional
+            Whether to overwrite an existing MGXS library file.
+        mgxs_path : str, optional
+            Path to the mgxs.h5 library file.
+        correction : str, optional
+            Transport correction to apply to the MGXS. Options are None and
+            "P0".
+        source_energy : openmc.stats.Univariate, optional
+            Energy distribution to use when generating MGXS data, replacing any
+            existing sources in the model. In all cases, a discrete source that
+            is uniform over all energy groups is created (strength = 0.01) to
+            ensure that total cross sections are generated for all energy
+            groups. In the case that the user has provided a source_energy
+            distribution as an argument, an additional source (strength = 0.99)
+            is created using that energy distribution. If the user has not
+            provided a source_energy distribution, but the model has sources
+            defined, and all of those sources are of IndependentSource type,
+            then additional sources are created based on the model's existing
+            sources, keeping their energy distributions but replacing their
+            spatial/angular distributions, with their combined strength being
+            0.99. If the user has not provided a source_energy distribution and
+            no sources are defined on the model and the run mode is
+            'eigenvalue', then a default Watt spectrum source (strength = 0.99)
+            is added. Note that this argument is only used when using the
+            "stochastic_slab" or "infinite_medium" MGXS generation methods.
+        temperatures : Sequence[float], optional
+            A list of temperatures to generate MGXS at. Each infinite material region
+            is isothermal at a given temperature data point for cross
+            section generation.
+        temperature_settings : dict, optional
+            A dictionary of temperature settings to use when generating MGXS.
+            Valid entries for temperature_settings are the same as the valid
+            entries in openmc.Settings.temperature_settings.
+        """
+        if not isinstance(groups, openmc.mgxs.EnergyGroups):
+            groups = openmc.mgxs.EnergyGroups(groups)
+
+        # Do all work (including MGXS generation) in a temporary directory
+        # to avoid polluting the working directory with residual XML files
+        with TemporaryDirectory() as tmpdir:
+
+            # Determine if there are DAGMC universes in the model. If so, we need to synchronize
+            # the dagmc materials with cells.
+            # TODO: Can this be done without having to init/finalize?
+            for univ in self.geometry.get_all_universes().values():
+                if isinstance(univ, openmc.DAGMCUniverse):
+                    # Initialize in stochastic volume mode (non-transport mode)
+                    # This mode doesn't require
+                    # valid transport settings like particles/batches
+                    original_run_mode = self.settings.run_mode
+                    self.settings.run_mode = 'volume'
+                    self.init_lib(directory=tmpdir)
+                    self.sync_dagmc_universes()
+                    self.finalize_lib()
+                    # Restore original run mode
+                    self.settings.run_mode = original_run_mode
+                    break
+
+            # Temporarily replace each material's name with a unique, valid HDF5
+            # dataset name (its name plus ID) for use as its MGXS library entry
+            # and macroscopic. The ID keeps the name unique even when materials
+            # share a name; the original names are restored at the end.
+            original_names = [material.name for material in self.materials]
+            for material in self.materials:
+                base = material.name if material.name and material.name.strip() \
+                    else "material"
+                material.name = re.sub(r'[^a-zA-Z0-9]', '_', base) + f"_{material.id}"
+
+            # If needed, generate the needed MGXS data library file
+            if not Path(mgxs_path).is_file() or overwrite_mgxs_library:
+                if method == "infinite_medium":
+                    self._generate_infinite_medium_mgxs(
+                        groups, nparticles, mgxs_path, correction, tmpdir, source_energy,
+                        temperatures, temperature_settings)
+                elif method == "material_wise":
+                    self._generate_material_wise_mgxs(
+                        groups, nparticles, mgxs_path, correction, tmpdir,
+                        temperatures, temperature_settings)
+                elif method == "stochastic_slab":
+                    self._generate_stochastic_slab_mgxs(
+                        groups, nparticles, mgxs_path, correction, tmpdir, source_energy,
+                        temperatures, temperature_settings)
+                else:
+                    raise ValueError(
+                        f'MGXS generation method "{method}" not recognized')
+            else:
+                print(f'Existing MGXS library file "{mgxs_path}" will be used')
+
+            # Convert all continuous energy materials to multigroup
+            self.materials.cross_sections = mgxs_path
+            for material in self.materials:
+                material.set_density('macro', 1.0)
+                material._nuclides = []
+                material._sab = []
+                material.add_macroscopic(material.name)
+
+            self.settings.energy_mode = 'multi-group'
+
+            # Restore the user's original material names.
+            for material, name in zip(self.materials, original_names):
+                material.name = name
+
+    def convert_to_random_ray(self):
+        """Convert a multigroup model to use random ray.
+
+        This method determines values for the needed settings and adds them to
+        the settings.random_ray dictionary so as to enable random ray mode. The
+        settings that are populated are:
+
+        - 'ray_source' (openmc.IndependentSource): Where random ray starting
+          points are sampled from.
+        - 'distance_inactive' (float): The "dead zone" distance at the beginning
+          of the ray.
+        - 'distance_active' (float): The "active" distance of the ray
+        - 'particles' (int): Number of rays to simulate
+
+        The method will determine reasonable defaults for each of the above
+        variables based on analysis of the model's geometry. The function will
+        have no effect if the random ray dictionary is already defined in the
+        model settings.
+        """
+        # If the random ray dictionary is already set, don't overwrite it
+        if self.settings.random_ray:
+            warnings.warn("Random ray conversion skipped as "
+                          "settings.random_ray dictionary is already set.")
+            return
+
+        if self.settings.energy_mode != 'multi-group':
+            raise ValueError(
+                "Random ray conversion failed: energy mode must be "
+                "'multi-group'. Use convert_to_multigroup() first."
+            )
+
+        # Helper function for detecting infinity
+        def _replace_infinity(value):
+            if np.isinf(value):
+                return 1.0 if value > 0 else -1.0
+            return value
+
+        # Get a bounding box for sampling rays. We can utilize the geometry's bounding box
+        # though for 2D problems we need to detect the infinities and replace them with an
+        # arbitrary finite value.
+        bounding_box = self.geometry.bounding_box
+        lower_left = [_replace_infinity(v) for v in bounding_box.lower_left]
+        upper_right = [_replace_infinity(v) for v in bounding_box.upper_right]
+        uniform_dist_ray = openmc.stats.Box(lower_left, upper_right)
+        rr_source = openmc.IndependentSource(space=uniform_dist_ray)
+        self.settings.random_ray['ray_source'] = rr_source
+
+        # For the dead zone and active length, a reasonable guess is the larger of either:
+        # 1) The maximum chord length through the geometry (as defined by its bounding box)
+        # 2) 30 cm
+        # Then, set the active length to be 5x longer than the dead zone length, for the sake of efficiency.
+        chord_length = np.array(upper_right) - np.array(lower_left)
+        max_length = max(np.linalg.norm(chord_length), 30.0)
+
+        self.settings.random_ray['distance_inactive'] = max_length
+        self.settings.random_ray['distance_active'] = 5 * max_length
+
+        # Take a wild guess as to how many rays are needed
+        self.settings.particles = 2 * int(max_length)
+
+    def keff_search(
+        self,
+        func: ModelModifier,
+        x0: float,
+        x1: float,
+        target: float = 1.0,
+        k_tol: float = 1e-4,
+        sigma_final: float = 3e-4,
+        p: float = 0.5,
+        q: float = 0.95,
+        memory: int = 4,
+        x_min: float | None = None,
+        x_max: float | None = None,
+        b0: int | None = None,
+        b_min: int = 20,
+        b_max: int | None = None,
+        maxiter: int = 50,
+        output: bool = False,
+        func_kwargs: dict[str, Any] | None = None,
+        run_kwargs: dict[str, Any] | None = None,
+    ) -> SearchResult:
+        r"""Perform a keff search on a model parametrized by a single variable.
+
+        This method uses the GRsecant method described in a paper by `Price and
+        Roskoff <https://doi.org/10.1016/j.pnucene.2023.104731>`_. The GRsecant
+        method is a modification of the secant method that accounts for
+        uncertainties in the function evaluations. The method uses a weighted
+        linear fit of the most recent function evaluations to predict the next
+        point to evaluate. It also adaptively changes the number of batches to
+        meet the target uncertainty value at each iteration.
+
+        The target uncertainty for iteration :math:`n+1` is determined by the
+        following equation (following Eq. (8) in the paper):
+
+        .. math::
+            \sigma_{i+1} = q \sigma_\text{final} \left ( \frac{ \min \left \{
+            \left\lvert k_i - k_\text{target} \right\rvert : k=0,1,\dots,n
+            \right \} }{k_\text{tol}} \right )^p
+
+        where :math:`q` is a multiplicative factor less than 1, given as the
+        ``sigma_factor`` parameter below.
+
+        Parameters
+        ----------
+        func : ModelModifier
+            Function that takes the parameter to be searched and makes a
+            modification to the model.
+        x0 : float
+            First guess for the parameter passed to `func`
+        x1 : float
+            Second guess for the parameter passed to `func`
+        target : float, optional
+            keff value to search for
+        k_tol : float, optional
+            Stopping criterion on the function value; the absolute value must be
+            within ``k_tol`` of zero to be accepted.
+        sigma_final : float, optional
+            Maximum accepted k-effective uncertainty for the stopping criterion.
+        p : float, optional
+            Exponent used in the stopping criterion.
+        q : float, optional
+            Multiplicative factor used in the stopping criterion.
+        memory : int, optional
+            Number of most-recent points used in the weighted linear fit of
+            ``f(x) = a + b x`` to predict the next point.
+        x_min : float, optional
+            Minimum allowed value for the parameter ``x``.
+        x_max : float, optional
+            Maximum allowed value for the parameter ``x``.
+        b0 : int, optional
+            Number of active batches to use for the initial function
+            evaluations. If None, uses the model's current setting.
+        b_min : int, optional
+            Minimum number of active batches to use in a function evaluation.
+        b_max : int, optional
+            Maximum number of active batches to use in a function evaluation.
+        maxiter : int, optional
+            Maximum number of iterations to perform.
+        output : bool, optional
+            Whether or not to display output showing iteration progress.
+        func_kwargs : dict, optional
+            Keyword-based arguments to pass to the `func` function.
+        run_kwargs : dict, optional
+            Keyword arguments to pass to :meth:`openmc.Model.run` or
+            :meth:`openmc.lib.run`.
+
+        Returns
+        -------
+        SearchResult
+            Result object containing the estimated root (parameter value) and
+            evaluation history (parameters, means, standard deviations, and
+            batches), plus convergence status and termination reason.
+
+        """
+        import openmc.lib
+
+        check_type('model modifier', func, Callable)
+        check_type('target', target, Real)
+        if memory < 2:
+            raise ValueError("memory must be ≥ 2")
+        func_kwargs = {} if func_kwargs is None else dict(func_kwargs)
+        run_kwargs = {} if run_kwargs is None else dict(run_kwargs)
+        run_kwargs.setdefault('output', False)
+
+        # Create lists to store the history of evaluations
+        xs: list[float] = []
+        fs: list[float] = []
+        ss: list[float] = []
+        gs: list[int] = []
+        count = 0
+
+        # Helper function to evaluate f and store results
+        def eval_at(x: float, batches: int) -> tuple[float, float]:
+            # Modify the model with the current guess
+            func(x, **func_kwargs)
+
+            # Change the number of batches and run the model
+            batches += self.settings.inactive
+            if openmc.lib.is_initialized:
+                openmc.lib.settings.set_batches(batches)
+                openmc.lib.reset()
+                openmc.lib.run(**run_kwargs)
+                sp_filepath = f'statepoint.{batches}.h5'
+            else:
+                self.settings.batches = batches
+                sp_filepath = self.run(**run_kwargs)
+
+            # Extract keff and its uncertainty
+            with openmc.StatePoint(sp_filepath) as sp:
+                keff = sp.keff
+
+            if output:
+                nonlocal count
+                count += 1
+                print(f'Iteration {count}: {batches=}, {x=:.6g}, {keff=:.5f}')
+
+            xs.append(float(x))
+            fs.append(float(keff.n - target))
+            ss.append(float(keff.s))
+            gs.append(int(batches))
+            return fs[-1], ss[-1]
+
+        # Default b0 to current model settings if not explicitly provided
+        if b0 is None:
+            b0 = self.settings.batches - self.settings.inactive
+
+        # Perform the search (inlined GRsecant) in a temporary directory
+        with TemporaryDirectory() as tmpdir:
+            if not openmc.lib.is_initialized:
+                run_kwargs.setdefault('cwd', tmpdir)
+
+            # ---- Seed with two evaluations
+            f0, s0 = eval_at(x0, b0)
+            if abs(f0) <= k_tol and s0 <= sigma_final:
+                return SearchResult(x0, xs, fs, ss, gs, True, "converged")
+            f1, s1 = eval_at(x1, b0)
+            if abs(f1) <= k_tol and s1 <= sigma_final:
+                return SearchResult(x1, xs, fs, ss, gs, True, "converged")
+
+            for _ in range(maxiter - 2):
+                # ------ Step 1: propose next x via GRsecant
+                m = min(memory, len(xs))
+
+                # Perform a curve fit on f(x) = a + bx accounting for
+                # uncertainties. This is equivalent to minimizing the function
+                # in Equation (A.14)
+                (a, b), _ = curve_fit(
+                    lambda x, a, b: a + b*x,
+                    xs[-m:], fs[-m:], sigma=ss[-m:], absolute_sigma=True
+                )
+                x_new = float(-a / b)
+
+                # Clamp x_new to the bounds if provided
+                if x_min is not None:
+                    x_new = max(x_new, x_min)
+                if x_max is not None:
+                    x_new = min(x_new, x_max)
+
+                # ------ Step 2: choose target σ for next run (Eq. 8 + clamp)
+
+                min_abs_f = float(np.min(np.abs(fs)))
+                base = q * sigma_final
+                ratio = min_abs_f / k_tol if k_tol > 0 else 1.0
+                sig = base * (ratio ** p)
+                sig_target = max(sig, base)
+
+                # ------ Step 3: choose generations to hit σ_target (Appendix C)
+
+                # Use at least two past points for regression
+                if len(gs) >= 2 and np.var(np.log(gs)) > 0.0:
+                    # Perform a curve fit based on Eq. (C.3) to solve for ln(k).
+                    # Note that unlike in the paper, we do not leave r as an
+                    # undetermined parameter and choose r=0.5.
+                    (ln_k,), _ = curve_fit(
+                        lambda ln_b, ln_k: ln_k - 0.5*ln_b,
+                        np.log(gs[-4:]), np.log(ss[-4:]),
+                    )
+                    k = float(np.exp(ln_k))
+                else:
+                    k = float(ss[-1] * math.sqrt(gs[-1]))
+
+                b_new = (k / sig_target) ** 2
+
+                # Clamp and round up to integer
+                b_new = max(b_min, math.ceil(b_new))
+                if b_max is not None:
+                    b_new = min(b_new, b_max)
+
+                # Evaluate at proposed x with batches determined above
+                f_new, s_new = eval_at(x_new, b_new)
+
+                # Termination based on both criteria (|f| and σ)
+                if abs(f_new) <= k_tol and s_new <= sigma_final:
+                    return SearchResult(x_new, xs, fs, ss, gs, True, "converged")
+
+            return SearchResult(xs[-1], xs, fs, ss, gs, False, "maxiter")
+
+
+@dataclass
+class SearchResult:
+    """Result of a GRsecant keff search.
+
+    Attributes
+    ----------
+    root : float
+        Estimated parameter value where f(x) = 0 at termination.
+    parameters : list[float]
+        Parameter values (x) evaluated during the search, in order.
+    keffs : list[float]
+        Estimated keff values for each evaluation.
+    stdevs : list[float]
+        One-sigma uncertainties of keff for each evaluation.
+    batches : list[int]
+        Number of active batches used for each evaluation.
+    converged : bool
+        Whether both |f| <= k_tol and sigma <= sigma_final were met.
+    flag : str
+        Reason for termination (e.g., "converged", "maxiter").
+    """
+    root: float
+    parameters: list[float] = field(repr=False)
+    means: list[float] = field(repr=False)
+    stdevs: list[float] = field(repr=False)
+    batches: list[int] = field(repr=False)
+    converged: bool
+    flag: str
+
+    @property
+    def function_calls(self) -> int:
+        """Number of function evaluations performed."""
+        return len(self.parameters)
+
+    @property
+    def total_batches(self) -> int:
+        """Total number of active batches used across all evaluations."""
+        return sum(self.batches)
diff --git a/openmc/particle_restart.py b/openmc/particle_restart.py
index 317d8761b..44864107d 100644
--- a/openmc/particle_restart.py
+++ b/openmc/particle_restart.py
@@ -1,6 +1,7 @@
 import h5py
 
 import openmc.checkvalue as cv
+from .particle_type import ParticleType
 
 _VERSION_PARTICLE_RESTART = 2
 
@@ -28,8 +29,8 @@ class Particle:
         Type of simulation (criticality or fixed source)
     id : long
         Identifier of the particle
-    type : int
-        Particle type (1 = neutron, 2 = photon, 3 = electron, 4 = positron)
+    type : openmc.ParticleType
+        Particle type
     weight : float
         Weight of the particle
     energy : float
@@ -52,7 +53,7 @@ class Particle:
             self.energy = f['energy'][()]
             self.generations_per_batch = f['generations_per_batch'][()]
             self.id = f['id'][()]
-            self.type = f['type'][()]
+            self.type = ParticleType(f['type'][()])
             self.n_particles = f['n_particles'][()]
             self.run_mode = f['run_mode'][()].decode()
             self.uvw = f['uvw'][()]
diff --git a/openmc/particle_type.py b/openmc/particle_type.py
new file mode 100644
index 000000000..9d7713443
--- /dev/null
+++ b/openmc/particle_type.py
@@ -0,0 +1,228 @@
+from numbers import Integral
+
+from openmc.data import gnds_name, zam, ATOMIC_SYMBOL
+
+
+_PDG_NAME = {
+    2112: 'neutron',
+    22: 'photon',
+    11: 'electron',
+    -11: 'positron',
+    2212: 'H1',
+}
+
+_ALIAS_PDG = {
+    'neutron': 2112,
+    'n': 2112,
+    'photon': 22,
+    'gamma': 22,
+    'electron': 11,
+    'positron': -11,
+    'proton': 2212,
+    'p': 2212,
+    'h1': 2212,
+    'deuteron': 1000010020,
+    'd': 1000010020,
+    'h2': 1000010020,
+    'triton': 1000010030,
+    't': 1000010030,
+    'h3': 1000010030,
+    'alpha': 1000020040,
+    'he4': 1000020040,
+}
+
+_LEGACY_PARTICLE_INDEX = {
+    0: 2112,
+    1: 22,
+    2: 11,
+    3: -11,
+}
+
+
+class ParticleType:
+    """Particle type defined by a PDG number.
+
+    ParticleType uses the Particle Data Group (PDG) Monte Carlo numbering scheme
+    to uniquely identify particle types. This includes elementary particles
+    (neutrons, photons, etc.) and nuclear codes for isotopes.
+
+    Parameters
+    ----------
+    value : str, int, or ParticleType
+        The particle identifier. Can be:
+
+        - A string name (e.g., 'neutron', 'photon', 'He4', 'U235')
+        - An integer PDG number (e.g., 2112 for neutron)
+        - A string with PDG prefix (e.g., 'pdg:2112')
+        - An existing ParticleType instance
+
+    Attributes
+    ----------
+    pdg_number : int
+        The PDG number for this particle type
+    zam : tuple of int or None
+        For nuclear particles, the (Z, A, m) tuple where Z is atomic number,
+        A is mass number, and m is metastable state. None for elementary particles.
+    is_nucleus : bool
+        Whether this particle is a nucleus (ion)
+
+    Examples
+    --------
+    >>> neutron = ParticleType('neutron')
+    >>> neutron.pdg_number
+    2112
+    >>> he4 = ParticleType('He4')
+    >>> he4.zam
+    (2, 4, 0)
+    >>> ParticleType(2112) == ParticleType('neutron')
+    True
+
+    """
+
+    __slots__ = ('_pdg_number',)
+
+    def __init__(self, value: 'str | int | ParticleType'):
+        if isinstance(value, ParticleType):
+            pdg = value._pdg_number
+        elif isinstance(value, str):
+            pdg = self._pdg_number_from_string(value)
+        elif isinstance(value, Integral):
+            pdg = int(value)
+            # Handle legacy particle indices (0, 1, 2, 3)
+            if pdg in _LEGACY_PARTICLE_INDEX:
+                pdg = _LEGACY_PARTICLE_INDEX[pdg]
+        else:
+            raise TypeError(f"Cannot create ParticleType from {type(value).__name__}")
+
+        self._pdg_number = pdg
+
+    def __eq__(self, other):
+        if isinstance(other, ParticleType):
+            return self._pdg_number == other._pdg_number
+        if isinstance(other, Integral):
+            return self._pdg_number == int(other)
+        if isinstance(other, str):
+            try:
+                return self._pdg_number == ParticleType(other)._pdg_number
+            except (ValueError, TypeError):
+                return False
+        return NotImplemented
+
+    def __hash__(self) -> int:
+        return hash(self._pdg_number)
+
+    def __int__(self) -> int:
+        return self._pdg_number
+
+    @property
+    def pdg_number(self) -> int:
+        return self._pdg_number
+
+    @staticmethod
+    def _pdg_number_from_string(value: str) -> int:
+        """Parse a string to get a PDG number.
+
+        Parameters
+        ----------
+        value : str
+            Particle identifier string
+
+        Returns
+        -------
+        int
+            PDG number
+
+        Raises
+        ------
+        ValueError
+            If string cannot be parsed as a valid particle identifier
+
+        """
+        s = value.strip()
+        if not s:
+            raise ValueError('Particle identifier cannot be empty.')
+
+        lower = s.lower()
+        if lower.startswith('pdg:'):
+            code_str = lower[4:]
+            try:
+                return int(code_str)
+            except ValueError:
+                raise ValueError(f'Invalid PDG number: {code_str}')
+
+        if lower in _ALIAS_PDG:
+            return _ALIAS_PDG[lower]
+
+        # Assume it is a GNDS nuclide name
+        Z, A, m = zam(s)
+        if Z <= 0 or Z > 999 or A <= 0 or A > 999 or m < 0 or m > 9:
+            raise ValueError('Invalid Z/A/m for nuclear PDG number.')
+        return 1000000000 + Z * 10000 + A * 10 + m
+
+    def __repr__(self) -> str:
+        return f'<ParticleType: {str(self)} (PDG={self._pdg_number})>'
+
+    def __str__(self) -> str:
+        """Return a canonical string representation of the particle type.
+
+        Returns
+        -------
+        str
+            Canonical name (e.g., 'neutron', 'He4', 'pdg:12345')
+
+        """
+        if self._pdg_number in _PDG_NAME:
+            return _PDG_NAME[self._pdg_number]
+
+        if (zam_tuple := self.zam) is not None:
+            Z, A, m = zam_tuple
+            if Z <= 0 or Z > max(ATOMIC_SYMBOL) or A <= 0 or A > 999:
+                raise ValueError(f"Invalid nuclear PDG number: {self._pdg_number}")
+            return gnds_name(Z, A, m)
+
+        return f'pdg:{self._pdg_number}'
+
+    @property
+    def zam(self) -> 'tuple[int, int, int] | None':
+        """Return the (Z, A, m) tuple for nuclear particles.
+
+        Returns
+        -------
+        tuple of int or None
+            For nuclear particles, returns (Z, A, m) where Z is atomic number,
+            A is mass number, and m is metastable state. Returns None for
+            elementary particles.
+
+        """
+        if self._pdg_number < 1000000000:
+            return None
+        Z = (self._pdg_number // 10000) % 1000
+        A = (self._pdg_number // 10) % 1000
+        m = self._pdg_number % 10
+        if Z <= 0 or A <= 0:
+            return None
+        else:
+            return (Z, A, m)
+
+    @property
+    def is_nucleus(self) -> bool:
+        """Return whether this particle is a nucleus.
+
+        Returns
+        -------
+        bool
+            True if the particle is a nucleus (ion), False otherwise
+
+        """
+        return self.zam is not None
+
+
+# Define common particle constants
+ParticleType.NEUTRON = ParticleType(2112)
+ParticleType.PHOTON = ParticleType(22)
+ParticleType.ELECTRON = ParticleType(11)
+ParticleType.POSITRON = ParticleType(-11)
+ParticleType.PROTON = ParticleType(2212)
+ParticleType.DEUTERON = ParticleType(1000010020)
+ParticleType.TRITON = ParticleType(1000010030)
+ParticleType.ALPHA = ParticleType(1000020040)
diff --git a/openmc/plots.py b/openmc/plots.py
index 40fcf0c78..531184095 100644
--- a/openmc/plots.py
+++ b/openmc/plots.py
@@ -1,6 +1,8 @@
-from collections.abc import Iterable, Mapping
+from collections.abc import Iterable, Mapping, Sequence
 from numbers import Integral, Real
 from pathlib import Path
+from textwrap import dedent
+import warnings
 
 import h5py
 import lxml.etree as ET
@@ -10,11 +12,13 @@ import openmc
 import openmc.checkvalue as cv
 from openmc.checkvalue import PathLike
 
-from ._xml import clean_indentation, get_elem_tuple, reorder_attributes, get_text
+from ._xml import clean_indentation, get_elem_list, get_text
 from .mixin import IDManagerMixin
 
 _BASES = {'xy', 'xz', 'yz'}
 
+_BASIS_INDICES = {'xy': (0, 1, 2), 'xz': (0, 2, 1), 'yz': (1, 2, 0)}
+
 _SVG_COLORS = {
     'aliceblue': (240, 248, 255),
     'antiquewhite': (250, 235, 215),
@@ -165,7 +169,8 @@ _SVG_COLORS = {
     'yellowgreen': (154, 205, 50)
 }
 
-_PLOT_PARAMS = """
+_PLOT_PARAMS = dedent("""\
+
         Parameters
         ----------
         origin : iterable of float
@@ -178,11 +183,12 @@ _PLOT_PARAMS = """
             ascertain the plot width. Defaults to (10, 10) if the bounding box
             contains inf values.
         pixels : Iterable of int or int
-            If iterable of ints provided then this directly sets the number of
-            pixels to use in each basis direction. If int provided then this
-            sets the total number of pixels in the plot and the number of
-            pixels in each basis direction is calculated from this total and
-            the image aspect ratio.
+            If an iterable of ints is provided then this directly sets the
+            number of pixels to use in each basis direction. If a single int
+            is provided then this sets the total number of pixels in the plot
+            and the number of pixels in each basis direction is calculated
+            from this total and the image aspect ratio based on the width
+            argument.
         basis : {'xy', 'xz', 'yz'}
             The basis directions for the plot
         color_by : {'cell', 'material'}
@@ -191,13 +197,13 @@ _PLOT_PARAMS = """
             Assigns colors to specific materials or cells. Keys are instances of
             :class:`Cell` or :class:`Material` and values are RGB 3-tuples, RGBA
             4-tuples, or strings indicating SVG color names. Red, green, blue,
-            and alpha should all be floats in the range [0.0, 1.0], for example:
+            and alpha should all be integers in the range [0, 255], for example:
 
             .. code-block:: python
 
                 # Make water blue
                 water = openmc.Cell(fill=h2o)
-                universe.plot(..., colors={water: (0., 0., 1.))
+                universe.plot(..., colors={water: (0, 0, 255))
         seed : int
             Seed for the random number generator
         openmc_exec : str
@@ -246,7 +252,7 @@ _PLOT_PARAMS = """
         -------
         matplotlib.axes.Axes
             Axes containing resulting image
-"""
+""")
 
 
 # Decorator for consistently adding plot parameters to docstrings (Model.plot,
@@ -349,6 +355,86 @@ def voxel_to_vtk(voxel_file: PathLike, output: PathLike = 'plot.vti'):
     return output
 
 
+def id_map_to_rgb(
+    id_map: np.ndarray,
+    color_by: str = 'cell',
+    colors: dict | None = None,
+    overlap_color: Sequence[int] | str = (255, 0, 0)
+) -> np.ndarray:
+    """Convert ID map array to RGB image array.
+
+    Parameters
+    ----------
+    id_map : numpy.ndarray
+        Array with shape (v_pixels, h_pixels, 3) containing cell IDs,
+        cell instances, and material IDs
+    color_by : {'cell', 'material'}
+        Whether to color by cell or material
+    colors : dict, optional
+        Dictionary mapping cells/materials to colors
+    overlap_color : sequence of int or str, optional
+        Color to use for overlaps. Defaults to red (255, 0, 0).
+
+    Returns
+    -------
+    numpy.ndarray
+        RGB image array with shape (v_pixels, h_pixels, 3) with values
+        in range [0, 1] for matplotlib
+    """
+    # Initialize RGB array with white background (values between 0 and 1 for matplotlib)
+    img = np.ones(id_map.shape, dtype=float)
+
+    # Get the appropriate index based on color_by
+    if color_by == 'cell':
+        id_index = 0  # Cell IDs are in the first channel
+    elif color_by == 'material':
+        id_index = 2  # Material IDs are in the third channel
+    else:
+        raise ValueError("color_by must be either 'cell' or 'material'")
+
+    # Get all unique IDs in the plot
+    unique_ids = np.unique(id_map[:, :, id_index])
+
+    # Generate default colors if not provided
+    if colors is None:
+        colors = {}
+
+    # Convert colors dict to use IDs as keys
+    color_map = {}
+    for key, color in colors.items():
+        if isinstance(key, (openmc.Cell, openmc.Material)):
+            color_map[key.id] = color
+        else:
+            color_map[key] = color
+
+    # Generate random colors for IDs not in color_map
+    rng = np.random.RandomState(1)
+    for uid in unique_ids:
+        if uid > 0 and uid not in color_map:
+            color_map[uid] = rng.randint(0, 256, (3,))
+
+    # Apply colors to each pixel
+    for uid in unique_ids:
+        if uid == -1:  # Background/void
+            continue
+        elif uid == -3:  # Overlap (only present if color_overlaps was True)
+            if isinstance(overlap_color, str):
+                rgb = _SVG_COLORS[overlap_color.lower()]
+            else:
+                rgb = overlap_color
+            mask = id_map[:, :, id_index] == uid
+            img[mask] = np.array(rgb) / 255.0
+        elif uid in color_map:
+            color = color_map[uid]
+            if isinstance(color, str):
+                rgb = _SVG_COLORS[color.lower()]
+            else:
+                rgb = color
+            mask = id_map[:, :, id_index] == uid
+            img[mask] = np.array(rgb) / 255.0
+
+    return img
+
 class PlotBase(IDManagerMixin):
     """
     Parameters
@@ -434,7 +520,7 @@ class PlotBase(IDManagerMixin):
 
     @filename.setter
     def filename(self, filename):
-        cv.check_type('filename', filename, str)
+        cv.check_type('filename', filename, (str, PathLike))
         self._filename = filename
 
     @property
@@ -621,14 +707,15 @@ class PlotBase(IDManagerMixin):
         return element
 
 
-class Plot(PlotBase):
-    """Definition of a finite region of space to be plotted.
+class SlicePlot(PlotBase):
+    """Definition of a 2D slice plot of the geometry.
 
-    OpenMC is capable of generating two-dimensional slice plots, or
-    three-dimensional voxel or projection plots. Colors that are used in plots can be given as
-    RGB tuples, e.g. (255, 255, 255) would be white, or by a string indicating a
+    Colors that are used in plots can be given as RGB tuples, e.g.
+    (255, 255, 255) would be white, or by a string indicating a
     valid `SVG color <https://www.w3.org/TR/SVG11/types.html#ColorKeywords>`_.
 
+    .. versionadded:: 0.15.4
+
     Parameters
     ----------
     plot_id : int
@@ -643,7 +730,7 @@ class Plot(PlotBase):
     name : str
         Name of the plot
     pixels : Iterable of int
-        Number of pixels to use in each direction
+        Number of pixels to use in each direction (2 values)
     filename : str
         Path to write the plot to
     color_by : {'cell', 'material'}
@@ -666,11 +753,9 @@ class Plot(PlotBase):
     level : int
         Universe depth to plot at
     width : Iterable of float
-        Width of the plot in each basis direction
+        Width of the plot in each basis direction (2 values)
     origin : tuple or list of ndarray
-        Origin (center) of the plot
-    type : {'slice', 'voxel'}
-        The type of the plot
+        Origin (center) of the plot (3 values)
     basis : {'xy', 'xz', 'yz'}
         The basis directions for the plot
     meshlines : dict
@@ -683,10 +768,37 @@ class Plot(PlotBase):
         super().__init__(plot_id, name)
         self._width = [4.0, 4.0]
         self._origin = [0., 0., 0.]
-        self._type = 'slice'
         self._basis = 'xy'
         self._meshlines = None
 
+    @property
+    def type(self):
+        warnings.warn(
+            "The 'type' attribute is deprecated and will be removed in a future version. "
+            "This is a SlicePlot instance.",
+            FutureWarning, stacklevel=2
+        )
+        return 'slice'
+
+    @type.setter
+    def type(self, value):
+        raise TypeError(
+            "Setting plot.type is no longer supported. "
+            "Use openmc.SlicePlot() for 2D slice plots or openmc.VoxelPlot() for 3D voxel plots."
+        )
+
+    @property
+    def pixels(self):
+        return self._pixels
+
+    @pixels.setter
+    def pixels(self, pixels):
+        cv.check_type('plot pixels', pixels, Iterable, Integral)
+        cv.check_length('plot pixels', pixels, 2, 2)
+        for dim in pixels:
+            cv.check_greater_than('plot pixels', dim, 0)
+        self._pixels = pixels
+
     @property
     def width(self):
         return self._width
@@ -694,7 +806,7 @@ class Plot(PlotBase):
     @width.setter
     def width(self, width):
         cv.check_type('plot width', width, Iterable, Real)
-        cv.check_length('plot width', width, 2, 3)
+        cv.check_length('plot width', width, 2, 2)
         self._width = width
 
     @property
@@ -707,15 +819,6 @@ class Plot(PlotBase):
         cv.check_length('plot origin', origin, 3)
         self._origin = origin
 
-    @property
-    def type(self):
-        return self._type
-
-    @type.setter
-    def type(self, plottype):
-        cv.check_value('plot type', plottype, ['slice', 'voxel'])
-        self._type = plottype
-
     @property
     def basis(self):
         return self._basis
@@ -758,11 +861,10 @@ class Plot(PlotBase):
         self._meshlines = meshlines
 
     def __repr__(self):
-        string = 'Plot\n'
+        string = 'SlicePlot\n'
         string += '{: <16}=\t{}\n'.format('\tID', self._id)
         string += '{: <16}=\t{}\n'.format('\tName', self._name)
         string += '{: <16}=\t{}\n'.format('\tFilename', self._filename)
-        string += '{: <16}=\t{}\n'.format('\tType', self._type)
         string += '{: <16}=\t{}\n'.format('\tBasis', self._basis)
         string += '{: <16}=\t{}\n'.format('\tWidth', self._width)
         string += '{: <16}=\t{}\n'.format('\tOrigin', self._origin)
@@ -878,7 +980,7 @@ class Plot(PlotBase):
                 self._colors[domain] = (r, g, b)
 
     def to_xml_element(self):
-        """Return XML representation of the slice/voxel plot
+        """Return XML representation of the slice plot
 
         Returns
         -------
@@ -888,10 +990,8 @@ class Plot(PlotBase):
         """
 
         element = super().to_xml_element()
-        element.set("type", self._type)
-
-        if self._type == 'slice':
-            element.set("basis", self._basis)
+        element.set("type", "slice")
+        element.set("basis", self._basis)
 
         subelement = ET.SubElement(element, "origin")
         subelement.text = ' '.join(map(str, self._origin))
@@ -937,68 +1037,63 @@ class Plot(PlotBase):
 
         Returns
         -------
-        openmc.Plot
-            Plot object
+        openmc.SlicePlot
+            SlicePlot object
 
         """
-        plot_id = int(elem.get("id"))
+        plot_id = int(get_text(elem, "id"))
         name = get_text(elem, 'name', '')
         plot = cls(plot_id, name)
         if "filename" in elem.keys():
-            plot.filename = elem.get("filename")
-        plot.color_by = elem.get("color_by")
-        plot.type = elem.get("type")
-        if plot.type == 'slice':
-            plot.basis = elem.get("basis")
+            plot.filename = get_text(elem, "filename")
+        plot.color_by = get_text(elem, "color_by")
+        plot.basis = get_text(elem, "basis")
 
-        plot.origin = get_elem_tuple(elem, "origin", float)
-        plot.width = get_elem_tuple(elem, "width", float)
-        plot.pixels = get_elem_tuple(elem, "pixels")
-        plot._background = get_elem_tuple(elem, "background")
+        plot.origin = tuple(get_elem_list(elem, "origin", float))
+        plot.width = tuple(get_elem_list(elem, "width", float))
+        plot.pixels = tuple(get_elem_list(elem, "pixels"))
+        background = get_elem_list(elem, "background")
+        if background is not None:
+            plot._background = tuple(background)
 
         # Set plot colors
         colors = {}
         for color_elem in elem.findall("color"):
-            uid = int(color_elem.get("id"))
-            colors[uid] = tuple([int(x)
-                                for x in color_elem.get("rgb").split()])
+            uid = int(get_text(color_elem, "id"))
+            colors[uid] = tuple(get_elem_list(color_elem, "rgb", int))
         plot.colors = colors
 
         # Set masking information
         mask_elem = elem.find("mask")
         if mask_elem is not None:
-            plot.mask_components = [
-                int(x) for x in mask_elem.get("components").split()]
-            background = mask_elem.get("background")
+            plot.mask_components = get_elem_list(mask_elem, "components", int)
+            background = get_elem_list(mask_elem, "background", int)
             if background is not None:
-                plot.mask_background = tuple(
-                    [int(x) for x in background.split()])
+                plot.mask_background = tuple(background)
 
         # show overlaps
-        overlap_elem = elem.find("show_overlaps")
-        if overlap_elem is not None:
-            plot.show_overlaps = (overlap_elem.text in ('true', '1'))
-        overlap_color = get_elem_tuple(elem, "overlap_color")
+        overlap = get_text(elem, "show_overlaps")
+        if overlap is not None:
+            plot.show_overlaps = (overlap in ('true', '1'))
+        overlap_color = get_elem_list(elem, "overlap_color", int)
         if overlap_color is not None:
-            plot.overlap_color = overlap_color
+            plot.overlap_color = tuple(overlap_color)
 
         # Set universe level
-        level = elem.find("level")
+        level = get_text(elem, "level")
         if level is not None:
-            plot.level = int(level.text)
+            plot.level = int(level)
 
         # Set meshlines
         mesh_elem = elem.find("meshlines")
         if mesh_elem is not None:
-            meshlines = {'type': mesh_elem.get('meshtype')}
+            meshlines = {'type': get_text(mesh_elem, "meshtype")}
             if 'id' in mesh_elem.keys():
-                meshlines['id'] = int(mesh_elem.get('id'))
+                meshlines['id'] = int(get_text(mesh_elem, "id"))
             if 'linewidth' in mesh_elem.keys():
-                meshlines['linewidth'] = int(mesh_elem.get('linewidth'))
+                meshlines['linewidth'] = int(get_text(mesh_elem, "linewidth"))
             if 'color' in mesh_elem.keys():
-                meshlines['color'] = tuple(
-                    [int(x) for x in mesh_elem.get('color').split()]
-                )
+                meshlines['color'] = tuple(get_elem_list(mesh_elem, "color", int))
             plot.meshlines = meshlines
 
         return plot
@@ -1034,9 +1129,215 @@ class Plot(PlotBase):
         # Return produced image
         return _get_plot_image(self, cwd)
 
+
+
+class VoxelPlot(PlotBase):
+    """Definition of a 3D voxel plot of the geometry.
+
+    Colors that are used in plots can be given as RGB tuples, e.g.
+    (255, 255, 255) would be white, or by a string indicating a
+    valid `SVG color <https://www.w3.org/TR/SVG11/types.html#ColorKeywords>`_.
+
+    .. versionadded:: 0.15.1
+
+    Parameters
+    ----------
+    plot_id : int
+        Unique identifier for the plot
+    name : str
+        Name of the plot
+
+    Attributes
+    ----------
+    id : int
+        Unique identifier
+    name : str
+        Name of the plot
+    pixels : Iterable of int
+        Number of pixels to use in each direction (3 values)
+    filename : str
+        Path to write the plot to
+    color_by : {'cell', 'material'}
+        Indicate whether the plot should be colored by cell or by material
+    background : Iterable of int or str
+        Color of the background
+    mask_components : Iterable of openmc.Cell or openmc.Material or int
+        The cells or materials (or corresponding IDs) to mask
+    mask_background : Iterable of int or str
+        Color to apply to all cells/materials listed in mask_components
+    show_overlaps : bool
+        Indicate whether or not overlapping regions are shown
+    overlap_color : Iterable of int or str
+        Color to apply to overlapping regions
+    colors : dict
+        Dictionary indicating that certain cells/materials should be
+        displayed with a particular color. The keys can be of type
+        :class:`~openmc.Cell`, :class:`~openmc.Material`, or int (ID for a
+        cell/material).
+    level : int
+        Universe depth to plot at
+    width : Iterable of float
+        Width of the plot in each dimension (3 values)
+    origin : tuple or list of ndarray
+        Origin (center) of the plot (3 values)
+
+    """
+
+    def __init__(self, plot_id=None, name=''):
+        super().__init__(plot_id, name)
+        self._width = [4.0, 4.0, 4.0]
+        self._origin = [0., 0., 0.]
+        self._pixels = [400, 400, 400]
+
+    @property
+    def pixels(self):
+        return self._pixels
+
+    @pixels.setter
+    def pixels(self, pixels):
+        cv.check_type('plot pixels', pixels, Iterable, Integral)
+        cv.check_length('plot pixels', pixels, 3, 3)
+        for dim in pixels:
+            cv.check_greater_than('plot pixels', dim, 0)
+        self._pixels = pixels
+
+    @property
+    def width(self):
+        return self._width
+
+    @width.setter
+    def width(self, width):
+        cv.check_type('plot width', width, Iterable, Real)
+        cv.check_length('plot width', width, 3, 3)
+        self._width = width
+
+    @property
+    def origin(self):
+        return self._origin
+
+    @origin.setter
+    def origin(self, origin):
+        cv.check_type('plot origin', origin, Iterable, Real)
+        cv.check_length('plot origin', origin, 3)
+        self._origin = origin
+
+    def __repr__(self):
+        string = 'VoxelPlot\n'
+        string += '{: <16}=\t{}\n'.format('\tID', self._id)
+        string += '{: <16}=\t{}\n'.format('\tName', self._name)
+        string += '{: <16}=\t{}\n'.format('\tFilename', self._filename)
+        string += '{: <16}=\t{}\n'.format('\tWidth', self._width)
+        string += '{: <16}=\t{}\n'.format('\tOrigin', self._origin)
+        string += '{: <16}=\t{}\n'.format('\tPixels', self._pixels)
+        string += '{: <16}=\t{}\n'.format('\tColor by', self._color_by)
+        string += '{: <16}=\t{}\n'.format('\tBackground', self._background)
+        string += '{: <16}=\t{}\n'.format('\tMask components',
+                                          self._mask_components)
+        string += '{: <16}=\t{}\n'.format('\tMask background',
+                                          self._mask_background)
+        string += '{: <16}=\t{}\n'.format('\tOverlap Color',
+                                          self._overlap_color)
+        string += '{: <16}=\t{}\n'.format('\tColors', self._colors)
+        string += '{: <16}=\t{}\n'.format('\tLevel', self._level)
+        return string
+
+    def to_xml_element(self):
+        """Return XML representation of the voxel plot
+
+        Returns
+        -------
+        element : lxml.etree._Element
+            XML element containing plot data
+
+        """
+
+        element = super().to_xml_element()
+        element.set("type", "voxel")
+
+        subelement = ET.SubElement(element, "origin")
+        subelement.text = ' '.join(map(str, self._origin))
+
+        subelement = ET.SubElement(element, "width")
+        subelement.text = ' '.join(map(str, self._width))
+
+        if self._colors:
+            self._colors_to_xml(element)
+
+        if self._show_overlaps:
+            subelement = ET.SubElement(element, "show_overlaps")
+            subelement.text = "true"
+
+            if self._overlap_color is not None:
+                color = self._overlap_color
+                if isinstance(color, str):
+                    color = _SVG_COLORS[color.lower()]
+                subelement = ET.SubElement(element, "overlap_color")
+                subelement.text = ' '.join(str(x) for x in color)
+
+        return element
+
+    @classmethod
+    def from_xml_element(cls, elem):
+        """Generate plot object from an XML element
+
+        Parameters
+        ----------
+        elem : lxml.etree._Element
+            XML element
+
+        Returns
+        -------
+        openmc.VoxelPlot
+            VoxelPlot object
+
+        """
+        plot_id = int(get_text(elem, "id"))
+        name = get_text(elem, 'name', '')
+        plot = cls(plot_id, name)
+        if "filename" in elem.keys():
+            plot.filename = get_text(elem, "filename")
+        plot.color_by = get_text(elem, "color_by")
+
+        plot.origin = tuple(get_elem_list(elem, "origin", float))
+        plot.width = tuple(get_elem_list(elem, "width", float))
+        plot.pixels = tuple(get_elem_list(elem, "pixels"))
+        background = get_elem_list(elem, "background")
+        if background is not None:
+            plot._background = tuple(background)
+
+        # Set plot colors
+        colors = {}
+        for color_elem in elem.findall("color"):
+            uid = int(get_text(color_elem, "id"))
+            colors[uid] = tuple(get_elem_list(color_elem, "rgb", int))
+        plot.colors = colors
+
+        # Set masking information
+        mask_elem = elem.find("mask")
+        if mask_elem is not None:
+            plot.mask_components = get_elem_list(mask_elem, "components", int)
+            background = get_elem_list(mask_elem, "background", int)
+            if background is not None:
+                plot.mask_background = tuple(background)
+
+        # show overlaps
+        overlap = get_text(elem, "show_overlaps")
+        if overlap is not None:
+            plot.show_overlaps = (overlap in ('true', '1'))
+        overlap_color = get_elem_list(elem, "overlap_color", int)
+        if overlap_color is not None:
+            plot.overlap_color = tuple(overlap_color)
+
+        # Set universe level
+        level = get_text(elem, "level")
+        if level is not None:
+            plot.level = int(level)
+
+        return plot
+
     def to_vtk(self, output: PathLike | None = None,
                openmc_exec: str = 'openmc', cwd: str = '.'):
-        """Render plot as an voxel image
+        """Render plot as a voxel image
 
         This method runs OpenMC in plotting mode to produce a .vti file.
 
@@ -1057,10 +1358,6 @@ class Plot(PlotBase):
             Path of the .vti file produced
 
         """
-        if self.type != 'voxel':
-            raise ValueError(
-                'Generating a VTK file only works for voxel plots')
-
         # Create plots.xml
         Plots([self]).export_to_xml(cwd)
 
@@ -1080,6 +1377,20 @@ class Plot(PlotBase):
         return voxel_to_vtk(h5_voxel_file, output)
 
 
+def Plot(plot_id=None, name=''):
+    """Legacy Plot class for backward compatibility.
+
+    .. deprecated:: 0.15.4
+        Use :class:`SlicePlot` for 2D slice plots or :class:`VoxelPlot` for 3D voxel plots.
+
+    """
+    warnings.warn(
+        "The Plot class is deprecated. Use SlicePlot for 2D slice plots "
+        "or VoxelPlot for 3D voxel plots.", FutureWarning
+    )
+    return SlicePlot(plot_id, name)
+
+
 class RayTracePlot(PlotBase):
     """Definition of a camera's view of OpenMC geometry
 
@@ -1256,38 +1567,39 @@ class RayTracePlot(PlotBase):
         None
         """
 
-        if "filename" in elem.keys():
-            self.filename = elem.get("filename")
-        self.color_by = elem.get("color_by")
+        filename = get_text(elem, "filename")
+        if filename is not None:
+            self.filename = filename
+        self.color_by = get_text(elem, "color_by")
 
-        horizontal_fov = elem.find("horizontal_field_of_view")
+        horizontal_fov = get_text(elem, "horizontal_field_of_view")
         if horizontal_fov is not None:
-            self.horizontal_field_of_view = float(horizontal_fov.text)
+            self.horizontal_field_of_view = float(horizontal_fov)
 
-        if (tmp := elem.find("orthographic_width")) is not None:
-            self.orthographic_width = float(tmp)
+        orthographic_width = get_text(elem, "orthographic_width")
+        if orthographic_width is not None:
+            self.orthographic_width = float(orthographic_width)
 
-        self.pixels = get_elem_tuple(elem, "pixels")
-        self.camera_position = get_elem_tuple(elem, "camera_position", float)
-        self.look_at = get_elem_tuple(elem, "look_at", float)
+        self.pixels = tuple(get_elem_list(elem, "pixels", int))
+        self.camera_position = tuple(get_elem_list(elem, "camera_position", float))
+        self.look_at = tuple(get_elem_list(elem, "look_at", float))
 
-        if elem.find("background") is not None:
-            self.background = get_elem_tuple(elem, "background")
+        background = get_elem_list(elem, "background", int)
+        if background is not None:
+            self.background = tuple(background)
 
         # Set masking information
         if (mask_elem := elem.find("mask")) is not None:
-            mask_components = [int(x)
-                               for x in mask_elem.get("components").split()]
+            mask_components = get_elem_list(mask_elem, "components", int)
             # TODO: set mask components(needs geometry information)
-            background = mask_elem.get("background")
+            background = get_elem_list(mask_elem, "background", int)
             if background is not None:
-                self.mask_background = tuple(
-                    [int(x) for x in background.split()])
+                self.mask_background = tuple(background)
 
         # Set universe level
-        level = elem.find("level")
+        level = get_text(elem, "level")
         if level is not None:
-            self.level = int(level.text)
+            self.level = int(level)
 
 
 class WireframeRayTracePlot(RayTracePlot):
@@ -1512,7 +1824,7 @@ class WireframeRayTracePlot(RayTracePlot):
 
         """
 
-        plot_id = int(elem.get("id"))
+        plot_id = int(get_text(elem, "id"))
         plot_name = get_text(elem, 'name', '')
         plot = cls(plot_id, plot_name)
         plot.type = "wireframe_raytrace"
@@ -1520,19 +1832,18 @@ class WireframeRayTracePlot(RayTracePlot):
         plot._read_xml_attributes(elem)
 
         # Attempt to get wireframe thickness.May not be present
-        wireframe_thickness = elem.find("wireframe_thickness")
+        wireframe_thickness = get_text(elem, "wireframe_thickness")
         if wireframe_thickness is not None:
-            plot.wireframe_thickness = int(wireframe_thickness.text)
-        wireframe_color = elem.get("wireframe_color")
+            plot.wireframe_thickness = int(wireframe_thickness)
+        wireframe_color = get_elem_list(elem, "wireframe_color", int)
         if wireframe_color:
-            plot.wireframe_color = [int(item) for item in wireframe_color]
+            plot.wireframe_color = wireframe_color
 
         # Set plot colors
         for color_elem in elem.findall("color"):
-            uid = int(color_elem.get("id"))
-            plot.colors[uid] = tuple(int(i)
-                                     for i in get_text(color_elem, 'rgb').split())
-            plot.xs[uid] = float(color_elem.get("xs"))
+            uid = int(get_text(color_elem, "id"))
+            plot.colors[uid] = tuple(get_elem_list(color_elem, "rgb", int))
+            plot.xs[uid] = float(get_text(color_elem, "xs"))
 
         return plot
 
@@ -1690,15 +2001,17 @@ class SolidRayTracePlot(RayTracePlot):
 
     def _read_phong_attributes(self, elem):
         """Read attributes specific to the Phong plot from an XML element"""
-        if elem.find('light_position') is not None:
-            self.light_position = get_elem_tuple(elem, 'light_position', float)
+        light_position = get_elem_list(elem, 'light_position', float)
+        if light_position is not None:
+            self.light_position = tuple(light_position)
 
-        diffuse_fraction = elem.find('diffuse_fraction')
+        diffuse_fraction = get_text(elem, "diffuse_fraction")
         if diffuse_fraction is not None:
-            self.diffuse_fraction = float(diffuse_fraction.text)
+            self.diffuse_fraction = float(diffuse_fraction)
 
-        if elem.find('opaque_ids') is not None:
-            self.opaque_domains = list(get_elem_tuple(elem, 'opaque_ids', int))
+        opaque_domains = get_elem_list(elem, 'opaque_ids', int)
+        if opaque_domains is not None:
+            self.opaque_domains = opaque_domains
 
     @classmethod
     def from_xml_element(cls, elem):
@@ -1716,7 +2029,7 @@ class SolidRayTracePlot(RayTracePlot):
 
         """
 
-        plot_id = int(elem.get("id"))
+        plot_id = int(get_text(elem, "id"))
         plot_name = get_text(elem, 'name', '')
         plot = cls(plot_id, plot_name)
         plot.type = "solid_raytrace"
@@ -1726,23 +2039,23 @@ class SolidRayTracePlot(RayTracePlot):
 
         # Set plot colors
         for color_elem in elem.findall("color"):
-            uid = color_elem.get("id")
-            plot.colors[uid] = get_elem_tuple(color_elem, "rgb")
+            uid = get_text(color_elem, "id")
+            plot.colors[uid] = tuple(get_elem_list(color_elem, "rgb", int))
 
         return plot
 
 
 class Plots(cv.CheckedList):
-    """Collection of Plots used for an OpenMC simulation.
+    """Collection of plots used for an OpenMC simulation.
 
     This class corresponds directly to the plots.xml input file. It can be
     thought of as a normal Python list where each member is inherits from
     :class:`PlotBase`. It behaves like a list as the following example
     demonstrates:
 
-    >>> xz_plot = openmc.Plot()
-    >>> big_plot = openmc.Plot()
-    >>> small_plot = openmc.Plot()
+    >>> xz_plot = openmc.SlicePlot()
+    >>> big_plot = openmc.VoxelPlot()
+    >>> small_plot = openmc.SlicePlot()
     >>> p = openmc.Plots((xz_plot, big_plot))
     >>> p.append(small_plot)
     >>> small_plot = p.pop()
@@ -1778,7 +2091,7 @@ class Plots(cv.CheckedList):
         ----------
         index : int
             Index in list
-        plot : openmc.Plot
+        plot : openmc.PlotBase
             Plot to insert
 
         """
@@ -1854,8 +2167,6 @@ class Plots(cv.CheckedList):
 
         # Clean the indentation in the file to be user-readable
         clean_indentation(self._plots_file)
-        # TODO: Remove when support is Python 3.8+
-        reorder_attributes(self._plots_file)
 
         return self._plots_file
 
@@ -1896,13 +2207,18 @@ class Plots(cv.CheckedList):
         # Generate each plot
         plots = cls()
         for e in elem.findall('plot'):
-            plot_type = e.get('type')
+            plot_type = get_text(e, "type")
             if plot_type == 'wireframe_raytrace':
                 plots.append(WireframeRayTracePlot.from_xml_element(e))
             elif plot_type == 'solid_raytrace':
                 plots.append(SolidRayTracePlot.from_xml_element(e))
-            elif plot_type in ('slice', 'voxel'):
-                plots.append(Plot.from_xml_element(e))
+            elif plot_type == 'slice':
+                plots.append(SlicePlot.from_xml_element(e))
+            elif plot_type == 'voxel':
+                plots.append(VoxelPlot.from_xml_element(e))
+            elif plot_type is None:
+                # For backward compatibility, assume slice if no type specified
+                plots.append(SlicePlot.from_xml_element(e))
             else:
                 raise ValueError("Unknown plot type: {}".format(plot_type))
         return plots
diff --git a/openmc/plotter.py b/openmc/plotter.py
index 85c4963a7..abd8ab6dd 100644
--- a/openmc/plotter.py
+++ b/openmc/plotter.py
@@ -413,7 +413,8 @@ def _calculate_cexs_nuclide(this, types, temperature=294., sab_name=None,
 
         # Prep S(a,b) data if needed
         if sab_name:
-            sab = openmc.data.ThermalScattering.from_hdf5(sab_name)
+            sab = openmc.data.ThermalScattering.from_hdf5(
+                library.get_by_material(sab_name, data_type='thermal')['path'])
             # Obtain the nearest temperature
             if strT in sab.temperatures:
                 sabT = strT
@@ -500,7 +501,7 @@ def _calculate_cexs_nuclide(this, types, temperature=294., sab_name=None,
                     elif ncrystal_cfg:
                         import NCrystal
                         nc_scatter = NCrystal.createScatter(ncrystal_cfg)
-                        nc_func = nc_scatter.crossSectionNonOriented
+                        nc_func = nc_scatter.xsect
                         nc_emax = 5 # eV # this should be obtained from NCRYSTAL_MAX_ENERGY
                         energy_grid = np.union1d(np.geomspace(min(energy_grid),
                                                               1.1*nc_emax,
@@ -640,14 +641,13 @@ def _calculate_cexs_elem_mat(this, types, temperature=294.,
             sab = openmc.data.ThermalScattering.from_hdf5(
                 library.get_by_material(sab_name, data_type='thermal')['path'])
             for nuc in sab.nuclides:
-                sabs[nuc] = library.get_by_material(sab_name,
-                        data_type='thermal')['path']
+                sabs[nuc] = sab_name
     else:
         if sab_name:
-            sab = openmc.data.ThermalScattering.from_hdf5(sab_name)
+            sab = openmc.data.ThermalScattering.from_hdf5(
+                library.get_by_material(sab_name, data_type='thermal')['path'])
             for nuc in sab.nuclides:
-                sabs[nuc] = library.get_by_material(sab_name,
-                        data_type='thermal')['path']
+                sabs[nuc] = sab_name
 
     # Now we can create the data sets to be plotted
     xs = {}
@@ -655,8 +655,8 @@ def _calculate_cexs_elem_mat(this, types, temperature=294.,
     for nuclide in nuclides.items():
         name = nuclide[0]
         nuc = nuclide[1]
-        sab_tab = sabs[name]
-        temp_E, temp_xs = calculate_cexs(nuc, types, T, sab_tab, cross_sections,
+        sab_name = sabs[name]
+        temp_E, temp_xs = calculate_cexs(nuc, types, T, sab_name, cross_sections,
                                          ncrystal_cfg=ncrystal_cfg
                                          )
         E.append(temp_E)
diff --git a/openmc/settings.py b/openmc/settings.py
index 17c24ba81..8120eb073 100644
--- a/openmc/settings.py
+++ b/openmc/settings.py
@@ -4,19 +4,20 @@ import itertools
 from math import ceil
 from numbers import Integral, Real
 from pathlib import Path
+import traceback
 
 import lxml.etree as ET
-
+import warnings
 import openmc
 import openmc.checkvalue as cv
 from openmc.checkvalue import PathLike
 from openmc.stats.multivariate import MeshSpatial
-from ._xml import clean_indentation, get_text, reorder_attributes
+from ._xml import clean_indentation, get_elem_list, get_text
 from .mesh import _read_meshes, RegularMesh, MeshBase
 from .source import SourceBase, MeshSource, IndependentSource
 from .utility_funcs import input_path
 from .volume import VolumeCalculation
-from .weight_windows import WeightWindows, WeightWindowGenerator
+from .weight_windows import WeightWindows, WeightWindowGenerator, WeightWindowsList
 
 
 class RunMode(Enum):
@@ -40,6 +41,10 @@ class Settings:
 
     Attributes
     ----------
+    atomic_relaxation : bool
+        Whether to simulate the atomic relaxation cascade (fluorescence photons
+        and Auger electrons) following photoelectric and incoherent scattering
+        interactions.
     batches : int
         Number of batches to simulate
     confidence_intervals : bool
@@ -47,6 +52,25 @@ class Settings:
         half-width of the 95% two-sided confidence interval. If False,
         uncertainties on tally results will be reported as the sample standard
         deviation.
+    collision_track : dict
+        Options for writing collision information. Acceptable keys are:
+
+        :max_collisions: Maximum number of collisions to be banked per file. (int)
+        :max_collision_track_files: Maximum number of collision_track files. (int)
+        :mcpl: Output in the form of an MCPL-file. (bool)
+        :cell_ids: List of cell IDs to define cells in which collisions should be banked. (list of int)
+        :universe_ids: List of universe IDs to define universes in which collisions should be banked. (list of int)
+        :material_ids: List of material IDs to define materials in which collisions should be banked. (list of int)
+        :nuclides: List of nuclides to define nuclides in which collisions should be banked.
+                    (ex: ["I135m", "U233"] ). (list of str)
+        :reactions: List of reaction to define specific reactions that should be banked
+                    (ex: ["(n,fission)", 2, "(n,2n)"] ). (list of str or int)
+        :deposited_E_threshold: Number to define the minimum deposited energy during
+                     per collision to trigger banking. (float)
+    create_delayed_neutrons : bool
+        Whether delayed neutrons are created in fission.
+
+        .. versionadded:: 0.13.3
     create_fission_neutrons : bool
         Indicate whether fission neutrons should be created or not.
     cutoff : dict
@@ -84,8 +108,15 @@ class Settings:
         history-based parallelism.
 
         .. versionadded:: 0.12
+    free_gas_threshold : float
+        Energy multiplier (in units of :math:`kT`) below which the free gas
+        scattering treatment is applied for elastic scattering. If not
+        specified, a value of 400.0 is used.
     generations_per_batch : int
         Number of generations per batch
+    ifp_n_generation : int
+        Number of generations to consider for the Iterated Fission Probability
+        method.
     max_lost_particles : int
         Maximum number of lost particles
 
@@ -125,6 +156,10 @@ class Settings:
         Maximum number of times a particle can split during a history
 
         .. versionadded:: 0.13
+    max_secondaries : int
+        Maximum secondary bank size
+
+        .. versionadded:: 0.15.3
     max_tracks : int
         Maximum number of tracks written to a track file (per MPI process).
 
@@ -152,11 +187,14 @@ class Settings:
        Initial seed for randomly generated plot colors.
     ptables : bool
         Determine whether probability tables are used.
+    properties_file : PathLike
+        Location of the properties file to load cell temperatures/densities
+        and material densities
     random_ray : dict
         Options for configuring the random ray solver. Acceptable keys are:
 
         :distance_inactive:
-            Indicates the total active distance in [cm] a ray should travel
+            Indicates the total inactive distance in [cm] a ray should travel
         :distance_active:
             Indicates the total active distance in [cm] a ray should travel
         :ray_source:
@@ -180,13 +218,31 @@ class Settings:
             default is 'False'.
         :sample_method:
             Sampling method for the ray starting location and direction of
-            travel. Options are `prng` (default) or 'halton`.
+            travel. Options are `prng` (default), `halton`, or `s2`. `s2`
+            modifies the `prng` sampling method such that rays are sampled
+            with directions (-1, 0, 0) or (1, 0, 0). This is used for verification
+            against analytic transport benchmarks which are often derivied with
+            a reduced angular domain.
         :source_region_meshes:
             List of tuples where each tuple contains a mesh and a list of
             domains. Each domain is an instance of openmc.Material, openmc.Cell,
             or openmc.Universe. The mesh will be applied to the listed domains
             to subdivide source regions so as to improve accuracy and/or conform
             with tally meshes.
+        :diagonal_stabilization_rho:
+            The rho factor for use with diagonal stabilization. This technique is
+            applied when negative diagonal (in-group) elements are detected in
+            the scattering matrix of input MGXS data, which is a common feature
+            of transport corrected MGXS data. The default is 1.0, which ensures
+            no negative diagonal elements are present in the iteration matrix and
+            thus stabilizes the simulation. A value of 0.0 will disable diagonal
+            stabilization. Values between 0.0 and 1.0 will apply a degree of
+            stabilization, which may be desirable as stronger diagonal stabilization
+            also tends to dampen the convergence rate of the solver, thus requiring
+            more iterations to converge.
+        :adjoint_source:
+            Source object used to define localized adjoint source/detector response 
+            function.
 
         .. versionadded:: 0.15.0
     resonance_scattering : dict
@@ -204,10 +260,21 @@ class Settings:
         The type of calculation to perform (default is 'eigenvalue')
     seed : int
         Seed for the linear congruential pseudorandom number generator
-    stride : int
-        Number of random numbers allocated for each source particle history
+    shared_secondary_bank : bool
+        Whether to use a shared secondary particle bank. When enabled,
+        secondary particles are collected into a global bank, sorted for
+        reproducibility, and load-balanced across MPI ranks between
+        generations. If not specified, the shared secondary bank is
+        enabled automatically for fixed-source simulations with weight
+        windows active, and disabled otherwise.
+
+        .. versionadded:: 0.15.4
     source : Iterable of openmc.SourceBase
         Distribution of source sites in space, angle, and energy
+    source_rejection_fraction : float
+        Minimum fraction of source sites that must be accepted when applying
+        rejection sampling based on constraints. If not specified, the default
+        value is 0.05.
     sourcepoint : dict
         Options for writing source points. Acceptable keys are:
 
@@ -221,6 +288,8 @@ class Settings:
         Options for writing state points. Acceptable keys are:
 
         :batches: list of batches at which to write statepoint files
+    stride : int
+        Number of random numbers allocated for each source particle history
     surf_source_read : dict
         Options for reading surface source points. Acceptable keys are:
 
@@ -243,6 +312,14 @@ class Settings:
         :cellto: Cell ID used to determine if particles crossing identified
                  surfaces are to be banked. Particles going to this declared
                  cell will be banked (int)
+    surface_grazing_cutoff : float
+        Surface flux cosine cutoff. If not specified, the default value is
+        0.001. For more information, see the surface tally section in the theory
+        manual.
+    surface_grazing_ratio : float
+        Surface flux cosine substitution ratio. If not specified, the default
+        value is 0.5. For more information, see the surface tally section in the
+        theory manual.
     survival_biasing : bool
         Indicate whether survival biasing is to be used
     tabular_legendre : dict
@@ -295,7 +372,7 @@ class Settings:
         described in :ref:`verbosity`.
     volume_calculations : VolumeCalculation or iterable of VolumeCalculation
         Stochastic volume calculation specifications
-    weight_windows : WeightWindows or iterable of WeightWindows
+    weight_windows : WeightWindowsList
         Weight windows to use for variance reduction
 
         .. versionadded:: 0.13
@@ -309,10 +386,6 @@ class Settings:
 
         .. versionadded:: 0.14.0
 
-    create_delayed_neutrons : bool
-        Whether delayed neutrons are created in fission.
-
-        .. versionadded:: 0.13.3
     weight_windows_on : bool
         Whether weight windows are enabled
 
@@ -343,16 +416,22 @@ class Settings:
 
         # Source subelement
         self._source = cv.CheckedList(SourceBase, 'source distributions')
+        self._source_rejection_fraction = None
 
         self._confidence_intervals = None
         self._electron_treatment = None
         self._photon_transport = None
+        self._atomic_relaxation = None
         self._plot_seed = None
         self._ptables = None
+        self._properties_file = None
         self._uniform_source_sampling = None
         self._seed = None
         self._stride = None
+        self._surface_grazing_cutoff = None
+        self._surface_grazing_ratio = None
         self._survival_biasing = None
+        self._free_gas_threshold = None
 
         # Shannon entropy mesh
         self._entropy_mesh = None
@@ -364,6 +443,12 @@ class Settings:
 
         self._output = None
 
+        # Iterated Fission Probability
+        self._ifp_n_generation = None
+
+        # Collision track feature
+        self._collision_track = {}
+
         # Output options
         self._statepoint = {}
         self._sourcepoint = {}
@@ -402,13 +487,16 @@ class Settings:
         self._max_particles_in_flight = None
         self._max_particle_events = None
         self._write_initial_source = None
-        self._weight_windows = cv.CheckedList(WeightWindows, 'weight windows')
-        self._weight_window_generators = cv.CheckedList(WeightWindowGenerator, 'weight window generators')
+        self._weight_windows = WeightWindowsList()
+        self._weight_window_generators = cv.CheckedList(
+            WeightWindowGenerator, 'weight window generators')
         self._weight_windows_on = None
+        self._shared_secondary_bank = None
         self._weight_windows_file = None
         self._weight_window_checkpoints = {}
         self._max_history_splits = None
         self._max_tracks = None
+        self._max_secondaries = None
         self._use_decay_photons = None
 
         self._random_ray = {}
@@ -416,6 +504,16 @@ class Settings:
         for key, value in kwargs.items():
             setattr(self, key, value)
 
+    def __setattr__(self, name: str, value):
+        if not name.startswith('_'):
+            try:
+                getattr(self, name)
+            except AttributeError as e:
+                msg, = traceback.format_exception_only(e)
+                msg = msg.strip().split(maxsplit=1)[-1]
+                warnings.warn(msg, stacklevel=2)
+        super().__setattr__(name, value)
+
     @property
     def run_mode(self) -> str:
         return self._run_mode.value
@@ -443,8 +541,9 @@ class Settings:
 
     @generations_per_batch.setter
     def generations_per_batch(self, generations_per_batch: int):
-        cv.check_type('generations per patch', generations_per_batch, Integral)
-        cv.check_greater_than('generations per batch', generations_per_batch, 0)
+        cv.check_type('generations per batch', generations_per_batch, Integral)
+        cv.check_greater_than('generations per batch',
+                              generations_per_batch, 0)
         self._generations_per_batch = generations_per_batch
 
     @property
@@ -474,7 +573,8 @@ class Settings:
     @rel_max_lost_particles.setter
     def rel_max_lost_particles(self, rel_max_lost_particles: float):
         cv.check_type('rel_max_lost_particles', rel_max_lost_particles, Real)
-        cv.check_greater_than('rel_max_lost_particles', rel_max_lost_particles, 0)
+        cv.check_greater_than('rel_max_lost_particles',
+                              rel_max_lost_particles, 0)
         cv.check_less_than('rel_max_lost_particles', rel_max_lost_particles, 1)
         self._rel_max_lost_particles = rel_max_lost_particles
 
@@ -484,8 +584,10 @@ class Settings:
 
     @max_write_lost_particles.setter
     def max_write_lost_particles(self, max_write_lost_particles: int):
-        cv.check_type('max_write_lost_particles', max_write_lost_particles, Integral)
-        cv.check_greater_than('max_write_lost_particles', max_write_lost_particles, 0)
+        cv.check_type('max_write_lost_particles',
+                      max_write_lost_particles, Integral)
+        cv.check_greater_than('max_write_lost_particles',
+                              max_write_lost_particles, 0)
         self._max_write_lost_particles = max_write_lost_particles
 
     @property
@@ -506,12 +608,12 @@ class Settings:
     def keff_trigger(self, keff_trigger: dict):
         if not isinstance(keff_trigger, dict):
             msg = f'Unable to set a trigger on keff from "{keff_trigger}" ' \
-                  'which is not a Python dictionary'
+                'which is not a Python dictionary'
             raise ValueError(msg)
 
         elif 'type' not in keff_trigger:
             msg = f'Unable to set a trigger on keff from "{keff_trigger}" ' \
-                  'which does not have a "type" key'
+                'which does not have a "type" key'
             raise ValueError(msg)
 
         elif keff_trigger['type'] not in ['variance', 'std_dev', 'rel_err']:
@@ -521,7 +623,7 @@ class Settings:
 
         elif 'threshold' not in keff_trigger:
             msg = f'Unable to set a trigger on keff from "{keff_trigger}" ' \
-                  'which does not have a "threshold" key'
+                'which does not have a "threshold" key'
             raise ValueError(msg)
 
         elif not isinstance(keff_trigger['threshold'], Real):
@@ -538,7 +640,7 @@ class Settings:
     @energy_mode.setter
     def energy_mode(self, energy_mode: str):
         cv.check_value('energy mode', energy_mode,
-                    ['continuous-energy', 'multi-group'])
+                       ['continuous-energy', 'multi-group'])
         self._energy_mode = energy_mode
 
     @property
@@ -561,7 +663,8 @@ class Settings:
     def source(self, source: SourceBase | Iterable[SourceBase]):
         if not isinstance(source, MutableSequence):
             source = [source]
-        self._source = cv.CheckedList(SourceBase, 'source distributions', source)
+        self._source = cv.CheckedList(
+            SourceBase, 'source distributions', source)
 
     @property
     def confidence_intervals(self) -> bool:
@@ -578,9 +681,19 @@ class Settings:
 
     @electron_treatment.setter
     def electron_treatment(self, electron_treatment: str):
-        cv.check_value('electron treatment', electron_treatment, ['led', 'ttb'])
+        cv.check_value('electron treatment',
+                       electron_treatment, ['led', 'ttb'])
         self._electron_treatment = electron_treatment
 
+    @property
+    def atomic_relaxation(self) -> bool:
+        return self._atomic_relaxation
+
+    @atomic_relaxation.setter
+    def atomic_relaxation(self, atomic_relaxation: bool):
+        cv.check_type('atomic relaxation', atomic_relaxation, bool)
+        self._atomic_relaxation = atomic_relaxation
+
     @property
     def ptables(self) -> bool:
         return self._ptables
@@ -638,6 +751,27 @@ class Settings:
         cv.check_greater_than('random number generator stride', stride, 0)
         self._stride = stride
 
+    @property
+    def surface_grazing_cutoff(self) -> float:
+        return self._surface_grazing_cutoff
+
+    @surface_grazing_cutoff.setter
+    def surface_grazing_cutoff(self, surface_grazing_cutoff: float):
+        cv.check_type('surface grazing cutoff', surface_grazing_cutoff, float)
+        cv.check_greater_than('surface grazing cutoff', surface_grazing_cutoff, 0.0)
+        cv.check_less_than('surface grazing cutoff', surface_grazing_cutoff, 1.0)
+        self._surface_grazing_cutoff = surface_grazing_cutoff
+
+    @property
+    def surface_grazing_ratio(self) -> float:
+        return self._surface_grazing_ratio
+
+    @surface_grazing_ratio.setter
+    def surface_grazing_ratio(self, surface_grazing_ratio: float):
+        cv.check_type('surface grazing ratio', surface_grazing_ratio, float)
+        cv.check_greater_than('surface grazing ratio', surface_grazing_ratio, 0.0)
+        self._surface_grazing_ratio = surface_grazing_ratio
+
     @property
     def survival_biasing(self) -> bool:
         return self._survival_biasing
@@ -672,7 +806,8 @@ class Settings:
     @trigger_max_batches.setter
     def trigger_max_batches(self, trigger_max_batches: int):
         cv.check_type('trigger maximum batches', trigger_max_batches, Integral)
-        cv.check_greater_than('trigger maximum batches', trigger_max_batches, 0)
+        cv.check_greater_than('trigger maximum batches',
+                              trigger_max_batches, 0)
         self._trigger_max_batches = trigger_max_batches
 
     @property
@@ -681,8 +816,10 @@ class Settings:
 
     @trigger_batch_interval.setter
     def trigger_batch_interval(self, trigger_batch_interval: int):
-        cv.check_type('trigger batch interval', trigger_batch_interval, Integral)
-        cv.check_greater_than('trigger batch interval', trigger_batch_interval, 0)
+        cv.check_type('trigger batch interval',
+                      trigger_batch_interval, Integral)
+        cv.check_greater_than('trigger batch interval',
+                              trigger_batch_interval, 0)
         self._trigger_batch_interval = trigger_batch_interval
 
     @property
@@ -766,19 +903,22 @@ class Settings:
 
     @surf_source_write.setter
     def surf_source_write(self, surf_source_write: dict):
-        cv.check_type("surface source writing options", surf_source_write, Mapping)
+        cv.check_type("surface source writing options",
+                      surf_source_write, Mapping)
         for key, value in surf_source_write.items():
             cv.check_value(
                 "surface source writing key",
                 key,
-                ("surface_ids", "max_particles", "max_source_files", "mcpl", "cell", "cellfrom", "cellto"),
+                ("surface_ids", "max_particles", "max_source_files",
+                 "mcpl", "cell", "cellfrom", "cellto"),
             )
             if key == "surface_ids":
                 cv.check_type(
                     "surface ids for source banking", value, Iterable, Integral
                 )
                 for surf_id in value:
-                    cv.check_greater_than("surface id for source banking", surf_id, 0)
+                    cv.check_greater_than(
+                        "surface id for source banking", surf_id, 0)
 
             elif key == "mcpl":
                 cv.check_type("write to an MCPL-format file", value, bool)
@@ -795,6 +935,79 @@ class Settings:
 
         self._surf_source_write = surf_source_write
 
+    @property
+    def collision_track(self) -> dict:
+        return self._collision_track
+
+    @collision_track.setter
+    def collision_track(self, collision_track: dict):
+        cv.check_type('Collision tracking options', collision_track, Mapping)
+        for key, value in collision_track.items():
+            cv.check_value('collision_track key', key,
+                           ('cell_ids', 'reactions', 'universe_ids', 'material_ids', 'nuclides',
+                            'deposited_E_threshold', 'max_collisions', 'max_collision_track_files', 'mcpl'))
+            if key == 'cell_ids':
+                cv.check_type('cell ids for collision tracking data banking', value,
+                              Iterable, Integral)
+                for cell_id in value:
+                    cv.check_greater_than('cell id for collision tracking data banking',
+                                          cell_id, 0)
+            elif key == 'reactions':
+                cv.check_type('MT numbers for collision tracking data banking', value,
+                              Iterable)
+                for reaction in value:
+                    if isinstance(reaction, int):
+                        cv.check_greater_than(
+                            'MT number for collision tracking data banking', reaction, 0
+                        )
+                    elif isinstance(reaction, str):
+                        # check against allowed strings? so far let C++ code handle it
+                        pass
+                    else:
+                        raise TypeError(
+                            f"MT number for collision tracking data banking must be a positive int or string, "
+                            f"got {type(reaction).__name__}")
+            elif key == 'universe_ids':
+                cv.check_type('universe ids for collision tracking data banking', value,
+                              Iterable, Integral)
+                for universe_id in value:
+                    cv.check_greater_than('universe id for collision tracking data banking',
+                                          universe_id, 0)
+            elif key == 'material_ids':
+                cv.check_type('material ids for collision tracking data banking', value,
+                              Iterable, Integral)
+                for material_id in value:
+                    cv.check_greater_than('material id for collision tracking data banking',
+                                          material_id, 0)
+            elif key == 'nuclides':
+                cv.check_type('nuclides for collision tracking data banking', value,
+                              Iterable, str)
+                for nuclide in value:
+                    # If nuclide name doesn't look valid, give a warning
+                    try:
+                        openmc.data.zam(nuclide)
+                    except ValueError:
+                        warnings.warn(f"Nuclide {nuclide} is not valid")
+            elif key == 'deposited_E_threshold':
+                cv.check_type('Deposited Energy Threshold for collision tracking data banking',
+                              value, Real)
+                cv.check_greater_than('Deposited Energy Threshold for collision tracking data banking',
+                                      value, 0)
+            elif key == 'max_collisions':
+                cv.check_type('maximum collisions banks per file',
+                              value, Integral)
+                cv.check_greater_than('maximum collisions banks in collision tracking',
+                                      value, 0)
+            elif key == 'max_collision_track_files':
+                cv.check_type('maximum collisions banks',
+                              value, Integral)
+                cv.check_greater_than('maximum number of collision_track files ',
+                                      value, 0)
+            elif key == 'mcpl':
+                cv.check_type('write to an MCPL-format file', value, bool)
+
+        self._collision_track = collision_track
+
     @property
     def no_reduce(self) -> bool:
         return self._no_reduce
@@ -815,6 +1028,17 @@ class Settings:
         cv.check_less_than('verbosity', verbosity, 10, True)
         self._verbosity = verbosity
 
+    @property
+    def ifp_n_generation(self) -> int:
+        return self._ifp_n_generation
+
+    @ifp_n_generation.setter
+    def ifp_n_generation(self, ifp_n_generation: int):
+        if ifp_n_generation is not None:
+            cv.check_type("number of generations", ifp_n_generation, Integral)
+            cv.check_greater_than("number of generations", ifp_n_generation, 0)
+        self._ifp_n_generation = ifp_n_generation
+
     @property
     def tabular_legendre(self) -> dict:
         return self._tabular_legendre
@@ -860,6 +1084,18 @@ class Settings:
 
         self._temperature = temperature
 
+    @property
+    def properties_file(self) -> PathLike | None:
+        return self._properties_file
+
+    @properties_file.setter
+    def properties_file(self, value: PathLike | None):
+        if value is None:
+            self._properties_file = None
+        else:
+            cv.check_type('properties file', value, PathLike)
+            self._properties_file = input_path(value)
+
     @property
     def trace(self) -> Iterable:
         return self._trace
@@ -900,7 +1136,7 @@ class Settings:
     def cutoff(self, cutoff: dict):
         if not isinstance(cutoff, Mapping):
             msg = f'Unable to set cutoff from "{cutoff}" which is not a '\
-                  'Python dictionary'
+                'Python dictionary'
             raise ValueError(msg)
         for key in cutoff:
             if key == 'weight':
@@ -918,7 +1154,7 @@ class Settings:
                 cv.check_greater_than('energy cutoff', cutoff[key], 0.0)
             else:
                 msg = f'Unable to set cutoff to "{key}" which is unsupported ' \
-                      'by OpenMC'
+                    'by OpenMC'
 
         self._cutoff = cutoff
 
@@ -1062,14 +1298,14 @@ class Settings:
         self._write_initial_source = value
 
     @property
-    def weight_windows(self) -> list[WeightWindows]:
+    def weight_windows(self) -> WeightWindowsList:
         return self._weight_windows
 
     @weight_windows.setter
-    def weight_windows(self, value: WeightWindows | Iterable[WeightWindows]):
-        if not isinstance(value, MutableSequence):
+    def weight_windows(self, value: WeightWindows | Sequence[WeightWindows]):
+        if not isinstance(value, Sequence):
             value = [value]
-        self._weight_windows = cv.CheckedList(WeightWindows, 'weight windows', value)
+        self._weight_windows = WeightWindowsList(value)
 
     @property
     def weight_windows_on(self) -> bool:
@@ -1080,6 +1316,15 @@ class Settings:
         cv.check_type('weight windows on', value, bool)
         self._weight_windows_on = value
 
+    @property
+    def shared_secondary_bank(self) -> bool:
+        return self._shared_secondary_bank
+
+    @shared_secondary_bank.setter
+    def shared_secondary_bank(self, value: bool):
+        cv.check_type('shared secondary bank', value, bool)
+        self._shared_secondary_bank = value
+
     @property
     def weight_window_checkpoints(self) -> dict:
         return self._weight_window_checkpoints
@@ -1087,12 +1332,14 @@ class Settings:
     @weight_window_checkpoints.setter
     def weight_window_checkpoints(self, weight_window_checkpoints: dict):
         for key in weight_window_checkpoints.keys():
-            cv.check_value('weight_window_checkpoints', key, ('collision', 'surface'))
+            cv.check_value('weight_window_checkpoints',
+                           key, ('collision', 'surface'))
         self._weight_window_checkpoints = weight_window_checkpoints
 
     @property
     def max_splits(self):
-        raise AttributeError('max_splits has been deprecated. Please use max_history_splits instead')
+        raise AttributeError(
+            'max_splits has been deprecated. Please use max_history_splits instead')
 
     @property
     def max_history_splits(self) -> int:
@@ -1104,6 +1351,16 @@ class Settings:
         cv.check_greater_than('max particle splits', value, 0)
         self._max_history_splits = value
 
+    @property
+    def max_secondaries(self) -> int:
+        return self._max_secondaries
+
+    @max_secondaries.setter
+    def max_secondaries(self, value: int):
+        cv.check_type('maximum secondary bank size', value, Integral)
+        cv.check_greater_than('max secondary bank size', value, 0)
+        self._max_secondaries = value
+
     @property
     def max_tracks(self) -> int:
         return self._max_tracks
@@ -1119,9 +1376,12 @@ class Settings:
         return self._weight_windows_file
 
     @weight_windows_file.setter
-    def weight_windows_file(self, value: PathLike):
-        cv.check_type('weight windows file', value, PathLike)
-        self._weight_windows_file = input_path(value)
+    def weight_windows_file(self, value: PathLike | None):
+        if value is None:
+            self._weight_windows_file = None
+        else:
+            cv.check_type('weight windows file', value, PathLike)
+            self._weight_windows_file = input_path(value)
 
     @property
     def weight_window_generators(self) -> list[WeightWindowGenerator]:
@@ -1131,7 +1391,8 @@ class Settings:
     def weight_window_generators(self, wwgs):
         if not isinstance(wwgs, MutableSequence):
             wwgs = [wwgs]
-        self._weight_window_generators = cv.CheckedList(WeightWindowGenerator, 'weight window generators', wwgs)
+        self._weight_window_generators = cv.CheckedList(
+            WeightWindowGenerator, 'weight window generators', wwgs)
 
     @property
     def random_ray(self) -> dict:
@@ -1168,7 +1429,8 @@ class Settings:
                 for mesh, domains in value:
                     cv.check_type('mesh', mesh, MeshBase)
                     cv.check_type('domains', domains, Iterable)
-                    valid_types = (openmc.Material, openmc.Cell, openmc.Universe)
+                    valid_types = (openmc.Material,
+                                   openmc.Cell, openmc.Universe)
                     for domain in domains:
                         if not isinstance(domain, valid_types):
                             raise ValueError(
@@ -1176,7 +1438,19 @@ class Settings:
                                 'openmc.Material, openmc.Cell, or openmc.Universe.')
             elif key == 'sample_method':
                 cv.check_value('sample method', value,
-                               ('prng', 'halton'))
+                               ('prng', 'halton', 's2'))
+            elif key == 'diagonal_stabilization_rho':
+                cv.check_type('diagonal stabilization rho', value, Real)
+                cv.check_greater_than('diagonal stabilization rho',
+                                      value, 0.0, True)
+            elif key == 'adjoint_source':
+                if not isinstance(value, MutableSequence):
+                    value = [value]
+                for source in value:
+                    if not isinstance(source, SourceBase):
+                        raise ValueError(
+                            f'Invalid adjoint source type: {type(source)}. '
+                            'Expected openmc.SourceBase.')
             else:
                 raise ValueError(f'Unable to set random ray to "{key}" which is '
                                  'unsupported by OpenMC')
@@ -1192,6 +1466,31 @@ class Settings:
         cv.check_type('use decay photons', value, bool)
         self._use_decay_photons = value
 
+    @property
+    def source_rejection_fraction(self) -> float:
+        return self._source_rejection_fraction
+
+    @source_rejection_fraction.setter
+    def source_rejection_fraction(self, source_rejection_fraction: float):
+        cv.check_type('source_rejection_fraction',
+                      source_rejection_fraction, Real)
+        cv.check_greater_than('source_rejection_fraction',
+                              source_rejection_fraction, 0)
+        cv.check_less_than('source_rejection_fraction',
+                           source_rejection_fraction, 1)
+        self._source_rejection_fraction = source_rejection_fraction
+
+    @property
+    def free_gas_threshold(self) -> float | None:
+        return self._free_gas_threshold
+
+    @free_gas_threshold.setter
+    def free_gas_threshold(self, free_gas_threshold: float | None):
+        if free_gas_threshold is not None:
+            cv.check_type('free gas threshold', free_gas_threshold, Real)
+            cv.check_greater_than('free gas threshold', free_gas_threshold, 0.0)
+        self._free_gas_threshold = free_gas_threshold
+
     def _create_run_mode_subelement(self, root):
         elem = ET.SubElement(root, "run_mode")
         elem.text = self._run_mode.value
@@ -1343,6 +1642,45 @@ class Settings:
                     subelement = ET.SubElement(element, key)
                     subelement.text = str(self._surf_source_write[key])
 
+    def _create_collision_track_subelement(self, root):
+        if self._collision_track:
+            element = ET.SubElement(root, "collision_track")
+            if 'cell_ids' in self._collision_track:
+                subelement = ET.SubElement(element, "cell_ids")
+                subelement.text = ' '.join(
+                    str(x) for x in self._collision_track['cell_ids'])
+            if 'reactions' in self._collision_track:
+                subelement = ET.SubElement(element, "reactions")
+                subelement.text = ' '.join(
+                    str(x) for x in self._collision_track['reactions'])
+            if 'universe_ids' in self._collision_track:
+                subelement = ET.SubElement(element, "universe_ids")
+                subelement.text = ' '.join(
+                    str(x) for x in self._collision_track['universe_ids'])
+            if 'material_ids' in self._collision_track:
+                subelement = ET.SubElement(element, "material_ids")
+                subelement.text = ' '.join(
+                    str(x) for x in self._collision_track['material_ids'])
+            if 'nuclides' in self._collision_track:
+                subelement = ET.SubElement(element, "nuclides")
+                subelement.text = ' '.join(
+                    str(x) for x in self._collision_track['nuclides'])
+            if 'deposited_E_threshold' in self._collision_track:
+                subelement = ET.SubElement(element, "deposited_E_threshold")
+                subelement.text = str(
+                    self._collision_track['deposited_E_threshold'])
+            if 'max_collisions' in self._collision_track:
+                subelement = ET.SubElement(element, "max_collisions")
+                subelement.text = str(self._collision_track['max_collisions'])
+            if 'max_collision_track_files' in self._collision_track:
+                subelement = ET.SubElement(
+                    element, "max_collision_track_files")
+                subelement.text = str(
+                    self._collision_track['max_collision_track_files'])
+            if 'mcpl' in self._collision_track:
+                subelement = ET.SubElement(element, "mcpl")
+                subelement.text = str(self._collision_track['mcpl']).lower()
+
     def _create_confidence_intervals(self, root):
         if self._confidence_intervals is not None:
             element = ET.SubElement(root, "confidence_intervals")
@@ -1353,6 +1691,11 @@ class Settings:
             element = ET.SubElement(root, "electron_treatment")
             element.text = str(self._electron_treatment)
 
+    def _create_atomic_relaxation_subelement(self, root):
+        if self._atomic_relaxation is not None:
+            element = ET.SubElement(root, "atomic_relaxation")
+            element.text = str(self._atomic_relaxation).lower()
+
     def _create_photon_transport_subelement(self, root):
         if self._photon_transport is not None:
             element = ET.SubElement(root, "photon_transport")
@@ -1378,6 +1721,16 @@ class Settings:
             element = ET.SubElement(root, "stride")
             element.text = str(self._stride)
 
+    def _create_surface_grazing_cutoff_subelement(self, root):
+        if self._surface_grazing_cutoff is not None:
+            element = ET.SubElement(root, "surface_grazing_cutoff")
+            element.text = str(self._surface_grazing_cutoff)
+
+    def _create_surface_grazing_ratio_subelement(self, root):
+        if self._surface_grazing_ratio is not None:
+            element = ET.SubElement(root, "surface_grazing_ratio")
+            element.text = str(self._surface_grazing_ratio)
+
     def _create_survival_biasing_subelement(self, root):
         if self._survival_biasing is not None:
             element = ET.SubElement(root, "survival_biasing")
@@ -1398,8 +1751,8 @@ class Settings:
         # use default heuristic for entropy mesh if not set by user
         if self.entropy_mesh.dimension is None:
             if self.particles is None:
-                raise RuntimeError("Number of particles must be set in order to " \
-                    "use entropy mesh dimension heuristic")
+                raise RuntimeError("Number of particles must be set in order to "
+                                   "use entropy mesh dimension heuristic")
             else:
                 n = ceil((self.particles / 20.0)**(1.0 / 3.0))
                 d = len(self.entropy_mesh.lower_left)
@@ -1440,6 +1793,11 @@ class Settings:
             element = ET.SubElement(root, "no_reduce")
             element.text = str(self._no_reduce).lower()
 
+    def _create_ifp_n_generation_subelement(self, root):
+        if self._ifp_n_generation is not None:
+            element = ET.SubElement(root, "ifp_n_generation")
+            element.text = str(self._ifp_n_generation)
+
     def _create_tabular_legendre_subelements(self, root):
         if self.tabular_legendre:
             element = ET.SubElement(root, "tabular_legendre")
@@ -1460,6 +1818,12 @@ class Settings:
                 else:
                     element.text = str(value)
 
+    def _create_properties_file_element(self, root):
+        if self.properties_file is not None:
+            element = ET.Element("properties_file")
+            element.text = str(self.properties_file)
+            root.append(element)
+
     def _create_trace_subelement(self, root):
         if self._trace is not None:
             element = ET.SubElement(root, "trace")
@@ -1484,7 +1848,8 @@ class Settings:
         path = f"./mesh[@id='{self.ufs_mesh.id}']"
         if root.find(path) is None:
             root.append(self.ufs_mesh.to_xml_element())
-            if mesh_memo is not None: mesh_memo.add(self.ufs_mesh.id)
+            if mesh_memo is not None:
+                mesh_memo.add(self.ufs_mesh.id)
 
     def _create_use_decay_photons_subelement(self, root):
         if self._use_decay_photons is not None:
@@ -1573,10 +1938,16 @@ class Settings:
                 if mesh_memo is not None:
                     mesh_memo.add(ww.mesh.id)
 
+    def _create_weight_windows_on_subelement(self, root):
         if self._weight_windows_on is not None:
             elem = ET.SubElement(root, "weight_windows_on")
             elem.text = str(self._weight_windows_on).lower()
 
+    def _create_shared_secondary_bank_subelement(self, root):
+        if self._shared_secondary_bank is not None:
+            elem = ET.SubElement(root, "shared_secondary_bank")
+            elem.text = str(self._shared_secondary_bank).lower()
+
     def _create_weight_window_generators_subelement(self, root, mesh_memo=None):
         if not self.weight_window_generators:
             return
@@ -1589,9 +1960,12 @@ class Settings:
             if mesh_memo is not None and wwg.mesh.id in mesh_memo:
                 continue
 
-            root.append(wwg.mesh.to_xml_element())
-            if mesh_memo is not None:
-                mesh_memo.add(wwg.mesh.id)
+            # See if a <mesh> element already exists -- if not, add it
+            path = f"./mesh[@id='{wwg.mesh.id}']"
+            if root.find(path) is None:
+                root.append(wwg.mesh.to_xml_element())
+                if mesh_memo is not None:
+                    mesh_memo.add(wwg.mesh.id)
 
     def _create_weight_windows_file_element(self, root):
         if self.weight_windows_file is not None:
@@ -1606,17 +1980,24 @@ class Settings:
 
         if 'collision' in self._weight_window_checkpoints:
             subelement = ET.SubElement(element, "collision")
-            subelement.text = str(self._weight_window_checkpoints['collision']).lower()
+            subelement.text = str(
+                self._weight_window_checkpoints['collision']).lower()
 
         if 'surface' in self._weight_window_checkpoints:
             subelement = ET.SubElement(element, "surface")
-            subelement.text = str(self._weight_window_checkpoints['surface']).lower()
+            subelement.text = str(
+                self._weight_window_checkpoints['surface']).lower()
 
     def _create_max_history_splits_subelement(self, root):
         if self._max_history_splits is not None:
             elem = ET.SubElement(root, "max_history_splits")
             elem.text = str(self._max_history_splits)
 
+    def _create_max_secondaries_subelement(self, root):
+        if self._max_secondaries is not None:
+            elem = ET.SubElement(root, "max_secondaries")
+            elem.text = str(self._max_secondaries)
+
     def _create_max_tracks_subelement(self, root):
         if self._max_tracks is not None:
             elem = ET.SubElement(root, "max_tracks")
@@ -1627,8 +2008,13 @@ class Settings:
             element = ET.SubElement(root, "random_ray")
             for key, value in self._random_ray.items():
                 if key == 'ray_source' and isinstance(value, SourceBase):
+                    subelement = ET.SubElement(element, 'ray_source')
                     source_element = value.to_xml_element()
-                    element.append(source_element)
+                    if source_element.find('bias') is not None:
+                        raise RuntimeError(
+                            "Ray source distributions should not be biased.")
+                    subelement.append(source_element)
+
                 elif key == 'source_region_meshes':
                     subelement = ET.SubElement(element, 'source_region_meshes')
                     for mesh, domains in value:
@@ -1637,14 +2023,45 @@ class Settings:
                         for domain in domains:
                             domain_elem = ET.SubElement(mesh_elem, 'domain')
                             domain_elem.set('id', str(domain.id))
-                            domain_elem.set('type', domain.__class__.__name__.lower())
+                            domain_elem.set(
+                                'type', domain.__class__.__name__.lower())
                         if mesh_memo is not None and mesh.id not in mesh_memo:
+                            domain_elem.set('type', domain.__class__.__name__.lower())
+                        # See if a <mesh> element already exists -- if not, add it
+                        path = f"./mesh[@id='{mesh.id}']"
+                        if root.find(path) is None:
                             root.append(mesh.to_xml_element())
-                            mesh_memo.add(mesh.id)
+                            if mesh_memo is not None:    
+                                mesh_memo.add(mesh.id)
+                elif key == 'adjoint_source':
+                    subelement = ET.SubElement(element, 'adjoint_source')
+                    # Check that all entries are valid SourceBase instances, in case 
+                    # the random_ray setter was not used to populate dict entries.
+                    if not isinstance(value, MutableSequence):
+                        value = [value]
+                    for source in value:
+                        if not isinstance(source, SourceBase):
+                            raise ValueError(
+                                f'Invalid adjoint source type: {type(source)}. '
+                                'Expected openmc.SourceBase.')
+                        subelement.append(source.to_xml_element())
+                elif isinstance(value, bool):
+                    subelement = ET.SubElement(element, key)
+                    subelement.text = str(value).lower()
                 else:
                     subelement = ET.SubElement(element, key)
                     subelement.text = str(value)
 
+    def _create_source_rejection_fraction_subelement(self, root):
+        if self._source_rejection_fraction is not None:
+            element = ET.SubElement(root, "source_rejection_fraction")
+            element.text = str(self._source_rejection_fraction)
+
+    def _create_free_gas_threshold_subelement(self, root):
+        if self._free_gas_threshold is not None:
+            element = ET.SubElement(root, "free_gas_threshold")
+            element.text = str(self._free_gas_threshold)
+
     def _eigenvalue_from_xml_element(self, root):
         elem = root.find('eigenvalue')
         if elem is not None:
@@ -1730,23 +2147,21 @@ class Settings:
     def _statepoint_from_xml_element(self, root):
         elem = root.find('state_point')
         if elem is not None:
-            text = get_text(elem, 'batches')
-            if text is not None:
-                self.statepoint['batches'] = [int(x) for x in text.split()]
+            batches = get_elem_list(elem, "batches", int)
+            if batches is not None:
+                self.statepoint['batches'] = batches
 
     def _sourcepoint_from_xml_element(self, root):
         elem = root.find('source_point')
         if elem is not None:
             for key in ('separate', 'write', 'overwrite_latest', 'batches', 'mcpl'):
-                value = get_text(elem, key)
-                if value is not None:
-                    if key in ('separate', 'write', 'mcpl'):
-                        value = value in ('true', '1')
-                    elif key == 'overwrite_latest':
-                        value = value in ('true', '1')
+                if key in ('separate', 'write', 'mcpl', 'overwrite_latest'):
+                    value = get_text(elem, key) in ('true', '1')
+                    if key == 'overwrite_latest':
                         key = 'overwrite'
-                    else:
-                        value = [int(x) for x in value.split()]
+                else:
+                    value = get_elem_list(elem, key, int)
+                if value is not None:
                     self.sourcepoint[key] = value
 
     def _surf_source_read_from_xml_element(self, root):
@@ -1763,16 +2178,36 @@ class Settings:
         if elem is None:
             return
         for key in ('surface_ids', 'max_particles', 'max_source_files', 'mcpl', 'cell', 'cellto', 'cellfrom'):
-            value = get_text(elem, key)
+            if key == 'surface_ids':
+                value = get_elem_list(elem, key, int)
+            else:
+                value = get_text(elem, key)
             if value is not None:
-                if key == 'surface_ids':
-                    value = [int(x) for x in value.split()]
-                elif key == 'mcpl':
+                if key == 'mcpl':
                     value = value in ('true', '1')
                 elif key in ('max_particles', 'max_source_files', 'cell', 'cellfrom', 'cellto'):
                     value = int(value)
                 self.surf_source_write[key] = value
 
+    def _collision_track_from_xml_element(self, root):
+        elem = root.find('collision_track')
+        if elem is not None:
+            for key in ('cell_ids', 'reactions', 'universe_ids', 'material_ids', 'nuclides',
+                        'deposited_E_threshold', 'max_collisions', "max_collision_track_files", 'mcpl'):
+                value = get_text(elem, key)
+                if value is not None:
+                    if key in ('cell_ids', 'universe_ids', 'material_ids'):
+                        value = [int(x) for x in value.split()]
+                    elif key in ('reactions', 'nuclides'):
+                        value = value.split()
+                    elif key in ('max_collisions', 'max_collision_track_files'):
+                        value = int(value)
+                    elif key == 'deposited_E_threshold':
+                        value = float(value)
+                    elif key == 'mcpl':
+                        value = value in ('true', '1')
+                    self.collision_track[key] = value
+
     def _confidence_intervals_from_xml_element(self, root):
         text = get_text(root, 'confidence_intervals')
         if text is not None:
@@ -1783,6 +2218,11 @@ class Settings:
         if text is not None:
             self.electron_treatment = text
 
+    def _atomic_relaxation_from_xml_element(self, root):
+        text = get_text(root, 'atomic_relaxation')
+        if text is not None:
+            self.atomic_relaxation = text in ('true', '1')
+
     def _energy_mode_from_xml_element(self, root):
         text = get_text(root, 'energy_mode')
         if text is not None:
@@ -1823,6 +2263,16 @@ class Settings:
         if text is not None:
             self.stride = int(text)
 
+    def _surface_grazing_cutoff_from_xml_element(self, root):
+        text = get_text(root, 'surface_grazing_cutoff')
+        if text is not None:
+            self.surface_grazing_cutoff = float(text)
+
+    def _surface_grazing_ratio_from_xml_element(self, root):
+        text = get_text(root, 'surface_grazing_ratio')
+        if text is not None:
+            self.surface_grazing_ratio = float(text)
+
     def _survival_biasing_from_xml_element(self, root):
         text = get_text(root, 'survival_biasing')
         if text is not None:
@@ -1873,6 +2323,11 @@ class Settings:
         if text is not None:
             self.verbosity = int(text)
 
+    def _ifp_n_generation_from_xml_element(self, root):
+        text = get_text(root, 'ifp_n_generation')
+        if text is not None:
+            self.ifp_n_generation = int(text)
+
     def _tabular_legendre_from_xml_element(self, root):
         elem = root.find('tabular_legendre')
         if elem is not None:
@@ -1892,22 +2347,26 @@ class Settings:
         text = get_text(root, 'temperature_method')
         if text is not None:
             self.temperature['method'] = text
-        text = get_text(root, 'temperature_range')
+        text = get_elem_list(root, "temperature_range", float)
         if text is not None:
-            self.temperature['range'] = [float(x) for x in text.split()]
+            self.temperature['range'] = text
         text = get_text(root, 'temperature_multipole')
         if text is not None:
             self.temperature['multipole'] = text in ('true', '1')
 
-    def _trace_from_xml_element(self, root):
-        text = get_text(root, 'trace')
+    def _properties_file_from_xml_element(self, root):
+        text = get_text(root, 'properties_file')
         if text is not None:
-            self.trace = [int(x) for x in text.split()]
+            self.properties_file = text
+
+    def _trace_from_xml_element(self, root):
+        text = get_elem_list(root, "trace", int)
+        if text is not None:
+            self.trace = text
 
     def _track_from_xml_element(self, root):
-        text = get_text(root, 'track')
-        if text is not None:
-            values = [int(x) for x in text.split()]
+        values = get_elem_list(root, "track", int)
+        if values is not None:
             self.track = list(zip(values[::3], values[1::3], values[2::3]))
 
     def _ufs_mesh_from_xml_element(self, root, meshes):
@@ -1924,14 +2383,15 @@ class Settings:
         if elem is not None:
             keys = ('enable', 'method', 'energy_min', 'energy_max', 'nuclides')
             for key in keys:
-                value = get_text(elem, key)
+                if key == 'nuclides':
+                    value = get_elem_list(elem, key, str)
+                else:
+                    value = get_text(elem, key)
                 if value is not None:
                     if key == 'enable':
                         value = value in ('true', '1')
                     elif key in ('energy_min', 'energy_max'):
                         value = float(value)
-                    elif key == 'nuclides':
-                        value = value.split()
                     self.resonance_scattering[key] = value
 
     def _create_fission_neutrons_from_xml_element(self, root):
@@ -1989,10 +2449,21 @@ class Settings:
             ww = WeightWindows.from_xml_element(elem, meshes)
             self.weight_windows.append(ww)
 
+    def _weight_windows_on_from_xml_element(self, root):
         text = get_text(root, 'weight_windows_on')
         if text is not None:
             self.weight_windows_on = text in ('true', '1')
 
+    def _shared_secondary_bank_from_xml_element(self, root):
+        text = get_text(root, 'shared_secondary_bank')
+        if text is not None:
+            self.shared_secondary_bank = text in ('true', '1')
+
+    def _weight_windows_file_from_xml_element(self, root):
+        text = get_text(root, 'weight_windows_file')
+        if text is not None:
+            self.weight_windows_file = text
+
     def _weight_window_checkpoints_from_xml_element(self, root):
         elem = root.find('weight_window_checkpoints')
         if elem is None:
@@ -2008,20 +2479,29 @@ class Settings:
         if text is not None:
             self.max_history_splits = int(text)
 
+    def _max_secondaries_from_xml_element(self, root):
+        text = get_text(root, 'max_secondaries')
+        if text is not None:
+            self.max_secondaries = int(text)
+
     def _max_tracks_from_xml_element(self, root):
         text = get_text(root, 'max_tracks')
         if text is not None:
             self.max_tracks = int(text)
 
-    def _random_ray_from_xml_element(self, root):
+    def _random_ray_from_xml_element(self, root, meshes=None):
         elem = root.find('random_ray')
         if elem is not None:
             self.random_ray = {}
             for child in elem:
-                if child.tag in ('distance_inactive', 'distance_active'):
+                if child.tag in ('distance_inactive', 'distance_active', 'diagonal_stabilization_rho'):
                     self.random_ray[child.tag] = float(child.text)
-                elif child.tag == 'source':
-                    source = SourceBase.from_xml_element(child)
+                elif child.tag == 'ray_source':
+                    source_element = child.find('source')
+                    source = SourceBase.from_xml_element(source_element)
+                    if child.find('bias') is not None:
+                        raise RuntimeError(
+                            "Ray source distributions should not be biased.")
                     self.random_ray['ray_source'] = source
                 elif child.tag == 'volume_estimator':
                     self.random_ray['volume_estimator'] = child.text
@@ -2035,16 +2515,26 @@ class Settings:
                     self.random_ray['adjoint'] = (
                         child.text in ('true', '1')
                     )
+                elif child.tag == 'adjoint_source':
+                    self.random_ray['adjoint_source'] = []
+                    for subelem in child.findall('source'):
+                        src = SourceBase.from_xml_element(subelem)
+                        # add newly constructed source object to the list
+                        self.random_ray['adjoint_source'].append(src)
                 elif child.tag == 'sample_method':
                     self.random_ray['sample_method'] = child.text
                 elif child.tag == 'source_region_meshes':
                     self.random_ray['source_region_meshes'] = []
                     for mesh_elem in child.findall('mesh'):
-                        mesh = MeshBase.from_xml_element(mesh_elem)
+                        mesh_id = int(get_text(mesh_elem, 'id'))
+                        if meshes and mesh_id in meshes:
+                            mesh = meshes[mesh_id]
+                        else:
+                            mesh = MeshBase.from_xml_element(mesh_elem)
                         domains = []
                         for domain_elem in mesh_elem.findall('domain'):
-                            domain_id = int(domain_elem.get('id'))
-                            domain_type = domain_elem.get('type')
+                            domain_id = int(get_text(domain_elem, "id"))
+                            domain_type = get_text(domain_elem, "type")
                             if domain_type == 'material':
                                 domain = openmc.Material(domain_id)
                             elif domain_type == 'cell':
@@ -2052,13 +2542,24 @@ class Settings:
                             elif domain_type == 'universe':
                                 domain = openmc.Universe(domain_id)
                             domains.append(domain)
-                        self.random_ray['source_region_meshes'].append((mesh, domains))
+                        self.random_ray['source_region_meshes'].append(
+                            (mesh, domains))
 
     def _use_decay_photons_from_xml_element(self, root):
         text = get_text(root, 'use_decay_photons')
         if text is not None:
             self.use_decay_photons = text in ('true', '1')
 
+    def _source_rejection_fraction_from_xml_element(self, root):
+        text = get_text(root, 'source_rejection_fraction')
+        if text is not None:
+            self.source_rejection_fraction = float(text)
+
+    def _free_gas_threshold_from_xml_element(self, root):
+        text = get_text(root, 'free_gas_threshold')
+        if text is not None:
+            self.free_gas_threshold = float(text)
+
     def to_xml_element(self, mesh_memo=None):
         """Create a 'settings' element to be written to an XML file.
 
@@ -2085,8 +2586,10 @@ class Settings:
         self._create_sourcepoint_subelement(element)
         self._create_surf_source_read_subelement(element)
         self._create_surf_source_write_subelement(element)
+        self._create_collision_track_subelement(element)
         self._create_confidence_intervals(element)
         self._create_electron_treatment_subelement(element)
+        self._create_atomic_relaxation_subelement(element)
         self._create_energy_mode_subelement(element)
         self._create_max_order_subelement(element)
         self._create_photon_transport_subelement(element)
@@ -2095,14 +2598,18 @@ class Settings:
         self._create_ptables_subelement(element)
         self._create_seed_subelement(element)
         self._create_stride_subelement(element)
+        self._create_surface_grazing_cutoff_subelement(element)
+        self._create_surface_grazing_ratio_subelement(element)
         self._create_survival_biasing_subelement(element)
         self._create_cutoff_subelement(element)
         self._create_entropy_mesh_subelement(element, mesh_memo)
         self._create_trigger_subelement(element)
         self._create_no_reduce_subelement(element)
         self._create_verbosity_subelement(element)
+        self._create_ifp_n_generation_subelement(element)
         self._create_tabular_legendre_subelements(element)
         self._create_temperature_subelements(element)
+        self._create_properties_file_element(element)
         self._create_trace_subelement(element)
         self._create_track_subelement(element)
         self._create_ufs_mesh_subelement(element, mesh_memo)
@@ -2118,17 +2625,21 @@ class Settings:
         self._create_log_grid_bins_subelement(element)
         self._create_write_initial_source_subelement(element)
         self._create_weight_windows_subelement(element, mesh_memo)
+        self._create_weight_windows_on_subelement(element)
+        self._create_shared_secondary_bank_subelement(element)
         self._create_weight_window_generators_subelement(element, mesh_memo)
         self._create_weight_windows_file_element(element)
         self._create_weight_window_checkpoints_subelement(element)
         self._create_max_history_splits_subelement(element)
         self._create_max_tracks_subelement(element)
+        self._create_max_secondaries_subelement(element)
         self._create_random_ray_subelement(element, mesh_memo)
         self._create_use_decay_photons_subelement(element)
+        self._create_source_rejection_fraction_subelement(element)
+        self._create_free_gas_threshold_subelement(element)
 
         # Clean the indentation in the file to be user-readable
         clean_indentation(element)
-        reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
 
         return element
 
@@ -2194,8 +2705,10 @@ class Settings:
         settings._sourcepoint_from_xml_element(elem)
         settings._surf_source_read_from_xml_element(elem)
         settings._surf_source_write_from_xml_element(elem)
+        settings._collision_track_from_xml_element(elem)
         settings._confidence_intervals_from_xml_element(elem)
         settings._electron_treatment_from_xml_element(elem)
+        settings._atomic_relaxation_from_xml_element(elem)
         settings._energy_mode_from_xml_element(elem)
         settings._max_order_from_xml_element(elem)
         settings._photon_transport_from_xml_element(elem)
@@ -2204,14 +2717,18 @@ class Settings:
         settings._ptables_from_xml_element(elem)
         settings._seed_from_xml_element(elem)
         settings._stride_from_xml_element(elem)
+        settings._surface_grazing_cutoff_from_xml_element(elem)
+        settings._surface_grazing_ratio_from_xml_element(elem)
         settings._survival_biasing_from_xml_element(elem)
         settings._cutoff_from_xml_element(elem)
         settings._entropy_mesh_from_xml_element(elem, meshes)
         settings._trigger_from_xml_element(elem)
         settings._no_reduce_from_xml_element(elem)
         settings._verbosity_from_xml_element(elem)
+        settings._ifp_n_generation_from_xml_element(elem)
         settings._tabular_legendre_from_xml_element(elem)
         settings._temperature_from_xml_element(elem)
+        settings._properties_file_from_xml_element(elem)
         settings._trace_from_xml_element(elem)
         settings._track_from_xml_element(elem)
         settings._ufs_mesh_from_xml_element(elem, meshes)
@@ -2226,12 +2743,18 @@ class Settings:
         settings._log_grid_bins_from_xml_element(elem)
         settings._write_initial_source_from_xml_element(elem)
         settings._weight_windows_from_xml_element(elem, meshes)
+        settings._weight_windows_on_from_xml_element(elem)
+        settings._shared_secondary_bank_from_xml_element(elem)
+        settings._weight_windows_file_from_xml_element(elem)
         settings._weight_window_generators_from_xml_element(elem, meshes)
         settings._weight_window_checkpoints_from_xml_element(elem)
         settings._max_history_splits_from_xml_element(elem)
         settings._max_tracks_from_xml_element(elem)
-        settings._random_ray_from_xml_element(elem)
+        settings._max_secondaries_from_xml_element(elem)
+        settings._random_ray_from_xml_element(elem, meshes)
         settings._use_decay_photons_from_xml_element(elem)
+        settings._source_rejection_fraction_from_xml_element(elem)
+        settings._free_gas_threshold_from_xml_element(elem)
 
         return settings
 
diff --git a/openmc/source.py b/openmc/source.py
index 878f52c3b..5947540d6 100644
--- a/openmc/source.py
+++ b/openmc/source.py
@@ -1,12 +1,10 @@
 from __future__ import annotations
 from abc import ABC, abstractmethod
 from collections.abc import Iterable, Sequence
-from enum import IntEnum
-from numbers import Real
+from numbers import Integral, Real
 from pathlib import Path
 import warnings
 from typing import Any
-from pathlib import Path
 
 import lxml.etree as ET
 import numpy as np
@@ -18,8 +16,10 @@ import openmc.checkvalue as cv
 from openmc.checkvalue import PathLike
 from openmc.stats.multivariate import UnitSphere, Spatial
 from openmc.stats.univariate import Univariate
-from ._xml import get_text
+from ._xml import get_elem_list, get_text
 from .mesh import MeshBase, StructuredMesh, UnstructuredMesh
+from .particle_type import ParticleType
+from .statepoint import _VERSION_STATEPOINT
 from .utility_funcs import input_path
 
 
@@ -46,7 +46,7 @@ class SourceBase(ABC):
 
     Attributes
     ----------
-    type : {'independent', 'file', 'compiled', 'mesh'}
+    type : {'independent', 'file', 'compiled', 'mesh', 'tokamak'}
         Indicator of source type.
     strength : float
         Strength of the source
@@ -107,10 +107,12 @@ class SourceBase(ABC):
                 cv.check_type('fissionable', value, bool)
                 self._constraints['fissionable'] = value
             elif key == 'rejection_strategy':
-                cv.check_value('rejection strategy', value, ('resample', 'kill'))
+                cv.check_value('rejection strategy',
+                               value, ('resample', 'kill'))
                 self._constraints['rejection_strategy'] = value
             else:
-                raise ValueError('Unknown key in constraints dictionary: {key}')
+                raise ValueError(
+                    f'Unknown key in constraints dictionary: {key}')
 
     @abstractmethod
     def populate_xml_element(self, element):
@@ -144,13 +146,16 @@ class SourceBase(ABC):
                 dt_elem = ET.SubElement(constraints_elem, "domain_type")
                 dt_elem.text = constraints["domain_type"]
                 id_elem = ET.SubElement(constraints_elem, "domain_ids")
-                id_elem.text = ' '.join(str(uid) for uid in constraints["domain_ids"])
+                id_elem.text = ' '.join(str(uid)
+                                        for uid in constraints["domain_ids"])
             if "time_bounds" in constraints:
                 dt_elem = ET.SubElement(constraints_elem, "time_bounds")
-                dt_elem.text = ' '.join(str(t) for t in constraints["time_bounds"])
+                dt_elem.text = ' '.join(str(t)
+                                        for t in constraints["time_bounds"])
             if "energy_bounds" in constraints:
                 dt_elem = ET.SubElement(constraints_elem, "energy_bounds")
-                dt_elem.text = ' '.join(str(E) for E in constraints["energy_bounds"])
+                dt_elem.text = ' '.join(str(E)
+                                        for E in constraints["energy_bounds"])
             if "fissionable" in constraints:
                 dt_elem = ET.SubElement(constraints_elem, "fissionable")
                 dt_elem.text = str(constraints["fissionable"]).lower()
@@ -198,8 +203,11 @@ class SourceBase(ABC):
                 return FileSource.from_xml_element(elem)
             elif source_type == 'mesh':
                 return MeshSource.from_xml_element(elem, meshes)
+            elif source_type == 'tokamak':
+                return TokamakSource.from_xml_element(elem)
             else:
-                raise ValueError(f'Source type {source_type} is not recognized')
+                raise ValueError(
+                    f'Source type {source_type} is not recognized')
 
     @staticmethod
     def _get_constraints(elem: ET.Element) -> dict[str, Any]:
@@ -210,7 +218,7 @@ class SourceBase(ABC):
         constraints = {}
         domain_type = get_text(elem, "domain_type")
         if domain_type is not None:
-            domain_ids = [int(x) for x in get_text(elem, "domain_ids").split()]
+            domain_ids = get_elem_list(elem, "domain_ids", int)
 
             # Instantiate some throw-away domains that are used by the
             # constructor to assign IDs
@@ -224,13 +232,13 @@ class SourceBase(ABC):
                     domains = [openmc.Universe(uid) for uid in domain_ids]
             constraints['domains'] = domains
 
-        time_bounds = get_text(elem, "time_bounds")
+        time_bounds = get_elem_list(elem, "time_bounds", float)
         if time_bounds is not None:
-            constraints['time_bounds'] = [float(x) for x in time_bounds.split()]
+            constraints['time_bounds'] = time_bounds
 
-        energy_bounds = get_text(elem, "energy_bounds")
+        energy_bounds = get_elem_list(elem, "energy_bounds", float)
         if energy_bounds is not None:
-            constraints['energy_bounds'] = [float(x) for x in energy_bounds.split()]
+            constraints['energy_bounds'] = energy_bounds
 
         fissionable = get_text(elem, "fissionable")
         if fissionable is not None:
@@ -260,8 +268,8 @@ class IndependentSource(SourceBase):
         time distribution of source sites
     strength : float
         Strength of the source
-    particle : {'neutron', 'photon'}
-        Source particle type
+    particle : str or int or openmc.ParticleType
+        Source particle type (name, PDG number, or type)
     domains : iterable of openmc.Cell, openmc.Material, or openmc.Universe
         Domains to reject based on, i.e., if a sampled spatial location is not
         within one of these domains, it will be rejected.
@@ -297,10 +305,9 @@ class IndependentSource(SourceBase):
     type : str
         Indicator of source type: 'independent'
 
-    .. versionadded:: 0.14.0
-
-    particle : {'neutron', 'photon'}
-        Source particle type
+        .. versionadded:: 0.14.0
+    particle : str or int or openmc.ParticleType
+        Source particle type (alias, PDG number, or GNDS nuclide name)
     constraints : dict
         Constraints on sampled source particles. Valid keys include
         'domain_type', 'domain_ids', 'time_bounds', 'energy_bounds',
@@ -315,8 +322,9 @@ class IndependentSource(SourceBase):
         energy: openmc.stats.Univariate | None = None,
         time: openmc.stats.Univariate | None = None,
         strength: float = 1.0,
-        particle: str = 'neutron',
-        domains: Sequence[openmc.Cell | openmc.Material | openmc.Universe] | None = None,
+        particle: str | int | ParticleType = 'neutron',
+        domains: Sequence[openmc.Cell | openmc.Material |
+                          openmc.Universe] | None = None,
         constraints: dict[str, Any] | None = None
     ):
         if domains is not None:
@@ -399,13 +407,12 @@ class IndependentSource(SourceBase):
         self._time = time
 
     @property
-    def particle(self):
+    def particle(self) -> ParticleType:
         return self._particle
 
     @particle.setter
     def particle(self, particle):
-        cv.check_value('source particle', particle, ['neutron', 'photon'])
-        self._particle = particle
+        self._particle = ParticleType(particle)
 
     def populate_xml_element(self, element):
         """Add necessary source information to an XML element
@@ -416,7 +423,7 @@ class IndependentSource(SourceBase):
             XML element containing source data
 
         """
-        element.set("particle", self.particle)
+        element.set("particle", str(self.particle))
         if self.space is not None:
             element.append(self.space.to_xml_element())
         if self.angle is not None:
@@ -526,11 +533,12 @@ class MeshSource(SourceBase):
         'fissionable', and 'rejection_strategy'.
 
     """
+
     def __init__(
             self,
             mesh: MeshBase,
             sources: Sequence[SourceBase],
-            constraints: dict[str, Any] | None  = None,
+            constraints: dict[str, Any] | None = None,
     ):
         super().__init__(strength=None, constraints=constraints)
         self.mesh = mesh
@@ -564,10 +572,10 @@ class MeshSource(SourceBase):
         s = np.asarray(s)
 
         if isinstance(self.mesh, StructuredMesh):
-            if s.size != self.mesh.num_mesh_cells:
+            if s.size != self.mesh.n_elements:
                 raise ValueError(
                     f'The length of the source array ({s.size}) does not match '
-                    f'the number of mesh elements ({self.mesh.num_mesh_cells}).')
+                    f'the number of mesh elements ({self.mesh.n_elements}).')
 
             # If user gave a multidimensional array, flatten in the order
             # of the mesh indices
@@ -576,7 +584,8 @@ class MeshSource(SourceBase):
 
         elif isinstance(self.mesh, UnstructuredMesh):
             if s.ndim > 1:
-                raise ValueError('Sources must be a 1-D array for unstructured mesh')
+                raise ValueError(
+                    'Sources must be a 1-D array for unstructured mesh')
 
         self._sources = s
         for src in self._sources:
@@ -645,7 +654,8 @@ class MeshSource(SourceBase):
         mesh_id = int(get_text(elem, 'mesh'))
         mesh = meshes[mesh_id]
 
-        sources = [SourceBase.from_xml_element(e) for e in elem.iterchildren('source')]
+        sources = [SourceBase.from_xml_element(
+            e) for e in elem.iterchildren('source')]
         constraints = cls._get_constraints(elem)
         return cls(mesh, sources, constraints=constraints)
 
@@ -655,7 +665,8 @@ def Source(*args, **kwargs):
     A function for backward compatibility of sources. Will be removed in the
     future. Please update to IndependentSource.
     """
-    warnings.warn("This class is deprecated in favor of 'IndependentSource'", FutureWarning)
+    warnings.warn(
+        "This class is deprecated in favor of 'IndependentSource'", FutureWarning)
     return openmc.IndependentSource(*args, **kwargs)
 
 
@@ -702,6 +713,7 @@ class CompiledSource(SourceBase):
         'fissionable', and 'rejection_strategy'.
 
     """
+
     def __init__(
         self,
         library: PathLike,
@@ -886,75 +898,430 @@ class FileSource(SourceBase):
         return cls(**kwargs)
 
 
-class ParticleType(IntEnum):
-    """
-    IntEnum class representing a particle type. Type
-    values mirror those found in the C++ class.
-    """
-    NEUTRON = 0
-    PHOTON = 1
-    ELECTRON = 2
-    POSITRON = 3
+class TokamakSource(SourceBase):
+    r"""A source representing neutron emission from a tokamak plasma.
 
-    @classmethod
-    def from_string(cls, value: str):
-        """
-        Constructs a ParticleType instance from a string.
+    This source samples neutron positions from a tokamak plasma geometry using
+    Miller-style flux surface parameterization. The user provides an emission
+    profile S(r/a) as a function of normalized minor radius, along with one or
+    more energy distributions.
 
-        Parameters
-        ----------
-        value : str
-            The string representation of the particle type.
+    The flux surface parameterization is
+
+    .. math::
+
+        \begin{aligned}
+        R &= R_0 + r \cos\left(\alpha + \delta \sin\alpha\right)
+             + \Delta \left[1 - \left(\frac{r}{a}\right)^2\right] \\
+        Z &= Z_\mathrm{shift} + \kappa r \sin\alpha
+        \end{aligned}
+
+    where :math:`R_0` is major radius, :math:`a` is minor radius,
+    :math:`\kappa` is elongation, :math:`\delta` is triangularity,
+    :math:`\Delta` is the Shafranov shift, and :math:`Z_\mathrm{shift}` is
+    the vertical shift.
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    major_radius : float
+        Major radius R0 in [cm]
+    minor_radius : float
+        Minor radius a in [cm]
+    elongation : float
+        Plasma elongation κ (must be > 0)
+    triangularity : float
+        Plasma triangularity δ (must be in [-1, 1])
+    shafranov_shift : float
+        Shafranov shift Δ in [cm] (must be >= 0 and < a/2)
+    r_over_a : numpy.ndarray
+        Normalized minor radius grid points, must start at 0 and end at 1
+    emission_density : numpy.ndarray
+        Emission density S(r) at each r/a point (arbitrary units, must be >= 0).
+        Values are linearly interpolated between grid points and refined on an
+        internal grid for radial sampling. Must have the same length as
+        ``r_over_a`` and contain at least one positive value.
+    energy : openmc.stats.Univariate or Sequence[openmc.stats.Univariate]
+        Energy distribution(s). Either a single distribution used at all radii,
+        or one distribution per ``r_over_a`` grid point. When one distribution
+        per grid point is given, the energy of a sampled particle is drawn from
+        one of the two distributions bracketing its sampled radius, selected
+        stochastically with probability proportional to the proximity of the
+        radius to each grid point (stochastic interpolation).
+    time : openmc.stats.Univariate, optional
+        Time distribution of the source. If None, particles are born at
+        :math:`t=0`, matching the default behavior of
+        :class:`openmc.IndependentSource`.
+    phi_start : float
+        Starting toroidal angle in [rad] (default: 0)
+    phi_extent : float
+        Toroidal angle extent in [rad] (default: 2π)
+    n_alpha : int
+        Number of poloidal angle grid points for CDF sampling (default: 101)
+    vertical_shift : float
+        Vertical shift of the plasma center in [cm] (default: 0)
+    strength : float
+        Strength of the source (default: 1.0)
+    constraints : dict
+        Constraints on sampled source particles. See :class:`SourceBase` for
+        valid keys and values.
+
+    Attributes
+    ----------
+    major_radius : float
+        Major radius R0 in [cm]
+    minor_radius : float
+        Minor radius a in [cm]
+    elongation : float
+        Plasma elongation κ
+    triangularity : float
+        Plasma triangularity δ
+    shafranov_shift : float
+        Shafranov shift Δ in [cm]
+    r_over_a : numpy.ndarray
+        Normalized minor radius grid points
+    emission_density : numpy.ndarray
+        Emission density S(r) at each r/a point
+    energy : list of openmc.stats.Univariate
+        Energy distribution(s)
+    time : openmc.stats.Univariate or None
+        Time distribution of the source
+    phi_start : float
+        Starting toroidal angle in [rad]
+    phi_extent : float
+        Toroidal angle extent in [rad]
+    n_alpha : int
+        Number of poloidal angle grid points
+    vertical_shift : float
+        Vertical shift of the plasma center in [cm]
+    strength : float
+        Strength of the source
+    type : str
+        Indicator of source type: 'tokamak'
+    constraints : dict
+        Constraints on sampled source particles
+
+    """
+
+    def __init__(
+        self,
+        major_radius: float,
+        minor_radius: float,
+        elongation: float,
+        triangularity: float,
+        shafranov_shift: float,
+        r_over_a: Sequence[float],
+        emission_density: Sequence[float],
+        energy: Univariate | Sequence[Univariate],
+        time: Univariate | None = None,
+        phi_start: float = 0.0,
+        phi_extent: float = 2.0 * np.pi,
+        n_alpha: int = 101,
+        vertical_shift: float = 0.0,
+        strength: float = 1.0,
+        constraints: dict[str, Any] | None = None
+    ):
+        super().__init__(strength=strength, constraints=constraints)
+        self.major_radius = major_radius
+        self.minor_radius = minor_radius
+        self.elongation = elongation
+        self.triangularity = triangularity
+        self.shafranov_shift = shafranov_shift
+        self.r_over_a = r_over_a
+        self.emission_density = emission_density
+        self.phi_start = phi_start
+        self.phi_extent = phi_extent
+        self.n_alpha = n_alpha
+        self.vertical_shift = vertical_shift
+        self.energy = energy
+        self.time = time
+
+        self._validate()
+
+    def _validate(self):
+        """Validate relationships between tokamak source parameters."""
+        if self.minor_radius >= self.major_radius:
+            raise ValueError(
+                f"minor_radius ({self.minor_radius}) must be smaller than "
+                f"major_radius ({self.major_radius})")
+        if self.shafranov_shift >= 0.5 * self.minor_radius:
+            raise ValueError(
+                f"shafranov_shift ({self.shafranov_shift}) must be smaller "
+                f"than half the minor_radius ({0.5 * self.minor_radius})")
+        if len(self.emission_density) != len(self.r_over_a):
+            raise ValueError(
+                f"emission_density (length {len(self.emission_density)}) must "
+                f"have the same length as r_over_a (length {len(self.r_over_a)})")
+        if not np.any(self.emission_density > 0.0):
+            raise ValueError("emission_density must contain a positive value")
+        if len(self.energy) not in (1, len(self.r_over_a)):
+            raise ValueError(
+                f"Number of energy distributions ({len(self.energy)}) must be "
+                f"either 1 or equal to the number of r_over_a grid points "
+                f"({len(self.r_over_a)})")
+
+    @property
+    def type(self) -> str:
+        return "tokamak"
+
+    @property
+    def major_radius(self) -> float:
+        return self._major_radius
+
+    @major_radius.setter
+    def major_radius(self, value: float):
+        cv.check_type('major radius', value, Real)
+        cv.check_greater_than('major radius', value, 0.0)
+        self._major_radius = value
+
+    @property
+    def minor_radius(self) -> float:
+        return self._minor_radius
+
+    @minor_radius.setter
+    def minor_radius(self, value: float):
+        cv.check_type('minor radius', value, Real)
+        cv.check_greater_than('minor radius', value, 0.0)
+        self._minor_radius = value
+
+    @property
+    def elongation(self) -> float:
+        return self._elongation
+
+    @elongation.setter
+    def elongation(self, value: float):
+        cv.check_type('elongation', value, Real)
+        cv.check_greater_than('elongation', value, 0.0)
+        self._elongation = value
+
+    @property
+    def triangularity(self) -> float:
+        return self._triangularity
+
+    @triangularity.setter
+    def triangularity(self, value: float):
+        cv.check_type('triangularity', value, Real)
+        cv.check_greater_than('triangularity', value, -1.0, equality=True)
+        cv.check_less_than('triangularity', value, 1.0, equality=True)
+        self._triangularity = value
+
+    @property
+    def shafranov_shift(self) -> float:
+        return self._shafranov_shift
+
+    @shafranov_shift.setter
+    def shafranov_shift(self, value: float):
+        cv.check_type('Shafranov shift', value, Real)
+        cv.check_greater_than('Shafranov shift', value, 0.0, equality=True)
+        self._shafranov_shift = value
+
+    @property
+    def r_over_a(self) -> np.ndarray:
+        return self._r_over_a
+
+    @r_over_a.setter
+    def r_over_a(self, value: Sequence[float]):
+        value = np.asarray(value, dtype=float)
+        if value.ndim != 1 or len(value) < 2:
+            raise ValueError("r_over_a must be a 1-D array with at least 2 points")
+        if value[0] != 0.0:
+            raise ValueError("r_over_a must start at 0")
+        if value[-1] != 1.0:
+            raise ValueError("r_over_a must end at 1")
+        if not np.all(np.diff(value) > 0):
+            raise ValueError("r_over_a must be strictly increasing")
+        self._r_over_a = value
+
+    @property
+    def emission_density(self) -> np.ndarray:
+        return self._emission_density
+
+    @emission_density.setter
+    def emission_density(self, value: Sequence[float]):
+        value = np.asarray(value, dtype=float)
+        if value.ndim != 1:
+            raise ValueError("emission_density must be a 1-D array")
+        if np.any(value < 0):
+            raise ValueError("emission_density values cannot be negative")
+        self._emission_density = value
+
+    @property
+    def energy(self) -> list[Univariate]:
+        return self._energy
+
+    @energy.setter
+    def energy(self, value: Univariate | Sequence[Univariate]):
+        if isinstance(value, Univariate):
+            self._energy = [value]
+        else:
+            cv.check_iterable_type('energy distributions', value, Univariate)
+            self._energy = list(value)
+
+    @property
+    def time(self) -> Univariate | None:
+        return self._time
+
+    @time.setter
+    def time(self, value: Univariate | None):
+        if value is not None:
+            cv.check_type('time distribution', value, Univariate)
+        self._time = value
+
+    @property
+    def phi_start(self) -> float:
+        return self._phi_start
+
+    @phi_start.setter
+    def phi_start(self, value: float):
+        cv.check_type('phi_start', value, Real)
+        self._phi_start = value
+
+    @property
+    def phi_extent(self) -> float:
+        return self._phi_extent
+
+    @phi_extent.setter
+    def phi_extent(self, value: float):
+        cv.check_type('phi_extent', value, Real)
+        cv.check_greater_than('phi_extent', value, 0.0)
+        cv.check_less_than('phi_extent', value, 2.0 * np.pi, equality=True)
+        self._phi_extent = value
+
+    @property
+    def n_alpha(self) -> int:
+        return self._n_alpha
+
+    @n_alpha.setter
+    def n_alpha(self, value: int):
+        cv.check_type('n_alpha', value, Integral)
+        cv.check_greater_than('n_alpha', value, 2)
+        if value < 51:
+            warnings.warn(
+                "n_alpha values below 51 may introduce noticeable "
+                "discretization bias in tokamak source sampling", stacklevel=2)
+        self._n_alpha = value
+
+    @property
+    def vertical_shift(self) -> float:
+        return self._vertical_shift
+
+    @vertical_shift.setter
+    def vertical_shift(self, value: float):
+        cv.check_type('vertical shift', value, Real)
+        self._vertical_shift = value
+
+    def populate_xml_element(self, element):
+        """Add necessary tokamak source information to an XML element
 
         Returns
         -------
-        The corresponding ParticleType instance.
+        element : lxml.etree._Element
+            XML element containing source data
+
         """
-        try:
-            return cls[value.upper()]
-        except KeyError:
-            raise ValueError(f"Invalid string for creation of {cls.__name__}: {value}")
+        self._validate()
+
+        # Geometry parameters
+        ET.SubElement(element, "major_radius").text = str(self.major_radius)
+        ET.SubElement(element, "minor_radius").text = str(self.minor_radius)
+        ET.SubElement(element, "elongation").text = str(self.elongation)
+        ET.SubElement(element, "triangularity").text = str(self.triangularity)
+        ET.SubElement(element, "shafranov_shift").text = str(self.shafranov_shift)
+
+        # Toroidal angle bounds
+        ET.SubElement(element, "phi_start").text = str(self.phi_start)
+        ET.SubElement(element, "phi_extent").text = str(self.phi_extent)
+
+        # Poloidal sampling resolution
+        ET.SubElement(element, "n_alpha").text = str(self.n_alpha)
+
+        # Vertical shift
+        if self.vertical_shift != 0.0:
+            ET.SubElement(element, "vertical_shift").text = str(self.vertical_shift)
+
+        # Emission profile
+        ET.SubElement(element, "r_over_a").text = ' '.join(str(r) for r in self.r_over_a)
+        ET.SubElement(element, "emission_density").text = ' '.join(str(s) for s in self.emission_density)
+
+        # Energy distribution(s)
+        for dist in self.energy:
+            element.append(dist.to_xml_element('energy'))
+
+        # Time distribution
+        if self.time is not None:
+            element.append(self.time.to_xml_element('time'))
 
     @classmethod
-    def from_pdg_number(cls, pdg_number: int) -> ParticleType:
-        """Constructs a ParticleType instance from a PDG number.
-
-        The Particle Data Group at LBNL publishes a Monte Carlo particle
-        numbering scheme as part of the `Review of Particle Physics
-        <10.1103/PhysRevD.110.030001>`_. This method maps PDG numbers to the
-        corresponding :class:`ParticleType`.
+    def from_xml_element(cls, elem: ET.Element) -> TokamakSource:
+        """Generate tokamak source from an XML element
 
         Parameters
         ----------
-        pdg_number : int
-            The PDG number of the particle type.
+        elem : lxml.etree._Element
+            XML element
 
         Returns
         -------
-        The corresponding ParticleType instance.
-        """
-        try:
-            return {
-                2112: ParticleType.NEUTRON,
-                22: ParticleType.PHOTON,
-                11: ParticleType.ELECTRON,
-                -11: ParticleType.POSITRON,
-            }[pdg_number]
-        except KeyError:
-            raise ValueError(f"Unrecognized PDG number: {pdg_number}")
+        openmc.TokamakSource
+            Source generated from XML element
 
-    def __repr__(self) -> str:
         """
-        Returns a string representation of the ParticleType instance.
+        # Read geometry parameters
+        major_radius = float(get_text(elem, 'major_radius'))
+        minor_radius = float(get_text(elem, 'minor_radius'))
+        elongation = float(get_text(elem, 'elongation'))
+        triangularity = float(get_text(elem, 'triangularity'))
+        shafranov_shift = float(get_text(elem, 'shafranov_shift'))
 
-        Returns:
-            str: The lowercase name of the ParticleType instance.
-        """
-        return self.name.lower()
+        # Read optional parameters
+        phi_start_text = get_text(elem, 'phi_start')
+        phi_start = float(phi_start_text) if phi_start_text else 0.0
 
-    # needed for < Python 3.11
-    def __str__(self) -> str:
-        return self.__repr__()
+        phi_extent_text = get_text(elem, 'phi_extent')
+        phi_extent = float(phi_extent_text) if phi_extent_text else 2.0 * np.pi
+
+        n_alpha_text = get_text(elem, 'n_alpha')
+        n_alpha = int(n_alpha_text) if n_alpha_text else 101
+
+        vertical_shift_text = get_text(elem, 'vertical_shift')
+        vertical_shift = float(vertical_shift_text) if vertical_shift_text else 0.0
+
+        # Read emission profile
+        r_over_a = np.array([float(x) for x in get_text(elem, 'r_over_a').split()])
+        emission_density = np.array([float(x) for x in get_text(elem, 'emission_density').split()])
+
+        # Read energy distributions
+        energy = [Univariate.from_xml_element(e) for e in elem.findall('energy')]
+        if len(energy) == 1:
+            energy = energy[0]
+
+        # Read time distribution
+        time_elem = elem.find('time')
+        time = Univariate.from_xml_element(time_elem) if time_elem is not None else None
+
+        # Read constraints and strength
+        constraints = cls._get_constraints(elem)
+        strength_text = get_text(elem, 'strength')
+        strength = float(strength_text) if strength_text else 1.0
+
+        return cls(
+            major_radius=major_radius,
+            minor_radius=minor_radius,
+            elongation=elongation,
+            triangularity=triangularity,
+            shafranov_shift=shafranov_shift,
+            r_over_a=r_over_a,
+            emission_density=emission_density,
+            energy=energy,
+            time=time,
+            phi_start=phi_start,
+            phi_extent=phi_extent,
+            n_alpha=n_alpha,
+            vertical_shift=vertical_shift,
+            strength=strength,
+            constraints=constraints
+        )
 
 
 class SourceParticle:
@@ -979,10 +1346,11 @@ class SourceParticle:
         Delayed group particle was created in (neutrons only)
     surf_id : int
         Surface ID where particle is at, if any.
-    particle : ParticleType
-        Type of the particle
+    particle : ParticleType or str or int
+        Type of the particle (type, name, or PDG number)
 
     """
+
     def __init__(
         self,
         r: Iterable[float] = (0., 0., 0.),
@@ -992,7 +1360,7 @@ class SourceParticle:
         wgt: float = 1.0,
         delayed_group: int = 0,
         surf_id: int = 0,
-        particle: ParticleType = ParticleType.NEUTRON
+        particle: ParticleType | str | int = ParticleType.NEUTRON
     ):
 
         self.r = tuple(r)
@@ -1004,9 +1372,16 @@ class SourceParticle:
         self.surf_id = surf_id
         self.particle = particle
 
+    @property
+    def particle(self) -> ParticleType:
+        return self._particle
+
+    @particle.setter
+    def particle(self, particle):
+        self._particle = ParticleType(particle)
+
     def __repr__(self):
-        name = self.particle.name.lower()
-        return f'<SourceParticle: {name} at E={self.E:.6e} eV>'
+        return f'<SourceParticle: {str(self.particle)} at E={self.E:.6e} eV>'
 
     def to_tuple(self) -> tuple:
         """Return source particle attributes as a tuple
@@ -1018,7 +1393,7 @@ class SourceParticle:
 
         """
         return (self.r, self.u, self.E, self.time, self.wgt,
-                self.delayed_group, self.surf_id, self.particle.value)
+                self.delayed_group, self.surf_id, self.particle.pdg_number)
 
 
 def write_source_file(
@@ -1041,7 +1416,8 @@ def write_source_file(
     openmc.SourceParticle
 
     """
-    cv.check_iterable_type("source particles", source_particles, SourceParticle)
+    cv.check_iterable_type(
+        "source particles", source_particles, SourceParticle)
     pl = ParticleList(source_particles)
     pl.export_to_hdf5(filename, **kwargs)
 
@@ -1101,11 +1477,7 @@ class ParticleList(list):
         particles = []
         with mcpl.MCPLFile(filename) as f:
             for particle in f.particles:
-                # Determine particle type based on the PDG number
-                try:
-                    particle_type = ParticleType.from_pdg_number(particle.pdgcode)
-                except ValueError:
-                    particle_type = "UNKNOWN"
+                particle_type = ParticleType(particle.pdgcode)
 
                 # Create a source particle instance. Note that MCPL stores
                 # energy in MeV and time in ms.
@@ -1163,7 +1535,7 @@ class ParticleList(list):
         # Extract the attributes of the source particles into a list of tuples
         data = [(sp.r[0], sp.r[1], sp.r[2], sp.u[0], sp.u[1], sp.u[2],
                  sp.E, sp.time, sp.wgt, sp.delayed_group, sp.surf_id,
-                 sp.particle.name.lower()) for sp in self]
+                 str(sp.particle)) for sp in self]
 
         # Define the column names for the DataFrame
         columns = ['x', 'y', 'z', 'u_x', 'u_y', 'u_z', 'E', 'time', 'wgt',
@@ -1210,6 +1582,7 @@ class ParticleList(list):
         kwargs.setdefault('mode', 'w')
         with h5py.File(filename, **kwargs) as fh:
             fh.attrs['filetype'] = np.bytes_("source")
+            fh.attrs['version'] = np.array([_VERSION_STATEPOINT, 2])
             fh.create_dataset('source_bank', data=arr, dtype=source_dtype)
 
 
@@ -1240,3 +1613,133 @@ def read_source_file(filename: PathLike) -> ParticleList:
         return ParticleList.from_hdf5(filename)
     else:
         return ParticleList.from_mcpl(filename)
+
+
+def read_collision_track_hdf5(filename):
+    """Read a collision track file in HDF5 format.
+
+    Parameters
+    ----------
+    filename : str or path-like
+        Path to the HDF5 collision track file.
+
+    Returns
+    -------
+    numpy.ndarray
+        Structured array containing collision track data.
+
+    See Also
+    --------
+    read_collision_track_mcpl
+    read_collision_track_file
+    """
+
+    with h5py.File(filename, 'r') as file:
+        data = file['collision_track_bank'][:]
+
+    return data
+
+
+def read_collision_track_mcpl(file_path):
+    """Read a collision track file in MCPL format.
+
+    Parameters
+    ----------
+    file_path : str or path-like
+        Path to the MCPL collision track file.
+
+    Returns
+    -------
+    numpy.ndarray
+        Structured array of particle collision track information, including
+        position, direction, energy, weight, reaction data, and identifiers.
+
+    See Also
+    --------
+    read_collision_track_hdf5
+    read_collision_track_file
+    """
+    import mcpl
+    myfile = mcpl.MCPLFile(file_path)
+    data = {
+        'r': [],  # for position (x, y, z)
+        'u': [],  # for direction (ux, uy, uz)
+        'E': [], 'dE': [], 'time': [],
+        'wgt': [], 'event_mt': [], 'delayed_group': [],
+        'cell_id': [], 'nuclide_id': [], 'material_id': [],
+        'universe_id': [], 'n_collision': [], 'particle': [],
+        'parent_id': [], 'progeny_id': []
+    }
+
+    # Read and collect data from the MCPL file
+    for i, p in enumerate(myfile.particles):
+        if f'blob_{i}' in myfile.blobs:
+            blob_data = myfile.blobs[f'blob_{i}']
+            decoded_str = blob_data.decode('utf-8')
+            pairs = decoded_str.split(';')
+            values_dict = {k.strip(): v.strip()
+                           for k, v in (pair.split(':') for pair in pairs if pair.strip())}
+
+            data['r'].append((p.x, p.y, p.z))  # Append as tuple
+            data['u'].append((p.ux, p.uy, p.uz))  # Append as tuple
+            data['E'].append(p.ekin * 1e6)
+            data['dE'].append(float(values_dict.get('dE', 0)))
+            data['time'].append(p.time * 1e-3)
+            data['wgt'].append(p.weight)
+            data['event_mt'].append(int(values_dict.get('event_mt', 0)))
+            data['delayed_group'].append(
+                int(values_dict.get('delayed_group', 0)))
+            data['cell_id'].append(int(values_dict.get('cell_id', 0)))
+            data['nuclide_id'].append(int(values_dict.get('nuclide_id', 0)))
+            data['material_id'].append(int(values_dict.get('material_id', 0)))
+            data['universe_id'].append(int(values_dict.get('universe_id', 0)))
+            data['n_collision'].append(int(values_dict.get('n_collision', 0)))
+            data['particle'].append(ParticleType(p.pdgcode))
+            data['parent_id'].append(int(values_dict.get('parent_id', 0)))
+            data['progeny_id'].append(int(values_dict.get('progeny_id', 0)))
+
+    dtypes = [
+        ('r', [('x', 'f8'), ('y', 'f8'), ('z', 'f8')]),
+        ('u', [('x', 'f8'), ('y', 'f8'), ('z', 'f8')]),
+        ('E', 'f8'), ('dE', 'f8'), ('time', 'f8'), ('wgt', 'f8'),
+        ('event_mt', 'f8'), ('delayed_group', 'i4'), ('cell_id', 'i4'),
+        ('nuclide_id', 'i4'), ('material_id', 'i4'), ('universe_id', 'i4'),
+        ('n_collision', 'i4'), ('particle', 'i4'),
+        ('parent_id', 'i8'), ('progeny_id', 'i8')
+    ]
+
+    structured_array = np.zeros(len(data['r']), dtype=dtypes)
+    for key in data:
+        structured_array[key] = data[key]  # Assign data
+
+    return structured_array
+
+
+def read_collision_track_file(filename):
+    """Read a collision track file (HDF5 or MCPL) and return its data.
+
+    Parameters
+    ----------
+    filename : str or path-like
+        Path to the collision track file to read. Must end with
+        ``.h5`` or ``.mcpl``.
+
+    Returns
+    -------
+    numpy.ndarray
+        Structured array containing collision track data.
+
+    See Also
+    --------
+    read_collision_track_hdf5
+    read_collision_track_mcpl
+    """
+
+    filename = Path(filename)
+    if filename.suffix not in ('.h5', '.mcpl'):
+        raise ValueError('Collision track file must have a .h5 or .mcpl extension.')
+
+    if filename.suffix == '.h5':
+        return read_collision_track_hdf5(filename)
+    else:
+        return read_collision_track_mcpl(filename)
diff --git a/openmc/statepoint.py b/openmc/statepoint.py
index 715becf48..a10ec3a83 100644
--- a/openmc/statepoint.py
+++ b/openmc/statepoint.py
@@ -1,4 +1,5 @@
 from datetime import datetime
+from collections import namedtuple
 import glob
 import re
 import os
@@ -8,6 +9,7 @@ import h5py
 import numpy as np
 from pathlib import Path
 from uncertainties import ufloat
+from uncertainties.unumpy import uarray
 
 import openmc
 import openmc.checkvalue as cv
@@ -15,6 +17,9 @@ import openmc.checkvalue as cv
 _VERSION_STATEPOINT = 18
 
 
+KineticsParameters = namedtuple("KineticsParameters", ["generation_time", "beta_effective"])
+
+
 class StatePoint:
     """State information on a simulation at a certain point in time (at the end
     of a given batch). Statepoints can be used to analyze tally results as well
@@ -429,6 +434,10 @@ class StatePoint:
                     if "multiply_density" in group.attrs:
                         tally.multiply_density = group.attrs["multiply_density"].item() > 0
 
+                    # Check if tally has higher_moments attribute
+                    if 'higher_moments' in group.attrs:
+                        tally.higher_moments = bool(group.attrs['higher_moments'][()])
+
                     # Read the number of realizations
                     n_realizations = group['n_realizations'][()]
 
@@ -531,7 +540,7 @@ class StatePoint:
     def get_tally(self, scores=[], filters=[], nuclides=[],
                   name=None, id=None, estimator=None, exact_filters=False,
                   exact_nuclides=False, exact_scores=False,
-                  multiply_density=None, derivative=None):
+                  multiply_density=None, derivative=None, filter_type=None):
         """Finds and returns a Tally object with certain properties.
 
         This routine searches the list of Tallies and returns the first Tally
@@ -575,6 +584,9 @@ class StatePoint:
             to the same value as this parameter.
         derivative : openmc.TallyDerivative, optional
             TallyDerivative object to match.
+        filter_type : type, optional
+            If not None, the Tally must have at least one Filter that is an
+            instance of this type. For example `openmc.MeshFilter`.
 
         Returns
         -------
@@ -648,6 +660,10 @@ class StatePoint:
                 if not contains_filters:
                     continue
 
+            if filter_type is not None:
+                if not any(isinstance(f, filter_type) for f in test_tally.filters):
+                    continue
+
             # Determine if Tally has the queried Nuclide(s)
             if nuclides:
                 if not all(nuclide in test_tally.nuclides for nuclide in nuclides):
@@ -707,6 +723,59 @@ class StatePoint:
                     cell = cells[cell_id]
                     if not cell._paths:
                         summary.geometry.determine_paths()
-                    tally_filter.paths = cell.paths
+                    tally_filter._paths = cell.paths
 
         self._summary = summary
+
+    def get_kinetics_parameters(self) -> KineticsParameters:
+        """Get kinetics parameters from IFP tallies.
+
+        This method searches the tallies in the statepoint for the tallies
+        required to compute kinetics parameters using the Iterated Fission
+        Probability (IFP) method.
+
+        Returns
+        -------
+        KineticsParameters
+            A named tuple containing the generation time and effective delayed
+            neutron fraction. If the necessary tallies for one or both
+            parameters are not found, that parameter is returned as None.
+
+        """
+
+        denom_tally = None
+        gen_time_tally = None
+        beta_tally = None
+        for tally in self.tallies.values():
+            if 'ifp-denominator' in tally.scores:
+                denom_tally = self.get_tally(scores=['ifp-denominator'])
+            if 'ifp-time-numerator' in tally.scores:
+                gen_time_tally = self.get_tally(scores=['ifp-time-numerator'])
+            if 'ifp-beta-numerator' in tally.scores:
+                beta_tally = self.get_tally(scores=['ifp-beta-numerator'])
+
+        if denom_tally is None:
+            return KineticsParameters(None, None)
+
+        def get_ufloat(tally, score):
+            return uarray(tally.get_values(scores=[score]),
+                          tally.get_values(scores=[score], value='std_dev'))
+
+        denom_values = get_ufloat(denom_tally, 'ifp-denominator')
+        if gen_time_tally is None:
+            generation_time = None
+        else:
+            gen_time_values = get_ufloat(gen_time_tally, 'ifp-time-numerator')
+            gen_time_values /= denom_values*self.keff
+            generation_time = gen_time_values.flatten()[0]
+
+        if beta_tally is None:
+            beta_effective = None
+        else:
+            beta_values = get_ufloat(beta_tally, 'ifp-beta-numerator')
+            beta_values /= denom_values
+            beta_effective = beta_values.flatten()
+            if beta_effective.size == 1:
+                beta_effective = beta_effective[0]
+
+        return KineticsParameters(generation_time, beta_effective)
diff --git a/openmc/stats/multivariate.py b/openmc/stats/multivariate.py
index cd474fa9b..d48697037 100644
--- a/openmc/stats/multivariate.py
+++ b/openmc/stats/multivariate.py
@@ -10,9 +10,9 @@ import numpy as np
 
 import openmc
 import openmc.checkvalue as cv
-from .._xml import get_text
+from .._xml import get_elem_list, get_text
 from ..mesh import MeshBase
-from .univariate import PowerLaw, Uniform, Univariate
+from .univariate import PowerLaw, Uniform, Univariate, delta_function
 
 
 class UnitSphere(ABC):
@@ -79,6 +79,9 @@ class PolarAzimuthal(UnitSphere):
     reference_uvw : Iterable of float
         Direction from which polar angle is measured. Defaults to the positive
         z-direction.
+    reference_vwu : Iterable of float
+        Direction from which azimuthal angle is measured. Defaults to the positive
+        x-direction.
 
     Attributes
     ----------
@@ -89,8 +92,9 @@ class PolarAzimuthal(UnitSphere):
 
     """
 
-    def __init__(self, mu=None, phi=None, reference_uvw=(0., 0., 1.)):
+    def __init__(self, mu=None, phi=None, reference_uvw=(0., 0., 1.), reference_vwu=(1., 0., 0.)):
         super().__init__(reference_uvw)
+        self.reference_vwu = reference_vwu
         if mu is not None:
             self.mu = mu
         else:
@@ -101,6 +105,20 @@ class PolarAzimuthal(UnitSphere):
         else:
             self.phi = Uniform(0., 2*pi)
 
+    @property
+    def reference_vwu(self):
+        return self._reference_vwu
+
+    @reference_vwu.setter
+    def reference_vwu(self, vwu):
+        cv.check_type('reference v direction', vwu, Iterable, Real)
+        vwu = np.asarray(vwu)
+        uvw = self.reference_uvw
+        cv.check_greater_than('reference v direction must not be parallel to reference u direction', np.linalg.norm(np.cross(vwu,uvw)), 1e-6*np.linalg.norm(vwu))
+        vwu -= vwu.dot(uvw)*uvw
+        cv.check_less_than('reference v direction must be orthogonal to reference u direction', np.abs(vwu.dot(uvw)), 1e-6)
+        self._reference_vwu = vwu/np.linalg.norm(vwu)
+
     @property
     def mu(self):
         return self._mu
@@ -119,19 +137,30 @@ class PolarAzimuthal(UnitSphere):
         cv.check_type('azimuthal angle', phi, Univariate)
         self._phi = phi
 
-    def to_xml_element(self):
+    def to_xml_element(self, element_name: str = None):
         """Return XML representation of the angular distribution
 
+        Parameters
+        ----------
+        element_name : str, optional
+            XML element name
+
         Returns
         -------
         element : lxml.etree._Element
             XML element containing angular distribution data
 
         """
-        element = ET.Element('angle')
+        if element_name is not None:
+            element = ET.Element(element_name)
+        else:
+            element = ET.Element('angle')
+
         element.set("type", "mu-phi")
         if self.reference_uvw is not None:
             element.set("reference_uvw", ' '.join(map(str, self.reference_uvw)))
+        if self.reference_vwu is not None:
+            element.set("reference_vwu", ' '.join(map(str, self.reference_vwu)))
         element.append(self.mu.to_xml_element('mu'))
         element.append(self.phi.to_xml_element('phi'))
         return element
@@ -152,19 +181,53 @@ class PolarAzimuthal(UnitSphere):
 
         """
         mu_phi = cls()
-        uvw = get_text(elem, 'reference_uvw')
+        uvw = get_elem_list(elem, "reference_uvw", float)
         if uvw is not None:
-            mu_phi.reference_uvw = [float(x) for x in uvw.split()]
+            mu_phi.reference_uvw = uvw
+        vwu = get_elem_list(elem, "reference_vwu", float)
+        if vwu is not None:
+            mu_phi.reference_vwu = vwu
         mu_phi.mu = Univariate.from_xml_element(elem.find('mu'))
         mu_phi.phi = Univariate.from_xml_element(elem.find('phi'))
         return mu_phi
 
 
 class Isotropic(UnitSphere):
-    """Isotropic angular distribution."""
+    """Isotropic angular distribution.
 
-    def __init__(self):
+    Parameters
+    ----------
+    bias : openmc.stats.PolarAzimuthal, optional
+        Distribution for biased sampling.
+
+    Attributes
+    ----------
+    bias : openmc.stats.PolarAzimuthal or None
+        Distribution for biased sampling
+
+    """
+
+    def __init__(self, bias: PolarAzimuthal | None = None):
         super().__init__()
+        self.bias = bias
+
+    @property
+    def bias(self):
+        return self._bias
+
+    @bias.setter
+    def bias(self, bias):
+        cv.check_type('Biasing distribution', bias, PolarAzimuthal, none_ok=True)
+        if bias is not None:
+            if (bias.mu.bias is not None) or (bias.phi.bias is not None):
+                raise RuntimeError('Biasing distributions should not have their own bias.')
+            elif (bias.mu.support != (-1., 1.)
+                or not np.all(np.isclose(bias.phi.support, (0., 2*np.pi)))):
+                raise ValueError("Biasing distribution for an isotropic "
+                                 "distribution should be supported on "
+                                 "mu=(-1.0,1.0) and phi=(0.0,2*pi).")
+
+        self._bias = bias
 
     def to_xml_element(self):
         """Return XML representation of the isotropic distribution
@@ -177,6 +240,15 @@ class Isotropic(UnitSphere):
         """
         element = ET.Element('angle')
         element.set("type", "isotropic")
+
+        if self.bias is not None:
+            bias_dist = self.bias
+            if (bias_dist.mu.bias is not None) or (bias_dist.phi.bias is not None):
+                raise RuntimeError('Biasing distributions should not have their own bias!')
+            else:
+                bias_elem = self.bias.to_xml_element("bias")
+                element.append(bias_elem)
+
         return element
 
     @classmethod
@@ -194,7 +266,13 @@ class Isotropic(UnitSphere):
             Isotropic distribution generated from XML element
 
         """
-        return cls()
+        bias_elem = elem.find('bias')
+        if bias_elem is not None:
+            bias_dist = PolarAzimuthal.from_xml_element(bias_elem)
+            return cls(bias=bias_dist)
+        else:
+            return cls()
+
 
 
 class Monodirectional(UnitSphere):
@@ -246,9 +324,9 @@ class Monodirectional(UnitSphere):
 
         """
         monodirectional = cls()
-        uvw = get_text(elem, 'reference_uvw')
+        uvw = get_elem_list(elem, "reference_uvw", float)
         if uvw is not None:
-            monodirectional.reference_uvw = [float(x) for x in uvw.split()]
+            monodirectional.reference_uvw = uvw
         return monodirectional
 
 
@@ -504,7 +582,7 @@ class SphericalIndependent(Spatial):
         r = Univariate.from_xml_element(elem.find('r'))
         cos_theta = Univariate.from_xml_element(elem.find('cos_theta'))
         phi = Univariate.from_xml_element(elem.find('phi'))
-        origin = [float(x) for x in elem.get('origin').split()]
+        origin = get_elem_list(elem, "origin", float)
         return cls(r, cos_theta, phi, origin=origin)
 
 
@@ -532,6 +610,10 @@ class CylindricalIndependent(Spatial):
     origin: Iterable of float, optional
         coordinates (x0, y0, z0) of the center of the cylindrical reference
         frame. Defaults to (0.0, 0.0, 0.0)
+    r_dir : Iterable of float, optional
+        Unit vector of the cylinder r axis at phi=0.
+    z_dir : Iterable of float, optional
+        Unit vector of the cylinder z axis direction.
 
     Attributes
     ----------
@@ -545,14 +627,21 @@ class CylindricalIndependent(Spatial):
     origin: Iterable of float, optional
         coordinates (x0, y0, z0) of the center of the cylindrical reference
         frame. Defaults to (0.0, 0.0, 0.0)
+    r_dir : Iterable of float, optional
+        Unit vector of the cylinder r axis at phi=0.
+    z_dir : Iterable of float, optional
+        Unit vector of the cylinder z axis direction.
 
     """
 
-    def __init__(self, r, phi, z, origin=(0.0, 0.0, 0.0)):
+    def __init__(self, r, phi, z, origin=(0.0, 0.0, 0.0), r_dir=(1.0, 0.0, 0.0),
+                 z_dir=(0.0, 0.0, 1.0)):
         self.r = r
         self.phi = phi
         self.z = z
         self.origin = origin
+        self.z_dir = z_dir
+        self.r_dir = r_dir
 
     @property
     def r(self):
@@ -591,6 +680,33 @@ class CylindricalIndependent(Spatial):
         origin = np.asarray(origin)
         self._origin = origin
 
+    @property
+    def z_dir(self):
+        return self._z_dir
+
+    @z_dir.setter
+    def z_dir(self, z_dir):
+        cv.check_type('z-axis direction', z_dir, Iterable, Real)
+        z_dir = np.array(z_dir)
+        norm = np.linalg.norm(z_dir)
+        cv.check_greater_than('z-axis direction magnitude', norm, 0.0)
+        z_dir /= norm
+        self._z_dir = z_dir
+
+    @property
+    def r_dir(self):
+        return self._r_dir
+
+    @r_dir.setter
+    def r_dir(self, r_dir):
+        cv.check_type('r-axis direction', r_dir, Iterable, Real)
+        r_dir = np.array(r_dir)
+        r_dir -= np.dot(r_dir, self.z_dir) * self.z_dir
+        norm = np.linalg.norm(r_dir)
+        cv.check_greater_than('r-axis direction magnitude', norm, 0.0)
+        r_dir /= norm
+        self._r_dir = r_dir
+
     def to_xml_element(self):
         """Return XML representation of the spatial distribution
 
@@ -605,7 +721,12 @@ class CylindricalIndependent(Spatial):
         element.append(self.r.to_xml_element('r'))
         element.append(self.phi.to_xml_element('phi'))
         element.append(self.z.to_xml_element('z'))
-        element.set("origin", ' '.join(map(str, self.origin)))
+        if not np.allclose(self.origin, [0., 0., 0.]):
+            element.set("origin", ' '.join(map(str, self.origin)))
+        if not np.allclose(self.r_dir, [1., 0., 0.]):
+            element.set("r_dir", ' '.join(map(str, self.r_dir)))
+        if not np.allclose(self.z_dir, [0., 0., 1.]):
+            element.set("z_dir", ' '.join(map(str, self.z_dir)))
         return element
 
     @classmethod
@@ -626,8 +747,10 @@ class CylindricalIndependent(Spatial):
         r = Univariate.from_xml_element(elem.find('r'))
         phi = Univariate.from_xml_element(elem.find('phi'))
         z = Univariate.from_xml_element(elem.find('z'))
-        origin = [float(x) for x in elem.get('origin').split()]
-        return cls(r, phi, z, origin=origin)
+        origin = get_elem_list(elem, "origin", float) or [0.0, 0.0, 0.0]
+        r_dir = get_elem_list(elem, "r_dir", float) or [1.0, 0.0, 0.0]
+        z_dir = get_elem_list(elem, "z_dir", float) or [0.0, 0.0, 1.0]
+        return cls(r, phi, z, origin=origin, r_dir=r_dir, z_dir=z_dir)
 
 
 class MeshSpatial(Spatial):
@@ -649,6 +772,9 @@ class MeshSpatial(Spatial):
     volume_normalized : bool, optional
         Whether or not the strengths will be multiplied by element volumes at
         runtime. Default is True.
+    bias : iterable of float, optional
+        An iterable of values giving the selection weights assigned to each
+        element during biased sampling.
 
     Attributes
     ----------
@@ -659,12 +785,16 @@ class MeshSpatial(Spatial):
     volume_normalized : bool
         Whether or not the strengths will be multiplied by element volumes at
         runtime.
+    bias : numpy.ndarray or None
+        Distribution for biased sampling
     """
 
-    def __init__(self, mesh, strengths=None, volume_normalized=True):
+    def __init__(self, mesh, strengths=None, volume_normalized=True,
+                 bias: Sequence[float] | None = None):
         self.mesh = mesh
         self.strengths = strengths
         self.volume_normalized = volume_normalized
+        self.bias = bias
 
     @property
     def mesh(self):
@@ -697,6 +827,23 @@ class MeshSpatial(Spatial):
         else:
             self._strengths = None
 
+    @property
+    def bias(self):
+        return self._bias
+
+    @bias.setter
+    def bias(self, given_bias):
+        if given_bias is not None:
+            cv.check_type('Biasing strengths array', given_bias, Iterable, Real)
+            bias_array = np.asarray(given_bias, dtype=float).flatten()
+            if bias_array.size != self.strengths.size:
+                raise ValueError(
+                    'Bias strengths array must have same size as strengths array.')
+            else:
+                self._bias = bias_array
+        else:
+            self._bias = None
+
     @property
     def num_strength_bins(self):
         if self.strengths is None:
@@ -722,6 +869,9 @@ class MeshSpatial(Spatial):
             subelement = ET.SubElement(element, 'strengths')
             subelement.text = ' '.join(str(e) for e in self.strengths)
 
+        if self.bias is not None:
+            Univariate._append_array_bias_to_xml(self, element)
+
         return element
 
     @classmethod
@@ -743,19 +893,16 @@ class MeshSpatial(Spatial):
 
         """
 
-        mesh_id = int(elem.get('mesh_id'))
+        mesh_id = int(get_text(elem, "mesh_id"))
 
         # check if this mesh has been read in from another location already
         if mesh_id not in meshes:
             raise ValueError(f'Could not locate mesh with ID "{mesh_id}"')
 
-        volume_normalized = elem.get("volume_normalized")
         volume_normalized = get_text(elem, 'volume_normalized').lower() == 'true'
-        strengths = get_text(elem, 'strengths')
-        if strengths is not None:
-            strengths = [float(b) for b in get_text(elem, 'strengths').split()]
-
-        return cls(meshes[mesh_id], strengths, volume_normalized)
+        strengths = get_elem_list(elem, 'strengths', float)
+        bias_strengths = Univariate._read_array_bias_from_xml(elem)
+        return cls(meshes[mesh_id], strengths, volume_normalized, bias=bias_strengths)
 
 
 class PointCloud(Spatial):
@@ -773,6 +920,9 @@ class PointCloud(Spatial):
     strengths : iterable of float, optional
         An iterable of values that represents the relative probabilty of each
         point.
+    bias : iterable of float, optional
+        An iterable of values representing the relative probability of each
+        point under biased sampling.
 
     Attributes
     ----------
@@ -780,15 +930,19 @@ class PointCloud(Spatial):
         The points in space to be sampled with shape (N, 3)
     strengths : numpy.ndarray or None
         An array of relative probabilities for each mesh point
+    bias : numpy.ndarray or None
+        An array of relative probabilities for biased sampling of mesh points
     """
 
     def __init__(
         self,
         positions: Sequence[Sequence[float]],
-        strengths: Sequence[float] | None = None
+        strengths: Sequence[float] | None = None,
+        bias: Sequence[float] | None = None
     ):
         self.positions = positions
         self.strengths = strengths
+        self.bias = bias
 
     @property
     def positions(self) -> np.ndarray:
@@ -817,6 +971,23 @@ class PointCloud(Spatial):
                 raise ValueError('strengths must have the same length as positions')
         self._strengths = strengths
 
+    @property
+    def bias(self):
+        return self._bias
+
+    @bias.setter
+    def bias(self, given_bias):
+        if given_bias is not None:
+            cv.check_type('Biasing strengths array', given_bias, Iterable, Real)
+            bias_array = np.asarray(given_bias, dtype=float).flatten()
+            if bias_array.size != self.strengths.size:
+                raise ValueError(
+                    'Bias strengths array must have same size as strengths array.')
+            else:
+                self._bias = bias_array
+        else:
+            self._bias = None
+
     @property
     def num_strength_bins(self) -> int:
         if self.strengths is None:
@@ -842,6 +1013,9 @@ class PointCloud(Spatial):
             subelement = ET.SubElement(element, 'strengths')
             subelement.text = ' '.join(str(e) for e in self.strengths)
 
+        if self.bias is not None:
+            Univariate._append_array_bias_to_xml(self, element)
+
         return element
 
     @classmethod
@@ -860,14 +1034,12 @@ class PointCloud(Spatial):
 
 
         """
-        coord_data = get_text(elem, 'coords')
-        positions = np.array([float(b) for b in coord_data.split()]).reshape((-1, 3))
+        coord_data = get_elem_list(elem, 'coords', float)
+        positions = np.array(coord_data).reshape((-1, 3))
 
-        strengths = get_text(elem, 'strengths')
-        if strengths is not None:
-            strengths = [float(b) for b in strengths.split()]
-
-        return cls(positions, strengths)
+        strengths = get_elem_list(elem, 'strengths', float)
+        bias_strengths = Univariate._read_array_bias_from_xml(elem)
+        return cls(positions, strengths, bias=bias_strengths)
 
 
 class Box(Spatial):
@@ -979,7 +1151,7 @@ class Box(Spatial):
 
         """
         only_fissionable = get_text(elem, 'type') == 'fission'
-        params = [float(x) for x in get_text(elem, 'parameters').split()]
+        params = get_elem_list(elem, "parameters", float)
         lower_left = params[:len(params)//2]
         upper_right = params[len(params)//2:]
         return cls(lower_left, upper_right, only_fissionable)
@@ -1046,7 +1218,7 @@ class Point(Spatial):
             Point distribution generated from XML element
 
         """
-        xyz = [float(x) for x in get_text(elem, 'parameters').split()]
+        xyz = get_elem_list(elem, "parameters", float)
         return cls(xyz)
 
 
@@ -1092,3 +1264,49 @@ def spherical_uniform(
     phis_dist = Uniform(phis[0], phis[1])
 
     return SphericalIndependent(r_dist, cos_thetas_dist, phis_dist, origin)
+
+
+def cylindrical_uniform(
+    r_outer: float,
+    height: float,
+    r_inner: float = 0.0,
+    phis: Sequence[float] = (0., 2*pi),
+    **kwargs,
+):
+    """Return a uniform spatial distribution over a cylindrical shell.
+
+    This function provides a uniform spatial distribution over a cylindrical
+    shell between `r_inner` and `r_outer`. When `height` is zero, a delta
+    function is used for the z-distribution, giving a uniform distribution over
+    a flat ring (annulus) at z=0 in the local coordinate frame. Optionally, the
+    range of angles can be restricted by the `phis` argument.
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    r_outer : float
+        Outer radius of the cylindrical shell in [cm]
+    height : float
+        Height of the cylindrical shell in [cm]. When 0, the distribution is a
+        flat ring at z=0 in the local frame.
+    r_inner : float
+        Inner radius of the cylindrical shell in [cm]
+    phis : iterable of float
+        Starting and ending phi coordinates (azimuthal angle) in radians in a
+        reference frame centered at `origin`.
+    **kwargs
+        Keyword arguments passed directly to
+        :class:`~openmc.stats.CylindricalIndependent` (e.g., ``origin``,
+        ``r_dir``, ``z_dir``).
+
+    Returns
+    -------
+    openmc.stats.CylindricalIndependent
+        Uniform distribution over the cylindrical shell
+    """
+
+    r_dist = PowerLaw(r_inner, r_outer, 1)
+    phis_dist = Uniform(phis[0], phis[1])
+    z_dist = delta_function(0.0) if height == 0.0 else Uniform(-height/2, height/2)
+    return CylindricalIndependent(r_dist, phis_dist, z_dist, **kwargs)
diff --git a/openmc/stats/univariate.py b/openmc/stats/univariate.py
index e0475bf78..1b18bcb17 100644
--- a/openmc/stats/univariate.py
+++ b/openmc/stats/univariate.py
@@ -1,18 +1,23 @@
 from __future__ import annotations
-import math
 from abc import ABC, abstractmethod
 from collections import defaultdict
 from collections.abc import Iterable, Sequence
-from copy import deepcopy
+from functools import cache
+from math import sqrt, pi, exp, log
 from numbers import Real
+from pathlib import Path
 from warnings import warn
 
 import lxml.etree as ET
 import numpy as np
 from scipy.integrate import trapezoid
+from scipy.special import exprel, hyp1f1, lambertw
+import scipy
 
 import openmc.checkvalue as cv
-from .._xml import get_text
+from openmc.data import atomic_mass, NEUTRON_MASS
+import openmc.data
+from .._xml import get_elem_list, get_text
 from ..mixin import EqualityMixin
 
 _INTERPOLATION_SCHEMES = {
@@ -24,13 +29,71 @@ _INTERPOLATION_SCHEMES = {
 }
 
 
+def exprel2(x):
+    """Evaluate 2*(exp(x)-1-x)/x^2 without loss of precision near 0"""
+    return hyp1f1(1, 3, x)
+
+
+def log1prel(x):
+    """Evaluate log(1+x)/x without loss of precision near 0"""
+    return np.where(np.abs(x) < 1e-16, 1.0, np.log1p(x) / x)
+
+
 class Univariate(EqualityMixin, ABC):
     """Probability distribution of a single random variable.
 
     The Univariate class is an abstract class that can be derived to implement a
     specific probability distribution.
 
+    Parameters
+    ----------
+    bias : Iterable of float, optional
+        Distribution or discrete probabilities for biased sampling or discrete
+        probabilities for biased sampling.
+
     """
+    def __init__(self, bias: Univariate | Sequence[float] | None = None):
+        self.bias = bias
+
+    @property
+    def bias(self):
+        return self._bias
+
+    @bias.setter
+    def bias(self, bias):
+        check_bias_support(self, bias)
+        self._bias = bias
+
+    def _append_bias_to_xml(self, element: ET.Element) -> None:
+        """Append bias distribution element to XML if present."""
+        if self.bias is not None:
+            if self.bias.bias is not None:
+                raise RuntimeError('Biasing distributions should not have their own bias.')
+            bias_elem = self.bias.to_xml_element("bias")
+            element.append(bias_elem)
+
+    @classmethod
+    def _read_bias_from_xml(cls, elem: ET.Element):
+        """Read bias distribution from XML element if present."""
+        bias_elem = elem.find('bias')
+        if bias_elem is not None:
+            return Univariate.from_xml_element(bias_elem)
+        return None
+
+    def _append_array_bias_to_xml(self, element: ET.Element) -> None:
+        """Append array-based bias probabilities to XML."""
+        if self.bias is not None:
+            bias_elem = ET.SubElement(element, "bias")
+            bias_elem.text = ' '.join(map(str, self.bias))
+
+    @classmethod
+    def _read_array_bias_from_xml(cls, elem: ET.Element):
+        """Read array-based bias probabilities from XML."""
+        bias_elem = elem.find('bias')
+        if bias_elem is not None:
+            return get_elem_list(elem, "bias", float)
+        return None
+
     @abstractmethod
     def to_xml_element(self, element_name):
         return ''
@@ -57,18 +120,19 @@ class Univariate(EqualityMixin, ABC):
             return Normal.from_xml_element(elem)
         elif distribution == 'muir':
             # Support older files where Muir had its own class
-            params = [float(x) for x in get_text(elem, 'parameters').split()]
-            return muir(*params)
+            return muir(*get_elem_list(elem, "parameters", float))
         elif distribution == 'tabular':
             return Tabular.from_xml_element(elem)
         elif distribution == 'legendre':
             return Legendre.from_xml_element(elem)
         elif distribution == 'mixture':
             return Mixture.from_xml_element(elem)
+        elif distribution == 'decay_spectrum':
+            return DecaySpectrum.from_xml_element(elem)
 
     @abstractmethod
-    def sample(n_samples: int = 1, seed: int | None = None):
-        """Sample the univariate distribution
+    def _sample_unbiased(self, n_samples: int = 1, seed: int | None = None):
+        """Sample without bias handling.
 
         Parameters
         ----------
@@ -80,10 +144,35 @@ class Univariate(EqualityMixin, ABC):
         Returns
         -------
         numpy.ndarray
-            A 1-D array of sampled values
+            The array of sampled values
         """
         pass
 
+    def sample(self, n_samples: int = 1, seed: int | None = None):
+        """Sample the univariate distribution, handling biasing automatically.
+
+        Parameters
+        ----------
+        n_samples : int
+            Number of sampled values to generate
+        seed : int or None
+            Initial random number seed.
+
+        Returns
+        -------
+        tuple of numpy.ndarray
+            A tuple of (samples, weights)
+        """
+        if self.bias is None:
+            x = self._sample_unbiased(n_samples, seed)
+            return x, np.ones_like(x)
+        else:
+            if self.bias.bias is not None:
+                raise RuntimeError('Biasing distributions should not have their own bias.')
+            x, _ = self.bias.sample(n_samples=n_samples, seed=seed)
+            weight = self.evaluate(x) / self.bias.evaluate(x)
+            return x, weight
+
     def integral(self):
         """Return integral of distribution
 
@@ -96,6 +185,36 @@ class Univariate(EqualityMixin, ABC):
         """
         return 1.0
 
+    @abstractmethod
+    def evaluate(self, x: float | Sequence[float]):
+        """Evaluate the probability density at the provided value.
+
+        Parameters
+        ----------
+        x : float or sequence of float
+            Location to evaluate p(x)
+
+        Returns
+        -------
+        float or numpy.ndarray
+            Value of p(x)
+        """
+        pass
+
+    @property
+    @abstractmethod
+    def support(self):
+        """Return the support of the probability distribution.
+
+        Returns
+        -------
+        set or tuple of float or dict
+            Returns the set of unique points assigned probability mass in a
+            discrete distribution, the sampling interval for a continuous
+            distribution, or a dictionary storing the discrete and continuous
+            parts of the support of a mixed random variable
+        """
+        pass
 
 def _intensity_clip(intensity: Sequence[float], tolerance: float = 1e-6) -> np.ndarray:
     """Clip low-importance points from an array of intensities.
@@ -149,6 +268,9 @@ class Discrete(Univariate):
         Values of the random variable
     p : Iterable of float
         Discrete probability for each value
+    bias : Iterable of float, optional
+        Alternative discrete probabilities for biased sampling. Defaults to
+        None for unbiased sampling.
 
     Attributes
     ----------
@@ -156,12 +278,18 @@ class Discrete(Univariate):
         Values of the random variable
     p : numpy.ndarray
         Discrete probability for each value
+    support : set
+        Values of the random variable over which the distribution is
+        nonzero-valued
+    bias : numpy.ndarray or None
+        Discrete probabilities for biased sampling
 
     """
 
-    def __init__(self, x, p):
+    def __init__(self, x, p, bias=None):
         self.x = x
         self.p = p
+        super().__init__(bias)
 
     def __len__(self):
         return len(self.x)
@@ -190,10 +318,42 @@ class Discrete(Univariate):
             cv.check_greater_than('discrete probability', pk, 0.0, True)
         self._p = np.array(p, dtype=float)
 
+    @property
+    def support(self):
+        return set(np.unique(self._x))
+
+    @Univariate.bias.setter
+    def bias(self, bias):
+        if bias is None:
+            self._bias = bias
+        else:
+            if isinstance(bias, Real):
+                bias = [bias]
+            cv.check_type('discrete bias probabilities', bias, Iterable, Real)
+            for bk in bias:
+                cv.check_greater_than('discrete probability', bk, 0.0, True)
+            if len(bias) != len(self.x):
+                raise RuntimeError("Discrete distribution has unequal number of "
+                                   "biased and unbiased probability entries.")
+            self._bias = np.array(bias, dtype=float)
+
     def cdf(self):
         return np.insert(np.cumsum(self.p), 0, 0.0)
 
     def sample(self, n_samples=1, seed=None):
+        if self.bias is None:
+            samples = self._sample_unbiased(n_samples, seed)
+            return samples, np.ones_like(samples)
+        else:
+            rng = np.random.RandomState(seed)
+            p = self.p / self.p.sum()
+            b = self.bias / self.bias.sum()
+            indices = rng.choice(self.x.size, n_samples, p=b)
+            biased_sample = self.x[indices]
+            wgt = p[indices] / b[indices]
+            return biased_sample, wgt
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
         rng = np.random.RandomState(seed)
         p = self.p / self.p.sum()
         return rng.choice(self.x, n_samples, p=p)
@@ -203,6 +363,9 @@ class Discrete(Univariate):
         norm = sum(self.p)
         self.p = [val / norm for val in self.p]
 
+    def evaluate(self, x):
+        raise NotImplementedError
+
     def to_xml_element(self, element_name):
         """Return XML representation of the discrete distribution
 
@@ -222,7 +385,7 @@ class Discrete(Univariate):
 
         params = ET.SubElement(element, "parameters")
         params.text = ' '.join(map(str, self.x)) + ' ' + ' '.join(map(str, self.p))
-
+        self._append_array_bias_to_xml(element)
         return element
 
     @classmethod
@@ -240,16 +403,17 @@ class Discrete(Univariate):
             Discrete distribution generated from XML element
 
         """
-        params = [float(x) for x in get_text(elem, 'parameters').split()]
+        params = get_elem_list(elem, "parameters", float)
         x = params[:len(params)//2]
         p = params[len(params)//2:]
-        return cls(x, p)
+        bias_dist = cls._read_array_bias_from_xml(elem)
+        return cls(x, p, bias=bias_dist)
 
     @classmethod
     def merge(
         cls,
         dists: Sequence[Discrete],
-        probs: Sequence[int]
+        probs: Sequence[float]
     ):
         """Merge multiple discrete distributions into a single distribution
 
@@ -271,18 +435,52 @@ class Discrete(Univariate):
         if len(dists) != len(probs):
             raise ValueError("Number of distributions and probabilities must match.")
 
+        biasing = False
+        for d in dists:
+            if d.bias is not None:
+                # If we find that at least one distribution is biased, all
+                # distributions which are not biased will be assigned their
+                # default probability vector as a "bias" so that biased
+                # sampling can occur on the merged distribution.
+                biasing = True
+                break
+
         # Combine distributions accounting for duplicate x values
         x_merged = set()
         p_merged = defaultdict(float)
-        for dist, p_dist in zip(dists, probs):
-            for x, p in zip(dist.x, dist.p):
-                x_merged.add(x)
-                p_merged[x] += p*p_dist
+        new_bias = None
 
-        # Create values and probabilities as arrays
-        x_arr = np.array(sorted(x_merged))
+        if biasing:
+            b_merged = defaultdict(float)
+
+            # Generate any missing bias distributions
+            dists = dists.copy()
+            for i, d in enumerate(dists):
+                if d.bias is None:
+                    dists[i] = Discrete(d.x, d.p, bias=d.p)
+
+            for dist, p_dist in zip(dists, probs):
+                for x, p, b in zip(dist.x, dist.p, dist.bias):
+                    x_merged.add(x)
+                    p_merged[x] += p*p_dist
+                    b_merged[x] += b*p_dist
+
+            # Create values and bias probabilities as arrays
+            x_arr = np.array(sorted(x_merged))
+            new_bias = np.array([b_merged[x] for x in x_arr])
+
+        else:
+            for dist, p_dist in zip(dists, probs):
+                for x, p in zip(dist.x, dist.p):
+                    x_merged.add(x)
+                    p_merged[x] += p*p_dist
+
+            # Create values as array
+            x_arr = np.array(sorted(x_merged))
+
+        # Create probabilities as array
         p_arr = np.array([p_merged[x] for x in x_arr])
-        return cls(x_arr, p_arr)
+        return cls(x_arr, p_arr, new_bias)
 
     def integral(self):
         """Return integral of distribution
@@ -296,6 +494,20 @@ class Discrete(Univariate):
         """
         return np.sum(self.p)
 
+    def mean(self) -> float:
+        """Return mean of the discrete distribution
+
+        The mean is the weighted average of the discrete values.
+
+        .. versionadded:: 0.15.3
+
+        Returns
+        -------
+        float
+            Mean of discrete distribution
+        """
+        return np.sum(self.x * self.p) / np.sum(self.p)
+
     def clip(self, tolerance: float = 1e-6, inplace: bool = False) -> Discrete:
         r"""Remove low-importance points from discrete distribution.
 
@@ -305,6 +517,9 @@ class Discrete(Univariate):
         function will remove any low-importance points such that :math:`\sum_i
         x_i p_i` is preserved to within some threshold.
 
+        For biased distributions, clipping should be performed before the bias
+        probabilities are added.
+
         .. versionadded:: 0.14.0
 
         Parameters
@@ -319,6 +534,10 @@ class Discrete(Univariate):
         Discrete distribution with low-importance points removed
 
         """
+        if self.bias is not None:
+            raise RuntimeError("Biased discrete distributions should be clipped "
+                               "before applying bias.")
+
         cv.check_less_than("tolerance", tolerance, 1.0, equality=True)
         cv.check_greater_than("tolerance", tolerance, 0.0, equality=True)
 
@@ -369,6 +588,8 @@ class Uniform(Univariate):
         Lower bound of the sampling interval. Defaults to zero.
     b : float, optional
         Upper bound of the sampling interval. Defaults to unity.
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
@@ -376,12 +597,19 @@ class Uniform(Univariate):
         Lower bound of the sampling interval
     b : float
         Upper bound of the sampling interval
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
 
     """
 
-    def __init__(self, a: float = 0.0, b: float = 1.0):
+    def __init__(self, a: float = 0.0, b: float = 1.0,
+                 bias: Univariate | None = None):
         self.a = a
         self.b = b
+        super().__init__(bias)
 
     def __len__(self):
         return 2
@@ -404,16 +632,37 @@ class Uniform(Univariate):
         cv.check_type('Uniform b', b, Real)
         self._b = b
 
+    @property
+    def support(self):
+        return (self._a, self._b)
+
     def to_tabular(self):
+        if self.bias is not None:
+            raise RuntimeError("to_tabular() is not permitted for biased distributions.")
         prob = 1./(self.b - self.a)
         t = Tabular([self.a, self.b], [prob, prob], 'histogram')
         t.c = [0., 1.]
         return t
 
-    def sample(self, n_samples=1, seed=None):
+    def _sample_unbiased(self, n_samples=1, seed=None):
         rng = np.random.RandomState(seed)
         return rng.uniform(self.a, self.b, n_samples)
 
+    def evaluate(self, x):
+        return np.where((self.a <= x) & (x <= self.b), 1/(self.b - self.a), 0.0)
+
+    def mean(self) -> float:
+        """Return mean of the uniform distribution
+
+        .. versionadded:: 0.15.3
+
+        Returns
+        -------
+        float
+            Mean of uniform distribution
+        """
+        return 0.5 * (self.a + self.b)
+
     def to_xml_element(self, element_name: str):
         """Return XML representation of the uniform distribution
 
@@ -431,6 +680,7 @@ class Uniform(Univariate):
         element = ET.Element(element_name)
         element.set("type", "uniform")
         element.set("parameters", f'{self.a} {self.b}')
+        self._append_bias_to_xml(element)
         return element
 
     @classmethod
@@ -448,8 +698,9 @@ class Uniform(Univariate):
             Uniform distribution generated from XML element
 
         """
-        params = get_text(elem, 'parameters').split()
-        return cls(*map(float, params))
+        params = get_elem_list(elem, "parameters", float)
+        bias_dist = cls._read_bias_from_xml(elem)
+        return cls(*params, bias=bias_dist)
 
 
 class PowerLaw(Univariate):
@@ -468,6 +719,8 @@ class PowerLaw(Univariate):
     n : float, optional
         Power law exponent. Defaults to zero, which is equivalent to a uniform
         distribution.
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
@@ -477,13 +730,23 @@ class PowerLaw(Univariate):
         Upper bound of the sampling interval
     n : float
         Power law exponent
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
 
     """
 
-    def __init__(self, a: float = 0.0, b: float = 1.0, n: float = 0.):
+    def __init__(self, a: float = 0.0, b: float = 1.0, n: float = 0.,
+                 bias: Univariate | None = None):
+        if a >= b:
+            raise ValueError(
+                "Lower bound of sampling interval must be less than upper bound.")
         self.a = a
         self.b = b
         self.n = n
+        super().__init__(bias)
 
     def __len__(self):
         return 3
@@ -495,6 +758,9 @@ class PowerLaw(Univariate):
     @a.setter
     def a(self, a):
         cv.check_type('interval lower bound', a, Real)
+        if a < 0:
+            raise ValueError(
+                "PowerLaw sampling is restricted to positive-valued intervals.")
         self._a = a
 
     @property
@@ -504,6 +770,9 @@ class PowerLaw(Univariate):
     @b.setter
     def b(self, b):
         cv.check_type('interval upper bound', b, Real)
+        if b < 0:
+            raise ValueError(
+                "PowerLaw sampling is restricted to positive-valued intervals.")
         self._b = b
 
     @property
@@ -515,7 +784,11 @@ class PowerLaw(Univariate):
         cv.check_type('power law exponent', n, Real)
         self._n = n
 
-    def sample(self, n_samples=1, seed=None):
+    @property
+    def support(self):
+        return (self._a, self._b)
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
         rng = np.random.RandomState(seed)
         xi = rng.random(n_samples)
         pwr = self.n + 1
@@ -523,6 +796,10 @@ class PowerLaw(Univariate):
         span = self.b**pwr - offset
         return np.power(offset + xi * span, 1/pwr)
 
+    def evaluate(self, x):
+        c = (self.n + 1)/(self.b**(self.n + 1) - self.a**(self.n + 1))
+        return np.where((self.a <= x) & (x <= self.b), c * np.abs(x)**self.n, 0.0)
+
     def to_xml_element(self, element_name: str):
         """Return XML representation of the power law distribution
 
@@ -540,6 +817,7 @@ class PowerLaw(Univariate):
         element = ET.Element(element_name)
         element.set("type", "powerlaw")
         element.set("parameters", f'{self.a} {self.b} {self.n}')
+        self._append_bias_to_xml(element)
         return element
 
     @classmethod
@@ -557,8 +835,9 @@ class PowerLaw(Univariate):
             Distribution generated from XML element
 
         """
-        params = get_text(elem, 'parameters').split()
-        return cls(*map(float, params))
+        params = get_elem_list(elem, "parameters", float)
+        bias_dist = cls._read_bias_from_xml(elem)
+        return cls(*map(float, params), bias=bias_dist)
 
 
 class Maxwell(Univariate):
@@ -572,16 +851,24 @@ class Maxwell(Univariate):
     ----------
     theta : float
         Effective temperature for distribution in eV
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
     theta : float
         Effective temperature for distribution in eV
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
 
     """
 
-    def __init__(self, theta):
+    def __init__(self, theta, bias: Univariate | None = None):
         self.theta = theta
+        super().__init__(bias)
 
     def __len__(self):
         return 1
@@ -596,7 +883,11 @@ class Maxwell(Univariate):
         cv.check_greater_than('Maxwell temperature', theta, 0.0)
         self._theta = theta
 
-    def sample(self, n_samples=1, seed=None):
+    @property
+    def support(self):
+        return (0.0, np.inf)
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
         rng = np.random.RandomState(seed)
         return self.sample_maxwell(self.theta, n_samples, rng=rng)
 
@@ -604,9 +895,10 @@ class Maxwell(Univariate):
     def sample_maxwell(t, n_samples: int, rng=None):
         if rng is None:
             rng = np.random.default_rng()
-        r1, r2, r3 = rng.random((3, n_samples))
-        c = np.cos(0.5 * np.pi * r3)
-        return -t * (np.log(r1) + np.log(r2) * c * c)
+        return rng.gamma(1.5, t, n_samples)
+
+    def evaluate(self, E):
+        return scipy.stats.gamma.pdf(E, 1.5, scale=self.theta)
 
     def to_xml_element(self, element_name: str):
         """Return XML representation of the Maxwellian distribution
@@ -625,6 +917,7 @@ class Maxwell(Univariate):
         element = ET.Element(element_name)
         element.set("type", "maxwell")
         element.set("parameters", str(self.theta))
+        self._append_bias_to_xml(element)
         return element
 
     @classmethod
@@ -643,7 +936,8 @@ class Maxwell(Univariate):
 
         """
         theta = float(get_text(elem, 'parameters'))
-        return cls(theta)
+        bias_dist = cls._read_bias_from_xml(elem)
+        return cls(theta, bias=bias_dist)
 
 
 class Watt(Univariate):
@@ -659,6 +953,8 @@ class Watt(Univariate):
         First parameter of distribution in units of eV
     b : float
         Second parameter of distribution in units of 1/eV
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
@@ -666,12 +962,18 @@ class Watt(Univariate):
         First parameter of distribution in units of eV
     b : float
         Second parameter of distribution in units of 1/eV
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
 
     """
 
-    def __init__(self, a=0.988e6, b=2.249e-6):
+    def __init__(self, a=0.988e6, b=2.249e-6, bias: Univariate | None = None):
         self.a = a
         self.b = b
+        super().__init__(bias)
 
     def __len__(self):
         return 2
@@ -696,13 +998,21 @@ class Watt(Univariate):
         cv.check_greater_than('Watt b', b, 0.0)
         self._b = b
 
-    def sample(self, n_samples=1, seed=None):
+    @property
+    def support(self):
+        return (0.0, np.inf)
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
         rng = np.random.RandomState(seed)
         w = Maxwell.sample_maxwell(self.a, n_samples, rng=rng)
         u = rng.uniform(-1., 1., n_samples)
         aab = self.a * self.a * self.b
         return w + 0.25*aab + u*np.sqrt(aab*w)
 
+    def evaluate(self, E):
+        c = 2.0/(sqrt(pi * self.b) * (self.a**1.5) * exp(self.a*self.b/4))
+        return c*np.exp(-E/self.a)*np.sinh(np.sqrt(self.b*E))
+
     def to_xml_element(self, element_name: str):
         """Return XML representation of the Watt distribution
 
@@ -720,6 +1030,7 @@ class Watt(Univariate):
         element = ET.Element(element_name)
         element.set("type", "watt")
         element.set("parameters", f'{self.a} {self.b}')
+        self._append_bias_to_xml(element)
         return element
 
     @classmethod
@@ -737,23 +1048,35 @@ class Watt(Univariate):
             Watt distribution generated from XML element
 
         """
-        params = get_text(elem, 'parameters').split()
-        return cls(*map(float, params))
+        params = get_elem_list(elem, "parameters", float)
+        bias_dist = cls._read_bias_from_xml(elem)
+        return cls(*map(float, params), bias=bias_dist)
 
 
 class Normal(Univariate):
-    r"""Normally distributed sampling.
+    r"""Normally distributed sampling with optional truncation.
 
-    The Normal Distribution is characterized by two parameters
-    :math:`\mu` and :math:`\sigma` and has density function
-    :math:`p(X) dX = 1/(\sqrt{2\pi}\sigma) e^{-(X-\mu)^2/(2\sigma^2)}`
+    The normal distribution is characterized by parameters :math:`\mu` and
+    :math:`\sigma` and has density function :math:`p(X) = 1/(\sqrt{2\pi}\sigma)
+    e^{-(X-\mu)^2/(2\sigma^2)}`. When truncated to the interval [lower, upper],
+    the distribution is renormalized so that the PDF integrates to 1 over the
+    truncation interval.
+
+    .. versionchanged:: 0.15.4
+        Added optional truncation bounds via `lower` and `upper` parameters.
 
     Parameters
     ----------
     mean_value : float
-        Mean value of the  distribution
+        Mean value of the distribution
     std_dev : float
         Standard deviation of the Normal distribution
+    lower : float, optional
+        Lower truncation bound. Defaults to -infinity (no lower bound).
+    upper : float, optional
+        Upper truncation bound. Defaults to +infinity (no upper bound).
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
@@ -761,13 +1084,29 @@ class Normal(Univariate):
         Mean of the Normal distribution
     std_dev : float
         Standard deviation of the Normal distribution
+    lower : float
+        Lower truncation bound
+    upper : float
+        Upper truncation bound
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
     """
 
-    def __init__(self, mean_value, std_dev):
+    def __init__(self, mean_value, std_dev, lower=-np.inf, upper=np.inf,
+                 bias: Univariate | None = None):
         self.mean_value = mean_value
         self.std_dev = std_dev
+        self.lower = lower
+        self.upper = upper
+        self._compute_normalization()
+        super().__init__(bias)
 
     def __len__(self):
+        if self._is_truncated:
+            return 4
         return 2
 
     @property
@@ -789,9 +1128,69 @@ class Normal(Univariate):
         cv.check_greater_than('Normal std_dev', std_dev, 0.0)
         self._std_dev = std_dev
 
-    def sample(self, n_samples=1, seed=None):
+    @property
+    def lower(self):
+        return self._lower
+
+    @lower.setter
+    def lower(self, lower):
+        cv.check_type('Normal lower bound', lower, Real)
+        self._lower = lower
+
+    @property
+    def upper(self):
+        return self._upper
+
+    @upper.setter
+    def upper(self, upper):
+        cv.check_type('Normal upper bound', upper, Real)
+        self._upper = upper
+
+    def _compute_normalization(self):
+        """Compute normalization factor for truncated distribution."""
+        # Check if truncation bounds are finite
+        self._is_truncated = (self._lower > -np.inf or self._upper < np.inf)
+
+        if self._lower >= self._upper:
+            raise ValueError("Normal distribution lower bound must be less "
+                             "than upper bound.")
+
+        if self._is_truncated:
+            alpha = (self._lower - self._mean_value) / self._std_dev
+            beta = (self._upper - self._mean_value) / self._std_dev
+            cdf_diff = scipy.stats.norm.cdf(beta) - scipy.stats.norm.cdf(alpha)
+            if cdf_diff <= 0:
+                raise ValueError("Truncation bounds exclude entire distribution")
+            self._norm_factor = 1.0 / cdf_diff
+        else:
+            self._norm_factor = 1.0
+
+    @property
+    def support(self):
+        return (self._lower, self._upper)
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
         rng = np.random.RandomState(seed)
-        return rng.normal(self.mean_value, self.std_dev, n_samples)
+        if not self._is_truncated:
+            return rng.normal(self.mean_value, self.std_dev, n_samples)
+        else:
+            # Use scipy's truncated normal for efficient direct sampling
+            a = (self._lower - self._mean_value) / self._std_dev
+            b = (self._upper - self._mean_value) / self._std_dev
+            return scipy.stats.truncnorm.rvs(
+                a, b, loc=self._mean_value, scale=self._std_dev,
+                size=n_samples, random_state=rng
+            )
+
+    def evaluate(self, x):
+        """Evaluate PDF at x, returning normalized value for truncated dist."""
+        x = np.asarray(x)
+        f = scipy.stats.norm.pdf(x, self.mean_value, self.std_dev)
+        if self._is_truncated:
+            # PDF is zero outside bounds
+            in_bounds = (x >= self._lower) & (x <= self._upper)
+            f = np.where(in_bounds, f * self._norm_factor, 0.0)
+        return f
 
     def to_xml_element(self, element_name: str):
         """Return XML representation of the Normal distribution
@@ -804,12 +1203,17 @@ class Normal(Univariate):
         Returns
         -------
         element : lxml.etree._Element
-            XML element containing Watt distribution data
+            XML element containing Normal distribution data
 
         """
         element = ET.Element(element_name)
         element.set("type", "normal")
-        element.set("parameters", f'{self.mean_value} {self.std_dev}')
+        if self._is_truncated:
+            element.set("parameters",
+                f'{self.mean_value} {self.std_dev} {self.lower} {self.upper}')
+        else:
+            element.set("parameters", f'{self.mean_value} {self.std_dev}')
+        self._append_bias_to_xml(element)
         return element
 
     @classmethod
@@ -827,11 +1231,15 @@ class Normal(Univariate):
             Normal distribution generated from XML element
 
         """
-        params = get_text(elem, 'parameters').split()
-        return cls(*map(float, params))
+        params = get_elem_list(elem, "parameters", float)
+        bias_dist = cls._read_bias_from_xml(elem)
+        if len(params) == 4:
+            return cls(params[0], params[1], params[2], params[3], bias=bias_dist)
+        else:
+            return cls(params[0], params[1], bias=bias_dist)
 
 
-def muir(e0: float, m_rat: float, kt: float):
+def muir(e0: float, m_rat: float, kt: float, bias: Univariate | None = None):
     """Generate a Muir energy spectrum
 
     The Muir energy spectrum is a normal distribution, but for convenience
@@ -849,6 +1257,8 @@ def muir(e0: float, m_rat: float, kt: float):
         Ratio of the sum of the masses of the reaction inputs to 1 amu
     kt : float
          Ion temperature for the Muir distribution in [eV]
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Returns
     -------
@@ -857,8 +1267,8 @@ def muir(e0: float, m_rat: float, kt: float):
 
     """
     # https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-05411-MS
-    std_dev = math.sqrt(2 * e0 * kt / m_rat)
-    return Normal(e0, std_dev)
+    std_dev = sqrt(2 * e0 * kt / m_rat)
+    return Normal(e0, std_dev, bias=bias)
 
 
 # Retain deprecated name for the time being
@@ -872,6 +1282,138 @@ def Muir(*args, **kwargs):
     return muir(*args, **kwargs)
 
 
+def fusion_neutron_spectrum(
+    ion_temp: float,
+    reactants: str = 'DD',
+    bias: Univariate | None = None
+) -> Normal:
+    r"""Return a Gaussian energy distribution for fusion neutron emission.
+
+    Computes the mean energy and spectral width of the neutron energy spectrum
+    from thermonuclear fusion reactions in a plasma with Maxwellian ion velocity
+    distributions. The mean neutron energy is calculated as
+
+    .. math::
+
+        \langle E_n \rangle = E_0 + \Delta E_\text{th}
+
+    where :math:`E_0` is the neutron energy at zero ion temperature and
+    :math:`\Delta E_\text{th}` is the thermal peak shift due to the motion of
+    the reacting ions. The spectral width is characterized by the FWHM:
+
+    .. math::
+
+        W_{1/2} = \omega_0 (1 + \delta_\omega) \sqrt{T_i}
+
+    where :math:`\omega_0` is the width at the :math:`T_i \to 0` limit and
+    :math:`\delta_\omega` is a temperature-dependent correction term. Both
+    :math:`\Delta E_\text{th}` and :math:`\delta_\omega` are evaluated using
+    interpolation formulas from `Ballabio et al.
+    <https://doi.org/10.1088/0029-5515/38/11/310>`_: Table III for :math:`0 <
+    T_i \le 40` keV and Table IV for :math:`40 < T_i < 100` keV. The returned
+    distribution is a normal (Gaussian) approximation to the spectrum.
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    ion_temp : float
+        Ion temperature of the plasma in [eV].
+    reactants : {'DD', 'DT'}
+        Fusion reactants. 'DD' corresponds to the D(d,n)\ :sup:`3`\ He reaction
+        and 'DT' to the T(d,n)\ :math:`\alpha` reaction.
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
+
+    Returns
+    -------
+    openmc.stats.Normal
+        Normal distribution with mean and standard deviation corresponding to
+        the first and second moments of the fusion neutron energy spectrum. Both
+        the mean and standard deviation are in [eV].
+
+    """
+    if ion_temp < 0.0 or ion_temp > 100e3:
+        raise ValueError("Ion temperature must be between 0 and 100 keV.")
+
+    # Formulas from doi:10.1088/0029-5515/38/11/310
+    mn = NEUTRON_MASS
+    md = atomic_mass('H2')
+    ev_per_c2 = 931.49410372*1e6
+    if reactants == 'DD':
+        mhe3 = atomic_mass('He3')
+        Q = (md + md - mhe3 - mn)*ev_per_c2
+        E_n = mhe3/(mhe3 + mn)*Q
+        w0 = 82.542
+
+        # Low-T constants for peak shift (Table III)
+        a1 = 4.69515
+        a2 = -0.040729
+        a3 = 0.47
+        a4 = 0.81844
+
+        # Low-T constants for width correction (Table III)
+        b1 = 1.7013e-3
+        b2 = 0.16888
+        b3 = 0.49
+        b4 = 7.9460e-4
+
+        # High-T constants for peak shift (Table IV)
+        a5 = 18.225
+        a6 = 2.1525
+
+        # High-T constants for width correction (Table IV)
+        b5 = 8.4619e-3
+        b6 = 8.3241e-4
+
+    elif reactants == 'DT':
+        mt = atomic_mass('H3')
+        ma = atomic_mass('He4')
+        Q = (md + mt - ma - mn)*ev_per_c2
+        E_n = ma/(ma + mn)*Q
+        w0 = 177.259
+
+        # Low-T constants for peak shift (Table III)
+        a1 = 5.30509
+        a2 = 2.4736e-3
+        a3 = 1.84
+        a4 = 1.3818
+
+        # Low-T constants for width correction (Table III)
+        b1 = 5.1068e-4
+        b2 = 7.6223e-3
+        b3 = 1.78
+        b4 = 8.7691e-5
+
+        # High-T constants for peak shift (Table IV)
+        a5 = 37.771
+        a6 = 0.92181
+
+        # High-T constants for width correction (Table IV)
+        b5 = 2.0199e-3
+        b6 = 5.9501e-5
+    else:
+        raise ValueError("Invalid reactants specified. Must be 'DD' or 'DT'.")
+
+    # Ion temperature in keV
+    T = ion_temp * 1e-3
+
+    if T <= 40.0:
+        # Low-temperature interpolation (Table III, 0 < T_i <= 40 keV)
+        Delta_E = a1/(1 + a2*T**a3)*T**(2/3) + a4*T
+        delta_w = b1/(1 + b2*T**b3)*T**(2/3) + b4*T
+    else:
+        # High-temperature interpolation (Table IV, 40 < T_i < 100 keV)
+        Delta_E = a5 + a6*T
+        delta_w = b5 + b6*T
+
+    # Calculate FWHM
+    fwhm = (w0*(1 + delta_w) * sqrt(T))*1e3
+
+    sigma = fwhm / (2*sqrt(2*log(2)))
+    return Normal(E_n + Delta_E * 1e3, sigma, bias=bias)
+
+
 class Tabular(Univariate):
     """Piecewise continuous probability distribution.
 
@@ -892,6 +1434,8 @@ class Tabular(Univariate):
         points. Defaults to 'linear-linear'.
     ignore_negative : bool
         Ignore negative probabilities
+    bias : openmc.stats.Univariate, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
@@ -902,6 +1446,11 @@ class Tabular(Univariate):
     interpolation : {'histogram', 'linear-linear', 'linear-log', 'log-linear', 'log-log'}
         Indicates how the density function is interpolated between tabulated
         points. Defaults to 'linear-linear'.
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
 
     Notes
     -----
@@ -919,7 +1468,8 @@ class Tabular(Univariate):
             x: Sequence[float],
             p: Sequence[float],
             interpolation: str = 'linear-linear',
-            ignore_negative: bool = False
+            ignore_negative: bool = False,
+            bias: Univariate | None = None
         ):
         self.interpolation = interpolation
 
@@ -941,6 +1491,7 @@ class Tabular(Univariate):
 
         self._x = x
         self._p = p
+        super().__init__(bias)
 
     def __len__(self):
         return self.p.size
@@ -962,6 +1513,10 @@ class Tabular(Univariate):
         cv.check_value('interpolation', interpolation, _INTERPOLATION_SCHEMES)
         self._interpolation = interpolation
 
+    @property
+    def support(self):
+        return (self._x[0], self._x[-1])
+
     def cdf(self):
         c = np.zeros_like(self.x)
         x = self.x
@@ -971,51 +1526,71 @@ class Tabular(Univariate):
             c[1:] = p[:x.size-1] * np.diff(x)
         elif self.interpolation == 'linear-linear':
             c[1:] = 0.5 * (p[:-1] + p[1:]) * np.diff(x)
+        elif self.interpolation == "linear-log":
+            m = np.diff(p) / np.diff(np.log(x))
+            c[1:] = p[:-1] * np.diff(x) + m * (
+                x[1:] * (np.diff(np.log(x)) - 1.0) + x[:-1]
+            )
+        elif self.interpolation == "log-linear":
+            m = np.diff(np.log(p)) / np.diff(x)
+            c[1:] = p[:-1] * np.diff(x) * exprel(m * np.diff(x))
+        elif self.interpolation == "log-log":
+            m = np.diff(np.log(x * p)) / np.diff(np.log(x))
+            c[1:] = (x * p)[:-1] * np.diff(np.log(x)) * exprel(m * np.diff(np.log(x)))
         else:
-            raise NotImplementedError('Can only generate CDFs for tabular '
-                                      'distributions using histogram or '
-                                      'linear-linear interpolation')
-
+            raise NotImplementedError(
+                f"Cannot generate CDFs for tabular "
+                f"distributions using {self.interpolation} interpolation"
+            )
 
         return np.cumsum(c)
 
     def mean(self):
         """Compute the mean of the tabular distribution"""
-        if self.interpolation == 'linear-linear':
-            mean = 0.0
-            for i in range(1, len(self.x)):
-                y_min = self.p[i-1]
-                y_max = self.p[i]
-                x_min = self.x[i-1]
-                x_max = self.x[i]
 
-                m = (y_max - y_min) / (x_max - x_min)
+        # use normalized probabilities when computing mean
+        p = self.p / self.cdf().max()
+        x = self.x
+        x_min = x[:-1]
+        x_max = x[1:]
+        p_min = p[: x.size - 1]
 
-                exp_val = (1./3.) * m * (x_max**3 - x_min**3)
-                exp_val += 0.5 * m * x_min * (x_min**2 - x_max**2)
-                exp_val += 0.5 * y_min * (x_max**2 - x_min**2)
-                mean += exp_val
-
-        elif self.interpolation == 'histogram':
-            x_l = self.x[:-1]
-            x_r = self.x[1:]
-            p_l = self.p[:self.x.size-1]
-            mean = (0.5 * (x_l + x_r) * (x_r - x_l) * p_l).sum()
+        if self.interpolation == "linear-linear":
+            m = np.diff(p) / np.diff(x)
+            mean = ((1.0 / 3.0) * m * np.diff(x**3)
+                    + 0.5 * (p_min - m * x_min) * np.diff(x**2)).sum()
+        elif self.interpolation == "linear-log":
+            m = np.diff(p) / np.diff(np.log(x))
+            mean = (
+                (1.0 / 4.0) * m * x_min**2
+                * ((x_max / x_min)**2 * (2 * np.diff(np.log(x)) - 1) + 1)
+                + 0.5 * p_min * np.diff(x**2)
+            ).sum()
+        elif self.interpolation == "log-linear":
+            m = np.diff(np.log(p)) / np.diff(x)
+            mean = (p_min * (
+                np.diff(x) ** 2
+                * ((0.5 * exprel2(m * np.diff(x)) * (m * np.diff(x) - 1) + 1))
+                + np.diff(x) * x_min * exprel(m * np.diff(x)))
+            ).sum()
+        elif self.interpolation == "log-log":
+            m = np.diff(np.log(p)) / np.diff(np.log(x))
+            mean = (p_min * x_min**2 * np.diff(np.log(x))
+                    * exprel((m + 2) * np.diff(np.log(x)))).sum()
+        elif self.interpolation == "histogram":
+            mean = (0.5 * (x_min + x_max) * np.diff(x) * p_min).sum()
         else:
-            raise NotImplementedError('Can only compute mean for tabular '
-                                      'distributions using histogram '
-                                      'or linear-linear interpolation.')
-
-        # Normalize for when integral of distribution is not 1
-        mean /= self.integral()
-
+            raise NotImplementedError(
+                f"Cannot compute mean for tabular "
+                f"distributions using {self.interpolation} interpolation"
+            )
         return mean
 
     def normalize(self):
         """Normalize the probabilities stored on the distribution"""
         self._p /= self.cdf().max()
 
-    def sample(self, n_samples: int = 1, seed: int | None = None):
+    def _sample_unbiased(self, n_samples: int = 1, seed: int | None = None):
         rng = np.random.RandomState(seed)
         xi = rng.random(n_samples)
 
@@ -1067,15 +1642,93 @@ class Tabular(Univariate):
             quad[quad < 0.0] = 0.0
             m[non_zero] = x_i[non_zero] + (np.sqrt(quad) - p_i[non_zero]) / m[non_zero]
             samples_out = m
+        elif self.interpolation == "linear-log":
+            # get variable and probability values for the
+            # next entry
+            x_i1 = self.x[cdf_idx + 1]
+            p_i1 = p[cdf_idx + 1]
+            # compute slope between entries
+            m = (p_i1 - p_i) / np.log(x_i1 / x_i)
+            # set values for zero slope
+            zero = m == 0.0
+            m[zero] = x_i[zero] + (xi[zero] - c_i[zero]) / p_i[zero]
 
+            positive = m > 0
+            negative = m < 0
+            a = p_i / m - 1
+            m[positive] = (
+                x_i
+                * ((xi - c_i) / (m * x_i) + a)
+                / np.real(lambertw((((xi - c_i) / (m * x_i) + a)) * np.exp(a)))
+            )[positive]
+            m[negative] = (
+                x_i
+                * ((xi - c_i) / (m * x_i) + a)
+                / np.real(lambertw((((xi - c_i) / (m * x_i) + a)) * np.exp(a), -1.0))
+            )[negative]
+            samples_out = m
+        elif self.interpolation == "log-linear":
+            # get variable and probability values for the
+            # next entry
+            x_i1 = self.x[cdf_idx + 1]
+            p_i1 = p[cdf_idx + 1]
+            # compute slope between entries
+            m = np.log(p_i1 / p_i) / (x_i1 - x_i)
+            f = (xi - c_i) / p_i
+
+            samples_out = x_i + f * log1prel(m * f)
+        elif self.interpolation == "log-log":
+            # get variable and probability values for the
+            # next entry
+            x_i1 = self.x[cdf_idx + 1]
+            p_i1 = p[cdf_idx + 1]
+            # compute slope between entries
+            m = np.log((x_i1 * p_i1) / (x_i * p_i)) / np.log(x_i1 / x_i)
+            f = (xi - c_i) / (x_i * p_i)
+
+            samples_out = x_i * np.exp(f * log1prel(m * f))
         else:
-            raise NotImplementedError('Can only sample tabular distributions '
-                                      'using histogram or '
-                                      'linear-linear interpolation')
+            raise NotImplementedError(
+                f"Cannot sample tabular distributions "
+                f"for {self.inteprolation} interpolation "
+            )
 
         assert all(samples_out < self.x[-1])
         return samples_out
 
+    def sample(self, n_samples: int = 1, seed: int | None = None):
+        if self.bias is None:
+            samples = self._sample_unbiased(n_samples, seed)
+            return samples, np.ones_like(samples)
+        else:
+            if self.bias.bias is not None:
+                raise RuntimeError('Biasing distributions should not have their own bias.')
+            biased_sample, _ = self.bias.sample(n_samples=n_samples, seed=seed)
+            self.normalize()  # must have normalized probabilities to apply correct weights
+            wgt = np.array([self.evaluate(s) / self.bias.evaluate(s) for s in biased_sample])
+            return biased_sample, wgt
+
+    def evaluate(self, x):
+        if self.interpolation == 'linear-linear':
+            i = np.searchsorted(self.x, x, side='left') - 1
+            if i < 0 or i >= len(self.p) - 1:
+                return 0.0
+            x0, x1 = self.x[i], self.x[i + 1]
+            p0, p1 = self.p[i], self.p[i + 1]
+            t = (x - x0) / (x1 - x0)
+            return (1 - t) * p0 + t * p1
+
+        elif self.interpolation == 'histogram':
+            i = np.searchsorted(self.x, x, side='right') - 1
+            if i < 0 or i >= len(self.p):
+                return 0.0
+            return self.p[i]
+
+        else:
+            raise NotImplementedError('Can only evaluate tabular '
+                                      'distributions using histogram '
+                                      'or linear-linear interpolation.')
+
     def to_xml_element(self, element_name: str):
         """Return XML representation of the tabular distribution
 
@@ -1096,7 +1749,7 @@ class Tabular(Univariate):
 
         params = ET.SubElement(element, "parameters")
         params.text = ' '.join(map(str, self.x)) + ' ' + ' '.join(map(str, self.p))
-
+        self._append_bias_to_xml(element)
         return element
 
     @classmethod
@@ -1115,11 +1768,12 @@ class Tabular(Univariate):
 
         """
         interpolation = get_text(elem, 'interpolation')
-        params = [float(x) for x in get_text(elem, 'parameters').split()]
+        params = get_elem_list(elem, "parameters", float)
         m = (len(params) + 1)//2  # +1 for when len(params) is odd
         x = params[:m]
         p = params[m:]
-        return cls(x, p, interpolation)
+        bias_dist = cls._read_bias_from_xml(elem)
+        return cls(x, p, interpolation, bias=bias_dist)
 
     def integral(self):
         """Return integral of distribution
@@ -1135,9 +1789,22 @@ class Tabular(Univariate):
             return np.sum(np.diff(self.x) * self.p[:self.x.size-1])
         elif self.interpolation == 'linear-linear':
             return trapezoid(self.p, self.x)
+        elif self.interpolation == "linear-log":
+            m = np.diff(self.p) / np.diff(np.log(self.x))
+            return np.sum(
+                self.p[:-1] * np.diff(self.x)
+                + m * (self.x[1:] * (np.diff(np.log(self.x)) - 1.0) + self.x[:-1])
+            )
+        elif self.interpolation == "log-linear":
+            m = np.diff(np.log(self.p)) / np.diff(self.x)
+            return np.sum(self.p[:-1] * np.diff(self.x) * exprel(m * np.diff(self.x)))
+        elif self.interpolation == "log-log":
+            m = np.diff(np.log(self.p)) / np.diff(np.log(self.x))
+            return np.sum(self.p[:-1] * self.x[:-1] * np.diff(np.log(self.x))
+                          * exprel((m + 1) * np.diff(np.log(self.x))))
         else:
             raise NotImplementedError(
-                f'integral() not supported for {self.inteprolation} interpolation')
+                f'integral() not supported for {self.interpolation} interpolation')
 
 
 class Legendre(Univariate):
@@ -1149,16 +1816,24 @@ class Legendre(Univariate):
     coefficients : Iterable of Real
         Expansion coefficients :math:`a_\ell`. Note that the :math:`(2\ell +
         1)/2` factor should not be included.
+    bias : openmc.stats.Univariate or None, optional
+        Distribution for biased sampling.
 
     Attributes
     ----------
     coefficients : Iterable of Real
         Expansion coefficients :math:`a_\ell`. Note that the :math:`(2\ell +
         1)/2` factor should not be included.
+    support : tuple of float
+        A 2-tuple (lower, upper) defining the interval over which the
+        distribution is nonzero-valued
+    bias : openmc.stats.Univariate or None
+        Distribution for biased sampling
 
     """
 
-    def __init__(self, coefficients: Sequence[float]):
+    def __init__(self, coefficients: Sequence[float], bias: Univariate | None = None):
+        super().__init__(bias)
         self.coefficients = coefficients
         self._legendre_poly = None
 
@@ -1182,9 +1857,19 @@ class Legendre(Univariate):
     def coefficients(self, coefficients):
         self._coefficients = np.asarray(coefficients)
 
+    @property
+    def support(self):
+        raise NotImplementedError
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
+        raise NotImplementedError
+
     def sample(self, n_samples=1, seed=None):
         raise NotImplementedError
 
+    def evaluate(self, x):
+        raise NotImplementedError
+
     def to_xml_element(self, element_name):
         raise NotImplementedError
 
@@ -1202,6 +1887,9 @@ class Mixture(Univariate):
         Probability of selecting a particular distribution
     distribution : Iterable of Univariate
         List of distributions with corresponding probabilities
+    bias : Iterable of Real, optional
+        Probability of selecting a particular distribution under biased
+        sampling
 
     Attributes
     ----------
@@ -1209,14 +1897,20 @@ class Mixture(Univariate):
         Probability of selecting a particular distribution
     distribution : Iterable of Univariate
         List of distributions with corresponding probabilities
+    support : dict
+        Dictionary containing discrete and continuous parts of the support
+    bias : numpy.ndarray or None
+        Probability of selecting each distribution under biased sampling
 
     """
 
     def __init__(
         self,
         probability: Sequence[float],
-        distribution: Sequence[Univariate]
+        distribution: Sequence[Univariate],
+        bias: Sequence[float] | None = None
     ):
+        super().__init__(bias)
         self.probability = probability
         self.distribution = distribution
 
@@ -1246,10 +1940,52 @@ class Mixture(Univariate):
                       Iterable, Univariate)
         self._distribution = distribution
 
+    @Univariate.bias.setter
+    def bias(self, bias):
+        if bias is None:
+            self._bias = bias
+        else:
+            cv.check_type('biased mixture distribution probabilities', bias,
+                          Iterable, Real)
+            for b in bias:
+                cv.check_greater_than('biased mixture distribution probabilities',
+                                      b, 0.0, True)
+            self._bias = np.array(bias, dtype=float)
+
+    @property
+    def support(self):
+        discrete_points = set()
+        intervals = []
+
+        for dist in self.distribution:
+            if isinstance(dist, Discrete):
+                discrete_points |= dist.support
+            else:
+                intervals.append(tuple(dist.support))
+
+        if intervals:
+            # simplify union by combining intervals when able
+            sorted_intervals = sorted(intervals, key=lambda x: x[0])
+            merged = [sorted_intervals[0]]
+
+            for current in sorted_intervals[1:]:
+                prev_start, prev_end = merged[-1]
+                curr_start, curr_end = current
+
+                if curr_start <= prev_end:
+                    merged[-1] = (prev_start, max(prev_end, curr_end))
+                else:
+                    merged.append(current)
+
+            intervals = merged
+
+        return {"discrete": discrete_points, "continuous": intervals}
+
     def cdf(self):
         return np.insert(np.cumsum(self.probability), 0, 0.0)
 
-    def sample(self, n_samples=1, seed=None):
+    def _sample_unbiased(self, n_samples=1, seed=None):
+        # Mixture uses internal bias mechanism, not base class bias
         rng = np.random.RandomState(seed)
 
         # Get probability of each distribution accounting for its intensity
@@ -1262,11 +1998,49 @@ class Mixture(Univariate):
 
         # Draw samples from the distributions sampled above
         out = np.empty_like(idx, dtype=float)
+        out_wgt = np.empty_like(idx, dtype=float)
         for i in np.unique(idx):
             n_dist_samples = np.count_nonzero(idx == i)
-            samples = self.distribution[i].sample(n_dist_samples)
+            samples, weights = self.distribution[i].sample(n_dist_samples)
             out[idx == i] = samples
-        return out
+            out_wgt[idx == i] = weights
+        return out, out_wgt
+
+    def sample(self, n_samples=1, seed=None):
+        # Mixture uses internal bias mechanism, not base class bias
+        if self.bias is None:
+            return self._sample_unbiased(n_samples, seed)
+
+        rng = np.random.RandomState(seed)
+
+        # Get probability of each distribution accounting for its intensity
+        p = np.array([prob*dist.integral() for prob, dist in
+                      zip(self.probability, self.distribution)])
+        p /= p.sum()
+
+        b = np.array([prob*dist.integral() for prob, dist in
+                      zip(self.bias, self.distribution)])
+        b /= b.sum()
+
+        # Sample from the distributions using biased probabilities
+        idx = rng.choice(range(len(self.distribution)), n_samples, p=b)
+        idx_wgt = np.ones(n_samples)
+        for i in np.unique(idx):
+            idx_wgt[idx == i] = p[i]/b[i]
+
+        # Draw samples from the distributions sampled above
+        out = np.empty_like(idx, dtype=float)
+        out_wgt = np.empty_like(idx, dtype=float)
+        for i in np.unique(idx):
+            n_dist_samples = np.count_nonzero(idx == i)
+            samples, weights = self.distribution[i].sample(n_dist_samples)
+            out[idx == i] = samples
+            out_wgt[idx == i] = weights * idx_wgt[idx == i]
+        return out, out_wgt
+
+    def evaluate(self, x):
+        raise NotImplementedError(
+            "evaluate() is undefined for Mixture distributions")
 
     def normalize(self):
         """Normalize the probabilities stored on the distribution"""
@@ -1297,6 +2071,7 @@ class Mixture(Univariate):
           data.set("probability", str(p))
           data.append(d.to_xml_element("dist"))
 
+        self._append_array_bias_to_xml(element)
         return element
 
     @classmethod
@@ -1322,7 +2097,8 @@ class Mixture(Univariate):
             probability.append(float(get_text(pair, 'probability')))
             distribution.append(Univariate.from_xml_element(pair.find("dist")))
 
-        return cls(probability, distribution)
+        bias_dist = cls._read_array_bias_from_xml(elem)
+        return cls(probability, distribution, bias=bias_dist)
 
     def integral(self):
         """Return integral of the distribution
@@ -1339,6 +2115,30 @@ class Mixture(Univariate):
             for p, dist in zip(self.probability, self.distribution)
         ])
 
+    def mean(self) -> float:
+        """Return mean of the mixture distribution
+
+        The mean is the weighted average of the means of the component
+        distributions, weighted by probability * integral.
+
+        .. versionadded:: 0.15.3
+
+        Returns
+        -------
+        float
+            Mean of the mixture distribution
+        """
+        # Weight each component by its probability and integral
+        weights = [p*dist.integral() for p, dist in
+                   zip(self.probability, self.distribution)]
+        total_weight = sum(weights)
+
+        if total_weight == 0:
+            return 0.0
+
+        return sum([w*dist.mean() for w, dist in
+                   zip(weights, self.distribution)]) / total_weight
+
     def clip(self, tolerance: float = 1e-6, inplace: bool = False) -> Mixture:
         r"""Remove low-importance points / distributions
 
@@ -1361,14 +2161,14 @@ class Mixture(Univariate):
         Distribution with low-importance points / distributions removed
 
         """
-        # Determine integral of original distribution to compare later
-        original_integral = self.integral()
+        # Calculate mean * integral for original distribution to compare later.
+        original_mean_integral = self.mean() * self.integral()
 
         # Determine indices for any distributions that contribute non-negligibly
-        # to overall intensity
-        intensities = [prob*dist.integral() for prob, dist in
-                       zip(self.probability, self.distribution)]
-        indices = _intensity_clip(intensities, tolerance=tolerance)
+        # to overall mean * integral
+        mean_integrals = [prob*dist.mean()*dist.integral() for prob, dist in
+                          zip(self.probability, self.distribution)]
+        indices = _intensity_clip(mean_integrals, tolerance=tolerance)
 
         # Clip mixture of distributions
         probability = self.probability[indices]
@@ -1389,64 +2189,433 @@ class Mixture(Univariate):
             # Create new distribution
             new_dist = type(self)(probability, distribution)
 
-        # Show warning if integral of new distribution is not within
-        # tolerance of original
-        diff = (original_integral - new_dist.integral())/original_integral
+        # Show warning if mean * integral of new distribution is not within
+        # tolerance of original. For energy distributions, mean * integral
+        # represents total energy.
+        new_mean_integral = new_dist.mean() * new_dist.integral()
+        diff = (original_mean_integral - new_mean_integral)/original_mean_integral
         if diff > tolerance:
-            warn("Clipping mixture distribution resulted in an integral that is "
-                    f"lower by a fraction of {diff} when tolerance={tolerance}.")
+            warn("Clipping mixture distribution resulted in a mean*integral "
+                 f"that is lower by a fraction of {diff} when tolerance={tolerance}.")
 
         return new_dist
 
 
+class DecaySpectrum(Univariate):
+    """Energy distribution from decay photon spectra of a mixture of nuclides.
+
+    This distribution stores nuclide names, their atom densities, and the volume
+    of the region. When written to XML and read by the C++ solver, the nuclide
+    names are resolved against the depletion chain to obtain the decay photon
+    energy spectra and decay constants. The resulting distribution is a mixture
+    of per-nuclide photon spectra weighted by absolute activity. The volume is
+    necessary so that the C++ solver can compute the total photon emission rate
+    in [photons/s], which is used as the source strength.
+
+    .. versionadded:: 0.15.4
+
+    Parameters
+    ----------
+    nuclides : dict
+        Dictionary mapping nuclide name (str) to atom density (float) in units
+        of [atom/b-cm].
+    volume : float
+        Volume of the source region in [cm³]. Used together with atom densities
+        to compute the absolute photon emission rate.
+
+    Attributes
+    ----------
+    nuclides : dict
+        Dictionary mapping nuclide name to atom density in [atom/b-cm].
+    volume : float
+        Volume of the source region in [cm³].
+
+    """
+
+    def __init__(self, nuclides: dict[str, float], volume: float):
+        super().__init__(bias=None)
+        self._dist_cache = None
+        self._dist_cache_key = None
+        self.nuclides = nuclides
+        self.volume = volume
+
+    def __len__(self):
+        return len(self.nuclides)
+
+    @property
+    def nuclides(self):
+        return self._nuclides
+
+    @nuclides.setter
+    def nuclides(self, nuclides):
+        cv.check_type('nuclides', nuclides, dict)
+        for name, density in nuclides.items():
+            cv.check_type('nuclide name', name, str)
+            cv.check_type(f'atom density for {name}', density, Real)
+            cv.check_greater_than(f'atom density for {name}', density, 0.0, True)
+        self._nuclides = dict(nuclides)
+        self._dist_cache = None
+        self._dist_cache_key = None
+
+    @property
+    def volume(self):
+        return self._volume
+
+    @volume.setter
+    def volume(self, volume):
+        cv.check_type('volume', volume, Real)
+        cv.check_greater_than('volume', volume, 0.0)
+        self._volume = float(volume)
+        self._dist_cache = None
+        self._dist_cache_key = None
+
+    @staticmethod
+    def _chain_file_cache_key():
+        """Return a hashable key for the active depletion chain."""
+        chain_file = openmc.config.get('chain_file')
+        if chain_file is None:
+            return None
+
+        path = Path(chain_file).resolve()
+        try:
+            stat = path.stat()
+        except OSError:
+            return (path, None, None)
+        return (path, stat.st_mtime, stat.st_size)
+
+    def to_distribution(self):
+        """Convert to a concrete distribution using decay chain data.
+
+        Builds a combined photon energy distribution by looking up each nuclide
+        in the depletion chain via :func:`openmc.data.decay_photon_energy` and
+        weighting by absolute atom count (``density * 1e24 * volume``). The
+        result is cached on the object; the cache is invalidated automatically
+        when :attr:`nuclides` or :attr:`volume` are reassigned.
+
+        Requires ``openmc.config['chain_file']`` to be set.
+
+        Returns
+        -------
+        openmc.stats.Univariate or None
+            Combined photon energy distribution, or ``None`` if no nuclide in
+            :attr:`nuclides` has a photon source in the chain.
+
+        """
+        chain_key = self._chain_file_cache_key()
+        if self._dist_cache is not None and self._dist_cache_key == chain_key:
+            return self._dist_cache
+
+        dists = []
+        weights = []
+        for name, density in self.nuclides.items():
+            dist = openmc.data.decay_photon_energy(name)
+            if dist is not None:
+                dists.append(dist)
+                weights.append(density * 1e24 * self.volume)
+
+        if not dists:
+            return None
+
+        self._dist_cache = combine_distributions(dists, weights)
+        self._dist_cache_key = chain_key
+        return self._dist_cache
+
+    def to_xml_element(self, element_name: str):
+        """Return XML representation of the decay photon distribution
+
+        Parameters
+        ----------
+        element_name : str
+            XML element name
+
+        Returns
+        -------
+        element : lxml.etree._Element
+            XML element containing decay photon distribution data
+
+        """
+        element = ET.Element(element_name)
+        element.set("type", "decay_spectrum")
+        element.set("volume", str(self.volume))
+        nuclides = ET.SubElement(element, "nuclides")
+        nuclides.text = ' '.join(self.nuclides)
+        parameters = ET.SubElement(element, "parameters")
+        parameters.text = ' '.join(str(density) for density in self.nuclides.values())
+        return element
+
+    @classmethod
+    def from_xml_element(cls, elem: ET.Element):
+        """Generate decay photon distribution from an XML element
+
+        Parameters
+        ----------
+        elem : lxml.etree._Element
+            XML element
+
+        Returns
+        -------
+        openmc.stats.DecaySpectrum
+            Decay photon distribution generated from XML element
+
+        """
+        volume = float(elem.get('volume'))
+        names = get_elem_list(elem, 'nuclides', str)
+        densities = get_elem_list(elem, 'parameters', float)
+        nuclides = dict(zip(names, densities))
+        return cls(nuclides, volume)
+
+    def _sample_unbiased(self, n_samples=1, seed=None):
+        dist = self.to_distribution()
+        if dist is None:
+            raise RuntimeError(
+                "DecaySpectrum._sample_unbiased requires chain data but none "
+                "was found. Ensure openmc.config['chain_file'] is set and the "
+                "chain contains photon sources for the nuclides present."
+            )
+        return dist.sample(n_samples, seed)[0]
+
+    def integral(self):
+        """Return integral of the distribution
+
+        Returns the total photon emission rate in [photons/s] by delegating to
+        :meth:`to_distribution`. Returns ``0.0`` when no chain data is
+        available (e.g., ``openmc.config['chain_file']`` is not set).
+
+        Returns
+        -------
+        float
+            Total photon emission rate in [photons/s], or ``0.0`` if chain
+            data is unavailable.
+        """
+        try:
+            dist = self.to_distribution()
+        except Exception:
+            return 0.0
+        if dist is None:
+            return 0.0
+        return dist.integral()
+
+    @staticmethod
+    @cache
+    def _photon_integral(nuclide: str, chain_key) -> float | None:
+        """Return the per-atom photon emission integral for a nuclide"""
+        dist = openmc.data.decay_photon_energy(nuclide)
+        return dist.integral() if dist is not None else None
+
+    def clip(self, tolerance: float = 1e-9, inplace: bool = False):
+        """Remove nuclides with negligible contribution to photon emission.
+
+        Nuclides that are stable or have no photon source in the depletion
+        chain are removed unconditionally.  The remaining nuclides are ranked
+        by their photon emission rate (proportional to
+        ``atom_density * decay_constant * photon_yield``) and the least
+        important are discarded until the cumulative discarded fraction of the
+        total emission rate exceeds *tolerance*.
+
+        Requires ``openmc.config['chain_file']`` to be set.
+
+        Parameters
+        ----------
+        tolerance : float
+            Maximum fraction of total photon emission rate that may be
+            discarded.
+        inplace : bool
+            Whether to modify the current object in-place or return a new one.
+
+        Returns
+        -------
+        openmc.stats.DecaySpectrum
+            Distribution with negligible nuclides removed.
+
+        """
+        # Compute per-nuclide emission rate; drop non-emitters
+        emitting_names = []
+        emitting_densities = []
+        rates = []
+        chain_key = self._chain_file_cache_key()
+        for name, density in self.nuclides.items():
+            integral = DecaySpectrum._photon_integral(name, chain_key)
+            if integral is None:
+                continue
+            emitting_names.append(name)
+            emitting_densities.append(density)
+            rates.append(density * self.volume * integral)
+
+        if not emitting_names:
+            new_nuclides = {}
+        else:
+            indices = _intensity_clip(rates, tolerance=tolerance)
+            new_nuclides = {
+                emitting_names[i]: emitting_densities[i] for i in indices
+            }
+
+        if inplace:
+            self._nuclides = new_nuclides
+            self._dist_cache = None
+            self._dist_cache_key = None
+            return self
+        return type(self)(new_nuclides, self.volume)
+
+    @property
+    def support(self):
+        return (0.0, np.inf)
+
+    def evaluate(self, x):
+        """Evaluate the probability density at a given value.
+
+        Delegates to the combined distribution built from chain data. Raises
+        ``NotImplementedError`` if the combined distribution is a
+        :class:`~openmc.stats.Mixture` (which does not support
+        ``evaluate()``).
+
+        Parameters
+        ----------
+        x : float
+            Value at which to evaluate the PDF.
+
+        Returns
+        -------
+        float
+            Probability density at *x*.
+        """
+        dist = self.to_distribution()
+        if dist is None:
+            raise RuntimeError(
+                "DecaySpectrum.evaluate requires chain data. Ensure "
+                "openmc.config['chain_file'] is set."
+            )
+        return dist.evaluate(x)
+
+    def mean(self):
+        """Return the mean of the distribution.
+
+        Delegates to the combined distribution built from chain data.
+
+        Returns
+        -------
+        float
+            Mean photon energy in [eV].
+        """
+        dist = self.to_distribution()
+        if dist is None:
+            raise RuntimeError(
+                "DecaySpectrum.mean requires chain data. Ensure "
+                "openmc.config['chain_file'] is set."
+            )
+        return dist.mean()
+
+
 def combine_distributions(
-    dists: Sequence[Univariate],
+    dists: Sequence[Discrete | Tabular | Mixture],
     probs: Sequence[float]
 ):
     """Combine distributions with specified probabilities
 
     This function can be used to combine multiple instances of
-    :class:`~openmc.stats.Discrete` and `~openmc.stats.Tabular`. Multiple
-    discrete distributions are merged into a single distribution and the
-    remainder of the distributions are put into a :class:`~openmc.stats.Mixture`
-    distribution.
+    :class:`~openmc.stats.Discrete`, :class:`~openmc.stats.Tabular` and
+    :class:`~openmc.stats.Mixture` of them. Multiple discrete distributions are
+    merged into a single distribution and the remainder of the distributions are
+    put into a :class:`~openmc.stats.Mixture` distribution.
 
     .. versionadded:: 0.13.1
 
     Parameters
     ----------
-    dists : iterable of openmc.stats.Univariate
+    dists : sequence of openmc.stats.Discrete, openmc.stats.Tabular, or openmc.stats.Mixture
         Distributions to combine
-    probs : iterable of float
+    probs : sequence of float
         Probability (or intensity) of each distribution
 
     """
-    # Get copy of distribution list so as not to modify the argument
-    dist_list = deepcopy(dists)
+    new_probs = []
+    new_dists = []
+    for i, dist in enumerate(dists):
+        cv.check_type(f'dists[{i}]', dist, (Discrete, Tabular, Mixture))
+        cv.check_type(f'probs[{i}]', probs[i], Real)
+        cv.check_greater_than(f'probs[{i}]', probs[i], 0.0)
+        if isinstance(dist, Mixture):
+            if dist.bias is not None:
+                warn("A Mixture distribution with a bias specified was passed "
+                     "to combine_distributions. The bias will be discarded "
+                     "during flattening.")
+            for j, d in enumerate(dist.distribution):
+                cv.check_type(f'dists[{i}].distribution[{j}]', d, (Discrete, Tabular))
+                new_probs.append(probs[i]*dist.probability[j])
+                new_dists.append(d)
+        else:
+            new_probs.append(probs[i])
+            new_dists.append(dist)
+
+    probs = new_probs
+    dists = new_dists
 
     # Get list of discrete/continuous distribution indices
-    discrete_index = [i for i, d in enumerate(dist_list) if isinstance(d, Discrete)]
-    cont_index = [i for i, d in enumerate(dist_list) if isinstance(d, Tabular)]
+    discrete_index = [i for i, d in enumerate(dists) if isinstance(d, Discrete)]
+    cont_index = [i for i, d in enumerate(dists) if isinstance(d, Tabular)]
 
-    # Apply probabilites to continuous distributions
-    for i in cont_index:
-        dist = dist_list[i]
-        dist._p *= probs[i]
+    cont_dists = [dists[i] for i in cont_index]
+    cont_probs = [probs[i] for i in cont_index]
 
     if discrete_index:
         # Create combined discrete distribution
-        dist_discrete = [dist_list[i] for i in discrete_index]
+        dist_discrete = [dists[i] for i in discrete_index]
         discrete_probs = [probs[i] for i in discrete_index]
         combined_dist = Discrete.merge(dist_discrete, discrete_probs)
+        if cont_index:
+            return Mixture(cont_probs + [1.0], cont_dists + [combined_dist])
+        else:
+            return combined_dist
+    else:
+        if len(cont_dists) == 1:
+            dist = cont_dists[0]
+            return Tabular(dist.x, dist.p * cont_probs[0],
+                           dist.interpolation, bias=dist.bias)
+        else:
+            return Mixture(cont_probs, cont_dists)
 
-        # Replace multiple discrete distributions with merged
-        for idx in reversed(discrete_index):
-            dist_list.pop(idx)
-        dist_list.append(combined_dist)
 
-    # Combine discrete and continuous if present
-    if len(dist_list) > 1:
-        probs = [1.0]*len(dist_list)
-        dist_list[:] = [Mixture(probs, dist_list.copy())]
+def check_bias_support(parent: Univariate, bias: Univariate | None):
+    """Ensure that bias distributions share the support of the univariate
+    distribution they are biasing.
 
-    return dist_list[0]
+    Parameters
+    ----------
+    parent : openmc.stats.Univariate
+        Distributions to be biased
+    bias : openmc.stats.Univariate or None
+        Proposed bias distribution
+
+    """
+    if bias is None:
+        return
+
+    def mismatch_error(err_type, msg):
+        raise err_type(f"Support of parent {type(parent).__name__} and bias "
+                       f"{type(bias).__name__} distributions do not match. "
+                       f"{msg}")
+
+    p_sup, b_sup = parent.support, bias.support
+
+    if isinstance(p_sup, set) or isinstance(b_sup, set):
+        raise RuntimeError("Discrete distributions cannot be used as biasing "
+                           "distributions or be biased by another Univariate "
+                           "distribution. Instead, assign a vector of "
+                           "alternate probabilities to the bias attribute.")
+
+    elif isinstance(p_sup, dict) or isinstance (b_sup, dict):
+        raise RuntimeError("Mixture distributions cannot be used as biasing "
+                           "distributions or be biased by another Univariate "
+                           "distribution. Instead, instantiate the Mixture "
+                           "object using biased member distributions, or "
+                           "assign a vector of alternative probabilities to "
+                           "the bias attribute.")
+
+    elif isinstance(p_sup, tuple):
+        if isinstance(b_sup, tuple):
+            if p_sup != b_sup:
+                mismatch_error(ValueError, "")
+        else:
+            mismatch_error(TypeError, "Incompatible support types.")
+
+    else:
+        raise TypeError("Unrecognized type for parent distribution support")
diff --git a/openmc/summary.py b/openmc/summary.py
index 6423f8ce1..ca5cfadd7 100644
--- a/openmc/summary.py
+++ b/openmc/summary.py
@@ -127,11 +127,23 @@ class Summary:
             self._fast_materials[material.id] = material
 
     def _read_surfaces(self):
+        periodic_surface_ids = set()
         for group in self._f['geometry/surfaces'].values():
             surface = openmc.Surface.from_hdf5(group)
             # surface may be None for DAGMC surfaces
             if surface:
                 self._fast_surfaces[surface.id] = surface
+                if surface.boundary_type == "periodic":
+                    periodic_surface_ids.add(surface.id)
+
+        # Assign periodic surfaces when information is in file
+        for surface_id in periodic_surface_ids:
+            group = self._f[f'geometry/surfaces/surface {surface_id}']
+            surface = self._fast_surfaces[surface_id]
+            if 'periodic_surface_id' in group:
+                periodic_surface_id = int(group['periodic_surface_id'][()])
+                surface.periodic_surface = self._fast_surfaces[periodic_surface_id]
+
 
     def _read_cells(self):
 
diff --git a/openmc/surface.py b/openmc/surface.py
index 840c3125d..c2afeb613 100644
--- a/openmc/surface.py
+++ b/openmc/surface.py
@@ -13,6 +13,7 @@ from .checkvalue import check_type, check_value, check_length, check_greater_tha
 from .mixin import IDManagerMixin, IDWarning
 from .region import Region, Intersection, Union
 from .bounding_box import BoundingBox
+from ._xml import get_elem_list, get_text
 
 
 _BOUNDARY_TYPES = {'transmission', 'vacuum', 'reflective', 'periodic', 'white'}
@@ -37,10 +38,13 @@ class SurfaceCoefficient:
     value : float or str
         Value of the coefficient (float) or the name of the coefficient that
         it is equivalent to (str).
+    positive : bool
+        Does the surface coefficient must be positive. Defaults to False.
 
     """
-    def __init__(self, value):
+    def __init__(self, value, positive=False):
         self.value = value
+        self.positive = positive
 
     def __get__(self, instance, owner=None):
         if instance is None:
@@ -55,6 +59,8 @@ class SurfaceCoefficient:
         if isinstance(self.value, Real):
             raise AttributeError('This coefficient is read-only')
         check_type(f'{self.value} coefficient', value, Real)
+        if self.positive:
+            check_greater_than(f'{self.value} coefficient', value, 0.0)
         instance._coefficients[self.value] = value
 
 
@@ -122,9 +128,8 @@ class Surface(IDManagerMixin, ABC):
     boundary_type : {'transmission', 'vacuum', 'reflective', 'periodic', 'white'}, optional
         Boundary condition that defines the behavior for particles hitting the
         surface. Defaults to transmissive boundary condition where particles
-        freely pass through the surface. Note that periodic boundary conditions
-        can only be applied to x-, y-, and z-planes, and only axis-aligned
-        periodicity is supported.
+        freely pass through the surface. Note that only axis-aligned
+        periodicity is supported around the x-, y-, and z-axes.
     albedo : float, optional
         Albedo of the surfaces as a ratio of particle weight after interaction
         with the surface to the initial weight. Values must be positive. Only
@@ -151,6 +156,7 @@ class Surface(IDManagerMixin, ABC):
 
     """
 
+    min_id = 1
     next_id = 1
     used_ids = set()
     _atol = 1.e-12
@@ -451,17 +457,17 @@ class Surface(IDManagerMixin, ABC):
         """
 
         # Determine appropriate class
-        surf_type = elem.get('type')
+        surf_type = get_text(elem, "type")
         cls = _SURFACE_CLASSES[surf_type]
 
         # Determine ID, boundary type, boundary albedo, coefficients
         kwargs = {}
-        kwargs['surface_id'] = int(elem.get('id'))
-        kwargs['boundary_type'] = elem.get('boundary', 'transmission')
+        kwargs['surface_id'] = int(get_text(elem, "id"))
+        kwargs['boundary_type'] = get_text(elem, "boundary", "transmission")
         if kwargs['boundary_type'] in _ALBEDO_BOUNDARIES:
-            kwargs['albedo'] = float(elem.get('albedo', 1.0))
-        kwargs['name'] = elem.get('name')
-        coeffs = [float(x) for x in elem.get('coeffs').split()]
+            kwargs['albedo'] = float(get_text(elem, "albedo", 1.0))
+        kwargs['name'] = get_text(elem, "name")
+        coeffs = get_elem_list(elem, "coeffs", float)
         kwargs.update(dict(zip(cls._coeff_keys, coeffs)))
 
         return cls(**kwargs)
@@ -821,8 +827,7 @@ class XPlane(PlaneMixin, Surface):
     boundary_type : {'transmission', 'vacuum', 'reflective', 'periodic', 'white'}, optional
         Boundary condition that defines the behavior for particles hitting the
         surface. Defaults to transmissive boundary condition where particles
-        freely pass through the surface. Only axis-aligned periodicity is
-        supported, i.e., x-planes can only be paired with x-planes.
+        freely pass through the surface.
     albedo : float, optional
         Albedo of the surfaces as a ratio of particle weight after interaction
         with the surface to the initial weight. Values must be positive. Only
@@ -886,8 +891,7 @@ class YPlane(PlaneMixin, Surface):
     boundary_type : {'transmission', 'vacuum', 'reflective', 'periodic', 'white'}, optional
         Boundary condition that defines the behavior for particles hitting the
         surface. Defaults to transmissive boundary condition where particles
-        freely pass through the surface. Only axis-aligned periodicity is
-        supported, i.e., y-planes can only be paired with y-planes.
+        freely pass through the surface.
     albedo : float, optional
         Albedo of the surfaces as a ratio of particle weight after interaction
         with the surface to the initial weight. Values must be positive. Only
@@ -951,8 +955,7 @@ class ZPlane(PlaneMixin, Surface):
     boundary_type : {'transmission', 'vacuum', 'reflective', 'periodic', 'white'}, optional
         Boundary condition that defines the behavior for particles hitting the
         surface. Defaults to transmissive boundary condition where particles
-        freely pass through the surface. Only axis-aligned periodicity is
-        supported, i.e., z-planes can only be paired with z-planes.
+        freely pass through the surface.
     albedo : float, optional
         Albedo of the surfaces as a ratio of particle weight after interaction
         with the surface to the initial weight. Values must be positive. Only
@@ -1263,7 +1266,7 @@ class Cylinder(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r = SurfaceCoefficient('r')
+    r = SurfaceCoefficient('r', positive=True)
     dx = SurfaceCoefficient('dx')
     dy = SurfaceCoefficient('dy')
     dz = SurfaceCoefficient('dz')
@@ -1429,7 +1432,7 @@ class XCylinder(QuadricMixin, Surface):
     x0 = SurfaceCoefficient(0.)
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r = SurfaceCoefficient('r')
+    r = SurfaceCoefficient('r', positive=True)
     dx = SurfaceCoefficient(1.)
     dy = SurfaceCoefficient(0.)
     dz = SurfaceCoefficient(0.)
@@ -1527,7 +1530,7 @@ class YCylinder(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient(0.)
     z0 = SurfaceCoefficient('z0')
-    r = SurfaceCoefficient('r')
+    r = SurfaceCoefficient('r', positive=True)
     dx = SurfaceCoefficient(0.)
     dy = SurfaceCoefficient(1.)
     dz = SurfaceCoefficient(0.)
@@ -1625,7 +1628,7 @@ class ZCylinder(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient(0.)
-    r = SurfaceCoefficient('r')
+    r = SurfaceCoefficient('r', positive=True)
     dx = SurfaceCoefficient(0.)
     dy = SurfaceCoefficient(0.)
     dz = SurfaceCoefficient(1.)
@@ -1725,7 +1728,7 @@ class Sphere(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r = SurfaceCoefficient('r')
+    r = SurfaceCoefficient('r', positive=True)
 
     def _get_base_coeffs(self):
         x0, y0, z0, r = self.x0, self.y0, self.z0, self.r
@@ -1851,7 +1854,7 @@ class Cone(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r2 = SurfaceCoefficient('r2')
+    r2 = SurfaceCoefficient('r2', positive=True)
     dx = SurfaceCoefficient('dx')
     dy = SurfaceCoefficient('dy')
     dz = SurfaceCoefficient('dz')
@@ -1987,7 +1990,7 @@ class XCone(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r2 = SurfaceCoefficient('r2')
+    r2 = SurfaceCoefficient('r2', positive=True)
     dx = SurfaceCoefficient(1.)
     dy = SurfaceCoefficient(0.)
     dz = SurfaceCoefficient(0.)
@@ -2089,7 +2092,7 @@ class YCone(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r2 = SurfaceCoefficient('r2')
+    r2 = SurfaceCoefficient('r2', positive=True)
     dx = SurfaceCoefficient(0.)
     dy = SurfaceCoefficient(1.)
     dz = SurfaceCoefficient(0.)
@@ -2191,7 +2194,7 @@ class ZCone(QuadricMixin, Surface):
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    r2 = SurfaceCoefficient('r2')
+    r2 = SurfaceCoefficient('r2', positive=True)
     dx = SurfaceCoefficient(0.)
     dy = SurfaceCoefficient(0.)
     dz = SurfaceCoefficient(1.)
@@ -2294,9 +2297,9 @@ class TorusMixin:
     x0 = SurfaceCoefficient('x0')
     y0 = SurfaceCoefficient('y0')
     z0 = SurfaceCoefficient('z0')
-    a = SurfaceCoefficient('a')
-    b = SurfaceCoefficient('b')
-    c = SurfaceCoefficient('c')
+    a = SurfaceCoefficient('a', positive=True)
+    b = SurfaceCoefficient('b', positive=True)
+    c = SurfaceCoefficient('c', positive=True)
 
     def translate(self, vector, inplace=False):
         surf = self if inplace else self.clone()
diff --git a/openmc/tallies.py b/openmc/tallies.py
index ebb313551..ceced4255 100644
--- a/openmc/tallies.py
+++ b/openmc/tallies.py
@@ -3,6 +3,7 @@ from collections.abc import Iterable, MutableSequence
 import copy
 from functools import partial, reduce, wraps
 from itertools import product
+from math import sqrt, log
 from numbers import Integral, Real
 import operator
 from pathlib import Path
@@ -11,11 +12,29 @@ import lxml.etree as ET
 import h5py
 import numpy as np
 import pandas as pd
-import scipy.sparse as sps
+from scipy.stats import chi2, norm
 
 import openmc
 import openmc.checkvalue as cv
-from ._xml import clean_indentation, reorder_attributes, get_text
+from openmc.filter import (
+    Filter, 
+    DistribcellFilter, 
+    EnergyFunctionFilter, 
+    DelayedGroupFilter, 
+    FilterMeta, 
+    MeshFilter,
+    MeshBornFilter,
+)
+from openmc.arithmetic import (
+    CrossFilter, 
+    AggregateFilter,
+    CrossScore,
+    AggregateScore,
+    CrossNuclide,
+    AggregateNuclide,
+)
+from ._sparse_compat import lil_array
+from ._xml import clean_indentation, get_elem_list, get_text
 from .mixin import IDManagerMixin
 from .mesh import MeshBase
 
@@ -29,9 +48,9 @@ _PRODUCT_TYPES = ['tensor', 'entrywise']
 
 # The following indicate acceptable types when setting Tally.scores,
 # Tally.nuclides, and Tally.filters
-_SCORE_CLASSES = (str, openmc.CrossScore, openmc.AggregateScore)
-_NUCLIDE_CLASSES = (str, openmc.CrossNuclide, openmc.AggregateNuclide)
-_FILTER_CLASSES = (openmc.Filter, openmc.CrossFilter, openmc.AggregateFilter)
+_SCORE_CLASSES = (str, CrossScore, AggregateScore)
+_NUCLIDE_CLASSES = (str, CrossNuclide, AggregateNuclide)
+_FILTER_CLASSES = (Filter, CrossFilter, AggregateFilter)
 
 # Valid types of estimators
 ESTIMATOR_TYPES = {'tracklength', 'collision', 'analog'}
@@ -48,6 +67,18 @@ class Tally(IDManagerMixin):
         will automatically be assigned
     name : str, optional
         Name of the tally. If not specified, the name is the empty string.
+    scores : list of str, optional
+        List of scores, e.g. ['flux', 'fission']
+    filters : list of openmc.Filter, optional
+        List of filters for the tally
+    nuclides : list of str, optional
+        List of nuclides to score results for
+    estimator : {'analog', 'tracklength', 'collision'}, optional
+        Type of estimator for the tally
+    triggers : list of openmc.Trigger, optional
+        List of tally triggers
+    derivative : openmc.TallyDerivative, optional
+        A material perturbation derivative to apply to all scores in the tally
 
     Attributes
     ----------
@@ -91,10 +122,24 @@ class Tally(IDManagerMixin):
     sum_sq : numpy.ndarray
         An array containing the sum of each independent realization squared for
         each bin
+    sum_third : numpy.ndarray
+        An array containing the sum of each independent realization to the third power for
+        each bin
+    sum_fourth : numpy.ndarray
+        An array containing the sum of each independent realization to the fourth power for
+        each bin
     mean : numpy.ndarray
         An array containing the sample mean for each bin
     std_dev : numpy.ndarray
         An array containing the sample standard deviation for each bin
+    vov : numpy.ndarray
+        An array containing the variance of the variance for each tally bin
+    higher_moments : bool
+        Whether or not the tally accumulates the sums third and fourth to compute higher-order moments
+    figure_of_merit : numpy.ndarray
+        An array containing the figure of merit for each bin
+
+        .. versionadded:: 0.15.3
     derived : bool
         Whether or not the tally is derived from one or more other tallies
     sparse : bool
@@ -108,7 +153,9 @@ class Tally(IDManagerMixin):
     next_id = 1
     used_ids = set()
 
-    def __init__(self, tally_id=None, name=''):
+    def __init__(self, tally_id=None, name='', scores=None, filters=None,
+                 nuclides=None, estimator=None, triggers=None,
+                 derivative=None):
         # Initialize Tally class attributes
         self.id = tally_id
         self.name = name
@@ -125,8 +172,13 @@ class Tally(IDManagerMixin):
 
         self._sum = None
         self._sum_sq = None
+        self._sum_third = None
+        self._sum_fourth = None
         self._mean = None
         self._std_dev = None
+        self._vov = None
+        self._higher_moments = False
+        self._simulation_time = None
         self._with_batch_statistics = False
         self._derived = False
         self._sparse = False
@@ -134,6 +186,19 @@ class Tally(IDManagerMixin):
         self._sp_filename = None
         self._results_read = False
 
+        if filters is not None:
+            self.filters = filters
+        if nuclides is not None:
+            self.nuclides = nuclides
+        if scores is not None:
+            self.scores = scores
+        if estimator is not None:
+            self.estimator = estimator
+        if triggers is not None:
+            self.triggers = triggers
+        if derivative is not None:
+            self.derivative = derivative
+
     def __eq__(self, other):
         if other.id != self.id:
             return False
@@ -216,6 +281,15 @@ class Tally(IDManagerMixin):
         cv.check_type('multiply density', value, bool)
         self._multiply_density = value
 
+    @property
+    def higher_moments(self) -> bool:
+        return self._higher_moments
+
+    @higher_moments.setter
+    def higher_moments(self, value):
+        cv.check_type("higher_moments", value, bool)
+        self._higher_moments = value
+
     @property
     def filters(self):
         return self._filters
@@ -260,7 +334,7 @@ class Tally(IDManagerMixin):
 
     @property
     def num_nuclides(self):
-        return len(self._nuclides)
+        return max(len(self._nuclides), 1)
 
     @property
     def scores(self):
@@ -363,9 +437,19 @@ class Tally(IDManagerMixin):
             group = f[f'tallies/tally {self.id}']
             self._num_realizations = int(group['n_realizations'][()])
 
+            for filt in self.filters:
+                if isinstance(filt, DistribcellFilter):
+                    filter_group = f[f'tallies/filters/filter {filt.id}']
+                    filt._num_bins = int(filter_group['n_bins'][()])
+
             # Update nuclides
             nuclide_names = group['nuclides'][()]
             self._nuclides = [name.decode().strip() for name in nuclide_names]
+            # Check for higher_moments attribute
+            if "higher_moments" in group.attrs:
+                self._higher_moments = bool(group.attrs["higher_moments"][()])
+            else:
+                self._higher_moments = False
 
             # Extract Tally data from the file
             data = group['results']
@@ -380,10 +464,28 @@ class Tally(IDManagerMixin):
             self._sum = sum_
             self._sum_sq = sum_sq
 
+            if self._higher_moments:
+                # Extract additional Tally data when higher moments enabled
+                sum_third = data[:, :, 2]
+                sum_fourth = data[:, :, 3]
+
+                # Reshape the results arrays
+                sum_third = np.reshape(sum_third, self.shape)
+                sum_fourth = np.reshape(sum_fourth, self.shape)
+
+                # Set the additional data for this Tally
+                self._sum_third = sum_third
+                self._sum_fourth = sum_fourth
+
             # Convert NumPy arrays to SciPy sparse LIL matrices
             if self.sparse:
-                self._sum = sps.lil_matrix(self._sum.flatten(), self._sum.shape)
-                self._sum_sq = sps.lil_matrix(self._sum_sq.flatten(), self._sum_sq.shape)
+                self._sum = lil_array(self._sum.flatten(), self._sum.shape)
+                self._sum_sq = lil_array(self._sum_sq.flatten(), self._sum_sq.shape)
+                self._sum_third = lil_array(self._sum_third.flatten(), self._sum_third.shape)
+                self._sum_fourth = lil_array(self._sum_fourth.flatten(), self._sum_fourth.shape)
+
+            # Read simulation time (needed for figure of merit)
+            self._simulation_time = f["runtime"]["simulation"][()]
 
         # Indicate that Tally results have been read
         self._results_read = True
@@ -420,6 +522,52 @@ class Tally(IDManagerMixin):
         cv.check_type('sum_sq', sum_sq, Iterable)
         self._sum_sq = sum_sq
 
+    @property
+    @ensure_results
+    def sum_third(self):
+        if not self._higher_moments:
+            raise ValueError(
+                "Higher moments have not been enabled for this tally. To make "
+                "higher moments available, set the higher_moments attribute to "
+                "True before running a simulation."
+            )
+
+        if not self._sp_filename or self.derived:
+            return None
+
+        if self.sparse:
+            return np.reshape(self._sum_third.toarray(), self.shape)
+        else:
+            return self._sum_third
+
+    @sum_third.setter
+    def sum_third(self, sum_third):
+        cv.check_type("sum_third", sum_third, Iterable)
+        self._sum_third = sum_third
+
+    @property
+    @ensure_results
+    def sum_fourth(self):
+        if not self._higher_moments:
+            raise ValueError(
+                "Higher moments have not been enabled for this tally. To make "
+                "higher moments available, set the higher_moments attribute to "
+                "True before running a simulation."
+            )
+
+        if not self._sp_filename or self.derived:
+            return None
+
+        if self.sparse:
+            return np.reshape(self._sum_fourth.toarray(), self.shape)
+        else:
+            return self._sum_fourth
+
+    @sum_fourth.setter
+    def sum_fourth(self, sum_fourth):
+        cv.check_type("sum_fourth", sum_fourth, Iterable)
+        self._sum_fourth = sum_fourth
+
     @property
     def mean(self):
         if self._mean is None:
@@ -430,8 +578,7 @@ class Tally(IDManagerMixin):
 
             # Convert NumPy array to SciPy sparse LIL matrix
             if self.sparse:
-                self._mean = sps.lil_matrix(self._mean.flatten(),
-                                            self._mean.shape)
+                self._mean = lil_array(self._mean.flatten(), self._mean.shape)
 
         if self.sparse:
             return np.reshape(self._mean.toarray(), self.shape)
@@ -452,8 +599,7 @@ class Tally(IDManagerMixin):
 
             # Convert NumPy array to SciPy sparse LIL matrix
             if self.sparse:
-                self._std_dev = sps.lil_matrix(self._std_dev.flatten(),
-                                               self._std_dev.shape)
+                self._std_dev = lil_array(self._std_dev.flatten(), self._std_dev.shape)
 
             self.with_batch_statistics = True
 
@@ -462,6 +608,359 @@ class Tally(IDManagerMixin):
         else:
             return self._std_dev
 
+    @property
+    def vov(self):
+        if self._vov is None:
+            n = self.num_realizations
+            sum1 = self.sum
+            sum2 = self.sum_sq
+            sum3 = self.sum_third
+            sum4 = self.sum_fourth
+            self._vov = np.zeros_like(sum1, dtype=float)
+
+            # Calculate the variance of the variance (Eq. 2.232 in
+            # https://doi.org/10.2172/2372634)
+            numerator = (sum4 - (4.0*sum3*sum1)/n
+                        + (6.0*sum2*(sum1**2))/(n**2)
+                        - (3.0*(sum1)**4)/(n**3))
+            denominator = (sum2 - (1.0/n)*(sum1**2))**2
+
+            mask = denominator > 0.0
+
+            self._vov[mask] = numerator[mask]/denominator[mask] - 1.0/n
+
+            if self.sparse:
+                self._vov = lil_array(self._vov.flatten(), self._vov.shape)
+
+        if self.sparse:
+            return np.reshape(self._vov.toarray(), self.shape)
+        else:
+            return self._vov
+
+    @property
+    def m2(self):
+        n = self.num_realizations
+        return self.sum_sq/n - self.mean**2
+
+    @property
+    def m3(self):
+        n = self.num_realizations
+        mean = self.mean
+        sum2 = self.sum_sq/n
+        sum3 = self.sum_third/n
+
+        return sum3 - 3.0*mean*sum2 + 2.0*mean**3
+
+    @property
+    def m4(self):
+        n = self.num_realizations
+        mean = self.mean
+        sum2 = self.sum_sq/n
+        sum3 = self.sum_third/n
+        sum4 = self.sum_fourth/n
+
+        return sum4 - 4.0*mean*sum3 + 6.0*(mean**2)*sum2 - 3.0*mean**4
+
+    def skew(self, bias=False) -> np.ndarray:
+        """Return the sample skewness of each tally bin.
+
+        This method computes and returns the unadjusted or adjusted
+        Fisher-Pearson coefficient of skewness.
+
+        Parameters
+        ----------
+        bias : bool
+            If False, calculations are corrected for bias and the adjusted
+            Fisher-Pearson skewness (:math:`G_1`) is returned. If True,
+            calculations are not corrected for bias and the unadjusted skewness
+            (:math:`g_1`) is returned.
+
+        Returns
+        -------
+        float
+            The skewness of each tally bin
+        """
+        n = self.num_realizations
+        m2 = self.m2
+        m3 = self.m3
+
+        with np.errstate(divide="ignore", invalid="ignore"):
+            g1 = np.where(m2 > 0.0, m3/(m2**1.5), 0.0)
+
+        if bias:
+            return g1
+        else:
+            if n <= 2:
+                raise ValueError("Insufficient number of independent realizations"
+                                 f"for bias-corrected skewness: need n >= 3, got {n=}.")
+            else:
+                return sqrt(n*(n - 1))/(n - 2)*g1
+
+    def kurtosis(self, fisher=True, bias=False) -> np.ndarray:
+        r"""Return the sample kurtosis of each tally bin.
+
+        This method computes and returns the sample kurtosis using either
+        Pearson's or Fisher's definition, with or without finite-sample bias
+        correction. The value returned depends on the `bias` and `fisher`
+        arguments as follows:
+
+        - **bias=True, fisher=False**: Returns :math:`b_2` (Pearson's kurtosis)
+          This is the raw fourth standardized moment: :math:`m_4/m_2^2`. For a
+          normal distribution, :math:`b_2\approx 3`.
+
+        - **bias=True, fisher=True**: Returns :math:`g_2` (excess kurtosis) This
+          is :math:`b_2 - 3`, centered at 0 for normal distributions. Positive
+          values indicate heavier tails, negative values lighter tails.
+
+        - **bias=False, fisher=True** (default): Returns :math:`G_2` (adjusted
+          excess kurtosis). This applies finite-sample bias correction to
+          :math:`g_2`. This is the recommended estimator for statistical
+          inference.
+
+        - **bias=False, fisher=False**: Returns bias-corrected Pearson's
+          kurtosis. This is :math:`G_2 + 3`.
+
+        Parameters
+        ----------
+        fisher : bool, optional
+            If True (default), Fisher's definition is used (excess kurtosis). If
+            False, Pearson's definition is used.
+        bias : bool, optional
+            If False (default), calculations are corrected for statistical bias
+            using finite-sample adjustments. If True, calculations use the
+            biased estimator (population formulas).
+
+        Returns
+        -------
+        numpy.ndarray
+            The kurtosis of each tally bin
+
+        """
+        n = self.num_realizations
+        m2 = self.m2
+        m4 = self.m4
+
+        with np.errstate(divide="ignore", invalid="ignore"):
+            b2 = np.where(m2 > 0.0, m4/(m2**2), 0.0)
+        g2 = b2 - 3.0
+
+        if bias:
+            # Biased estimator (g2 or b2)
+            return g2 if fisher else b2
+        else:
+            # Unbiased estimator with finite-sample correction
+            if n <= 3:
+                raise ValueError("Insufficient number of independent realizations"
+                                 f"for bias-corrected kurtosis: need n >= 4, got {n=}.")
+            else:
+                G2 = ((n - 1)/((n - 2)*(n - 3)))*((n + 1)*g2 + 6.0)
+                return G2 if fisher else G2 + 3.0
+
+    def skewtest(self, alternative: str = "two-sided"):
+        """Perform D'Agostino and Pearson's test for skewness.
+
+        This method tests the null hypothesis that the skewness of the
+        population that the sample was drawn from is the same as that of a
+        corresponding normal distribution.
+
+        Parameters
+        ----------
+        alternative : {'two-sided', 'less', 'greater'}, optional
+            Defines the alternative hypothesis. The following options are
+            available:
+
+            * 'two-sided': the skewness of the distribution is different from
+              that of the normal distribution (i.e., non-zero)
+            * 'less': the skewness of the distribution is less than that of the
+              normal distribution
+            * 'greater': the skewness of the distribution is greater than that
+              of the normal distribution
+
+        Returns
+        -------
+        statistic : np.ndarray
+            The computed z-score for the skewness test for each tally bin
+        pvalue : np.ndarray
+            The p-value for the hypothesis test for each tally bin
+
+        Notes
+        -----
+        This test is based on `D'Agostino and Pearson's test
+        <https://doi.org/10.1093/biomet/60.3.613>`_. The test requires at least
+        8 realizations to produce valid results.
+
+        """
+        n = self.num_realizations
+        if n < 8:
+            raise ValueError("Skewness test is not well-defined for n < 8.")
+
+        g1 = self.skew(bias=True)
+
+        # --- Z1 (skewness) ---
+        y = g1 * sqrt(((n + 1.0)*(n + 3.0))/(6.0*(n - 2.0)))
+        beta2 = (3.0*(n**2 + 27.0*n - 70.0)*(n + 1.0)*(n + 3.0)
+                )/((n - 2.0)*(n + 5.0)*(n + 7.0)*(n + 9.0))
+        W2 = -1.0 + sqrt(2.0*(beta2 - 1.0))
+        delta = 1.0 / sqrt(log(sqrt(W2)))
+        alpha = sqrt(2.0 / (W2 - 1.0))
+        Zb1 = np.where(
+            y >= 0.0,
+            delta*np.log((y/alpha) + np.sqrt((y/alpha)**2 + 1.0)),
+            -delta*np.log((-y/alpha) + np.sqrt((y/alpha)**2 + 1.0))
+        )
+
+        # p-value
+        if alternative == "two-sided":
+            p = 2.0 * (1.0 - norm.cdf(np.abs(Zb1)))
+        elif alternative == "greater":
+            p = 1.0 - norm.cdf(Zb1)
+        elif alternative == "less":
+            p = norm.cdf(Zb1)
+        else:
+            raise ValueError("alternative must be 'two-sided', 'greater', or 'less'")
+
+        return Zb1, p
+
+    def kurtosistest(self, alternative: str = "two-sided"):
+        """Perform D'Agostino and Pearson's test for kurtosis.
+
+        This method tests the null hypothesis that the kurtosis of the
+        population that the sample was drawn from is the same as that of a
+        corresponding normal distribution.
+
+        Parameters
+        ----------
+        alternative : {'two-sided', 'less', 'greater'}, optional
+            Defines the alternative hypothesis. Default is 'two-sided'. The
+            following options are available:
+
+            * 'two-sided': the kurtosis of the distribution is different from
+              that of the normal distribution
+            * 'less': the kurtosis of the distribution is less than that of the
+              normal distribution
+            * 'greater': the kurtosis of the distribution is greater than that
+              of the normal distribution
+
+        Returns
+        -------
+        statistic : np.ndarray
+            The computed z-score for the kurtosis test for each tally bin
+        pvalue : np.ndarray
+            The p-value for the hypothesis test for each tally bin
+
+        Raises
+        ------
+        ValueError
+            If the number of realizations is less than 20, or if an invalid
+            alternative hypothesis is specified.
+
+        Notes
+        -----
+        This test is based on `D'Agostino and Pearson's test
+        <https://doi.org/10.1093/biomet/60.3.613>`_. The test is typically
+        recommended for at least 20 realizations to produce valid results.
+
+        """
+        n = self.num_realizations
+        if n < 20:
+            raise ValueError("Kurtosis test is typically recommended for n >= 20.")
+
+        b2 = self.kurtosis(bias=True, fisher=False)
+
+        # --- Z2 (kurtosis) ---
+        mean_b2 = 3.0 * (n - 1.0) / (n + 1.0)
+        var_b2 = (24.0*n*(n - 2.0)*(n - 3.0)/(
+                    (n + 1.0)**2*(n + 3.0)*(n + 5.0)))
+        x = (b2 - mean_b2)/np.sqrt(var_b2)
+        moment = ((6.0*(n**2 - 5.0*n + 2.0))/((n + 7.0)*(n + 9.0))
+                    )*sqrt((6.0*(n + 3.0)*(n + 5.0))/(n*(n - 2.0)*(n - 3.0)))
+        A = 6.0 + (8.0/moment)*((2.0/moment) + sqrt(1.0 + 4.0/(moment**2)))
+        Zb2 = (1.0- 2.0/(9.0*A) - ((1.0 - 2.0/A) / (1.0 + (x
+                )*sqrt(2.0/(A - 4.0))))**(1.0/3.0)) / sqrt(2.0/(9.0*A))
+
+        # p-value
+        if alternative == "two-sided":
+            p = 2.0 * (1.0 - norm.cdf(np.abs(Zb2)))
+        elif alternative == "greater":
+            p = 1.0 - norm.cdf(Zb2)
+        elif alternative == "less":
+            p = norm.cdf(Zb2)
+        else:
+            raise ValueError("alternative must be 'two-sided', 'greater', or 'less'")
+
+        return Zb2, p
+
+    def normaltest(self, alternative: str = "two-sided"):
+        """Perform D'Agostino and Pearson's omnibus test for normality.
+
+        This method tests the null hypothesis that a sample comes from a normal
+        distribution. It combines skewness and kurtosis to produce an omnibus
+        test of normality.
+
+        Parameters
+        ----------
+        alternative : {'two-sided', 'less', 'greater'}, optional
+            Defines the alternative hypothesis used for the component skewness
+            and kurtosis tests. Default is 'two-sided'. The following options
+            are available:
+
+            * 'two-sided': the distribution is different from normal
+            * 'less': used for the component tests
+            * 'greater': used for the component tests
+
+        Returns
+        -------
+        statistic : np.ndarray
+            The computed z-score for the normality test for each tally bin
+        pvalue : np.ndarray
+            The p-value for the hypothesis test for each tally bin
+
+        Raises
+        ------
+        ValueError
+            If the number of realizations is less than 20, or if an invalid
+            alternative hypothesis is specified.
+
+        Notes
+        -----
+        This test combines a test for skewness and a test for kurtosis to
+        produce an `omnibus test <https://doi.org/10.1093/biomet/60.3.613>`_.
+        The test statistic is:
+
+        .. math::
+
+            K^2 = Z_1^2 + Z_2^2
+
+        where :math:`Z_1` is the z-score from the skewness test and :math:`Z_2`
+        is the z-score from the kurtosis test. This statistic follows a
+        chi-square distribution with 2 degrees of freedom.
+
+        The test requires at least 20 realizations to produce valid results.
+
+        """
+        n = self.num_realizations
+        if n < 20:
+            raise ValueError("normaltest requires n >= 20 (per D'Agostino-Pearson).")
+
+        # Use the component tests
+        Z1, _ = self.skewtest(alternative)
+        Z2, _ = self.kurtosistest(alternative)
+
+        # Combine as chi-square with df=2 since we have skewness and kurtosis
+        K2 = Z1*Z1 + Z2*Z2
+        p = chi2.sf(K2, df=2)
+        return K2, p
+
+    @property
+    def figure_of_merit(self):
+        mean = self.mean
+        std_dev = self.std_dev
+        fom = np.zeros_like(mean)
+        nonzero = np.abs(mean) > 0
+        rel_err = std_dev[nonzero] / mean[nonzero]
+        fom[nonzero] = 1.0 / (rel_err**2 * self._simulation_time)
+        return fom
+
     @property
     def with_batch_statistics(self):
         return self._with_batch_statistics
@@ -506,16 +1005,17 @@ class Tally(IDManagerMixin):
         # Convert NumPy arrays to SciPy sparse LIL matrices
         if sparse and not self.sparse:
             if self._sum is not None:
-                self._sum = sps.lil_matrix(self._sum.flatten(), self._sum.shape)
+                self._sum = lil_array(self._sum.flatten(), self._sum.shape)
             if self._sum_sq is not None:
-                self._sum_sq = sps.lil_matrix(self._sum_sq.flatten(),
-                                              self._sum_sq.shape)
+                self._sum_sq = lil_array(self._sum_sq.flatten(), self._sum_sq.shape)
+            if self._sum_third is not None:
+                self._sum_third = lil_array(self._sum_third.flatten(), self._sum_third.shape)
+            if self._sum_fourth is not None:
+                self._sum_fourth = lil_array(self._sum_fourth.flatten(), self._sum_fourth.shape)
             if self._mean is not None:
-                self._mean = sps.lil_matrix(self._mean.flatten(),
-                                            self._mean.shape)
+                self._mean = lil_array(self._mean.flatten(), self._mean.shape)
             if self._std_dev is not None:
-                self._std_dev = sps.lil_matrix(self._std_dev.flatten(),
-                                               self._std_dev.shape)
+                self._std_dev = lil_array(self._std_dev.flatten(), self._std_dev.shape)
 
             self._sparse = True
 
@@ -525,6 +1025,10 @@ class Tally(IDManagerMixin):
                 self._sum = np.reshape(self._sum.toarray(), self.shape)
             if self._sum_sq is not None:
                 self._sum_sq = np.reshape(self._sum_sq.toarray(), self.shape)
+            if self._sum_third is not None:
+                self._sum_third = np.reshape(self._sum_third.toarray(), self.shape)
+            if self._sum_fourth is not None:
+                self._sum_fourth = np.reshape(self._sum_fourth.toarray(), self.shape)
             if self._mean is not None:
                 self._mean = np.reshape(self._mean.toarray(), self.shape)
             if self._std_dev is not None:
@@ -602,8 +1106,8 @@ class Tally(IDManagerMixin):
             return False
 
         # Return False if only one tally has a delayed group filter
-        tally1_dg = self.contains_filter(openmc.DelayedGroupFilter)
-        tally2_dg = other.contains_filter(openmc.DelayedGroupFilter)
+        tally1_dg = self.contains_filter(DelayedGroupFilter)
+        tally2_dg = other.contains_filter(DelayedGroupFilter)
         if tally1_dg != tally2_dg:
             return False
 
@@ -851,6 +1355,34 @@ class Tally(IDManagerMixin):
 
             merged_tally._sum_sq = np.reshape(merged_sum_sq, merged_tally.shape)
 
+        # Concatenate sum_third arrays if present in both tallies
+        if self._sum_third is not None and other._sum_third is not None:
+            self_sum_third = self.get_reshaped_data(value="sum_third")
+            other_sum_third = other_copy.get_reshaped_data(value="sum_third")
+
+            if join_right:
+                merged_sum_third = np.concatenate((self_sum_third, other_sum_third),
+                                                  axis=merge_axis)
+            else:
+                merged_sum_third = np.concatenate((other_sum_third, self_sum_third),
+                                                  axis=merge_axis)
+
+            merged_tally._sum_third = np.reshape(merged_sum_third, merged_tally.shape)
+
+        # Concatenate sum_fourth arrays if present in both tallies
+        if self._sum_fourth is not None and other._sum_fourth is not None:
+            self_sum_fourth = self.get_reshaped_data(value="sum_fourth")
+            other_sum_fourth = other_copy.get_reshaped_data(value="sum_fourth")
+
+            if join_right:
+                merged_sum_fourth = np.concatenate((self_sum_fourth, other_sum_fourth),
+                                                   axis=merge_axis)
+            else:
+                merged_sum_fourth = np.concatenate((other_sum_fourth, self_sum_fourth),
+                                                   axis=merge_axis)
+
+            merged_tally._sum_fourth = np.reshape(merged_sum_fourth, merged_tally.shape)
+
         # Concatenate mean arrays if present in both tallies
         if self.mean is not None and other.mean is not None:
             self_mean = self.get_reshaped_data(value='mean')
@@ -940,6 +1472,11 @@ class Tally(IDManagerMixin):
             subelement = ET.SubElement(element, "derivative")
             subelement.text = str(self.derivative.id)
 
+        # Optional higher moments accumulation
+        if self.higher_moments:
+            subelement = ET.SubElement(element, "higher_moments")
+            subelement.text = str(self.higher_moments).lower()
+
         return element
 
     def add_results(self, statepoint: cv.PathLike | openmc.StatePoint):
@@ -966,8 +1503,12 @@ class Tally(IDManagerMixin):
         # point are based on the current statepoint file
         self._sum = None
         self._sum_sq = None
+        self._sum_third = None
+        self._sum_fourth = None
         self._mean = None
         self._std_dev = None
+        self._vov = None
+        self._higher_moments = False
         self._num_realizations = 0
         self._results_read = False
 
@@ -988,8 +1529,8 @@ class Tally(IDManagerMixin):
             Tally object
 
         """
-        tally_id = int(elem.get('id'))
-        name = elem.get('name', '')
+        tally_id = int(get_text(elem, "id"))
+        name = get_text(elem, "name", "")
         tally = cls(tally_id=tally_id, name=name)
 
         text = get_text(elem, 'multiply_density')
@@ -997,25 +1538,24 @@ class Tally(IDManagerMixin):
             tally.multiply_density = text in ('true', '1')
 
         # Read filters
-        filters_elem = elem.find('filters')
-        if filters_elem is not None:
-            filter_ids = [int(x) for x in filters_elem.text.split()]
+        filter_ids = get_elem_list(elem, "filters", int)
+        if filter_ids is not None:
             tally.filters = [kwargs['filters'][uid] for uid in filter_ids]
 
         # Read nuclides
-        nuclides_elem = elem.find('nuclides')
-        if nuclides_elem is not None:
-            tally.nuclides = nuclides_elem.text.split()
+        nuclides = get_elem_list(elem, "nuclides", str)
+        if nuclides is not None:
+            tally.nuclides = nuclides
 
         # Read scores
-        scores_elem = elem.find('scores')
-        if scores_elem is not None:
-            tally.scores = scores_elem.text.split()
+        scores = get_elem_list(elem, "scores", str)
+        if scores is not None:
+            tally.scores = scores
 
         # Set estimator
-        estimator_elem = elem.find('estimator')
-        if estimator_elem is not None:
-            tally.estimator = estimator_elem.text
+        estimator = get_text(elem, "estimator")
+        if estimator is not None:
+            tally.estimator = estimator
 
         # Read triggers
         tally.triggers = [
@@ -1024,9 +1564,9 @@ class Tally(IDManagerMixin):
         ]
 
         # Read tally derivative
-        deriv_elem = elem.find('derivative')
-        if deriv_elem is not None:
-            deriv_id = int(deriv_elem.text)
+        deriv = get_text(elem, "derivative")
+        if deriv is not None:
+            deriv_id = int(deriv)
             tally.derivative = kwargs['derivatives'][deriv_id]
 
         return tally
@@ -1079,7 +1619,7 @@ class Tally(IDManagerMixin):
 
             # Also check to see if the desired filter is wrapped up in an
             # aggregate
-            elif isinstance(test_filter, openmc.AggregateFilter):
+            elif isinstance(test_filter, AggregateFilter):
                 if isinstance(test_filter.aggregate_filter, filter_type):
                     return test_filter
 
@@ -1181,7 +1721,7 @@ class Tally(IDManagerMixin):
 
         """
 
-        cv.check_type('filters', filters, Iterable, openmc.FilterMeta)
+        cv.check_type('filters', filters, Iterable, FilterMeta)
         cv.check_type('filter_bins', filter_bins, Iterable, tuple)
 
         # If user did not specify any specific Filters, use them all
@@ -1264,7 +1804,7 @@ class Tally(IDManagerMixin):
         """
 
         for score in scores:
-            if not isinstance(score, (str, openmc.CrossScore)):
+            if not isinstance(score, (str, CrossScore)):
                 msg = f'Unable to get score indices for score "{score}" in ' \
                       f'ID="{self.id}" since it is not a string or CrossScore ' \
                       'Tally'
@@ -1338,7 +1878,9 @@ class Tally(IDManagerMixin):
            (value == 'std_dev' and self.std_dev is None) or \
            (value == 'rel_err' and self.mean is None) or \
            (value == 'sum' and self.sum is None) or \
-           (value == 'sum_sq' and self.sum_sq is None):
+           (value == 'sum_sq' and self.sum_sq is None) or \
+           (value == "sum_third" and self.sum_third is None) or \
+           (value == "sum_fourth" and self.sum_fourth is None):
             msg = f'The Tally ID="{self.id}" has no data to return'
             raise ValueError(msg)
 
@@ -1361,10 +1903,14 @@ class Tally(IDManagerMixin):
             data = self.sum[indices]
         elif value == 'sum_sq':
             data = self.sum_sq[indices]
+        elif value == "sum_third":
+            data = self.sum_third[indices]
+        elif value == "sum_fourth":
+            data = self.sum_fourth[indices]
         else:
             msg = f'Unable to return results from Tally ID="{value}" since ' \
                   f'the requested value "{self.id}" is not \'mean\', ' \
-                  '\'std_dev\', \'rel_err\', \'sum\', or \'sum_sq\''
+                  '\'std_dev\', \'rel_err\', \'sum\', \'sum_sq\', \'sum_third\' or \'sum_fourth\''
             raise LookupError(msg)
 
         return data
@@ -1455,9 +2001,9 @@ class Tally(IDManagerMixin):
             column_name = 'score'
 
             for score in self.scores:
-                if isinstance(score, (str, openmc.CrossScore)):
+                if isinstance(score, (str, CrossScore)):
                     scores.append(str(score))
-                elif isinstance(score, openmc.AggregateScore):
+                elif isinstance(score, AggregateScore):
                     scores.append(score.name)
                     column_name = f'{score.aggregate_op}(score)'
 
@@ -1480,7 +2026,7 @@ class Tally(IDManagerMixin):
 
         # Expand the columns into Pandas MultiIndices for readability
         if pd.__version__ >= '0.16':
-            columns = copy.deepcopy(df.columns.values)
+            columns = copy.deepcopy(list(df.columns.values))
 
             # Convert all elements in columns list to tuples
             for i, column in enumerate(columns):
@@ -1557,7 +2103,7 @@ class Tally(IDManagerMixin):
         for i, f in enumerate(self.filters):
             if expand_dims:
                 # Mesh filter indices are backwards so we need to flip them
-                if isinstance(f, openmc.MeshFilter):
+                if type(f) in {MeshFilter, MeshBornFilter}:
                     fshape = f.shape[::-1]
                     new_shape += fshape
                     idx0, idx1 = i, i + len(fshape) - 1
@@ -1744,7 +2290,7 @@ class Tally(IDManagerMixin):
         else:
             all_filters = [self_copy.filters, other_copy.filters]
             for self_filter, other_filter in product(*all_filters):
-                new_filter = openmc.CrossFilter(self_filter, other_filter,
+                new_filter = CrossFilter(self_filter, other_filter,
                                                 binary_op)
                 new_tally.filters.append(new_filter)
 
@@ -1755,7 +2301,7 @@ class Tally(IDManagerMixin):
         else:
             all_nuclides = [self_copy.nuclides, other_copy.nuclides]
             for self_nuclide, other_nuclide in product(*all_nuclides):
-                new_nuclide = openmc.CrossNuclide(self_nuclide, other_nuclide,
+                new_nuclide = CrossNuclide(self_nuclide, other_nuclide,
                                                   binary_op)
                 new_tally.nuclides.append(new_nuclide)
 
@@ -1766,9 +2312,9 @@ class Tally(IDManagerMixin):
                 if score1 == score2:
                     return score1
                 else:
-                    return openmc.CrossScore(score1, score2, binary_op)
+                    return CrossScore(score1, score2, binary_op)
             else:
-                return openmc.CrossScore(score1, score2, binary_op)
+                return CrossScore(score1, score2, binary_op)
 
         # Add scores to the new tally
         if score_product == 'entrywise':
@@ -1977,16 +2523,16 @@ class Tally(IDManagerMixin):
 
         # Construct lists of tuples for the bins in each of the two filters
         filters = [type(filter1), type(filter2)]
-        if isinstance(filter1, openmc.DistribcellFilter):
+        if isinstance(filter1, DistribcellFilter):
             filter1_bins = [b for b in range(filter1.num_bins)]
-        elif isinstance(filter1, openmc.EnergyFunctionFilter):
+        elif isinstance(filter1, EnergyFunctionFilter):
             filter1_bins = [None]
         else:
             filter1_bins = filter1.bins
 
-        if isinstance(filter2, openmc.DistribcellFilter):
+        if isinstance(filter2, DistribcellFilter):
             filter2_bins = [b for b in range(filter2.num_bins)]
-        elif isinstance(filter2, openmc.EnergyFunctionFilter):
+        elif isinstance(filter2, EnergyFunctionFilter):
             filter2_bins = [None]
         else:
             filter2_bins = filter2.bins
@@ -2119,11 +2665,11 @@ class Tally(IDManagerMixin):
             raise ValueError(msg)
 
         # Check that the scores are valid
-        if not isinstance(score1, (str, openmc.CrossScore)):
+        if not isinstance(score1, (str, CrossScore)):
             msg = 'Unable to swap score1 "{}" in Tally ID="{}" since it is ' \
                   'not a string or CrossScore'.format(score1, self.id)
             raise ValueError(msg)
-        elif not isinstance(score2, (str, openmc.CrossScore)):
+        elif not isinstance(score2, (str, CrossScore)):
             msg = 'Unable to swap score2 "{}" in Tally ID="{}" since it is ' \
                   'not a string or CrossScore'.format(score2, self.id)
             raise ValueError(msg)
@@ -2694,6 +3240,16 @@ class Tally(IDManagerMixin):
             new_sum_sq = self.get_values(scores, filters, filter_bins,
                                          nuclides, 'sum_sq')
             new_tally.sum_sq = new_sum_sq
+        if not self.derived and self._sum_third is not None:
+            new_sum_third = self.get_values(
+                scores, filters, filter_bins, nuclides, "sum_third"
+            )
+            new_tally._sum_third = new_sum_third
+        if not self.derived and self._sum_fourth is not None:
+            new_sum_fourth = self.get_values(
+                scores, filters, filter_bins, nuclides, "sum_fourth"
+            )
+            new_tally._sum_fourth = new_sum_fourth
         if self.mean is not None:
             new_mean = self.get_values(scores, filters, filter_bins,
                                        nuclides, 'mean')
@@ -2757,7 +3313,7 @@ class Tally(IDManagerMixin):
                 new_filter.bins = [f.bins[i] for i in bin_indices]
 
                 # Set number of bins manually for mesh/distribcell filters
-                if filter_type is openmc.DistribcellFilter:
+                if filter_type is DistribcellFilter:
                     new_filter._num_bins = f._num_bins
 
                 # Replace existing filter with new one
@@ -2823,16 +3379,16 @@ class Tally(IDManagerMixin):
         std_dev = self.get_reshaped_data(value='std_dev')
 
         # Sum across any filter bins specified by the user
-        if isinstance(filter_type, openmc.FilterMeta):
+        if isinstance(filter_type, FilterMeta):
             find_filter = self.find_filter(filter_type)
 
             # If user did not specify filter bins, sum across all bins
             if len(filter_bins) == 0:
                 bin_indices = np.arange(find_filter.num_bins)
 
-                if isinstance(find_filter, openmc.DistribcellFilter):
+                if isinstance(find_filter, DistribcellFilter):
                     filter_bins = np.arange(find_filter.num_bins)
-                elif isinstance(find_filter, openmc.EnergyFunctionFilter):
+                elif isinstance(find_filter, EnergyFunctionFilter):
                     filter_bins = [None]
                 else:
                     filter_bins = find_filter.bins
@@ -2861,7 +3417,7 @@ class Tally(IDManagerMixin):
 
                     # Add AggregateFilter to the tally sum
                     if not remove_filter:
-                        filter_sum = openmc.AggregateFilter(self_filter,
+                        filter_sum = AggregateFilter(self_filter,
                             [tuple(filter_bins)], 'sum')
                         tally_sum.filters.append(filter_sum)
 
@@ -2884,7 +3440,7 @@ class Tally(IDManagerMixin):
             std_dev = np.sqrt(std_dev)
 
             # Add AggregateNuclide to the tally sum
-            nuclide_sum = openmc.AggregateNuclide(nuclides, 'sum')
+            nuclide_sum = AggregateNuclide(nuclides, 'sum')
             tally_sum.nuclides.append(nuclide_sum)
 
         # Add a copy of this tally's nuclides to the tally sum
@@ -2902,7 +3458,7 @@ class Tally(IDManagerMixin):
             std_dev = np.sqrt(std_dev)
 
             # Add AggregateScore to the tally sum
-            score_sum = openmc.AggregateScore(scores, 'sum')
+            score_sum = AggregateScore(scores, 'sum')
             tally_sum.scores.append(score_sum)
 
         # Add a copy of this tally's scores to the tally sum
@@ -2975,16 +3531,16 @@ class Tally(IDManagerMixin):
         std_dev = self.get_reshaped_data(value='std_dev')
 
         # Average across any filter bins specified by the user
-        if isinstance(filter_type, openmc.FilterMeta):
+        if isinstance(filter_type, FilterMeta):
             find_filter = self.find_filter(filter_type)
 
             # If user did not specify filter bins, average across all bins
             if len(filter_bins) == 0:
                 bin_indices = np.arange(find_filter.num_bins)
 
-                if isinstance(find_filter, openmc.DistribcellFilter):
+                if isinstance(find_filter, DistribcellFilter):
                     filter_bins = np.arange(find_filter.num_bins)
-                elif isinstance(find_filter, openmc.EnergyFunctionFilter):
+                elif isinstance(find_filter, EnergyFunctionFilter):
                     filter_bins = [None]
                 else:
                     filter_bins = find_filter.bins
@@ -3014,7 +3570,7 @@ class Tally(IDManagerMixin):
 
                     # Add AggregateFilter to the tally avg
                     if not remove_filter:
-                        filter_sum = openmc.AggregateFilter(self_filter,
+                        filter_sum = AggregateFilter(self_filter,
                             [tuple(filter_bins)], 'avg')
                         tally_avg.filters.append(filter_sum)
 
@@ -3038,7 +3594,7 @@ class Tally(IDManagerMixin):
             std_dev = np.sqrt(std_dev)
 
             # Add AggregateNuclide to the tally avg
-            nuclide_avg = openmc.AggregateNuclide(nuclides, 'avg')
+            nuclide_avg = AggregateNuclide(nuclides, 'avg')
             tally_avg.nuclides.append(nuclide_avg)
 
         # Add a copy of this tally's nuclides to the tally avg
@@ -3057,7 +3613,7 @@ class Tally(IDManagerMixin):
             std_dev = np.sqrt(std_dev)
 
             # Add AggregateScore to the tally avg
-            score_sum = openmc.AggregateScore(scores, 'avg')
+            score_sum = AggregateScore(scores, 'avg')
             tally_avg.scores.append(score_sum)
 
         # Add a copy of this tally's scores to the tally avg
@@ -3134,6 +3690,12 @@ class Tally(IDManagerMixin):
         if not self.derived and self.sum_sq is not None:
             new_tally._sum_sq = np.zeros(new_tally.shape, dtype=np.float64)
             new_tally._sum_sq[diag_indices, :, :] = self.sum_sq
+        if not self.derived and self._sum_third is not None:
+            new_tally._sum_third = np.zeros(new_tally.shape, dtype=np.float64)
+            new_tally._sum_third[diag_indices, :, :] = self.sum_third
+        if not self.derived and self._sum_fourth is not None:
+            new_tally._sum_fourth = np.zeros(new_tally.shape, dtype=np.float64)
+            new_tally._sum_fourth[diag_indices, :, :] = self.sum_fourth
         if self.mean is not None:
             new_tally._mean = np.zeros(new_tally.shape, dtype=np.float64)
             new_tally._mean[diag_indices, :, :] = self.mean
@@ -3184,43 +3746,17 @@ class Tallies(cv.CheckedList):
             if possible. Defaults to False.
 
         """
-        if not isinstance(tally, Tally):
-            msg = f'Unable to add a non-Tally "{tally}" to the Tallies instance'
-            raise TypeError(msg)
-
         if merge:
-            merged = False
-
             # Look for a tally to merge with this one
             for i, tally2 in enumerate(self):
-
                 # If a mergeable tally is found
                 if tally2.can_merge(tally):
                     # Replace tally2 with the merged tally
                     merged_tally = tally2.merge(tally)
                     self[i] = merged_tally
-                    merged = True
-                    break
+                    return
 
-            # If no mergeable tally was found, simply add this tally
-            if not merged:
-                super().append(tally)
-
-        else:
-            super().append(tally)
-
-    def insert(self, index, item):
-        """Insert tally before index
-
-        Parameters
-        ----------
-        index : int
-            Index in list
-        item : openmc.Tally
-            Tally to insert
-
-        """
-        super().insert(index, item)
+        super().append(tally)
 
     def merge_tallies(self):
         """Merge any mergeable tallies together. Note that n-way merges are
@@ -3267,7 +3803,7 @@ class Tallies(cv.CheckedList):
         already_written = memo if memo else set()
         for tally in self:
             for f in tally.filters:
-                if isinstance(f, openmc.MeshFilter):
+                if isinstance(f, MeshFilter):
                     if f.mesh.id in already_written:
                         continue
                     if len(f.mesh.name) > 0:
@@ -3311,7 +3847,6 @@ class Tallies(cv.CheckedList):
 
         # Clean the indentation in the file to be user-readable
         clean_indentation(element)
-        reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
 
         return element
 
@@ -3363,7 +3898,7 @@ class Tallies(cv.CheckedList):
         # Read filter elements
         filters = {}
         for e in elem.findall('filter'):
-            filter = openmc.Filter.from_xml_element(e, meshes=meshes)
+            filter = Filter.from_xml_element(e, meshes=meshes)
             filters[filter.id] = filter
 
         # Read derivative elements
diff --git a/openmc/tally_derivative.py b/openmc/tally_derivative.py
index ff918c1b3..f7ba5dce5 100644
--- a/openmc/tally_derivative.py
+++ b/openmc/tally_derivative.py
@@ -4,6 +4,7 @@ import lxml.etree as ET
 
 import openmc.checkvalue as cv
 from .mixin import EqualityMixin, IDManagerMixin
+from ._xml import get_text
 
 
 class TallyDerivative(EqualityMixin, IDManagerMixin):
@@ -122,8 +123,8 @@ class TallyDerivative(EqualityMixin, IDManagerMixin):
             Tally derivative object
 
         """
-        derivative_id = int(elem.get("id"))
-        variable = elem.get("variable")
-        material = int(elem.get("material"))
-        nuclide = elem.get("nuclide") if variable == "nuclide_density" else None
+        derivative_id = int(get_text(elem, "id"))
+        variable = get_text(elem, "variable")
+        material = int(get_text(elem, "material"))
+        nuclide = get_text(elem, "nuclide") if variable == "nuclide_density" else None
         return cls(derivative_id, variable, material, nuclide)
diff --git a/openmc/tracks.py b/openmc/tracks.py
index 61e5a7244..44e3c0eba 100644
--- a/openmc/tracks.py
+++ b/openmc/tracks.py
@@ -4,7 +4,8 @@ from collections.abc import Sequence
 import h5py
 
 from .checkvalue import check_filetype_version
-from .source import SourceParticle, ParticleType
+from .particle_type import ParticleType
+from .source import SourceParticle
 
 from pathlib import Path
 
@@ -25,7 +26,7 @@ states : numpy.ndarray
 
 """
 def _particle_track_repr(self):
-    return f"<ParticleTrack: {self.particle}, {len(self.states)} states>"
+    return f"<ParticleTrack: {str(self.particle)}, {len(self.states)} states>"
 ParticleTrack.__repr__ = _particle_track_repr
 
 
@@ -92,8 +93,8 @@ class Track(Sequence):
 
         Parameters
         ----------
-        particle : {'neutron', 'photon', 'electron', 'positron'}
-            Matching particle type
+        particle : str or int or openmc.ParticleType
+            Matching particle type (name, PDG number, or type)
         state_filter : function
             Function that takes a state (structured datatype) and returns a bool
             depending on some criteria.
@@ -126,7 +127,7 @@ class Track(Sequence):
         for t in self:
             # Check for matching particle
             if particle is not None:
-                if t.particle.name.lower() != particle:
+                if t.particle != ParticleType(particle):
                     continue
 
             # Apply arbitrary state filter
@@ -184,7 +185,7 @@ class Track(Sequence):
     def sources(self):
         sources = []
         for particle_track in self:
-            particle_type = ParticleType(particle_track.particle)
+            particle_type = particle_track.particle
             state = particle_track.states[0]
             sources.append(
                 SourceParticle(
@@ -296,16 +297,16 @@ class Tracks(list):
                 for state in pt.states:
                     points.InsertNextPoint(state['r'])
 
-            # Create VTK line and assign points to line.
-            n = pt.states.size
-            line = vtk.vtkPolyLine()
-            line.GetPointIds().SetNumberOfIds(n)
-            for i in range(n):
-                line.GetPointIds().SetId(i, point_offset + i)
-            point_offset += n
+                # Create VTK line and assign points to line.
+                n = pt.states.size
+                line = vtk.vtkPolyLine()
+                line.GetPointIds().SetNumberOfIds(n)
+                for i in range(n):
+                    line.GetPointIds().SetId(i, point_offset + i)
+                point_offset += n
 
-            # Add line to cell array
-            cells.InsertNextCell(line)
+                # Add line to cell array
+                cells.InsertNextCell(line)
 
         data = vtk.vtkPolyData()
         data.SetPoints(points)
diff --git a/openmc/trigger.py b/openmc/trigger.py
index be2537e3a..70b6b7a03 100644
--- a/openmc/trigger.py
+++ b/openmc/trigger.py
@@ -5,6 +5,7 @@ import lxml.etree as ET
 
 import openmc.checkvalue as cv
 from .mixin import EqualityMixin
+from ._xml import get_elem_list, get_text
 
 
 class Trigger(EqualityMixin):
@@ -129,16 +130,16 @@ class Trigger(EqualityMixin):
 
         """
         # Generate trigger object
-        trigger_type = elem.get("type")
-        threshold = float(elem.get("threshold"))
-        ignore_zeros = str(elem.get("ignore_zeros", "false")).lower()
+        trigger_type = get_text(elem, "type")
+        threshold = float(get_text(elem, "threshold"))
+        ignore_zeros = str(get_text(elem, "ignore_zeros", "false")).lower()
         # Try to convert to bool. Let Trigger error out on instantiation.
         ignore_zeros = ignore_zeros in ('true', '1')
         trigger = cls(trigger_type, threshold, ignore_zeros)
 
         # Add scores if present
-        scores = elem.get("scores")
+        scores = get_elem_list(elem, "scores", str)
         if scores is not None:
-            trigger.scores = scores.split()
+            trigger.scores = scores
 
         return trigger
diff --git a/openmc/universe.py b/openmc/universe.py
index 02b794dee..0e64693ba 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -337,14 +337,6 @@ class UniverseBase(ABC, IDManagerMixin):
         """
         model = openmc.Model()
         model.geometry = openmc.Geometry(self)
-
-        # Determine whether any materials contains macroscopic data and if
-        # so, set energy mode accordingly
-        for mat in self.get_all_materials().values():
-            if mat._macroscopic is not None:
-                model.settings.energy_mode = 'multi-group'
-                break
-
         return model.plot(*args, **kwargs)
 
     def get_nuclides(self):
diff --git a/openmc/utility_funcs.py b/openmc/utility_funcs.py
index da9f73b16..935a58985 100644
--- a/openmc/utility_funcs.py
+++ b/openmc/utility_funcs.py
@@ -3,6 +3,8 @@ import os
 from pathlib import Path
 from tempfile import TemporaryDirectory
 
+import h5py
+
 import openmc
 from .checkvalue import PathLike
 
@@ -57,3 +59,20 @@ def input_path(filename: PathLike) -> Path:
         return Path(filename).resolve()
     else:
         return Path(filename)
+
+
+@contextmanager
+def h5py_file_or_group(group_or_filename: PathLike | h5py.Group, *args, **kwargs):
+    """Context manager for opening an HDF5 file or using an existing group
+
+    Parameters
+    ----------
+    group_or_filename : path-like or h5py.Group
+        Path to HDF5 file, or group from an existing HDF5 file
+
+    """
+    if isinstance(group_or_filename, h5py.Group):
+        yield group_or_filename
+    else:
+        with h5py.File(group_or_filename, *args, **kwargs) as f:
+            yield f
diff --git a/openmc/volume.py b/openmc/volume.py
index df19def1e..c44adf98a 100644
--- a/openmc/volume.py
+++ b/openmc/volume.py
@@ -10,7 +10,7 @@ from uncertainties import ufloat
 
 import openmc
 import openmc.checkvalue as cv
-from openmc._xml import get_text
+from openmc._xml import get_elem_list, get_text
 
 _VERSION_VOLUME = 1
 
@@ -375,13 +375,10 @@ class VolumeCalculation:
 
         """
         domain_type = get_text(elem, "domain_type")
-        domain_ids = get_text(elem, "domain_ids").split()
-        ids = [int(x) for x in domain_ids]
+        ids = get_elem_list(elem, "domain_ids", int)
         samples = int(get_text(elem, "samples"))
-        lower_left = get_text(elem, "lower_left").split()
-        lower_left = tuple([float(x) for x in lower_left])
-        upper_right = get_text(elem, "upper_right").split()
-        upper_right = tuple([float(x) for x in upper_right])
+        lower_left = tuple(get_elem_list(elem, "lower_left", float))
+        upper_right = tuple(get_elem_list(elem, "upper_right", float))
 
         # Instantiate some throw-away domains that are used by the constructor
         # to assign IDs
diff --git a/openmc/waste.py b/openmc/waste.py
new file mode 100644
index 000000000..80cfc0adc
--- /dev/null
+++ b/openmc/waste.py
@@ -0,0 +1,286 @@
+from __future__ import annotations
+
+import openmc
+from openmc.data import half_life
+
+
+def _waste_classification(mat: openmc.Material, metal: bool = True) -> str:
+    """Classify a material for near-surface waste disposal.
+
+    This method determines a waste classification for a material based on the
+    NRC regulations (10 CFR 61.55).
+
+    Parameters
+    ----------
+    mat : openmc.Material
+        The material to classify.
+    metal : bool, optional
+        Whether or not the material is in metal form. This changes the
+        acceptable limits in Tables 1 and 2 for certain nuclides.
+
+    Returns
+    -------
+    str
+        The waste disposal classification, which can be "Class A", "Class B",
+        "Class C", or "GTCC" (greater than class C).
+
+    """
+    # Determine metrics based on Tables 1 and 2 using sum of fractions rule for
+    # mixture of radionuclides from §61.55(a)(7)
+    ratio1 = _waste_disposal_rating(mat, 'NRC_long', metal=metal)
+    ratio2 = [
+        _waste_disposal_rating(mat, 'NRC_short_A', metal=metal),
+        _waste_disposal_rating(mat, 'NRC_short_B', metal=metal),
+        _waste_disposal_rating(mat, 'NRC_short_C', metal=metal),
+    ]
+
+    # Determine which nuclides are present in Table 1 and Table 2
+    table1_nuclides_present = (ratio1 > 0.0)
+    table2_nuclides_present = any(x > 0.0 for x in ratio2)
+
+    # Helper function for classifying based on Table 2
+    def classify_table2(col1, col2, col3):
+        if col1 < 1.0:
+            return "Class A"
+        elif col2 < 1.0:
+            return "Class B"
+        elif col3 < 1.0:
+            return "Class C"
+        else:
+            return "GTCC"
+
+    if table1_nuclides_present and table2_nuclides_present:
+        # Classification based on §61.55(a)(5)
+        if ratio1 < 0.1:
+            return classify_table2(*ratio2)
+        elif ratio1 < 1.0:
+            return "Class C" if ratio2[2] < 1.0 else "GTCC"
+        else:
+            return "GTCC"
+
+    elif table1_nuclides_present:
+        # Classification based on §61.55(a)(3)
+        if ratio1 < 0.1:
+            return "Class A"
+        elif ratio1 < 1.0:
+            return "Class C"
+        else:
+            return "GTCC"
+
+    elif table2_nuclides_present:
+        # Classification based on §61.55(a)(4)
+        return classify_table2(*ratio2)
+
+    else:
+        # Classification based on §61.55(a)(6)
+        return "Class A"
+
+
+def _waste_disposal_rating(
+    mat: openmc.Material,
+    limits: str | dict[str, float] = 'Fetter',
+    metal: bool = False,
+    by_nuclide: bool = False,
+) -> float | dict[str, float]:
+    """Return the waste disposal rating for a material.
+
+    This method returns a waste disposal rating for the material based on a set
+    of specific activity limits. The waste disposal rating is a single number
+    that represents the sum of the ratios of the specific activity for each
+    radionuclide in the material against a nuclide-specific limit. A value less
+    than 1.0 indicates that the material "meets" the limits whereas a value
+    greater than 1.0 exceeds the limits.
+
+    Parameters
+    ----------
+    mat : openmc.Material
+        The material to classify.
+    limits : str or dict, optional
+        The name of a predefined set of specific activity limits or a dictionary
+        that contains specific activity limits for radionuclides, where keys are
+        nuclide names and values are activities in units of [Ci/m3]. The
+        predefined options are:
+
+        - 'Fetter': Uses limits from Fetter et al. (1990)
+        - 'NRC_long': Uses the 10 CFR 61.55 limits for long-lived radionuclides
+        - 'NRC_short_A': Uses the 10 CFR 61.55 class A limits for short-lived
+          radionuclides
+        - 'NRC_short_B': Uses the 10 CFR 61.55 class B limits for short-lived
+          radionuclides
+        - 'NRC_short_C': Uses the 10 CFR 61.55 class C limits for short-lived
+          radionuclides
+    metal : bool, optional
+        Whether or not the material is in metal form (only applicable for NRC
+        based limits)
+    by_nuclide : bool, optional
+        Whether to return the waste disposal rating for each nuclide in the
+        material. If True, a dictionary is returned where the keys are the
+        nuclide names and the values are the waste disposal ratings for each
+        nuclide. If False, a single float value is returned that represents the
+        overall waste disposal rating for the material.
+
+    Returns
+    -------
+    float or dict
+        The waste disposal rating for the material or its constituent nuclides.
+
+    """
+    if limits == 'Fetter':
+        # Specific activity limits for radionuclides with half-lives between 5
+        # years and 1e12 years from Table 2 in Fetter
+        limits = {
+            "Be10": 5.0e3,
+            "C14": 6.0e2,
+            "Al26": 9.0e-2,
+            "Si32": 6.0e2,
+            "Cl36": 1.0e1,
+            "Ar39": 2.0e4,
+            "Ar42": 2.0e4,
+            "K40": 2.0e0,
+            "Ca41": 1.0e4,
+            "Ti44": 2.0e2,
+            "Fe60": 1.0e-1,
+            "Co60": 3.0e8,
+            "Ni59": 9.0e2,
+            "Ni63": 7.0e5,
+            "Se79": 5.0e1,
+            "Kr81": 3.0e1,
+            "Sr90": 8.0e5,
+            "Nb91": 2.0e2,
+            "Nb92": 2.0e-1,
+            "Nb94": 2.0e-1,
+            "Mo93": 4.0e3,
+            "Tc97": 4.0e-1,
+            "Tc98": 1.0e-2,
+            "Tc99": 6.0e-2,
+            "Pd107": 9.0e2,
+            "Ag108_m1": 3.0e0,
+            "Sn121_m1": 7.0e5,
+            "Sn126": 1.0e-1,
+            "I129": 2.0e0,
+            "Cs137": 5.0e4,
+            "Ba133": 2.0e8,
+            "La137": 2.0e2,
+            "Sm151": 5.0e7,
+            "Eu150_m1": 3.0e3,
+            "Eu152": 3.0e5,
+            "Eu154": 5.0e6,
+            "Gd148": 2.0e5,
+            "Gd150": 2.0e3,
+            "Tb157": 5.0e3,
+            "Tb158": 4.0e0,
+            "Dy154": 1.0e3,
+            "Ho166_m1": 2.0e-1,
+            "Hf178_m1": 9.0e3,
+            "Hf182": 2.0e-1,
+            "Re186_m1": 2.0e1,
+            "Ir192_m1": 1.0e0,
+            "Pt193": 2.0e8,
+            "Hg194": 5.0e-1,
+            "Pb202": 6.0e-1,
+            "Pb210": 3.0e7,
+            "Bi207": 9.0e3,
+            "Bi208": 8.0e-2,
+            "Bi210_m1": 1.0e0,
+            "Po209": 3.0e3,
+            "Ra226": 1.0e-1,
+            "Ra228": 3.0e7,
+            "Ac227": 5.0e5,
+            "Th229": 2.0e0,
+            "Th230": 3.0e-1,
+            "Th232": 1.0e-1,
+            "Pa231": 7.0e-1,
+            "U232": 3.0e1,
+            "U233": 2.0e1,
+            "U234": 9.0e1,
+            "U235": 2.0e0,
+            "Np236": 1.0e0,
+            "Np237": 1.0e0,
+            "Pu238": 7.0e4,
+            "Pu239": 1.0e3,
+            "Pu240": 1.0e3,
+            "Pu241": 2.0e3,
+            "Pu242": 1.0e3,
+            "Pu244": 9.0e-1,
+            "Am241": 5.0e1,
+            "Am242_m1": 3.0e2,
+            "Am243": 2.0e0,
+            "Cm243": 6.0e2,
+            "Cm244": 5.0e5,
+            "Cm245": 5.0e0,
+            "Cm246": 8.0e2,
+            "Cm248": 8.0e2,
+        }
+
+    elif limits == 'NRC_long':
+        # Specific activity limits for long-lived radionuclides from Table 1 in
+        # 10 CFR 61.55 in Ci/m3.
+        limits = {
+            'C14': 8.0,
+            'Tc99': 3.0,
+            'I129': 0.08,
+        }
+        if metal:
+            limits['C14'] = 80.0
+            limits['Ni59'] = 220.0
+            limits['Nb94'] = 0.2
+
+        # Convert values in nCi/g to Ci/m3
+        factor = (1e6 * mat.get_mass_density()) / 1e9
+        limits.update({
+            'Pu241': 3500.0 * factor,
+            'Cm242': 20000.0 * factor,
+            'Np237': 100.0 * factor,
+            'Pu238': 100.0 * factor,
+            'Pu239': 100.0 * factor,
+            'Pu240': 100.0 * factor,
+            'Pu242': 100.0 * factor,
+            'Pu244': 100.0 * factor,
+            'Am241': 100.0 * factor,
+            'Am243': 100.0 * factor,
+            'Cm243': 100.0 * factor,
+            'Cm244': 100.0 * factor,
+            'Cm245': 100.0 * factor,
+            'Cm246': 100.0 * factor,
+            'Cm247': 100.0 * factor,
+            'Cm248': 100.0 * factor,
+            'Bk247': 100.0 * factor,
+            'Cf249': 100.0 * factor,
+            'Cf250': 100.0 * factor,
+            'Cf251': 100.0 * factor,
+        })
+
+    elif limits == 'NRC_short_A':
+        # Get Class A specific activity limits for short-lived radionuclides
+        # from Table 2 in 10 CFR 61.55
+        limits = {
+            'H3': 40.0,
+            'Co60': 700.0,
+            'Ni63': 35.0 if metal else 3.5,
+            'Sr90': 0.04,
+            'Cs137': 1.0
+        }
+
+        # Add radionuclides with half-lives < 5 years to limits for class A
+        five_years = 60.0 * 60.0 * 24.0 * 365.25 * 5.0
+        for nuc in mat.get_nuclides():
+            if half_life(nuc) is not None and half_life(nuc) < five_years:
+                limits[nuc] = 700.0
+
+    elif limits == 'NRC_short_B':
+        # Get Class B specific activity limits for short-lived radionuclides
+        # from Table 2 in 10 CFR 61.55
+        limits = {'Ni63': 700.0 if metal else 70.0, 'Sr90': 150.0, 'Cs137': 44.0}
+
+    elif limits == 'NRC_short_C':
+        # Get Class C specific activity limits for short-lived radionuclides
+        # from Table 2 in 10 CFR 61.55
+        limits = {'Ni63': 7000.0 if metal else 700.0, 'Sr90': 7000.0, 'Cs137': 4600.0}
+
+    # Calculate the sum of the fractions of the activity of each radionuclide
+    # compared to the specified limits
+    ratio = {}
+    for nuc, ci_m3 in mat.get_activity(units="Ci/m3", by_nuclide=True).items():
+        if nuc in limits:
+            ratio[nuc] = ci_m3 / limits[nuc]
+    return ratio if by_nuclide else sum(ratio.values())
diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index d52b81925..63af2596e 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -1,6 +1,8 @@
 from __future__ import annotations
 from numbers import Real, Integral
 from collections.abc import Iterable, Sequence
+from pathlib import Path
+from typing import Self
 import warnings
 
 import lxml.etree as ET
@@ -8,12 +10,13 @@ import numpy as np
 import h5py
 
 import openmc
-from openmc.filter import _PARTICLES
 from openmc.mesh import MeshBase, RectilinearMesh, CylindricalMesh, SphericalMesh, UnstructuredMesh
+from openmc.tallies import Tallies
 import openmc.checkvalue as cv
 from openmc.checkvalue import PathLike
-from ._xml import get_text, clean_indentation
+from ._xml import get_elem_list, get_text, clean_indentation
 from .mixin import IDManagerMixin
+from .particle_type import ParticleType
 
 
 class WeightWindows(IDManagerMixin):
@@ -48,7 +51,7 @@ class WeightWindows(IDManagerMixin):
         A list of values for which each successive pair constitutes a range of
         energies in [eV] for a single bin. If no energy bins are provided, the
         maximum and minimum energy for the data available at runtime.
-    particle_type : {'neutron', 'photon'}
+    particle_type : str or int or openmc.ParticleType
         Particle type the weight windows apply to
     survival_ratio : float
         Ratio of the survival weight to the lower weight window bound for
@@ -90,7 +93,7 @@ class WeightWindows(IDManagerMixin):
     survival_ratio : float
         Ratio of the survival weight to the lower weight window bound for
         rouletting
-    max_lower_bound_ratio: float
+    max_lower_bound_ratio : float
         Maximum allowed ratio of a particle's weight to the weight window's
         lower bound. (Default: 1.0)
     max_split : int
@@ -113,8 +116,8 @@ class WeightWindows(IDManagerMixin):
         upper_ww_bounds: Iterable[float] | None = None,
         upper_bound_ratio: float | None = None,
         energy_bounds: Iterable[Real] | None = None,
-        particle_type: str = 'neutron',
-        survival_ratio: float = 3,
+        particle_type: str | int | openmc.ParticleType = 'neutron',
+        survival_ratio: float = 3.0,
         max_lower_bound_ratio: float | None = None,
         max_split: int = 10,
         weight_cutoff: float = 1.e-38,
@@ -210,13 +213,15 @@ class WeightWindows(IDManagerMixin):
         self._mesh = mesh
 
     @property
-    def particle_type(self) -> str:
+    def particle_type(self) -> ParticleType:
         return self._particle_type
 
     @particle_type.setter
-    def particle_type(self, pt: str):
-        cv.check_value('Particle type', pt, _PARTICLES)
-        self._particle_type = pt
+    def particle_type(self, pt):
+        ptype = ParticleType(pt)
+        if ptype not in {ParticleType.NEUTRON, ParticleType.PHOTON}:
+            raise ValueError("Weight windows can only be applied for neutrons or photons")
+        self._particle_type = ptype
 
     @property
     def energy_bounds(self) -> Iterable[Real]:
@@ -326,7 +331,7 @@ class WeightWindows(IDManagerMixin):
         subelement.text = str(self.mesh.id)
 
         subelement = ET.SubElement(element, 'particle_type')
-        subelement.text = self.particle_type
+        subelement.text = str(self.particle_type)
 
         if self.energy_bounds is not None:
             subelement = ET.SubElement(element, 'energy_bounds')
@@ -354,7 +359,7 @@ class WeightWindows(IDManagerMixin):
         return element
 
     @classmethod
-    def from_xml_element(cls, elem: ET.Element, meshes: dict[int, MeshBase]) -> WeightWindows:
+    def from_xml_element(cls, elem: ET.Element, meshes: dict[int, MeshBase]) -> Self:
         """Generate weight window settings from an XML element
 
         Parameters
@@ -376,9 +381,9 @@ class WeightWindows(IDManagerMixin):
         mesh = meshes[mesh_id]
 
         # Read all other parameters
-        lower_ww_bounds = [float(l) for l in get_text(elem, 'lower_ww_bounds').split()]
-        upper_ww_bounds = [float(u) for u in get_text(elem, 'upper_ww_bounds').split()]
-        e_bounds = [float(b) for b in get_text(elem, 'energy_bounds').split()]
+        lower_ww_bounds = get_elem_list(elem, "lower_ww_bounds", float)
+        upper_ww_bounds = get_elem_list(elem, "upper_ww_bounds", float)
+        e_bounds = get_elem_list(elem, "energy_bounds", float)
         particle_type = get_text(elem, 'particle_type')
         survival_ratio = float(get_text(elem, 'survival_ratio'))
 
@@ -408,7 +413,7 @@ class WeightWindows(IDManagerMixin):
         )
 
     @classmethod
-    def from_hdf5(cls, group: h5py.Group, meshes: dict[int, MeshBase]) -> WeightWindows:
+    def from_hdf5(cls, group: h5py.Group, meshes: dict[int, MeshBase]) -> Self:
         """Create weight windows from HDF5 group
 
         Parameters
@@ -458,7 +463,7 @@ class WeightWindows(IDManagerMixin):
         )
 
 
-def wwinp_to_wws(path: PathLike) -> list[WeightWindows]:
+def wwinp_to_wws(path: PathLike) -> WeightWindowsList:
     """Create WeightWindows instances from a wwinp file
 
     .. versionadded:: 0.13.1
@@ -470,190 +475,13 @@ def wwinp_to_wws(path: PathLike) -> list[WeightWindows]:
 
     Returns
     -------
-    list of openmc.WeightWindows
+    WeightWindowsList
     """
-
-    with open(path) as wwinp:
-        # BLOCK 1
-        header = wwinp.readline().split(None, 4)
-        # read file type, time-dependence, number of
-        # particles, mesh type and problem identifier
-        _if, iv, ni, nr = [int(x) for x in header[:4]]
-
-        # header value checks
-        if _if != 1:
-            raise ValueError(f'Found incorrect file type, if: {_if}')
-
-        if iv > 1:
-            # read number of time bins for each particle, 'nt(1...ni)'
-            nt = np.fromstring(wwinp.readline(), sep=' ', dtype=int)
-
-            # raise error if time bins are present for now
-            raise ValueError('Time-dependent weight windows '
-                             'are not yet supported')
-        else:
-            nt = ni * [1]
-
-        # read number of energy bins for each particle, 'ne(1...ni)'
-        ne = np.fromstring(wwinp.readline(), sep=' ', dtype=int)
-
-        # read coarse mesh dimensions and lower left corner
-        mesh_description = np.fromstring(wwinp.readline(), sep=' ')
-        nfx, nfy, nfz = mesh_description[:3].astype(int)
-        xyz0 = mesh_description[3:]
-
-        # read cylindrical and spherical mesh vectors if present
-        if nr == 16:
-            # read number of coarse bins
-            line_arr = np.fromstring(wwinp.readline(), sep=' ')
-            ncx, ncy, ncz = line_arr[:3].astype(int)
-            # read polar vector (x1, y1, z1)
-            xyz1 = line_arr[3:]
-            # read azimuthal vector (x2, y2, z2)
-            line_arr = np.fromstring(wwinp.readline(), sep=' ')
-            xyz2 = line_arr[:3]
-
-            # Get polar and azimuthal axes
-            polar_axis = xyz1 - xyz0
-            azimuthal_axis = xyz2 - xyz0
-
-            # Check for polar axis other than (0, 0, 1)
-            norm = np.linalg.norm(polar_axis)
-            if not np.isclose(polar_axis[2]/norm, 1.0):
-                raise NotImplementedError('Polar axis not aligned to z-axis not supported')
-
-            # Check for azimuthal axis other than (1, 0, 0)
-            norm = np.linalg.norm(azimuthal_axis)
-            if not np.isclose(azimuthal_axis[0]/norm, 1.0):
-                raise NotImplementedError('Azimuthal axis not aligned to x-axis not supported')
-
-            # read geometry type
-            nwg = int(line_arr[-1])
-
-        elif nr == 10:
-            # read rectilinear data:
-            # number of coarse mesh bins and mesh type
-            ncx, ncy, ncz, nwg = \
-                np.fromstring(wwinp.readline(), sep=' ').astype(int)
-        else:
-            raise RuntimeError(f'Invalid mesh description (nr) found: {nr}')
-
-        # read BLOCK 2 and BLOCK 3 data into a single array
-        ww_data = np.fromstring(wwinp.read(), sep=' ')
-
-    # extract mesh data from the ww_data array
-    start_idx = 0
-
-    # first values in the mesh definition arrays are the first
-    # coordinate of the grid
-    end_idx = start_idx + 1 + 3 * ncx
-    i0, i_vals = ww_data[start_idx], ww_data[start_idx+1:end_idx]
-    start_idx = end_idx
-
-    end_idx = start_idx + 1 + 3 * ncy
-    j0, j_vals = ww_data[start_idx], ww_data[start_idx+1:end_idx]
-    start_idx = end_idx
-
-    end_idx = start_idx + 1 + 3 * ncz
-    k0, k_vals = ww_data[start_idx], ww_data[start_idx+1:end_idx]
-    start_idx = end_idx
-
-    # mesh consistency checks
-    if nr == 16 and nwg == 1 or nr == 10 and nwg != 1:
-        raise ValueError(f'Mesh description in header ({nr}) '
-                         f'does not match the mesh type ({nwg})')
-
-    if nr == 10 and (xyz0 != (i0, j0, k0)).any():
-        raise ValueError(f'Mesh origin in the header ({xyz0}) '
-                         f' does not match the origin in the mesh '
-                         f' description ({i0, j0, k0})')
-
-    # create openmc mesh object
-    grids = []
-    mesh_definition = [(i0, i_vals, nfx), (j0, j_vals, nfy), (k0, k_vals, nfz)]
-    for grid0, grid_vals, n_pnts in mesh_definition:
-        # file spec checks for the mesh definition
-        if (grid_vals[2::3] != 1.0).any():
-            raise ValueError('One or more mesh ratio value, qx, '
-                             'is not equal to one')
-
-        s = int(grid_vals[::3].sum())
-        if s != n_pnts:
-            raise ValueError(f'Sum of the fine bin entries, {s}, does '
-                             f'not match the number of fine bins, {n_pnts}')
-
-        # extend the grid based on the next coarse bin endpoint, px
-        # and the number of fine bins in the coarse bin, sx
-        intervals = grid_vals.reshape(-1, 3)
-        coords = [grid0]
-        for sx, px, qx in intervals:
-            coords += np.linspace(coords[-1], px, int(sx + 1)).tolist()[1:]
-
-        grids.append(np.array(coords))
-
-    if nwg == 1:
-        mesh = RectilinearMesh()
-        mesh.x_grid, mesh.y_grid, mesh.z_grid = grids
-    elif nwg == 2:
-        mesh = CylindricalMesh(
-            r_grid=grids[0],
-            z_grid=grids[1],
-            phi_grid=grids[2],
-            origin = xyz0,
-        )
-    elif nwg == 3:
-        mesh = SphericalMesh(
-            r_grid=grids[0],
-            theta_grid=grids[1],
-            phi_grid=grids[2],
-            origin = xyz0
-        )
-
-    # extract weight window values from array
-    wws = []
-    for ne_i, nt_i, particle_type in zip(ne, nt, ('neutron', 'photon')):
-        # no information to read for this particle if
-        # either the energy bins or time bins are empty
-        if ne_i == 0 or nt_i == 0:
-            continue
-
-        if iv > 1:
-            # time bins are parsed but unused for now
-            end_idx = start_idx + nt_i
-            time_bounds = ww_data[start_idx:end_idx]
-            np.insert(time_bounds, (0,), (0.0,))
-            start_idx = end_idx
-
-        # read energy boundaries
-        end_idx = start_idx + ne_i
-        energy_bounds = np.insert(ww_data[start_idx:end_idx], (0,), (0.0,))
-        # convert from MeV to eV
-        energy_bounds *= 1e6
-        start_idx = end_idx
-
-        # read weight window values
-        end_idx = start_idx + (nfx * nfy * nfz) * nt_i * ne_i
-
-        # read values and reshape according to ordering
-        # slowest to fastest: t, e, z, y, x
-        # reorder with transpose since our ordering is x, y, z, e, t
-        ww_shape = (nt_i, ne_i, nfz, nfy, nfx)
-        ww_values = ww_data[start_idx:end_idx].reshape(ww_shape).T
-        # Only use first time bin since we don't support time dependent weight
-        # windows yet.
-        ww_values = ww_values[:, :, :, :, 0]
-        start_idx = end_idx
-
-        # create a weight window object
-        ww = WeightWindows(id=None,
-                           mesh=mesh,
-                           lower_ww_bounds=ww_values,
-                           upper_bound_ratio=5.0,
-                           energy_bounds=energy_bounds,
-                           particle_type=particle_type)
-        wws.append(ww)
-
-    return wws
+    warnings.warn(
+        "This function is deprecated in favor of 'WeightWindowsList.from_wwinp'",
+        FutureWarning
+    )
+    return WeightWindowsList.from_wwinp(path)
 
 
 class WeightWindowGenerator:
@@ -668,10 +496,12 @@ class WeightWindowGenerator:
         A list of values for which each successive pair constitutes a range of
         energies in [eV] for a single bin. If no energy bins are provided, the
         maximum and minimum energy for the data available at runtime.
-    particle_type : {'neutron', 'photon'}
+    particle_type : str or int or openmc.ParticleType
         Particle type the weight windows apply to
     method : {'magic', 'fw_cadis'}
         The weight window generation methodology applied during an update.
+    targets : :class:`openmc.Tallies` or iterable of int
+        Target tallies for local variance reduction via FW-CADIS.
     max_realizations : int
         The upper limit for number of tally realizations when generating weight
         windows.
@@ -687,10 +517,12 @@ class WeightWindowGenerator:
     energy_bounds : Iterable of Real
         A list of values for which each successive pair constitutes a range of
         energies in [eV] for a single bin
-    particle_type : {'neutron', 'photon'}
+    particle_type : openmc.ParticleType
         Particle type the weight windows apply to
     method : {'magic', 'fw_cadis'}
         The weight window generation methodology applied during an update.
+    targets : :class:`openmc.Tallies` or numpy.ndarray
+        Target tallies for local variance reduction via FW-CADIS.
     max_realizations : int
         The upper limit for number of tally realizations when generating weight
         windows.
@@ -702,14 +534,15 @@ class WeightWindowGenerator:
         Whether or not to apply weight windows on the fly.
     """
 
-    _MAGIC_PARAMS = {'value': str, 'threshold': float, 'ratio': float}
+    _WWG_PARAMS = {'value': str, 'threshold': float, 'ratio': float}
 
     def __init__(
         self,
         mesh: openmc.MeshBase,
         energy_bounds: Sequence[float] | None = None,
-        particle_type: str = 'neutron',
+        particle_type: str | int | openmc.ParticleType = 'neutron',
         method: str = 'magic',
+        targets: openmc.Tallies | Iterable[int] | None = None,
         max_realizations: int = 1,
         update_interval: int = 1,
         on_the_fly: bool = True
@@ -722,6 +555,7 @@ class WeightWindowGenerator:
             self.energy_bounds = energy_bounds
         self.particle_type = particle_type
         self.method = method
+        self.targets = targets
         self.max_realizations = max_realizations
         self.update_interval = update_interval
         self.on_the_fly = on_the_fly
@@ -729,7 +563,7 @@ class WeightWindowGenerator:
     def __repr__(self):
         string = type(self).__name__ + '\n'
         string += f'\t{"Mesh":<20}=\t{self.mesh.id}\n'
-        string += f'\t{"Particle:":<20}=\t{self.particle_type}\n'
+        string += f'\t{"Particle:":<20}=\t{str(self.particle_type)}\n'
         string += f'\t{"Energy Bounds:":<20}=\t{self.energy_bounds}\n'
         string += f'\t{"Method":<20}=\t{self.method}\n'
         string += f'\t{"Max Realizations:":<20}=\t{self.max_realizations}\n'
@@ -760,13 +594,15 @@ class WeightWindowGenerator:
         self._energy_bounds = eb
 
     @property
-    def particle_type(self) -> str:
+    def particle_type(self) -> ParticleType:
         return self._particle_type
 
     @particle_type.setter
-    def particle_type(self, pt: str):
-        cv.check_value('particle type', pt, ('neutron', 'photon'))
-        self._particle_type = pt
+    def particle_type(self, pt):
+        ptype = ParticleType(pt)
+        if ptype not in {ParticleType.NEUTRON, ParticleType.PHOTON}:
+            raise ValueError("Weight windows can only be applied for neutrons or photons")
+        self._particle_type = ptype
 
     @property
     def method(self) -> str:
@@ -782,6 +618,22 @@ class WeightWindowGenerator:
                 self._check_update_parameters()
             except (TypeError, KeyError):
                 warnings.warn(f'Update parameters are invalid for the "{m}" method.')
+    
+    @property
+    def targets(self) -> openmc.Tallies:
+        return self._targets
+
+    @targets.setter
+    def targets(self, t):
+        if t is None:
+            self._targets = t
+        else:
+            cv.check_type('Local FW-CADIS target tallies', t, Iterable)
+            cv.check_greater_than('Local FW-CADIS target tallies', len(t), 0)
+            if not isinstance(t, openmc.Tallies):
+                cv.check_iterable_type('Local FW-CADIS target tallies', t, int)
+                t = np.asarray(list(t), dtype=int)
+            self._targets = t
 
     @property
     def max_realizations(self) -> int:
@@ -809,13 +661,13 @@ class WeightWindowGenerator:
 
     def _check_update_parameters(self, params: dict):
         if self.method == 'magic' or self.method == 'fw_cadis':
-            check_params = self._MAGIC_PARAMS
+            check_params = self._WWG_PARAMS
 
         for key, val in params.items():
             if key not in check_params:
                 raise ValueError(f'Invalid param "{key}" for {self.method} '
                                   'weight window generation')
-            cv.check_type(f'weight window generation param: "{key}"', val, self._MAGIC_PARAMS[key])
+            cv.check_type(f'weight window generation param: "{key}"', val, self._WWG_PARAMS[key])
 
     @update_parameters.setter
     def update_parameters(self, params: dict):
@@ -852,7 +704,7 @@ class WeightWindowGenerator:
             The update parameters as-read from the XML node (keys: str, values: str)
         """
         if method == 'magic' or method == 'fw_cadis':
-            check_params = cls._MAGIC_PARAMS
+            check_params = cls._WWG_PARAMS
 
         for param, param_type in check_params.items():
             if param in update_parameters:
@@ -869,7 +721,7 @@ class WeightWindowGenerator:
             subelement = ET.SubElement(element, 'energy_bounds')
             subelement.text = ' '.join(str(e) for e in self.energy_bounds)
         particle_elem = ET.SubElement(element, 'particle_type')
-        particle_elem.text = self.particle_type
+        particle_elem.text = str(self.particle_type)
         realizations_elem = ET.SubElement(element, 'max_realizations')
         realizations_elem.text = str(self.max_realizations)
         update_interval_elem = ET.SubElement(element, 'update_interval')
@@ -878,6 +730,20 @@ class WeightWindowGenerator:
         otf_elem.text = str(self.on_the_fly).lower()
         method_elem = ET.SubElement(element, 'method')
         method_elem.text = self.method
+        if self.targets is not None:
+            if self.method != 'fw_cadis':
+                raise ValueError(
+                    "FW-CADIS update method is required in order to use " \
+                    "target tallies for WeightWindowGenerator.")
+            elif isinstance(self.targets, openmc.Tallies):
+                raise RuntimeError(
+                    "FW-CADIS target tallies must be checked to ensure they are " \
+                    "present on model.tallies. Use model.export_to_xml() or " \
+                    "model.export_to_model_xml() to link FW-CADIS target tallies.")
+            else:
+                targets_elem = ET.SubElement(element, 'targets')
+                targets_elem.text = ' '.join(str(tally_id) for tally_id in self.targets)
+
         if self.update_parameters is not None:
             self._update_parameters_subelement(element)
 
@@ -886,7 +752,7 @@ class WeightWindowGenerator:
         return element
 
     @classmethod
-    def from_xml_element(cls, elem: ET.Element, meshes: dict) -> WeightWindowGenerator:
+    def from_xml_element(cls, elem: ET.Element, meshes: dict) -> Self:
         """
         Create a weight window generation object from an XML element
 
@@ -904,11 +770,8 @@ class WeightWindowGenerator:
 
         mesh_id = int(get_text(elem, 'mesh'))
         mesh = meshes[mesh_id]
-
-        if (energy_bounds := get_text(elem, 'energy_bounds')) is not None:
-            energy_bounds = [float(x) for x in energy_bounds.split()]
-        else:
-            energy_bounds = None
+        
+        energy_bounds = get_elem_list(elem, "energy_bounds", float)
         particle_type = get_text(elem, 'particle_type')
 
         wwg = cls(mesh, energy_bounds, particle_type)
@@ -917,6 +780,14 @@ class WeightWindowGenerator:
         wwg.update_interval = int(get_text(elem, 'update_interval'))
         wwg.on_the_fly = bool(get_text(elem, 'on_the_fly'))
         wwg.method = get_text(elem, 'method')
+        targets_elem = elem.find('targets')
+        if targets_elem is not None:
+            if wwg.method != 'fw_cadis':
+                raise ValueError(
+                    "FW-CADIS update method is required in order to use " \
+                    "target tallies for WeightWindowGenerator.")
+            else:
+                wwg.targets = get_elem_list(elem, "targets")
 
         if elem.find('update_parameters') is not None:
             update_parameters = {}
@@ -929,8 +800,8 @@ class WeightWindowGenerator:
 
         return wwg
 
-def hdf5_to_wws(path='weight_windows.h5'):
-    """Create WeightWindows instances from a weight windows HDF5 file
+def hdf5_to_wws(path='weight_windows.h5') -> WeightWindowsList:
+    """Create a WeightWindowsList from a weight windows HDF5 file
 
     .. versionadded:: 0.14.0
 
@@ -941,13 +812,280 @@ def hdf5_to_wws(path='weight_windows.h5'):
 
     Returns
     -------
-    list of openmc.WeightWindows
+    WeightWindowsList
     """
+    warnings.warn(
+        "This function is deprecated in favor of 'WeightWindowsList.from_hdf5'",
+        FutureWarning
+    )
+    return WeightWindowsList.from_hdf5(path)
 
-    with h5py.File(path) as h5_file:
-        # read in all of the meshes in the mesh node
-        meshes = {}
-        for mesh_group in h5_file['meshes']:
-            mesh = MeshBase.from_hdf5(h5_file['meshes'][mesh_group])
-            meshes[mesh.id] = mesh
-        return [WeightWindows.from_hdf5(ww, meshes) for ww in h5_file['weight_windows'].values()]
+
+class WeightWindowsList(list):
+    """A list of WeightWindows objects.
+
+    .. versionadded:: 0.15.3
+
+    Parameters
+    ----------
+    iterable : iterable of openmc.WeightWindows
+        An iterable of WeightWindows objects to initialize the list with
+
+    """
+    def __init__(self, iterable: Iterable[WeightWindows] = ()):
+        super().__init__(iterable)
+
+    @classmethod
+    def from_hdf5(cls, path: PathLike = 'weight_windows.h5') -> Self:
+        """Create WeightWindowsList from a weight windows HDF5 file.
+
+        Parameters
+        ----------
+        path : PathLike
+            Path to the weight windows hdf5 file
+
+        Returns
+        -------
+        WeightWindowsList
+            A list of WeightWindows objects read from the file
+        """
+
+        with h5py.File(path) as h5_file:
+            # read in all of the meshes in the mesh node
+            meshes = {}
+            for mesh_group in h5_file['meshes']:
+                mesh = MeshBase.from_hdf5(h5_file['meshes'][mesh_group])
+                meshes[mesh.id] = mesh
+            wws = [
+                WeightWindows.from_hdf5(ww, meshes)
+                for ww in h5_file['weight_windows'].values()
+            ]
+
+        return cls(wws)
+
+    @classmethod
+    def from_wwinp(cls, path: PathLike) -> Self:
+        """Create WeightWindowsList from a wwinp file.
+
+        Parameters
+        ----------
+        path : PathLike
+            Path to the wwinp file
+
+        Returns
+        -------
+        WeightWindowsList
+            A list of WeightWindows objects read from the file
+        """
+
+        with open(path) as wwinp:
+            # BLOCK 1
+            header = wwinp.readline().split(None, 4)
+            # read file type, time-dependence, number of
+            # particles, mesh type and problem identifier
+            _if, iv, ni, nr = [int(x) for x in header[:4]]
+
+            # header value checks
+            if _if != 1:
+                raise ValueError(f'Found incorrect file type, if: {_if}')
+
+            if iv > 1:
+                # read number of time bins for each particle, 'nt(1...ni)'
+                nt = np.fromstring(wwinp.readline(), sep=' ', dtype=int)
+
+                # raise error if time bins are present for now
+                raise ValueError('Time-dependent weight windows '
+                                'are not yet supported')
+            else:
+                nt = ni * [1]
+
+            # read number of energy bins for each particle, 'ne(1...ni)'
+            ne = np.fromstring(wwinp.readline(), sep=' ', dtype=int)
+
+            # read coarse mesh dimensions and lower left corner
+            mesh_description = np.fromstring(wwinp.readline(), sep=' ')
+            nfx, nfy, nfz = mesh_description[:3].astype(int)
+            xyz0 = mesh_description[3:]
+
+            # read cylindrical and spherical mesh vectors if present
+            if nr == 16:
+                # read number of coarse bins
+                line_arr = np.fromstring(wwinp.readline(), sep=' ')
+                ncx, ncy, ncz = line_arr[:3].astype(int)
+                # read polar vector (x1, y1, z1)
+                xyz1 = line_arr[3:]
+                # read azimuthal vector (x2, y2, z2)
+                line_arr = np.fromstring(wwinp.readline(), sep=' ')
+                xyz2 = line_arr[:3]
+
+                # Get polar and azimuthal axes
+                polar_axis = xyz1 - xyz0
+                azimuthal_axis = xyz2 - xyz0
+
+                # Check for polar axis other than (0, 0, 1)
+                norm = np.linalg.norm(polar_axis)
+                if not np.isclose(polar_axis[2]/norm, 1.0):
+                    raise NotImplementedError('Polar axis not aligned to z-axis not supported')
+
+                # Check for azimuthal axis other than (1, 0, 0)
+                norm = np.linalg.norm(azimuthal_axis)
+                if not np.isclose(azimuthal_axis[0]/norm, 1.0):
+                    raise NotImplementedError('Azimuthal axis not aligned to x-axis not supported')
+
+                # read geometry type
+                nwg = int(line_arr[-1])
+
+            elif nr == 10:
+                # read rectilinear data:
+                # number of coarse mesh bins and mesh type
+                ncx, ncy, ncz, nwg = \
+                    np.fromstring(wwinp.readline(), sep=' ').astype(int)
+            else:
+                raise RuntimeError(f'Invalid mesh description (nr) found: {nr}')
+
+            # read BLOCK 2 and BLOCK 3 data into a single array
+            ww_data = np.fromstring(wwinp.read(), sep=' ')
+
+        # extract mesh data from the ww_data array
+        start_idx = 0
+
+        # first values in the mesh definition arrays are the first
+        # coordinate of the grid
+        end_idx = start_idx + 1 + 3 * ncx
+        i0, i_vals = ww_data[start_idx], ww_data[start_idx+1:end_idx]
+        start_idx = end_idx
+
+        end_idx = start_idx + 1 + 3 * ncy
+        j0, j_vals = ww_data[start_idx], ww_data[start_idx+1:end_idx]
+        start_idx = end_idx
+
+        end_idx = start_idx + 1 + 3 * ncz
+        k0, k_vals = ww_data[start_idx], ww_data[start_idx+1:end_idx]
+        start_idx = end_idx
+
+        # mesh consistency checks
+        if nr == 16 and nwg == 1 or nr == 10 and nwg != 1:
+            raise ValueError(f'Mesh description in header ({nr}) '
+                            f'does not match the mesh type ({nwg})')
+
+        if nr == 10 and (xyz0 != (i0, j0, k0)).any():
+            raise ValueError(f'Mesh origin in the header ({xyz0}) '
+                            f' does not match the origin in the mesh '
+                            f' description ({i0, j0, k0})')
+
+        # create openmc mesh object
+        grids = []
+        mesh_definition = [(i0, i_vals, nfx), (j0, j_vals, nfy), (k0, k_vals, nfz)]
+        for grid0, grid_vals, n_pnts in mesh_definition:
+            # file spec checks for the mesh definition
+            if (grid_vals[2::3] != 1.0).any():
+                raise ValueError('One or more mesh ratio value, qx, '
+                                'is not equal to one')
+
+            s = int(grid_vals[::3].sum())
+            if s != n_pnts:
+                raise ValueError(f'Sum of the fine bin entries, {s}, does '
+                                f'not match the number of fine bins, {n_pnts}')
+
+            # extend the grid based on the next coarse bin endpoint, px
+            # and the number of fine bins in the coarse bin, sx
+            intervals = grid_vals.reshape(-1, 3)
+            coords = [grid0]
+            for sx, px, qx in intervals:
+                coords += np.linspace(coords[-1], px, int(sx + 1)).tolist()[1:]
+
+            grids.append(np.array(coords))
+
+        if nwg == 1:
+            mesh = RectilinearMesh()
+            mesh.x_grid, mesh.y_grid, mesh.z_grid = grids
+        elif nwg == 2:
+            mesh = CylindricalMesh(
+                r_grid=grids[0],
+                z_grid=grids[1],
+                phi_grid=grids[2],
+                origin = xyz0,
+            )
+        elif nwg == 3:
+            mesh = SphericalMesh(
+                r_grid=grids[0],
+                theta_grid=grids[1],
+                phi_grid=grids[2],
+                origin = xyz0
+            )
+
+        # extract weight window values from array
+        wws = cls()
+        for ne_i, nt_i, particle_type in zip(ne, nt, ('neutron', 'photon')):
+            # no information to read for this particle if
+            # either the energy bins or time bins are empty
+            if ne_i == 0 or nt_i == 0:
+                continue
+
+            if iv > 1:
+                # time bins are parsed but unused for now
+                end_idx = start_idx + nt_i
+                time_bounds = ww_data[start_idx:end_idx]
+                np.insert(time_bounds, (0,), (0.0,))
+                start_idx = end_idx
+
+            # read energy boundaries
+            end_idx = start_idx + ne_i
+            energy_bounds = np.insert(ww_data[start_idx:end_idx], (0,), (0.0,))
+            # convert from MeV to eV
+            energy_bounds *= 1e6
+            start_idx = end_idx
+
+            # read weight window values
+            end_idx = start_idx + (nfx * nfy * nfz) * nt_i * ne_i
+
+            # read values and reshape according to ordering
+            # slowest to fastest: t, e, z, y, x
+            # reorder with transpose since our ordering is x, y, z, e, t
+            ww_shape = (nt_i, ne_i, nfz, nfy, nfx)
+            ww_values = ww_data[start_idx:end_idx].reshape(ww_shape).T
+            # Only use first time bin since we don't support time dependent weight
+            # windows yet.
+            ww_values = ww_values[:, :, :, :, 0]
+            start_idx = end_idx
+
+            # create a weight window object
+            ww = WeightWindows(id=None,
+                            mesh=mesh,
+                            lower_ww_bounds=ww_values,
+                            upper_bound_ratio=5.0,
+                            energy_bounds=energy_bounds,
+                            particle_type=particle_type)
+            wws.append(ww)
+
+        return wws
+
+    def export_to_hdf5(self, path: PathLike = 'weight_windows.h5', **init_kwargs):
+        """Write weight windows to an HDF5 file.
+
+        Parameters
+        ----------
+        path : PathLike
+            Path to the file to write weight windows to
+        **init_kwargs
+            Keyword arguments passed to :func:`openmc.lib.init`
+
+        """
+        import openmc.lib
+        cv.check_type('path', path, PathLike)
+
+        # Create a temporary model with the weight windows
+        model = openmc.Model()
+        sph = openmc.Sphere(boundary_type='vacuum')
+        cell = openmc.Cell(region=-sph)
+        model.geometry = openmc.Geometry([cell])
+        model.settings.weight_windows = self
+        model.settings.particles = 100
+        model.settings.batches = 1
+
+        # Get absolute path before moving to temporary directory
+        path = Path(path).resolve()
+
+        # Load the model with openmc.lib and then export it to an HDF5 file
+        with openmc.lib.TemporarySession(model, **init_kwargs):
+            openmc.lib.export_weight_windows(path)
diff --git a/pyproject.toml b/pyproject.toml
index 6e8ed798e..c769fb7d7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -9,7 +9,7 @@ authors = [
 ]
 description = "OpenMC"
 dynamic = ["version"]
-requires-python = ">=3.11"
+requires-python = ">=3.12"
 license = {file = "LICENSE"}
 classifiers = [
     "Development Status :: 4 - Beta",
@@ -21,9 +21,9 @@ classifiers = [
     "Topic :: Scientific/Engineering",
     "Programming Language :: C++",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
+    "Programming Language :: Python :: 3.14",
 ]
 dependencies = [
     "numpy",
@@ -34,22 +34,29 @@ dependencies = [
     "pandas",
     "lxml",
     "uncertainties",
-    "setuptools",
     "endf",
 ]
 
 [project.optional-dependencies]
 depletion-mpi = ["mpi4py"]
 docs = [
-    "sphinx==5.0.2",
+    "breathe",
+    "sphinx",
     "sphinxcontrib-katex",
     "sphinx-numfig",
     "jupyter",
     "sphinxcontrib-svg2pdfconverter",
-    "sphinx-rtd-theme==1.0.0"
+    "sphinx-rtd-theme"
 ]
-test = ["packaging", "pytest", "pytest-cov", "colorama", "openpyxl"]
-ci = ["cpp-coveralls", "coveralls"]
+test = [
+    "packaging",
+    "pytest",
+    "pytest-cov>=4.0",
+    "pytest-rerunfailures",
+    "colorama",
+    "openpyxl",
+]
+ci = ["coverage>=7.4", "gcovr>=7.2"]
 vtk = ["vtk"]
 
 [project.urls]
@@ -63,7 +70,7 @@ include = ['openmc*']
 exclude = ['tests*']
 
 [tool.setuptools.package-data]
-"openmc.data.effective_dose" = ["**/*.txt"]
+"openmc.data.dose" = ["**/*.txt", "*.h5"]
 "openmc.data" = ["*.txt", "*.DAT", "*.json", "*.h5"]
 "openmc.lib" = ["libopenmc.dylib", "libopenmc.so"]
 
diff --git a/src/atomic_mass.cpp b/src/atomic_mass.cpp
new file mode 100644
index 000000000..0e63c9603
--- /dev/null
+++ b/src/atomic_mass.cpp
@@ -0,0 +1,3569 @@
+#include "openmc/atomic_mass.h"
+
+namespace openmc {
+
+// Atomic masses in [u] from AME2020 and CODATA 2018
+std::unordered_map<int32_t, double> ATOMIC_MASS = {
+  {11, MASS_ELECTRON},
+  {22, 0.0},
+  {2112, MASS_NEUTRON},
+  {2212, MASS_PROTON},
+  {1000010020, MASS_DEUTRON},
+  {1000020030, MASS_HELION},
+  {1000020040, MASS_ALPHA},
+  {1000010030, 3.01604928132},
+  {1000030030, 3.030775},
+  {1000010040, 4.026431867},
+  {1000030040, 4.027185561},
+  {1000010050, 5.035311492},
+  {1000020050, 5.012057224},
+  {1000030050, 5.0125378},
+  {1000040050, 5.03987},
+  {1000010060, 6.044955437},
+  {1000020060, 6.018885889},
+  {1000030060, 6.01512288742},
+  {1000040060, 6.0197264090000004},
+  {1000050060, 6.0508},
+  {1000010070, 7.052749},
+  {1000020070, 7.027990652},
+  {1000030070, 7.01600343426},
+  {1000040070, 7.016928714},
+  {1000050070, 7.029712},
+  {1000020080, 8.033934388},
+  {1000030080, 8.022486244},
+  {1000040080, 8.005305102},
+  {1000050080, 8.024607315},
+  {1000060080, 8.037643039},
+  {1000020090, 9.043946414},
+  {1000030090, 9.026790191},
+  {1000040090, 9.012183062},
+  {1000050090, 9.013329645},
+  {1000060090, 9.031037202},
+  {1000020100, 10.052815306},
+  {1000030100, 10.035483453},
+  {1000040100, 10.013534692},
+  {1000050100, 10.012936862},
+  {1000060100, 10.016853217},
+  {1000070100, 10.04165354},
+  {1000030110, 11.043723581},
+  {1000040110, 11.02166108},
+  {1000050110, 11.009305166},
+  {1000060110, 11.011432597},
+  {1000070110, 11.026157593},
+  {1000080110, 11.051249828},
+  {1000030120, 12.052613942},
+  {1000040120, 12.026922082},
+  {1000050120, 12.014352638},
+  {1000060120, 12.0},
+  {1000070120, 12.01861318},
+  {1000080120, 12.034367726},
+  {1000030130, 13.061171503},
+  {1000040130, 13.036134506},
+  {1000050130, 13.017779981},
+  {1000060130, 13.00335483534},
+  {1000070130, 13.005738609},
+  {1000080130, 13.024815435},
+  {1000090130, 13.045121},
+  {1000040140, 14.04289292},
+  {1000050140, 14.02540401},
+  {1000060140, 14.00324198862},
+  {1000070140, 14.00307400425},
+  {1000080140, 14.008596706},
+  {1000090140, 14.034315196},
+  {1000040150, 15.053490215},
+  {1000050150, 15.031087023},
+  {1000060150, 15.010599256},
+  {1000070150, 15.00010889827},
+  {1000080150, 15.003065636},
+  {1000090150, 15.017785139},
+  {1000100150, 15.043172977},
+  {1000040160, 16.061672036},
+  {1000050160, 16.039841045},
+  {1000060160, 16.014701255},
+  {1000070160, 16.006101925},
+  {1000080160, 15.99491461926},
+  {1000090160, 16.011460278},
+  {1000100160, 16.02575086},
+  {1000050170, 17.046931399},
+  {1000060170, 17.02257865},
+  {1000070170, 17.008448876},
+  {1000080170, 16.99913175595},
+  {1000090170, 17.002095237},
+  {1000100170, 17.017713962},
+  {1000110170, 17.037273},
+  {1000050180, 18.055601683},
+  {1000060180, 18.02675193},
+  {1000070180, 18.014077563},
+  {1000080180, 17.99915961214},
+  {1000090180, 18.000937324},
+  {1000100180, 18.005708696},
+  {1000110180, 18.026879388},
+  {1000050190, 19.064166},
+  {1000060190, 19.034797594},
+  {1000070190, 19.017022389},
+  {1000080190, 19.003577969},
+  {1000090190, 18.99840316207},
+  {1000100190, 19.001880906},
+  {1000110190, 19.013880264},
+  {1000120190, 19.03417992},
+  {1000050200, 20.074505644},
+  {1000060200, 20.040261732},
+  {1000070200, 20.023367295},
+  {1000080200, 20.004075357},
+  {1000090200, 19.999981252},
+  {1000100200, 19.99244017525},
+  {1000110200, 20.007354301},
+  {1000120200, 20.018763075},
+  {1000050210, 21.084147485},
+  {1000060210, 21.049},
+  {1000070210, 21.027087573},
+  {1000080210, 21.008654948},
+  {1000090210, 20.999948893},
+  {1000100210, 20.993846685},
+  {1000110210, 20.997654459},
+  {1000120210, 21.011705764},
+  {1000130210, 21.029082},
+  {1000060220, 22.05755399},
+  {1000070220, 22.034100918},
+  {1000080220, 22.009965744},
+  {1000090220, 22.002998812},
+  {1000100220, 21.991385113},
+  {1000110220, 21.994437547},
+  {1000120220, 21.999570597},
+  {1000130220, 22.01954},
+  {1000140220, 22.036114},
+  {1000060230, 23.06889},
+  {1000070230, 23.039421},
+  {1000080230, 23.015696686},
+  {1000090230, 23.003526875},
+  {1000100230, 22.994466905},
+  {1000110230, 22.98976928195},
+  {1000120230, 22.994123768},
+  {1000130230, 23.007244351},
+  {1000140230, 23.025711},
+  {1000070240, 24.05039},
+  {1000080240, 24.019861},
+  {1000090240, 24.00809937},
+  {1000100240, 23.993610649},
+  {1000110240, 23.990963012},
+  {1000120240, 23.985041689},
+  {1000130240, 23.999947598},
+  {1000140240, 24.01153543},
+  {1000150240, 24.036522},
+  {1000070250, 25.0601},
+  {1000080250, 25.029338919},
+  {1000090250, 25.012167727},
+  {1000100250, 24.997814797},
+  {1000110250, 24.989953974},
+  {1000120250, 24.985836966},
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+  {1001002510, 251.08154513},
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+  {1001002520, 252.082466019},
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+  {1001012530, 253.087143},
+  {1001022530, 253.09056278},
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+  {1001022540, 254.090954211},
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+  {1000992550, 255.090273504},
+  {1001002550, 255.089963495},
+  {1001012550, 255.091081702},
+  {1001022550, 255.093196439},
+  {1001032550, 255.096562399},
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+  {1001052550, 255.106919},
+  {1000982560, 256.093442},
+  {1000992560, 256.093597},
+  {1001002560, 256.091771699},
+  {1001012560, 256.093888},
+  {1001022560, 256.094281912},
+  {1001032560, 256.098494024},
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+  {1001032570, 257.09948},
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+  {1001002590, 259.100596},
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+  {1001022590, 259.100998364},
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+  {1001002600, 260.102809},
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+  {1001062610, 261.115948135},
+  {1001072610, 261.121395733},
+  {1001012620, 262.109144},
+  {1001022620, 262.107463},
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+  {1001092720, 272.143298},
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+  {1001072730, 273.140294},
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+  {1001112730, 273.153393},
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+  {1001112740, 274.155247},
+  {1001072750, 275.145766},
+  {1001082750, 275.14653},
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+  {1001112750, 275.156088},
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+  {1001082760, 276.148348},
+  {1001092760, 276.151705},
+  {1001102760, 276.153022},
+  {1001112760, 276.158226},
+  {1001122760, 276.161418},
+  {1001072770, 277.151477},
+  {1001082770, 277.151772},
+  {1001092770, 277.153525},
+  {1001102770, 277.155763},
+  {1001112770, 277.159322},
+  {1001122770, 277.163535},
+  {1001072780, 278.154988},
+  {1001082780, 278.153753},
+  {1001092780, 278.156487},
+  {1001102780, 278.157007},
+  {1001112780, 278.16159},
+  {1001122780, 278.164083},
+  {1001132780, 278.170725},
+  {1001082790, 279.157274},
+  {1001092790, 279.158439},
+  {1001102790, 279.159984},
+  {1001112790, 279.16288},
+  {1001122790, 279.166422},
+  {1001132790, 279.171187},
+  {1001082800, 280.159335},
+  {1001092800, 280.161579},
+  {1001102800, 280.161375},
+  {1001112800, 280.165204},
+  {1001122800, 280.167102},
+  {1001132800, 280.173098},
+  {1001092810, 281.163608},
+  {1001102810, 281.164545},
+  {1001112810, 281.166757},
+  {1001122810, 281.169563},
+  {1001132810, 281.17371},
+  {1001092820, 282.166888},
+  {1001102820, 282.166174},
+  {1001112820, 282.169343},
+  {1001122820, 282.170507},
+  {1001132820, 282.17577},
+  {1001102830, 283.169437},
+  {1001112830, 283.171101},
+  {1001122830, 283.173202},
+  {1001132830, 283.176666},
+  {1001102840, 284.171187},
+  {1001112840, 284.173882},
+  {1001122840, 284.17436},
+  {1001132840, 284.178843},
+  {1001142840, 284.181192},
+  {1001112850, 285.175771},
+  {1001122850, 285.177227},
+  {1001132850, 285.180106},
+  {1001142850, 285.183503},
+  {1001112860, 286.178756},
+  {1001122860, 286.178691},
+  {1001132860, 286.182456},
+  {1001142860, 286.184226},
+  {1001122870, 287.181826},
+  {1001132870, 287.184064},
+  {1001142870, 287.18672},
+  {1001152870, 287.19082},
+  {1001122880, 288.183501},
+  {1001132880, 288.186764},
+  {1001142880, 288.187781},
+  {1001152880, 288.192879},
+  {1001132890, 289.188461},
+  {1001142890, 289.190517},
+  {1001152890, 289.193971},
+  {1001162890, 289.198023},
+  {1001132900, 290.191429},
+  {1001142900, 290.191875},
+  {1001152900, 290.196235},
+  {1001162900, 290.198635},
+  {1001142910, 291.194848},
+  {1001152910, 291.197725},
+  {1001162910, 291.201014},
+  {1001172910, 291.205748},
+  {1001152920, 292.200323},
+  {1001162920, 292.201969},
+  {1001172920, 292.207861},
+  {1001162930, 293.204583},
+  {1001172930, 293.208727},
+  {1001182930, 293.213423},
+  {1001172940, 294.21084},
+  {1001182940, 294.213979},
+  {1001182950, 295.216178},
+};
+
+} // namespace openmc
diff --git a/src/bank.cpp b/src/bank.cpp
index 8d00d5440..5b12b48fd 100644
--- a/src/bank.cpp
+++ b/src/bank.cpp
@@ -1,11 +1,14 @@
 #include "openmc/bank.h"
 #include "openmc/capi.h"
 #include "openmc/error.h"
+#include "openmc/ifp.h"
 #include "openmc/message_passing.h"
 #include "openmc/simulation.h"
 #include "openmc/vector.h"
 
 #include <cstdint>
+#include <limits>
+#include <numeric>
 
 namespace openmc {
 
@@ -19,6 +22,8 @@ vector<SourceSite> source_bank;
 
 SharedArray<SourceSite> surf_source_bank;
 
+SharedArray<CollisionTrackSite> collision_track_bank;
+
 // The fission bank is allocated as a SharedArray, rather than a vector, as it
 // will be shared by all threads in the simulation. It will be allocated to a
 // fixed maximum capacity in the init_fission_bank() function. Then, Elements
@@ -26,11 +31,26 @@ SharedArray<SourceSite> surf_source_bank;
 // function.
 SharedArray<SourceSite> fission_bank;
 
+vector<vector<int>> ifp_source_delayed_group_bank;
+
+vector<vector<double>> ifp_source_lifetime_bank;
+
+vector<vector<int>> ifp_fission_delayed_group_bank;
+
+vector<vector<double>> ifp_fission_lifetime_bank;
+
 // Each entry in this vector corresponds to the number of progeny produced
 // this generation for the particle located at that index. This vector is
 // used to efficiently sort the fission bank after each iteration.
 vector<int64_t> progeny_per_particle;
 
+// When shared secondary bank mode is enabled, secondaries produced during
+// transport are collected in the write bank. When a secondary generation is
+// complete, write is moved to read for transport, and a new empty write bank
+// is created. This repeats until no secondaries remain.
+SharedArray<SourceSite> shared_secondary_bank_read;
+SharedArray<SourceSite> shared_secondary_bank_write;
+
 } // namespace simulation
 
 //==============================================================================
@@ -41,8 +61,15 @@ void free_memory_bank()
 {
   simulation::source_bank.clear();
   simulation::surf_source_bank.clear();
+  simulation::collision_track_bank.clear();
   simulation::fission_bank.clear();
   simulation::progeny_per_particle.clear();
+  simulation::ifp_source_delayed_group_bank.clear();
+  simulation::ifp_source_lifetime_bank.clear();
+  simulation::ifp_fission_delayed_group_bank.clear();
+  simulation::ifp_fission_lifetime_bank.clear();
+  simulation::shared_secondary_bank_read.clear();
+  simulation::shared_secondary_bank_write.clear();
 }
 
 void init_fission_bank(int64_t max)
@@ -51,13 +78,13 @@ void init_fission_bank(int64_t max)
   simulation::progeny_per_particle.resize(simulation::work_per_rank);
 }
 
-// Performs an O(n) sort on the fission bank, by leveraging
+// Performs an O(n) sort on a fission or secondary bank, by leveraging
 // the parent_id and progeny_id fields of banked particles. See the following
 // paper for more details:
 // "Reproducibility and Monte Carlo Eigenvalue Calculations," F.B. Brown and
 // T.M. Sutton, 1992 ANS Annual Meeting, Transactions of the American Nuclear
 // Society, Volume 65, Page 235.
-void sort_fission_bank()
+void sort_bank(SharedArray<SourceSite>& bank, bool is_fission_bank)
 {
   // Ensure we don't read off the end of the array if we ran with 0 particles
   if (simulation::progeny_per_particle.size() == 0) {
@@ -66,47 +93,211 @@ void sort_fission_bank()
 
   // Perform exclusive scan summation to determine starting indices in fission
   // bank for each parent particle id
-  int64_t tmp = simulation::progeny_per_particle[0];
-  simulation::progeny_per_particle[0] = 0;
-  for (int64_t i = 1; i < simulation::progeny_per_particle.size(); i++) {
-    int64_t value = simulation::progeny_per_particle[i - 1] + tmp;
-    tmp = simulation::progeny_per_particle[i];
-    simulation::progeny_per_particle[i] = value;
-  }
-
-  // TODO: C++17 introduces the exclusive_scan() function which could be
-  // used to replace everything above this point in this function.
+  std::exclusive_scan(simulation::progeny_per_particle.begin(),
+    simulation::progeny_per_particle.end(),
+    simulation::progeny_per_particle.begin(), 0);
 
   // We need a scratch vector to make permutation of the fission bank into
   // sorted order easy. Under normal usage conditions, the fission bank is
   // over provisioned, so we can use that as scratch space.
   SourceSite* sorted_bank;
   vector<SourceSite> sorted_bank_holder;
+  vector<vector<int>> sorted_ifp_delayed_group_bank;
+  vector<vector<double>> sorted_ifp_lifetime_bank;
 
   // If there is not enough space, allocate a temporary vector and point to it
-  if (simulation::fission_bank.size() >
-      simulation::fission_bank.capacity() / 2) {
-    sorted_bank_holder.resize(simulation::fission_bank.size());
+  if (bank.size() > bank.capacity() / 2) {
+    sorted_bank_holder.resize(bank.size());
     sorted_bank = sorted_bank_holder.data();
   } else { // otherwise, point sorted_bank to unused portion of the fission bank
-    sorted_bank = &simulation::fission_bank[simulation::fission_bank.size()];
+    sorted_bank = bank.data() + bank.size();
   }
 
-  // Use parent and progeny indices to sort fission bank
-  for (int64_t i = 0; i < simulation::fission_bank.size(); i++) {
-    const auto& site = simulation::fission_bank[i];
-    int64_t offset = site.parent_id - 1 - simulation::work_index[mpi::rank];
-    int64_t idx = simulation::progeny_per_particle[offset] + site.progeny_id;
-    if (idx >= simulation::fission_bank.size()) {
+  if (settings::ifp_on && is_fission_bank) {
+    allocate_temporary_vector_ifp(
+      sorted_ifp_delayed_group_bank, sorted_ifp_lifetime_bank);
+  }
+
+  // Use parent and progeny indices to sort bank
+  for (int64_t i = 0; i < bank.size(); i++) {
+    const auto& site = bank[i];
+    if (site.parent_id < 0 ||
+        site.parent_id >=
+          static_cast<int64_t>(simulation::progeny_per_particle.size())) {
+      fatal_error(fmt::format("Invalid parent_id {} for banked site (expected "
+                              "range [0, {})).",
+        site.parent_id, simulation::progeny_per_particle.size()));
+    }
+    int64_t idx =
+      simulation::progeny_per_particle[site.parent_id] + site.progeny_id;
+    if (idx < 0 || idx >= bank.size()) {
       fatal_error("Mismatch detected between sum of all particle progeny and "
-                  "shared fission bank size.");
+                  "bank size during sorting.");
     }
     sorted_bank[idx] = site;
+    if (settings::ifp_on && is_fission_bank) {
+      copy_ifp_data_from_fission_banks(
+        i, sorted_ifp_delayed_group_bank[idx], sorted_ifp_lifetime_bank[idx]);
+    }
   }
 
   // Copy sorted bank into the fission bank
-  std::copy(sorted_bank, sorted_bank + simulation::fission_bank.size(),
-    simulation::fission_bank.data());
+  std::copy(sorted_bank, sorted_bank + bank.size(), bank.data());
+  if (settings::ifp_on && is_fission_bank) {
+    copy_ifp_data_to_fission_banks(
+      sorted_ifp_delayed_group_bank.data(), sorted_ifp_lifetime_bank.data());
+  }
+}
+
+// This function redistributes SourceSite particles across MPI ranks to
+// achieve load balancing while preserving the global ordering of particles.
+//
+// GUARANTEES:
+// -----------
+// 1. Global Order Preservation: After redistribution, each rank holds a
+//    contiguous slice of the original global ordering. For example, if the
+//    input across 3 ranks was:
+//      - Rank 0: IDs 0-4
+//      - Rank 1: IDs 5-6
+//      - Rank 2: IDs 7-200
+//    Then after redistribution (assuming ~67 particles per rank):
+//      - Rank 0: IDs 0-66   (contiguous)
+//      - Rank 1: IDs 67-133 (contiguous)
+//      - Rank 2: IDs 134-200 (contiguous)
+//    The global ordering is always preserved - no rank will ever hold
+//    non-contiguous ID ranges like "0-4 and 100-200".
+//
+// 2. Even Load Balancing: Particles are distributed as evenly as possible.
+//    If total % n_procs != 0, the first 'remainder' ranks each get one extra
+//    particle (i.e., floor division with remainder distributed to lower
+//    ranks). This follows the same logic as calculate_work().
+//
+// HOW IT WORKS:
+// -------------
+// The algorithm uses overlap-based redistribution:
+// 1. Each rank's current data occupies a range [cumulative_before[rank],
+//    cumulative_before[rank+1]) in the global index space.
+// 2. Each rank's target data should occupy [cumulative_target[rank],
+//    cumulative_target[rank+1]) in the same global index space.
+// 3. For each pair of (source_rank, dest_rank), we calculate the overlap
+//    between what source_rank currently has and what dest_rank needs.
+// 4. MPI_Alltoallv transfers exactly these overlapping regions, with
+//    displacements ensuring data lands at the correct position in the
+//    receiving buffer.
+//
+// EDGE CASES HANDLED:
+// -------------------
+// - Single rank (n_procs == 1): Returns immediately with local size, no MPI.
+// - Empty total (all ranks have 0 particles): Returns 0 immediately.
+// - Imbalanced input (e.g., one rank has all particles): Works correctly;
+//   that rank will send portions to all other ranks based on target ranges.
+// - Non-divisible totals: First 'remainder' ranks get one extra particle.
+int64_t synchronize_global_secondary_bank(
+  SharedArray<SourceSite>& shared_secondary_bank)
+{
+  // Get current size of local bank
+  int64_t local_size = shared_secondary_bank.size();
+
+  if (mpi::n_procs == 1) {
+    return local_size;
+  }
+
+#ifdef OPENMC_MPI
+  // Gather all sizes to all ranks
+  vector<int64_t> all_sizes(mpi::n_procs);
+  MPI_Allgather(&local_size, 1, MPI_INT64_T, all_sizes.data(), 1, MPI_INT64_T,
+    mpi::intracomm);
+
+  // Calculate total and check for empty case
+  int64_t total = 0;
+  for (int64_t size : all_sizes) {
+    total += size;
+  }
+
+  // If we don't have any items to distribute, return
+  if (total == 0) {
+    return total;
+  }
+
+  int64_t base_count = total / mpi::n_procs;
+  int64_t remainder = total % mpi::n_procs;
+
+  // Calculate target size for each rank
+  // First 'remainder' ranks get base_count + 1, rest get base_count
+  vector<int64_t> target_sizes(mpi::n_procs);
+  for (int i = 0; i < mpi::n_procs; ++i) {
+    target_sizes[i] = base_count + (i < remainder ? 1 : 0);
+  }
+
+  // Calculate send and receive counts in terms of SourceSite objects
+  // (not bytes)
+  vector<int> send_counts(mpi::n_procs, 0);
+  vector<int> recv_counts(mpi::n_procs, 0);
+  vector<int> send_displs(mpi::n_procs, 0);
+  vector<int> recv_displs(mpi::n_procs, 0);
+
+  // Calculate cumulative positions (starting index for each rank in the
+  // global array)
+  vector<int64_t> cumulative_before(mpi::n_procs + 1, 0);
+  vector<int64_t> cumulative_target(mpi::n_procs + 1, 0);
+  for (int i = 0; i < mpi::n_procs; ++i) {
+    cumulative_before[i + 1] = cumulative_before[i] + all_sizes[i];
+    cumulative_target[i + 1] = cumulative_target[i] + target_sizes[i];
+  }
+
+  // Determine send and receive amounts for each rank
+  int64_t my_start = cumulative_before[mpi::rank];
+  int64_t my_end = cumulative_before[mpi::rank + 1];
+  int64_t my_target_start = cumulative_target[mpi::rank];
+  int64_t my_target_end = cumulative_target[mpi::rank + 1];
+
+  for (int r = 0; r < mpi::n_procs; ++r) {
+    // Send: overlap between my current range and rank r's target range
+    int64_t send_overlap_start = std::max(my_start, cumulative_target[r]);
+    int64_t send_overlap_end = std::min(my_end, cumulative_target[r + 1]);
+    if (send_overlap_start < send_overlap_end) {
+      int64_t count = send_overlap_end - send_overlap_start;
+      int64_t displ = send_overlap_start - my_start;
+      if (count > std::numeric_limits<int>::max() ||
+          displ > std::numeric_limits<int>::max()) {
+        fatal_error("Secondary bank size exceeds MPI_Alltoallv int limit.");
+      }
+      send_counts[r] = static_cast<int>(count);
+      send_displs[r] = static_cast<int>(displ);
+    }
+
+    // Recv: overlap between rank r's current range and my target range
+    int64_t recv_overlap_start =
+      std::max(cumulative_before[r], my_target_start);
+    int64_t recv_overlap_end =
+      std::min(cumulative_before[r + 1], my_target_end);
+    if (recv_overlap_start < recv_overlap_end) {
+      int64_t count = recv_overlap_end - recv_overlap_start;
+      int64_t displ = recv_overlap_start - my_target_start;
+      if (count > std::numeric_limits<int>::max() ||
+          displ > std::numeric_limits<int>::max()) {
+        fatal_error("Secondary bank size exceeds MPI_Alltoallv int limit.");
+      }
+      recv_counts[r] = static_cast<int>(count);
+      recv_displs[r] = static_cast<int>(displ);
+    }
+  }
+
+  // Prepare receive buffer with target size
+  SharedArray<SourceSite> new_bank(target_sizes[mpi::rank]);
+
+  // Perform all-to-all redistribution using the custom MPI type
+  MPI_Alltoallv(shared_secondary_bank.data(), send_counts.data(),
+    send_displs.data(), mpi::source_site, new_bank.data(), recv_counts.data(),
+    recv_displs.data(), mpi::source_site, mpi::intracomm);
+
+  // Replace old bank with redistributed data
+  shared_secondary_bank = std::move(new_bank);
+
+  return total;
+#else
+  return local_size;
+#endif
 }
 
 //==============================================================================
diff --git a/src/boundary_condition.cpp b/src/boundary_condition.cpp
index 7216ac896..d166f1247 100644
--- a/src/boundary_condition.cpp
+++ b/src/boundary_condition.cpp
@@ -38,6 +38,9 @@ void VacuumBC::handle_particle(Particle& p, const Surface& surf) const
 void ReflectiveBC::handle_particle(Particle& p, const Surface& surf) const
 {
   Direction u = surf.reflect(p.r(), p.u(), &p);
+
+  // normalize reflected u to ensure no floating point error leads to
+  // unnormalized directions
   u /= u.norm();
 
   // Handle the effects of the surface albedo on the particle's weight.
@@ -53,6 +56,9 @@ void ReflectiveBC::handle_particle(Particle& p, const Surface& surf) const
 void WhiteBC::handle_particle(Particle& p, const Surface& surf) const
 {
   Direction u = surf.diffuse_reflect(p.r(), p.u(), p.current_seed());
+
+  // normalize outgoing u to ensure no floating point error leads to
+  // unnormalized directions
   u /= u.norm();
 
   // Handle the effects of the surface albedo on the particle's weight.
@@ -129,23 +135,8 @@ TranslationalPeriodicBC::TranslationalPeriodicBC(int i_surf, int j_surf)
 void TranslationalPeriodicBC::handle_particle(
   Particle& p, const Surface& surf) const
 {
-  int i_particle_surf = p.surface_index();
-
-  // Figure out which of the two BC surfaces were struck then find the
-  // particle's new location and surface.
-  Position new_r;
-  int new_surface;
-  if (i_particle_surf == i_surf_) {
-    new_r = p.r() + translation_;
-    new_surface = p.surface() > 0 ? j_surf_ + 1 : -(j_surf_ + 1);
-  } else if (i_particle_surf == j_surf_) {
-    new_r = p.r() - translation_;
-    new_surface = p.surface() > 0 ? i_surf_ + 1 : -(i_surf_ + 1);
-  } else {
-    throw std::runtime_error(
-      "Called BoundaryCondition::handle_particle after "
-      "hitting a surface, but that surface is not recognized by the BC.");
-  }
+  auto new_r = p.r() + translation_;
+  int new_surface = p.surface() > 0 ? j_surf_ + 1 : -(j_surf_ + 1);
 
   // Handle the effects of the surface albedo on the particle's weight.
   BoundaryCondition::handle_albedo(p, surf);
@@ -158,63 +149,45 @@ void TranslationalPeriodicBC::handle_particle(
 // RotationalPeriodicBC implementation
 //==============================================================================
 
-RotationalPeriodicBC::RotationalPeriodicBC(int i_surf, int j_surf)
-  : PeriodicBC(i_surf, j_surf)
+RotationalPeriodicBC::RotationalPeriodicBC(
+  int i_surf, int j_surf, PeriodicAxis axis)
+  : PeriodicBC(std::abs(i_surf) - 1, std::abs(j_surf) - 1)
 {
   Surface& surf1 {*model::surfaces[i_surf_]};
   Surface& surf2 {*model::surfaces[j_surf_]};
 
-  // Check the type of the first surface
-  bool surf1_is_xyplane;
-  if (const auto* ptr = dynamic_cast<const SurfaceXPlane*>(&surf1)) {
-    surf1_is_xyplane = true;
-  } else if (const auto* ptr = dynamic_cast<const SurfaceYPlane*>(&surf1)) {
-    surf1_is_xyplane = true;
-  } else if (const auto* ptr = dynamic_cast<const SurfacePlane*>(&surf1)) {
-    surf1_is_xyplane = false;
-  } else {
-    throw std::invalid_argument(fmt::format(
-      "Surface {} is an invalid type for "
-      "rotational periodic BCs. Only x-planes, y-planes, or general planes "
-      "(that are perpendicular to z) are supported for these BCs.",
-      surf1.id_));
+  // below convention for right handed coordinate system
+  switch (axis) {
+  case x:
+    zero_axis_idx_ = 0; // x component of plane must be zero
+    axis_1_idx_ = 1;    // y component independent
+    axis_2_idx_ = 2;    // z component dependent
+    break;
+  case y:
+    zero_axis_idx_ = 1; // y component of plane must be zero
+    axis_1_idx_ = 2;    // z component independent
+    axis_2_idx_ = 0;    // x component dependent
+    break;
+  case z:
+    zero_axis_idx_ = 2; // z component of plane must be zero
+    axis_1_idx_ = 0;    // x component independent
+    axis_2_idx_ = 1;    // y component dependent
+    break;
+  default:
+    throw std::invalid_argument(
+      fmt::format("You've specified an axis that is not x, y, or z."));
   }
 
-  // Check the type of the second surface
-  bool surf2_is_xyplane;
-  if (const auto* ptr = dynamic_cast<const SurfaceXPlane*>(&surf2)) {
-    surf2_is_xyplane = true;
-  } else if (const auto* ptr = dynamic_cast<const SurfaceYPlane*>(&surf2)) {
-    surf2_is_xyplane = true;
-  } else if (const auto* ptr = dynamic_cast<const SurfacePlane*>(&surf2)) {
-    surf2_is_xyplane = false;
-  } else {
-    throw std::invalid_argument(fmt::format(
-      "Surface {} is an invalid type for "
-      "rotational periodic BCs. Only x-planes, y-planes, or general planes "
-      "(that are perpendicular to z) are supported for these BCs.",
-      surf2.id_));
-  }
+  Direction ax = {0.0, 0.0, 0.0};
+  ax[zero_axis_idx_] = 1.0;
+
+  auto i_sign = std::copysign(1, i_surf);
+  auto j_sign = -std::copysign(1, j_surf);
 
   // Compute the surface normal vectors and make sure they are perpendicular
-  // to the z-axis
-  Direction norm1 = surf1.normal({0, 0, 0});
-  Direction norm2 = surf2.normal({0, 0, 0});
-  if (std::abs(norm1.z) > FP_PRECISION) {
-    throw std::invalid_argument(fmt::format(
-      "Rotational periodic BCs are only "
-      "supported for rotations about the z-axis, but surface {} is not "
-      "perpendicular to the z-axis.",
-      surf1.id_));
-  }
-  if (std::abs(norm2.z) > FP_PRECISION) {
-    throw std::invalid_argument(fmt::format(
-      "Rotational periodic BCs are only "
-      "supported for rotations about the z-axis, but surface {} is not "
-      "perpendicular to the z-axis.",
-      surf2.id_));
-  }
-
+  // to the correct axis
+  Direction norm1 = i_sign * surf1.normal({0, 0, 0});
+  Direction norm2 = j_sign * surf2.normal({0, 0, 0});
   // Make sure both surfaces intersect the origin
   if (std::abs(surf1.evaluate({0, 0, 0})) > FP_COINCIDENT) {
     throw std::invalid_argument(fmt::format(
@@ -231,15 +204,15 @@ RotationalPeriodicBC::RotationalPeriodicBC(int i_surf, int j_surf)
       surf2.id_));
   }
 
-  // Compute the BC rotation angle.  Here it is assumed that both surface
-  // normal vectors point inwards---towards the valid geometry region.
-  // Consequently, the rotation angle is not the difference between the two
-  // normals, but is instead the difference between one normal and one
-  // anti-normal.  (An incident ray on one surface must be an outgoing ray on
-  // the other surface after rotation hence the anti-normal.)
-  double theta1 = std::atan2(norm1.y, norm1.x);
-  double theta2 = std::atan2(norm2.y, norm2.x) + PI;
-  angle_ = theta2 - theta1;
+  // Compute the signed rotation angle about the periodic axis. Note that
+  // (n1×n2)·a = |n1||n2|sin(θ) and n1·n2 = |n1||n2|cos(θ), where a is the axis
+  // of rotation.
+  auto c = norm1.cross(norm2);
+  angle_ = std::atan2(c.dot(ax), norm1.dot(norm2));
+
+  // If the normals point in the same general direction, the surface sense
+  // should change when crossing the boundary
+  flip_sense_ = (i_sign * j_sign > 0.0);
 
   // Warn the user if the angle does not evenly divide a circle
   double rem = std::abs(std::remainder((2 * PI / angle_), 1.0));
@@ -254,34 +227,29 @@ RotationalPeriodicBC::RotationalPeriodicBC(int i_surf, int j_surf)
 void RotationalPeriodicBC::handle_particle(
   Particle& p, const Surface& surf) const
 {
-  int i_particle_surf = p.surface_index();
+  int new_surface = p.surface() > 0 ? -(j_surf_ + 1) : j_surf_ + 1;
+  if (flip_sense_)
+    new_surface = -new_surface;
 
-  // Figure out which of the two BC surfaces were struck to figure out if a
-  // forward or backward rotation is required.  Specify the other surface as
-  // the particle's new surface.
-  double theta;
-  int new_surface;
-  if (i_particle_surf == i_surf_) {
-    theta = angle_;
-    new_surface = p.surface() > 0 ? -(j_surf_ + 1) : j_surf_ + 1;
-  } else if (i_particle_surf == j_surf_) {
-    theta = -angle_;
-    new_surface = p.surface() > 0 ? -(i_surf_ + 1) : i_surf_ + 1;
-  } else {
-    throw std::runtime_error(
-      "Called BoundaryCondition::handle_particle after "
-      "hitting a surface, but that surface is not recognized by the BC.");
-  }
-
-  // Rotate the particle's position and direction about the z-axis.
+  // Rotate the particle's position and direction.
   Position r = p.r();
   Direction u = p.u();
-  double cos_theta = std::cos(theta);
-  double sin_theta = std::sin(theta);
-  Position new_r = {
-    cos_theta * r.x - sin_theta * r.y, sin_theta * r.x + cos_theta * r.y, r.z};
-  Direction new_u = {
-    cos_theta * u.x - sin_theta * u.y, sin_theta * u.x + cos_theta * u.y, u.z};
+  double cos_theta = std::cos(angle_);
+  double sin_theta = std::sin(angle_);
+
+  Position new_r;
+  new_r[zero_axis_idx_] = r[zero_axis_idx_];
+  new_r[axis_1_idx_] = cos_theta * r[axis_1_idx_] - sin_theta * r[axis_2_idx_];
+  new_r[axis_2_idx_] = sin_theta * r[axis_1_idx_] + cos_theta * r[axis_2_idx_];
+
+  Direction new_u;
+  new_u[zero_axis_idx_] = u[zero_axis_idx_];
+  new_u[axis_1_idx_] = cos_theta * u[axis_1_idx_] - sin_theta * u[axis_2_idx_];
+  new_u[axis_2_idx_] = sin_theta * u[axis_1_idx_] + cos_theta * u[axis_2_idx_];
+
+  // normalize new_u to ensure no floating point error leads to unnormalized
+  // directions
+  new_u /= new_u.norm();
 
   // Handle the effects of the surface albedo on the particle's weight.
   BoundaryCondition::handle_albedo(p, surf);
diff --git a/src/bremsstrahlung.cpp b/src/bremsstrahlung.cpp
index d22d6392a..ec1088101 100644
--- a/src/bremsstrahlung.cpp
+++ b/src/bremsstrahlung.cpp
@@ -6,7 +6,7 @@
 #include "openmc/search.h"
 #include "openmc/settings.h"
 
-#include "xtensor/xmath.hpp"
+#include "openmc/tensor.h"
 
 namespace openmc {
 
@@ -16,8 +16,8 @@ namespace openmc {
 
 namespace data {
 
-xt::xtensor<double, 1> ttb_e_grid;
-xt::xtensor<double, 1> ttb_k_grid;
+tensor::Tensor<double> ttb_e_grid;
+tensor::Tensor<double> ttb_k_grid;
 vector<Bremsstrahlung> ttb;
 
 } // namespace data
@@ -31,13 +31,13 @@ void thick_target_bremsstrahlung(Particle& p, double* E_lost)
   if (p.material() == MATERIAL_VOID)
     return;
 
-  int photon = static_cast<int>(ParticleType::photon);
+  int photon = ParticleType::photon().transport_index();
   if (p.E() < settings::energy_cutoff[photon])
     return;
 
   // Get bremsstrahlung data for this material and particle type
   BremsstrahlungData* mat;
-  if (p.type() == ParticleType::positron) {
+  if (p.type() == ParticleType::positron()) {
     mat = &model::materials[p.material()]->ttb_->positron;
   } else {
     mat = &model::materials[p.material()]->ttb_->electron;
@@ -112,8 +112,14 @@ void thick_target_bremsstrahlung(Particle& p, double* E_lost)
                std::pow(a * (c - c_l) / (std::exp(w_l) * p_l) + 1.0, 1.0 / a);
 
     if (w > settings::energy_cutoff[photon]) {
+      // If the energy of the secondary photon is larger than the remaining
+      // energy of the primary particle, adjust it to the remaining energy
+      if (*E_lost + w > p.E()) {
+        w = p.E() - *E_lost;
+      }
+
       // Create secondary photon
-      p.create_secondary(p.wgt(), p.u(), w, ParticleType::photon);
+      p.create_secondary(p.wgt(), p.u(), w, ParticleType::photon());
       *E_lost += w;
     }
   }
diff --git a/src/cell.cpp b/src/cell.cpp
index 4b2699229..06bc29a66 100644
--- a/src/cell.cpp
+++ b/src/cell.cpp
@@ -40,6 +40,11 @@ vector<unique_ptr<Cell>> cells;
 // Cell implementation
 //==============================================================================
 
+int32_t Cell::n_instances() const
+{
+  return model::universes[universe_]->n_instances_;
+}
+
 void Cell::set_rotation(const vector<double>& rot)
 {
   if (fill_ == C_NONE) {
@@ -52,7 +57,7 @@ void Cell::set_rotation(const vector<double>& rot)
     fatal_error(fmt::format("Non-3D rotation vector applied to cell {}", id_));
   }
 
-  // Compute and store the rotation matrix.
+  // Compute and store the inverse rotation matrix for the angles given.
   rotation_.clear();
   rotation_.reserve(rot.size() == 9 ? 9 : 12);
   if (rot.size() == 3) {
@@ -96,6 +101,25 @@ double Cell::temperature(int32_t instance) const
   }
 }
 
+double Cell::density_mult(int32_t instance) const
+{
+  if (instance >= 0) {
+    return density_mult_.size() == 1 ? density_mult_.at(0)
+                                     : density_mult_.at(instance);
+  } else {
+    return density_mult_[0];
+  }
+}
+
+double Cell::density(int32_t instance) const
+{
+  const int32_t mat_index = material(instance);
+  if (mat_index == MATERIAL_VOID)
+    return 0.0;
+
+  return density_mult(instance) * model::materials[mat_index]->density_gpcc();
+}
+
 void Cell::set_temperature(double T, int32_t instance, bool set_contained)
 {
   if (settings::temperature_method == TemperatureMethod::INTERPOLATION) {
@@ -115,8 +139,8 @@ void Cell::set_temperature(double T, int32_t instance, bool set_contained)
   if (type_ == Fill::MATERIAL) {
     if (instance >= 0) {
       // If temperature vector is not big enough, resize it first
-      if (sqrtkT_.size() != n_instances_)
-        sqrtkT_.resize(n_instances_, sqrtkT_[0]);
+      if (sqrtkT_.size() != n_instances())
+        sqrtkT_.resize(n_instances(), sqrtkT_[0]);
 
       // Set temperature for the corresponding instance
       sqrtkT_.at(instance) = std::sqrt(K_BOLTZMANN * T);
@@ -146,6 +170,47 @@ void Cell::set_temperature(double T, int32_t instance, bool set_contained)
   }
 }
 
+void Cell::set_density(double density, int32_t instance, bool set_contained)
+{
+  if (type_ != Fill::MATERIAL && !set_contained) {
+    fatal_error(
+      fmt::format("Attempted to set the density multiplier of cell {} "
+                  "which is not filled by a material.",
+        id_));
+  }
+
+  if (type_ == Fill::MATERIAL) {
+    const int32_t mat_index = material(instance);
+    if (mat_index == MATERIAL_VOID)
+      return;
+
+    if (instance >= 0) {
+      // If density multiplier vector is not big enough, resize it first
+      if (density_mult_.size() != n_instances())
+        density_mult_.resize(n_instances(), density_mult_[0]);
+
+      // Set density multiplier for the corresponding instance
+      density_mult_.at(instance) =
+        density / model::materials[mat_index]->density_gpcc();
+    } else {
+      // Set density multiplier for all instances
+      for (auto& x : density_mult_) {
+        x = density / model::materials[mat_index]->density_gpcc();
+      }
+    }
+  } else {
+    auto contained_cells = this->get_contained_cells(instance);
+    for (const auto& entry : contained_cells) {
+      auto& cell = model::cells[entry.first];
+      assert(cell->type_ == Fill::MATERIAL);
+      auto& instances = entry.second;
+      for (auto instance : instances) {
+        cell->set_density(density, instance);
+      }
+    }
+  }
+}
+
 void Cell::export_properties_hdf5(hid_t group) const
 {
   // Create a group for this cell.
@@ -157,6 +222,15 @@ void Cell::export_properties_hdf5(hid_t group) const
     temps.push_back(sqrtkT_val * sqrtkT_val / K_BOLTZMANN);
   write_dataset(cell_group, "temperature", temps);
 
+  // Write density for one or more cell instances
+  if (type_ == Fill::MATERIAL && material_.size() > 0) {
+    vector<double> density;
+    for (int32_t i = 0; i < density_mult_.size(); ++i)
+      density.push_back(this->density(i));
+
+    write_dataset(cell_group, "density", density);
+  }
+
   close_group(cell_group);
 }
 
@@ -170,8 +244,8 @@ void Cell::import_properties_hdf5(hid_t group)
 
   // Ensure number of temperatures makes sense
   auto n_temps = temps.size();
-  if (n_temps > 1 && n_temps != n_instances_) {
-    throw std::runtime_error(fmt::format(
+  if (n_temps > 1 && n_temps != n_instances()) {
+    fatal_error(fmt::format(
       "Number of temperatures for cell {} doesn't match number of instances",
       id_));
   }
@@ -183,6 +257,25 @@ void Cell::import_properties_hdf5(hid_t group)
     this->set_temperature(temps[i], i);
   }
 
+  // Read densities
+  if (object_exists(cell_group, "density")) {
+    vector<double> density;
+    read_dataset(cell_group, "density", density);
+
+    // Ensure number of densities makes sense
+    auto n_density = density.size();
+    if (n_density > 1 && n_density != n_instances()) {
+      fatal_error(fmt::format("Number of densities for cell {} "
+                              "doesn't match number of instances",
+        id_));
+    }
+
+    // Set densities.
+    for (int32_t i = 0; i < n_density; ++i) {
+      this->set_density(density[i], i);
+    }
+  }
+
   close_group(cell_group);
 }
 
@@ -222,6 +315,8 @@ void Cell::to_hdf5(hid_t cell_group) const
       temps.push_back(sqrtkT_val * sqrtkT_val / K_BOLTZMANN);
     write_dataset(group, "temperature", temps);
 
+    write_dataset(group, "density_mult", density_mult_);
+
   } else if (type_ == Fill::UNIVERSE) {
     write_dataset(group, "fill_type", "universe");
     write_dataset(group, "fill", model::universes[fill_]->id_);
@@ -245,6 +340,63 @@ void Cell::to_hdf5(hid_t cell_group) const
   close_group(group);
 }
 
+//==============================================================================
+// XML parsing helpers for <cell> nodes
+//==============================================================================
+
+vector<int32_t> parse_cell_material_xml(pugi::xml_node node, int32_t cell_id)
+{
+  vector<std::string> mats {
+    get_node_array<std::string>(node, "material", true)};
+  if (mats.empty()) {
+    fatal_error(fmt::format(
+      "An empty material element was specified for cell {}", cell_id));
+  }
+  vector<int32_t> material;
+  material.reserve(mats.size());
+  for (const auto& mat : mats) {
+    if (mat == "void") {
+      material.push_back(MATERIAL_VOID);
+    } else {
+      material.push_back(std::stoi(mat));
+    }
+  }
+  return material;
+}
+
+vector<double> parse_cell_temperature_xml(pugi::xml_node node, int32_t cell_id)
+{
+  auto temperatures = get_node_array<double>(node, "temperature");
+  if (temperatures.empty()) {
+    fatal_error(fmt::format(
+      "An empty temperature element was specified for cell {}", cell_id));
+  }
+  for (auto T : temperatures) {
+    if (T < 0) {
+      fatal_error(fmt::format(
+        "Cell {} was specified with a negative temperature", cell_id));
+    }
+  }
+  return temperatures;
+}
+
+vector<double> parse_cell_density_xml(pugi::xml_node node, int32_t cell_id)
+{
+  auto densities = get_node_array<double>(node, "density");
+  if (densities.empty()) {
+    fatal_error(fmt::format(
+      "An empty density element was specified for cell {}", cell_id));
+  }
+  for (auto rho : densities) {
+    if (rho <= 0) {
+      fatal_error(fmt::format(
+        "Cell {} was specified with a density less than or equal to zero",
+        cell_id));
+    }
+  }
+  return densities;
+}
+
 //==============================================================================
 // CSGCell implementation
 //==============================================================================
@@ -295,26 +447,12 @@ CSGCell::CSGCell(pugi::xml_node cell_node)
   // universe), more than one material (distribmats), and some materials may
   // be "void".
   if (material_present) {
-    vector<std::string> mats {
-      get_node_array<std::string>(cell_node, "material", true)};
-    if (mats.size() > 0) {
-      material_.reserve(mats.size());
-      for (std::string mat : mats) {
-        if (mat.compare("void") == 0) {
-          material_.push_back(MATERIAL_VOID);
-        } else {
-          material_.push_back(std::stoi(mat));
-        }
-      }
-    } else {
-      fatal_error(fmt::format(
-        "An empty material element was specified for cell {}", id_));
-    }
+    material_ = parse_cell_material_xml(cell_node, id_);
   }
 
   // Read the temperature element which may be distributed like materials.
   if (check_for_node(cell_node, "temperature")) {
-    sqrtkT_ = get_node_array<double>(cell_node, "temperature");
+    sqrtkT_ = parse_cell_temperature_xml(cell_node, id_);
     sqrtkT_.shrink_to_fit();
 
     // Make sure this is a material-filled cell.
@@ -325,20 +463,41 @@ CSGCell::CSGCell(pugi::xml_node cell_node)
         id_));
     }
 
-    // Make sure all temperatures are non-negative.
-    for (auto T : sqrtkT_) {
-      if (T < 0) {
-        fatal_error(fmt::format(
-          "Cell {} was specified with a negative temperature", id_));
-      }
-    }
-
     // Convert to sqrt(k*T).
     for (auto& T : sqrtkT_) {
       T = std::sqrt(K_BOLTZMANN * T);
     }
   }
 
+  // Read the density element which can be distributed similar to temperature.
+  // These get assigned to the density multiplier, requiring a division by
+  // the material density.
+  // Note: calculating the actual density multiplier is deferred until materials
+  // are finalized. density_mult_ contains the true density in the meantime.
+  if (check_for_node(cell_node, "density")) {
+    density_mult_ = parse_cell_density_xml(cell_node, id_);
+    density_mult_.shrink_to_fit();
+
+    // Make sure this is a material-filled cell.
+    if (material_.size() == 0) {
+      fatal_error(fmt::format(
+        "Cell {} was specified with a density but no material. Density"
+        "specification is only valid for cells filled with a material.",
+        id_));
+    }
+
+    // Make sure this is a non-void material.
+    for (auto mat_id : material_) {
+      if (mat_id == MATERIAL_VOID) {
+        fatal_error(fmt::format(
+          "Cell {} was specified with a density, but contains a void "
+          "material. Density specification is only valid for cells "
+          "filled with a non-void material.",
+          id_));
+      }
+    }
+  }
+
   // Read the region specification.
   std::string region_spec;
   if (check_for_node(cell_node, "region")) {
@@ -527,7 +686,7 @@ Region::Region(std::string region_spec, int32_t cell_id)
       if (token == OP_UNION) {
         simple_ = false;
         // Ensure intersections have precedence over unions
-        add_precedence();
+        enforce_precedence();
         break;
       }
     }
@@ -570,7 +729,7 @@ void Region::apply_demorgan(
 //! precedence than unions using parentheses.
 //==============================================================================
 
-int64_t Region::add_parentheses(int64_t start)
+void Region::add_parentheses(int64_t start)
 {
   int32_t start_token = expression_[start];
   // Add left parenthesis and set new position to be after parenthesis
@@ -579,14 +738,6 @@ int64_t Region::add_parentheses(int64_t start)
   }
   expression_.insert(expression_.begin() + start - 1, OP_LEFT_PAREN);
 
-  // Keep track of return iterator distance. If we don't encounter a left
-  // parenthesis, we return an iterator corresponding to wherever the right
-  // parenthesis is inserted. If a left parenthesis is encountered, an iterator
-  // corresponding to the left parenthesis is returned. Also note that we keep
-  // track of a *distance* instead of an iterator because the underlying memory
-  // allocation may change.
-  std::size_t return_it_dist = 0;
-
   // Add right parenthesis
   // While the start iterator is within the bounds of infix
   while (start + 1 < expression_.size()) {
@@ -600,7 +751,6 @@ int64_t Region::add_parentheses(int64_t start)
       // in the region, when the operator is an intersection then include the
       // operator and next surface
       if (expression_[start] == OP_LEFT_PAREN) {
-        return_it_dist = start;
         int depth = 1;
         do {
           start++;
@@ -617,54 +767,73 @@ int64_t Region::add_parentheses(int64_t start)
           --start;
         }
         expression_.insert(expression_.begin() + start, OP_RIGHT_PAREN);
-        if (return_it_dist > 0) {
-          return return_it_dist;
-        } else {
-          return start - 1;
-        }
+        return;
       }
     }
   }
-  // If we get here a right parenthesis hasn't been placed,
-  // return iterator
+  // If we get here a right parenthesis hasn't been placed
   expression_.push_back(OP_RIGHT_PAREN);
-  if (return_it_dist > 0) {
-    return return_it_dist;
-  } else {
-    return start - 1;
-  }
 }
 
+//==============================================================================
+//! Add parentheses to enforce operator precedence in region expressions
+//!
+//! This function ensures that intersection operators have higher precedence
+//! than union operators by adding parentheses where needed. For example:
+//!   "1 2 | 3" becomes "(1 2) | 3"
+//!   "1 | 2 3" becomes "1 | (2 3)"
+//!
+//! The algorithm uses stacks to track the current operator type and its
+//! position at each parenthesis depth level. When it encounters a different
+//! operator at the same depth, it adds parentheses to group the
+//! higher-precedence operations.
 //==============================================================================
 
-void Region::add_precedence()
+void Region::enforce_precedence()
 {
-  int32_t current_op = 0;
-  std::size_t current_dist = 0;
+  // Stack tracking the operator type at each depth (0 = no operator seen yet)
+  vector<int32_t> op_stack = {0};
 
-  for (int64_t i = 0; i < expression_.size(); i++) {
+  // Stack tracking where the operator sequence started at each depth
+  vector<std::size_t> pos_stack = {0};
+
+  for (int64_t i = 0; i < expression_.size(); ++i) {
     int32_t token = expression_[i];
 
-    if (token == OP_UNION || token == OP_INTERSECTION) {
-      if (current_op == 0) {
-        // Set the current operator if is hasn't been set
-        current_op = token;
-        current_dist = i;
-      } else if (token != current_op) {
-        // If the current operator doesn't match the token, add parenthesis to
-        // assert precedence
-        if (current_op == OP_INTERSECTION) {
-          i = add_parentheses(current_dist);
-        } else {
-          i = add_parentheses(i);
-        }
-        current_op = 0;
-        current_dist = 0;
+    if (token == OP_LEFT_PAREN) {
+      // Entering a new parenthesis level - push new tracking state
+      op_stack.push_back(0);
+      pos_stack.push_back(0);
+      continue;
+    } else if (token == OP_RIGHT_PAREN) {
+      // Exiting a parenthesis level - pop tracking state (keep at least one)
+      if (op_stack.size() > 1) {
+        op_stack.pop_back();
+        pos_stack.pop_back();
+      }
+      continue;
+    }
+
+    if (token == OP_UNION || token == OP_INTERSECTION) {
+      if (op_stack.back() == 0) {
+        // First operator at this depth - record it and its position
+        op_stack.back() = token;
+        pos_stack.back() = i;
+      } else if (token != op_stack.back()) {
+        // Encountered a different operator at the same depth - need to add
+        // parentheses to enforce precedence. Intersection has higher
+        // precedence, so we parenthesize the intersection terms.
+        if (op_stack.back() == OP_INTERSECTION) {
+          add_parentheses(pos_stack.back());
+        } else {
+          add_parentheses(i);
+        }
+
+        // Restart the scan since we modified the expression
+        i = -1; // Will be incremented to 0 by the for loop
+        op_stack = {0};
+        pos_stack = {0};
       }
-    } else if (token > OP_COMPLEMENT) {
-      // If the token is a parenthesis reset the current operator
-      current_op = 0;
-      current_dist = 0;
     }
   }
 }
@@ -1025,7 +1194,7 @@ void populate_universes()
       model::universes.back()->cells_.push_back(index_cell);
       model::universe_map[uid] = model::universes.size() - 1;
     } else {
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
       // Skip implicit complement cells for now
       Universe* univ = model::universes[it->second].get();
       DAGUniverse* dag_univ = dynamic_cast<DAGUniverse*>(univ);
@@ -1124,6 +1293,24 @@ extern "C" int openmc_cell_set_temperature(
   return 0;
 }
 
+extern "C" int openmc_cell_set_density(
+  int32_t index, double density, const int32_t* instance, bool set_contained)
+{
+  if (index < 0 || index >= model::cells.size()) {
+    strcpy(openmc_err_msg, "Index in cells array is out of bounds.");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  int32_t instance_index = instance ? *instance : -1;
+  try {
+    model::cells[index]->set_density(density, instance_index, set_contained);
+  } catch (const std::exception& e) {
+    set_errmsg(e.what());
+    return OPENMC_E_UNASSIGNED;
+  }
+  return 0;
+}
+
 extern "C" int openmc_cell_get_temperature(
   int32_t index, const int32_t* instance, double* T)
 {
@@ -1142,6 +1329,36 @@ extern "C" int openmc_cell_get_temperature(
   return 0;
 }
 
+extern "C" int openmc_cell_get_density(
+  int32_t index, const int32_t* instance, double* density)
+{
+  if (index < 0 || index >= model::cells.size()) {
+    strcpy(openmc_err_msg, "Index in cells array is out of bounds.");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  int32_t instance_index = instance ? *instance : -1;
+  try {
+    if (model::cells[index]->type_ != Fill::MATERIAL) {
+      fatal_error(
+        fmt::format("Cell {}, instance {} is not filled with a material.",
+          model::cells[index]->id_, instance_index));
+    }
+
+    int32_t mat_index = model::cells[index]->material(instance_index);
+    if (mat_index == MATERIAL_VOID) {
+      *density = 0.0;
+    } else {
+      *density = model::cells[index]->density_mult(instance_index) *
+                 model::materials[mat_index]->density_gpcc();
+    }
+  } catch (const std::exception& e) {
+    set_errmsg(e.what());
+    return OPENMC_E_UNASSIGNED;
+  }
+  return 0;
+}
+
 //! Get the bounding box of a cell
 extern "C" int openmc_cell_bounding_box(
   const int32_t index, double* llc, double* urc)
@@ -1153,14 +1370,14 @@ extern "C" int openmc_cell_bounding_box(
   bbox = c->bounding_box();
 
   // set lower left corner values
-  llc[0] = bbox.xmin;
-  llc[1] = bbox.ymin;
-  llc[2] = bbox.zmin;
+  llc[0] = bbox.min.x;
+  llc[1] = bbox.min.y;
+  llc[2] = bbox.min.z;
 
   // set upper right corner values
-  urc[0] = bbox.xmax;
-  urc[1] = bbox.ymax;
-  urc[2] = bbox.zmax;
+  urc[0] = bbox.max.x;
+  urc[1] = bbox.max.y;
+  urc[2] = bbox.max.z;
 
   return 0;
 }
@@ -1314,10 +1531,10 @@ vector<ParentCell> Cell::find_parent_cells(
   bool cell_found = false;
   for (auto it = coords.begin(); it != coords.end(); it++) {
     const auto& coord = *it;
-    const auto& cell = model::cells[coord.cell];
+    const auto& cell = model::cells[coord.cell()];
     // if the cell at this level matches the current cell, stop adding to the
     // stack
-    if (coord.cell == model::cell_map[this->id_]) {
+    if (coord.cell() == model::cell_map[this->id_]) {
       cell_found = true;
       break;
     }
@@ -1327,10 +1544,10 @@ vector<ParentCell> Cell::find_parent_cells(
     int lattice_idx = C_NONE;
     if (cell->type_ == Fill::LATTICE) {
       const auto& next_coord = *(it + 1);
-      lattice_idx = model::lattices[next_coord.lattice]->get_flat_index(
-        next_coord.lattice_i);
+      lattice_idx = model::lattices[next_coord.lattice()]->get_flat_index(
+        next_coord.lattice_index());
     }
-    stack.push(coord.universe, {coord.cell, lattice_idx});
+    stack.push(coord.universe(), {coord.cell(), lattice_idx});
   }
 
   // if this loop finished because the cell was found and
@@ -1618,7 +1835,7 @@ extern "C" int openmc_cell_get_num_instances(
     set_errmsg("Index in cells array is out of bounds.");
     return OPENMC_E_OUT_OF_BOUNDS;
   }
-  *num_instances = model::cells[index]->n_instances_;
+  *num_instances = model::cells[index]->n_instances();
   return 0;
 }
 
diff --git a/src/chain.cpp b/src/chain.cpp
index e279d1f59..3915c016c 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -42,7 +42,7 @@ ChainNuclide::ChainNuclide(pugi::xml_node node)
       branching_ratio =
         std::stod(get_node_value(reaction_node, "branching_ratio"));
     }
-    int mt = reaction_type(rx_name);
+    int mt = reaction_mt(rx_name);
     reaction_products_[mt].push_back({rx_target, branching_ratio});
   }
 
@@ -70,8 +70,15 @@ ChainNuclide::~ChainNuclide()
 void DecayPhotonAngleEnergy::sample(
   double E_in, double& E_out, double& mu, uint64_t* seed) const
 {
-  E_out = photon_energy_->sample(seed);
-  mu = Uniform(-1., 1.).sample(seed);
+  E_out = photon_energy_->sample(seed).first;
+  mu = Uniform(-1., 1.).sample(seed).first;
+}
+
+double DecayPhotonAngleEnergy::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  E_out = photon_energy_->sample(seed).first;
+  return 0.5;
 }
 
 //==============================================================================
@@ -91,6 +98,8 @@ vector<unique_ptr<ChainNuclide>> chain_nuclides;
 
 void read_chain_file_xml()
 {
+  free_memory_chain();
+
   char* chain_file_path = std::getenv("OPENMC_CHAIN_FILE");
   if (!chain_file_path) {
     return;
@@ -113,4 +122,10 @@ void read_chain_file_xml()
   }
 }
 
+void free_memory_chain()
+{
+  data::chain_nuclides.clear();
+  data::chain_nuclide_map.clear();
+}
+
 } // namespace openmc
diff --git a/src/cmfd_solver.cpp b/src/cmfd_solver.cpp
index 943042f67..714a5bf3a 100644
--- a/src/cmfd_solver.cpp
+++ b/src/cmfd_solver.cpp
@@ -5,7 +5,7 @@
 #ifdef _OPENMP
 #include <omp.h>
 #endif
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/bank.h"
 #include "openmc/capi.h"
@@ -36,7 +36,7 @@ double spectral;
 
 int nx, ny, nz, ng;
 
-xt::xtensor<int, 2> indexmap;
+tensor::Tensor<int> indexmap;
 
 int use_all_threads;
 
@@ -79,15 +79,14 @@ int get_cmfd_energy_bin(const double E)
 // COUNT_BANK_SITES bins fission sites according to CMFD mesh and energy
 //==============================================================================
 
-xt::xtensor<double, 1> count_bank_sites(
-  xt::xtensor<int, 1>& bins, bool* outside)
+tensor::Tensor<double> count_bank_sites(
+  tensor::Tensor<int>& bins, bool* outside)
 {
   // Determine shape of array for counts
   std::size_t cnt_size = cmfd::nx * cmfd::ny * cmfd::nz * cmfd::ng;
-  vector<std::size_t> cnt_shape = {cnt_size};
 
   // Create array of zeros
-  xt::xarray<double> cnt {cnt_shape, 0.0};
+  tensor::Tensor<double> cnt = tensor::zeros<double>({cnt_size});
   bool outside_ = false;
 
   auto bank_size = simulation::source_bank.size();
@@ -113,29 +112,22 @@ xt::xtensor<double, 1> count_bank_sites(
     bins[i] = mesh_bin * cmfd::ng + energy_bin;
   }
 
-  // Create copy of count data. Since ownership will be acquired by xtensor,
-  // std::allocator must be used to avoid Valgrind mismatched free() / delete
-  // warnings.
   int total = cnt.size();
-  double* cnt_reduced = std::allocator<double> {}.allocate(total);
+  tensor::Tensor<double> counts = tensor::zeros<double>({cnt_size});
 
 #ifdef OPENMC_MPI
   // collect values from all processors
   MPI_Reduce(
-    cnt.data(), cnt_reduced, total, MPI_DOUBLE, MPI_SUM, 0, mpi::intracomm);
+    cnt.data(), counts.data(), total, MPI_DOUBLE, MPI_SUM, 0, mpi::intracomm);
 
   // Check if there were sites outside the mesh for any processor
   MPI_Reduce(&outside_, outside, 1, MPI_C_BOOL, MPI_LOR, 0, mpi::intracomm);
 
 #else
-  std::copy(cnt.data(), cnt.data() + total, cnt_reduced);
+  std::copy(cnt.data(), cnt.data() + total, counts.data());
   *outside = outside_;
 #endif
 
-  // Adapt reduced values in array back into an xarray
-  auto arr = xt::adapt(cnt_reduced, total, xt::acquire_ownership(), cnt_shape);
-  xt::xarray<double> counts = arr;
-
   return counts;
 }
 
@@ -151,19 +143,19 @@ extern "C" void openmc_cmfd_reweight(
   std::size_t src_size = cmfd::nx * cmfd::ny * cmfd::nz * cmfd::ng;
 
   // count bank sites for CMFD mesh, store bins in bank_bins for reweighting
-  xt::xtensor<int, 1> bank_bins({bank_size}, 0);
+  tensor::Tensor<int> bank_bins = tensor::zeros<int>({bank_size});
   bool sites_outside;
-  xt::xtensor<double, 1> sourcecounts =
+  tensor::Tensor<double> sourcecounts =
     count_bank_sites(bank_bins, &sites_outside);
 
   // Compute CMFD weightfactors
-  xt::xtensor<double, 1> weightfactors = xt::xtensor<double, 1>({src_size}, 1.);
+  tensor::Tensor<double> weightfactors = tensor::ones<double>({src_size});
   if (mpi::master) {
     if (sites_outside) {
       fatal_error("Source sites outside of the CMFD mesh");
     }
 
-    double norm = xt::sum(sourcecounts)() / cmfd::norm;
+    double norm = sourcecounts.sum() / cmfd::norm;
     for (int i = 0; i < src_size; i++) {
       if (sourcecounts[i] > 0 && cmfd_src[i] > 0) {
         weightfactors[i] = cmfd_src[i] * norm / sourcecounts[i];
@@ -561,7 +553,7 @@ void free_memory_cmfd()
   cmfd::indices.clear();
   cmfd::egrid.clear();
 
-  // Resize xtensors to be empty
+  // Resize tensors to be empty
   cmfd::indexmap.resize({0});
 
   // Set pointers to null
diff --git a/src/collision_track.cpp b/src/collision_track.cpp
new file mode 100644
index 000000000..2e749007f
--- /dev/null
+++ b/src/collision_track.cpp
@@ -0,0 +1,242 @@
+#include "openmc/collision_track.h"
+
+#include <algorithm>
+#include <string>
+
+#include <fmt/format.h>
+
+#include "openmc/bank.h"
+#include "openmc/bank_io.h"
+#include "openmc/cell.h"
+#include "openmc/constants.h"
+#include "openmc/error.h"
+#include "openmc/file_utils.h"
+#include "openmc/hdf5_interface.h"
+#include "openmc/material.h"
+#include "openmc/mcpl_interface.h"
+#include "openmc/message_passing.h"
+#include "openmc/nuclide.h"
+#include "openmc/output.h"
+#include "openmc/particle.h"
+#include "openmc/settings.h"
+#include "openmc/simulation.h"
+#include "openmc/universe.h"
+
+#ifdef OPENMC_MPI
+#include <mpi.h>
+#endif
+
+namespace openmc {
+
+namespace {
+
+hid_t h5_collision_track_banktype()
+{
+  hid_t postype = H5Tcreate(H5T_COMPOUND, sizeof(Position));
+  H5Tinsert(postype, "x", HOFFSET(Position, x), H5T_NATIVE_DOUBLE);
+  H5Tinsert(postype, "y", HOFFSET(Position, y), H5T_NATIVE_DOUBLE);
+  H5Tinsert(postype, "z", HOFFSET(Position, z), H5T_NATIVE_DOUBLE);
+
+  hid_t banktype = H5Tcreate(H5T_COMPOUND, sizeof(CollisionTrackSite));
+
+  H5Tinsert(banktype, "r", HOFFSET(CollisionTrackSite, r), postype);
+  H5Tinsert(banktype, "u", HOFFSET(CollisionTrackSite, u), postype);
+  H5Tinsert(banktype, "E", HOFFSET(CollisionTrackSite, E), H5T_NATIVE_DOUBLE);
+  H5Tinsert(banktype, "dE", HOFFSET(CollisionTrackSite, dE), H5T_NATIVE_DOUBLE);
+  H5Tinsert(
+    banktype, "time", HOFFSET(CollisionTrackSite, time), H5T_NATIVE_DOUBLE);
+  H5Tinsert(
+    banktype, "wgt", HOFFSET(CollisionTrackSite, wgt), H5T_NATIVE_DOUBLE);
+  H5Tinsert(banktype, "event_mt", HOFFSET(CollisionTrackSite, event_mt),
+    H5T_NATIVE_INT);
+  H5Tinsert(banktype, "delayed_group",
+    HOFFSET(CollisionTrackSite, delayed_group), H5T_NATIVE_INT);
+  H5Tinsert(
+    banktype, "cell_id", HOFFSET(CollisionTrackSite, cell_id), H5T_NATIVE_INT);
+  H5Tinsert(banktype, "nuclide_id", HOFFSET(CollisionTrackSite, nuclide_id),
+    H5T_NATIVE_INT);
+  H5Tinsert(banktype, "material_id", HOFFSET(CollisionTrackSite, material_id),
+    H5T_NATIVE_INT);
+  H5Tinsert(banktype, "universe_id", HOFFSET(CollisionTrackSite, universe_id),
+    H5T_NATIVE_INT);
+  H5Tinsert(banktype, "n_collision", HOFFSET(CollisionTrackSite, n_collision),
+    H5T_NATIVE_INT);
+  H5Tinsert(banktype, "particle", HOFFSET(CollisionTrackSite, particle),
+    H5T_NATIVE_INT);
+  H5Tinsert(banktype, "parent_id", HOFFSET(CollisionTrackSite, parent_id),
+    H5T_NATIVE_INT64);
+  H5Tinsert(banktype, "progeny_id", HOFFSET(CollisionTrackSite, progeny_id),
+    H5T_NATIVE_INT64);
+  H5Tclose(postype);
+  return banktype;
+}
+
+void write_collision_track_bank(hid_t group_id,
+  openmc::span<CollisionTrackSite> collision_track_bank,
+  const openmc::vector<int64_t>& bank_index)
+{
+  hid_t banktype = h5_collision_track_banktype();
+#ifdef OPENMC_MPI
+  write_bank_dataset("collision_track_bank", group_id, collision_track_bank,
+    bank_index, banktype, banktype, mpi::collision_track_site);
+#else
+  write_bank_dataset("collision_track_bank", group_id, collision_track_bank,
+    bank_index, banktype, banktype);
+#endif
+
+  H5Tclose(banktype);
+}
+
+void write_h5_collision_track(const char* filename,
+  openmc::span<CollisionTrackSite> collision_track_bank,
+  const openmc::vector<int64_t>& bank_index)
+{
+#ifdef PHDF5
+  bool parallel = true;
+#else
+  bool parallel = false;
+#endif
+
+  if (!filename)
+    fatal_error("write_h5_collision_track filename needs a nonempty name.");
+
+  std::string filename_(filename);
+  const auto extension = get_file_extension(filename_);
+  if (extension.empty()) {
+    filename_.append(".h5");
+  } else if (extension != "h5") {
+    warning("write_h5_collision_track was passed a file extension differing "
+            "from .h5, but an hdf5 file will be written.");
+  }
+
+  hid_t file_id;
+  if (mpi::master || parallel) {
+    file_id = file_open(filename_.c_str(), 'w', true);
+
+    // Write filetype and version info
+    write_attribute(file_id, "filetype", "collision_track");
+    write_attribute(file_id, "version", VERSION_COLLISION_TRACK);
+  }
+
+  write_collision_track_bank(file_id, collision_track_bank, bank_index);
+
+  if (mpi::master || parallel)
+    file_close(file_id);
+}
+
+} // namespace
+
+bool should_record_event(int id_cell, int mt_event, const std::string& nuclide,
+  int id_universe, int id_material, double energy_loss)
+{
+  auto matches_filter = [](const auto& filter_set, const auto& value) {
+    return filter_set.empty() || filter_set.count(value) > 0;
+  };
+
+  const auto& cfg = settings::collision_track_config;
+  return simulation::current_batch > settings::n_inactive &&
+         !simulation::collision_track_bank.full() &&
+         matches_filter(cfg.cell_ids, id_cell) &&
+         matches_filter(cfg.mt_numbers, mt_event) &&
+         matches_filter(cfg.universe_ids, id_universe) &&
+         matches_filter(cfg.material_ids, id_material) &&
+         matches_filter(cfg.nuclides, nuclide) &&
+         (cfg.deposited_energy_threshold == 0 ||
+           cfg.deposited_energy_threshold < energy_loss);
+}
+
+void collision_track_reserve_bank()
+{
+  simulation::collision_track_bank.reserve(
+    settings::collision_track_config.max_collisions);
+}
+
+void collision_track_flush_bank()
+{
+  const auto& cfg = settings::collision_track_config;
+  if (simulation::ct_current_file > cfg.max_files)
+    return;
+
+  bool last_batch = (simulation::current_batch == settings::n_batches);
+  if (!simulation::collision_track_bank.full() && !last_batch)
+    return;
+
+  auto size = simulation::collision_track_bank.size();
+  if (size == 0 && !last_batch)
+    return;
+
+  auto collision_track_work_index = mpi::calculate_parallel_index_vector(size);
+  openmc::span<CollisionTrackSite> collisiontrackbankspan(
+    simulation::collision_track_bank.begin(), size);
+
+  std::string ext = cfg.mcpl_write ? "mcpl" : "h5";
+  auto filename = fmt::format("{}collision_track.{}.{}", settings::path_output,
+    simulation::ct_current_file, ext);
+
+  if (cfg.max_files == 1 || (simulation::ct_current_file == 1 && last_batch)) {
+    filename = settings::path_output + "collision_track." + ext;
+  }
+  write_message("Creating {}...", filename, 4);
+
+  if (cfg.mcpl_write) {
+    write_mcpl_collision_track(
+      filename.c_str(), collisiontrackbankspan, collision_track_work_index);
+  } else {
+    write_h5_collision_track(
+      filename.c_str(), collisiontrackbankspan, collision_track_work_index);
+  }
+
+  simulation::collision_track_bank.clear();
+  if (!last_batch && cfg.max_files >= 1) {
+    collision_track_reserve_bank();
+  }
+  ++simulation::ct_current_file;
+}
+
+void collision_track_record(Particle& particle)
+{
+  int cell_index = particle.lowest_coord().cell();
+  if (cell_index == C_NONE)
+    return;
+
+  int cell_id = model::cells[cell_index]->id_;
+  std::string nuclide {};
+  int nuclide_id = 0;
+  if (particle.event_nuclide() != NUCLIDE_NONE) {
+    const auto* nuclide_ptr = data::nuclides[particle.event_nuclide()].get();
+    nuclide = nuclide_ptr->name_;
+    nuclide_id = nuclide_ptr->particle_type().pdg_number();
+  }
+  int universe_id = model::universes[particle.lowest_coord().universe()]->id_;
+  double delta_E = particle.E_last() - particle.E();
+  int material_index = particle.material();
+  if (material_index == C_NONE)
+    return;
+
+  int material_id = model::materials[material_index]->id_;
+
+  if (!should_record_event(cell_id, particle.event_mt(), nuclide, universe_id,
+        material_id, delta_E))
+    return;
+
+  CollisionTrackSite site;
+  site.r = particle.r();
+  site.u = particle.u();
+  site.E = particle.E_last();
+  site.dE = delta_E;
+  site.time = particle.time();
+  site.wgt = particle.wgt();
+  site.event_mt = particle.event_mt();
+  site.delayed_group = particle.delayed_group();
+  site.cell_id = cell_id;
+  site.nuclide_id = nuclide_id;
+  site.material_id = material_id;
+  site.universe_id = universe_id;
+  site.n_collision = particle.n_collision();
+  site.particle = particle.type();
+  site.parent_id = particle.id();
+  site.progeny_id = particle.n_progeny();
+  simulation::collision_track_bank.thread_safe_append(site);
+}
+
+} // namespace openmc
diff --git a/src/cross_sections.cpp b/src/cross_sections.cpp
index b1bfde03d..ec9da5b8a 100644
--- a/src/cross_sections.cpp
+++ b/src/cross_sections.cpp
@@ -254,7 +254,7 @@ void read_ce_cross_sections(const vector<vector<double>>& nuc_temps,
   if (settings::photon_transport &&
       settings::electron_treatment == ElectronTreatment::TTB) {
     // Take logarithm of energies since they are log-log interpolated
-    data::ttb_e_grid = xt::log(data::ttb_e_grid);
+    data::ttb_e_grid = tensor::log(data::ttb_e_grid);
   }
 
   // Show minimum/maximum temperature
diff --git a/src/dagmc.cpp b/src/dagmc.cpp
index 134360886..a5cc71b52 100644
--- a/src/dagmc.cpp
+++ b/src/dagmc.cpp
@@ -13,7 +13,7 @@
 #include "openmc/settings.h"
 #include "openmc/string_utils.h"
 
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
 #include "uwuw.hpp"
 #endif
 #include <fmt/core.h>
@@ -26,13 +26,13 @@
 
 namespace openmc {
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 const bool DAGMC_ENABLED = true;
 #else
 const bool DAGMC_ENABLED = false;
 #endif
 
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
 const bool UWUW_ENABLED = true;
 #else
 const bool UWUW_ENABLED = false;
@@ -40,7 +40,7 @@ const bool UWUW_ENABLED = false;
 
 } // namespace openmc
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 
 namespace openmc {
 
@@ -50,6 +50,10 @@ namespace openmc {
 
 DAGUniverse::DAGUniverse(pugi::xml_node node)
 {
+  MaterialOverrides material_overrides;
+  TemperatureOverrides temperature_overrides;
+  DensityOverrides density_overrides;
+
   if (check_for_node(node, "id")) {
     id_ = std::stoi(get_node_value(node, "id"));
   } else {
@@ -76,24 +80,77 @@ DAGUniverse::DAGUniverse(pugi::xml_node node)
     adjust_material_ids_ = get_node_value_bool(node, "auto_mat_ids");
   }
 
-  // get material assignment overloading
-  if (check_for_node(node, "material_overrides")) {
-    auto mat_node = node.child("material_overrides");
-    // loop over all subelements (each subelement corresponds to a material)
-    for (pugi::xml_node cell_node : mat_node.children("cell_override")) {
-      // Store assignment reference name
-      int32_t ref_assignment = std::stoi(get_node_value(cell_node, "id"));
+  // Get material assignment overrides from nested DAGMC cell elements.
+  if (node.child("cell")) {
+    for (pugi::xml_node cell_node : node.children("cell")) {
+      if (!check_for_node(cell_node, "id")) {
+        fatal_error(
+          "Must specify id for each DAGMC cell override in <dagmc_universe>.");
+      }
 
-      // Get mat name for each assignement instances
-      vector<int32_t> instance_mats =
-        get_node_array<int32_t>(cell_node, "material_ids");
+      int32_t cell_id = std::stoi(get_node_value(cell_node, "id"));
 
-      // Store mat name for each instances
-      material_overrides_.emplace(ref_assignment, instance_mats);
+      if (check_for_node(cell_node, "region")) {
+        fatal_error(fmt::format(
+          "DAGMC cell {} override cannot specify a region.", cell_id));
+      }
+      if (check_for_node(cell_node, "fill")) {
+        fatal_error(fmt::format(
+          "DAGMC cell {} override currently only supports material fills.",
+          cell_id));
+      }
+      if (check_for_node(cell_node, "universe")) {
+        fatal_error(fmt::format(
+          "DAGMC cell {} override cannot specify a universe.", cell_id));
+      }
+      if (check_for_node(cell_node, "translation") ||
+          check_for_node(cell_node, "rotation")) {
+        fatal_error(fmt::format(
+          "DAGMC cell {} override does not support translation or rotation.",
+          cell_id));
+      }
+      if (!check_for_node(cell_node, "material")) {
+        fatal_error(fmt::format(
+          "DAGMC cell {} override must specify material.", cell_id));
+      }
+
+      auto inserted = material_overrides.emplace(
+        cell_id, parse_cell_material_xml(cell_node, cell_id));
+      if (!inserted.second) {
+        fatal_error(fmt::format(
+          "Duplicate DAGMC cell override specified for cell {}", cell_id));
+      }
+
+      if (check_for_node(cell_node, "temperature")) {
+        temperature_overrides.emplace(
+          cell_id, parse_cell_temperature_xml(cell_node, cell_id));
+      }
+
+      if (check_for_node(cell_node, "density")) {
+        density_overrides.emplace(
+          cell_id, parse_cell_density_xml(cell_node, cell_id));
+      }
+    }
+  } else if (check_for_node(node, "material_overrides")) {
+    if (node.child("cell")) {
+      fatal_error("DAGMCUniverse cannot specify both <material_overrides> and "
+                  "<cell> sub-elements. Use <cell> elements only.");
+    }
+    warning("DAGMCUniverse <material_overrides> is deprecated. Use nested "
+            "<cell> elements under <dagmc_universe> instead.");
+    for (pugi::xml_node co :
+      node.child("material_overrides").children("cell_override")) {
+      int32_t cell_id = std::stoi(get_node_value(co, "id"));
+      std::istringstream iss(co.child("material_ids").text().get());
+      vector<int32_t> mats;
+      for (std::string s; iss >> s;) {
+        mats.push_back(s == "void" ? MATERIAL_VOID : std::stoi(s));
+      }
+      material_overrides.emplace(cell_id, mats);
     }
   }
 
-  initialize();
+  initialize(material_overrides, temperature_overrides, density_overrides);
 }
 
 DAGUniverse::DAGUniverse(
@@ -110,9 +167,12 @@ DAGUniverse::DAGUniverse(std::shared_ptr<moab::DagMC> dagmc_ptr,
   : dagmc_instance_(dagmc_ptr), filename_(filename),
     adjust_geometry_ids_(auto_geom_ids), adjust_material_ids_(auto_mat_ids)
 {
+  MaterialOverrides material_overrides;
+  TemperatureOverrides temperature_overrides;
+  DensityOverrides density_overrides;
   set_id();
   init_metadata();
-  init_geometry();
+  init_geometry(material_overrides, temperature_overrides, density_overrides);
 }
 
 void DAGUniverse::set_id()
@@ -131,7 +191,16 @@ void DAGUniverse::set_id()
 
 void DAGUniverse::initialize()
 {
-#ifdef OPENMC_UWUW
+  MaterialOverrides material_overrides;
+  TemperatureOverrides temperature_overrides;
+  initialize(material_overrides, temperature_overrides);
+}
+
+void DAGUniverse::initialize(const MaterialOverrides& material_overrides,
+  const TemperatureOverrides& temperature_overrides,
+  const DensityOverrides& density_overrides)
+{
+#ifdef OPENMC_UWUW_ENABLED
   // read uwuw materials from the .h5m file if present
   read_uwuw_materials();
 #endif
@@ -140,7 +209,7 @@ void DAGUniverse::initialize()
 
   init_metadata();
 
-  init_geometry();
+  init_geometry(material_overrides, temperature_overrides, density_overrides);
 }
 
 void DAGUniverse::init_dagmc()
@@ -176,7 +245,9 @@ void DAGUniverse::init_metadata()
   MB_CHK_ERR_CONT(rval);
 }
 
-void DAGUniverse::init_geometry()
+void DAGUniverse::init_geometry(const MaterialOverrides& material_overrides,
+  const TemperatureOverrides& temperature_overrides,
+  const DensityOverrides& density_overrides)
 {
   moab::ErrorCode rval;
 
@@ -202,6 +273,9 @@ void DAGUniverse::init_geometry()
                : dagmc_instance_->id_by_index(3, c->dag_index());
     c->universe_ = this->id_;
     c->fill_ = C_NONE; // no fill, single universe
+    if (dagmc_instance_->is_implicit_complement(vol_handle)) {
+      c->name_ = "implicit complement";
+    }
 
     auto in_map = model::cell_map.find(c->id_);
     if (in_map == model::cell_map.end()) {
@@ -230,16 +304,68 @@ void DAGUniverse::init_geometry()
     if (mat_str == "graveyard") {
       graveyard = vol_handle;
     }
-    // material void checks
-    if (mat_str == "void" || mat_str == "vacuum" || mat_str == "graveyard") {
+    if (material_overrides.count(c->id_)) {
+      override_assign_material(c, material_overrides);
+    } else if (mat_str == "void" || mat_str == "vacuum" ||
+               mat_str == "graveyard") {
       c->material_.push_back(MATERIAL_VOID);
+    } else if (uses_uwuw()) {
+      uwuw_assign_material(vol_handle, c);
     } else {
-      if (material_overrides_.count(c->id_)) {
-        override_assign_material(c);
-      } else if (uses_uwuw()) {
-        uwuw_assign_material(vol_handle, c);
-      } else {
-        legacy_assign_material(mat_str, c);
+      legacy_assign_material(mat_str, c);
+    }
+
+    if (temperature_overrides.count(c->id_)) {
+      if (c->material_.empty() || c->material_[0] == MATERIAL_VOID) {
+        fatal_error(fmt::format("DAGMC cell {} was specified with a "
+                                "temperature but no non-void material.",
+          c->id_));
+      }
+
+      c->sqrtkT_.clear();
+      const auto& temp_overrides = temperature_overrides.at(c->id_);
+      c->sqrtkT_.reserve(temp_overrides.size());
+      for (auto T : temp_overrides) {
+        c->sqrtkT_.push_back(std::sqrt(K_BOLTZMANN * T));
+      }
+
+      if (settings::verbosity >= 10) {
+        std::stringstream override_values;
+        for (size_t i = 0; i < temp_overrides.size(); ++i) {
+          if (i > 0) {
+            override_values << " ";
+          }
+          override_values << temp_overrides[i];
+        }
+        auto msg = fmt::format("Overriding DAGMC cell {} property "
+                               "'temperature [K]' with value(s): {}",
+          c->id_, override_values.str());
+        write_message(msg, 10);
+      }
+    }
+
+    if (density_overrides.count(c->id_)) {
+      if (c->material_.empty() || c->material_[0] == MATERIAL_VOID) {
+        fatal_error(fmt::format("DAGMC cell {} was specified with a density "
+                                "but no non-void material.",
+          c->id_));
+      }
+      // density_mult_ holds the true density until materials are finalized,
+      // at which point it is converted to a proper multiplier (same as CSG).
+      c->density_mult_ = density_overrides.at(c->id_);
+
+      if (settings::verbosity >= 10) {
+        const auto& dens = density_overrides.at(c->id_);
+        std::stringstream override_values;
+        for (size_t i = 0; i < dens.size(); ++i) {
+          if (i > 0)
+            override_values << " ";
+          override_values << dens[i];
+        }
+        write_message(fmt::format("Overriding DAGMC cell {} property "
+                                  "'density [g/cm³]' with value(s): {}",
+                        c->id_, override_values.str()),
+          10);
       }
     }
 
@@ -252,18 +378,21 @@ void DAGUniverse::init_geometry()
       continue;
     }
 
-    // assign cell temperature
-    const auto& mat = model::materials[model::material_map.at(c->material_[0])];
-    if (dagmc_instance_->has_prop(vol_handle, "temp")) {
-      rval = dagmc_instance_->prop_value(vol_handle, "temp", temp_value);
-      MB_CHK_ERR_CONT(rval);
-      double temp = std::stod(temp_value);
-      c->sqrtkT_.push_back(std::sqrt(K_BOLTZMANN * temp));
-    } else if (mat->temperature() > 0.0) {
-      c->sqrtkT_.push_back(std::sqrt(K_BOLTZMANN * mat->temperature()));
-    } else {
-      c->sqrtkT_.push_back(
-        std::sqrt(K_BOLTZMANN * settings::temperature_default));
+    // assign cell temperature if not explicitly overridden
+    if (c->sqrtkT_.empty()) {
+      const auto& mat =
+        model::materials[model::material_map.at(c->material_[0])];
+      if (dagmc_instance_->has_prop(vol_handle, "temp")) {
+        rval = dagmc_instance_->prop_value(vol_handle, "temp", temp_value);
+        MB_CHK_ERR_CONT(rval);
+        double temp = std::stod(temp_value);
+        c->sqrtkT_.push_back(std::sqrt(K_BOLTZMANN * temp));
+      } else if (mat->temperature() > 0.0) {
+        c->sqrtkT_.push_back(std::sqrt(K_BOLTZMANN * mat->temperature()));
+      } else {
+        c->sqrtkT_.push_back(
+          std::sqrt(K_BOLTZMANN * settings::temperature_default));
+      }
     }
 
     model::cells.emplace_back(std::move(c));
@@ -430,7 +559,7 @@ bool DAGUniverse::find_cell(GeometryState& p) const
   // cells, place it in the implicit complement
   bool found = Universe::find_cell(p);
   if (!found && model::universe_map[this->id_] != model::root_universe) {
-    p.lowest_coord().cell = implicit_complement_idx();
+    p.lowest_coord().cell() = implicit_complement_idx();
     found = true;
   }
   return found;
@@ -456,16 +585,16 @@ void DAGUniverse::to_hdf5(hid_t universes_group) const
 
 bool DAGUniverse::uses_uwuw() const
 {
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
   return uwuw_ && !uwuw_->material_library.empty();
 #else
   return false;
-#endif // OPENMC_UWUW
+#endif // OPENMC_UWUW_ENABLED
 }
 
 std::string DAGUniverse::get_uwuw_materials_xml() const
 {
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
   if (!uses_uwuw()) {
     throw std::runtime_error("This DAGMC Universe does not use UWUW materials");
   }
@@ -485,12 +614,12 @@ std::string DAGUniverse::get_uwuw_materials_xml() const
   return ss.str();
 #else
   fatal_error("DAGMC was not configured with UWUW.");
-#endif // OPENMC_UWUW
+#endif // OPENMC_UWUW_ENABLED
 }
 
 void DAGUniverse::write_uwuw_materials_xml(const std::string& outfile) const
 {
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
   if (!uses_uwuw()) {
     throw std::runtime_error(
       "This DAGMC universe does not use UWUW materials.");
@@ -503,7 +632,7 @@ void DAGUniverse::write_uwuw_materials_xml(const std::string& outfile) const
   mats_xml.close();
 #else
   fatal_error("DAGMC was not configured with UWUW.");
-#endif // OPENMC_UWUW
+#endif // OPENMC_UWUW_ENABLED
 }
 
 void DAGUniverse::legacy_assign_material(
@@ -565,7 +694,7 @@ void DAGUniverse::legacy_assign_material(
 
 void DAGUniverse::read_uwuw_materials()
 {
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
   // If no filename was provided, don't read UWUW materials
   if (filename_ == "")
     return;
@@ -605,13 +734,13 @@ void DAGUniverse::read_uwuw_materials()
   }
 #else
   fatal_error("DAGMC was not configured with UWUW.");
-#endif // OPENMC_UWUW
+#endif // OPENMC_UWUW_ENABLED
 }
 
 void DAGUniverse::uwuw_assign_material(
   moab::EntityHandle vol_handle, std::unique_ptr<DAGCell>& c) const
 {
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
   // lookup material in uwuw if present
   std::string uwuw_mat = dmd_ptr->volume_material_property_data_eh[vol_handle];
   if (uwuw_->material_library.count(uwuw_mat) != 0) {
@@ -627,10 +756,11 @@ void DAGUniverse::uwuw_assign_material(
   }
 #else
   fatal_error("DAGMC was not configured with UWUW.");
-#endif // OPENMC_UWUW
+#endif // OPENMC_UWUW_ENABLED
 }
 
-void DAGUniverse::override_assign_material(std::unique_ptr<DAGCell>& c) const
+void DAGUniverse::override_assign_material(std::unique_ptr<DAGCell>& c,
+  const MaterialOverrides& material_overrides) const
 {
   // if Cell ID matches an override key, use it to override the material
   // assignment else if UWUW is used, get the material assignment from the DAGMC
@@ -638,17 +768,30 @@ void DAGUniverse::override_assign_material(std::unique_ptr<DAGCell>& c) const
   // Notify User that an override is being applied on a DAGMCCell
   write_message(fmt::format("Applying override for DAGMCCell {}", c->id_), 8);
 
+  const auto& mat_overrides = material_overrides.at(c->id_);
   if (settings::verbosity >= 10) {
-    auto msg = fmt::format("Assigning DAGMC cell {} material(s) based on "
-                           "override information (see input XML).",
-      c->id_);
+    std::stringstream override_values;
+    for (size_t i = 0; i < mat_overrides.size(); ++i) {
+      if (i > 0) {
+        override_values << " ";
+      }
+      if (mat_overrides[i] == MATERIAL_VOID) {
+        override_values << "void";
+      } else {
+        override_values << mat_overrides[i];
+      }
+    }
+    auto msg = fmt::format("Overriding DAGMC cell {} property 'material' "
+                           "with value(s): {}",
+      c->id_, override_values.str());
     write_message(msg, 10);
   }
 
   // Override the material assignment for each cell instance using the legacy
   // assignement
-  for (auto mat_id : material_overrides_.at(c->id_)) {
-    if (model::material_map.find(mat_id) == model::material_map.end()) {
+  for (auto mat_id : mat_overrides) {
+    if (mat_id != MATERIAL_VOID &&
+        model::material_map.find(mat_id) == model::material_map.end()) {
       fatal_error(fmt::format(
         "Material with ID '{}' not found for DAGMC cell {}", mat_id, c->id_));
     }
@@ -672,11 +815,11 @@ std::pair<double, int32_t> DAGCell::distance(
     p->last_dir() = u;
     p->history().reset();
   }
-  if (on_surface == 0) {
+  if (on_surface == SURFACE_NONE) {
     p->history().reset();
   }
 
-  const auto& univ = model::universes[p->lowest_coord().universe];
+  const auto& univ = model::universes[p->lowest_coord().universe()];
 
   DAGUniverse* dag_univ = static_cast<DAGUniverse*>(univ.get());
   if (!dag_univ)
@@ -753,7 +896,7 @@ BoundingBox DAGCell::bounding_box() const
   double min[3], max[3];
   rval = dagmc_ptr_->getobb(vol, min, max);
   MB_CHK_ERR_CONT(rval);
-  return {min[0], max[0], min[1], max[1], min[2], max[2]};
+  return {{min[0], min[1], min[2]}, {max[0], max[1], max[2]}};
 }
 
 //==============================================================================
@@ -926,4 +1069,4 @@ int32_t next_cell(int32_t surf, int32_t curr_cell, int32_t univ);
 
 } // namespace openmc
 
-#endif // DAGMC
+#endif // OPENMC_DAGMC_ENABLED
diff --git a/src/distribution.cpp b/src/distribution.cpp
index ca00cbde4..cbdd13127 100644
--- a/src/distribution.cpp
+++ b/src/distribution.cpp
@@ -7,15 +7,90 @@
 #include <numeric>   // for accumulate
 #include <stdexcept> // for runtime_error
 #include <string>    // for string, stod
+#include <unordered_set>
 
+#include "openmc/chain.h"
+#include "openmc/constants.h"
 #include "openmc/error.h"
 #include "openmc/math_functions.h"
 #include "openmc/random_dist.h"
 #include "openmc/random_lcg.h"
 #include "openmc/xml_interface.h"
 
+namespace {
+std::unordered_set<std::string> decay_spectrum_missing_chain_nuclides;
+}
+
 namespace openmc {
 
+//==============================================================================
+// Helper function for computing importance weights from biased sampling
+//==============================================================================
+
+vector<double> compute_importance_weights(
+  const vector<double>& p, const vector<double>& b)
+{
+  std::size_t n = p.size();
+
+  // Normalize original probabilities
+  double sum_p = std::accumulate(p.begin(), p.end(), 0.0);
+  vector<double> p_norm(n);
+  for (std::size_t i = 0; i < n; ++i) {
+    p_norm[i] = p[i] / sum_p;
+  }
+
+  // Normalize bias probabilities
+  double sum_b = std::accumulate(b.begin(), b.end(), 0.0);
+  vector<double> b_norm(n);
+  for (std::size_t i = 0; i < n; ++i) {
+    b_norm[i] = b[i] / sum_b;
+  }
+
+  // Compute importance weights
+  vector<double> weights(n);
+  for (std::size_t i = 0; i < n; ++i) {
+    weights[i] = (b_norm[i] == 0.0) ? INFTY : p_norm[i] / b_norm[i];
+  }
+  return weights;
+}
+
+std::pair<double, double> Distribution::sample(uint64_t* seed) const
+{
+  if (bias_) {
+    // Sample from the bias distribution and compute importance weight
+    double val = bias_->sample_unbiased(seed);
+    double wgt = this->evaluate(val) / bias_->evaluate(val);
+    return {val, wgt};
+  } else {
+    // Unbiased sampling: return sampled value with weight 1.0
+    double val = sample_unbiased(seed);
+    return {val, 1.0};
+  }
+}
+
+// PDF evaluation not supported for all distribution types
+double Distribution::evaluate(double x) const
+{
+  throw std::runtime_error(
+    "PDF evaluation not implemented for this distribution type.");
+}
+
+void Distribution::read_bias_from_xml(pugi::xml_node node)
+{
+  if (check_for_node(node, "bias")) {
+    pugi::xml_node bias_node = node.child("bias");
+
+    if (check_for_node(bias_node, "bias")) {
+      openmc::fatal_error(
+        "Distribution has a bias distribution with its own bias distribution. "
+        "Please ensure bias distributions do not have their own bias.");
+    }
+
+    UPtrDist bias = distribution_from_xml(bias_node);
+    this->set_bias(std::move(bias));
+  }
+}
+
 //==============================================================================
 // DiscreteIndex implementation
 //==============================================================================
@@ -36,7 +111,6 @@ DiscreteIndex::DiscreteIndex(span<const double> p)
 void DiscreteIndex::assign(span<const double> p)
 {
   prob_.assign(p.begin(), p.end());
-
   this->init_alias();
 }
 
@@ -115,24 +189,55 @@ void DiscreteIndex::normalize()
 // Discrete implementation
 //==============================================================================
 
-Discrete::Discrete(pugi::xml_node node) : di_(node)
+Discrete::Discrete(pugi::xml_node node)
 {
   auto params = get_node_array<double>(node, "parameters");
-
   std::size_t n = params.size() / 2;
 
+  // First half is x values, second half is probabilities
   x_.assign(params.begin(), params.begin() + n);
+  const double* p = params.data() + n;
+
+  // Check for bias
+  if (check_for_node(node, "bias")) {
+    // Get bias probabilities
+    auto bias_params = get_node_array<double>(node, "bias");
+    if (bias_params.size() != n) {
+      openmc::fatal_error(
+        "Size mismatch: Attempted to bias Discrete distribution with " +
+        std::to_string(n) + " probability entries using a bias with " +
+        std::to_string(bias_params.size()) +
+        " entries. Please ensure distributions have the same size.");
+    }
+
+    // Compute importance weights
+    vector<double> p_vec(p, p + n);
+    weight_ = compute_importance_weights(p_vec, bias_params);
+
+    // Initialize DiscreteIndex with bias probabilities for sampling
+    di_.assign(bias_params);
+  } else {
+    // Unbiased case: weight_ stays empty
+    di_.assign({p, n});
+  }
 }
 
 Discrete::Discrete(const double* x, const double* p, size_t n) : di_({p, n})
 {
-
   x_.assign(x, x + n);
 }
 
-double Discrete::sample(uint64_t* seed) const
+std::pair<double, double> Discrete::sample(uint64_t* seed) const
 {
-  return x_[di_.sample(seed)];
+  size_t idx = di_.sample(seed);
+  double wgt = weight_.empty() ? 1.0 : weight_[idx];
+  return {x_[idx], wgt};
+}
+
+double Discrete::sample_unbiased(uint64_t* seed) const
+{
+  size_t idx = di_.sample(seed);
+  return x_[idx];
 }
 
 //==============================================================================
@@ -149,13 +254,26 @@ Uniform::Uniform(pugi::xml_node node)
 
   a_ = params.at(0);
   b_ = params.at(1);
+
+  read_bias_from_xml(node);
 }
 
-double Uniform::sample(uint64_t* seed) const
+double Uniform::sample_unbiased(uint64_t* seed) const
 {
   return a_ + prn(seed) * (b_ - a_);
 }
 
+double Uniform::evaluate(double x) const
+{
+  if (x <= a()) {
+    return 0.0;
+  } else if (x >= b()) {
+    return 0.0;
+  } else {
+    return 1 / (b() - a());
+  }
+}
+
 //==============================================================================
 // PowerLaw implementation
 //==============================================================================
@@ -175,9 +293,24 @@ PowerLaw::PowerLaw(pugi::xml_node node)
   offset_ = std::pow(a, n + 1);
   span_ = std::pow(b, n + 1) - offset_;
   ninv_ = 1 / (n + 1);
+
+  read_bias_from_xml(node);
 }
 
-double PowerLaw::sample(uint64_t* seed) const
+double PowerLaw::evaluate(double x) const
+{
+  if (x <= a()) {
+    return 0.0;
+  } else if (x >= b()) {
+    return 0.0;
+  } else {
+    int pwr = n() + 1;
+    double norm = pwr / span_;
+    return norm * std::pow(std::fabs(x), n());
+  }
+}
+
+double PowerLaw::sample_unbiased(uint64_t* seed) const
 {
   return std::pow(offset_ + prn(seed) * span_, ninv_);
 }
@@ -189,13 +322,21 @@ double PowerLaw::sample(uint64_t* seed) const
 Maxwell::Maxwell(pugi::xml_node node)
 {
   theta_ = std::stod(get_node_value(node, "parameters"));
+
+  read_bias_from_xml(node);
 }
 
-double Maxwell::sample(uint64_t* seed) const
+double Maxwell::sample_unbiased(uint64_t* seed) const
 {
   return maxwell_spectrum(theta_, seed);
 }
 
+double Maxwell::evaluate(double x) const
+{
+  double c = (2.0 / SQRT_PI) * std::pow(theta_, -1.5);
+  return c * std::sqrt(x) * std::exp(-x / theta_);
+}
+
 //==============================================================================
 // Watt implementation
 //==============================================================================
@@ -209,31 +350,111 @@ Watt::Watt(pugi::xml_node node)
 
   a_ = params.at(0);
   b_ = params.at(1);
+
+  read_bias_from_xml(node);
 }
 
-double Watt::sample(uint64_t* seed) const
+double Watt::sample_unbiased(uint64_t* seed) const
 {
   return watt_spectrum(a_, b_, seed);
 }
 
+double Watt::evaluate(double x) const
+{
+  double c =
+    2.0 / (std::sqrt(PI * b_) * std::pow(a_, 1.5) * std::exp(a_ * b_ / 4.0));
+  return c * std::exp(-x / a_) * std::sinh(std::sqrt(b_ * x));
+}
+
 //==============================================================================
 // Normal implementation
 //==============================================================================
+
+Normal::Normal(double mean_value, double std_dev, double lower, double upper)
+  : mean_value_ {mean_value}, std_dev_ {std_dev}, lower_ {lower}, upper_ {upper}
+{
+  compute_normalization();
+}
+
 Normal::Normal(pugi::xml_node node)
 {
   auto params = get_node_array<double>(node, "parameters");
-  if (params.size() != 2) {
+  if (params.size() != 2 && params.size() != 4) {
     openmc::fatal_error("Normal energy distribution must have two "
-                        "parameters specified.");
+                        "parameters (mean, std_dev) or four parameters "
+                        "(mean, std_dev, lower, upper) specified.");
   }
 
   mean_value_ = params.at(0);
   std_dev_ = params.at(1);
+
+  // Optional truncation bounds
+  if (params.size() == 4) {
+    lower_ = params.at(2);
+    upper_ = params.at(3);
+  } else {
+    lower_ = -INFTY;
+    upper_ = INFTY;
+  }
+
+  compute_normalization();
+  read_bias_from_xml(node);
 }
 
-double Normal::sample(uint64_t* seed) const
+void Normal::compute_normalization()
 {
-  return normal_variate(mean_value_, std_dev_, seed);
+  // Validate bounds
+  if (lower_ >= upper_) {
+    openmc::fatal_error(
+      "Normal distribution lower bound must be less than upper bound.");
+  }
+
+  // Check if truncation bounds are finite
+  is_truncated_ = (lower_ > -INFTY || upper_ < INFTY);
+
+  if (is_truncated_) {
+    double alpha = (lower_ - mean_value_) / std_dev_;
+    double beta = (upper_ - mean_value_) / std_dev_;
+    double cdf_diff = standard_normal_cdf(beta) - standard_normal_cdf(alpha);
+
+    if (cdf_diff <= 0.0) {
+      openmc::fatal_error(
+        "Normal distribution truncation bounds exclude entire distribution.");
+    }
+    norm_factor_ = 1.0 / cdf_diff;
+  } else {
+    norm_factor_ = 1.0;
+  }
+}
+
+double Normal::sample_unbiased(uint64_t* seed) const
+{
+  if (!is_truncated_) {
+    return normal_variate(mean_value_, std_dev_, seed);
+  }
+
+  // Rejection sampling for truncated normal
+  double x;
+  do {
+    x = normal_variate(mean_value_, std_dev_, seed);
+  } while (x < lower_ || x > upper_);
+  return x;
+}
+
+double Normal::evaluate(double x) const
+{
+  // Return 0 outside truncation bounds
+  if (x < lower_ || x > upper_) {
+    return 0.0;
+  }
+
+  // Standard normal PDF value
+  double pdf = (1.0 / (std::sqrt(2.0 * PI) * std_dev_)) *
+               std::exp(-std::pow((x - mean_value_), 2.0) /
+                        (2.0 * std::pow(std_dev_, 2.0)));
+
+  // Apply normalization for truncation
+  return pdf * norm_factor_;
 }
 
 //==============================================================================
@@ -248,6 +469,10 @@ Tabular::Tabular(pugi::xml_node node)
       interp_ = Interpolation::histogram;
     } else if (temp == "linear-linear") {
       interp_ = Interpolation::lin_lin;
+    } else if (temp == "log-linear") {
+      interp_ = Interpolation::log_lin;
+    } else if (temp == "log-log") {
+      interp_ = Interpolation::log_log;
     } else {
       openmc::fatal_error(
         "Unsupported interpolation type for distribution: " + temp);
@@ -266,6 +491,8 @@ Tabular::Tabular(pugi::xml_node node)
   const double* x = params.data();
   const double* p = x + n;
   init(x, p, n);
+
+  read_bias_from_xml(node);
 }
 
 Tabular::Tabular(const double* x, const double* p, int n, Interpolation interp,
@@ -282,13 +509,6 @@ void Tabular::init(
   std::copy(x, x + n, std::back_inserter(x_));
   std::copy(p, p + n, std::back_inserter(p_));
 
-  // Check interpolation parameter
-  if (interp_ != Interpolation::histogram &&
-      interp_ != Interpolation::lin_lin) {
-    openmc::fatal_error("Only histogram and linear-linear interpolation "
-                        "for tabular distribution is supported.");
-  }
-
   // Calculate cumulative distribution function
   if (c) {
     std::copy(c, c + n, std::back_inserter(c_));
@@ -300,6 +520,18 @@ void Tabular::init(
         c_[i] = c_[i - 1] + p_[i - 1] * (x_[i] - x_[i - 1]);
       } else if (interp_ == Interpolation::lin_lin) {
         c_[i] = c_[i - 1] + 0.5 * (p_[i - 1] + p_[i]) * (x_[i] - x_[i - 1]);
+      } else if (interp_ == Interpolation::log_lin) {
+        double m = std::log(p_[i] / p_[i - 1]) / (x_[i] - x_[i - 1]);
+        c_[i] = c_[i - 1] + p_[i - 1] * (x_[i] - x_[i - 1]) *
+                              exprel(m * (x_[i] - x_[i - 1]));
+      } else if (interp_ == Interpolation::log_log) {
+        double m = std::log((x_[i] * p_[i]) / (x_[i - 1] * p_[i - 1])) /
+                   std::log(x_[i] / x_[i - 1]);
+        c_[i] = c_[i - 1] + x_[i - 1] * p_[i - 1] *
+                              std::log(x_[i] / x_[i - 1]) *
+                              exprel(m * std::log(x_[i] / x_[i - 1]));
+      } else {
+        UNREACHABLE();
       }
     }
   }
@@ -314,20 +546,15 @@ void Tabular::init(
   }
 }
 
-double Tabular::sample(uint64_t* seed) const
+double Tabular::sample_unbiased(uint64_t* seed) const
 {
   // Sample value of CDF
   double c = prn(seed);
 
   // Find first CDF bin which is above the sampled value
-  double c_i = c_[0];
-  int i;
-  std::size_t n = c_.size();
-  for (i = 0; i < n - 1; ++i) {
-    if (c <= c_[i + 1])
-      break;
-    c_i = c_[i + 1];
-  }
+  auto c_iter = std::lower_bound(c_.begin() + 1, c_.end(), c);
+  int i = std::distance(c_.begin(), c_iter) - 1;
+  double c_i = c_[i];
 
   // Determine bounding PDF values
   double x_i = x_[i];
@@ -340,7 +567,7 @@ double Tabular::sample(uint64_t* seed) const
     } else {
       return x_i;
     }
-  } else {
+  } else if (interp_ == Interpolation::lin_lin) {
     // Linear-linear interpolation
     double x_i1 = x_[i + 1];
     double p_i1 = p_[i + 1];
@@ -353,6 +580,52 @@ double Tabular::sample(uint64_t* seed) const
              (std::sqrt(std::max(0.0, p_i * p_i + 2 * m * (c - c_i))) - p_i) /
                m;
     }
+  } else if (interp_ == Interpolation::log_lin) {
+    // Log-linear interpolation
+    double x_i1 = x_[i + 1];
+    double p_i1 = p_[i + 1];
+
+    double m = std::log(p_i1 / p_i) / (x_i1 - x_i);
+    double f = (c - c_i) / p_i;
+    return x_i + f * log1prel(m * f);
+  } else if (interp_ == Interpolation::log_log) {
+    // Log-Log interpolation
+    double x_i1 = x_[i + 1];
+    double p_i1 = p_[i + 1];
+
+    double m = std::log((x_i1 * p_i1) / (x_i * p_i)) / std::log(x_i1 / x_i);
+    double f = (c - c_i) / (p_i * x_i);
+    return x_i * std::exp(f * log1prel(m * f));
+  } else {
+    UNREACHABLE();
+  }
+}
+
+double Tabular::evaluate(double x) const
+{
+  int i;
+
+  if (interp_ == Interpolation::histogram) {
+    i = std::upper_bound(x_.begin(), x_.end(), x) - x_.begin() - 1;
+    if (i < 0 || i >= static_cast<int>(p_.size())) {
+      return 0.0;
+    } else {
+      return p_[i];
+    }
+  } else {
+    i = std::lower_bound(x_.begin(), x_.end(), x) - x_.begin() - 1;
+
+    if (i < 0 || i >= static_cast<int>(p_.size()) - 1) {
+      return 0.0;
+    } else {
+      double x0 = x_[i];
+      double x1 = x_[i + 1];
+      double p0 = p_[i];
+      double p1 = p_[i + 1];
+
+      double t = (x - x0) / (x1 - x0);
+      return (1 - t) * p0 + t * p1;
+    }
   }
 }
 
@@ -360,7 +633,7 @@ double Tabular::sample(uint64_t* seed) const
 // Equiprobable implementation
 //==============================================================================
 
-double Equiprobable::sample(uint64_t* seed) const
+double Equiprobable::sample_unbiased(uint64_t* seed) const
 {
   std::size_t n = x_.size();
 
@@ -372,13 +645,27 @@ double Equiprobable::sample(uint64_t* seed) const
   return xl + ((n - 1) * r - i) * (xr - xl);
 }
 
+double Equiprobable::evaluate(double x) const
+{
+  double x_min = *std::min_element(x_.begin(), x_.end());
+  double x_max = *std::max_element(x_.begin(), x_.end());
+
+  if (x < x_min || x > x_max) {
+    return 0.0;
+  } else {
+    return 1.0 / (x_max - x_min);
+  }
+}
+
 //==============================================================================
 // Mixture implementation
 //==============================================================================
 
 Mixture::Mixture(pugi::xml_node node)
 {
-  double cumsum = 0.0;
+  vector<double> probabilities;
+
+  // First pass: collect distributions and their probabilities
   for (pugi::xml_node pair : node.children("pair")) {
     // Check that required data exists
     if (!pair.attribute("probability"))
@@ -386,39 +673,60 @@ Mixture::Mixture(pugi::xml_node node)
     if (!pair.child("dist"))
       fatal_error("Mixture pair element does not have a distribution.");
 
-    // cummulative sum of probabilities
+    // Get probability and distribution
     double p = std::stod(pair.attribute("probability").value());
-
-    // Save cummulative probability and distribution
     auto dist = distribution_from_xml(pair.child("dist"));
-    cumsum += p * dist->integral();
 
-    distribution_.push_back(std::make_pair(cumsum, std::move(dist)));
+    // Weight probability by the distribution's integral
+    double weighted_prob = p * dist->integral();
+    probabilities.push_back(weighted_prob);
+    distribution_.push_back(std::move(dist));
   }
 
-  // Save integral of distribution
-  integral_ = cumsum;
+  // Save sum of weighted probabilities
+  integral_ = std::accumulate(probabilities.begin(), probabilities.end(), 0.0);
 
-  // Normalize cummulative probabilities to 1
-  for (auto& pair : distribution_) {
-    pair.first /= cumsum;
+  std::size_t n = probabilities.size();
+
+  // Check for bias
+  if (check_for_node(node, "bias")) {
+    // Get bias probabilities
+    auto bias_params = get_node_array<double>(node, "bias");
+    if (bias_params.size() != n) {
+      openmc::fatal_error(
+        "Size mismatch: Attempted to bias Mixture distribution with " +
+        std::to_string(n) + " components using a bias with " +
+        std::to_string(bias_params.size()) +
+        " entries. Please ensure distributions have the same size.");
+    }
+
+    // Compute importance weights
+    weight_ = compute_importance_weights(probabilities, bias_params);
+
+    // Initialize DiscreteIndex with bias probabilities for sampling
+    di_.assign(bias_params);
+  } else {
+    // Unbiased case: weight_ stays empty
+    di_.assign(probabilities);
   }
 }
 
-double Mixture::sample(uint64_t* seed) const
+std::pair<double, double> Mixture::sample(uint64_t* seed) const
 {
-  // Sample value of CDF
-  const double p = prn(seed);
-
-  // find matching distribution
-  const auto it = std::lower_bound(distribution_.cbegin(), distribution_.cend(),
-    p, [](const DistPair& pair, double p) { return pair.first < p; });
-
-  // This should not happen. Catch it
-  assert(it != distribution_.cend());
+  size_t idx = di_.sample(seed);
 
   // Sample the chosen distribution
-  return it->second->sample(seed);
+  auto [val, sub_wgt] = distribution_[idx]->sample(seed);
+
+  // Multiply by component selection weight
+  double mix_wgt = weight_.empty() ? 1.0 : weight_[idx];
+  return {val, mix_wgt * sub_wgt};
+}
+
+double Mixture::sample_unbiased(uint64_t* seed) const
+{
+  size_t idx = di_.sample(seed);
+  return distribution_[idx]->sample(seed).first;
 }
 
 //==============================================================================
@@ -451,6 +759,8 @@ UPtrDist distribution_from_xml(pugi::xml_node node)
     dist = UPtrDist {new Tabular(node)};
   } else if (type == "mixture") {
     dist = UPtrDist {new Mixture(node)};
+  } else if (type == "decay_spectrum") {
+    dist = UPtrDist {new DecaySpectrum(node)};
   } else if (type == "muir") {
     openmc::fatal_error(
       "'muir' distributions are now specified using the openmc.stats.muir() "
@@ -461,4 +771,120 @@ UPtrDist distribution_from_xml(pugi::xml_node node)
   return dist;
 }
 
+//==============================================================================
+// DecaySpectrum implementation
+//==============================================================================
+
+DecaySpectrum::DecaySpectrum(pugi::xml_node node)
+{
+  // Read the region volume [cm^3] needed for absolute emission rate
+  if (!check_for_node(node, "volume"))
+    fatal_error("DecaySpectrum: 'volume' attribute is required.");
+  double volume = std::stod(get_node_value(node, "volume"));
+
+  // Read nuclide names and atom densities from XML
+  vector<int> nuclide_indices;
+  vector<double> atoms;
+  auto names = get_node_array<std::string>(node, "nuclides");
+  auto densities = get_node_array<double>(node, "parameters");
+  if (names.size() != densities.size()) {
+    fatal_error("DecaySpectrum nuclides and parameters must have the same "
+                "length.");
+  }
+
+  for (size_t i = 0; i < names.size(); ++i) {
+    const auto& name = names[i];
+    double density = densities[i];
+
+    // Look up nuclide in the depletion chain
+    auto it = data::chain_nuclide_map.find(name);
+    if (it == data::chain_nuclide_map.end()) {
+      if (decay_spectrum_missing_chain_nuclides.insert(name).second) {
+        warning("Nuclide '" + name +
+                "' appears in a DecaySpectrum source but is not present in "
+                "the depletion chain; it will be ignored.");
+      }
+      continue;
+    }
+
+    int nuclide_index = it->second;
+    const auto& chain_nuc = data::chain_nuclides[nuclide_index];
+    const Distribution* photon_dist = chain_nuc->photon_energy();
+    if (!photon_dist)
+      continue;
+
+    // Skip non-positive densities and warn if negative
+    if (density <= 0.0) {
+      if (density < 0.0) {
+        warning("Nuclide '" + name +
+                "' has a negative density in a DecaySpectrum source; it will "
+                "be ignored.");
+      }
+      continue;
+    }
+
+    // atoms = density [atom/b-cm] * 1e24 [b/cm^2] * volume [cm^3]
+    double atoms_i = density * 1.0e24 * volume;
+
+    nuclide_indices.push_back(nuclide_index);
+    atoms.push_back(atoms_i);
+  }
+
+  init(std::move(nuclide_indices), atoms);
+}
+
+void DecaySpectrum::init(
+  vector<int> nuclide_indices, const vector<double>& atoms)
+{
+  if (nuclide_indices.size() != atoms.size()) {
+    fatal_error("DecaySpectrum nuclide index and atoms arrays must have "
+                "the same length.");
+  }
+
+  vector<double> probs;
+  probs.reserve(nuclide_indices.size());
+  for (size_t i = 0; i < nuclide_indices.size(); ++i) {
+    // Distribution integral is in [photons/s/atom]; multiplying by atoms gives
+    // the total emission rate [photons/s] for this nuclide.
+    const auto* dist =
+      data::chain_nuclides[nuclide_indices[i]]->photon_energy();
+    probs.push_back(atoms[i] * dist->integral());
+  }
+
+  nuclide_indices_ = std::move(nuclide_indices);
+  integral_ = std::accumulate(probs.begin(), probs.end(), 0.0);
+  if (nuclide_indices_.empty() || integral_ <= 0.0) {
+    fatal_error("DecaySpectrum source did not resolve any nuclides with decay "
+                "photon spectra and positive atom densities. Ensure "
+                "OPENMC_CHAIN_FILE is set and matches the nuclides in the "
+                "source definition.");
+  }
+  di_.assign(probs);
+}
+
+DecaySpectrum::Sample DecaySpectrum::sample_with_parent(uint64_t* seed) const
+{
+  size_t idx = di_.sample(seed);
+  int parent_nuclide = nuclide_indices_[idx];
+  const auto* dist = data::chain_nuclides[parent_nuclide]->photon_energy();
+  auto [energy, weight] = dist->sample(seed);
+  return {energy, weight, parent_nuclide};
+}
+
+std::pair<double, double> DecaySpectrum::sample(uint64_t* seed) const
+{
+  auto sample = sample_with_parent(seed);
+  return {sample.energy, sample.weight};
+}
+
+double DecaySpectrum::integral() const
+{
+  return integral_;
+}
+
+double DecaySpectrum::sample_unbiased(uint64_t* seed) const
+{
+  return sample_with_parent(seed).energy;
+}
+
 } // namespace openmc
diff --git a/src/distribution_angle.cpp b/src/distribution_angle.cpp
index 5e92b794a..ecb5961f6 100644
--- a/src/distribution_angle.cpp
+++ b/src/distribution_angle.cpp
@@ -2,11 +2,11 @@
 
 #include <cmath> // for abs, copysign
 
-#include "xtensor/xarray.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/endf.h"
 #include "openmc/hdf5_interface.h"
+#include "openmc/math_functions.h"
 #include "openmc/random_lcg.h"
 #include "openmc/search.h"
 #include "openmc/vector.h" // for vector
@@ -29,7 +29,7 @@ AngleDistribution::AngleDistribution(hid_t group)
   hid_t dset = open_dataset(group, "mu");
   read_attribute(dset, "offsets", offsets);
   read_attribute(dset, "interpolation", interp);
-  xt::xarray<double> temp;
+  tensor::Tensor<double> temp;
   read_dataset(dset, temp);
   close_dataset(dset);
 
@@ -40,13 +40,13 @@ AngleDistribution::AngleDistribution(hid_t group)
     if (i < n_energy - 1) {
       n = offsets[i + 1] - j;
     } else {
-      n = temp.shape()[1] - j;
+      n = temp.shape(1) - j;
     }
 
     // Create and initialize tabular distribution
-    auto xs = xt::view(temp, 0, xt::range(j, j + n));
-    auto ps = xt::view(temp, 1, xt::range(j, j + n));
-    auto cs = xt::view(temp, 2, xt::range(j, j + n));
+    tensor::View<double> xs = temp.slice(0, tensor::range(j, j + n));
+    tensor::View<double> ps = temp.slice(1, tensor::range(j, j + n));
+    tensor::View<double> cs = temp.slice(2, tensor::range(j, j + n));
     vector<double> x {xs.begin(), xs.end()};
     vector<double> p {ps.begin(), ps.end()};
     vector<double> c {cs.begin(), cs.end()};
@@ -64,30 +64,17 @@ AngleDistribution::AngleDistribution(hid_t group)
 
 double AngleDistribution::sample(double E, uint64_t* seed) const
 {
-  // Determine number of incoming energies
-  auto n = energy_.size();
-
-  // Find energy bin and calculate interpolation factor -- if the energy is
-  // outside the range of the tabulated energies, choose the first or last bins
+  // Find energy bin and calculate interpolation factor
   int i;
   double r;
-  if (E < energy_[0]) {
-    i = 0;
-    r = 0.0;
-  } else if (E > energy_[n - 1]) {
-    i = n - 2;
-    r = 1.0;
-  } else {
-    i = lower_bound_index(energy_.begin(), energy_.end(), E);
-    r = (E - energy_[i]) / (energy_[i + 1] - energy_[i]);
-  }
+  get_energy_index(energy_, E, i, r);
 
   // Sample between the ith and (i+1)th bin
   if (r > prn(seed))
     ++i;
 
   // Sample i-th distribution
-  double mu = distribution_[i]->sample(seed);
+  double mu = distribution_[i]->sample(seed).first;
 
   // Make sure mu is in range [-1,1] and return
   if (std::abs(mu) > 1.0)
@@ -95,4 +82,19 @@ double AngleDistribution::sample(double E, uint64_t* seed) const
   return mu;
 }
 
+double AngleDistribution::evaluate(double E, double mu) const
+{
+  // Find energy bin and calculate interpolation factor
+  int i;
+  double r;
+  get_energy_index(energy_, E, i, r);
+
+  double pdf = 0.0;
+  if (r > 0.0)
+    pdf += r * distribution_[i + 1]->evaluate(mu);
+  if (r < 1.0)
+    pdf += (1.0 - r) * distribution_[i]->evaluate(mu);
+  return pdf;
+}
+
 } // namespace openmc
diff --git a/src/distribution_energy.cpp b/src/distribution_energy.cpp
index a4a5ce9e1..2f8e6cf1a 100644
--- a/src/distribution_energy.cpp
+++ b/src/distribution_energy.cpp
@@ -4,7 +4,7 @@
 #include <cstddef>   // for size_t
 #include <iterator>  // for back_inserter
 
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/endf.h"
 #include "openmc/hdf5_interface.h"
@@ -60,11 +60,11 @@ ContinuousTabular::ContinuousTabular(hid_t group)
   hid_t dset = open_dataset(group, "energy");
 
   // Get interpolation parameters
-  xt::xarray<int> temp;
+  tensor::Tensor<int> temp;
   read_attribute(dset, "interpolation", temp);
 
-  auto temp_b = xt::view(temp, 0); // view of breakpoints
-  auto temp_i = xt::view(temp, 1); // view of interpolation parameters
+  tensor::View<int> temp_b = temp.slice(0); // breakpoints
+  tensor::View<int> temp_i = temp.slice(1); // interpolation parameters
 
   std::copy(temp_b.begin(), temp_b.end(), std::back_inserter(breakpoints_));
   for (const auto i : temp_i)
@@ -85,7 +85,7 @@ ContinuousTabular::ContinuousTabular(hid_t group)
   read_attribute(dset, "interpolation", interp);
   read_attribute(dset, "n_discrete_lines", n_discrete);
 
-  xt::xarray<double> eout;
+  tensor::Tensor<double> eout;
   read_dataset(dset, eout);
   close_dataset(dset);
 
@@ -96,7 +96,7 @@ ContinuousTabular::ContinuousTabular(hid_t group)
     if (i < n_energy - 1) {
       n = offsets[i + 1] - j;
     } else {
-      n = eout.shape()[1] - j;
+      n = eout.shape(1) - j;
     }
 
     // Assign interpolation scheme and number of discrete lines
@@ -105,15 +105,15 @@ ContinuousTabular::ContinuousTabular(hid_t group)
     d.n_discrete = n_discrete[i];
 
     // Copy data
-    d.e_out = xt::view(eout, 0, xt::range(j, j + n));
-    d.p = xt::view(eout, 1, xt::range(j, j + n));
+    d.e_out = eout.slice(0, tensor::range(j, j + n));
+    d.p = eout.slice(1, tensor::range(j, j + n));
 
     // To get answers that match ACE data, for now we still use the tabulated
     // CDF values that were passed through to the HDF5 library. At a later
     // time, we can remove the CDF values from the HDF5 library and
     // reconstruct them using the PDF
     if (true) {
-      d.c = xt::view(eout, 2, xt::range(j, j + n));
+      d.c = eout.slice(2, tensor::range(j, j + n));
     } else {
       // Calculate cumulative distribution function -- discrete portion
       for (int k = 0; k < d.n_discrete; ++k) {
diff --git a/src/distribution_multi.cpp b/src/distribution_multi.cpp
index b7b3efe52..47785649b 100644
--- a/src/distribution_multi.cpp
+++ b/src/distribution_multi.cpp
@@ -1,6 +1,6 @@
 #include "openmc/distribution_multi.h"
 
-#include <algorithm> // for move
+#include <algorithm> // for move, clamp
 #include <cmath>     // for sqrt, sin, cos, max
 
 #include "openmc/constants.h"
@@ -20,7 +20,7 @@ unique_ptr<UnitSphereDistribution> UnitSphereDistribution::create(
   if (check_for_node(node, "type"))
     type = get_node_value(node, "type", true, true);
   if (type == "isotropic") {
-    return UPtrAngle {new Isotropic()};
+    return UPtrAngle {new Isotropic(node)};
   } else if (type == "monodirectional") {
     return UPtrAngle {new Monodirectional(node)};
   } else if (type == "mu-phi") {
@@ -44,6 +44,7 @@ UnitSphereDistribution::UnitSphereDistribution(pugi::xml_node node)
       fatal_error("Angular distribution reference direction must have "
                   "three parameters specified.");
     u_ref_ = Direction(u_ref.data());
+    u_ref_ /= u_ref_.norm();
   }
 }
 
@@ -58,6 +59,16 @@ PolarAzimuthal::PolarAzimuthal(Direction u, UPtrDist mu, UPtrDist phi)
 PolarAzimuthal::PolarAzimuthal(pugi::xml_node node)
   : UnitSphereDistribution {node}
 {
+  // Read reference directional unit vector
+  if (check_for_node(node, "reference_vwu")) {
+    auto v_ref = get_node_array<double>(node, "reference_vwu");
+    if (v_ref.size() != 3)
+      fatal_error("Angular distribution reference v direction must have "
+                  "three parameters specified.");
+    v_ref_ = Direction(v_ref.data());
+    v_ref_ /= v_ref_.norm();
+  }
+  w_ref_ = u_ref_.cross(v_ref_);
   if (check_for_node(node, "mu")) {
     pugi::xml_node node_dist = node.child("mu");
     mu_ = distribution_from_xml(node_dist);
@@ -73,23 +84,79 @@ PolarAzimuthal::PolarAzimuthal(pugi::xml_node node)
   }
 }
 
-Direction PolarAzimuthal::sample(uint64_t* seed) const
+std::pair<Direction, double> PolarAzimuthal::sample(uint64_t* seed) const
+{
+  return sample_impl(seed, false);
+}
+
+std::pair<Direction, double> PolarAzimuthal::sample_as_bias(
+  uint64_t* seed) const
+{
+  return sample_impl(seed, true);
+}
+
+std::pair<Direction, double> PolarAzimuthal::sample_impl(
+  uint64_t* seed, bool return_pdf) const
 {
   // Sample cosine of polar angle
-  double mu = mu_->sample(seed);
-  if (mu == 1.0)
-    return u_ref_;
+  auto [mu, mu_wgt] = mu_->sample(seed);
 
   // Sample azimuthal angle
-  double phi = phi_->sample(seed);
+  auto [phi, phi_wgt] = phi_->sample(seed);
 
-  return rotate_angle(u_ref_, mu, &phi, seed);
+  // Compute either the PDF value or the importance weight
+  double weight =
+    return_pdf ? (mu_->evaluate(mu) * phi_->evaluate(phi)) : (mu_wgt * phi_wgt);
+
+  if (mu == 1.0)
+    return {u_ref_, weight};
+  if (mu == -1.0)
+    return {-u_ref_, weight};
+
+  double f = std::sqrt(1 - mu * mu);
+  return {mu * u_ref_ + f * std::cos(phi) * v_ref_ + f * std::sin(phi) * w_ref_,
+    weight};
+}
+
+double PolarAzimuthal::evaluate(Direction u) const
+{
+  double mu = std::clamp(u.dot(u_ref_), -1.0, 1.0);
+  double phi = 0.0;
+  double sin_theta_sq = std::max(0.0, 1.0 - mu * mu);
+  if (sin_theta_sq > 0.0) {
+    double sin_theta = std::sqrt(sin_theta_sq);
+    double cos_phi = u.dot(v_ref_) / sin_theta;
+    double sin_phi = u.dot(w_ref_) / sin_theta;
+    phi = std::atan2(sin_phi, cos_phi);
+    if (phi < 0.0)
+      phi += 2.0 * PI;
+  }
+  return mu_->evaluate(mu) * phi_->evaluate(phi);
 }
 
 //==============================================================================
 // Isotropic implementation
 //==============================================================================
 
+Isotropic::Isotropic(pugi::xml_node node) : UnitSphereDistribution {node}
+{
+  if (check_for_node(node, "bias")) {
+    pugi::xml_node bias_node = node.child("bias");
+    std::string bias_type = get_node_value(bias_node, "type", true, true);
+    if (bias_type != "mu-phi") {
+      openmc::fatal_error(
+        "Isotropic distributions may only be biased by a PolarAzimuthal.");
+    }
+    auto bias = std::make_unique<PolarAzimuthal>(bias_node);
+    if (bias->mu()->bias() || bias->phi()->bias()) {
+      openmc::fatal_error(
+        "Attempted to bias Isotropic distribution with a biased PolarAzimuthal "
+        "distribution. Please ensure bias distributions are unbiased.");
+    }
+    this->set_bias(std::move(bias));
+  }
+}
+
 Direction isotropic_direction(uint64_t* seed)
 {
   double phi = uniform_distribution(0., 2.0 * PI, seed);
@@ -98,18 +165,28 @@ Direction isotropic_direction(uint64_t* seed)
     std::sqrt(1.0 - mu * mu) * std::sin(phi)};
 }
 
-Direction Isotropic::sample(uint64_t* seed) const
+std::pair<Direction, double> Isotropic::sample(uint64_t* seed) const
 {
-  return isotropic_direction(seed);
+  if (bias()) {
+    auto [val, eval] = bias()->sample_as_bias(seed);
+    return {val, 1.0 / (4.0 * PI * eval)};
+  } else {
+    return {isotropic_direction(seed), 1.0};
+  }
+}
+
+double Isotropic::evaluate(Direction u) const
+{
+  return 1.0 / (4.0 * PI);
 }
 
 //==============================================================================
 // Monodirectional implementation
 //==============================================================================
 
-Direction Monodirectional::sample(uint64_t* seed) const
+std::pair<Direction, double> Monodirectional::sample(uint64_t* seed) const
 {
-  return u_ref_;
+  return {u_ref_, 1.0};
 }
 
 } // namespace openmc
diff --git a/src/distribution_spatial.cpp b/src/distribution_spatial.cpp
index d2b0f413b..5a3452422 100644
--- a/src/distribution_spatial.cpp
+++ b/src/distribution_spatial.cpp
@@ -80,9 +80,13 @@ CartesianIndependent::CartesianIndependent(pugi::xml_node node)
   }
 }
 
-Position CartesianIndependent::sample(uint64_t* seed) const
+std::pair<Position, double> CartesianIndependent::sample(uint64_t* seed) const
 {
-  return {x_->sample(seed), y_->sample(seed), z_->sample(seed)};
+  auto [x_val, x_wgt] = x_->sample(seed);
+  auto [y_val, y_wgt] = y_->sample(seed);
+  auto [z_val, z_wgt] = z_->sample(seed);
+  Position xi {x_val, y_val, z_val};
+  return {xi, x_wgt * y_wgt * z_wgt};
 }
 
 //==============================================================================
@@ -137,16 +141,51 @@ CylindricalIndependent::CylindricalIndependent(pugi::xml_node node)
     // If no coordinates were specified, default to (0, 0, 0)
     origin_ = {0.0, 0.0, 0.0};
   }
+
+  // Read cylinder z_dir
+  if (check_for_node(node, "z_dir")) {
+    auto z_dir = get_node_array<double>(node, "z_dir");
+    if (z_dir.size() == 3) {
+      z_dir_ = z_dir;
+      z_dir_ /= z_dir_.norm();
+    } else {
+      fatal_error("z_dir for cylindrical source distribution must be length 3");
+    }
+  } else {
+    // If no z_dir was specified, default to (0, 0, 1)
+    z_dir_ = {0.0, 0.0, 1.0};
+  }
+
+  // Read cylinder r_dir
+  if (check_for_node(node, "r_dir")) {
+    auto r_dir = get_node_array<double>(node, "r_dir");
+    if (r_dir.size() == 3) {
+      r_dir_ = r_dir;
+      r_dir_ /= r_dir_.norm();
+    } else {
+      fatal_error("r_dir for cylindrical source distribution must be length 3");
+    }
+  } else {
+    // If no r_dir was specified, default to (1, 0, 0)
+    r_dir_ = {1.0, 0.0, 0.0};
+  }
+
+  if (r_dir_.dot(z_dir_) > 1e-12)
+    fatal_error("r_dir must be perpendicular to z_dir");
+
+  auto phi_dir = z_dir_.cross(r_dir_);
+  phi_dir /= phi_dir.norm();
+  phi_dir_ = phi_dir;
 }
 
-Position CylindricalIndependent::sample(uint64_t* seed) const
+std::pair<Position, double> CylindricalIndependent::sample(uint64_t* seed) const
 {
-  double r = r_->sample(seed);
-  double phi = phi_->sample(seed);
-  double x = r * cos(phi) + origin_.x;
-  double y = r * sin(phi) + origin_.y;
-  double z = z_->sample(seed) + origin_.z;
-  return {x, y, z};
+  auto [r, r_wgt] = r_->sample(seed);
+  auto [phi, phi_wgt] = phi_->sample(seed);
+  auto [z, z_wgt] = z_->sample(seed);
+  Position xi =
+    r * (cos(phi) * r_dir_ + sin(phi) * phi_dir_) + z * z_dir_ + origin_;
+  return {xi, r_wgt * phi_wgt * z_wgt};
 }
 
 //==============================================================================
@@ -203,16 +242,17 @@ SphericalIndependent::SphericalIndependent(pugi::xml_node node)
   }
 }
 
-Position SphericalIndependent::sample(uint64_t* seed) const
+std::pair<Position, double> SphericalIndependent::sample(uint64_t* seed) const
 {
-  double r = r_->sample(seed);
-  double cos_theta = cos_theta_->sample(seed);
-  double phi = phi_->sample(seed);
+  auto [r, r_wgt] = r_->sample(seed);
+  auto [cos_theta, cos_theta_wgt] = cos_theta_->sample(seed);
+  auto [phi, phi_wgt] = phi_->sample(seed);
   // sin(theta) by sin**2 + cos**2 = 1
   double x = r * std::sqrt(1 - cos_theta * cos_theta) * cos(phi) + origin_.x;
   double y = r * std::sqrt(1 - cos_theta * cos_theta) * sin(phi) + origin_.y;
   double z = r * cos_theta + origin_.z;
-  return {x, y, z};
+  Position xi {x, y, z};
+  return {xi, r_wgt * cos_theta_wgt * phi_wgt};
 }
 
 //==============================================================================
@@ -222,9 +262,11 @@ Position SphericalIndependent::sample(uint64_t* seed) const
 MeshSpatial::MeshSpatial(pugi::xml_node node)
 {
 
-  if (get_node_value(node, "type", true, true) != "mesh") {
-    fatal_error(fmt::format(
-      "Incorrect spatial type '{}' for a MeshSpatial distribution"));
+  auto spatial_type = get_node_value(node, "type", true, true);
+  if (spatial_type != "mesh") {
+    fatal_error(
+      fmt::format("Incorrect spatial type '{}' for a MeshSpatial distribution",
+        spatial_type));
   }
 
   // No in-tet distributions implemented, could include distributions for the
@@ -260,6 +302,37 @@ MeshSpatial::MeshSpatial(pugi::xml_node node)
   }
 
   elem_idx_dist_.assign(strengths);
+
+  if (check_for_node(node, "bias")) {
+    pugi::xml_node bias_node = node.child("bias");
+
+    if (check_for_node(bias_node, "strengths")) {
+      std::vector<double> bias_strengths(n_bins, 1.0);
+      bias_strengths = get_node_array<double>(node, "strengths");
+
+      if (bias_strengths.size() != n_bins) {
+        fatal_error(
+          fmt::format("Number of entries in the bias strengths array {} does "
+                      "not match the number of entities in mesh {} ({}).",
+            bias_strengths.size(), mesh_id, n_bins));
+      }
+
+      if (get_node_value_bool(node, "volume_normalized")) {
+        for (int i = 0; i < n_bins; i++) {
+          bias_strengths[i] *= this->mesh()->volume(i);
+        }
+      }
+
+      // Compute importance weights
+      weight_ = compute_importance_weights(strengths, bias_strengths);
+
+      // Re-initialize DiscreteIndex with bias strengths for sampling
+      elem_idx_dist_.assign(bias_strengths);
+    } else {
+      fatal_error(fmt::format(
+        "Bias node for mesh {} found without strengths array.", mesh_id));
+    }
+  }
 }
 
 MeshSpatial::MeshSpatial(int32_t mesh_idx, span<const double> strengths)
@@ -295,9 +368,11 @@ std::pair<int32_t, Position> MeshSpatial::sample_mesh(uint64_t* seed) const
   return {elem_idx, mesh()->sample_element(elem_idx, seed)};
 }
 
-Position MeshSpatial::sample(uint64_t* seed) const
+std::pair<Position, double> MeshSpatial::sample(uint64_t* seed) const
 {
-  return this->sample_mesh(seed).second;
+  auto [elem_idx, u] = this->sample_mesh(seed);
+  double wgt = weight_.empty() ? 1.0 : weight_[elem_idx];
+  return {u, wgt};
 }
 
 //==============================================================================
@@ -328,6 +403,31 @@ PointCloud::PointCloud(pugi::xml_node node)
   }
 
   point_idx_dist_.assign(strengths);
+
+  if (check_for_node(node, "bias")) {
+    pugi::xml_node bias_node = node.child("bias");
+
+    if (check_for_node(bias_node, "strengths")) {
+      std::vector<double> bias_strengths(point_cloud_.size(), 1.0);
+      bias_strengths = get_node_array<double>(node, "strengths");
+
+      if (bias_strengths.size() != point_cloud_.size()) {
+        fatal_error(
+          fmt::format("Number of entries in the bias strengths array {} does "
+                      "not match the number of spatial points provided {}.",
+            bias_strengths.size(), point_cloud_.size()));
+      }
+
+      // Compute importance weights
+      weight_ = compute_importance_weights(strengths, bias_strengths);
+
+      // Re-initialize DiscreteIndex with bias strengths for sampling
+      point_idx_dist_.assign(bias_strengths);
+    } else {
+      fatal_error(
+        fmt::format("Bias node for PointCloud found without strengths array."));
+    }
+  }
 }
 
 PointCloud::PointCloud(
@@ -337,10 +437,11 @@ PointCloud::PointCloud(
   point_idx_dist_.assign(strengths);
 }
 
-Position PointCloud::sample(uint64_t* seed) const
+std::pair<Position, double> PointCloud::sample(uint64_t* seed) const
 {
   int32_t index = point_idx_dist_.sample(seed);
-  return point_cloud_[index];
+  double wgt = weight_.empty() ? 1.0 : weight_[index];
+  return {point_cloud_[index], wgt};
 }
 
 //==============================================================================
@@ -360,10 +461,15 @@ SpatialBox::SpatialBox(pugi::xml_node node, bool fission)
   upper_right_ = Position {params[3], params[4], params[5]};
 }
 
-Position SpatialBox::sample(uint64_t* seed) const
+SpatialBox::SpatialBox(Position lower_left, Position upper_right, bool fission)
+  : lower_left_(lower_left), upper_right_(upper_right),
+    only_fissionable_(fission)
+{}
+
+std::pair<Position, double> SpatialBox::sample(uint64_t* seed) const
 {
   Position xi {prn(seed), prn(seed), prn(seed)};
-  return lower_left_ + xi * (upper_right_ - lower_left_);
+  return {lower_left_ + xi * (upper_right_ - lower_left_), 1.0};
 }
 
 //==============================================================================
@@ -382,9 +488,9 @@ SpatialPoint::SpatialPoint(pugi::xml_node node)
   r_ = Position {params.data()};
 }
 
-Position SpatialPoint::sample(uint64_t* seed) const
+std::pair<Position, double> SpatialPoint::sample(uint64_t* seed) const
 {
-  return r_;
+  return {r_, 1.0};
 }
 
 } // namespace openmc
diff --git a/src/eigenvalue.cpp b/src/eigenvalue.cpp
index 8669d76f9..bc8dcc9ea 100644
--- a/src/eigenvalue.cpp
+++ b/src/eigenvalue.cpp
@@ -1,9 +1,6 @@
 #include "openmc/eigenvalue.h"
 
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xmath.hpp"
-#include "xtensor/xtensor.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/array.h"
 #include "openmc/bank.h"
@@ -11,6 +8,7 @@
 #include "openmc/constants.h"
 #include "openmc/error.h"
 #include "openmc/hdf5_interface.h"
+#include "openmc/ifp.h"
 #include "openmc/math_functions.h"
 #include "openmc/mesh.h"
 #include "openmc/message_passing.h"
@@ -38,7 +36,7 @@ namespace simulation {
 double keff_generation;
 array<double, 2> k_sum;
 vector<double> entropy;
-xt::xtensor<double, 1> source_frac;
+tensor::Tensor<double> source_frac;
 
 } // namespace simulation
 
@@ -126,54 +124,54 @@ void synchronize_bank()
       "No fission sites banked on MPI rank " + std::to_string(mpi::rank));
   }
 
-  // Make sure all processors start at the same point for random sampling. Then
-  // skip ahead in the sequence using the starting index in the 'global'
-  // fission bank for each processor.
-
-  int64_t id = simulation::total_gen + overall_generation();
-  uint64_t seed = init_seed(id, STREAM_TRACKING);
-  advance_prn_seed(start, &seed);
-
-  // Determine how many fission sites we need to sample from the source bank
-  // and the probability for selecting a site.
-
-  int64_t sites_needed;
-  if (total < settings::n_particles) {
-    sites_needed = settings::n_particles % total;
-  } else {
-    sites_needed = settings::n_particles;
-  }
-  double p_sample = static_cast<double>(sites_needed) / total;
-
   simulation::time_bank_sample.start();
 
-  // ==========================================================================
-  // SAMPLE N_PARTICLES FROM FISSION BANK AND PLACE IN TEMP_SITES
-
   // Allocate temporary source bank -- we don't really know how many fission
   // sites were created, so overallocate by a factor of 3
   int64_t index_temp = 0;
+
   vector<SourceSite> temp_sites(3 * simulation::work_per_rank);
 
-  for (int64_t i = 0; i < simulation::fission_bank.size(); i++) {
-    const auto& site = simulation::fission_bank[i];
+  // Temporary banks for IFP
+  vector<vector<int>> temp_delayed_groups;
+  vector<vector<double>> temp_lifetimes;
+  if (settings::ifp_on) {
+    resize_ifp_data(
+      temp_delayed_groups, temp_lifetimes, 3 * simulation::work_per_rank);
+  }
 
-    // If there are less than n_particles particles banked, automatically add
-    // int(n_particles/total) sites to temp_sites. For example, if you need
-    // 1000 and 300 were banked, this would add 3 source sites per banked site
-    // and the remaining 100 would be randomly sampled.
-    if (total < settings::n_particles) {
-      for (int64_t j = 1; j <= settings::n_particles / total; ++j) {
-        temp_sites[index_temp] = site;
-        ++index_temp;
-      }
-    }
+  // ==========================================================================
+  // SAMPLE N_PARTICLES FROM FISSION BANK AND PLACE IN TEMP_SITES
 
-    // Randomly sample sites needed
-    if (prn(&seed) < p_sample) {
-      temp_sites[index_temp] = site;
-      ++index_temp;
+  // We use Uniform Combing method to exactly get the targeted particle size
+  // [https://doi.org/10.1080/00295639.2022.2091906]
+
+  // Make sure all processors use the same random number seed.
+  int64_t id = simulation::total_gen + overall_generation();
+  uint64_t seed = init_seed(id, STREAM_TRACKING);
+
+  // Comb specification
+  double teeth_distance = static_cast<double>(total) / settings::n_particles;
+  double teeth_offset = prn(&seed) * teeth_distance;
+
+  // First and last hitting tooth
+  int64_t end = start + simulation::fission_bank.size();
+  int64_t tooth_start = std::ceil((start - teeth_offset) / teeth_distance);
+  int64_t tooth_end = std::floor((end - teeth_offset) / teeth_distance) + 1;
+
+  // Locally comb particles in fission_bank
+  double tooth = tooth_start * teeth_distance + teeth_offset;
+  for (int64_t i = tooth_start; i < tooth_end; i++) {
+    int64_t idx = std::floor(tooth) - start;
+    temp_sites[index_temp] = simulation::fission_bank[idx];
+    if (settings::ifp_on) {
+      copy_ifp_data_from_fission_banks(
+        idx, temp_delayed_groups[index_temp], temp_lifetimes[index_temp]);
     }
+    ++index_temp;
+
+    // Next tooth
+    tooth += teeth_distance;
   }
 
   // At this point, the sampling of source sites is done and now we need to
@@ -188,40 +186,17 @@ void synchronize_bank()
   MPI_Exscan(&index_temp, &start, 1, MPI_INT64_T, MPI_SUM, mpi::intracomm);
   finish = start + index_temp;
 
+  // TODO: protect for MPI_Exscan at rank 0
+
   // Allocate space for bank_position if this hasn't been done yet
-  int64_t bank_position[mpi::n_procs];
-  MPI_Allgather(
-    &start, 1, MPI_INT64_T, bank_position, 1, MPI_INT64_T, mpi::intracomm);
+  std::vector<int64_t> bank_position(mpi::n_procs);
+  MPI_Allgather(&start, 1, MPI_INT64_T, bank_position.data(), 1, MPI_INT64_T,
+    mpi::intracomm);
 #else
   start = 0;
   finish = index_temp;
 #endif
 
-  // Now that the sampling is complete, we need to ensure that we have exactly
-  // n_particles source sites. The way this is done in a reproducible manner is
-  // to adjust only the source sites on the last processor.
-
-  if (mpi::rank == mpi::n_procs - 1) {
-    if (finish > settings::n_particles) {
-      // If we have extra sites sampled, we will simply discard the extra
-      // ones on the last processor
-      index_temp = settings::n_particles - start;
-
-    } else if (finish < settings::n_particles) {
-      // If we have too few sites, repeat sites from the very end of the
-      // fission bank
-      sites_needed = settings::n_particles - finish;
-      for (int i = 0; i < sites_needed; ++i) {
-        int i_bank = simulation::fission_bank.size() - sites_needed + i;
-        temp_sites[index_temp] = simulation::fission_bank[i_bank];
-        ++index_temp;
-      }
-    }
-
-    // the last processor should not be sending sites to right
-    finish = simulation::work_index[mpi::rank + 1];
-  }
-
   simulation::time_bank_sample.stop();
   simulation::time_bank_sendrecv.start();
 
@@ -229,15 +204,32 @@ void synchronize_bank()
   // ==========================================================================
   // SEND BANK SITES TO NEIGHBORS
 
+  // IFP number of generation
+  int ifp_n_generation;
+  if (settings::ifp_on) {
+    broadcast_ifp_n_generation(
+      ifp_n_generation, temp_delayed_groups, temp_lifetimes);
+  }
+
   int64_t index_local = 0;
   vector<MPI_Request> requests;
 
+  // IFP send buffers
+  vector<int> send_delayed_groups;
+  vector<double> send_lifetimes;
+
   if (start < settings::n_particles) {
     // Determine the index of the processor which has the first part of the
     // source_bank for the local processor
     int neighbor = upper_bound_index(
       simulation::work_index.begin(), simulation::work_index.end(), start);
 
+    // Resize IFP send buffers
+    if (settings::ifp_on && mpi::n_procs > 1) {
+      resize_ifp_data(send_delayed_groups, send_lifetimes,
+        ifp_n_generation * 3 * simulation::work_per_rank);
+    }
+
     while (start < finish) {
       // Determine the number of sites to send
       int64_t n =
@@ -250,6 +242,16 @@ void synchronize_bank()
         MPI_Isend(&temp_sites[index_local], static_cast<int>(n),
           mpi::source_site, neighbor, mpi::rank, mpi::intracomm,
           &requests.back());
+
+        if (settings::ifp_on) {
+          // Send IFP data
+          if (is_beta_effective_or_both())
+            send_ifp_info(index_local, n, ifp_n_generation, neighbor, requests,
+              temp_delayed_groups, send_delayed_groups);
+          if (is_generation_time_or_both())
+            send_ifp_info(index_local, n, ifp_n_generation, neighbor, requests,
+              temp_lifetimes, send_lifetimes);
+        }
       }
 
       // Increment all indices
@@ -271,6 +273,11 @@ void synchronize_bank()
   start = simulation::work_index[mpi::rank];
   index_local = 0;
 
+  // IFP receive buffers
+  vector<int> recv_delayed_groups;
+  vector<double> recv_lifetimes;
+  vector<DeserializationInfo> deserialization_info;
+
   // Determine what process has the source sites that will need to be stored at
   // the beginning of this processor's source bank.
 
@@ -279,7 +286,13 @@ void synchronize_bank()
     neighbor = mpi::n_procs - 1;
   } else {
     neighbor =
-      upper_bound_index(bank_position, bank_position + mpi::n_procs, start);
+      upper_bound_index(bank_position.begin(), bank_position.end(), start);
+  }
+
+  // Resize IFP receive buffers
+  if (settings::ifp_on && mpi::n_procs > 1) {
+    resize_ifp_data(recv_delayed_groups, recv_lifetimes,
+      ifp_n_generation * simulation::work_per_rank);
   }
 
   while (start < simulation::work_index[mpi::rank + 1]) {
@@ -301,13 +314,28 @@ void synchronize_bank()
       MPI_Irecv(&simulation::source_bank[index_local], static_cast<int>(n),
         mpi::source_site, neighbor, neighbor, mpi::intracomm, &requests.back());
 
+      if (settings::ifp_on) {
+        // Receive IFP data
+        if (is_beta_effective_or_both())
+          receive_ifp_data(index_local, n, ifp_n_generation, neighbor, requests,
+            recv_delayed_groups, deserialization_info);
+        if (is_generation_time_or_both())
+          receive_ifp_data(index_local, n, ifp_n_generation, neighbor, requests,
+            recv_lifetimes, deserialization_info);
+      }
+
     } else {
-      // If the source sites are on this procesor, we can simply copy them
+      // If the source sites are on this processor, we can simply copy them
       // from the temp_sites bank
 
       index_temp = start - bank_position[mpi::rank];
       std::copy(&temp_sites[index_temp], &temp_sites[index_temp + n],
         &simulation::source_bank[index_local]);
+
+      if (settings::ifp_on) {
+        copy_partial_ifp_data_to_source_banks(
+          index_temp, n, index_local, temp_delayed_groups, temp_lifetimes);
+      }
     }
 
     // Increment all indices
@@ -323,9 +351,21 @@ void synchronize_bank()
   int n_request = requests.size();
   MPI_Waitall(n_request, requests.data(), MPI_STATUSES_IGNORE);
 
+  if (settings::ifp_on) {
+    if (is_beta_effective_or_both())
+      deserialize_ifp_info(ifp_n_generation, recv_delayed_groups,
+        simulation::ifp_source_delayed_group_bank, deserialization_info);
+    if (is_generation_time_or_both())
+      deserialize_ifp_info(ifp_n_generation, recv_lifetimes,
+        simulation::ifp_source_lifetime_bank, deserialization_info);
+  }
+
 #else
   std::copy(temp_sites.data(), temp_sites.data() + settings::n_particles,
     simulation::source_bank.begin());
+  if (settings::ifp_on) {
+    copy_complete_ifp_data_to_source_banks(temp_delayed_groups, temp_lifetimes);
+  }
 #endif
 
   simulation::time_bank_sendrecv.stop();
@@ -371,6 +411,16 @@ void calculate_average_keff()
         t_value *
         std::sqrt(
           (simulation::k_sum[1] / n - std::pow(simulation::keff, 2)) / (n - 1));
+
+      // In some cases (such as an infinite medium problem), random ray
+      // may estimate k exactly and in an unvarying manner between iterations.
+      // In this case, the floating point roundoff between the division and the
+      // power operations may cause an extremely small negative value to occur
+      // inside the sqrt operation, leading to NaN. If this occurs, we check for
+      // it and set the std dev to zero.
+      if (!std::isfinite(simulation::keff_std)) {
+        simulation::keff_std = 0.0;
+      }
     }
   }
 }
@@ -399,7 +449,7 @@ int openmc_get_keff(double* k_combined)
   const auto& gt = simulation::global_tallies;
 
   array<double, 3> kv {};
-  xt::xtensor<double, 2> cov = xt::zeros<double>({3, 3});
+  tensor::Tensor<double> cov = tensor::zeros<double>({3, 3});
   kv[0] = gt(GlobalTally::K_COLLISION, TallyResult::SUM) / n;
   kv[1] = gt(GlobalTally::K_ABSORPTION, TallyResult::SUM) / n;
   kv[2] = gt(GlobalTally::K_TRACKLENGTH, TallyResult::SUM) / n;
@@ -538,7 +588,7 @@ void shannon_entropy()
 {
   // Get source weight in each mesh bin
   bool sites_outside;
-  xt::xtensor<double, 1> p =
+  tensor::Tensor<double> p =
     simulation::entropy_mesh->count_sites(simulation::fission_bank.data(),
       simulation::fission_bank.size(), &sites_outside);
 
@@ -550,7 +600,7 @@ void shannon_entropy()
 
   if (mpi::master) {
     // Normalize to total weight of bank sites
-    p /= xt::sum(p);
+    p /= p.sum();
 
     // Sum values to obtain Shannon entropy
     double H = 0.0;
@@ -574,7 +624,7 @@ void ufs_count_sites()
 
     std::size_t n = simulation::ufs_mesh->n_bins();
     double vol_frac = simulation::ufs_mesh->volume_frac_;
-    simulation::source_frac = xt::xtensor<double, 1>({n}, vol_frac);
+    simulation::source_frac = tensor::Tensor<double>({n}, vol_frac);
 
   } else {
     // count number of source sites in each ufs mesh cell
@@ -596,7 +646,7 @@ void ufs_count_sites()
 #endif
 
     // Normalize to total weight to get fraction of source in each cell
-    double total = xt::sum(simulation::source_frac)();
+    double total = simulation::source_frac.sum();
     simulation::source_frac /= total;
 
     // Since the total starting weight is not equal to n_particles, we need to
diff --git a/src/endf.cpp b/src/endf.cpp
index c0c1d2e7e..8a9c5e48a 100644
--- a/src/endf.cpp
+++ b/src/endf.cpp
@@ -5,8 +5,7 @@
 #include <iterator>  // for back_inserter
 #include <stdexcept> // for runtime_error
 
-#include "xtensor/xarray.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/array.h"
 #include "openmc/constants.h"
@@ -88,6 +87,83 @@ bool is_inelastic_scatter(int mt)
   }
 }
 
+bool mt_matches(int event_mt, int target_mt)
+{
+  // Direct match
+  if (event_mt == target_mt)
+    return true;
+
+  // Check if event_mt is a component of target_mt summation reaction
+  switch (target_mt) {
+  case TOTAL_XS:
+    return event_mt == ELASTIC || mt_matches(event_mt, N_NONELASTIC);
+
+  case N_NONELASTIC: {
+    static constexpr int components[] = {4, 5, 11, 16, 17, 22, 23, 24, 25, 27,
+      28, 29, 30, 32, 33, 34, 35, 36, 37, 41, 42, 44, 45, 152, 153, 154, 156,
+      157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171,
+      172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 183, 184, 185, 186, 187,
+      188, 189, 190, 194, 195, 196, 198, 199, 200};
+    for (int mt : components) {
+      if (mt_matches(event_mt, mt))
+        return true;
+    }
+    return false;
+  }
+
+  case N_LEVEL:
+    // Inelastic scattering levels
+    return event_mt >= 50 && event_mt <= N_NC;
+
+  case N_2N:
+    // (n,2n) to excited states
+    return event_mt >= N_2N0 && event_mt <= N_2NC;
+
+  case N_FISSION:
+    return is_fission(event_mt);
+
+  case 27:
+    return is_fission(event_mt) || is_disappearance(event_mt);
+
+  case N_DISAPPEAR: {
+    return is_disappearance(event_mt);
+  }
+
+  case N_P:
+    // (n,p) to excited states
+    return event_mt >= N_P0 && event_mt <= N_PC;
+
+  case N_D:
+    // (n,d) to excited states
+    return event_mt >= N_D0 && event_mt <= N_DC;
+
+  case N_T:
+    // (n,t) to excited states
+    return event_mt >= N_T0 && event_mt <= N_TC;
+
+  case N_3HE:
+    // (n,3He) to excited states
+    return event_mt >= N_3HE0 && event_mt <= N_3HEC;
+
+  case N_A:
+    // (n,alpha) to excited states
+    return event_mt >= N_A0 && event_mt <= N_AC;
+
+  case 501:
+    return event_mt == 502 || event_mt == 504 || mt_matches(event_mt, 516) ||
+           mt_matches(event_mt, 522);
+
+  case PAIR_PROD:
+    return event_mt == PAIR_PROD_ELEC || event_mt == PAIR_PROD_NUC;
+
+  case PHOTOELECTRIC:
+    return event_mt >= 534 && event_mt < 573;
+
+  default:
+    return false;
+  }
+}
+
 unique_ptr<Function1D> read_function(hid_t group, const char* name)
 {
   hid_t obj_id = open_object(group, name);
@@ -153,11 +229,11 @@ Tabulated1D::Tabulated1D(hid_t dset)
   for (const auto i : int_temp)
     int_.push_back(int2interp(i));
 
-  xt::xarray<double> arr;
+  tensor::Tensor<double> arr;
   read_dataset(dset, arr);
 
-  auto xs = xt::view(arr, 0);
-  auto ys = xt::view(arr, 1);
+  tensor::View<double> xs = arr.slice(0);
+  tensor::View<double> ys = arr.slice(1);
 
   std::copy(xs.begin(), xs.end(), std::back_inserter(x_));
   std::copy(ys.begin(), ys.end(), std::back_inserter(y_));
@@ -229,12 +305,12 @@ double Tabulated1D::operator()(double x) const
 CoherentElasticXS::CoherentElasticXS(hid_t dset)
 {
   // Read 2D array from dataset
-  xt::xarray<double> arr;
+  tensor::Tensor<double> arr;
   read_dataset(dset, arr);
 
   // Get views for Bragg edges and structure factors
-  auto E = xt::view(arr, 0);
-  auto s = xt::view(arr, 1);
+  tensor::View<double> E = arr.slice(0);
+  tensor::View<double> s = arr.slice(1);
 
   // Copy Bragg edges and partial sums of structure factors
   std::copy(E.begin(), E.end(), std::back_inserter(bragg_edges_));
diff --git a/src/error.cpp b/src/error.cpp
index 566950a97..f99f5935f 100644
--- a/src/error.cpp
+++ b/src/error.cpp
@@ -110,23 +110,26 @@ void write_message(const std::string& message, int level)
 
 void fatal_error(const std::string& message, int err)
 {
+#pragma omp critical(FatalError)
+  {
 #ifdef _POSIX_VERSION
-  // Make output red if user is in a terminal
-  if (isatty(STDERR_FILENO)) {
-    std::cerr << "\033[0;31m";
-  }
+    // Make output red if user is in a terminal
+    if (isatty(STDERR_FILENO)) {
+      std::cerr << "\033[0;31m";
+    }
 #endif
 
-  // Write error message
-  std::cerr << " ERROR: ";
-  output(message, std::cerr, 8);
+    // Write error message
+    std::cerr << " ERROR: ";
+    output(message, std::cerr, 8);
 
 #ifdef _POSIX_VERSION
-  // Reset color for terminal
-  if (isatty(STDERR_FILENO)) {
-    std::cerr << "\033[0m";
-  }
+    // Reset color for terminal
+    if (isatty(STDERR_FILENO)) {
+      std::cerr << "\033[0m";
+    }
 #endif
+  }
 
 #ifdef OPENMC_MPI
   MPI_Abort(mpi::intracomm, err);
diff --git a/src/event.cpp b/src/event.cpp
index ae914c8be..2d436bb9d 100644
--- a/src/event.cpp
+++ b/src/event.cpp
@@ -1,5 +1,9 @@
 #include "openmc/event.h"
+
+#include "openmc/bank.h"
+#include "openmc/error.h"
 #include "openmc/material.h"
+#include "openmc/settings.h"
 #include "openmc/simulation.h"
 #include "openmc/timer.h"
 
@@ -63,7 +67,8 @@ void process_init_events(int64_t n_particles, int64_t source_offset)
   simulation::time_event_init.start();
 #pragma omp parallel for schedule(runtime)
   for (int64_t i = 0; i < n_particles; i++) {
-    initialize_history(simulation::particles[i], source_offset + i + 1);
+    initialize_particle_track(
+      simulation::particles[i], source_offset + i + 1, false);
     dispatch_xs_event(i);
   }
   simulation::time_event_init.stop();
@@ -73,17 +78,17 @@ void process_calculate_xs_events(SharedArray<EventQueueItem>& queue)
 {
   simulation::time_event_calculate_xs.start();
 
-  // TODO: If using C++17, perform a parallel sort of the queue
-  // by particle type, material type, and then energy, in order to
-  // improve cache locality and reduce thread divergence on GPU. Prior
-  // to C++17, std::sort is a serial only operation, which in this case
-  // makes it too slow to be practical for most test problems.
+  // TODO: If using C++17, we could perform a parallel sort of the queue by
+  // particle type, material type, and then energy, in order to improve cache
+  // locality and reduce thread divergence on GPU. However, the parallel
+  // algorithms typically require linking against an additional library (Intel
+  // TBB). Prior to C++17, std::sort is a serial only operation, which in this
+  // case makes it too slow to be practical for most test problems.
   //
   // std::sort(std::execution::par_unseq, queue.data(), queue.data() +
   // queue.size());
 
   int64_t offset = simulation::advance_particle_queue.size();
-  ;
 
 #pragma omp parallel for schedule(runtime)
   for (int64_t i = 0; i < queue.size(); i++) {
@@ -114,7 +119,7 @@ void process_advance_particle_events()
     p.event_advance();
     if (!p.alive())
       continue;
-    if (p.collision_distance() > p.boundary().distance) {
+    if (p.collision_distance() > p.boundary().distance()) {
       simulation::surface_crossing_queue.thread_safe_append({p, buffer_idx});
     } else {
       simulation::collision_queue.thread_safe_append({p, buffer_idx});
@@ -135,7 +140,7 @@ void process_surface_crossing_events()
     int64_t buffer_idx = simulation::surface_crossing_queue[i].idx;
     Particle& p = simulation::particles[buffer_idx];
     p.event_cross_surface();
-    p.event_revive_from_secondary();
+    p.event_check_limit_and_revive();
     if (p.alive())
       dispatch_xs_event(buffer_idx);
   }
@@ -154,7 +159,7 @@ void process_collision_events()
     int64_t buffer_idx = simulation::collision_queue[i].idx;
     Particle& p = simulation::particles[buffer_idx];
     p.event_collide();
-    p.event_revive_from_secondary();
+    p.event_check_limit_and_revive();
     if (p.alive())
       dispatch_xs_event(buffer_idx);
   }
@@ -175,4 +180,45 @@ void process_death_events(int64_t n_particles)
   simulation::time_event_death.stop();
 }
 
+void process_transport_events()
+{
+  while (true) {
+    int64_t max = std::max({simulation::calculate_fuel_xs_queue.size(),
+      simulation::calculate_nonfuel_xs_queue.size(),
+      simulation::advance_particle_queue.size(),
+      simulation::surface_crossing_queue.size(),
+      simulation::collision_queue.size()});
+
+    if (max == 0) {
+      break;
+    } else if (max == simulation::calculate_fuel_xs_queue.size()) {
+      process_calculate_xs_events(simulation::calculate_fuel_xs_queue);
+    } else if (max == simulation::calculate_nonfuel_xs_queue.size()) {
+      process_calculate_xs_events(simulation::calculate_nonfuel_xs_queue);
+    } else if (max == simulation::advance_particle_queue.size()) {
+      process_advance_particle_events();
+    } else if (max == simulation::surface_crossing_queue.size()) {
+      process_surface_crossing_events();
+    } else if (max == simulation::collision_queue.size()) {
+      process_collision_events();
+    }
+  }
+}
+
+void process_init_secondary_events(int64_t n_particles, int64_t offset,
+  const SharedArray<SourceSite>& shared_secondary_bank)
+{
+  simulation::time_event_init.start();
+#pragma omp parallel for schedule(runtime)
+  for (int64_t i = 0; i < n_particles; i++) {
+    initialize_particle_track(simulation::particles[i], offset + i + 1, true);
+    const SourceSite& site = shared_secondary_bank[offset + i];
+    simulation::particles[i].event_revive_from_secondary(site);
+    if (simulation::particles[i].alive()) {
+      dispatch_xs_event(i);
+    }
+  }
+  simulation::time_event_init.stop();
+}
+
 } // namespace openmc
diff --git a/src/external/quartic_solver.cpp b/src/external/quartic_solver.cpp
index 915020ffa..71bdb25c3 100644
--- a/src/external/quartic_solver.cpp
+++ b/src/external/quartic_solver.cpp
@@ -1,5 +1,5 @@
-#include <algorithm>
 #define _USE_MATH_DEFINES // to make M_PI declared in Intel and MSVC compilers
+#include <algorithm>
 #include <cmath>
 #include <complex>
 #include <cstdlib>
diff --git a/src/finalize.cpp b/src/finalize.cpp
index ed3d0e7f4..fd891d9dd 100644
--- a/src/finalize.cpp
+++ b/src/finalize.cpp
@@ -2,7 +2,9 @@
 
 #include "openmc/bank.h"
 #include "openmc/capi.h"
+#include "openmc/chain.h"
 #include "openmc/cmfd_solver.h"
+#include "openmc/collision_track.h"
 #include "openmc/constants.h"
 #include "openmc/cross_sections.h"
 #include "openmc/dagmc.h"
@@ -13,6 +15,7 @@
 #include "openmc/material.h"
 #include "openmc/mesh.h"
 #include "openmc/message_passing.h"
+#include "openmc/mgxs_interface.h"
 #include "openmc/nuclide.h"
 #include "openmc/photon.h"
 #include "openmc/plot.h"
@@ -28,7 +31,7 @@
 #include "openmc/volume_calc.h"
 #include "openmc/weight_windows.h"
 
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 namespace openmc {
 
@@ -42,6 +45,7 @@ void free_memory()
   free_memory_photon();
   free_memory_settings();
   free_memory_thermal();
+  free_memory_chain();
   library_clear();
   nuclides_clear();
   free_memory_source();
@@ -76,6 +80,7 @@ int openmc_finalize()
   // Reset global variables
   settings::assume_separate = false;
   settings::check_overlaps = false;
+  settings::collision_track_config = CollisionTrackConfig {};
   settings::confidence_intervals = false;
   settings::create_fission_neutrons = true;
   settings::create_delayed_neutrons = true;
@@ -85,6 +90,7 @@ int openmc_finalize()
   settings::time_cutoff = {INFTY, INFTY, INFTY, INFTY};
   settings::entropy_on = false;
   settings::event_based = false;
+  settings::free_gas_threshold = 400.0;
   settings::gen_per_batch = 1;
   settings::legendre_to_tabular = true;
   settings::legendre_to_tabular_points = -1;
@@ -92,6 +98,7 @@ int openmc_finalize()
   settings::max_lost_particles = 10;
   settings::max_order = 0;
   settings::max_particles_in_flight = 100000;
+  settings::max_secondaries = 10000;
   settings::max_particle_events = 1'000'000;
   settings::max_history_splits = 10'000'000;
   settings::max_tracks = 1000;
@@ -118,7 +125,11 @@ int openmc_finalize()
   settings::restart_run = false;
   settings::run_CE = true;
   settings::run_mode = RunMode::UNSET;
+  settings::surface_grazing_cutoff = 0.001;
+  settings::surface_grazing_ratio = 0.5;
+  settings::solver_type = SolverType::MONTE_CARLO;
   settings::source_latest = false;
+  settings::source_rejection_fraction = 0.05;
   settings::source_separate = false;
   settings::source_write = true;
   settings::ssw_cell_id = C_NONE;
@@ -131,13 +142,16 @@ int openmc_finalize()
   settings::temperature_multipole = false;
   settings::temperature_range = {0.0, 0.0};
   settings::temperature_tolerance = 10.0;
+  settings::properties_file.clear();
   settings::trigger_on = false;
   settings::trigger_predict = false;
   settings::trigger_batch_interval = 1;
   settings::uniform_source_sampling = false;
   settings::ufs_on = false;
   settings::urr_ptables_on = true;
-  settings::verbosity = 7;
+  settings::use_decay_photons = false;
+  settings::use_shared_secondary_bank = false;
+  settings::verbosity = -1;
   settings::weight_cutoff = 0.25;
   settings::weight_survive = 1.0;
   settings::weight_windows_file.clear();
@@ -153,10 +167,11 @@ int openmc_finalize()
   simulation::entropy_mesh = nullptr;
   simulation::ufs_mesh = nullptr;
 
-  data::energy_max = {INFTY, INFTY};
-  data::energy_min = {0.0, 0.0};
+  data::energy_max = {INFTY, INFTY, INFTY, INFTY};
+  data::energy_min = {0.0, 0.0, 0.0, 0.0};
   data::temperature_min = 0.0;
   data::temperature_max = INFTY;
+  data::mg = {};
   model::root_universe = -1;
   model::plotter_seed = 1;
   openmc::openmc_set_seed(DEFAULT_SEED);
@@ -165,17 +180,21 @@ int openmc_finalize()
   // Deallocate arrays
   free_memory();
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
   settings::libmesh_init.reset();
 #endif
 
   // Free all MPI types
 #ifdef OPENMC_MPI
-  if (mpi::source_site != MPI_DATATYPE_NULL)
+  if (mpi::source_site != MPI_DATATYPE_NULL) {
     MPI_Type_free(&mpi::source_site);
+  }
+  if (mpi::collision_track_site != MPI_DATATYPE_NULL) {
+    MPI_Type_free(&mpi::collision_track_site);
+  }
 #endif
 
-  openmc_reset_random_ray();
+  openmc_finalize_random_ray();
 
   return 0;
 }
@@ -183,7 +202,6 @@ int openmc_finalize()
 int openmc_reset()
 {
 
-  model::universe_cell_counts.clear();
   model::universe_level_counts.clear();
 
   for (auto& t : model::tallies) {
@@ -192,7 +210,7 @@ int openmc_reset()
 
   // Reset global tallies
   simulation::n_realizations = 0;
-  xt::view(simulation::global_tallies, xt::all()) = 0.0;
+  simulation::global_tallies.fill(0.0);
 
   simulation::k_col_abs = 0.0;
   simulation::k_col_tra = 0.0;
@@ -203,6 +221,7 @@ int openmc_reset()
   settings::cmfd_run = false;
 
   simulation::n_lost_particles = 0;
+  simulation::simulation_tracks_completed = 0;
 
   return 0;
 }
diff --git a/src/geometry.cpp b/src/geometry.cpp
index 9e975c00d..bf654f4f9 100644
--- a/src/geometry.cpp
+++ b/src/geometry.cpp
@@ -26,39 +26,64 @@ int n_coord_levels;
 
 vector<int64_t> overlap_check_count;
 
+vector<OverlapKey> overlap_keys;
+std::unordered_map<OverlapKey, int, OverlapKeyHash> overlap_key_index;
+
 } // namespace model
 
 //==============================================================================
 // Non-member functions
 //==============================================================================
 
-bool check_cell_overlap(GeometryState& p, bool error)
+int check_cell_overlap(GeometryState& p, bool error)
 {
   int n_coord = p.n_coord();
 
+  // If no overlap found, return a nonphysical index
+  int overlap_index = -1;
+
   // Loop through each coordinate level
   for (int j = 0; j < n_coord; j++) {
-    Universe& univ = *model::universes[p.coord(j).universe];
+    Universe& univ = *model::universes[p.coord(j).universe()];
 
     // Loop through each cell on this level
     for (auto index_cell : univ.cells_) {
       Cell& c = *model::cells[index_cell];
-      if (c.contains(p.coord(j).r, p.coord(j).u, p.surface())) {
-        if (index_cell != p.coord(j).cell) {
+      if (c.contains(p.coord(j).r(), p.coord(j).u(), p.surface())) {
+#pragma omp atomic
+        ++model::overlap_check_count[index_cell];
+        if (index_cell != p.coord(j).cell()) {
           if (error) {
             fatal_error(
               fmt::format("Overlapping cells detected: {}, {} on universe {}",
-                c.id_, model::cells[p.coord(j).cell]->id_, univ.id_));
+                c.id_, model::cells[p.coord(j).cell()]->id_, univ.id_));
           }
-          return true;
+
+          // With no fatal error (plotter is calling), now adds overlaps and
+          // ensures order does not matter when making overlap key
+          int cell_a = model::cells[index_cell]->id_;
+          int cell_b = model::cells[p.coord(j).cell()]->id_;
+          int a = std::min(cell_a, cell_b);
+          int b = std::max(cell_a, cell_b);
+          OverlapKey key {univ.id_, a, b};
+#pragma omp critical(overlap_key_update)
+          {
+            auto it = model::overlap_key_index.find(key);
+            if (it != model::overlap_key_index.end()) {
+              overlap_index = it->second; // already exists, reuse index
+            } else {
+              int idx = int(model::overlap_keys.size());
+              model::overlap_keys.push_back(key);
+              model::overlap_key_index[key] = idx;
+              overlap_index = idx;
+            }
+          }
+          break;
         }
-#pragma omp atomic
-        ++model::overlap_check_count[index_cell];
       }
     }
   }
-
-  return false;
+  return overlap_index;
 }
 
 //==============================================================================
@@ -73,7 +98,7 @@ int cell_instance_at_level(const GeometryState& p, int level)
   }
 
   // determine the cell instance
-  Cell& c {*model::cells[p.coord(level).cell]};
+  Cell& c {*model::cells[p.coord(level).cell()]};
 
   // quick exit if this cell doesn't have distribcell instances
   if (c.distribcell_index_ == C_NONE)
@@ -82,13 +107,13 @@ int cell_instance_at_level(const GeometryState& p, int level)
   // compute the cell's instance
   int instance = 0;
   for (int i = 0; i < level; i++) {
-    const auto& c_i {*model::cells[p.coord(i).cell]};
+    const auto& c_i {*model::cells[p.coord(i).cell()]};
     if (c_i.type_ == Fill::UNIVERSE) {
       instance += c_i.offset_[c.distribcell_index_];
     } else if (c_i.type_ == Fill::LATTICE) {
       instance += c_i.offset_[c.distribcell_index_];
-      auto& lat {*model::lattices[p.coord(i + 1).lattice]};
-      const auto& i_xyz {p.coord(i + 1).lattice_i};
+      auto& lat {*model::lattices[p.coord(i + 1).lattice()]};
+      const auto& i_xyz {p.coord(i + 1).lattice_index()};
       if (lat.are_valid_indices(i_xyz)) {
         instance += lat.offset(c.distribcell_index_, i_xyz);
       }
@@ -111,7 +136,7 @@ bool find_cell_inner(
       i_cell = *it;
 
       // Make sure the search cell is in the same universe.
-      int i_universe = p.lowest_coord().universe;
+      int i_universe = p.lowest_coord().universe();
       if (model::cells[i_cell]->universe_ != i_universe)
         continue;
 
@@ -120,7 +145,7 @@ bool find_cell_inner(
       Direction u {p.u_local()};
       auto surf = p.surface();
       if (model::cells[i_cell]->contains(r, u, surf)) {
-        p.lowest_coord().cell = i_cell;
+        p.lowest_coord().cell() = i_cell;
         found = true;
         break;
       }
@@ -146,7 +171,7 @@ bool find_cell_inner(
     // code below this conditional, we set i_cell back to C_NONE to indicate
     // that.
     if (i_cell == C_NONE) {
-      int i_universe = p.lowest_coord().universe;
+      int i_universe = p.lowest_coord().universe();
       const auto& univ {model::universes[i_universe]};
       found = univ->find_cell(p);
     }
@@ -154,7 +179,7 @@ bool find_cell_inner(
     if (!found) {
       return found;
     }
-    i_cell = p.lowest_coord().cell;
+    i_cell = p.lowest_coord().cell();
 
     // Announce the cell that the particle is entering.
     if (found && verbose) {
@@ -172,11 +197,13 @@ bool find_cell_inner(
         p.cell_instance() = cell_instance_at_level(p, p.n_coord() - 1);
       }
 
-      // Set the material and temperature.
+      // Set the material, temperature and density multiplier.
       p.material_last() = p.material();
       p.material() = c.material(p.cell_instance());
       p.sqrtkT_last() = p.sqrtkT();
       p.sqrtkT() = c.sqrtkT(p.cell_instance());
+      p.density_mult_last() = p.density_mult();
+      p.density_mult() = c.density_mult(p.cell_instance());
 
       return true;
 
@@ -186,14 +213,14 @@ bool find_cell_inner(
 
       // Set the lower coordinate level universe.
       auto& coord {p.coord(p.n_coord())};
-      coord.universe = c.fill_;
+      coord.universe() = c.fill_;
 
       // Set the position and direction.
-      coord.r = p.r_local();
-      coord.u = p.u_local();
+      coord.r() = p.r_local();
+      coord.u() = p.u_local();
 
       // Apply translation.
-      coord.r -= c.translation_;
+      coord.r() -= c.translation_;
 
       // Apply rotation.
       if (!c.rotation_.empty()) {
@@ -208,11 +235,11 @@ bool find_cell_inner(
 
       // Set the position and direction.
       auto& coord {p.coord(p.n_coord())};
-      coord.r = p.r_local();
-      coord.u = p.u_local();
+      coord.r() = p.r_local();
+      coord.u() = p.u_local();
 
       // Apply translation.
-      coord.r -= c.translation_;
+      coord.r() -= c.translation_;
 
       // Apply rotation.
       if (!c.rotation_.empty()) {
@@ -220,21 +247,21 @@ bool find_cell_inner(
       }
 
       // Determine lattice indices.
-      auto& i_xyz {coord.lattice_i};
-      lat.get_indices(coord.r, coord.u, i_xyz);
+      auto& i_xyz {coord.lattice_index()};
+      lat.get_indices(coord.r(), coord.u(), i_xyz);
 
       // Get local position in appropriate lattice cell
-      coord.r = lat.get_local_position(coord.r, i_xyz);
+      coord.r() = lat.get_local_position(coord.r(), i_xyz);
 
       // Set lattice indices.
-      coord.lattice = c.fill_;
+      coord.lattice() = c.fill_;
 
       // Set the lower coordinate level universe.
       if (lat.are_valid_indices(i_xyz)) {
-        coord.universe = lat[i_xyz];
+        coord.universe() = lat[i_xyz];
       } else {
         if (lat.outer_ != NO_OUTER_UNIVERSE) {
-          coord.universe = lat.outer_;
+          coord.universe() = lat.outer_;
         } else {
           p.mark_as_lost(fmt::format(
             "Particle {} left lattice {}, but it has no outer definition.",
@@ -261,7 +288,7 @@ bool neighbor_list_find_cell(GeometryState& p, bool verbose)
 
   // Get the cell this particle was in previously.
   auto coord_lvl = p.n_coord() - 1;
-  auto i_cell = p.coord(coord_lvl).cell;
+  auto i_cell = p.coord(coord_lvl).cell();
   Cell& c {*model::cells[i_cell]};
 
   // Search for the particle in that cell's neighbor list.  Return if we
@@ -275,15 +302,15 @@ bool neighbor_list_find_cell(GeometryState& p, bool verbose)
   // neighboring cell.
   found = find_cell_inner(p, nullptr, verbose);
   if (found)
-    c.neighbors_.push_back(p.coord(coord_lvl).cell);
+    c.neighbors_.push_back(p.coord(coord_lvl).cell());
   return found;
 }
 
 bool exhaustive_find_cell(GeometryState& p, bool verbose)
 {
-  int i_universe = p.lowest_coord().universe;
+  int i_universe = p.lowest_coord().universe();
   if (i_universe == C_NONE) {
-    p.coord(0).universe = model::root_universe;
+    p.coord(0).universe() = model::root_universe;
     p.n_coord() = 1;
     i_universe = model::root_universe;
   }
@@ -299,32 +326,32 @@ bool exhaustive_find_cell(GeometryState& p, bool verbose)
 void cross_lattice(GeometryState& p, const BoundaryInfo& boundary, bool verbose)
 {
   auto& coord {p.lowest_coord()};
-  auto& lat {*model::lattices[coord.lattice]};
+  auto& lat {*model::lattices[coord.lattice()]};
 
   if (verbose) {
     write_message(
       fmt::format("    Crossing lattice {}. Current position ({},{},{}). r={}",
-        lat.id_, coord.lattice_i[0], coord.lattice_i[1], coord.lattice_i[2],
-        p.r()),
+        lat.id_, coord.lattice_index()[0], coord.lattice_index()[1],
+        coord.lattice_index()[2], p.r()),
       1);
   }
 
   // Set the lattice indices.
-  coord.lattice_i[0] += boundary.lattice_translation[0];
-  coord.lattice_i[1] += boundary.lattice_translation[1];
-  coord.lattice_i[2] += boundary.lattice_translation[2];
+  coord.lattice_index()[0] += boundary.lattice_translation()[0];
+  coord.lattice_index()[1] += boundary.lattice_translation()[1];
+  coord.lattice_index()[2] += boundary.lattice_translation()[2];
 
   // Set the new coordinate position.
   const auto& upper_coord {p.coord(p.n_coord() - 2)};
-  const auto& cell {model::cells[upper_coord.cell]};
-  Position r = upper_coord.r;
+  const auto& cell {model::cells[upper_coord.cell()]};
+  Position r = upper_coord.r();
   r -= cell->translation_;
   if (!cell->rotation_.empty()) {
     r = r.rotate(cell->rotation_);
   }
-  p.r_local() = lat.get_local_position(r, coord.lattice_i);
+  p.r_local() = lat.get_local_position(r, coord.lattice_index());
 
-  if (!lat.are_valid_indices(coord.lattice_i)) {
+  if (!lat.are_valid_indices(coord.lattice_index())) {
     // The particle is outside the lattice.  Search for it from the base coords.
     p.n_coord() = 1;
     bool found = exhaustive_find_cell(p);
@@ -337,7 +364,7 @@ void cross_lattice(GeometryState& p, const BoundaryInfo& boundary, bool verbose)
 
   } else {
     // Find cell in next lattice element.
-    p.lowest_coord().universe = lat[coord.lattice_i];
+    p.lowest_coord().universe() = lat[coord.lattice_index()];
     bool found = exhaustive_find_cell(p);
 
     if (!found) {
@@ -367,9 +394,9 @@ BoundaryInfo distance_to_boundary(GeometryState& p)
   // Loop over each coordinate level.
   for (int i = 0; i < p.n_coord(); i++) {
     const auto& coord {p.coord(i)};
-    const Position& r {coord.r};
-    const Direction& u {coord.u};
-    Cell& c {*model::cells[coord.cell]};
+    const Position& r {coord.r()};
+    const Direction& u {coord.u()};
+    Cell& c {*model::cells[coord.cell()]};
 
     // Find the oncoming surface in this cell and the distance to it.
     auto surface_distance = c.distance(r, u, p.surface(), &p);
@@ -377,24 +404,24 @@ BoundaryInfo distance_to_boundary(GeometryState& p)
     level_surf_cross = surface_distance.second;
 
     // Find the distance to the next lattice tile crossing.
-    if (coord.lattice != C_NONE) {
-      auto& lat {*model::lattices[coord.lattice]};
+    if (coord.lattice() != C_NONE) {
+      auto& lat {*model::lattices[coord.lattice()]};
       // TODO: refactor so both lattice use the same position argument (which
       // also means the lat.type attribute can be removed)
       std::pair<double, array<int, 3>> lattice_distance;
       switch (lat.type_) {
       case LatticeType::rect:
-        lattice_distance = lat.distance(r, u, coord.lattice_i);
+        lattice_distance = lat.distance(r, u, coord.lattice_index());
         break;
       case LatticeType::hex:
-        auto& cell_above {model::cells[p.coord(i - 1).cell]};
-        Position r_hex {p.coord(i - 1).r};
+        auto& cell_above {model::cells[p.coord(i - 1).cell()]};
+        Position r_hex {p.coord(i - 1).r()};
         r_hex -= cell_above->translation_;
-        if (coord.rotated) {
+        if (coord.rotated()) {
           r_hex = r_hex.rotate(cell_above->rotation_);
         }
-        r_hex.z = coord.r.z;
-        lattice_distance = lat.distance(r_hex, u, coord.lattice_i);
+        r_hex.z = coord.r().z;
+        lattice_distance = lat.distance(r_hex, u, coord.lattice_index());
         break;
       }
       d_lat = lattice_distance.first;
@@ -410,7 +437,7 @@ BoundaryInfo distance_to_boundary(GeometryState& p)
     // If the boundary on this coordinate level is coincident with a boundary on
     // a higher level then we need to make sure that the higher level boundary
     // is selected.  This logic must consider floating point precision.
-    double& d = info.distance;
+    double& d = info.distance();
     if (d_surf < d_lat - FP_COINCIDENT) {
       if (d == INFINITY || (d - d_surf) / d >= FP_REL_PRECISION) {
         // Update closest distance
@@ -421,29 +448,29 @@ BoundaryInfo distance_to_boundary(GeometryState& p)
         // have to explicitly check which half-space the particle would be
         // traveling into if the surface is crossed
         if (c.is_simple() || d == INFTY) {
-          info.surface = level_surf_cross;
+          info.surface() = level_surf_cross;
         } else {
           Position r_hit = r + d_surf * u;
           Surface& surf {*model::surfaces[std::abs(level_surf_cross) - 1]};
           Direction norm = surf.normal(r_hit);
           if (u.dot(norm) > 0) {
-            info.surface = std::abs(level_surf_cross);
+            info.surface() = std::abs(level_surf_cross);
           } else {
-            info.surface = -std::abs(level_surf_cross);
+            info.surface() = -std::abs(level_surf_cross);
           }
         }
 
-        info.lattice_translation[0] = 0;
-        info.lattice_translation[1] = 0;
-        info.lattice_translation[2] = 0;
-        info.coord_level = i + 1;
+        info.lattice_translation()[0] = 0;
+        info.lattice_translation()[1] = 0;
+        info.lattice_translation()[2] = 0;
+        info.coord_level() = i + 1;
       }
     } else {
       if (d == INFINITY || (d - d_lat) / d >= FP_REL_PRECISION) {
         d = d_lat;
-        info.surface = SURFACE_NONE;
-        info.lattice_translation = level_lat_trans;
-        info.coord_level = i + 1;
+        info.surface() = SURFACE_NONE;
+        info.lattice_translation() = level_lat_trans;
+        info.coord_level() = i + 1;
       }
     }
   }
@@ -468,7 +495,7 @@ extern "C" int openmc_find_cell(
     return OPENMC_E_GEOMETRY;
   }
 
-  *index = geom_state.lowest_coord().cell;
+  *index = geom_state.lowest_coord().cell();
   *instance = geom_state.cell_instance();
   return 0;
 }
@@ -478,14 +505,14 @@ extern "C" int openmc_global_bounding_box(double* llc, double* urc)
   auto bbox = model::universes.at(model::root_universe)->bounding_box();
 
   // set lower left corner values
-  llc[0] = bbox.xmin;
-  llc[1] = bbox.ymin;
-  llc[2] = bbox.zmin;
+  llc[0] = bbox.min.x;
+  llc[1] = bbox.min.y;
+  llc[2] = bbox.min.z;
 
   // set upper right corner values
-  urc[0] = bbox.xmax;
-  urc[1] = bbox.ymax;
-  urc[2] = bbox.zmax;
+  urc[0] = bbox.max.x;
+  urc[1] = bbox.max.y;
+  urc[2] = bbox.max.z;
 
   return 0;
 }
diff --git a/src/geometry_aux.cpp b/src/geometry_aux.cpp
index 2f8a55743..a740740c1 100644
--- a/src/geometry_aux.cpp
+++ b/src/geometry_aux.cpp
@@ -25,21 +25,9 @@
 namespace openmc {
 
 namespace model {
-std::unordered_map<int32_t, std::unordered_map<int32_t, int32_t>>
-  universe_cell_counts;
 std::unordered_map<int32_t, int32_t> universe_level_counts;
 } // namespace model
 
-// adds the cell counts of universe b to universe a
-void update_universe_cell_count(int32_t a, int32_t b)
-{
-  auto& universe_a_counts = model::universe_cell_counts[a];
-  const auto& universe_b_counts = model::universe_cell_counts[b];
-  for (const auto& it : universe_b_counts) {
-    universe_a_counts[it.first] += it.second;
-  }
-}
-
 void read_geometry_xml()
 {
   // Display output message
@@ -67,8 +55,13 @@ void read_geometry_xml()
 void read_geometry_xml(pugi::xml_node root)
 {
   // Read surfaces, cells, lattice
-  read_surfaces(root);
+  std::set<std::pair<int, int>> periodic_pairs;
+  std::unordered_map<int, double> albedo_map;
+  std::unordered_map<int, int> periodic_sense_map;
+
+  read_surfaces(root, periodic_pairs, albedo_map, periodic_sense_map);
   read_cells(root);
+  prepare_boundary_conditions(periodic_pairs, albedo_map, periodic_sense_map);
   read_lattices(root);
 
   // Check to make sure a boundary condition was applied to at least one
@@ -207,6 +200,24 @@ void assign_temperatures()
 
 //==============================================================================
 
+void finalize_cell_densities()
+{
+  for (auto& c : model::cells) {
+    // Convert to density multipliers.
+    if (!c->density_mult_.empty()) {
+      for (int32_t instance = 0; instance < c->density_mult_.size();
+           ++instance) {
+        c->density_mult_[instance] /=
+          model::materials[c->material(instance)]->density_gpcc();
+      }
+    } else {
+      c->density_mult_ = {1.0};
+    }
+  }
+}
+
+//==============================================================================
+
 void get_temperatures(
   vector<vector<double>>& nuc_temps, vector<vector<double>>& thermal_temps)
 {
@@ -263,7 +274,7 @@ void finalize_geometry()
 {
   // Perform some final operations to set up the geometry
   adjust_indices();
-  count_cell_instances(model::root_universe);
+  count_universe_instances();
   partition_universes();
 
   // Assign temperatures to cells that don't have temperatures already assigned
@@ -356,35 +367,49 @@ void prepare_distribcell(const std::vector<int32_t>* user_distribcells)
     Cell& c {*model::cells[i]};
 
     if (c.material_.size() > 1) {
-      if (c.material_.size() != c.n_instances_) {
+      if (c.material_.size() != c.n_instances()) {
         fatal_error(fmt::format(
           "Cell {} was specified with {} materials but has {} distributed "
           "instances. The number of materials must equal one or the number "
           "of instances.",
-          c.id_, c.material_.size(), c.n_instances_));
+          c.id_, c.material_.size(), c.n_instances()));
       }
     }
 
     if (c.sqrtkT_.size() > 1) {
-      if (c.sqrtkT_.size() != c.n_instances_) {
+      if (c.sqrtkT_.size() != c.n_instances()) {
         fatal_error(fmt::format(
           "Cell {} was specified with {} temperatures but has {} distributed "
           "instances. The number of temperatures must equal one or the number "
           "of instances.",
-          c.id_, c.sqrtkT_.size(), c.n_instances_));
+          c.id_, c.sqrtkT_.size(), c.n_instances()));
+      }
+    }
+
+    if (c.density_mult_.size() > 1) {
+      if (c.density_mult_.size() != c.n_instances()) {
+        fatal_error(fmt::format("Cell {} was specified with {} density "
+                                "multipliers but has {} distributed "
+                                "instances. The number of density multipliers "
+                                "must equal one or the number "
+                                "of instances.",
+          c.id_, c.density_mult_.size(), c.n_instances()));
       }
     }
   }
 
   // Search through universes for material cells and assign each one a
-  // unique distribcell array index.
-  int distribcell_index = 0;
+  // distribcell array index according to the containing universe.
   vector<int32_t> target_univ_ids;
   for (const auto& u : model::universes) {
     for (auto idx : u->cells_) {
       if (distribcells.find(idx) != distribcells.end()) {
-        model::cells[idx]->distribcell_index_ = distribcell_index++;
-        target_univ_ids.push_back(u->id_);
+        if (!contains(target_univ_ids, u->id_)) {
+          target_univ_ids.push_back(u->id_);
+        }
+        model::cells[idx]->distribcell_index_ =
+          std::find(target_univ_ids.begin(), target_univ_ids.end(), u->id_) -
+          target_univ_ids.begin();
       }
     }
   }
@@ -419,8 +444,7 @@ void prepare_distribcell(const std::vector<int32_t>* user_distribcells)
         } else if (c.type_ == Fill::LATTICE) {
           c.offset_[map] = offset;
           Lattice& lat = *model::lattices[c.fill_];
-          offset +=
-            lat.fill_offset_table(offset, target_univ_id, map, univ_count_memo);
+          offset += lat.fill_offset_table(target_univ_id, map, univ_count_memo);
         }
       }
     }
@@ -429,32 +453,12 @@ void prepare_distribcell(const std::vector<int32_t>* user_distribcells)
 
 //==============================================================================
 
-void count_cell_instances(int32_t univ_indx)
+void count_universe_instances()
 {
-  const auto univ_counts = model::universe_cell_counts.find(univ_indx);
-  if (univ_counts != model::universe_cell_counts.end()) {
-    for (const auto& it : univ_counts->second) {
-      model::cells[it.first]->n_instances_ += it.second;
-    }
-  } else {
-    for (int32_t cell_indx : model::universes[univ_indx]->cells_) {
-      Cell& c = *model::cells[cell_indx];
-      ++c.n_instances_;
-      model::universe_cell_counts[univ_indx][cell_indx] += 1;
-
-      if (c.type_ == Fill::UNIVERSE) {
-        // This cell contains another universe.  Recurse into that universe.
-        count_cell_instances(c.fill_);
-        update_universe_cell_count(univ_indx, c.fill_);
-      } else if (c.type_ == Fill::LATTICE) {
-        // This cell contains a lattice.  Recurse into the lattice universes.
-        Lattice& lat = *model::lattices[c.fill_];
-        for (auto it = lat.begin(); it != lat.end(); ++it) {
-          count_cell_instances(*it);
-          update_universe_cell_count(univ_indx, *it);
-        }
-      }
-    }
+  for (auto& univ : model::universes) {
+    std::unordered_map<int32_t, int32_t> univ_count_memo;
+    univ->n_instances_ = count_universe_instances(
+      model::root_universe, univ->id_, univ_count_memo);
   }
 }
 
@@ -528,19 +532,16 @@ std::string distribcell_path_inner(int32_t target_cell, int32_t map,
 
     // Material cells don't contain other cells so ignore them.
     if (c.type_ != Fill::MATERIAL) {
-      int32_t temp_offset;
-      if (c.type_ == Fill::UNIVERSE) {
-        temp_offset =
-          offset + c.offset_[map]; // TODO: should also apply to lattice fills?
-      } else {
+      int32_t temp_offset = offset + c.offset_[map];
+      if (c.type_ == Fill::LATTICE) {
         Lattice& lat = *model::lattices[c.fill_];
         int32_t indx = lat.universes_.size() * map + lat.begin().indx_;
-        temp_offset = offset + lat.offsets_[indx];
+        temp_offset += lat.offsets_[indx];
       }
 
       // The desired cell is the first cell that gives an offset smaller or
       // equal to the target offset.
-      if (temp_offset <= target_offset - c.offset_[map])
+      if (temp_offset <= target_offset)
         break;
     }
   }
@@ -570,12 +571,12 @@ std::string distribcell_path_inner(int32_t target_cell, int32_t map,
     path << "l" << lat.id_;
     for (ReverseLatticeIter it = lat.rbegin(); it != lat.rend(); ++it) {
       int32_t indx = lat.universes_.size() * map + it.indx_;
-      int32_t temp_offset = offset + lat.offsets_[indx];
-      if (temp_offset <= target_offset - c.offset_[map]) {
+      int32_t temp_offset = offset + lat.offsets_[indx] + c.offset_[map];
+      if (temp_offset <= target_offset) {
         offset = temp_offset;
         path << "(" << lat.index_to_string(it.indx_) << ")->";
-        path << distribcell_path_inner(target_cell, map, target_offset,
-          *model::universes[*it], offset + c.offset_[map]);
+        path << distribcell_path_inner(
+          target_cell, map, target_offset, *model::universes[*it], offset);
         return path.str();
       }
     }
diff --git a/src/hdf5_interface.cpp b/src/hdf5_interface.cpp
index 27b750b93..00c6a4399 100644
--- a/src/hdf5_interface.cpp
+++ b/src/hdf5_interface.cpp
@@ -4,8 +4,7 @@
 #include <stdexcept>
 #include <string>
 
-#include "xtensor/xarray.hpp"
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include "hdf5.h"
@@ -92,7 +91,7 @@ void get_shape_attr(hid_t obj_id, const char* name, hsize_t* dims)
   H5Aclose(attr);
 }
 
-hid_t create_group(hid_t parent_id, char const* name)
+hid_t create_group(hid_t parent_id, const char* name)
 {
   hid_t out = H5Gcreate(parent_id, name, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
   if (out < 0) {
@@ -225,8 +224,7 @@ void get_name(hid_t obj_id, std::string& name)
 {
   size_t size = 1 + H5Iget_name(obj_id, nullptr, 0);
   name.resize(size);
-  // TODO: switch to name.data() when using C++17
-  H5Iget_name(obj_id, &name[0], size);
+  H5Iget_name(obj_id, name.data(), size);
 }
 
 int get_num_datasets(hid_t group_id)
@@ -467,22 +465,19 @@ void read_dataset_lowlevel(hid_t obj_id, const char* name, hid_t mem_type_id,
 }
 
 template<>
-void read_dataset(hid_t dset, xt::xarray<std::complex<double>>& arr, bool indep)
+void read_dataset(
+  hid_t dset, tensor::Tensor<std::complex<double>>& tensor, bool indep)
 {
   // Get shape of dataset
   vector<hsize_t> shape = object_shape(dset);
 
-  // Allocate new array to read data into
-  std::size_t size = 1;
-  for (const auto x : shape)
-    size *= x;
-  vector<std::complex<double>> buffer(size);
+  // Resize tensor and read data directly
+  vector<size_t> tshape(shape.begin(), shape.end());
+  tensor.resize(tshape);
 
-  // Read data from attribute
-  read_complex(dset, nullptr, buffer.data(), indep);
-
-  // Adapt into xarray
-  arr = xt::adapt(buffer, shape);
+  // Read data from dataset
+  read_complex(dset, nullptr,
+    reinterpret_cast<std::complex<double>*>(tensor.data()), indep);
 }
 
 void read_double(hid_t obj_id, const char* name, double* buffer, bool indep)
@@ -537,14 +532,14 @@ void read_complex(
   H5Tclose(complex_id);
 }
 
-void read_tally_results(
-  hid_t group_id, hsize_t n_filter, hsize_t n_score, double* results)
+void read_tally_results(hid_t group_id, hsize_t n_filter, hsize_t n_score,
+  hsize_t n_results, double* results)
 {
   // Create dataspace for hyperslab in memory
   constexpr int ndim = 3;
-  hsize_t dims[ndim] {n_filter, n_score, 3};
+  hsize_t dims[ndim] {n_filter, n_score, n_results};
   hsize_t start[ndim] {0, 0, 1};
-  hsize_t count[ndim] {n_filter, n_score, 2};
+  hsize_t count[ndim] {n_filter, n_score, n_results - 1};
   hid_t memspace = H5Screate_simple(ndim, dims, nullptr);
   H5Sselect_hyperslab(memspace, H5S_SELECT_SET, start, nullptr, count, nullptr);
 
@@ -687,15 +682,15 @@ void write_string(
     group_id, 0, nullptr, buffer.length(), name, buffer.c_str(), indep);
 }
 
-void write_tally_results(
-  hid_t group_id, hsize_t n_filter, hsize_t n_score, const double* results)
+void write_tally_results(hid_t group_id, hsize_t n_filter, hsize_t n_score,
+  hsize_t n_results, const double* results)
 {
   // Set dimensions of sum/sum_sq hyperslab to store
   constexpr int ndim = 3;
-  hsize_t count[ndim] {n_filter, n_score, 2};
+  hsize_t count[ndim] {n_filter, n_score, n_results - 1};
 
   // Set dimensions of results array
-  hsize_t dims[ndim] {n_filter, n_score, 3};
+  hsize_t dims[ndim] {n_filter, n_score, n_results};
   hsize_t start[ndim] {0, 0, 1};
   hid_t memspace = H5Screate_simple(ndim, dims, nullptr);
   H5Sselect_hyperslab(memspace, H5S_SELECT_SET, start, nullptr, count, nullptr);
diff --git a/src/ifp.cpp b/src/ifp.cpp
new file mode 100644
index 000000000..f0f98b8ef
--- /dev/null
+++ b/src/ifp.cpp
@@ -0,0 +1,135 @@
+#include "openmc/ifp.h"
+
+#include "openmc/bank.h"
+#include "openmc/message_passing.h"
+#include "openmc/particle.h"
+#include "openmc/particle_data.h"
+#include "openmc/settings.h"
+#include "openmc/simulation.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+bool is_beta_effective_or_both()
+{
+  if (settings::ifp_parameter == IFPParameter::BetaEffective ||
+      settings::ifp_parameter == IFPParameter::Both) {
+    return true;
+  }
+  return false;
+}
+
+bool is_generation_time_or_both()
+{
+  if (settings::ifp_parameter == IFPParameter::GenerationTime ||
+      settings::ifp_parameter == IFPParameter::Both) {
+    return true;
+  }
+  return false;
+}
+
+void ifp(const Particle& p, int64_t idx)
+{
+  if (is_beta_effective_or_both()) {
+    const auto& delayed_groups =
+      simulation::ifp_source_delayed_group_bank[p.current_work()];
+    simulation::ifp_fission_delayed_group_bank[idx] =
+      _ifp(p.delayed_group(), delayed_groups);
+  }
+  if (is_generation_time_or_both()) {
+    const auto& lifetimes =
+      simulation::ifp_source_lifetime_bank[p.current_work()];
+    simulation::ifp_fission_lifetime_bank[idx] = _ifp(p.lifetime(), lifetimes);
+  }
+}
+
+void resize_simulation_ifp_banks()
+{
+  resize_ifp_data(simulation::ifp_source_delayed_group_bank,
+    simulation::ifp_source_lifetime_bank, simulation::work_per_rank);
+  resize_ifp_data(simulation::ifp_fission_delayed_group_bank,
+    simulation::ifp_fission_lifetime_bank, 3 * simulation::work_per_rank);
+}
+
+void copy_ifp_data_from_fission_banks(
+  int i_bank, vector<int>& delayed_groups, vector<double>& lifetimes)
+{
+  if (is_beta_effective_or_both()) {
+    delayed_groups = simulation::ifp_fission_delayed_group_bank[i_bank];
+  }
+  if (is_generation_time_or_both()) {
+    lifetimes = simulation::ifp_fission_lifetime_bank[i_bank];
+  }
+}
+
+#ifdef OPENMC_MPI
+void broadcast_ifp_n_generation(int& n_generation,
+  const vector<vector<int>>& delayed_groups,
+  const vector<vector<double>>& lifetimes)
+{
+  if (mpi::rank == 0) {
+    if (is_beta_effective_or_both()) {
+      n_generation = static_cast<int>(delayed_groups[0].size());
+    } else {
+      n_generation = static_cast<int>(lifetimes[0].size());
+    }
+  }
+  MPI_Bcast(&n_generation, 1, MPI_INT, 0, mpi::intracomm);
+}
+
+void copy_partial_ifp_data_to_source_banks(int64_t idx, int n, int64_t i_bank,
+  const vector<vector<int>>& delayed_groups,
+  const vector<vector<double>>& lifetimes)
+{
+  if (is_beta_effective_or_both()) {
+    std::copy(&delayed_groups[idx], &delayed_groups[idx + n],
+      &simulation::ifp_source_delayed_group_bank[i_bank]);
+  }
+  if (is_generation_time_or_both()) {
+    std::copy(&lifetimes[idx], &lifetimes[idx + n],
+      &simulation::ifp_source_lifetime_bank[i_bank]);
+  }
+}
+#endif
+
+void copy_complete_ifp_data_to_source_banks(
+  const vector<vector<int>>& delayed_groups,
+  const vector<vector<double>>& lifetimes)
+{
+  if (is_beta_effective_or_both()) {
+    std::copy(delayed_groups.data(),
+      delayed_groups.data() + settings::n_particles,
+      simulation::ifp_source_delayed_group_bank.begin());
+  }
+  if (is_generation_time_or_both()) {
+    std::copy(lifetimes.data(), lifetimes.data() + settings::n_particles,
+      simulation::ifp_source_lifetime_bank.begin());
+  }
+}
+
+void allocate_temporary_vector_ifp(
+  vector<vector<int>>& delayed_groups, vector<vector<double>>& lifetimes)
+{
+  if (is_beta_effective_or_both()) {
+    delayed_groups.resize(simulation::fission_bank.size());
+  }
+  if (is_generation_time_or_both()) {
+    lifetimes.resize(simulation::fission_bank.size());
+  }
+}
+
+void copy_ifp_data_to_fission_banks(const vector<int>* const delayed_groups_ptr,
+  const vector<double>* lifetimes_ptr)
+{
+  if (is_beta_effective_or_both()) {
+    std::copy(delayed_groups_ptr,
+      delayed_groups_ptr + simulation::fission_bank.size(),
+      simulation::ifp_fission_delayed_group_bank.data());
+  }
+  if (is_generation_time_or_both()) {
+    std::copy(lifetimes_ptr, lifetimes_ptr + simulation::fission_bank.size(),
+      simulation::ifp_fission_lifetime_bank.data());
+  }
+}
+
+} // namespace openmc
diff --git a/src/initialize.cpp b/src/initialize.cpp
index c5803976b..78b414f78 100644
--- a/src/initialize.cpp
+++ b/src/initialize.cpp
@@ -38,7 +38,7 @@
 #include "openmc/vector.h"
 #include "openmc/weight_windows.h"
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 #include "libmesh/libmesh.h"
 #endif
 
@@ -64,7 +64,7 @@ int openmc_init(int argc, char* argv[], const void* intracomm)
   if (err)
     return err;
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
   const int n_threads = num_threads();
   // initialize libMesh if it hasn't been initialized already
   // (if initialized externally, the libmesh_init object needs to be provided
@@ -118,6 +118,10 @@ int openmc_init(int argc, char* argv[], const void* intracomm)
   if (!read_model_xml())
     read_separate_xml_files();
 
+  if (!settings::properties_file.empty()) {
+    openmc_properties_import(settings::properties_file.c_str());
+  }
+
   // Reset locale to previous state
   if (std::setlocale(LC_ALL, prev_locale.c_str()) == NULL) {
     fatal_error("Cannot reset locale.");
@@ -157,7 +161,7 @@ void initialize_mpi(MPI_Comm intracomm)
 
   // Create bank datatype
   SourceSite b;
-  MPI_Aint disp[11];
+  MPI_Aint disp[14];
   MPI_Get_address(&b.r, &disp[0]);
   MPI_Get_address(&b.u, &disp[1]);
   MPI_Get_address(&b.E, &disp[2]);
@@ -169,15 +173,67 @@ void initialize_mpi(MPI_Comm intracomm)
   MPI_Get_address(&b.parent_nuclide, &disp[8]);
   MPI_Get_address(&b.parent_id, &disp[9]);
   MPI_Get_address(&b.progeny_id, &disp[10]);
-  for (int i = 10; i >= 0; --i) {
+  MPI_Get_address(&b.wgt_born, &disp[11]);
+  MPI_Get_address(&b.wgt_ww_born, &disp[12]);
+  MPI_Get_address(&b.n_split, &disp[13]);
+  for (int i = 13; i >= 0; --i) {
     disp[i] -= disp[0];
   }
 
-  int blocks[] {3, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1};
-  MPI_Datatype types[] {MPI_DOUBLE, MPI_DOUBLE, MPI_DOUBLE, MPI_DOUBLE,
-    MPI_DOUBLE, MPI_INT, MPI_INT, MPI_INT, MPI_INT, MPI_LONG, MPI_LONG};
-  MPI_Type_create_struct(11, blocks, disp, types, &mpi::source_site);
+  // Block counts for each field
+  int blocks[] = {3, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
+
+  // Types for each field
+  MPI_Datatype types[] = {
+    MPI_DOUBLE,  // r (3 doubles)
+    MPI_DOUBLE,  // u (3 doubles)
+    MPI_DOUBLE,  // E
+    MPI_DOUBLE,  // time
+    MPI_DOUBLE,  // wgt
+    MPI_INT,     // delayed_group
+    MPI_INT,     // surf_id
+    MPI_INT,     // particle (enum)
+    MPI_INT,     // parent_nuclide
+    MPI_INT64_T, // parent_id
+    MPI_INT64_T, // progeny_id
+    MPI_DOUBLE,  // wgt_born
+    MPI_DOUBLE,  // wgt_ww_born
+    MPI_INT64_T  // n_split
+  };
+
+  MPI_Type_create_struct(14, blocks, disp, types, &mpi::source_site);
   MPI_Type_commit(&mpi::source_site);
+
+  CollisionTrackSite bc;
+  MPI_Aint dispc[16];
+  MPI_Get_address(&bc.r, &dispc[0]);             // double
+  MPI_Get_address(&bc.u, &dispc[1]);             // double
+  MPI_Get_address(&bc.E, &dispc[2]);             // double
+  MPI_Get_address(&bc.dE, &dispc[3]);            // double
+  MPI_Get_address(&bc.time, &dispc[4]);          // double
+  MPI_Get_address(&bc.wgt, &dispc[5]);           // double
+  MPI_Get_address(&bc.event_mt, &dispc[6]);      // int
+  MPI_Get_address(&bc.delayed_group, &dispc[7]); // int
+  MPI_Get_address(&bc.cell_id, &dispc[8]);       // int
+  MPI_Get_address(&bc.nuclide_id, &dispc[9]);    // int
+  MPI_Get_address(&bc.material_id, &dispc[10]);  // int
+  MPI_Get_address(&bc.universe_id, &dispc[11]);  // int
+  MPI_Get_address(&bc.n_collision, &dispc[12]);  // int
+  MPI_Get_address(&bc.particle, &dispc[13]);     // int
+  MPI_Get_address(&bc.parent_id, &dispc[14]);    // int64_t
+  MPI_Get_address(&bc.progeny_id, &dispc[15]);   // int64_t
+  for (int i = 15; i >= 0; --i) {
+    dispc[i] -= dispc[0];
+  }
+
+  int blocksc[] = {3, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
+  MPI_Datatype typesc[] = {MPI_DOUBLE, MPI_DOUBLE, MPI_DOUBLE, MPI_DOUBLE,
+    MPI_DOUBLE, MPI_DOUBLE, MPI_INT, MPI_INT, MPI_INT, MPI_INT, MPI_INT,
+    MPI_INT, MPI_INT, MPI_INT, MPI_INT64_T, MPI_INT64_T};
+
+  MPI_Type_create_struct(
+    16, blocksc, dispc, typesc, &mpi::collision_track_site);
+  MPI_Type_commit(&mpi::collision_track_site);
 }
 #endif // OPENMC_MPI
 
@@ -195,6 +251,15 @@ int parse_command_line(int argc, char* argv[])
         i += 1;
         settings::n_particles = std::stoll(argv[i]);
 
+      } else if (arg == "-q" || arg == "--verbosity") {
+        i += 1;
+        settings::verbosity = std::stoi(argv[i]);
+        if (settings::verbosity > 10 || settings::verbosity < 1) {
+          auto msg = fmt::format("Invalid verbosity: {}.", settings::verbosity);
+          strcpy(openmc_err_msg, msg.c_str());
+          return OPENMC_E_INVALID_ARGUMENT;
+        }
+
       } else if (arg == "-e" || arg == "--event") {
         settings::event_based = true;
       } else if (arg == "-r" || arg == "--restart") {
@@ -256,6 +321,11 @@ int parse_command_line(int argc, char* argv[])
         settings::run_mode = RunMode::VOLUME;
       } else if (arg == "-s" || arg == "--threads") {
         // Read number of threads
+        if (i + 1 >= argc) {
+          std::string msg {"Number of threads not specified."};
+          strcpy(openmc_err_msg, msg.c_str());
+          return OPENMC_E_INVALID_ARGUMENT;
+        }
         i += 1;
 
 #ifdef _OPENMP
@@ -317,6 +387,28 @@ int parse_command_line(int argc, char* argv[])
   return 0;
 }
 
+// TODO: Pulse-height tallies require per-history scoring across the full
+// particle tree (parent + all descendants). The shared secondary bank
+// transports each secondary as an independent Particle, breaking this
+// assumption. A proper fix would defer pulse-height scoring: save
+// (root_source_id, cell, pht_storage) per particle, then aggregate by
+// root_source_id after all secondary generations complete before scoring
+// into the histogram. For now, disable shared secondary when pulse-height
+// tallies are present.
+static void check_pulse_height_compatibility()
+{
+  if (settings::use_shared_secondary_bank) {
+    for (const auto& t : model::tallies) {
+      if (t->type_ == TallyType::PULSE_HEIGHT) {
+        settings::use_shared_secondary_bank = false;
+        warning("Pulse-height tallies are not yet compatible with the shared "
+                "secondary bank. Disabling shared secondary bank.");
+        break;
+      }
+    }
+  }
+}
+
 bool read_model_xml()
 {
   std::string model_filename = settings::path_input;
@@ -345,8 +437,10 @@ bool read_model_xml()
   auto settings_root = root.child("settings");
 
   // Verbosity
-  if (check_for_node(settings_root, "verbosity")) {
+  if (check_for_node(settings_root, "verbosity") && settings::verbosity == -1) {
     settings::verbosity = std::stoi(get_node_value(settings_root, "verbosity"));
+  } else if (settings::verbosity == -1) {
+    settings::verbosity = 7;
   }
 
   // To this point, we haven't displayed any output since we didn't know what
@@ -359,6 +453,10 @@ bool read_model_xml()
   write_message(
     fmt::format("Reading model XML file '{}' ...", model_filename), 5);
 
+  // Read chain data before settings so DecaySpectrum source distributions can
+  // resolve nuclides while sources are constructed.
+  read_chain_file_xml();
+
   read_settings_xml(settings_root);
 
   // If other XML files are present, display warning
@@ -374,9 +472,6 @@ bool read_model_xml()
     }
   }
 
-  // Read data from chain file
-  read_chain_file_xml();
-
   // Read materials and cross sections
   if (!check_for_node(root, "materials")) {
     fatal_error(fmt::format(
@@ -401,9 +496,15 @@ bool read_model_xml()
   // Finalize cross sections having assigned temperatures
   finalize_cross_sections();
 
+  // Compute cell density multipliers now that material densities
+  // have been finalized (from geometry_aux.h)
+  finalize_cell_densities();
+
   if (check_for_node(root, "tallies"))
     read_tallies_xml(root.child("tallies"));
 
+  check_pulse_height_compatibility();
+
   // Initialize distribcell_filters
   prepare_distribcell();
 
@@ -425,14 +526,15 @@ bool read_model_xml()
 
 void read_separate_xml_files()
 {
+  // Read chain data before settings so DecaySpectrum source distributions can
+  // resolve nuclides while sources are constructed.
+  read_chain_file_xml();
+
   read_settings_xml();
   if (settings::run_mode != RunMode::PLOTTING) {
     read_cross_sections_xml();
   }
 
-  // Read data from chain file
-  read_chain_file_xml();
-
   read_materials_xml();
   read_geometry_xml();
 
@@ -441,8 +543,15 @@ void read_separate_xml_files()
 
   // Finalize cross sections having assigned temperatures
   finalize_cross_sections();
+
+  // Compute cell density multipliers now that material densities
+  // have been finalized (from geometry_aux.h)
+  finalize_cell_densities();
+
   read_tallies_xml();
 
+  check_pulse_height_compatibility();
+
   // Initialize distribcell_filters
   prepare_distribcell();
 
diff --git a/src/lattice.cpp b/src/lattice.cpp
index 73005ad09..9e56c58e8 100644
--- a/src/lattice.cpp
+++ b/src/lattice.cpp
@@ -10,6 +10,7 @@
 #include "openmc/geometry.h"
 #include "openmc/geometry_aux.h"
 #include "openmc/hdf5_interface.h"
+#include "openmc/math_functions.h"
 #include "openmc/string_utils.h"
 #include "openmc/vector.h"
 #include "openmc/xml_interface.h"
@@ -103,8 +104,8 @@ void Lattice::adjust_indices()
 
 //==============================================================================
 
-int32_t Lattice::fill_offset_table(int32_t offset, int32_t target_univ_id,
-  int map, std::unordered_map<int32_t, int32_t>& univ_count_memo)
+int32_t Lattice::fill_offset_table(int32_t target_univ_id, int map,
+  std::unordered_map<int32_t, int32_t>& univ_count_memo)
 {
   // If the offsets have already been determined for this "map", don't bother
   // recalculating all of them and just return the total offset. Note that the
@@ -119,6 +120,7 @@ int32_t Lattice::fill_offset_table(int32_t offset, int32_t target_univ_id,
            count_universe_instances(last_univ, target_univ_id, univ_count_memo);
   }
 
+  int32_t offset = 0;
   for (LatticeIter it = begin(); it != end(); ++it) {
     offsets_[map * universes_.size() + it.indx_] = offset;
     offset += count_universe_instances(*it, target_univ_id, univ_count_memo);
@@ -260,47 +262,33 @@ std::pair<double, array<int, 3>> RectLattice::distance(
   double x0 {copysign(0.5 * pitch_[0], u.x)};
   double y0 {copysign(0.5 * pitch_[1], u.y)};
 
-  // Left and right sides
-  double d {INFTY};
-  array<int, 3> lattice_trans;
-  if ((std::abs(x - x0) > FP_PRECISION) && u.x != 0) {
-    d = (x0 - x) / u.x;
-    if (u.x > 0) {
-      lattice_trans = {1, 0, 0};
-    } else {
-      lattice_trans = {-1, 0, 0};
-    }
-  }
-
-  // Front and back sides
-  if ((std::abs(y - y0) > FP_PRECISION) && u.y != 0) {
-    double this_d = (y0 - y) / u.y;
-    if (this_d < d) {
-      d = this_d;
-      if (u.y > 0) {
-        lattice_trans = {0, 1, 0};
-      } else {
-        lattice_trans = {0, -1, 0};
-      }
-    }
-  }
-
-  // Top and bottom sides
+  // Evaluate the distance to each oncoming edge independently. Comparing these
+  // distances directly (rather than reconstructing the crossing position)
+  // avoids the floating-point cancellation that occurs for large pitches.
+  double dx = u.x != 0.0 ? (x0 - x) / u.x : INFTY;
+  double dy = u.y != 0.0 ? (y0 - y) / u.y : INFTY;
+  double dz = INFTY;
   if (is_3d_) {
     double z0 {copysign(0.5 * pitch_[2], u.z)};
-    if ((std::abs(z - z0) > FP_PRECISION) && u.z != 0) {
-      double this_d = (z0 - z) / u.z;
-      if (this_d < d) {
-        d = this_d;
-        if (u.z > 0) {
-          lattice_trans = {0, 0, 1};
-        } else {
-          lattice_trans = {0, 0, -1};
-        }
-      }
-    }
+    dz = u.z != 0.0 ? (z0 - z) / u.z : INFTY;
   }
 
+  // The distance to the nearest lattice boundary is the smallest axial
+  // distance.
+  double d = std::min({dx, dy, dz});
+
+  // Determine which lattice boundaries are being crossed. The axis attaining
+  // the minimum is exactly equal to d, so at least one translation is always
+  // set for a finite crossing; a near-equal second axis indicates a corner
+  // crossing.
+  array<int, 3> lattice_trans = {0, 0, 0};
+  if (isclose(d, dx, FP_COINCIDENT, FP_PRECISION))
+    lattice_trans[0] = copysign(1, u.x);
+  if (isclose(d, dy, FP_COINCIDENT, FP_PRECISION))
+    lattice_trans[1] = copysign(1, u.y);
+  if (is_3d_ && isclose(d, dz, FP_COINCIDENT, FP_PRECISION))
+    lattice_trans[2] = copysign(1, u.z);
+
   return {d, lattice_trans};
 }
 
@@ -361,6 +349,26 @@ Position RectLattice::get_local_position(
 
 //==============================================================================
 
+Direction RectLattice::get_normal(
+  const array<int, 3>& i_xyz, bool& is_valid) const
+{
+  is_valid = false;
+  Direction dir = {0.0, 0.0, 0.0};
+  if ((std::abs(i_xyz[0]) == 1) && (i_xyz[1] == 0) && (i_xyz[2] == 0)) {
+    is_valid = true;
+    dir[0] = std::copysign(1.0, i_xyz[0]);
+  } else if ((i_xyz[0] == 0) && (std::abs(i_xyz[1]) == 1) && (i_xyz[2] == 0)) {
+    is_valid = true;
+    dir[1] = std::copysign(1.0, i_xyz[1]);
+  } else if ((i_xyz[0] == 0) && (i_xyz[1] == 0) && (std::abs(i_xyz[2]) == 1)) {
+    is_valid = true;
+    dir[2] = std::copysign(1.0, i_xyz[2]);
+  }
+  return dir;
+}
+
+//==============================================================================
+
 int32_t& RectLattice::offset(int map, const array<int, 3>& i_xyz)
 {
   return offsets_[n_cells_[0] * n_cells_[1] * n_cells_[2] * map +
@@ -1007,6 +1015,91 @@ Position HexLattice::get_local_position(
 
 //==============================================================================
 
+Direction HexLattice::get_normal(
+  const array<int, 3>& i_xyz, bool& is_valid) const
+{
+  // Short description of the direction vectors used here.  The beta, gamma, and
+  // delta vectors point towards the flat sides of each hexagonal tile.
+  // Y - orientation:
+  //   basis0 = (1, 0)
+  //   basis1 = (-1/sqrt(3), 1)   = +120 degrees from basis0
+  //   beta   = (sqrt(3)/2, 1/2)  = +30 degrees from basis0
+  //   gamma  = (sqrt(3)/2, -1/2) = -60 degrees from beta
+  //   delta  = (0, 1)            = +60 degrees from beta
+  // X - orientation:
+  //   basis0 = (1/sqrt(3), -1)
+  //   basis1 = (0, 1)            = +120 degrees from basis0
+  //   beta   = (1, 0)            = +30 degrees from basis0
+  //   gamma  = (1/2, -sqrt(3)/2) = -60 degrees from beta
+  //   delta  = (1/2, sqrt(3)/2)  = +60 degrees from beta
+
+  is_valid = false;
+  Direction dir = {0.0, 0.0, 0.0};
+  if ((i_xyz[0] == 0) && (i_xyz[1] == 0) && (std::abs(i_xyz[2]) == 1)) {
+    is_valid = true;
+    dir[2] = std::copysign(1.0, i_xyz[2]);
+  } else if ((i_xyz[2] == 0) &&
+             std::max({std::abs(i_xyz[0]), std::abs(i_xyz[1]),
+               std::abs(i_xyz[0] + i_xyz[1])}) == 1) {
+    is_valid = true;
+    // beta direction
+    if ((i_xyz[0] == 1) && (i_xyz[1] == 0)) {
+      if (orientation_ == Orientation::y) {
+        dir[0] = 0.5 * std::sqrt(3.0);
+        dir[1] = 0.5;
+      } else {
+        dir[0] = 1.0;
+        dir[1] = 0.0;
+      }
+    } else if ((i_xyz[0] == -1) && (i_xyz[1] == 0)) {
+      if (orientation_ == Orientation::y) {
+        dir[0] = -0.5 * std::sqrt(3.0);
+        dir[1] = -0.5;
+      } else {
+        dir[0] = -1.0;
+        dir[1] = 0.0;
+      }
+      // gamma direction
+    } else if ((i_xyz[0] == 1) && (i_xyz[1] == -1)) {
+      if (orientation_ == Orientation::y) {
+        dir[0] = 0.5 * std::sqrt(3.0);
+        dir[1] = -0.5;
+      } else {
+        dir[0] = 0.5;
+        dir[1] = -0.5 * std::sqrt(3.0);
+      }
+    } else if ((i_xyz[0] == -1) && (i_xyz[1] == 1)) {
+      if (orientation_ == Orientation::y) {
+        dir[0] = -0.5 * std::sqrt(3.0);
+        dir[1] = 0.5;
+      } else {
+        dir[0] = -0.5;
+        dir[1] = 0.5 * std::sqrt(3.0);
+      }
+      // delta direction
+    } else if ((i_xyz[0] == 0) && (i_xyz[1] == 1)) {
+      if (orientation_ == Orientation::y) {
+        dir[0] = 0.0;
+        dir[1] = 1.0;
+      } else {
+        dir[0] = 0.5;
+        dir[1] = 0.5 * std::sqrt(3.0);
+      }
+    } else if ((i_xyz[0] == 0) && (i_xyz[1] == -1)) {
+      if (orientation_ == Orientation::y) {
+        dir[0] = 0.0;
+        dir[1] = -1.0;
+      } else {
+        dir[0] = -0.5;
+        dir[1] = -0.5 * std::sqrt(3.0);
+      }
+    }
+  }
+  return dir;
+}
+
+//==============================================================================
+
 bool HexLattice::is_valid_index(int indx) const
 {
   int nx {2 * n_rings_ - 1};
diff --git a/src/material.cpp b/src/material.cpp
index a1937aca4..21b11b8b9 100644
--- a/src/material.cpp
+++ b/src/material.cpp
@@ -8,9 +8,7 @@
 #include <string>
 #include <unordered_set>
 
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xoperation.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/capi.h"
 #include "openmc/container_util.h"
@@ -216,7 +214,7 @@ Material::Material(pugi::xml_node node)
   // allocate arrays in Material object
   auto n = names.size();
   nuclide_.reserve(n);
-  atom_density_ = xt::empty<double>({n});
+  atom_density_ = tensor::Tensor<double>({n});
   if (settings::photon_transport)
     element_.reserve(n);
 
@@ -290,14 +288,14 @@ Material::Material(pugi::xml_node node)
 
   // Check to make sure either all atom percents or all weight percents are
   // given
-  if (!(xt::all(atom_density_ >= 0.0) || xt::all(atom_density_ <= 0.0))) {
+  if (!((atom_density_ >= 0.0).all() || (atom_density_ <= 0.0).all())) {
     fatal_error(
       "Cannot mix atom and weight percents in material " + std::to_string(id_));
   }
 
   // Determine density if it is a sum value
   if (sum_density)
-    density_ = xt::sum(atom_density_)();
+    density_ = atom_density_.sum();
 
   if (check_for_node(node, "temperature")) {
     temperature_ = std::stod(get_node_value(node, "temperature"));
@@ -435,7 +433,7 @@ void Material::normalize_density()
   // determine normalized atom percents. if given atom percents, this is
   // straightforward. if given weight percents, the value is w/awr and is
   // divided by sum(w/awr)
-  atom_density_ /= xt::sum(atom_density_)();
+  atom_density_ /= atom_density_.sum();
 
   // Change density in g/cm^3 to atom/b-cm. Since all values are now in
   // atom percent, the sum needs to be re-evaluated as 1/sum(x*awr)
@@ -454,12 +452,15 @@ void Material::normalize_density()
   // Calculate nuclide atom densities
   atom_density_ *= density_;
 
-  // Calculate density in g/cm^3.
+  // Calculate density in [g/cm^3] and charge density in [e/b-cm]
   density_gpcc_ = 0.0;
+  charge_density_ = 0.0;
   for (int i = 0; i < nuclide_.size(); ++i) {
     int i_nuc = nuclide_[i];
     double awr = settings::run_CE ? data::nuclides[i_nuc]->awr_ : 1.0;
+    int z = settings::run_CE ? data::nuclides[i_nuc]->Z_ : 0.0;
     density_gpcc_ += atom_density_(i) * awr * MASS_NEUTRON / N_AVOGADRO;
+    charge_density_ += atom_density_(i) * z;
   }
 }
 
@@ -638,14 +639,14 @@ void Material::init_bremsstrahlung()
     bool positron = (particle == 1);
 
     // Allocate arrays for TTB data
-    ttb->pdf = xt::zeros<double>({n_e, n_e});
-    ttb->cdf = xt::zeros<double>({n_e, n_e});
-    ttb->yield = xt::empty<double>({n_e});
+    ttb->pdf = tensor::zeros<double>({n_e, n_e});
+    ttb->cdf = tensor::zeros<double>({n_e, n_e});
+    ttb->yield = tensor::zeros<double>({n_e});
 
     // Allocate temporary arrays
-    xt::xtensor<double, 1> stopping_power_collision({n_e}, 0.0);
-    xt::xtensor<double, 1> stopping_power_radiative({n_e}, 0.0);
-    xt::xtensor<double, 2> dcs({n_e, n_k}, 0.0);
+    auto stopping_power_collision = tensor::zeros<double>({n_e});
+    auto stopping_power_radiative = tensor::zeros<double>({n_e});
+    auto dcs = tensor::zeros<double>({n_e, n_k});
 
     double Z_eq_sq = 0.0;
     double sum_density = 0.0;
@@ -695,18 +696,18 @@ void Material::init_bremsstrahlung()
                    1.0595e-3 * std::pow(t, 5) + 7.0568e-5 * std::pow(t, 6) -
                    1.808e-6 * std::pow(t, 7));
         stopping_power_radiative(i) *= r;
-        auto dcs_i = xt::view(dcs, i, xt::all());
+        tensor::View<double> dcs_i = dcs.slice(i);
         dcs_i *= r;
       }
     }
 
     // Total material stopping power
-    xt::xtensor<double, 1> stopping_power =
+    tensor::Tensor<double> stopping_power =
       stopping_power_collision + stopping_power_radiative;
 
     // Loop over photon energies
-    xt::xtensor<double, 1> f({n_e}, 0.0);
-    xt::xtensor<double, 1> z({n_e}, 0.0);
+    auto f = tensor::zeros<double>({n_e});
+    auto z = tensor::zeros<double>({n_e});
     for (int i = 0; i < n_e - 1; ++i) {
       double w = data::ttb_e_grid(i);
 
@@ -777,14 +778,15 @@ void Material::init_bremsstrahlung()
       // Loop over photon energies
       double c = 0.0;
       for (int i = 0; i < j; ++i) {
-        // Integrate the CDF from the PDF using the trapezoidal rule in log-log
-        // space
+        // Integrate the CDF from the PDF using the fact that the PDF is linear
+        // in log-log space
         double w_l = std::log(data::ttb_e_grid(i));
         double w_r = std::log(data::ttb_e_grid(i + 1));
         double x_l = std::log(ttb->pdf(j, i));
         double x_r = std::log(ttb->pdf(j, i + 1));
-
-        c += 0.5 * (w_r - w_l) * (std::exp(w_l + x_l) + std::exp(w_r + x_r));
+        double beta = (x_r - x_l) / (w_r - w_l);
+        double a = beta + 1.0;
+        c += std::exp(w_l + x_l) / a * std::expm1(a * (w_r - w_l));
         ttb->cdf(j, i + 1) = c;
       }
 
@@ -793,7 +795,8 @@ void Material::init_bremsstrahlung()
     }
 
     // Use logarithm of number yield since it is log-log interpolated
-    ttb->yield = xt::where(ttb->yield > 0.0, xt::log(ttb->yield), -500.0);
+    ttb->yield =
+      tensor::where(ttb->yield > 0.0, tensor::log(ttb->yield), -500.0);
   }
 }
 
@@ -815,9 +818,9 @@ void Material::calculate_xs(Particle& p) const
   p.macro_xs().fission = 0.0;
   p.macro_xs().nu_fission = 0.0;
 
-  if (p.type() == ParticleType::neutron) {
+  if (p.type().is_neutron()) {
     this->calculate_neutron_xs(p);
-  } else if (p.type() == ParticleType::photon) {
+  } else if (p.type().is_photon()) {
     this->calculate_photon_xs(p);
   }
 }
@@ -825,7 +828,7 @@ void Material::calculate_xs(Particle& p) const
 void Material::calculate_neutron_xs(Particle& p) const
 {
   // Find energy index on energy grid
-  int neutron = static_cast<int>(ParticleType::neutron);
+  int neutron = ParticleType::neutron().transport_index();
   int i_grid =
     std::log(p.E() / data::energy_min[neutron]) / simulation::log_spacing;
 
@@ -886,7 +889,7 @@ void Material::calculate_neutron_xs(Particle& p) const
     // ADD TO MACROSCOPIC CROSS SECTION
 
     // Copy atom density of nuclide in material
-    double atom_density = atom_density_(i);
+    double atom_density = this->atom_density(i, p.density_mult());
 
     // Add contributions to cross sections
     p.macro_xs().total += atom_density * micro.total;
@@ -921,7 +924,7 @@ void Material::calculate_photon_xs(Particle& p) const
     // ADD TO MACROSCOPIC CROSS SECTION
 
     // Copy atom density of nuclide in material
-    double atom_density = atom_density_(i);
+    double atom_density = this->atom_density(i, p.density_mult());
 
     // Add contributions to material macroscopic cross sections
     p.macro_xs().total += atom_density * micro.total;
@@ -975,18 +978,21 @@ void Material::set_density(double density, const std::string& units)
     density_ = density;
 
     // Determine normalized atom percents
-    double sum_percent = xt::sum(atom_density_)();
+    double sum_percent = atom_density_.sum();
     atom_density_ /= sum_percent;
 
     // Recalculate nuclide atom densities based on given density
     atom_density_ *= density;
 
-    // Calculate density in g/cm^3.
+    // Calculate density in g/cm^3 and charge density in [e/b-cm]
     density_gpcc_ = 0.0;
+    charge_density_ = 0.0;
     for (int i = 0; i < nuclide_.size(); ++i) {
       int i_nuc = nuclide_[i];
       double awr = data::nuclides[i_nuc]->awr_;
+      int z = settings::run_CE ? data::nuclides[i_nuc]->Z_ : 0.0;
       density_gpcc_ += atom_density_(i) * awr * MASS_NEUTRON / N_AVOGADRO;
+      charge_density_ += atom_density_(i) * z;
     }
   } else if (units == "g/cm3" || units == "g/cc") {
     // Determine factor by which to change densities
@@ -997,6 +1003,7 @@ void Material::set_density(double density, const std::string& units)
     density_gpcc_ = density;
     density_ *= f;
     atom_density_ *= f;
+    charge_density_ *= f;
   } else {
     throw std::invalid_argument {
       "Invalid units '" + std::string(units.data()) + "' specified."};
@@ -1012,7 +1019,7 @@ void Material::set_densities(
 
   if (n != nuclide_.size()) {
     nuclide_.resize(n);
-    atom_density_ = xt::zeros<double>({n});
+    atom_density_ = tensor::zeros<double>({n});
     if (settings::photon_transport)
       element_.resize(n);
   }
@@ -1173,8 +1180,8 @@ void Material::add_nuclide(const std::string& name, double density)
   auto n = nuclide_.size();
 
   // Create copy of atom_density_ array with one extra entry
-  xt::xtensor<double, 1> atom_density = xt::zeros<double>({n});
-  xt::view(atom_density, xt::range(0, n - 1)) = atom_density_;
+  tensor::Tensor<double> atom_density = tensor::zeros<double>({n});
+  atom_density.slice(tensor::range(0, n - 1)) = atom_density_;
   atom_density(n - 1) = density;
   atom_density_ = atom_density;
 
diff --git a/src/math_functions.cpp b/src/math_functions.cpp
index 5469b56c8..ddacc2bd9 100644
--- a/src/math_functions.cpp
+++ b/src/math_functions.cpp
@@ -1,5 +1,7 @@
 #include "openmc/math_functions.h"
 
+#include <limits> // for numeric_limits
+
 #include "openmc/external/Faddeeva.hh"
 
 #include "openmc/constants.h"
@@ -95,6 +97,13 @@ double t_percentile(double p, int df)
   return t;
 }
 
+double standard_normal_cdf(double z)
+{
+  // Use the complementary error function to compute the standard normal CDF
+  // Phi(z) = 0.5 * (1 + erf(z / sqrt(2))) = 0.5 * erfc(-z / sqrt(2))
+  return 0.5 * std::erfc(-z / std::sqrt(2.0));
+}
+
 void calc_pn_c(int n, double x, double pnx[])
 {
   pnx[0] = 1.;
@@ -919,4 +928,85 @@ std::complex<double> w_derivative(std::complex<double> z, int order)
   }
 }
 
+double exprel(double x)
+{
+  if (std::abs(x) < 1e-16)
+    return 1.0;
+  else {
+    return std::expm1(x) / x;
+  }
+}
+
+double log1prel(double x)
+{
+  if (std::abs(x) < 1e-16)
+    return 1.0;
+  else {
+    return std::log1p(x) / x;
+  }
+}
+
+double cyl_bessel_j(int n, double x)
+{
+  // Handle negative arguments via the parity relation
+  // J_n(-x) = (-1)^n J_n(x); std::cyl_bessel_j has a domain error for x < 0.
+  double sign = 1.0;
+  if (x < 0.0) {
+    x = -x;
+    if (n % 2 == 1)
+      sign = -1.0;
+  }
+
+#if defined(__cpp_lib_math_special_functions) &&                               \
+  __cpp_lib_math_special_functions >= 201603L
+  return sign * std::cyl_bessel_j(static_cast<double>(n), x);
+#else
+  // Ascending power series (e.g., Abramowitz & Stegun eq. 9.1.10):
+  //   J_n(x) = sum_{m=0}^inf (-1)^m / (m! (m+n)!) * (x/2)^(2m+n)
+  // The term ratio is -(x/2)^2 / (m*(m+n)), so for |x| <= 2 the series
+  // converges to machine precision within ~20 terms.
+  double half_x = 0.5 * x;
+
+  // First term: (x/2)^n / n!
+  double term = 1.0;
+  for (int k = 1; k <= n; ++k) {
+    term *= half_x / k;
+  }
+
+  double sum = term;
+  double neg_half_x_sq = -half_x * half_x;
+  for (int m = 1; m <= 50; ++m) {
+    term *= neg_half_x_sq / (m * (m + n));
+    sum += term;
+    if (std::abs(term) <=
+        std::numeric_limits<double>::epsilon() * std::abs(sum))
+      break;
+  }
+  return sign * sum;
+#endif
+}
+
+// Helper function to get index and interpolation function on an incident energy
+// grid
+void get_energy_index(
+  const vector<double>& energies, double E, int& i, double& f)
+{
+  // Get index and interpolation factor for linear-linear energy grid
+  i = 0;
+  f = 0.0;
+  if (E >= energies.front()) {
+    i = lower_bound_index(energies.begin(), energies.end(), E);
+    if (i + 1 < energies.size())
+      f = (E - energies[i]) / (energies[i + 1] - energies[i]);
+  }
+}
+
+// Return true if two floating-point values are approximately equal within a
+// combined relative and absolute tolerance.
+bool isclose(double a, double b, double rel_tol, double abs_tol)
+{
+  return std::abs(a - b) <=
+         std::max(rel_tol * std::max(std::abs(a), std::abs(b)), abs_tol);
+}
+
 } // namespace openmc
diff --git a/src/mcpl_interface.cpp b/src/mcpl_interface.cpp
index 83ef63320..30c41ec5a 100644
--- a/src/mcpl_interface.cpp
+++ b/src/mcpl_interface.cpp
@@ -11,217 +11,636 @@
 
 #include <fmt/core.h>
 
-#ifdef OPENMC_MCPL
-#include <mcpl.h>
+#include <cctype>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <memory>
+#include <mutex>
+#include <sstream>
+#include <stdexcept>
+#include <string>
+
+#ifdef _WIN32
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#else
+#include <dlfcn.h>
 #endif
 
+// WARNING: These declarations MUST EXACTLY MATCH the structure and function
+// signatures of the libmcpl being loaded at runtime. Any discrepancy will
+// likely lead to crashes or incorrect behavior. This is a maintenance risk.
+// MCPL 2.2.0
+
+#pragma pack(push, 1)
+struct mcpl_particle_repr_t {
+  double ekin;
+  double polarisation[3];
+  double position[3];
+  double direction[3];
+  double time;
+  double weight;
+  int32_t pdgcode;
+  uint32_t userflags;
+};
+#pragma pack(pop)
+
+// Opaque struct definitions replicating the MCPL C-API to ensure ABI
+// compatibility without including mcpl.h. These must be kept in sync.
+struct mcpl_file_t {
+  void* internal;
+};
+struct mcpl_outfile_t {
+  void* internal;
+};
+
+// Function pointer types for the dynamically loaded MCPL library
+using mcpl_open_file_fpt = mcpl_file_t* (*)(const char* filename);
+using mcpl_hdr_nparticles_fpt = uint64_t (*)(mcpl_file_t* file_handle);
+using mcpl_read_fpt = const mcpl_particle_repr_t* (*)(mcpl_file_t* file_handle);
+using mcpl_close_file_fpt = void (*)(mcpl_file_t* file_handle);
+
+using mcpl_hdr_add_data_fpt = void (*)(mcpl_outfile_t* file_handle,
+  const char* key, uint32_t datalength, const char* data);
+using mcpl_create_outfile_fpt = mcpl_outfile_t* (*)(const char* filename);
+using mcpl_hdr_set_srcname_fpt = void (*)(
+  mcpl_outfile_t* outfile_handle, const char* srcname);
+using mcpl_add_particle_fpt = void (*)(
+  mcpl_outfile_t* outfile_handle, const mcpl_particle_repr_t* particle);
+using mcpl_close_outfile_fpt = void (*)(mcpl_outfile_t* outfile_handle);
+using mcpl_hdr_add_stat_sum_fpt = void (*)(
+  mcpl_outfile_t* outfile_handle, const char* key, double value);
+
 namespace openmc {
 
-//==============================================================================
-// Constants
-//==============================================================================
-
-#ifdef OPENMC_MCPL
-const bool MCPL_ENABLED = true;
+#ifdef _WIN32
+using LibraryHandleType = HMODULE;
 #else
-const bool MCPL_ENABLED = false;
+using LibraryHandleType = void*;
 #endif
 
-//==============================================================================
-// Functions
-//==============================================================================
-
-#ifdef OPENMC_MCPL
-SourceSite mcpl_particle_to_site(const mcpl_particle_t* particle)
+std::string get_last_library_error()
 {
-  SourceSite site;
+#ifdef _WIN32
+  DWORD error_code = GetLastError();
+  if (error_code == 0)
+    return "No error reported by system."; // More accurate than "No error."
+  LPSTR message_buffer = nullptr;
+  size_t size =
+    FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM |
+                     FORMAT_MESSAGE_IGNORE_INSERTS,
+      NULL, error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+      (LPSTR)&message_buffer, 0, NULL);
+  std::string message(message_buffer, size);
+  LocalFree(message_buffer);
+  while (
+    !message.empty() && (message.back() == '\n' || message.back() == '\r')) {
+    message.pop_back();
+  }
+  return message;
+#else
+  const char* err = dlerror();
+  return err ? std::string(err) : "No error reported by dlerror.";
+#endif
+}
 
-  switch (particle->pdgcode) {
-  case 2112:
-    site.particle = ParticleType::neutron;
-    break;
-  case 22:
-    site.particle = ParticleType::photon;
-    break;
-  case 11:
-    site.particle = ParticleType::electron;
-    break;
-  case -11:
-    site.particle = ParticleType::positron;
-    break;
+struct McplApi {
+  mcpl_open_file_fpt open_file;
+  mcpl_hdr_nparticles_fpt hdr_nparticles;
+  mcpl_read_fpt read;
+  mcpl_close_file_fpt close_file;
+  mcpl_create_outfile_fpt create_outfile;
+  mcpl_hdr_set_srcname_fpt hdr_set_srcname;
+  mcpl_hdr_add_data_fpt hdr_add_data;
+  mcpl_add_particle_fpt add_particle;
+  mcpl_close_outfile_fpt close_outfile;
+  mcpl_hdr_add_stat_sum_fpt hdr_add_stat_sum;
+
+  explicit McplApi(LibraryHandleType lib_handle)
+  {
+    if (!lib_handle)
+      throw std::runtime_error(
+        "MCPL library handle is null during API binding.");
+
+    auto load_symbol_platform = [lib_handle](const char* name) {
+      void* sym = nullptr;
+#ifdef _WIN32
+      sym = (void*)GetProcAddress(lib_handle, name);
+#else
+      sym = dlsym(lib_handle, name);
+#endif
+      if (!sym) {
+        throw std::runtime_error(
+          fmt::format("Failed to load MCPL symbol '{}': {}", name,
+            get_last_library_error()));
+      }
+      return sym;
+    };
+
+    open_file = reinterpret_cast<mcpl_open_file_fpt>(
+      load_symbol_platform("mcpl_open_file"));
+    hdr_nparticles = reinterpret_cast<mcpl_hdr_nparticles_fpt>(
+      load_symbol_platform("mcpl_hdr_nparticles"));
+    read = reinterpret_cast<mcpl_read_fpt>(load_symbol_platform("mcpl_read"));
+    close_file = reinterpret_cast<mcpl_close_file_fpt>(
+      load_symbol_platform("mcpl_close_file"));
+    create_outfile = reinterpret_cast<mcpl_create_outfile_fpt>(
+      load_symbol_platform("mcpl_create_outfile"));
+    hdr_set_srcname = reinterpret_cast<mcpl_hdr_set_srcname_fpt>(
+      load_symbol_platform("mcpl_hdr_set_srcname"));
+    add_particle = reinterpret_cast<mcpl_add_particle_fpt>(
+      load_symbol_platform("mcpl_add_particle"));
+    close_outfile = reinterpret_cast<mcpl_close_outfile_fpt>(
+      load_symbol_platform("mcpl_close_outfile"));
+
+    // Try to load mcpl_hdr_add_data (available in MCPL >= 2.1.0)
+    // Set to nullptr if not available for graceful fallback
+    try {
+      hdr_add_data = reinterpret_cast<mcpl_hdr_add_data_fpt>(
+        load_symbol_platform("mcpl_hdr_add_data"));
+    } catch (const std::runtime_error&) {
+      hdr_add_data = nullptr;
+    }
+
+    // Try to load mcpl_hdr_add_stat_sum (available in MCPL >= 2.1.0)
+    // Set to nullptr if not available for graceful fallback
+    try {
+      hdr_add_stat_sum = reinterpret_cast<mcpl_hdr_add_stat_sum_fpt>(
+        load_symbol_platform("mcpl_hdr_add_stat_sum"));
+    } catch (const std::runtime_error&) {
+      hdr_add_stat_sum = nullptr;
+    }
+  }
+};
+
+static LibraryHandleType g_mcpl_lib_handle = nullptr;
+static std::unique_ptr<McplApi> g_mcpl_api;
+static bool g_mcpl_init_attempted = false;
+static bool g_mcpl_successfully_loaded = false;
+static std::string g_mcpl_load_error_msg;
+static std::once_flag g_mcpl_init_flag;
+
+void append_error(std::string& existing_msg, const std::string& new_error)
+{
+  if (!existing_msg.empty()) {
+    existing_msg += "; ";
+  }
+  existing_msg += new_error;
+}
+
+void initialize_mcpl_interface_impl()
+{
+  g_mcpl_init_attempted = true;
+  g_mcpl_load_error_msg.clear();
+
+  // Try mcpl-config
+  if (!g_mcpl_lib_handle) {
+    FILE* pipe = nullptr;
+#ifdef _WIN32
+    pipe = _popen("mcpl-config --show libpath", "r");
+#else
+    pipe = popen("mcpl-config --show libpath 2>/dev/null", "r");
+#endif
+    if (pipe) {
+      char buffer[512];
+      if (fgets(buffer, sizeof(buffer), pipe) != nullptr) {
+        std::string shlibpath = buffer;
+        // Remove trailing whitespace
+        while (!shlibpath.empty() &&
+               std::isspace(static_cast<unsigned char>(shlibpath.back()))) {
+          shlibpath.pop_back();
+        }
+
+        if (!shlibpath.empty()) {
+#ifdef _WIN32
+          g_mcpl_lib_handle = LoadLibraryA(shlibpath.c_str());
+#else
+          g_mcpl_lib_handle = dlopen(shlibpath.c_str(), RTLD_LAZY);
+#endif
+          if (!g_mcpl_lib_handle) {
+            append_error(
+              g_mcpl_load_error_msg, fmt::format("From mcpl-config ({}): {}",
+                                       shlibpath, get_last_library_error()));
+          }
+        }
+      }
+#ifdef _WIN32
+      _pclose(pipe);
+#else
+      pclose(pipe);
+#endif
+    } else { // pipe failed to open
+      append_error(g_mcpl_load_error_msg,
+        "mcpl-config command not found or failed to execute");
+    }
   }
 
+  // Try standard library names
+  if (!g_mcpl_lib_handle) {
+#ifdef _WIN32
+    const char* standard_names[] = {"mcpl.dll", "libmcpl.dll"};
+#else
+    const char* standard_names[] = {"libmcpl.so", "libmcpl.dylib"};
+#endif
+    for (const char* name : standard_names) {
+#ifdef _WIN32
+      g_mcpl_lib_handle = LoadLibraryA(name);
+#else
+      g_mcpl_lib_handle = dlopen(name, RTLD_LAZY);
+#endif
+      if (g_mcpl_lib_handle)
+        break;
+    }
+    if (!g_mcpl_lib_handle) {
+      append_error(
+        g_mcpl_load_error_msg, fmt::format("Using standard names (e.g. {}): {}",
+                                 standard_names[0], get_last_library_error()));
+    }
+  }
+
+  if (!g_mcpl_lib_handle) {
+    if (mpi::master) {
+      warning(fmt::format("MCPL library could not be loaded. MCPL-dependent "
+                          "features will be unavailable. Load attempts: {}",
+        g_mcpl_load_error_msg.empty()
+          ? "No specific error during load attempts."
+          : g_mcpl_load_error_msg));
+    }
+    g_mcpl_successfully_loaded = false;
+    return;
+  }
+
+  try {
+    g_mcpl_api = std::make_unique<McplApi>(g_mcpl_lib_handle);
+    g_mcpl_successfully_loaded = true;
+    // Do not call dlclose/FreeLibrary at exit. Leaking the handle is safer
+    // and standard practice for libraries used for the application's lifetime.
+  } catch (const std::runtime_error& e) {
+    append_error(g_mcpl_load_error_msg,
+      fmt::format(
+        "MCPL library loaded, but failed to bind symbols: {}", e.what()));
+    if (mpi::master) {
+      warning(g_mcpl_load_error_msg);
+    }
+#ifdef _WIN32
+    FreeLibrary(g_mcpl_lib_handle);
+#else
+    dlclose(g_mcpl_lib_handle);
+#endif
+    g_mcpl_lib_handle = nullptr;
+    g_mcpl_successfully_loaded = false;
+  }
+}
+
+void initialize_mcpl_interface_if_needed()
+{
+  std::call_once(g_mcpl_init_flag, initialize_mcpl_interface_impl);
+}
+
+bool is_mcpl_interface_available()
+{
+  initialize_mcpl_interface_if_needed();
+  return g_mcpl_successfully_loaded;
+}
+
+inline void ensure_mcpl_ready_or_fatal()
+{
+  initialize_mcpl_interface_if_needed();
+  if (!g_mcpl_successfully_loaded) {
+    fatal_error("MCPL functionality is required, but the MCPL library is not "
+                "available or failed to initialize. Please ensure MCPL is "
+                "installed and its library can be found (e.g., via PATH on "
+                "Windows, LD_LIBRARY_PATH on Linux, or DYLD_LIBRARY_PATH on "
+                "macOS). You can often install MCPL with 'pip install mcpl' or "
+                "'conda install mcpl'.");
+  }
+}
+
+SourceSite mcpl_particle_to_site(const mcpl_particle_repr_t* particle_repr)
+{
+  SourceSite site;
+  site.particle = ParticleType {particle_repr->pdgcode};
+
   // Copy position and direction
-  site.r.x = particle->position[0];
-  site.r.y = particle->position[1];
-  site.r.z = particle->position[2];
-  site.u.x = particle->direction[0];
-  site.u.y = particle->direction[1];
-  site.u.z = particle->direction[2];
-
+  site.r.x = particle_repr->position[0];
+  site.r.y = particle_repr->position[1];
+  site.r.z = particle_repr->position[2];
+  site.u.x = particle_repr->direction[0];
+  site.u.y = particle_repr->direction[1];
+  site.u.z = particle_repr->direction[2];
   // MCPL stores kinetic energy in [MeV], time in [ms]
-  site.E = particle->ekin * 1e6;
-  site.time = particle->time * 1e-3;
-  site.wgt = particle->weight;
-
+  site.E = particle_repr->ekin * 1e6;
+  site.time = particle_repr->time * 1e-3;
+  site.wgt = particle_repr->weight;
   return site;
 }
-#endif
-
-//==============================================================================
 
 vector<SourceSite> mcpl_source_sites(std::string path)
 {
+  ensure_mcpl_ready_or_fatal();
   vector<SourceSite> sites;
 
-#ifdef OPENMC_MCPL
-  // Open MCPL file and determine number of particles
-  auto mcpl_file = mcpl_open_file(path.c_str());
-  size_t n_sites = mcpl_hdr_nparticles(mcpl_file);
+  mcpl_file_t* mcpl_file = g_mcpl_api->open_file(path.c_str());
+  if (!mcpl_file) {
+    fatal_error(fmt::format("MCPL: Could not open file '{}'. It might be "
+                            "missing, inaccessible, or not a valid MCPL file.",
+      path));
+  }
 
-  for (int i = 0; i < n_sites; i++) {
-    // Extract particle from mcpl-file, checking if it is a neutron, photon,
-    // electron, or positron. Otherwise skip.
-    const mcpl_particle_t* particle;
-    int pdg = 0;
-    while (pdg != 2112 && pdg != 22 && pdg != 11 && pdg != -11) {
-      particle = mcpl_read(mcpl_file);
-      pdg = particle->pdgcode;
+  size_t n_particles_in_file = g_mcpl_api->hdr_nparticles(mcpl_file);
+  if (n_particles_in_file > 0) {
+    sites.reserve(n_particles_in_file);
+  }
+
+  for (size_t i = 0; i < n_particles_in_file; ++i) {
+    const mcpl_particle_repr_t* p_repr = g_mcpl_api->read(mcpl_file);
+    if (!p_repr) {
+      warning(fmt::format("MCPL: Read error or unexpected end of file '{}' "
+                          "after reading {} of {} expected particles.",
+        path, sites.size(), n_particles_in_file));
+      break;
     }
-
-    // Convert to source site and add to vector
-    sites.push_back(mcpl_particle_to_site(particle));
+    sites.push_back(mcpl_particle_to_site(p_repr));
   }
 
-  // Check that some sites were read
+  g_mcpl_api->close_file(mcpl_file);
+
   if (sites.empty()) {
-    fatal_error("MCPL file contained no neutron, photon, electron, or positron "
-                "source particles.");
+    if (n_particles_in_file > 0) {
+      fatal_error(fmt::format(
+        "MCPL file '{}' contained {} particles, but no particles could be "
+        "read.",
+        path, n_particles_in_file));
+    } else {
+      fatal_error(fmt::format(
+        "MCPL file '{}' is empty or contains no particle data.", path));
+    }
   }
-
-  mcpl_close_file(mcpl_file);
-#else
-  fatal_error(
-    "Your build of OpenMC does not support reading MCPL source files.");
-#endif
-
   return sites;
 }
 
-//==============================================================================
-
-#ifdef OPENMC_MCPL
-void write_mcpl_source_bank(mcpl_outfile_t file_id,
-  span<SourceSite> source_bank, const vector<int64_t>& bank_index)
+void write_mcpl_source_bank_internal(mcpl_outfile_t* file_id,
+  span<SourceSite> local_source_bank,
+  const vector<int64_t>& bank_index_all_ranks)
 {
-  int64_t dims_size = settings::n_particles;
-  int64_t count_size = simulation::work_per_rank;
-
   if (mpi::master) {
-    // Particles are writeen to disk from the master node only
+    if (!file_id) {
+      fatal_error("MCPL: Internal error - master rank called "
+                  "write_mcpl_source_bank_internal with null file_id.");
+    }
+    vector<SourceSite> receive_buffer;
 
-    // Save source bank sites since the array is overwritten below
+    for (int rank_idx = 0; rank_idx < mpi::n_procs; ++rank_idx) {
+      size_t num_sites_on_rank = static_cast<size_t>(
+        bank_index_all_ranks[rank_idx + 1] - bank_index_all_ranks[rank_idx]);
+      if (num_sites_on_rank == 0)
+        continue;
+
+      span<const SourceSite> sites_to_write;
 #ifdef OPENMC_MPI
-    vector<SourceSite> temp_source {source_bank.begin(), source_bank.end()};
-#endif
-
-    // loop over the other nodes and receive data - then write those.
-    for (int i = 0; i < mpi::n_procs; ++i) {
-      // number of particles for node node i
-      size_t count[] {static_cast<size_t>(bank_index[i + 1] - bank_index[i])};
-
-#ifdef OPENMC_MPI
-      if (i > 0)
-        MPI_Recv(source_bank.data(), count[0], mpi::source_site, i, i,
-          mpi::intracomm, MPI_STATUS_IGNORE);
-#endif
-      // now write the source_bank data again.
-      for (const auto& site : source_bank) {
-        // particle is now at the iterator
-        // write it to the mcpl-file
-        mcpl_particle_t p;
-        p.position[0] = site.r.x;
-        p.position[1] = site.r.y;
-        p.position[2] = site.r.z;
-
-        // mcpl requires that the direction vector is unit length
-        // which is also the case in openmc
-        p.direction[0] = site.u.x;
-        p.direction[1] = site.u.y;
-        p.direction[2] = site.u.z;
-
-        // MCPL stores kinetic energy in [MeV], time in [ms]
-        p.ekin = site.E * 1e-6;
-        p.time = site.time * 1e3;
-        p.weight = site.wgt;
-
-        switch (site.particle) {
-        case ParticleType::neutron:
-          p.pdgcode = 2112;
-          break;
-        case ParticleType::photon:
-          p.pdgcode = 22;
-          break;
-        case ParticleType::electron:
-          p.pdgcode = 11;
-          break;
-        case ParticleType::positron:
-          p.pdgcode = -11;
-          break;
+      if (rank_idx == mpi::rank) {
+        sites_to_write = openmc::span<const SourceSite>(
+          local_source_bank.data(), num_sites_on_rank);
+      } else {
+        if (receive_buffer.size() < num_sites_on_rank) {
+          receive_buffer.resize(num_sites_on_rank);
         }
-
-        mcpl_add_particle(file_id, &p);
+        MPI_Recv(receive_buffer.data(), num_sites_on_rank, mpi::source_site,
+          rank_idx, rank_idx, mpi::intracomm, MPI_STATUS_IGNORE);
+        sites_to_write = openmc::span<const SourceSite>(
+          receive_buffer.data(), num_sites_on_rank);
+      }
+#else
+      sites_to_write = openmc::span<const SourceSite>(
+        local_source_bank.data(), num_sites_on_rank);
+#endif
+      for (const auto& site : sites_to_write) {
+        mcpl_particle_repr_t p_repr {};
+        p_repr.position[0] = site.r.x;
+        p_repr.position[1] = site.r.y;
+        p_repr.position[2] = site.r.z;
+        p_repr.direction[0] = site.u.x;
+        p_repr.direction[1] = site.u.y;
+        p_repr.direction[2] = site.u.z;
+        p_repr.ekin = site.E * 1e-6;
+        p_repr.time = site.time * 1e3;
+        p_repr.weight = site.wgt;
+        p_repr.pdgcode = site.particle.pdg_number();
+        g_mcpl_api->add_particle(file_id, &p_repr);
       }
     }
-#ifdef OPENMC_MPI
-    // Restore state of source bank
-    std::copy(temp_source.begin(), temp_source.end(), source_bank.begin());
-#endif
   } else {
 #ifdef OPENMC_MPI
-    MPI_Send(source_bank.data(), count_size, mpi::source_site, 0, mpi::rank,
-      mpi::intracomm);
+    if (!local_source_bank.empty()) {
+      MPI_Send(local_source_bank.data(), local_source_bank.size(),
+        mpi::source_site, 0, mpi::rank, mpi::intracomm);
+    }
 #endif
   }
 }
-#endif
-
-//==============================================================================
 
 void write_mcpl_source_point(const char* filename, span<SourceSite> source_bank,
   const vector<int64_t>& bank_index)
 {
+  ensure_mcpl_ready_or_fatal();
+
   std::string filename_(filename);
   const auto extension = get_file_extension(filename_);
-  if (extension == "") {
+  if (extension.empty()) {
     filename_.append(".mcpl");
   } else if (extension != "mcpl") {
-    warning("write_mcpl_source_point was passed a file extension differing "
-            "from .mcpl, but an mcpl file will be written.");
+    warning(fmt::format("Specified filename '{}' has an extension '.{}', but "
+                        "an MCPL file (.mcpl) will be written using this name.",
+      filename, extension));
   }
 
-#ifdef OPENMC_MCPL
-  mcpl_outfile_t file_id;
+  mcpl_outfile_t* file_id = nullptr;
 
-  std::string line;
   if (mpi::master) {
-    file_id = mcpl_create_outfile(filename_.c_str());
+    file_id = g_mcpl_api->create_outfile(filename_.c_str());
+    if (!file_id) {
+      fatal_error(fmt::format(
+        "MCPL: Failed to create output file '{}'. Check permissions and path.",
+        filename_));
+    }
+    std::string src_line;
     if (VERSION_DEV) {
-      line = fmt::format("OpenMC {0}.{1}.{2}-dev{3}", VERSION_MAJOR,
+      src_line = fmt::format("OpenMC {}.{}.{}-dev{}", VERSION_MAJOR,
         VERSION_MINOR, VERSION_RELEASE, VERSION_COMMIT_COUNT);
     } else {
-      line = fmt::format(
-        "OpenMC {0}.{1}.{2}", VERSION_MAJOR, VERSION_MINOR, VERSION_RELEASE);
+      src_line = fmt::format(
+        "OpenMC {}.{}.{}", VERSION_MAJOR, VERSION_MINOR, VERSION_RELEASE);
+    }
+    g_mcpl_api->hdr_set_srcname(file_id, src_line.c_str());
+
+    // Initialize stat:sum with -1 to indicate incomplete file (issue #3514)
+    // This follows MCPL >= 2.1.0 convention for tracking simulation statistics
+    // The -1 value indicates "not available" if file creation is interrupted
+    if (g_mcpl_api->hdr_add_stat_sum) {
+      // Using key "openmc_np1" following tkittel's recommendation
+      // Initial value of -1 prevents misleading values in case of crashes
+      g_mcpl_api->hdr_add_stat_sum(file_id, "openmc_np1", -1.0);
     }
-    mcpl_hdr_set_srcname(file_id, line.c_str());
   }
 
-  write_mcpl_source_bank(file_id, source_bank, bank_index);
+  write_mcpl_source_bank_internal(file_id, source_bank, bank_index);
 
   if (mpi::master) {
-    mcpl_close_outfile(file_id);
+    if (file_id) {
+      // Update stat:sum with actual particle count before closing (issue #3514)
+      // This represents the original number of source particles in the
+      // simulation (not the number of particles in the file)
+      if (g_mcpl_api->hdr_add_stat_sum) {
+        // Calculate total source particles from active batches
+        // Per issue #3514: this should be the original number of source
+        // particles, not the number written to the file
+        int64_t total_source_particles =
+          static_cast<int64_t>(settings::n_batches - settings::n_inactive) *
+          settings::gen_per_batch * settings::n_particles;
+        // Update with actual count - this overwrites the initial -1 value
+        g_mcpl_api->hdr_add_stat_sum(
+          file_id, "openmc_np1", static_cast<double>(total_source_particles));
+      }
+
+      g_mcpl_api->close_outfile(file_id);
+    }
   }
+}
+
+// Collision track feature with MCPL
+void write_mcpl_collision_track_internal(mcpl_outfile_t* file_id,
+  span<CollisionTrackSite> collision_track_bank,
+  const vector<int64_t>& bank_index_all_ranks)
+{
+  if (mpi::master) {
+    if (!file_id) {
+      fatal_error("MCPL: Internal error - master rank called "
+                  "write_mcpl_source_bank_internal with null file_id.");
+    }
+    vector<CollisionTrackSite> receive_buffer;
+    vector<CollisionTrackSite> all_sites;
+    all_sites.reserve(static_cast<size_t>(bank_index_all_ranks.back()));
+    vector<std::string> all_blobs;
+    all_blobs.reserve(static_cast<size_t>(bank_index_all_ranks.back()));
+
+    for (int rank_idx = 0; rank_idx < mpi::n_procs; ++rank_idx) {
+      size_t num_sites_on_rank = static_cast<size_t>(
+        bank_index_all_ranks[rank_idx + 1] - bank_index_all_ranks[rank_idx]);
+      if (num_sites_on_rank == 0)
+        continue;
+
+      span<const CollisionTrackSite> sites_to_process;
+#ifdef OPENMC_MPI
+      if (rank_idx == mpi::rank) {
+        sites_to_process = openmc::span<const CollisionTrackSite>(
+          collision_track_bank.data(), num_sites_on_rank);
+      } else {
+        receive_buffer.resize(num_sites_on_rank);
+        MPI_Recv(receive_buffer.data(), num_sites_on_rank,
+          mpi::collision_track_site, rank_idx, rank_idx, mpi::intracomm,
+          MPI_STATUS_IGNORE);
+        sites_to_process = openmc::span<const CollisionTrackSite>(
+          receive_buffer.data(), num_sites_on_rank);
+      }
+#else
+      sites_to_process = openmc::span<const CollisionTrackSite>(
+        collision_track_bank.data(), num_sites_on_rank);
 #endif
+
+      for (const auto& site : sites_to_process) {
+        std::ostringstream custom_data_stream;
+        custom_data_stream << " dE : " << site.dE
+                           << " ; event_mt : " << site.event_mt
+                           << " ; delayed_group : " << site.delayed_group
+                           << " ; cell_id : " << site.cell_id
+                           << " ; nuclide_id : " << site.nuclide_id
+                           << " ; material_id : " << site.material_id
+                           << " ; universe_id : " << site.universe_id
+                           << " ; n_collision : " << site.n_collision
+                           << " ; parent_id : " << site.parent_id
+                           << " ; progeny_id : " << site.progeny_id;
+
+        all_blobs.push_back(custom_data_stream.str());
+        all_sites.push_back(site);
+      }
+    }
+
+    for (size_t idx = 0; idx < all_blobs.size(); ++idx) {
+      const auto& blob = all_blobs[idx];
+      std::string key = "blob_" + std::to_string(idx);
+      g_mcpl_api->hdr_add_data(file_id, key.c_str(), blob.size(), blob.c_str());
+    }
+
+    for (const auto& site : all_sites) {
+      mcpl_particle_repr_t p_repr {};
+      p_repr.position[0] = site.r.x;
+      p_repr.position[1] = site.r.y;
+      p_repr.position[2] = site.r.z;
+      p_repr.direction[0] = site.u.x;
+      p_repr.direction[1] = site.u.y;
+      p_repr.direction[2] = site.u.z;
+      p_repr.ekin = site.E * 1e-6;
+      p_repr.time = site.time * 1e3;
+      p_repr.weight = site.wgt;
+      p_repr.pdgcode = site.particle.pdg_number();
+      g_mcpl_api->add_particle(file_id, &p_repr);
+    }
+  } else {
+#ifdef OPENMC_MPI
+    if (!collision_track_bank.empty()) {
+      MPI_Send(collision_track_bank.data(), collision_track_bank.size(),
+        mpi::collision_track_site, 0, mpi::rank, mpi::intracomm);
+    }
+#endif
+  }
+}
+
+void write_mcpl_collision_track(const char* filename,
+  span<CollisionTrackSite> collision_track_bank,
+  const vector<int64_t>& bank_index)
+{
+  ensure_mcpl_ready_or_fatal();
+
+  std::string filename_(filename);
+  const auto extension = get_file_extension(filename_);
+  if (extension.empty()) {
+    filename_.append(".mcpl");
+  } else if (extension != "mcpl") {
+    warning(fmt::format("Specified filename '{}' has an extension '.{}', but "
+                        "an MCPL file (.mcpl) will be written using this name.",
+      filename, extension));
+  }
+
+  mcpl_outfile_t* file_id = nullptr;
+
+  if (mpi::master) {
+    file_id = g_mcpl_api->create_outfile(filename_.c_str());
+    if (!file_id) {
+      fatal_error(fmt::format(
+        "MCPL: Failed to create output file '{}'. Check permissions and path.",
+        filename_));
+    }
+    std::string src_line;
+    if (VERSION_DEV) {
+      src_line = fmt::format("OpenMC {}.{}.{}-dev{}", VERSION_MAJOR,
+        VERSION_MINOR, VERSION_RELEASE, VERSION_COMMIT_COUNT);
+    } else {
+      src_line = fmt::format(
+        "OpenMC {}.{}.{}", VERSION_MAJOR, VERSION_MINOR, VERSION_RELEASE);
+    }
+
+    g_mcpl_api->hdr_set_srcname(file_id, src_line.c_str());
+  }
+  write_mcpl_collision_track_internal(
+    file_id, collision_track_bank, bank_index);
+
+  if (mpi::master) {
+    if (file_id) {
+      g_mcpl_api->close_outfile(file_id);
+    }
+  }
 }
 
 } // namespace openmc
diff --git a/src/mesh.cpp b/src/mesh.cpp
index 101da7468..b05560d2a 100644
--- a/src/mesh.cpp
+++ b/src/mesh.cpp
@@ -1,9 +1,12 @@
 #include "openmc/mesh.h"
 #include <algorithm> // for copy, equal, min, min_element
 #include <cassert>
-#define _USE_MATH_DEFINES // to make M_PI declared in Intel and MSVC compilers
-#include <cmath>          // for ceil
-#include <cstddef>        // for size_t
+#include <cmath>   // for ceil
+#include <cstddef> // for size_t
+#include <cstdint> // for uint64_t
+#include <cstring> // for memcpy
+#include <limits>
+#include <numeric> // for accumulate
 #include <string>
 
 #ifdef _MSC_VER
@@ -14,13 +17,7 @@
 #include "mpi.h"
 #endif
 
-#include "xtensor/xadapt.hpp"
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xeval.hpp"
-#include "xtensor/xmath.hpp"
-#include "xtensor/xsort.hpp"
-#include "xtensor/xtensor.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h> // for fmt
 
 #include "openmc/capi.h"
@@ -47,13 +44,14 @@
 #include "openmc/volume_calc.h"
 #include "openmc/xml_interface.h"
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 #include "libmesh/mesh_modification.h"
 #include "libmesh/mesh_tools.h"
 #include "libmesh/numeric_vector.h"
+#include "libmesh/replicated_mesh.h"
 #endif
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 #include "moab/FileOptions.hpp"
 #endif
 
@@ -63,7 +61,7 @@ namespace openmc {
 // Global variables
 //==============================================================================
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 const bool LIBMESH_ENABLED = true;
 #else
 const bool LIBMESH_ENABLED = false;
@@ -80,7 +78,7 @@ vector<unique_ptr<Mesh>> meshes;
 
 } // namespace model
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 namespace settings {
 unique_ptr<libMesh::LibMeshInit> libmesh_init;
 const libMesh::Parallel::Communicator* libmesh_comm {nullptr};
@@ -139,6 +137,63 @@ inline bool atomic_cas_int32(int32_t* ptr, int32_t& expected, int32_t desired)
 #endif
 }
 
+// Helper function equivalent to std::bit_cast in C++20
+template<typename To, typename From>
+inline To bit_cast_value(const From& value)
+{
+  To out;
+  std::memcpy(&out, &value, sizeof(To));
+  return out;
+}
+
+inline void atomic_update_double(double* ptr, double value, bool is_min)
+{
+#if defined(__GNUC__) || defined(__clang__)
+  using may_alias_uint64_t [[gnu::may_alias]] = uint64_t;
+  auto* bits_ptr = reinterpret_cast<may_alias_uint64_t*>(ptr);
+  uint64_t current_bits = __atomic_load_n(bits_ptr, __ATOMIC_SEQ_CST);
+  double current = bit_cast_value<double>(current_bits);
+  while (is_min ? (value < current) : (value > current)) {
+    uint64_t desired_bits = bit_cast_value<uint64_t>(value);
+    uint64_t expected_bits = current_bits;
+    if (__atomic_compare_exchange_n(bits_ptr, &expected_bits, desired_bits,
+          false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) {
+      return;
+    }
+    current_bits = expected_bits;
+    current = bit_cast_value<double>(current_bits);
+  }
+
+#elif defined(_MSC_VER)
+  auto* bits_ptr = reinterpret_cast<volatile long long*>(ptr);
+  long long current_bits = *bits_ptr;
+  double current = bit_cast_value<double>(current_bits);
+  while (is_min ? (value < current) : (value > current)) {
+    long long desired_bits = bit_cast_value<long long>(value);
+    long long old_bits =
+      _InterlockedCompareExchange64(bits_ptr, desired_bits, current_bits);
+    if (old_bits == current_bits) {
+      return;
+    }
+    current_bits = old_bits;
+    current = bit_cast_value<double>(current_bits);
+  }
+
+#else
+#error "No compare-and-swap implementation available for this compiler."
+#endif
+}
+
+inline void atomic_max_double(double* ptr, double value)
+{
+  atomic_update_double(ptr, value, false);
+}
+
+inline void atomic_min_double(double* ptr, double value)
+{
+  atomic_update_double(ptr, value, true);
+}
+
 namespace detail {
 
 //==============================================================================
@@ -146,7 +201,7 @@ namespace detail {
 //==============================================================================
 
 void MaterialVolumes::add_volume(
-  int index_elem, int index_material, double volume)
+  int index_elem, int index_material, double volume, const BoundingBox* bbox)
 {
   // This method handles adding elements to the materials hash table,
   // implementing open addressing with linear probing. Consistency across
@@ -165,10 +220,18 @@ void MaterialVolumes::add_volume(
     // Non-atomic read of current material
     int32_t current_val = *slot_ptr;
 
-    // Found the desired material; accumulate volume
+    // Found the desired material; accumulate volume and bbox
     if (current_val == index_material) {
 #pragma omp atomic
       this->volumes(index_elem, slot) += volume;
+      if (bbox) {
+        atomic_min_double(&this->bboxes(index_elem, slot, 0), bbox->min.x);
+        atomic_min_double(&this->bboxes(index_elem, slot, 1), bbox->min.y);
+        atomic_min_double(&this->bboxes(index_elem, slot, 2), bbox->min.z);
+        atomic_max_double(&this->bboxes(index_elem, slot, 3), bbox->max.x);
+        atomic_max_double(&this->bboxes(index_elem, slot, 4), bbox->max.y);
+        atomic_max_double(&this->bboxes(index_elem, slot, 5), bbox->max.z);
+      }
       return;
     }
 
@@ -184,6 +247,14 @@ void MaterialVolumes::add_volume(
       if (claimed_slot || (expected_val == index_material)) {
 #pragma omp atomic
         this->volumes(index_elem, slot) += volume;
+        if (bbox) {
+          atomic_min_double(&this->bboxes(index_elem, slot, 0), bbox->min.x);
+          atomic_min_double(&this->bboxes(index_elem, slot, 1), bbox->min.y);
+          atomic_min_double(&this->bboxes(index_elem, slot, 2), bbox->min.z);
+          atomic_max_double(&this->bboxes(index_elem, slot, 3), bbox->max.x);
+          atomic_max_double(&this->bboxes(index_elem, slot, 4), bbox->max.y);
+          atomic_max_double(&this->bboxes(index_elem, slot, 5), bbox->max.z);
+        }
         return;
       }
     }
@@ -194,7 +265,7 @@ void MaterialVolumes::add_volume(
 }
 
 void MaterialVolumes::add_volume_unsafe(
-  int index_elem, int index_material, double volume)
+  int index_elem, int index_material, double volume, const BoundingBox* bbox)
 {
   // Linear probe
   for (int attempt = 0; attempt < table_size_; ++attempt) {
@@ -206,9 +277,23 @@ void MaterialVolumes::add_volume_unsafe(
     // Read current material
     int32_t current_val = this->materials(index_elem, slot);
 
-    // Found the desired material; accumulate volume
+    // Found the desired material; accumulate volume and bbox
     if (current_val == index_material) {
       this->volumes(index_elem, slot) += volume;
+      if (bbox) {
+        this->bboxes(index_elem, slot, 0) =
+          std::min(this->bboxes(index_elem, slot, 0), bbox->min.x);
+        this->bboxes(index_elem, slot, 1) =
+          std::min(this->bboxes(index_elem, slot, 1), bbox->min.y);
+        this->bboxes(index_elem, slot, 2) =
+          std::min(this->bboxes(index_elem, slot, 2), bbox->min.z);
+        this->bboxes(index_elem, slot, 3) =
+          std::max(this->bboxes(index_elem, slot, 3), bbox->max.x);
+        this->bboxes(index_elem, slot, 4) =
+          std::max(this->bboxes(index_elem, slot, 4), bbox->max.y);
+        this->bboxes(index_elem, slot, 5) =
+          std::max(this->bboxes(index_elem, slot, 5), bbox->max.z);
+      }
       return;
     }
 
@@ -216,6 +301,20 @@ void MaterialVolumes::add_volume_unsafe(
     if (current_val == EMPTY) {
       this->materials(index_elem, slot) = index_material;
       this->volumes(index_elem, slot) += volume;
+      if (bbox) {
+        this->bboxes(index_elem, slot, 0) =
+          std::min(this->bboxes(index_elem, slot, 0), bbox->min.x);
+        this->bboxes(index_elem, slot, 1) =
+          std::min(this->bboxes(index_elem, slot, 1), bbox->min.y);
+        this->bboxes(index_elem, slot, 2) =
+          std::min(this->bboxes(index_elem, slot, 2), bbox->min.z);
+        this->bboxes(index_elem, slot, 3) =
+          std::max(this->bboxes(index_elem, slot, 3), bbox->max.x);
+        this->bboxes(index_elem, slot, 4) =
+          std::max(this->bboxes(index_elem, slot, 4), bbox->max.y);
+        this->bboxes(index_elem, slot, 5) =
+          std::max(this->bboxes(index_elem, slot, 5), bbox->max.z);
+      }
       return;
     }
   }
@@ -230,6 +329,42 @@ void MaterialVolumes::add_volume_unsafe(
 // Mesh implementation
 //==============================================================================
 
+template<typename T>
+const std::unique_ptr<Mesh>& Mesh::create(
+  T dataset, const std::string& mesh_type, const std::string& mesh_library)
+{
+  // Determine mesh type. Add to model vector and map
+  if (mesh_type == RegularMesh::mesh_type) {
+    model::meshes.push_back(make_unique<RegularMesh>(dataset));
+  } else if (mesh_type == RectilinearMesh::mesh_type) {
+    model::meshes.push_back(make_unique<RectilinearMesh>(dataset));
+  } else if (mesh_type == CylindricalMesh::mesh_type) {
+    model::meshes.push_back(make_unique<CylindricalMesh>(dataset));
+  } else if (mesh_type == SphericalMesh::mesh_type) {
+    model::meshes.push_back(make_unique<SphericalMesh>(dataset));
+#ifdef OPENMC_DAGMC_ENABLED
+  } else if (mesh_type == UnstructuredMesh::mesh_type &&
+             mesh_library == MOABMesh::mesh_lib_type) {
+    model::meshes.push_back(make_unique<MOABMesh>(dataset));
+#endif
+#ifdef OPENMC_LIBMESH_ENABLED
+  } else if (mesh_type == UnstructuredMesh::mesh_type &&
+             mesh_library == LibMesh::mesh_lib_type) {
+    model::meshes.push_back(make_unique<LibMesh>(dataset));
+#endif
+  } else if (mesh_type == UnstructuredMesh::mesh_type) {
+    fatal_error("Unstructured mesh support is not enabled or the mesh "
+                "library is invalid.");
+  } else {
+    fatal_error(fmt::format("Invalid mesh type: {}", mesh_type));
+  }
+
+  // Map ID to position in vector
+  model::mesh_map[model::meshes.back()->id_] = model::meshes.size() - 1;
+
+  return model::meshes.back();
+}
+
 Mesh::Mesh(pugi::xml_node node)
 {
   // Read mesh id
@@ -238,6 +373,17 @@ Mesh::Mesh(pugi::xml_node node)
     name_ = get_node_value(node, "name");
 }
 
+Mesh::Mesh(hid_t group)
+{
+  // Read mesh ID
+  read_attribute(group, "id", id_);
+
+  // Read mesh name
+  if (object_exists(group, "name")) {
+    read_dataset(group, "name", name_);
+  }
+}
+
 void Mesh::set_id(int32_t id)
 {
   assert(id >= 0 || id == C_NONE);
@@ -265,7 +411,13 @@ void Mesh::set_id(int32_t id)
 
   // Update ID and entry in the mesh map
   id_ = id;
-  model::mesh_map[id] = model::meshes.size() - 1;
+
+  // find the index of this mesh in the model::meshes vector
+  // (search in reverse because this mesh was likely just added to the vector)
+  auto it = std::find_if(model::meshes.rbegin(), model::meshes.rend(),
+    [this](const std::unique_ptr<Mesh>& mesh) { return mesh.get() == this; });
+
+  model::mesh_map[id] = std::distance(model::meshes.begin(), it.base()) - 1;
 }
 
 vector<double> Mesh::volumes() const
@@ -279,6 +431,12 @@ vector<double> Mesh::volumes() const
 
 void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
   int32_t* materials, double* volumes) const
+{
+  this->material_volumes(nx, ny, nz, table_size, materials, volumes, nullptr);
+}
+
+void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
+  int32_t* materials, double* volumes, double* bboxes) const
 {
   if (mpi::master) {
     header("MESH MATERIAL VOLUMES CALCULATION", 7);
@@ -297,7 +455,8 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
   timer.start();
 
   // Create object for keeping track of materials/volumes
-  detail::MaterialVolumes result(materials, volumes, table_size);
+  detail::MaterialVolumes result(materials, volumes, bboxes, table_size);
+  bool compute_bboxes = bboxes != nullptr;
 
   // Determine bounding box
   auto bbox = this->bounding_box();
@@ -305,9 +464,10 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
   std::array<int, 3> n_rays = {nx, ny, nz};
 
   // Determine effective width of rays
-  Position width((nx > 0) ? (bbox.xmax - bbox.xmin) / nx : 0.0,
-    (ny > 0) ? (bbox.ymax - bbox.ymin) / ny : 0.0,
-    (nz > 0) ? (bbox.zmax - bbox.zmin) / nz : 0.0);
+  Position width = bbox.max - bbox.min;
+  width.x = (nx > 0) ? width.x / nx : 0.0;
+  width.y = (ny > 0) ? width.y / ny : 0.0;
+  width.z = (nz > 0) ? width.z / nz : 0.0;
 
   // Set flag for mesh being contained within model
   bool out_of_model = false;
@@ -315,26 +475,26 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
 #pragma omp parallel
   {
     // Preallocate vector for mesh indices and length fractions and particle
-    std::vector<int> bins;
-    std::vector<double> length_fractions;
+    vector<int> bins;
+    vector<double> length_fractions;
     Particle p;
 
     SourceSite site;
     site.E = 1.0;
-    site.particle = ParticleType::neutron;
+    site.particle = ParticleType::neutron();
 
     for (int axis = 0; axis < 3; ++axis) {
       // Set starting position and direction
       site.r = {0.0, 0.0, 0.0};
-      site.r[axis] = bbox.min()[axis];
+      site.r[axis] = bbox.min[axis];
       site.u = {0.0, 0.0, 0.0};
       site.u[axis] = 1.0;
 
       // Determine width of rays and number of rays in other directions
       int ax1 = (axis + 1) % 3;
       int ax2 = (axis + 2) % 3;
-      double min1 = bbox.min()[ax1];
-      double min2 = bbox.min()[ax2];
+      double min1 = bbox.min[ax1];
+      double min2 = bbox.min[ax2];
       double d1 = width[ax1];
       double d2 = width[ax2];
       int n1 = n_rays[ax1];
@@ -368,24 +528,24 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
 
           // Set birth cell attribute
           if (p.cell_born() == C_NONE)
-            p.cell_born() = p.lowest_coord().cell;
+            p.cell_born() = p.lowest_coord().cell();
 
           // Initialize last cells from current cell
           for (int j = 0; j < p.n_coord(); ++j) {
-            p.cell_last(j) = p.coord(j).cell;
+            p.cell_last(j) = p.coord(j).cell();
           }
           p.n_coord_last() = p.n_coord();
 
           while (true) {
             // Ray trace from r_start to r_end
             Position r0 = p.r();
-            double max_distance = bbox.max()[axis] - r0[axis];
+            double max_distance = bbox.max[axis] - r0[axis];
 
             // Find the distance to the nearest boundary
             BoundaryInfo boundary = distance_to_boundary(p);
 
             // Advance particle forward
-            double distance = std::min(boundary.distance, max_distance);
+            double distance = std::min(boundary.distance(), max_distance);
             p.move_distance(distance);
 
             // Determine what mesh elements were crossed by particle
@@ -398,12 +558,36 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
             if (i_material != C_NONE) {
               i_material = model::materials[i_material]->id();
             }
+            double cumulative_frac = 0.0;
             for (int i_bin = 0; i_bin < bins.size(); i_bin++) {
               int mesh_index = bins[i_bin];
               double length = distance * length_fractions[i_bin];
+              double volume = length * d1 * d2;
 
-              // Add volume to result
-              result.add_volume(mesh_index, i_material, length * d1 * d2);
+              if (compute_bboxes) {
+                double axis_start = r0[axis] + distance * cumulative_frac;
+                double axis_end = axis_start + length;
+                cumulative_frac += length_fractions[i_bin];
+
+                Position contrib_min = site.r;
+                Position contrib_max = site.r;
+
+                contrib_min[ax1] = site.r[ax1] - 0.5 * d1;
+                contrib_max[ax1] = site.r[ax1] + 0.5 * d1;
+                contrib_min[ax2] = site.r[ax2] - 0.5 * d2;
+                contrib_max[ax2] = site.r[ax2] + 0.5 * d2;
+                contrib_min[axis] = std::min(axis_start, axis_end);
+                contrib_max[axis] = std::max(axis_start, axis_end);
+
+                BoundingBox contrib_bbox {contrib_min, contrib_max};
+                contrib_bbox &= bbox;
+
+                result.add_volume(
+                  mesh_index, i_material, volume, &contrib_bbox);
+              } else {
+                // Add volume to result
+                result.add_volume(mesh_index, i_material, volume);
+              }
             }
 
             if (distance == max_distance)
@@ -411,17 +595,17 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
 
             // cross next geometric surface
             for (int j = 0; j < p.n_coord(); ++j) {
-              p.cell_last(j) = p.coord(j).cell;
+              p.cell_last(j) = p.coord(j).cell();
             }
             p.n_coord_last() = p.n_coord();
 
             // Set surface that particle is on and adjust coordinate levels
-            p.surface() = boundary.surface;
-            p.n_coord() = boundary.coord_level;
+            p.surface() = boundary.surface();
+            p.n_coord() = boundary.coord_level();
 
-            if (boundary.lattice_translation[0] != 0 ||
-                boundary.lattice_translation[1] != 0 ||
-                boundary.lattice_translation[2] != 0) {
+            if (boundary.lattice_translation()[0] != 0 ||
+                boundary.lattice_translation()[1] != 0 ||
+                boundary.lattice_translation()[2] != 0) {
               // Particle crosses lattice boundary
               cross_lattice(p, boundary);
             } else {
@@ -450,10 +634,15 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
   // Combine results from multiple MPI processes
   if (mpi::n_procs > 1) {
     int total = this->n_bins() * table_size;
+    int total_bbox = total * 6;
     if (mpi::master) {
       // Allocate temporary buffer for receiving data
-      std::vector<int32_t> mats(total);
-      std::vector<double> vols(total);
+      vector<int32_t> mats(total);
+      vector<double> vols(total);
+      vector<double> recv_bboxes;
+      if (compute_bboxes) {
+        recv_bboxes.resize(total_bbox);
+      }
 
       for (int i = 1; i < mpi::n_procs; ++i) {
         // Receive material indices and volumes from process i
@@ -461,6 +650,10 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
           MPI_STATUS_IGNORE);
         MPI_Recv(vols.data(), total, MPI_DOUBLE, i, i, mpi::intracomm,
           MPI_STATUS_IGNORE);
+        if (compute_bboxes) {
+          MPI_Recv(recv_bboxes.data(), total_bbox, MPI_DOUBLE, i, i,
+            mpi::intracomm, MPI_STATUS_IGNORE);
+        }
 
         // Combine with existing results; we can call thread unsafe version of
         // add_volume because each thread is operating on a different element
@@ -469,7 +662,18 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
           for (int k = 0; k < table_size; ++k) {
             int index = index_elem * table_size + k;
             if (mats[index] != EMPTY) {
-              result.add_volume_unsafe(index_elem, mats[index], vols[index]);
+              if (compute_bboxes) {
+                int bbox_index = index * 6;
+                BoundingBox slot_bbox {
+                  {recv_bboxes[bbox_index + 0], recv_bboxes[bbox_index + 1],
+                    recv_bboxes[bbox_index + 2]},
+                  {recv_bboxes[bbox_index + 3], recv_bboxes[bbox_index + 4],
+                    recv_bboxes[bbox_index + 5]}};
+                result.add_volume_unsafe(
+                  index_elem, mats[index], vols[index], &slot_bbox);
+              } else {
+                result.add_volume_unsafe(index_elem, mats[index], vols[index]);
+              }
             }
           }
         }
@@ -478,6 +682,9 @@ void Mesh::material_volumes(int nx, int ny, int nz, int table_size,
       // Send material indices and volumes to process 0
       MPI_Send(materials, total, MPI_INT32_T, 0, mpi::rank, mpi::intracomm);
       MPI_Send(volumes, total, MPI_DOUBLE, 0, mpi::rank, mpi::intracomm);
+      if (compute_bboxes) {
+        MPI_Send(bboxes, total_bbox, MPI_DOUBLE, 0, mpi::rank, mpi::intracomm);
+      }
     }
   }
 
@@ -560,11 +767,9 @@ std::string StructuredMesh::bin_label(int bin) const
   }
 }
 
-xt::xtensor<int, 1> StructuredMesh::get_x_shape() const
+tensor::Tensor<int> StructuredMesh::get_shape_tensor() const
 {
-  // because method is const, shape_ is const as well and can't be adapted
-  auto tmp_shape = shape_;
-  return xt::adapt(tmp_shape, {n_dimension_});
+  return tensor::Tensor<int>(shape_.data(), static_cast<size_t>(n_dimension_));
 }
 
 Position StructuredMesh::sample_element(
@@ -590,6 +795,8 @@ Position StructuredMesh::sample_element(
 
 UnstructuredMesh::UnstructuredMesh(pugi::xml_node node) : Mesh(node)
 {
+  n_dimension_ = 3;
+
   // check the mesh type
   if (check_for_node(node, "type")) {
     auto temp = get_node_value(node, "type", true, true);
@@ -625,6 +832,46 @@ UnstructuredMesh::UnstructuredMesh(pugi::xml_node node) : Mesh(node)
   }
 }
 
+UnstructuredMesh::UnstructuredMesh(hid_t group) : Mesh(group)
+{
+  n_dimension_ = 3;
+
+  // check the mesh type
+  if (object_exists(group, "type")) {
+    std::string temp;
+    read_dataset(group, "type", temp);
+    if (temp != mesh_type) {
+      fatal_error(fmt::format("Invalid mesh type: {}", temp));
+    }
+  }
+
+  // check if a length unit multiplier was specified
+  if (object_exists(group, "length_multiplier")) {
+    read_dataset(group, "length_multiplier", length_multiplier_);
+  }
+
+  // get the filename of the unstructured mesh to load
+  if (object_exists(group, "filename")) {
+    read_dataset(group, "filename", filename_);
+    if (!file_exists(filename_)) {
+      fatal_error("Mesh file '" + filename_ + "' does not exist!");
+    }
+  } else {
+    fatal_error(fmt::format(
+      "No filename supplied for unstructured mesh with ID: {}", id_));
+  }
+
+  if (attribute_exists(group, "options")) {
+    read_attribute(group, "options", options_);
+  }
+
+  // check if mesh tally data should be written with
+  // statepoint files
+  if (attribute_exists(group, "output")) {
+    read_attribute(group, "output", output_);
+  }
+}
+
 void UnstructuredMesh::determine_bounds()
 {
   double xmin = INFTY;
@@ -707,10 +954,11 @@ void UnstructuredMesh::to_hdf5_inner(hid_t mesh_group) const
     write_dataset(mesh_group, "length_multiplier", length_multiplier_);
 
   // write vertex coordinates
-  xt::xtensor<double, 2> vertices({static_cast<size_t>(this->n_vertices()), 3});
+  tensor::Tensor<double> vertices(
+    {static_cast<size_t>(this->n_vertices()), static_cast<size_t>(3)});
   for (int i = 0; i < this->n_vertices(); i++) {
     auto v = this->vertex(i);
-    xt::view(vertices, i, xt::all()) = xt::xarray<double>({v.x, v.y, v.z});
+    vertices.slice(i) = {v.x, v.y, v.z};
   }
   write_dataset(mesh_group, "vertices", vertices);
 
@@ -718,8 +966,10 @@ void UnstructuredMesh::to_hdf5_inner(hid_t mesh_group) const
 
   // write element types and connectivity
   vector<double> volumes;
-  xt::xtensor<int, 2> connectivity({static_cast<size_t>(this->n_bins()), 8});
-  xt::xtensor<int, 2> elem_types({static_cast<size_t>(this->n_bins()), 1});
+  tensor::Tensor<int> connectivity(
+    {static_cast<size_t>(this->n_bins()), static_cast<size_t>(8)});
+  tensor::Tensor<int> elem_types(
+    {static_cast<size_t>(this->n_bins()), static_cast<size_t>(1)});
   for (int i = 0; i < this->n_bins(); i++) {
     auto conn = this->connectivity(i);
 
@@ -727,21 +977,18 @@ void UnstructuredMesh::to_hdf5_inner(hid_t mesh_group) const
 
     // write linear tet element
     if (conn.size() == 4) {
-      xt::view(elem_types, i, xt::all()) =
-        static_cast<int>(ElementType::LINEAR_TET);
-      xt::view(connectivity, i, xt::all()) =
-        xt::xarray<int>({conn[0], conn[1], conn[2], conn[3], -1, -1, -1, -1});
+      elem_types.slice(i) = static_cast<int>(ElementType::LINEAR_TET);
+      connectivity.slice(i) = {
+        conn[0], conn[1], conn[2], conn[3], -1, -1, -1, -1};
       // write linear hex element
     } else if (conn.size() == 8) {
-      xt::view(elem_types, i, xt::all()) =
-        static_cast<int>(ElementType::LINEAR_HEX);
-      xt::view(connectivity, i, xt::all()) = xt::xarray<int>({conn[0], conn[1],
-        conn[2], conn[3], conn[4], conn[5], conn[6], conn[7]});
+      elem_types.slice(i) = static_cast<int>(ElementType::LINEAR_HEX);
+      connectivity.slice(i) = {
+        conn[0], conn[1], conn[2], conn[3], conn[4], conn[5], conn[6], conn[7]};
     } else {
       num_elem_skipped++;
-      xt::view(elem_types, i, xt::all()) =
-        static_cast<int>(ElementType::UNSUPPORTED);
-      xt::view(connectivity, i, xt::all()) = -1;
+      elem_types.slice(i) = static_cast<int>(ElementType::UNSUPPORTED);
+      connectivity.slice(i) = -1;
     }
   }
 
@@ -833,16 +1080,29 @@ int StructuredMesh::get_bin(Position r) const
 
 int StructuredMesh::n_bins() const
 {
-  return std::accumulate(
-    shape_.begin(), shape_.begin() + n_dimension_, 1, std::multiplies<>());
+  // Bin indices are stored as 32-bit ints in the tally system.
+  int64_t n = 1;
+  for (int i = 0; i < n_dimension_; ++i)
+    n *= shape_[i];
+  if (n > std::numeric_limits<int>::max()) {
+    fatal_error(fmt::format(
+      "Mesh {} has too many bins ({}) for 32-bit tally indexing", id_, n));
+  }
+  return static_cast<int>(n);
 }
 
 int StructuredMesh::n_surface_bins() const
 {
-  return 4 * n_dimension_ * n_bins();
+  // Surface bin indices are stored as 32-bit ints in the tally system.
+  int64_t n = static_cast<int64_t>(n_bins()) * 4 * n_dimension_;
+  if (n > std::numeric_limits<int>::max()) {
+    fatal_error(fmt::format(
+      "Mesh {} has too many surface bins ({}) for tally indexing", id_, n));
+  }
+  return static_cast<int>(n);
 }
 
-xt::xtensor<double, 1> StructuredMesh::count_sites(
+tensor::Tensor<double> StructuredMesh::count_sites(
   const SourceSite* bank, int64_t length, bool* outside) const
 {
   // Determine shape of array for counts
@@ -850,7 +1110,7 @@ xt::xtensor<double, 1> StructuredMesh::count_sites(
   vector<std::size_t> shape = {m};
 
   // Create array of zeros
-  xt::xarray<double> cnt {shape, 0.0};
+  auto cnt = tensor::zeros<double>(shape);
   bool outside_ = false;
 
   for (int64_t i = 0; i < length; i++) {
@@ -869,31 +1129,25 @@ xt::xtensor<double, 1> StructuredMesh::count_sites(
     cnt(mesh_bin) += site.wgt;
   }
 
-  // Create copy of count data. Since ownership will be acquired by xtensor,
-  // std::allocator must be used to avoid Valgrind mismatched free() / delete
-  // warnings.
+  // Create reduced count data
+  auto counts = tensor::zeros<double>(shape);
   int total = cnt.size();
-  double* cnt_reduced = std::allocator<double> {}.allocate(total);
 
 #ifdef OPENMC_MPI
   // collect values from all processors
   MPI_Reduce(
-    cnt.data(), cnt_reduced, total, MPI_DOUBLE, MPI_SUM, 0, mpi::intracomm);
+    cnt.data(), counts.data(), total, MPI_DOUBLE, MPI_SUM, 0, mpi::intracomm);
 
   // Check if there were sites outside the mesh for any processor
   if (outside) {
     MPI_Reduce(&outside_, outside, 1, MPI_C_BOOL, MPI_LOR, 0, mpi::intracomm);
   }
 #else
-  std::copy(cnt.data(), cnt.data() + total, cnt_reduced);
+  std::copy(cnt.data(), cnt.data() + total, counts.data());
   if (outside)
     *outside = outside_;
 #endif
 
-  // Adapt reduced values in array back into an xarray
-  auto arr = xt::adapt(cnt_reduced, total, xt::acquire_ownership(), shape);
-  xt::xarray<double> counts = arr;
-
   return counts;
 }
 
@@ -915,6 +1169,11 @@ void StructuredMesh::raytrace_mesh(
   if (total_distance == 0.0 && settings::solver_type != SolverType::RANDOM_RAY)
     return;
 
+  // keep a copy of the original global position to pass to get_indices,
+  // which performs its own transformation to local coordinates
+  Position global_r = r0;
+  Position local_r = local_coords(r0);
+
   const int n = n_dimension_;
 
   // Flag if position is inside the mesh
@@ -925,7 +1184,7 @@ void StructuredMesh::raytrace_mesh(
 
   // Calculate index of current cell. Offset the position a tiny bit in
   // direction of flight
-  MeshIndex ijk = get_indices(r0 + TINY_BIT * u, in_mesh);
+  MeshIndex ijk = get_indices(global_r + TINY_BIT * u, in_mesh);
 
   // if track is very short, assume that it is completely inside one cell.
   // Only the current cell will score and no surfaces
@@ -936,16 +1195,10 @@ void StructuredMesh::raytrace_mesh(
     return;
   }
 
-  // translate start and end positions,
-  // this needs to come after the get_indices call because it does its own
-  // translation
-  local_coords(r0);
-  local_coords(r1);
-
   // Calculate initial distances to next surfaces in all three dimensions
   std::array<MeshDistance, 3> distances;
   for (int k = 0; k < n; ++k) {
-    distances[k] = distance_to_grid_boundary(ijk, k, r0, u, 0.0);
+    distances[k] = distance_to_grid_boundary(ijk, k, local_r, u, 0.0);
   }
 
   // Loop until r = r1 is eventually reached
@@ -975,7 +1228,7 @@ void StructuredMesh::raytrace_mesh(
       // The two other directions are still valid!
       ijk[k] = distances[k].next_index;
       distances[k] =
-        distance_to_grid_boundary(ijk, k, r0, u, traveled_distance);
+        distance_to_grid_boundary(ijk, k, local_r, u, traveled_distance);
 
       // Check if we have left the interior of the mesh
       in_mesh = ((ijk[k] >= 1) && (ijk[k] <= shape_[k]));
@@ -990,7 +1243,7 @@ void StructuredMesh::raytrace_mesh(
       // For all directions outside the mesh, find the distance that we need
       // to travel to reach the next surface. Use the largest distance, as
       // only this will cross all outer surfaces.
-      int k_max {0};
+      int k_max {-1};
       for (int k = 0; k < n; ++k) {
         if ((ijk[k] < 1 || ijk[k] > shape_[k]) &&
             (distances[k].distance > traveled_distance)) {
@@ -998,6 +1251,10 @@ void StructuredMesh::raytrace_mesh(
           k_max = k;
         }
       }
+      // Assure some distance is traveled
+      if (k_max == -1) {
+        traveled_distance += TINY_BIT;
+      }
 
       // If r1 is not inside the mesh, exit here
       if (traveled_distance >= total_distance)
@@ -1005,14 +1262,14 @@ void StructuredMesh::raytrace_mesh(
 
       // Calculate the new cell index and update all distances to next
       // surfaces.
-      ijk = get_indices(r0 + (traveled_distance + TINY_BIT) * u, in_mesh);
+      ijk = get_indices(global_r + (traveled_distance + TINY_BIT) * u, in_mesh);
       for (int k = 0; k < n; ++k) {
         distances[k] =
-          distance_to_grid_boundary(ijk, k, r0, u, traveled_distance);
+          distance_to_grid_boundary(ijk, k, local_r, u, traveled_distance);
       }
 
       // If inside the mesh, Tally inward current
-      if (in_mesh)
+      if (in_mesh && k_max >= 0)
         tally.surface(ijk, k_max, !distances[k_max].max_surface, true);
     }
   }
@@ -1081,6 +1338,72 @@ void StructuredMesh::surface_bins_crossed(
 // RegularMesh implementation
 //==============================================================================
 
+int RegularMesh::set_grid()
+{
+  tensor::Tensor<int> shape(shape_.data(), static_cast<size_t>(n_dimension_));
+
+  // Check that dimensions are all greater than zero
+  if ((shape <= 0).any()) {
+    set_errmsg("All entries for a regular mesh dimensions "
+               "must be positive.");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  // Make sure lower_left and dimension match
+  if (lower_left_.size() != n_dimension_) {
+    set_errmsg("Number of entries in lower_left must be the same "
+               "as the regular mesh dimensions.");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+  if (width_.size() > 0) {
+
+    // Check to ensure width has same dimensions
+    if (width_.size() != n_dimension_) {
+      set_errmsg("Number of entries on width must be the same as "
+                 "the regular mesh dimensions.");
+      return OPENMC_E_INVALID_ARGUMENT;
+    }
+
+    // Check for negative widths
+    if ((width_ < 0.0).any()) {
+      set_errmsg("Cannot have a negative width on a regular mesh.");
+      return OPENMC_E_INVALID_ARGUMENT;
+    }
+
+    // Set width and upper right coordinate
+    upper_right_ = lower_left_ + shape * width_;
+
+  } else if (upper_right_.size() > 0) {
+
+    // Check to ensure upper_right_ has same dimensions
+    if (upper_right_.size() != n_dimension_) {
+      set_errmsg("Number of entries on upper_right must be the "
+                 "same as the regular mesh dimensions.");
+      return OPENMC_E_INVALID_ARGUMENT;
+    }
+
+    // Check that upper-right is above lower-left
+    if ((upper_right_ < lower_left_).any()) {
+      set_errmsg(
+        "The upper_right coordinates of a regular mesh must be greater than "
+        "the lower_left coordinates.");
+      return OPENMC_E_INVALID_ARGUMENT;
+    }
+
+    // Set width
+    width_ = (upper_right_ - lower_left_) / shape;
+  }
+
+  // Set material volumes
+  volume_frac_ = 1.0 / shape.prod();
+
+  element_volume_ = 1.0;
+  for (int i = 0; i < n_dimension_; i++) {
+    element_volume_ *= width_[i];
+  }
+  return 0;
+}
+
 RegularMesh::RegularMesh(pugi::xml_node node) : StructuredMesh {node}
 {
   // Determine number of dimensions for mesh
@@ -1088,84 +1411,75 @@ RegularMesh::RegularMesh(pugi::xml_node node) : StructuredMesh {node}
     fatal_error("Must specify <dimension> on a regular mesh.");
   }
 
-  xt::xtensor<int, 1> shape = get_node_xarray<int>(node, "dimension");
+  tensor::Tensor<int> shape = get_node_tensor<int>(node, "dimension");
   int n = n_dimension_ = shape.size();
   if (n != 1 && n != 2 && n != 3) {
     fatal_error("Mesh must be one, two, or three dimensions.");
   }
   std::copy(shape.begin(), shape.end(), shape_.begin());
 
-  // Check that dimensions are all greater than zero
-  if (xt::any(shape <= 0)) {
-    fatal_error("All entries on the <dimension> element for a tally "
-                "mesh must be positive.");
-  }
-
   // Check for lower-left coordinates
   if (check_for_node(node, "lower_left")) {
     // Read mesh lower-left corner location
-    lower_left_ = get_node_xarray<double>(node, "lower_left");
+    lower_left_ = get_node_tensor<double>(node, "lower_left");
   } else {
     fatal_error("Must specify <lower_left> on a mesh.");
   }
 
-  // Make sure lower_left and dimension match
-  if (shape.size() != lower_left_.size()) {
-    fatal_error("Number of entries on <lower_left> must be the same "
-                "as the number of entries on <dimension>.");
-  }
-
   if (check_for_node(node, "width")) {
     // Make sure one of upper-right or width were specified
     if (check_for_node(node, "upper_right")) {
       fatal_error("Cannot specify both <upper_right> and <width> on a mesh.");
     }
 
-    width_ = get_node_xarray<double>(node, "width");
-
-    // Check to ensure width has same dimensions
-    auto n = width_.size();
-    if (n != lower_left_.size()) {
-      fatal_error("Number of entries on <width> must be the same as "
-                  "the number of entries on <lower_left>.");
-    }
-
-    // Check for negative widths
-    if (xt::any(width_ < 0.0)) {
-      fatal_error("Cannot have a negative <width> on a tally mesh.");
-    }
-
-    // Set width and upper right coordinate
-    upper_right_ = xt::eval(lower_left_ + shape * width_);
+    width_ = get_node_tensor<double>(node, "width");
 
   } else if (check_for_node(node, "upper_right")) {
-    upper_right_ = get_node_xarray<double>(node, "upper_right");
 
-    // Check to ensure width has same dimensions
-    auto n = upper_right_.size();
-    if (n != lower_left_.size()) {
-      fatal_error("Number of entries on <upper_right> must be the "
-                  "same as the number of entries on <lower_left>.");
-    }
+    upper_right_ = get_node_tensor<double>(node, "upper_right");
 
-    // Check that upper-right is above lower-left
-    if (xt::any(upper_right_ < lower_left_)) {
-      fatal_error("The <upper_right> coordinates must be greater than "
-                  "the <lower_left> coordinates on a tally mesh.");
-    }
-
-    // Set width
-    width_ = xt::eval((upper_right_ - lower_left_) / shape);
   } else {
     fatal_error("Must specify either <upper_right> or <width> on a mesh.");
   }
 
-  // Set material volumes
-  volume_frac_ = 1.0 / xt::prod(shape)();
+  if (int err = set_grid()) {
+    fatal_error(openmc_err_msg);
+  }
+}
 
-  element_volume_ = 1.0;
-  for (int i = 0; i < n_dimension_; i++) {
-    element_volume_ *= width_[i];
+RegularMesh::RegularMesh(hid_t group) : StructuredMesh {group}
+{
+  // Determine number of dimensions for mesh
+  if (!object_exists(group, "dimension")) {
+    fatal_error("Must specify <dimension> on a regular mesh.");
+  }
+
+  tensor::Tensor<int> shape;
+  read_dataset(group, "dimension", shape);
+  int n = n_dimension_ = shape.size();
+  if (n != 1 && n != 2 && n != 3) {
+    fatal_error("Mesh must be one, two, or three dimensions.");
+  }
+  std::copy(shape.begin(), shape.end(), shape_.begin());
+
+  // Check for lower-left coordinates
+  if (object_exists(group, "lower_left")) {
+    // Read mesh lower-left corner location
+    read_dataset(group, "lower_left", lower_left_);
+  } else {
+    fatal_error("Must specify lower_left dataset on a mesh.");
+  }
+
+  if (object_exists(group, "upper_right")) {
+
+    read_dataset(group, "upper_right", upper_right_);
+
+  } else {
+    fatal_error("Must specify either upper_right dataset on a mesh.");
+  }
+
+  if (int err = set_grid()) {
+    fatal_error(openmc_err_msg);
   }
 }
 
@@ -1208,6 +1522,7 @@ StructuredMesh::MeshDistance RegularMesh::distance_to_grid_boundary(
     d.next_index--;
     d.distance = (negative_grid_boundary(ijk, i) - r0[i]) / u[i];
   }
+
   return d;
 }
 
@@ -1254,13 +1569,13 @@ std::pair<vector<double>, vector<double>> RegularMesh::plot(
 
 void RegularMesh::to_hdf5_inner(hid_t mesh_group) const
 {
-  write_dataset(mesh_group, "dimension", get_x_shape());
+  write_dataset(mesh_group, "dimension", get_shape_tensor());
   write_dataset(mesh_group, "lower_left", lower_left_);
   write_dataset(mesh_group, "upper_right", upper_right_);
   write_dataset(mesh_group, "width", width_);
 }
 
-xt::xtensor<double, 1> RegularMesh::count_sites(
+tensor::Tensor<double> RegularMesh::count_sites(
   const SourceSite* bank, int64_t length, bool* outside) const
 {
   // Determine shape of array for counts
@@ -1268,7 +1583,7 @@ xt::xtensor<double, 1> RegularMesh::count_sites(
   vector<std::size_t> shape = {m};
 
   // Create array of zeros
-  xt::xarray<double> cnt {shape, 0.0};
+  auto cnt = tensor::zeros<double>(shape);
   bool outside_ = false;
 
   for (int64_t i = 0; i < length; i++) {
@@ -1287,31 +1602,25 @@ xt::xtensor<double, 1> RegularMesh::count_sites(
     cnt(mesh_bin) += site.wgt;
   }
 
-  // Create copy of count data. Since ownership will be acquired by xtensor,
-  // std::allocator must be used to avoid Valgrind mismatched free() / delete
-  // warnings.
+  // Create reduced count data
+  auto counts = tensor::zeros<double>(shape);
   int total = cnt.size();
-  double* cnt_reduced = std::allocator<double> {}.allocate(total);
 
 #ifdef OPENMC_MPI
   // collect values from all processors
   MPI_Reduce(
-    cnt.data(), cnt_reduced, total, MPI_DOUBLE, MPI_SUM, 0, mpi::intracomm);
+    cnt.data(), counts.data(), total, MPI_DOUBLE, MPI_SUM, 0, mpi::intracomm);
 
   // Check if there were sites outside the mesh for any processor
   if (outside) {
     MPI_Reduce(&outside_, outside, 1, MPI_C_BOOL, MPI_LOR, 0, mpi::intracomm);
   }
 #else
-  std::copy(cnt.data(), cnt.data() + total, cnt_reduced);
+  std::copy(cnt.data(), cnt.data() + total, counts.data());
   if (outside)
     *outside = outside_;
 #endif
 
-  // Adapt reduced values in array back into an xarray
-  auto arr = xt::adapt(cnt_reduced, total, xt::acquire_ownership(), shape);
-  xt::xarray<double> counts = arr;
-
   return counts;
 }
 
@@ -1337,6 +1646,19 @@ RectilinearMesh::RectilinearMesh(pugi::xml_node node) : StructuredMesh {node}
   }
 }
 
+RectilinearMesh::RectilinearMesh(hid_t group) : StructuredMesh {group}
+{
+  n_dimension_ = 3;
+
+  read_dataset(group, "x_grid", grid_[0]);
+  read_dataset(group, "y_grid", grid_[1]);
+  read_dataset(group, "z_grid", grid_[2]);
+
+  if (int err = set_grid()) {
+    fatal_error(openmc_err_msg);
+  }
+}
+
 const std::string RectilinearMesh::mesh_type = "rectilinear";
 
 std::string RectilinearMesh::get_mesh_type() const
@@ -1472,6 +1794,19 @@ CylindricalMesh::CylindricalMesh(pugi::xml_node node)
   }
 }
 
+CylindricalMesh::CylindricalMesh(hid_t group) : PeriodicStructuredMesh {group}
+{
+  n_dimension_ = 3;
+  read_dataset(group, "r_grid", grid_[0]);
+  read_dataset(group, "phi_grid", grid_[1]);
+  read_dataset(group, "z_grid", grid_[2]);
+  read_dataset(group, "origin", origin_);
+
+  if (int err = set_grid()) {
+    fatal_error(openmc_err_msg);
+  }
+}
+
 const std::string CylindricalMesh::mesh_type = "cylindrical";
 
 std::string CylindricalMesh::get_mesh_type() const
@@ -1482,7 +1817,7 @@ std::string CylindricalMesh::get_mesh_type() const
 StructuredMesh::MeshIndex CylindricalMesh::get_indices(
   Position r, bool& in_mesh) const
 {
-  local_coords(r);
+  r = local_coords(r);
 
   Position mapped_r;
   mapped_r[0] = std::hypot(r.x, r.y);
@@ -1493,7 +1828,7 @@ StructuredMesh::MeshIndex CylindricalMesh::get_indices(
   } else {
     mapped_r[1] = std::atan2(r.y, r.x);
     if (mapped_r[1] < 0)
-      mapped_r[1] += 2 * M_PI;
+      mapped_r[1] += 2 * PI;
   }
 
   MeshIndex idx = StructuredMesh::get_indices(mapped_r, in_mesh);
@@ -1552,18 +1887,19 @@ double CylindricalMesh::find_r_crossing(
   const double inv_denominator = 1.0 / denominator;
 
   const double p = (u.x * r.x + u.y * r.y) * inv_denominator;
-  double c = r.x * r.x + r.y * r.y - r0 * r0;
-  double D = p * p - c * inv_denominator;
+  double R = std::sqrt(r.x * r.x + r.y * r.y);
+  double D = p * p - (R - r0) * (R + r0) * inv_denominator;
 
   if (D < 0.0)
     return INFTY;
 
   D = std::sqrt(D);
 
-  // the solution -p - D is always smaller as -p + D : Check this one first
-  if (std::abs(c) <= RADIAL_MESH_TOL)
+  // Particle is already on the shell surface; avoid spurious crossing
+  if (std::abs(R - r0) <= RADIAL_MESH_TOL * (1.0 + std::abs(r0)))
     return INFTY;
 
+  // Check -p - D first because it is always smaller as -p + D
   if (-p - D > l)
     return -p - D;
   if (-p + D > l)
@@ -1630,23 +1966,21 @@ StructuredMesh::MeshDistance CylindricalMesh::distance_to_grid_boundary(
   const MeshIndex& ijk, int i, const Position& r0, const Direction& u,
   double l) const
 {
-  Position r = r0 - origin_;
-
   if (i == 0) {
 
     return std::min(
-      MeshDistance(ijk[i] + 1, true, find_r_crossing(r, u, l, ijk[i])),
-      MeshDistance(ijk[i] - 1, false, find_r_crossing(r, u, l, ijk[i] - 1)));
+      MeshDistance(ijk[i] + 1, true, find_r_crossing(r0, u, l, ijk[i])),
+      MeshDistance(ijk[i] - 1, false, find_r_crossing(r0, u, l, ijk[i] - 1)));
 
   } else if (i == 1) {
 
     return std::min(MeshDistance(sanitize_phi(ijk[i] + 1), true,
-                      find_phi_crossing(r, u, l, ijk[i])),
+                      find_phi_crossing(r0, u, l, ijk[i])),
       MeshDistance(sanitize_phi(ijk[i] - 1), false,
-        find_phi_crossing(r, u, l, ijk[i] - 1)));
+        find_phi_crossing(r0, u, l, ijk[i] - 1)));
 
   } else {
-    return find_z_crossing(r, u, l, ijk[i]);
+    return find_z_crossing(r0, u, l, ijk[i]);
   }
 }
 
@@ -1752,6 +2086,20 @@ SphericalMesh::SphericalMesh(pugi::xml_node node)
   }
 }
 
+SphericalMesh::SphericalMesh(hid_t group) : PeriodicStructuredMesh {group}
+{
+  n_dimension_ = 3;
+
+  read_dataset(group, "r_grid", grid_[0]);
+  read_dataset(group, "theta_grid", grid_[1]);
+  read_dataset(group, "phi_grid", grid_[2]);
+  read_dataset(group, "origin", origin_);
+
+  if (int err = set_grid()) {
+    fatal_error(openmc_err_msg);
+  }
+}
+
 const std::string SphericalMesh::mesh_type = "spherical";
 
 std::string SphericalMesh::get_mesh_type() const
@@ -1762,7 +2110,7 @@ std::string SphericalMesh::get_mesh_type() const
 StructuredMesh::MeshIndex SphericalMesh::get_indices(
   Position r, bool& in_mesh) const
 {
-  local_coords(r);
+  r = local_coords(r);
 
   Position mapped_r;
   mapped_r[0] = r.norm();
@@ -1774,7 +2122,7 @@ StructuredMesh::MeshIndex SphericalMesh::get_indices(
     mapped_r[1] = std::acos(r.z / mapped_r.x);
     mapped_r[2] = std::atan2(r.y, r.x);
     if (mapped_r[2] < 0)
-      mapped_r[2] += 2 * M_PI;
+      mapped_r[2] += 2 * PI;
   }
 
   MeshIndex idx = StructuredMesh::get_indices(mapped_r, in_mesh);
@@ -1797,7 +2145,8 @@ Position SphericalMesh::sample_element(
   double phi_min = this->phi(ijk[2] - 1);
   double phi_max = this->phi(ijk[2]);
 
-  double cos_theta = uniform_distribution(theta_min, theta_max, seed);
+  double cos_theta =
+    uniform_distribution(std::cos(theta_min), std::cos(theta_max), seed);
   double sin_theta = std::sin(std::acos(cos_theta));
   double phi = uniform_distribution(phi_min, phi_max, seed);
   double r_min_cub = std::pow(r_min, 3);
@@ -1824,15 +2173,16 @@ double SphericalMesh::find_r_crossing(
   if (r0 == 0.0)
     return INFTY;
   const double p = r.dot(u);
-  double c = r.dot(r) - r0 * r0;
-  double D = p * p - c;
+  double R = r.norm();
+  double D = p * p - (R - r0) * (R + r0);
 
-  if (std::abs(c) <= RADIAL_MESH_TOL)
+  // Particle is already on the shell surface; avoid spurious crossing
+  if (std::abs(R - r0) <= RADIAL_MESH_TOL * (1.0 + std::abs(r0)))
     return INFTY;
 
   if (D >= 0.0) {
     D = std::sqrt(D);
-    // the solution -p - D is always smaller as -p + D : Check this one first
+    // Check -p - D first because it is always smaller as -p + D
     if (-p - D > l)
       return -p - D;
     if (-p + D > l)
@@ -2131,14 +2481,14 @@ extern "C" int openmc_add_unstructured_mesh(
   std::string mesh_file(filename);
   bool valid_lib = false;
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   if (lib_name == MOABMesh::mesh_lib_type) {
     model::meshes.push_back(std::move(make_unique<MOABMesh>(mesh_file)));
     valid_lib = true;
   }
 #endif
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
   if (lib_name == LibMesh::mesh_lib_type) {
     model::meshes.push_back(std::move(make_unique<LibMesh>(mesh_file)));
     valid_lib = true;
@@ -2219,26 +2569,26 @@ extern "C" int openmc_mesh_bounding_box(int32_t index, double* ll, double* ur)
   BoundingBox bbox = model::meshes[index]->bounding_box();
 
   // set lower left corner values
-  ll[0] = bbox.xmin;
-  ll[1] = bbox.ymin;
-  ll[2] = bbox.zmin;
+  ll[0] = bbox.min.x;
+  ll[1] = bbox.min.y;
+  ll[2] = bbox.min.z;
 
   // set upper right corner values
-  ur[0] = bbox.xmax;
-  ur[1] = bbox.ymax;
-  ur[2] = bbox.zmax;
+  ur[0] = bbox.max.x;
+  ur[1] = bbox.max.y;
+  ur[2] = bbox.max.z;
   return 0;
 }
 
 extern "C" int openmc_mesh_material_volumes(int32_t index, int nx, int ny,
-  int nz, int table_size, int32_t* materials, double* volumes)
+  int nz, int table_size, int32_t* materials, double* volumes, double* bboxes)
 {
   if (int err = check_mesh(index))
     return err;
 
   try {
     model::meshes[index]->material_volumes(
-      nx, ny, nz, table_size, materials, volumes);
+      nx, ny, nz, table_size, materials, volumes, bboxes);
   } catch (const std::exception& e) {
     set_errmsg(e.what());
     if (starts_with(e.what(), "Mesh")) {
@@ -2342,7 +2692,7 @@ extern "C" int openmc_regular_mesh_get_params(
     return err;
   RegularMesh* m = dynamic_cast<RegularMesh*>(model::meshes[index].get());
 
-  if (m->lower_left_.dimension() == 0) {
+  if (m->lower_left_.empty()) {
     set_errmsg("Mesh parameters have not been set.");
     return OPENMC_E_ALLOCATE;
   }
@@ -2369,17 +2719,17 @@ extern "C" int openmc_regular_mesh_set_params(
 
   vector<std::size_t> shape = {static_cast<std::size_t>(n)};
   if (ll && ur) {
-    m->lower_left_ = xt::adapt(ll, n, xt::no_ownership(), shape);
-    m->upper_right_ = xt::adapt(ur, n, xt::no_ownership(), shape);
-    m->width_ = (m->upper_right_ - m->lower_left_) / m->get_x_shape();
+    m->lower_left_ = tensor::Tensor<double>(ll, n);
+    m->upper_right_ = tensor::Tensor<double>(ur, n);
+    m->width_ = (m->upper_right_ - m->lower_left_) / m->get_shape_tensor();
   } else if (ll && width) {
-    m->lower_left_ = xt::adapt(ll, n, xt::no_ownership(), shape);
-    m->width_ = xt::adapt(width, n, xt::no_ownership(), shape);
-    m->upper_right_ = m->lower_left_ + m->get_x_shape() * m->width_;
+    m->lower_left_ = tensor::Tensor<double>(ll, n);
+    m->width_ = tensor::Tensor<double>(width, n);
+    m->upper_right_ = m->lower_left_ + m->get_shape_tensor() * m->width_;
   } else if (ur && width) {
-    m->upper_right_ = xt::adapt(ur, n, xt::no_ownership(), shape);
-    m->width_ = xt::adapt(width, n, xt::no_ownership(), shape);
-    m->lower_left_ = m->upper_right_ - m->get_x_shape() * m->width_;
+    m->upper_right_ = tensor::Tensor<double>(ur, n);
+    m->width_ = tensor::Tensor<double>(width, n);
+    m->lower_left_ = m->upper_right_ - m->get_shape_tensor() * m->width_;
   } else {
     set_errmsg("At least two parameters must be specified.");
     return OPENMC_E_INVALID_ARGUMENT;
@@ -2389,7 +2739,7 @@ extern "C" int openmc_regular_mesh_set_params(
 
   // TODO: incorporate this into method in RegularMesh that can be called from
   // here and from constructor
-  m->volume_frac_ = 1.0 / xt::prod(m->get_x_shape())();
+  m->volume_frac_ = 1.0 / m->get_shape_tensor().prod();
   m->element_volume_ = 1.0;
   for (int i = 0; i < m->n_dimension_; i++) {
     m->element_volume_ *= m->width_[i];
@@ -2438,7 +2788,7 @@ int openmc_structured_mesh_get_grid_impl(int32_t index, double** grid_x,
     return err;
   C* m = dynamic_cast<C*>(model::meshes[index].get());
 
-  if (m->lower_left_.dimension() == 0) {
+  if (m->lower_left_.empty()) {
     set_errmsg("Mesh parameters have not been set.");
     return OPENMC_E_ALLOCATE;
   }
@@ -2506,7 +2856,7 @@ extern "C" int openmc_spherical_mesh_set_grid(int32_t index,
     index, grid_x, nx, grid_y, ny, grid_z, nz);
 }
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 
 const std::string MOABMesh::mesh_lib_type = "moab";
 
@@ -2515,8 +2865,15 @@ MOABMesh::MOABMesh(pugi::xml_node node) : UnstructuredMesh(node)
   initialize();
 }
 
-MOABMesh::MOABMesh(const std::string& filename, double length_multiplier)
+MOABMesh::MOABMesh(hid_t group) : UnstructuredMesh(group)
 {
+  initialize();
+}
+
+MOABMesh::MOABMesh(const std::string& filename, double length_multiplier)
+  : UnstructuredMesh()
+{
+  n_dimension_ = 3;
   filename_ = filename;
   set_length_multiplier(length_multiplier);
   initialize();
@@ -3208,11 +3565,20 @@ void MOABMesh::write(const std::string& base_filename) const
 
 #endif
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
 
 const std::string LibMesh::mesh_lib_type = "libmesh";
 
-LibMesh::LibMesh(pugi::xml_node node) : UnstructuredMesh(node), adaptive_(false)
+LibMesh::LibMesh(pugi::xml_node node) : UnstructuredMesh(node)
+{
+  // filename_ and length_multiplier_ will already be set by the
+  // UnstructuredMesh constructor
+  set_mesh_pointer_from_filename(filename_);
+  set_length_multiplier(length_multiplier_);
+  initialize();
+}
+
+LibMesh::LibMesh(hid_t group) : UnstructuredMesh(group)
 {
   // filename_ and length_multiplier_ will already be set by the
   // UnstructuredMesh constructor
@@ -3223,9 +3589,8 @@ LibMesh::LibMesh(pugi::xml_node node) : UnstructuredMesh(node), adaptive_(false)
 
 // create the mesh from a pointer to a libMesh Mesh
 LibMesh::LibMesh(libMesh::MeshBase& input_mesh, double length_multiplier)
-  : adaptive_(input_mesh.n_active_elem() != input_mesh.n_elem())
 {
-  if (!dynamic_cast<libMesh::ReplicatedMesh*>(&input_mesh)) {
+  if (!input_mesh.is_replicated()) {
     fatal_error("At present LibMesh tallies require a replicated mesh. Please "
                 "ensure 'input_mesh' is a libMesh::ReplicatedMesh.");
   }
@@ -3237,8 +3602,8 @@ LibMesh::LibMesh(libMesh::MeshBase& input_mesh, double length_multiplier)
 
 // create the mesh from an input file
 LibMesh::LibMesh(const std::string& filename, double length_multiplier)
-  : adaptive_(false)
 {
+  n_dimension_ = 3;
   set_mesh_pointer_from_filename(filename);
   set_length_multiplier(length_multiplier);
   initialize();
@@ -3273,9 +3638,6 @@ void LibMesh::initialize()
   // assuming that unstructured meshes used in OpenMC are 3D
   n_dimension_ = 3;
 
-  if (length_multiplier_ > 0.0) {
-    libMesh::MeshTools::Modification::scale(*m_, length_multiplier_);
-  }
   // if OpenMC is managing the libMesh::MeshBase instance, prepare the mesh.
   // Otherwise assume that it is prepared by its owning application
   if (unique_m_) {
@@ -3299,27 +3661,19 @@ void LibMesh::initialize()
   auto first_elem = *m_->elements_begin();
   first_element_id_ = first_elem->id();
 
-  // if the mesh is adaptive elements aren't guaranteed by libMesh to be
-  // contiguous in ID space, so we need to map from bin indices (defined over
-  // active elements) to global dof ids
-  if (adaptive_) {
-    bin_to_elem_map_.reserve(m_->n_active_elem());
-    elem_to_bin_map_.resize(m_->n_elem(), -1);
-    for (auto it = m_->active_elements_begin(); it != m_->active_elements_end();
-         it++) {
-      auto elem = *it;
-
-      bin_to_elem_map_.push_back(elem->id());
-      elem_to_bin_map_[elem->id()] = bin_to_elem_map_.size() - 1;
-    }
-  }
-
   // bounding box for the mesh for quick rejection checks
   bbox_ = libMesh::MeshTools::create_bounding_box(*m_);
   libMesh::Point ll = bbox_.min();
   libMesh::Point ur = bbox_.max();
-  lower_left_ = {ll(0), ll(1), ll(2)};
-  upper_right_ = {ur(0), ur(1), ur(2)};
+  if (length_multiplier_ > 0.0) {
+    lower_left_ = {length_multiplier_ * ll(0), length_multiplier_ * ll(1),
+      length_multiplier_ * ll(2)};
+    upper_right_ = {length_multiplier_ * ur(0), length_multiplier_ * ur(1),
+      length_multiplier_ * ur(2)};
+  } else {
+    lower_left_ = {ll(0), ll(1), ll(2)};
+    upper_right_ = {ur(0), ur(1), ur(2)};
+  }
 }
 
 // Sample position within a tet for LibMesh type tets
@@ -3329,18 +3683,27 @@ Position LibMesh::sample_element(int32_t bin, uint64_t* seed) const
   // Get tet vertex coordinates from LibMesh
   std::array<Position, 4> tet_verts;
   for (int i = 0; i < elem.n_nodes(); i++) {
-    auto node_ref = elem.node_ref(i);
+    const auto& node_ref = elem.node_ref(i);
     tet_verts[i] = {node_ref(0), node_ref(1), node_ref(2)};
   }
   // Samples position within tet using Barycentric coordinates
-  return this->sample_tet(tet_verts, seed);
+  Position sampled_position = this->sample_tet(tet_verts, seed);
+  if (length_multiplier_ > 0.0) {
+    return length_multiplier_ * sampled_position;
+  } else {
+    return sampled_position;
+  }
 }
 
 Position LibMesh::centroid(int bin) const
 {
   const auto& elem = this->get_element_from_bin(bin);
   auto centroid = elem.vertex_average();
-  return {centroid(0), centroid(1), centroid(2)};
+  if (length_multiplier_ > 0.0) {
+    return length_multiplier_ * Position(centroid(0), centroid(1), centroid(2));
+  } else {
+    return {centroid(0), centroid(1), centroid(2)};
+  }
 }
 
 int LibMesh::n_vertices() const
@@ -3350,8 +3713,12 @@ int LibMesh::n_vertices() const
 
 Position LibMesh::vertex(int vertex_id) const
 {
-  const auto node_ref = m_->node_ref(vertex_id);
-  return {node_ref(0), node_ref(1), node_ref(2)};
+  const auto& node_ref = m_->node_ref(vertex_id);
+  if (length_multiplier_ > 0.0) {
+    return length_multiplier_ * Position(node_ref(0), node_ref(1), node_ref(2));
+  } else {
+    return {node_ref(0), node_ref(1), node_ref(2)};
+  }
 }
 
 std::vector<int> LibMesh::connectivity(int elem_id) const
@@ -3371,7 +3738,7 @@ std::string LibMesh::library() const
 
 int LibMesh::n_bins() const
 {
-  return m_->n_active_elem();
+  return m_->n_elem();
 }
 
 int LibMesh::n_surface_bins() const
@@ -3394,14 +3761,6 @@ int LibMesh::n_surface_bins() const
 
 void LibMesh::add_score(const std::string& var_name)
 {
-  if (adaptive_) {
-    warning(fmt::format(
-      "Exodus output cannot be provided as unstructured mesh {} is adaptive.",
-      this->id_));
-
-    return;
-  }
-
   if (!equation_systems_) {
     build_eqn_sys();
   }
@@ -3437,14 +3796,6 @@ void LibMesh::remove_scores()
 void LibMesh::set_score_data(const std::string& var_name,
   const vector<double>& values, const vector<double>& std_dev)
 {
-  if (adaptive_) {
-    warning(fmt::format(
-      "Exodus output cannot be provided as unstructured mesh {} is adaptive.",
-      this->id_));
-
-    return;
-  }
-
   if (!equation_systems_) {
     build_eqn_sys();
   }
@@ -3488,14 +3839,6 @@ void LibMesh::set_score_data(const std::string& var_name,
 
 void LibMesh::write(const std::string& filename) const
 {
-  if (adaptive_) {
-    warning(fmt::format(
-      "Exodus output cannot be provided as unstructured mesh {} is adaptive.",
-      this->id_));
-
-    return;
-  }
-
   write_message(fmt::format(
     "Writing file: {}.e for unstructured mesh {}", filename, this->id_));
   libMesh::ExodusII_IO exo(*m_);
@@ -3516,6 +3859,11 @@ int LibMesh::get_bin(Position r) const
   // look-up a tet using the point locator
   libMesh::Point p(r.x, r.y, r.z);
 
+  if (length_multiplier_ > 0.0) {
+    // Scale the point down
+    p /= length_multiplier_;
+  }
+
   // quick rejection check
   if (!bbox_.contains_point(p)) {
     return -1;
@@ -3529,8 +3877,7 @@ int LibMesh::get_bin(Position r) const
 
 int LibMesh::get_bin_from_element(const libMesh::Elem* elem) const
 {
-  int bin =
-    adaptive_ ? elem_to_bin_map_[elem->id()] : elem->id() - first_element_id_;
+  int bin = elem->id() - first_element_id_;
   if (bin >= n_bins() || bin < 0) {
     fatal_error(fmt::format("Invalid bin: {}", bin));
   }
@@ -3545,15 +3892,105 @@ std::pair<vector<double>, vector<double>> LibMesh::plot(
 
 const libMesh::Elem& LibMesh::get_element_from_bin(int bin) const
 {
-  return adaptive_ ? m_->elem_ref(bin_to_elem_map_.at(bin)) : m_->elem_ref(bin);
+  return m_->elem_ref(bin);
 }
 
 double LibMesh::volume(int bin) const
 {
-  return this->get_element_from_bin(bin).volume();
+  return this->get_element_from_bin(bin).volume() * length_multiplier_ *
+         length_multiplier_ * length_multiplier_;
 }
 
-#endif // LIBMESH
+AdaptiveLibMesh::AdaptiveLibMesh(libMesh::MeshBase& input_mesh,
+  double length_multiplier,
+  const std::set<libMesh::subdomain_id_type>& block_ids)
+  : LibMesh(input_mesh, length_multiplier), block_ids_(block_ids),
+    block_restrict_(!block_ids_.empty()),
+    num_active_(
+      block_restrict_
+        ? std::distance(m_->active_subdomain_set_elements_begin(block_ids_),
+            m_->active_subdomain_set_elements_end(block_ids_))
+        : m_->n_active_elem())
+{
+  // if the mesh is adaptive elements aren't guaranteed by libMesh to be
+  // contiguous in ID space, so we need to map from bin indices (defined over
+  // active elements) to global dof ids
+  bin_to_elem_map_.reserve(num_active_);
+  elem_to_bin_map_.resize(m_->n_elem(), -1);
+  auto begin = block_restrict_
+                 ? m_->active_subdomain_set_elements_begin(block_ids_)
+                 : m_->active_elements_begin();
+  auto end = block_restrict_ ? m_->active_subdomain_set_elements_end(block_ids_)
+                             : m_->active_elements_end();
+  for (const auto& elem : libMesh::as_range(begin, end)) {
+    bin_to_elem_map_.push_back(elem->id());
+    elem_to_bin_map_[elem->id()] = bin_to_elem_map_.size() - 1;
+  }
+}
+
+int AdaptiveLibMesh::n_bins() const
+{
+  return num_active_;
+}
+
+void AdaptiveLibMesh::add_score(const std::string& var_name)
+{
+  warning(fmt::format(
+    "Exodus output cannot be provided as unstructured mesh {} is adaptive.",
+    this->id_));
+}
+
+void AdaptiveLibMesh::set_score_data(const std::string& var_name,
+  const vector<double>& values, const vector<double>& std_dev)
+{
+  warning(fmt::format(
+    "Exodus output cannot be provided as unstructured mesh {} is adaptive.",
+    this->id_));
+}
+
+void AdaptiveLibMesh::write(const std::string& filename) const
+{
+  warning(fmt::format(
+    "Exodus output cannot be provided as unstructured mesh {} is adaptive.",
+    this->id_));
+}
+
+int AdaptiveLibMesh::get_bin(Position r) const
+{
+  // look-up a tet using the point locator
+  libMesh::Point p(r.x, r.y, r.z);
+
+  if (length_multiplier_ > 0.0) {
+    // Scale the point down
+    p /= length_multiplier_;
+  }
+
+  // quick rejection check
+  if (!bbox_.contains_point(p)) {
+    return -1;
+  }
+
+  const auto& point_locator = pl_.at(thread_num());
+
+  const auto elem_ptr = (*point_locator)(p, &block_ids_);
+  return elem_ptr ? get_bin_from_element(elem_ptr) : -1;
+}
+
+int AdaptiveLibMesh::get_bin_from_element(const libMesh::Elem* elem) const
+{
+  int bin = elem_to_bin_map_[elem->id()];
+  if (bin >= n_bins() || bin < 0) {
+    fatal_error(fmt::format("Invalid bin: {}", bin));
+  }
+  return bin;
+}
+
+const libMesh::Elem& AdaptiveLibMesh::get_element_from_bin(int bin) const
+{
+  return m_->elem_ref(bin_to_elem_map_.at(bin));
+}
+
+#endif // OPENMC_LIBMESH_ENABLED
 
 //==============================================================================
 // Non-member functions
@@ -3593,34 +4030,51 @@ void read_meshes(pugi::xml_node root)
       mesh_lib = get_node_value(node, "library", true, true);
     }
 
-    // Read mesh and add to vector
-    if (mesh_type == RegularMesh::mesh_type) {
-      model::meshes.push_back(make_unique<RegularMesh>(node));
-    } else if (mesh_type == RectilinearMesh::mesh_type) {
-      model::meshes.push_back(make_unique<RectilinearMesh>(node));
-    } else if (mesh_type == CylindricalMesh::mesh_type) {
-      model::meshes.push_back(make_unique<CylindricalMesh>(node));
-    } else if (mesh_type == SphericalMesh::mesh_type) {
-      model::meshes.push_back(make_unique<SphericalMesh>(node));
-#ifdef DAGMC
-    } else if (mesh_type == UnstructuredMesh::mesh_type &&
-               mesh_lib == MOABMesh::mesh_lib_type) {
-      model::meshes.push_back(make_unique<MOABMesh>(node));
-#endif
-#ifdef LIBMESH
-    } else if (mesh_type == UnstructuredMesh::mesh_type &&
-               mesh_lib == LibMesh::mesh_lib_type) {
-      model::meshes.push_back(make_unique<LibMesh>(node));
-#endif
-    } else if (mesh_type == UnstructuredMesh::mesh_type) {
-      fatal_error("Unstructured mesh support is not enabled or the mesh "
-                  "library is invalid.");
-    } else {
-      fatal_error("Invalid mesh type: " + mesh_type);
+    Mesh::create(node, mesh_type, mesh_lib);
+  }
+}
+
+void read_meshes(hid_t group)
+{
+  std::unordered_set<int> mesh_ids;
+
+  std::vector<int> ids;
+  read_attribute(group, "ids", ids);
+
+  for (auto id : ids) {
+
+    // Check to make sure multiple meshes in the same file don't share IDs
+    if (contains(mesh_ids, id)) {
+      fatal_error(fmt::format("Two or more meshes use the same unique ID "
+                              "'{}' in the same HDF5 input file",
+        id));
+    }
+    mesh_ids.insert(id);
+
+    // If we've already read a mesh with the same ID in a *different* file,
+    // assume it is the same here
+    if (model::mesh_map.find(id) != model::mesh_map.end()) {
+      warning(fmt::format("Mesh with ID={} appears in multiple files.", id));
+      continue;
     }
 
-    // Map ID to position in vector
-    model::mesh_map[model::meshes.back()->id_] = model::meshes.size() - 1;
+    std::string name = fmt::format("mesh {}", id);
+    hid_t mesh_group = open_group(group, name.c_str());
+
+    std::string mesh_type;
+    if (object_exists(mesh_group, "type")) {
+      read_dataset(mesh_group, "type", mesh_type);
+    } else {
+      mesh_type = "regular";
+    }
+
+    // determine the mesh library to use
+    std::string mesh_lib;
+    if (object_exists(mesh_group, "library")) {
+      read_dataset(mesh_group, "library", mesh_lib);
+    }
+
+    Mesh::create(mesh_group, mesh_type, mesh_lib);
   }
 }
 
diff --git a/src/message_passing.cpp b/src/message_passing.cpp
index 374c1aa72..cf6113ed6 100644
--- a/src/message_passing.cpp
+++ b/src/message_passing.cpp
@@ -10,6 +10,7 @@ bool master {true};
 #ifdef OPENMC_MPI
 MPI_Comm intracomm {MPI_COMM_NULL};
 MPI_Datatype source_site {MPI_DATATYPE_NULL};
+MPI_Datatype collision_track_site {MPI_DATATYPE_NULL};
 #endif
 
 extern "C" bool openmc_master()
@@ -38,6 +39,14 @@ vector<int64_t> calculate_parallel_index_vector(int64_t size)
   return result;
 }
 
+#ifdef OPENMC_MPI
+// Specializations of the MPITypeMap template struct
+template<>
+const MPI_Datatype MPITypeMap<int>::mpi_type = MPI_INT;
+template<>
+const MPI_Datatype MPITypeMap<double>::mpi_type = MPI_DOUBLE;
+#endif
+
 } // namespace mpi
 
 } // namespace openmc
diff --git a/src/mgxs.cpp b/src/mgxs.cpp
index eae3e5817..1dc090ab9 100644
--- a/src/mgxs.cpp
+++ b/src/mgxs.cpp
@@ -5,15 +5,13 @@
 #include <cstdlib>
 #include <sstream>
 
-#include "xtensor/xadapt.hpp"
-#include "xtensor/xmath.hpp"
-#include "xtensor/xsort.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include "openmc/error.h"
 #include "openmc/math_functions.h"
 #include "openmc/mgxs_interface.h"
+#include "openmc/nuclide.h"
 #include "openmc/random_lcg.h"
 #include "openmc/settings.h"
 #include "openmc/string_utils.h"
@@ -32,8 +30,7 @@ void Mgxs::init(const std::string& in_name, double in_awr,
   // Set the metadata
   name = in_name;
   awr = in_awr;
-  // TODO: Remove adapt when in_KTs is an xtensor
-  kTs = xt::adapt(in_kTs);
+  kTs = tensor::Tensor<double>(in_kTs.data(), in_kTs.size());
   fissionable = in_fissionable;
   scatter_format = in_scatter_format;
   xs.resize(in_kTs.size());
@@ -72,7 +69,7 @@ void Mgxs::metadata_from_hdf5(hid_t xs_id, const vector<double>& temperature,
   }
   get_datasets(kT_group, dset_names);
   vector<size_t> shape = {num_temps};
-  xt::xarray<double> temps_available(shape);
+  tensor::Tensor<double> temps_available(shape);
   for (int i = 0; i < num_temps; i++) {
     read_double(kT_group, dset_names[i], &temps_available[i], true);
 
@@ -85,6 +82,13 @@ void Mgxs::metadata_from_hdf5(hid_t xs_id, const vector<double>& temperature,
   delete[] dset_names;
   std::sort(temps_available.begin(), temps_available.end());
 
+  // Set the global upper and lower interpolation bounds to avoid errors
+  // involving C-API functions.
+  data::temperature_min =
+    std::min(data::temperature_min, temps_available.front());
+  data::temperature_max =
+    std::max(data::temperature_max, temps_available.back());
+
   // If only one temperature is available, lets just use nearest temperature
   // interpolation
   if ((num_temps == 1) &&
@@ -99,7 +103,9 @@ void Mgxs::metadata_from_hdf5(hid_t xs_id, const vector<double>& temperature,
   case TemperatureMethod::NEAREST:
     // Determine actual temperatures to read
     for (const auto& T : temperature) {
-      auto i_closest = xt::argmin(xt::abs(temps_available - T))[0];
+      // Determine the closest temperature value
+      auto i_closest = tensor::abs(temps_available - T).argmin();
+
       double temp_actual = temps_available[i_closest];
       if (std::fabs(temp_actual - T) < settings::temperature_tolerance) {
         if (std::find(temps_to_read.begin(), temps_to_read.end(),
@@ -333,7 +339,7 @@ Mgxs::Mgxs(const std::string& in_name, const vector<double>& mat_kTs,
     for (int m = 0; m < micros.size(); m++) {
       switch (settings::temperature_method) {
       case TemperatureMethod::NEAREST: {
-        micro_t[m] = xt::argmin(xt::abs(micros[m]->kTs - temp_desired))[0];
+        micro_t[m] = tensor::abs(micros[m]->kTs - temp_desired).argmin();
         auto temp_actual = micros[m]->kTs[micro_t[m]];
 
         if (std::abs(temp_actual - temp_desired) >=
@@ -346,7 +352,7 @@ Mgxs::Mgxs(const std::string& in_name, const vector<double>& mat_kTs,
       case TemperatureMethod::INTERPOLATION:
         // Get a list of bounding temperatures for each actual temperature
         // present in the model
-        for (int k = 0; k < micros[m]->kTs.shape()[0] - 1; k++) {
+        for (int k = 0; k < micros[m]->kTs.shape(0) - 1; k++) {
           if ((micros[m]->kTs[k] <= temp_desired) &&
               (temp_desired < micros[m]->kTs[k + 1])) {
             micro_t[m] = k;
@@ -359,7 +365,7 @@ Mgxs::Mgxs(const std::string& in_name, const vector<double>& mat_kTs,
           }
         }
       } // end switch
-    }   // end microscopic temperature loop
+    } // end microscopic temperature loop
 
     // Now combine the microscopic data at each relevant temperature
     // We will do this by treating the multiple temperatures of a nuclide as
@@ -452,7 +458,7 @@ double Mgxs::get_xs(MgxsType xstype, int gin, const int* gout, const double* mu,
         val = xs_t->delayed_nu_fission(a, *dg, gin);
       } else {
         val = 0.;
-        for (int d = 0; d < xs_t->delayed_nu_fission.shape()[1]; d++) {
+        for (int d = 0; d < xs_t->delayed_nu_fission.shape(1); d++) {
           val += xs_t->delayed_nu_fission(a, d, gin);
         }
       }
@@ -467,7 +473,7 @@ double Mgxs::get_xs(MgxsType xstype, int gin, const int* gout, const double* mu,
       } else {
         // provide an outgoing group-wise sum
         val = 0.;
-        for (int g = 0; g < xs_t->chi_prompt.shape()[2]; g++) {
+        for (int g = 0; g < xs_t->chi_prompt.shape(2); g++) {
           val += xs_t->chi_prompt(a, gin, g);
         }
       }
@@ -486,13 +492,13 @@ double Mgxs::get_xs(MgxsType xstype, int gin, const int* gout, const double* mu,
       } else {
         if (dg != nullptr) {
           val = 0.;
-          for (int g = 0; g < xs_t->delayed_nu_fission.shape()[2]; g++) {
+          for (int g = 0; g < xs_t->delayed_nu_fission.shape(2); g++) {
             val += xs_t->delayed_nu_fission(a, *dg, gin, g);
           }
         } else {
           val = 0.;
-          for (int g = 0; g < xs_t->delayed_nu_fission.shape()[2]; g++) {
-            for (int d = 0; d < xs_t->delayed_nu_fission.shape()[3]; d++) {
+          for (int g = 0; g < xs_t->delayed_nu_fission.shape(2); g++) {
+            for (int d = 0; d < xs_t->delayed_nu_fission.shape(3); d++) {
               val += xs_t->delayed_nu_fission(a, d, gin, g);
             }
           }
@@ -504,6 +510,8 @@ double Mgxs::get_xs(MgxsType xstype, int gin, const int* gout, const double* mu,
     break;
   case MgxsType::INVERSE_VELOCITY:
     val = xs_t->inverse_velocity(a, gin);
+    if (!(val > 0))
+      val = data::mg.default_inverse_velocity_[gin];
     break;
   case MgxsType::DECAY_RATE:
     if (dg != nullptr) {
@@ -603,10 +611,12 @@ void Mgxs::calculate_xs(Particle& p)
   }
   int temperature = p.mg_xs_cache().t;
   int angle = p.mg_xs_cache().a;
-  p.macro_xs().total = xs[temperature].total(angle, p.g());
-  p.macro_xs().absorption = xs[temperature].absorption(angle, p.g());
+  p.macro_xs().total = xs[temperature].total(angle, p.g()) * p.density_mult();
+  p.macro_xs().absorption =
+    xs[temperature].absorption(angle, p.g()) * p.density_mult();
   p.macro_xs().nu_fission =
-    fissionable ? xs[temperature].nu_fission(angle, p.g()) : 0.;
+    fissionable ? xs[temperature].nu_fission(angle, p.g()) * p.density_mult()
+                : 0.;
 }
 
 //==============================================================================
@@ -626,7 +636,7 @@ bool Mgxs::equiv(const Mgxs& that)
 
 int Mgxs::get_temperature_index(double sqrtkT) const
 {
-  return xt::argmin(xt::abs(kTs - sqrtkT * sqrtkT))[0];
+  return tensor::abs(kTs - sqrtkT * sqrtkT).argmin();
 }
 
 //==============================================================================
diff --git a/src/mgxs_interface.cpp b/src/mgxs_interface.cpp
index ed734d401..865c56580 100644
--- a/src/mgxs_interface.cpp
+++ b/src/mgxs_interface.cpp
@@ -169,13 +169,13 @@ vector<vector<double>> MgxsInterface::get_mat_kTs()
         continue;
 
       // Get temperature of cell (rounding to nearest integer)
-      double sqrtkT =
-        cell->sqrtkT_.size() == 1 ? cell->sqrtkT_[j] : cell->sqrtkT_[0];
-      double kT = sqrtkT * sqrtkT;
+      for (int k = 0; k < cell->sqrtkT_.size(); ++k) {
+        double kT = cell->sqrtkT_[k] * cell->sqrtkT_[k];
 
-      // Add temperature if it hasn't already been added
-      if (!contains(kTs[i_material], kT)) {
-        kTs[i_material].push_back(kT);
+        // Add temperature if it hasn't already been added
+        if (!contains(kTs[i_material], kT)) {
+          kTs[i_material].push_back(kT);
+        }
       }
     }
   }
@@ -237,6 +237,19 @@ void MgxsInterface::read_header(const std::string& path_cross_sections)
                 "library file!");
   }
 
+  // Calculate approximate default inverse velocity data
+  for (int i = 0; i < energy_bins_.size() - 1; ++i) {
+    double e_min = std::max(energy_bins_[i + 1], 1e-5);
+    double e_max = energy_bins_[i];
+    double alpha = 1.0 / (C_LIGHT * std::log(e_max / e_min));
+    double k_max = std::sqrt(1 + 2.0 * MASS_NEUTRON_EV / e_max);
+    double k_min = std::sqrt(1 + 2.0 * MASS_NEUTRON_EV / e_min);
+    double inv_v =
+      alpha * (2.0 * (std::atanh(1.0 / k_max) - std::atanh(1.0 / k_min)) -
+                (k_max - k_min));
+    default_inverse_velocity_.push_back(inv_v);
+  }
+
   // Close MGXS HDF5 file
   file_close(file_id);
 }
@@ -244,7 +257,7 @@ void MgxsInterface::read_header(const std::string& path_cross_sections)
 void put_mgxs_header_data_to_globals()
 {
   // Get the minimum and maximum energies
-  int neutron = static_cast<int>(ParticleType::neutron);
+  int neutron = ParticleType::neutron().transport_index();
   data::energy_min[neutron] = data::mg.energy_bins_.back();
   data::energy_max[neutron] = data::mg.energy_bins_.front();
 
diff --git a/src/nuclide.cpp b/src/nuclide.cpp
index eaac2f756..17d6e952c 100644
--- a/src/nuclide.cpp
+++ b/src/nuclide.cpp
@@ -17,8 +17,7 @@
 
 #include <fmt/core.h>
 
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include <algorithm> // for sort, min_element
 #include <cassert>
@@ -31,8 +30,8 @@ namespace openmc {
 //==============================================================================
 
 namespace data {
-array<double, 2> energy_min {0.0, 0.0};
-array<double, 2> energy_max {INFTY, INFTY};
+array<double, 4> energy_min {0.0, 0.0, 0.0, 0.0};
+array<double, 4> energy_max {INFTY, INFTY, INFTY, INFTY};
 double temperature_min {INFTY};
 double temperature_max {0.0};
 std::unordered_map<std::string, int> nuclide_map;
@@ -361,8 +360,7 @@ void Nuclide::create_derived(
 {
   for (const auto& grid : grid_) {
     // Allocate and initialize cross section
-    array<size_t, 2> shape {grid.energy.size(), 5};
-    xs_.emplace_back(shape, 0.0);
+    xs_.push_back(tensor::zeros<double>({grid.energy.size(), 5}));
   }
 
   reaction_index_.fill(C_NONE);
@@ -375,11 +373,10 @@ void Nuclide::create_derived(
     for (int t = 0; t < kTs_.size(); ++t) {
       int j = rx->xs_[t].threshold;
       int n = rx->xs_[t].value.size();
-      auto xs = xt::adapt(rx->xs_[t].value);
-      auto pprod = xt::view(xs_[t], xt::range(j, j + n), XS_PHOTON_PROD);
-
+      auto xs = tensor::Tensor<double>(
+        rx->xs_[t].value.data(), rx->xs_[t].value.size());
       for (const auto& p : rx->products_) {
-        if (p.particle_ == ParticleType::photon) {
+        if (p.particle_.is_photon()) {
           for (int k = 0; k < n; ++k) {
             double E = grid_[t].energy[k + j];
 
@@ -396,7 +393,7 @@ void Nuclide::create_derived(
               }
             }
 
-            pprod[k] += f * xs[k] * (*p.yield_)(E);
+            xs_[t](j + k, XS_PHOTON_PROD) += f * xs[k] * (*p.yield_)(E);
           }
         }
       }
@@ -406,20 +403,17 @@ void Nuclide::create_derived(
         continue;
 
       // Add contribution to total cross section
-      auto total = xt::view(xs_[t], xt::range(j, j + n), XS_TOTAL);
-      total += xs;
+      xs_[t].slice(tensor::range(j, j + n), XS_TOTAL) += xs;
 
       // Add contribution to absorption cross section
-      auto absorption = xt::view(xs_[t], xt::range(j, j + n), XS_ABSORPTION);
       if (is_disappearance(rx->mt_)) {
-        absorption += xs;
+        xs_[t].slice(tensor::range(j, j + n), XS_ABSORPTION) += xs;
       }
 
       if (is_fission(rx->mt_)) {
         fissionable_ = true;
-        auto fission = xt::view(xs_[t], xt::range(j, j + n), XS_FISSION);
-        fission += xs;
-        absorption += xs;
+        xs_[t].slice(tensor::range(j, j + n), XS_FISSION) += xs;
+        xs_[t].slice(tensor::range(j, j + n), XS_ABSORPTION) += xs;
 
         // Keep track of fission reactions
         if (t == 0) {
@@ -501,7 +495,7 @@ void Nuclide::create_derived(
 
 void Nuclide::init_grid()
 {
-  int neutron = static_cast<int>(ParticleType::neutron);
+  int neutron = ParticleType::neutron().transport_index();
   double E_min = data::energy_min[neutron];
   double E_max = data::energy_max[neutron];
   int M = settings::n_log_bins;
@@ -510,7 +504,7 @@ void Nuclide::init_grid()
   double spacing = std::log(E_max / E_min) / M;
 
   // Create equally log-spaced energy grid
-  auto umesh = xt::linspace(0.0, M * spacing, M + 1);
+  auto umesh = tensor::linspace(0.0, M * spacing, M + 1);
 
   for (auto& grid : grid_) {
     // Resize array for storing grid indices
@@ -552,7 +546,7 @@ double Nuclide::nu(double E, EmissionMode mode, int group) const
         for (int i = 1; i < rx->products_.size(); ++i) {
           // Skip any non-neutron products
           const auto& product = rx->products_[i];
-          if (product.particle_ != ParticleType::neutron)
+          if (!product.particle_.is_neutron())
             continue;
 
           // Evaluate yield
@@ -1114,7 +1108,7 @@ extern "C" size_t nuclides_size()
 extern "C" int openmc_load_nuclide(const char* name, const double* temps, int n)
 {
   if (data::nuclide_map.find(name) == data::nuclide_map.end() ||
-      data::nuclide_map.at(name) >= data::elements.size()) {
+      data::nuclide_map.at(name) >= data::nuclides.size()) {
     LibraryKey key {Library::Type::neutron, name};
     const auto& it = data::library_map.find(key);
     if (it == data::library_map.end()) {
@@ -1215,7 +1209,6 @@ extern "C" int openmc_nuclide_collapse_rate(int index, int MT,
     *xs = data::nuclides[index]->collapse_rate(
       MT, temperature, {energy, energy + n + 1}, {flux, flux + n});
   } catch (const std::out_of_range& e) {
-    fmt::print("Caught error\n");
     set_errmsg(e.what());
     return OPENMC_E_OUT_OF_BOUNDS;
   }
diff --git a/src/output.cpp b/src/output.cpp
index e20868efb..04a8caa60 100644
--- a/src/output.cpp
+++ b/src/output.cpp
@@ -17,7 +17,7 @@
 #ifdef _OPENMP
 #include <omp.h>
 #endif
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/capi.h"
 #include "openmc/cell.h"
@@ -44,6 +44,12 @@
 
 namespace openmc {
 
+#ifdef OPENMC_ENABLE_STRICT_FP
+const bool STRICT_FP_ENABLED = true;
+#else
+const bool STRICT_FP_ENABLED = false;
+#endif
+
 //==============================================================================
 
 void title()
@@ -75,7 +81,7 @@ void title()
   // Write version information
   fmt::print(
     "                 | The OpenMC Monte Carlo Code\n"
-    "       Copyright | 2011-2025 MIT, UChicago Argonne LLC, and contributors\n"
+    "       Copyright | 2011-2026 MIT, UChicago Argonne LLC, and contributors\n"
     "         License | https://docs.openmc.org/en/latest/license.html\n"
     "         Version | {}.{}.{}{}{}\n",
     VERSION_MAJOR, VERSION_MINOR, VERSION_RELEASE, VERSION_DEV ? "-dev" : "",
@@ -155,21 +161,21 @@ std::string time_stamp()
 void print_particle(Particle& p)
 {
   // Display particle type and ID.
-  switch (p.type()) {
-  case ParticleType::neutron:
+  switch (p.type().pdg_number()) {
+  case PDG_NEUTRON:
     fmt::print("Neutron ");
     break;
-  case ParticleType::photon:
+  case PDG_PHOTON:
     fmt::print("Photon ");
     break;
-  case ParticleType::electron:
+  case PDG_ELECTRON:
     fmt::print("Electron ");
     break;
-  case ParticleType::positron:
+  case PDG_POSITRON:
     fmt::print("Positron ");
     break;
   default:
-    fmt::print("Unknown Particle ");
+    fmt::print("Particle {} ", p.type().str());
   }
   fmt::print("{}\n", p.id());
 
@@ -177,25 +183,26 @@ void print_particle(Particle& p)
   for (auto i = 0; i < p.n_coord(); i++) {
     fmt::print("  Level {}\n", i);
 
-    if (p.coord(i).cell != C_NONE) {
-      const Cell& c {*model::cells[p.coord(i).cell]};
+    if (p.coord(i).cell() != C_NONE) {
+      const Cell& c {*model::cells[p.coord(i).cell()]};
       fmt::print("    Cell             = {}\n", c.id_);
     }
 
-    if (p.coord(i).universe != C_NONE) {
-      const Universe& u {*model::universes[p.coord(i).universe]};
+    if (p.coord(i).universe() != C_NONE) {
+      const Universe& u {*model::universes[p.coord(i).universe()]};
       fmt::print("    Universe         = {}\n", u.id_);
     }
 
-    if (p.coord(i).lattice != C_NONE) {
-      const Lattice& lat {*model::lattices[p.coord(i).lattice]};
+    if (p.coord(i).lattice() != C_NONE) {
+      const Lattice& lat {*model::lattices[p.coord(i).lattice()]};
       fmt::print("    Lattice          = {}\n", lat.id_);
-      fmt::print("    Lattice position = ({},{},{})\n", p.coord(i).lattice_i[0],
-        p.coord(i).lattice_i[1], p.coord(i).lattice_i[2]);
+      fmt::print("    Lattice position = ({},{},{})\n",
+        p.coord(i).lattice_index()[0], p.coord(i).lattice_index()[1],
+        p.coord(i).lattice_index()[2]);
     }
 
-    fmt::print("    r = {}\n", p.coord(i).r);
-    fmt::print("    u = {}\n", p.coord(i).u);
+    fmt::print("    r = {}\n", p.coord(i).r());
+    fmt::print("    u = {}\n", p.coord(i).u());
   }
 
   // Display miscellaneous info.
@@ -280,6 +287,7 @@ void print_usage()
       "  -t, --track            Write tracks for all particles (up to "
       "max_tracks)\n"
       "  -e, --event            Run using event-based parallelism\n"
+      "  -q, --verbosity        Output verbosity\n"
       "  -v, --version          Show version information\n"
       "  -h, --help             Show this message\n");
   }
@@ -293,7 +301,7 @@ void print_version()
     fmt::print("OpenMC version {}.{}.{}{}{}\n", VERSION_MAJOR, VERSION_MINOR,
       VERSION_RELEASE, VERSION_DEV ? "-dev" : "", VERSION_COMMIT_COUNT);
     fmt::print("Commit hash: {}\n", VERSION_COMMIT_HASH);
-    fmt::print("Copyright (c) 2011-2025 MIT, UChicago Argonne LLC, and "
+    fmt::print("Copyright (c) 2011-2026 MIT, UChicago Argonne LLC, and "
                "contributors\nMIT/X license at "
                "<https://docs.openmc.org/en/latest/license.html>\n");
   }
@@ -313,8 +321,8 @@ void print_build_info()
   std::string png(n);
   std::string profiling(n);
   std::string coverage(n);
-  std::string mcpl(n);
   std::string uwuw(n);
+  std::string strict_fp(n);
 
 #ifdef PHDF5
   phdf5 = y;
@@ -322,15 +330,12 @@ void print_build_info()
 #ifdef OPENMC_MPI
   mpi = y;
 #endif
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   dagmc = y;
 #endif
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
   libmesh = y;
 #endif
-#ifdef OPENMC_MCPL
-  mcpl = y;
-#endif
 #ifdef USE_LIBPNG
   png = y;
 #endif
@@ -340,9 +345,12 @@ void print_build_info()
 #ifdef COVERAGEBUILD
   coverage = y;
 #endif
-#ifdef OPENMC_UWUW
+#ifdef OPENMC_UWUW_ENABLED
   uwuw = y;
 #endif
+#ifdef OPENMC_ENABLE_STRICT_FP
+  strict_fp = y;
+#endif
 
   // Wraps macro variables in quotes
 #define STRINGIFY(x) STRINGIFY2(x)
@@ -357,10 +365,10 @@ void print_build_info()
     fmt::print("PNG support:           {}\n", png);
     fmt::print("DAGMC support:         {}\n", dagmc);
     fmt::print("libMesh support:       {}\n", libmesh);
-    fmt::print("MCPL support:          {}\n", mcpl);
     fmt::print("Coverage testing:      {}\n", coverage);
     fmt::print("Profiling flags:       {}\n", profiling);
     fmt::print("UWUW support:          {}\n", uwuw);
+    fmt::print("Strict FP:             {}\n", strict_fp);
   }
 }
 
@@ -493,6 +501,14 @@ void print_runtime()
     show_rate("Calculation Rate (inactive)", speed_inactive);
   }
   show_rate("Calculation Rate (active)", speed_active);
+
+  // Display track rate when weight windows are enabled
+  if (settings::weight_windows_on) {
+    double speed_tracks =
+      simulation::simulation_tracks_completed / time_active.elapsed();
+    fmt::print(
+      " {:<33} = {:.6} tracks/second\n", "Track Rate (active)", speed_tracks);
+  }
 }
 
 //==============================================================================
@@ -593,6 +609,9 @@ const std::unordered_map<int, const char*> score_names = {
   {SCORE_FISS_Q_RECOV, "Recoverable fission power"},
   {SCORE_CURRENT, "Current"},
   {SCORE_PULSE_HEIGHT, "pulse-height"},
+  {SCORE_IFP_TIME_NUM, "IFP lifetime numerator"},
+  {SCORE_IFP_BETA_NUM, "IFP delayed fraction numerator"},
+  {SCORE_IFP_DENOM, "IFP common denominator"},
 };
 
 //! Create an ASCII output file showing all tally results.
@@ -602,8 +621,15 @@ void write_tallies()
   if (model::tallies.empty())
     return;
 
+  // Tag tallies.out written during the forward solve of an adjoint run
+  const char* forward =
+    (FlatSourceDomain::solve_ == RandomRaySolve::FORWARD_FOR_ADJOINT)
+      ? "forward."
+      : "";
+
   // Set filename for tallies_out
-  std::string filename = fmt::format("{}tallies.out", settings::path_output);
+  std::string filename =
+    fmt::format("{}tallies.{}out", settings::path_output, forward);
 
   // Open the tallies.out file.
   std::ofstream tallies_out;
diff --git a/src/particle.cpp b/src/particle.cpp
index c51011d6b..10e8f213d 100644
--- a/src/particle.cpp
+++ b/src/particle.cpp
@@ -8,11 +8,13 @@
 #include "openmc/bank.h"
 #include "openmc/capi.h"
 #include "openmc/cell.h"
+#include "openmc/collision_track.h"
 #include "openmc/constants.h"
 #include "openmc/dagmc.h"
 #include "openmc/error.h"
 #include "openmc/geometry.h"
 #include "openmc/hdf5_interface.h"
+#include "openmc/lattice.h"
 #include "openmc/material.h"
 #include "openmc/message_passing.h"
 #include "openmc/mgxs_interface.h"
@@ -32,7 +34,7 @@
 #include "openmc/track_output.h"
 #include "openmc/weight_windows.h"
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
 #include "DagMC.hpp"
 #endif
 
@@ -44,34 +46,35 @@ namespace openmc {
 
 double Particle::speed() const
 {
-  // Determine mass in eV/c^2
-  double mass;
-  switch (this->type()) {
-  case ParticleType::neutron:
-    mass = MASS_NEUTRON_EV;
-    break;
-  case ParticleType::photon:
-    mass = 0.0;
-    break;
-  case ParticleType::electron:
-  case ParticleType::positron:
-    mass = MASS_ELECTRON_EV;
-    break;
-  }
+  if (settings::run_CE) {
+    // Determine mass in eV/c^2
+    double mass = this->mass();
 
-  if (this->E() < 1.0e-9 * mass) {
-    // If the energy is much smaller than the mass, revert to non-relativistic
-    // formula. The 1e-9 criterion is specifically chosen as the point below
-    // which the error from using the non-relativistic formula is less than the
-    // round-off eror when using the relativistic formula (see analysis at
-    // https://gist.github.com/paulromano/da3b473fe3df33de94b265bdff0c7817)
-    return C_LIGHT * std::sqrt(2 * this->E() / mass);
+    // Equivalent to C * sqrt(1-(m/(m+E))^2) without problem at E<<m:
+    return C_LIGHT * std::sqrt(this->E() * (this->E() + 2 * mass)) /
+           (this->E() + mass);
   } else {
-    // Calculate inverse of Lorentz factor
-    const double inv_gamma = mass / (this->E() + mass);
+    auto mat = this->material();
+    if (mat == MATERIAL_VOID)
+      return 1.0 / data::mg.default_inverse_velocity_[this->g()];
+    auto& macro_xs = data::mg.macro_xs_[mat];
+    int macro_t = this->mg_xs_cache().t;
+    int macro_a = macro_xs.get_angle_index(this->u());
+    return 1.0 / macro_xs.get_xs(
+                   MgxsType::INVERSE_VELOCITY, this->g(), macro_t, macro_a);
+  }
+}
 
-    // Calculate speed via v = c * sqrt(1 - γ^-2)
-    return C_LIGHT * std::sqrt(1 - inv_gamma * inv_gamma);
+double Particle::mass() const
+{
+  switch (type().pdg_number()) {
+  case PDG_NEUTRON:
+    return MASS_NEUTRON_EV;
+  case PDG_ELECTRON:
+  case PDG_POSITRON:
+    return MASS_ELECTRON_EV;
+  default:
+    return this->type().mass() * AMU_EV;
   }
 }
 
@@ -80,11 +83,18 @@ bool Particle::create_secondary(
 {
   // If energy is below cutoff for this particle, don't create secondary
   // particle
-  if (E < settings::energy_cutoff[static_cast<int>(type)]) {
+  int idx = type.transport_index();
+  if (idx == C_NONE) {
+    return false;
+  }
+  if (E < settings::energy_cutoff[idx]) {
     return false;
   }
 
-  auto& bank = secondary_bank().emplace_back();
+  // Increment number of secondaries created (for ParticleProductionFilter)
+  n_secondaries()++;
+
+  SourceSite bank;
   bank.particle = type;
   bank.wgt = wgt;
   bank.r = r();
@@ -92,18 +102,45 @@ bool Particle::create_secondary(
   bank.E = settings::run_CE ? E : g();
   bank.time = time();
   bank_second_E() += bank.E;
+  bank.parent_id = current_work();
+  if (settings::use_shared_secondary_bank) {
+    bank.progeny_id = n_progeny()++;
+  }
+  bank.wgt_born = wgt_born();
+  bank.wgt_ww_born = wgt_ww_born();
+  bank.n_split = n_split();
+
+  local_secondary_bank().emplace_back(bank);
   return true;
 }
 
 void Particle::split(double wgt)
 {
-  auto& bank = secondary_bank().emplace_back();
+  SourceSite bank;
   bank.particle = type();
   bank.wgt = wgt;
   bank.r = r();
   bank.u = u();
   bank.E = settings::run_CE ? E() : g();
   bank.time = time();
+
+  // Convert signed index to a signed surface ID
+  if (surface() == SURFACE_NONE) {
+    bank.surf_id = SURFACE_NONE;
+  } else {
+    int surf_id = model::surfaces[surface_index()]->id_;
+    bank.surf_id = (surface() > 0) ? surf_id : -surf_id;
+  }
+
+  bank.wgt_born = wgt_born();
+  bank.wgt_ww_born = wgt_ww_born();
+  bank.n_split = n_split();
+  bank.parent_id = current_work();
+  if (settings::use_shared_secondary_bank) {
+    bank.progeny_id = n_progeny()++;
+  }
+
+  local_secondary_bank().emplace_back(bank);
 }
 
 void Particle::from_source(const SourceSite* src)
@@ -116,6 +153,10 @@ void Particle::from_source(const SourceSite* src)
   n_collision() = 0;
   fission() = false;
   zero_flux_derivs();
+  lifetime() = 0.0;
+#ifdef OPENMC_DAGMC_ENABLED
+  history().reset();
+#endif
 
   // Copy attributes from source bank site
   type() = src->particle;
@@ -139,6 +180,17 @@ void Particle::from_source(const SourceSite* src)
   time() = src->time;
   time_last() = src->time;
   parent_nuclide() = src->parent_nuclide;
+  delayed_group() = src->delayed_group;
+
+  // Convert signed surface ID to signed index
+  if (src->surf_id != SURFACE_NONE) {
+    int index_plus_one = model::surface_map[std::abs(src->surf_id)] + 1;
+    surface() = (src->surf_id > 0) ? index_plus_one : -index_plus_one;
+  }
+
+  wgt_born() = src->wgt_born;
+  wgt_ww_born() = src->wgt_ww_born;
+  n_split() = src->n_split;
 }
 
 void Particle::event_calculate_xs()
@@ -161,7 +213,7 @@ void Particle::event_calculate_xs()
   // If the cell hasn't been determined based on the particle's location,
   // initiate a search for the current cell. This generally happens at the
   // beginning of the history and again for any secondary particles
-  if (lowest_coord().cell == C_NONE) {
+  if (lowest_coord().cell() == C_NONE) {
     if (!exhaustive_find_cell(*this)) {
       mark_as_lost(
         "Could not find the cell containing particle " + std::to_string(id()));
@@ -170,11 +222,11 @@ void Particle::event_calculate_xs()
 
     // Set birth cell attribute
     if (cell_born() == C_NONE)
-      cell_born() = lowest_coord().cell;
+      cell_born() = lowest_coord().cell();
 
     // Initialize last cells from current cell
     for (int j = 0; j < n_coord(); ++j) {
-      cell_last(j) = coord(j).cell;
+      cell_last(j) = coord(j).cell();
     }
     n_coord_last() = n_coord();
   }
@@ -189,7 +241,8 @@ void Particle::event_calculate_xs()
   // Calculate microscopic and macroscopic cross sections
   if (material() != MATERIAL_VOID) {
     if (settings::run_CE) {
-      if (material() != material_last() || sqrtkT() != sqrtkT_last()) {
+      if (material() != material_last() || sqrtkT() != sqrtkT_last() ||
+          density_mult() != density_mult_last()) {
         // If the material is the same as the last material and the
         // temperature hasn't changed, we don't need to lookup cross
         // sections again.
@@ -218,35 +271,33 @@ void Particle::event_advance()
   boundary() = distance_to_boundary(*this);
 
   // Sample a distance to collision
-  if (type() == ParticleType::electron || type() == ParticleType::positron) {
-    collision_distance() = 0.0;
+  if (type() == ParticleType::electron() ||
+      type() == ParticleType::positron()) {
+    collision_distance() = material() == MATERIAL_VOID ? INFINITY : 0.0;
   } else if (macro_xs().total == 0.0) {
     collision_distance() = INFINITY;
   } else {
     collision_distance() = -std::log(prn(current_seed())) / macro_xs().total;
   }
 
-  // Select smaller of the two distances
-  double distance = std::min(boundary().distance, collision_distance());
+  double speed = this->speed();
+  double time_cutoff = settings::time_cutoff[type().transport_index()];
+  double distance_cutoff =
+    (time_cutoff < INFTY) ? (time_cutoff - time()) * speed : INFTY;
+
+  // Select smaller of the three distances
+  double distance =
+    std::min({boundary().distance(), collision_distance(), distance_cutoff});
 
   // Advance particle in space and time
-  // Short-term solution until the surface source is revised and we can use
-  // this->move_distance(distance)
-  for (int j = 0; j < n_coord(); ++j) {
-    coord(j).r += distance * coord(j).u;
-  }
-  this->time() += distance / this->speed();
+  this->move_distance(distance);
+  double dt = distance / speed;
+  this->time() += dt;
+  this->lifetime() += dt;
 
-  // Kill particle if its time exceeds the cutoff
-  bool hit_time_boundary = false;
-  double time_cutoff = settings::time_cutoff[static_cast<int>(type())];
-  if (time() > time_cutoff) {
-    double dt = time() - time_cutoff;
-    time() = time_cutoff;
-
-    double push_back_distance = speed() * dt;
-    this->move_distance(-push_back_distance);
-    hit_time_boundary = true;
+  // Score timed track-length tallies
+  if (!model::active_timed_tracklength_tallies.empty()) {
+    score_timed_tracklength_tally(*this, distance);
   }
 
   // Score track-length tallies
@@ -255,8 +306,7 @@ void Particle::event_advance()
   }
 
   // Score track-length estimate of k-eff
-  if (settings::run_mode == RunMode::EIGENVALUE &&
-      type() == ParticleType::neutron) {
+  if (settings::run_mode == RunMode::EIGENVALUE && type().is_neutron()) {
     keff_tally_tracklength() += wgt() * distance * macro_xs().nu_fission;
   }
 
@@ -266,7 +316,7 @@ void Particle::event_advance()
   }
 
   // Set particle weight to zero if it hit the time boundary
-  if (hit_time_boundary) {
+  if (distance == distance_cutoff) {
     wgt() = 0.0;
   }
 }
@@ -275,50 +325,69 @@ void Particle::event_cross_surface()
 {
   // Saving previous cell data
   for (int j = 0; j < n_coord(); ++j) {
-    cell_last(j) = coord(j).cell;
+    cell_last(j) = coord(j).cell();
   }
   n_coord_last() = n_coord();
 
   // Set surface that particle is on and adjust coordinate levels
-  surface() = boundary().surface;
-  n_coord() = boundary().coord_level;
+  surface() = boundary().surface();
+  n_coord() = boundary().coord_level();
 
-  if (boundary().lattice_translation[0] != 0 ||
-      boundary().lattice_translation[1] != 0 ||
-      boundary().lattice_translation[2] != 0) {
+  if (boundary().lattice_translation()[0] != 0 ||
+      boundary().lattice_translation()[1] != 0 ||
+      boundary().lattice_translation()[2] != 0) {
     // Particle crosses lattice boundary
 
+    int i_lattice = coord(boundary().coord_level() - 1).lattice();
     bool verbose = settings::verbosity >= 10 || trace();
     cross_lattice(*this, boundary(), verbose);
     event() = TallyEvent::LATTICE;
-  } else {
-    // Particle crosses surface
-    const auto& surf {model::surfaces[surface_index()].get()};
-    // If BC, add particle to surface source before crossing surface
-    if (surf->surf_source_ && surf->bc_) {
-      add_surf_source_to_bank(*this, *surf);
+
+    // Score cell to cell partial currents
+    if (!model::active_surface_tallies.empty()) {
+      auto& lat {*model::lattices[i_lattice]};
+      bool is_valid;
+      Direction normal =
+        lat.get_normal(boundary().lattice_translation(), is_valid);
+      if (is_valid) {
+        normal /= normal.norm();
+        score_surface_tally(*this, model::active_surface_tallies, normal);
+      }
     }
-    this->cross_surface(*surf);
+
+  } else {
+
+    const auto& surf {*model::surfaces[surface_index()].get()};
+
+    // Particle crosses surface
+    // If BC, add particle to surface source before crossing surface
+    if (surf.surf_source_ && surf.bc_) {
+      add_surf_source_to_bank(*this, surf);
+    }
+    this->cross_surface(surf);
     // If no BC, add particle to surface source after crossing surface
-    if (surf->surf_source_ && !surf->bc_) {
-      add_surf_source_to_bank(*this, *surf);
+    if (surf.surf_source_ && !surf.bc_) {
+      add_surf_source_to_bank(*this, surf);
     }
     if (settings::weight_window_checkpoint_surface) {
       apply_weight_windows(*this);
     }
     event() = TallyEvent::SURFACE;
-  }
-  // Score cell to cell partial currents
-  if (!model::active_surface_tallies.empty()) {
-    score_surface_tally(*this, model::active_surface_tallies);
+
+    // Score cell to cell partial currents
+    if (!model::active_surface_tallies.empty()) {
+      Direction normal = surf.normal(r());
+      normal /= normal.norm();
+      score_surface_tally(*this, model::active_surface_tallies, normal);
+    }
   }
 }
 
 void Particle::event_collide()
 {
+
   // Score collision estimate of keff
-  if (settings::run_mode == RunMode::EIGENVALUE &&
-      type() == ParticleType::neutron) {
+  if (settings::run_mode == RunMode::EIGENVALUE && type().is_neutron()) {
     keff_tally_collision() += wgt() * macro_xs().nu_fission / macro_xs().total;
   }
 
@@ -327,7 +396,7 @@ void Particle::event_collide()
   // pre-collision direction to figure out what mesh surfaces were crossed
 
   if (!model::active_meshsurf_tallies.empty())
-    score_surface_tally(*this, model::active_meshsurf_tallies);
+    score_meshsurface_tally(*this, model::active_meshsurf_tallies);
 
   // Clear surface component
   surface() = SURFACE_NONE;
@@ -338,6 +407,11 @@ void Particle::event_collide()
     collision_mg(*this);
   }
 
+  // Collision track feature to recording particle interaction
+  if (settings::collision_track) {
+    collision_track_record(*this);
+  }
+
   // Score collision estimator tallies -- this is done after a collision
   // has occurred rather than before because we need information on the
   // outgoing energy for any tallies with an outgoing energy filter
@@ -351,8 +425,7 @@ void Particle::event_collide()
     }
   }
 
-  if (!model::active_pulse_height_tallies.empty() &&
-      type() == ParticleType::photon) {
+  if (!model::active_pulse_height_tallies.empty() && type().is_photon()) {
     pht_collision_energy();
   }
 
@@ -360,6 +433,11 @@ void Particle::event_collide()
   n_bank() = 0;
   bank_second_E() = 0.0;
   wgt_bank() = 0.0;
+
+  // Clear number of secondaries in this collision. This is
+  // distinct from the number of created neutrons n_bank() above!
+  n_secondaries() = 0;
+
   zero_delayed_bank();
 
   // Reset fission logical
@@ -375,14 +453,14 @@ void Particle::event_collide()
   // Set all directions to base level -- right now, after a collision, only
   // the base level directions are changed
   for (int j = 0; j < n_coord() - 1; ++j) {
-    if (coord(j + 1).rotated) {
+    if (coord(j + 1).rotated()) {
       // If next level is rotated, apply rotation matrix
-      const auto& m {model::cells[coord(j).cell]->rotation_};
-      const auto& u {coord(j).u};
-      coord(j + 1).u = u.rotate(m);
+      const auto& m {model::cells[coord(j).cell()]->rotation_};
+      const auto& u {coord(j).u()};
+      coord(j + 1).u() = u.rotate(m);
     } else {
       // Otherwise, copy this level's direction
-      coord(j + 1).u = coord(j).u;
+      coord(j + 1).u() = coord(j).u();
     }
   }
 
@@ -390,89 +468,116 @@ void Particle::event_collide()
   if (!model::active_tallies.empty())
     score_collision_derivative(*this);
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   history().reset();
 #endif
 }
 
-void Particle::event_revive_from_secondary()
+void Particle::event_revive_from_secondary(const SourceSite& site)
+{
+  // Write final position for the previous track (skip if this is a freshly
+  // constructed particle with no prior track, e.g., Phase 2 of shared
+  // secondary transport)
+  if (write_track() && n_event() > 0) {
+    write_particle_track(*this);
+  }
+
+  from_source(&site);
+
+  n_event() = 0;
+  if (!settings::use_shared_secondary_bank) {
+    n_tracks()++;
+  }
+  bank_second_E() = 0.0;
+
+  // Subtract secondary particle energy from interim pulse-height results.
+  // In shared secondary mode, this subtraction was already done on the parent
+  // particle during create_secondary(), so skip it here.
+  if (!settings::use_shared_secondary_bank &&
+      !model::active_pulse_height_tallies.empty() && this->type().is_photon()) {
+    // Since the birth cell of the particle has not been set we
+    // have to determine it before the energy of the secondary particle can be
+    // removed from the pulse-height of this cell.
+    if (lowest_coord().cell() == C_NONE) {
+      bool verbose = settings::verbosity >= 10 || trace();
+      if (!exhaustive_find_cell(*this, verbose)) {
+        mark_as_lost("Could not find the cell containing particle " +
+                     std::to_string(id()));
+        return;
+      }
+      // Set birth cell attribute
+      if (cell_born() == C_NONE)
+        cell_born() = lowest_coord().cell();
+
+      // Initialize last cells from current cell
+      for (int j = 0; j < n_coord(); ++j) {
+        cell_last(j) = coord(j).cell();
+      }
+      n_coord_last() = n_coord();
+    }
+    pht_secondary_particles();
+  }
+
+  // Enter new particle in particle track file
+  if (write_track())
+    add_particle_track(*this);
+}
+
+void Particle::event_check_limit_and_revive()
 {
   // If particle has too many events, display warning and kill it
-  ++n_event();
+  n_event()++;
   if (n_event() == settings::max_particle_events) {
     warning("Particle " + std::to_string(id()) +
             " underwent maximum number of events.");
     wgt() = 0.0;
   }
 
-  // Check for secondary particles if this particle is dead
-  if (!alive()) {
-    // Write final position for this particle
-    if (write_track()) {
-      write_particle_track(*this);
-    }
-
-    // If no secondary particles, break out of event loop
-    if (secondary_bank().empty())
-      return;
-
-    from_source(&secondary_bank().back());
-    secondary_bank().pop_back();
-    n_event() = 0;
-    bank_second_E() = 0.0;
-
-    // Subtract secondary particle energy from interim pulse-height results
-    if (!model::active_pulse_height_tallies.empty() &&
-        this->type() == ParticleType::photon) {
-      // Since the birth cell of the particle has not been set we
-      // have to determine it before the energy of the secondary particle can be
-      // removed from the pulse-height of this cell.
-      if (lowest_coord().cell == C_NONE) {
-        bool verbose = settings::verbosity >= 10 || trace();
-        if (!exhaustive_find_cell(*this, verbose)) {
-          mark_as_lost("Could not find the cell containing particle " +
-                       std::to_string(id()));
-          return;
-        }
-        // Set birth cell attribute
-        if (cell_born() == C_NONE)
-          cell_born() = lowest_coord().cell;
-
-        // Initialize last cells from current cell
-        for (int j = 0; j < n_coord(); ++j) {
-          cell_last(j) = coord(j).cell;
-        }
-        n_coord_last() = n_coord();
-      }
-      pht_secondary_particles();
-    }
-
-    // Enter new particle in particle track file
-    if (write_track())
-      add_particle_track(*this);
+  // In non-shared-secondary mode, revive from local secondary bank
+  if (!alive() && !settings::use_shared_secondary_bank &&
+      !local_secondary_bank().empty()) {
+    SourceSite& site = local_secondary_bank().back();
+    event_revive_from_secondary(site);
+    local_secondary_bank().pop_back();
   }
 }
 
 void Particle::event_death()
 {
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   history().reset();
 #endif
 
   // Finish particle track output.
   if (write_track()) {
+    write_particle_track(*this);
     finalize_particle_track(*this);
   }
 
-// Contribute tally reduction variables to global accumulator
+  // Contribute tally reduction variables to global accumulator
+  const auto k_absorption = keff_tally_absorption();
+  const auto k_collision = keff_tally_collision();
+  const auto k_tracklength = keff_tally_tracklength();
+  const auto leakage = keff_tally_leakage();
+
+  if (settings::run_mode == RunMode::EIGENVALUE) {
+    if (k_absorption != 0.0) {
 #pragma omp atomic
-  global_tally_absorption += keff_tally_absorption();
+      global_tally_absorption += k_absorption;
+    }
+    if (k_collision != 0.0) {
 #pragma omp atomic
-  global_tally_collision += keff_tally_collision();
+      global_tally_collision += k_collision;
+    }
+    if (k_tracklength != 0.0) {
 #pragma omp atomic
-  global_tally_tracklength += keff_tally_tracklength();
+      global_tally_tracklength += k_tracklength;
+    }
+  }
+  if (leakage != 0.0) {
 #pragma omp atomic
-  global_tally_leakage += keff_tally_leakage();
+    global_tally_leakage += leakage;
+  }
 
   // Reset particle tallies once accumulated
   keff_tally_absorption() = 0.0;
@@ -484,11 +589,17 @@ void Particle::event_death()
     score_pulse_height_tally(*this, model::active_pulse_height_tallies);
   }
 
+  // Accumulate track count for this particle history
+  if (!settings::use_shared_secondary_bank) {
+#pragma omp atomic
+    simulation::simulation_tracks_completed += n_tracks();
+  }
+
   // Record the number of progeny created by this particle.
   // This data will be used to efficiently sort the fission bank.
-  if (settings::run_mode == RunMode::EIGENVALUE) {
-    int64_t offset = id() - 1 - simulation::work_index[mpi::rank];
-    simulation::progeny_per_particle[offset] = n_progeny();
+  if (settings::run_mode == RunMode::EIGENVALUE ||
+      settings::use_shared_secondary_bank) {
+    simulation::progeny_per_particle[current_work()] = n_progeny();
   }
 }
 
@@ -498,7 +609,7 @@ void Particle::pht_collision_energy()
 
   // determine index of cell in pulse_height_cells
   auto it = std::find(model::pulse_height_cells.begin(),
-    model::pulse_height_cells.end(), lowest_coord().cell);
+    model::pulse_height_cells.end(), lowest_coord().cell());
 
   if (it != model::pulse_height_cells.end()) {
     int index = std::distance(model::pulse_height_cells.begin(), it);
@@ -506,7 +617,7 @@ void Particle::pht_collision_energy()
 
     // If the energy of the particle is below the cutoff, it will not be sampled
     // so its energy is added to the pulse-height in the cell
-    int photon = static_cast<int>(ParticleType::photon);
+    int photon = ParticleType::photon().transport_index();
     if (E() < settings::energy_cutoff[photon]) {
       pht_storage()[index] += E();
     }
@@ -535,13 +646,14 @@ void Particle::cross_surface(const Surface& surf)
   }
 
 // if we're crossing a CSG surface, make sure the DAG history is reset
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   if (surf.geom_type() == GeometryType::CSG)
     history().reset();
 #endif
 
   // Handle any applicable boundary conditions.
-  if (surf.bc_ && settings::run_mode != RunMode::PLOTTING) {
+  if (surf.bc_ && settings::run_mode != RunMode::PLOTTING &&
+      settings::run_mode != RunMode::VOLUME) {
     surf.bc_->handle_particle(*this, surf);
     return;
   }
@@ -549,17 +661,18 @@ void Particle::cross_surface(const Surface& surf)
   // ==========================================================================
   // SEARCH NEIGHBOR LISTS FOR NEXT CELL
 
-#ifdef DAGMC
+#ifdef OPENMC_DAGMC_ENABLED
   // in DAGMC, we know what the next cell should be
   if (surf.geom_type() == GeometryType::DAG) {
     int32_t i_cell = next_cell(surface_index(), cell_last(n_coord() - 1),
-                       lowest_coord().universe) -
+                       lowest_coord().universe()) -
                      1;
-    // save material and temp
+    // save material, temperature, and density multiplier
     material_last() = material();
     sqrtkT_last() = sqrtkT();
+    density_mult_last() = density_mult();
     // set new cell value
-    lowest_coord().cell = i_cell;
+    lowest_coord().cell() = i_cell;
     auto& cell = model::cells[i_cell];
 
     cell_instance() = 0;
@@ -568,6 +681,7 @@ void Particle::cross_surface(const Surface& surf)
 
     material() = cell->material(cell_instance());
     sqrtkT() = cell->sqrtkT(cell_instance());
+    density_mult() = cell->density_mult(cell_instance());
     return;
   }
 #endif
@@ -617,7 +731,7 @@ void Particle::cross_vacuum_bc(const Surface& surf)
     // physically moving the particle forward slightly
 
     r() += TINY_BIT * u();
-    score_surface_tally(*this, model::active_meshsurf_tallies);
+    score_meshsurface_tally(*this, model::active_meshsurf_tallies);
   }
 
   // Score to global leakage tally
@@ -649,13 +763,15 @@ void Particle::cross_reflective_bc(const Surface& surf, Direction new_u)
   // with a mesh boundary
 
   if (!model::active_surface_tallies.empty()) {
-    score_surface_tally(*this, model::active_surface_tallies);
+    Direction normal = surf.normal(r());
+    normal /= normal.norm();
+    score_surface_tally(*this, model::active_surface_tallies, normal);
   }
 
   if (!model::active_meshsurf_tallies.empty()) {
     Position r {this->r()};
     this->r() -= TINY_BIT * u();
-    score_surface_tally(*this, model::active_meshsurf_tallies);
+    score_meshsurface_tally(*this, model::active_meshsurf_tallies);
     this->r() = r;
   }
 
@@ -663,7 +779,7 @@ void Particle::cross_reflective_bc(const Surface& surf, Direction new_u)
   u() = new_u;
 
   // Reassign particle's cell and surface
-  coord(0).cell = cell_last(0);
+  coord(0).cell() = cell_last(0);
   surface() = -surface();
 
   // If a reflective surface is coincident with a lattice or universe
@@ -705,7 +821,7 @@ void Particle::cross_periodic_bc(
   if (!model::active_meshsurf_tallies.empty()) {
     Position r {this->r()};
     this->r() -= TINY_BIT * u();
-    score_surface_tally(*this, model::active_meshsurf_tallies);
+    score_meshsurface_tally(*this, model::active_meshsurf_tallies);
     this->r() = r;
   }
 
@@ -722,9 +838,7 @@ void Particle::cross_periodic_bc(
   if (!neighbor_list_find_cell(*this)) {
     mark_as_lost("Couldn't find particle after hitting periodic "
                  "boundary on surface " +
-                 std::to_string(surf.id_) +
-                 ". The normal vector "
-                 "of one periodic surface may need to be reversed.");
+                 std::to_string(surf.id_) + ".");
     return;
   }
 
@@ -804,30 +918,29 @@ void Particle::write_restart() const
       break;
     }
     write_dataset(file_id, "id", id());
-    write_dataset(file_id, "type", static_cast<int>(type()));
+    write_dataset(file_id, "type", type().pdg_number());
 
+    // Get source site data for the particle that got lost
     int64_t i = current_work();
+    SourceSite site;
     if (settings::run_mode == RunMode::EIGENVALUE) {
-      // take source data from primary bank for eigenvalue simulation
-      write_dataset(file_id, "weight", simulation::source_bank[i - 1].wgt);
-      write_dataset(file_id, "energy", simulation::source_bank[i - 1].E);
-      write_dataset(file_id, "xyz", simulation::source_bank[i - 1].r);
-      write_dataset(file_id, "uvw", simulation::source_bank[i - 1].u);
-      write_dataset(file_id, "time", simulation::source_bank[i - 1].time);
+      site = simulation::source_bank[i];
+    } else if (settings::run_mode == RunMode::FIXED_SOURCE &&
+               settings::use_shared_secondary_bank &&
+               i < simulation::shared_secondary_bank_read.size()) {
+      site = simulation::shared_secondary_bank_read[i];
     } else if (settings::run_mode == RunMode::FIXED_SOURCE) {
-      // re-sample using rng random number seed used to generate source particle
-      int64_t id = (simulation::total_gen + overall_generation() - 1) *
-                     settings::n_particles +
-                   simulation::work_index[mpi::rank] + i;
+      // Re-sample using the same seed used to generate the source particle.
+      // current_work() is 0-indexed, compute_particle_id expects 1-indexed.
+      int64_t id = compute_transport_seed(compute_particle_id(i + 1));
       uint64_t seed = init_seed(id, STREAM_SOURCE);
-      // re-sample source site
-      auto site = sample_external_source(&seed);
-      write_dataset(file_id, "weight", site.wgt);
-      write_dataset(file_id, "energy", site.E);
-      write_dataset(file_id, "xyz", site.r);
-      write_dataset(file_id, "uvw", site.u);
-      write_dataset(file_id, "time", site.time);
+      site = sample_external_source(&seed);
     }
+    write_dataset(file_id, "weight", site.wgt);
+    write_dataset(file_id, "energy", site.E);
+    write_dataset(file_id, "xyz", site.r);
+    write_dataset(file_id, "uvw", site.u);
+    write_dataset(file_id, "time", site.time);
 
     // Close file
     file_close(file_id);
@@ -842,10 +955,12 @@ void Particle::update_neutron_xs(
 
   // If the cache doesn't match, recalculate micro xs
   if (this->E() != micro.last_E || this->sqrtkT() != micro.last_sqrtkT ||
-      i_sab != micro.index_sab || sab_frac != micro.sab_frac) {
+      i_sab != micro.index_sab || sab_frac != micro.sab_frac ||
+      ncrystal_xs != micro.ncrystal_xs) {
     data::nuclides[i_nuclide]->calculate_xs(i_sab, i_grid, sab_frac, *this);
 
     // If NCrystal is being used, update micro cross section cache
+    micro.ncrystal_xs = ncrystal_xs;
     if (ncrystal_xs >= 0.0) {
       data::nuclides[i_nuclide]->calculate_elastic_xs(*this);
       ncrystal_update_micro(ncrystal_xs, micro);
@@ -856,37 +971,6 @@ void Particle::update_neutron_xs(
 //==============================================================================
 // Non-method functions
 //==============================================================================
-
-std::string particle_type_to_str(ParticleType type)
-{
-  switch (type) {
-  case ParticleType::neutron:
-    return "neutron";
-  case ParticleType::photon:
-    return "photon";
-  case ParticleType::electron:
-    return "electron";
-  case ParticleType::positron:
-    return "positron";
-  }
-  UNREACHABLE();
-}
-
-ParticleType str_to_particle_type(std::string str)
-{
-  if (str == "neutron") {
-    return ParticleType::neutron;
-  } else if (str == "photon") {
-    return ParticleType::photon;
-  } else if (str == "electron") {
-    return ParticleType::electron;
-  } else if (str == "positron") {
-    return ParticleType::positron;
-  } else {
-    throw std::invalid_argument {fmt::format("Invalid particle name: {}", str)};
-  }
-}
-
 void add_surf_source_to_bank(Particle& p, const Surface& surf)
 {
   if (simulation::current_batch <= settings::n_inactive ||
@@ -924,7 +1008,7 @@ void add_surf_source_to_bank(Particle& p, const Surface& surf)
     // Check if the cell of interest has been entered
     bool entered = false;
     for (int i = 0; i < p.n_coord(); ++i) {
-      if (p.coord(i).cell == cell_idx) {
+      if (p.coord(i).cell() == cell_idx) {
         entered = true;
       }
     }
diff --git a/src/particle_data.cpp b/src/particle_data.cpp
index fef8359a3..370ca12e4 100644
--- a/src/particle_data.cpp
+++ b/src/particle_data.cpp
@@ -32,20 +32,20 @@ void GeometryState::mark_as_lost(const std::stringstream& message)
 
 void LocalCoord::rotate(const vector<double>& rotation)
 {
-  r = r.rotate(rotation);
-  u = u.rotate(rotation);
-  rotated = true;
+  r_ = r_.rotate(rotation);
+  u_ = u_.rotate(rotation);
+  rotated_ = true;
 }
 
 void LocalCoord::reset()
 {
-  cell = C_NONE;
-  universe = C_NONE;
-  lattice = C_NONE;
-  lattice_i[0] = 0;
-  lattice_i[1] = 0;
-  lattice_i[2] = 0;
-  rotated = false;
+  cell_ = C_NONE;
+  universe_ = C_NONE;
+  lattice_ = C_NONE;
+  lattice_index_[0] = 0;
+  lattice_index_[1] = 0;
+  lattice_index_[2] = 0;
+  rotated_ = false;
 }
 
 GeometryState::GeometryState()
@@ -64,29 +64,29 @@ void GeometryState::advance_to_boundary_from_void()
 
   for (auto c_i : root_universe->cells_) {
     auto dist =
-      model::cells.at(c_i)->distance(root_coord.r, root_coord.u, 0, this);
-    if (dist.first < boundary().distance) {
-      boundary().distance = dist.first;
-      boundary().surface = dist.second;
+      model::cells.at(c_i)->distance(root_coord.r(), root_coord.u(), 0, this);
+    if (dist.first < boundary().distance()) {
+      boundary().distance() = dist.first;
+      boundary().surface() = dist.second;
     }
   }
 
   // if no intersection or near-infinite intersection, reset
   // boundary information
-  if (boundary().distance > 1e300) {
-    boundary().distance = INFTY;
-    boundary().surface = SURFACE_NONE;
+  if (boundary().distance() > 1e300) {
+    boundary().distance() = INFTY;
+    boundary().surface() = SURFACE_NONE;
     return;
   }
 
   // move the particle up to (and just past) the boundary
-  move_distance(boundary().distance + TINY_BIT);
+  move_distance(boundary().distance() + TINY_BIT);
 }
 
 void GeometryState::move_distance(double length)
 {
   for (int j = 0; j < n_coord(); ++j) {
-    coord(j).r += length * coord(j).u;
+    coord(j).r() += length * coord(j).u();
   }
 }
 
@@ -123,7 +123,7 @@ TrackState ParticleData::get_track_state() const
   state.E = this->E();
   state.time = this->time();
   state.wgt = this->wgt();
-  state.cell_id = model::cells[this->lowest_coord().cell]->id_;
+  state.cell_id = model::cells[this->lowest_coord().cell()]->id_;
   state.cell_instance = this->cell_instance();
   if (this->material() != MATERIAL_VOID) {
     state.material_id = model::materials[material()]->id_;
diff --git a/src/particle_restart.cpp b/src/particle_restart.cpp
index 6b7778211..c226d51ec 100644
--- a/src/particle_restart.cpp
+++ b/src/particle_restart.cpp
@@ -1,6 +1,7 @@
 #include "openmc/particle_restart.h"
 
 #include "openmc/array.h"
+#include "openmc/bank.h"
 #include "openmc/constants.h"
 #include "openmc/hdf5_interface.h"
 #include "openmc/mgxs_interface.h"
@@ -31,6 +32,16 @@ void read_particle_restart(Particle& p, RunMode& previous_run_mode)
   hid_t file_id = file_open(settings::path_particle_restart, 'r');
 
   // Read data from file
+  bool legacy_particle_codes = true;
+  if (attribute_exists(file_id, "version")) {
+    array<int, 2> version;
+    read_attribute(file_id, "version", version);
+    if (version[0] > VERSION_PARTICLE_RESTART[0] ||
+        (version[0] == VERSION_PARTICLE_RESTART[0] && version[1] >= 1)) {
+      legacy_particle_codes = false;
+    }
+  }
+
   read_dataset(file_id, "current_batch", simulation::current_batch);
   read_dataset(file_id, "generations_per_batch", settings::gen_per_batch);
   read_dataset(file_id, "current_generation", simulation::current_gen);
@@ -45,7 +56,8 @@ void read_particle_restart(Particle& p, RunMode& previous_run_mode)
   read_dataset(file_id, "id", p.id());
   int type;
   read_dataset(file_id, "type", type);
-  p.type() = static_cast<ParticleType>(type);
+  p.type() = legacy_particle_codes ? legacy_particle_index_to_type(type)
+                                   : ParticleType {type};
   read_dataset(file_id, "weight", p.wgt());
   read_dataset(file_id, "energy", p.E());
   read_dataset(file_id, "xyz", p.r());
@@ -95,20 +107,16 @@ void run_particle_restart()
   // Set all tallies to 0 for now (just tracking errors)
   model::tallies.clear();
 
-  // Compute random number seed
-  int64_t particle_seed;
-  switch (previous_run_mode) {
-  case RunMode::EIGENVALUE:
-  case RunMode::FIXED_SOURCE:
-    particle_seed = (simulation::total_gen + overall_generation() - 1) *
-                      settings::n_particles +
-                    p.id();
-    break;
-  default:
-    throw std::runtime_error {
-      "Unexpected run mode: " +
-      std::to_string(static_cast<int>(previous_run_mode))};
+  // Allocate progeny_per_particle if needed for shared secondary mode
+  // (event_death() writes to this array). Set current_work to 0 since we
+  // only have one particle being restarted.
+  if (settings::use_shared_secondary_bank) {
+    p.current_work() = 0;
+    simulation::progeny_per_particle.resize(1, 0);
   }
+
+  // Compute random number seed
+  int64_t particle_seed = compute_transport_seed(p.id());
   init_particle_seeds(particle_seed, p.seeds());
 
   // Force calculation of cross-sections by setting last energy to zero
diff --git a/src/particle_type.cpp b/src/particle_type.cpp
new file mode 100644
index 000000000..fe8f9fe2a
--- /dev/null
+++ b/src/particle_type.cpp
@@ -0,0 +1,246 @@
+#include "openmc/particle_type.h"
+
+#include <algorithm>
+#include <cctype>
+#include <stdexcept>
+
+#include "openmc/string_utils.h"
+
+namespace openmc {
+namespace {
+
+constexpr const char* ATOMIC_SYMBOL[] = {"", "H", "He", "Li", "Be", "B", "C",
+  "N", "O", "F", "Ne", "Na", "Mg", "Al", "Si", "P", "S", "Cl", "Ar", "K", "Ca",
+  "Sc", "Ti", "V", "Cr", "Mn", "Fe", "Co", "Ni", "Cu", "Zn", "Ga", "Ge", "As",
+  "Se", "Br", "Kr", "Rb", "Sr", "Y", "Zr", "Nb", "Mo", "Tc", "Ru", "Rh", "Pd",
+  "Ag", "Cd", "In", "Sn", "Sb", "Te", "I", "Xe", "Cs", "Ba", "La", "Ce", "Pr",
+  "Nd", "Pm", "Sm", "Eu", "Gd", "Tb", "Dy", "Ho", "Er", "Tm", "Yb", "Lu", "Hf",
+  "Ta", "W", "Re", "Os", "Ir", "Pt", "Au", "Hg", "Tl", "Pb", "Bi", "Po", "At",
+  "Rn", "Fr", "Ra", "Ac", "Th", "Pa", "U", "Np", "Pu", "Am", "Cm", "Bk", "Cf",
+  "Es", "Fm", "Md", "No", "Lr", "Rf", "Db", "Sg", "Bh", "Hs", "Mt", "Ds", "Rg",
+  "Cn", "Nh", "Fl", "Mc", "Lv", "Ts", "Og"};
+
+constexpr int MAX_Z =
+  static_cast<int>(sizeof(ATOMIC_SYMBOL) / sizeof(ATOMIC_SYMBOL[0])) - 1;
+
+bool is_integer_string(const std::string& s)
+{
+  if (s.empty())
+    return false;
+  size_t i = 0;
+  if (s[0] == '-' || s[0] == '+') {
+    if (s.size() == 1)
+      return false;
+    i = 1;
+  }
+  for (; i < s.size(); ++i) {
+    if (!std::isdigit(static_cast<unsigned char>(s[i])))
+      return false;
+  }
+  return true;
+}
+
+int atomic_number_from_symbol(std::string_view symbol)
+{
+  for (int z = 1; z <= MAX_Z; ++z) {
+    if (symbol == ATOMIC_SYMBOL[z]) {
+      return z;
+    }
+  }
+  return 0;
+}
+
+bool parse_gnds_nuclide(std::string_view name, int& Z, int& A, int& m)
+{
+  if (name.empty())
+    return false;
+
+  size_t pos = 0;
+  if (!std::isupper(static_cast<unsigned char>(name[pos])))
+    return false;
+
+  std::string symbol;
+  symbol += name[pos++];
+  if (pos < name.size() &&
+      std::islower(static_cast<unsigned char>(name[pos]))) {
+    symbol += name[pos++];
+  }
+
+  if (pos >= name.size() ||
+      !std::isdigit(static_cast<unsigned char>(name[pos]))) {
+    return false;
+  }
+
+  size_t a_start = pos;
+  while (
+    pos < name.size() && std::isdigit(static_cast<unsigned char>(name[pos]))) {
+    ++pos;
+  }
+  A = std::stoi(std::string {name.substr(a_start, pos - a_start)});
+  if (A <= 0 || A > 999)
+    return false;
+
+  m = 0;
+  if (pos < name.size()) {
+    if (name[pos] != '_' || pos + 2 >= name.size() || name[pos + 1] != 'm') {
+      return false;
+    }
+    pos += 2;
+    size_t m_start = pos;
+    while (pos < name.size() &&
+           std::isdigit(static_cast<unsigned char>(name[pos]))) {
+      ++pos;
+    }
+    if (m_start == pos)
+      return false;
+    m = std::stoi(std::string {name.substr(m_start, pos - m_start)});
+    if (m < 0 || m > 9)
+      return false;
+  }
+
+  if (pos != name.size())
+    return false;
+
+  Z = atomic_number_from_symbol(symbol);
+  return Z != 0;
+}
+
+// Helper to convert nuclear PDG number to nuclide name
+std::string nuclide_name_from_pdg(int32_t pdg)
+{
+  int32_t code = pdg;
+  int m = code % 10;
+  int A = (code / 10) % 1000;
+  int Z = (code / 10000) % 1000;
+
+  if (Z <= 0 || Z > MAX_Z || A <= 0 || A > 999) {
+    throw std::invalid_argument {
+      "Invalid nuclear PDG number: " + std::to_string(pdg)};
+  }
+
+  std::string name = ATOMIC_SYMBOL[Z] + std::to_string(A);
+  if (m > 0) {
+    name += "_m" + std::to_string(m);
+  }
+  return name;
+}
+
+} // namespace
+
+//==============================================================================
+// ParticleType member function implementations
+//==============================================================================
+
+ParticleType::ParticleType(std::string_view str)
+{
+  std::string s {str};
+  strtrim(s);
+  if (s.empty()) {
+    throw std::invalid_argument {"Particle string is empty."};
+  }
+
+  std::string lower = s;
+  to_lower(lower);
+
+  // Check for pdg: prefix
+  if (starts_with(lower, "pdg:")) {
+    std::string value_str = lower.substr(4);
+    if (!is_integer_string(value_str)) {
+      throw std::invalid_argument {"Invalid PDG number: " + value_str};
+    }
+    pdg_number_ = std::stoi(value_str);
+    return;
+  }
+
+  // Check for known particle names
+  if (lower == "neutron" || lower == "n") {
+    pdg_number_ = PDG_NEUTRON;
+    return;
+  }
+  if (lower == "photon" || lower == "gamma") {
+    pdg_number_ = PDG_PHOTON;
+    return;
+  }
+  if (lower == "electron") {
+    pdg_number_ = PDG_ELECTRON;
+    return;
+  }
+  if (lower == "positron") {
+    pdg_number_ = PDG_POSITRON;
+    return;
+  }
+  if (lower == "proton" || lower == "p" || lower == "h1") {
+    pdg_number_ = PDG_PROTON;
+    return;
+  }
+  if (lower == "deuteron" || lower == "d" || lower == "h2") {
+    pdg_number_ = PDG_DEUTERON;
+    return;
+  }
+  if (lower == "triton" || lower == "t" || lower == "h3") {
+    pdg_number_ = PDG_TRITON;
+    return;
+  }
+  if (lower == "alpha" || lower == "he4") {
+    pdg_number_ = PDG_ALPHA;
+    return;
+  }
+
+  // Check for integer string
+  if (is_integer_string(s)) {
+    pdg_number_ = std::stoi(s);
+    return;
+  }
+
+  // Try to parse as GNDS nuclide name
+  int Z = 0;
+  int A = 0;
+  int m = 0;
+  if (!parse_gnds_nuclide(s, Z, A, m)) {
+    throw std::invalid_argument {"Invalid nuclide name: " + s};
+  }
+  pdg_number_ = 1000000000 + Z * 10000 + A * 10 + m;
+}
+
+std::string ParticleType::str() const
+{
+  if (pdg_number_ == PDG_NEUTRON)
+    return "neutron";
+  if (pdg_number_ == PDG_PHOTON)
+    return "photon";
+  if (pdg_number_ == PDG_ELECTRON)
+    return "electron";
+  if (pdg_number_ == PDG_POSITRON)
+    return "positron";
+  if (pdg_number_ == PDG_PROTON)
+    return "proton";
+
+  if (is_nucleus()) {
+    return nuclide_name_from_pdg(pdg_number_);
+  }
+
+  return "pdg:" + std::to_string(pdg_number_);
+}
+
+//==============================================================================
+// Free function implementations
+//==============================================================================
+
+ParticleType legacy_particle_index_to_type(int index)
+{
+  switch (index) {
+  case 0:
+    return ParticleType {PDG_NEUTRON};
+  case 1:
+    return ParticleType {PDG_PHOTON};
+  case 2:
+    return ParticleType {PDG_ELECTRON};
+  case 3:
+    return ParticleType {PDG_POSITRON};
+  default:
+    throw std::invalid_argument {
+      "Invalid legacy particle index: " + std::to_string(index)};
+  }
+}
+
+} // namespace openmc
diff --git a/src/photon.cpp b/src/photon.cpp
index 4b2fb4197..6bbdc928f 100644
--- a/src/photon.cpp
+++ b/src/photon.cpp
@@ -13,11 +13,7 @@
 #include "openmc/search.h"
 #include "openmc/settings.h"
 
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xmath.hpp"
-#include "xtensor/xoperation.hpp"
-#include "xtensor/xslice.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include <cmath>
 #include <fmt/core.h>
@@ -33,7 +29,7 @@ constexpr int PhotonInteraction::MAX_STACK_SIZE;
 
 namespace data {
 
-xt::xtensor<double, 1> compton_profile_pz;
+tensor::Tensor<double> compton_profile_pz;
 
 std::unordered_map<std::string, int> element_map;
 vector<unique_ptr<PhotonInteraction>> elements;
@@ -46,8 +42,6 @@ vector<unique_ptr<PhotonInteraction>> elements;
 
 PhotonInteraction::PhotonInteraction(hid_t group)
 {
-  using namespace xt::placeholders;
-
   // Set index of element in global vector
   index_ = data::elements.size();
 
@@ -96,7 +90,7 @@ PhotonInteraction::PhotonInteraction(hid_t group)
     read_dataset(rgroup, "xs", pair_production_electron_);
     close_group(rgroup);
   } else {
-    pair_production_electron_ = xt::zeros_like(energy_);
+    pair_production_electron_ = tensor::zeros_like(energy_);
   }
 
   // Read pair production
@@ -105,7 +99,7 @@ PhotonInteraction::PhotonInteraction(hid_t group)
     read_dataset(rgroup, "xs", pair_production_nuclear_);
     close_group(rgroup);
   } else {
-    pair_production_nuclear_ = xt::zeros_like(energy_);
+    pair_production_nuclear_ = tensor::zeros_like(energy_);
   }
 
   // Read photoelectric
@@ -119,7 +113,7 @@ PhotonInteraction::PhotonInteraction(hid_t group)
     read_dataset(rgroup, "xs", heating_);
     close_group(rgroup);
   } else {
-    heating_ = xt::zeros_like(energy_);
+    heating_ = tensor::zeros_like(energy_);
   }
 
   // Read subshell photoionization cross section and atomic relaxation data
@@ -133,7 +127,7 @@ PhotonInteraction::PhotonInteraction(hid_t group)
   }
 
   shells_.resize(n_shell);
-  cross_sections_ = xt::zeros<double>({energy_.size(), n_shell});
+  cross_sections_ = tensor::zeros<double>({energy_.size(), n_shell});
 
   // Create mapping from designator to index
   std::unordered_map<int, int> shell_map;
@@ -160,26 +154,26 @@ PhotonInteraction::PhotonInteraction(hid_t group)
 
     hid_t tgroup = open_group(rgroup, designator.c_str());
 
-    // Read binding energy energy and number of electrons if atomic relaxation
-    // data is present
+    // Read binding energy if atomic relaxation data is present
     if (attribute_exists(tgroup, "binding_energy")) {
       has_atomic_relaxation_ = true;
       read_attribute(tgroup, "binding_energy", shell.binding_energy);
-      read_attribute(tgroup, "num_electrons", shell.n_electrons);
     }
 
     // Read subshell cross section
-    xt::xtensor<double, 1> xs;
+    tensor::Tensor<double> xs;
     dset = open_dataset(tgroup, "xs");
     read_attribute(dset, "threshold_idx", shell.threshold);
     close_dataset(dset);
     read_dataset(tgroup, "xs", xs);
 
     auto cross_section =
-      xt::view(cross_sections_, xt::range(shell.threshold, _), i);
-    cross_section = xt::where(xs > 0, xt::log(xs), 0);
+      cross_sections_.slice(tensor::range(static_cast<size_t>(shell.threshold),
+                              cross_sections_.shape(0)),
+        i);
+    cross_section = tensor::where(xs > 0, tensor::log(xs), 0);
 
-    if (object_exists(tgroup, "transitions")) {
+    if (settings::atomic_relaxation && object_exists(tgroup, "transitions")) {
       // Determine dimensions of transitions
       dset = open_dataset(tgroup, "transitions");
       auto dims = object_shape(dset);
@@ -187,11 +181,12 @@ PhotonInteraction::PhotonInteraction(hid_t group)
 
       int n_transition = dims[0];
       if (n_transition > 0) {
-        xt::xtensor<double, 2> matrix;
+        tensor::Tensor<double> matrix;
         read_dataset(tgroup, "transitions", matrix);
 
         // Transition probability normalization
-        double norm = xt::sum(xt::col(matrix, 3))();
+        double norm =
+          tensor::Tensor<double>(matrix.slice(tensor::all, 3)).sum();
 
         shell.transitions.resize(n_transition);
         for (int j = 0; j < n_transition; ++j) {
@@ -210,9 +205,8 @@ PhotonInteraction::PhotonInteraction(hid_t group)
   // Check the maximum size of the atomic relaxation stack
   auto max_size = this->calc_max_stack_size();
   if (max_size > MAX_STACK_SIZE && mpi::master) {
-    warning(fmt::format(
-      "The subshell vacancy stack in atomic relaxation can grow up to {}, but "
-      "the stack size limit is set to {}.",
+    warning(fmt::format("The subshell vacancy stack in atomic relaxation can "
+                        "grow up to {}, but the stack size limit is set to {}.",
       max_size, MAX_STACK_SIZE));
   }
 
@@ -221,7 +215,7 @@ PhotonInteraction::PhotonInteraction(hid_t group)
 
   // Read electron shell PDF and binding energies
   read_dataset(rgroup, "num_electrons", electron_pdf_);
-  electron_pdf_ /= xt::sum(electron_pdf_);
+  electron_pdf_ /= electron_pdf_.sum();
   read_dataset(rgroup, "binding_energy", binding_energy_);
 
   // Read Compton profiles
@@ -233,10 +227,32 @@ PhotonInteraction::PhotonInteraction(hid_t group)
   }
   close_group(rgroup);
 
+  // Map Compton subshell data to atomic relaxation data by finding the
+  // subshell with the equivalent binding energy
+  if (settings::atomic_relaxation && has_atomic_relaxation_) {
+    auto is_close = [](double a, double b) {
+      return std::abs(a - b) / a < FP_REL_PRECISION;
+    };
+    subshell_map_ = tensor::Tensor<int>(binding_energy_.shape(), -1);
+    for (int i = 0; i < binding_energy_.size(); ++i) {
+      double E_b = binding_energy_[i];
+      if (i < n_shell && is_close(E_b, shells_[i].binding_energy)) {
+        subshell_map_[i] = i;
+      } else {
+        for (int j = 0; j < n_shell; ++j) {
+          if (is_close(E_b, shells_[j].binding_energy)) {
+            subshell_map_[i] = j;
+            break;
+          }
+        }
+      }
+    }
+  }
+
   // Create Compton profile CDF
   auto n_profile = data::compton_profile_pz.size();
   auto n_shell_compton = profile_pdf_.shape(0);
-  profile_cdf_ = xt::empty<double>({n_shell_compton, n_profile});
+  profile_cdf_ = tensor::Tensor<double>({n_shell_compton, n_profile});
   for (int i = 0; i < n_shell_compton; ++i) {
     double c = 0.0;
     profile_cdf_(i, 0) = 0.0;
@@ -255,11 +271,11 @@ PhotonInteraction::PhotonInteraction(hid_t group)
     // Read bremsstrahlung scaled DCS
     rgroup = open_group(group, "bremsstrahlung");
     read_dataset(rgroup, "dcs", dcs_);
-    auto n_e = dcs_.shape()[0];
-    auto n_k = dcs_.shape()[1];
+    auto n_e = dcs_.shape(0);
+    auto n_k = dcs_.shape(1);
 
     // Get energy grids used for bremsstrahlung DCS and for stopping powers
-    xt::xtensor<double, 1> electron_energy;
+    tensor::Tensor<double> electron_energy;
     read_dataset(rgroup, "electron_energy", electron_energy);
     if (data::ttb_k_grid.size() == 0) {
       read_dataset(rgroup, "photon_energy", data::ttb_k_grid);
@@ -272,7 +288,7 @@ PhotonInteraction::PhotonInteraction(hid_t group)
     close_group(rgroup);
 
     // Truncate the bremsstrahlung data at the cutoff energy
-    int photon = static_cast<int>(ParticleType::photon);
+    int photon = ParticleType::photon().transport_index();
     const auto& E {electron_energy};
     double cutoff = settings::energy_cutoff[photon];
     if (cutoff > E(0)) {
@@ -284,12 +300,12 @@ PhotonInteraction::PhotonInteraction(hid_t group)
                  (std::log(E(i_grid + 1)) - std::log(E(i_grid)));
 
       // Interpolate bremsstrahlung DCS at the cutoff energy and truncate
-      xt::xtensor<double, 2> dcs({n_e - i_grid, n_k});
+      tensor::Tensor<double> dcs({n_e - i_grid, n_k});
       for (int i = 0; i < n_k; ++i) {
         double y = std::exp(
           std::log(dcs_(i_grid, i)) +
           f * (std::log(dcs_(i_grid + 1, i)) - std::log(dcs_(i_grid, i))));
-        auto col_i = xt::view(dcs, xt::all(), i);
+        tensor::View<double> col_i = dcs.slice(tensor::all, i);
         col_i(0) = y;
         for (int j = i_grid + 1; j < n_e; ++j) {
           col_i(j - i_grid) = dcs_(j, i);
@@ -297,9 +313,11 @@ PhotonInteraction::PhotonInteraction(hid_t group)
       }
       dcs_ = dcs;
 
-      xt::xtensor<double, 1> frst {cutoff};
-      electron_energy = xt::concatenate(xt::xtuple(
-        frst, xt::view(electron_energy, xt::range(i_grid + 1, n_e))));
+      tensor::Tensor<double> frst({static_cast<size_t>(1)});
+      frst(0) = cutoff;
+      tensor::Tensor<double> rest(electron_energy.slice(
+        tensor::range(i_grid + 1, electron_energy.size())));
+      electron_energy = tensor::concatenate(frst, rest);
     }
 
     // Set incident particle energy grid
@@ -308,7 +326,8 @@ PhotonInteraction::PhotonInteraction(hid_t group)
     }
 
     // Calculate the radiative stopping power
-    stopping_power_radiative_ = xt::empty<double>({data::ttb_e_grid.size()});
+    stopping_power_radiative_ =
+      tensor::Tensor<double>({data::ttb_e_grid.size()});
     for (int i = 0; i < data::ttb_e_grid.size(); ++i) {
       // Integrate over reduced photon energy
       double c = 0.0;
@@ -333,14 +352,15 @@ PhotonInteraction::PhotonInteraction(hid_t group)
   // values below exp(-499) we store the log as -900, for which exp(-900)
   // evaluates to zero.
   double limit = std::exp(-499.0);
-  energy_ = xt::log(energy_);
-  coherent_ = xt::where(coherent_ > limit, xt::log(coherent_), -900.0);
-  incoherent_ = xt::where(incoherent_ > limit, xt::log(incoherent_), -900.0);
-  photoelectric_total_ = xt::where(
-    photoelectric_total_ > limit, xt::log(photoelectric_total_), -900.0);
-  pair_production_total_ = xt::where(
-    pair_production_total_ > limit, xt::log(pair_production_total_), -900.0);
-  heating_ = xt::where(heating_ > limit, xt::log(heating_), -900.0);
+  energy_ = tensor::log(energy_);
+  coherent_ = tensor::where(coherent_ > limit, tensor::log(coherent_), -900.0);
+  incoherent_ =
+    tensor::where(incoherent_ > limit, tensor::log(incoherent_), -900.0);
+  photoelectric_total_ = tensor::where(
+    photoelectric_total_ > limit, tensor::log(photoelectric_total_), -900.0);
+  pair_production_total_ = tensor::where(pair_production_total_ > limit,
+    tensor::log(pair_production_total_), -900.0);
+  heating_ = tensor::where(heating_ > limit, tensor::log(heating_), -900.0);
 }
 
 PhotonInteraction::~PhotonInteraction()
@@ -423,13 +443,6 @@ void PhotonInteraction::compton_scatter(double alpha, bool doppler,
         double E_out;
         this->compton_doppler(alpha, *mu, &E_out, i_shell, seed);
         *alpha_out = E_out / MASS_ELECTRON_EV;
-
-        // It's possible for the Compton profile data to have more shells than
-        // there are in the ENDF data. Make sure the shell index doesn't end up
-        // out of bounds.
-        if (*i_shell >= shells_.size()) {
-          *i_shell = -1;
-        }
       } else {
         *i_shell = -1;
       }
@@ -498,7 +511,7 @@ void PhotonInteraction::compton_doppler(
     c = prn(seed) * c_max;
 
     // Determine pz corresponding to sampled cdf value
-    auto cdf_shell = xt::view(profile_cdf_, shell, xt::all());
+    tensor::View<const double> cdf_shell = profile_cdf_.slice(shell);
     int i = lower_bound_index(cdf_shell.cbegin(), cdf_shell.cend(), c);
     double pz_l = data::compton_profile_pz(i);
     double pz_r = data::compton_profile_pz(i + 1);
@@ -594,8 +607,8 @@ void PhotonInteraction::calculate_xs(Particle& p) const
 
   // Calculate microscopic photoelectric cross section
   xs.photoelectric = 0.0;
-  const auto& xs_lower = xt::row(cross_sections_, i_grid);
-  const auto& xs_upper = xt::row(cross_sections_, i_grid + 1);
+  tensor::View<const double> xs_lower = cross_sections_.slice(i_grid);
+  tensor::View<const double> xs_upper = cross_sections_.slice(i_grid + 1);
 
   for (int i = 0; i < xs_upper.size(); ++i)
     if (xs_lower(i) != 0)
@@ -770,8 +783,9 @@ void PhotonInteraction::pair_production(double alpha, double* E_electron,
 
 void PhotonInteraction::atomic_relaxation(int i_shell, Particle& p) const
 {
-  // Return if no atomic relaxation data is present
-  if (!has_atomic_relaxation_)
+  // Return if no atomic relaxation data is present or if the binding energy is
+  // larger than the incident particle energy
+  if (!has_atomic_relaxation_ || shells_[i_shell].binding_energy > p.E())
     return;
 
   // Stack for unprocessed holes left by transitioning electrons
@@ -790,7 +804,7 @@ void PhotonInteraction::atomic_relaxation(int i_shell, Particle& p) const
     if (shell.transitions.empty()) {
       Direction u = isotropic_direction(p.current_seed());
       double E = shell.binding_energy;
-      p.create_secondary(p.wgt(), u, E, ParticleType::photon);
+      p.create_secondary(p.wgt(), u, E, ParticleType::photon());
       continue;
     }
 
@@ -818,12 +832,13 @@ void PhotonInteraction::atomic_relaxation(int i_shell, Particle& p) const
       holes[n_holes++] = transition.secondary_subshell;
 
       // Create auger electron
-      p.create_secondary(p.wgt(), u, transition.energy, ParticleType::electron);
+      p.create_secondary(
+        p.wgt(), u, transition.energy, ParticleType::electron());
     } else {
       // Radiative transition -- get X-ray energy
 
       // Create fluorescent photon
-      p.create_secondary(p.wgt(), u, transition.energy, ParticleType::photon);
+      p.create_secondary(p.wgt(), u, transition.energy, ParticleType::photon());
     }
   }
 }
diff --git a/src/physics.cpp b/src/physics.cpp
index 72c04a5ca..4a8b4d368 100644
--- a/src/physics.cpp
+++ b/src/physics.cpp
@@ -8,6 +8,7 @@
 #include "openmc/eigenvalue.h"
 #include "openmc/endf.h"
 #include "openmc/error.h"
+#include "openmc/ifp.h"
 #include "openmc/material.h"
 #include "openmc/math_functions.h"
 #include "openmc/message_passing.h"
@@ -29,9 +30,9 @@
 
 #include <fmt/core.h>
 
+#include "openmc/tensor.h"
 #include <algorithm> // for max, min, max_element
 #include <cmath>     // for sqrt, exp, log, abs, copysign
-#include <xtensor/xview.hpp>
 
 namespace openmc {
 
@@ -43,29 +44,41 @@ void collision(Particle& p)
 {
   // Add to collision counter for particle
   ++(p.n_collision());
+  p.secondary_bank_index() = p.local_secondary_bank().size();
 
   // Sample reaction for the material the particle is in
-  switch (p.type()) {
-  case ParticleType::neutron:
+  switch (p.type().pdg_number()) {
+  case PDG_NEUTRON:
     sample_neutron_reaction(p);
     break;
-  case ParticleType::photon:
+  case PDG_PHOTON:
     sample_photon_reaction(p);
     break;
-  case ParticleType::electron:
+  case PDG_ELECTRON:
     sample_electron_reaction(p);
     break;
-  case ParticleType::positron:
+  case PDG_POSITRON:
     sample_positron_reaction(p);
     break;
+  default:
+    fatal_error("Unsupported particle PDG for collision sampling.");
   }
 
-  if (settings::weight_window_checkpoint_collision)
-    apply_weight_windows(p);
+  if (settings::weight_windows_on) {
+    auto [ww_found, ww] = search_weight_window(p);
+    if (!ww_found && p.type() == ParticleType::neutron()) {
+      // if the weight window is not valid, apply russian roulette for neutrons
+      // (regardless of weight window collision checkpoint setting)
+      apply_russian_roulette(p);
+    } else if (settings::weight_window_checkpoint_collision) {
+      // if collision checkpointing is on, apply weight window
+      apply_weight_window(p, ww);
+    }
+  }
 
   // Kill particle if energy falls below cutoff
-  int type = static_cast<int>(p.type());
-  if (p.E() < settings::energy_cutoff[type]) {
+  int type = p.type().transport_index();
+  if (type != C_NONE && p.E() < settings::energy_cutoff[type]) {
     p.wgt() = 0.0;
   }
 
@@ -74,11 +87,11 @@ void collision(Particle& p)
     std::string msg;
     if (p.event() == TallyEvent::KILL) {
       msg = fmt::format("    Killed. Energy = {} eV.", p.E());
-    } else if (p.type() == ParticleType::neutron) {
+    } else if (p.type().is_neutron()) {
       msg = fmt::format("    {} with {}. Energy = {} eV.",
         reaction_name(p.event_mt()), data::nuclides[p.event_nuclide()]->name_,
         p.E());
-    } else if (p.type() == ParticleType::photon) {
+    } else if (p.type().is_photon()) {
       msg = fmt::format("    {} with {}. Energy = {} eV.",
         reaction_name(p.event_mt()),
         to_element(data::nuclides[p.event_nuclide()]->name_), p.E());
@@ -114,13 +127,15 @@ void sample_neutron_reaction(Particle& p)
 
       // Make sure particle population doesn't grow out of control for
       // subcritical multiplication problems.
-      if (p.secondary_bank().size() >= 10000) {
+      if (p.local_secondary_bank().size() >= settings::max_secondaries &&
+          !settings::use_shared_secondary_bank) {
         fatal_error(
           "The secondary particle bank appears to be growing without "
           "bound. You are likely running a subcritical multiplication problem "
           "with k-effective close to or greater than one.");
       }
     }
+    p.event_mt() = rx.mt_;
   }
 
   // Create secondary photons
@@ -151,18 +166,9 @@ void sample_neutron_reaction(Particle& p)
     advance_prn_seed(data::nuclides.size(), &p.seeds(STREAM_URR_PTABLE));
   }
 
-  // Play russian roulette if survival biasing is turned on
-  if (settings::survival_biasing) {
-    // if survival normalization is on, use normalized weight cutoff and
-    // normalized weight survive
-    if (settings::survival_normalization) {
-      if (p.wgt() < settings::weight_cutoff * p.wgt_born()) {
-        russian_roulette(p, settings::weight_survive * p.wgt_born());
-      }
-    } else if (p.wgt() < settings::weight_cutoff) {
-      russian_roulette(p, settings::weight_survive);
-    }
-  }
+  // Play russian roulette if there are no weight windows
+  if (!settings::weight_windows_on)
+    apply_russian_roulette(p);
 }
 
 void create_fission_sites(Particle& p, int i_nuclide, const Reaction& rx)
@@ -206,16 +212,26 @@ void create_fission_sites(Particle& p, int i_nuclide, const Reaction& rx)
     // Initialize fission site object with particle data
     SourceSite site;
     site.r = p.r();
-    site.particle = ParticleType::neutron;
+    site.particle = ParticleType::neutron();
     site.time = p.time();
     site.wgt = 1. / weight;
-    site.parent_id = p.id();
-    site.progeny_id = p.n_progeny()++;
     site.surf_id = 0;
 
     // Sample delayed group and angle/energy for fission reaction
     sample_fission_neutron(i_nuclide, rx, &site, p);
 
+    // Reject site if it exceeds time cutoff
+    if (site.delayed_group > 0) {
+      double t_cutoff = settings::time_cutoff[site.particle.transport_index()];
+      if (site.time > t_cutoff) {
+        continue;
+      }
+    }
+
+    // Set parent and progeny IDs
+    site.parent_id = p.current_work();
+    site.progeny_id = p.n_progeny()++;
+
     // Store fission site in bank
     if (use_fission_bank) {
       int64_t idx = simulation::fission_bank.thread_safe_append(site);
@@ -233,16 +249,21 @@ void create_fission_sites(Particle& p, int i_nuclide, const Reaction& rx)
         // Break out of loop as no more sites can be added to fission bank
         break;
       }
+      // Iterated Fission Probability (IFP) method
+      if (settings::ifp_on) {
+        ifp(p, idx);
+      }
     } else {
-      p.secondary_bank().push_back(site);
+      site.wgt_born = p.wgt_born();
+      site.wgt_ww_born = p.wgt_ww_born();
+      site.n_split = p.n_split();
+      p.local_secondary_bank().push_back(site);
+      p.n_secondaries()++;
     }
 
-    // Set the delayed group on the particle as well
-    p.delayed_group() = site.delayed_group;
-
     // Increment the number of neutrons born delayed
-    if (p.delayed_group() > 0) {
-      nu_d[p.delayed_group() - 1]++;
+    if (site.delayed_group > 0) {
+      nu_d[site.delayed_group - 1]++;
     }
 
     // Write fission particles to nuBank
@@ -276,7 +297,7 @@ void sample_photon_reaction(Particle& p)
   // Kill photon if below energy cutoff -- an extra check is made here because
   // photons with energy below the cutoff may have been produced by neutrons
   // reactions or atomic relaxation
-  int photon = static_cast<int>(ParticleType::photon);
+  int photon = ParticleType::photon().transport_index();
   if (p.E() < settings::energy_cutoff[photon]) {
     p.E() = 0.0;
     p.wgt() = 0.0;
@@ -326,20 +347,21 @@ void sample_photon_reaction(Particle& p)
     // Create Compton electron
     double phi = uniform_distribution(0., 2.0 * PI, p.current_seed());
     double E_electron = (alpha - alpha_out) * MASS_ELECTRON_EV - e_b;
-    int electron = static_cast<int>(ParticleType::electron);
+    int electron = ParticleType::electron().transport_index();
     if (E_electron >= settings::energy_cutoff[electron]) {
       double mu_electron = (alpha - alpha_out * p.mu()) /
                            std::sqrt(alpha * alpha + alpha_out * alpha_out -
                                      2.0 * alpha * alpha_out * p.mu());
       Direction u = rotate_angle(p.u(), mu_electron, &phi, p.current_seed());
-      p.create_secondary(p.wgt(), u, E_electron, ParticleType::electron);
+      p.create_secondary(p.wgt(), u, E_electron, ParticleType::electron());
     }
 
-    // TODO: Compton subshell data does not match atomic relaxation data
-    // Allow electrons to fill orbital and produce auger electrons
-    // and fluorescent photons
-    if (i_shell >= 0) {
-      element.atomic_relaxation(i_shell, p);
+    // Allow electrons to fill orbital and produce Auger electrons and
+    // fluorescent photons. Since Compton subshell data does not match atomic
+    // relaxation data, use the mapping between the data to find the subshell
+    if (settings::atomic_relaxation && element.has_atomic_relaxation_ &&
+        i_shell >= 0 && element.subshell_map_[i_shell] >= 0) {
+      element.atomic_relaxation(element.subshell_map_[i_shell], p);
     }
 
     phi += PI;
@@ -358,8 +380,9 @@ void sample_photon_reaction(Particle& p)
     // cross sections
     int i_grid = micro.index_grid;
     double f = micro.interp_factor;
-    const auto& xs_lower = xt::row(element.cross_sections_, i_grid);
-    const auto& xs_upper = xt::row(element.cross_sections_, i_grid + 1);
+    tensor::View<const double> xs_lower = element.cross_sections_.slice(i_grid);
+    tensor::View<const double> xs_upper =
+      element.cross_sections_.slice(i_grid + 1);
 
     for (int i_shell = 0; i_shell < element.shells_.size(); ++i_shell) {
       const auto& shell {element.shells_[i_shell]};
@@ -405,11 +428,13 @@ void sample_photon_reaction(Particle& p)
         u.z = std::sqrt(1.0 - mu * mu) * std::sin(phi);
 
         // Create secondary electron
-        p.create_secondary(p.wgt(), u, E_electron, ParticleType::electron);
+        p.create_secondary(p.wgt(), u, E_electron, ParticleType::electron());
 
         // Allow electrons to fill orbital and produce auger electrons
         // and fluorescent photons
-        element.atomic_relaxation(i_shell, p);
+        if (settings::atomic_relaxation) {
+          element.atomic_relaxation(i_shell, p);
+        }
         p.event() = TallyEvent::ABSORB;
         p.event_mt() = 533 + shell.index_subshell;
         p.wgt() = 0.0;
@@ -430,12 +455,11 @@ void sample_photon_reaction(Particle& p)
 
     // Create secondary electron
     Direction u = rotate_angle(p.u(), mu_electron, nullptr, p.current_seed());
-    p.create_secondary(p.wgt(), u, E_electron, ParticleType::electron);
+    p.create_secondary(p.wgt(), u, E_electron, ParticleType::electron());
 
     // Create secondary positron
     u = rotate_angle(p.u(), mu_positron, nullptr, p.current_seed());
-    p.create_secondary(p.wgt(), u, E_positron, ParticleType::positron);
-
+    p.create_secondary(p.wgt(), u, E_positron, ParticleType::positron());
     p.event() = TallyEvent::ABSORB;
     p.event_mt() = PAIR_PROD;
     p.wgt() = 0.0;
@@ -470,8 +494,8 @@ void sample_positron_reaction(Particle& p)
   Direction u = isotropic_direction(p.current_seed());
 
   // Create annihilation photon pair traveling in opposite directions
-  p.create_secondary(p.wgt(), u, MASS_ELECTRON_EV, ParticleType::photon);
-  p.create_secondary(p.wgt(), -u, MASS_ELECTRON_EV, ParticleType::photon);
+  p.create_secondary(p.wgt(), u, MASS_ELECTRON_EV, ParticleType::photon());
+  p.create_secondary(p.wgt(), -u, MASS_ELECTRON_EV, ParticleType::photon());
 
   p.E() = 0.0;
   p.wgt() = 0.0;
@@ -491,7 +515,7 @@ int sample_nuclide(Particle& p)
   for (int i = 0; i < n; ++i) {
     // Get atom density
     int i_nuclide = mat->nuclide_[i];
-    double atom_density = mat->atom_density_[i];
+    double atom_density = mat->atom_density(i, p.density_mult());
 
     // Increment probability to compare to cutoff
     prob += atom_density * p.neutron_xs(i_nuclide).total;
@@ -516,7 +540,7 @@ int sample_element(Particle& p)
   for (int i = 0; i < mat->element_.size(); ++i) {
     // Find atom density
     int i_element = mat->element_[i];
-    double atom_density = mat->atom_density_[i];
+    double atom_density = mat->atom_density(i, p.density_mult());
 
     // Determine microscopic cross section
     double sigma = atom_density * p.photon_xs(i_element).total;
@@ -596,7 +620,7 @@ void sample_photon_product(
       continue;
 
     for (int j = 0; j < rx->products_.size(); ++j) {
-      if (rx->products_[j].particle_ == ParticleType::photon) {
+      if (rx->products_[j].particle_.is_photon()) {
         // For fission, artificially increase the photon yield to account
         // for delayed photons
         double f = 1.0;
@@ -651,7 +675,9 @@ void absorption(Particle& p, int i_nuclide)
 
       p.wgt() = 0.0;
       p.event() = TallyEvent::ABSORB;
-      p.event_mt() = N_DISAPPEAR;
+      if (!p.fission()) {
+        p.event_mt() = N_DISAPPEAR;
+      }
     }
   }
 }
@@ -839,7 +865,7 @@ Direction sample_target_velocity(const Nuclide& nuc, double E, Direction u,
 
     // otherwise, use free gas model
   } else {
-    if (E >= FREE_GAS_THRESHOLD * kT && nuc.awr_ > 1.0) {
+    if (E >= settings::free_gas_threshold * kT && nuc.awr_ > 1.0) {
       return {};
     } else {
       sampling_method = ResScatMethod::cxs;
@@ -1064,6 +1090,10 @@ void sample_fission_neutron(
     // set the delayed group for the particle born from fission
     site->delayed_group = group;
 
+    // Sample time of emission based on decay constant of precursor
+    double decay_rate = rx.products_[site->delayed_group].decay_rate_;
+    site->time -= std::log(prn(p.current_seed())) / decay_rate;
+
   } else {
     // ====================================================================
     // PROMPT NEUTRON SAMPLED
@@ -1079,7 +1109,7 @@ void sample_fission_neutron(
     rx.products_[site->delayed_group].sample(E_in, site->E, mu, seed);
 
     // resample if energy is greater than maximum neutron energy
-    constexpr int neutron = static_cast<int>(ParticleType::neutron);
+    int neutron = ParticleType::neutron().transport_index();
     if (site->E < data::energy_max[neutron])
       break;
 
@@ -1139,7 +1169,7 @@ void inelastic_scatter(const Nuclide& nuc, const Reaction& rx, Particle& p)
   if (std::floor(yield) == yield && yield > 0) {
     // If yield is integral, create exactly that many secondary particles
     for (int i = 0; i < static_cast<int>(std::round(yield)) - 1; ++i) {
-      p.create_secondary(p.wgt(), p.u(), p.E(), ParticleType::neutron);
+      p.create_secondary(p.wgt(), p.u(), p.E(), ParticleType::neutron());
     }
   } else {
     // Otherwise, change weight of particle based on yield
@@ -1195,11 +1225,22 @@ void sample_secondary_photons(Particle& p, int i_nuclide)
     }
 
     // Create the secondary photon
-    bool created_photon = p.create_secondary(wgt, u, E, ParticleType::photon);
+    bool created_photon = p.create_secondary(wgt, u, E, ParticleType::photon());
+
+    // Pre-add photon energy to pht_storage so pht_secondary_particles()
+    // subtraction results in net zero
+    if (created_photon && !model::active_pulse_height_tallies.empty()) {
+      auto it = std::find(model::pulse_height_cells.begin(),
+        model::pulse_height_cells.end(), p.lowest_coord().cell());
+      if (it != model::pulse_height_cells.end()) {
+        int index = std::distance(model::pulse_height_cells.begin(), it);
+        p.pht_storage()[index] += E;
+      }
+    }
 
     // Tag secondary particle with parent nuclide
     if (created_photon && settings::use_decay_photons) {
-      p.secondary_bank().back().parent_nuclide =
+      p.local_secondary_bank().back().parent_nuclide =
         rx->products_[i_product].parent_nuclide_;
     }
   }
diff --git a/src/physics_common.cpp b/src/physics_common.cpp
index e25ae6b97..10760ce2d 100644
--- a/src/physics_common.cpp
+++ b/src/physics_common.cpp
@@ -18,4 +18,20 @@ void russian_roulette(Particle& p, double weight_survive)
   }
 }
 
+void apply_russian_roulette(Particle& p)
+{
+  // Exit if survival biasing is turned off
+  if (!settings::survival_biasing)
+    return;
+
+  // if survival normalization is on, use normalized weight cutoff and
+  // normalized weight survive
+  if (settings::survival_normalization) {
+    if (p.wgt() < settings::weight_cutoff * p.wgt_born()) {
+      russian_roulette(p, settings::weight_survive * p.wgt_born());
+    }
+  } else if (p.wgt() < settings::weight_cutoff) {
+    russian_roulette(p, settings::weight_survive);
+  }
+}
 } // namespace openmc
diff --git a/src/physics_mg.cpp b/src/physics_mg.cpp
index c97a3d6f3..212ba765c 100644
--- a/src/physics_mg.cpp
+++ b/src/physics_mg.cpp
@@ -2,7 +2,7 @@
 
 #include <stdexcept>
 
-#include "xtensor/xarray.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include "openmc/bank.h"
@@ -27,12 +27,22 @@ void collision_mg(Particle& p)
 {
   // Add to the collision counter for the particle
   p.n_collision()++;
+  p.secondary_bank_index() = p.local_secondary_bank().size();
 
   // Sample the reaction type
   sample_reaction(p);
 
-  if (settings::weight_window_checkpoint_collision)
-    apply_weight_windows(p);
+  if (settings::weight_windows_on) {
+    auto [ww_found, ww] = search_weight_window(p);
+    if (!ww_found && p.type() == ParticleType::neutron()) {
+      // if the weight window is not valid, apply russian roulette
+      // (regardless of weight window collision checkpoint setting)
+      apply_russian_roulette(p);
+    } else if (settings::weight_window_checkpoint_collision) {
+      // if collision checkpointing is on, apply weight window
+      apply_weight_window(p, ww);
+    }
+  }
 
   // Display information about collision
   if ((settings::verbosity >= 10) || p.trace()) {
@@ -66,18 +76,9 @@ void sample_reaction(Particle& p)
   // Sample a scattering event to determine the energy of the exiting neutron
   scatter(p);
 
-  // Play Russian roulette if survival biasing is turned on
-  if (settings::survival_biasing) {
-    // if survival normalization is applicable, use normalized weight cutoff and
-    // normalized weight survive
-    if (settings::survival_normalization) {
-      if (p.wgt() < settings::weight_cutoff * p.wgt_born()) {
-        russian_roulette(p, settings::weight_survive * p.wgt_born());
-      }
-    } else if (p.wgt() < settings::weight_cutoff) {
-      russian_roulette(p, settings::weight_survive);
-    }
-  }
+  // Play russian roulette if there are no weight windows
+  if (!settings::weight_windows_on)
+    apply_russian_roulette(p);
 }
 
 void scatter(Particle& p)
@@ -136,10 +137,9 @@ void create_fission_sites(Particle& p)
     // Initialize fission site object with particle data
     SourceSite site;
     site.r = p.r();
-    site.particle = ParticleType::neutron;
+    site.particle = ParticleType::neutron();
+    site.time = p.time();
     site.wgt = 1. / weight;
-    site.parent_id = p.id();
-    site.progeny_id = p.n_progeny()++;
 
     // Sample the cosine of the angle, assuming fission neutrons are emitted
     // isotropically
@@ -164,6 +164,24 @@ void create_fission_sites(Particle& p)
     // of the code, 0 is prompt.
     site.delayed_group = dg + 1;
 
+    // If delayed product production, sample time of emission
+    if (dg != -1) {
+      auto& macro_xs = data::mg.macro_xs_[p.material()];
+      double decay_rate =
+        macro_xs.get_xs(MgxsType::DECAY_RATE, 0, nullptr, nullptr, &dg, 0, 0);
+      site.time -= std::log(prn(p.current_seed())) / decay_rate;
+
+      // Reject site if it exceeds time cutoff
+      double t_cutoff = settings::time_cutoff[site.particle.transport_index()];
+      if (site.time > t_cutoff) {
+        continue;
+      }
+    }
+
+    // Set parent and progeny ID
+    site.parent_id = p.current_work();
+    site.progeny_id = p.n_progeny()++;
+
     // Store fission site in bank
     if (use_fission_bank) {
       int64_t idx = simulation::fission_bank.thread_safe_append(site);
@@ -182,7 +200,11 @@ void create_fission_sites(Particle& p)
         break;
       }
     } else {
-      p.secondary_bank().push_back(site);
+      site.wgt_born = p.wgt_born();
+      site.wgt_ww_born = p.wgt_ww_born();
+      site.n_split = p.n_split();
+      p.local_secondary_bank().push_back(site);
+      p.n_secondaries()++;
     }
 
     // Set the delayed group on the particle as well
diff --git a/src/plot.cpp b/src/plot.cpp
index dbc25b21e..5c4eb9460 100644
--- a/src/plot.cpp
+++ b/src/plot.cpp
@@ -1,20 +1,19 @@
 #include "openmc/plot.h"
 
 #include <algorithm>
-#define _USE_MATH_DEFINES // to make M_PI declared in Intel and MSVC compilers
 #include <cmath>
 #include <cstdio>
 #include <fstream>
 #include <sstream>
 
-#include "xtensor/xmanipulation.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 #include <fmt/ostream.h>
 #ifdef USE_LIBPNG
 #include <png.h>
 #endif
 
+#include "openmc/cell.h"
 #include "openmc/constants.h"
 #include "openmc/container_util.h"
 #include "openmc/dagmc.h"
@@ -33,6 +32,7 @@
 #include "openmc/settings.h"
 #include "openmc/simulation.h"
 #include "openmc/string_utils.h"
+#include "openmc/tallies/filter.h"
 
 namespace openmc {
 
@@ -44,56 +44,123 @@ constexpr int PLOT_LEVEL_LOWEST {-1}; //!< lower bound on plot universe level
 constexpr int32_t NOT_FOUND {-2};
 constexpr int32_t OVERLAP {-3};
 
-IdData::IdData(size_t h_res, size_t v_res) : data_({v_res, h_res, 3}, NOT_FOUND)
+IdData::IdData(size_t h_res, size_t v_res, bool /*include_filter*/)
+  : data_({v_res, h_res, 3}, NOT_FOUND)
 {}
 
-void IdData::set_value(size_t y, size_t x, const GeometryState& p, int level)
+void IdData::set_value(size_t y, size_t x, const Particle& p, int level,
+  Filter* /*filter*/, FilterMatch* /*match*/)
 {
   // set cell data
   if (p.n_coord() <= level) {
     data_(y, x, 0) = NOT_FOUND;
     data_(y, x, 1) = NOT_FOUND;
   } else {
-    data_(y, x, 0) = model::cells.at(p.coord(level).cell)->id_;
+    data_(y, x, 0) = model::cells.at(p.coord(level).cell())->id_;
     data_(y, x, 1) = level == p.n_coord() - 1
                        ? p.cell_instance()
                        : cell_instance_at_level(p, level);
   }
 
   // set material data
-  Cell* c = model::cells.at(p.lowest_coord().cell).get();
+  Cell* c = model::cells.at(p.lowest_coord().cell()).get();
   if (p.material() == MATERIAL_VOID) {
     data_(y, x, 2) = MATERIAL_VOID;
-    return;
   } else if (c->type_ == Fill::MATERIAL) {
     Material* m = model::materials.at(p.material()).get();
     data_(y, x, 2) = m->id_;
   }
 }
 
-void IdData::set_overlap(size_t y, size_t x)
+void IdData::set_overlap(size_t y, size_t x, int /*overlap_idx*/)
 {
-  xt::view(data_, y, x, xt::all()) = OVERLAP;
+  for (size_t k = 0; k < data_.shape(2); ++k)
+    data_(y, x, k) = OVERLAP;
 }
 
-PropertyData::PropertyData(size_t h_res, size_t v_res)
+PropertyData::PropertyData(size_t h_res, size_t v_res, bool /*include_filter*/)
   : data_({v_res, h_res, 2}, NOT_FOUND)
 {}
 
-void PropertyData::set_value(
-  size_t y, size_t x, const GeometryState& p, int level)
+void PropertyData::set_value(size_t y, size_t x, const Particle& p, int level,
+  Filter* /*filter*/, FilterMatch* /*match*/)
 {
-  Cell* c = model::cells.at(p.lowest_coord().cell).get();
+  Cell* c = model::cells.at(p.lowest_coord().cell()).get();
   data_(y, x, 0) = (p.sqrtkT() * p.sqrtkT()) / K_BOLTZMANN;
+  data_(y, x, 1) = c->density(p.cell_instance());
+}
+
+void PropertyData::set_overlap(size_t y, size_t x, int /*overlap_idx*/)
+{
+  data_(y, x) = OVERLAP;
+}
+
+//==============================================================================
+// RasterData implementation
+//==============================================================================
+
+RasterData::RasterData(size_t h_res, size_t v_res, bool include_filter)
+  : id_data_({v_res, h_res, include_filter ? 4u : 3u}, NOT_FOUND),
+    property_data_({v_res, h_res, 2}, static_cast<double>(NOT_FOUND)),
+    include_filter_(include_filter)
+{}
+
+void RasterData::set_value(size_t y, size_t x, const Particle& p, int level,
+  Filter* filter, FilterMatch* match)
+{
+  // set cell data
+  if (p.n_coord() <= level) {
+    id_data_(y, x, 0) = NOT_FOUND;
+    id_data_(y, x, 1) = NOT_FOUND;
+  } else {
+    id_data_(y, x, 0) = model::cells.at(p.coord(level).cell())->id_;
+    id_data_(y, x, 1) = level == p.n_coord() - 1
+                          ? p.cell_instance()
+                          : cell_instance_at_level(p, level);
+  }
+
+  // set material data
+  Cell* c = model::cells.at(p.lowest_coord().cell()).get();
+  if (p.material() == MATERIAL_VOID) {
+    id_data_(y, x, 2) = MATERIAL_VOID;
+  } else if (c->type_ == Fill::MATERIAL) {
+    Material* m = model::materials.at(p.material()).get();
+    id_data_(y, x, 2) = m->id_;
+  }
+
+  // set filter index (only if filter is being used)
+  if (include_filter_ && filter) {
+    filter->get_all_bins(p, TallyEstimator::COLLISION, *match);
+    if (match->bins_.empty()) {
+      id_data_(y, x, 3) = -1;
+    } else {
+      id_data_(y, x, 3) = match->bins_[0];
+    }
+    match->bins_.clear();
+    match->weights_.clear();
+  }
+
+  // set temperature (in K)
+  property_data_(y, x, 0) = (p.sqrtkT() * p.sqrtkT()) / K_BOLTZMANN;
+
+  // set density (g/cm³)
   if (c->type_ != Fill::UNIVERSE && p.material() != MATERIAL_VOID) {
     Material* m = model::materials.at(p.material()).get();
-    data_(y, x, 1) = m->density_gpcc_;
+    property_data_(y, x, 1) = c->density(p.cell_instance());
   }
 }
 
-void PropertyData::set_overlap(size_t y, size_t x)
+void RasterData::set_overlap(size_t y, size_t x, int overlap_idx)
 {
-  data_(y, x) = OVERLAP;
+  // Set cell, instance, and material to OVERLAP, but preserve filter bin for
+  // tally plotting. Cell encodes the overlap index as a negative number so that
+  // it can be used to look up overlap information in the plotter.
+  id_data_(y, x, 0) = OVERLAP - overlap_idx - 1;
+  id_data_(y, x, 1) = OVERLAP;
+  id_data_(y, x, 2) = OVERLAP;
+
+  property_data_(y, x, 0) = OVERLAP;
+  property_data_(y, x, 1) = OVERLAP;
 }
 
 //==============================================================================
@@ -123,11 +190,21 @@ extern "C" int openmc_plot_geometry()
   return 0;
 }
 
+void PlottableInterface::write_image(const ImageData& data) const
+{
+#ifdef USE_LIBPNG
+  output_png(path_plot(), data);
+#else
+  output_ppm(path_plot(), data);
+#endif
+}
+
 void Plot::create_output() const
 {
   if (PlotType::slice == type_) {
     // create 2D image
-    create_image();
+    ImageData image = create_image();
+    write_image(image);
   } else if (PlotType::voxel == type_) {
     // create voxel file for 3D viewing
     create_voxel();
@@ -170,9 +247,9 @@ void Plot::print_info() const
       fmt::print("Basis: YZ\n");
       break;
     }
-    fmt::print("Pixels: {} {}\n", pixels_[0], pixels_[1]);
+    fmt::print("Pixels: {} {}\n", pixels()[0], pixels()[1]);
   } else if (PlotType::voxel == type_) {
-    fmt::print("Voxels: {} {} {}\n", pixels_[0], pixels_[1], pixels_[2]);
+    fmt::print("Voxels: {} {} {}\n", pixels()[0], pixels()[1], pixels()[2]);
   }
 }
 
@@ -200,8 +277,11 @@ void read_plots_xml()
 void read_plots_xml(pugi::xml_node root)
 {
   for (auto node : root.children("plot")) {
-    std::string id_string = get_node_value(node, "id", true);
-    int id = std::stoi(id_string);
+    std::string plot_desc = "<auto>";
+    if (check_for_node(node, "id")) {
+      plot_desc = get_node_value(node, "id", true);
+    }
+
     if (check_for_node(node, "type")) {
       std::string type_str = get_node_value(node, "type", true);
       if (type_str == "slice") {
@@ -216,12 +296,12 @@ void read_plots_xml(pugi::xml_node root)
       } else if (type_str == "solid_raytrace") {
         model::plots.emplace_back(std::make_unique<SolidRayTracePlot>(node));
       } else {
-        fatal_error(
-          fmt::format("Unsupported plot type '{}' in plot {}", type_str, id));
+        fatal_error(fmt::format(
+          "Unsupported plot type '{}' in plot {}", type_str, plot_desc));
       }
       model::plot_map[model::plots.back()->id()] = model::plots.size() - 1;
     } else {
-      fatal_error(fmt::format("Must specify plot type in plot {}", id));
+      fatal_error(fmt::format("Must specify plot type in plot {}", plot_desc));
     }
   }
 }
@@ -234,11 +314,10 @@ void free_memory_plot()
 
 // creates an image based on user input from a plots.xml <plot>
 // specification in the PNG/PPM format
-void Plot::create_image() const
+ImageData Plot::create_image() const
 {
-
-  size_t width = pixels_[0];
-  size_t height = pixels_[1];
+  size_t width = pixels()[0];
+  size_t height = pixels()[1];
 
   ImageData data({width, height}, not_found_);
 
@@ -275,30 +354,48 @@ void Plot::create_image() const
     draw_mesh_lines(data);
   }
 
-// create image file
-#ifdef USE_LIBPNG
-  output_png(path_plot(), data);
-#else
-  output_ppm(path_plot(), data);
-#endif
+  return data;
 }
 
 void PlottableInterface::set_id(pugi::xml_node plot_node)
 {
-  // Copy data into plots
+  int id {C_NONE};
   if (check_for_node(plot_node, "id")) {
-    id_ = std::stoi(get_node_value(plot_node, "id"));
-  } else {
-    fatal_error("Must specify plot id in plots XML file.");
+    id = std::stoi(get_node_value(plot_node, "id"));
   }
 
-  // Check to make sure 'id' hasn't been used
-  if (model::plot_map.find(id_) != model::plot_map.end()) {
-    fatal_error(
-      fmt::format("Two or more plots use the same unique ID: {}", id_));
+  try {
+    set_id(id);
+  } catch (const std::runtime_error& e) {
+    fatal_error(e.what());
   }
 }
 
+void PlottableInterface::set_id(int id)
+{
+  if (id < 0 && id != C_NONE) {
+    throw std::runtime_error {fmt::format("Invalid plot ID: {}", id)};
+  }
+
+  if (id == C_NONE) {
+    id = 1;
+    for (const auto& p : model::plots) {
+      id = std::max(id, p->id() + 1);
+    }
+  }
+
+  if (id_ == id)
+    return;
+
+  // Check to make sure this ID doesn't already exist
+  if (model::plot_map.find(id) != model::plot_map.end()) {
+    throw std::runtime_error {
+      fmt::format("Two or more plots use the same unique ID: {}", id)};
+  }
+
+  id_ = id;
+}
+
 // Checks if png or ppm is already present
 bool file_extension_present(
   const std::string& filename, const std::string& extension)
@@ -348,17 +445,17 @@ void Plot::set_output_path(pugi::xml_node plot_node)
   vector<int> pxls = get_node_array<int>(plot_node, "pixels");
   if (PlotType::slice == type_) {
     if (pxls.size() == 2) {
-      pixels_[0] = pxls[0];
-      pixels_[1] = pxls[1];
+      pixels()[0] = pxls[0];
+      pixels()[1] = pxls[1];
     } else {
       fatal_error(
         fmt::format("<pixels> must be length 2 in slice plot {}", id()));
     }
   } else if (PlotType::voxel == type_) {
     if (pxls.size() == 3) {
-      pixels_[0] = pxls[0];
-      pixels_[1] = pxls[1];
-      pixels_[2] = pxls[2];
+      pixels()[0] = pxls[0];
+      pixels()[1] = pxls[1];
+      pixels()[2] = pxls[2];
     } else {
       fatal_error(
         fmt::format("<pixels> must be length 3 in voxel plot {}", id()));
@@ -419,6 +516,22 @@ void Plot::set_width(pugi::xml_node plot_node)
     if (pl_width.size() == 2) {
       width_.x = pl_width[0];
       width_.y = pl_width[1];
+      switch (basis_) {
+      case PlotBasis::xy:
+        u_span_ = {width_.x, 0.0, 0.0};
+        v_span_ = {0.0, width_.y, 0.0};
+        break;
+      case PlotBasis::xz:
+        u_span_ = {width_.x, 0.0, 0.0};
+        v_span_ = {0.0, 0.0, width_.y};
+        break;
+      case PlotBasis::yz:
+        u_span_ = {0.0, width_.x, 0.0};
+        v_span_ = {0.0, 0.0, width_.y};
+        break;
+      default:
+        UNREACHABLE();
+      }
     } else {
       fatal_error(
         fmt::format("<width> must be length 2 in slice plot {}", id()));
@@ -447,23 +560,31 @@ void PlottableInterface::set_universe(pugi::xml_node plot_node)
   }
 }
 
-void PlottableInterface::set_default_colors(pugi::xml_node plot_node)
+void PlottableInterface::set_color_by(pugi::xml_node plot_node)
 {
-  // Copy plot color type and initialize all colors randomly
+  // Copy plot color type
   std::string pl_color_by = "cell";
   if (check_for_node(plot_node, "color_by")) {
     pl_color_by = get_node_value(plot_node, "color_by", true);
   }
   if ("cell" == pl_color_by) {
     color_by_ = PlotColorBy::cells;
-    colors_.resize(model::cells.size());
   } else if ("material" == pl_color_by) {
     color_by_ = PlotColorBy::mats;
-    colors_.resize(model::materials.size());
   } else {
     fatal_error(fmt::format(
       "Unsupported plot color type '{}' in plot {}", pl_color_by, id()));
   }
+}
+
+void PlottableInterface::set_default_colors()
+{
+  // Copy plot color type and initialize all colors randomly
+  if (PlotColorBy::cells == color_by_) {
+    colors_.resize(model::cells.size());
+  } else if (PlotColorBy::mats == color_by_) {
+    colors_.resize(model::materials.size());
+  }
 
   for (auto& c : colors_) {
     c = random_color();
@@ -710,7 +831,8 @@ PlottableInterface::PlottableInterface(pugi::xml_node plot_node)
   set_id(plot_node);
   set_bg_color(plot_node);
   set_universe(plot_node);
-  set_default_colors(plot_node);
+  set_color_by(plot_node);
+  set_default_colors();
   set_user_colors(plot_node);
   set_mask(plot_node);
   set_overlap_color(plot_node);
@@ -725,7 +847,7 @@ Plot::Plot(pugi::xml_node plot_node, PlotType type)
   set_width(plot_node);
   set_meshlines(plot_node);
   slice_level_ = level_; // Copy level employed in SlicePlotBase::get_map
-  slice_color_overlaps_ = color_overlaps_;
+  show_overlaps_ = color_overlaps_;
 }
 
 //==============================================================================
@@ -743,14 +865,14 @@ void output_ppm(const std::string& filename, const ImageData& data)
 
   // Write header
   of << "P6\n";
-  of << data.shape()[0] << " " << data.shape()[1] << "\n";
+  of << data.shape(0) << " " << data.shape(1) << "\n";
   of << "255\n";
   of.close();
 
   of.open(fname, std::ios::binary | std::ios::app);
   // Write color for each pixel
-  for (int y = 0; y < data.shape()[1]; y++) {
-    for (int x = 0; x < data.shape()[0]; x++) {
+  for (int y = 0; y < data.shape(1); y++) {
+    for (int x = 0; x < data.shape(0); x++) {
       RGBColor rgb = data(x, y);
       of << rgb.red << rgb.green << rgb.blue;
     }
@@ -782,8 +904,8 @@ void output_png(const std::string& filename, const ImageData& data)
   png_init_io(png_ptr, fp);
 
   // Write header (8 bit colour depth)
-  int width = data.shape()[0];
-  int height = data.shape()[1];
+  int width = data.shape(0);
+  int height = data.shape(1);
   png_set_IHDR(png_ptr, info_ptr, width, height, 8, PNG_COLOR_TYPE_RGB,
     PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
   png_write_info(png_ptr, info_ptr);
@@ -822,23 +944,39 @@ void Plot::draw_mesh_lines(ImageData& data) const
   rgb = meshlines_color_;
 
   int ax1, ax2;
+  Position expected_u {};
+  Position expected_v {};
   switch (basis_) {
   case PlotBasis::xy:
     ax1 = 0;
     ax2 = 1;
+    expected_u = {width_[0], 0.0, 0.0};
+    expected_v = {0.0, width_[1], 0.0};
     break;
   case PlotBasis::xz:
     ax1 = 0;
     ax2 = 2;
+    expected_u = {width_[0], 0.0, 0.0};
+    expected_v = {0.0, 0.0, width_[1]};
     break;
   case PlotBasis::yz:
     ax1 = 1;
     ax2 = 2;
+    expected_u = {0.0, width_[0], 0.0};
+    expected_v = {0.0, 0.0, width_[1]};
     break;
   default:
     UNREACHABLE();
   }
 
+  // Meshlines rely on axis-aligned indexing in global coordinates.
+  constexpr double rel_tol {1e-12};
+  double span_tol = rel_tol * (1.0 + u_span_.norm() + v_span_.norm());
+  if ((u_span_ - expected_u).norm() > span_tol ||
+      (v_span_ - expected_v).norm() > span_tol) {
+    fatal_error("Meshlines are only supported for axis-aligned slice plots.");
+  }
+
   Position ll_plot {origin_};
   Position ur_plot {origin_};
 
@@ -857,27 +995,27 @@ void Plot::draw_mesh_lines(ImageData& data) const
   int ax2_min, ax2_max;
   if (axis_lines.second.size() > 0) {
     double frac = (axis_lines.second.back() - ll_plot[ax2]) / width[ax2];
-    ax2_min = (1.0 - frac) * pixels_[1];
+    ax2_min = (1.0 - frac) * pixels()[1];
     if (ax2_min < 0)
       ax2_min = 0;
     frac = (axis_lines.second.front() - ll_plot[ax2]) / width[ax2];
-    ax2_max = (1.0 - frac) * pixels_[1];
-    if (ax2_max > pixels_[1])
-      ax2_max = pixels_[1];
+    ax2_max = (1.0 - frac) * pixels()[1];
+    if (ax2_max > pixels()[1])
+      ax2_max = pixels()[1];
   } else {
     ax2_min = 0;
-    ax2_max = pixels_[1];
+    ax2_max = pixels()[1];
   }
 
   // Iterate across the first axis and draw lines.
   for (auto ax1_val : axis_lines.first) {
     double frac = (ax1_val - ll_plot[ax1]) / width[ax1];
-    int ax1_ind = frac * pixels_[0];
+    int ax1_ind = frac * pixels()[0];
     for (int ax2_ind = ax2_min; ax2_ind < ax2_max; ++ax2_ind) {
       for (int plus = 0; plus <= meshlines_width_; plus++) {
-        if (ax1_ind + plus >= 0 && ax1_ind + plus < pixels_[0])
+        if (ax1_ind + plus >= 0 && ax1_ind + plus < pixels()[0])
           data(ax1_ind + plus, ax2_ind) = rgb;
-        if (ax1_ind - plus >= 0 && ax1_ind - plus < pixels_[0])
+        if (ax1_ind - plus >= 0 && ax1_ind - plus < pixels()[0])
           data(ax1_ind - plus, ax2_ind) = rgb;
       }
     }
@@ -887,27 +1025,27 @@ void Plot::draw_mesh_lines(ImageData& data) const
   int ax1_min, ax1_max;
   if (axis_lines.first.size() > 0) {
     double frac = (axis_lines.first.front() - ll_plot[ax1]) / width[ax1];
-    ax1_min = frac * pixels_[0];
+    ax1_min = frac * pixels()[0];
     if (ax1_min < 0)
       ax1_min = 0;
     frac = (axis_lines.first.back() - ll_plot[ax1]) / width[ax1];
-    ax1_max = frac * pixels_[0];
-    if (ax1_max > pixels_[0])
-      ax1_max = pixels_[0];
+    ax1_max = frac * pixels()[0];
+    if (ax1_max > pixels()[0])
+      ax1_max = pixels()[0];
   } else {
     ax1_min = 0;
-    ax1_max = pixels_[0];
+    ax1_max = pixels()[0];
   }
 
   // Iterate across the second axis and draw lines.
   for (auto ax2_val : axis_lines.second) {
     double frac = (ax2_val - ll_plot[ax2]) / width[ax2];
-    int ax2_ind = (1.0 - frac) * pixels_[1];
+    int ax2_ind = (1.0 - frac) * pixels()[1];
     for (int ax1_ind = ax1_min; ax1_ind < ax1_max; ++ax1_ind) {
       for (int plus = 0; plus <= meshlines_width_; plus++) {
-        if (ax2_ind + plus >= 0 && ax2_ind + plus < pixels_[1])
+        if (ax2_ind + plus >= 0 && ax2_ind + plus < pixels()[1])
           data(ax1_ind, ax2_ind + plus) = rgb;
-        if (ax2_ind - plus >= 0 && ax2_ind - plus < pixels_[1])
+        if (ax2_ind - plus >= 0 && ax2_ind - plus < pixels()[1])
           data(ax1_ind, ax2_ind - plus) = rgb;
       }
     }
@@ -928,9 +1066,9 @@ void Plot::create_voxel() const
 {
   // compute voxel widths in each direction
   array<double, 3> vox;
-  vox[0] = width_[0] / static_cast<double>(pixels_[0]);
-  vox[1] = width_[1] / static_cast<double>(pixels_[1]);
-  vox[2] = width_[2] / static_cast<double>(pixels_[2]);
+  vox[0] = width_[0] / static_cast<double>(pixels()[0]);
+  vox[1] = width_[1] / static_cast<double>(pixels()[1]);
+  vox[2] = width_[2] / static_cast<double>(pixels()[2]);
 
   // initial particle position
   Position ll = origin_ - width_ / 2.;
@@ -952,30 +1090,30 @@ void Plot::create_voxel() const
 
   // Write current date and time
   write_attribute(file_id, "date_and_time", time_stamp().c_str());
-  array<int, 3> pixels;
-  std::copy(pixels_.begin(), pixels_.end(), pixels.begin());
-  write_attribute(file_id, "num_voxels", pixels);
+  array<int, 3> h5_pixels;
+  std::copy(pixels().begin(), pixels().end(), h5_pixels.begin());
+  write_attribute(file_id, "num_voxels", h5_pixels);
   write_attribute(file_id, "voxel_width", vox);
   write_attribute(file_id, "lower_left", ll);
 
   // Create dataset for voxel data -- note that the dimensions are reversed
   // since we want the order in the file to be z, y, x
   hsize_t dims[3];
-  dims[0] = pixels_[2];
-  dims[1] = pixels_[1];
-  dims[2] = pixels_[0];
+  dims[0] = pixels()[2];
+  dims[1] = pixels()[1];
+  dims[2] = pixels()[0];
   hid_t dspace, dset, memspace;
   voxel_init(file_id, &(dims[0]), &dspace, &dset, &memspace);
 
   SlicePlotBase pltbase;
-  pltbase.width_ = width_;
   pltbase.origin_ = origin_;
-  pltbase.basis_ = PlotBasis::xy;
-  pltbase.pixels_ = pixels_;
-  pltbase.slice_color_overlaps_ = color_overlaps_;
+  pltbase.u_span_ = {width_.x, 0.0, 0.0};
+  pltbase.v_span_ = {0.0, width_.y, 0.0};
+  pltbase.pixels() = pixels();
+  pltbase.show_overlaps_ = color_overlaps_;
 
   ProgressBar pb;
-  for (int z = 0; z < pixels_[2]; z++) {
+  for (int z = 0; z < pixels()[2]; z++) {
     // update z coordinate
     pltbase.origin_.z = ll.z + z * vox[2];
 
@@ -984,16 +1122,21 @@ void Plot::create_voxel() const
 
     // select only cell/material ID data and flip the y-axis
     int idx = color_by_ == PlotColorBy::cells ? 0 : 2;
-    xt::xtensor<int32_t, 2> data_slice =
-      xt::view(ids.data_, xt::all(), xt::all(), idx);
-    xt::xtensor<int32_t, 2> data_flipped = xt::flip(data_slice, 0);
+    // Extract 2D slice at index idx from 3D data
+    size_t rows = ids.data_.shape(0);
+    size_t cols = ids.data_.shape(1);
+    tensor::Tensor<int32_t> data_slice({rows, cols});
+    for (size_t r = 0; r < rows; ++r)
+      for (size_t c = 0; c < cols; ++c)
+        data_slice(r, c) = ids.data_(r, c, idx);
+    tensor::Tensor<int32_t> data_flipped = data_slice.flip(0);
 
     // Write to HDF5 dataset
     voxel_write_slice(z, dspace, dset, memspace, data_flipped.data());
 
     // update progress bar
     pb.set_value(
-      100. * static_cast<double>(z + 1) / static_cast<double>((pixels_[2])));
+      100. * static_cast<double>(z + 1) / static_cast<double>((pixels()[2])));
   }
 
   voxel_finalize(dspace, dset, memspace);
@@ -1052,7 +1195,10 @@ RayTracePlot::RayTracePlot(pugi::xml_node node) : PlottableInterface(node)
       check_for_node(node, "field_of_view"))
     fatal_error("orthographic_width and field_of_view are mutually exclusive "
                 "parameters.");
+}
 
+void RayTracePlot::update_view()
+{
   // Get centerline vector for camera-to-model. We create vectors around this
   // that form a pixel array, and then trace rays along that.
   auto up = up_ / up_.norm();
@@ -1079,6 +1225,7 @@ WireframeRayTracePlot::WireframeRayTracePlot(pugi::xml_node node)
   set_wireframe_thickness(node);
   set_wireframe_ids(node);
   set_wireframe_color(node);
+  update_view();
 }
 
 void WireframeRayTracePlot::set_wireframe_color(pugi::xml_node plot_node)
@@ -1180,10 +1327,10 @@ std::pair<Position, Direction> RayTracePlot::get_pixel_ray(
   int horiz, int vert) const
 {
   // Compute field of view in radians
-  constexpr double DEGREE_TO_RADIAN = M_PI / 180.0;
+  constexpr double DEGREE_TO_RADIAN = PI / 180.0;
   double horiz_fov_radians = horizontal_field_of_view_ * DEGREE_TO_RADIAN;
-  double p0 = static_cast<double>(pixels_[0]);
-  double p1 = static_cast<double>(pixels_[1]);
+  double p0 = static_cast<double>(pixels()[0]);
+  double p1 = static_cast<double>(pixels()[1]);
   double vert_fov_radians = horiz_fov_radians * p1 / p0;
 
   // focal_plane_dist can be changed to alter the perspective distortion
@@ -1219,16 +1366,17 @@ std::pair<Position, Direction> RayTracePlot::get_pixel_ray(
   return result;
 }
 
-void WireframeRayTracePlot::create_output() const
+ImageData WireframeRayTracePlot::create_image() const
 {
-  size_t width = pixels_[0];
-  size_t height = pixels_[1];
+  size_t width = pixels()[0];
+  size_t height = pixels()[1];
   ImageData data({width, height}, not_found_);
 
   // This array marks where the initial wireframe was drawn. We convolve it with
   // a filter that gets adjusted with the wireframe thickness in order to
   // thicken the lines.
-  xt::xtensor<int, 2> wireframe_initial({width, height}, 0);
+  tensor::Tensor<int> wireframe_initial(
+    {static_cast<size_t>(width), static_cast<size_t>(height)}, 0);
 
   /* Holds all of the track segments for the current rendered line of pixels.
    * old_segments holds a copy of this_line_segments from the previous line.
@@ -1245,11 +1393,11 @@ void WireframeRayTracePlot::create_output() const
   std::vector<std::vector<std::vector<TrackSegment>>> this_line_segments(
     n_threads);
   for (int t = 0; t < n_threads; ++t) {
-    this_line_segments[t].resize(pixels_[0]);
+    this_line_segments[t].resize(pixels()[0]);
   }
 
   // The last thread writes to this, and the first thread reads from it.
-  std::vector<std::vector<TrackSegment>> old_segments(pixels_[0]);
+  std::vector<std::vector<TrackSegment>> old_segments(pixels()[0]);
 
 #pragma omp parallel
   {
@@ -1257,7 +1405,7 @@ void WireframeRayTracePlot::create_output() const
     const int tid = thread_num();
 
     int vert = tid;
-    for (int iter = 0; iter <= pixels_[1] / n_threads; iter++) {
+    for (int iter = 0; iter <= pixels()[1] / n_threads; iter++) {
 
       // Save bottom line of current work chunk to compare against later. This
       // used to be inside the below if block, but it causes a spurious line to
@@ -1266,9 +1414,9 @@ void WireframeRayTracePlot::create_output() const
       if (tid == n_threads - 1)
         old_segments = this_line_segments[n_threads - 1];
 
-      if (vert < pixels_[1]) {
+      if (vert < pixels()[1]) {
 
-        for (int horiz = 0; horiz < pixels_[0]; ++horiz) {
+        for (int horiz = 0; horiz < pixels()[0]; ++horiz) {
 
           // RayTracePlot implements camera ray generation
           std::pair<Position, Direction> ru = get_pixel_ray(horiz, vert);
@@ -1330,7 +1478,7 @@ void WireframeRayTracePlot::create_output() const
       // Now that the horizontal line has finished rendering, we can fill in
       // wireframe entries that require comparison among all the threads. Hence
       // the omp barrier being used. It has to be OUTSIDE any if blocks!
-      if (vert < pixels_[1]) {
+      if (vert < pixels()[1]) {
         // Loop over horizontal pixels, checking intersection stack of upper
         // neighbor
 
@@ -1340,7 +1488,7 @@ void WireframeRayTracePlot::create_output() const
         else
           top_cmp = &this_line_segments[tid - 1];
 
-        for (int horiz = 0; horiz < pixels_[0]; ++horiz) {
+        for (int horiz = 0; horiz < pixels()[0]; ++horiz) {
           if (!trackstack_equivalent(
                 this_line_segments[tid][horiz], (*top_cmp)[horiz])) {
             wireframe_initial(horiz, vert) = 1;
@@ -1357,8 +1505,8 @@ void WireframeRayTracePlot::create_output() const
   } // end omp parallel
 
   // Now thicken the wireframe lines and apply them to our image
-  for (int vert = 0; vert < pixels_[1]; ++vert) {
-    for (int horiz = 0; horiz < pixels_[0]; ++horiz) {
+  for (int vert = 0; vert < pixels()[1]; ++vert) {
+    for (int horiz = 0; horiz < pixels()[0]; ++horiz) {
       if (wireframe_initial(horiz, vert)) {
         if (wireframe_thickness_ == 1)
           data(horiz, vert) = wireframe_color_;
@@ -1369,19 +1517,21 @@ void WireframeRayTracePlot::create_output() const
             if (i * i + j * j < wireframe_thickness_ * wireframe_thickness_) {
 
               // Check if wireframe pixel is out of bounds
-              int w_i = std::max(std::min(horiz + i, pixels_[0] - 1), 0);
-              int w_j = std::max(std::min(vert + j, pixels_[1] - 1), 0);
+              int w_i = std::max(std::min(horiz + i, pixels()[0] - 1), 0);
+              int w_j = std::max(std::min(vert + j, pixels()[1] - 1), 0);
               data(w_i, w_j) = wireframe_color_;
             }
       }
     }
   }
 
-#ifdef USE_LIBPNG
-  output_png(path_plot(), data);
-#else
-  output_ppm(path_plot(), data);
-#endif
+  return data;
+}
+
+void WireframeRayTracePlot::create_output() const
+{
+  ImageData data = create_image();
+  write_image(data);
 }
 
 void RayTracePlot::print_info() const
@@ -1391,7 +1541,7 @@ void RayTracePlot::print_info() const
   fmt::print("Look at: {} {} {}\n", look_at_.x, look_at_.y, look_at_.z);
   fmt::print(
     "Horizontal field of view: {} degrees\n", horizontal_field_of_view_);
-  fmt::print("Pixels: {} {}\n", pixels_[0], pixels_[1]);
+  fmt::print("Pixels: {} {}\n", pixels()[0], pixels()[1]);
 }
 
 void WireframeRayTracePlot::print_info() const
@@ -1473,8 +1623,8 @@ void RayTracePlot::set_pixels(pugi::xml_node node)
   if (pxls.size() != 2)
     fatal_error(
       fmt::format("<pixels> must be length 2 in projection plot {}", id()));
-  pixels_[0] = pxls[0];
-  pixels_[1] = pxls[1];
+  pixels()[0] = pxls[0];
+  pixels()[1] = pxls[1];
 }
 
 void RayTracePlot::set_camera_position(pugi::xml_node node)
@@ -1521,6 +1671,7 @@ SolidRayTracePlot::SolidRayTracePlot(pugi::xml_node node) : RayTracePlot(node)
   set_opaque_ids(node);
   set_diffuse_fraction(node);
   set_light_position(node);
+  update_view();
 }
 
 void SolidRayTracePlot::print_info() const
@@ -1529,15 +1680,15 @@ void SolidRayTracePlot::print_info() const
   RayTracePlot::print_info();
 }
 
-void SolidRayTracePlot::create_output() const
+ImageData SolidRayTracePlot::create_image() const
 {
-  size_t width = pixels_[0];
-  size_t height = pixels_[1];
+  size_t width = pixels()[0];
+  size_t height = pixels()[1];
   ImageData data({width, height}, not_found_);
 
 #pragma omp parallel for schedule(dynamic) collapse(2)
-  for (int horiz = 0; horiz < pixels_[0]; ++horiz) {
-    for (int vert = 0; vert < pixels_[1]; ++vert) {
+  for (int horiz = 0; horiz < pixels()[0]; ++horiz) {
+    for (int vert = 0; vert < pixels()[1]; ++vert) {
       // RayTracePlot implements camera ray generation
       std::pair<Position, Direction> ru = get_pixel_ray(horiz, vert);
       PhongRay ray(ru.first, ru.second, *this);
@@ -1546,11 +1697,13 @@ void SolidRayTracePlot::create_output() const
     }
   }
 
-#ifdef USE_LIBPNG
-  output_png(path_plot(), data);
-#else
-  output_ppm(path_plot(), data);
-#endif
+  return data;
+}
+
+void SolidRayTracePlot::create_output() const
+{
+  ImageData data = create_image();
+  write_image(data);
 }
 
 void SolidRayTracePlot::set_opaque_ids(pugi::xml_node node)
@@ -1594,144 +1747,6 @@ void SolidRayTracePlot::set_diffuse_fraction(pugi::xml_node node)
   }
 }
 
-void Ray::compute_distance()
-{
-  boundary() = distance_to_boundary(*this);
-}
-
-void Ray::trace()
-{
-  // To trace the ray from its origin all the way through the model, we have
-  // to proceed in two phases. In the first, the ray may or may not be found
-  // inside the model. If the ray is already in the model, phase one can be
-  // skipped. Otherwise, the ray has to be advanced to the boundary of the
-  // model where all the cells are defined. Importantly, this is assuming that
-  // the model is convex, which is a very reasonable assumption for any
-  // radiation transport model.
-  //
-  // After phase one is done, we can starting tracing from cell to cell within
-  // the model. This step can use neighbor lists to accelerate the ray tracing.
-
-  // Attempt to initialize the particle. We may have to enter a loop to move
-  // it up to the edge of the model.
-  bool inside_cell = exhaustive_find_cell(*this, settings::verbosity >= 10);
-
-  // Advance to the boundary of the model
-  while (!inside_cell) {
-    advance_to_boundary_from_void();
-    inside_cell = exhaustive_find_cell(*this, settings::verbosity >= 10);
-
-    // If true this means no surface was intersected. See cell.cpp and search
-    // for numeric_limits to see where we return it.
-    if (surface() == std::numeric_limits<int>::max()) {
-      warning(fmt::format("Lost a ray, r = {}, u = {}", r(), u()));
-      return;
-    }
-
-    // Exit this loop and enter into cell-to-cell ray tracing (which uses
-    // neighbor lists)
-    if (inside_cell)
-      break;
-
-    // if there is no intersection with the model, we're done
-    if (boundary().surface == SURFACE_NONE)
-      return;
-
-    event_counter_++;
-    if (event_counter_ > MAX_INTERSECTIONS) {
-      warning("Likely infinite loop in ray traced plot");
-      return;
-    }
-  }
-
-  // Call the specialized logic for this type of ray. This is for the
-  // intersection for the first intersection if we had one.
-  if (boundary().surface != SURFACE_NONE) {
-    // set the geometry state's surface attribute to be used for
-    // surface normal computation
-    surface() = boundary().surface;
-    on_intersection();
-    if (stop_)
-      return;
-  }
-
-  // reset surface attribute to zero after the first intersection so that it
-  // doesn't perturb surface crossing logic from here on out
-  surface() = 0;
-
-  // This is the ray tracing loop within the model. It exits after exiting
-  // the model, which is equivalent to assuming that the model is convex.
-  // It would be nice to factor out the on_intersection at the end of this
-  // loop and then do "while (inside_cell)", but we can't guarantee it's
-  // on a surface in that case. There might be some other way to set it
-  // up that is perhaps a little more elegant, but this is what works just
-  // fine.
-  while (true) {
-
-    compute_distance();
-
-    // There are no more intersections to process
-    // if we hit the edge of the model, so stop
-    // the particle in that case. Also, just exit
-    // if a negative distance was somehow computed.
-    if (boundary().distance == INFTY || boundary().distance == INFINITY ||
-        boundary().distance < 0) {
-      return;
-    }
-
-    // See below comment where call_on_intersection is checked in an
-    // if statement for an explanation of this.
-    bool call_on_intersection {true};
-    if (boundary().distance < 10 * TINY_BIT) {
-      call_on_intersection = false;
-    }
-
-    // DAGMC surfaces expect us to go a little bit further than the advance
-    // distance to properly check cell inclusion.
-    boundary().distance += TINY_BIT;
-
-    // Advance particle, prepare for next intersection
-    for (int lev = 0; lev < n_coord(); ++lev) {
-      coord(lev).r += boundary().distance * coord(lev).u;
-    }
-    surface() = boundary().surface;
-    n_coord_last() = n_coord();
-    n_coord() = boundary().coord_level;
-    if (boundary().lattice_translation[0] != 0 ||
-        boundary().lattice_translation[1] != 0 ||
-        boundary().lattice_translation[2] != 0) {
-      cross_lattice(*this, boundary(), settings::verbosity >= 10);
-    }
-
-    // Record how far the ray has traveled
-    traversal_distance_ += boundary().distance;
-    inside_cell = neighbor_list_find_cell(*this, settings::verbosity >= 10);
-
-    // Call the specialized logic for this type of ray. Note that we do not
-    // call this if the advance distance is very small. Unfortunately, it seems
-    // darn near impossible to get the particle advanced to the model boundary
-    // and through it without sometimes accidentally calling on_intersection
-    // twice. This incorrectly shades the region as occluded when it might not
-    // actually be. By screening out intersection distances smaller than a
-    // threshold 10x larger than the scoot distance used to advance up to the
-    // model boundary, we can avoid that situation.
-    if (call_on_intersection) {
-      on_intersection();
-      if (stop_)
-        return;
-    }
-
-    if (!inside_cell)
-      return;
-
-    event_counter_++;
-    if (event_counter_ > MAX_INTERSECTIONS) {
-      warning("Likely infinite loop in ray traced plot");
-      return;
-    }
-  }
-}
-
 void ProjectionRay::on_intersection()
 {
   // This records a tuple with the following info
@@ -1743,7 +1758,7 @@ void ProjectionRay::on_intersection()
   line_segments_.emplace_back(
     plot_.color_by_ == PlottableInterface::PlotColorBy::mats
       ? material()
-      : lowest_coord().cell,
+      : lowest_coord().cell(),
     traversal_distance_, boundary().surface_index());
 }
 
@@ -1752,7 +1767,7 @@ void PhongRay::on_intersection()
   // Check if we hit an opaque material or cell
   int hit_id = plot_.color_by_ == PlottableInterface::PlotColorBy::mats
                  ? material()
-                 : lowest_coord().cell;
+                 : lowest_coord().cell();
 
   // If we are reflected and have advanced beyond the camera,
   // the ray is done. This is checked here because we should
@@ -1771,7 +1786,10 @@ void PhongRay::on_intersection()
     // the normal or the diffuse lighting contribution
     reflected_ = true;
     result_color_ = plot_.colors_[hit_id];
-    Direction to_light = plot_.light_location_ - r();
+    // The ray has been advanced slightly past the boundary. Use an
+    // approximation to the actual hit point for stable normal/lighting.
+    Position r_hit = r() - TINY_BIT * u();
+    Direction to_light = plot_.light_location_ - r_hit;
     to_light /= to_light.norm();
 
     // TODO
@@ -1792,14 +1810,24 @@ void PhongRay::on_intersection()
     // Get surface pointer
     const auto& surf = model::surfaces.at(surface_index());
 
-    Direction normal = surf->normal(r_local());
+    // The crossed surface may be on a higher coordinate level than the
+    // innermost local coordinates, so we check the surface's coordinate level
+    // to find the appropriate coordinate level to use for the normal
+    // calculation
+    int surf_level = boundary().coord_level() - 1;
+    // ensure surface level is within bounds of current coordinate stack
+    surf_level = std::max(0, std::min(surf_level, n_coord() - 1));
+
+    Position r_hit_level =
+      coord(surf_level).r() - TINY_BIT * coord(surf_level).u();
+    Direction normal = surf->normal(r_hit_level);
     normal /= normal.norm();
 
-    // Need to apply translations to find the normal vector in
+    // Need to apply rotations to find the normal vector in
     // the base level universe's coordinate system.
-    for (int lev = n_coord() - 2; lev >= 0; --lev) {
-      if (coord(lev + 1).rotated) {
-        const Cell& c {*model::cells[coord(lev).cell]};
+    for (int lev = surf_level - 1; lev >= 0; --lev) {
+      if (coord(lev + 1).rotated()) {
+        const Cell& c {*model::cells[coord(lev).cell()]};
         normal = normal.inverse_rotate(c.rotation_);
       }
     }
@@ -1864,6 +1892,12 @@ void PhongRay::on_intersection()
 
 extern "C" int openmc_id_map(const void* plot, int32_t* data_out)
 {
+  static bool warned {false};
+  if (!warned) {
+    warning("openmc_id_map is deprecated and will be removed in a future "
+            "release. Use openmc_slice_data.");
+    warned = true;
+  }
 
   auto plt = reinterpret_cast<const SlicePlotBase*>(plot);
   if (!plt) {
@@ -1871,7 +1905,7 @@ extern "C" int openmc_id_map(const void* plot, int32_t* data_out)
     return OPENMC_E_INVALID_ARGUMENT;
   }
 
-  if (plt->slice_color_overlaps_ && model::overlap_check_count.size() == 0) {
+  if (plt->show_overlaps_ && model::overlap_check_count.size() == 0) {
     model::overlap_check_count.resize(model::cells.size());
   }
 
@@ -1885,14 +1919,20 @@ extern "C" int openmc_id_map(const void* plot, int32_t* data_out)
 
 extern "C" int openmc_property_map(const void* plot, double* data_out)
 {
+  static bool warned {false};
+  if (!warned) {
+    warning("openmc_property_map is deprecated and will be removed in a future "
+            "release. Use openmc_slice_data.");
+    warned = true;
+  }
 
   auto plt = reinterpret_cast<const SlicePlotBase*>(plot);
   if (!plt) {
-    set_errmsg("Invalid slice pointer passed to openmc_id_map");
+    set_errmsg("Invalid slice pointer passed to openmc_property_map");
     return OPENMC_E_INVALID_ARGUMENT;
   }
 
-  if (plt->slice_color_overlaps_ && model::overlap_check_count.size() == 0) {
+  if (plt->show_overlaps_ && model::overlap_check_count.size() == 0) {
     model::overlap_check_count.resize(model::cells.size());
   }
 
@@ -1904,4 +1944,678 @@ extern "C" int openmc_property_map(const void* plot, double* data_out)
   return 0;
 }
 
+extern "C" int openmc_slice_data(const double origin[3], const double u_span[3],
+  const double v_span[3], const size_t pixels[2], bool color_overlaps,
+  int level, int32_t filter_index, int32_t* geom_data, double* property_data)
+{
+  // Validate span vectors
+  Direction u_span_pos {u_span[0], u_span[1], u_span[2]};
+  Direction v_span_pos {v_span[0], v_span[1], v_span[2]};
+  double u_norm = u_span_pos.norm();
+  double v_norm = v_span_pos.norm();
+  if (u_norm == 0.0 || v_norm == 0.0) {
+    set_errmsg("Slice span vectors must be non-zero.");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  constexpr double ORTHO_REL_TOL = 1e-10;
+  double dot = u_span_pos.dot(v_span_pos);
+  if (std::abs(dot) > ORTHO_REL_TOL * u_norm * v_norm) {
+    set_errmsg("Slice span vectors must be orthogonal.");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  // Validate filter index if provided
+  if (filter_index >= 0) {
+    if (int err = verify_filter(filter_index))
+      return err;
+  }
+
+  // Initialize overlap check vector if needed
+  if (color_overlaps && model::overlap_check_count.size() == 0) {
+    model::overlap_check_count.resize(model::cells.size());
+  }
+
+  try {
+    // Create a temporary SlicePlotBase object to reuse get_map logic
+    SlicePlotBase plot_params;
+    plot_params.origin_ = Position {origin[0], origin[1], origin[2]};
+    plot_params.u_span_ = u_span_pos;
+    plot_params.v_span_ = v_span_pos;
+    plot_params.pixels_[0] = pixels[0];
+    plot_params.pixels_[1] = pixels[1];
+    plot_params.show_overlaps_ = color_overlaps;
+    plot_params.slice_level_ = level;
+
+    // Clear overlap data structures on new slice call
+    model::overlap_keys.clear();
+    model::overlap_key_index.clear();
+
+    // Use get_map<RasterData> to generate data
+    auto data = plot_params.get_map<RasterData>(filter_index);
+    std::copy(data.id_data_.begin(), data.id_data_.end(), geom_data);
+
+    // Copy property data if requested
+    if (property_data != nullptr) {
+      std::copy(
+        data.property_data_.begin(), data.property_data_.end(), property_data);
+    }
+  } catch (const std::exception& e) {
+    set_errmsg(e.what());
+    return OPENMC_E_UNASSIGNED;
+  }
+
+  return 0;
+}
+
+// Gets the number of overlaps that we need data for
+extern "C" int openmc_slice_data_overlap_count(size_t* count)
+{
+  if (!count) {
+    set_errmsg("Null pointer passed for overlap count.");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+  *count = model::overlap_keys.size();
+
+  return 0;
+}
+
+// Plotter pre-allocates array size based on what is returned with
+// overlap_count; populates an array of size 3*count
+extern "C" int openmc_slice_data_overlap_info(
+  size_t count, int32_t* overlap_info)
+{
+  for (size_t i = 0; i < count; ++i) {
+    overlap_info[i * 3] = model::overlap_keys[i].universe_id;
+    overlap_info[i * 3 + 1] = model::overlap_keys[i].cell1_id;
+    overlap_info[i * 3 + 2] = model::overlap_keys[i].cell2_id;
+  }
+
+  return 0;
+}
+
+extern "C" int openmc_get_plot_index(int32_t id, int32_t* index)
+{
+  auto it = model::plot_map.find(id);
+  if (it == model::plot_map.end()) {
+    set_errmsg("No plot exists with ID=" + std::to_string(id) + ".");
+    return OPENMC_E_INVALID_ID;
+  }
+
+  *index = it->second;
+  return 0;
+}
+
+extern "C" int openmc_plot_get_id(int32_t index, int32_t* id)
+{
+  if (index < 0 || index >= model::plots.size()) {
+    set_errmsg("Index in plots array is out of bounds.");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  *id = model::plots[index]->id();
+  return 0;
+}
+
+extern "C" int openmc_plot_set_id(int32_t index, int32_t id)
+{
+  if (index < 0 || index >= model::plots.size()) {
+    set_errmsg("Index in plots array is out of bounds.");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  if (id < 0 && id != C_NONE) {
+    set_errmsg("Invalid plot ID.");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  auto* plot = model::plots[index].get();
+  int32_t old_id = plot->id();
+  if (id == old_id)
+    return 0;
+
+  model::plot_map.erase(old_id);
+  try {
+    plot->set_id(id);
+  } catch (const std::runtime_error& e) {
+    model::plot_map[old_id] = index;
+    set_errmsg(e.what());
+    return OPENMC_E_INVALID_ID;
+  }
+  model::plot_map[plot->id()] = index;
+  return 0;
+}
+
+extern "C" size_t openmc_plots_size()
+{
+  return model::plots.size();
+}
+
+int map_phong_domain_id(
+  const SolidRayTracePlot* plot, int32_t id, int32_t* index_out)
+{
+  if (!plot || !index_out) {
+    set_errmsg("Invalid plot pointer passed to map_phong_domain_id");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  if (plot->color_by_ == PlottableInterface::PlotColorBy::mats) {
+    auto it = model::material_map.find(id);
+    if (it == model::material_map.end()) {
+      set_errmsg("Invalid material ID for SolidRayTracePlot");
+      return OPENMC_E_INVALID_ID;
+    }
+    *index_out = it->second;
+    return 0;
+  }
+
+  if (plot->color_by_ == PlottableInterface::PlotColorBy::cells) {
+    auto it = model::cell_map.find(id);
+    if (it == model::cell_map.end()) {
+      set_errmsg("Invalid cell ID for SolidRayTracePlot");
+      return OPENMC_E_INVALID_ID;
+    }
+    *index_out = it->second;
+    return 0;
+  }
+
+  set_errmsg("Unsupported color_by for SolidRayTracePlot");
+  return OPENMC_E_INVALID_TYPE;
+}
+
+int get_solidraytrace_plot_by_index(int32_t index, SolidRayTracePlot** plot)
+{
+  if (!plot) {
+    set_errmsg("Null output pointer passed to get_solidraytrace_plot_by_index");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  if (index < 0 || index >= model::plots.size()) {
+    set_errmsg("Index in plots array is out of bounds.");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  auto* plottable = model::plots[index].get();
+  auto* solid_plot = dynamic_cast<SolidRayTracePlot*>(plottable);
+  if (!solid_plot) {
+    set_errmsg("Plot at index=" + std::to_string(index) +
+               " is not a solid raytrace plot.");
+    return OPENMC_E_INVALID_TYPE;
+  }
+
+  *plot = solid_plot;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_create(int32_t* index)
+{
+  if (!index) {
+    set_errmsg(
+      "Null output pointer passed to openmc_solidraytrace_plot_create");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  try {
+    auto new_plot = std::make_unique<SolidRayTracePlot>();
+    new_plot->set_id();
+    int32_t new_plot_id = new_plot->id();
+#ifdef USE_LIBPNG
+    new_plot->path_plot() = fmt::format("plot_{}.png", new_plot_id);
+#else
+    new_plot->path_plot() = fmt::format("plot_{}.ppm", new_plot_id);
+#endif
+    int32_t new_plot_index = model::plots.size();
+    model::plots.emplace_back(std::move(new_plot));
+    model::plot_map[new_plot_id] = new_plot_index;
+    *index = new_plot_index;
+  } catch (const std::exception& e) {
+    set_errmsg(e.what());
+    return OPENMC_E_ALLOCATE;
+  }
+
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_pixels(
+  int32_t index, int32_t* width, int32_t* height)
+{
+  if (!width || !height) {
+    set_errmsg(
+      "Invalid arguments passed to openmc_solidraytrace_plot_get_pixels");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  *width = plt->pixels()[0];
+  *height = plt->pixels()[1];
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_pixels(
+  int32_t index, int32_t width, int32_t height)
+{
+  if (width <= 0 || height <= 0) {
+    set_errmsg(
+      "Invalid arguments passed to openmc_solidraytrace_plot_set_pixels");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->pixels()[0] = width;
+  plt->pixels()[1] = height;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_color_by(
+  int32_t index, int32_t* color_by)
+{
+  if (!color_by) {
+    set_errmsg(
+      "Invalid arguments passed to openmc_solidraytrace_plot_get_color_by");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  if (plt->color_by_ == PlottableInterface::PlotColorBy::mats) {
+    *color_by = 0;
+  } else if (plt->color_by_ == PlottableInterface::PlotColorBy::cells) {
+    *color_by = 1;
+  } else {
+    set_errmsg("Unsupported color_by for SolidRayTracePlot");
+    return OPENMC_E_INVALID_TYPE;
+  }
+
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_color_by(
+  int32_t index, int32_t color_by)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  if (color_by == 0) {
+    plt->color_by_ = PlottableInterface::PlotColorBy::mats;
+  } else if (color_by == 1) {
+    plt->color_by_ = PlottableInterface::PlotColorBy::cells;
+  } else {
+    set_errmsg("Invalid color_by value for SolidRayTracePlot");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_default_colors(int32_t index)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->set_default_colors();
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_all_opaque(int32_t index)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->opaque_ids().clear();
+  if (plt->color_by_ == PlottableInterface::PlotColorBy::mats) {
+    for (int32_t i = 0; i < model::materials.size(); ++i) {
+      plt->opaque_ids().insert(i);
+    }
+    return 0;
+  }
+
+  if (plt->color_by_ == PlottableInterface::PlotColorBy::cells) {
+    for (int32_t i = 0; i < model::cells.size(); ++i) {
+      plt->opaque_ids().insert(i);
+    }
+    return 0;
+  }
+
+  set_errmsg("Unsupported color_by for SolidRayTracePlot");
+  return OPENMC_E_INVALID_TYPE;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_opaque(
+  int32_t index, int32_t id, bool visible)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  int32_t domain_index = -1;
+  err = map_phong_domain_id(plt, id, &domain_index);
+  if (err)
+    return err;
+
+  if (visible) {
+    plt->opaque_ids().insert(domain_index);
+  } else {
+    plt->opaque_ids().erase(domain_index);
+  }
+
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_color(
+  int32_t index, int32_t id, uint8_t r, uint8_t g, uint8_t b)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  int32_t domain_index = -1;
+  err = map_phong_domain_id(plt, id, &domain_index);
+  if (err)
+    return err;
+
+  if (domain_index < 0 ||
+      static_cast<size_t>(domain_index) >= plt->colors_.size()) {
+    set_errmsg("Color index out of range for SolidRayTracePlot");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  plt->colors_[domain_index] = RGBColor(r, g, b);
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_camera_position(
+  int32_t index, double* x, double* y, double* z)
+{
+  if (!x || !y || !z) {
+    set_errmsg("Invalid arguments passed to "
+               "openmc_solidraytrace_plot_get_camera_position");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  const auto& camera_position = plt->camera_position();
+  *x = camera_position.x;
+  *y = camera_position.y;
+  *z = camera_position.z;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_camera_position(
+  int32_t index, double x, double y, double z)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->camera_position() = {x, y, z};
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_look_at(
+  int32_t index, double* x, double* y, double* z)
+{
+  if (!x || !y || !z) {
+    set_errmsg(
+      "Invalid arguments passed to openmc_solidraytrace_plot_get_look_at");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  const auto& look_at = plt->look_at();
+  *x = look_at.x;
+  *y = look_at.y;
+  *z = look_at.z;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_look_at(
+  int32_t index, double x, double y, double z)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->look_at() = {x, y, z};
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_up(
+  int32_t index, double* x, double* y, double* z)
+{
+  if (!x || !y || !z) {
+    set_errmsg("Invalid arguments passed to openmc_solidraytrace_plot_get_up");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  const auto& up = plt->up();
+  *x = up.x;
+  *y = up.y;
+  *z = up.z;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_up(
+  int32_t index, double x, double y, double z)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->up() = {x, y, z};
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_light_position(
+  int32_t index, double* x, double* y, double* z)
+{
+  if (!x || !y || !z) {
+    set_errmsg("Invalid arguments passed to "
+               "openmc_solidraytrace_plot_get_light_position");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  const auto& light_position = plt->light_location();
+  *x = light_position.x;
+  *y = light_position.y;
+  *z = light_position.z;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_light_position(
+  int32_t index, double x, double y, double z)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->light_location() = {x, y, z};
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_fov(int32_t index, double* fov)
+{
+  if (!fov) {
+    set_errmsg("Invalid arguments passed to openmc_solidraytrace_plot_get_fov");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  *fov = plt->horizontal_field_of_view();
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_fov(int32_t index, double fov)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->horizontal_field_of_view() = fov;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_update_view(int32_t index)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  plt->update_view();
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_create_image(
+  int32_t index, uint8_t* data_out, int32_t width, int32_t height)
+{
+  if (!data_out || width <= 0 || height <= 0) {
+    set_errmsg(
+      "Invalid arguments passed to openmc_solidraytrace_plot_create_image");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  if (plt->pixels()[0] != width || plt->pixels()[1] != height) {
+    set_errmsg(
+      "Requested image size does not match SolidRayTracePlot pixel settings");
+    return OPENMC_E_INVALID_SIZE;
+  }
+
+  ImageData data = plt->create_image();
+  if (static_cast<int32_t>(data.shape()[0]) != width ||
+      static_cast<int32_t>(data.shape()[1]) != height) {
+    set_errmsg("Unexpected image size from SolidRayTracePlot create_image");
+    return OPENMC_E_INVALID_SIZE;
+  }
+
+  for (int32_t y = 0; y < height; ++y) {
+    for (int32_t x = 0; x < width; ++x) {
+      const auto& color = data(x, y);
+      size_t idx = (static_cast<size_t>(y) * width + x) * 3;
+      data_out[idx + 0] = color.red;
+      data_out[idx + 1] = color.green;
+      data_out[idx + 2] = color.blue;
+    }
+  }
+
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_color(
+  int32_t index, int32_t id, uint8_t* r, uint8_t* g, uint8_t* b)
+{
+  if (!r || !g || !b) {
+    set_errmsg(
+      "Invalid arguments passed to openmc_solidraytrace_plot_get_color");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  int32_t domain_index = -1;
+  err = map_phong_domain_id(plt, id, &domain_index);
+  if (err)
+    return err;
+
+  if (domain_index < 0 ||
+      static_cast<size_t>(domain_index) >= plt->colors_.size()) {
+    set_errmsg("Color index out of range for SolidRayTracePlot");
+    return OPENMC_E_OUT_OF_BOUNDS;
+  }
+
+  const auto& color = plt->colors_[domain_index];
+  *r = color.red;
+  *g = color.green;
+  *b = color.blue;
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_get_diffuse_fraction(
+  int32_t index, double* diffuse_fraction)
+{
+  if (!diffuse_fraction) {
+    set_errmsg("Invalid arguments passed to "
+               "openmc_solidraytrace_plot_get_diffuse_fraction");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  *diffuse_fraction = plt->diffuse_fraction();
+  return 0;
+}
+
+extern "C" int openmc_solidraytrace_plot_set_diffuse_fraction(
+  int32_t index, double diffuse_fraction)
+{
+  SolidRayTracePlot* plt = nullptr;
+  int err = get_solidraytrace_plot_by_index(index, &plt);
+  if (err)
+    return err;
+
+  if (diffuse_fraction < 0.0 || diffuse_fraction > 1.0) {
+    set_errmsg("Diffuse fraction must be between 0 and 1");
+    return OPENMC_E_INVALID_ARGUMENT;
+  }
+
+  plt->diffuse_fraction() = diffuse_fraction;
+  return 0;
+}
+
 } // namespace openmc
diff --git a/src/random_lcg.cpp b/src/random_lcg.cpp
index 29457569b..f2a81fc1c 100644
--- a/src/random_lcg.cpp
+++ b/src/random_lcg.cpp
@@ -12,6 +12,45 @@ constexpr uint64_t prn_mult {6364136223846793005ULL}; // multiplication
 constexpr uint64_t prn_add {1442695040888963407ULL};  // additive factor, c
 uint64_t prn_stride {DEFAULT_STRIDE}; // stride between particles
 
+namespace {
+
+struct SkipAheadCoefficients {
+  uint64_t multiplier;
+  uint64_t increment;
+};
+
+SkipAheadCoefficients future_seed_coefficients(uint64_t n)
+{
+  // The algorithm here to determine the parameters used to skip ahead is
+  // described in F. Brown, "Random Number Generation with Arbitrary Stride,"
+  // Trans. Am. Nucl. Soc. (Nov. 1994). This algorithm is able to skip ahead in
+  // O(log2(N)) operations instead of O(N). Basically, it computes parameters G
+  // and C which can then be used to find x_N = G*x_0 + C mod 2^M.
+
+  // Initialize constants
+  uint64_t g {prn_mult};
+  uint64_t c {prn_add};
+  uint64_t g_new {1};
+  uint64_t c_new {0};
+
+  while (n > 0) {
+    // Check if the least significant bit is 1.
+    if (n & 1) {
+      g_new *= g;
+      c_new = c_new * g + c;
+    }
+    c *= (g + 1);
+    g *= g;
+
+    // Move bits right, dropping least significant bit.
+    n >>= 1;
+  }
+
+  return {g_new, c_new};
+}
+
+} // namespace
+
 //==============================================================================
 // PRN
 //==============================================================================
@@ -69,9 +108,10 @@ uint64_t init_seed(int64_t id, int offset)
 
 void init_particle_seeds(int64_t id, uint64_t* seeds)
 {
+  auto [multiplier, increment] =
+    future_seed_coefficients(static_cast<uint64_t>(id) * prn_stride);
   for (int i = 0; i < N_STREAMS; i++) {
-    seeds[i] =
-      future_seed(static_cast<uint64_t>(id) * prn_stride, master_seed + i);
+    seeds[i] = multiplier * (master_seed + i) + increment;
   }
 }
 
@@ -90,33 +130,9 @@ void advance_prn_seed(int64_t n, uint64_t* seed)
 
 uint64_t future_seed(uint64_t n, uint64_t seed)
 {
-  // The algorithm here to determine the parameters used to skip ahead is
-  // described in F. Brown, "Random Number Generation with Arbitrary Stride,"
-  // Trans. Am. Nucl. Soc. (Nov. 1994). This algorithm is able to skip ahead in
-  // O(log2(N)) operations instead of O(N). Basically, it computes parameters G
-  // and C which can then be used to find x_N = G*x_0 + C mod 2^M.
-
-  // Initialize constants
-  uint64_t g {prn_mult};
-  uint64_t c {prn_add};
-  uint64_t g_new {1};
-  uint64_t c_new {0};
-
-  while (n > 0) {
-    // Check if the least significant bit is 1.
-    if (n & 1) {
-      g_new *= g;
-      c_new = c_new * g + c;
-    }
-    c *= (g + 1);
-    g *= g;
-
-    // Move bits right, dropping least significant bit.
-    n >>= 1;
-  }
-
   // With G and C, we can now find the new seed.
-  return g_new * seed + c_new;
+  auto [multiplier, increment] = future_seed_coefficients(n);
+  return multiplier * seed + increment;
 }
 
 //==============================================================================
diff --git a/src/random_ray/flat_source_domain.cpp b/src/random_ray/flat_source_domain.cpp
index 33651574f..83128fdaa 100644
--- a/src/random_ray/flat_source_domain.cpp
+++ b/src/random_ray/flat_source_domain.cpp
@@ -18,6 +18,7 @@
 #include "openmc/weight_windows.h"
 
 #include <cstdio>
+#include <numeric>
 
 namespace openmc {
 
@@ -29,9 +30,13 @@ namespace openmc {
 RandomRayVolumeEstimator FlatSourceDomain::volume_estimator_ {
   RandomRayVolumeEstimator::HYBRID};
 bool FlatSourceDomain::volume_normalized_flux_tallies_ {false};
-bool FlatSourceDomain::adjoint_ {false};
+bool FlatSourceDomain::adjoint_requested_ {false};
+RandomRaySolve FlatSourceDomain::solve_ {RandomRaySolve::FORWARD};
+bool FlatSourceDomain::fw_cadis_local_ {false};
+double FlatSourceDomain::diagonal_stabilization_rho_ {1.0};
 std::unordered_map<int, vector<std::pair<Source::DomainType, int>>>
   FlatSourceDomain::mesh_domain_map_;
+std::vector<size_t> FlatSourceDomain::fw_cadis_local_targets_;
 
 FlatSourceDomain::FlatSourceDomain() : negroups_(data::mg.num_energy_groups_)
 {
@@ -45,31 +50,13 @@ FlatSourceDomain::FlatSourceDomain() : negroups_(data::mg.num_energy_groups_)
       source_region_offsets_.push_back(-1);
     } else {
       source_region_offsets_.push_back(base_source_regions);
-      base_source_regions += c->n_instances_;
+      base_source_regions += c->n_instances();
     }
   }
 
   // Initialize source regions.
   bool is_linear = RandomRay::source_shape_ != RandomRaySourceShape::FLAT;
   source_regions_ = SourceRegionContainer(negroups_, is_linear);
-  source_regions_.assign(
-    base_source_regions, SourceRegion(negroups_, is_linear));
-
-  // Initialize materials
-  int64_t source_region_id = 0;
-  for (int i = 0; i < model::cells.size(); i++) {
-    Cell& cell = *model::cells[i];
-    if (cell.type_ == Fill::MATERIAL) {
-      for (int j = 0; j < cell.n_instances_; j++) {
-        source_regions_.material(source_region_id++) = cell.material(j);
-      }
-    }
-  }
-
-  // Sanity check
-  if (source_region_id != base_source_regions) {
-    fatal_error("Unexpected number of source regions");
-  }
 
   // Initialize tally volumes
   if (volume_normalized_flux_tallies_) {
@@ -80,8 +67,7 @@ FlatSourceDomain::FlatSourceDomain() : negroups_(data::mg.num_energy_groups_)
 
       // Create a new 2D tensor with the same size as the first
       // two dimensions of the 3D tensor
-      tally_volumes_[i] =
-        xt::xtensor<double, 2>::from_shape({shape[0], shape[1]});
+      tally_volumes_[i] = tensor::Tensor<double>({shape[0], shape[1]});
     }
   }
 
@@ -117,54 +103,63 @@ void FlatSourceDomain::accumulate_iteration_flux()
   }
 }
 
-// Compute new estimate of scattering + fission sources in each source region
-// based on the flux estimate from the previous iteration.
-void FlatSourceDomain::update_neutron_source(double k_eff)
+void FlatSourceDomain::update_single_neutron_source(SourceRegionHandle& srh)
 {
-  simulation::time_update_src.start();
-
-  double inverse_k_eff = 1.0 / k_eff;
-
-// Reset all source regions to zero (important for void regions)
-#pragma omp parallel for
-  for (int64_t se = 0; se < n_source_elements(); se++) {
-    source_regions_.source(se) = 0.0;
+  // Reset all source regions to zero (important for void regions)
+  for (int g = 0; g < negroups_; g++) {
+    srh.source(g) = 0.0;
   }
 
   // Add scattering + fission source
-#pragma omp parallel for
-  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
-    int material = source_regions_.material(sr);
-    if (material == MATERIAL_VOID) {
-      continue;
-    }
+  int material = srh.material();
+  int temp = srh.temperature_idx();
+  double density_mult = srh.density_mult();
+  if (material != MATERIAL_VOID) {
+    double inverse_k_eff = 1.0 / k_eff_;
+    const int material_offset = (material * ntemperature_ + temp) * negroups_;
+    const int scatter_offset =
+      (material * ntemperature_ + temp) * negroups_ * negroups_;
     for (int g_out = 0; g_out < negroups_; g_out++) {
-      double sigma_t = sigma_t_[material * negroups_ + g_out];
+      double sigma_t = sigma_t_[material_offset + g_out] * density_mult;
       double scatter_source = 0.0;
       double fission_source = 0.0;
 
       for (int g_in = 0; g_in < negroups_; g_in++) {
-        double scalar_flux = source_regions_.scalar_flux_old(sr, g_in);
+        double scalar_flux = srh.scalar_flux_old(g_in);
         double sigma_s =
-          sigma_s_[material * negroups_ * negroups_ + g_out * negroups_ + g_in];
-        double nu_sigma_f = nu_sigma_f_[material * negroups_ + g_in];
-        double chi = chi_[material * negroups_ + g_out];
+          sigma_s_[scatter_offset + g_out * negroups_ + g_in] * density_mult;
+        double nu_sigma_f = nu_sigma_f_[material_offset + g_in] * density_mult;
+        double chi = chi_[material_offset + g_out];
 
         scatter_source += sigma_s * scalar_flux;
-        fission_source += nu_sigma_f * scalar_flux * chi;
+        if (settings::create_fission_neutrons) {
+          fission_source += nu_sigma_f * scalar_flux * chi;
+        }
       }
-      source_regions_.source(sr, g_out) =
+      srh.source(g_out) =
         (scatter_source + fission_source * inverse_k_eff) / sigma_t;
     }
   }
 
   // Add external source if in fixed source mode
   if (settings::run_mode == RunMode::FIXED_SOURCE) {
-#pragma omp parallel for
-    for (int64_t se = 0; se < n_source_elements(); se++) {
-      source_regions_.source(se) += source_regions_.external_source(se);
+    for (int g = 0; g < negroups_; g++) {
+      srh.source(g) += srh.external_source(g);
     }
   }
+}
+
+// Compute new estimate of scattering + fission sources in each source region
+// based on the flux estimate from the previous iteration.
+void FlatSourceDomain::update_all_neutron_sources()
+{
+  simulation::time_update_src.start();
+
+#pragma omp parallel for
+  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
+    SourceRegionHandle srh = source_regions_.get_source_region_handle(sr);
+    update_single_neutron_source(srh);
+  }
 
   simulation::time_update_src.stop();
 }
@@ -203,13 +198,18 @@ void FlatSourceDomain::set_flux_to_flux_plus_source(
   int64_t sr, double volume, int g)
 {
   int material = source_regions_.material(sr);
+  int temp = source_regions_.temperature_idx(sr);
   if (material == MATERIAL_VOID) {
     source_regions_.scalar_flux_new(sr, g) /= volume;
-    source_regions_.scalar_flux_new(sr, g) +=
-      0.5f * source_regions_.external_source(sr, g) *
-      source_regions_.volume_sq(sr);
+    if (settings::run_mode == RunMode::FIXED_SOURCE) {
+      source_regions_.scalar_flux_new(sr, g) +=
+        0.5f * source_regions_.external_source(sr, g) *
+        source_regions_.volume_sq(sr);
+    }
   } else {
-    double sigma_t = sigma_t_[source_regions_.material(sr) * negroups_ + g];
+    double sigma_t =
+      sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+      source_regions_.density_mult(sr);
     source_regions_.scalar_flux_new(sr, g) /= (sigma_t * volume);
     source_regions_.scalar_flux_new(sr, g) += source_regions_.source(sr, g);
   }
@@ -295,12 +295,19 @@ int64_t FlatSourceDomain::add_source_to_scalar_flux()
         // in the cell we will use the previous iteration's flux estimate. This
         // injects a small degree of correlation into the simulation, but this
         // is going to be trivial when the miss rate is a few percent or less.
-        if (source_regions_.external_source_present(sr) && !adjoint_) {
+        if (source_regions_.external_source_present(sr)) {
           set_flux_to_old_flux(sr, g);
         } else {
           set_flux_to_source(sr, g);
         }
       }
+      // Halt if NaN implosion is detected
+      if (!std::isfinite(source_regions_.scalar_flux_new(sr, g))) {
+        fatal_error("A source region scalar flux is not finite. "
+                    "This indicates a numerical instability in the "
+                    "simulation. Consider increasing ray density or adjusting "
+                    "the source region mesh.");
+      }
     }
   }
 
@@ -310,7 +317,7 @@ int64_t FlatSourceDomain::add_source_to_scalar_flux()
 
 // Generates new estimate of k_eff based on the differences between this
 // iteration's estimate of the scalar flux and the last iteration's estimate.
-double FlatSourceDomain::compute_k_eff(double k_eff_old) const
+void FlatSourceDomain::compute_k_eff()
 {
   double fission_rate_old = 0;
   double fission_rate_new = 0;
@@ -328,6 +335,7 @@ double FlatSourceDomain::compute_k_eff(double k_eff_old) const
     }
 
     int material = source_regions_.material(sr);
+    int temp = source_regions_.temperature_idx(sr);
     if (material == MATERIAL_VOID) {
       continue;
     }
@@ -336,7 +344,9 @@ double FlatSourceDomain::compute_k_eff(double k_eff_old) const
     double sr_fission_source_new = 0;
 
     for (int g = 0; g < negroups_; g++) {
-      double nu_sigma_f = nu_sigma_f_[material * negroups_ + g];
+      double nu_sigma_f =
+        nu_sigma_f_[(material * ntemperature_ + temp) * negroups_ + g] *
+        source_regions_.density_mult(sr);
       sr_fission_source_old +=
         nu_sigma_f * source_regions_.scalar_flux_old(sr, g);
       sr_fission_source_new +=
@@ -355,7 +365,7 @@ double FlatSourceDomain::compute_k_eff(double k_eff_old) const
     p[sr] = sr_fission_source_new;
   }
 
-  double k_eff_new = k_eff_old * (fission_rate_new / fission_rate_old);
+  double k_eff_new = k_eff_ * (fission_rate_new / fission_rate_old);
 
   double H = 0.0;
   // defining an inverse sum for better performance
@@ -375,7 +385,8 @@ double FlatSourceDomain::compute_k_eff(double k_eff_old) const
   // Adds entropy value to shared entropy vector in openmc namespace.
   simulation::entropy.push_back(H);
 
-  return k_eff_new;
+  fission_rate_ = fission_rate_new;
+  k_eff_ = k_eff_new;
 }
 
 // This function is responsible for generating a mapping between random
@@ -411,7 +422,12 @@ double FlatSourceDomain::compute_k_eff(double k_eff_old) const
 // be passed back to the caller to alert them that this function doesn't
 // need to be called for the remainder of the simulation.
 
-void FlatSourceDomain::convert_source_regions_to_tallies()
+// It takes as an argument the starting index in the source region array,
+// and it will operate from that index until the end of the array. This
+// is useful as it can be called for both explicit user source regions or
+// when a source region mesh is overlaid.
+
+void FlatSourceDomain::convert_source_regions_to_tallies(int64_t start_sr_id)
 {
   openmc::simulation::time_tallies.start();
 
@@ -420,7 +436,7 @@ void FlatSourceDomain::convert_source_regions_to_tallies()
 
 // Attempt to generate mapping for all source regions
 #pragma omp parallel for
-  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
+  for (int64_t sr = start_sr_id; sr < n_source_regions(); sr++) {
 
     // If this source region has not been hit by a ray yet, then
     // we aren't going to be able to map it, so skip it.
@@ -458,7 +474,7 @@ void FlatSourceDomain::convert_source_regions_to_tallies()
       // Loop over all active tallies. This logic is essentially identical
       // to what happens when scanning for applicable tallies during
       // MC transport.
-      for (auto i_tally : model::active_tallies) {
+      for (int i_tally = 0; i_tally < model::tallies.size(); i_tally++) {
         Tally& tally {*model::tallies[i_tally]};
 
         // Initialize an iterator over valid filter bin combinations.
@@ -520,18 +536,40 @@ void FlatSourceDomain::reset_tally_volumes()
 // simulation
 double FlatSourceDomain::compute_fixed_source_normalization_factor() const
 {
-  // If we are not in fixed source mode, then there are no external sources
-  // so no normalization is needed.
-  if (settings::run_mode != RunMode::FIXED_SOURCE || adjoint_) {
+  // Eigenvalue mode normalization
+  if (settings::run_mode == RunMode::EIGENVALUE) {
+    // Normalize fluxes by total number of fission neutrons produced. This
+    // ensures consistent scaling of the eigenvector such that its magnitude is
+    // comparable to the eigenvector produced by the Monte Carlo solver.
+    // Multiplying by the eigenvalue is unintuitive, but it is necessary.
+    // If the eigenvalue is 1.2, per starting source neutron, you will
+    // generate 1.2 neutrons. Thus if we normalize to generating only ONE
+    // neutron in total for the whole domain, then we don't actually have enough
+    // flux to generate the required 1.2 neutrons. We only know the flux
+    // required to generate 1 neutron (which would have required less than one
+    // starting neutron). Thus, you have to scale the flux up by the eigenvalue
+    // such that 1.2 neutrons are generated, so as to be consistent with the
+    // bookkeeping in MC which is all done per starting source neutron (not per
+    // neutron produced).
+    return k_eff_ / (fission_rate_ * simulation_volume_);
+  }
+
+  // If we are in adjoint mode of a fixed source problem, the external
+  // source is already normalized, such that all resulting fluxes are
+  // also normalized.
+  if (solve_ == RandomRaySolve::ADJOINT) {
     return 1.0;
   }
 
+  // Fixed source mode normalization
+
   // Step 1 is to sum over all source regions and energy groups to get the
   // total external source strength in the simulation.
   double simulation_external_source_strength = 0.0;
 #pragma omp parallel for reduction(+ : simulation_external_source_strength)
   for (int64_t sr = 0; sr < n_source_regions(); sr++) {
     int material = source_regions_.material(sr);
+    int temp = source_regions_.temperature_idx(sr);
     double volume = source_regions_.volume(sr) * simulation_volume_;
     for (int g = 0; g < negroups_; g++) {
       // For non-void regions, we store the external source pre-divided by
@@ -539,7 +577,8 @@ double FlatSourceDomain::compute_fixed_source_normalization_factor() const
       // to get the total source strength in the expected units.
       double sigma_t = 1.0;
       if (material != MATERIAL_VOID) {
-        sigma_t = sigma_t_[material * negroups_ + g];
+        sigma_t = sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+                  source_regions_.density_mult(sr);
       }
       simulation_external_source_strength +=
         source_regions_.external_source(sr, g) * sigma_t * volume;
@@ -596,6 +635,8 @@ void FlatSourceDomain::random_ray_tally()
     double volume = source_regions_.volume(sr) * simulation_volume_;
 
     double material = source_regions_.material(sr);
+    int temp = source_regions_.temperature_idx(sr);
+    double density_mult = source_regions_.density_mult(sr);
     for (int g = 0; g < negroups_; g++) {
       double flux =
         source_regions_.scalar_flux_new(sr, g) * source_normalization_factor;
@@ -611,19 +652,28 @@ void FlatSourceDomain::random_ray_tally()
 
         case SCORE_TOTAL:
           if (material != MATERIAL_VOID) {
-            score = flux * volume * sigma_t_[material * negroups_ + g];
+            score =
+              flux * volume *
+              sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+              density_mult;
           }
           break;
 
         case SCORE_FISSION:
           if (material != MATERIAL_VOID) {
-            score = flux * volume * sigma_f_[material * negroups_ + g];
+            score =
+              flux * volume *
+              sigma_f_[(material * ntemperature_ + temp) * negroups_ + g] *
+              density_mult;
           }
           break;
 
         case SCORE_NU_FISSION:
           if (material != MATERIAL_VOID) {
-            score = flux * volume * nu_sigma_f_[material * negroups_ + g];
+            score =
+              flux * volume *
+              nu_sigma_f_[(material * ntemperature_ + temp) * negroups_ + g] *
+              density_mult;
           }
           break;
 
@@ -631,13 +681,19 @@ void FlatSourceDomain::random_ray_tally()
           score = 1.0;
           break;
 
+        case SCORE_KAPPA_FISSION:
+          score =
+            flux * volume *
+            kappa_fission_[(material * ntemperature_ + temp) * negroups_ + g] *
+            density_mult;
+          break;
+
         default:
           fatal_error("Invalid score specified in tallies.xml. Only flux, "
-                      "total, fission, nu-fission, and events are supported in "
-                      "random ray mode.");
+                      "total, fission, nu-fission, kappa-fission, and events "
+                      "are supported in random ray mode.");
           break;
         }
-
         // Apply score to the appropriate tally bin
         Tally& tally {*model::tallies[task.tally_idx]};
 #pragma omp atomic
@@ -669,7 +725,7 @@ void FlatSourceDomain::random_ray_tally()
     for (int i = 0; i < model::tallies.size(); i++) {
       Tally& tally {*model::tallies[i]};
 #pragma omp parallel for
-      for (int bin = 0; bin < tally.n_filter_bins(); bin++) {
+      for (int64_t bin = 0; bin < tally.n_filter_bins(); bin++) {
         for (int score_idx = 0; score_idx < tally.n_scores(); score_idx++) {
           auto score_type = tally.scores_[score_idx];
           if (score_type == SCORE_FLUX) {
@@ -711,21 +767,21 @@ void FlatSourceDomain::output_to_vtk() const
   print_plot();
 
   // Outer loop over plots
-  for (int p = 0; p < model::plots.size(); p++) {
+  for (int plt = 0; plt < model::plots.size(); plt++) {
 
     // Get handle to OpenMC plot object and extract params
-    Plot* openmc_plot = dynamic_cast<Plot*>(model::plots[p].get());
+    Plot* openmc_plot = dynamic_cast<Plot*>(model::plots[plt].get());
 
     // Random ray plots only support voxel plots
     if (!openmc_plot) {
       warning(fmt::format("Plot {} is invalid plot type -- only voxel plotting "
                           "is allowed in random ray mode.",
-        p));
+        plt));
       continue;
     } else if (openmc_plot->type_ != Plot::PlotType::voxel) {
       warning(fmt::format("Plot {} is invalid plot type -- only voxel plotting "
                           "is allowed in random ray mode.",
-        p));
+        plt));
       continue;
     }
 
@@ -740,6 +796,12 @@ void FlatSourceDomain::output_to_vtk() const
     double z_delta = width.z / Nz;
     std::string filename = openmc_plot->path_plot();
 
+    // Tag plots written during the forward solve of an adjoint run
+    if (solve_ == RandomRaySolve::FORWARD_FOR_ADJOINT) {
+      auto dot = filename.find_last_of('.');
+      filename = filename.substr(0, dot) + ".forward" + filename.substr(dot);
+    }
+
     // Perform sanity checks on file size
     uint64_t bytes = Nx * Ny * Nz * (negroups_ + 1 + 1 + 1) * sizeof(float);
     write_message(5, "Processing plot {}: {}... (Estimated size is {} MB)",
@@ -779,30 +841,18 @@ void FlatSourceDomain::output_to_vtk() const
             continue;
           }
 
-          int i_cell = p.lowest_coord().cell;
-          int64_t sr = source_region_offsets_[i_cell] + p.cell_instance();
-          if (RandomRay::mesh_subdivision_enabled_) {
-            int mesh_idx = base_source_regions_.mesh(sr);
-            int mesh_bin;
-            if (mesh_idx == C_NONE) {
-              mesh_bin = 0;
-            } else {
-              mesh_bin = model::meshes[mesh_idx]->get_bin(p.r());
-            }
-            SourceRegionKey sr_key {sr, mesh_bin};
-            auto it = source_region_map_.find(sr_key);
-            if (it != source_region_map_.end()) {
-              sr = it->second;
-            } else {
-              sr = -1;
-            }
+          SourceRegionKey sr_key = lookup_source_region_key(p);
+          int64_t sr = -1;
+          auto it = source_region_map_.find(sr_key);
+          if (it != source_region_map_.end()) {
+            sr = it->second;
           }
 
           voxel_indices[z * Ny * Nx + y * Nx + x] = sr;
           voxel_positions[z * Ny * Nx + y * Nx + x] = sample;
 
           if (variance_reduction::weight_windows.size() == 1) {
-            WeightWindow ww =
+            auto [ww_found, ww] =
               variance_reduction::weight_windows[0]->get_weight_window(p);
             float weight = ww.lower_weight;
             weight_windows[z * Ny * Nx + y * Nx + x] = weight;
@@ -899,11 +949,14 @@ void FlatSourceDomain::output_to_vtk() const
         float total_fission = 0.0;
         if (fsr >= 0) {
           int mat = source_regions_.material(fsr);
+          int temp = source_regions_.temperature_idx(fsr);
           if (mat != MATERIAL_VOID) {
             for (int g = 0; g < negroups_; g++) {
               int64_t source_element = fsr * negroups_ + g;
               float flux = evaluate_flux_at_point(voxel_positions[i], fsr, g);
-              double sigma_f = sigma_f_[mat * negroups_ + g];
+              double sigma_f =
+                sigma_f_[(mat * ntemperature_ + temp) * negroups_ + g] *
+                source_regions_.density_mult(fsr);
               total_fission += sigma_f * flux;
             }
           }
@@ -916,10 +969,19 @@ void FlatSourceDomain::output_to_vtk() const
       std::fprintf(plot, "LOOKUP_TABLE default\n");
       for (int i = 0; i < Nx * Ny * Nz; i++) {
         int64_t fsr = voxel_indices[i];
+        int mat = source_regions_.material(fsr);
+        int temp = source_regions_.temperature_idx(fsr);
         float total_external = 0.0f;
         if (fsr >= 0) {
           for (int g = 0; g < negroups_; g++) {
-            total_external += source_regions_.external_source(fsr, g);
+            // External sources are already divided by sigma_t, so we need to
+            // multiply it back to get the true external source.
+            double sigma_t = 1.0;
+            if (mat != MATERIAL_VOID) {
+              sigma_t = sigma_t_[(mat * ntemperature_ + temp) * negroups_ + g] *
+                        source_regions_.density_mult(fsr);
+            }
+            total_external += source_regions_.external_source(fsr, g) * sigma_t;
           }
         }
         total_external = convert_to_big_endian<float>(total_external);
@@ -945,23 +1007,28 @@ void FlatSourceDomain::output_to_vtk() const
 }
 
 void FlatSourceDomain::apply_external_source_to_source_region(
-  Discrete* discrete, double strength_factor, int64_t sr)
+  int src_idx, SourceRegionHandle& srh)
 {
-  source_regions_.external_source_present(sr) = 1;
-
+  auto s =
+    (solve_ == RandomRaySolve::ADJOINT && !model::adjoint_sources.empty())
+      ? model::adjoint_sources[src_idx].get()
+      : model::external_sources[src_idx].get();
+  auto is = dynamic_cast<IndependentSource*>(s);
+  auto discrete = dynamic_cast<Discrete*>(is->energy());
+  double strength_factor = is->strength();
   const auto& discrete_energies = discrete->x();
   const auto& discrete_probs = discrete->prob();
 
+  srh.external_source_present() = 1;
+
   for (int i = 0; i < discrete_energies.size(); i++) {
     int g = data::mg.get_group_index(discrete_energies[i]);
-    source_regions_.external_source(sr, g) +=
-      discrete_probs[i] * strength_factor;
+    srh.external_source(g) += discrete_probs[i] * strength_factor;
   }
 }
 
 void FlatSourceDomain::apply_external_source_to_cell_instances(int32_t i_cell,
-  Discrete* discrete, double strength_factor, int target_material_id,
-  const vector<int32_t>& instances)
+  int src_idx, int target_material_id, const vector<int32_t>& instances)
 {
   Cell& cell = *model::cells[i_cell];
 
@@ -979,30 +1046,28 @@ void FlatSourceDomain::apply_external_source_to_cell_instances(int32_t i_cell,
     if (target_material_id == C_NONE ||
         cell_material_id == target_material_id) {
       int64_t source_region = source_region_offsets_[i_cell] + j;
-      apply_external_source_to_source_region(
-        discrete, strength_factor, source_region);
+      external_volumetric_source_map_[source_region].push_back(src_idx);
     }
   }
 }
 
 void FlatSourceDomain::apply_external_source_to_cell_and_children(
-  int32_t i_cell, Discrete* discrete, double strength_factor,
-  int32_t target_material_id)
+  int32_t i_cell, int src_idx, int32_t target_material_id)
 {
   Cell& cell = *model::cells[i_cell];
 
   if (cell.type_ == Fill::MATERIAL) {
-    vector<int> instances(cell.n_instances_);
+    vector<int> instances(cell.n_instances());
     std::iota(instances.begin(), instances.end(), 0);
     apply_external_source_to_cell_instances(
-      i_cell, discrete, strength_factor, target_material_id, instances);
+      i_cell, src_idx, target_material_id, instances);
   } else if (target_material_id == C_NONE) {
     std::unordered_map<int32_t, vector<int32_t>> cell_instance_list =
       cell.get_contained_cells(0, nullptr);
     for (const auto& pair : cell_instance_list) {
       int32_t i_child_cell = pair.first;
-      apply_external_source_to_cell_instances(i_child_cell, discrete,
-        strength_factor, target_material_id, pair.second);
+      apply_external_source_to_cell_instances(
+        i_child_cell, src_idx, target_material_id, pair.second);
     }
   }
 }
@@ -1018,55 +1083,77 @@ void FlatSourceDomain::count_external_source_regions()
   }
 }
 
-void FlatSourceDomain::convert_external_sources()
+void FlatSourceDomain::convert_external_sources(bool use_adjoint_sources)
 {
+  // Determine whether forward or (local) adjoint sources are desired
+  const auto& sources =
+    use_adjoint_sources ? model::adjoint_sources : model::external_sources;
+
   // Loop over external sources
-  for (int es = 0; es < model::external_sources.size(); es++) {
-    Source* s = model::external_sources[es].get();
+  for (int es = 0; es < sources.size(); es++) {
+
+    // Extract source information
+    Source* s = sources[es].get();
     IndependentSource* is = dynamic_cast<IndependentSource*>(s);
     Discrete* energy = dynamic_cast<Discrete*>(is->energy());
     const std::unordered_set<int32_t>& domain_ids = is->domain_ids();
-
     double strength_factor = is->strength();
 
-    if (is->domain_type() == Source::DomainType::MATERIAL) {
-      for (int32_t material_id : domain_ids) {
-        for (int i_cell = 0; i_cell < model::cells.size(); i_cell++) {
-          apply_external_source_to_cell_and_children(
-            i_cell, energy, strength_factor, material_id);
+    // If there is no domain constraint specified, then this must be a point
+    // source. In this case, we need to find the source region that contains the
+    // point source and apply or relate it to the external source.
+    if (is->domain_ids().size() == 0) {
+
+      // Extract the point source coordinate and find the base source region at
+      // that point
+      auto sp = dynamic_cast<SpatialPoint*>(is->space());
+      GeometryState gs;
+      gs.r() = sp->r();
+      gs.r_last() = sp->r();
+      gs.u() = {1.0, 0.0, 0.0};
+      bool found = exhaustive_find_cell(gs);
+      if (!found) {
+        fatal_error(fmt::format("Could not find cell containing external "
+                                "point source at {}",
+          sp->r()));
+      }
+      SourceRegionKey key = lookup_source_region_key(gs);
+
+      // With the source region and mesh bin known, we can use the
+      // accompanying SourceRegionKey as a key into a map that stores the
+      // corresponding external source index for the point source. Notably, we
+      // do not actually apply the external source to any source regions here,
+      // as if mesh subdivision is enabled, they haven't actually been
+      // discovered & initilized yet. When discovered, they will read from the
+      // external_source_map to determine if there are any external source
+      // terms that should be applied.
+      external_point_source_map_[key].push_back(es);
+
+    } else {
+      // If not a point source, then use the volumetric domain constraints to
+      // determine which source regions to apply the external source to.
+      if (is->domain_type() == Source::DomainType::MATERIAL) {
+        for (int32_t material_id : domain_ids) {
+          for (int i_cell = 0; i_cell < model::cells.size(); i_cell++) {
+            apply_external_source_to_cell_and_children(i_cell, es, material_id);
+          }
         }
-      }
-    } else if (is->domain_type() == Source::DomainType::CELL) {
-      for (int32_t cell_id : domain_ids) {
-        int32_t i_cell = model::cell_map[cell_id];
-        apply_external_source_to_cell_and_children(
-          i_cell, energy, strength_factor, C_NONE);
-      }
-    } else if (is->domain_type() == Source::DomainType::UNIVERSE) {
-      for (int32_t universe_id : domain_ids) {
-        int32_t i_universe = model::universe_map[universe_id];
-        Universe& universe = *model::universes[i_universe];
-        for (int32_t i_cell : universe.cells_) {
-          apply_external_source_to_cell_and_children(
-            i_cell, energy, strength_factor, C_NONE);
+      } else if (is->domain_type() == Source::DomainType::CELL) {
+        for (int32_t cell_id : domain_ids) {
+          int32_t i_cell = model::cell_map[cell_id];
+          apply_external_source_to_cell_and_children(i_cell, es, C_NONE);
+        }
+      } else if (is->domain_type() == Source::DomainType::UNIVERSE) {
+        for (int32_t universe_id : domain_ids) {
+          int32_t i_universe = model::universe_map[universe_id];
+          Universe& universe = *model::universes[i_universe];
+          for (int32_t i_cell : universe.cells_) {
+            apply_external_source_to_cell_and_children(i_cell, es, C_NONE);
+          }
         }
       }
     }
   } // End loop over external sources
-
-// Divide the fixed source term by sigma t (to save time when applying each
-// iteration)
-#pragma omp parallel for
-  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
-    int material = source_regions_.material(sr);
-    if (material == MATERIAL_VOID) {
-      continue;
-    }
-    for (int g = 0; g < negroups_; g++) {
-      double sigma_t = sigma_t_[material * negroups_ + g];
-      source_regions_.external_source(sr, g) /= sigma_t;
-    }
-  }
 }
 
 void FlatSourceDomain::flux_swap()
@@ -1078,68 +1165,142 @@ void FlatSourceDomain::flatten_xs()
 {
   // Temperature and angle indices, if using multiple temperature
   // data sets and/or anisotropic data sets.
-  // TODO: Currently assumes we are only using single temp/single angle data.
-  const int t = 0;
+  // TODO: Currently assumes we are only using single angle data.
   const int a = 0;
 
   n_materials_ = data::mg.macro_xs_.size();
-  for (auto& m : data::mg.macro_xs_) {
-    for (int g_out = 0; g_out < negroups_; g_out++) {
-      if (m.exists_in_model) {
-        double sigma_t =
-          m.get_xs(MgxsType::TOTAL, g_out, NULL, NULL, NULL, t, a);
-        sigma_t_.push_back(sigma_t);
+  ntemperature_ = 1;
+  for (int i = 0; i < n_materials_; i++) {
+    ntemperature_ =
+      std::max(ntemperature_, data::mg.macro_xs_[i].n_temperature_points());
+  }
 
-        double nu_Sigma_f =
-          m.get_xs(MgxsType::NU_FISSION, g_out, NULL, NULL, NULL, t, a);
-        nu_sigma_f_.push_back(nu_Sigma_f);
+  for (int i = 0; i < n_materials_; i++) {
+    auto& m = data::mg.macro_xs_[i];
+    for (int t = 0; t < ntemperature_; t++) {
+      for (int g_out = 0; g_out < negroups_; g_out++) {
+        if (m.exists_in_model && t < m.n_temperature_points()) {
+          double sigma_t =
+            m.get_xs(MgxsType::TOTAL, g_out, NULL, NULL, NULL, t, a);
+          sigma_t_.push_back(sigma_t);
 
-        double sigma_f =
-          m.get_xs(MgxsType::FISSION, g_out, NULL, NULL, NULL, t, a);
-        sigma_f_.push_back(sigma_f);
+          if (sigma_t < MINIMUM_MACRO_XS) {
+            Material* mat = model::materials[i].get();
+            warning(fmt::format(
+              "Material \"{}\" (id: {}) has a group {} total cross section "
+              "({:.3e}) below the minimum threshold "
+              "({:.3e}). Material will be treated as pure void.",
+              mat->name(), mat->id(), g_out, sigma_t, MINIMUM_MACRO_XS));
+          }
 
-        double chi =
-          m.get_xs(MgxsType::CHI_PROMPT, g_out, &g_out, NULL, NULL, t, a);
-        chi_.push_back(chi);
+          double nu_sigma_f =
+            m.get_xs(MgxsType::NU_FISSION, g_out, NULL, NULL, NULL, t, a);
+          nu_sigma_f_.push_back(nu_sigma_f);
 
-        for (int g_in = 0; g_in < negroups_; g_in++) {
-          double sigma_s =
-            m.get_xs(MgxsType::NU_SCATTER, g_in, &g_out, NULL, NULL, t, a);
-          sigma_s_.push_back(sigma_s);
-        }
-      } else {
-        sigma_t_.push_back(0);
-        nu_sigma_f_.push_back(0);
-        sigma_f_.push_back(0);
-        chi_.push_back(0);
-        for (int g_in = 0; g_in < negroups_; g_in++) {
-          sigma_s_.push_back(0);
+          double sigma_f =
+            m.get_xs(MgxsType::FISSION, g_out, NULL, NULL, NULL, t, a);
+          sigma_f_.push_back(sigma_f);
+
+          double chi =
+            m.get_xs(MgxsType::CHI_PROMPT, g_out, &g_out, NULL, NULL, t, a);
+          if (!std::isfinite(chi)) {
+            // MGXS interface may return NaN in some cases, such as when
+            // material is fissionable but has very small sigma_f.
+            chi = 0.0;
+          }
+          chi_.push_back(chi);
+
+          double kappa_fission =
+            m.get_xs(MgxsType::KAPPA_FISSION, g_out, NULL, NULL, NULL, t, a);
+          kappa_fission_.push_back(kappa_fission);
+
+          for (int g_in = 0; g_in < negroups_; g_in++) {
+            double sigma_s =
+              m.get_xs(MgxsType::NU_SCATTER, g_in, &g_out, NULL, NULL, t, a);
+            sigma_s_.push_back(sigma_s);
+            // For transport corrected XS data, diagonal elements may be
+            // negative. In this case, set a flag to enable transport
+            // stabilization for the simulation.
+            if (g_out == g_in && sigma_s < 0.0)
+              is_transport_stabilization_needed_ = true;
+          }
+        } else {
+          sigma_t_.push_back(0);
+          nu_sigma_f_.push_back(0);
+          sigma_f_.push_back(0);
+          chi_.push_back(0);
+          kappa_fission_.push_back(0);
+          for (int g_in = 0; g_in < negroups_; g_in++) {
+            sigma_s_.push_back(0);
+          }
         }
       }
     }
   }
 }
 
-void FlatSourceDomain::set_adjoint_sources(const vector<double>& forward_flux)
+void FlatSourceDomain::set_fw_adjoint_sources()
 {
-  // Set the external source to 1/forward_flux. If the forward flux is negative
-  // or zero, set the adjoint source to zero, as this is likely a very small
-  // source region that we don't need to bother trying to vector particles
-  // towards. Flux negativity in random ray is not related to the flux being
-  // small in magnitude, but rather due to the source region being physically
-  // small in volume and thus having a noisy flux estimate.
+  // Set the adjoint external source to 1/forward_flux. If the forward flux is
+  // negative, zero, or extremely close to zero, set the adjoint source to zero,
+  // as this is likely a very small source region that we don't need to bother
+  // trying to vector particles towards. In the case of flux "being extremely
+  // close to zero", we define this as being a fixed fraction of the maximum
+  // forward flux, below which we assume the flux would be physically
+  // undetectable.
+
+  // First, find the maximum forward flux value
+  double max_flux = 0.0;
+#pragma omp parallel for reduction(max : max_flux)
+  for (int64_t se = 0; se < n_source_elements(); se++) {
+    double flux = source_regions_.scalar_flux_final(se);
+    if (flux > max_flux) {
+      max_flux = flux;
+    }
+  }
+
+  // Then, compute the adjoint source for each source region
 #pragma omp parallel for
   for (int64_t sr = 0; sr < n_source_regions(); sr++) {
     for (int g = 0; g < negroups_; g++) {
-      double flux = forward_flux[sr * negroups_ + g];
-      if (flux <= 0.0) {
+      double flux = source_regions_.scalar_flux_final(sr, g);
+      if (flux <= ZERO_FLUX_CUTOFF * max_flux) {
         source_regions_.external_source(sr, g) = 0.0;
       } else {
         source_regions_.external_source(sr, g) = 1.0 / flux;
+        if (!std::isfinite(source_regions_.external_source(sr, g))) {
+          // If the flux is NaN or Inf, set the adjoint source to zero
+          source_regions_.external_source(sr, g) = 0.0;
+        }
       }
       if (flux > 0.0) {
         source_regions_.external_source_present(sr) = 1;
       }
+      source_regions_.scalar_flux_final(sr, g) = 0.0;
+    }
+  }
+
+  // "Small" source regions in OpenMC are defined as those that are hit by
+  // MIN_HITS_PER_BATCH rays or fewer each batch. These regions typically have
+  // very small volumes combined with a low aspect ratio, and are often
+  // generated when applying a source region mesh that clips the edge of a
+  // curved surface. As perhaps only a few rays will visit these regions over
+  // the entire forward simulation, the forward flux estimates are extremely
+  // noisy and unreliable. In some cases, the noise may make the forward fluxes
+  // extremely low, leading to unphysically large adjoint source terms,
+  // resulting in weight windows that aggressively try to drive particles
+  // towards these regions. To fix this, we simply filter out any "small" source
+  // regions from consideration. If a source region is "small", we
+  // set its adjoint source to zero. This adds negligible bias to the adjoint
+  // flux solution, as the true total adjoint source contribution from small
+  // regions is likely to be negligible.
+#pragma omp parallel for
+  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
+    if (source_regions_.is_small(sr)) {
+      for (int g = 0; g < negroups_; g++) {
+        source_regions_.external_source(sr, g) = 0.0;
+      }
+      source_regions_.external_source_present(sr) = 0;
     }
   }
 
@@ -1148,29 +1309,113 @@ void FlatSourceDomain::set_adjoint_sources(const vector<double>& forward_flux)
 #pragma omp parallel for
   for (int64_t sr = 0; sr < n_source_regions(); sr++) {
     int material = source_regions_.material(sr);
+    int temp = source_regions_.temperature_idx(sr);
     if (material == MATERIAL_VOID) {
       continue;
     }
     for (int g = 0; g < negroups_; g++) {
-      double sigma_t = sigma_t_[material * negroups_ + g];
+      double sigma_t =
+        sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+        source_regions_.density_mult(sr);
       source_regions_.external_source(sr, g) /= sigma_t;
+      if (!std::isfinite(source_regions_.external_source(sr, g))) {
+        // If the flux is NaN or Inf, set the adjoint source to zero
+        source_regions_.external_source(sr, g) = 0.0;
+      }
     }
   }
+
+  if (fw_cadis_local_) {
+// Only external sources that have a non-mesh type tally task should remain
+// non-zero. Everything else gets zero'd out.
+#pragma omp parallel for
+    for (int64_t sr = 0; sr < n_source_regions(); sr++) {
+
+      // If there is already no external source, don't need to do anything
+      if (source_regions_.external_source_present(sr) == 0) {
+        continue;
+      }
+
+      // If there is an adjoint source term here, then we need to check it.
+
+      // We will track if ANY group has a valid local FW-CADIS source term
+      bool has_any_sources = false;
+
+      // Now, loop over groups
+      for (int g = 0; g < negroups_; g++) {
+
+        // If there are no tally tasks associated with this source element
+        // then it is not a local FW-CADIS source, so we continue to the next
+        // group
+        if (source_regions_.tally_task(sr, g).empty()) {
+          source_regions_.external_source(sr, g) = 0.0;
+          continue;
+        }
+
+        // If there are tally tasks, we can through them and check if
+        // any of them are local FW-CADIS targets.
+
+        // We track if ANY of the tasks are local FW-CADIS target tallies
+        bool local_fw_cadis_target_region = false;
+
+        // Now we loop through
+        for (const auto& task : source_regions_.tally_task(sr, g)) {
+          Tally& tally {*model::tallies[task.tally_idx]};
+          const auto t_id = tally.id();
+
+          // Search for target tallies
+          if (std::find(fw_cadis_local_targets_.begin(),
+                fw_cadis_local_targets_.end(),
+                t_id) != fw_cadis_local_targets_.end()) {
+            local_fw_cadis_target_region = true;
+            break;
+          }
+        }
+
+        // If ANY of the tasks is a local FW-CADIS target,
+        // Then we keep the source term and set that this
+        // source region has a valid FW-CADIS source term.
+        // Otherwise, we zero out the source term.
+        if (local_fw_cadis_target_region) {
+          has_any_sources = true;
+        } else {
+          source_regions_.external_source(sr, g) = 0.0;
+        }
+      } // End loop over groups
+
+      // If there were any valid FW-CADIS source terms for any
+      // of the groups, then the SR as a whole counts as a source
+      if (has_any_sources) {
+        source_regions_.external_source_present(sr) = 1;
+      } else {
+        source_regions_.external_source_present(sr) = 0;
+      }
+    } // End loop over source regions
+  } // End local FW-CADIS logic
+}
+
+void FlatSourceDomain::set_local_adjoint_sources()
+{
+  // Set the external source to user-specified adjoint sources.
+  convert_external_sources(true);
 }
 
 void FlatSourceDomain::transpose_scattering_matrix()
 {
   // Transpose the inner two dimensions for each material
+#pragma omp parallel for
   for (int m = 0; m < n_materials_; ++m) {
-    int material_offset = m * negroups_ * negroups_;
-    for (int i = 0; i < negroups_; ++i) {
-      for (int j = i + 1; j < negroups_; ++j) {
-        // Calculate indices of the elements to swap
-        int idx1 = material_offset + i * negroups_ + j;
-        int idx2 = material_offset + j * negroups_ + i;
+    for (int t = 0; t < ntemperature_; t++) {
+      int material_offset = (m * ntemperature_ + t) * negroups_ * negroups_;
+      for (int i = 0; i < negroups_; ++i) {
+        for (int j = i + 1; j < negroups_; ++j) {
+          // Calculate indices of the elements to swap
+          int idx1 = material_offset + i * negroups_ + j;
+          int idx2 = material_offset + j * negroups_ + i;
 
-        // Swap the elements to transpose the matrix
-        std::swap(sigma_s_[idx1], sigma_s_[idx2]);
+          // Swap the elements to transpose the matrix
+          std::swap(sigma_s_[idx1], sigma_s_[idx2]);
+        }
       }
     }
   }
@@ -1203,13 +1448,14 @@ void FlatSourceDomain::apply_mesh_to_cell_instances(int32_t i_cell,
     if ((target_material_id == C_NONE && !is_target_void) ||
         cell_material_id == target_material_id) {
       int64_t sr = source_region_offsets_[i_cell] + j;
-      if (source_regions_.mesh(sr) != C_NONE) {
-        // print out the source region that is broken:
+      // Check if the key is already present in the mesh_map_
+      if (mesh_map_.find(sr) != mesh_map_.end()) {
         fatal_error(fmt::format("Source region {} already has mesh idx {} "
                                 "applied, but trying to apply mesh idx {}",
-          sr, source_regions_.mesh(sr), mesh_idx));
+          sr, mesh_map_[sr], mesh_idx));
       }
-      source_regions_.mesh(sr) = mesh_idx;
+      // If the SR has not already been assigned, then we can write to it
+      mesh_map_[sr] = mesh_idx;
     }
   }
 }
@@ -1220,12 +1466,12 @@ void FlatSourceDomain::apply_mesh_to_cell_and_children(int32_t i_cell,
   Cell& cell = *model::cells[i_cell];
 
   if (cell.type_ == Fill::MATERIAL) {
-    vector<int> instances(cell.n_instances_);
+    vector<int> instances(cell.n_instances());
     std::iota(instances.begin(), instances.end(), 0);
     apply_mesh_to_cell_instances(
       i_cell, mesh_idx, target_material_id, instances, is_target_void);
   } else if (target_material_id == C_NONE && !is_target_void) {
-    for (int j = 0; j < cell.n_instances_; j++) {
+    for (int j = 0; j < cell.n_instances(); j++) {
       std::unordered_map<int32_t, vector<int32_t>> cell_instance_list =
         cell.get_contained_cells(j, nullptr);
       for (const auto& pair : cell_instance_list) {
@@ -1279,18 +1525,9 @@ void FlatSourceDomain::apply_meshes()
   }
 }
 
-void FlatSourceDomain::prepare_base_source_regions()
-{
-  std::swap(source_regions_, base_source_regions_);
-  source_regions_.negroups() = base_source_regions_.negroups();
-  source_regions_.is_linear() = base_source_regions_.is_linear();
-}
-
 SourceRegionHandle FlatSourceDomain::get_subdivided_source_region_handle(
-  int64_t sr, int mesh_bin, Position r, double dist, Direction u)
+  SourceRegionKey sr_key, Position r, Direction u)
 {
-  SourceRegionKey sr_key {sr, mesh_bin};
-
   // Case 1: Check if the source region key is already present in the permanent
   // map. This is the most common condition, as any source region visited in a
   // previous power iteration will already be present in the permanent map. If
@@ -1352,9 +1589,8 @@ SourceRegionHandle FlatSourceDomain::get_subdivided_source_region_handle(
   gs.r() = r + TINY_BIT * u;
   gs.u() = {1.0, 0.0, 0.0};
   exhaustive_find_cell(gs);
-  int gs_i_cell = gs.lowest_coord().cell;
-  int64_t sr_found = source_region_offsets_[gs_i_cell] + gs.cell_instance();
-  if (sr_found != sr) {
+  int64_t sr_found = lookup_base_source_region_idx(gs);
+  if (sr_found != sr_key.base_source_region_id) {
     discovered_source_regions_.unlock(sr_key);
     SourceRegionHandle handle;
     handle.is_numerical_fp_artifact_ = true;
@@ -1362,9 +1598,9 @@ SourceRegionHandle FlatSourceDomain::get_subdivided_source_region_handle(
   }
 
   // Sanity check on mesh bin
-  int mesh_idx = base_source_regions_.mesh(sr);
+  int mesh_idx = lookup_mesh_idx(sr_key.base_source_region_id);
   if (mesh_idx == C_NONE) {
-    if (mesh_bin != 0) {
+    if (sr_key.mesh_bin != 0) {
       discovered_source_regions_.unlock(sr_key);
       SourceRegionHandle handle;
       handle.is_numerical_fp_artifact_ = true;
@@ -1373,7 +1609,7 @@ SourceRegionHandle FlatSourceDomain::get_subdivided_source_region_handle(
   } else {
     Mesh* mesh = model::meshes[mesh_idx].get();
     int bin_found = mesh->get_bin(r + TINY_BIT * u);
-    if (bin_found != mesh_bin) {
+    if (bin_found != sr_key.mesh_bin) {
       discovered_source_regions_.unlock(sr_key);
       SourceRegionHandle handle;
       handle.is_numerical_fp_artifact_ = true;
@@ -1385,14 +1621,94 @@ SourceRegionHandle FlatSourceDomain::get_subdivided_source_region_handle(
   // condition only occurs the first time the source region is discovered
   // (typically in the first power iteration). In this case, we need to handle
   // creation of the new source region and its storage into the parallel map.
-  // The new source region is created by copying the base source region, so as
-  // to inherit material, external source, and some flux properties etc. We
-  // also pass the base source region id to allow the new source region to
-  // know which base source region it is derived from.
-  SourceRegion* sr_ptr = discovered_source_regions_.emplace(
-    sr_key, {base_source_regions_.get_source_region_handle(sr), sr});
-  discovered_source_regions_.unlock(sr_key);
+  // Additionally, we need to determine the source region's material, initialize
+  // the starting scalar flux guess, and apply any known external sources.
+
+  // Call the basic constructor for the source region and store in the parallel
+  // map.
+  bool is_linear = RandomRay::source_shape_ != RandomRaySourceShape::FLAT;
+  SourceRegion* sr_ptr =
+    discovered_source_regions_.emplace(sr_key, {negroups_, is_linear});
   SourceRegionHandle handle {*sr_ptr};
+
+  // Determine the material
+  int gs_i_cell = gs.lowest_coord().cell();
+  Cell& cell = *model::cells[gs_i_cell];
+  int material = cell.material(gs.cell_instance());
+  int temp = 0;
+
+  // If material total XS is extremely low, just set it to void to avoid
+  // problems with 1/Sigma_t
+  if (material != MATERIAL_VOID) {
+    temp = data::mg.macro_xs_[material].get_temperature_index(
+      cell.sqrtkT(gs.cell_instance()));
+    for (int g = 0; g < negroups_; g++) {
+      double sigma_t =
+        sigma_t_[(material * ntemperature_ + temp) * negroups_ + g];
+      if (sigma_t < MINIMUM_MACRO_XS) {
+        material = MATERIAL_VOID;
+        temp = 0;
+        break;
+      }
+    }
+  }
+
+  handle.material() = material;
+  handle.temperature_idx() = temp;
+
+  handle.density_mult() = cell.density_mult(gs.cell_instance());
+
+  // Store the mesh index (if any) assigned to this source region
+  handle.mesh() = mesh_idx;
+
+  if (settings::run_mode == RunMode::FIXED_SOURCE) {
+    // Determine if there are any volumetric sources, and apply them.
+    // Volumetric sources are specifc only to the base SR idx.
+    auto it_vol =
+      external_volumetric_source_map_.find(sr_key.base_source_region_id);
+    if (it_vol != external_volumetric_source_map_.end()) {
+      const vector<int>& vol_sources = it_vol->second;
+      for (int src_idx : vol_sources) {
+        apply_external_source_to_source_region(src_idx, handle);
+      }
+    }
+
+    // Determine if there are any point sources, and apply them.
+    // Point sources are specific to the source region key.
+    auto it_point = external_point_source_map_.find(sr_key);
+    if (it_point != external_point_source_map_.end()) {
+      const vector<int>& point_sources = it_point->second;
+      for (int src_idx : point_sources) {
+        apply_external_source_to_source_region(src_idx, handle);
+      }
+    }
+
+    // Divide external source term by sigma_t
+    if (material != C_NONE) {
+      for (int g = 0; g < negroups_; g++) {
+        double sigma_t =
+          sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+          handle.density_mult();
+        handle.external_source(g) /= sigma_t;
+      }
+    }
+  }
+
+  // Compute the combined source term
+  update_single_neutron_source(handle);
+
+  // Unlock the parallel map. Note: we may be tempted to release
+  // this lock earlier, and then just use the source region's lock to protect
+  // the flux/source initialization stages above. However, the rest of the code
+  // only protects updates to the new flux and volume fields, and assumes that
+  // the source is constant for the duration of transport. Thus, using just the
+  // source region's lock by itself would result in other threads potentially
+  // reading from the source before it is computed, as they won't use the lock
+  // when only reading from the SR's source. It would be expensive to protect
+  // those operations, whereas generating the SR is only done once, so we just
+  // hold the map's bucket lock until the source region is fully initialized.
+  discovered_source_regions_.unlock(sr_key);
+
   return handle;
 }
 
@@ -1412,6 +1728,9 @@ void FlatSourceDomain::finalize_discovered_source_regions()
     // order due to shared memory threading.
     std::sort(keys.begin(), keys.end());
 
+    // Remember the index of the first new source region
+    int64_t start_sr_id = source_regions_.n_source_regions();
+
     // Append the source regions in the sorted key order.
     for (const auto& key : keys) {
       const SourceRegion& sr = discovered_source_regions_[key];
@@ -1419,12 +1738,115 @@ void FlatSourceDomain::finalize_discovered_source_regions()
       source_regions_.push_back(sr);
     }
 
-    // If any new source regions were discovered, we need to update the
-    // tally mapping between source regions and tally bins.
-    mapped_all_tallies_ = false;
+    // Map all new source regions to tallies
+    convert_source_regions_to_tallies(start_sr_id);
   }
 
   discovered_source_regions_.clear();
 }
 
+// This is the "diagonal stabilization" technique developed by Gunow et al. in:
+//
+// Geoffrey Gunow, Benoit Forget, Kord Smith, Stabilization of multi-group
+// neutron transport with transport-corrected cross-sections, Annals of Nuclear
+// Energy, Volume 126, 2019, Pages 211-219, ISSN 0306-4549,
+// https://doi.org/10.1016/j.anucene.2018.10.036.
+void FlatSourceDomain::apply_transport_stabilization()
+{
+  // Don't do anything if all in-group scattering
+  // cross sections are positive
+  if (!is_transport_stabilization_needed_) {
+    return;
+  }
+
+  // Apply the stabilization factor to all source elements
+#pragma omp parallel for
+  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
+    int material = source_regions_.material(sr);
+    int temp = source_regions_.temperature_idx(sr);
+    double density_mult = source_regions_.density_mult(sr);
+    if (material == MATERIAL_VOID) {
+      continue;
+    }
+    for (int g = 0; g < negroups_; g++) {
+      // Only apply stabilization if the diagonal (in-group) scattering XS is
+      // negative
+      double sigma_s =
+        sigma_s_[((material * ntemperature_ + temp) * negroups_ + g) *
+                   negroups_ +
+                 g] *
+        density_mult;
+      if (sigma_s < 0.0) {
+        double sigma_t =
+          sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+          density_mult;
+        double phi_new = source_regions_.scalar_flux_new(sr, g);
+        double phi_old = source_regions_.scalar_flux_old(sr, g);
+
+        // Equation 18 in the above Gunow et al. 2019 paper. For a default
+        // rho of 1.0, this ensures there are no negative diagonal elements
+        // in the iteration matrix. A lesser rho could be used (or exposed
+        // as a user input parameter) to reduce the negative impact on
+        // convergence rate though would need to be experimentally tested to see
+        // if it doesn't become unstable. rho = 1.0 is good as it gives the
+        // highest assurance of stability, and the impacts on convergence rate
+        // are pretty mild.
+        double D = diagonal_stabilization_rho_ * sigma_s / sigma_t;
+
+        // Equation 16 in the above Gunow et al. 2019 paper
+        source_regions_.scalar_flux_new(sr, g) =
+          (phi_new - D * phi_old) / (1.0 - D);
+      }
+    }
+  }
+}
+
+// Determines the base source region index (i.e., a material filled cell
+// instance) that corresponds to a particular location in the geometry. Requires
+// that the "gs" object passed in has already been initialized and has called
+// find_cell etc.
+int64_t FlatSourceDomain::lookup_base_source_region_idx(
+  const GeometryState& gs) const
+{
+  int i_cell = gs.lowest_coord().cell();
+  int64_t sr = source_region_offsets_[i_cell] + gs.cell_instance();
+  return sr;
+}
+
+// Determines the index of the mesh (if any) that has been applied
+// to a particular base source region index.
+int FlatSourceDomain::lookup_mesh_idx(int64_t sr) const
+{
+  int mesh_idx = C_NONE;
+  auto mesh_it = mesh_map_.find(sr);
+  if (mesh_it != mesh_map_.end()) {
+    mesh_idx = mesh_it->second;
+  }
+  return mesh_idx;
+}
+
+// Determines the source region key that corresponds to a particular location in
+// the geometry. This takes into account both the base source region index as
+// well as the mesh bin if a mesh is applied to this source region for
+// subdivision.
+SourceRegionKey FlatSourceDomain::lookup_source_region_key(
+  const GeometryState& gs) const
+{
+  int64_t sr = lookup_base_source_region_idx(gs);
+  int64_t mesh_bin = lookup_mesh_bin(sr, gs.r());
+  return SourceRegionKey {sr, mesh_bin};
+}
+
+// Determines the mesh bin that corresponds to a particular base source region
+// index and position.
+int64_t FlatSourceDomain::lookup_mesh_bin(int64_t sr, Position r) const
+{
+  int mesh_idx = lookup_mesh_idx(sr);
+  int mesh_bin = 0;
+  if (mesh_idx != C_NONE) {
+    mesh_bin = model::meshes[mesh_idx]->get_bin(r);
+  }
+  return mesh_bin;
+}
+
 } // namespace openmc
diff --git a/src/random_ray/linear_source_domain.cpp b/src/random_ray/linear_source_domain.cpp
index 81412164e..b4701ed1f 100644
--- a/src/random_ray/linear_source_domain.cpp
+++ b/src/random_ray/linear_source_domain.cpp
@@ -34,28 +34,25 @@ void LinearSourceDomain::batch_reset()
   }
 }
 
-void LinearSourceDomain::update_neutron_source(double k_eff)
+void LinearSourceDomain::update_single_neutron_source(SourceRegionHandle& srh)
 {
-  simulation::time_update_src.start();
-
-  double inverse_k_eff = 1.0 / k_eff;
-
-// Reset all source regions to zero (important for void regions)
-#pragma omp parallel for
-  for (int64_t se = 0; se < n_source_elements(); se++) {
-    source_regions_.source(se) = 0.0;
+  // Reset all source regions to zero (important for void regions)
+  for (int g = 0; g < negroups_; g++) {
+    srh.source(g) = 0.0;
   }
 
-#pragma omp parallel for
-  for (int64_t sr = 0; sr < n_source_regions(); sr++) {
-    int material = source_regions_.material(sr);
-    if (material == MATERIAL_VOID) {
-      continue;
-    }
-    MomentMatrix invM = source_regions_.mom_matrix(sr).inverse();
+  // Add scattering + fission source
+  int material = srh.material();
+  int temp = srh.temperature_idx();
+  double density_mult = srh.density_mult();
+  if (material != MATERIAL_VOID) {
+    double inverse_k_eff = 1.0 / k_eff_;
+    MomentMatrix invM = srh.mom_matrix().inverse();
 
     for (int g_out = 0; g_out < negroups_; g_out++) {
-      double sigma_t = sigma_t_[material * negroups_ + g_out];
+      double sigma_t =
+        sigma_t_[(material * ntemperature_ + temp) * negroups_ + g_out] *
+        density_mult;
 
       double scatter_flat = 0.0f;
       double fission_flat = 0.0f;
@@ -64,24 +61,32 @@ void LinearSourceDomain::update_neutron_source(double k_eff)
 
       for (int g_in = 0; g_in < negroups_; g_in++) {
         // Handles for the flat and linear components of the flux
-        double flux_flat = source_regions_.scalar_flux_old(sr, g_in);
-        MomentArray flux_linear = source_regions_.flux_moments_old(sr, g_in);
+        double flux_flat = srh.scalar_flux_old(g_in);
+        MomentArray flux_linear = srh.flux_moments_old(g_in);
 
         // Handles for cross sections
         double sigma_s =
-          sigma_s_[material * negroups_ * negroups_ + g_out * negroups_ + g_in];
-        double nu_sigma_f = nu_sigma_f_[material * negroups_ + g_in];
-        double chi = chi_[material * negroups_ + g_out];
+          sigma_s_[((material * ntemperature_ + temp) * negroups_ + g_out) *
+                     negroups_ +
+                   g_in] *
+          density_mult;
+        double nu_sigma_f =
+          nu_sigma_f_[(material * ntemperature_ + temp) * negroups_ + g_in] *
+          density_mult;
+        double chi =
+          chi_[(material * ntemperature_ + temp) * negroups_ + g_out];
 
         // Compute source terms for flat and linear components of the flux
         scatter_flat += sigma_s * flux_flat;
-        fission_flat += nu_sigma_f * flux_flat * chi;
         scatter_linear += sigma_s * flux_linear;
-        fission_linear += nu_sigma_f * flux_linear * chi;
+        if (settings::create_fission_neutrons) {
+          fission_flat += nu_sigma_f * flux_flat * chi;
+          fission_linear += nu_sigma_f * flux_linear * chi;
+        }
       }
 
       // Compute the flat source term
-      source_regions_.source(sr, g_out) =
+      srh.source(g_out) =
         (scatter_flat + fission_flat * inverse_k_eff) / sigma_t;
 
       // Compute the linear source terms. In the first 10 iterations when the
@@ -91,25 +96,21 @@ void LinearSourceDomain::update_neutron_source(double k_eff)
       // very small/noisy or have poorly developed spatial moments, so we zero
       // the source gradients (effectively making this a flat source region
       // temporarily), so as to improve stability.
-      if (simulation::current_batch > 10 &&
-          source_regions_.source(sr, g_out) >= 0.0) {
-        source_regions_.source_gradients(sr, g_out) =
+      if (simulation::current_batch > 10 && srh.source(g_out) >= 0.0) {
+        srh.source_gradients(g_out) =
           invM * ((scatter_linear + fission_linear * inverse_k_eff) / sigma_t);
       } else {
-        source_regions_.source_gradients(sr, g_out) = {0.0, 0.0, 0.0};
+        srh.source_gradients(g_out) = {0.0, 0.0, 0.0};
       }
     }
   }
 
+  // Add external source if in fixed source mode
   if (settings::run_mode == RunMode::FIXED_SOURCE) {
-// Add external source to flat source term if in fixed source mode
-#pragma omp parallel for
-    for (int64_t se = 0; se < n_source_elements(); se++) {
-      source_regions_.source(se) += source_regions_.external_source(se);
+    for (int g = 0; g < negroups_; g++) {
+      srh.source(g) += srh.external_source(g);
     }
   }
-
-  simulation::time_update_src.stop();
 }
 
 void LinearSourceDomain::normalize_scalar_flux_and_volumes(
diff --git a/src/random_ray/random_ray.cpp b/src/random_ray/random_ray.cpp
index 9a9d1394a..dde5023e4 100644
--- a/src/random_ray/random_ray.cpp
+++ b/src/random_ray/random_ray.cpp
@@ -10,6 +10,8 @@
 #include "openmc/settings.h"
 #include "openmc/simulation.h"
 
+#include <numeric>
+
 #include "openmc/distribution_spatial.h"
 #include "openmc/random_dist.h"
 #include "openmc/source.h"
@@ -237,7 +239,6 @@ double RandomRay::distance_inactive_;
 double RandomRay::distance_active_;
 unique_ptr<Source> RandomRay::ray_source_;
 RandomRaySourceShape RandomRay::source_shape_ {RandomRaySourceShape::FLAT};
-bool RandomRay::mesh_subdivision_enabled_ {false};
 RandomRaySampleMethod RandomRay::sample_method_ {RandomRaySampleMethod::PRNG};
 
 RandomRay::RandomRay()
@@ -265,6 +266,12 @@ uint64_t RandomRay::transport_history_based_single_ray()
     if (!alive())
       break;
     event_cross_surface();
+    // If ray has too many events, display warning and kill it
+    if (n_event() >= settings::max_particle_events) {
+      warning("Ray " + std::to_string(id()) +
+              " underwent maximum number of events, terminating ray.");
+      wgt() = 0.0;
+    }
   }
 
   return n_event();
@@ -273,11 +280,15 @@ uint64_t RandomRay::transport_history_based_single_ray()
 // Transports ray across a single source region
 void RandomRay::event_advance_ray()
 {
+  // If geometry debug mode is on, check for cell overlaps
+  if (settings::check_overlaps)
+    check_cell_overlap(*this);
+
   // Find the distance to the nearest boundary
   boundary() = distance_to_boundary(*this);
-  double distance = boundary().distance;
+  double distance = boundary().distance();
 
-  if (distance <= 0.0) {
+  if (distance < 0.0) {
     mark_as_lost("Negative transport distance detected for particle " +
                  std::to_string(id()));
     return;
@@ -324,77 +335,66 @@ void RandomRay::event_advance_ray()
 
   // Advance particle
   for (int j = 0; j < n_coord(); ++j) {
-    coord(j).r += distance * coord(j).u;
+    coord(j).r() += distance * coord(j).u();
   }
 }
 
 void RandomRay::attenuate_flux(double distance, bool is_active, double offset)
 {
-  // Determine source region index etc.
-  int i_cell = lowest_coord().cell;
-
-  // The base source region is the spatial region index
-  int64_t sr = domain_->source_region_offsets_[i_cell] + cell_instance();
+  // Lookup base source region index
+  int64_t sr = domain_->lookup_base_source_region_idx(*this);
 
   // Perform ray tracing across mesh
-  if (mesh_subdivision_enabled_) {
-    // Determine the mesh index for the base source region, if any
-    int mesh_idx = domain_->base_source_regions_.mesh(sr);
+  // Determine the mesh index for the base source region, if any
+  int mesh_idx = domain_->lookup_mesh_idx(sr);
 
-    if (mesh_idx == C_NONE) {
-      // If there's no mesh being applied to this cell, then
-      // we just attenuate the flux as normal, and set
-      // the mesh bin to 0
-      attenuate_flux_inner(distance, is_active, sr, 0, r());
-    } else {
-      // If there is a mesh being applied to this cell, then
-      // we loop over all the bin crossings and attenuate
-      // separately.
-      Mesh* mesh = model::meshes[mesh_idx].get();
-
-      // We adjust the start and end positions of the ray slightly
-      // to accomodate for floating point precision issues that tend
-      // to occur at mesh boundaries that overlap with geometry lattice
-      // boundaries.
-      Position start = r() + (offset + TINY_BIT) * u();
-      Position end = start + (distance - 2.0 * TINY_BIT) * u();
-      double reduced_distance = (end - start).norm();
-
-      // Ray trace through the mesh and record bins and lengths
-      mesh_bins_.resize(0);
-      mesh_fractional_lengths_.resize(0);
-      mesh->bins_crossed(start, end, u(), mesh_bins_, mesh_fractional_lengths_);
-
-      // Loop over all mesh bins and attenuate flux
-      for (int b = 0; b < mesh_bins_.size(); b++) {
-        double physical_length = reduced_distance * mesh_fractional_lengths_[b];
-        attenuate_flux_inner(
-          physical_length, is_active, sr, mesh_bins_[b], start);
-        start += physical_length * u();
-      }
-    }
+  if (mesh_idx == C_NONE) {
+    // If there's no mesh being applied to this cell, then
+    // we just attenuate the flux as normal, and set
+    // the mesh bin to 0
+    attenuate_flux_inner(distance, is_active, sr, 0, r());
   } else {
-    attenuate_flux_inner(distance, is_active, sr, C_NONE, r());
+    // If there is a mesh being applied to this cell, then
+    // we loop over all the bin crossings and attenuate
+    // separately.
+    Mesh* mesh = model::meshes[mesh_idx].get();
+
+    // We adjust the start and end positions of the ray slightly
+    // to accomodate for floating point precision issues that tend
+    // to occur at mesh boundaries that overlap with geometry lattice
+    // boundaries.
+    Position start = r() + (offset + TINY_BIT) * u();
+    Position end = start + (distance - 2.0 * TINY_BIT) * u();
+    double reduced_distance = (end - start).norm();
+
+    // Ray trace through the mesh and record bins and lengths
+    mesh_bins_.resize(0);
+    mesh_fractional_lengths_.resize(0);
+    mesh->bins_crossed(start, end, u(), mesh_bins_, mesh_fractional_lengths_);
+
+    // Loop over all mesh bins and attenuate flux
+    for (int b = 0; b < mesh_bins_.size(); b++) {
+      double physical_length = reduced_distance * mesh_fractional_lengths_[b];
+      attenuate_flux_inner(
+        physical_length, is_active, sr, mesh_bins_[b], start);
+      start += physical_length * u();
+    }
   }
 }
 
 void RandomRay::attenuate_flux_inner(
   double distance, bool is_active, int64_t sr, int mesh_bin, Position r)
 {
+  SourceRegionKey sr_key {sr, mesh_bin};
   SourceRegionHandle srh;
-  if (mesh_subdivision_enabled_) {
-    srh = domain_->get_subdivided_source_region_handle(
-      sr, mesh_bin, r, distance, u());
-    if (srh.is_numerical_fp_artifact_) {
-      return;
-    }
-  } else {
-    srh = domain_->source_regions_.get_source_region_handle(sr);
+  srh = domain_->get_subdivided_source_region_handle(sr_key, r, u());
+  if (srh.is_numerical_fp_artifact_) {
+    return;
   }
 
   switch (source_shape_) {
   case RandomRaySourceShape::FLAT:
-    if (this->material() == MATERIAL_VOID) {
+    if (srh.material() == MATERIAL_VOID) {
       attenuate_flux_flat_source_void(srh, distance, is_active, r);
     } else {
       attenuate_flux_flat_source(srh, distance, is_active, r);
@@ -402,7 +402,7 @@ void RandomRay::attenuate_flux_inner(
     break;
   case RandomRaySourceShape::LINEAR:
   case RandomRaySourceShape::LINEAR_XY:
-    if (this->material() == MATERIAL_VOID) {
+    if (srh.material() == MATERIAL_VOID) {
       attenuate_flux_linear_source_void(srh, distance, is_active, r);
     } else {
       attenuate_flux_linear_source(srh, distance, is_active, r);
@@ -433,11 +433,14 @@ void RandomRay::attenuate_flux_flat_source(
   n_event()++;
 
   // Get material
-  int material = this->material();
+  int material = srh.material();
+  int temp = srh.temperature_idx();
 
   // MOC incoming flux attenuation + source contribution/attenuation equation
   for (int g = 0; g < negroups_; g++) {
-    float sigma_t = domain_->sigma_t_[material * negroups_ + g];
+    float sigma_t =
+      domain_->sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+      srh.density_mult();
     float tau = sigma_t * distance;
     float exponential = cjosey_exponential(tau); // exponential = 1 - exp(-tau)
     float new_delta_psi = (angular_flux_[g] - srh.source(g)) * exponential;
@@ -484,7 +487,7 @@ void RandomRay::attenuate_flux_flat_source_void(
   // The number of geometric intersections is counted for reporting purposes
   n_event()++;
 
-  int material = this->material();
+  int material = srh.material();
 
   // If ray is in the active phase (not in dead zone), make contributions to
   // source region bookkeeping
@@ -518,8 +521,10 @@ void RandomRay::attenuate_flux_flat_source_void(
   }
 
   // Add source to incoming angular flux, assuming void region
-  for (int g = 0; g < negroups_; g++) {
-    angular_flux_[g] += srh.external_source(g) * distance;
+  if (settings::run_mode == RunMode::FIXED_SOURCE) {
+    for (int g = 0; g < negroups_; g++) {
+      angular_flux_[g] += srh.external_source(g) * distance;
+    }
   }
 }
 
@@ -529,7 +534,8 @@ void RandomRay::attenuate_flux_linear_source(
   // The number of geometric intersections is counted for reporting purposes
   n_event()++;
 
-  int material = this->material();
+  int material = srh.material();
+  int temp = srh.temperature_idx();
 
   Position& centroid = srh.centroid();
   Position midpoint = r + u() * (distance / 2.0);
@@ -558,7 +564,9 @@ void RandomRay::attenuate_flux_linear_source(
   for (int g = 0; g < negroups_; g++) {
 
     // Compute tau, the optical thickness of the ray segment
-    float sigma_t = domain_->sigma_t_[material * negroups_ + g];
+    float sigma_t =
+      domain_->sigma_t_[(material * ntemperature_ + temp) * negroups_ + g] *
+      srh.density_mult();
     float tau = sigma_t * distance;
 
     // If tau is very small, set it to zero to avoid numerical issues.
@@ -688,7 +696,10 @@ void RandomRay::attenuate_flux_linear_source_void(
   // transport through a void region is greatly simplified. Here we
   // compute the updated flux moments.
   for (int g = 0; g < negroups_; g++) {
-    float spatial_source = srh.external_source(g);
+    float spatial_source = 0.f;
+    if (settings::run_mode == RunMode::FIXED_SOURCE) {
+      spatial_source = srh.external_source(g);
+    }
     float new_delta_psi = (angular_flux_[g] - spatial_source) * distance;
     float h1 = 0.5f;
     h1 = h1 * angular_flux_[g];
@@ -750,14 +761,17 @@ void RandomRay::attenuate_flux_linear_source_void(
   }
 
   // Add source to incoming angular flux, assuming void region
-  for (int g = 0; g < negroups_; g++) {
-    angular_flux_[g] += srh.external_source(g) * distance;
+  if (settings::run_mode == RunMode::FIXED_SOURCE) {
+    for (int g = 0; g < negroups_; g++) {
+      angular_flux_[g] += srh.external_source(g) * distance;
+    }
   }
 }
 
 void RandomRay::initialize_ray(uint64_t ray_id, FlatSourceDomain* domain)
 {
   domain_ = domain;
+  ntemperature_ = domain->ntemperature_;
 
   // Reset particle event counter
   n_event() = 0;
@@ -778,17 +792,18 @@ void RandomRay::initialize_ray(uint64_t ray_id, FlatSourceDomain* domain)
   case RandomRaySampleMethod::HALTON:
     site = sample_halton();
     break;
+  case RandomRaySampleMethod::S2:
+    site = sample_s2();
+    break;
   default:
     fatal_error("Unknown sample method for random ray transport.");
   }
 
-  site.E = lower_bound_index(
-    data::mg.rev_energy_bins_.begin(), data::mg.rev_energy_bins_.end(), site.E);
-  site.E = negroups_ - site.E - 1.;
+  site.E = 0.0;
   this->from_source(&site);
 
   // Locate ray
-  if (lowest_coord().cell == C_NONE) {
+  if (lowest_coord().cell() == C_NONE) {
     if (!exhaustive_find_cell(*this)) {
       this->mark_as_lost(
         "Could not find the cell containing particle " + std::to_string(id()));
@@ -796,30 +811,15 @@ void RandomRay::initialize_ray(uint64_t ray_id, FlatSourceDomain* domain)
 
     // Set birth cell attribute
     if (cell_born() == C_NONE)
-      cell_born() = lowest_coord().cell;
+      cell_born() = lowest_coord().cell();
   }
 
+  SourceRegionKey sr_key = domain_->lookup_source_region_key(*this);
+  SourceRegionHandle srh =
+    domain_->get_subdivided_source_region_handle(sr_key, r(), u());
+
   // Initialize ray's starting angular flux to starting location's isotropic
   // source
-  int i_cell = lowest_coord().cell;
-  int64_t sr = domain_->source_region_offsets_[i_cell] + cell_instance();
-
-  SourceRegionHandle srh;
-  if (mesh_subdivision_enabled_) {
-    int mesh_idx = domain_->base_source_regions_.mesh(sr);
-    int mesh_bin;
-    if (mesh_idx == C_NONE) {
-      mesh_bin = 0;
-    } else {
-      Mesh* mesh = model::meshes[mesh_idx].get();
-      mesh_bin = mesh->get_bin(r());
-    }
-    srh =
-      domain_->get_subdivided_source_region_handle(sr, mesh_bin, r(), 0.0, u());
-  } else {
-    srh = domain_->source_regions_.get_source_region_handle(sr);
-  }
-
   if (!srh.is_numerical_fp_artifact_) {
     for (int g = 0; g < negroups_; g++) {
       angular_flux_[g] = srh.source(g);
@@ -877,4 +877,27 @@ SourceSite RandomRay::sample_halton()
   return site;
 }
 
+SourceSite RandomRay::sample_s2()
+{
+  // set random number seed
+  int64_t particle_seed =
+    (simulation::current_batch - 1) * settings::n_particles + id();
+  init_particle_seeds(particle_seed, seeds());
+  stream() = STREAM_TRACKING;
+
+  // Get spatial component of the ray_source_
+  SpatialDistribution* space =
+    dynamic_cast<IndependentSource*>(RandomRay::ray_source_.get())->space();
+
+  SourceSite site;
+
+  // Sample spatial distribution
+  site.r = space->sample(current_seed()).first;
+
+  // Sample either left or right for S2 (flashlight) transport.
+  site.u = {prn(current_seed()) < 0.5 ? -1.0 : 1.0, 0.0, 0.0};
+
+  return site;
+}
+
 } // namespace openmc
diff --git a/src/random_ray/random_ray_simulation.cpp b/src/random_ray/random_ray_simulation.cpp
index bf01d2f57..00fff99a7 100644
--- a/src/random_ray/random_ray_simulation.cpp
+++ b/src/random_ray/random_ray_simulation.cpp
@@ -1,5 +1,6 @@
 #include "openmc/random_ray/random_ray_simulation.h"
 
+#include "openmc/capi.h"
 #include "openmc/eigenvalue.h"
 #include "openmc/geometry.h"
 #include "openmc/message_passing.h"
@@ -22,124 +23,6 @@ namespace openmc {
 // Non-member functions
 //==============================================================================
 
-void openmc_run_random_ray()
-{
-  //////////////////////////////////////////////////////////
-  // Run forward simulation
-  //////////////////////////////////////////////////////////
-
-  // Check if adjoint calculation is needed. If it is, we will run the forward
-  // calculation first and then the adjoint calculation later.
-  bool adjoint_needed = FlatSourceDomain::adjoint_;
-
-  // Configure the domain for forward simulation
-  FlatSourceDomain::adjoint_ = false;
-
-  // If we're going to do an adjoint simulation afterwards, report that this is
-  // the initial forward flux solve.
-  if (adjoint_needed && mpi::master)
-    header("FORWARD FLUX SOLVE", 3);
-
-  // Initialize OpenMC general data structures
-  openmc_simulation_init();
-
-  // Validate that inputs meet requirements for random ray mode
-  if (mpi::master)
-    validate_random_ray_inputs();
-
-  // Declare forward flux so that it can be saved for later adjoint simulation
-  vector<double> forward_flux;
-  SourceRegionContainer forward_source_regions;
-  SourceRegionContainer forward_base_source_regions;
-  std::unordered_map<SourceRegionKey, int64_t, SourceRegionKey::HashFunctor>
-    forward_source_region_map;
-
-  {
-    // Initialize Random Ray Simulation Object
-    RandomRaySimulation sim;
-
-    // Initialize fixed sources, if present
-    sim.apply_fixed_sources_and_mesh_domains();
-
-    // Begin main simulation timer
-    simulation::time_total.start();
-
-    // Execute random ray simulation
-    sim.simulate();
-
-    // End main simulation timer
-    simulation::time_total.stop();
-
-    // Normalize and save the final forward flux
-    sim.domain()->serialize_final_fluxes(forward_flux);
-
-    double source_normalization_factor =
-      sim.domain()->compute_fixed_source_normalization_factor() /
-      (settings::n_batches - settings::n_inactive);
-
-#pragma omp parallel for
-    for (uint64_t i = 0; i < forward_flux.size(); i++) {
-      forward_flux[i] *= source_normalization_factor;
-    }
-
-    forward_source_regions = sim.domain()->source_regions_;
-    forward_source_region_map = sim.domain()->source_region_map_;
-    forward_base_source_regions = sim.domain()->base_source_regions_;
-
-    // Finalize OpenMC
-    openmc_simulation_finalize();
-
-    // Output all simulation results
-    sim.output_simulation_results();
-  }
-
-  //////////////////////////////////////////////////////////
-  // Run adjoint simulation (if enabled)
-  //////////////////////////////////////////////////////////
-
-  if (adjoint_needed) {
-    reset_timers();
-
-    // Configure the domain for adjoint simulation
-    FlatSourceDomain::adjoint_ = true;
-
-    if (mpi::master)
-      header("ADJOINT FLUX SOLVE", 3);
-
-    // Initialize OpenMC general data structures
-    openmc_simulation_init();
-
-    // Initialize Random Ray Simulation Object
-    RandomRaySimulation adjoint_sim;
-
-    // Initialize adjoint fixed sources, if present
-    adjoint_sim.prepare_fixed_sources_adjoint(forward_flux,
-      forward_source_regions, forward_base_source_regions,
-      forward_source_region_map);
-
-    // Transpose scattering matrix
-    adjoint_sim.domain()->transpose_scattering_matrix();
-
-    // Swap nu_sigma_f and chi
-    adjoint_sim.domain()->nu_sigma_f_.swap(adjoint_sim.domain()->chi_);
-
-    // Begin main simulation timer
-    simulation::time_total.start();
-
-    // Execute random ray simulation
-    adjoint_sim.simulate();
-
-    // End main simulation timer
-    simulation::time_total.stop();
-
-    // Finalize OpenMC
-    openmc_simulation_finalize();
-
-    // Output all simulation results
-    adjoint_sim.output_simulation_results();
-  }
-}
-
 // Enforces restrictions on inputs in random ray mode.  While there are
 // many features that don't make sense in random ray mode, and are therefore
 // unsupported, we limit our testing/enforcement operations only to inputs
@@ -158,11 +41,12 @@ void validate_random_ray_inputs()
       case SCORE_FISSION:
       case SCORE_NU_FISSION:
       case SCORE_EVENTS:
+      case SCORE_KAPPA_FISSION:
         break;
       default:
         fatal_error(
-          "Invalid score specified. Only flux, total, fission, nu-fission, and "
-          "event scores are supported in random ray mode.");
+          "Invalid score specified. Only flux, total, fission, nu-fission, "
+          "kappa-fission, and event scores are supported in random ray mode.");
       }
     }
 
@@ -195,10 +79,6 @@ void validate_random_ray_inputs()
       fatal_error("Anisotropic MGXS detected. Only isotropic XS data sets "
                   "supported in random ray mode.");
     }
-    if (material.get_xsdata().size() > 1) {
-      fatal_error("Non-isothermal MGXS detected. Only isothermal XS data sets "
-                  "supported in random ray mode.");
-    }
     for (int g = 0; g < data::mg.num_energy_groups_; g++) {
       if (material.exists_in_model) {
         // Temperature and angle indices, if using multiple temperature
@@ -281,10 +161,19 @@ void validate_random_ray_inputs()
           "allowed in random ray mode.");
       }
 
-      // Validate that a domain ID was specified
-      if (is->domain_ids().size() == 0) {
-        fatal_error("Fixed sources must be specified by domain "
-                    "id (cell, material, or universe) in random ray mode.");
+      // Validate that a domain ID was specified OR that it is a point source
+      auto sp = dynamic_cast<SpatialPoint*>(is->space());
+      if (is->domain_ids().size() == 0 && !sp) {
+        fatal_error("Fixed sources must be point source or spatially "
+                    "constrained by domain id (cell, material, or universe) in "
+                    "random ray mode.");
+      } else if (is->domain_ids().size() > 0 && sp) {
+        // If both a domain constraint and a point source location are
+        // specified, notify user that domain constraint takes precedence.
+        warning("Fixed source has both a domain constraint and a point "
+                "type spatial distribution. The domain constraint takes "
+                "precedence in random ray mode -- point source coordinate "
+                "will be ignored.");
       }
 
       // Check that a discrete energy distribution was used
@@ -298,25 +187,76 @@ void validate_random_ray_inputs()
     }
   }
 
+  // Validate adjoint sources
+  ///////////////////////////////////////////////////////////////////
+  if (FlatSourceDomain::adjoint_requested_ && !model::adjoint_sources.empty()) {
+    for (int i = 0; i < model::adjoint_sources.size(); i++) {
+      Source* s = model::adjoint_sources[i].get();
+
+      // Check for independent source
+      IndependentSource* is = dynamic_cast<IndependentSource*>(s);
+
+      if (!is) {
+        fatal_error(
+          "Only IndependentSource adjoint source types are allowed in "
+          "random ray mode");
+      }
+
+      // Check for isotropic source
+      UnitSphereDistribution* angle_dist = is->angle();
+      Isotropic* id = dynamic_cast<Isotropic*>(angle_dist);
+      if (!id) {
+        fatal_error(
+          "Invalid source definition -- only isotropic adjoint sources are "
+          "allowed in random ray mode.");
+      }
+
+      // Validate that a domain ID was specified OR that it is a point source
+      auto sp = dynamic_cast<SpatialPoint*>(is->space());
+      if (is->domain_ids().size() == 0 && !sp) {
+        fatal_error("Adjoint sources must be point source or spatially "
+                    "constrained by domain id (cell, material, or universe) in "
+                    "random ray mode.");
+      } else if (is->domain_ids().size() > 0 && sp) {
+        // If both a domain constraint and a point source location are
+        // specified, notify user that domain constraint takes precedence.
+        warning("Adjoint source has both a domain constraint and a point "
+                "type spatial distribution. The domain constraint takes "
+                "precedence in random ray mode -- point source coordinate "
+                "will be ignored.");
+      }
+
+      // Check that a discrete energy distribution was used
+      Distribution* d = is->energy();
+      Discrete* dd = dynamic_cast<Discrete*>(d);
+      if (!dd) {
+        fatal_error(
+          "Only discrete (multigroup) energy distributions are allowed for "
+          "adjoint sources in random ray mode.");
+      }
+    }
+  }
+
   // Validate plotting files
   ///////////////////////////////////////////////////////////////////
   for (int p = 0; p < model::plots.size(); p++) {
 
     // Get handle to OpenMC plot object
-    Plot* openmc_plot = dynamic_cast<Plot*>(model::plots[p].get());
+    const auto& openmc_plottable = model::plots[p];
+    Plot* openmc_plot = dynamic_cast<Plot*>(openmc_plottable.get());
 
     // Random ray plots only support voxel plots
     if (!openmc_plot) {
       warning(fmt::format(
         "Plot {} will not be used for end of simulation data plotting -- only "
         "voxel plotting is allowed in random ray mode.",
-        p));
+        openmc_plottable->id()));
       continue;
     } else if (openmc_plot->type_ != Plot::PlotType::voxel) {
       warning(fmt::format(
         "Plot {} will not be used for end of simulation data plotting -- only "
         "voxel plotting is allowed in random ray mode.",
-        p));
+        openmc_plottable->id()));
       continue;
     }
   }
@@ -336,25 +276,26 @@ void validate_random_ray_inputs()
   // when generating weight windows with FW-CADIS and an overlaid mesh.
   ///////////////////////////////////////////////////////////////////
   if (RandomRay::source_shape_ == RandomRaySourceShape::LINEAR &&
-      RandomRay::mesh_subdivision_enabled_ &&
       variance_reduction::weight_windows.size() > 0) {
     warning(
       "Linear sources may result in negative fluxes in small source regions "
       "generated by mesh subdivision. Negative sources may result in low "
-      "quality FW-CADIS weight windows. We recommend you use flat source mode "
-      "when generating weight windows with an overlaid mesh tally.");
+      "quality FW-CADIS weight windows. We recommend you use flat source "
+      "mode when generating weight windows with an overlaid mesh tally.");
   }
 }
 
-void openmc_reset_random_ray()
+void openmc_finalize_random_ray()
 {
   FlatSourceDomain::volume_estimator_ = RandomRayVolumeEstimator::HYBRID;
   FlatSourceDomain::volume_normalized_flux_tallies_ = false;
-  FlatSourceDomain::adjoint_ = false;
+  FlatSourceDomain::adjoint_requested_ = false;
+  FlatSourceDomain::solve_ = RandomRaySolve::FORWARD;
+  FlatSourceDomain::fw_cadis_local_ = false;
+  FlatSourceDomain::fw_cadis_local_targets_.clear();
   FlatSourceDomain::mesh_domain_map_.clear();
   RandomRay::ray_source_.reset();
   RandomRay::source_shape_ = RandomRaySourceShape::FLAT;
-  RandomRay::mesh_subdivision_enabled_ = false;
   RandomRay::sample_method_ = RandomRaySampleMethod::PRNG;
 }
 
@@ -392,33 +333,65 @@ RandomRaySimulation::RandomRaySimulation()
 
 void RandomRaySimulation::apply_fixed_sources_and_mesh_domains()
 {
+  domain_->apply_meshes();
   if (settings::run_mode == RunMode::FIXED_SOURCE) {
     // Transfer external source user inputs onto random ray source regions
-    domain_->convert_external_sources();
+    domain_->convert_external_sources(false);
     domain_->count_external_source_regions();
   }
-  domain_->apply_meshes();
 }
 
-void RandomRaySimulation::prepare_fixed_sources_adjoint(
-  vector<double>& forward_flux, SourceRegionContainer& forward_source_regions,
-  SourceRegionContainer& forward_base_source_regions,
-  std::unordered_map<SourceRegionKey, int64_t, SourceRegionKey::HashFunctor>&
-    forward_source_region_map)
+void RandomRaySimulation::prepare_fw_fixed_sources_adjoint()
+{
+  // Prepare adjoint fixed sources using forward flux
+  domain_->source_regions_.adjoint_reset();
+  if (settings::run_mode == RunMode::FIXED_SOURCE) {
+    domain_->set_fw_adjoint_sources();
+  }
+}
+
+void RandomRaySimulation::prepare_local_fixed_sources_adjoint()
 {
   if (settings::run_mode == RunMode::FIXED_SOURCE) {
-    if (RandomRay::mesh_subdivision_enabled_) {
-      domain_->source_regions_ = forward_source_regions;
-      domain_->source_region_map_ = forward_source_region_map;
-      domain_->base_source_regions_ = forward_base_source_regions;
-      domain_->source_regions_.adjoint_reset();
-    }
-    domain_->set_adjoint_sources(forward_flux);
+    domain_->set_local_adjoint_sources();
   }
 }
 
+void RandomRaySimulation::prepare_adjoint_simulation(bool from_forward)
+{
+  reset_timers();
+
+  if (mpi::master)
+    header("ADJOINT FLUX SOLVE", 3);
+
+  if (from_forward) {
+    // The forward solve has already run. Re-initialize OpenMC's general data
+    // structures for the adjoint solve and derive the adjoint source from the
+    // forward flux.
+    openmc_simulation_init();
+
+    prepare_fw_fixed_sources_adjoint();
+  } else {
+    // Initialize adjoint fixed sources
+    domain_->apply_meshes();
+    prepare_local_fixed_sources_adjoint();
+    domain_->count_external_source_regions();
+  }
+
+  domain_->k_eff_ = 1.0;
+
+  // Transpose scattering matrix
+  domain_->transpose_scattering_matrix();
+
+  // Swap nu_sigma_f and chi
+  domain_->nu_sigma_f_.swap(domain_->chi_);
+}
+
 void RandomRaySimulation::simulate()
 {
+  // Begin main simulation timer
+  simulation::time_total.start();
+
   // Random ray power iteration loop
   while (simulation::current_batch < settings::n_batches) {
     // Initialize the current batch
@@ -433,22 +406,18 @@ void RandomRaySimulation::simulate()
       simulation::total_weight = 1.0;
 
       // Update source term (scattering + fission)
-      domain_->update_neutron_source(k_eff_);
+      domain_->update_all_neutron_sources();
 
-      // Reset scalar fluxes, iteration volume tallies, and region hit flags to
-      // zero
+      // Reset scalar fluxes, iteration volume tallies, and region hit flags
+      // to zero
       domain_->batch_reset();
 
-      // At the beginning of the simulation, if mesh subvivision is in use, we
+      // At the beginning of the simulation, if mesh subdivision is in use, we
       // need to swap the main source region container into the base container,
       // as the main source region container will be used to hold the true
       // subdivided source regions. The base container will therefore only
       // contain the external source region information, the mesh indices,
       // material properties, and initial guess values for the flux/source.
-      if (RandomRay::mesh_subdivision_enabled_ &&
-          simulation::current_batch == 1 && !FlatSourceDomain::adjoint_) {
-        domain_->prepare_base_source_regions();
-      }
 
       // Start timer for transport
       simulation::time_transport.start();
@@ -464,11 +433,9 @@ void RandomRaySimulation::simulate()
 
       simulation::time_transport.stop();
 
-      // If using mesh subdivision, add any newly discovered source regions
-      // to the main source region container.
-      if (RandomRay::mesh_subdivision_enabled_) {
-        domain_->finalize_discovered_source_regions();
-      }
+      // Add any newly discovered source regions to the main source region
+      // container.
+      domain_->finalize_discovered_source_regions();
 
       // Normalize scalar flux and update volumes
       domain_->normalize_scalar_flux_and_volumes(
@@ -477,12 +444,15 @@ void RandomRaySimulation::simulate()
       // Add source to scalar flux, compute number of FSR hits
       int64_t n_hits = domain_->add_source_to_scalar_flux();
 
+      // Apply transport stabilization factors
+      domain_->apply_transport_stabilization();
+
       if (settings::run_mode == RunMode::EIGENVALUE) {
         // Compute random ray k-eff
-        k_eff_ = domain_->compute_k_eff(k_eff_);
+        domain_->compute_k_eff();
 
         // Store random ray k-eff into OpenMC's native k-eff variable
-        global_tally_tracklength = k_eff_;
+        global_tally_tracklength = domain_->k_eff_;
       }
 
       // Execute all tallying tasks, if this is an active batch
@@ -492,11 +462,6 @@ void RandomRaySimulation::simulate()
         // estimate
         domain_->accumulate_iteration_flux();
 
-        // Generate mapping between source regions and tallies
-        if (!domain_->mapped_all_tallies_) {
-          domain_->convert_source_regions_to_tallies();
-        }
-
         // Use above mapping to contribute FSR flux data to appropriate
         // tallies
         domain_->random_ray_tally();
@@ -506,13 +471,35 @@ void RandomRaySimulation::simulate()
       domain_->flux_swap();
 
       // Check for any obvious insabilities/nans/infs
-      instability_check(n_hits, k_eff_, avg_miss_rate_);
+      instability_check(n_hits, domain_->k_eff_, avg_miss_rate_);
     } // End MPI master work
 
     // Finalize the current batch
     finalize_generation();
     finalize_batch();
   } // End random ray power iteration loop
+
+  domain_->count_external_source_regions();
+
+  // End main simulation timer
+  simulation::time_total.stop();
+
+  // Normalize and save the final forward flux
+  double source_normalization_factor =
+    domain_->compute_fixed_source_normalization_factor() /
+    (settings::n_batches - settings::n_inactive);
+
+#pragma omp parallel for
+  for (uint64_t se = 0; se < domain_->n_source_elements(); se++) {
+    domain_->source_regions_.scalar_flux_final(se) *=
+      source_normalization_factor;
+  }
+
+  // Finalize OpenMC
+  openmc_simulation_finalize();
+
+  // Output all simulation results
+  output_simulation_results();
 }
 
 void RandomRaySimulation::output_simulation_results() const
@@ -553,7 +540,7 @@ void RandomRaySimulation::instability_check(
     }
 
     if (k_eff > 10.0 || k_eff < 0.01 || !(std::isfinite(k_eff))) {
-      fatal_error("Instability detected");
+      fatal_error(fmt::format("Instability detected: k-eff = {:.5f}", k_eff));
     }
   }
 }
@@ -576,17 +563,23 @@ void RandomRaySimulation::print_results_random_ray(
     header("Simulation Statistics", 4);
     fmt::print(
       " Total Iterations                  = {}\n", settings::n_batches);
-    fmt::print(" Flat Source Regions (FSRs)        = {}\n", n_source_regions);
     fmt::print(
-      " FSRs Containing External Sources  = {}\n", n_external_source_regions);
+      " Number of Rays per Iteration      = {}\n", settings::n_particles);
+    fmt::print(" Inactive Distance                 = {} cm\n",
+      RandomRay::distance_inactive_);
+    fmt::print(" Active Distance                   = {} cm\n",
+      RandomRay::distance_active_);
+    fmt::print(" Source Regions (SRs)              = {}\n", n_source_regions);
+    fmt::print(
+      " SRs Containing External Sources   = {}\n", n_external_source_regions);
     fmt::print(" Total Geometric Intersections     = {:.4e}\n",
       static_cast<double>(total_geometric_intersections));
     fmt::print("   Avg per Iteration               = {:.4e}\n",
       static_cast<double>(total_geometric_intersections) / settings::n_batches);
-    fmt::print("   Avg per Iteration per FSR       = {:.2f}\n",
+    fmt::print("   Avg per Iteration per SR        = {:.2f}\n",
       static_cast<double>(total_geometric_intersections) /
         static_cast<double>(settings::n_batches) / n_source_regions);
-    fmt::print(" Avg FSR Miss Rate per Iteration   = {:.4f}%\n", avg_miss_rate);
+    fmt::print(" Avg SR Miss Rate per Iteration    = {:.4f}%\n", avg_miss_rate);
     fmt::print(" Energy Groups                     = {}\n", negroups);
     fmt::print(
       " Total Integrations                = {:.4e}\n", total_integrations);
@@ -609,9 +602,46 @@ void RandomRaySimulation::print_results_random_ray(
     }
     fmt::print(" Volume Estimator Type             = {}\n", estimator);
 
-    std::string adjoint_true = (FlatSourceDomain::adjoint_) ? "ON" : "OFF";
+    std::string adjoint_true =
+      (FlatSourceDomain::solve_ == RandomRaySolve::ADJOINT) ? "ON" : "OFF";
     fmt::print(" Adjoint Flux Mode                 = {}\n", adjoint_true);
 
+    std::string shape;
+    switch (RandomRay::source_shape_) {
+    case RandomRaySourceShape::FLAT:
+      shape = "Flat";
+      break;
+    case RandomRaySourceShape::LINEAR:
+      shape = "Linear";
+      break;
+    case RandomRaySourceShape::LINEAR_XY:
+      shape = "Linear XY";
+      break;
+    default:
+      fatal_error("Invalid random ray source shape");
+    }
+    fmt::print(" Source Shape                      = {}\n", shape);
+    std::string sample_method;
+    switch (RandomRay::sample_method_) {
+    case RandomRaySampleMethod::PRNG:
+      sample_method = "PRNG";
+      break;
+    case RandomRaySampleMethod::HALTON:
+      sample_method = "Halton";
+      break;
+    case RandomRaySampleMethod::S2:
+      sample_method = "PRNG S2";
+      break;
+    }
+    fmt::print(" Sample Method                     = {}\n", sample_method);
+
+    if (domain_->is_transport_stabilization_needed_) {
+      fmt::print(" Transport XS Stabilization Used   = YES (rho = {:.3f})\n",
+        FlatSourceDomain::diagonal_stabilization_rho_);
+    } else {
+      fmt::print(" Transport XS Stabilization Used   = NO\n");
+    }
+
     header("Timing Statistics", 4);
     show_time("Total time for initialization", time_initialize.elapsed());
     show_time("Reading cross sections", time_read_xs.elapsed(), 1);
@@ -638,3 +668,56 @@ void RandomRaySimulation::print_results_random_ray(
 }
 
 } // namespace openmc
+
+//==============================================================================
+// C API functions
+//==============================================================================
+
+void openmc_run_random_ray()
+{
+  using namespace openmc;
+
+  // Determine which solves to run. If adjoint results are requested and no
+  // user-defined adjoint source is present, an initial forward solve is needed
+  // to construct the adjoint source from the forward flux (FW-CADIS). If the
+  // user has defined an adjoint source, the forward solve is skipped and only
+  // the adjoint solve is run.
+  const bool run_adjoint = FlatSourceDomain::adjoint_requested_;
+  const bool have_adjoint_source = !model::adjoint_sources.empty();
+  const bool run_forward = !(run_adjoint && have_adjoint_source);
+
+  // Set the initial solve type
+  if (!run_forward) {
+    FlatSourceDomain::solve_ = RandomRaySolve::ADJOINT;
+  } else if (run_adjoint) {
+    FlatSourceDomain::solve_ = RandomRaySolve::FORWARD_FOR_ADJOINT;
+  } else {
+    FlatSourceDomain::solve_ = RandomRaySolve::FORWARD;
+  }
+
+  // Initialize OpenMC general data structures
+  openmc_simulation_init();
+
+  // Validate that inputs meet requirements for random ray mode
+  if (mpi::master)
+    validate_random_ray_inputs();
+
+  // Initialize Random Ray Simulation Object
+  RandomRaySimulation sim;
+
+  // Run the forward solve
+  if (run_forward) {
+    // When an adjoint solve follows, report this as the initial forward solve
+    if (run_adjoint && mpi::master)
+      header("FORWARD FLUX SOLVE", 3);
+    sim.apply_fixed_sources_and_mesh_domains();
+    sim.simulate();
+  }
+
+  // Run the adjoint solve
+  if (run_adjoint) {
+    FlatSourceDomain::solve_ = RandomRaySolve::ADJOINT;
+    sim.prepare_adjoint_simulation(run_forward);
+    sim.simulate();
+  }
+}
diff --git a/src/random_ray/source_region.cpp b/src/random_ray/source_region.cpp
index 6c0df0157..78543c5ab 100644
--- a/src/random_ray/source_region.cpp
+++ b/src/random_ray/source_region.cpp
@@ -11,6 +11,7 @@ namespace openmc {
 //==============================================================================
 SourceRegionHandle::SourceRegionHandle(SourceRegion& sr)
   : negroups_(sr.scalar_flux_old_.size()), material_(&sr.material_),
+    temperature_idx_(&sr.temperature_idx_), density_mult_(&sr.density_mult_),
     is_small_(&sr.is_small_), n_hits_(&sr.n_hits_),
     is_linear_(sr.source_gradients_.size() > 0), lock_(&sr.lock_),
     volume_(&sr.volume_), volume_t_(&sr.volume_t_), volume_sq_(&sr.volume_sq_),
@@ -48,7 +49,7 @@ SourceRegion::SourceRegion(int negroups, bool is_linear)
   }
 
   scalar_flux_new_.assign(negroups, 0.0);
-  source_.resize(negroups);
+  source_.assign(negroups, 0.0);
   scalar_flux_final_.assign(negroups, 0.0);
 
   tally_task_.resize(negroups);
@@ -60,25 +61,6 @@ SourceRegion::SourceRegion(int negroups, bool is_linear)
   }
 }
 
-SourceRegion::SourceRegion(const SourceRegionHandle& handle, int64_t parent_sr)
-  : SourceRegion(handle.negroups_, handle.is_linear_)
-{
-  material_ = handle.material();
-  mesh_ = handle.mesh();
-  parent_sr_ = parent_sr;
-  for (int g = 0; g < scalar_flux_new_.size(); g++) {
-    scalar_flux_old_[g] = handle.scalar_flux_old(g);
-    source_[g] = handle.source(g);
-  }
-
-  if (settings::run_mode == RunMode::FIXED_SOURCE) {
-    external_source_present_ = handle.external_source_present();
-    for (int g = 0; g < scalar_flux_new_.size(); g++) {
-      external_source_[g] = handle.external_source(g);
-    }
-  }
-}
-
 //==============================================================================
 // SourceRegionContainer implementation
 //==============================================================================
@@ -89,6 +71,8 @@ void SourceRegionContainer::push_back(const SourceRegion& sr)
 
   // Scalar fields
   material_.push_back(sr.material_);
+  temperature_idx_.push_back(sr.temperature_idx_);
+  density_mult_.push_back(sr.density_mult_);
   is_small_.push_back(sr.is_small_);
   n_hits_.push_back(sr.n_hits_);
   lock_.push_back(sr.lock_);
@@ -142,6 +126,8 @@ void SourceRegionContainer::assign(
   // Clear existing data
   n_source_regions_ = 0;
   material_.clear();
+  temperature_idx_.clear();
+  density_mult_.clear();
   is_small_.clear();
   n_hits_.clear();
   lock_.clear();
@@ -199,6 +185,8 @@ SourceRegionHandle SourceRegionContainer::get_source_region_handle(int64_t sr)
   SourceRegionHandle handle;
   handle.negroups_ = negroups();
   handle.material_ = &material(sr);
+  handle.temperature_idx_ = &temperature_idx(sr);
+  handle.density_mult_ = &density_mult(sr);
   handle.is_small_ = &is_small(sr);
   handle.n_hits_ = &n_hits(sr);
   handle.is_linear_ = is_linear();
@@ -217,7 +205,11 @@ SourceRegionHandle SourceRegionContainer::get_source_region_handle(int64_t sr)
   handle.scalar_flux_old_ = &scalar_flux_old(sr, 0);
   handle.scalar_flux_new_ = &scalar_flux_new(sr, 0);
   handle.source_ = &source(sr, 0);
-  handle.external_source_ = &external_source(sr, 0);
+  if (settings::run_mode == RunMode::FIXED_SOURCE) {
+    handle.external_source_ = &external_source(sr, 0);
+  } else {
+    handle.external_source_ = nullptr;
+  }
   handle.scalar_flux_final_ = &scalar_flux_final(sr, 0);
   handle.tally_task_ = &tally_task(sr, 0);
 
@@ -255,15 +247,14 @@ void SourceRegionContainer::adjoint_reset()
     MomentMatrix {0.0, 0.0, 0.0, 0.0, 0.0, 0.0});
   std::fill(mom_matrix_t_.begin(), mom_matrix_t_.end(),
     MomentMatrix {0.0, 0.0, 0.0, 0.0, 0.0, 0.0});
-  for (auto& task_set : volume_task_) {
-    task_set.clear();
+  if (settings::run_mode == RunMode::FIXED_SOURCE) {
+    std::fill(scalar_flux_old_.begin(), scalar_flux_old_.end(), 0.0);
+  } else {
+    std::fill(scalar_flux_old_.begin(), scalar_flux_old_.end(), 1.0);
   }
-  std::fill(scalar_flux_old_.begin(), scalar_flux_old_.end(), 0.0);
   std::fill(scalar_flux_new_.begin(), scalar_flux_new_.end(), 0.0);
-  std::fill(scalar_flux_final_.begin(), scalar_flux_final_.end(), 0.0);
   std::fill(source_.begin(), source_.end(), 0.0f);
   std::fill(external_source_.begin(), external_source_.end(), 0.0f);
-
   std::fill(source_gradients_.begin(), source_gradients_.end(),
     MomentArray {0.0, 0.0, 0.0});
   std::fill(flux_moments_old_.begin(), flux_moments_old_.end(),
@@ -272,10 +263,6 @@ void SourceRegionContainer::adjoint_reset()
     MomentArray {0.0, 0.0, 0.0});
   std::fill(flux_moments_t_.begin(), flux_moments_t_.end(),
     MomentArray {0.0, 0.0, 0.0});
-
-  for (auto& task_set : tally_task_) {
-    task_set.clear();
-  }
 }
 
 } // namespace openmc
diff --git a/src/ray.cpp b/src/ray.cpp
new file mode 100644
index 000000000..3d848e3a3
--- /dev/null
+++ b/src/ray.cpp
@@ -0,0 +1,168 @@
+#include "openmc/ray.h"
+
+#include "openmc/error.h"
+#include "openmc/geometry.h"
+#include "openmc/settings.h"
+
+namespace openmc {
+
+void Ray::compute_distance()
+{
+  boundary() = distance_to_boundary(*this);
+}
+
+void Ray::trace()
+{
+  // To trace the ray from its origin all the way through the model, we have
+  // to proceed in two phases. In the first, the ray may or may not be found
+  // inside the model. If the ray is already in the model, phase one can be
+  // skipped. Otherwise, the ray has to be advanced to the boundary of the
+  // model where all the cells are defined. Importantly, this is assuming that
+  // the model is convex, which is a very reasonable assumption for any
+  // radiation transport model.
+  //
+  // After phase one is done, we can starting tracing from cell to cell within
+  // the model. This step can use neighbor lists to accelerate the ray tracing.
+
+  bool inside_cell;
+  // Check for location if the particle is already known
+  if (lowest_coord().cell() == C_NONE) {
+    // The geometry position of the particle is either unknown or outside of the
+    // edge of the model.
+    if (lowest_coord().universe() == C_NONE) {
+      // Attempt to initialize the particle. We may have to
+      // enter a loop to move it up to the edge of the model.
+      inside_cell = exhaustive_find_cell(*this, settings::verbosity >= 10);
+    } else {
+      // It has been already calculated that the current position is outside of
+      // the edge of the model.
+      inside_cell = false;
+    }
+  } else {
+    // Availability of the cell means that the particle is located inside the
+    // edge.
+    inside_cell = true;
+  }
+
+  // Advance to the boundary of the model
+  while (!inside_cell) {
+    advance_to_boundary_from_void();
+    inside_cell = exhaustive_find_cell(*this, settings::verbosity >= 10);
+
+    // If true this means no surface was intersected. See cell.cpp and search
+    // for numeric_limits to see where we return it.
+    if (surface() == std::numeric_limits<int>::max()) {
+      warning(fmt::format("Lost a ray, r = {}, u = {}", r(), u()));
+      return;
+    }
+
+    // Exit this loop and enter into cell-to-cell ray tracing (which uses
+    // neighbor lists)
+    if (inside_cell)
+      break;
+
+    // if there is no intersection with the model, we're done
+    if (boundary().surface() == SURFACE_NONE)
+      return;
+
+    event_counter_++;
+    if (event_counter_ > MAX_INTERSECTIONS) {
+      warning("Likely infinite loop in ray traced plot");
+      return;
+    }
+  }
+
+  // Call the specialized logic for this type of ray. This is for the
+  // intersection for the first intersection if we had one.
+  if (boundary().surface() != SURFACE_NONE) {
+    // set the geometry state's surface attribute to be used for
+    // surface normal computation
+    surface() = boundary().surface();
+    on_intersection();
+    if (stop_)
+      return;
+  }
+
+  // reset surface attribute to zero after the first intersection so that it
+  // doesn't perturb surface crossing logic from here on out
+  surface() = 0;
+
+  // This is the ray tracing loop within the model. It exits after exiting
+  // the model, which is equivalent to assuming that the model is convex.
+  // It would be nice to factor out the on_intersection at the end of this
+  // loop and then do "while (inside_cell)", but we can't guarantee it's
+  // on a surface in that case. There might be some other way to set it
+  // up that is perhaps a little more elegant, but this is what works just
+  // fine.
+  while (true) {
+
+    compute_distance();
+
+    // There are no more intersections to process
+    // if we hit the edge of the model, so stop
+    // the particle in that case. Also, just exit
+    // if a negative distance was somehow computed.
+    if (boundary().distance() == INFTY || boundary().distance() == INFINITY ||
+        boundary().distance() < 0) {
+      return;
+    }
+
+    // See below comment where call_on_intersection is checked in an
+    // if statement for an explanation of this.
+    bool call_on_intersection {true};
+    if (boundary().distance() < 10 * TINY_BIT) {
+      call_on_intersection = false;
+    }
+
+    // DAGMC surfaces expect us to go a little bit further than the advance
+    // distance to properly check cell inclusion.
+    boundary().distance() += TINY_BIT;
+
+    // Advance particle, prepare for next intersection
+    for (int lev = 0; lev < n_coord(); ++lev) {
+      coord(lev).r() += boundary().distance() * coord(lev).u();
+    }
+    surface() = boundary().surface();
+    // Initialize last cells from the current cell, because the cell() variable
+    // does not contain the data for the case of a single-segment ray
+    for (int j = 0; j < n_coord(); ++j) {
+      cell_last(j) = coord(j).cell();
+    }
+    n_coord_last() = n_coord();
+    n_coord() = boundary().coord_level();
+    if (boundary().lattice_translation()[0] != 0 ||
+        boundary().lattice_translation()[1] != 0 ||
+        boundary().lattice_translation()[2] != 0) {
+      cross_lattice(*this, boundary(), settings::verbosity >= 10);
+    }
+
+    // Record how far the ray has traveled
+    traversal_distance_ += boundary().distance();
+    inside_cell = neighbor_list_find_cell(*this, settings::verbosity >= 10);
+
+    // Call the specialized logic for this type of ray. Note that we do not
+    // call this if the advance distance is very small. Unfortunately, it seems
+    // darn near impossible to get the particle advanced to the model boundary
+    // and through it without sometimes accidentally calling on_intersection
+    // twice. This incorrectly shades the region as occluded when it might not
+    // actually be. By screening out intersection distances smaller than a
+    // threshold 10x larger than the scoot distance used to advance up to the
+    // model boundary, we can avoid that situation.
+    if (call_on_intersection) {
+      on_intersection();
+      if (stop_)
+        return;
+    }
+
+    if (!inside_cell)
+      return;
+
+    event_counter_++;
+    if (event_counter_ > MAX_INTERSECTIONS) {
+      warning("Likely infinite loop in ray traced plot");
+      return;
+    }
+  }
+}
+
+} // namespace openmc
diff --git a/src/reaction.cpp b/src/reaction.cpp
index 971473438..c02e0cc40 100644
--- a/src/reaction.cpp
+++ b/src/reaction.cpp
@@ -70,9 +70,8 @@ Reaction::Reaction(
 
   if (settings::use_decay_photons) {
     // Remove photon products for D1S method
-    products_.erase(
-      std::remove_if(products_.begin(), products_.end(),
-        [](const auto& p) { return p.particle_ == ParticleType::photon; }),
+    products_.erase(std::remove_if(products_.begin(), products_.end(),
+                      [](const auto& p) { return p.particle_.is_photon(); }),
       products_.end());
 
     // Determine product for D1S method
@@ -202,6 +201,9 @@ std::unordered_map<int, std::string> REACTION_NAME_MAP {
   {SCORE_FISS_Q_PROMPT, "fission-q-prompt"},
   {SCORE_FISS_Q_RECOV, "fission-q-recoverable"},
   {SCORE_PULSE_HEIGHT, "pulse-height"},
+  {SCORE_IFP_TIME_NUM, "ifp-time-numerator"},
+  {SCORE_IFP_BETA_NUM, "ifp-beta-numerator"},
+  {SCORE_IFP_DENOM, "ifp-denominator"},
   // Normal ENDF-based reactions
   {TOTAL_XS, "(n,total)"},
   {ELASTIC, "(n,elastic)"},
@@ -307,6 +309,7 @@ std::unordered_map<int, std::string> REACTION_NAME_MAP {
   {N_XA, "(n,Xa)"},
   {HEATING, "heating"},
   {DAMAGE_ENERGY, "damage-energy"},
+  {PHOTON_TOTAL, "photon-total"},
   {COHERENT, "coherent-scatter"},
   {INCOHERENT, "incoherent-scatter"},
   {PAIR_PROD_ELEC, "pair-production-electron"},
@@ -344,13 +347,24 @@ void initialize_maps()
   // Create photoelectric subshells
   for (int mt = 534; mt <= 572; ++mt) {
     REACTION_NAME_MAP[mt] =
-      fmt::format("photoelectric, {} subshell", SUBSHELLS[mt - 534]);
+      fmt::format("photoelectric-{}", SUBSHELLS[mt - 534]);
   }
 
   // Invert name map to create type map
   for (const auto& kv : REACTION_NAME_MAP) {
     REACTION_TYPE_MAP[kv.second] = kv.first;
   }
+
+  // Alternate names
+  REACTION_TYPE_MAP["elastic"] = ELASTIC;
+  REACTION_TYPE_MAP["n2n"] = N_2N;
+  REACTION_TYPE_MAP["n3n"] = N_3N;
+  REACTION_TYPE_MAP["n4n"] = N_4N;
+  REACTION_TYPE_MAP["H1-production"] = N_XP;
+  REACTION_TYPE_MAP["H2-production"] = N_XD;
+  REACTION_TYPE_MAP["H3-production"] = N_XT;
+  REACTION_TYPE_MAP["He3-production"] = N_X3HE;
+  REACTION_TYPE_MAP["He4-production"] = N_XA;
 }
 
 std::string reaction_name(int mt)
@@ -368,62 +382,42 @@ std::string reaction_name(int mt)
   }
 }
 
-int reaction_type(std::string name)
+int reaction_tally_mt(std::string name)
 {
-  // Initialize remainder of name map and all of type map
+  // All "total" scores should map to the special SCORE_TOTAL
+  if (name == "total" || name == "(n,total)" || name == "photon-total")
+    return SCORE_TOTAL;
+
+  // All fission scores should map to the special SCORE_FISSION
+  if (name == "fission" || name == "(n,fission)")
+    return SCORE_FISSION;
+
+  // Delegate everything else to reaction_mt()
+  return reaction_mt(name);
+}
+
+int reaction_mt(const std::string& name)
+{
+  // Initialize maps if needed
   if (REACTION_TYPE_MAP.empty())
     initialize_maps();
 
-  // (n,total) exists in REACTION_TYPE_MAP for MT=1, but we need this to return
-  // the special SCORE_TOTAL score
-  if (name == "(n,total)")
-    return SCORE_TOTAL;
-
-  // Check if type map has an entry for this reaction name
+  // Look up directly in type map (no score indirection)
   auto it = REACTION_TYPE_MAP.find(name);
   if (it != REACTION_TYPE_MAP.end()) {
-    return it->second;
+    int mt = it->second;
+    return mt;
   }
 
-  // Alternate names for several reactions
-  if (name == "elastic") {
-    return ELASTIC;
-  } else if (name == "n2n") {
-    return N_2N;
-  } else if (name == "n3n") {
-    return N_3N;
-  } else if (name == "n4n") {
-    return N_4N;
-  } else if (name == "H1-production") {
-    return N_XP;
-  } else if (name == "H2-production") {
-    return N_XD;
-  } else if (name == "H3-production") {
-    return N_XT;
-  } else if (name == "He3-production") {
-    return N_X3HE;
-  } else if (name == "He4-production") {
-    return N_XA;
-  }
-
-  // Assume the given string is a reaction MT number.  Make sure it's a natural
-  // number then return.
+  // Assume the given string is an MT number
   int MT = 0;
   try {
     MT = std::stoi(name);
   } catch (const std::invalid_argument& ex) {
-    throw std::invalid_argument(
-      "Invalid tally score \"" + name +
-      "\". See the docs "
-      "for details: "
-      "https://docs.openmc.org/en/stable/usersguide/tallies.html#scores");
+    throw std::invalid_argument("Unknown reaction name \"" + name + "\".");
   }
   if (MT < 1)
-    throw std::invalid_argument(
-      "Invalid tally score \"" + name +
-      "\". See the docs "
-      "for details: "
-      "https://docs.openmc.org/en/stable/usersguide/tallies.html#scores");
+    throw std::invalid_argument("Unknown reaction name \"" + name + "\".");
   return MT;
 }
 
diff --git a/src/reaction_product.cpp b/src/reaction_product.cpp
index 3ba2c0cfb..a1c937861 100644
--- a/src/reaction_product.cpp
+++ b/src/reaction_product.cpp
@@ -1,5 +1,6 @@
 #include "openmc/reaction_product.h"
 
+#include <cassert>
 #include <string> // for string
 
 #include <fmt/core.h>
@@ -26,7 +27,7 @@ ReactionProduct::ReactionProduct(hid_t group)
   // Read particle type
   std::string temp;
   read_attribute(group, "particle", temp);
-  particle_ = str_to_particle_type(temp);
+  particle_ = ParticleType {temp};
 
   // Read emission mode and decay rate
   read_attribute(group, "emission_mode", temp);
@@ -42,7 +43,7 @@ ReactionProduct::ReactionProduct(hid_t group)
   if (emission_mode_ == EmissionMode::delayed) {
     if (attribute_exists(group, "decay_rate")) {
       read_attribute(group, "decay_rate", decay_rate_);
-    } else if (particle_ == ParticleType::neutron) {
+    } else if (particle_.is_neutron()) {
       warning(fmt::format("Decay rate doesn't exist for delayed neutron "
                           "emission ({}).",
         object_name(group)));
@@ -85,7 +86,7 @@ ReactionProduct::ReactionProduct(hid_t group)
 
 ReactionProduct::ReactionProduct(const ChainNuclide::Product& product)
 {
-  particle_ = ParticleType::photon;
+  particle_ = ParticleType::photon();
   emission_mode_ = EmissionMode::delayed;
 
   // Get chain nuclide object for radionuclide
@@ -106,9 +107,10 @@ ReactionProduct::ReactionProduct(const ChainNuclide::Product& product)
     make_unique<DecayPhotonAngleEnergy>(chain_nuc->photon_energy()));
 }
 
-void ReactionProduct::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+AngleEnergy& ReactionProduct::sample_dist(double E_in, uint64_t* seed) const
 {
+  assert(!distribution_.empty());
+
   auto n = applicability_.size();
   if (n > 1) {
     double prob = 0.0;
@@ -118,15 +120,24 @@ void ReactionProduct::sample(
       prob += applicability_[i](E_in);
 
       // If i-th distribution is sampled, sample energy from the distribution
-      if (c <= prob) {
-        distribution_[i]->sample(E_in, E_out, mu, seed);
-        break;
-      }
+      if (c <= prob)
+        return *distribution_[i];
     }
-  } else {
-    // If only one distribution is present, go ahead and sample it
-    distribution_[0]->sample(E_in, E_out, mu, seed);
   }
+
+  return *distribution_.back();
+}
+
+void ReactionProduct::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  sample_dist(E_in, seed).sample(E_in, E_out, mu, seed);
+}
+
+double ReactionProduct::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  return sample_dist(E_in, seed).sample_energy_and_pdf(E_in, mu, E_out, seed);
 }
 
 } // namespace openmc
diff --git a/src/scattdata.cpp b/src/scattdata.cpp
index 21b18cbd9..9aa09956d 100644
--- a/src/scattdata.cpp
+++ b/src/scattdata.cpp
@@ -4,8 +4,7 @@
 #include <cmath>
 #include <numeric>
 
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/constants.h"
 #include "openmc/error.h"
@@ -19,8 +18,8 @@ namespace openmc {
 // ScattData base-class methods
 //==============================================================================
 
-void ScattData::base_init(int order, const xt::xtensor<int, 1>& in_gmin,
-  const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_energy,
+void ScattData::base_init(int order, const tensor::Tensor<int>& in_gmin,
+  const tensor::Tensor<int>& in_gmax, const double_2dvec& in_energy,
   const double_2dvec& in_mult)
 {
   size_t groups = in_energy.size();
@@ -63,23 +62,26 @@ void ScattData::base_init(int order, const xt::xtensor<int, 1>& in_gmin,
 
 void ScattData::base_combine(size_t max_order, size_t order_dim,
   const vector<ScattData*>& those_scatts, const vector<double>& scalars,
-  xt::xtensor<int, 1>& in_gmin, xt::xtensor<int, 1>& in_gmax,
+  tensor::Tensor<int>& in_gmin, tensor::Tensor<int>& in_gmax,
   double_2dvec& sparse_mult, double_3dvec& sparse_scatter)
 {
   size_t groups = those_scatts[0]->energy.size();
 
   // Now allocate and zero our storage spaces
-  xt::xtensor<double, 3> this_nuscatt_matrix({groups, groups, order_dim}, 0.);
-  xt::xtensor<double, 2> this_nuscatt_P0({groups, groups}, 0.);
-  xt::xtensor<double, 2> this_scatt_P0({groups, groups}, 0.);
-  xt::xtensor<double, 2> this_mult({groups, groups}, 1.);
+  tensor::Tensor<double> this_nuscatt_matrix =
+    tensor::zeros<double>({groups, groups, order_dim});
+  tensor::Tensor<double> this_nuscatt_P0 =
+    tensor::zeros<double>({groups, groups});
+  tensor::Tensor<double> this_scatt_P0 =
+    tensor::zeros<double>({groups, groups});
+  tensor::Tensor<double> this_mult = tensor::ones<double>({groups, groups});
 
   // Build the dense scattering and multiplicity matrices
   for (int i = 0; i < those_scatts.size(); i++) {
     ScattData* that = those_scatts[i];
 
     // Build the dense matrix for that object
-    xt::xtensor<double, 3> that_matrix = that->get_matrix(max_order);
+    tensor::Tensor<double> that_matrix = that->get_matrix(max_order);
 
     // Now add that to this for the nu-scatter matrix
     this_nuscatt_matrix += scalars[i] * that_matrix;
@@ -97,7 +99,7 @@ void ScattData::base_combine(size_t max_order, size_t order_dim,
 
   // Now we have the dense nuscatt and scatt, we can easily compute the
   // multiplicity matrix by dividing the two and fixing any nans
-  this_mult = xt::nan_to_num(this_nuscatt_P0 / this_scatt_P0);
+  this_mult = tensor::nan_to_num(this_nuscatt_P0 / this_scatt_P0);
 
   // We have the data, now we need to convert to a jagged array and then use
   // the initialize function to store it on the object.
@@ -106,7 +108,7 @@ void ScattData::base_combine(size_t max_order, size_t order_dim,
     int gmin_;
     for (gmin_ = 0; gmin_ < groups; gmin_++) {
       bool non_zero = false;
-      for (int l = 0; l < this_nuscatt_matrix.shape()[2]; l++) {
+      for (int l = 0; l < this_nuscatt_matrix.shape(2); l++) {
         if (this_nuscatt_matrix(gin, gmin_, l) != 0.) {
           non_zero = true;
           break;
@@ -118,7 +120,7 @@ void ScattData::base_combine(size_t max_order, size_t order_dim,
     int gmax_;
     for (gmax_ = groups - 1; gmax_ >= 0; gmax_--) {
       bool non_zero = false;
-      for (int l = 0; l < this_nuscatt_matrix.shape()[2]; l++) {
+      for (int l = 0; l < this_nuscatt_matrix.shape(2); l++) {
         if (this_nuscatt_matrix(gin, gmax_, l) != 0.) {
           non_zero = true;
           break;
@@ -143,8 +145,8 @@ void ScattData::base_combine(size_t max_order, size_t order_dim,
     sparse_mult[gin].resize(gmax_ - gmin_ + 1);
     int i_gout = 0;
     for (int gout = gmin_; gout <= gmax_; gout++) {
-      sparse_scatter[gin][i_gout].resize(this_nuscatt_matrix.shape()[2]);
-      for (int l = 0; l < this_nuscatt_matrix.shape()[2]; l++) {
+      sparse_scatter[gin][i_gout].resize(this_nuscatt_matrix.shape(2));
+      for (int l = 0; l < this_nuscatt_matrix.shape(2); l++) {
         sparse_scatter[gin][i_gout][l] = this_nuscatt_matrix(gin, gout, l);
       }
       sparse_mult[gin][i_gout] = this_mult(gin, gout);
@@ -227,8 +229,8 @@ double ScattData::get_xs(
 // ScattDataLegendre methods
 //==============================================================================
 
-void ScattDataLegendre::init(const xt::xtensor<int, 1>& in_gmin,
-  const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+void ScattDataLegendre::init(const tensor::Tensor<int>& in_gmin,
+  const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
   const double_3dvec& coeffs)
 {
   size_t groups = coeffs.size();
@@ -239,7 +241,7 @@ void ScattDataLegendre::init(const xt::xtensor<int, 1>& in_gmin,
 
   // Get the scattering cross section value by summing the un-normalized P0
   // coefficient in the variable matrix over all outgoing groups.
-  scattxs = xt::zeros<double>({groups});
+  scattxs = tensor::zeros<double>({groups});
   for (int gin = 0; gin < groups; gin++) {
     int num_groups = in_gmax[gin] - in_gmin[gin] + 1;
     for (int i_gout = 0; i_gout < num_groups; i_gout++) {
@@ -386,8 +388,8 @@ void ScattDataLegendre::combine(
 
   size_t groups = those_scatts[0]->energy.size();
 
-  xt::xtensor<int, 1> in_gmin({groups}, 0);
-  xt::xtensor<int, 1> in_gmax({groups}, 0);
+  tensor::Tensor<int> in_gmin({groups}, 0);
+  tensor::Tensor<int> in_gmax({groups}, 0);
   double_3dvec sparse_scatter(groups);
   double_2dvec sparse_mult(groups);
 
@@ -404,12 +406,13 @@ void ScattDataLegendre::combine(
 
 //==============================================================================
 
-xt::xtensor<double, 3> ScattDataLegendre::get_matrix(size_t max_order)
+tensor::Tensor<double> ScattDataLegendre::get_matrix(size_t max_order)
 {
   // Get the sizes and initialize the data to 0
   size_t groups = energy.size();
   size_t order_dim = max_order + 1;
-  xt::xtensor<double, 3> matrix({groups, groups, order_dim}, 0.);
+  tensor::Tensor<double> matrix =
+    tensor::zeros<double>({groups, groups, order_dim});
 
   for (int gin = 0; gin < groups; gin++) {
     for (int i_gout = 0; i_gout < energy[gin].size(); i_gout++) {
@@ -427,8 +430,8 @@ xt::xtensor<double, 3> ScattDataLegendre::get_matrix(size_t max_order)
 // ScattDataHistogram methods
 //==============================================================================
 
-void ScattDataHistogram::init(const xt::xtensor<int, 1>& in_gmin,
-  const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+void ScattDataHistogram::init(const tensor::Tensor<int>& in_gmin,
+  const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
   const double_3dvec& coeffs)
 {
   size_t groups = coeffs.size();
@@ -439,7 +442,7 @@ void ScattDataHistogram::init(const xt::xtensor<int, 1>& in_gmin,
 
   // Get the scattering cross section value by summing the distribution
   // over all the histogram bins in angle and outgoing energy groups
-  scattxs = xt::zeros<double>({groups});
+  scattxs = tensor::zeros<double>({groups});
   for (int gin = 0; gin < groups; gin++) {
     for (int i_gout = 0; i_gout < matrix[gin].size(); i_gout++) {
       scattxs[gin] += std::accumulate(
@@ -468,7 +471,7 @@ void ScattDataHistogram::init(const xt::xtensor<int, 1>& in_gmin,
   ScattData::base_init(order, in_gmin, in_gmax, in_energy, in_mult);
 
   // Build the angular distribution mu values
-  mu = xt::linspace(-1., 1., order + 1);
+  mu = tensor::linspace(-1., 1., order + 1);
   dmu = 2. / order;
 
   // Calculate f(mu) and integrate it so we can avoid rejection sampling
@@ -513,7 +516,7 @@ double ScattDataHistogram::calc_f(int gin, int gout, double mu)
     int imu;
     if (mu == 1.) {
       // use size -2 to have the index one before the end
-      imu = this->mu.shape()[0] - 2;
+      imu = this->mu.shape(0) - 2;
     } else {
       imu = std::floor((mu + 1.) / dmu + 1.) - 1;
     }
@@ -559,13 +562,13 @@ void ScattDataHistogram::sample(
 
 //==============================================================================
 
-xt::xtensor<double, 3> ScattDataHistogram::get_matrix(size_t max_order)
+tensor::Tensor<double> ScattDataHistogram::get_matrix(size_t max_order)
 {
   // Get the sizes and initialize the data to 0
   size_t groups = energy.size();
   // We ignore the requested order for Histogram and Tabular representations
   size_t order_dim = get_order();
-  xt::xtensor<double, 3> matrix({groups, groups, order_dim}, 0);
+  tensor::Tensor<double> matrix({groups, groups, order_dim}, 0);
 
   for (int gin = 0; gin < groups; gin++) {
     for (int i_gout = 0; i_gout < energy[gin].size(); i_gout++) {
@@ -600,8 +603,8 @@ void ScattDataHistogram::combine(
 
   size_t groups = those_scatts[0]->energy.size();
 
-  xt::xtensor<int, 1> in_gmin({groups}, 0);
-  xt::xtensor<int, 1> in_gmax({groups}, 0);
+  tensor::Tensor<int> in_gmin({groups}, 0);
+  tensor::Tensor<int> in_gmax({groups}, 0);
   double_3dvec sparse_scatter(groups);
   double_2dvec sparse_mult(groups);
 
@@ -620,8 +623,8 @@ void ScattDataHistogram::combine(
 // ScattDataTabular methods
 //==============================================================================
 
-void ScattDataTabular::init(const xt::xtensor<int, 1>& in_gmin,
-  const xt::xtensor<int, 1>& in_gmax, const double_2dvec& in_mult,
+void ScattDataTabular::init(const tensor::Tensor<int>& in_gmin,
+  const tensor::Tensor<int>& in_gmax, const double_2dvec& in_mult,
   const double_3dvec& coeffs)
 {
   size_t groups = coeffs.size();
@@ -631,12 +634,12 @@ void ScattDataTabular::init(const xt::xtensor<int, 1>& in_gmin,
   double_3dvec matrix = coeffs;
 
   // Build the angular distribution mu values
-  mu = xt::linspace(-1., 1., order);
+  mu = tensor::linspace(-1., 1., order);
   dmu = 2. / (order - 1);
 
   // Get the scattering cross section value by integrating the distribution
   // over all mu points and then combining over all outgoing groups
-  scattxs = xt::zeros<double>({groups});
+  scattxs = tensor::zeros<double>({groups});
   for (int gin = 0; gin < groups; gin++) {
     for (int i_gout = 0; i_gout < matrix[gin].size(); i_gout++) {
       for (int imu = 1; imu < order; imu++) {
@@ -713,7 +716,7 @@ double ScattDataTabular::calc_f(int gin, int gout, double mu)
     int imu;
     if (mu == 1.) {
       // use size -2 to have the index one before the end
-      imu = this->mu.shape()[0] - 2;
+      imu = this->mu.shape(0) - 2;
     } else {
       imu = std::floor((mu + 1.) / dmu + 1.) - 1;
     }
@@ -734,7 +737,7 @@ void ScattDataTabular::sample(
   sample_energy(gin, gout, i_gout, seed);
 
   // Determine the outgoing cosine bin
-  int NP = this->mu.shape()[0];
+  int NP = this->mu.shape(0);
   double xi = prn(seed);
 
   double c_k = dist[gin][i_gout][0];
@@ -776,13 +779,14 @@ void ScattDataTabular::sample(
 
 //==============================================================================
 
-xt::xtensor<double, 3> ScattDataTabular::get_matrix(size_t max_order)
+tensor::Tensor<double> ScattDataTabular::get_matrix(size_t max_order)
 {
   // Get the sizes and initialize the data to 0
   size_t groups = energy.size();
   // We ignore the requested order for Histogram and Tabular representations
   size_t order_dim = get_order();
-  xt::xtensor<double, 3> matrix({groups, groups, order_dim}, 0.);
+  tensor::Tensor<double> matrix =
+    tensor::zeros<double>({groups, groups, order_dim});
 
   for (int gin = 0; gin < groups; gin++) {
     for (int i_gout = 0; i_gout < energy[gin].size(); i_gout++) {
@@ -816,8 +820,8 @@ void ScattDataTabular::combine(
 
   size_t groups = those_scatts[0]->energy.size();
 
-  xt::xtensor<int, 1> in_gmin({groups}, 0);
-  xt::xtensor<int, 1> in_gmax({groups}, 0);
+  tensor::Tensor<int> in_gmin({groups}, 0);
+  tensor::Tensor<int> in_gmax({groups}, 0);
   double_3dvec sparse_scatter(groups);
   double_2dvec sparse_mult(groups);
 
@@ -854,7 +858,7 @@ void convert_legendre_to_tabular(ScattDataLegendre& leg, ScattDataTabular& tab)
   tab.scattxs = leg.scattxs;
 
   // Build mu and dmu
-  tab.mu = xt::linspace(-1., 1., n_mu);
+  tab.mu = tensor::linspace(-1., 1., n_mu);
   tab.dmu = 2. / (n_mu - 1);
 
   // Calculate f(mu) and integrate it so we can avoid rejection sampling
diff --git a/src/secondary_correlated.cpp b/src/secondary_correlated.cpp
index 0e4891dd3..32701791a 100644
--- a/src/secondary_correlated.cpp
+++ b/src/secondary_correlated.cpp
@@ -5,11 +5,11 @@
 #include <cstddef>  // for size_t
 #include <iterator> // for back_inserter
 
-#include "xtensor/xarray.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/endf.h"
 #include "openmc/hdf5_interface.h"
+#include "openmc/math_functions.h"
 #include "openmc/random_lcg.h"
 #include "openmc/search.h"
 
@@ -25,11 +25,11 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
   hid_t dset = open_dataset(group, "energy");
 
   // Get interpolation parameters
-  xt::xarray<int> temp;
+  tensor::Tensor<int> temp;
   read_attribute(dset, "interpolation", temp);
 
-  auto temp_b = xt::view(temp, 0); // view of breakpoints
-  auto temp_i = xt::view(temp, 1); // view of interpolation parameters
+  tensor::View<int> temp_b = temp.slice(0); // breakpoints
+  tensor::View<int> temp_i = temp.slice(1); // interpolation parameters
 
   std::copy(temp_b.begin(), temp_b.end(), std::back_inserter(breakpoints_));
   for (const auto i : temp_i)
@@ -50,12 +50,12 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
   read_attribute(dset, "interpolation", interp);
   read_attribute(dset, "n_discrete_lines", n_discrete);
 
-  xt::xarray<double> eout;
+  tensor::Tensor<double> eout;
   read_dataset(dset, eout);
   close_dataset(dset);
 
   // Read angle distributions
-  xt::xarray<double> mu;
+  tensor::Tensor<double> mu;
   read_dataset(group, "mu", mu);
 
   for (int i = 0; i < n_energy; ++i) {
@@ -65,7 +65,7 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
     if (i < n_energy - 1) {
       n = offsets[i + 1] - j;
     } else {
-      n = eout.shape()[1] - j;
+      n = eout.shape(1) - j;
     }
 
     // Assign interpolation scheme and number of discrete lines
@@ -74,9 +74,9 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
     d.n_discrete = n_discrete[i];
 
     // Copy data
-    d.e_out = xt::view(eout, 0, xt::range(j, j + n));
-    d.p = xt::view(eout, 1, xt::range(j, j + n));
-    d.c = xt::view(eout, 2, xt::range(j, j + n));
+    d.e_out = eout.slice(0, tensor::range(j, j + n));
+    d.p = eout.slice(1, tensor::range(j, j + n));
+    d.c = eout.slice(2, tensor::range(j, j + n));
 
     // To get answers that match ACE data, for now we still use the tabulated
     // CDF values that were passed through to the HDF5 library. At a later
@@ -118,10 +118,10 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
       // Determine offset and size of distribution
       int offset_mu = std::lround(eout(4, offsets[i] + j));
       int m;
-      if (offsets[i] + j + 1 < eout.shape()[1]) {
+      if (offsets[i] + j + 1 < eout.shape(1)) {
         m = std::lround(eout(4, offsets[i] + j + 1)) - offset_mu;
       } else {
-        m = mu.shape()[1] - offset_mu;
+        m = mu.shape(1) - offset_mu;
       }
 
       // For incoherent inelastic thermal scattering, the angle distributions
@@ -132,9 +132,12 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
         interp_mu = 1;
 
       auto interp = int2interp(interp_mu);
-      auto xs = xt::view(mu, 0, xt::range(offset_mu, offset_mu + m));
-      auto ps = xt::view(mu, 1, xt::range(offset_mu, offset_mu + m));
-      auto cs = xt::view(mu, 2, xt::range(offset_mu, offset_mu + m));
+      tensor::View<double> xs =
+        mu.slice(0, tensor::range(offset_mu, offset_mu + m));
+      tensor::View<double> ps =
+        mu.slice(1, tensor::range(offset_mu, offset_mu + m));
+      tensor::View<double> cs =
+        mu.slice(2, tensor::range(offset_mu, offset_mu + m));
 
       vector<double> x {xs.begin(), xs.end()};
       vector<double> p {ps.begin(), ps.end()};
@@ -152,25 +155,13 @@ CorrelatedAngleEnergy::CorrelatedAngleEnergy(hid_t group)
     distribution_.push_back(std::move(d));
   } // incoming energies
 }
-
-void CorrelatedAngleEnergy::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+Distribution& CorrelatedAngleEnergy::sample_dist(
+  double E_in, double& E_out, uint64_t* seed) const
 {
-  // Find energy bin and calculate interpolation factor -- if the energy is
-  // outside the range of the tabulated energies, choose the first or last bins
-  auto n_energy_in = energy_.size();
+  // Find energy bin and calculate interpolation factor
   int i;
   double r;
-  if (E_in < energy_[0]) {
-    i = 0;
-    r = 0.0;
-  } else if (E_in > energy_[n_energy_in - 1]) {
-    i = n_energy_in - 2;
-    r = 1.0;
-  } else {
-    i = lower_bound_index(energy_.begin(), energy_.end(), E_in);
-    r = (E_in - energy_[i]) / (energy_[i + 1] - energy_[i]);
-  }
+  get_energy_index(energy_, E_in, i, r);
 
   // Sample between the ith and [i+1]th bin
   int l = r > prn(seed) ? i + 1 : i;
@@ -257,10 +248,22 @@ void CorrelatedAngleEnergy::sample(
   // Find correlated angular distribution for closest outgoing energy bin
   if (r1 - c_k < c_k1 - r1 ||
       distribution_[l].interpolation == Interpolation::histogram) {
-    mu = distribution_[l].angle[k]->sample(seed);
+    return *distribution_[l].angle[k];
   } else {
-    mu = distribution_[l].angle[k + 1]->sample(seed);
+    return *distribution_[l].angle[k + 1];
   }
 }
 
+void CorrelatedAngleEnergy::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  mu = sample_dist(E_in, E_out, seed).sample(seed).first;
+}
+
+double CorrelatedAngleEnergy::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  return sample_dist(E_in, E_out, seed).evaluate(mu);
+}
+
 } // namespace openmc
diff --git a/src/secondary_kalbach.cpp b/src/secondary_kalbach.cpp
index 4a7878343..018ce1c8a 100644
--- a/src/secondary_kalbach.cpp
+++ b/src/secondary_kalbach.cpp
@@ -5,10 +5,10 @@
 #include <cstddef>   // for size_t
 #include <iterator>  // for back_inserter
 
-#include "xtensor/xarray.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/hdf5_interface.h"
+#include "openmc/math_functions.h"
 #include "openmc/random_dist.h"
 #include "openmc/random_lcg.h"
 #include "openmc/search.h"
@@ -26,11 +26,11 @@ KalbachMann::KalbachMann(hid_t group)
   hid_t dset = open_dataset(group, "energy");
 
   // Get interpolation parameters
-  xt::xarray<int> temp;
+  tensor::Tensor<int> temp;
   read_attribute(dset, "interpolation", temp);
 
-  auto temp_b = xt::view(temp, 0); // view of breakpoints
-  auto temp_i = xt::view(temp, 1); // view of interpolation parameters
+  tensor::View<int> temp_b = temp.slice(0); // breakpoints
+  tensor::View<int> temp_i = temp.slice(1); // interpolation parameters
 
   std::copy(temp_b.begin(), temp_b.end(), std::back_inserter(breakpoints_));
   for (const auto i : temp_i)
@@ -51,7 +51,7 @@ KalbachMann::KalbachMann(hid_t group)
   read_attribute(dset, "interpolation", interp);
   read_attribute(dset, "n_discrete_lines", n_discrete);
 
-  xt::xarray<double> eout;
+  tensor::Tensor<double> eout;
   read_dataset(dset, eout);
   close_dataset(dset);
 
@@ -62,7 +62,7 @@ KalbachMann::KalbachMann(hid_t group)
     if (i < n_energy - 1) {
       n = offsets[i + 1] - j;
     } else {
-      n = eout.shape()[1] - j;
+      n = eout.shape(1) - j;
     }
 
     // Assign interpolation scheme and number of discrete lines
@@ -71,11 +71,11 @@ KalbachMann::KalbachMann(hid_t group)
     d.n_discrete = n_discrete[i];
 
     // Copy data
-    d.e_out = xt::view(eout, 0, xt::range(j, j + n));
-    d.p = xt::view(eout, 1, xt::range(j, j + n));
-    d.c = xt::view(eout, 2, xt::range(j, j + n));
-    d.r = xt::view(eout, 3, xt::range(j, j + n));
-    d.a = xt::view(eout, 4, xt::range(j, j + n));
+    d.e_out = eout.slice(0, tensor::range(j, j + n));
+    d.p = eout.slice(1, tensor::range(j, j + n));
+    d.c = eout.slice(2, tensor::range(j, j + n));
+    d.r = eout.slice(3, tensor::range(j, j + n));
+    d.a = eout.slice(4, tensor::range(j, j + n));
 
     // To get answers that match ACE data, for now we still use the tabulated
     // CDF values that were passed through to the HDF5 library. At a later
@@ -114,24 +114,13 @@ KalbachMann::KalbachMann(hid_t group)
   } // incoming energies
 }
 
-void KalbachMann::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+void KalbachMann::sample_params(
+  double E_in, double& E_out, double& km_a, double& km_r, uint64_t* seed) const
 {
-  // Find energy bin and calculate interpolation factor -- if the energy is
-  // outside the range of the tabulated energies, choose the first or last bins
-  auto n_energy_in = energy_.size();
+  // Find energy bin and calculate interpolation factor
   int i;
   double r;
-  if (E_in < energy_[0]) {
-    i = 0;
-    r = 0.0;
-  } else if (E_in > energy_[n_energy_in - 1]) {
-    i = n_energy_in - 2;
-    r = 1.0;
-  } else {
-    i = lower_bound_index(energy_.begin(), energy_.end(), E_in);
-    r = (E_in - energy_[i]) / (energy_[i + 1] - energy_[i]);
-  }
+  get_energy_index(energy_, E_in, i, r);
 
   // Sample between the ith and [i+1]th bin
   int l = r > prn(seed) ? i + 1 : i;
@@ -181,7 +170,6 @@ void KalbachMann::sample(
 
   double E_l_k = distribution_[l].e_out[k];
   double p_l_k = distribution_[l].p[k];
-  double km_r, km_a;
   if (distribution_[l].interpolation == Interpolation::histogram) {
     // Histogram interpolation
     if (p_l_k > 0.0 && k >= n_discrete) {
@@ -227,6 +215,13 @@ void KalbachMann::sample(
       E_out = E_1 + (E_out - E_i1_1) * (E_K - E_1) / (E_i1_K - E_i1_1);
     }
   }
+}
+
+void KalbachMann::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  double km_r, km_a;
+  sample_params(E_in, E_out, km_a, km_r, seed);
 
   // Sampled correlated angle from Kalbach-Mann parameters
   if (prn(seed) > km_r) {
@@ -237,5 +232,15 @@ void KalbachMann::sample(
     mu = std::log(r1 * std::exp(km_a) + (1.0 - r1) * std::exp(-km_a)) / km_a;
   }
 }
+double KalbachMann::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  double km_r, km_a;
+  sample_params(E_in, E_out, km_a, km_r, seed);
+
+  // https://docs.openmc.org/en/latest/methods/neutron_physics.html#equation-KM-pdf-angle
+  return km_a / (2 * std::sinh(km_a)) *
+         (std::cosh(km_a * mu) + km_r * std::sinh(km_a * mu));
+}
 
 } // namespace openmc
diff --git a/src/secondary_nbody.cpp b/src/secondary_nbody.cpp
index da0bb81c4..72f0b0b92 100644
--- a/src/secondary_nbody.cpp
+++ b/src/secondary_nbody.cpp
@@ -22,13 +22,8 @@ NBodyPhaseSpace::NBodyPhaseSpace(hid_t group)
   read_attribute(group, "q_value", Q_);
 }
 
-void NBodyPhaseSpace::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+double NBodyPhaseSpace::sample_energy(double E_in, uint64_t* seed) const
 {
-  // By definition, the distribution of the angle is isotropic for an N-body
-  // phase space distribution
-  mu = uniform_distribution(-1., 1., seed);
-
   // Determine E_max parameter
   double Ap = mass_ratio_;
   double E_max = (Ap - 1.0) / Ap * (A_ / (A_ + 1.0) * E_in + Q_);
@@ -59,12 +54,29 @@ void NBodyPhaseSpace::sample(
         std::log(r5) * std::pow(std::cos(PI / 2.0 * r6), 2);
     break;
   default:
-    throw std::runtime_error {"N-body phase space with >5 bodies."};
+    fatal_error("N-body phase space with >5 bodies.");
   }
 
   // Now determine v and E_out
   double v = x / (x + y);
-  E_out = E_max * v;
+  return E_max * v;
+}
+
+void NBodyPhaseSpace::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  // By definition, the distribution of the angle is isotropic for an N-body
+  // phase space distribution
+  mu = uniform_distribution(-1., 1., seed);
+
+  E_out = sample_energy(E_in, seed);
+}
+
+double NBodyPhaseSpace::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  E_out = sample_energy(E_in, seed);
+  return 0.5;
 }
 
 } // namespace openmc
diff --git a/src/secondary_thermal.cpp b/src/secondary_thermal.cpp
index 8b9e8737c..b0f601809 100644
--- a/src/secondary_thermal.cpp
+++ b/src/secondary_thermal.cpp
@@ -1,44 +1,42 @@
 #include "openmc/secondary_thermal.h"
 
 #include "openmc/hdf5_interface.h"
+#include "openmc/math_functions.h"
 #include "openmc/random_lcg.h"
 #include "openmc/search.h"
+#include "openmc/vector.h"
 
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include <cassert>
 #include <cmath> // for log, exp
 
 namespace openmc {
 
-// Helper function to get index on incident energy grid
-void get_energy_index(
-  const vector<double>& energies, double E, int& i, double& f)
-{
-  // Get index and interpolation factor for elastic grid
-  i = 0;
-  f = 0.0;
-  if (E >= energies.front()) {
-    i = lower_bound_index(energies.begin(), energies.end(), E);
-    if (i + 1 < energies.size())
-      f = (E - energies[i]) / (energies[i + 1] - energies[i]);
-  }
-}
-
 //==============================================================================
 // CoherentElasticAE implementation
 //==============================================================================
 
-CoherentElasticAE::CoherentElasticAE(const CoherentElasticXS& xs) : xs_ {xs} {}
+CoherentElasticAE::CoherentElasticAE(const CoherentElasticXS& xs) : xs_ {xs}
+{
+  const auto& bragg = xs_.bragg_edges();
+  auto n = bragg.size();
+  bragg_edges_ = tensor::Tensor<double>(bragg.data(), n);
+
+  const auto& factors = xs_.factors();
+  factors_diff_ = tensor::zeros<double>({n});
+  factors_diff_.slice(0) = factors[0];
+  for (int i = 1; i < n; ++i) {
+    factors_diff_.slice(i) = factors[i] - factors[i - 1];
+  }
+}
 
 void CoherentElasticAE::sample(
   double E_in, double& E_out, double& mu, uint64_t* seed) const
 {
   // Energy doesn't change in elastic scattering (ENDF-102, Eq. 7-1)
   E_out = E_in;
-
   const auto& energies {xs_.bragg_edges()};
-
   assert(E_in >= energies.front());
 
   const int i = lower_bound_index(energies.begin(), energies.end(), E_in);
@@ -55,6 +53,25 @@ void CoherentElasticAE::sample(
   mu = 1.0 - 2.0 * energies[k] / E_in;
 }
 
+double CoherentElasticAE::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  // Energy doesn't change in elastic scattering (ENDF-102, Eq. 7-1)
+  E_out = E_in;
+  const auto& factors = xs_.factors();
+
+  if (E_in < bragg_edges_.front())
+    return 0.0;
+
+  const int i =
+    lower_bound_index(bragg_edges_.begin(), bragg_edges_.end(), E_in);
+  double E = 0.5 * (1 - mu) * E_in;
+  double C = 0.5 * E_in / factors[i];
+
+  return C * get_pdf_discrete(bragg_edges_.slice(tensor::range(i + 1)),
+               factors_diff_.slice(tensor::range(i + 1)), E, 0.0, E_in);
+}
+
 //==============================================================================
 // IncoherentElasticAE implementation
 //==============================================================================
@@ -67,12 +84,21 @@ IncoherentElasticAE::IncoherentElasticAE(hid_t group)
 void IncoherentElasticAE::sample(
   double E_in, double& E_out, double& mu, uint64_t* seed) const
 {
+  E_out = E_in;
+
   // Sample angle by inverting the distribution in ENDF-102, Eq. 7.4
   double c = 2 * E_in * debye_waller_;
   mu = std::log(1.0 + prn(seed) * (std::exp(2.0 * c) - 1)) / c - 1.0;
-
-  // Energy doesn't change in elastic scattering (ENDF-102, Eq. 7.4)
+}
+double IncoherentElasticAE::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
   E_out = E_in;
+
+  // Sample angle by inverting the distribution in ENDF-102, Eq. 7.4
+  double c = 2 * E_in * debye_waller_;
+  double A = c / (1 - std::exp(-2.0 * c)); // normalization factor
+  return A * std::exp(-c * (1 - mu));
 }
 
 //==============================================================================
@@ -98,7 +124,7 @@ void IncoherentElasticAEDiscrete::sample(
   // incoming energies.
 
   // Sample outgoing cosine bin
-  int n_mu = mu_out_.shape()[1];
+  int n_mu = mu_out_.shape(1);
   int k = prn(seed) * n_mu;
 
   // Rather than use the sampled discrete mu directly, it is smeared over
@@ -129,6 +155,20 @@ void IncoherentElasticAEDiscrete::sample(
   E_out = E_in;
 }
 
+double IncoherentElasticAEDiscrete::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  // Get index and interpolation factor for elastic grid
+  int i;
+  double f;
+  get_energy_index(energy_, E_in, i, f);
+  // Energy doesn't change in elastic scattering
+  E_out = E_in;
+
+  return get_pdf_discrete_interpolated(
+    mu_out_.slice(i, tensor::all), mu_out_.slice(i + 1, tensor::all), f, mu);
+}
+
 //==============================================================================
 // IncoherentInelasticAEDiscrete implementation
 //==============================================================================
@@ -142,8 +182,8 @@ IncoherentInelasticAEDiscrete::IncoherentInelasticAEDiscrete(
   read_dataset(group, "skewed", skewed_);
 }
 
-void IncoherentInelasticAEDiscrete::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+void IncoherentInelasticAEDiscrete::sample_params(
+  double E_in, double& E_out, int& j, uint64_t* seed) const
 {
   // Get index and interpolation factor for inelastic grid
   int i;
@@ -157,8 +197,7 @@ void IncoherentInelasticAEDiscrete::sample(
   // for the second and second to last bins, relative to a normal bin
   // probability of 1). Otherwise, each bin is equally probable.
 
-  int j;
-  int n = energy_out_.shape()[1];
+  int n = energy_out_.shape(1);
   if (!skewed_) {
     // All bins equally likely
     j = prn(seed) * n;
@@ -189,9 +228,21 @@ void IncoherentInelasticAEDiscrete::sample(
 
   // Outgoing energy
   E_out = (1 - f) * E_ij + f * E_i1j;
+}
+
+void IncoherentInelasticAEDiscrete::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  // Get index and interpolation factor for inelastic grid
+  int i;
+  double f;
+  get_energy_index(energy_, E_in, i, f);
+
+  int j;
+  sample_params(E_in, E_out, j, seed);
 
   // Sample outgoing cosine bin
-  int m = mu_out_.shape()[2];
+  int m = mu_out_.shape(2);
   int k = prn(seed) * m;
 
   // Determine outgoing cosine corresponding to E_in[i] and E_in[i+1]
@@ -202,6 +253,20 @@ void IncoherentInelasticAEDiscrete::sample(
   mu = (1 - f) * mu_ijk + f * mu_i1jk;
 }
 
+double IncoherentInelasticAEDiscrete::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  // Get index and interpolation factor for inelastic grid
+  int i;
+  double f;
+  get_energy_index(energy_, E_in, i, f);
+  int j;
+  sample_params(E_in, E_out, j, seed);
+
+  return get_pdf_discrete_interpolated(mu_out_.slice(i, j, tensor::all),
+    mu_out_.slice(i + 1, j, tensor::all), f, mu);
+}
+
 //==============================================================================
 // IncoherentInelasticAE implementation
 //==============================================================================
@@ -231,11 +296,11 @@ IncoherentInelasticAE::IncoherentInelasticAE(hid_t group)
         // On first pass, allocate space for angles
         if (j == 0) {
           auto n_mu = adist->x().size();
-          d.mu = xt::empty<double>({d.n_e_out, n_mu});
+          d.mu = tensor::Tensor<double>({d.n_e_out, n_mu});
         }
 
         // Copy outgoing angles
-        auto mu_j = xt::view(d.mu, j);
+        tensor::View<double> mu_j = d.mu.slice(j);
         std::copy(adist->x().begin(), adist->x().end(), mu_j.begin());
       }
     }
@@ -244,24 +309,23 @@ IncoherentInelasticAE::IncoherentInelasticAE(hid_t group)
   }
 }
 
-void IncoherentInelasticAE::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+void IncoherentInelasticAE::sample_params(
+  double E_in, double& E_out, double& f, int& l, int& j, uint64_t* seed) const
 {
   // Get index and interpolation factor for inelastic grid
   int i;
-  double f;
-  get_energy_index(energy_, E_in, i, f);
+  double f0;
+  get_energy_index(energy_, E_in, i, f0);
 
   // Pick closer energy based on interpolation factor
-  int l = f > 0.5 ? i + 1 : i;
+  l = f0 > 0.5 ? i + 1 : i;
 
   // Determine outgoing energy bin
   // (First reset n_energy_out to the right value)
-  auto n = distribution_[l].n_e_out;
+  int n = distribution_[l].n_e_out;
   double r1 = prn(seed);
   double c_j = distribution_[l].e_out_cdf[0];
   double c_j1;
-  std::size_t j;
   for (j = 0; j < n - 1; ++j) {
     c_j1 = distribution_[l].e_out_cdf[j + 1];
     if (r1 < c_j1)
@@ -299,15 +363,23 @@ void IncoherentInelasticAE::sample(
     E_out += E_in - E_l;
   }
 
+  f = (r1 - c_j) / (c_j1 - c_j);
+}
+void IncoherentInelasticAE::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  double f;
+  int l, j;
+  sample_params(E_in, E_out, f, l, j, seed);
+
   // Sample outgoing cosine bin
-  int n_mu = distribution_[l].mu.shape()[1];
+  int n_mu = distribution_[l].mu.shape(1);
   std::size_t k = prn(seed) * n_mu;
 
   // Rather than use the sampled discrete mu directly, it is smeared over
   // a bin of width 0.5*min(mu[k] - mu[k-1], mu[k+1] - mu[k]) centered on the
   // discrete mu value itself.
   const auto& mu_l = distribution_[l].mu;
-  f = (r1 - c_j) / (c_j1 - c_j);
 
   // Interpolate kth mu value between distributions at energies j and j+1
   mu = mu_l(j, k) + f * (mu_l(j + 1, k) - mu_l(j, k));
@@ -331,6 +403,19 @@ void IncoherentInelasticAE::sample(
   mu += std::min(mu - mu_left, mu_right - mu) * (prn(seed) - 0.5);
 }
 
+double IncoherentInelasticAE::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  double f;
+  int l, j;
+  sample_params(E_in, E_out, f, l, j, seed);
+
+  const auto& mu_l = distribution_[l].mu;
+
+  return get_pdf_discrete_interpolated(
+    mu_l.slice(j, tensor::all), mu_l.slice(j + 1, tensor::all), f, mu);
+}
+
 //==============================================================================
 // MixedElasticAE implementation
 //==============================================================================
@@ -353,18 +438,30 @@ MixedElasticAE::MixedElasticAE(
   close_group(incoherent_group);
 }
 
-void MixedElasticAE::sample(
-  double E_in, double& E_out, double& mu, uint64_t* seed) const
+const AngleEnergy& MixedElasticAE::sample_dist(
+  double E_in, uint64_t* seed) const
 {
   // Evaluate coherent and incoherent elastic cross sections
   double xs_coh = coherent_xs_(E_in);
   double xs_incoh = incoherent_xs_(E_in);
 
   if (prn(seed) * (xs_coh + xs_incoh) < xs_coh) {
-    coherent_dist_.sample(E_in, E_out, mu, seed);
+    return coherent_dist_;
   } else {
-    incoherent_dist_->sample(E_in, E_out, mu, seed);
+    return *incoherent_dist_;
   }
 }
 
+void MixedElasticAE::sample(
+  double E_in, double& E_out, double& mu, uint64_t* seed) const
+{
+  sample_dist(E_in, seed).sample(E_in, E_out, mu, seed);
+}
+
+double MixedElasticAE::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  return sample_dist(E_in, seed).sample_energy_and_pdf(E_in, mu, E_out, seed);
+}
+
 } // namespace openmc
diff --git a/src/secondary_uncorrelated.cpp b/src/secondary_uncorrelated.cpp
index 5cbb76fb9..ec2af7102 100644
--- a/src/secondary_uncorrelated.cpp
+++ b/src/secondary_uncorrelated.cpp
@@ -65,4 +65,22 @@ void UncorrelatedAngleEnergy::sample(
   E_out = energy_->sample(E_in, seed);
 }
 
+double UncorrelatedAngleEnergy::sample_energy_and_pdf(
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  // Sample outgoing energy
+  if (energy_ != nullptr) {
+    E_out = energy_->sample(E_in, seed);
+  } else {
+    E_out = E_in;
+  }
+
+  if (!angle_.empty()) {
+    return angle_.evaluate(E_in, mu);
+  } else {
+    // no angle distribution given => assume isotropic for all energies
+    return 0.5;
+  }
+}
+
 } // namespace openmc
diff --git a/src/settings.cpp b/src/settings.cpp
index 49b6590ec..15a7b9c27 100644
--- a/src/settings.cpp
+++ b/src/settings.cpp
@@ -11,6 +11,7 @@
 #endif
 
 #include "openmc/capi.h"
+#include "openmc/collision_track.h"
 #include "openmc/constants.h"
 #include "openmc/container_util.h"
 #include "openmc/distribution.h"
@@ -26,6 +27,7 @@
 #include "openmc/plot.h"
 #include "openmc/random_lcg.h"
 #include "openmc/random_ray/random_ray.h"
+#include "openmc/reaction.h"
 #include "openmc/simulation.h"
 #include "openmc/source.h"
 #include "openmc/string_utils.h"
@@ -45,6 +47,7 @@ namespace settings {
 // Default values for boolean flags
 bool assume_separate {false};
 bool check_overlaps {false};
+bool collision_track {false};
 bool cmfd_run {false};
 bool confidence_intervals {false};
 bool create_delayed_neutrons {true};
@@ -52,12 +55,14 @@ bool create_fission_neutrons {true};
 bool delayed_photon_scaling {true};
 bool entropy_on {false};
 bool event_based {false};
+bool ifp_on {false};
 bool legendre_to_tabular {true};
 bool material_cell_offsets {true};
 bool output_summary {true};
 bool output_tallies {true};
 bool particle_restart_run {false};
 bool photon_transport {false};
+bool atomic_relaxation {true};
 bool reduce_tallies {true};
 bool res_scat_on {false};
 bool restart_run {false};
@@ -78,6 +83,7 @@ bool uniform_source_sampling {false};
 bool ufs_on {false};
 bool urr_ptables_on {true};
 bool use_decay_photons {false};
+bool use_shared_secondary_bank {false};
 bool weight_windows_on {false};
 bool weight_window_checkpoint_surface {false};
 bool weight_window_checkpoint_collision {true};
@@ -92,6 +98,7 @@ std::string path_sourcepoint;
 std::string path_statepoint;
 const char* path_statepoint_c {path_statepoint.c_str()};
 std::string weight_windows_file;
+std::string properties_file;
 
 int32_t n_inactive {0};
 int32_t max_lost_particles {10};
@@ -106,11 +113,14 @@ int max_particle_events {1000000};
 ElectronTreatment electron_treatment {ElectronTreatment::TTB};
 array<double, 4> energy_cutoff {0.0, 1000.0, 0.0, 0.0};
 array<double, 4> time_cutoff {INFTY, INFTY, INFTY, INFTY};
+int ifp_n_generation {-1};
+IFPParameter ifp_parameter {IFPParameter::None};
 int legendre_to_tabular_points {C_NONE};
 int max_order {0};
 int n_log_bins {8000};
 int n_batches;
 int n_max_batches;
+int max_secondaries {10000};
 int max_history_splits {10'000'000};
 int max_tracks {1000};
 ResScatMethod res_scat_method {ResScatMethod::rvs};
@@ -121,11 +131,16 @@ RunMode run_mode {RunMode::UNSET};
 SolverType solver_type {SolverType::MONTE_CARLO};
 std::unordered_set<int> sourcepoint_batch;
 std::unordered_set<int> statepoint_batch;
+double source_rejection_fraction {0.05};
+double free_gas_threshold {400.0};
 std::unordered_set<int> source_write_surf_id;
+CollisionTrackConfig collision_track_config {};
 int64_t ssw_max_particles;
 int64_t ssw_max_files;
 int64_t ssw_cell_id {C_NONE};
 SSWCellType ssw_cell_type {SSWCellType::None};
+double surface_grazing_cutoff {0.001};
+double surface_grazing_ratio {0.5};
 TemperatureMethod temperature_method {TemperatureMethod::NEAREST};
 double temperature_tolerance {10.0};
 double temperature_default {293.6};
@@ -135,7 +150,7 @@ int trace_gen;
 int64_t trace_particle;
 vector<array<int, 3>> track_identifiers;
 int trigger_batch_interval {1};
-int verbosity {7};
+int verbosity {-1};
 double weight_cutoff {0.25};
 double weight_survive {1.0};
 
@@ -266,8 +281,9 @@ void get_run_parameters(pugi::xml_node node_base)
     } else {
       fatal_error("Specify random ray inactive distance in settings XML");
     }
-    if (check_for_node(random_ray_node, "source")) {
-      xml_node source_node = random_ray_node.child("source");
+    if (check_for_node(random_ray_node, "ray_source")) {
+      xml_node ray_source_node = random_ray_node.child("ray_source");
+      xml_node source_node = ray_source_node.child("source");
       // Get point to list of <source> elements and make sure there is at least
       // one
       RandomRay::ray_source_ = Source::create(source_node);
@@ -306,7 +322,7 @@ void get_run_parameters(pugi::xml_node node_base)
         get_node_value_bool(random_ray_node, "volume_normalized_flux_tallies");
     }
     if (check_for_node(random_ray_node, "adjoint")) {
-      FlatSourceDomain::adjoint_ =
+      FlatSourceDomain::adjoint_requested_ =
         get_node_value_bool(random_ray_node, "adjoint");
     }
     if (check_for_node(random_ray_node, "sample_method")) {
@@ -316,6 +332,8 @@ void get_run_parameters(pugi::xml_node node_base)
         RandomRay::sample_method_ = RandomRaySampleMethod::PRNG;
       } else if (temp_str == "halton") {
         RandomRay::sample_method_ = RandomRaySampleMethod::HALTON;
+      } else if (temp_str == "s2") {
+        RandomRay::sample_method_ = RandomRaySampleMethod::S2;
       } else {
         fatal_error("Unrecognized sample method: " + temp_str);
       }
@@ -341,10 +359,25 @@ void get_run_parameters(pugi::xml_node node_base)
           }
           FlatSourceDomain::mesh_domain_map_[mesh_id].emplace_back(
             type, domain_id);
-          RandomRay::mesh_subdivision_enabled_ = true;
         }
       }
     }
+    if (check_for_node(random_ray_node, "diagonal_stabilization_rho")) {
+      FlatSourceDomain::diagonal_stabilization_rho_ = std::stod(
+        get_node_value(random_ray_node, "diagonal_stabilization_rho"));
+      if (FlatSourceDomain::diagonal_stabilization_rho_ < 0.0 ||
+          FlatSourceDomain::diagonal_stabilization_rho_ > 1.0) {
+        fatal_error("Random ray diagonal stabilization rho factor must be "
+                    "between 0 and 1");
+      }
+    }
+    if (check_for_node(random_ray_node, "adjoint_source")) {
+      pugi::xml_node adj_source_node = random_ray_node.child("adjoint_source");
+      for (pugi::xml_node source_node : adj_source_node.children("source")) {
+        // Find any local adjoint sources
+        model::adjoint_sources.push_back(Source::create(source_node));
+      }
+    }
   }
 }
 
@@ -378,8 +411,10 @@ void read_settings_xml()
   xml_node root = doc.document_element();
 
   // Verbosity
-  if (check_for_node(root, "verbosity")) {
+  if (check_for_node(root, "verbosity") && verbosity == -1) {
     verbosity = std::stoi(get_node_value(root, "verbosity"));
+  } else if (verbosity == -1) {
+    verbosity = 7;
   }
 
   // To this point, we haven't displayed any output since we didn't know what
@@ -527,6 +562,20 @@ void read_settings_xml(pugi::xml_node root)
     } else if (rel_max_lost_particles <= 0.0 || rel_max_lost_particles >= 1.0) {
       fatal_error("Relative max lost particles must be between zero and one.");
     }
+
+    // Check for user value for the number of generation of the Iterated Fission
+    // Probability (IFP) method
+    if (check_for_node(root, "ifp_n_generation")) {
+      ifp_n_generation = std::stoi(get_node_value(root, "ifp_n_generation"));
+      if (ifp_n_generation <= 0) {
+        fatal_error("'ifp_n_generation' must be greater than 0.");
+      }
+      // Avoid tallying 0 if IFP logs are not complete when active cycles start
+      if (ifp_n_generation > n_inactive) {
+        fatal_error("'ifp_n_generation' must be lower than or equal to the "
+                    "number of inactive cycles.");
+      }
+    }
   }
 
   // Copy plotting random number seed if specified
@@ -569,6 +618,11 @@ void read_settings_xml(pugi::xml_node root)
     }
   }
 
+  // Check for atomic relaxation
+  if (check_for_node(root, "atomic_relaxation")) {
+    atomic_relaxation = get_node_value_bool(root, "atomic_relaxation");
+  }
+
   // Number of bins for logarithmic grid
   if (check_for_node(root, "log_grid_bins")) {
     n_log_bins = std::stoi(get_node_value(root, "log_grid_bins"));
@@ -608,13 +662,6 @@ void read_settings_xml(pugi::xml_node root)
     model::external_sources.push_back(make_unique<FileSource>(path));
   }
 
-  // Build probability mass function for sampling external sources
-  vector<double> source_strengths;
-  for (auto& s : model::external_sources) {
-    source_strengths.push_back(s->strength());
-  }
-  model::external_sources_probability.assign(source_strengths);
-
   // If no source specified, default to isotropic point source at origin with
   // Watt spectrum. No default source is needed in random ray mode.
   if (model::external_sources.empty() &&
@@ -627,11 +674,36 @@ void read_settings_xml(pugi::xml_node root)
       UPtrDist {new Discrete(T, p, 1)}));
   }
 
+  // Build probability mass function for sampling external sources
+  vector<double> source_strengths;
+  for (auto& s : model::external_sources) {
+    source_strengths.push_back(s->strength());
+  }
+  model::external_sources_probability.assign(source_strengths);
+
   // Check if we want to write out source
   if (check_for_node(root, "write_initial_source")) {
     write_initial_source = get_node_value_bool(root, "write_initial_source");
   }
 
+  // Get relative number of lost particles
+  if (check_for_node(root, "source_rejection_fraction")) {
+    source_rejection_fraction =
+      std::stod(get_node_value(root, "source_rejection_fraction"));
+  }
+
+  if (check_for_node(root, "free_gas_threshold")) {
+    free_gas_threshold = std::stod(get_node_value(root, "free_gas_threshold"));
+  }
+
+  // Surface grazing
+  if (check_for_node(root, "surface_grazing_cutoff"))
+    surface_grazing_cutoff =
+      std::stod(get_node_value(root, "surface_grazing_cutoff"));
+  if (check_for_node(root, "surface_grazing_ratio"))
+    surface_grazing_ratio =
+      std::stod(get_node_value(root, "surface_grazing_ratio"));
+
   // Survival biasing
   if (check_for_node(root, "survival_biasing")) {
     survival_biasing = get_node_value_bool(root, "survival_biasing");
@@ -689,6 +761,14 @@ void read_settings_xml(pugi::xml_node root)
     }
   }
 
+  // read properties from file
+  if (check_for_node(root, "properties_file")) {
+    properties_file = get_node_value(root, "properties_file");
+    if (!file_exists(properties_file)) {
+      fatal_error(fmt::format("File '{}' does not exist.", properties_file));
+    }
+  }
+
   // Particle trace
   if (check_for_node(root, "trace")) {
     auto temp = get_node_array<int64_t>(root, "trace");
@@ -825,12 +905,6 @@ void read_settings_xml(pugi::xml_node root)
     }
     if (check_for_node(node_sp, "mcpl")) {
       source_mcpl_write = get_node_value_bool(node_sp, "mcpl");
-
-      // Make sure MCPL support is enabled
-      if (source_mcpl_write && !MCPL_ENABLED) {
-        fatal_error(
-          "Your build of OpenMC does not support writing MCPL source files.");
-      }
     }
     if (check_for_node(node_sp, "overwrite_latest")) {
       source_latest = get_node_value_bool(node_sp, "overwrite_latest");
@@ -883,12 +957,6 @@ void read_settings_xml(pugi::xml_node root)
 
     if (check_for_node(node_ssw, "mcpl")) {
       surf_mcpl_write = get_node_value_bool(node_ssw, "mcpl");
-
-      // Make sure MCPL support is enabled
-      if (surf_mcpl_write && !MCPL_ENABLED) {
-        fatal_error("Your build of OpenMC does not support writing MCPL "
-                    "surface source files.");
-      }
     }
     // Get cell information
     if (check_for_node(node_ssw, "cell")) {
@@ -913,8 +981,72 @@ void read_settings_xml(pugi::xml_node root)
     }
   }
 
-  // If source is not separate and is to be written out in the statepoint file,
-  // make sure that the sourcepoint batch numbers are contained in the
+  // Check if the user has specified to write specific collisions
+  if (check_for_node(root, "collision_track")) {
+    settings::collision_track = true;
+    // Get collision track node
+    xml_node node_ct = root.child("collision_track");
+    collision_track_config = CollisionTrackConfig {};
+
+    // Determine cell ids at which crossing particles are to be banked
+    if (check_for_node(node_ct, "cell_ids")) {
+      auto temp = get_node_array<int>(node_ct, "cell_ids");
+      for (const auto& b : temp) {
+        collision_track_config.cell_ids.insert(b);
+      }
+    }
+    if (check_for_node(node_ct, "reactions")) {
+      auto temp = get_node_array<std::string>(node_ct, "reactions");
+      for (const auto& b : temp) {
+        int reaction_int = reaction_mt(b);
+        if (reaction_int > 0) {
+          collision_track_config.mt_numbers.insert(reaction_int);
+        }
+      }
+    }
+    if (check_for_node(node_ct, "universe_ids")) {
+      auto temp = get_node_array<int>(node_ct, "universe_ids");
+      for (const auto& b : temp) {
+        collision_track_config.universe_ids.insert(b);
+      }
+    }
+    if (check_for_node(node_ct, "material_ids")) {
+      auto temp = get_node_array<int>(node_ct, "material_ids");
+      for (const auto& b : temp) {
+        collision_track_config.material_ids.insert(b);
+      }
+    }
+    if (check_for_node(node_ct, "nuclides")) {
+      auto temp = get_node_array<std::string>(node_ct, "nuclides");
+      for (const auto& b : temp) {
+        collision_track_config.nuclides.insert(b);
+      }
+    }
+    if (check_for_node(node_ct, "deposited_E_threshold")) {
+      collision_track_config.deposited_energy_threshold =
+        std::stod(get_node_value(node_ct, "deposited_E_threshold"));
+    }
+    // Get maximum number of particles to be banked per collision
+    if (check_for_node(node_ct, "max_collisions")) {
+      collision_track_config.max_collisions =
+        std::stoll(get_node_value(node_ct, "max_collisions"));
+    } else {
+      warning("A maximum number of collisions needs to be specified. "
+              "By default the code sets 'max_collisions' parameter equals to "
+              "1000.");
+    }
+    // Get maximum number of collision_track files to be created
+    if (check_for_node(node_ct, "max_collision_track_files")) {
+      collision_track_config.max_files =
+        std::stoll(get_node_value(node_ct, "max_collision_track_files"));
+    }
+    if (check_for_node(node_ct, "mcpl")) {
+      collision_track_config.mcpl_write = get_node_value_bool(node_ct, "mcpl");
+    }
+  }
+
+  // If source is not separate and is to be written out in the statepoint
+  // file, make sure that the sourcepoint batch numbers are contained in the
   // statepoint list
   if (!source_separate) {
     for (const auto& b : sourcepoint_batch) {
@@ -1115,6 +1247,7 @@ void read_settings_xml(pugi::xml_node root)
   // read weight windows from file
   if (check_for_node(root, "weight_windows_file")) {
     weight_windows_file = get_node_value(root, "weight_windows_file");
+    weight_windows_on = true;
   }
 
   // read settings for weight windows value, this will override
@@ -1123,6 +1256,11 @@ void read_settings_xml(pugi::xml_node root)
     weight_windows_on = get_node_value_bool(root, "weight_windows_on");
   }
 
+  if (check_for_node(root, "max_secondaries")) {
+    settings::max_secondaries =
+      std::stoi(get_node_value(root, "max_secondaries"));
+  }
+
   if (check_for_node(root, "max_history_splits")) {
     settings::max_history_splits =
       std::stoi(get_node_value(root, "max_history_splits"));
@@ -1140,14 +1278,24 @@ void read_settings_xml(pugi::xml_node root)
       variance_reduction::weight_windows_generators.emplace_back(
         std::make_unique<WeightWindowsGenerator>(node_wwg));
     }
-    // if any of the weight windows are intended to be generated otf, make sure
-    // they're applied
+    // if any of the weight windows are intended to be generated otf, make
+    // sure they're applied
     for (const auto& wwg : variance_reduction::weight_windows_generators) {
       if (wwg->on_the_fly_) {
         settings::weight_windows_on = true;
         break;
       }
     }
+    // If any weight window generators have local FW-CADIS target tallies,
+    // user-defined adjoint sources cannot be used at the same time.
+    if (!model::adjoint_sources.empty()) {
+      for (const auto& wwg : variance_reduction::weight_windows_generators) {
+        if (!wwg->targets_.empty()) {
+          fatal_error("Cannot use both user-defined adjoint sources and "
+                      "FW-CADIS target tallies at the same time.");
+        }
+      }
+    }
   }
 
   // Set up weight window checkpoints
@@ -1163,10 +1311,38 @@ void read_settings_xml(pugi::xml_node root)
     }
   }
 
+  if (weight_windows_on) {
+    if (!weight_window_checkpoint_surface &&
+        !weight_window_checkpoint_collision)
+      fatal_error(
+        "Weight Windows are enabled but there are no valid checkpoints.");
+  }
+
   if (check_for_node(root, "use_decay_photons")) {
     settings::use_decay_photons =
       get_node_value_bool(root, "use_decay_photons");
   }
+
+  // If weight windows are on, also enable shared secondary bank (unless
+  // explicitly disabled by user).
+  if (check_for_node(root, "shared_secondary_bank")) {
+    bool val = get_node_value_bool(root, "shared_secondary_bank");
+    if (val && run_mode == RunMode::EIGENVALUE) {
+      warning(
+        "Shared secondary bank is not supported in eigenvalue calculations. "
+        "Setting will be ignored.");
+    } else {
+      settings::use_shared_secondary_bank = val;
+    }
+  } else if (settings::weight_windows_on) {
+    if (run_mode == RunMode::EIGENVALUE) {
+      warning(
+        "Shared secondary bank is not supported in eigenvalue calculations. "
+        "Particle local secondary banks will be used instead.");
+    } else if (run_mode == RunMode::FIXED_SOURCE) {
+      settings::use_shared_secondary_bank = true;
+    }
+  }
 }
 
 void free_memory_settings()
@@ -1189,11 +1365,6 @@ extern "C" int openmc_set_n_batches(
     return OPENMC_E_INVALID_ARGUMENT;
   }
 
-  if (simulation::current_batch >= n_batches) {
-    set_errmsg("Number of batches must be greater than current batch.");
-    return OPENMC_E_INVALID_ARGUMENT;
-  }
-
   if (!settings::trigger_on) {
     // Set n_batches and n_max_batches to same value
     settings::n_batches = n_batches;
diff --git a/src/simulation.cpp b/src/simulation.cpp
index 74a7dbe63..03f40a726 100644
--- a/src/simulation.cpp
+++ b/src/simulation.cpp
@@ -2,19 +2,22 @@
 
 #include "openmc/bank.h"
 #include "openmc/capi.h"
+#include "openmc/collision_track.h"
 #include "openmc/container_util.h"
 #include "openmc/eigenvalue.h"
 #include "openmc/error.h"
 #include "openmc/event.h"
 #include "openmc/geometry_aux.h"
+#include "openmc/ifp.h"
 #include "openmc/material.h"
-#include "openmc/mcpl_interface.h"
 #include "openmc/message_passing.h"
 #include "openmc/nuclide.h"
+#include "openmc/openmp_interface.h"
 #include "openmc/output.h"
 #include "openmc/particle.h"
 #include "openmc/photon.h"
 #include "openmc/random_lcg.h"
+#include "openmc/random_ray/flat_source_domain.h"
 #include "openmc/settings.h"
 #include "openmc/source.h"
 #include "openmc/state_point.h"
@@ -29,7 +32,7 @@
 #ifdef _OPENMP
 #include <omp.h>
 #endif
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #ifdef OPENMC_MPI
 #include <mpi.h>
@@ -39,6 +42,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <numeric>
 #include <string>
 
 //==============================================================================
@@ -85,7 +89,7 @@ int openmc_simulation_init()
   }
 
   // Determine how much work each process should do
-  calculate_work();
+  calculate_work(settings::n_particles);
 
   // Allocate source, fission and surface source banks.
   allocate_banks();
@@ -117,9 +121,11 @@ int openmc_simulation_init()
   // Reset global variables -- this is done before loading state point (as that
   // will potentially populate k_generation and entropy)
   simulation::current_batch = 0;
+  simulation::ct_current_file = 1;
   simulation::ssw_current_file = 1;
   simulation::k_generation.clear();
   simulation::entropy.clear();
+  reset_source_rejection_counters();
   openmc_reset();
 
   // If this is a restart run, load the state point data and binary source
@@ -197,9 +203,12 @@ int openmc_simulation_finalize()
   if (settings::output_tallies && mpi::master)
     write_tallies();
 
-  // If weight window generators are present in this simulation,
-  // write a weight windows file
-  if (variance_reduction::weight_windows_generators.size() > 0) {
+  // If weight window generators are present in this simulation, write a
+  // weight windows file. This is skipped during the forward solve of an
+  // adjoint (FW-CADIS) run, where only the adjoint-derived weight windows
+  // are meaningful.
+  if (variance_reduction::weight_windows_generators.size() > 0 &&
+      FlatSourceDomain::solve_ != RandomRaySolve::FORWARD_FOR_ADJOINT) {
     openmc_weight_windows_export();
   }
 
@@ -211,6 +220,20 @@ int openmc_simulation_finalize()
   // Stop timers and show timing statistics
   simulation::time_finalize.stop();
   simulation::time_total.stop();
+
+#ifdef OPENMC_MPI
+  // Reduce track count across ranks for correct reporting. In shared secondary
+  // bank mode, all ranks already have the global count; in non-shared mode,
+  // each rank only has its own count.
+  if (settings::weight_windows_on && !settings::use_shared_secondary_bank) {
+    int64_t total_tracks;
+    MPI_Reduce(&simulation::simulation_tracks_completed, &total_tracks, 1,
+      MPI_INT64_T, MPI_SUM, 0, mpi::intracomm);
+    if (mpi::master)
+      simulation::simulation_tracks_completed = total_tracks;
+  }
+#endif
+
   if (mpi::master) {
     if (settings::solver_type != SolverType::RANDOM_RAY) {
       if (settings::verbosity >= 6)
@@ -251,9 +274,17 @@ int openmc_next_batch(int* status)
 
     // Transport loop
     if (settings::event_based) {
-      transport_event_based();
+      if (settings::use_shared_secondary_bank) {
+        transport_event_based_shared_secondary();
+      } else {
+        transport_event_based();
+      }
     } else {
-      transport_history_based();
+      if (settings::use_shared_secondary_bank) {
+        transport_history_based_shared_secondary();
+      } else {
+        transport_history_based();
+      }
     }
 
     // Accumulate time for transport
@@ -296,6 +327,7 @@ namespace openmc {
 
 namespace simulation {
 
+int ct_current_file;
 int current_batch;
 int current_gen;
 bool initialized {false};
@@ -320,8 +352,98 @@ const RegularMesh* ufs_mesh {nullptr};
 vector<double> k_generation;
 vector<int64_t> work_index;
 
+int64_t simulation_tracks_completed {0};
+
 } // namespace simulation
 
+namespace {
+
+//! Collect thread-local secondary banks into the shared secondary bank in
+//! sorted order.
+//!
+//! \param thread_banks  Secondary banks produced by each OpenMP thread
+void collect_sorted_history_secondary_banks(
+  vector<vector<SourceSite>>& thread_banks)
+{
+  // Count the total number of all secondary sites produced
+  int64_t n_collected = 0;
+  for (const auto& bank : thread_banks) {
+    n_collected += bank.size();
+  }
+
+  // Count the expected number of progeny from per-parent progeny counts
+  int64_t n_progeny = 0;
+  for (int64_t count : simulation::progeny_per_particle) {
+    n_progeny += count;
+  }
+
+  if (n_collected != n_progeny) {
+    fatal_error("Mismatch detected between sum of all particle progeny and "
+                "secondary bank size during collection.");
+  }
+
+  // Convert per-parent progeny counts to offsets into the sorted bank
+  std::exclusive_scan(simulation::progeny_per_particle.begin(),
+    simulation::progeny_per_particle.end(),
+    simulation::progeny_per_particle.begin(), 0);
+
+  // Allocate the shared bank once for the complete generation
+  simulation::shared_secondary_bank_write.resize(0);
+  simulation::shared_secondary_bank_write.extend_uninitialized(n_progeny);
+
+  // Place each secondary according to its parent and progeny identifiers
+  for (const auto& bank : thread_banks) {
+    for (const auto& site : bank) {
+      if (site.parent_id < 0 ||
+          site.parent_id >=
+            static_cast<int64_t>(simulation::progeny_per_particle.size())) {
+        fatal_error(fmt::format("Invalid parent_id {} for banked site "
+                                "(expected range [0, {})).",
+          site.parent_id, simulation::progeny_per_particle.size()));
+      }
+      int64_t idx =
+        simulation::progeny_per_particle[site.parent_id] + site.progeny_id;
+      if (idx < 0 || idx >= n_progeny) {
+        fatal_error("Mismatch detected between sum of all particle progeny and "
+                    "secondary bank size during collection.");
+      }
+      simulation::shared_secondary_bank_write[idx] = site;
+    }
+  }
+}
+
+//! Collect particle-local secondary banks into the shared secondary bank.
+//!
+//! \param n_particles  Number of particles in the active event-based buffer
+void collect_event_secondary_banks(int64_t n_particles)
+{
+  // Compute offsets for each particle's local secondary bank.
+  vector<int64_t> offsets(n_particles);
+  int64_t total = 0;
+  for (int64_t i = 0; i < n_particles; ++i) {
+    offsets[i] = total;
+    total += simulation::particles[i].local_secondary_bank().size();
+  }
+
+  // Extend the shared bank once for all collected secondaries
+  int64_t bank_offset =
+    simulation::shared_secondary_bank_write.extend_uninitialized(total);
+
+  // Copy each local bank into its assigned range and clear the local storage
+#pragma omp parallel for schedule(static)
+  for (int64_t i = 0; i < n_particles; ++i) {
+    auto& local_bank = simulation::particles[i].local_secondary_bank();
+    if (!local_bank.empty()) {
+      std::copy(local_bank.cbegin(), local_bank.cend(),
+        simulation::shared_secondary_bank_write.data() + bank_offset +
+          offsets[i]);
+      local_bank.clear();
+    }
+  }
+}
+
+} // namespace
+
 //==============================================================================
 // Non-member functions
 //==============================================================================
@@ -335,12 +457,22 @@ void allocate_banks()
 
     // Allocate fission bank
     init_fission_bank(3 * simulation::work_per_rank);
+
+    // Allocate IFP bank
+    if (settings::ifp_on) {
+      resize_simulation_ifp_banks();
+    }
   }
 
   if (settings::surf_source_write) {
     // Allocate surface source bank
     simulation::surf_source_bank.reserve(settings::ssw_max_particles);
   }
+
+  if (settings::collision_track) {
+    // Allocate collision track bank
+    collision_track_reserve_bank();
+  }
 }
 
 void initialize_batch()
@@ -400,7 +532,7 @@ void finalize_batch()
 
   // Reset global tally results
   if (simulation::current_batch <= settings::n_inactive) {
-    xt::view(simulation::global_tallies, xt::all()) = 0.0;
+    simulation::global_tallies.fill(0.0);
     simulation::n_realizations = 0;
   }
 
@@ -484,6 +616,10 @@ void finalize_batch()
       ++simulation::ssw_current_file;
     }
   }
+  // Write collision track file if requested
+  if (settings::collision_track) {
+    collision_track_flush_bank();
+  }
 }
 
 void initialize_generation()
@@ -529,7 +665,7 @@ void finalize_generation()
     // If using shared memory, stable sort the fission bank (by parent IDs)
     // so as to allow for reproducibility regardless of which order particles
     // are run in.
-    sort_fission_bank();
+    sort_bank(simulation::fission_bank, true);
 
     // Distribute fission bank across processors evenly
     synchronize_bank();
@@ -553,26 +689,35 @@ void finalize_generation()
   }
 }
 
-void initialize_history(Particle& p, int64_t index_source)
+void sample_source_particle(Particle& p, int64_t index_source)
 {
-  // set defaults
+  // Sample a particle from the source bank
   if (settings::run_mode == RunMode::EIGENVALUE) {
-    // set defaults for eigenvalue simulations from primary bank
     p.from_source(&simulation::source_bank[index_source - 1]);
   } else if (settings::run_mode == RunMode::FIXED_SOURCE) {
     // initialize random number seed
-    int64_t id = (simulation::total_gen + overall_generation() - 1) *
-                   settings::n_particles +
-                 simulation::work_index[mpi::rank] + index_source;
+    int64_t id = compute_transport_seed(compute_particle_id(index_source));
     uint64_t seed = init_seed(id, STREAM_SOURCE);
     // sample from external source distribution or custom library then set
     auto site = sample_external_source(&seed);
     p.from_source(&site);
   }
-  p.current_work() = index_source;
+}
+
+void initialize_particle_track(
+  Particle& p, int64_t index_source, bool is_secondary)
+{
+  // Note: index_source is 1-based (first particle = 1), but current_work() is
+  // stored as 0-based for direct use as an array index into
+  // progeny_per_particle, source_bank, ifp banks, etc.
+  if (!is_secondary) {
+    sample_source_particle(p, index_source);
+  }
+
+  p.current_work() = index_source - 1;
 
   // set identifier for particle
-  p.id() = simulation::work_index[mpi::rank] + index_source;
+  p.id() = compute_particle_id(index_source);
 
   // set progeny count to zero
   p.n_progeny() = 0;
@@ -580,6 +725,9 @@ void initialize_history(Particle& p, int64_t index_source)
   // Reset particle event counter
   p.n_event() = 0;
 
+  // Initialize track counter (1 for this primary/secondary track)
+  p.n_tracks() = 1;
+
   // Reset split counter
   p.n_split() = 0;
 
@@ -593,9 +741,7 @@ void initialize_history(Particle& p, int64_t index_source)
   std::fill(p.pht_storage().begin(), p.pht_storage().end(), 0);
 
   // set random number seed
-  int64_t particle_seed =
-    (simulation::total_gen + overall_generation() - 1) * settings::n_particles +
-    p.id();
+  int64_t particle_seed = compute_transport_seed(p.id());
   init_particle_seeds(particle_seed, p.seeds());
 
   // set particle trace
@@ -608,14 +754,22 @@ void initialize_history(Particle& p, int64_t index_source)
   // Set particle track.
   p.write_track() = check_track_criteria(p);
 
+  // Set the particle's initial weight window value.
+  if (!is_secondary) {
+    p.wgt_ww_born() = -1.0;
+    apply_weight_windows(p);
+  }
+
   // Display message if high verbosity or trace is on
   if (settings::verbosity >= 9 || p.trace()) {
     write_message("Simulating Particle {}", p.id());
   }
 
-// Add particle's starting weight to count for normalizing tallies later
+  // Add particle's starting weight to count for normalizing tallies later
+  if (!is_secondary) {
 #pragma omp atomic
-  simulation::total_weight += p.wgt();
+    simulation::total_weight += p.wgt();
+  }
 
   // Force calculation of cross-sections by setting last energy to zero
   if (settings::run_CE) {
@@ -633,13 +787,34 @@ int overall_generation()
   return settings::gen_per_batch * (current_batch - 1) + current_gen;
 }
 
-void calculate_work()
+int64_t compute_particle_id(int64_t index_source)
+{
+  if (settings::use_shared_secondary_bank) {
+    return simulation::work_index[mpi::rank] + index_source +
+           simulation::simulation_tracks_completed;
+  } else {
+    return simulation::work_index[mpi::rank] + index_source;
+  }
+}
+
+int64_t compute_transport_seed(int64_t particle_id)
+{
+  if (settings::use_shared_secondary_bank) {
+    return particle_id;
+  } else {
+    return (simulation::total_gen + overall_generation() - 1) *
+             settings::n_particles +
+           particle_id;
+  }
+}
+
+void calculate_work(int64_t n_particles)
 {
   // Determine minimum amount of particles to simulate on each processor
-  int64_t min_work = settings::n_particles / mpi::n_procs;
+  int64_t min_work = n_particles / mpi::n_procs;
 
   // Determine number of processors that have one extra particle
-  int64_t remainder = settings::n_particles % mpi::n_procs;
+  int64_t remainder = n_particles % mpi::n_procs;
 
   int64_t i_bank = 0;
   simulation::work_index.resize(mpi::n_procs + 1);
@@ -661,11 +836,12 @@ void calculate_work()
 void initialize_data()
 {
   // Determine minimum/maximum energy for incident neutron/photon data
-  data::energy_max = {INFTY, INFTY};
-  data::energy_min = {0.0, 0.0};
+  data::energy_max = {INFTY, INFTY, INFTY, INFTY};
+  data::energy_min = {0.0, 0.0, 0.0, 0.0};
+
   for (const auto& nuc : data::nuclides) {
     if (nuc->grid_.size() >= 1) {
-      int neutron = static_cast<int>(ParticleType::neutron);
+      int neutron = ParticleType::neutron().transport_index();
       data::energy_min[neutron] =
         std::max(data::energy_min[neutron], nuc->grid_[0].energy.front());
       data::energy_max[neutron] =
@@ -676,7 +852,7 @@ void initialize_data()
   if (settings::photon_transport) {
     for (const auto& elem : data::elements) {
       if (elem->energy_.size() >= 1) {
-        int photon = static_cast<int>(ParticleType::photon);
+        int photon = ParticleType::photon().transport_index();
         int n = elem->energy_.size();
         data::energy_min[photon] =
           std::max(data::energy_min[photon], std::exp(elem->energy_(1)));
@@ -689,12 +865,22 @@ void initialize_data()
       // Determine if minimum/maximum energy for bremsstrahlung is greater/less
       // than the current minimum/maximum
       if (data::ttb_e_grid.size() >= 1) {
-        int photon = static_cast<int>(ParticleType::photon);
+        int photon = ParticleType::photon().transport_index();
+        int electron = ParticleType::electron().transport_index();
+        int positron = ParticleType::positron().transport_index();
         int n_e = data::ttb_e_grid.size();
+
+        const std::vector<int> charged = {electron, positron};
+        for (auto t : charged) {
+          data::energy_min[t] = std::exp(data::ttb_e_grid(1));
+          data::energy_max[t] = std::exp(data::ttb_e_grid(n_e - 1));
+        }
+
         data::energy_min[photon] =
-          std::max(data::energy_min[photon], std::exp(data::ttb_e_grid(1)));
-        data::energy_max[photon] = std::min(
-          data::energy_max[photon], std::exp(data::ttb_e_grid(n_e - 1)));
+          std::max(data::energy_min[photon], data::energy_min[electron]);
+
+        data::energy_max[photon] =
+          std::min(data::energy_max[photon], data::energy_max[electron]);
       }
     }
   }
@@ -705,7 +891,7 @@ void initialize_data()
     // grid has not been allocated
     if (nuc->grid_.size() > 0) {
       double max_E = nuc->grid_[0].energy.back();
-      int neutron = static_cast<int>(ParticleType::neutron);
+      int neutron = ParticleType::neutron().transport_index();
       if (max_E == data::energy_max[neutron]) {
         write_message(7, "Maximum neutron transport energy: {} eV for {}",
           data::energy_max[neutron], nuc->name_);
@@ -722,7 +908,7 @@ void initialize_data()
   for (auto& nuc : data::nuclides) {
     nuc->init_grid();
   }
-  int neutron = static_cast<int>(ParticleType::neutron);
+  int neutron = ParticleType::neutron().transport_index();
   simulation::log_spacing =
     std::log(data::energy_max[neutron] / data::energy_min[neutron]) /
     settings::n_log_bins;
@@ -777,27 +963,146 @@ void transport_history_based_single_particle(Particle& p)
       p.event_advance();
     }
     if (p.alive()) {
-      if (p.collision_distance() > p.boundary().distance) {
+      if (p.collision_distance() > p.boundary().distance()) {
         p.event_cross_surface();
       } else if (p.alive()) {
         p.event_collide();
       }
     }
-    p.event_revive_from_secondary();
+    p.event_check_limit_and_revive();
   }
   p.event_death();
 }
 
 void transport_history_based()
 {
-#pragma omp parallel for schedule(runtime)
-  for (int64_t i_work = 1; i_work <= simulation::work_per_rank; ++i_work) {
+#pragma omp parallel
+  {
     Particle p;
-    initialize_history(p, i_work);
-    transport_history_based_single_particle(p);
+#pragma omp for schedule(runtime)
+    for (int64_t i_work = 1; i_work <= simulation::work_per_rank; ++i_work) {
+      initialize_particle_track(p, i_work, false);
+      transport_history_based_single_particle(p);
+    }
   }
 }
 
+// The shared secondary bank transport algorithm works in two phases. In the
+// first phase, all primary particles are sampled then transported, and their
+// secondary particles are deposited into a shared secondary bank. The second
+// phase occurs in a loop, where all secondary tracks in the shared secondary
+// bank are transported. Any secondary particles generated during this phase are
+// deposited back into the shared secondary bank. The shared secondary bank is
+// sorted for consistent ordering and load balanced across MPI ranks. This loop
+// continues until there are no more secondary tracks left to transport.
+void transport_history_based_shared_secondary()
+{
+  // Clear shared secondary banks from any prior use
+  simulation::shared_secondary_bank_read.clear();
+  simulation::shared_secondary_bank_write.clear();
+
+  if (mpi::master) {
+    write_message(fmt::format(" Primary source          particles: {}",
+                    settings::n_particles),
+      6);
+  }
+
+  simulation::progeny_per_particle.resize(simulation::work_per_rank);
+  std::fill(simulation::progeny_per_particle.begin(),
+    simulation::progeny_per_particle.end(), 0);
+
+  vector<vector<SourceSite>> thread_banks(num_threads());
+
+  // Phase 1: Transport primary particles and deposit first generation of
+  // secondaries in the shared secondary bank
+#pragma omp parallel
+  {
+    auto& thread_bank = thread_banks[thread_num()];
+    Particle p;
+
+#pragma omp for schedule(runtime)
+    for (int64_t i = 1; i <= simulation::work_per_rank; i++) {
+      initialize_particle_track(p, i, false);
+      transport_history_based_single_particle(p);
+      for (auto& site : p.local_secondary_bank()) {
+        thread_bank.push_back(site);
+      }
+      p.local_secondary_bank().clear();
+    }
+  }
+  collect_sorted_history_secondary_banks(thread_banks);
+  thread_banks.clear();
+
+  simulation::simulation_tracks_completed += settings::n_particles;
+
+  // Phase 2: Now that the secondary bank has been populated, enter loop over
+  // all secondary generations
+  int n_generation_depth = 1;
+  int64_t alive_secondary = 1;
+  while (alive_secondary) {
+
+    // Synchronize the shared secondary bank amongst all MPI ranks, such
+    // that each MPI rank has an approximately equal number of secondary
+    // tracks. Also reports the total number of secondaries alive across
+    // all MPI ranks.
+    alive_secondary = synchronize_global_secondary_bank(
+      simulation::shared_secondary_bank_write);
+
+    // Recalculate work for each MPI rank based on number of alive secondary
+    // tracks
+    calculate_work(alive_secondary);
+
+    // Display the number of secondary tracks in this generation. This
+    // is useful for user monitoring so as to see if the secondary population is
+    // exploding and to determine how many generations of secondaries are being
+    // transported.
+    if (mpi::master) {
+      write_message(fmt::format(" Secondary generation {:<2}    tracks: {}",
+                      n_generation_depth, alive_secondary),
+        6);
+    }
+
+    simulation::shared_secondary_bank_read =
+      std::move(simulation::shared_secondary_bank_write);
+    simulation::shared_secondary_bank_write = SharedArray<SourceSite>();
+    simulation::progeny_per_particle.resize(
+      simulation::shared_secondary_bank_read.size());
+    std::fill(simulation::progeny_per_particle.begin(),
+      simulation::progeny_per_particle.end(), 0);
+    thread_banks.resize(num_threads());
+
+    // Transport all secondary tracks from the shared secondary bank
+#pragma omp parallel
+    {
+      auto& thread_bank = thread_banks[thread_num()];
+      Particle p;
+
+#pragma omp for schedule(runtime)
+      for (int64_t i = 1; i <= simulation::shared_secondary_bank_read.size();
+           i++) {
+        initialize_particle_track(p, i, true);
+        SourceSite& site = simulation::shared_secondary_bank_read[i - 1];
+        p.event_revive_from_secondary(site);
+        transport_history_based_single_particle(p);
+        for (auto& secondary_site : p.local_secondary_bank()) {
+          thread_bank.push_back(secondary_site);
+        }
+        p.local_secondary_bank().clear();
+      }
+    } // End of transport loop over tracks in shared secondary bank
+    simulation::shared_secondary_bank_write =
+      std::move(simulation::shared_secondary_bank_read);
+    simulation::shared_secondary_bank_read = SharedArray<SourceSite>();
+    collect_sorted_history_secondary_banks(thread_banks);
+    thread_banks.clear();
+    n_generation_depth++;
+    simulation::simulation_tracks_completed += alive_secondary;
+  } // End of loop over secondary generations
+
+  // Reset work so that fission bank etc works correctly
+  calculate_work(settings::n_particles);
+}
+
 void transport_event_based()
 {
   int64_t remaining_work = simulation::work_per_rank;
@@ -815,33 +1120,7 @@ void transport_event_based()
 
     // Initialize all particle histories for this subiteration
     process_init_events(n_particles, source_offset);
-
-    // Event-based transport loop
-    while (true) {
-      // Determine which event kernel has the longest queue
-      int64_t max = std::max({simulation::calculate_fuel_xs_queue.size(),
-        simulation::calculate_nonfuel_xs_queue.size(),
-        simulation::advance_particle_queue.size(),
-        simulation::surface_crossing_queue.size(),
-        simulation::collision_queue.size()});
-
-      // Execute event with the longest queue
-      if (max == 0) {
-        break;
-      } else if (max == simulation::calculate_fuel_xs_queue.size()) {
-        process_calculate_xs_events(simulation::calculate_fuel_xs_queue);
-      } else if (max == simulation::calculate_nonfuel_xs_queue.size()) {
-        process_calculate_xs_events(simulation::calculate_nonfuel_xs_queue);
-      } else if (max == simulation::advance_particle_queue.size()) {
-        process_advance_particle_events();
-      } else if (max == simulation::surface_crossing_queue.size()) {
-        process_surface_crossing_events();
-      } else if (max == simulation::collision_queue.size()) {
-        process_collision_events();
-      }
-    }
-
-    // Execute death event for all particles
+    process_transport_events();
     process_death_events(n_particles);
 
     // Adjust remaining work and source offset variables
@@ -850,4 +1129,110 @@ void transport_event_based()
   }
 }
 
+void transport_event_based_shared_secondary()
+{
+  // Clear shared secondary banks from any prior use
+  simulation::shared_secondary_bank_read.clear();
+  simulation::shared_secondary_bank_write.clear();
+
+  if (mpi::master) {
+    write_message(fmt::format(" Primary source          particles: {}",
+                    settings::n_particles),
+      6);
+  }
+
+  simulation::progeny_per_particle.resize(simulation::work_per_rank);
+  std::fill(simulation::progeny_per_particle.begin(),
+    simulation::progeny_per_particle.end(), 0);
+
+  // Phase 1: Transport primary particles using event-based processing and
+  // deposit first generation of secondaries in the shared secondary bank
+  int64_t remaining_work = simulation::work_per_rank;
+  int64_t source_offset = 0;
+
+  while (remaining_work > 0) {
+    int64_t n_particles =
+      std::min(remaining_work, settings::max_particles_in_flight);
+
+    process_init_events(n_particles, source_offset);
+    process_transport_events();
+    process_death_events(n_particles);
+
+    collect_event_secondary_banks(n_particles);
+
+    remaining_work -= n_particles;
+    source_offset += n_particles;
+  }
+
+  simulation::simulation_tracks_completed += settings::n_particles;
+
+  // Phase 2: Now that the secondary bank has been populated, enter loop over
+  // all secondary generations
+  int n_generation_depth = 1;
+  int64_t alive_secondary = 1;
+  while (alive_secondary) {
+
+    // Sort the shared secondary bank by parent ID then progeny ID to
+    // ensure reproducibility.
+    sort_bank(simulation::shared_secondary_bank_write, false);
+
+    // Synchronize the shared secondary bank amongst all MPI ranks, such
+    // that each MPI rank has an approximately equal number of secondary
+    // tracks.
+    alive_secondary = synchronize_global_secondary_bank(
+      simulation::shared_secondary_bank_write);
+
+    // Recalculate work for each MPI rank based on number of alive secondary
+    // tracks
+    calculate_work(alive_secondary);
+
+    if (mpi::master) {
+      write_message(fmt::format(" Secondary generation {:<2}    tracks: {}",
+                      n_generation_depth, alive_secondary),
+        6);
+    }
+
+    simulation::shared_secondary_bank_read =
+      std::move(simulation::shared_secondary_bank_write);
+    simulation::shared_secondary_bank_write = SharedArray<SourceSite>();
+    simulation::progeny_per_particle.resize(
+      simulation::shared_secondary_bank_read.size());
+    std::fill(simulation::progeny_per_particle.begin(),
+      simulation::progeny_per_particle.end(), 0);
+
+    // Ensure particle buffer is large enough for this secondary generation
+    int64_t sec_buffer_length = std::min(
+      static_cast<int64_t>(simulation::shared_secondary_bank_read.size()),
+      settings::max_particles_in_flight);
+    if (sec_buffer_length >
+        static_cast<int64_t>(simulation::particles.size())) {
+      init_event_queues(sec_buffer_length);
+    }
+
+    // Transport secondary tracks using event-based processing
+    int64_t sec_remaining = simulation::shared_secondary_bank_read.size();
+    int64_t sec_offset = 0;
+
+    while (sec_remaining > 0) {
+      int64_t n_particles =
+        std::min(sec_remaining, settings::max_particles_in_flight);
+
+      process_init_secondary_events(
+        n_particles, sec_offset, simulation::shared_secondary_bank_read);
+      process_transport_events();
+      process_death_events(n_particles);
+
+      collect_event_secondary_banks(n_particles);
+
+      sec_remaining -= n_particles;
+      sec_offset += n_particles;
+    } // End of subiteration loop over secondary tracks
+    n_generation_depth++;
+    simulation::simulation_tracks_completed += alive_secondary;
+  } // End of loop over secondary generations
+
+  // Reset work so that fission bank etc works correctly
+  calculate_work(settings::n_particles);
+}
+
 } // namespace openmc
diff --git a/src/source.cpp b/src/source.cpp
index c1b4ce260..12bbdf84e 100644
--- a/src/source.cpp
+++ b/src/source.cpp
@@ -4,29 +4,34 @@
 #define HAS_DYNAMIC_LINKING
 #endif
 
-#include <utility> // for move
+#include <algorithm> // for max
+#include <cmath>     // for sin, cos, abs
+#include <utility>   // for move
 
 #ifdef HAS_DYNAMIC_LINKING
 #include <dlfcn.h> // for dlopen, dlsym, dlclose, dlerror
 #endif
 
-#include "xtensor/xadapt.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include "openmc/bank.h"
 #include "openmc/capi.h"
 #include "openmc/cell.h"
+#include "openmc/constants.h"
 #include "openmc/container_util.h"
 #include "openmc/error.h"
 #include "openmc/file_utils.h"
 #include "openmc/geometry.h"
 #include "openmc/hdf5_interface.h"
 #include "openmc/material.h"
+#include "openmc/math_functions.h"
 #include "openmc/mcpl_interface.h"
 #include "openmc/memory.h"
 #include "openmc/message_passing.h"
 #include "openmc/mgxs_interface.h"
 #include "openmc/nuclide.h"
+#include "openmc/random_dist.h"
 #include "openmc/random_lcg.h"
 #include "openmc/search.h"
 #include "openmc/settings.h"
@@ -37,6 +42,23 @@
 
 namespace openmc {
 
+std::atomic<int64_t> source_n_accept {0};
+std::atomic<int64_t> source_n_reject {0};
+
+namespace {
+
+void validate_particle_type(ParticleType type, const std::string& context)
+{
+  if (type.is_transportable())
+    return;
+
+  fatal_error(
+    fmt::format("Unsupported source particle type '{}' (PDG {}) in {}.",
+      type.str(), type.pdg_number(), context));
+}
+
+} // namespace
+
 //==============================================================================
 // Global variables
 //==============================================================================
@@ -45,6 +67,8 @@ namespace model {
 
 vector<unique_ptr<Source>> external_sources;
 
+vector<unique_ptr<Source>> adjoint_sources;
+
 DiscreteIndex external_sources_probability;
 
 } // namespace model
@@ -81,6 +105,8 @@ unique_ptr<Source> Source::create(pugi::xml_node node)
       return make_unique<CompiledSourceWrapper>(node);
     } else if (source_type == "mesh") {
       return make_unique<MeshSource>(node);
+    } else if (source_type == "tokamak") {
+      return make_unique<TokamakSource>(node);
     } else {
       fatal_error(fmt::format("Invalid source type '{}' found.", source_type));
     }
@@ -157,12 +183,28 @@ void Source::read_constraints(pugi::xml_node node)
   }
 }
 
+void check_rejection_fraction(int64_t n_reject, int64_t n_accept)
+{
+  // Don't check unless we've hit a minimum number of total sites rejected
+  if (n_reject < EXTSRC_REJECT_THRESHOLD)
+    return;
+
+  // Compute fraction of accepted sites and compare against minimum
+  double fraction = static_cast<double>(n_accept) / n_reject;
+  if (fraction <= settings::source_rejection_fraction) {
+    fatal_error(fmt::format(
+      "Too few source sites satisfied the constraints (minimum source "
+      "rejection fraction = {}). Please check your source definition or "
+      "set a lower value of Settings.source_rejection_fraction.",
+      settings::source_rejection_fraction));
+  }
+}
+
 SourceSite Source::sample_with_constraints(uint64_t* seed) const
 {
   bool accepted = false;
-  static int n_reject = 0;
-  static int n_accept = 0;
-  SourceSite site;
+  int64_t n_local_reject = 0;
+  SourceSite site {};
 
   while (!accepted) {
     // Sample a source site without considering constraints yet
@@ -176,12 +218,12 @@ SourceSite Source::sample_with_constraints(uint64_t* seed) const
                  satisfies_energy_constraints(site.E) &&
                  satisfies_time_constraints(site.time);
       if (!accepted) {
-        ++n_reject;
-        if (n_reject >= EXTSRC_REJECT_THRESHOLD &&
-            static_cast<double>(n_accept) / n_reject <=
-              EXTSRC_REJECT_FRACTION) {
-          fatal_error("More than 95% of external source sites sampled were "
-                      "rejected. Please check your source definition.");
+        ++n_local_reject;
+
+        // Check per-particle rejection limit
+        if (n_local_reject >= MAX_SOURCE_REJECTIONS_PER_SAMPLE) {
+          fatal_error("Exceeded maximum number of source rejections per "
+                      "sample. Please check your source definition.");
         }
 
         // For the "kill" strategy, accept particle but set weight to 0 so that
@@ -194,8 +236,13 @@ SourceSite Source::sample_with_constraints(uint64_t* seed) const
     }
   }
 
-  // Increment number of accepted samples
-  ++n_accept;
+  // Flush local rejection count, update accept counter, and check overall
+  // rejection fraction
+  if (n_local_reject > 0) {
+    source_n_reject += n_local_reject;
+  }
+  ++source_n_accept;
+  check_rejection_fraction(source_n_reject, source_n_accept);
 
   return site;
 }
@@ -226,14 +273,17 @@ bool Source::satisfies_spatial_constraints(Position r) const
   if (!domain_ids_.empty()) {
     if (domain_type_ == DomainType::MATERIAL) {
       auto mat_index = geom_state.material();
-      if (mat_index != MATERIAL_VOID) {
+      if (mat_index == MATERIAL_VOID) {
+        accepted = false;
+      } else {
         accepted = contains(domain_ids_, model::materials[mat_index]->id());
       }
     } else {
       for (int i = 0; i < geom_state.n_coord(); i++) {
-        auto id = (domain_type_ == DomainType::CELL)
-                    ? model::cells[geom_state.coord(i).cell]->id_
-                    : model::universes[geom_state.coord(i).universe]->id_;
+        auto id =
+          (domain_type_ == DomainType::CELL)
+            ? model::cells[geom_state.coord(i).cell()].get()->id_
+            : model::universes[geom_state.coord(i).universe()].get()->id_;
         if ((accepted = contains(domain_ids_, id)))
           break;
       }
@@ -268,16 +318,15 @@ IndependentSource::IndependentSource(pugi::xml_node node) : Source(node)
 {
   // Check for particle type
   if (check_for_node(node, "particle")) {
-    auto temp_str = get_node_value(node, "particle", true, true);
-    if (temp_str == "neutron") {
-      particle_ = ParticleType::neutron;
-    } else if (temp_str == "photon") {
-      particle_ = ParticleType::photon;
+    auto temp_str = get_node_value(node, "particle", false, true);
+    particle_ = ParticleType(temp_str);
+    if (particle_ == ParticleType::photon() ||
+        particle_ == ParticleType::electron() ||
+        particle_ == ParticleType::positron()) {
       settings::photon_transport = true;
-    } else {
-      fatal_error(std::string("Unknown source particle type: ") + temp_str);
     }
   }
+  validate_particle_type(particle_, "IndependentSource");
 
   // Check for external source file
   if (check_for_node(node, "file")) {
@@ -312,6 +361,19 @@ IndependentSource::IndependentSource(pugi::xml_node node) : Source(node)
     if (check_for_node(node, "energy")) {
       pugi::xml_node node_dist = node.child("energy");
       energy_ = distribution_from_xml(node_dist);
+
+      // For decay photon sources, use the absolute photon emission rate in
+      // [photons/s] as the source strength
+      if (dynamic_cast<DecaySpectrum*>(energy_.get())) {
+        if (strength_ != 1.0) {
+          warning(fmt::format(
+            "Source strength of {} is ignored because the source uses a "
+            "DecaySpectrum energy distribution. The source strength will be "
+            "set from the DecaySpectrum emission rate.",
+            strength_));
+        }
+        strength_ = energy_->integral();
+      }
     } else {
       // Default to a Watt spectrum with parameters 0.988 MeV and 2.249 MeV^-1
       energy_ = UPtrDist {new Watt(0.988e6, 2.249e-6)};
@@ -332,75 +394,93 @@ IndependentSource::IndependentSource(pugi::xml_node node) : Source(node)
 
 SourceSite IndependentSource::sample(uint64_t* seed) const
 {
-  SourceSite site;
+  SourceSite site {};
   site.particle = particle_;
+  double r_wgt = 1.0;
+  double E_wgt = 1.0;
 
   // Repeat sampling source location until a good site has been accepted
   bool accepted = false;
-  static int n_reject = 0;
-  static int n_accept = 0;
+  int64_t n_local_reject = 0;
 
   while (!accepted) {
 
     // Sample spatial distribution
-    site.r = space_->sample(seed);
+    auto [r, r_wgt_temp] = space_->sample(seed);
+    site.r = r;
+    r_wgt = r_wgt_temp;
 
     // Check if sampled position satisfies spatial constraints
     accepted = satisfies_spatial_constraints(site.r);
 
     // Check for rejection
     if (!accepted) {
-      ++n_reject;
-      if (n_reject >= EXTSRC_REJECT_THRESHOLD &&
-          static_cast<double>(n_accept) / n_reject <= EXTSRC_REJECT_FRACTION) {
-        fatal_error("More than 95% of external source sites sampled were "
-                    "rejected. Please check your external source's spatial "
-                    "definition.");
+      ++n_local_reject;
+      if (n_local_reject >= MAX_SOURCE_REJECTIONS_PER_SAMPLE) {
+        fatal_error("Exceeded maximum number of source rejections per "
+                    "sample. Please check your source definition.");
       }
     }
   }
 
   // Sample angle
-  site.u = angle_->sample(seed);
+  auto [u, u_wgt] = angle_->sample(seed);
+  site.u = u;
+
+  site.wgt = r_wgt * u_wgt;
 
   // Sample energy and time for neutron and photon sources
   if (settings::solver_type != SolverType::RANDOM_RAY) {
     // Check for monoenergetic source above maximum particle energy
-    auto p = static_cast<int>(particle_);
+    auto p = particle_.transport_index();
     auto energy_ptr = dynamic_cast<Discrete*>(energy_.get());
+    auto decay_spectrum = dynamic_cast<DecaySpectrum*>(energy_.get());
     if (energy_ptr) {
-      auto energies = xt::adapt(energy_ptr->x());
-      if (xt::any(energies > data::energy_max[p])) {
+      auto energies =
+        tensor::Tensor<double>(energy_ptr->x().data(), energy_ptr->x().size());
+      if ((energies > data::energy_max[p]).any()) {
         fatal_error("Source energy above range of energies of at least "
                     "one cross section table");
       }
     }
 
     while (true) {
-      // Sample energy spectrum
-      site.E = energy_->sample(seed);
+      // Sample energy spectrum. For decay photon sources, also get the parent
+      // nuclide index to store in the source site for tallying purposes.
+      if (decay_spectrum) {
+        auto sample = decay_spectrum->sample_with_parent(seed);
+        site.E = sample.energy;
+        E_wgt = sample.weight;
+        site.parent_nuclide = sample.parent_nuclide;
+      } else {
+        auto [E, E_wgt_temp] = energy_->sample(seed);
+        site.E = E;
+        E_wgt = E_wgt_temp;
+      }
 
       // Resample if energy falls above maximum particle energy
-      if (site.E < data::energy_max[p] and
+      if (site.E < data::energy_max[p] &&
           (satisfies_energy_constraints(site.E)))
         break;
 
-      n_reject++;
-      if (n_reject >= EXTSRC_REJECT_THRESHOLD &&
-          static_cast<double>(n_accept) / n_reject <= EXTSRC_REJECT_FRACTION) {
-        fatal_error(
-          "More than 95% of external source sites sampled were "
-          "rejected. Please check your external source energy spectrum "
-          "definition.");
+      ++n_local_reject;
+      if (n_local_reject >= MAX_SOURCE_REJECTIONS_PER_SAMPLE) {
+        fatal_error("Exceeded maximum number of source rejections per "
+                    "sample. Please check your source definition.");
       }
     }
 
     // Sample particle creation time
-    site.time = time_->sample(seed);
+    auto [time, time_wgt] = time_->sample(seed);
+    site.time = time;
+
+    site.wgt *= (E_wgt * time_wgt);
   }
 
-  // Increment number of accepted samples
-  ++n_accept;
+  // Flush local rejection count into global counter
+  if (n_local_reject > 0) {
+    source_n_reject += n_local_reject;
+  }
 
   return site;
 }
@@ -412,11 +492,7 @@ SourceSite IndependentSource::sample(uint64_t* seed) const
 FileSource::FileSource(pugi::xml_node node) : Source(node)
 {
   auto path = get_node_value(node, "file", false, true);
-  if (ends_with(path, ".mcpl") || ends_with(path, ".mcpl.gz")) {
-    sites_ = mcpl_source_sites(path);
-  } else {
-    this->load_sites_from_file(path);
-  }
+  load_sites_from_file(path);
 }
 
 FileSource::FileSource(const std::string& path)
@@ -426,30 +502,45 @@ FileSource::FileSource(const std::string& path)
 
 void FileSource::load_sites_from_file(const std::string& path)
 {
-  // Check if source file exists
-  if (!file_exists(path)) {
-    fatal_error(fmt::format("Source file '{}' does not exist.", path));
+  // If MCPL file, use the dedicated file reader
+  if (ends_with(path, ".mcpl") || ends_with(path, ".mcpl.gz")) {
+    sites_ = mcpl_source_sites(path);
+  } else {
+    // Check if source file exists
+    if (!file_exists(path)) {
+      fatal_error(fmt::format("Source file '{}' does not exist.", path));
+    }
+
+    write_message(6, "Reading source file from {}...", path);
+
+    // Open the binary file
+    hid_t file_id = file_open(path, 'r', true);
+
+    // Check to make sure this is a source file
+    std::string filetype;
+    read_attribute(file_id, "filetype", filetype);
+    if (filetype != "source" && filetype != "statepoint") {
+      fatal_error("Specified starting source file not a source file type.");
+    }
+
+    // Read in the source particles
+    read_source_bank(file_id, sites_, false);
+
+    // Close file
+    file_close(file_id);
   }
 
-  // Read the source from a binary file instead of sampling from some
-  // assumed source distribution
-  write_message(6, "Reading source file from {}...", path);
-
-  // Open the binary file
-  hid_t file_id = file_open(path, 'r', true);
-
-  // Check to make sure this is a source file
-  std::string filetype;
-  read_attribute(file_id, "filetype", filetype);
-  if (filetype != "source" && filetype != "statepoint") {
-    fatal_error("Specified starting source file not a source file type.");
+  // Make sure particles in source file have valid types. If any particle is a
+  // photon, electron, or positron, enable photon transport so that the
+  // appropriate cross sections are loaded.
+  for (const auto& site : this->sites_) {
+    validate_particle_type(site.particle, "FileSource");
+    if (site.particle == ParticleType::photon() ||
+        site.particle == ParticleType::electron() ||
+        site.particle == ParticleType::positron()) {
+      settings::photon_transport = true;
+    }
   }
-
-  // Read in the source particles
-  read_source_bank(file_id, sites_, false);
-
-  // Close file
-  file_close(file_id);
 }
 
 SourceSite FileSource::sample(uint64_t* seed) const
@@ -523,6 +614,15 @@ CompiledSourceWrapper::~CompiledSourceWrapper()
 #endif
 }
 
+//==============================================================================
+// MeshElementSpatial implementation
+//==============================================================================
+
+std::pair<Position, double> MeshElementSpatial::sample(uint64_t* seed) const
+{
+  return {model::meshes[mesh_index_]->sample_element(elem_index_, seed), 1.0};
+}
+
 //==============================================================================
 // MeshSource implementation
 //==============================================================================
@@ -537,10 +637,28 @@ MeshSource::MeshSource(pugi::xml_node node) : Source(node)
   // read all source distributions and populate strengths vector for MeshSpatial
   // object
   for (auto source_node : node.children("source")) {
-    sources_.emplace_back(Source::create(source_node));
+    auto src = Source::create(source_node);
+    if (auto ptr = dynamic_cast<IndependentSource*>(src.get())) {
+      src.release();
+      sources_.emplace_back(ptr);
+    } else {
+      fatal_error(
+        "The source assigned to each element must be an IndependentSource.");
+    }
     strengths.push_back(sources_.back()->strength());
   }
 
+  // Set spatial distributions for each mesh element
+  for (int elem_index = 0; elem_index < sources_.size(); ++elem_index) {
+    sources_[elem_index]->set_space(
+      std::make_unique<MeshElementSpatial>(mesh_idx, elem_index));
+  }
+
+  // Make sure sources use valid particle types
+  for (const auto& src : sources_) {
+    validate_particle_type(src->particle_type(), "MeshSource");
+  }
+
   // the number of source distributions should either be one or equal to the
   // number of mesh elements
   if (sources_.size() > 1 && sources_.size() != mesh->n_bins()) {
@@ -554,28 +672,476 @@ MeshSource::MeshSource(pugi::xml_node node) : Source(node)
 
 SourceSite MeshSource::sample(uint64_t* seed) const
 {
-  // Sample the CDF defined in initialization above
+  // Sample a mesh element based on the relative strengths
   int32_t element = space_->sample_element_index(seed);
 
-  // Sample position and apply rejection on spatial domains
-  Position r;
-  do {
-    r = space_->mesh()->sample_element(element, seed);
-  } while (!this->satisfies_spatial_constraints(r));
+  // Sample the distribution for the specific mesh element; note that the
+  // spatial distribution has been set for each element using MeshElementSpatial
+  return source(element)->sample_with_constraints(seed);
+}
 
-  SourceSite site;
-  while (true) {
-    // Sample source for the chosen element and replace the position
-    site = source(element)->sample_with_constraints(seed);
-    site.r = r;
+//==============================================================================
+// TokamakSource implementation
+//==============================================================================
 
-    // Apply other rejections
-    if (satisfies_energy_constraints(site.E) &&
-        satisfies_time_constraints(site.time)) {
+TokamakSource::TokamakSource(pugi::xml_node node) : Source(node)
+{
+  // Read geometry parameters
+  major_radius_ = std::stod(get_node_value(node, "major_radius"));
+  minor_radius_ = std::stod(get_node_value(node, "minor_radius"));
+  elongation_ = std::stod(get_node_value(node, "elongation"));
+  triangularity_ = std::stod(get_node_value(node, "triangularity"));
+  shafranov_shift_ = std::stod(get_node_value(node, "shafranov_shift"));
+
+  // Read optional vertical shift
+  if (check_for_node(node, "vertical_shift")) {
+    vertical_shift_ = std::stod(get_node_value(node, "vertical_shift"));
+  } else {
+    vertical_shift_ = 0.0;
+  }
+
+  // Read optional toroidal angle bounds
+  if (check_for_node(node, "phi_start")) {
+    phi_start_ = std::stod(get_node_value(node, "phi_start"));
+  } else {
+    phi_start_ = 0.0;
+  }
+  if (check_for_node(node, "phi_extent")) {
+    phi_extent_ = std::stod(get_node_value(node, "phi_extent"));
+  } else {
+    phi_extent_ = 2.0 * PI;
+  }
+  if (check_for_node(node, "n_alpha")) {
+    n_alpha_ = std::stoi(get_node_value(node, "n_alpha"));
+  } else {
+    n_alpha_ = 101; // Default
+  }
+
+  // Read emission profile
+  r_over_a_ = get_node_array<double>(node, "r_over_a");
+  emission_density_ = get_node_array<double>(node, "emission_density");
+
+  // Read energy distribution(s)
+  for (auto energy_node : node.children("energy")) {
+    energy_dists_.push_back(distribution_from_xml(energy_node));
+  }
+
+  // Read optional time distribution; default to a delta distribution at t=0
+  // for the same behavior as IndependentSource
+  if (check_for_node(node, "time")) {
+    time_ = distribution_from_xml(node.child("time"));
+  } else {
+    double T[] {0.0};
+    double p[] {1.0};
+    time_ = UPtrDist {new Discrete {T, p, 1}};
+  }
+
+  // Validate inputs
+  if (emission_density_.size() != r_over_a_.size()) {
+    fatal_error("TokamakSource: emission_density and r_over_a must have the "
+                "same length.");
+  }
+  if (r_over_a_.size() < 2) {
+    fatal_error(
+      "TokamakSource: At least 2 radial points are required for profiles.");
+  }
+  if (r_over_a_.front() != 0.0) {
+    fatal_error("TokamakSource: r_over_a must start at 0.");
+  }
+  if (r_over_a_.back() != 1.0) {
+    fatal_error("TokamakSource: r_over_a must end at 1.");
+  }
+  for (size_t i = 1; i < r_over_a_.size(); ++i) {
+    if (r_over_a_[i] <= r_over_a_[i - 1]) {
+      fatal_error("TokamakSource: r_over_a must be strictly increasing.");
+    }
+  }
+  for (size_t i = 0; i < emission_density_.size(); ++i) {
+    if (emission_density_[i] < 0.0) {
+      fatal_error("TokamakSource: emission_density values cannot be negative.");
+    }
+  }
+  if (major_radius_ <= 0.0) {
+    fatal_error("TokamakSource: major_radius must be > 0.");
+  }
+  if (minor_radius_ <= 0.0) {
+    fatal_error("TokamakSource: minor_radius must be > 0.");
+  }
+  if (minor_radius_ >= major_radius_) {
+    fatal_error("TokamakSource: minor_radius must be less than major_radius.");
+  }
+  if (elongation_ <= 0.0) {
+    fatal_error("TokamakSource: elongation must be > 0.");
+  }
+  if (triangularity_ < -1.0 || triangularity_ > 1.0) {
+    fatal_error("TokamakSource: triangularity must be in the range [-1, 1].");
+  }
+  if (shafranov_shift_ < 0.0) {
+    fatal_error("TokamakSource: shafranov_shift must be >= 0.");
+  }
+  if (shafranov_shift_ >= 0.5 * minor_radius_) {
+    fatal_error("TokamakSource: shafranov_shift must be less than half the "
+                "minor radius.");
+  }
+  if (phi_extent_ <= 0.0 || phi_extent_ > 2.0 * PI) {
+    fatal_error("TokamakSource: phi_extent must be > 0 and <= 2*pi.");
+  }
+  if (n_alpha_ <= 2) {
+    fatal_error("TokamakSource: n_alpha must be > 2.");
+  }
+  if (n_alpha_ < 51) {
+    warning("TokamakSource: n_alpha values below 51 may introduce noticeable "
+            "discretization bias in source sampling.");
+  }
+  if (energy_dists_.empty()) {
+    fatal_error("TokamakSource: At least one energy distribution is required.");
+  }
+  if (energy_dists_.size() != 1 && energy_dists_.size() != r_over_a_.size()) {
+    fatal_error("TokamakSource: energy distributions must be either 1 (for all "
+                "r) or match the number of r_over_a points.");
+  }
+
+  // Compute normalized geometry parameters
+  epsilon_ = minor_radius_ / major_radius_;
+  delta_tilde_ = shafranov_shift_ / minor_radius_;
+
+  // Initialize isotropic angular distribution
+  angle_ = UPtrAngle {new Isotropic()};
+
+  precompute_sampling_distributions();
+}
+
+void TokamakSource::precompute_sampling_distributions()
+{
+  // Use precomputed normalized geometry parameters
+  double eps = epsilon_;    // Inverse aspect ratio (a/R0)
+  double Dt = delta_tilde_; // Normalized Shafranov shift (Delta/a)
+  double delta = triangularity_;
+
+  //==========================================================================
+  // RADIAL CDF (computed first since it's simpler and sampled first)
+  //==========================================================================
+  // The marginal radial PDF is obtained by analytically integrating the joint
+  // distribution f(r_tilde, alpha) over alpha. The result is:
+  //
+  //   p(r_tilde) ~ S(r_tilde) * [(1 + eps*Dt)*r_tilde
+  //                              - (3/8)*c1*eps*r_tilde^2
+  //                              - 2*eps*Dt*r_tilde^3]
+  //
+  // where the Bessel function coefficients are:
+  //   c0 = J_0(delta) + J_2(delta)
+  //   c1 = (J_1(2*delta) + J_3(2*delta)) / c0
+  //
+  // For delta -> 0, c0 -> 1 and c1 -> 0, giving the circular cross-section
+  // limit.
+
+  // Compute Bessel function coefficients. openmc::cyl_bessel_j handles
+  // negative arguments (negative triangularity) via the parity relation
+  // J_n(-x) = (-1)^n * J_n(x).
+  double J0_d = cyl_bessel_j(0, delta);
+  double J2_d = cyl_bessel_j(2, delta);
+  double J1_2d = cyl_bessel_j(1, 2.0 * delta);
+  double J3_2d = cyl_bessel_j(3, 2.0 * delta);
+  double c0 = J0_d + J2_d;
+  double c1 = (J1_2d + J3_2d) / c0;
+
+  // Coefficients for the radial polynomial: A*r - B*r^2 - C*r^3
+  radial_poly_a_ = 1.0 + eps * Dt;
+  radial_poly_b_ = 0.375 * c1 * eps; // 3/8 * c1 * eps
+  radial_poly_c_ = 2.0 * eps * Dt;
+
+  // Build a refined radial grid that retains the user-specified grid points.
+  // The emission density is interpreted as linear-linear between those points.
+  constexpr int MIN_SUBINTERVALS = 8;
+  constexpr double MAX_GRID_SPACING = 1.0e-3;
+  vector<double> radial_grid {r_over_a_.front()};
+  vector<double> radial_emission {emission_density_.front()};
+  for (size_t i = 1; i < r_over_a_.size(); ++i) {
+    double r_lo = r_over_a_[i - 1];
+    double r_hi = r_over_a_[i];
+    double s_lo = emission_density_[i - 1];
+    double s_hi = emission_density_[i];
+    int n_subintervals = std::max(MIN_SUBINTERVALS,
+      static_cast<int>(std::ceil((r_hi - r_lo) / MAX_GRID_SPACING)));
+    for (int j = 1; j <= n_subintervals; ++j) {
+      double t = static_cast<double>(j) / n_subintervals;
+      radial_grid.push_back(r_lo + t * (r_hi - r_lo));
+      radial_emission.push_back(s_lo + t * (s_hi - s_lo));
+    }
+  }
+
+  vector<double> radial_pdf(radial_grid.size());
+  for (size_t i = 0; i < radial_grid.size(); ++i) {
+    double r = radial_grid[i];
+    // p(r) ~ S(r) * [A*r - B*r^2 - C*r^3]
+    double geometric_factor =
+      radial_poly_a_ * r - radial_poly_b_ * r * r - radial_poly_c_ * r * r * r;
+    radial_pdf[i] = radial_emission[i] * std::max(0.0, geometric_factor);
+  }
+
+  // Check that the refined profile contains positive probability mass before
+  // constructing the normalized tabular distribution.
+  double total = 0.0;
+  for (size_t i = 1; i < radial_grid.size(); ++i) {
+    total += 0.5 * (radial_pdf[i - 1] + radial_pdf[i]) *
+             (radial_grid[i] - radial_grid[i - 1]);
+  }
+  if (total <= 0.0) {
+    fatal_error(
+      "TokamakSource: Integrated emission density is zero or negative. "
+      "Check emission_density profile.");
+  }
+  radial_dist_ = make_unique<Tabular>(radial_grid.data(), radial_pdf.data(),
+    radial_grid.size(), Interpolation::lin_lin);
+
+  //==========================================================================
+  // POLOIDAL CDFs (for conditional sampling of alpha given r)
+  //==========================================================================
+  // The conditional distribution P(alpha | r) is a mixture:
+  //   P(alpha | r) ~ sum_k w_k(r) * I_hat_k * p_k(alpha)
+  // where:
+  //   - w_k(r) are the "dynamic" Bernstein weight functions (depend on r)
+  //   - I_hat_k are the "static" normalized integrals (precomputed constants)
+  //   - p_k(alpha) are the normalized basis distributions (precomputed CDFs)
+  //
+  // The static weights I_hat_k = I_k / (2*pi*c0) are:
+  //   I_hat_0 = 1 + eps*Dt
+  //   I_hat_1 = 1 + eps*Dt - (3/16)*c1*eps
+  //   I_hat_2 = 1 - (3/8)*c1*eps
+  //   I_hat_3 = 1 + eps*Dt
+  //   I_hat_4 = 1 + (1/2)*eps*Dt - (3/16)*c1*eps
+  //   I_hat_5 = 1 - eps*Dt - (3/8)*c1*eps
+
+  // Compute static weights analytically
+  poloidal_integrals_[0] = 1.0 + eps * Dt;
+  poloidal_integrals_[1] = 1.0 + eps * Dt - 0.1875 * c1 * eps; // 3/16 = 0.1875
+  poloidal_integrals_[2] = 1.0 - 0.375 * c1 * eps;             // 3/8 = 0.375
+  poloidal_integrals_[3] = 1.0 + eps * Dt;
+  poloidal_integrals_[4] = 1.0 + 0.5 * eps * Dt - 0.1875 * c1 * eps;
+  poloidal_integrals_[5] = 1.0 - eps * Dt - 0.375 * c1 * eps;
+
+  // Build the alpha grid on [0, pi] (half domain due to up-down symmetry)
+  int n_alpha = n_alpha_;
+  vector<double> alpha_grid(n_alpha);
+  double dalpha = PI / (n_alpha - 1);
+  for (int i = 0; i < n_alpha; ++i) {
+    alpha_grid[i] = i * dalpha;
+  }
+
+  // Compute basis function values g_k(alpha) for tabular distributions
+  // Using Bernstein form:
+  //   R_tilde = b0*(1-r)^2 + 2*b1*r*(1-r) + b2*r^2
+  //   J_tilde = b3*(1-r) + b4*r
+  // with:
+  //   b0(alpha) = 1 + eps*Dt
+  //   b1(alpha) = b0 + (eps/2)*cos(psi),  psi = alpha + delta*sin(alpha)
+  //   b2(alpha) = 1 + eps*cos(psi)
+  //   b3(alpha) = cos(delta*sin(alpha))
+  //               + (delta/4)*(cos(alpha - delta*sin(alpha))
+  //                          - cos(3*alpha + delta*sin(alpha)))
+  //   b4(alpha) = b3(alpha) - 2*Dt*cos(alpha)
+
+  array<vector<double>, N_POLOIDAL_BASIS> basis;
+  for (int k = 0; k < N_POLOIDAL_BASIS; ++k) {
+    basis[k].resize(n_alpha);
+  }
+
+  for (int i = 0; i < n_alpha; ++i) {
+    double alpha = alpha_grid[i];
+    double sin_alpha = std::sin(alpha);
+    double cos_alpha = std::cos(alpha);
+    double delta_sin_alpha = delta * sin_alpha;
+    double psi = alpha + delta_sin_alpha;
+    double cos_psi = std::cos(psi);
+
+    // Bernstein coefficients b0-b4
+    double b0 = 1.0 + eps * Dt;
+    double b1 = b0 + 0.5 * eps * cos_psi;
+    double b2 = 1.0 + eps * cos_psi;
+    double b3 =
+      std::cos(delta_sin_alpha) + 0.25 * delta *
+                                    (std::cos(alpha - delta_sin_alpha) -
+                                      std::cos(3.0 * alpha + delta_sin_alpha));
+    double b4 = b3 - 2.0 * Dt * cos_alpha;
+
+    // 6 basis functions g_k(alpha) = b_i * b_j
+    basis[0][i] = b0 * b3; // w0 = (1-r)^3
+    basis[1][i] = b1 * b3; // w1 = 2*r*(1-r)^2
+    basis[2][i] = b2 * b3; // w2 = r^2*(1-r)
+    basis[3][i] = b0 * b4; // w3 = r*(1-r)^2
+    basis[4][i] = b1 * b4; // w4 = 2*r^2*(1-r)
+    basis[5][i] = b2 * b4; // w5 = r^3
+  }
+
+  // Build a linear-linear distribution for each basis function p_k(alpha)
+  for (int k = 0; k < N_POLOIDAL_BASIS; ++k) {
+    poloidal_dists_[k] = make_unique<Tabular>(
+      alpha_grid.data(), basis[k].data(), n_alpha, Interpolation::lin_lin);
+  }
+}
+
+double TokamakSource::sample_r_over_a(uint64_t* seed) const
+{
+  return radial_dist_->sample(seed).first;
+}
+
+double TokamakSource::mixture_weight(int k, double r) const
+{
+  double s = 1.0 - r;
+  switch (k) {
+  case 0:
+    return s * s * s * poloidal_integrals_[0];
+  case 1:
+    return 2.0 * r * s * s * poloidal_integrals_[1];
+  case 2:
+    return r * r * s * poloidal_integrals_[2];
+  case 3:
+    return r * s * s * poloidal_integrals_[3];
+  case 4:
+    return 2.0 * r * r * s * poloidal_integrals_[4];
+  case 5:
+    return r * r * r * poloidal_integrals_[5];
+  default:
+    UNREACHABLE();
+  }
+}
+
+double TokamakSource::sample_poloidal_angle(double r_norm, uint64_t* seed) const
+{
+  // Sample from the conditional distribution P(alpha | r_tilde) using
+  // mixture sampling with 6 precomputed basis distributions.
+  //
+  // The conditional is: P(alpha | r) ~ sum_k w_k(r) * I_hat_k * p_k(alpha)
+  // where:
+  //   - w_k(r) are the "dynamic" Bernstein weight functions
+  //   - I_hat_k are the "static" normalized integrals (precomputed in
+  //   poloidal_integrals_)
+  //   - p_k(alpha) are the normalized, precomputed basis distributions
+  //
+  // The normalization sum_k w_k(r) * I_hat_k equals the radial geometric
+  // polynomial evaluated at r, which is known analytically.
+  //
+  // Algorithm:
+  // 1. Compute total from analytical normalization
+  // 2. Lazily evaluate mixture weights with early exit to select component k
+  // 3. Sample alpha from the selected basis distribution
+
+  // Analytical normalization: sum_k w_k(r) * I_hat_k
+  double total =
+    radial_poly_a_ - radial_poly_b_ * r_norm - radial_poly_c_ * r_norm * r_norm;
+  double xi = prn(seed) * total;
+
+  // Sample component via lazy evaluation with early exit
+  // Order optimized for peaked emission profiles: 0, 1, 4, 5, 3, 2
+  constexpr int order[] = {0, 1, 4, 5, 3, 2};
+  double cumsum = 0.0;
+  int component = order[N_POLOIDAL_BASIS - 1];
+  for (int i = 0; i < N_POLOIDAL_BASIS; ++i) {
+    cumsum += mixture_weight(order[i], r_norm);
+    if (xi < cumsum) {
+      component = order[i];
       break;
     }
   }
 
+  // Sample alpha from [0, pi]
+  double alpha = poloidal_dists_[component]->sample(seed).first;
+
+  // Exploit up-down symmetry: randomly flip to [pi, 2*pi] with 50% probability
+  // This is equivalent to flipping the sign of Z in the final position
+  if (prn(seed) >= 0.5) {
+    alpha = 2.0 * PI - alpha;
+  }
+  return alpha;
+}
+
+std::pair<double, double> TokamakSource::sample_energy(
+  double r_norm, uint64_t* seed) const
+{
+  if (energy_dists_.size() == 1) {
+    // Single distribution for all r
+    return energy_dists_[0]->sample(seed);
+  }
+
+  // Multiple distributions: stochastic selection between bracketing r points
+  // Find the interval containing r_norm
+  size_t i = lower_bound_index(r_over_a_.begin(), r_over_a_.end(), r_norm);
+
+  // Handle boundary cases
+  if (i >= energy_dists_.size() - 1) {
+    return energy_dists_.back()->sample(seed);
+  }
+
+  // Stochastic interpolation: randomly select one of the two bracketing
+  // distributions based on distance to each
+  double t = (r_norm - r_over_a_[i]) / (r_over_a_[i + 1] - r_over_a_[i]);
+  size_t idx = (prn(seed) < t) ? i + 1 : i;
+  return energy_dists_[idx]->sample(seed);
+}
+
+Position TokamakSource::flux_to_cartesian(
+  double r, double alpha, double phi) const
+{
+  // Flux surface parameterization:
+  // R = R0 + r*cos(alpha + delta*sin(alpha)) + Delta*(1 - (r/a)^2)
+  // Z = kappa * r * sin(alpha)
+  // x = R * cos(phi)
+  // y = R * sin(phi)
+  // z = Z
+
+  double psi = alpha + triangularity_ * std::sin(alpha);
+  double r_over_a_sq = (r * r) / (minor_radius_ * minor_radius_);
+
+  double R =
+    major_radius_ + r * std::cos(psi) + shafranov_shift_ * (1.0 - r_over_a_sq);
+  double Z = elongation_ * r * std::sin(alpha);
+
+  double x = R * std::cos(phi);
+  double y = R * std::sin(phi);
+  double z = Z;
+
+  return {x, y, z};
+}
+
+SourceSite TokamakSource::sample(uint64_t* seed) const
+{
+  SourceSite site;
+  site.particle = ParticleType::neutron();
+  site.wgt = 1.0;
+  site.delayed_group = 0;
+
+  // 1. Sample r/a from radial CDF
+  double r_norm = sample_r_over_a(seed);
+  double r = r_norm * minor_radius_;
+
+  // 2. Sample poloidal angle from conditional distribution P(alpha|r)
+  double alpha = sample_poloidal_angle(r_norm, seed);
+
+  // 3. Sample toroidal angle uniformly in [phi_start, phi_start + phi_extent]
+  double phi = phi_start_ + phi_extent_ * prn(seed);
+
+  // 4. Convert to Cartesian coordinates
+  site.r = flux_to_cartesian(r, alpha, phi);
+
+  // 4a. Apply vertical shift if non-zero
+  if (vertical_shift_ != 0.0) {
+    site.r.z += vertical_shift_;
+  }
+
+  // 5. Sample isotropic direction
+  site.u = angle_->sample(seed).first;
+
+  // 6. Sample energy from distribution(s), applying the importance weight so
+  // that biased distributions are handled correctly
+  auto [E, E_wgt] = sample_energy(r_norm, seed);
+  site.E = E;
+
+  // 7. Sample particle creation time
+  auto [time, time_wgt] = time_->sample(seed);
+  site.time = time;
+
+  site.wgt *= E_wgt * time_wgt;
+
   return site;
 }
 
@@ -613,9 +1179,10 @@ SourceSite sample_external_source(uint64_t* seed)
 {
   // Sample from among multiple source distributions
   int i = 0;
-  if (model::external_sources.size() > 1) {
+  int n_sources = model::external_sources.size();
+  if (n_sources > 1) {
     if (settings::uniform_source_sampling) {
-      i = prn(seed) * model::external_sources.size();
+      i = prn(seed) * n_sources;
     } else {
       i = model::external_sources_probability.sample(seed);
     }
@@ -624,9 +1191,13 @@ SourceSite sample_external_source(uint64_t* seed)
   // Sample source site from i-th source distribution
   SourceSite site {model::external_sources[i]->sample_with_constraints(seed)};
 
-  // Set particle creation weight
-  if (settings::uniform_source_sampling) {
-    site.wgt *= model::external_sources[i]->strength();
+  // For uniform source sampling, multiply the weight by the ratio of the actual
+  // probability of sampling source i to the biased probability of sampling
+  // source i, which is (strength_i / total_strength) / (1 / n)
+  if (n_sources > 1 && settings::uniform_source_sampling) {
+    double total_strength = model::external_sources_probability.integral();
+    site.wgt *=
+      model::external_sources[i]->strength() * n_sources / total_strength;
   }
 
   // If running in MG, convert site.E to group
@@ -642,6 +1213,14 @@ SourceSite sample_external_source(uint64_t* seed)
 void free_memory_source()
 {
   model::external_sources.clear();
+  model::adjoint_sources.clear();
+  reset_source_rejection_counters();
+}
+
+void reset_source_rejection_counters()
+{
+  source_n_accept = 0;
+  source_n_reject = 0;
 }
 
 //==============================================================================
@@ -662,8 +1241,15 @@ extern "C" int openmc_sample_external_source(
   }
 
   auto sites_array = static_cast<SourceSite*>(sites);
+
+  // Derive independent per-particle seeds from the base seed so that
+  // each iteration has its own RNG state for thread-safe parallel sampling.
+  uint64_t base_seed = *seed;
+
+#pragma omp parallel for schedule(static)
   for (size_t i = 0; i < n; ++i) {
-    sites_array[i] = sample_external_source(seed);
+    uint64_t particle_seed = init_seed(base_seed + i, STREAM_SOURCE);
+    sites_array[i] = sample_external_source(&particle_seed);
   }
   return 0;
 }
diff --git a/src/state_point.cpp b/src/state_point.cpp
index 3b822715c..02c68bf60 100644
--- a/src/state_point.cpp
+++ b/src/state_point.cpp
@@ -4,11 +4,11 @@
 #include <cstdint> // for int64_t
 #include <string>
 
-#include "xtensor/xbuilder.hpp" // for empty_like
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include "openmc/bank.h"
+#include "openmc/bank_io.h"
 #include "openmc/capi.h"
 #include "openmc/constants.h"
 #include "openmc/eigenvalue.h"
@@ -21,6 +21,8 @@
 #include "openmc/mgxs_interface.h"
 #include "openmc/nuclide.h"
 #include "openmc/output.h"
+#include "openmc/particle_type.h"
+#include "openmc/random_ray/flat_source_domain.h"
 #include "openmc/settings.h"
 #include "openmc/simulation.h"
 #include "openmc/tallies/derivative.h"
@@ -45,9 +47,15 @@ extern "C" int openmc_statepoint_write(const char* filename, bool* write_source)
     // Determine width for zero padding
     int w = std::to_string(settings::n_max_batches).size();
 
+    // Tag statepoints written during the forward solve of an adjoint run
+    const char* forward =
+      (FlatSourceDomain::solve_ == RandomRaySolve::FORWARD_FOR_ADJOINT)
+        ? "forward."
+        : "";
+
     // Set filename for state point
-    filename_ = fmt::format("{0}statepoint.{1:0{2}}.h5", settings::path_output,
-      simulation::current_batch, w);
+    filename_ = fmt::format("{0}statepoint.{3}{1:0{2}}.h5",
+      settings::path_output, simulation::current_batch, w, forward);
   }
 
   // If a file name was specified, ensure it has .h5 file extension
@@ -201,6 +209,12 @@ extern "C" int openmc_statepoint_write(const char* filename, bool* write_source)
           write_attribute(tally_group, "multiply_density", 0);
         }
 
+        if (tally->higher_moments()) {
+          write_attribute(tally_group, "higher_moments", 1);
+        } else {
+          write_attribute(tally_group, "higher_moments", 0);
+        }
+
         if (tally->estimator_ == TallyEstimator::ANALOG) {
           write_dataset(tally_group, "estimator", "analog");
         } else if (tally->estimator_ == TallyEstimator::TRACKLENGTH) {
@@ -264,12 +278,13 @@ extern "C" int openmc_statepoint_write(const char* filename, bool* write_source)
         for (const auto& tally : model::tallies) {
           if (!tally->writable_)
             continue;
-          // Write sum and sum_sq for each bin
+
+          // Write results for each bin
           std::string name = "tally " + std::to_string(tally->id_);
           hid_t tally_group = open_group(tallies_group, name.c_str());
           auto& results = tally->results_;
-          write_tally_results(tally_group, results.shape()[0],
-            results.shape()[1], results.data());
+          write_tally_results(tally_group, results.shape(0), results.shape(1),
+            results.shape(2), results.data());
           close_group(tally_group);
         }
       } else {
@@ -340,7 +355,7 @@ extern "C" int openmc_statepoint_write(const char* filename, bool* write_source)
       file_close(file_id);
   }
 
-#if defined(LIBMESH) || defined(DAGMC)
+#if defined(OPENMC_LIBMESH_ENABLED) || defined(OPENMC_DAGMC_ENABLED)
   // write unstructured mesh tally files
   write_unstructured_mesh_results();
 #endif
@@ -436,7 +451,8 @@ extern "C" int openmc_statepoint_load(const char* filename)
   // Read batch number to restart at
   read_dataset(file_id, "current_batch", simulation::restart_batch);
 
-  if (simulation::restart_batch >= settings::n_max_batches) {
+  if (settings::restart_run &&
+      simulation::restart_batch >= settings::n_max_batches) {
     warning(fmt::format(
       "The number of batches specified for simulation ({}) is smaller "
       "than or equal to the number of batches in the restart statepoint file "
@@ -507,8 +523,9 @@ extern "C" int openmc_statepoint_load(const char* filename)
           tally->writable_ = false;
         } else {
           auto& results = tally->results_;
-          read_tally_results(tally_group, results.shape()[0],
-            results.shape()[1], results.data());
+          read_tally_results(tally_group, results.shape(0), results.shape(1),
+            results.shape(2), results.data());
+
           read_dataset(tally_group, "n_realizations", tally->n_realizations_);
           close_group(tally_group);
         }
@@ -544,7 +561,7 @@ extern "C" int openmc_statepoint_load(const char* filename)
   return 0;
 }
 
-hid_t h5banktype()
+hid_t h5banktype(bool memory)
 {
   // Create compound type for position
   hid_t postype = H5Tcreate(H5T_COMPOUND, sizeof(struct Position));
@@ -559,7 +576,10 @@ hid_t h5banktype()
   // - openmc/statepoint.py
   // - docs/source/io_formats/statepoint.rst
   // - docs/source/io_formats/source.rst
-  hid_t banktype = H5Tcreate(H5T_COMPOUND, sizeof(struct SourceSite));
+  auto n = sizeof(SourceSite);
+  if (!memory)
+    n = 2 * sizeof(struct Position) + 3 * sizeof(double) + 3 * sizeof(int);
+  hid_t banktype = H5Tcreate(H5T_COMPOUND, n);
   H5Tinsert(banktype, "r", HOFFSET(SourceSite, r), postype);
   H5Tinsert(banktype, "u", HOFFSET(SourceSite, u), postype);
   H5Tinsert(banktype, "E", HOFFSET(SourceSite, E), H5T_NATIVE_DOUBLE);
@@ -579,8 +599,16 @@ void write_source_point(std::string filename, span<SourceSite> source_bank,
   const vector<int64_t>& bank_index, bool use_mcpl)
 {
   std::string ext = use_mcpl ? "mcpl" : "h5";
+
+  int total_surf_particles = source_bank.size();
+#ifdef OPENMC_MPI
+  int num_particles = source_bank.size();
+  MPI_Allreduce(
+    &num_particles, &total_surf_particles, 1, MPI_INT, MPI_SUM, mpi::intracomm);
+#endif
+
   write_message("Creating source file {}.{} with {} particles ...", filename,
-    ext, source_bank.size(), 5);
+    ext, total_surf_particles, 5);
 
   // Dispatch to appropriate function based on file type
   if (use_mcpl) {
@@ -619,6 +647,7 @@ void write_h5_source_point(const char* filename, span<SourceSite> source_bank,
   if (mpi::master || parallel) {
     file_id = file_open(filename_.c_str(), 'w', true);
     write_attribute(file_id, "filetype", "source");
+    write_attribute(file_id, "version", VERSION_STATEPOINT);
   }
 
   // Get pointer to source bank and write to file
@@ -631,96 +660,19 @@ void write_h5_source_point(const char* filename, span<SourceSite> source_bank,
 void write_source_bank(hid_t group_id, span<SourceSite> source_bank,
   const vector<int64_t>& bank_index)
 {
-  hid_t banktype = h5banktype();
-
-  // Set total and individual process dataspace sizes for source bank
-  int64_t dims_size = bank_index.back();
-  int64_t count_size = bank_index[mpi::rank + 1] - bank_index[mpi::rank];
-
-#ifdef PHDF5
-  // Set size of total dataspace for all procs and rank
-  hsize_t dims[] {static_cast<hsize_t>(dims_size)};
-  hid_t dspace = H5Screate_simple(1, dims, nullptr);
-  hid_t dset = H5Dcreate(group_id, "source_bank", banktype, dspace, H5P_DEFAULT,
-    H5P_DEFAULT, H5P_DEFAULT);
-
-  // Create another data space but for each proc individually
-  hsize_t count[] {static_cast<hsize_t>(count_size)};
-  hid_t memspace = H5Screate_simple(1, count, nullptr);
-
-  // Select hyperslab for this dataspace
-  hsize_t start[] {static_cast<hsize_t>(bank_index[mpi::rank])};
-  H5Sselect_hyperslab(dspace, H5S_SELECT_SET, start, nullptr, count, nullptr);
-
-  // Set up the property list for parallel writing
-  hid_t plist = H5Pcreate(H5P_DATASET_XFER);
-  H5Pset_dxpl_mpio(plist, H5FD_MPIO_COLLECTIVE);
-
-  // Write data to file in parallel
-  H5Dwrite(dset, banktype, memspace, dspace, plist, source_bank.data());
-
-  // Free resources
-  H5Sclose(dspace);
-  H5Sclose(memspace);
-  H5Dclose(dset);
-  H5Pclose(plist);
+  hid_t membanktype = h5banktype(true);
+  hid_t filebanktype = h5banktype(false);
 
+#ifdef OPENMC_MPI
+  write_bank_dataset("source_bank", group_id, source_bank, bank_index,
+    membanktype, filebanktype, mpi::source_site);
 #else
-
-  if (mpi::master) {
-    // Create dataset big enough to hold all source sites
-    hsize_t dims[] {static_cast<hsize_t>(dims_size)};
-    hid_t dspace = H5Screate_simple(1, dims, nullptr);
-    hid_t dset = H5Dcreate(group_id, "source_bank", banktype, dspace,
-      H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
-
-    // Save source bank sites since the array is overwritten below
-#ifdef OPENMC_MPI
-    vector<SourceSite> temp_source {source_bank.begin(), source_bank.end()};
+  write_bank_dataset("source_bank", group_id, source_bank, bank_index,
+    membanktype, filebanktype);
 #endif
 
-    for (int i = 0; i < mpi::n_procs; ++i) {
-      // Create memory space
-      hsize_t count[] {static_cast<hsize_t>(bank_index[i + 1] - bank_index[i])};
-      hid_t memspace = H5Screate_simple(1, count, nullptr);
-
-#ifdef OPENMC_MPI
-      // Receive source sites from other processes
-      if (i > 0)
-        MPI_Recv(source_bank.data(), count[0], mpi::source_site, i, i,
-          mpi::intracomm, MPI_STATUS_IGNORE);
-#endif
-
-      // Select hyperslab for this dataspace
-      dspace = H5Dget_space(dset);
-      hsize_t start[] {static_cast<hsize_t>(bank_index[i])};
-      H5Sselect_hyperslab(
-        dspace, H5S_SELECT_SET, start, nullptr, count, nullptr);
-
-      // Write data to hyperslab
-      H5Dwrite(
-        dset, banktype, memspace, dspace, H5P_DEFAULT, source_bank.data());
-
-      H5Sclose(memspace);
-      H5Sclose(dspace);
-    }
-
-    // Close all ids
-    H5Dclose(dset);
-
-#ifdef OPENMC_MPI
-    // Restore state of source bank
-    std::copy(temp_source.begin(), temp_source.end(), source_bank.begin());
-#endif
-  } else {
-#ifdef OPENMC_MPI
-    MPI_Send(source_bank.data(), count_size, mpi::source_site, 0, mpi::rank,
-      mpi::intracomm);
-#endif
-  }
-#endif
-
-  H5Tclose(banktype);
+  H5Tclose(membanktype);
+  H5Tclose(filebanktype);
 }
 
 // Determine member names of a compound HDF5 datatype
@@ -741,7 +693,17 @@ std::string dtype_member_names(hid_t dtype_id)
 void read_source_bank(
   hid_t group_id, vector<SourceSite>& sites, bool distribute)
 {
-  hid_t banktype = h5banktype();
+  bool legacy_particle_codes = true;
+  if (attribute_exists(group_id, "version")) {
+    array<int, 2> version;
+    read_attribute(group_id, "version", version);
+    if (version[0] > VERSION_STATEPOINT[0] ||
+        (version[0] == VERSION_STATEPOINT[0] && version[1] >= 2)) {
+      legacy_particle_codes = false;
+    }
+  }
+
+  hid_t banktype = h5banktype(true);
 
   // Open the dataset
   hid_t dset = H5Dopen(group_id, "source_bank", H5P_DEFAULT);
@@ -802,6 +764,12 @@ void read_source_bank(
     H5Sclose(memspace);
   H5Dclose(dset);
   H5Tclose(banktype);
+
+  if (legacy_particle_codes) {
+    for (auto& site : sites) {
+      site.particle = legacy_particle_index_to_type(site.particle.pdg_number());
+    }
+  }
 }
 
 void write_unstructured_mesh_results()
@@ -873,7 +841,7 @@ void write_unstructured_mesh_results()
           // construct result vectors
           vector<double> mean_vec(umesh->n_bins()),
             std_dev_vec(umesh->n_bins());
-          for (int j = 0; j < tally->results_.shape()[0]; j++) {
+          for (int j = 0; j < tally->results_.shape(0); j++) {
             // get the volume for this bin
             double volume = umesh->volume(j);
             // compute the mean
@@ -935,7 +903,7 @@ void write_tally_results_nr(hid_t file_id)
 
 #ifdef OPENMC_MPI
   // Reduce global tallies
-  xt::xtensor<double, 2> gt_reduced = xt::empty_like(gt);
+  tensor::Tensor<double> gt_reduced({N_GLOBAL_TALLIES, 3});
   MPI_Reduce(gt.data(), gt_reduced.data(), gt.size(), MPI_DOUBLE, MPI_SUM, 0,
     mpi::intracomm);
 
@@ -964,13 +932,18 @@ void write_tally_results_nr(hid_t file_id)
       write_attribute(file_id, "tallies_present", 1);
     }
 
-    // Get view of accumulated tally values
-    auto values_view = xt::view(t->results_, xt::all(), xt::all(),
-      xt::range(static_cast<int>(TallyResult::SUM),
-        static_cast<int>(TallyResult::SUM_SQ) + 1));
-
-    // Make copy of tally values in contiguous array
-    xt::xtensor<double, 3> values = values_view;
+    // Copy the SUM and SUM_SQ columns from the tally results into a
+    // contiguous array for MPI reduction
+    const int r_start = static_cast<int>(TallyResult::SUM);
+    const int r_end = static_cast<int>(TallyResult::SUM_SQ) + 1;
+    const size_t r_count = r_end - r_start;
+    const size_t ni = t->results_.shape(0);
+    const size_t nj = t->results_.shape(1);
+    tensor::Tensor<double> values({ni, nj, r_count});
+    for (size_t i = 0; i < ni; i++)
+      for (size_t j = 0; j < nj; j++)
+        for (size_t r = 0; r < r_count; r++)
+          values(i, j, r) = t->results_(i, j, r_start + r);
 
     if (mpi::master) {
       // Open group for tally
@@ -984,23 +957,27 @@ void write_tally_results_nr(hid_t file_id)
         MPI_SUM, 0, mpi::intracomm);
 #endif
 
-      // At the end of the simulation, store the results back in the
-      // regular TallyResults array
+      // At the end of the simulation, store the reduced results back
+      // into the tally results array
       if (simulation::current_batch == settings::n_max_batches ||
           simulation::satisfy_triggers) {
-        values_view = values;
+        for (size_t i = 0; i < ni; i++)
+          for (size_t j = 0; j < nj; j++)
+            for (size_t r = 0; r < r_count; r++)
+              t->results_(i, j, r_start + r) = values(i, j, r);
       }
 
-      // Put in temporary tally result
-      xt::xtensor<double, 3> results_copy = xt::zeros_like(t->results_);
-      auto copy_view = xt::view(results_copy, xt::all(), xt::all(),
-        xt::range(static_cast<int>(TallyResult::SUM),
-          static_cast<int>(TallyResult::SUM_SQ) + 1));
-      copy_view = values;
+      // Put reduced values into a full-sized copy for writing to HDF5
+      tensor::Tensor<double> results_copy = tensor::zeros_like(t->results_);
+      for (size_t i = 0; i < ni; i++)
+        for (size_t j = 0; j < nj; j++)
+          for (size_t r = 0; r < r_count; r++)
+            results_copy(i, j, r_start + r) = values(i, j, r);
 
       // Write reduced tally results to file
       auto shape = results_copy.shape();
-      write_tally_results(tally_group, shape[0], shape[1], results_copy.data());
+      write_tally_results(
+        tally_group, shape[0], shape[1], shape[2], results_copy.data());
 
       close_group(tally_group);
     } else {
diff --git a/src/surface.cpp b/src/surface.cpp
index 0002275c3..81b756dea 100644
--- a/src/surface.cpp
+++ b/src/surface.cpp
@@ -9,6 +9,7 @@
 #include <fmt/core.h>
 
 #include "openmc/array.h"
+#include "openmc/cell.h"
 #include "openmc/container_util.h"
 #include "openmc/error.h"
 #include "openmc/external/quartic_solver.h"
@@ -172,6 +173,19 @@ void Surface::to_hdf5(hid_t group_id) const
     if (bc_) {
       write_string(surf_group, "boundary_type", bc_->type(), false);
       bc_->to_hdf5(surf_group);
+
+      // write periodic surface ID
+      if (bc_->type() == "periodic") {
+        auto pbc = dynamic_cast<PeriodicBC*>(bc_.get());
+        Surface& surf1 {*model::surfaces[pbc->i_surf()]};
+        Surface& surf2 {*model::surfaces[pbc->j_surf()]};
+
+        if (id_ == surf1.id_) {
+          write_dataset(surf_group, "periodic_surface_id", surf2.id_);
+        } else {
+          write_dataset(surf_group, "periodic_surface_id", surf1.id_);
+        }
+      }
     } else {
       write_string(surf_group, "boundary_type", "transmission", false);
     }
@@ -238,9 +252,9 @@ void SurfaceXPlane::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceXPlane::bounding_box(bool pos_side) const
 {
   if (pos_side) {
-    return {x0_, INFTY, -INFTY, INFTY, -INFTY, INFTY};
+    return {{x0_, -INFTY, -INFTY}, {INFTY, INFTY, INFTY}};
   } else {
-    return {-INFTY, x0_, -INFTY, INFTY, -INFTY, INFTY};
+    return {{-INFTY, -INFTY, -INFTY}, {x0_, INFTY, INFTY}};
   }
 }
 
@@ -278,9 +292,9 @@ void SurfaceYPlane::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceYPlane::bounding_box(bool pos_side) const
 {
   if (pos_side) {
-    return {-INFTY, INFTY, y0_, INFTY, -INFTY, INFTY};
+    return {{-INFTY, y0_, -INFTY}, {INFTY, INFTY, INFTY}};
   } else {
-    return {-INFTY, INFTY, -INFTY, y0_, -INFTY, INFTY};
+    return {{-INFTY, -INFTY, -INFTY}, {INFTY, y0_, INFTY}};
   }
 }
 
@@ -318,9 +332,9 @@ void SurfaceZPlane::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceZPlane::bounding_box(bool pos_side) const
 {
   if (pos_side) {
-    return {-INFTY, INFTY, -INFTY, INFTY, z0_, INFTY};
+    return {{-INFTY, -INFTY, z0_}, {INFTY, INFTY, INFTY}};
   } else {
-    return {-INFTY, INFTY, -INFTY, INFTY, -INFTY, z0_};
+    return {{-INFTY, -INFTY, -INFTY}, {INFTY, INFTY, z0_}};
   }
 }
 
@@ -479,8 +493,8 @@ void SurfaceXCylinder::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceXCylinder::bounding_box(bool pos_side) const
 {
   if (!pos_side) {
-    return {-INFTY, INFTY, y0_ - radius_, y0_ + radius_, z0_ - radius_,
-      z0_ + radius_};
+    return {{-INFTY, y0_ - radius_, z0_ - radius_},
+      {INFTY, y0_ + radius_, z0_ + radius_}};
   } else {
     return {};
   }
@@ -522,8 +536,8 @@ void SurfaceYCylinder::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceYCylinder::bounding_box(bool pos_side) const
 {
   if (!pos_side) {
-    return {x0_ - radius_, x0_ + radius_, -INFTY, INFTY, z0_ - radius_,
-      z0_ + radius_};
+    return {{x0_ - radius_, -INFTY, z0_ - radius_},
+      {x0_ + radius_, INFTY, z0_ + radius_}};
   } else {
     return {};
   }
@@ -566,8 +580,8 @@ void SurfaceZCylinder::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceZCylinder::bounding_box(bool pos_side) const
 {
   if (!pos_side) {
-    return {x0_ - radius_, x0_ + radius_, y0_ - radius_, y0_ + radius_, -INFTY,
-      INFTY};
+    return {{x0_ - radius_, y0_ - radius_, -INFTY},
+      {x0_ + radius_, y0_ + radius_, INFTY}};
   } else {
     return {};
   }
@@ -644,8 +658,8 @@ void SurfaceSphere::to_hdf5_inner(hid_t group_id) const
 BoundingBox SurfaceSphere::bounding_box(bool pos_side) const
 {
   if (!pos_side) {
-    return {x0_ - radius_, x0_ + radius_, y0_ - radius_, y0_ + radius_,
-      z0_ - radius_, z0_ + radius_};
+    return {{x0_ - radius_, y0_ - radius_, z0_ - radius_},
+      {x0_ + radius_, y0_ + radius_, z0_ + radius_}};
   } else {
     return {};
   }
@@ -1156,7 +1170,10 @@ Direction SurfaceZTorus::normal(Position r) const
 
 //==============================================================================
 
-void read_surfaces(pugi::xml_node node)
+void read_surfaces(pugi::xml_node node,
+  std::set<std::pair<int, int>>& periodic_pairs,
+  std::unordered_map<int, double>& albedo_map,
+  std::unordered_map<int, int>& periodic_sense_map)
 {
   // Count the number of surfaces
   int n_surfaces = 0;
@@ -1167,8 +1184,6 @@ void read_surfaces(pugi::xml_node node)
   // Loop over XML surface elements and populate the array.  Keep track of
   // periodic surfaces and their albedos.
   model::surfaces.reserve(n_surfaces);
-  std::set<std::pair<int, int>> periodic_pairs;
-  std::unordered_map<int, double> albedo_map;
   {
     pugi::xml_node surf_node;
     int i_surf;
@@ -1231,6 +1246,7 @@ void read_surfaces(pugi::xml_node node)
       if (check_for_node(surf_node, "boundary")) {
         std::string surf_bc = get_node_value(surf_node, "boundary", true, true);
         if (surf_bc == "periodic") {
+          periodic_sense_map[model::surfaces.back()->id_] = 0;
           // Check for surface albedo. Skip sanity check as it is already done
           // in the Surface class's constructor.
           if (check_for_node(surf_node, "albedo")) {
@@ -1262,6 +1278,28 @@ void read_surfaces(pugi::xml_node node)
         fmt::format("Two or more surfaces use the same unique ID: {}", id));
     }
   }
+}
+
+void prepare_boundary_conditions(std::set<std::pair<int, int>>& periodic_pairs,
+  std::unordered_map<int, double>& albedo_map,
+  std::unordered_map<int, int>& periodic_sense_map)
+{
+  // Fill the senses map for periodic surfaces
+  auto n_periodic = periodic_sense_map.size();
+  for (const auto& cell : model::cells) {
+    if (n_periodic == 0)
+      break; // Early exit once all periodic surfaces found
+
+    for (auto s : cell->surfaces()) {
+      auto surf_idx = std::abs(s) - 1;
+      auto id = model::surfaces[surf_idx]->id_;
+
+      if (periodic_sense_map.count(id)) {
+        periodic_sense_map[id] = std::copysign(1, s);
+        --n_periodic;
+      }
+    }
+  }
 
   // Resolve unpaired periodic surfaces.  A lambda function is used with
   // std::find_if to identify the unpaired surfaces.
@@ -1319,10 +1357,50 @@ void read_surfaces(pugi::xml_node node)
     // condition.  Otherwise, it is a rotational periodic BC.
     if (std::abs(1.0 - dot_prod) < FP_PRECISION) {
       surf1.bc_ = make_unique<TranslationalPeriodicBC>(i_surf, j_surf);
-      surf2.bc_ = make_unique<TranslationalPeriodicBC>(i_surf, j_surf);
+      surf2.bc_ = make_unique<TranslationalPeriodicBC>(j_surf, i_surf);
     } else {
-      surf1.bc_ = make_unique<RotationalPeriodicBC>(i_surf, j_surf);
-      surf2.bc_ = make_unique<RotationalPeriodicBC>(i_surf, j_surf);
+      // check that both normals have at least one 0 component
+      if (std::abs(norm1.x) > FP_PRECISION &&
+          std::abs(norm1.y) > FP_PRECISION &&
+          std::abs(norm1.z) > FP_PRECISION) {
+        fatal_error(fmt::format(
+          "The normal ({}) of the periodic surface ({}) does not contain any "
+          "component with a zero value. A RotationalPeriodicBC requires one "
+          "component which is zero for both plane normals.",
+          norm1, i_surf));
+      }
+      if (std::abs(norm2.x) > FP_PRECISION &&
+          std::abs(norm2.y) > FP_PRECISION &&
+          std::abs(norm2.z) > FP_PRECISION) {
+        fatal_error(fmt::format(
+          "The normal ({}) of the periodic surface ({}) does not contain any "
+          "component with a zero value. A RotationalPeriodicBC requires one "
+          "component which is zero for both plane normals.",
+          norm2, j_surf));
+      }
+      // find common zero component, which indicates the periodic axis
+      RotationalPeriodicBC::PeriodicAxis axis;
+      if (std::abs(norm1.x) <= FP_PRECISION &&
+          std::abs(norm2.x) <= FP_PRECISION) {
+        axis = RotationalPeriodicBC::PeriodicAxis::x;
+      } else if (std::abs(norm1.y) <= FP_PRECISION &&
+                 std::abs(norm2.y) <= FP_PRECISION) {
+        axis = RotationalPeriodicBC::PeriodicAxis::y;
+      } else if (std::abs(norm1.z) <= FP_PRECISION &&
+                 std::abs(norm2.z) <= FP_PRECISION) {
+        axis = RotationalPeriodicBC::PeriodicAxis::z;
+      } else {
+        fatal_error(fmt::format(
+          "There is no component which is 0.0 in both normal vectors. This "
+          "indicates that the two planes are not periodic about the X, Y, or Z "
+          "axis, which is not supported."));
+      }
+      auto i_sign = periodic_sense_map[periodic_pair.first];
+      auto j_sign = periodic_sense_map[periodic_pair.second];
+      surf1.bc_ = make_unique<RotationalPeriodicBC>(
+        i_sign * (i_surf + 1), j_sign * (j_surf + 1), axis);
+      surf2.bc_ = make_unique<RotationalPeriodicBC>(
+        j_sign * (j_surf + 1), i_sign * (i_surf + 1), axis);
     }
 
     // If albedo data is present in albedo map, set the boundary albedo.
diff --git a/src/tallies/filter.cpp b/src/tallies/filter.cpp
index 6430d9ae1..badb91077 100644
--- a/src/tallies/filter.cpp
+++ b/src/tallies/filter.cpp
@@ -25,17 +25,21 @@
 #include "openmc/tallies/filter_materialfrom.h"
 #include "openmc/tallies/filter_mesh.h"
 #include "openmc/tallies/filter_meshborn.h"
+#include "openmc/tallies/filter_meshmaterial.h"
 #include "openmc/tallies/filter_meshsurface.h"
 #include "openmc/tallies/filter_mu.h"
 #include "openmc/tallies/filter_musurface.h"
 #include "openmc/tallies/filter_parent_nuclide.h"
 #include "openmc/tallies/filter_particle.h"
+#include "openmc/tallies/filter_particle_production.h"
 #include "openmc/tallies/filter_polar.h"
+#include "openmc/tallies/filter_reaction.h"
 #include "openmc/tallies/filter_sph_harm.h"
 #include "openmc/tallies/filter_sptl_legendre.h"
 #include "openmc/tallies/filter_surface.h"
 #include "openmc/tallies/filter_time.h"
 #include "openmc/tallies/filter_universe.h"
+#include "openmc/tallies/filter_weight.h"
 #include "openmc/tallies/filter_zernike.h"
 #include "openmc/xml_interface.h"
 
@@ -132,6 +136,8 @@ Filter* Filter::create(const std::string& type, int32_t id)
     return Filter::create<MeshFilter>(id);
   } else if (type == "meshborn") {
     return Filter::create<MeshBornFilter>(id);
+  } else if (type == "meshmaterial") {
+    return Filter::create<MeshMaterialFilter>(id);
   } else if (type == "meshsurface") {
     return Filter::create<MeshSurfaceFilter>(id);
   } else if (type == "mu") {
@@ -142,8 +148,12 @@ Filter* Filter::create(const std::string& type, int32_t id)
     return Filter::create<ParentNuclideFilter>(id);
   } else if (type == "particle") {
     return Filter::create<ParticleFilter>(id);
+  } else if (type == "particleproduction") {
+    return Filter::create<ParticleProductionFilter>(id);
   } else if (type == "polar") {
     return Filter::create<PolarFilter>(id);
+  } else if (type == "reaction") {
+    return Filter::create<ReactionFilter>(id);
   } else if (type == "surface") {
     return Filter::create<SurfaceFilter>(id);
   } else if (type == "spatiallegendre") {
@@ -154,6 +164,8 @@ Filter* Filter::create(const std::string& type, int32_t id)
     return Filter::create<TimeFilter>(id);
   } else if (type == "universe") {
     return Filter::create<UniverseFilter>(id);
+  } else if (type == "weight") {
+    return Filter::create<WeightFilter>(id);
   } else if (type == "zernike") {
     return Filter::create<ZernikeFilter>(id);
   } else if (type == "zernikeradial") {
diff --git a/src/tallies/filter_cell.cpp b/src/tallies/filter_cell.cpp
index b545801ea..7a6394956 100644
--- a/src/tallies/filter_cell.cpp
+++ b/src/tallies/filter_cell.cpp
@@ -49,7 +49,7 @@ void CellFilter::get_all_bins(
   const Particle& p, TallyEstimator estimator, FilterMatch& match) const
 {
   for (int i = 0; i < p.n_coord(); i++) {
-    auto search = map_.find(p.coord(i).cell);
+    auto search = map_.find(p.coord(i).cell());
     if (search != map_.end()) {
       match.bins_.push_back(search->second);
       match.weights_.push_back(1.0);
diff --git a/src/tallies/filter_cell_instance.cpp b/src/tallies/filter_cell_instance.cpp
index 0634175dd..928bfb6c5 100644
--- a/src/tallies/filter_cell_instance.cpp
+++ b/src/tallies/filter_cell_instance.cpp
@@ -9,6 +9,7 @@
 #include "openmc/cell.h"
 #include "openmc/error.h"
 #include "openmc/geometry.h"
+#include "openmc/tensor.h"
 #include "openmc/xml_interface.h"
 
 namespace openmc {
@@ -73,7 +74,7 @@ void CellInstanceFilter::set_cell_instances(span<CellInstance> instances)
 void CellInstanceFilter::get_all_bins(
   const Particle& p, TallyEstimator estimator, FilterMatch& match) const
 {
-  int64_t index_cell = p.lowest_coord().cell;
+  int64_t index_cell = p.lowest_coord().cell();
   int64_t instance = p.cell_instance();
 
   if (cells_.count(index_cell) > 0) {
@@ -89,7 +90,7 @@ void CellInstanceFilter::get_all_bins(
     return;
 
   for (int i = 0; i < p.n_coord() - 1; i++) {
-    int64_t index_cell = p.coord(i).cell;
+    int64_t index_cell = p.coord(i).cell();
     // if this cell isn't used on the filter, move on
     if (cells_.count(index_cell) == 0)
       continue;
@@ -108,7 +109,7 @@ void CellInstanceFilter::to_statepoint(hid_t filter_group) const
 {
   Filter::to_statepoint(filter_group);
   size_t n = cell_instances_.size();
-  xt::xtensor<size_t, 2> data({n, 2});
+  tensor::Tensor<size_t> data({n, 2});
   for (int64_t i = 0; i < n; ++i) {
     const auto& x = cell_instances_[i];
     data(i, 0) = model::cells[x.index_cell]->id_;
diff --git a/src/tallies/filter_delayedgroup.cpp b/src/tallies/filter_delayedgroup.cpp
index 01e39e554..46adf529b 100644
--- a/src/tallies/filter_delayedgroup.cpp
+++ b/src/tallies/filter_delayedgroup.cpp
@@ -27,7 +27,7 @@ void DelayedGroupFilter::set_groups(span<int> groups)
     } else if (group > MAX_DELAYED_GROUPS) {
       throw std::invalid_argument {
         "Encountered delayedgroup bin with index " + std::to_string(group) +
-        " which is greater than MAX_DELATED_GROUPS (" +
+        " which is greater than MAX_DELAYED_GROUPS (" +
         std::to_string(MAX_DELAYED_GROUPS) + ")"};
     }
     groups_.push_back(group);
@@ -39,6 +39,10 @@ void DelayedGroupFilter::set_groups(span<int> groups)
 void DelayedGroupFilter::get_all_bins(
   const Particle& p, TallyEstimator estimator, FilterMatch& match) const
 {
+  // Note that the bin is set to zero here, but bins outside zero are
+  // tallied to regardless. This is because that logic has to be handled
+  // in the scoring code instead where looping over the delayed
+  // group takes place (tally_scoring.cpp).
   match.bins_.push_back(0);
   match.weights_.push_back(1.0);
 }
diff --git a/src/tallies/filter_distribcell.cpp b/src/tallies/filter_distribcell.cpp
index 821e843da..f511a6816 100644
--- a/src/tallies/filter_distribcell.cpp
+++ b/src/tallies/filter_distribcell.cpp
@@ -34,7 +34,7 @@ void DistribcellFilter::set_cell(int32_t cell)
   assert(cell >= 0);
   assert(cell < model::cells.size());
   cell_ = cell;
-  n_bins_ = model::cells[cell]->n_instances_;
+  n_bins_ = model::cells[cell]->n_instances();
 }
 
 void DistribcellFilter::get_all_bins(
@@ -43,18 +43,18 @@ void DistribcellFilter::get_all_bins(
   int offset = 0;
   auto distribcell_index = model::cells[cell_]->distribcell_index_;
   for (int i = 0; i < p.n_coord(); i++) {
-    auto& c {*model::cells[p.coord(i).cell]};
+    auto& c {*model::cells[p.coord(i).cell()]};
     if (c.type_ == Fill::UNIVERSE) {
       offset += c.offset_[distribcell_index];
     } else if (c.type_ == Fill::LATTICE) {
-      auto& lat {*model::lattices[p.coord(i + 1).lattice]};
-      const auto& i_xyz {p.coord(i + 1).lattice_i};
+      auto& lat {*model::lattices[p.coord(i + 1).lattice()]};
+      const auto& i_xyz {p.coord(i + 1).lattice_index()};
       if (lat.are_valid_indices(i_xyz)) {
         offset +=
           lat.offset(distribcell_index, i_xyz) + c.offset_[distribcell_index];
       }
     }
-    if (cell_ == p.coord(i).cell) {
+    if (cell_ == p.coord(i).cell()) {
       match.bins_.push_back(offset);
       match.weights_.push_back(1.0);
       return;
diff --git a/src/tallies/filter_energy.cpp b/src/tallies/filter_energy.cpp
index 0b954cce3..566b5710a 100644
--- a/src/tallies/filter_energy.cpp
+++ b/src/tallies/filter_energy.cpp
@@ -60,13 +60,8 @@ void EnergyFilter::get_all_bins(
   const Particle& p, TallyEstimator estimator, FilterMatch& match) const
 {
   if (p.g() != C_NONE && matches_transport_groups_) {
-    if (estimator == TallyEstimator::TRACKLENGTH) {
-      match.bins_.push_back(data::mg.num_energy_groups_ - p.g() - 1);
-    } else {
-      match.bins_.push_back(data::mg.num_energy_groups_ - p.g_last() - 1);
-    }
+    match.bins_.push_back(data::mg.num_energy_groups_ - p.g_last() - 1);
     match.weights_.push_back(1.0);
-
   } else {
     // Get the pre-collision energy of the particle.
     auto E = p.E_last();
diff --git a/src/tallies/filter_mesh.cpp b/src/tallies/filter_mesh.cpp
index 4edfbec4b..a0698992d 100644
--- a/src/tallies/filter_mesh.cpp
+++ b/src/tallies/filter_mesh.cpp
@@ -6,6 +6,7 @@
 #include "openmc/constants.h"
 #include "openmc/error.h"
 #include "openmc/mesh.h"
+#include "openmc/position.h"
 #include "openmc/xml_interface.h"
 
 namespace openmc {
@@ -30,6 +31,10 @@ void MeshFilter::from_xml(pugi::xml_node node)
   if (check_for_node(node, "translation")) {
     set_translation(get_node_array<double>(node, "translation"));
   }
+  // Read the rotation transform.
+  if (check_for_node(node, "rotation")) {
+    set_rotation(get_node_array<double>(node, "rotation"));
+  }
 }
 
 void MeshFilter::get_all_bins(
@@ -45,6 +50,12 @@ void MeshFilter::get_all_bins(
     last_r -= translation();
     r -= translation();
   }
+  // apply rotation if present
+  if (!rotation_.empty()) {
+    last_r = last_r.rotate(rotation_);
+    r = r.rotate(rotation_);
+    u = u.rotate(rotation_);
+  }
 
   if (estimator != TallyEstimator::TRACKLENGTH) {
     auto bin = model::meshes[mesh_]->get_bin(r);
@@ -65,6 +76,9 @@ void MeshFilter::to_statepoint(hid_t filter_group) const
   if (translated_) {
     write_dataset(filter_group, "translation", translation_);
   }
+  if (rotated_) {
+    write_dataset(filter_group, "rotation", rotation_);
+  }
 }
 
 std::string MeshFilter::text_label(int bin) const
@@ -93,6 +107,40 @@ void MeshFilter::set_translation(const double translation[3])
   this->set_translation({translation[0], translation[1], translation[2]});
 }
 
+void MeshFilter::set_rotation(const vector<double>& rot)
+{
+  rotated_ = true;
+
+  // Compute and store the inverse rotation matrix for the angles given.
+  rotation_.clear();
+  rotation_.reserve(rot.size() == 9 ? 9 : 12);
+  if (rot.size() == 3) {
+    double phi = -rot[0] * PI / 180.0;
+    double theta = -rot[1] * PI / 180.0;
+    double psi = -rot[2] * PI / 180.0;
+    rotation_.push_back(std::cos(theta) * std::cos(psi));
+    rotation_.push_back(-std::cos(phi) * std::sin(psi) +
+                        std::sin(phi) * std::sin(theta) * std::cos(psi));
+    rotation_.push_back(std::sin(phi) * std::sin(psi) +
+                        std::cos(phi) * std::sin(theta) * std::cos(psi));
+    rotation_.push_back(std::cos(theta) * std::sin(psi));
+    rotation_.push_back(std::cos(phi) * std::cos(psi) +
+                        std::sin(phi) * std::sin(theta) * std::sin(psi));
+    rotation_.push_back(-std::sin(phi) * std::cos(psi) +
+                        std::cos(phi) * std::sin(theta) * std::sin(psi));
+    rotation_.push_back(-std::sin(theta));
+    rotation_.push_back(std::sin(phi) * std::cos(theta));
+    rotation_.push_back(std::cos(phi) * std::cos(theta));
+
+    // When user specifies angles, write them at end of vector
+    rotation_.push_back(rot[0]);
+    rotation_.push_back(rot[1]);
+    rotation_.push_back(rot[2]);
+  } else {
+    std::copy(rot.begin(), rot.end(), std::back_inserter(rotation_));
+  }
+}
+
 //==============================================================================
 // C-API functions
 //==============================================================================
@@ -201,4 +249,48 @@ extern "C" int openmc_mesh_filter_set_translation(
   return 0;
 }
 
+//! Return the rotation matrix of a mesh filter
+extern "C" int openmc_mesh_filter_get_rotation(
+  int32_t index, double rot[], size_t* n)
+{
+  // Make sure this is a valid index to an allocated filter
+  if (int err = verify_filter(index))
+    return err;
+
+  // Check the filter type
+  const auto& filter = model::tally_filters[index];
+  if (filter->type() != FilterType::MESH) {
+    set_errmsg("Tried to get a rotation from a non-mesh filter.");
+    return OPENMC_E_INVALID_TYPE;
+  }
+  // Get rotation from the mesh filter and set value
+  auto mesh_filter = dynamic_cast<MeshFilter*>(filter.get());
+  *n = mesh_filter->rotation().size();
+  std::memcpy(rot, mesh_filter->rotation().data(),
+    *n * sizeof(mesh_filter->rotation()[0]));
+  return 0;
+}
+
+//! Set the flattened rotation matrix of a mesh filter
+extern "C" int openmc_mesh_filter_set_rotation(
+  int32_t index, const double rot[], size_t rot_len)
+{
+  // Make sure this is a valid index to an allocated filter
+  if (int err = verify_filter(index))
+    return err;
+
+  const auto& filter = model::tally_filters[index];
+  // Check the filter type
+  if (filter->type() != FilterType::MESH) {
+    set_errmsg("Tried to set a rotation from a non-mesh filter.");
+    return OPENMC_E_INVALID_TYPE;
+  }
+
+  // Get a pointer to the filter and downcast
+  auto mesh_filter = dynamic_cast<MeshFilter*>(filter.get());
+  std::vector<double> vec_rot(rot, rot + rot_len);
+  mesh_filter->set_rotation(vec_rot);
+  return 0;
+}
+
 } // namespace openmc
diff --git a/src/tallies/filter_meshmaterial.cpp b/src/tallies/filter_meshmaterial.cpp
new file mode 100644
index 000000000..b45bb4164
--- /dev/null
+++ b/src/tallies/filter_meshmaterial.cpp
@@ -0,0 +1,187 @@
+#include "openmc/tallies/filter_meshmaterial.h"
+
+#include <cassert>
+#include <utility> // for move
+
+#include <fmt/core.h>
+
+#include "openmc/capi.h"
+#include "openmc/constants.h"
+#include "openmc/container_util.h"
+#include "openmc/error.h"
+#include "openmc/material.h"
+#include "openmc/mesh.h"
+#include "openmc/tensor.h"
+#include "openmc/xml_interface.h"
+
+namespace openmc {
+
+void MeshMaterialFilter::from_xml(pugi::xml_node node)
+{
+  // Get mesh ID
+  auto mesh = get_node_array<int32_t>(node, "mesh");
+  if (mesh.size() != 1) {
+    fatal_error(
+      "Only one mesh can be specified per " + type_str() + " mesh filter.");
+  }
+
+  auto id = mesh[0];
+  auto search = model::mesh_map.find(id);
+  if (search == model::mesh_map.end()) {
+    fatal_error(
+      fmt::format("Could not find mesh {} specified on tally filter.", id));
+  }
+  set_mesh(search->second);
+
+  // Get pairs of (element index, material) and set the bins
+  auto bins = get_node_array<int32_t>(node, "bins");
+  this->set_bins(bins);
+
+  if (check_for_node(node, "translation")) {
+    set_translation(get_node_array<double>(node, "translation"));
+  }
+}
+
+void MeshMaterialFilter::set_bins(span<int32_t> bins)
+{
+  if (bins.size() % 2 != 0) {
+    fatal_error(
+      fmt::format("Size of mesh material bins is not even: {}", bins.size()));
+  }
+
+  // Create a vector of ElementMat pairs from the flat vector of bins
+  vector<ElementMat> element_mats;
+  for (int64_t i = 0; i < bins.size() / 2; ++i) {
+    int32_t element = bins[2 * i];
+    int32_t mat_id = bins[2 * i + 1];
+    auto search = model::material_map.find(mat_id);
+    if (search == model::material_map.end()) {
+      fatal_error(fmt::format(
+        "Could not find material {} specified on tally filter.", mat_id));
+    }
+    int32_t mat_index = search->second;
+    element_mats.push_back({element, mat_index});
+  }
+
+  this->set_bins(std::move(element_mats));
+}
+
+void MeshMaterialFilter::set_bins(vector<ElementMat>&& bins)
+{
+  // Swap internal bins_ with the provided vector to avoid copying
+  bins_.swap(bins);
+
+  // Clear and update the mapping and vector of materials
+  materials_.clear();
+  map_.clear();
+  for (std::size_t i = 0; i < bins_.size(); ++i) {
+    const auto& x = bins_[i];
+    assert(x.index_mat >= 0);
+    assert(x.index_mat < model::materials.size());
+    materials_.insert(x.index_mat);
+    map_[x] = i;
+  }
+
+  n_bins_ = bins_.size();
+}
+
+void MeshMaterialFilter::set_mesh(int32_t mesh)
+{
+  // perform any additional perparation for mesh tallies here
+  mesh_ = mesh;
+  model::meshes[mesh_]->prepare_for_point_location();
+}
+
+void MeshMaterialFilter::set_translation(const Position& translation)
+{
+  translated_ = true;
+  translation_ = translation;
+}
+
+void MeshMaterialFilter::set_translation(const double translation[3])
+{
+  this->set_translation({translation[0], translation[1], translation[2]});
+}
+
+void MeshMaterialFilter::get_all_bins(
+  const Particle& p, TallyEstimator estimator, FilterMatch& match) const
+{
+  // If current material is not in any bins, don't bother checking
+  if (!contains(materials_, p.material())) {
+    return;
+  }
+
+  Position last_r = p.r_last();
+  Position r = p.r();
+  Position u = p.u();
+
+  // apply translation if present
+  if (translated_) {
+    last_r -= translation();
+    r -= translation();
+  }
+
+  if (estimator != TallyEstimator::TRACKLENGTH) {
+    int32_t index_element = model::meshes[mesh_]->get_bin(r);
+    if (index_element >= 0) {
+      auto search = map_.find({index_element, p.material()});
+      if (search != map_.end()) {
+        match.bins_.push_back(search->second);
+        match.weights_.push_back(1.0);
+      }
+    }
+  } else {
+    // First determine which elements the particle crosses (may or may not
+    // actually match bins so we have to adjust bins_/weight_ after)
+    int32_t n_start = match.bins_.size();
+    model::meshes[mesh_]->bins_crossed(
+      last_r, r, u, match.bins_, match.weights_);
+    int32_t n_end = match.bins_.size();
+
+    // Go through bins and weights and check which ones are actually a match
+    // based on the (element, material) pair. For matches, overwrite the bin.
+    int i = 0;
+    for (int j = n_start; j < n_end; ++j) {
+      int32_t index_element = match.bins_[j];
+      double weight = match.weights_[j];
+      auto search = map_.find({index_element, p.material()});
+      if (search != map_.end()) {
+        match.bins_[n_start + i] = search->second;
+        match.weights_[n_start + i] = weight;
+        ++i;
+      }
+    }
+
+    // Resize the vectors to remove the unmatched bins
+    match.bins_.resize(n_start + i);
+  }
+}
+
+void MeshMaterialFilter::to_statepoint(hid_t filter_group) const
+{
+  Filter::to_statepoint(filter_group);
+  write_dataset(filter_group, "mesh", model::meshes[mesh_]->id_);
+
+  size_t n = bins_.size();
+  tensor::Tensor<size_t> data({n, 2});
+  for (int64_t i = 0; i < n; ++i) {
+    const auto& x = bins_[i];
+    data(i, 0) = x.index_element;
+    data(i, 1) = model::materials[x.index_mat]->id_;
+  }
+  write_dataset(filter_group, "bins", data);
+
+  if (translated_) {
+    write_dataset(filter_group, "translation", translation_);
+  }
+}
+
+std::string MeshMaterialFilter::text_label(int bin) const
+{
+  auto& x = bins_[bin];
+  auto& mesh = *model::meshes.at(mesh_);
+  return fmt::format("Mesh {}, {}, Material {}", mesh.id(),
+    mesh.bin_label(x.index_element), model::materials[x.index_mat]->id_);
+}
+
+} // namespace openmc
diff --git a/src/tallies/filter_particle.cpp b/src/tallies/filter_particle.cpp
index eef1d1e63..031068f3a 100644
--- a/src/tallies/filter_particle.cpp
+++ b/src/tallies/filter_particle.cpp
@@ -13,7 +13,7 @@ void ParticleFilter::from_xml(pugi::xml_node node)
   // Convert to vector of ParticleType
   vector<ParticleType> types;
   for (auto& p : particles) {
-    types.push_back(str_to_particle_type(p));
+    types.emplace_back(p);
   }
   this->set_particles(types);
 }
@@ -47,7 +47,7 @@ void ParticleFilter::to_statepoint(hid_t filter_group) const
   Filter::to_statepoint(filter_group);
   vector<std::string> particles;
   for (auto p : particles_) {
-    particles.push_back(particle_type_to_str(p));
+    particles.push_back(p.str());
   }
   write_dataset(filter_group, "bins", particles);
 }
@@ -55,10 +55,10 @@ void ParticleFilter::to_statepoint(hid_t filter_group) const
 std::string ParticleFilter::text_label(int bin) const
 {
   const auto& p = particles_.at(bin);
-  return fmt::format("Particle: {}", particle_type_to_str(p));
+  return fmt::format("Particle: {}", p.str());
 }
 
-extern "C" int openmc_particle_filter_get_bins(int32_t idx, int bins[])
+extern "C" int openmc_particle_filter_get_bins(int32_t idx, int32_t bins[])
 {
   if (int err = verify_filter(idx))
     return err;
@@ -68,7 +68,7 @@ extern "C" int openmc_particle_filter_get_bins(int32_t idx, int bins[])
   if (pf) {
     const auto& particles = pf->particles();
     for (int i = 0; i < particles.size(); i++) {
-      bins[i] = static_cast<int>(particles[i]);
+      bins[i] = particles[i].pdg_number();
     }
   } else {
     set_errmsg("The filter at the specified index is not a ParticleFilter");
diff --git a/src/tallies/filter_particle_production.cpp b/src/tallies/filter_particle_production.cpp
new file mode 100644
index 000000000..899809679
--- /dev/null
+++ b/src/tallies/filter_particle_production.cpp
@@ -0,0 +1,108 @@
+#include "openmc/tallies/filter_particle_production.h"
+
+#include <fmt/core.h>
+
+#include "openmc/search.h"
+#include "openmc/xml_interface.h"
+
+namespace openmc {
+
+//==============================================================================
+// ParticleProductionFilter implementation
+//==============================================================================
+
+void ParticleProductionFilter::get_all_bins(
+  const Particle& p, TallyEstimator estimator, FilterMatch& match) const
+{
+  int start_idx = p.secondary_bank_index();
+  int end_idx = start_idx + p.n_secondaries();
+
+  // Loop over secondary bank entries
+  for (int bank_idx = start_idx; bank_idx < end_idx; bank_idx++) {
+    const auto& site = p.local_secondary_bank(bank_idx);
+
+    // Find which particle-type slot this secondary belongs to
+    auto it = type_to_index_.find(site.particle.pdg_number());
+    if (it == type_to_index_.end())
+      continue;
+
+    int particle_idx = it->second;
+    if (energy_bins_.empty()) {
+      // No energy binning, just particle type
+      match.bins_.push_back(particle_idx);
+      match.weights_.push_back(site.wgt);
+    } else {
+      // Bin the energy
+      if (site.E >= energy_bins_.front() && site.E <= energy_bins_.back()) {
+        int n_energies = static_cast<int>(energy_bins_.size()) - 1;
+        auto energy_idx =
+          lower_bound_index(energy_bins_.begin(), energy_bins_.end(), site.E);
+        match.bins_.push_back(particle_idx * n_energies + energy_idx);
+        match.weights_.push_back(site.wgt);
+      }
+    }
+  }
+}
+
+std::string ParticleProductionFilter::text_label(int bin) const
+{
+  if (energy_bins_.empty()) {
+    return fmt::format("Secondary {}", secondary_types_.at(bin).str());
+  } else {
+    int n_energies = static_cast<int>(energy_bins_.size()) - 1;
+    int particle_idx = bin / n_energies;
+    int energy_idx = bin % n_energies;
+    return fmt::format("Secondary {}, Energy [{}, {})",
+      secondary_types_.at(particle_idx).str(), energy_bins_.at(energy_idx),
+      energy_bins_.at(energy_idx + 1));
+  }
+}
+
+void ParticleProductionFilter::from_xml(pugi::xml_node node)
+{
+  // Read energy bins if present (optional)
+  if (check_for_node(node, "energies")) {
+    auto bins = get_node_array<double>(node, "energies");
+    for (int64_t i = 1; i < bins.size(); ++i) {
+      if (bins[i] <= bins[i - 1]) {
+        throw std::runtime_error {
+          "Energy bins must be monotonically increasing."};
+      }
+    }
+    energy_bins_.assign(bins.begin(), bins.end());
+  }
+
+  // Read particle types (required)
+  auto names = get_node_array<std::string>(node, "particles");
+  for (const auto& name : names) {
+    int idx = secondary_types_.size();
+    secondary_types_.emplace_back(name);
+    type_to_index_[secondary_types_.back().pdg_number()] = idx;
+  }
+
+  // Compute total bins
+  if (energy_bins_.empty()) {
+    n_bins_ = secondary_types_.size();
+  } else {
+    n_bins_ = secondary_types_.size() * (energy_bins_.size() - 1);
+  }
+}
+
+void ParticleProductionFilter::to_statepoint(hid_t filter_group) const
+{
+  Filter::to_statepoint(filter_group);
+
+  // Write energy bins if present
+  if (!energy_bins_.empty()) {
+    write_dataset(filter_group, "energies", energy_bins_);
+  }
+
+  // Write particle types
+  vector<std::string> names;
+  for (const auto& pt : secondary_types_) {
+    names.push_back(pt.str());
+  }
+  write_dataset(filter_group, "particles", names);
+}
+
+} // namespace openmc
diff --git a/src/tallies/filter_reaction.cpp b/src/tallies/filter_reaction.cpp
new file mode 100644
index 000000000..8ee9f3ce8
--- /dev/null
+++ b/src/tallies/filter_reaction.cpp
@@ -0,0 +1,79 @@
+#include "openmc/tallies/filter_reaction.h"
+
+#include <fmt/core.h>
+
+#include "openmc/capi.h"
+#include "openmc/endf.h"
+#include "openmc/error.h"
+#include "openmc/reaction.h"
+#include "openmc/xml_interface.h"
+
+namespace openmc {
+
+//==============================================================================
+// ReactionFilter implementation
+//==============================================================================
+
+void ReactionFilter::from_xml(pugi::xml_node node)
+{
+  // Read bins as reaction name strings
+  auto bins_str = get_node_array<std::string>(node, "bins");
+
+  // Convert reaction names to MT numbers
+  vector<int> bins_mt;
+  bins_mt.reserve(bins_str.size());
+  for (const auto& name : bins_str) {
+    bins_mt.push_back(reaction_mt(name));
+  }
+
+  this->set_bins(bins_mt);
+}
+
+void ReactionFilter::set_bins(span<const int> bins)
+{
+  // Clear existing bins
+  bins_.clear();
+  bins_.reserve(bins.size());
+
+  // Copy bins and build lookup map
+  for (int64_t i = 0; i < bins.size(); ++i) {
+    bins_.push_back(bins[i]);
+  }
+
+  n_bins_ = bins_.size();
+}
+
+void ReactionFilter::get_all_bins(
+  const Particle& p, TallyEstimator estimator, FilterMatch& match) const
+{
+  // Get the event MT number from the particle
+  int event_mt = p.event_mt();
+
+  // Check each bin, considering summation rules
+  for (int64_t i = 0; i < bins_.size(); ++i) {
+    if (mt_matches(event_mt, bins_[i])) {
+      match.bins_.push_back(i);
+      match.weights_.push_back(1.0);
+    }
+  }
+}
+
+void ReactionFilter::to_statepoint(hid_t filter_group) const
+{
+  Filter::to_statepoint(filter_group);
+
+  // Write bins as reaction name strings for human readability
+  vector<std::string> names;
+  names.reserve(bins_.size());
+  for (auto mt : bins_) {
+    names.push_back(reaction_name(mt));
+  }
+  write_dataset(filter_group, "bins", names);
+}
+
+std::string ReactionFilter::text_label(int bin) const
+{
+  return reaction_name(bins_[bin]);
+}
+
+} // namespace openmc
diff --git a/src/tallies/filter_surface.cpp b/src/tallies/filter_surface.cpp
index 82f3d7178..4a636f1aa 100644
--- a/src/tallies/filter_surface.cpp
+++ b/src/tallies/filter_surface.cpp
@@ -52,11 +52,7 @@ void SurfaceFilter::get_all_bins(
   auto search = map_.find(p.surface_index());
   if (search != map_.end()) {
     match.bins_.push_back(search->second);
-    if (p.surface() < 0) {
-      match.weights_.push_back(-1.0);
-    } else {
-      match.weights_.push_back(1.0);
-    }
+    match.weights_.push_back(1.0);
   }
 }
 
diff --git a/src/tallies/filter_universe.cpp b/src/tallies/filter_universe.cpp
index 48114cfad..f4b22decd 100644
--- a/src/tallies/filter_universe.cpp
+++ b/src/tallies/filter_universe.cpp
@@ -48,7 +48,7 @@ void UniverseFilter::get_all_bins(
   const Particle& p, TallyEstimator estimator, FilterMatch& match) const
 {
   for (int i = 0; i < p.n_coord(); i++) {
-    auto search = map_.find(p.coord(i).universe);
+    auto search = map_.find(p.coord(i).universe());
     if (search != map_.end()) {
       match.bins_.push_back(search->second);
       match.weights_.push_back(1.0);
diff --git a/src/tallies/filter_weight.cpp b/src/tallies/filter_weight.cpp
new file mode 100644
index 000000000..31f4bd1bf
--- /dev/null
+++ b/src/tallies/filter_weight.cpp
@@ -0,0 +1,63 @@
+#include "openmc/tallies/filter_weight.h"
+
+#include <algorithm> // for is_sorted
+#include <stdexcept> // for runtime_error
+
+#include <fmt/core.h>
+
+#include "openmc/search.h"
+#include "openmc/xml_interface.h"
+
+namespace openmc {
+
+//==============================================================================
+// WeightFilter implementation
+//==============================================================================
+
+void WeightFilter::from_xml(pugi::xml_node node)
+{
+  auto bins = get_node_array<double>(node, "bins");
+  this->set_bins(bins);
+}
+
+void WeightFilter::set_bins(span<const double> bins)
+{
+  if (!std::is_sorted(bins.begin(), bins.end())) {
+    throw std::runtime_error {"Weight bins must be monotonically increasing."};
+  }
+
+  // Clear existing bins
+  bins_.clear();
+  bins_.reserve(bins.size());
+
+  // Copy bins
+  bins_.insert(bins_.end(), bins.begin(), bins.end());
+  n_bins_ = bins_.size() - 1;
+}
+
+void WeightFilter::get_all_bins(
+  const Particle& p, TallyEstimator estimator, FilterMatch& match) const
+{
+  // Get particle weight
+  double wgt = p.wgt_last();
+
+  // Bin the weight
+  if (wgt >= bins_.front() && wgt <= bins_.back()) {
+    auto bin = lower_bound_index(bins_.begin(), bins_.end(), wgt);
+    match.bins_.push_back(bin);
+    match.weights_.push_back(1.0);
+  }
+}
+
+void WeightFilter::to_statepoint(hid_t filter_group) const
+{
+  Filter::to_statepoint(filter_group);
+  write_dataset(filter_group, "bins", bins_);
+}
+
+std::string WeightFilter::text_label(int bin) const
+{
+  return fmt::format("Weight [{}, {}]", bins_[bin], bins_[bin + 1]);
+}
+
+} // namespace openmc
diff --git a/src/tallies/tally.cpp b/src/tallies/tally.cpp
index 96d684f71..085633247 100644
--- a/src/tallies/tally.cpp
+++ b/src/tallies/tally.cpp
@@ -2,6 +2,7 @@
 
 #include "openmc/array.h"
 #include "openmc/capi.h"
+#include "openmc/cell.h"
 #include "openmc/constants.h"
 #include "openmc/container_util.h"
 #include "openmc/error.h"
@@ -27,20 +28,21 @@
 #include "openmc/tallies/filter_legendre.h"
 #include "openmc/tallies/filter_mesh.h"
 #include "openmc/tallies/filter_meshborn.h"
+#include "openmc/tallies/filter_meshmaterial.h"
 #include "openmc/tallies/filter_meshsurface.h"
 #include "openmc/tallies/filter_particle.h"
 #include "openmc/tallies/filter_sph_harm.h"
 #include "openmc/tallies/filter_surface.h"
+#include "openmc/tallies/filter_time.h"
 #include "openmc/xml_interface.h"
 
-#include "xtensor/xadapt.hpp"
-#include "xtensor/xbuilder.hpp" // for empty_like
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
-#include <algorithm> // for max
+#include <algorithm> // for max, set_union
 #include <cassert>
-#include <cstddef> // for size_t
+#include <cstddef>  // for size_t
+#include <iterator> // for back_inserter
 #include <string>
 
 namespace openmc {
@@ -56,15 +58,17 @@ vector<unique_ptr<Tally>> tallies;
 vector<int> active_tallies;
 vector<int> active_analog_tallies;
 vector<int> active_tracklength_tallies;
+vector<int> active_timed_tracklength_tallies;
 vector<int> active_collision_tallies;
 vector<int> active_meshsurf_tallies;
 vector<int> active_surface_tallies;
 vector<int> active_pulse_height_tallies;
-vector<int> pulse_height_cells;
+vector<int32_t> pulse_height_cells;
+vector<double> time_grid;
 } // namespace model
 
 namespace simulation {
-xt::xtensor_fixed<double, xt::xshape<N_GLOBAL_TALLIES, 3>> global_tallies;
+tensor::StaticTensor2D<double, N_GLOBAL_TALLIES, 3> global_tallies;
 int32_t n_realizations {0};
 } // namespace simulation
 
@@ -102,6 +106,9 @@ Tally::Tally(pugi::xml_node node)
     multiply_density_ = get_node_value_bool(node, "multiply_density");
   }
 
+  if (check_for_node(node, "higher_moments")) {
+    higher_moments_ = get_node_value_bool(node, "higher_moments");
+  }
   // =======================================================================
   // READ DATA FOR FILTERS
 
@@ -163,6 +170,12 @@ Tally::Tally(pugi::xml_node node)
                filt_type == FilterType::ZERNIKE ||
                filt_type == FilterType::ZERNIKE_RADIAL) {
       estimator_ = TallyEstimator::COLLISION;
+    } else if (filt_type == FilterType::PARTICLE_PRODUCTION) {
+      estimator_ = TallyEstimator::ANALOG;
+    } else if (filt_type == FilterType::REACTION) {
+      if (estimator_ == TallyEstimator::TRACKLENGTH) {
+        estimator_ = TallyEstimator::COLLISION;
+      }
     }
   }
 
@@ -180,13 +193,71 @@ Tally::Tally(pugi::xml_node node)
     fatal_error(fmt::format("No scores specified on tally {}.", id_));
   }
 
+  // Set IFP if needed
+  if (!settings::ifp_on) {
+    // Determine if this tally has an IFP score
+    bool has_ifp_score = false;
+    for (int score : scores_) {
+      if (score == SCORE_IFP_TIME_NUM || score == SCORE_IFP_BETA_NUM ||
+          score == SCORE_IFP_DENOM) {
+        has_ifp_score = true;
+        break;
+      }
+    }
+
+    // Check for errors
+    if (has_ifp_score) {
+      if (settings::run_mode == RunMode::EIGENVALUE) {
+        if (settings::ifp_n_generation < 0) {
+          settings::ifp_n_generation = DEFAULT_IFP_N_GENERATION;
+          warning(fmt::format(
+            "{} generations will be used for IFP (default value). It can be "
+            "changed using the 'ifp_n_generation' settings.",
+            settings::ifp_n_generation));
+        }
+        if (settings::ifp_n_generation > settings::n_inactive) {
+          fatal_error("'ifp_n_generation' must be lower than or equal to the "
+                      "number of inactive cycles.");
+        }
+        settings::ifp_on = true;
+      } else if (settings::run_mode == RunMode::FIXED_SOURCE) {
+        fatal_error(
+          "Iterated Fission Probability can only be used in an eigenvalue "
+          "calculation.");
+      }
+    }
+  }
+
+  // Set IFP parameters if needed
+  if (settings::ifp_on) {
+    for (int score : scores_) {
+      switch (score) {
+      case SCORE_IFP_TIME_NUM:
+        if (settings::ifp_parameter == IFPParameter::None) {
+          settings::ifp_parameter = IFPParameter::GenerationTime;
+        } else if (settings::ifp_parameter == IFPParameter::BetaEffective) {
+          settings::ifp_parameter = IFPParameter::Both;
+        }
+        break;
+      case SCORE_IFP_BETA_NUM:
+      case SCORE_IFP_DENOM:
+        if (settings::ifp_parameter == IFPParameter::None) {
+          settings::ifp_parameter = IFPParameter::BetaEffective;
+        } else if (settings::ifp_parameter == IFPParameter::GenerationTime) {
+          settings::ifp_parameter = IFPParameter::Both;
+        }
+        break;
+      }
+    }
+  }
+
   // Check if tally is compatible with particle type
   if (!settings::photon_transport) {
     for (int score : scores_) {
       switch (score) {
       case SCORE_PULSE_HEIGHT:
-        fatal_error(
-          "For pulse-height tallies, photon transport needs to be activated.");
+        fatal_error("For pulse-height tallies, photon transport needs to be "
+                    "activated.");
         break;
       }
     }
@@ -223,12 +294,12 @@ Tally::Tally(pugi::xml_node node)
       const auto& f = model::tally_filters[particle_filter_index].get();
       auto pf = dynamic_cast<ParticleFilter*>(f);
       for (auto p : pf->particles()) {
-        if (p != ParticleType::neutron) {
+        if (!p.is_neutron()) {
           warning(fmt::format(
             "Particle filter other than NEUTRON used with "
             "photon transport turned off. All tallies for particle type {}"
             " will have no scores",
-            static_cast<int>(p)));
+            p.str()));
         }
       }
     }
@@ -260,7 +331,8 @@ Tally::Tally(pugi::xml_node node)
         if (has_energyout && i_nuc == -1) {
           fatal_error(fmt::format(
             "Error on tally {}: Cannot use a "
-            "'nuclide_density' or 'temperature' derivative on a tally with an "
+            "'nuclide_density' or 'temperature' derivative on a tally with "
+            "an "
             "outgoing energy filter and 'total' nuclide rate. Instead, tally "
             "each nuclide in the material individually.",
             id_));
@@ -318,7 +390,7 @@ Tally::Tally(pugi::xml_node node)
     }
   }
 
-#ifdef LIBMESH
+#ifdef OPENMC_LIBMESH_ENABLED
   // ensure a tracklength tally isn't used with a libMesh filter
   for (auto i : this->filters_) {
     auto df = dynamic_cast<MeshFilter*>(model::tally_filters[i].get());
@@ -429,22 +501,18 @@ void Tally::add_filter(Filter* filter)
     energyout_filter_ = filters_.size();
   } else if (filter->type() == FilterType::DELAYED_GROUP) {
     delayedgroup_filter_ = filters_.size();
-  } else if (filter->type() == FilterType::CELL) {
-    cell_filter_ = filters_.size();
-  } else if (filter->type() == FilterType::ENERGY) {
-    energy_filter_ = filters_.size();
   }
   filters_.push_back(filter_idx);
 }
 
 void Tally::set_strides()
 {
-  // Set the strides.  Filters are traversed in reverse so that the last filter
-  // has the shortest stride in memory and the first filter has the longest
-  // stride.
+  // Set the strides.  Filters are traversed in reverse so that the last
+  // filter has the shortest stride in memory and the first filter has the
+  // longest stride.
   auto n = filters_.size();
   strides_.resize(n, 0);
-  int stride = 1;
+  int64_t stride = 1;
   for (int i = n - 1; i >= 0; --i) {
     strides_[i] = stride;
     stride *= model::tally_filters[filters_[i]]->n_bins();
@@ -472,6 +540,8 @@ void Tally::set_scores(const vector<std::string>& scores)
   bool legendre_present = false;
   bool cell_present = false;
   bool cellfrom_present = false;
+  bool material_present = false;
+  bool materialfrom_present = false;
   bool surface_present = false;
   bool meshsurface_present = false;
   bool non_cell_energy_present = false;
@@ -488,23 +558,34 @@ void Tally::set_scores(const vector<std::string>& scores)
       cellfrom_present = true;
     } else if (filt->type() == FilterType::CELL) {
       cell_present = true;
+    } else if (filt->type() == FilterType::MATERIALFROM) {
+      materialfrom_present = true;
+    } else if (filt->type() == FilterType::MATERIAL) {
+      material_present = true;
     } else if (filt->type() == FilterType::SURFACE) {
       surface_present = true;
     } else if (filt->type() == FilterType::MESH_SURFACE) {
       meshsurface_present = true;
     }
   }
+  bool surface_types_present =
+    (surface_present || cellfrom_present || materialfrom_present);
+  bool non_meshsurface_types_present =
+    (surface_present || cell_present || cellfrom_present || material_present ||
+      materialfrom_present);
 
   // Iterate over the given scores.
   for (auto score_str : scores) {
-    // Make sure a delayed group filter wasn't used with an incompatible score.
+    // Make sure a delayed group filter wasn't used with an incompatible
+    // score.
     if (delayedgroup_filter_ != C_NONE) {
-      if (score_str != "delayed-nu-fission" && score_str != "decay-rate")
+      if (score_str != "delayed-nu-fission" && score_str != "decay-rate" &&
+          score_str != "ifp-beta-numerator")
         fatal_error("Cannot tally " + score_str + "with a delayedgroup filter");
     }
 
     // Determine integer code for score
-    int score = reaction_type(score_str);
+    int score = reaction_tally_mt(score_str);
 
     switch (score) {
     case SCORE_FLUX:
@@ -513,6 +594,12 @@ void Tally::set_scores(const vector<std::string>& scores)
           fatal_error("Cannot tally flux for an individual nuclide.");
       if (energyout_present)
         fatal_error("Cannot tally flux with an outgoing energy filter.");
+      if (surface_types_present) {
+        if (meshsurface_present)
+          fatal_error("OpenMC does not support mesh surface fluxes yet");
+        type_ = TallyType::SURFACE;
+        estimator_ = TallyEstimator::ANALOG;
+      }
       break;
 
     case SCORE_TOTAL:
@@ -545,47 +632,61 @@ void Tally::set_scores(const vector<std::string>& scores)
 
     case SCORE_CURRENT:
       // Check which type of current is desired: mesh or surface currents.
-      if (surface_present || cell_present || cellfrom_present) {
-        if (meshsurface_present)
+      if (meshsurface_present) {
+        if (non_meshsurface_types_present)
           fatal_error("Cannot tally mesh surface currents in the same tally as "
                       "normal surface currents");
-        type_ = TallyType::SURFACE;
-        estimator_ = TallyEstimator::ANALOG;
-      } else if (meshsurface_present) {
         type_ = TallyType::MESH_SURFACE;
       } else {
-        fatal_error("Cannot tally currents without surface type filters");
+        type_ = TallyType::SURFACE;
+        estimator_ = TallyEstimator::ANALOG;
       }
       break;
 
     case HEATING:
-      if (settings::photon_transport)
-        estimator_ = TallyEstimator::COLLISION;
+      if (settings::photon_transport) {
+        // Photon heating requires a collision estimator (analog energy
+        // balance). However, if the tally only scores neutrons, we can keep the
+        // tracklength estimator since neutron heating uses kerma coefficients
+        // that support tracklength scoring.
+        bool neutron_only = false;
+        for (auto i_filt : filters_) {
+          auto pf =
+            dynamic_cast<ParticleFilter*>(model::tally_filters[i_filt].get());
+          if (pf && pf->particles().size() == 1 &&
+              pf->particles()[0].is_neutron()) {
+            neutron_only = true;
+            break;
+          }
+        }
+        if (!neutron_only)
+          estimator_ = TallyEstimator::COLLISION;
+      }
       break;
 
-    case SCORE_PULSE_HEIGHT:
+    case SCORE_PULSE_HEIGHT: {
       if (non_cell_energy_present) {
         fatal_error("Pulse-height tallies are not compatible with filters "
                     "other than CellFilter and EnergyFilter");
       }
       type_ = TallyType::PULSE_HEIGHT;
-
-      // Collecting indices of all cells covered by the filters in the pulse
-      // height tally in global variable pulse_height_cells
-      for (const auto& i_filt : filters_) {
-        auto cell_filter =
-          dynamic_cast<CellFilter*>(model::tally_filters[i_filt].get());
-        if (cell_filter) {
-          const auto& cells = cell_filter->cells();
-          for (int i = 0; i < cell_filter->n_bins(); i++) {
-            int cell_index = cells[i];
-            if (!contains(model::pulse_height_cells, cell_index)) {
-              model::pulse_height_cells.push_back(cell_index);
-            }
-          }
-        }
+      // Collect all unique cell indices covered by this tally.
+      // If no CellFilter is present, all cells in the geometry are scored.
+      const auto* cell_filter_ptr = get_filter<CellFilter>();
+      int n = cell_filter_ptr ? cell_filter_ptr->n_bins()
+                              : static_cast<int>(model::cells.size());
+      for (int i = 0; i < n; ++i) {
+        int32_t cell_index = cell_filter_ptr ? cell_filter_ptr->cells()[i] : i;
+        if (!contains(model::pulse_height_cells, cell_index))
+          model::pulse_height_cells.push_back(cell_index);
       }
+      break;
+    }
 
+    case SCORE_IFP_TIME_NUM:
+    case SCORE_IFP_BETA_NUM:
+    case SCORE_IFP_DENOM:
+      estimator_ = TallyEstimator::COLLISION;
       break;
     }
 
@@ -611,15 +712,20 @@ void Tally::set_scores(const vector<std::string>& scores)
                     "in multi-group mode");
   }
 
-  // Make sure current scores are not mixed in with volumetric scores.
-  if (type_ == TallyType::SURFACE || type_ == TallyType::MESH_SURFACE) {
-    if (scores_.size() != 1)
-      fatal_error("Cannot tally other scores in the same tally as surface "
-                  "currents.");
+  // Make sure mesh surface tallies contain only current score.
+  if (meshsurface_present) {
+    if ((scores_[0] != SCORE_CURRENT) || (scores_.size() > 1))
+      fatal_error("Cannot tally score other than 'current' when using a "
+                  "mesh-surface filter.");
+  }
+
+  // Make sure surface tallies contain only surface type scores score.
+  if (type_ == TallyType::SURFACE) {
+    for (auto sc : scores_)
+      if ((sc != SCORE_CURRENT) && (sc != SCORE_FLUX))
+        fatal_error("Cannot tally scores other than 'current' or 'flux' "
+                    "when using surface filters.");
   }
-  if ((surface_present || meshsurface_present) && scores_[0] != SCORE_CURRENT)
-    fatal_error("Cannot tally score other than 'current' when using a surface "
-                "or mesh-surface filter.");
 }
 
 void Tally::set_nuclides(pugi::xml_node node)
@@ -715,7 +821,7 @@ void Tally::init_triggers(pugi::xml_node node)
       } else {
         int i_score = 0;
         for (; i_score < this->scores_.size(); ++i_score) {
-          if (this->scores_[i_score] == reaction_type(score_str))
+          if (this->scores_[i_score] == reaction_tally_mt(score_str))
             break;
         }
         if (i_score == this->scores_.size()) {
@@ -733,14 +839,20 @@ void Tally::init_triggers(pugi::xml_node node)
 void Tally::init_results()
 {
   int n_scores = scores_.size() * nuclides_.size();
-  results_ = xt::empty<double>({n_filter_bins_, n_scores, 3});
+  if (higher_moments_) {
+    results_ = tensor::Tensor<double>({static_cast<size_t>(n_filter_bins_),
+      static_cast<size_t>(n_scores), size_t {5}});
+  } else {
+    results_ = tensor::Tensor<double>({static_cast<size_t>(n_filter_bins_),
+      static_cast<size_t>(n_scores), size_t {3}});
+  }
 }
 
 void Tally::reset()
 {
   n_realizations_ = 0;
   if (results_.size() != 0) {
-    xt::view(results_, xt::all()) = 0.0;
+    results_.fill(0.0);
   }
 }
 
@@ -752,31 +864,52 @@ void Tally::accumulate()
   if (mpi::master || !settings::reduce_tallies) {
     // Calculate total source strength for normalization
     double total_source = 0.0;
-    if (settings::run_mode == RunMode::FIXED_SOURCE &&
-        !settings::uniform_source_sampling) {
-      for (const auto& s : model::external_sources) {
-        total_source += s->strength();
-      }
+    if (settings::run_mode == RunMode::FIXED_SOURCE) {
+      total_source = model::external_sources_probability.integral();
     } else {
       total_source = 1.0;
     }
 
+    // Determine number of particles contributing to tally
+    double contributing_particles = settings::reduce_tallies
+                                      ? settings::n_particles
+                                      : simulation::work_per_rank;
+
     // Account for number of source particles in normalization
     double norm =
-      total_source / (settings::n_particles * settings::gen_per_batch);
+      total_source / (contributing_particles * settings::gen_per_batch);
 
     if (settings::solver_type == SolverType::RANDOM_RAY) {
       norm = 1.0;
     }
 
-// Accumulate each result
+    // Accumulate each result
+    if (higher_moments_) {
 #pragma omp parallel for
-    for (int i = 0; i < results_.shape()[0]; ++i) {
-      for (int j = 0; j < results_.shape()[1]; ++j) {
-        double val = results_(i, j, TallyResult::VALUE) * norm;
-        results_(i, j, TallyResult::VALUE) = 0.0;
-        results_(i, j, TallyResult::SUM) += val;
-        results_(i, j, TallyResult::SUM_SQ) += val * val;
+      // filter bins (specific cell, energy bins)
+      for (int64_t i = 0; i < results_.shape(0); ++i) {
+        // score bins (flux, total reaction rate, fission reaction rate, etc.)
+        for (int j = 0; j < results_.shape(1); ++j) {
+          double val = results_(i, j, TallyResult::VALUE) * norm;
+          double val2 = val * val;
+          results_(i, j, TallyResult::VALUE) = 0.0;
+          results_(i, j, TallyResult::SUM) += val;
+          results_(i, j, TallyResult::SUM_SQ) += val2;
+          results_(i, j, TallyResult::SUM_THIRD) += val2 * val;
+          results_(i, j, TallyResult::SUM_FOURTH) += val2 * val2;
+        }
+      }
+    } else {
+#pragma omp parallel for
+      // filter bins (specific cell, energy bins)
+      for (int64_t i = 0; i < results_.shape(0); ++i) {
+        // score bins (flux, total reaction rate, fission reaction rate, etc.)
+        for (int j = 0; j < results_.shape(1); ++j) {
+          double val = results_(i, j, TallyResult::VALUE) * norm;
+          results_(i, j, TallyResult::VALUE) = 0.0;
+          results_(i, j, TallyResult::SUM) += val;
+          results_(i, j, TallyResult::SUM_SQ) += val * val;
+        }
       }
     }
   }
@@ -791,18 +924,18 @@ int Tally::score_index(const std::string& score) const
   return -1;
 }
 
-xt::xarray<double> Tally::get_reshaped_data() const
+tensor::Tensor<double> Tally::get_reshaped_data() const
 {
-  std::vector<uint64_t> shape;
+  vector<size_t> shape;
   for (auto f : filters()) {
     shape.push_back(model::tally_filters[f]->n_bins());
   }
 
   // add number of scores and nuclides to tally
-  shape.push_back(results_.shape()[1]);
-  shape.push_back(results_.shape()[2]);
+  shape.push_back(results_.shape(1));
+  shape.push_back(results_.shape(2));
 
-  xt::xarray<double> reshaped_results = results_;
+  tensor::Tensor<double> reshaped_results = results_;
   reshaped_results.reshape(shape);
   return reshaped_results;
 }
@@ -907,13 +1040,14 @@ void reduce_tally_results()
       // Skip any tallies that are not active
       auto& tally {model::tallies[i_tally]};
 
-      // Get view of accumulated tally values
-      auto values_view = xt::view(tally->results_, xt::all(), xt::all(),
-        static_cast<int>(TallyResult::VALUE));
+      // Extract 2D view of the VALUE column from the 3D results tensor,
+      // then copy into a contiguous array for MPI reduction
+      const int val_idx = static_cast<int>(TallyResult::VALUE);
+      tensor::View<double> val_view =
+        tally->results_.slice(tensor::all, tensor::all, val_idx);
+      tensor::Tensor<double> values(val_view);
 
-      // Make copy of tally values in contiguous array
-      xt::xtensor<double, 2> values = values_view;
-      xt::xtensor<double, 2> values_reduced = xt::empty_like(values);
+      tensor::Tensor<double> values_reduced(values.shape());
 
       // Reduce contiguous set of tally results
       MPI_Reduce(values.data(), values_reduced.data(), values.size(),
@@ -921,24 +1055,23 @@ void reduce_tally_results()
 
       // Transfer values on master and reset on other ranks
       if (mpi::master) {
-        values_view = values_reduced;
+        val_view = values_reduced;
       } else {
-        values_view = 0.0;
+        val_view = 0.0;
       }
     }
   }
 
-  // Note that global tallies are *always* reduced even when no_reduce option is
-  // on.
+  // Note that global tallies are *always* reduced even when no_reduce option
+  // is on.
 
-  // Get view of global tally values
+  // Get reference to global tallies
   auto& gt = simulation::global_tallies;
-  auto gt_values_view =
-    xt::view(gt, xt::all(), static_cast<int>(TallyResult::VALUE));
+  const int val_col = static_cast<int>(TallyResult::VALUE);
 
-  // Make copy of values in contiguous array
-  xt::xtensor<double, 1> gt_values = gt_values_view;
-  xt::xtensor<double, 1> gt_values_reduced = xt::empty_like(gt_values);
+  // Copy VALUE column into contiguous array for MPI reduction
+  tensor::Tensor<double> gt_values(gt.slice(tensor::all, val_col));
+  tensor::Tensor<double> gt_values_reduced({size_t {N_GLOBAL_TALLIES}});
 
   // Reduce contiguous data
   MPI_Reduce(gt_values.data(), gt_values_reduced.data(), N_GLOBAL_TALLIES,
@@ -946,9 +1079,9 @@ void reduce_tally_results()
 
   // Transfer values on master and reset on other ranks
   if (mpi::master) {
-    gt_values_view = gt_values_reduced;
+    gt.slice(tensor::all, val_col) = gt_values_reduced;
   } else {
-    gt_values_view = 0.0;
+    gt.slice(tensor::all, val_col) = 0.0;
   }
 
   // We also need to determine the total starting weight of particles from the
@@ -1008,21 +1141,59 @@ void accumulate_tallies()
   }
 }
 
+double distance_to_time_boundary(double time, double speed)
+{
+  if (model::time_grid.empty()) {
+    return INFTY;
+  } else if (time >= model::time_grid.back()) {
+    return INFTY;
+  } else {
+    double next_time =
+      *std::upper_bound(model::time_grid.begin(), model::time_grid.end(), time);
+    return (next_time - time) * speed;
+  }
+}
+
+//! Add new points to the global time grid
+//
+//! \param grid Vector of new time points to add
+void add_to_time_grid(vector<double> grid)
+{
+  if (grid.empty())
+    return;
+
+  // Create new vector with enough space to hold old and new grid points
+  vector<double> merged;
+  merged.reserve(model::time_grid.size() + grid.size());
+
+  // Merge and remove duplicates
+  std::set_union(model::time_grid.begin(), model::time_grid.end(), grid.begin(),
+    grid.end(), std::back_inserter(merged));
+
+  // Swap in the new grid
+  model::time_grid.swap(merged);
+}
+
 void setup_active_tallies()
 {
   model::active_tallies.clear();
   model::active_analog_tallies.clear();
   model::active_tracklength_tallies.clear();
+  model::active_timed_tracklength_tallies.clear();
   model::active_collision_tallies.clear();
   model::active_meshsurf_tallies.clear();
   model::active_surface_tallies.clear();
   model::active_pulse_height_tallies.clear();
+  model::time_grid.clear();
 
   for (auto i = 0; i < model::tallies.size(); ++i) {
     const auto& tally {*model::tallies[i]};
 
     if (tally.active_) {
       model::active_tallies.push_back(i);
+      bool mesh_present = (tally.get_filter<MeshFilter>() ||
+                           tally.get_filter<MeshMaterialFilter>());
+      auto time_filter = tally.get_filter<TimeFilter>();
       switch (tally.type_) {
 
       case TallyType::VOLUME:
@@ -1031,7 +1202,12 @@ void setup_active_tallies()
           model::active_analog_tallies.push_back(i);
           break;
         case TallyEstimator::TRACKLENGTH:
-          model::active_tracklength_tallies.push_back(i);
+          if (time_filter && mesh_present) {
+            model::active_timed_tracklength_tallies.push_back(i);
+            add_to_time_grid(time_filter->bins());
+          } else {
+            model::active_tracklength_tallies.push_back(i);
+          }
           break;
         case TallyEstimator::COLLISION:
           model::active_collision_tallies.push_back(i);
@@ -1067,10 +1243,12 @@ void free_memory_tally()
   model::active_tallies.clear();
   model::active_analog_tallies.clear();
   model::active_tracklength_tallies.clear();
+  model::active_timed_tracklength_tallies.clear();
   model::active_collision_tallies.clear();
   model::active_meshsurf_tallies.clear();
   model::active_surface_tallies.clear();
   model::active_pulse_height_tallies.clear();
+  model::time_grid.clear();
 
   model::tally_map.clear();
 }
@@ -1409,8 +1587,8 @@ extern "C" int openmc_tally_get_n_realizations(int32_t index, int32_t* n)
   return 0;
 }
 
-//! \brief Returns a pointer to a tally results array along with its shape. This
-//! allows a user to obtain in-memory tally results from Python directly.
+//! \brief Returns a pointer to a tally results array along with its shape.
+//! This allows a user to obtain in-memory tally results from Python directly.
 extern "C" int openmc_tally_results(
   int32_t index, double** results, size_t* shape)
 {
diff --git a/src/tallies/tally_scoring.cpp b/src/tallies/tally_scoring.cpp
index 02cb48567..241cbf008 100644
--- a/src/tallies/tally_scoring.cpp
+++ b/src/tallies/tally_scoring.cpp
@@ -4,6 +4,7 @@
 #include "openmc/capi.h"
 #include "openmc/constants.h"
 #include "openmc/error.h"
+#include "openmc/ifp.h"
 #include "openmc/material.h"
 #include "openmc/mgxs_interface.h"
 #include "openmc/nuclide.h"
@@ -13,12 +14,14 @@
 #include "openmc/settings.h"
 #include "openmc/simulation.h"
 #include "openmc/string_utils.h"
+#include "openmc/surface.h"
 #include "openmc/tallies/derivative.h"
 #include "openmc/tallies/filter.h"
 #include "openmc/tallies/filter_cell.h"
 #include "openmc/tallies/filter_delayedgroup.h"
 #include "openmc/tallies/filter_energy.h"
 
+#include <numeric>
 #include <string>
 
 namespace openmc {
@@ -155,7 +158,7 @@ void score_fission_delayed_dg(int i_tally, int d_bin, double score,
   dg_match.bins_[i_bin] = d_bin;
 
   // Determine the filter scoring index
-  auto filter_index = 0;
+  int64_t filter_index = 0;
   double filter_weight = 1.;
   for (auto i = 0; i < tally.filters().size(); ++i) {
     auto i_filt = tally.filters(i);
@@ -232,7 +235,7 @@ double score_fission_q(const Particle& p, int score_bin, const Tally& tally,
       double score {0.0};
       for (auto i = 0; i < material.nuclide_.size(); ++i) {
         auto j_nuclide = material.nuclide_[i];
-        auto atom_density = material.atom_density_(i);
+        auto atom_density = material.atom_density(i, p.density_mult());
         const Nuclide& nuc {*data::nuclides[j_nuclide]};
         score += get_nuc_fission_q(nuc, p, score_bin) * atom_density *
                  p.neutron_xs(j_nuclide).fission;
@@ -324,6 +327,56 @@ double score_neutron_heating(const Particle& p, const Tally& tally, double flux,
   return score;
 }
 
+//! Helper function to obtain reaction Q value for photons and charged particles
+double get_reaction_q_value(const Particle& p)
+{
+  if (p.type().is_photon() && p.event_mt() == PAIR_PROD) {
+    // pair production
+    return -2 * MASS_ELECTRON_EV;
+  } else if (p.type() == ParticleType::positron()) {
+    // positron annihilation
+    return 2 * MASS_ELECTRON_EV;
+  } else {
+    return 0.0;
+  }
+}
+
+//! Helper function to obtain particle heating [eV]
+
+double score_particle_heating(const Particle& p, const Tally& tally,
+  double flux, int rxn_bin, int i_nuclide, double atom_density)
+{
+  if (p.type().is_neutron())
+    return score_neutron_heating(
+      p, tally, flux, rxn_bin, i_nuclide, atom_density);
+  if (i_nuclide == -1 || i_nuclide == p.event_nuclide() ||
+      p.event_nuclide() == -1) {
+    // For pair production and positron annihilation, we need to account for the
+    // reaction Q value
+    double Q = get_reaction_q_value(p);
+
+    // Get the pre-collision energy of the particle.
+    auto E = p.E_last();
+
+    // The energy deposited is the sum of the incident energy and the reaction
+    // Q-value less the energy of any outgoing particles
+    double score = E + Q - p.E() - p.bank_second_E();
+
+    score *= p.wgt_last();
+
+    // if no event_nuclide (charged particle) scale energy deposition by
+    // fractional charge density
+    if (i_nuclide != -1 && p.event_nuclide() == -1) {
+      const auto& mat {model::materials[p.material()]};
+      int z = data::nuclides[i_nuclide]->Z_;
+      auto i = mat->mat_nuclide_index_[i_nuclide];
+      score *= (z * mat->atom_density_[i] / mat->charge_density());
+    }
+    return score;
+  }
+  return 0.0;
+}
+
 //! Helper function for nu-fission tallies with energyout filters.
 //
 //! In this case, we may need to score to multiple bins if there were multiple
@@ -396,7 +449,7 @@ void score_fission_eout(Particle& p, int i_tally, int i_score, int score_bin)
         (score_bin == SCORE_PROMPT_NU_FISSION && g == 0)) {
 
       // Find the filter scoring index for this filter combination
-      int filter_index = 0;
+      int64_t filter_index = 0;
       double filter_weight = 1.0;
       for (auto j = 0; j < tally.filters().size(); ++j) {
         auto i_filt = tally.filters(j);
@@ -444,7 +497,7 @@ void score_fission_eout(Particle& p, int i_tally, int i_score, int score_bin)
       } else {
 
         // Find the filter index and weight for this filter combination
-        int filter_index = 0;
+        int64_t filter_index = 0;
         double filter_weight = 1.;
         for (auto j = 0; j < tally.filters().size(); ++j) {
           auto i_filt = tally.filters(j);
@@ -525,16 +578,14 @@ double get_nuclide_xs(const Particle& p, int i_nuclide, int score_bin)
 //! collision estimator.
 
 void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
-  int filter_index, double filter_weight, int i_nuclide, double atom_density,
-  double flux)
+  int64_t filter_index, double filter_weight, int i_nuclide,
+  double atom_density, double flux)
 {
   Tally& tally {*model::tallies[i_tally]};
 
   // Get the pre-collision energy of the particle.
   auto E = p.E_last();
 
-  using Type = ParticleType;
-
   for (auto i = 0; i < tally.scores_.size(); ++i) {
     auto score_bin = tally.scores_[i];
     auto score_index = start_index + i;
@@ -547,9 +598,9 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
 
     case SCORE_TOTAL:
       if (i_nuclide >= 0) {
-        if (p.type() == Type::neutron) {
+        if (p.type().is_neutron()) {
           score = p.neutron_xs(i_nuclide).total * atom_density * flux;
-        } else if (p.type() == Type::photon) {
+        } else if (p.type().is_photon()) {
           score = p.photon_xs(i_nuclide).total * atom_density * flux;
         }
       } else {
@@ -558,7 +609,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_INVERSE_VELOCITY:
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // Score inverse velocity in units of s/cm.
@@ -566,11 +617,11 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_SCATTER:
-      if (p.type() != Type::neutron && p.type() != Type::photon)
+      if (!p.type().is_neutron() && !p.type().is_photon())
         continue;
 
       if (i_nuclide >= 0) {
-        if (p.type() == Type::neutron) {
+        if (p.type().is_neutron()) {
           const auto& micro = p.neutron_xs(i_nuclide);
           score = (micro.total - micro.absorption) * atom_density * flux;
         } else {
@@ -578,7 +629,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
           score = (micro.coherent + micro.incoherent) * atom_density * flux;
         }
       } else {
-        if (p.type() == Type::neutron) {
+        if (p.type().is_neutron()) {
           score = (p.macro_xs().total - p.macro_xs().absorption) * flux;
         } else {
           score = (p.macro_xs().coherent + p.macro_xs().incoherent) * flux;
@@ -587,11 +638,11 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_ABSORPTION:
-      if (p.type() != Type::neutron && p.type() != Type::photon)
+      if (!p.type().is_neutron() && !p.type().is_photon())
         continue;
 
       if (i_nuclide >= 0) {
-        if (p.type() == Type::neutron) {
+        if (p.type().is_neutron()) {
           score = p.neutron_xs(i_nuclide).absorption * atom_density * flux;
         } else {
           const auto& xs = p.photon_xs(i_nuclide);
@@ -599,7 +650,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
             (xs.total - xs.coherent - xs.incoherent) * atom_density * flux;
         }
       } else {
-        if (p.type() == Type::neutron) {
+        if (p.type().is_neutron()) {
           score = p.macro_xs().absorption * flux;
         } else {
           score =
@@ -645,7 +696,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
           const Material& material {*model::materials[p.material()]};
           for (auto i = 0; i < material.nuclide_.size(); ++i) {
             auto j_nuclide = material.nuclide_[i];
-            auto atom_density = material.atom_density_(i);
+            auto atom_density = material.atom_density(i, p.density_mult());
             score += p.neutron_xs(j_nuclide).fission *
                      data::nuclides[j_nuclide]->nu(
                        E, ReactionProduct::EmissionMode::prompt) *
@@ -692,7 +743,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
             const Material& material {*model::materials[p.material()]};
             for (auto i = 0; i < material.nuclide_.size(); ++i) {
               auto j_nuclide = material.nuclide_[i];
-              auto atom_density = material.atom_density_(i);
+              auto atom_density = material.atom_density(i, p.density_mult());
               // Tally each delayed group bin individually
               for (auto d_bin = 0; d_bin < filt.n_bins(); ++d_bin) {
                 auto d = filt.groups()[d_bin];
@@ -712,7 +763,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
             const Material& material {*model::materials[p.material()]};
             for (auto i = 0; i < material.nuclide_.size(); ++i) {
               auto j_nuclide = material.nuclide_[i];
-              auto atom_density = material.atom_density_(i);
+              auto atom_density = material.atom_density(i, p.density_mult());
               score += p.neutron_xs(j_nuclide).fission *
                        data::nuclides[j_nuclide]->nu(
                          E, ReactionProduct::EmissionMode::delayed) *
@@ -755,7 +806,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
           // this loop.
           for (auto d = 1; d < rxn.products_.size(); ++d) {
             const auto& product = rxn.products_[d];
-            if (product.particle_ != Type::neutron)
+            if (!product.particle_.is_neutron())
               continue;
 
             auto yield = nuc.nu(E, ReactionProduct::EmissionMode::delayed, d);
@@ -773,7 +824,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
             const Material& material {*model::materials[p.material()]};
             for (auto i = 0; i < material.nuclide_.size(); ++i) {
               auto j_nuclide = material.nuclide_[i];
-              auto atom_density = material.atom_density_(i);
+              auto atom_density = material.atom_density(i, p.density_mult());
               const auto& nuc {*data::nuclides[j_nuclide]};
               if (nuc.fissionable_) {
                 const auto& rxn {*nuc.fission_rx_[0]};
@@ -798,7 +849,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
             const Material& material {*model::materials[p.material()]};
             for (auto i = 0; i < material.nuclide_.size(); ++i) {
               auto j_nuclide = material.nuclide_[i];
-              auto atom_density = material.atom_density_(i);
+              auto atom_density = material.atom_density(i, p.density_mult());
               const auto& nuc {*data::nuclides[j_nuclide]};
               if (nuc.fissionable_) {
                 const auto& rxn {*nuc.fission_rx_[0]};
@@ -809,7 +860,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
                 // this loop.
                 for (auto d = 1; d < rxn.products_.size(); ++d) {
                   const auto& product = rxn.products_[d];
-                  if (product.particle_ != Type::neutron)
+                  if (!product.particle_.is_neutron())
                     continue;
 
                   auto yield =
@@ -842,7 +893,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
         const Material& material {*model::materials[p.material()]};
         for (auto i = 0; i < material.nuclide_.size(); ++i) {
           auto j_nuclide = material.nuclide_[i];
-          auto atom_density = material.atom_density_(i);
+          auto atom_density = material.atom_density(i, p.density_mult());
           const auto& nuc {*data::nuclides[j_nuclide]};
           if (nuc.fissionable_) {
             const auto& rxn {*nuc.fission_rx_[0]};
@@ -854,13 +905,12 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_EVENTS:
-// Simply count the number of scoring events
-#pragma omp atomic
-      tally.results_(filter_index, score_index, TallyResult::VALUE) += 1.0;
-      continue;
+      // Simply count the number of scoring events
+      score = 1.0;
+      break;
 
     case ELASTIC:
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       if (i_nuclide >= 0) {
@@ -873,7 +923,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
           const Material& material {*model::materials[p.material()]};
           for (auto i = 0; i < material.nuclide_.size(); ++i) {
             auto j_nuclide = material.nuclide_[i];
-            auto atom_density = material.atom_density_(i);
+            auto atom_density = material.atom_density(i, p.density_mult());
             if (p.neutron_xs(j_nuclide).elastic == CACHE_INVALID)
               data::nuclides[j_nuclide]->calculate_elastic_xs(p);
             score += p.neutron_xs(j_nuclide).elastic * atom_density * flux;
@@ -890,6 +940,64 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
         score_fission_q(p, score_bin, tally, flux, i_nuclide, atom_density);
       break;
 
+    case SCORE_IFP_TIME_NUM:
+      if (settings::ifp_on) {
+        if (p.type().is_neutron() && p.fission()) {
+          if (is_generation_time_or_both()) {
+            const auto& lifetimes =
+              simulation::ifp_source_lifetime_bank[p.current_work()];
+            if (lifetimes.size() == settings::ifp_n_generation) {
+              score = lifetimes[0] * p.wgt_last();
+            }
+          }
+        }
+      }
+      break;
+
+    case SCORE_IFP_BETA_NUM:
+      if (settings::ifp_on) {
+        if (p.type().is_neutron() && p.fission()) {
+          if (is_beta_effective_or_both()) {
+            const auto& delayed_groups =
+              simulation::ifp_source_delayed_group_bank[p.current_work()];
+            if (delayed_groups.size() == settings::ifp_n_generation) {
+              if (delayed_groups[0] > 0) {
+                score = p.wgt_last();
+                if (tally.delayedgroup_filter_ != C_NONE) {
+                  auto i_dg_filt = tally.filters()[tally.delayedgroup_filter_];
+                  const DelayedGroupFilter& filt {
+                    *dynamic_cast<DelayedGroupFilter*>(
+                      model::tally_filters[i_dg_filt].get())};
+                  score_fission_delayed_dg(i_tally, delayed_groups[0] - 1,
+                    score, score_index, p.filter_matches());
+                  continue;
+                }
+              }
+            }
+          }
+        }
+      }
+      break;
+
+    case SCORE_IFP_DENOM:
+      if (settings::ifp_on) {
+        if (p.type().is_neutron() && p.fission()) {
+          int ifp_data_size;
+          if (is_beta_effective_or_both()) {
+            ifp_data_size = static_cast<int>(
+              simulation::ifp_source_delayed_group_bank[p.current_work()]
+                .size());
+          } else {
+            ifp_data_size = static_cast<int>(
+              simulation::ifp_source_lifetime_bank[p.current_work()].size());
+          }
+          if (ifp_data_size == settings::ifp_n_generation) {
+            score = p.wgt_last();
+          }
+        }
+      }
+      break;
+
     case N_2N:
     case N_3N:
     case N_4N:
@@ -902,7 +1010,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
       if (!simulation::need_depletion_rx)
         goto default_case;
 
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       int m;
@@ -924,7 +1032,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
           const Material& material {*model::materials[p.material()]};
           for (auto i = 0; i < material.nuclide_.size(); ++i) {
             auto j_nuclide = material.nuclide_[i];
-            auto atom_density = material.atom_density_(i);
+            auto atom_density = material.atom_density(i, p.density_mult());
             score += p.neutron_xs(j_nuclide).reaction[m] * atom_density * flux;
           }
         }
@@ -935,7 +1043,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
     case INCOHERENT:
     case PHOTOELECTRIC:
     case PAIR_PROD:
-      if (p.type() != Type::photon)
+      if (!p.type().is_photon())
         continue;
 
       if (i_nuclide >= 0) {
@@ -956,23 +1064,8 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
       break;
 
     case HEATING:
-      if (p.type() == Type::neutron) {
-        score = score_neutron_heating(
-          p, tally, flux, HEATING, i_nuclide, atom_density);
-      } else {
-        if (i_nuclide == -1 || i_nuclide == p.event_nuclide()) {
-          // The energy deposited is the difference between the pre-collision
-          // and post-collision energy...
-          score = E - p.E();
-          // ...less the energy of any secondary particles since they will be
-          // transported individually later
-          score -= p.bank_second_E();
-
-          score *= p.wgt_last();
-        } else {
-          score = 0.0;
-        }
-      }
+      score = score_particle_heating(
+        p, tally, flux, HEATING, i_nuclide, atom_density);
       break;
 
     default:
@@ -980,7 +1073,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
 
       // The default block is really only meant for redundant neutron reactions
       // (e.g. 444, 901)
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // Any other cross section has to be calculated on-the-fly
@@ -993,7 +1086,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
         const Material& material {*model::materials[p.material()]};
         for (auto i = 0; i < material.nuclide_.size(); ++i) {
           auto j_nuclide = material.nuclide_[i];
-          auto atom_density = material.atom_density_(i);
+          auto atom_density = material.atom_density(i, p.density_mult());
           score +=
             get_nuclide_xs(p, j_nuclide, score_bin) * atom_density * flux;
         }
@@ -1019,8 +1112,8 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
 //! is not used for analog tallies.
 
 void score_general_ce_analog(Particle& p, int i_tally, int start_index,
-  int filter_index, double filter_weight, int i_nuclide, double atom_density,
-  double flux)
+  int64_t filter_index, double filter_weight, int i_nuclide,
+  double atom_density, double flux)
 {
   Tally& tally {*model::tallies[i_tally]};
 
@@ -1034,8 +1127,6 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
                             p.neutron_xs(p.event_nuclide()).total
                         : 0.0;
 
-  using Type = ParticleType;
-
   for (auto i = 0; i < tally.scores_.size(); ++i) {
     auto score_bin = tally.scores_[i];
     auto score_index = start_index + i;
@@ -1046,7 +1137,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       // All events score to a flux bin. We actually use a collision estimator
       // in place of an analog one since there is no way to count 'events'
       // exactly for the flux
-      if (p.type() == Type::neutron || p.type() == Type::photon) {
+      if (p.type().is_neutron() || p.type().is_photon()) {
         score = flux * p.wgt_last() / p.macro_xs().total;
       } else {
         score = 0.;
@@ -1060,7 +1151,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_INVERSE_VELOCITY:
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // All events score to an inverse velocity bin. We actually use a
@@ -1070,7 +1161,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_SCATTER:
-      if (p.type() != Type::neutron && p.type() != Type::photon)
+      if (!p.type().is_neutron() && !p.type().is_photon())
         continue;
 
       // Skip any event where the particle didn't scatter
@@ -1082,7 +1173,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_NU_SCATTER:
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // Only analog estimators are available.
@@ -1107,7 +1198,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_ABSORPTION:
-      if (p.type() != Type::neutron && p.type() != Type::photon)
+      if (!p.type().is_neutron() && !p.type().is_photon())
         continue;
 
       if (settings::survival_biasing) {
@@ -1336,7 +1427,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
             // this loop.
             for (auto d = 1; d < rxn.products_.size(); ++d) {
               const auto& product = rxn.products_[d];
-              if (product.particle_ != Type::neutron)
+              if (!product.particle_.is_neutron())
                 continue;
 
               auto yield = nuc.nu(E, ReactionProduct::EmissionMode::delayed, d);
@@ -1422,13 +1513,12 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_EVENTS:
-// Simply count the number of scoring events
-#pragma omp atomic
-      tally.results_(filter_index, score_index, TallyResult::VALUE) += 1.0;
-      continue;
+      // Simply count the number of scoring events
+      score = 1.0;
+      break;
 
     case ELASTIC:
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // Check if event MT matches
@@ -1457,7 +1547,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       if (!simulation::need_depletion_rx)
         goto default_case;
 
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // Check if the event MT matches
@@ -1470,7 +1560,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
     case INCOHERENT:
     case PHOTOELECTRIC:
     case PAIR_PROD:
-      if (p.type() != Type::photon)
+      if (!p.type().is_photon())
         continue;
 
       if (score_bin == PHOTOELECTRIC) {
@@ -1488,19 +1578,8 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
       break;
 
     case HEATING:
-      if (p.type() == Type::neutron) {
-        score = score_neutron_heating(
-          p, tally, flux, HEATING, i_nuclide, atom_density);
-      } else {
-        // The energy deposited is the difference between the pre-collision and
-        // post-collision energy...
-        score = E - p.E();
-        // ...less the energy of any secondary particles since they will be
-        // transported individually later
-        score -= p.bank_second_E();
-
-        score *= p.wgt_last();
-      }
+      score = score_particle_heating(
+        p, tally, flux, HEATING, i_nuclide, atom_density);
       break;
 
     default:
@@ -1508,7 +1587,7 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
 
       // The default block is really only meant for redundant neutron reactions
       // (e.g. 444, 901)
-      if (p.type() != Type::neutron)
+      if (!p.type().is_neutron())
         continue;
 
       // Any other score is assumed to be a MT number. Thus, we just need
@@ -1536,8 +1615,8 @@ void score_general_ce_analog(Particle& p, int i_tally, int start_index,
 //! argument is really just used for filter weights.
 
 void score_general_mg(Particle& p, int i_tally, int start_index,
-  int filter_index, double filter_weight, int i_nuclide, double atom_density,
-  double flux)
+  int64_t filter_index, double filter_weight, int i_nuclide,
+  double atom_density, double flux)
 {
   auto& tally {*model::tallies[i_tally]};
 
@@ -1549,8 +1628,7 @@ void score_general_mg(Particle& p, int i_tally, int start_index,
       tally.estimator_ == TallyEstimator::COLLISION) {
     if (settings::survival_biasing) {
       // Determine weight that was absorbed
-      wgt_absorb = p.wgt_last() * p.neutron_xs(p.event_nuclide()).absorption /
-                   p.neutron_xs(p.event_nuclide()).total;
+      wgt_absorb = p.wgt_last() * p.macro_xs().absorption / p.macro_xs().total;
 
       // Then we either are alive and had a scatter (and so g changed),
       // or are dead and g did not change
@@ -2212,10 +2290,9 @@ void score_general_mg(Particle& p, int i_tally, int start_index,
       break;
 
     case SCORE_EVENTS:
-// Simply count the number of scoring events
-#pragma omp atomic
-      tally.results_(filter_index, score_index, TallyResult::VALUE) += 1.0;
-      continue;
+      // Simply count the number of scoring events
+      score = 1.0;
+      break;
 
     default:
       continue;
@@ -2233,10 +2310,7 @@ void score_analog_tally_ce(Particle& p)
   // Since electrons/positrons are not transported, we assign a flux of zero.
   // Note that the heating score does NOT use the flux and will be non-zero for
   // electrons/positrons.
-  double flux =
-    (p.type() == ParticleType::neutron || p.type() == ParticleType::photon)
-      ? 1.0
-      : 0.0;
+  double flux = (p.type().is_neutron() || p.type().is_photon()) ? 1.0 : 0.0;
 
   for (auto i_tally : model::active_analog_tallies) {
     const Tally& tally {*model::tallies[i_tally]};
@@ -2308,7 +2382,8 @@ void score_analog_tally_mg(Particle& p)
             model::materials[p.material()]->mat_nuclide_index_[i_nuclide];
           if (j == C_NONE)
             continue;
-          atom_density = model::materials[p.material()]->atom_density_(j);
+          atom_density =
+            model::materials[p.material()]->atom_density(j, p.density_mult());
         }
 
         score_general_mg(p, i_tally, i * tally.scores_.size(), filter_index,
@@ -2329,15 +2404,13 @@ void score_analog_tally_mg(Particle& p)
     match.bins_present_ = false;
 }
 
-void score_tracklength_tally(Particle& p, double distance)
+void score_tracklength_tally_general(
+  Particle& p, double flux, const vector<int>& tallies)
 {
-  // Determine the tracklength estimate of the flux
-  double flux = p.wgt() * distance;
-
   // Set 'none' value for log union grid index
   int i_log_union = C_NONE;
 
-  for (auto i_tally : model::active_tracklength_tallies) {
+  for (auto i_tally : tallies) {
     const Tally& tally {*model::tallies[i_tally]};
 
     // Initialize an iterator over valid filter bin combinations.  If there are
@@ -2362,23 +2435,22 @@ void score_tracklength_tally(Particle& p, double distance)
           if (p.material() != MATERIAL_VOID) {
             const auto& mat = model::materials[p.material()];
             auto j = mat->mat_nuclide_index_[i_nuclide];
-            if (j == C_NONE) {
-              // Determine log union grid index
-              if (i_log_union == C_NONE) {
-                int neutron = static_cast<int>(ParticleType::neutron);
-                i_log_union = std::log(p.E() / data::energy_min[neutron]) /
-                              simulation::log_spacing;
-              }
-
-              // Update micro xs cache
-              if (!tally.multiply_density()) {
-                p.update_neutron_xs(i_nuclide, i_log_union);
-                atom_density = 1.0;
-              }
-            } else {
-              atom_density =
-                tally.multiply_density() ? mat->atom_density_(j) : 1.0;
+            if (j != C_NONE)
+              atom_density = mat->atom_density(j, p.density_mult());
+          }
+          if (atom_density > 0) {
+            if (!tally.multiply_density())
+              atom_density = 1.0;
+          } else if (!tally.multiply_density()) {
+            // Determine log union grid index
+            if (i_log_union == C_NONE) {
+              int neutron = ParticleType::neutron().transport_index();
+              i_log_union = std::log(p.E() / data::energy_min[neutron]) /
+                            simulation::log_spacing;
             }
+            // Update micro xs cache
+            p.update_neutron_xs(i_nuclide, i_log_union);
+            atom_density = 1.0;
           }
         }
 
@@ -2406,11 +2478,62 @@ void score_tracklength_tally(Particle& p, double distance)
     match.bins_present_ = false;
 }
 
+void score_timed_tracklength_tally(Particle& p, double total_distance)
+{
+  double speed = p.speed();
+  double total_dt = total_distance / speed;
+
+  // save particle last state
+  auto time_last = p.time_last();
+  auto r_last = p.r_last();
+
+  // move particle back
+  p.move_distance(-total_distance);
+  p.time() -= total_dt;
+  p.lifetime() -= total_dt;
+
+  double distance_traveled = 0.0;
+  while (distance_traveled < total_distance) {
+
+    double distance = std::min(distance_to_time_boundary(p.time(), speed),
+      total_distance - distance_traveled);
+    double dt = distance / speed;
+
+    // Save particle last state for tracklength tallies
+    p.time_last() = p.time();
+    p.r_last() = p.r();
+
+    // Advance particle in space and time
+    p.move_distance(distance);
+    p.time() += dt;
+    p.lifetime() += dt;
+
+    // Determine the tracklength estimate of the flux
+    double flux = p.wgt() * distance;
+
+    score_tracklength_tally_general(
+      p, flux, model::active_timed_tracklength_tallies);
+    distance_traveled += distance;
+  }
+
+  p.time_last() = time_last;
+  p.r_last() = r_last;
+}
+
+void score_tracklength_tally(Particle& p, double distance)
+{
+
+  // Determine the tracklength estimate of the flux
+  double flux = p.wgt() * distance;
+
+  score_tracklength_tally_general(p, flux, model::active_tracklength_tallies);
+}
+
 void score_collision_tally(Particle& p)
 {
   // Determine the collision estimate of the flux
   double flux = 0.0;
-  if (p.type() == ParticleType::neutron || p.type() == ParticleType::photon) {
+  if (p.type().is_neutron() || p.type().is_photon()) {
     flux = p.wgt_last() / p.macro_xs().total;
   }
 
@@ -2439,24 +2562,25 @@ void score_collision_tally(Particle& p)
 
         double atom_density = 0.;
         if (i_nuclide >= 0) {
-          const auto& mat = model::materials[p.material()];
-          auto j = mat->mat_nuclide_index_[i_nuclide];
-          if (j == C_NONE) {
+          if (p.material() != MATERIAL_VOID) {
+            const auto& mat = model::materials[p.material()];
+            auto j = mat->mat_nuclide_index_[i_nuclide];
+            if (j != C_NONE)
+              atom_density = mat->atom_density(j, p.density_mult());
+          }
+          if (atom_density > 0) {
+            if (!tally.multiply_density())
+              atom_density = 1.0;
+          } else if (!tally.multiply_density()) {
             // Determine log union grid index
             if (i_log_union == C_NONE) {
-              int neutron = static_cast<int>(ParticleType::neutron);
+              int neutron = ParticleType::neutron().transport_index();
               i_log_union = std::log(p.E() / data::energy_min[neutron]) /
                             simulation::log_spacing;
             }
-
             // Update micro xs cache
-            if (!tally.multiply_density()) {
-              p.update_neutron_xs(i_nuclide, i_log_union);
-              atom_density = 1.0;
-            }
-          } else {
-            atom_density =
-              tally.multiply_density() ? mat->atom_density_(j) : 1.0;
+            p.update_neutron_xs(i_nuclide, i_log_union);
+            atom_density = 1.0;
           }
         }
 
@@ -2484,7 +2608,7 @@ void score_collision_tally(Particle& p)
     match.bins_present_ = false;
 }
 
-void score_surface_tally(Particle& p, const vector<int>& tallies)
+void score_meshsurface_tally(Particle& p, const vector<int>& tallies)
 {
   double current = p.wgt_last();
 
@@ -2528,6 +2652,69 @@ void score_surface_tally(Particle& p, const vector<int>& tallies)
     match.bins_present_ = false;
 }
 
+void score_surface_tally(
+  Particle& p, const vector<int>& tallies, const Direction& normal)
+{
+  double wgt = p.wgt_last();
+
+  double mu = std::clamp(p.u().dot(normal), -1.0, 1.0);
+
+  // Sign for net current: +1 if crossing outward (in direction of normal),
+  // -1 if crossing inward
+  double current_sign = std::copysign(1.0, mu);
+
+  // Determine absolute cosine of angle between particle direction and surface
+  // normal, needed for the surface-crossing flux estimator.
+  double abs_mu = std::abs(mu);
+  if (abs_mu < settings::surface_grazing_cutoff)
+    abs_mu = settings::surface_grazing_ratio * settings::surface_grazing_cutoff;
+
+  for (auto i_tally : tallies) {
+    auto& tally {*model::tallies[i_tally]};
+
+    // Initialize an iterator over valid filter bin combinations. If there are
+    // no valid combinations, use a continue statement to ensure we skip the
+    // assume_separate break below.
+    auto filter_iter = FilterBinIter(tally, p);
+    auto end = FilterBinIter(tally, true, &p.filter_matches());
+    if (filter_iter == end)
+      continue;
+
+    // Loop over filter bins.
+    for (; filter_iter != end; ++filter_iter) {
+      auto filter_index = filter_iter.index_;
+      auto filter_weight = filter_iter.weight_;
+
+      // Loop over scores.
+      for (auto score_index = 0; score_index < tally.scores_.size();
+           ++score_index) {
+        auto score_bin = tally.scores_[score_index];
+        double score;
+        if (score_bin == SCORE_CURRENT) {
+          // Net current: weight carries the sign of the crossing direction.
+          score = wgt * current_sign;
+        } else {
+          // SCORE_FLUX: surface-crossing estimator phi_S = sum(w / |mu|).
+          score = wgt / abs_mu;
+        }
+#pragma omp atomic
+        tally.results_(filter_index, score_index, TallyResult::VALUE) +=
+          score * filter_weight;
+      }
+    }
+    // If the user has specified that we can assume all tallies are spatially
+    // separate, this implies that once a tally has been scored to, we needn't
+    // check the others. This cuts down on overhead when there are many
+    // tallies specified
+    if (settings::assume_separate)
+      break;
+  }
+
+  // Reset all the filter matches for the next tally event.
+  for (auto& match : p.filter_matches())
+    match.bins_present_ = false;
+}
+
 void score_pulse_height_tally(Particle& p, const vector<int>& tallies)
 {
   // The pulse height tally in OpenMC hijacks the logic of CellFilter and
@@ -2540,67 +2727,62 @@ void score_pulse_height_tally(Particle& p, const vector<int>& tallies)
 
   // Save original cell/energy information
   int orig_n_coord = p.n_coord();
-  int orig_cell = p.coord(0).cell;
+  int orig_cell = p.coord(0).cell();
   double orig_E_last = p.E_last();
 
+  // Set particle in top level
+  p.n_coord() = 1;
+
   for (auto i_tally : tallies) {
     auto& tally {*model::tallies[i_tally]};
 
-    // Determine all CellFilter in the tally
-    for (const auto& filter : tally.filters()) {
-      auto cell_filter =
-        dynamic_cast<CellFilter*>(model::tally_filters[filter].get());
-      if (cell_filter != nullptr) {
+    // Find CellFilter in the tally (if any) to determine cells to loop over
+    const auto* cell_filter = tally.get_filter<CellFilter>();
+    const auto& cells =
+      cell_filter ? cell_filter->cells() : model::pulse_height_cells;
 
-        const auto& cells = cell_filter->cells();
-        // Loop over all cells in the CellFilter
-        for (auto cell_index = 0; cell_index < cells.size(); ++cell_index) {
-          int cell_id = cells[cell_index];
+    for (auto cell_id : cells) {
+      // Temporarily change cell of particle
+      p.coord(0).cell() = cell_id;
 
-          // Temporarily change cell of particle
-          p.n_coord() = 1;
-          p.coord(0).cell = cell_id;
+      // Determine index of cell in model::pulse_height_cells
+      auto it = std::find(model::pulse_height_cells.begin(),
+        model::pulse_height_cells.end(), cell_id);
+      int index = std::distance(model::pulse_height_cells.begin(), it);
 
-          // Determine index of cell in model::pulse_height_cells
-          auto it = std::find(model::pulse_height_cells.begin(),
-            model::pulse_height_cells.end(), cell_id);
-          int index = std::distance(model::pulse_height_cells.begin(), it);
+      // Temporarily change energy of particle to pulse-height value
+      p.E_last() = p.pht_storage()[index];
 
-          // Temporarily change energy of particle to pulse-height value
-          p.E_last() = p.pht_storage()[index];
+      // Initialize an iterator over valid filter bin combinations. If
+      // there are no valid combinations, use a continue statement to ensure
+      // we skip the assume_separate break below.
+      auto filter_iter = FilterBinIter(tally, p);
+      auto end = FilterBinIter(tally, true, &p.filter_matches());
+      if (filter_iter != end) {
 
-          // Initialize an iterator over valid filter bin combinations. If
-          // there are no valid combinations, use a continue statement to ensure
-          // we skip the assume_separate break below.
-          auto filter_iter = FilterBinIter(tally, p);
-          auto end = FilterBinIter(tally, true, &p.filter_matches());
-          if (filter_iter == end)
-            continue;
+        // Loop over filter bins.
+        for (; filter_iter != end; ++filter_iter) {
+          auto filter_index = filter_iter.index_;
+          auto filter_weight = filter_iter.weight_;
 
-          // Loop over filter bins.
-          for (; filter_iter != end; ++filter_iter) {
-            auto filter_index = filter_iter.index_;
-            auto filter_weight = filter_iter.weight_;
-
-            // Loop over scores.
-            for (auto score_index = 0; score_index < tally.scores_.size();
-                 ++score_index) {
+          // Loop over scores.
+          for (auto score_index = 0; score_index < tally.scores_.size();
+               ++score_index) {
 #pragma omp atomic
-              tally.results_(filter_index, score_index, TallyResult::VALUE) +=
-                filter_weight;
-            }
+            tally.results_(filter_index, score_index, TallyResult::VALUE) +=
+              filter_weight;
           }
-
-          // Reset all the filter matches for the next tally event.
-          for (auto& match : p.filter_matches())
-            match.bins_present_ = false;
         }
       }
+
+      // Reset all the filter matches for the next tally event.
+      for (auto& match : p.filter_matches())
+        match.bins_present_ = false;
     }
-    // Restore cell/energy
-    p.n_coord() = orig_n_coord;
-    p.coord(0).cell = orig_cell;
-    p.E_last() = orig_E_last;
   }
+  // Restore cell/energy
+  p.n_coord() = orig_n_coord;
+  p.coord(0).cell() = orig_cell;
+  p.E_last() = orig_E_last;
 }
 } // namespace openmc
diff --git a/src/tallies/trigger.cpp b/src/tallies/trigger.cpp
index f1f83e298..7515f01a9 100644
--- a/src/tallies/trigger.cpp
+++ b/src/tallies/trigger.cpp
@@ -28,7 +28,7 @@ KTrigger keff_trigger;
 //==============================================================================
 
 std::pair<double, double> get_tally_uncertainty(
-  int i_tally, int score_index, int filter_index)
+  int i_tally, int score_index, int64_t filter_index)
 {
   const auto& tally {model::tallies[i_tally]};
 
@@ -71,7 +71,7 @@ void check_tally_triggers(double& ratio, int& tally_id, int& score)
         continue;
 
       const auto& results = t.results_;
-      for (auto filter_index = 0; filter_index < results.shape()[0];
+      for (int64_t filter_index = 0; filter_index < results.shape(0);
            ++filter_index) {
         // Compute the tally uncertainty metrics.
         auto uncert_pair =
diff --git a/src/thermal.cpp b/src/thermal.cpp
index cbe0983ed..edfbddf23 100644
--- a/src/thermal.cpp
+++ b/src/thermal.cpp
@@ -3,12 +3,7 @@
 #include <algorithm> // for sort, move, min, max, find
 #include <cmath>     // for round, sqrt, abs
 
-#include "xtensor/xarray.hpp"
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xmath.hpp"
-#include "xtensor/xsort.hpp"
-#include "xtensor/xtensor.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include "openmc/constants.h"
@@ -55,7 +50,7 @@ ThermalScattering::ThermalScattering(
   // Determine temperatures available
   auto dset_names = dataset_names(kT_group);
   auto n = dset_names.size();
-  auto temps_available = xt::empty<double>({n});
+  auto temps_available = tensor::Tensor<double>({n});
   for (int i = 0; i < dset_names.size(); ++i) {
     // Read temperature value
     double T;
@@ -82,7 +77,7 @@ ThermalScattering::ThermalScattering(
     // Determine actual temperatures to read
     for (const auto& T : temperature) {
 
-      auto i_closest = xt::argmin(xt::abs(temps_available - T))[0];
+      auto i_closest = tensor::abs(temps_available - T).argmin();
       auto temp_actual = temps_available[i_closest];
       if (std::abs(temp_actual - T) < settings::temperature_tolerance) {
         if (std::find(temps_to_read.begin(), temps_to_read.end(),
@@ -296,16 +291,21 @@ void ThermalData::calculate_xs(
   *inelastic = (*inelastic_.xs)(E);
 }
 
-void ThermalData::sample(const NuclideMicroXS& micro_xs, double E,
-  double* E_out, double* mu, uint64_t* seed)
+AngleEnergy& ThermalData::sample_dist(
+  const NuclideMicroXS& micro_xs, double E, uint64_t* seed) const
 {
   // Determine whether inelastic or elastic scattering will occur
   if (prn(seed) < micro_xs.thermal_elastic / micro_xs.thermal) {
-    elastic_.distribution->sample(E, *E_out, *mu, seed);
+    return *elastic_.distribution;
   } else {
-    inelastic_.distribution->sample(E, *E_out, *mu, seed);
+    return *inelastic_.distribution;
   }
+}
 
+void ThermalData::sample(const NuclideMicroXS& micro_xs, double E,
+  double* E_out, double* mu, uint64_t* seed) const
+{
+  sample_dist(micro_xs, E, seed).sample(E, *E_out, *mu, seed);
   // Because of floating-point roundoff, it may be possible for mu to be
   // outside of the range [-1,1). In these cases, we just set mu to exactly
   // -1 or 1
@@ -313,6 +313,13 @@ void ThermalData::sample(const NuclideMicroXS& micro_xs, double E,
     *mu = std::copysign(1.0, *mu);
 }
 
+double ThermalData::sample_energy_and_pdf(const NuclideMicroXS& micro_xs,
+  double E_in, double mu, double& E_out, uint64_t* seed) const
+{
+  return sample_dist(micro_xs, E_in, seed)
+    .sample_energy_and_pdf(E_in, mu, E_out, seed);
+}
+
 void free_memory_thermal()
 {
   data::thermal_scatt.clear();
diff --git a/src/track_output.cpp b/src/track_output.cpp
index f4344d50f..02e092236 100644
--- a/src/track_output.cpp
+++ b/src/track_output.cpp
@@ -8,7 +8,7 @@
 #include "openmc/simulation.h"
 #include "openmc/vector.h"
 
-#include "xtensor/xtensor.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 #include <hdf5.h>
 
@@ -122,7 +122,7 @@ void finalize_particle_track(Particle& p)
   int offset = 0;
   for (auto& track_i : p.tracks()) {
     offsets.push_back(offset);
-    particles.push_back(static_cast<int>(track_i.particle));
+    particles.push_back(track_i.particle.pdg_number());
     offset += track_i.states.size();
     tracks.insert(tracks.end(), track_i.states.begin(), track_i.states.end());
   }
diff --git a/src/universe.cpp b/src/universe.cpp
index b4ef6b4b2..e1db80b99 100644
--- a/src/universe.cpp
+++ b/src/universe.cpp
@@ -45,14 +45,14 @@ bool Universe::find_cell(GeometryState& p) const
   Position r {p.r_local()};
   Position u {p.u_local()};
   auto surf = p.surface();
-  int32_t i_univ = p.lowest_coord().universe;
+  int32_t i_univ = p.lowest_coord().universe();
 
   for (auto i_cell : cells) {
     if (model::cells[i_cell]->universe_ != i_univ)
       continue;
     // Check if this cell contains the particle
     if (model::cells[i_cell]->contains(r, u, surf)) {
-      p.lowest_coord().cell = i_cell;
+      p.lowest_coord().cell() = i_cell;
       return true;
     }
   }
@@ -61,7 +61,7 @@ bool Universe::find_cell(GeometryState& p) const
 
 BoundingBox Universe::bounding_box() const
 {
-  BoundingBox bbox = {INFTY, -INFTY, INFTY, -INFTY, INFTY, -INFTY};
+  BoundingBox bbox = BoundingBox::inverted();
   if (cells_.size() == 0) {
     return {};
   } else {
diff --git a/src/urr.cpp b/src/urr.cpp
index 02cef2280..b791eecb0 100644
--- a/src/urr.cpp
+++ b/src/urr.cpp
@@ -25,7 +25,7 @@ UrrData::UrrData(hid_t group_id)
   // Read URR tables. The HDF5 format is a little
   // different from how we want it laid out in memory.
   // This array used to be called "prob_".
-  xt::xtensor<double, 3> tmp_prob;
+  tensor::Tensor<double> tmp_prob;
   read_dataset(group_id, "table", tmp_prob);
   auto shape = tmp_prob.shape();
 
@@ -38,7 +38,7 @@ UrrData::UrrData(hid_t group_id)
   xs_values_.resize({n_energy, n_cdf_values});
 
   // Now fill in the values. Using manual loops here since we might
-  // not have fancy xtensor slicing code written for GPU tensors.
+  // not have fancy tensor slicing code written for GPU tensors.
   // The below enum gives how URR tables are laid out in our HDF5 tables.
   enum class URRTableParam {
     CUM_PROB,
diff --git a/src/volume_calc.cpp b/src/volume_calc.cpp
index 8b5c27f14..f675ae78a 100644
--- a/src/volume_calc.cpp
+++ b/src/volume_calc.cpp
@@ -17,8 +17,7 @@
 #include "openmc/timer.h"
 #include "openmc/xml_interface.h"
 
-#include "xtensor/xadapt.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 #include <fmt/core.h>
 
 #include <algorithm> // for copy
@@ -179,7 +178,7 @@ vector<VolumeCalculation::Result> VolumeCalculation::execute() const
         } else if (domain_type_ == TallyDomain::CELL) {
           for (int level = 0; level < p.n_coord(); ++level) {
             for (int i_domain = 0; i_domain < n; i_domain++) {
-              if (model::cells[p.coord(level).cell]->id_ ==
+              if (model::cells[p.coord(level).cell()]->id_ ==
                   domain_ids_[i_domain]) {
                 this->check_hit(
                   p.material(), indices[i_domain], hits[i_domain]);
@@ -190,7 +189,7 @@ vector<VolumeCalculation::Result> VolumeCalculation::execute() const
         } else if (domain_type_ == TallyDomain::UNIVERSE) {
           for (int level = 0; level < p.n_coord(); ++level) {
             for (int i_domain = 0; i_domain < n; ++i_domain) {
-              if (model::universes[p.coord(level).universe]->id_ ==
+              if (model::universes[p.coord(level).universe()]->id_ ==
                   domain_ids_[i_domain]) {
                 check_hit(p.material(), indices[i_domain], hits[i_domain]);
                 break;
@@ -242,7 +241,8 @@ vector<VolumeCalculation::Result> VolumeCalculation::execute() const
       // non-zero
       auto n_nuc =
         settings::run_CE ? data::nuclides.size() : data::mg.nuclides_.size();
-      xt::xtensor<double, 2> atoms({n_nuc, 2}, 0.0);
+      auto atoms =
+        tensor::zeros<double>({static_cast<size_t>(n_nuc), size_t {2}});
 
 #ifdef OPENMC_MPI
       if (mpi::master) {
@@ -452,9 +452,11 @@ void VolumeCalculation::to_hdf5(
     }
 
     // Create array of total # of atoms with uncertainty for each nuclide
-    xt::xtensor<double, 2> atom_data({n_nuc, 2});
-    xt::view(atom_data, xt::all(), 0) = xt::adapt(result.atoms);
-    xt::view(atom_data, xt::all(), 1) = xt::adapt(result.uncertainty);
+    tensor::Tensor<double> atom_data({static_cast<size_t>(n_nuc), size_t {2}});
+    for (size_t k = 0; k < static_cast<size_t>(n_nuc); ++k) {
+      atom_data(k, 0) = result.atoms[k];
+      atom_data(k, 1) = result.uncertainty[k];
+    }
 
     // Write results
     write_dataset(group_id, "nuclides", nucnames);
diff --git a/src/weight_windows.cpp b/src/weight_windows.cpp
index 0efa5a6ae..0d268ce17 100644
--- a/src/weight_windows.cpp
+++ b/src/weight_windows.cpp
@@ -6,12 +6,7 @@
 #include <set>
 #include <string>
 
-#include "xtensor/xindex_view.hpp"
-#include "xtensor/xio.hpp"
-#include "xtensor/xmasked_view.hpp"
-#include "xtensor/xnoalias.hpp"
-#include "xtensor/xstrided_view.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/error.h"
 #include "openmc/file_utils.h"
@@ -26,6 +21,7 @@
 #include "openmc/random_ray/flat_source_domain.h"
 #include "openmc/search.h"
 #include "openmc/settings.h"
+#include "openmc/simulation.h"
 #include "openmc/tallies/filter_energy.h"
 #include "openmc/tallies/filter_mesh.h"
 #include "openmc/tallies/filter_particle.h"
@@ -48,92 +44,6 @@ openmc::vector<unique_ptr<WeightWindowsGenerator>> weight_windows_generators;
 
 } // namespace variance_reduction
 
-//==============================================================================
-// Non-member functions
-//==============================================================================
-
-void apply_weight_windows(Particle& p)
-{
-  if (!settings::weight_windows_on)
-    return;
-
-  // WW on photon and neutron only
-  if (p.type() != ParticleType::neutron && p.type() != ParticleType::photon)
-    return;
-
-  // skip dead or no energy
-  if (p.E() <= 0 || !p.alive())
-    return;
-
-  bool in_domain = false;
-  // TODO: this is a linear search - should do something more clever
-  WeightWindow weight_window;
-  for (const auto& ww : variance_reduction::weight_windows) {
-    weight_window = ww->get_weight_window(p);
-    if (weight_window.is_valid())
-      break;
-  }
-  // particle is not in any of the ww domains, do nothing
-  if (!weight_window.is_valid())
-    return;
-
-  // get the paramters
-  double weight = p.wgt();
-
-  // first check to see if particle should be killed for weight cutoff
-  if (p.wgt() < weight_window.weight_cutoff) {
-    p.wgt() = 0.0;
-    return;
-  }
-
-  // check if particle is far above current weight window
-  // only do this if the factor is not already set on the particle and a
-  // maximum lower bound ratio is specified
-  if (p.ww_factor() == 0.0 && weight_window.max_lb_ratio > 1.0 &&
-      p.wgt() > weight_window.lower_weight * weight_window.max_lb_ratio) {
-    p.ww_factor() =
-      p.wgt() / (weight_window.lower_weight * weight_window.max_lb_ratio);
-  }
-
-  // move weight window closer to the particle weight if needed
-  if (p.ww_factor() > 1.0)
-    weight_window.scale(p.ww_factor());
-
-  // if particle's weight is above the weight window split until they are within
-  // the window
-  if (weight > weight_window.upper_weight) {
-    // do not further split the particle if above the limit
-    if (p.n_split() >= settings::max_history_splits)
-      return;
-
-    double n_split = std::ceil(weight / weight_window.upper_weight);
-    double max_split = weight_window.max_split;
-    n_split = std::min(n_split, max_split);
-
-    p.n_split() += n_split;
-
-    // Create secondaries and divide weight among all particles
-    int i_split = std::round(n_split);
-    for (int l = 0; l < i_split - 1; l++) {
-      p.split(weight / n_split);
-    }
-    // remaining weight is applied to current particle
-    p.wgt() = weight / n_split;
-
-  } else if (weight <= weight_window.lower_weight) {
-    // if the particle weight is below the window, play Russian roulette
-    double weight_survive =
-      std::min(weight * weight_window.max_split, weight_window.survival_weight);
-    russian_roulette(p, weight_survive);
-  } // else particle is in the window, continue as normal
-}
-
-void free_memory_weight_windows()
-{
-  variance_reduction::ww_map.clear();
-  variance_reduction::weight_windows.clear();
-}
-
 //==============================================================================
 // WeightWindowSettings implementation
 //==============================================================================
@@ -162,7 +72,7 @@ WeightWindows::WeightWindows(pugi::xml_node node)
 
   // get the particle type
   auto particle_type_str = std::string(get_node_value(node, "particle_type"));
-  particle_type_ = openmc::str_to_particle_type(particle_type_str);
+  particle_type_ = ParticleType {particle_type_str};
 
   // Determine associated mesh
   int32_t mesh_id = std::stoi(get_node_value(node, "mesh"));
@@ -235,7 +145,7 @@ WeightWindows* WeightWindows::from_hdf5(
 
   std::string particle_type;
   read_dataset(ww_group, "particle_type", particle_type);
-  wws->particle_type_ = openmc::str_to_particle_type(particle_type);
+  wws->particle_type_ = ParticleType {particle_type};
 
   read_dataset<double>(ww_group, "energy_bounds", wws->energy_bounds_);
 
@@ -248,8 +158,12 @@ WeightWindows* WeightWindows::from_hdf5(
   }
   wws->set_mesh(model::mesh_map[mesh_id]);
 
-  wws->lower_ww_ = xt::empty<double>(wws->bounds_size());
-  wws->upper_ww_ = xt::empty<double>(wws->bounds_size());
+  wws->lower_ww_ =
+    tensor::Tensor<double>({static_cast<size_t>(wws->bounds_size()[0]),
+      static_cast<size_t>(wws->bounds_size()[1])});
+  wws->upper_ww_ =
+    tensor::Tensor<double>({static_cast<size_t>(wws->bounds_size()[0]),
+      static_cast<size_t>(wws->bounds_size()[1])});
 
   read_dataset<double>(ww_group, "lower_ww_bounds", wws->lower_ww_);
   read_dataset<double>(ww_group, "upper_ww_bounds", wws->upper_ww_);
@@ -267,7 +181,10 @@ void WeightWindows::set_defaults()
 {
   // set energy bounds to the min/max energy supported by the data
   if (energy_bounds_.size() == 0) {
-    int p_type = static_cast<int>(particle_type_);
+    int p_type = particle_type_.transport_index();
+    if (p_type == C_NONE) {
+      fatal_error("Weight windows particle is not supported for transport.");
+    }
     energy_bounds_.push_back(data::energy_min[p_type]);
     energy_bounds_.push_back(data::energy_max[p_type]);
   }
@@ -281,9 +198,11 @@ void WeightWindows::allocate_ww_bounds()
       "Size of weight window bounds is zero for WeightWindows {}", id());
     warning(msg);
   }
-  lower_ww_ = xt::empty<double>(shape);
+  lower_ww_ = tensor::Tensor<double>(
+    {static_cast<size_t>(shape[0]), static_cast<size_t>(shape[1])});
   lower_ww_.fill(-1);
-  upper_ww_ = xt::empty<double>(shape);
+  upper_ww_ = tensor::Tensor<double>(
+    {static_cast<size_t>(shape[0]), static_cast<size_t>(shape[1])});
   upper_ww_.fill(-1);
 }
 
@@ -328,10 +247,9 @@ void WeightWindows::set_energy_bounds(span<const double> bounds)
 
 void WeightWindows::set_particle_type(ParticleType p_type)
 {
-  if (p_type != ParticleType::neutron && p_type != ParticleType::photon)
-    fatal_error(
-      fmt::format("Particle type '{}' cannot be applied to weight windows.",
-        particle_type_to_str(p_type)));
+  if (!p_type.is_neutron() && !p_type.is_photon())
+    fatal_error(fmt::format(
+      "Particle type '{}' cannot be applied to weight windows.", p_type.str()));
   particle_type_ = p_type;
 }
 
@@ -355,27 +273,28 @@ void WeightWindows::set_mesh(const Mesh* mesh)
   set_mesh(model::mesh_map[mesh->id_]);
 }
 
-WeightWindow WeightWindows::get_weight_window(const Particle& p) const
+std::pair<bool, WeightWindow> WeightWindows::get_weight_window(
+  const Particle& p) const
 {
   // check for particle type
   if (particle_type_ != p.type()) {
-    return {};
+    return {false, {}};
   }
 
+  // particle energy
+  double E = p.E();
+
+  // check to make sure energy is in range, expects sorted energy values
+  if (E < energy_bounds_.front() || E > energy_bounds_.back())
+    return {false, {}};
+
   // Get mesh index for particle's position
   const auto& mesh = this->mesh();
   int mesh_bin = mesh->get_bin(p.r());
 
   // particle is outside the weight window mesh
   if (mesh_bin < 0)
-    return {};
-
-  // particle energy
-  double E = p.E();
-
-  // check to make sure energy is in range, expects sorted energy values
-  if (E < energy_bounds_.front() || E > energy_bounds_.back())
-    return {};
+    return {false, {}};
 
   // get the mesh bin in energy group
   int energy_bin =
@@ -390,7 +309,7 @@ WeightWindow WeightWindows::get_weight_window(const Particle& p) const
   ww.max_lb_ratio = max_lb_ratio_;
   ww.max_split = max_split_;
   ww.weight_cutoff = weight_cutoff_;
-  return ww;
+  return {true, ww};
 }
 
 std::array<int, 2> WeightWindows::bounds_size() const
@@ -429,8 +348,8 @@ void WeightWindows::check_bounds(const T& bounds) const
   }
 }
 
-void WeightWindows::set_bounds(const xt::xtensor<double, 2>& lower_bounds,
-  const xt::xtensor<double, 2>& upper_bounds)
+void WeightWindows::set_bounds(const tensor::Tensor<double>& lower_bounds,
+  const tensor::Tensor<double>& upper_bounds)
 {
 
   this->check_bounds(lower_bounds, upper_bounds);
@@ -441,7 +360,7 @@ void WeightWindows::set_bounds(const xt::xtensor<double, 2>& lower_bounds,
 }
 
 void WeightWindows::set_bounds(
-  const xt::xtensor<double, 2>& lower_bounds, double ratio)
+  const tensor::Tensor<double>& lower_bounds, double ratio)
 {
   this->check_bounds(lower_bounds);
 
@@ -456,14 +375,16 @@ void WeightWindows::set_bounds(
 {
   check_bounds(lower_bounds, upper_bounds);
   auto shape = this->bounds_size();
-  lower_ww_ = xt::empty<double>(shape);
-  upper_ww_ = xt::empty<double>(shape);
+  lower_ww_ = tensor::Tensor<double>(
+    {static_cast<size_t>(shape[0]), static_cast<size_t>(shape[1])});
+  upper_ww_ = tensor::Tensor<double>(
+    {static_cast<size_t>(shape[0]), static_cast<size_t>(shape[1])});
 
-  // set new weight window values
-  xt::view(lower_ww_, xt::all()) =
-    xt::adapt(lower_bounds.data(), lower_ww_.shape());
-  xt::view(upper_ww_, xt::all()) =
-    xt::adapt(upper_bounds.data(), upper_ww_.shape());
+  // Copy weight window values from input spans into the tensors
+  std::copy(lower_bounds.data(), lower_bounds.data() + lower_ww_.size(),
+    lower_ww_.data());
+  std::copy(upper_bounds.data(), upper_bounds.data() + upper_ww_.size(),
+    upper_ww_.data());
 }
 
 void WeightWindows::set_bounds(span<const double> lower_bounds, double ratio)
@@ -471,14 +392,16 @@ void WeightWindows::set_bounds(span<const double> lower_bounds, double ratio)
   this->check_bounds(lower_bounds);
 
   auto shape = this->bounds_size();
-  lower_ww_ = xt::empty<double>(shape);
-  upper_ww_ = xt::empty<double>(shape);
+  lower_ww_ = tensor::Tensor<double>(
+    {static_cast<size_t>(shape[0]), static_cast<size_t>(shape[1])});
+  upper_ww_ = tensor::Tensor<double>(
+    {static_cast<size_t>(shape[0]), static_cast<size_t>(shape[1])});
 
-  // set new weight window values
-  xt::view(lower_ww_, xt::all()) =
-    xt::adapt(lower_bounds.data(), lower_ww_.shape());
-  xt::view(upper_ww_, xt::all()) =
-    xt::adapt(lower_bounds.data(), upper_ww_.shape());
+  // Copy lower bounds into both arrays, then scale upper by ratio
+  std::copy(lower_bounds.data(), lower_bounds.data() + lower_ww_.size(),
+    lower_ww_.data());
+  std::copy(lower_bounds.data(), lower_bounds.data() + upper_ww_.size(),
+    upper_ww_.data());
   upper_ww_ *= ratio;
 }
 
@@ -490,8 +413,18 @@ void WeightWindows::update_weights(const Tally* tally, const std::string& value,
   ///////////////////////////
   this->check_tally_update_compatibility(tally);
 
-  lower_ww_.fill(-1);
-  upper_ww_.fill(-1);
+  // Dimensions of weight window arrays
+  int e_bins = lower_ww_.shape(0);
+  int64_t mesh_bins = lower_ww_.shape(1);
+
+  // Initialize weight window arrays to -1.0 by default
+#pragma omp parallel for collapse(2) schedule(static)
+  for (int e = 0; e < e_bins; e++) {
+    for (int64_t m = 0; m < mesh_bins; m++) {
+      lower_ww_(e, m) = -1.0;
+      upper_ww_(e, m) = -1.0;
+    }
+  }
 
   // determine which value to use
   const std::set<std::string> allowed_values = {"mean", "rel_err"};
@@ -513,15 +446,17 @@ void WeightWindows::update_weights(const Tally* tally, const std::string& value,
   ///////////////////////////
   // Extract tally data
   //
-  // At the end of this section, the mean and rel_err array
-  // is a 2D view of tally data (n_e_groups, n_mesh_bins)
+  // At the end of this section, mean and rel_err are
+  // 2D tensors of tally data (n_e_groups, n_mesh_bins)
   //
   ///////////////////////////
 
-  // build a shape for a view of the tally results, this will always be
+  // build a shape for the tally results, this will always be
   // dimension 5 (3 filter dimensions, 1 score dimension, 1 results dimension)
-  std::array<int, 5> shape = {
-    1, 1, 1, tally->n_scores(), static_cast<int>(TallyResult::SIZE)};
+  // Look for the size of the last dimension of the results tensor
+  const auto& results = tally->results();
+  const int results_dim = static_cast<int>(results.shape(2));
+  std::array<int, 5> shape = {1, 1, 1, tally->n_scores(), results_dim};
 
   // set the shape for the filters applied on the tally
   for (int i = 0; i < tally->filters().size(); i++) {
@@ -557,42 +492,61 @@ void WeightWindows::update_weights(const Tally* tally, const std::string& value,
     std::find(filter_types.begin(), filter_types.end(), FilterType::MESH) -
     filter_types.begin();
 
-  // get a fully reshaped view of the tally according to tally ordering of
-  // filters
-  auto tally_values = xt::reshape_view(tally->results(), shape);
-
-  // get a that is (particle, energy, mesh, scores, values)
-  auto transposed_view = xt::transpose(tally_values, transpose);
-
-  // determine the dimension and index of the particle data
+  // determine the index of the particle within its filter
   int particle_idx = 0;
   if (tally->has_filter(FilterType::PARTICLE)) {
-    // get the particle filter
     auto pf = tally->get_filter<ParticleFilter>();
     const auto& particles = pf->particles();
 
-    // find the index of the particle that matches these weight windows
     auto p_it =
       std::find(particles.begin(), particles.end(), this->particle_type_);
-    // if the particle filter doesn't have particle data for the particle
-    // used on this weight windows instance, report an error
     if (p_it == particles.end()) {
       auto msg = fmt::format("Particle type '{}' not present on Filter {} for "
                              "Tally {} used to update WeightWindows {}",
-        particle_type_to_str(this->particle_type_), pf->id(), tally->id(),
-        this->id());
+        this->particle_type_.str(), pf->id(), tally->id(), this->id());
       fatal_error(msg);
     }
 
-    // use the index of the particle in the filter to down-select data later
     particle_idx = p_it - particles.begin();
   }
 
-  // down-select data based on particle and score
-  auto sum = xt::view(transposed_view, particle_idx, xt::all(), xt::all(),
-    score_index, static_cast<int>(TallyResult::SUM));
-  auto sum_sq = xt::view(transposed_view, particle_idx, xt::all(), xt::all(),
-    score_index, static_cast<int>(TallyResult::SUM_SQ));
+  // The tally results array is 3D: (n_filter_combos, n_scores, n_result_types).
+  // The first dimension is a row-major flattening of up to 3 filter dimensions
+  // (particle, energy, mesh) whose storage order depends on which filters the
+  // tally has. We need to map our desired indices (particle, energy, mesh)
+  // into the correct flat filter combination index.
+  //
+  // transpose[i] tells us which storage position holds dimension i:
+  //   i=0 -> particle, i=1 -> energy, i=2 -> mesh
+  // shape[j] gives the number of bins for filter storage position j.
+
+  // Row-major strides for the 3 filter dimensions
+  const int stride0 = shape[1] * shape[2];
+  const int stride1 = shape[2];
+
+  tensor::Tensor<double> sum(
+    {static_cast<size_t>(e_bins), static_cast<size_t>(mesh_bins)});
+  tensor::Tensor<double> sum_sq(
+    {static_cast<size_t>(e_bins), static_cast<size_t>(mesh_bins)});
+
+  const int i_sum = static_cast<int>(TallyResult::SUM);
+  const int i_sum_sq = static_cast<int>(TallyResult::SUM_SQ);
+
+  for (int e = 0; e < e_bins; e++) {
+    for (int64_t m = 0; m < mesh_bins; m++) {
+      // Place particle, energy, and mesh indices into their storage positions
+      std::array<int, 3> idx = {0, 0, 0};
+      idx[transpose[0]] = particle_idx;
+      idx[transpose[1]] = e;
+      idx[transpose[2]] = static_cast<int>(m);
+
+      // Compute flat filter combination index (row-major over filter dims)
+      int flat = idx[0] * stride0 + idx[1] * stride1 + idx[2];
+
+      sum(e, m) = results(flat, score_index, i_sum);
+      sum_sq(e, m) = results(flat, score_index, i_sum_sq);
+    }
+  }
   int n = tally->n_realizations_;
 
   //////////////////////////////////////////////
@@ -610,78 +564,117 @@ void WeightWindows::update_weights(const Tally* tally, const std::string& value,
   auto& new_bounds = this->lower_ww_;
   auto& rel_err = this->upper_ww_;
 
-  // noalias avoids memory allocation here
-  xt::noalias(new_bounds) = sum / n;
-
-  xt::noalias(rel_err) =
-    xt::sqrt(((sum_sq / n) - xt::square(new_bounds)) / (n - 1)) / new_bounds;
-  xt::filter(rel_err, sum <= 0.0).fill(INFTY);
-
-  if (value == "rel_err")
-    xt::noalias(new_bounds) = 1 / rel_err;
-
   // get mesh volumes
   auto mesh_vols = this->mesh()->volumes();
 
-  int e_bins = new_bounds.shape()[0];
-
-  if (method == WeightWindowUpdateMethod::MAGIC) {
-    // If we are computing weight windows with forward fluxes derived from a
-    // Monte Carlo or forward random ray solve, we use the MAGIC algorithm.
-    for (int e = 0; e < e_bins; e++) {
-      // select all
-      auto group_view = xt::view(new_bounds, e);
-
-      // divide by volume of mesh elements
-      for (int i = 0; i < group_view.size(); i++) {
-        group_view[i] /= mesh_vols[i];
+  // Calculate mean (new_bounds) and relative error
+#pragma omp parallel for collapse(2) schedule(static)
+  for (int e = 0; e < e_bins; e++) {
+    for (int64_t m = 0; m < mesh_bins; m++) {
+      // Calculate mean
+      new_bounds(e, m) = sum(e, m) / n;
+      // Calculate relative error
+      if (sum(e, m) > 0.0) {
+        double mean_val = new_bounds(e, m);
+        double variance = (sum_sq(e, m) / n - mean_val * mean_val) / (n - 1);
+        rel_err(e, m) = std::sqrt(variance) / mean_val;
+      } else {
+        rel_err(e, m) = INFTY;
       }
-
-      double group_max =
-        *std::max_element(group_view.begin(), group_view.end());
-      // normalize values in this energy group by the maximum value for this
-      // group
-      if (group_max > 0.0)
-        group_view /= 2.0 * group_max;
-    }
-  } else {
-    // If we are computing weight windows with adjoint fluxes derived from an
-    // adjoint random ray solve, we use the FW-CADIS algorithm.
-    for (int e = 0; e < e_bins; e++) {
-      // select all
-      auto group_view = xt::view(new_bounds, e);
-
-      // divide by volume of mesh elements
-      for (int i = 0; i < group_view.size(); i++) {
-        group_view[i] /= mesh_vols[i];
+      if (value == "rel_err") {
+        new_bounds(e, m) = 1.0 / rel_err(e, m);
       }
     }
-
-    // We take the inverse, but are careful not to divide by zero e.g. if some
-    // mesh bins are not reachable in the physical geometry.
-    xt::noalias(new_bounds) =
-      xt::where(xt::not_equal(new_bounds, 0.0), 1.0 / new_bounds, 0.0);
-    auto max_val = xt::amax(new_bounds)();
-    xt::noalias(new_bounds) = new_bounds / (2.0 * max_val);
-
-    // For bins that were missed, we use the minimum weight window value. This
-    // shouldn't matter except for plotting.
-    auto min_val = xt::amin(new_bounds)();
-    xt::noalias(new_bounds) =
-      xt::where(xt::not_equal(new_bounds, 0.0), new_bounds, min_val);
   }
 
-  // make sure that values where the mean is zero are set s.t. the weight window
-  // value will be ignored
-  xt::filter(new_bounds, sum <= 0.0).fill(-1.0);
+  // Divide by volume of mesh elements
+#pragma omp parallel for collapse(2) schedule(static)
+  for (int e = 0; e < e_bins; e++) {
+    for (int64_t m = 0; m < mesh_bins; m++) {
+      new_bounds(e, m) /= mesh_vols[m];
+    }
+  }
 
-  // make sure the weight windows are ignored for any locations where the
-  // relative error is higher than the specified relative error threshold
-  xt::filter(new_bounds, rel_err > threshold).fill(-1.0);
+  if (method == WeightWindowUpdateMethod::MAGIC) {
+    // For MAGIC, weight windows are proportional to the forward fluxes.
+    // We normalize weight windows independently for each energy group.
 
-  // update the bounds of this weight window class
-  // noalias avoids additional memory allocation
-  xt::noalias(upper_ww_) = ratio * lower_ww_;
+    // Find group maximum and normalize (per energy group)
+    for (int e = 0; e < e_bins; e++) {
+      double group_max = 0.0;
+
+      // Find maximum value across all elements in this energy group
+#pragma omp parallel for schedule(static) reduction(max : group_max)
+      for (int64_t m = 0; m < mesh_bins; m++) {
+        if (new_bounds(e, m) > group_max) {
+          group_max = new_bounds(e, m);
+        }
+      }
+
+      // Normalize values in this energy group by the maximum value
+      if (group_max > 0.0) {
+        double norm_factor = 1.0 / (2.0 * group_max);
+#pragma omp parallel for schedule(static)
+        for (int64_t m = 0; m < mesh_bins; m++) {
+          new_bounds(e, m) *= norm_factor;
+        }
+      }
+    }
+  } else {
+    // For (FW-)CADIS, weight windows are inversely proportional to the adjoint
+    // fluxes. We normalize the weight windows across all energy groups.
+#pragma omp parallel for collapse(2) schedule(static)
+    for (int e = 0; e < e_bins; e++) {
+      for (int64_t m = 0; m < mesh_bins; m++) {
+        // Take the inverse, but are careful not to divide by zero
+        if (new_bounds(e, m) != 0.0) {
+          new_bounds(e, m) = 1.0 / new_bounds(e, m);
+        } else {
+          new_bounds(e, m) = 0.0;
+        }
+      }
+    }
+
+    // Find the maximum value across all elements
+    double max_val = 0.0;
+#pragma omp parallel for collapse(2) schedule(static) reduction(max : max_val)
+    for (int e = 0; e < e_bins; e++) {
+      for (int64_t m = 0; m < mesh_bins; m++) {
+        if (new_bounds(e, m) > max_val) {
+          max_val = new_bounds(e, m);
+        }
+      }
+    }
+
+    // Parallel normalization
+    if (max_val > 0.0) {
+      double norm_factor = 1.0 / (2.0 * max_val);
+#pragma omp parallel for collapse(2) schedule(static)
+      for (int e = 0; e < e_bins; e++) {
+        for (int64_t m = 0; m < mesh_bins; m++) {
+          new_bounds(e, m) *= norm_factor;
+        }
+      }
+    }
+  }
+
+  // Final processing
+#pragma omp parallel for collapse(2) schedule(static)
+  for (int e = 0; e < e_bins; e++) {
+    for (int64_t m = 0; m < mesh_bins; m++) {
+      // Values where the mean is zero should be ignored
+      if (sum(e, m) <= 0.0) {
+        new_bounds(e, m) = -1.0;
+      }
+      // Values where the relative error is higher than the threshold should be
+      // ignored
+      else if (rel_err(e, m) > threshold) {
+        new_bounds(e, m) = -1.0;
+      }
+      // Set the upper bounds
+      upper_ww_(e, m) = ratio * lower_ww_(e, m);
+    }
+  }
 }
 
 void WeightWindows::check_tally_update_compatibility(const Tally* tally)
@@ -748,8 +741,7 @@ void WeightWindows::to_hdf5(hid_t group) const
   hid_t ww_group = create_group(group, fmt::format("weight_windows_{}", id()));
 
   write_dataset(ww_group, "mesh", this->mesh()->id());
-  write_dataset(
-    ww_group, "particle_type", openmc::particle_type_to_str(particle_type_));
+  write_dataset(ww_group, "particle_type", particle_type_.str());
   write_dataset(ww_group, "energy_bounds", energy_bounds_);
   write_dataset(ww_group, "lower_ww_bounds", lower_ww_);
   write_dataset(ww_group, "upper_ww_bounds", upper_ww_);
@@ -776,8 +768,8 @@ WeightWindowsGenerator::WeightWindowsGenerator(pugi::xml_node node)
         max_realizations_, active_batches);
     warning(msg);
   }
-  auto tmp_str = get_node_value(node, "particle_type", true, true);
-  auto particle_type = str_to_particle_type(tmp_str);
+  auto tmp_str = get_node_value(node, "particle_type", false, true);
+  auto particle_type = ParticleType {tmp_str};
 
   update_interval_ = std::stoi(get_node_value(node, "update_interval"));
   on_the_fly_ = get_node_value_bool(node, "on_the_fly");
@@ -786,7 +778,10 @@ WeightWindowsGenerator::WeightWindowsGenerator(pugi::xml_node node)
   if (check_for_node(node, "energy_bounds")) {
     e_bounds = get_node_array<double>(node, "energy_bounds");
   } else {
-    int p_type = static_cast<int>(particle_type);
+    int p_type = particle_type.transport_index();
+    if (p_type == C_NONE) {
+      fatal_error("Weight windows particle is not supported for transport.");
+    }
     e_bounds.push_back(data::energy_min[p_type]);
     e_bounds.push_back(data::energy_max[p_type]);
   }
@@ -796,7 +791,7 @@ WeightWindowsGenerator::WeightWindowsGenerator(pugi::xml_node node)
   if (method_string == "magic") {
     method_ = WeightWindowUpdateMethod::MAGIC;
     if (settings::solver_type == SolverType::RANDOM_RAY &&
-        FlatSourceDomain::adjoint_) {
+        FlatSourceDomain::adjoint_requested_) {
       fatal_error("Random ray weight window generation with MAGIC cannot be "
                   "done in adjoint mode.");
     }
@@ -805,7 +800,14 @@ WeightWindowsGenerator::WeightWindowsGenerator(pugi::xml_node node)
     if (settings::solver_type != SolverType::RANDOM_RAY) {
       fatal_error("FW-CADIS can only be run in random ray solver mode.");
     }
-    FlatSourceDomain::adjoint_ = true;
+    FlatSourceDomain::adjoint_requested_ = true;
+    if (check_for_node(node, "targets")) {
+      FlatSourceDomain::fw_cadis_local_ = true;
+      targets_ = get_node_array<size_t>(node, "targets");
+      FlatSourceDomain::fw_cadis_local_targets_.insert(
+        std::end(FlatSourceDomain::fw_cadis_local_targets_),
+        std::begin(targets_), std::end(targets_));
+    }
   } else {
     fatal_error(fmt::format(
       "Unknown weight window update method '{}' specified", method_string));
@@ -835,7 +837,8 @@ WeightWindowsGenerator::WeightWindowsGenerator(pugi::xml_node node)
       ratio_));
   if (ratio_ <= 1.0)
     fatal_error(fmt::format("Invalid weight window ratio '{}' (<= 1.0) "
-                            "specified for weight window generation"));
+                            "specified for weight window generation",
+      ratio_));
 
   // create a matching weight windows object
   auto wws = WeightWindows::create();
@@ -863,7 +866,8 @@ void WeightWindowsGenerator::create_tally()
   for (const auto& f : model::tally_filters) {
     if (f->type() == FilterType::MESH) {
       const auto* mesh_filter = dynamic_cast<MeshFilter*>(f.get());
-      if (mesh_filter->mesh() == mesh_idx && !mesh_filter->translated()) {
+      if (mesh_filter->mesh() == mesh_idx && !mesh_filter->translated() &&
+          !mesh_filter->rotated()) {
         ww_tally->add_filter(f.get());
         found_mesh_filter = true;
         break;
@@ -899,11 +903,17 @@ void WeightWindowsGenerator::update() const
 
   Tally* tally = model::tallies[tally_idx_].get();
 
-  // if we're beyond the number of max realizations or not at the corrrect
-  // update interval, skip the update
-  if (max_realizations_ < tally->n_realizations_ ||
-      tally->n_realizations_ % update_interval_ != 0)
+  // If in random ray mode, only update on the last batch
+  if (settings::solver_type == SolverType::RANDOM_RAY) {
+    if (simulation::current_batch != settings::n_batches) {
+      return;
+    }
+    // If in Monte Carlo mode and beyond the number of max realizations or
+    // not at the correct update interval, skip the update
+  } else if (max_realizations_ < tally->n_realizations_ ||
+             tally->n_realizations_ % update_interval_ != 0) {
     return;
+  }
 
   wws->update_weights(tally, tally_value_, threshold_, ratio_, method_);
 
@@ -920,6 +930,115 @@ void WeightWindowsGenerator::update() const
 // Non-member functions
 //==============================================================================
 
+std::pair<bool, WeightWindow> search_weight_window(const Particle& p)
+{
+  // TODO: this is a linear search - should do something more clever
+  for (const auto& ww : variance_reduction::weight_windows) {
+    auto [ww_found, weight_window] = ww->get_weight_window(p);
+    if (ww_found)
+      return {true, weight_window};
+  }
+  return {false, {}};
+}
+
+void apply_weight_windows(Particle& p)
+{
+  if (!settings::weight_windows_on)
+    return;
+
+  // WW on photon and neutron only
+  if (!p.type().is_neutron() && !p.type().is_photon())
+    return;
+
+  // skip dead or no energy
+  if (p.E() <= 0 || !p.alive())
+    return;
+
+  auto [ww_found, ww] = search_weight_window(p);
+  if (ww_found && ww.is_valid()) {
+    apply_weight_window(p, ww);
+  } else {
+    if (p.wgt_ww_born() == -1.0)
+      p.wgt_ww_born() = 1.0;
+  }
+}
+
+void apply_weight_window(Particle& p, WeightWindow weight_window)
+{
+  if (!weight_window.is_valid())
+    return;
+
+  // skip dead or no energy
+  if (p.E() <= 0 || !p.alive())
+    return;
+
+  // If particle has not yet had its birth weight window value set, set it to
+  // the current weight window.
+  if (p.wgt_ww_born() == -1.0)
+    p.wgt_ww_born() =
+      (weight_window.lower_weight + weight_window.upper_weight) / 2;
+
+  // Normalize weight windows based on particle's starting weight
+  // and the value of the weight window the particle was born in.
+  weight_window.scale(p.wgt_born() / p.wgt_ww_born());
+
+  // get the paramters
+  double weight = p.wgt();
+
+  // first check to see if particle should be killed for weight cutoff
+  if (p.wgt() < weight_window.weight_cutoff) {
+    p.wgt() = 0.0;
+    return;
+  }
+
+  // check if particle is far above current weight window
+  // only do this if the factor is not already set on the particle and a
+  // maximum lower bound ratio is specified
+  if (p.ww_factor() == 0.0 && weight_window.max_lb_ratio > 1.0 &&
+      p.wgt() > weight_window.lower_weight * weight_window.max_lb_ratio) {
+    p.ww_factor() =
+      p.wgt() / (weight_window.lower_weight * weight_window.max_lb_ratio);
+  }
+
+  // move weight window closer to the particle weight if needed
+  if (p.ww_factor() > 1.0)
+    weight_window.scale(p.ww_factor());
+
+  // if particle's weight is above the weight window split until they are within
+  // the window
+  if (weight > weight_window.upper_weight) {
+    // do not further split the particle if above the limit
+    if (p.n_split() >= settings::max_history_splits)
+      return;
+
+    double n_split = std::ceil(weight / weight_window.upper_weight);
+    double max_split = weight_window.max_split;
+    n_split = std::min(n_split, max_split);
+
+    p.n_split() += n_split;
+
+    // Create secondaries and divide weight among all particles
+    int i_split = std::round(n_split);
+    for (int l = 0; l < i_split - 1; l++) {
+      p.split(weight / n_split);
+    }
+    // remaining weight is applied to current particle
+    p.wgt() = weight / n_split;
+
+  } else if (weight <= weight_window.lower_weight) {
+    // if the particle weight is below the window, play Russian roulette
+    double weight_survive =
+      std::min(weight * weight_window.max_split, weight_window.survival_weight);
+    russian_roulette(p, weight_survive);
+  } // else particle is in the window, continue as normal
+}
+
+void free_memory_weight_windows()
+{
+  variance_reduction::ww_map.clear();
+  variance_reduction::weight_windows.clear();
+}
+
 void finalize_variance_reduction()
 {
   for (const auto& wwg : variance_reduction::weight_windows_generators) {
@@ -1033,23 +1152,25 @@ extern "C" int openmc_weight_windows_get_energy_bounds(
   return 0;
 }
 
-extern "C" int openmc_weight_windows_set_particle(int32_t index, int particle)
+extern "C" int openmc_weight_windows_set_particle(
+  int32_t index, int32_t particle)
 {
   if (int err = verify_ww_index(index))
     return err;
 
   const auto& wws = variance_reduction::weight_windows.at(index);
-  wws->set_particle_type(static_cast<ParticleType>(particle));
+  wws->set_particle_type(ParticleType {particle});
   return 0;
 }
 
-extern "C" int openmc_weight_windows_get_particle(int32_t index, int* particle)
+extern "C" int openmc_weight_windows_get_particle(
+  int32_t index, int32_t* particle)
 {
   if (int err = verify_ww_index(index))
     return err;
 
   const auto& wws = variance_reduction::weight_windows.at(index);
-  *particle = static_cast<int>(wws->particle_type());
+  *particle = wws->particle_type().pdg_number();
   return 0;
 }
 
@@ -1073,7 +1194,8 @@ extern "C" int openmc_weight_windows_set_bounds(int32_t index,
     return err;
 
   const auto& wws = variance_reduction::weight_windows[index];
-  wws->set_bounds({lower_bounds, size}, {upper_bounds, size});
+  wws->set_bounds(span<const double>(lower_bounds, size),
+    span<const double>(upper_bounds, size));
   return 0;
 }
 
@@ -1261,6 +1383,10 @@ extern "C" int openmc_weight_windows_import(const char* filename)
 
   hid_t weight_windows_group = open_group(ww_file, "weight_windows");
 
+  hid_t mesh_group = open_group(ww_file, "meshes");
+
+  read_meshes(mesh_group);
+
   std::vector<std::string> names = group_names(weight_windows_group);
 
   for (const auto& name : names) {
diff --git a/src/wmp.cpp b/src/wmp.cpp
index e9c4fb5e3..6f72e1ba4 100644
--- a/src/wmp.cpp
+++ b/src/wmp.cpp
@@ -33,22 +33,22 @@ WindowedMultipole::WindowedMultipole(hid_t group)
   // Read the "data" array.  Use its shape to figure out the number of poles
   // and residue types in this data.
   read_dataset(group, "data", data_);
-  int n_residues = data_.shape()[1] - 1;
+  int n_residues = data_.shape(1) - 1;
 
   // Check to see if this data includes fission residues.
   fissionable_ = (n_residues == 3);
 
   // Read the "windows" array and use its shape to figure out the number of
   // windows.
-  xt::xtensor<int, 2> windows;
+  tensor::Tensor<int> windows;
   read_dataset(group, "windows", windows);
-  int n_windows = windows.shape()[0];
+  int n_windows = windows.shape(0);
   windows -= 1; // Adjust to 0-based indices
 
   // Read the "broaden_poly" arrays.
-  xt::xtensor<bool, 1> broaden_poly;
+  tensor::Tensor<bool> broaden_poly;
   read_dataset(group, "broaden_poly", broaden_poly);
-  if (n_windows != broaden_poly.shape()[0]) {
+  if (n_windows != broaden_poly.shape(0)) {
     fatal_error("broaden_poly array shape is not consistent with the windows "
                 "array shape in WMP library for " +
                 name_ + ".");
@@ -56,12 +56,12 @@ WindowedMultipole::WindowedMultipole(hid_t group)
 
   // Read the "curvefit" array.
   read_dataset(group, "curvefit", curvefit_);
-  if (n_windows != curvefit_.shape()[0]) {
+  if (n_windows != curvefit_.shape(0)) {
     fatal_error("curvefit array shape is not consistent with the windows "
                 "array shape in WMP library for " +
                 name_ + ".");
   }
-  fit_order_ = curvefit_.shape()[1] - 1;
+  fit_order_ = curvefit_.shape(1) - 1;
 
   // Check the code is compiling to work with sufficiently high fit order
   if (fit_order_ + 1 > MAX_POLY_COEFFICIENTS) {
diff --git a/src/xsdata.cpp b/src/xsdata.cpp
index 1929f51e6..33d063a7b 100644
--- a/src/xsdata.cpp
+++ b/src/xsdata.cpp
@@ -5,10 +5,7 @@
 #include <cstdlib>
 #include <numeric>
 
-#include "xtensor/xbuilder.hpp"
-#include "xtensor/xindex_view.hpp"
-#include "xtensor/xmath.hpp"
-#include "xtensor/xview.hpp"
+#include "openmc/tensor.h"
 
 #include "openmc/constants.h"
 #include "openmc/error.h"
@@ -37,32 +34,32 @@ XsData::XsData(bool fissionable, AngleDistributionType scatter_format,
   }
   // allocate all [temperature][angle][in group] quantities
   vector<size_t> shape {n_ang, n_g_};
-  total = xt::zeros<double>(shape);
-  absorption = xt::zeros<double>(shape);
-  inverse_velocity = xt::zeros<double>(shape);
+  total = tensor::zeros<double>(shape);
+  absorption = tensor::zeros<double>(shape);
+  inverse_velocity = tensor::zeros<double>(shape);
   if (fissionable) {
-    fission = xt::zeros<double>(shape);
-    nu_fission = xt::zeros<double>(shape);
-    prompt_nu_fission = xt::zeros<double>(shape);
-    kappa_fission = xt::zeros<double>(shape);
+    fission = tensor::zeros<double>(shape);
+    nu_fission = tensor::zeros<double>(shape);
+    prompt_nu_fission = tensor::zeros<double>(shape);
+    kappa_fission = tensor::zeros<double>(shape);
   }
 
   // allocate decay_rate; [temperature][angle][delayed group]
   shape[1] = n_dg_;
-  decay_rate = xt::zeros<double>(shape);
+  decay_rate = tensor::zeros<double>(shape);
 
   if (fissionable) {
     shape = {n_ang, n_dg_, n_g_};
     // allocate delayed_nu_fission; [temperature][angle][delay group][in group]
-    delayed_nu_fission = xt::zeros<double>(shape);
+    delayed_nu_fission = tensor::zeros<double>(shape);
 
     // chi_prompt; [temperature][angle][in group][out group]
     shape = {n_ang, n_g_, n_g_};
-    chi_prompt = xt::zeros<double>(shape);
+    chi_prompt = tensor::zeros<double>(shape);
 
     // chi_delayed; [temperature][angle][delay group][in group][out group]
     shape = {n_ang, n_dg_, n_g_, n_g_};
-    chi_delayed = xt::zeros<double>(shape);
+    chi_delayed = tensor::zeros<double>(shape);
   }
 
   for (int a = 0; a < n_ang; a++) {
@@ -85,28 +82,30 @@ void XsData::from_hdf5(hid_t xsdata_grp, bool fissionable,
 {
   // Reconstruct the dimension information so it doesn't need to be passed
   size_t n_ang = n_pol * n_azi;
-  size_t energy_groups = total.shape()[1];
+  size_t energy_groups = total.shape(1);
 
   // Set the fissionable-specific data
   if (fissionable) {
     fission_from_hdf5(xsdata_grp, n_ang, is_isotropic);
   }
   // Get the non-fission-specific data
-  read_nd_vector(xsdata_grp, "decay-rate", decay_rate);
-  read_nd_vector(xsdata_grp, "absorption", absorption, true);
-  read_nd_vector(xsdata_grp, "inverse-velocity", inverse_velocity);
+  read_nd_tensor(xsdata_grp, "decay-rate", decay_rate);
+  read_nd_tensor(xsdata_grp, "absorption", absorption, true);
+  read_nd_tensor(xsdata_grp, "inverse-velocity", inverse_velocity);
 
   // Get scattering data
   scatter_from_hdf5(
     xsdata_grp, n_ang, scatter_format, final_scatter_format, order_data);
 
-  // Check absorption to ensure it is not 0 since it is often the
-  // denominator in tally methods
-  xt::filtration(absorption, xt::equal(absorption, 0.)) = 1.e-10;
+  // Replace zero absorption values with a small number to avoid
+  // division by zero in tally methods
+  for (size_t i = 0; i < absorption.size(); i++)
+    if (absorption.data()[i] == 0.0)
+      absorption.data()[i] = 1.e-10;
 
   // Get or calculate the total x/s
   if (object_exists(xsdata_grp, "total")) {
-    read_nd_vector(xsdata_grp, "total", total);
+    read_nd_tensor(xsdata_grp, "total", total);
   } else {
     for (size_t a = 0; a < n_ang; a++) {
       for (size_t gin = 0; gin < energy_groups; gin++) {
@@ -115,8 +114,11 @@ void XsData::from_hdf5(hid_t xsdata_grp, bool fissionable,
     }
   }
 
-  // Fix if total is 0, since it is in the denominator when tallying
-  xt::filtration(total, xt::equal(total, 0.)) = 1.e-10;
+  // Replace zero total cross sections with a small number to avoid
+  // division by zero in tally methods
+  for (size_t i = 0; i < total.size(); i++)
+    if (total.data()[i] == 0.0)
+      total.data()[i] = 1.e-10;
 }
 
 //==============================================================================
@@ -127,21 +129,30 @@ void XsData::fission_vector_beta_from_hdf5(
   // Data is provided as nu-fission and chi with a beta for delayed info
 
   // Get chi
-  xt::xtensor<double, 2> temp_chi({n_ang, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "chi", temp_chi, true);
+  tensor::Tensor<double> temp_chi = tensor::zeros<double>({n_ang, n_g_});
+  read_nd_tensor(xsdata_grp, "chi", temp_chi, true);
 
-  // Normalize chi by summing over the outgoing groups for each incoming angle
-  temp_chi /= xt::view(xt::sum(temp_chi, {1}), xt::all(), xt::newaxis());
+  // Normalize chi so it sums to 1 over outgoing groups for each angle
+  for (size_t a = 0; a < n_ang; a++) {
+    tensor::View<double> row = temp_chi.slice(a);
+    row /= row.sum();
+  }
 
-  // Now every incoming group in prompt_chi and delayed_chi is the normalized
-  // chi we just made
-  chi_prompt = xt::view(temp_chi, xt::all(), xt::newaxis(), xt::all());
-  chi_delayed =
-    xt::view(temp_chi, xt::all(), xt::newaxis(), xt::newaxis(), xt::all());
+  // Replicate the energy spectrum across all incoming groups — the
+  // spectrum is independent of the incoming neutron energy
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t gin = 0; gin < n_g_; gin++)
+      chi_prompt.slice(a, gin) = temp_chi.slice(a);
+
+  // Same spectrum for delayed neutrons, replicated across delayed groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t d = 0; d < n_dg_; d++)
+      for (size_t gin = 0; gin < n_g_; gin++)
+        chi_delayed.slice(a, d, gin) = temp_chi.slice(a);
 
   // Get nu-fission
-  xt::xtensor<double, 2> temp_nufiss({n_ang, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "nu-fission", temp_nufiss, true);
+  tensor::Tensor<double> temp_nufiss = tensor::zeros<double>({n_ang, n_g_});
+  read_nd_tensor(xsdata_grp, "nu-fission", temp_nufiss, true);
 
   // Get beta (strategy will depend upon the number of dimensions in beta)
   hid_t beta_dset = open_dataset(xsdata_grp, "beta");
@@ -151,26 +162,39 @@ void XsData::fission_vector_beta_from_hdf5(
   if (!is_isotropic)
     ndim_target += 2;
   if (beta_ndims == ndim_target) {
-    xt::xtensor<double, 2> temp_beta({n_ang, n_dg_}, 0.);
-    read_nd_vector(xsdata_grp, "beta", temp_beta, true);
+    tensor::Tensor<double> temp_beta = tensor::zeros<double>({n_ang, n_dg_});
+    read_nd_tensor(xsdata_grp, "beta", temp_beta, true);
 
-    // Set prompt_nu_fission = (1. - beta_total)*nu_fission
-    prompt_nu_fission = temp_nufiss * (1. - xt::sum(temp_beta, {1}));
+    // prompt_nu_fission = (1 - sum_of_beta) * nu_fission
+    auto beta_sum = temp_beta.sum(1);
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t g = 0; g < n_g_; g++)
+        prompt_nu_fission(a, g) = temp_nufiss(a, g) * (1.0 - beta_sum(a));
 
-    // Set delayed_nu_fission as beta * nu_fission
-    delayed_nu_fission =
-      xt::view(temp_beta, xt::all(), xt::all(), xt::newaxis()) *
-      xt::view(temp_nufiss, xt::all(), xt::newaxis(), xt::all());
+    // Delayed nu-fission is the outer product of the delayed neutron
+    // fraction (beta) and the fission production rate (nu-fission)
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg_; d++)
+        for (size_t g = 0; g < n_g_; g++)
+          delayed_nu_fission(a, d, g) = temp_beta(a, d) * temp_nufiss(a, g);
   } else if (beta_ndims == ndim_target + 1) {
-    xt::xtensor<double, 3> temp_beta({n_ang, n_dg_, n_g_}, 0.);
-    read_nd_vector(xsdata_grp, "beta", temp_beta, true);
+    tensor::Tensor<double> temp_beta =
+      tensor::zeros<double>({n_ang, n_dg_, n_g_});
+    read_nd_tensor(xsdata_grp, "beta", temp_beta, true);
 
-    // Set prompt_nu_fission = (1. - beta_total)*nu_fission
-    prompt_nu_fission = temp_nufiss * (1. - xt::sum(temp_beta, {1}));
+    // prompt_nu_fission = (1 - sum_of_beta) * nu_fission
+    // Here beta is energy-dependent, so sum over delayed groups (axis 1)
+    auto beta_sum = temp_beta.sum(1);
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t g = 0; g < n_g_; g++)
+        prompt_nu_fission(a, g) = temp_nufiss(a, g) * (1.0 - beta_sum(a, g));
 
-    // Set delayed_nu_fission as beta * nu_fission
-    delayed_nu_fission =
-      temp_beta * xt::view(temp_nufiss, xt::all(), xt::newaxis(), xt::all());
+    // Delayed nu-fission: beta is already energy-dependent [n_ang, n_dg, n_g],
+    // so scale each delayed group's beta by the total nu-fission for that group
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg_; d++)
+        for (size_t g = 0; g < n_g_; g++)
+          delayed_nu_fission(a, d, g) = temp_beta(a, d, g) * temp_nufiss(a, g);
   }
 }
 
@@ -179,29 +203,42 @@ void XsData::fission_vector_no_beta_from_hdf5(hid_t xsdata_grp, size_t n_ang)
   // Data is provided separately as prompt + delayed nu-fission and chi
 
   // Get chi-prompt
-  xt::xtensor<double, 2> temp_chi_p({n_ang, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "chi-prompt", temp_chi_p, true);
+  tensor::Tensor<double> temp_chi_p = tensor::zeros<double>({n_ang, n_g_});
+  read_nd_tensor(xsdata_grp, "chi-prompt", temp_chi_p, true);
 
-  // Normalize chi by summing over the outgoing groups for each incoming angle
-  temp_chi_p /= xt::view(xt::sum(temp_chi_p, {1}), xt::all(), xt::newaxis());
+  // Normalize prompt chi so it sums to 1 over outgoing groups for each angle
+  for (size_t a = 0; a < n_ang; a++) {
+    tensor::View<double> row = temp_chi_p.slice(a);
+    row /= row.sum();
+  }
 
   // Get chi-delayed
-  xt::xtensor<double, 3> temp_chi_d({n_ang, n_dg_, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "chi-delayed", temp_chi_d, true);
+  tensor::Tensor<double> temp_chi_d =
+    tensor::zeros<double>({n_ang, n_dg_, n_g_});
+  read_nd_tensor(xsdata_grp, "chi-delayed", temp_chi_d, true);
 
-  // Normalize chi by summing over the outgoing groups for each incoming angle
-  temp_chi_d /=
-    xt::view(xt::sum(temp_chi_d, {2}), xt::all(), xt::all(), xt::newaxis());
+  // Normalize delayed chi so it sums to 1 over outgoing groups for each
+  // angle and delayed group
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t d = 0; d < n_dg_; d++) {
+      tensor::View<double> row = temp_chi_d.slice(a, d);
+      row /= row.sum();
+    }
 
-  // Now assign the prompt and delayed chis by replicating for each incoming
-  // group
-  chi_prompt = xt::view(temp_chi_p, xt::all(), xt::newaxis(), xt::all());
-  chi_delayed =
-    xt::view(temp_chi_d, xt::all(), xt::all(), xt::newaxis(), xt::all());
+  // Replicate the prompt spectrum across all incoming groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t gin = 0; gin < n_g_; gin++)
+      chi_prompt.slice(a, gin) = temp_chi_p.slice(a);
+
+  // Replicate the delayed spectrum across all incoming groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t d = 0; d < n_dg_; d++)
+      for (size_t gin = 0; gin < n_g_; gin++)
+        chi_delayed.slice(a, d, gin) = temp_chi_d.slice(a, d);
 
   // Get prompt and delayed nu-fission directly
-  read_nd_vector(xsdata_grp, "prompt-nu-fission", prompt_nu_fission, true);
-  read_nd_vector(xsdata_grp, "delayed-nu-fission", delayed_nu_fission, true);
+  read_nd_tensor(xsdata_grp, "prompt-nu-fission", prompt_nu_fission, true);
+  read_nd_tensor(xsdata_grp, "delayed-nu-fission", delayed_nu_fission, true);
 }
 
 void XsData::fission_vector_no_delayed_from_hdf5(hid_t xsdata_grp, size_t n_ang)
@@ -210,17 +247,22 @@ void XsData::fission_vector_no_delayed_from_hdf5(hid_t xsdata_grp, size_t n_ang)
   // Therefore, the code only considers the data as prompt.
 
   // Get chi
-  xt::xtensor<double, 2> temp_chi({n_ang, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "chi", temp_chi, true);
+  tensor::Tensor<double> temp_chi = tensor::zeros<double>({n_ang, n_g_});
+  read_nd_tensor(xsdata_grp, "chi", temp_chi, true);
 
-  // Normalize chi by summing over the outgoing groups for each incoming angle
-  temp_chi /= xt::view(xt::sum(temp_chi, {1}), xt::all(), xt::newaxis());
+  // Normalize chi so it sums to 1 over outgoing groups for each angle
+  for (size_t a = 0; a < n_ang; a++) {
+    tensor::View<double> row = temp_chi.slice(a);
+    row /= row.sum();
+  }
 
-  // Now every incoming group in self.chi is the normalized chi we just made
-  chi_prompt = xt::view(temp_chi, xt::all(), xt::newaxis(), xt::all());
+  // Replicate the energy spectrum across all incoming groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t gin = 0; gin < n_g_; gin++)
+      chi_prompt.slice(a, gin) = temp_chi.slice(a);
 
   // Get nu-fission directly
-  read_nd_vector(xsdata_grp, "nu-fission", prompt_nu_fission, true);
+  read_nd_tensor(xsdata_grp, "nu-fission", prompt_nu_fission, true);
 }
 
 //==============================================================================
@@ -231,8 +273,9 @@ void XsData::fission_matrix_beta_from_hdf5(
   // Data is provided as nu-fission and chi with a beta for delayed info
 
   // Get nu-fission matrix
-  xt::xtensor<double, 3> temp_matrix({n_ang, n_g_, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "nu-fission", temp_matrix, true);
+  tensor::Tensor<double> temp_matrix =
+    tensor::zeros<double>({n_ang, n_g_, n_g_});
+  read_nd_tensor(xsdata_grp, "nu-fission", temp_matrix, true);
 
   // Get beta (strategy will depend upon the number of dimensions in beta)
   hid_t beta_dset = open_dataset(xsdata_grp, "beta");
@@ -242,65 +285,92 @@ void XsData::fission_matrix_beta_from_hdf5(
   if (!is_isotropic)
     ndim_target += 2;
   if (beta_ndims == ndim_target) {
-    xt::xtensor<double, 2> temp_beta({n_ang, n_dg_}, 0.);
-    read_nd_vector(xsdata_grp, "beta", temp_beta, true);
+    tensor::Tensor<double> temp_beta = tensor::zeros<double>({n_ang, n_dg_});
+    read_nd_tensor(xsdata_grp, "beta", temp_beta, true);
 
-    xt::xtensor<double, 1> temp_beta_sum({n_ang}, 0.);
-    temp_beta_sum = xt::sum(temp_beta, {1});
+    auto beta_sum = temp_beta.sum(1);
+    auto matrix_gout_sum = temp_matrix.sum(2);
 
-    // prompt_nu_fission is the sum of this matrix over outgoing groups and
-    // multiplied by (1 - beta_sum)
-    prompt_nu_fission = xt::sum(temp_matrix, {2}) * (1. - temp_beta_sum);
+    // prompt_nu_fission = sum_gout(matrix) * (1 - beta_total)
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t g = 0; g < n_g_; g++)
+        prompt_nu_fission(a, g) = matrix_gout_sum(a, g) * (1.0 - beta_sum(a));
 
-    // Store chi-prompt
-    chi_prompt =
-      xt::view(1.0 - temp_beta_sum, xt::all(), xt::newaxis(), xt::newaxis()) *
-      temp_matrix;
+    // chi_prompt = (1 - beta_total) * nu-fission matrix (unnormalized)
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t gin = 0; gin < n_g_; gin++)
+        for (size_t gout = 0; gout < n_g_; gout++)
+          chi_prompt(a, gin, gout) =
+            (1.0 - beta_sum(a)) * temp_matrix(a, gin, gout);
 
-    // delayed_nu_fission is the sum of this matrix over outgoing groups and
-    // multiplied by beta
-    delayed_nu_fission =
-      xt::view(temp_beta, xt::all(), xt::all(), xt::newaxis()) *
-      xt::view(xt::sum(temp_matrix, {2}), xt::all(), xt::newaxis(), xt::all());
+    // Delayed nu-fission is the outer product of the delayed neutron
+    // fraction (beta) and the total fission rate summed over outgoing groups
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg_; d++)
+        for (size_t g = 0; g < n_g_; g++)
+          delayed_nu_fission(a, d, g) = temp_beta(a, d) * matrix_gout_sum(a, g);
 
-    // Store chi-delayed
-    chi_delayed =
-      xt::view(temp_beta, xt::all(), xt::all(), xt::newaxis(), xt::newaxis()) *
-      xt::view(temp_matrix, xt::all(), xt::newaxis(), xt::all(), xt::all());
+    // chi_delayed = beta * nu-fission matrix, expanded across delayed groups
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg_; d++)
+        for (size_t gin = 0; gin < n_g_; gin++)
+          for (size_t gout = 0; gout < n_g_; gout++)
+            chi_delayed(a, d, gin, gout) =
+              temp_beta(a, d) * temp_matrix(a, gin, gout);
 
   } else if (beta_ndims == ndim_target + 1) {
-    xt::xtensor<double, 3> temp_beta({n_ang, n_dg_, n_g_}, 0.);
-    read_nd_vector(xsdata_grp, "beta", temp_beta, true);
+    tensor::Tensor<double> temp_beta =
+      tensor::zeros<double>({n_ang, n_dg_, n_g_});
+    read_nd_tensor(xsdata_grp, "beta", temp_beta, true);
 
-    xt::xtensor<double, 2> temp_beta_sum({n_ang, n_g_}, 0.);
-    temp_beta_sum = xt::sum(temp_beta, {1});
+    auto beta_sum = temp_beta.sum(1);
+    auto matrix_gout_sum = temp_matrix.sum(2);
 
-    // prompt_nu_fission is the sum of this matrix over outgoing groups and
-    // multiplied by (1 - beta_sum)
-    prompt_nu_fission = xt::sum(temp_matrix, {2}) * (1. - temp_beta_sum);
+    // prompt_nu_fission = sum_gout(matrix) * (1 - beta_total)
+    // Here beta is energy-dependent, so beta_sum is 2D [n_ang, n_g]
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t g = 0; g < n_g_; g++)
+        prompt_nu_fission(a, g) =
+          matrix_gout_sum(a, g) * (1.0 - beta_sum(a, g));
 
-    // Store chi-prompt
-    chi_prompt =
-      xt::view(1.0 - temp_beta_sum, xt::all(), xt::all(), xt::newaxis()) *
-      temp_matrix;
+    // chi_prompt = (1 - beta_sum) * nu-fission matrix (unnormalized)
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t gin = 0; gin < n_g_; gin++)
+        for (size_t gout = 0; gout < n_g_; gout++)
+          chi_prompt(a, gin, gout) =
+            (1.0 - beta_sum(a, gin)) * temp_matrix(a, gin, gout);
 
-    // delayed_nu_fission is the sum of this matrix over outgoing groups and
-    // multiplied by beta
-    delayed_nu_fission = temp_beta * xt::view(xt::sum(temp_matrix, {2}),
-                                       xt::all(), xt::newaxis(), xt::all());
+    // Delayed nu-fission: beta is energy-dependent [n_ang, n_dg, n_g],
+    // scale by total fission rate summed over outgoing groups
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg_; d++)
+        for (size_t g = 0; g < n_g_; g++)
+          delayed_nu_fission(a, d, g) =
+            temp_beta(a, d, g) * matrix_gout_sum(a, g);
 
-    // Store chi-delayed
-    chi_delayed =
-      xt::view(temp_beta, xt::all(), xt::all(), xt::all(), xt::newaxis()) *
-      xt::view(temp_matrix, xt::all(), xt::newaxis(), xt::all(), xt::all());
+    // chi_delayed = beta * nu-fission matrix, expanded across delayed groups
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg_; d++)
+        for (size_t gin = 0; gin < n_g_; gin++)
+          for (size_t gout = 0; gout < n_g_; gout++)
+            chi_delayed(a, d, gin, gout) =
+              temp_beta(a, d, gin) * temp_matrix(a, gin, gout);
   }
 
-  // Normalize both chis
-  chi_prompt /=
-    xt::view(xt::sum(chi_prompt, {2}), xt::all(), xt::all(), xt::newaxis());
+  // Normalize chi_prompt so it sums to 1 over outgoing groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t gin = 0; gin < n_g_; gin++) {
+      tensor::View<double> row = chi_prompt.slice(a, gin);
+      row /= row.sum();
+    }
 
-  chi_delayed /= xt::view(
-    xt::sum(chi_delayed, {3}), xt::all(), xt::all(), xt::all(), xt::newaxis());
+  // Normalize chi_delayed so it sums to 1 over outgoing groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t d = 0; d < n_dg_; d++)
+      for (size_t gin = 0; gin < n_g_; gin++) {
+        tensor::View<double> row = chi_delayed.slice(a, d, gin);
+        row /= row.sum();
+      }
 }
 
 void XsData::fission_matrix_no_beta_from_hdf5(hid_t xsdata_grp, size_t n_ang)
@@ -308,28 +378,36 @@ void XsData::fission_matrix_no_beta_from_hdf5(hid_t xsdata_grp, size_t n_ang)
   // Data is provided separately as prompt + delayed nu-fission and chi
 
   // Get the prompt nu-fission matrix
-  xt::xtensor<double, 3> temp_matrix_p({n_ang, n_g_, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "prompt-nu-fission", temp_matrix_p, true);
+  tensor::Tensor<double> temp_matrix_p =
+    tensor::zeros<double>({n_ang, n_g_, n_g_});
+  read_nd_tensor(xsdata_grp, "prompt-nu-fission", temp_matrix_p, true);
 
   // prompt_nu_fission is the sum over outgoing groups
-  prompt_nu_fission = xt::sum(temp_matrix_p, {2});
+  prompt_nu_fission = temp_matrix_p.sum(2);
 
-  // chi_prompt is this matrix but normalized over outgoing groups, which we
-  // have already stored in prompt_nu_fission
-  chi_prompt = temp_matrix_p /
-               xt::view(prompt_nu_fission, xt::all(), xt::all(), xt::newaxis());
+  // chi_prompt is the nu-fission matrix normalized over outgoing groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t gin = 0; gin < n_g_; gin++)
+      for (size_t gout = 0; gout < n_g_; gout++)
+        chi_prompt(a, gin, gout) =
+          temp_matrix_p(a, gin, gout) / prompt_nu_fission(a, gin);
 
   // Get the delayed nu-fission matrix
-  xt::xtensor<double, 4> temp_matrix_d({n_ang, n_dg_, n_g_, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "delayed-nu-fission", temp_matrix_d, true);
+  tensor::Tensor<double> temp_matrix_d =
+    tensor::zeros<double>({n_ang, n_dg_, n_g_, n_g_});
+  read_nd_tensor(xsdata_grp, "delayed-nu-fission", temp_matrix_d, true);
 
   // delayed_nu_fission is the sum over outgoing groups
-  delayed_nu_fission = xt::sum(temp_matrix_d, {3});
+  delayed_nu_fission = temp_matrix_d.sum(3);
 
-  // chi_prompt is this matrix but normalized over outgoing groups, which we
-  // have already stored in prompt_nu_fission
-  chi_delayed = temp_matrix_d / xt::view(delayed_nu_fission, xt::all(),
-                                  xt::all(), xt::all(), xt::newaxis());
+  // chi_delayed is the delayed nu-fission matrix normalized over outgoing
+  // groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t d = 0; d < n_dg_; d++)
+      for (size_t gin = 0; gin < n_g_; gin++)
+        for (size_t gout = 0; gout < n_g_; gout++)
+          chi_delayed(a, d, gin, gout) =
+            temp_matrix_d(a, d, gin, gout) / delayed_nu_fission(a, d, gin);
 }
 
 void XsData::fission_matrix_no_delayed_from_hdf5(hid_t xsdata_grp, size_t n_ang)
@@ -338,16 +416,19 @@ void XsData::fission_matrix_no_delayed_from_hdf5(hid_t xsdata_grp, size_t n_ang)
   // Therefore, the code only considers the data as prompt.
 
   // Get nu-fission matrix
-  xt::xtensor<double, 3> temp_matrix({n_ang, n_g_, n_g_}, 0.);
-  read_nd_vector(xsdata_grp, "nu-fission", temp_matrix, true);
+  tensor::Tensor<double> temp_matrix =
+    tensor::zeros<double>({n_ang, n_g_, n_g_});
+  read_nd_tensor(xsdata_grp, "nu-fission", temp_matrix, true);
 
   // prompt_nu_fission is the sum over outgoing groups
-  prompt_nu_fission = xt::sum(temp_matrix, {2});
+  prompt_nu_fission = temp_matrix.sum(2);
 
-  // chi_prompt is this matrix but normalized over outgoing groups, which we
-  // have already stored in prompt_nu_fission
-  chi_prompt = temp_matrix /
-               xt::view(prompt_nu_fission, xt::all(), xt::all(), xt::newaxis());
+  // chi_prompt is the nu-fission matrix normalized over outgoing groups
+  for (size_t a = 0; a < n_ang; a++)
+    for (size_t gin = 0; gin < n_g_; gin++)
+      for (size_t gout = 0; gout < n_g_; gout++)
+        chi_prompt(a, gin, gout) =
+          temp_matrix(a, gin, gout) / prompt_nu_fission(a, gin);
 }
 
 //==============================================================================
@@ -356,8 +437,8 @@ void XsData::fission_from_hdf5(
   hid_t xsdata_grp, size_t n_ang, bool is_isotropic)
 {
   // Get the fission and kappa_fission data xs; these are optional
-  read_nd_vector(xsdata_grp, "fission", fission);
-  read_nd_vector(xsdata_grp, "kappa-fission", kappa_fission);
+  read_nd_tensor(xsdata_grp, "fission", fission);
+  read_nd_tensor(xsdata_grp, "kappa-fission", kappa_fission);
 
   // Get the data; the strategy for doing so depends on if the data is provided
   // as a nu-fission matrix or a set of chi and nu-fission vectors
@@ -388,7 +469,7 @@ void XsData::fission_from_hdf5(
   if (n_dg_ == 0) {
     nu_fission = prompt_nu_fission;
   } else {
-    nu_fission = prompt_nu_fission + xt::sum(delayed_nu_fission, {1});
+    nu_fission = prompt_nu_fission + delayed_nu_fission.sum(1);
   }
 }
 
@@ -404,10 +485,10 @@ void XsData::scatter_from_hdf5(hid_t xsdata_grp, size_t n_ang,
   hid_t scatt_grp = open_group(xsdata_grp, "scatter_data");
 
   // Get the outgoing group boundary indices
-  xt::xtensor<int, 2> gmin({n_ang, n_g_}, 0.);
-  read_nd_vector(scatt_grp, "g_min", gmin, true);
-  xt::xtensor<int, 2> gmax({n_ang, n_g_}, 0.);
-  read_nd_vector(scatt_grp, "g_max", gmax, true);
+  tensor::Tensor<int> gmin = tensor::zeros<int>({n_ang, n_g_});
+  read_nd_tensor(scatt_grp, "g_min", gmin, true);
+  tensor::Tensor<int> gmax = tensor::zeros<int>({n_ang, n_g_});
+  read_nd_tensor(scatt_grp, "g_max", gmax, true);
 
   // Make gmin and gmax start from 0 vice 1 as they do in the library
   gmin -= 1;
@@ -415,11 +496,11 @@ void XsData::scatter_from_hdf5(hid_t xsdata_grp, size_t n_ang,
 
   // Now use this info to find the length of a vector to hold the flattened
   // data.
-  size_t length = order_data * xt::sum(gmax - gmin + 1)();
+  size_t length = order_data * (gmax - gmin + 1).sum();
 
   double_4dvec input_scatt(n_ang, double_3dvec(n_g_));
-  xt::xtensor<double, 1> temp_arr({length}, 0.);
-  read_nd_vector(scatt_grp, "scatter_matrix", temp_arr, true);
+  tensor::Tensor<double> temp_arr = tensor::zeros<double>({length});
+  read_nd_tensor(scatt_grp, "scatter_matrix", temp_arr, true);
 
   // Compare the number of orders given with the max order of the problem;
   // strip off the superfluous orders if needed
@@ -451,7 +532,7 @@ void XsData::scatter_from_hdf5(hid_t xsdata_grp, size_t n_ang,
   double_3dvec temp_mult(n_ang, double_2dvec(n_g_));
   if (object_exists(scatt_grp, "multiplicity_matrix")) {
     temp_arr.resize({length / order_data});
-    read_nd_vector(scatt_grp, "multiplicity_matrix", temp_arr);
+    read_nd_tensor(scatt_grp, "multiplicity_matrix", temp_arr);
 
     // convert the flat temp_arr to a jagged array for passing to scatt data
     size_t temp_idx = 0;
@@ -481,8 +562,8 @@ void XsData::scatter_from_hdf5(hid_t xsdata_grp, size_t n_ang,
       final_scatter_format == AngleDistributionType::TABULAR) {
     for (size_t a = 0; a < n_ang; a++) {
       ScattDataLegendre legendre_scatt;
-      xt::xtensor<int, 1> in_gmin = xt::view(gmin, a, xt::all());
-      xt::xtensor<int, 1> in_gmax = xt::view(gmax, a, xt::all());
+      tensor::Tensor<int> in_gmin(gmin.slice(a));
+      tensor::Tensor<int> in_gmax(gmax.slice(a));
 
       legendre_scatt.init(in_gmin, in_gmax, temp_mult[a], input_scatt[a]);
 
@@ -496,8 +577,8 @@ void XsData::scatter_from_hdf5(hid_t xsdata_grp, size_t n_ang,
     // We are sticking with the current representation
     // Initialize the ScattData object with this data
     for (size_t a = 0; a < n_ang; a++) {
-      xt::xtensor<int, 1> in_gmin = xt::view(gmin, a, xt::all());
-      xt::xtensor<int, 1> in_gmax = xt::view(gmax, a, xt::all());
+      tensor::Tensor<int> in_gmin(gmin.slice(a));
+      tensor::Tensor<int> in_gmax(gmax.slice(a));
       scatter[a]->init(in_gmin, in_gmax, temp_mult[a], input_scatt[a]);
     }
   }
@@ -519,33 +600,67 @@ void XsData::combine(
     if (i == 0) {
       inverse_velocity = that->inverse_velocity;
     }
-    if (that->prompt_nu_fission.shape()[0] > 0) {
+    if (!that->prompt_nu_fission.empty()) {
       nu_fission += scalar * that->nu_fission;
       prompt_nu_fission += scalar * that->prompt_nu_fission;
       kappa_fission += scalar * that->kappa_fission;
       fission += scalar * that->fission;
       delayed_nu_fission += scalar * that->delayed_nu_fission;
-      chi_prompt += scalar *
-                    xt::view(xt::sum(that->prompt_nu_fission, {1}), xt::all(),
-                      xt::newaxis(), xt::newaxis()) *
-                    that->chi_prompt;
-      chi_delayed += scalar *
-                     xt::view(xt::sum(that->delayed_nu_fission, {2}), xt::all(),
-                       xt::all(), xt::newaxis(), xt::newaxis()) *
-                     that->chi_delayed;
+      // Accumulate chi_prompt weighted by total prompt nu-fission
+      // (summed over energy groups) for this constituent
+      {
+        auto pnf_sum = that->prompt_nu_fission.sum(1);
+        size_t n_ang = chi_prompt.shape(0);
+        size_t n_g = chi_prompt.shape(1);
+        for (size_t a = 0; a < n_ang; a++)
+          for (size_t gin = 0; gin < n_g; gin++)
+            for (size_t gout = 0; gout < n_g; gout++)
+              chi_prompt(a, gin, gout) +=
+                scalar * pnf_sum(a) * that->chi_prompt(a, gin, gout);
+      }
+      // Accumulate chi_delayed weighted by total delayed nu-fission
+      // (summed over energy groups) for this constituent
+      {
+        auto dnf_sum = that->delayed_nu_fission.sum(2);
+        size_t n_ang = chi_delayed.shape(0);
+        size_t n_dg = chi_delayed.shape(1);
+        size_t n_g = chi_delayed.shape(2);
+        for (size_t a = 0; a < n_ang; a++)
+          for (size_t d = 0; d < n_dg; d++)
+            for (size_t gin = 0; gin < n_g; gin++)
+              for (size_t gout = 0; gout < n_g; gout++)
+                chi_delayed(a, d, gin, gout) +=
+                  scalar * dnf_sum(a, d) * that->chi_delayed(a, d, gin, gout);
+      }
     }
     decay_rate += scalar * that->decay_rate;
   }
 
-  // Ensure the chi_prompt and chi_delayed are normalized to 1 for each
-  // azimuthal angle and delayed group (for chi_delayed)
-  chi_prompt /=
-    xt::view(xt::sum(chi_prompt, {2}), xt::all(), xt::all(), xt::newaxis());
-  chi_delayed /= xt::view(
-    xt::sum(chi_delayed, {3}), xt::all(), xt::all(), xt::all(), xt::newaxis());
+  // Normalize chi_prompt so it sums to 1 over outgoing groups
+  {
+    size_t n_ang = chi_prompt.shape(0);
+    size_t n_g = chi_prompt.shape(1);
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t gin = 0; gin < n_g; gin++) {
+        tensor::View<double> row = chi_prompt.slice(a, gin);
+        row /= row.sum();
+      }
+  }
+  // Normalize chi_delayed so it sums to 1 over outgoing groups
+  {
+    size_t n_ang = chi_delayed.shape(0);
+    size_t n_dg = chi_delayed.shape(1);
+    size_t n_g = chi_delayed.shape(2);
+    for (size_t a = 0; a < n_ang; a++)
+      for (size_t d = 0; d < n_dg; d++)
+        for (size_t gin = 0; gin < n_g; gin++) {
+          tensor::View<double> row = chi_delayed.slice(a, d, gin);
+          row /= row.sum();
+        }
+  }
 
   // Allow the ScattData object to combine itself
-  for (size_t a = 0; a < total.shape()[0]; a++) {
+  for (size_t a = 0; a < total.shape(0); a++) {
     // Build vector of the scattering objects to incorporate
     vector<ScattData*> those_scatts(those_xs.size());
     for (size_t i = 0; i < those_xs.size(); i++) {
diff --git a/tests/conftest.py b/tests/conftest.py
index cd86da539..8dda9f756 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,8 +1,51 @@
+import os
+import hashlib
 import pytest
 import openmc
+import openmc.lib
 
 from tests.regression_tests import config as regression_config
 
+# MD5 hash of the official NNDC HDF5 cross_sections.xml file.
+# Generated via: md5sum /path/to/nndc_hdf5/cross_sections.xml
+_NNDC_XS_MD5 = "2d00773012eda670bc9f95d96a31c989"
+
+# Collected during pytest_configure, displayed at start and end of session
+_environment_warnings = []
+
+
+def _check_build_environment():
+    """Check STRICT_FP and cross section data, collecting any warnings."""
+    if not openmc.lib._strict_fp_enabled():
+        _environment_warnings.append(
+            "OpenMC was NOT built with -DOPENMC_ENABLE_STRICT_FP=on. "
+            "Regression test results may not match reference values due to "
+            "compiler floating-point optimizations. Rebuild with "
+            "-DOPENMC_ENABLE_STRICT_FP=on for reproducible results."
+        )
+
+    xs_path = os.environ.get("OPENMC_CROSS_SECTIONS")
+    if not xs_path:
+        _environment_warnings.append(
+            "OPENMC_CROSS_SECTIONS environment variable is not set. "
+            "Regression tests require the NNDC HDF5 cross section data."
+        )
+    elif not os.path.isfile(xs_path):
+        _environment_warnings.append(
+            f"OPENMC_CROSS_SECTIONS ({xs_path}) is not a valid file path. "
+            "Regression tests require the NNDC HDF5 cross section data."
+        )
+    else:
+        with open(xs_path, "rb") as f:
+            md5 = hashlib.md5(f.read()).hexdigest()
+        if md5 != _NNDC_XS_MD5:
+            _environment_warnings.append(
+                f"OPENMC_CROSS_SECTIONS ({xs_path}) does not match the "
+                "official NNDC HDF5 dataset. Regression tests expect the "
+                "NNDC data; results may differ with other cross section "
+                "libraries."
+            )
+
 
 def pytest_addoption(parser):
     parser.addoption('--exe')
@@ -20,6 +63,28 @@ def pytest_configure(config):
         if config.getoption(opt) is not None:
             regression_config[opt] = config.getoption(opt)
 
+    _check_build_environment()
+
+
+def _print_environment_warnings(terminalreporter):
+    """Print environment warnings as a visible section."""
+    if _environment_warnings:
+        terminalreporter.section("OpenMC Environment Warnings")
+        for msg in _environment_warnings:
+            terminalreporter.line(f"WARNING: {msg}", yellow=True)
+        terminalreporter.line("")
+
+
+def pytest_sessionstart(session):
+    """Print environment warnings at the start of the test session."""
+    _print_environment_warnings(session.config.pluginmanager.get_plugin(
+        "terminalreporter"))
+
+
+def pytest_terminal_summary(terminalreporter, exitstatus, config):
+    """Reprint environment warnings at the end so they aren't missed."""
+    _print_environment_warnings(terminalreporter)
+
 
 @pytest.fixture
 def run_in_tmpdir(tmpdir):
@@ -29,8 +94,28 @@ def run_in_tmpdir(tmpdir):
     finally:
         orig.chdir()
 
+@pytest.fixture(scope="module")
+def endf_data():
+    return os.environ['OPENMC_ENDF_DATA']
 
 @pytest.fixture(scope='session', autouse=True)
 def resolve_paths():
     with openmc.config.patch('resolve_paths', False):
         yield
+
+
+@pytest.fixture(scope='session', autouse=True)
+def disable_depletion_multiprocessing_under_mpi():
+    """Fork-based depletion multiprocessing may deadlock if MPI is active."""
+    if not regression_config['mpi']:
+        yield
+        return
+
+    from openmc.deplete import pool
+
+    original_setting = pool.USE_MULTIPROCESSING
+    pool.USE_MULTIPROCESSING = False
+    try:
+        yield
+    finally:
+        pool.USE_MULTIPROCESSING = original_setting
diff --git a/tests/cpp_unit_tests/CMakeLists.txt b/tests/cpp_unit_tests/CMakeLists.txt
index f0f5f2853..3eb88f023 100644
--- a/tests/cpp_unit_tests/CMakeLists.txt
+++ b/tests/cpp_unit_tests/CMakeLists.txt
@@ -4,6 +4,11 @@ set(TEST_NAMES
   test_tally
   test_interpolate
   test_math
+  test_mcpl_stat_sum
+  test_mesh
+  test_ray
+  test_region
+  test_tensor
   # Add additional unit test files here
 )
 
diff --git a/tests/cpp_unit_tests/test_distribution.cpp b/tests/cpp_unit_tests/test_distribution.cpp
index e1a212db5..479e7c8a7 100644
--- a/tests/cpp_unit_tests/test_distribution.cpp
+++ b/tests/cpp_unit_tests/test_distribution.cpp
@@ -1,4 +1,6 @@
 #include "openmc/distribution.h"
+#include "openmc/distribution_spatial.h"
+#include "openmc/position.h"
 #include "openmc/random_lcg.h"
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/matchers/catch_matchers_floating_point.hpp>
@@ -28,7 +30,7 @@ TEST_CASE("Test alias method sampling of a discrete distribution")
   int counter = 0;
 
   for (size_t i = 0; i < n_samples; i++) {
-    auto sample = dist.sample(&seed);
+    auto sample = dist.sample(&seed).first;
     std += sample * sample / n_samples;
     dist_mean += sample;
 
@@ -61,7 +63,7 @@ TEST_CASE("Test alias sampling method for pugixml constructor")
   // Initialize discrete distribution and seed
   openmc::Discrete dist(energy);
   uint64_t seed = openmc::init_seed(0, 0);
-  auto sample = dist.sample(&seed);
+  auto sample = dist.sample(&seed).first;
 
   // Assertions
   REQUIRE(dist.x().size() == 3);
@@ -79,3 +81,139 @@ TEST_CASE("Test alias sampling method for pugixml constructor")
     REQUIRE(dist.alias()[i] == correct_alias[i]);
   }
 }
+
+TEST_CASE("Test sampling a large linear-linear tabular distribution")
+{
+  constexpr int n_points = 10001;
+  constexpr int n_samples = 200000;
+  openmc::vector<double> x(n_points);
+  openmc::vector<double> p(n_points);
+  for (int i = 0; i < n_points; ++i) {
+    x[i] = static_cast<double>(i) / (n_points - 1);
+    p[i] = 2.0 * x[i];
+  }
+
+  openmc::Tabular dist(
+    x.data(), p.data(), n_points, openmc::Interpolation::lin_lin);
+  uint64_t seed = openmc::init_seed(0, 0);
+
+  double mean = 0.0;
+  for (int i = 0; i < n_samples; ++i) {
+    mean += dist.sample(&seed).first;
+  }
+  mean /= n_samples;
+
+  // The normalized PDF is 2x on [0, 1], which has a mean of 2/3.
+  REQUIRE_THAT(mean, Catch::Matchers::WithinAbs(2.0 / 3.0, 0.003));
+}
+
+TEST_CASE("Test construction of SpatialBox with parameters")
+{
+  openmc::Position ll {-1, -2, -3};
+  openmc::Position ur {30, 15, 5};
+  openmc::SpatialBox box(ll, ur);
+
+  REQUIRE(box.lower_left() == openmc::Position {-1, -2, -3});
+  REQUIRE(box.upper_right() == openmc::Position {30, 15, 5});
+  REQUIRE_FALSE(box.only_fissionable());
+}
+
+TEST_CASE("Test Normal distribution")
+{
+  // Test untruncated normal distribution
+  openmc::Normal normal_unbounded(0.0, 1.0);
+
+  // Check PDF at mean (should be 1/sqrt(2*pi) ≈ 0.3989)
+  REQUIRE_THAT(
+    normal_unbounded.evaluate(0.0), Catch::Matchers::WithinRel(0.3989, 0.001));
+
+  // Check that it's not truncated
+  REQUIRE_FALSE(normal_unbounded.is_truncated());
+
+  // Check accessors
+  REQUIRE(normal_unbounded.mean_value() == 0.0);
+  REQUIRE(normal_unbounded.std_dev() == 1.0);
+  REQUIRE(normal_unbounded.lower() == -openmc::INFTY);
+  REQUIRE(normal_unbounded.upper() == openmc::INFTY);
+}
+
+TEST_CASE("Test truncated Normal distribution")
+{
+  // Create a truncated normal: mean=0, std=1, bounds=[-1, 1]
+  openmc::Normal normal_truncated(0.0, 1.0, -1.0, 1.0);
+
+  // Check that it's truncated
+  REQUIRE(normal_truncated.is_truncated());
+
+  // Check accessors
+  REQUIRE(normal_truncated.lower() == -1.0);
+  REQUIRE(normal_truncated.upper() == 1.0);
+
+  // PDF should be zero outside bounds
+  REQUIRE(normal_truncated.evaluate(-2.0) == 0.0);
+  REQUIRE(normal_truncated.evaluate(2.0) == 0.0);
+
+  // PDF inside bounds should be higher than untruncated (due to
+  // renormalization)
+  openmc::Normal normal_unbounded(0.0, 1.0);
+  REQUIRE(normal_truncated.evaluate(0.0) > normal_unbounded.evaluate(0.0));
+
+  // The truncated PDF at mean should be approximately 0.3989 / 0.6827 ≈ 0.584
+  // (0.6827 is the probability mass of N(0,1) in [-1,1])
+  REQUIRE_THAT(
+    normal_truncated.evaluate(0.0), Catch::Matchers::WithinRel(0.584, 0.01));
+}
+
+TEST_CASE("Test truncated Normal sampling")
+{
+  constexpr int n_samples = 10000;
+  openmc::Normal normal_truncated(0.0, 1.0, -1.0, 1.0);
+  uint64_t seed = openmc::init_seed(0, 0);
+
+  // Sample and verify all samples are within bounds
+  for (int i = 0; i < n_samples; ++i) {
+    auto [x, w] = normal_truncated.sample(&seed);
+    REQUIRE(x >= -1.0);
+    REQUIRE(x <= 1.0);
+    REQUIRE(w == 1.0); // Unbiased sampling should have weight 1
+  }
+}
+
+TEST_CASE("Test one-sided truncated Normal")
+{
+  // Test lower-bounded only (positive half-normal)
+  openmc::Normal lower_bounded(0.0, 1.0, 0.0, openmc::INFTY);
+  REQUIRE(lower_bounded.is_truncated());
+  REQUIRE(lower_bounded.evaluate(-1.0) == 0.0);
+  REQUIRE(lower_bounded.evaluate(1.0) > 0.0);
+
+  // PDF at 0 should be approximately 2 * 0.3989 ≈ 0.798 (half-normal)
+  REQUIRE_THAT(
+    lower_bounded.evaluate(0.0), Catch::Matchers::WithinRel(0.798, 0.01));
+
+  // Test upper-bounded only
+  openmc::Normal upper_bounded(0.0, 1.0, -openmc::INFTY, 0.0);
+  REQUIRE(upper_bounded.is_truncated());
+  REQUIRE(upper_bounded.evaluate(1.0) == 0.0);
+  REQUIRE(upper_bounded.evaluate(-1.0) > 0.0);
+}
+
+TEST_CASE("Test Normal XML constructor with truncation")
+{
+  // XML doc node for truncated Normal
+  pugi::xml_document doc;
+  pugi::xml_node energy = doc.append_child("energy");
+  energy.append_child("type")
+    .append_child(pugi::node_pcdata)
+    .set_value("normal");
+  energy.append_child("parameters")
+    .append_child(pugi::node_pcdata)
+    .set_value("1.0e6 1.0e5 0.8e6 1.2e6");
+
+  openmc::Normal dist(energy);
+  REQUIRE(dist.mean_value() == 1.0e6);
+  REQUIRE(dist.std_dev() == 1.0e5);
+  REQUIRE(dist.lower() == 0.8e6);
+  REQUIRE(dist.upper() == 1.2e6);
+  REQUIRE(dist.is_truncated());
+}
diff --git a/tests/cpp_unit_tests/test_math.cpp b/tests/cpp_unit_tests/test_math.cpp
index 1ad7c4b70..7467aa1fe 100644
--- a/tests/cpp_unit_tests/test_math.cpp
+++ b/tests/cpp_unit_tests/test_math.cpp
@@ -46,6 +46,21 @@ TEST_CASE("Test t_percentile")
   }
 }
 
+TEST_CASE("Test cylindrical Bessel functions")
+{
+  constexpr double x = 2.0;
+  constexpr double expected[] {0.22389077914123567, 0.57672480775687339,
+    0.35283402861563773, 0.12894324947440205};
+
+  for (int n = 0; n < 4; ++n) {
+    REQUIRE_THAT(openmc::cyl_bessel_j(n, x),
+      Catch::Matchers::WithinRel(expected[n], 1.0e-14));
+    REQUIRE_THAT(openmc::cyl_bessel_j(n, -x),
+      Catch::Matchers::WithinRel(
+        (n % 2 == 0 ? 1.0 : -1.0) * expected[n], 1.0e-14));
+  }
+}
+
 TEST_CASE("Test calc_pn")
 {
   // The reference solutions come from scipy.special.eval_legendre
@@ -355,3 +370,24 @@ TEST_CASE("Test broaden_wmp_polynomials")
     REQUIRE_THAT(ref_val, Catch::Matchers::Approx(test_val));
   }
 }
+
+TEST_CASE("Test isclose")
+{
+  using openmc::isclose;
+
+  // Identical values are always close, regardless of tolerances.
+  REQUIRE(isclose(1.0, 1.0, 0.0, 0.0));
+  REQUIRE(isclose(0.0, 0.0, 0.0, 0.0));
+
+  // Absolute tolerance governs comparisons near zero.
+  REQUIRE(isclose(0.0, 1e-15, 0.0, 1e-14));
+  REQUIRE_FALSE(isclose(0.0, 1e-13, 0.0, 1e-14));
+
+  // Relative tolerance scales with the magnitude of the operands.
+  REQUIRE(isclose(1.0e12, 1.0e12 + 1.0, 1e-12, 0.0));
+  REQUIRE_FALSE(isclose(1.0e12, 1.0e12 + 10.0, 1e-12, 0.0));
+
+  // The looser of the two tolerances wins.
+  REQUIRE(isclose(1.0, 1.0 + 1e-13, 0.0, 1e-12));
+  REQUIRE(isclose(1.0e6, 1.0e6 + 1e-4, 1e-9, 0.0));
+}
diff --git a/tests/cpp_unit_tests/test_mcpl_stat_sum.cpp b/tests/cpp_unit_tests/test_mcpl_stat_sum.cpp
new file mode 100644
index 000000000..d0c26f26c
--- /dev/null
+++ b/tests/cpp_unit_tests/test_mcpl_stat_sum.cpp
@@ -0,0 +1,108 @@
+#include <catch2/catch_test_macros.hpp>
+#include <cstdio>
+#include <string>
+#include <vector>
+
+#include "openmc/bank.h"
+#include "openmc/mcpl_interface.h"
+
+// Test the MCPL stat:sum functionality (issue #3514)
+TEST_CASE("MCPL stat:sum field")
+{
+  // Check if MCPL interface is available
+  if (!openmc::is_mcpl_interface_available()) {
+    SKIP("MCPL library not available");
+  }
+
+  SECTION("stat:sum field is written to MCPL files")
+  {
+    // Create a temporary filename
+    std::string filename = "test_stat_sum.mcpl";
+
+    // Create some test particles
+    std::vector<openmc::SourceSite> source_bank(100);
+    std::vector<int64_t> bank_index = {0, 100}; // 100 particles total
+
+    // Initialize test particles
+    for (int i = 0; i < 100; ++i) {
+      source_bank[i].particle = openmc::ParticleType::neutron();
+      source_bank[i].r = {i * 0.1, i * 0.2, i * 0.3};
+      source_bank[i].u = {0.0, 0.0, 1.0};
+      source_bank[i].E = 2.0e6; // 2 MeV
+      source_bank[i].time = 0.0;
+      source_bank[i].wgt = 1.0;
+    }
+
+    // Write the MCPL file
+    openmc::write_mcpl_source_point(filename.c_str(), source_bank, bank_index);
+
+    // Verify the file was created
+    FILE* f = std::fopen(filename.c_str(), "r");
+    REQUIRE(f != nullptr);
+    std::fclose(f);
+
+    // Read the file back to check stat:sum
+    // Note: This would require mcpl_open_file and checking the header
+    // Since we can't easily read MCPL headers in C++ without the full MCPL API,
+    // we rely on the Python test to verify the actual content
+
+    // Clean up
+    std::remove(filename.c_str());
+  }
+
+  SECTION("stat:sum uses correct particle count")
+  {
+    std::string filename = "test_count.mcpl";
+
+    // Test with different particle counts
+    std::vector<int> test_counts = {1, 10, 100, 1000};
+
+    for (int count : test_counts) {
+      std::vector<openmc::SourceSite> source_bank(count);
+      std::vector<int64_t> bank_index = {0, count};
+
+      // Initialize particles
+      for (int i = 0; i < count; ++i) {
+        source_bank[i].particle = openmc::ParticleType::neutron();
+        source_bank[i].r = {0.0, 0.0, 0.0};
+        source_bank[i].u = {0.0, 0.0, 1.0};
+        source_bank[i].E = 1.0e6;
+        source_bank[i].time = 0.0;
+        source_bank[i].wgt = 1.0;
+      }
+
+      // Write MCPL file
+      openmc::write_mcpl_source_point(
+        filename.c_str(), source_bank, bank_index);
+
+      // The stat:sum should equal count (verified by Python test)
+      // Here we just verify the file was created successfully
+      FILE* f = std::fopen(filename.c_str(), "r");
+      REQUIRE(f != nullptr);
+      std::fclose(f);
+
+      // Clean up
+      std::remove(filename.c_str());
+    }
+  }
+
+  SECTION("stat:sum handles empty particle bank")
+  {
+    std::string filename = "test_empty.mcpl";
+
+    // Create empty particle bank
+    std::vector<openmc::SourceSite> source_bank;
+    std::vector<int64_t> bank_index = {0};
+
+    // This should still create a valid MCPL file with stat:sum = 0
+    openmc::write_mcpl_source_point(filename.c_str(), source_bank, bank_index);
+
+    // Verify file was created
+    FILE* f = std::fopen(filename.c_str(), "r");
+    REQUIRE(f != nullptr);
+    std::fclose(f);
+
+    // Clean up
+    std::remove(filename.c_str());
+  }
+}
diff --git a/tests/cpp_unit_tests/test_mesh.cpp b/tests/cpp_unit_tests/test_mesh.cpp
new file mode 100644
index 000000000..24c4f7737
--- /dev/null
+++ b/tests/cpp_unit_tests/test_mesh.cpp
@@ -0,0 +1,257 @@
+#include <cstdio>
+#include <iostream>
+#include <string>
+
+#include <catch2/catch_test_macros.hpp>
+#include <pugixml.hpp>
+
+#include "openmc/hdf5_interface.h"
+#include "openmc/mesh.h"
+
+using namespace openmc;
+
+TEST_CASE("Test mesh hdf5 roundtrip - regular")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <mesh id="1">
+            <dimension>3 4 5</dimension>
+            <lower_left>-2 -3 -5</lower_left>
+            <upper_right>2 3 5</upper_right>
+       </mesh>
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("mesh");
+
+  auto mesh = RegularMesh(root);
+
+  hid_t file_id = file_open("mesh.h5", 'w');
+
+  mesh.to_hdf5(file_id);
+
+  file_close(file_id);
+
+  hid_t file_id2 = file_open("mesh.h5", 'r');
+
+  hid_t group = open_group(file_id2, "mesh 1");
+
+  auto mesh2 = RegularMesh(group);
+
+  file_close(file_id2);
+
+  remove("mesh.h5");
+
+  REQUIRE(mesh2.shape_ == mesh.shape_);
+
+  REQUIRE(mesh2.lower_left() == mesh.lower_left());
+
+  REQUIRE(mesh2.upper_right() == mesh.upper_right());
+}
+
+TEST_CASE("Test mesh hdf5 roundtrip - rectilinear")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <mesh id="1" type="rectilinear">
+            <x_grid>0.0 1.0 5.0 10.0</x_grid>
+            <y_grid>-10.0 -5.0 0.0</y_grid>
+            <z_grid>-100.0 0.0 100.0</z_grid>
+        </mesh>
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("mesh");
+
+  auto mesh = RectilinearMesh(root);
+
+  hid_t file_id = file_open("mesh.h5", 'w');
+
+  mesh.to_hdf5(file_id);
+
+  file_close(file_id);
+
+  hid_t file_id2 = file_open("mesh.h5", 'r');
+
+  hid_t group = open_group(file_id2, "mesh 1");
+
+  auto mesh2 = RectilinearMesh(group);
+
+  file_close(file_id2);
+
+  remove("mesh.h5");
+
+  REQUIRE(mesh2.shape_ == mesh.shape_);
+
+  REQUIRE(mesh2.grid_ == mesh.grid_);
+}
+
+TEST_CASE("Test mesh hdf5 roundtrip - cylindrical")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <mesh id="1" type="cylindrical">
+            <r_grid>0.1 0.2 0.5 1.0</r_grid>
+            <phi_grid>0.0 6.283185307179586</phi_grid>
+            <z_grid>0.1 0.2 0.4 0.6 1.0</z_grid>
+            <origin>0 0 0</origin>
+        </mesh>
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("mesh");
+
+  auto mesh = CylindricalMesh(root);
+
+  hid_t file_id = file_open("mesh.h5", 'w');
+
+  mesh.to_hdf5(file_id);
+
+  file_close(file_id);
+
+  hid_t file_id2 = file_open("mesh.h5", 'r');
+
+  hid_t group = open_group(file_id2, "mesh 1");
+
+  auto mesh2 = CylindricalMesh(group);
+
+  file_close(file_id2);
+
+  remove("mesh.h5");
+
+  REQUIRE(mesh2.shape_ == mesh.shape_);
+
+  REQUIRE(mesh2.grid_ == mesh.grid_);
+}
+
+TEST_CASE("Test mesh hdf5 roundtrip - spherical")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <mesh id="1" type="spherical">
+            <r_grid>0.1 0.2 0.5 1.0</r_grid>
+            <theta_grid>0.0 3.141592653589793</theta_grid>
+            <phi_grid>0.0 6.283185307179586</phi_grid>
+            <origin>0.0 0.0 0.0</origin>
+        </mesh>'
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("mesh");
+
+  auto mesh = SphericalMesh(root);
+
+  hid_t file_id = file_open("mesh.h5", 'w');
+
+  mesh.to_hdf5(file_id);
+
+  file_close(file_id);
+
+  hid_t file_id2 = file_open("mesh.h5", 'r');
+
+  hid_t group = open_group(file_id2, "mesh 1");
+
+  auto mesh2 = SphericalMesh(group);
+
+  file_close(file_id2);
+
+  remove("mesh.h5");
+
+  REQUIRE(mesh2.shape_ == mesh.shape_);
+
+  REQUIRE(mesh2.grid_ == mesh.grid_);
+}
+
+TEST_CASE("Test multiple meshes HDF5 roundtrip - spherical")
+{
+  // The XML data as a string
+  std::string xml_string = R"(
+        <meshes>
+        <mesh id="1" type="spherical">
+            <r_grid>0.1 0.2 0.5 1.0</r_grid>
+            <theta_grid>0.0 3.141592653589793</theta_grid>
+            <phi_grid>0.0 6.283185307179586</phi_grid>
+            <origin>0.0 0.0 0.0</origin>
+        </mesh>
+         <mesh id="2">
+            <dimension>3 4 5</dimension>
+            <lower_left>-2 -3 -5</lower_left>
+            <upper_right>2 3 5</upper_right>
+       </mesh>
+       </meshes>
+    )";
+
+  // Create a pugixml document object
+  pugi::xml_document doc;
+
+  // Load the XML from the string
+  pugi::xml_parse_result result = doc.load_string(xml_string.c_str());
+
+  pugi::xml_node root = doc.child("meshes");
+
+  read_meshes(root);
+
+  const auto spherical_mesh_xml =
+    dynamic_cast<SphericalMesh*>(model::meshes[0].get());
+  const auto regular_mesh_xml =
+    dynamic_cast<RegularMesh*>(model::meshes[1].get());
+
+  hid_t file_id = file_open("meshes.h5", 'w');
+
+  hid_t root_group = create_group(file_id, "root");
+
+  open_group(file_id, "root");
+
+  meshes_to_hdf5(root_group);
+
+  close_group(root_group);
+
+  file_close(file_id);
+
+  hid_t file_id2 = file_open("meshes.h5", 'r');
+
+  hid_t root_group_read = open_group(file_id2, "root");
+
+  hid_t mesh_group_read = open_group(root_group_read, "meshes");
+
+  read_meshes(mesh_group_read);
+
+  // increment mesh IDs to avoid collision during read
+  for (auto& mesh : model::meshes) {
+    mesh->set_id(mesh->id() + 10);
+  }
+
+  const auto spherical_mesh_hdf5 = dynamic_cast<SphericalMesh*>(
+    model::meshes[model::mesh_map[spherical_mesh_xml->id_]].get());
+  const auto regular_mesh_hdf5 = dynamic_cast<RegularMesh*>(
+    model::meshes[model::mesh_map[regular_mesh_xml->id_]].get());
+
+  remove("meshes.h5");
+
+  REQUIRE(spherical_mesh_hdf5->shape_ == spherical_mesh_xml->shape_);
+  REQUIRE(spherical_mesh_hdf5->grid_ == spherical_mesh_xml->grid_);
+
+  REQUIRE(regular_mesh_hdf5->shape_ == regular_mesh_xml->shape_);
+  REQUIRE(regular_mesh_hdf5->lower_left() == regular_mesh_xml->lower_left());
+  REQUIRE(regular_mesh_hdf5->upper_right() == regular_mesh_xml->upper_right());
+}
diff --git a/tests/cpp_unit_tests/test_ray.cpp b/tests/cpp_unit_tests/test_ray.cpp
new file mode 100644
index 000000000..c043c4d25
--- /dev/null
+++ b/tests/cpp_unit_tests/test_ray.cpp
@@ -0,0 +1,117 @@
+#include <cmath>
+#include <utility>
+
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+#include <pugixml.hpp>
+
+#include "openmc/geometry.h"
+#include "openmc/geometry_aux.h"
+#include "openmc/ray.h"
+#include "openmc/settings.h"
+#include "openmc/surface.h"
+
+namespace {
+
+using Catch::Matchers::WithinAbs;
+
+constexpr double DISTANCE_TOLERANCE = 1.0e-12;
+
+class SphericalShellFixture {
+public:
+  SphericalShellFixture()
+    : run_mode_(openmc::settings::run_mode),
+      root_universe_(openmc::model::root_universe),
+      n_coord_levels_(openmc::model::n_coord_levels)
+  {
+    openmc::settings::run_mode = openmc::RunMode::PLOTTING;
+
+    constexpr auto geometry = R"(
+      <geometry>
+        <surface id="1" type="sphere" coeffs="0 0 0 1"/>
+        <surface id="2" type="sphere" coeffs="0 0 0 2"
+                 boundary="vacuum"/>
+        <cell id="1" material="void" region="-1"/>
+        <cell id="2" material="void" region="1 -2"/>
+      </geometry>
+    )";
+
+    auto result = document_.load_string(geometry);
+    REQUIRE(result);
+    openmc::read_geometry_xml(document_.document_element());
+    openmc::finalize_geometry();
+    openmc::finalize_cell_densities();
+  }
+
+  ~SphericalShellFixture()
+  {
+    openmc::free_memory_geometry();
+    openmc::free_memory_surfaces();
+    openmc::model::universe_level_counts.clear();
+    openmc::model::root_universe = root_universe_;
+    openmc::model::n_coord_levels = n_coord_levels_;
+    openmc::settings::run_mode = run_mode_;
+  }
+
+private:
+  pugi::xml_document document_;
+  openmc::RunMode run_mode_;
+  int root_universe_;
+  int n_coord_levels_;
+};
+
+class RecordingRay : public openmc::Ray {
+public:
+  using Ray::Ray;
+
+  void on_intersection() override
+  {
+    intersections.emplace_back(
+      traversal_distance_, boundary().surface_index() + 1);
+  }
+
+  openmc::vector<std::pair<double, int>> intersections;
+};
+
+} // namespace
+
+TEST_CASE_METHOD(SphericalShellFixture, "Trace a single chord through a sphere")
+{
+  constexpr double impact_parameter = 1.5;
+  const double half_chord =
+    std::sqrt(4.0 - impact_parameter * impact_parameter);
+
+  RecordingRay ray({-3.0, impact_parameter, 0.0}, {1.0, 0.0, 0.0});
+  ray.trace();
+
+  REQUIRE(ray.intersections.size() == 2);
+  CHECK(ray.intersections[0].second == 2);
+  CHECK_THAT(ray.intersections[0].first, WithinAbs(0.0, DISTANCE_TOLERANCE));
+  CHECK(ray.intersections[1].second == 2);
+  CHECK_THAT(ray.intersections[1].first,
+    WithinAbs(2.0 * half_chord, DISTANCE_TOLERANCE));
+}
+
+TEST_CASE_METHOD(
+  SphericalShellFixture, "Trace both segments of a spherical shell")
+{
+  constexpr double impact_parameter = 0.5;
+  const double outer = std::sqrt(4.0 - impact_parameter * impact_parameter);
+  const double inner = std::sqrt(1.0 - impact_parameter * impact_parameter);
+
+  RecordingRay ray({-3.0, impact_parameter, 0.0}, {1.0, 0.0, 0.0});
+  ray.trace();
+
+  REQUIRE(ray.intersections.size() == 4);
+  CHECK(ray.intersections[0].second == 2);
+  CHECK_THAT(ray.intersections[0].first, WithinAbs(0.0, DISTANCE_TOLERANCE));
+  CHECK(ray.intersections[1].second == 1);
+  CHECK_THAT(
+    ray.intersections[1].first, WithinAbs(outer - inner, DISTANCE_TOLERANCE));
+  CHECK(ray.intersections[2].second == 1);
+  CHECK_THAT(
+    ray.intersections[2].first, WithinAbs(outer + inner, DISTANCE_TOLERANCE));
+  CHECK(ray.intersections[3].second == 2);
+  CHECK_THAT(
+    ray.intersections[3].first, WithinAbs(2.0 * outer, DISTANCE_TOLERANCE));
+}
diff --git a/tests/cpp_unit_tests/test_region.cpp b/tests/cpp_unit_tests/test_region.cpp
new file mode 100644
index 000000000..b3d9a1427
--- /dev/null
+++ b/tests/cpp_unit_tests/test_region.cpp
@@ -0,0 +1,101 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "openmc/cell.h"
+#include "openmc/surface.h"
+
+#include <pugixml.hpp>
+
+namespace {
+
+// Helper class to set up and tear down test surfaces
+class SurfaceFixture {
+public:
+  SurfaceFixture()
+  {
+    pugi::xml_document doc;
+    pugi::xml_node surf_node = doc.append_child("surface");
+    surf_node.set_name("surface");
+    surf_node.append_attribute("id") = "0";
+    surf_node.append_attribute("type") = "x-plane";
+    surf_node.append_attribute("coeffs") = "1";
+
+    for (int i = 1; i < 10; ++i) {
+      surf_node.attribute("id") = i;
+      openmc::model::surfaces.push_back(
+        std::make_unique<openmc::SurfaceXPlane>(surf_node));
+      openmc::model::surface_map[i] = i - 1;
+    }
+  }
+
+  ~SurfaceFixture()
+  {
+    openmc::model::surfaces.clear();
+    openmc::model::surface_map.clear();
+  }
+};
+
+} // anonymous namespace
+
+TEST_CASE("Test region simplification")
+{
+  SurfaceFixture fixture;
+
+  SECTION("Original bug case from issue #3685")
+  {
+    // Input: "-1 2 (-3 4) | (-5 6)" was being incorrectly interpreted
+    auto region = openmc::Region("(-1 2 (-3 4) | (-5 6))", 0);
+    REQUIRE(region.str() == " ( ( -1 2 ( -3 4 ) ) | ( -5 6 ) )");
+  }
+
+  SECTION("Simple union - no extra parentheses needed")
+  {
+    auto region = openmc::Region("1 | 2", 0);
+    REQUIRE(region.str() == " 1 | 2");
+  }
+
+  SECTION("Intersection then union")
+  {
+    // Intersection should have higher precedence, so (1 2) grouped
+    auto region = openmc::Region("1 2 | 3", 0);
+    REQUIRE(region.str() == " ( 1 2 ) | 3");
+  }
+
+  SECTION("Union then intersection")
+  {
+    // The (2 3) intersection should be grouped
+    auto region = openmc::Region("1 | 2 3", 0);
+    REQUIRE(region.str() == " 1 | ( 2 3 )");
+  }
+
+  SECTION("Nested parentheses preserved")
+  {
+    // These parentheses are meaningful and should be preserved
+    auto region = openmc::Region("(1 | 2) (3 | 4)", 0);
+    REQUIRE(region.str() == " ( 1 | 2 ) ( 3 | 4 )");
+  }
+
+  SECTION("Deep nesting")
+  {
+    auto region = openmc::Region("((1 2) | (3 4)) 5", 0);
+    REQUIRE(region.str() == " ( ( 1 2 ) | ( 3 4 ) ) 5");
+  }
+
+  SECTION("Multiple unions")
+  {
+    auto region = openmc::Region("1 | 2 | 3", 0);
+    REQUIRE(region.str() == " 1 | 2 | 3");
+  }
+
+  SECTION("Multiple intersections")
+  {
+    auto region = openmc::Region("1 2 3", 0);
+    // Simple cell - no operators in output
+    REQUIRE(region.str() == " 1 2 3");
+  }
+
+  SECTION("Complex mixed expression")
+  {
+    auto region = openmc::Region("1 2 | 3 4 | 5 6", 0);
+    REQUIRE(region.str() == " ( 1 2 ) | ( 3 4 ) | ( 5 6 )");
+  }
+}
diff --git a/tests/cpp_unit_tests/test_tally.cpp b/tests/cpp_unit_tests/test_tally.cpp
index 964d30cc4..03322e764 100644
--- a/tests/cpp_unit_tests/test_tally.cpp
+++ b/tests/cpp_unit_tests/test_tally.cpp
@@ -1,3 +1,4 @@
+#include "openmc/tallies/filter_energy.h"
 #include "openmc/tallies/tally.h"
 #include <catch2/catch_test_macros.hpp>
 
@@ -46,10 +47,34 @@ TEST_CASE("Test add/set_filter")
   REQUIRE(model::filter_map[cell_filter->id()] == tally->filters(0));
   REQUIRE(model::filter_map[particle_filter->id()] == tally->filters(1));
 
-  // set filters with a duplicate filter, should only add the filter to the tally once
+  // set filters with a duplicate filter, should only add the filter to the
+  // tally once
   filters = {cell_filter, cell_filter};
   tally->set_filters(filters);
   REQUIRE(tally->filters().size() == 1);
   REQUIRE(model::filter_map[cell_filter->id()] == tally->filters(0));
+}
 
+// Regression test for 64-bit tally filter-bin counts (mesh x groups > 2^31).
+TEST_CASE("Tally filter-bin count does not overflow 32 bits")
+{
+  // Two energy filters whose bin counts multiply to 2.5e9, above INT32_MAX.
+  constexpr int64_t bins_per_filter = 50000;
+
+  // Only the bin count matters here, so the edge values are an arbitrary ramp.
+  std::vector<double> edges(bins_per_filter + 1);
+  for (int64_t i = 0; i < bins_per_filter + 1; ++i)
+    edges[i] = static_cast<double>(i);
+
+  Tally* tally = Tally::create();
+  for (int i = 0; i < 2; ++i) {
+    Filter* filter = Filter::create("energy");
+    dynamic_cast<EnergyFilter*>(filter)->set_bins(edges);
+    tally->add_filter(filter);
+  }
+  tally->set_strides();
+
+  // set_strides() previously accumulated this product in a 32-bit int.
+  REQUIRE(tally->n_filter_bins() == bins_per_filter * bins_per_filter);
+  REQUIRE(tally->n_filter_bins() > 2147483647);
 }
\ No newline at end of file
diff --git a/tests/cpp_unit_tests/test_tensor.cpp b/tests/cpp_unit_tests/test_tensor.cpp
new file mode 100644
index 000000000..0936d866a
--- /dev/null
+++ b/tests/cpp_unit_tests/test_tensor.cpp
@@ -0,0 +1,1008 @@
+#include <cmath>
+#include <vector>
+
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+
+#include "openmc/tensor.h"
+
+using namespace openmc;
+using namespace openmc::tensor;
+
+// ============================================================================
+// Tensor constructors
+// ============================================================================
+
+TEST_CASE("Tensor default constructor")
+{
+  Tensor<double> t;
+  REQUIRE(t.size() == 0);
+  REQUIRE(t.empty());
+  REQUIRE(t.shape().empty());
+}
+
+TEST_CASE("Tensor shape constructor")
+{
+  Tensor<double> t1({5});
+  REQUIRE(t1.size() == 5);
+  REQUIRE(t1.shape().size() == 1);
+  REQUIRE(t1.shape(0) == 5);
+
+  Tensor<double> t2({3, 4});
+  REQUIRE(t2.size() == 12);
+  REQUIRE(t2.shape().size() == 2);
+  REQUIRE(t2.shape(0) == 3);
+  REQUIRE(t2.shape(1) == 4);
+
+  Tensor<int> t3({2, 3, 4});
+  REQUIRE(t3.size() == 24);
+  REQUIRE(t3.shape().size() == 3);
+}
+
+TEST_CASE("Tensor shape + fill constructor")
+{
+  Tensor<double> t({2, 3}, 7.0);
+  REQUIRE(t.size() == 6);
+  for (size_t i = 0; i < t.size(); ++i)
+    REQUIRE(t[i] == 7.0);
+}
+
+TEST_CASE("Tensor pointer constructor")
+{
+  double vals[] = {1.0, 2.0, 3.0, 4.0};
+  Tensor<double> t(vals, 4);
+  REQUIRE(t.size() == 4);
+  REQUIRE(t.shape(0) == 4);
+  REQUIRE(t[0] == 1.0);
+  REQUIRE(t[1] == 2.0);
+  REQUIRE(t[2] == 3.0);
+  REQUIRE(t[3] == 4.0);
+}
+
+TEST_CASE("Tensor copy and move")
+{
+  Tensor<double> a({2, 3}, 5.0);
+  Tensor<double> b(a);
+  REQUIRE(b.size() == 6);
+  REQUIRE(b(0, 0) == 5.0);
+  // Modifying copy doesn't affect original
+  b(0, 0) = 99.0;
+  REQUIRE(a(0, 0) == 5.0);
+
+  Tensor<double> c(std::move(b));
+  REQUIRE(c(0, 0) == 99.0);
+  REQUIRE(c.size() == 6);
+}
+
+// ============================================================================
+// Tensor indexing
+// ============================================================================
+
+TEST_CASE("Tensor 1D indexing")
+{
+  Tensor<int> t({4}, 0);
+  t[0] = 10;
+  t[1] = 20;
+  t[2] = 30;
+  t[3] = 40;
+  REQUIRE(t(0) == 10);
+  REQUIRE(t(1) == 20);
+  REQUIRE(t(2) == 30);
+  REQUIRE(t(3) == 40);
+}
+
+TEST_CASE("Tensor 2D indexing (row-major)")
+{
+  // Layout: [[1, 2, 3], [4, 5, 6]]
+  Tensor<int> t({2, 3}, 0);
+  int val = 1;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      t(i, j) = val++;
+
+  REQUIRE(t(0, 0) == 1);
+  REQUIRE(t(0, 2) == 3);
+  REQUIRE(t(1, 0) == 4);
+  REQUIRE(t(1, 2) == 6);
+  // Flat index should match row-major order
+  REQUIRE(t[0] == 1);
+  REQUIRE(t[3] == 4);
+  REQUIRE(t[5] == 6);
+}
+
+TEST_CASE("Tensor 3D indexing")
+{
+  // 2x3x4 tensor
+  Tensor<int> t({2, 3, 4}, 0);
+  t(1, 2, 3) = 42;
+  // Flat index: 1*12 + 2*4 + 3 = 23
+  REQUIRE(t[23] == 42);
+  REQUIRE(t(1, 2, 3) == 42);
+}
+
+// ============================================================================
+// Tensor assignment
+// ============================================================================
+
+TEST_CASE("Tensor initializer_list assignment")
+{
+  Tensor<double> t;
+  t = {1.0, 2.0, 3.0};
+  REQUIRE(t.size() == 3);
+  REQUIRE(t.shape(0) == 3);
+  REQUIRE(t[0] == 1.0);
+  REQUIRE(t[2] == 3.0);
+}
+
+// ============================================================================
+// Tensor mutation
+// ============================================================================
+
+TEST_CASE("Tensor resize")
+{
+  Tensor<double> t({2, 3}, 1.0);
+  REQUIRE(t.size() == 6);
+  t.resize({4, 5});
+  REQUIRE(t.size() == 20);
+  REQUIRE(t.shape(0) == 4);
+  REQUIRE(t.shape(1) == 5);
+}
+
+TEST_CASE("Tensor reshape")
+{
+  Tensor<int> t({12}, 0);
+  for (size_t i = 0; i < 12; ++i)
+    t[i] = static_cast<int>(i);
+
+  t.reshape({3, 4});
+  REQUIRE(t.shape(0) == 3);
+  REQUIRE(t.shape(1) == 4);
+  REQUIRE(t.size() == 12);
+  // Data unchanged, just reinterpreted
+  REQUIRE(t(0, 0) == 0);
+  REQUIRE(t(1, 0) == 4);  // row 1, col 0 = flat index 4
+  REQUIRE(t(2, 3) == 11); // row 2, col 3 = flat index 11
+}
+
+TEST_CASE("Tensor fill")
+{
+  Tensor<double> t({3, 3}, 0.0);
+  t.fill(42.0);
+  for (size_t i = 0; i < t.size(); ++i)
+    REQUIRE(t[i] == 42.0);
+}
+
+// ============================================================================
+// Tensor iterators
+// ============================================================================
+
+TEST_CASE("Tensor iterators")
+{
+  Tensor<int> t({4}, 0);
+  t = {10, 20, 30, 40};
+  int sum = 0;
+  for (auto val : t)
+    sum += val;
+  REQUIRE(sum == 100);
+}
+
+// ============================================================================
+// Tensor reductions
+// ============================================================================
+
+TEST_CASE("Tensor sum (full)")
+{
+  Tensor<double> t({3}, 0.0);
+  t = {1.0, 2.0, 3.0};
+  REQUIRE(t.sum() == 6.0);
+}
+
+TEST_CASE("Tensor sum (axis) on 2D")
+{
+  // [[1, 2, 3],
+  //  [4, 5, 6]]
+  Tensor<int> t({2, 3}, 0);
+  int v = 1;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      t(i, j) = v++;
+
+  // Sum along axis 0 -> [5, 7, 9]
+  Tensor<int> s0 = t.sum(0);
+  REQUIRE(s0.size() == 3);
+  REQUIRE(s0[0] == 5);
+  REQUIRE(s0[1] == 7);
+  REQUIRE(s0[2] == 9);
+
+  // Sum along axis 1 -> [6, 15]
+  Tensor<int> s1 = t.sum(1);
+  REQUIRE(s1.size() == 2);
+  REQUIRE(s1[0] == 6);
+  REQUIRE(s1[1] == 15);
+}
+
+TEST_CASE("Tensor sum (axis) on 3D")
+{
+  // 2x3x2 tensor filled with sequential values 1..12
+  Tensor<int> t({2, 3, 2}, 0);
+  int v = 1;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      for (size_t k = 0; k < 2; ++k)
+        t(i, j, k) = v++;
+
+  // Sum along axis 1 (middle) -> 2x2, each sums 3 values
+  // [0,0]: t(0,0,0)+t(0,1,0)+t(0,2,0) = 1+3+5 = 9
+  // [0,1]: t(0,0,1)+t(0,1,1)+t(0,2,1) = 2+4+6 = 12
+  // [1,0]: t(1,0,0)+t(1,1,0)+t(1,2,0) = 7+9+11 = 27
+  // [1,1]: t(1,0,1)+t(1,1,1)+t(1,2,1) = 8+10+12 = 30
+  Tensor<int> s = t.sum(1);
+  REQUIRE(s.shape(0) == 2);
+  REQUIRE(s.shape(1) == 2);
+  REQUIRE(s(0, 0) == 9);
+  REQUIRE(s(0, 1) == 12);
+  REQUIRE(s(1, 0) == 27);
+  REQUIRE(s(1, 1) == 30);
+}
+
+TEST_CASE("Tensor prod")
+{
+  Tensor<int> t({4}, 0);
+  t = {1, 2, 3, 4};
+  REQUIRE(t.prod() == 24);
+}
+
+TEST_CASE("Tensor any and all")
+{
+  Tensor<bool> t({4}, false);
+  REQUIRE(!t.any());
+  REQUIRE(!t.all());
+
+  // Set one element true
+  t.data()[0] = true;
+  REQUIRE(t.any());
+  REQUIRE(!t.all());
+
+  // Set all true
+  for (size_t i = 0; i < t.size(); ++i)
+    t.data()[i] = true;
+  REQUIRE(t.any());
+  REQUIRE(t.all());
+}
+
+TEST_CASE("Tensor argmin")
+{
+  Tensor<double> t({5}, 0.0);
+  t = {3.0, 1.0, 4.0, 0.5, 2.0};
+  REQUIRE(t.argmin() == 3);
+}
+
+TEST_CASE("Tensor flip")
+{
+  Tensor<int> t({5}, 0);
+  t = {1, 2, 3, 4, 5};
+  Tensor<int> f = t.flip(0);
+  REQUIRE(f[0] == 5);
+  REQUIRE(f[1] == 4);
+  REQUIRE(f[2] == 3);
+  REQUIRE(f[3] == 2);
+  REQUIRE(f[4] == 1);
+}
+
+TEST_CASE("Tensor flip 2D")
+{
+  // [[1, 2], [3, 4], [5, 6]]
+  Tensor<int> t({3, 2}, 0);
+  t(0, 0) = 1;
+  t(0, 1) = 2;
+  t(1, 0) = 3;
+  t(1, 1) = 4;
+  t(2, 0) = 5;
+  t(2, 1) = 6;
+
+  // Flip axis 0 reverses rows -> [[5,6],[3,4],[1,2]]
+  Tensor<int> f = t.flip(0);
+  REQUIRE(f(0, 0) == 5);
+  REQUIRE(f(0, 1) == 6);
+  REQUIRE(f(1, 0) == 3);
+  REQUIRE(f(2, 0) == 1);
+}
+
+// ============================================================================
+// Tensor operators
+// ============================================================================
+
+TEST_CASE("Tensor scalar compound assignment")
+{
+  Tensor<double> t({3}, 0.0);
+  t = {2.0, 4.0, 6.0};
+
+  t += 1.0;
+  REQUIRE(t[0] == 3.0);
+  REQUIRE(t[1] == 5.0);
+
+  t -= 1.0;
+  REQUIRE(t[0] == 2.0);
+
+  t *= 3.0;
+  REQUIRE(t[0] == 6.0);
+  REQUIRE(t[1] == 12.0);
+
+  t /= 2.0;
+  REQUIRE(t[0] == 3.0);
+  REQUIRE(t[1] == 6.0);
+}
+
+TEST_CASE("Tensor element-wise arithmetic")
+{
+  Tensor<double> a({3}, 0.0);
+  Tensor<double> b({3}, 0.0);
+  a = {1.0, 2.0, 3.0};
+  b = {4.0, 5.0, 6.0};
+
+  Tensor<double> c = a + b;
+  REQUIRE(c[0] == 5.0);
+  REQUIRE(c[1] == 7.0);
+  REQUIRE(c[2] == 9.0);
+
+  c = a - b;
+  REQUIRE(c[0] == -3.0);
+
+  c = a / b;
+  REQUIRE(c[0] == 0.25);
+
+  c = a * b;
+  REQUIRE(c[0] == 4.0);
+}
+
+TEST_CASE("Tensor scalar arithmetic")
+{
+  Tensor<double> a({3}, 0.0);
+  a = {1.0, 2.0, 3.0};
+
+  Tensor<double> b = a + 10.0;
+  REQUIRE(b[0] == 11.0);
+  REQUIRE(b[2] == 13.0);
+
+  b = a - 1.0;
+  REQUIRE(b[0] == 0.0);
+
+  b = a * 2.0;
+  REQUIRE(b[0] == 2.0);
+  REQUIRE(b[2] == 6.0);
+
+  // Non-member scalar * tensor (commutativity)
+  b = 2.0 * a;
+  REQUIRE(b[0] == 2.0);
+  REQUIRE(b[2] == 6.0);
+
+  // Non-member scalar + tensor
+  b = 10.0 + a;
+  REQUIRE(b[0] == 11.0);
+}
+
+TEST_CASE("Tensor compound arithmetic with tensor")
+{
+  Tensor<double> a({3}, 0.0);
+  Tensor<double> b({3}, 0.0);
+  a = {1.0, 2.0, 3.0};
+  b = {10.0, 20.0, 30.0};
+  a += b;
+  REQUIRE(a[0] == 11.0);
+  REQUIRE(a[1] == 22.0);
+  REQUIRE(a[2] == 33.0);
+
+  a = {1.0, 2.0, 3.0};
+  b -= a;
+  REQUIRE(b[0] == 9.0);
+  REQUIRE(b[1] == 18.0);
+  REQUIRE(b[2] == 27.0);
+
+  b = {10.0, 20.0, 30.0};
+  a *= b;
+  REQUIRE(a[0] == 10.0);
+  REQUIRE(a[1] == 40.0);
+  REQUIRE(a[2] == 90.0);
+
+  a = {1.0, 2.0, 3.0};
+  b /= a;
+  REQUIRE(b[0] == 10.0);
+  REQUIRE(b[1] == 10.0);
+  REQUIRE(b[2] == 10.0);
+}
+
+TEST_CASE("Tensor comparison operators")
+{
+  Tensor<double> t({4}, 0.0);
+  t = {1.0, 2.0, 3.0, 4.0};
+
+  Tensor<bool> r = t < 3.0;
+  REQUIRE(r.data()[0] == true);
+  REQUIRE(r.data()[1] == true);
+  REQUIRE(r.data()[2] == false);
+  REQUIRE(r.data()[3] == false);
+
+  r = t >= 3.0;
+  REQUIRE(r.data()[0] == false);
+  REQUIRE(r.data()[2] == true);
+  REQUIRE(r.data()[3] == true);
+
+  r = t <= 2.0;
+  REQUIRE(r.data()[0] == true);
+  REQUIRE(r.data()[1] == true);
+  REQUIRE(r.data()[2] == false);
+
+  r = t > 3.0;
+  REQUIRE(r.data()[0] == false);
+  REQUIRE(r.data()[3] == true);
+}
+
+TEST_CASE("Tensor element-wise comparison")
+{
+  Tensor<double> a({3}, 0.0);
+  Tensor<double> b({3}, 0.0);
+  a = {1.0, 5.0, 3.0};
+  b = {2.0, 4.0, 3.0};
+
+  Tensor<bool> r = a < b;
+  REQUIRE(r.data()[0] == true);
+  REQUIRE(r.data()[1] == false);
+  REQUIRE(r.data()[2] == false);
+}
+
+TEST_CASE("Tensor mixed-type multiply")
+{
+  Tensor<int> a({3}, 0);
+  Tensor<double> b({3}, 0.0);
+  a = {2, 3, 4};
+  b = {1.5, 2.5, 3.5};
+
+  Tensor<double> c = a * b;
+  REQUIRE(c[0] == 3.0);
+  REQUIRE(c[1] == 7.5);
+  REQUIRE(c[2] == 14.0);
+}
+
+TEST_CASE("Tensor mixed-type divide")
+{
+  Tensor<double> a({3}, 0.0);
+  Tensor<int> b({3}, 0);
+  a = {10.0, 20.0, 30.0};
+  b = {2, 4, 5};
+
+  Tensor<double> c = a / b;
+  REQUIRE(c[0] == 5.0);
+  REQUIRE(c[1] == 5.0);
+  REQUIRE(c[2] == 6.0);
+}
+
+// ============================================================================
+// Tensor bool specialization
+// ============================================================================
+
+TEST_CASE("Tensor<bool> storage")
+{
+  // Tensor<bool> uses unsigned char internally to avoid std::vector<bool> proxy
+  Tensor<bool> t({4}, false);
+  t.data()[0] = true;
+  t.data()[2] = true;
+  REQUIRE(t.any());
+  REQUIRE(!t.all());
+  REQUIRE(t.data()[0] == true);
+  REQUIRE(t.data()[1] == false);
+}
+
+// ============================================================================
+// View (via Tensor accessors)
+// ============================================================================
+
+TEST_CASE("Tensor slice axis 0 (2D)")
+{
+  // [[1, 2, 3], [4, 5, 6]]
+  Tensor<int> t({2, 3}, 0);
+  int v = 1;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      t(i, j) = v++;
+
+  auto r0 = t.slice(0);
+  REQUIRE(r0.size() == 3);
+  REQUIRE(r0[0] == 1);
+  REQUIRE(r0[1] == 2);
+  REQUIRE(r0[2] == 3);
+
+  auto r1 = t.slice(1);
+  REQUIRE(r1[0] == 4);
+  REQUIRE(r1[1] == 5);
+  REQUIRE(r1[2] == 6);
+
+  // Writing through view modifies the tensor
+  r0[1] = 99;
+  REQUIRE(t(0, 1) == 99);
+}
+
+TEST_CASE("Tensor slice axis 1 (2D)")
+{
+  // [[1, 2], [3, 4], [5, 6]]
+  Tensor<int> t({3, 2}, 0);
+  t(0, 0) = 1;
+  t(0, 1) = 2;
+  t(1, 0) = 3;
+  t(1, 1) = 4;
+  t(2, 0) = 5;
+  t(2, 1) = 6;
+
+  auto c0 = t.slice(all, 0);
+  REQUIRE(c0.size() == 3);
+  REQUIRE(c0[0] == 1);
+  REQUIRE(c0[1] == 3);
+  REQUIRE(c0[2] == 5);
+
+  auto c1 = t.slice(all, 1);
+  REQUIRE(c1[0] == 2);
+  REQUIRE(c1[1] == 4);
+  REQUIRE(c1[2] == 6);
+
+  // Write through column view
+  c1[0] = 77;
+  REQUIRE(t(0, 1) == 77);
+}
+
+TEST_CASE("Tensor slice with range")
+{
+  Tensor<int> t({6}, 0);
+  t = {10, 20, 30, 40, 50, 60};
+
+  // range(start, end)
+  auto s = t.slice(range(1, 4));
+  REQUIRE(s.size() == 3);
+  REQUIRE(s[0] == 20);
+  REQUIRE(s[1] == 30);
+  REQUIRE(s[2] == 40);
+
+  // range(end) from start — range(3) means [0, 3)
+  auto s2 = t.slice(range(3));
+  REQUIRE(s2.size() == 3);
+  REQUIRE(s2[0] == 10);
+  REQUIRE(s2[2] == 30);
+
+  // range(start, SIZE_MAX) to end
+  auto s3 = t.slice(range(3, 6));
+  REQUIRE(s3.size() == 3);
+  REQUIRE(s3[0] == 40);
+  REQUIRE(s3[2] == 60);
+
+  // Write through slice
+  s[0] = 99;
+  REQUIRE(t[1] == 99);
+}
+
+TEST_CASE("Tensor flat view")
+{
+  Tensor<int> t({2, 3}, 0);
+  int v = 1;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      t(i, j) = v++;
+
+  auto f = t.flat();
+  REQUIRE(f.size() == 6);
+  REQUIRE(f[0] == 1);
+  REQUIRE(f[5] == 6);
+}
+
+TEST_CASE("Tensor slice on 3D")
+{
+  // 2x3x4 tensor
+  Tensor<int> t({2, 3, 4}, 0);
+  int v = 0;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      for (size_t k = 0; k < 4; ++k)
+        t(i, j, k) = v++;
+
+  // slice(1) -> fix axis 0 at 1 -> 3x4 view
+  auto s = t.slice(1);
+  REQUIRE(s.size() == 12);
+  // t(1,0,0) = 12, t(1,0,1) = 13, ...
+  REQUIRE(s(0, 0) == 12);
+  REQUIRE(s(0, 1) == 13);
+  REQUIRE(s(2, 3) == 23);
+
+  // slice(all, 2) -> fix axis 1 at 2 -> 2x4 view
+  auto s2 = t.slice(all, 2);
+  REQUIRE(s2.size() == 8);
+  // t(0,2,0)=8, t(0,2,1)=9, t(1,2,0)=20
+  REQUIRE(s2(0, 0) == 8);
+  REQUIRE(s2(0, 1) == 9);
+  REQUIRE(s2(1, 0) == 20);
+}
+
+TEST_CASE("Tensor multi-axis slice")
+{
+  // 2x3x4 tensor with sequential values
+  Tensor<int> t({2, 3, 4}, 0);
+  int v = 0;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      for (size_t k = 0; k < 4; ++k)
+        t(i, j, k) = v++;
+
+  // slice(1, 2) -> fix axes 0 and 1 -> 1D view of 4 elements
+  // Equivalent to numpy t[1, 2, :] -> t(1,2,0..3) = [20, 21, 22, 23]
+  auto s = t.slice(1, 2);
+  REQUIRE(s.size() == 4);
+  REQUIRE(s[0] == 20);
+  REQUIRE(s[1] == 21);
+  REQUIRE(s[3] == 23);
+
+  // slice(all, 1, range(1, 3)) -> keep axis 0, fix axis 1, range on axis 2
+  // Equivalent to numpy t[:, 1, 1:3]
+  // t(0,1,1)=5, t(0,1,2)=6, t(1,1,1)=17, t(1,1,2)=18
+  auto s2 = t.slice(all, 1, range(1, 3));
+  REQUIRE(s2.size() == 4);
+  REQUIRE(s2.ndim() == 2);
+  REQUIRE(s2(0, 0) == 5);
+  REQUIRE(s2(0, 1) == 6);
+  REQUIRE(s2(1, 0) == 17);
+  REQUIRE(s2(1, 1) == 18);
+
+  // slice(0, range(0, 2)) -> fix axis 0 at 0, range on axis 1
+  // Equivalent to numpy t[0, 0:2, :] -> shape (2, 4)
+  auto s3 = t.slice(0, range(0, 2));
+  REQUIRE(s3.ndim() == 2);
+  REQUIRE(s3.shape(0) == 2);
+  REQUIRE(s3.shape(1) == 4);
+  REQUIRE(s3(0, 0) == 0); // t(0,0,0)
+  REQUIRE(s3(1, 3) == 7); // t(0,1,3)
+}
+
+// ============================================================================
+// View assignment and arithmetic
+// ============================================================================
+
+TEST_CASE("View scalar assignment (fill)")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  auto r = t.slice(0);
+  r = 7.0;
+  REQUIRE(t(0, 0) == 7.0);
+  REQUIRE(t(0, 1) == 7.0);
+  REQUIRE(t(0, 2) == 7.0);
+  REQUIRE(t(1, 0) == 0.0); // Other row unchanged
+}
+
+TEST_CASE("View initializer_list assignment")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  auto r = t.slice(1);
+  r = {10.0, 20.0, 30.0};
+  REQUIRE(t(1, 0) == 10.0);
+  REQUIRE(t(1, 1) == 20.0);
+  REQUIRE(t(1, 2) == 30.0);
+}
+
+TEST_CASE("View copy assignment (deep copy)")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  t.slice(0) = {1.0, 2.0, 3.0};
+  t.slice(1) = {4.0, 5.0, 6.0};
+
+  // Copy row 0 into row 1
+  t.slice(1) = t.slice(0);
+  REQUIRE(t(1, 0) == 1.0);
+  REQUIRE(t(1, 1) == 2.0);
+  REQUIRE(t(1, 2) == 3.0);
+}
+
+TEST_CASE("View compound operators")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  t.slice(0) = {1.0, 2.0, 3.0};
+
+  t.slice(0) *= 2.0;
+  REQUIRE(t(0, 0) == 2.0);
+  REQUIRE(t(0, 1) == 4.0);
+
+  t.slice(0) /= 2.0;
+  REQUIRE(t(0, 0) == 1.0);
+  REQUIRE(t(0, 1) == 2.0);
+}
+
+TEST_CASE("View assignment from tensor")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  Tensor<double> vals({3}, 0.0);
+  vals = {7.0, 8.0, 9.0};
+
+  t.slice(1) = vals;
+  REQUIRE(t(1, 0) == 7.0);
+  REQUIRE(t(1, 1) == 8.0);
+  REQUIRE(t(1, 2) == 9.0);
+}
+
+TEST_CASE("View compound addition from tensor")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  t.slice(0) = {1.0, 2.0, 3.0};
+  Tensor<double> vals({3}, 0.0);
+  vals = {10.0, 20.0, 30.0};
+
+  t.slice(0) += vals;
+  REQUIRE(t(0, 0) == 11.0);
+  REQUIRE(t(0, 1) == 22.0);
+  REQUIRE(t(0, 2) == 33.0);
+}
+
+TEST_CASE("View sum")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  t.slice(0) = {1.0, 2.0, 3.0};
+  t.slice(1) = {4.0, 5.0, 6.0};
+
+  REQUIRE(t.slice(0).sum() == 6.0);
+  REQUIRE(t.slice(1).sum() == 15.0);
+}
+
+TEST_CASE("View iteration")
+{
+  Tensor<int> t({2, 3}, 0);
+  t.slice(0) = {1, 2, 3};
+
+  int sum = 0;
+  for (auto val : t.slice(0))
+    sum += val;
+  REQUIRE(sum == 6);
+}
+
+TEST_CASE("View sub-slice")
+{
+  Tensor<int> t({6}, 0);
+  t = {10, 20, 30, 40, 50, 60};
+
+  auto s = t.slice(range(1, 5));  // [20, 30, 40, 50]
+  auto ss = s.slice(range(1, 3)); // [30, 40]
+  REQUIRE(ss.size() == 2);
+  REQUIRE(ss[0] == 30);
+  REQUIRE(ss[1] == 40);
+}
+
+TEST_CASE("Tensor from View")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  t.slice(0) = {1.0, 2.0, 3.0};
+
+  // Construct a new tensor from a view (copies data)
+  Tensor<double> t2(t.slice(0));
+  REQUIRE(t2.size() == 3);
+  REQUIRE(t2[0] == 1.0);
+  REQUIRE(t2[2] == 3.0);
+
+  // Modifying the new tensor doesn't affect the original
+  t2[0] = 99.0;
+  REQUIRE(t(0, 0) == 1.0);
+}
+
+// ============================================================================
+// Const View
+// ============================================================================
+
+TEST_CASE("Const tensor produces const views")
+{
+  Tensor<double> t({2, 3}, 0.0);
+  int v = 1;
+  for (size_t i = 0; i < 2; ++i)
+    for (size_t j = 0; j < 3; ++j)
+      t(i, j) = v++;
+
+  const Tensor<double>& ct = t;
+  auto r = ct.slice(0); // View<const double>
+  REQUIRE(r[0] == 1.0);
+  REQUIRE(r[2] == 3.0);
+
+  auto c = ct.slice(all, 1);
+  REQUIRE(c[0] == 2.0);
+  REQUIRE(c[1] == 5.0);
+}
+
+// ============================================================================
+// StaticTensor2D
+// ============================================================================
+
+TEST_CASE("StaticTensor2D basics")
+{
+  StaticTensor2D<double, 3, 4> t;
+  REQUIRE(t.size() == 12);
+  REQUIRE(t.shape()[0] == 3);
+  REQUIRE(t.shape()[1] == 4);
+
+  // Default-initialized to zero
+  REQUIRE(t(0, 0) == 0.0);
+
+  t(1, 2) = 42.0;
+  REQUIRE(t(1, 2) == 42.0);
+  // Flat data: row 1, col 2 = index 1*4 + 2 = 6
+  REQUIRE(t.data()[6] == 42.0);
+}
+
+TEST_CASE("StaticTensor2D fill")
+{
+  StaticTensor2D<int, 2, 3> t;
+  t.fill(5);
+  for (size_t i = 0; i < t.size(); ++i)
+    REQUIRE(t.data()[i] == 5);
+}
+
+TEST_CASE("StaticTensor2D iteration")
+{
+  StaticTensor2D<int, 2, 3> t;
+  t.fill(1);
+  int sum = 0;
+  for (auto val : t)
+    sum += val;
+  REQUIRE(sum == 6);
+}
+
+TEST_CASE("StaticTensor2D slice")
+{
+  StaticTensor2D<int, 3, 2> t;
+  t(0, 0) = 1;
+  t(0, 1) = 2;
+  t(1, 0) = 3;
+  t(1, 1) = 4;
+  t(2, 0) = 5;
+  t(2, 1) = 6;
+
+  // slice(1) = row 1 (fix axis 0 at 1)
+  auto r1 = t.slice(1);
+  REQUIRE(r1.size() == 2);
+  REQUIRE(r1[0] == 3);
+  REQUIRE(r1[1] == 4);
+
+  // slice(all, 0) = column 0 (fix axis 1 at 0)
+  auto c0 = t.slice(all, 0);
+  REQUIRE(c0.size() == 3);
+  REQUIRE(c0[0] == 1);
+  REQUIRE(c0[1] == 3);
+  REQUIRE(c0[2] == 5);
+}
+
+TEST_CASE("StaticTensor2D flat view")
+{
+  StaticTensor2D<double, 2, 2> t;
+  t(0, 0) = 1.0;
+  t(0, 1) = 2.0;
+  t(1, 0) = 3.0;
+  t(1, 1) = 4.0;
+
+  auto f = t.flat();
+  REQUIRE(f.size() == 4);
+  f = 0.0;
+  REQUIRE(t(0, 0) == 0.0);
+  REQUIRE(t(1, 1) == 0.0);
+}
+
+// ============================================================================
+// Non-member functions
+// ============================================================================
+
+TEST_CASE("zeros")
+{
+  auto t = zeros<double>({3, 4});
+  REQUIRE(t.size() == 12);
+  for (size_t i = 0; i < t.size(); ++i)
+    REQUIRE(t[i] == 0.0);
+}
+
+TEST_CASE("zeros_like")
+{
+  Tensor<double> a({2, 5}, 7.0);
+  auto b = zeros_like(a);
+  REQUIRE(b.size() == 10);
+  REQUIRE(b.shape(0) == 2);
+  REQUIRE(b.shape(1) == 5);
+  for (size_t i = 0; i < b.size(); ++i)
+    REQUIRE(b[i] == 0.0);
+}
+
+TEST_CASE("full_like")
+{
+  Tensor<int> a({4}, 0);
+  auto b = full_like(a, 42);
+  REQUIRE(b.size() == 4);
+  for (size_t i = 0; i < b.size(); ++i)
+    REQUIRE(b[i] == 42);
+}
+
+TEST_CASE("linspace")
+{
+  auto t = linspace(0.0, 1.0, 5);
+  REQUIRE(t.size() == 5);
+  REQUIRE(t[0] == 0.0);
+  REQUIRE(t[4] == 1.0);
+  REQUIRE_THAT(t[1], Catch::Matchers::WithinRel(0.25, 1e-12));
+  REQUIRE_THAT(t[2], Catch::Matchers::WithinRel(0.5, 1e-12));
+  REQUIRE_THAT(t[3], Catch::Matchers::WithinRel(0.75, 1e-12));
+}
+
+TEST_CASE("concatenate")
+{
+  Tensor<int> a({3}, 0);
+  Tensor<int> b({2}, 0);
+  a = {1, 2, 3};
+  b = {4, 5};
+
+  auto c = concatenate(a, b);
+  REQUIRE(c.size() == 5);
+  REQUIRE(c[0] == 1);
+  REQUIRE(c[2] == 3);
+  REQUIRE(c[3] == 4);
+  REQUIRE(c[4] == 5);
+}
+
+TEST_CASE("log")
+{
+  Tensor<double> t({3}, 0.0);
+  t = {1.0, std::exp(1.0), std::exp(2.0)};
+
+  auto r = log(t);
+  REQUIRE_THAT(r[0], Catch::Matchers::WithinAbs(0.0, 1e-12));
+  REQUIRE_THAT(r[1], Catch::Matchers::WithinAbs(1.0, 1e-12));
+  REQUIRE_THAT(r[2], Catch::Matchers::WithinAbs(2.0, 1e-12));
+}
+
+TEST_CASE("abs")
+{
+  Tensor<double> t({4}, 0.0);
+  t = {-3.0, -1.0, 0.0, 2.0};
+
+  auto r = abs(t);
+  REQUIRE(r[0] == 3.0);
+  REQUIRE(r[1] == 1.0);
+  REQUIRE(r[2] == 0.0);
+  REQUIRE(r[3] == 2.0);
+}
+
+TEST_CASE("where")
+{
+  Tensor<bool> cond({4}, false);
+  cond.data()[0] = true;
+  cond.data()[2] = true;
+
+  Tensor<double> vals({4}, 0.0);
+  vals = {10.0, 20.0, 30.0, 40.0};
+
+  auto r = where(cond, vals, -1.0);
+  REQUIRE(r[0] == 10.0);
+  REQUIRE(r[1] == -1.0);
+  REQUIRE(r[2] == 30.0);
+  REQUIRE(r[3] == -1.0);
+}
+
+TEST_CASE("nan_to_num")
+{
+  Tensor<double> t({4}, 0.0);
+  t[0] = 1.0;
+  t[1] = std::nan("");
+  t[2] = std::numeric_limits<double>::infinity();
+  t[3] = -std::numeric_limits<double>::infinity();
+
+  auto r = nan_to_num(t);
+  REQUIRE(r[0] == 1.0);
+  REQUIRE(r[1] == 0.0);                                   // NaN -> 0
+  REQUIRE(r[2] == std::numeric_limits<double>::max());    // +inf -> max
+  REQUIRE(r[3] == std::numeric_limits<double>::lowest()); // -inf -> lowest
+}
+
+// ============================================================================
+// is_tensor trait
+// ============================================================================
+
+TEST_CASE("is_tensor trait")
+{
+  REQUIRE(is_tensor<Tensor<double>>::value);
+  REQUIRE(is_tensor<Tensor<int>>::value);
+  REQUIRE(is_tensor<StaticTensor2D<double, 3, 3>>::value);
+  REQUIRE(!is_tensor<double>::value);
+  REQUIRE(!is_tensor<std::vector<double>>::value);
+}
diff --git a/tests/dummy_operator.py b/tests/dummy_operator.py
index 1bb00129d..873633525 100644
--- a/tests/dummy_operator.py
+++ b/tests/dummy_operator.py
@@ -24,7 +24,7 @@ DepletionSolutionTuple = namedtuple(
 
 predictor_solution = DepletionSolutionTuple(
     PredictorIntegrator, np.array([1.0, 2.46847546272295, 4.11525874568034]),
-    np.array([1.0, 0.986431226850467, -0.0581692232513460]))
+    np.array([1.0, 0.986431226850467, 0.0]))
 
 
 cecm_solution = DepletionSolutionTuple(
@@ -243,4 +243,4 @@ class DummyOperator(TransportOperator):
             Maps cell name to index in global geometry.
 
         """
-        return self.volume, self.nuc_list, self.local_mats, self.burnable_mats
+        return self.volume, self.nuc_list, self.local_mats, self.burnable_mats, {"1": ""}
diff --git a/tests/fns_flux_709.npy b/tests/fns_flux_709.npy
new file mode 100644
index 000000000..62c612842
Binary files /dev/null and b/tests/fns_flux_709.npy differ
diff --git a/tests/regression_tests/adj_cell_rotation/inputs_true.dat b/tests/regression_tests/adj_cell_rotation/inputs_true.dat
index 7e2a0cf62..18c0552ce 100644
--- a/tests/regression_tests/adj_cell_rotation/inputs_true.dat
+++ b/tests/regression_tests/adj_cell_rotation/inputs_true.dat
@@ -1,35 +1,35 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cc" value="0.1"/>
-      <nuclide ao="0.1" name="H1"/>
+      <density value="0.1" units="g/cc"/>
+      <nuclide name="H1" ao="0.1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="2" region="1" universe="1"/>
-    <cell fill="1" id="3" region="2 -3 4 -5 6 -8" rotation="10 20 30" universe="2"/>
-    <cell fill="1" id="4" region="2 -3 4 -5 8 -7" translation="0 0 15" universe="2"/>
-    <surface coeffs="1.0 0.0 0.0 5.0" id="1" type="sphere"/>
-    <surface boundary="vacuum" coeffs="-7.5" id="2" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="7.5" id="3" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-7.5" id="4" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="7.5" id="5" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-7.5" id="6" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="22.5" id="7" type="z-plane"/>
-    <surface coeffs="7.5" id="8" type="z-plane"/>
+    <cell id="3" fill="1" region="2 -3 4 -5 6 -8" rotation="10 20 30" universe="2"/>
+    <cell id="4" fill="1" region="2 -3 4 -5 8 -7" translation="0 0 15" universe="2"/>
+    <surface id="1" type="sphere" coeffs="1.0 0.0 0.0 5.0"/>
+    <surface id="2" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-7.5"/>
+    <surface id="3" name="maximum x" type="x-plane" boundary="vacuum" coeffs="7.5"/>
+    <surface id="4" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-7.5"/>
+    <surface id="5" name="maximum y" type="y-plane" boundary="vacuum" coeffs="7.5"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="-7.5"/>
+    <surface id="7" type="z-plane" boundary="vacuum" coeffs="22.5"/>
+    <surface id="8" type="z-plane" coeffs="7.5"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>10000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4.0 -4.0 -4.0 4.0 4.0 4.0</parameters>
       </space>
diff --git a/tests/regression_tests/adj_cell_rotation/results_true.dat b/tests/regression_tests/adj_cell_rotation/results_true.dat
index b3df12e71..ddb1546b5 100644
--- a/tests/regression_tests/adj_cell_rotation/results_true.dat
+++ b/tests/regression_tests/adj_cell_rotation/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.403987E-01 1.514158E-03
+4.368327E-01 1.953533E-03
diff --git a/tests/regression_tests/albedo_box/results_true.dat b/tests/regression_tests/albedo_box/results_true.dat
index 0f571f245..dca80abcd 100644
--- a/tests/regression_tests/albedo_box/results_true.dat
+++ b/tests/regression_tests/albedo_box/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.590800E+00 4.251788E-03
+1.593206E+00 2.925742E-03
diff --git a/tests/regression_tests/asymmetric_lattice/inputs_true.dat b/tests/regression_tests/asymmetric_lattice/inputs_true.dat
index 302ef24c2..ee3d68907 100644
--- a/tests/regression_tests/asymmetric_lattice/inputs_true.dat
+++ b/tests/regression_tests/asymmetric_lattice/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UOX fuel">
-      <density units="g/cm3" value="10.062"/>
-      <nuclide ao="4.9476e-06" name="U234"/>
-      <nuclide ao="0.00048218" name="U235"/>
-      <nuclide ao="0.021504" name="U238"/>
-      <nuclide ao="1.0801e-08" name="Xe135"/>
-      <nuclide ao="0.045737" name="O16"/>
+    <material id="1" name="UOX fuel" depletable="true">
+      <density value="10.062" units="g/cm3"/>
+      <nuclide name="U234" ao="4.9476e-06"/>
+      <nuclide name="U235" ao="0.00048218"/>
+      <nuclide name="U238" ao="0.021504"/>
+      <nuclide name="Xe135" ao="1.0801e-08"/>
+      <nuclide name="O16" ao="0.045737"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="Cold borated water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="4" name="Hot borated water">
-      <density units="atom/b-cm" value="0.06614"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.06614" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="5" name="Reactor pressure vessel steel">
-      <density units="g/cm3" value="7.9"/>
+      <density value="7.9" units="g/cm3"/>
       <nuclide name="Fe54" wo="0.05437098"/>
       <nuclide name="Fe56" wo="0.88500663"/>
       <nuclide name="Fe57" wo="0.0208008"/>
@@ -47,7 +47,7 @@
       <nuclide name="Cu63" wo="0.0013696"/>
     </material>
     <material id="6" name="Lower radial reflector">
-      <density units="g/cm3" value="4.32"/>
+      <density value="4.32" units="g/cm3"/>
       <nuclide name="H1" wo="0.0095661"/>
       <nuclide name="O16" wo="0.0759107"/>
       <nuclide name="B10" wo="3.08409e-05"/>
@@ -62,7 +62,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="7" name="Upper radial reflector / Top plate region">
-      <density units="g/cm3" value="4.28"/>
+      <density value="4.28" units="g/cm3"/>
       <nuclide name="H1" wo="0.0086117"/>
       <nuclide name="O16" wo="0.0683369"/>
       <nuclide name="B10" wo="2.77638e-05"/>
@@ -77,7 +77,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="8" name="Bottom plate region">
-      <density units="g/cm3" value="7.184"/>
+      <density value="7.184" units="g/cm3"/>
       <nuclide name="H1" wo="0.0011505"/>
       <nuclide name="O16" wo="0.0091296"/>
       <nuclide name="B10" wo="3.70915e-06"/>
@@ -92,7 +92,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="9" name="Bottom nozzle region">
-      <density units="g/cm3" value="2.53"/>
+      <density value="2.53" units="g/cm3"/>
       <nuclide name="H1" wo="0.0245014"/>
       <nuclide name="O16" wo="0.1944274"/>
       <nuclide name="B10" wo="7.89917e-05"/>
@@ -107,7 +107,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="10" name="Top nozzle region">
-      <density units="g/cm3" value="1.746"/>
+      <density value="1.746" units="g/cm3"/>
       <nuclide name="H1" wo="0.035887"/>
       <nuclide name="O16" wo="0.2847761"/>
       <nuclide name="B10" wo="0.000115699"/>
@@ -122,7 +122,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="11" name="Top of fuel assemblies">
-      <density units="g/cm3" value="3.044"/>
+      <density value="3.044" units="g/cm3"/>
       <nuclide name="H1" wo="0.0162913"/>
       <nuclide name="O16" wo="0.1292776"/>
       <nuclide name="B10" wo="5.25228e-05"/>
@@ -135,7 +135,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="12" name="Bottom of fuel assemblies">
-      <density units="g/cm3" value="1.762"/>
+      <density value="1.762" units="g/cm3"/>
       <nuclide name="H1" wo="0.0292856"/>
       <nuclide name="O16" wo="0.2323919"/>
       <nuclide name="B10" wo="9.44159e-05"/>
@@ -149,7 +149,7 @@
     </material>
   </materials>
   <geometry>
-    <cell fill="202" id="1" region="9 -10 11 -12 13 -14" universe="0"/>
+    <cell id="1" fill="202" region="9 -10 11 -12 13 -14" universe="0"/>
     <cell id="27" material="1" region="-1" universe="3"/>
     <cell id="28" material="2" region="1 -2" universe="3"/>
     <cell id="29" material="4" region="2" universe="3"/>
@@ -157,7 +157,7 @@
     <cell id="31" material="2" region="3 -4" universe="4"/>
     <cell id="32" material="4" region="4" universe="4"/>
     <cell id="70" material="4" region="35 -36" universe="7"/>
-    <cell fill="101" id="80" region="35 -36" universe="8"/>
+    <cell id="80" fill="101" region="35 -36" universe="8"/>
     <lattice id="101" name="Fuel assembly (upper half)">
       <pitch>1.26 1.26</pitch>
       <dimension>17 17</dimension>
@@ -190,25 +190,25 @@
 8 8 8 
 7 7 7 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.41" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.475" id="2" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.56" id="3" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.62" id="4" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-32.13" id="9" type="x-plane"/>
-    <surface boundary="reflective" coeffs="32.13" id="10" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-32.13" id="11" type="y-plane"/>
-    <surface boundary="reflective" coeffs="32.13" id="12" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0" id="13" type="z-plane"/>
-    <surface boundary="reflective" coeffs="32.13" id="14" type="z-plane"/>
-    <surface coeffs="0.0" id="35" type="z-plane"/>
-    <surface coeffs="183.0" id="36" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.41"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.475"/>
+    <surface id="3" type="z-cylinder" coeffs="0.0 0.0 0.56"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.62"/>
+    <surface id="9" type="x-plane" boundary="reflective" coeffs="-32.13"/>
+    <surface id="10" type="x-plane" boundary="reflective" coeffs="32.13"/>
+    <surface id="11" type="y-plane" boundary="reflective" coeffs="-32.13"/>
+    <surface id="12" type="y-plane" boundary="reflective" coeffs="32.13"/>
+    <surface id="13" type="z-plane" boundary="reflective" coeffs="0"/>
+    <surface id="14" type="z-plane" boundary="reflective" coeffs="32.13"/>
+    <surface id="35" type="z-plane" coeffs="0.0"/>
+    <surface id="36" type="z-plane" coeffs="183.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-32 -32 0 32 32 32</parameters>
       </space>
diff --git a/tests/regression_tests/asymmetric_lattice/results_true.dat b/tests/regression_tests/asymmetric_lattice/results_true.dat
index 741327d80..3d6b610a6 100644
--- a/tests/regression_tests/asymmetric_lattice/results_true.dat
+++ b/tests/regression_tests/asymmetric_lattice/results_true.dat
@@ -1 +1 @@
-b0ca1fb0436732188b1a199b3250ca9a33782f8fc379b0f7ff9c582e0c794b0a0470df063cafd0b05e802b26f61eaaf9ff5c0a8a672a933246acf49eed3ebf9f
\ No newline at end of file
+cc76769636be4f681137598cf366e978d7347425a1dfa1b293d17a28381b2b62595fb7f0d2f126dd06972ff9e79089a18dd53aba45fa2b1f316515b91fe6495a
\ No newline at end of file
diff --git a/tests/regression_tests/statepoint_batch/__init__.py b/tests/regression_tests/atomic_relaxation/__init__.py
similarity index 100%
rename from tests/regression_tests/statepoint_batch/__init__.py
rename to tests/regression_tests/atomic_relaxation/__init__.py
diff --git a/tests/regression_tests/atomic_relaxation/inputs_true.dat b/tests/regression_tests/atomic_relaxation/inputs_true.dat
new file mode 100644
index 000000000..637e04285
--- /dev/null
+++ b/tests/regression_tests/atomic_relaxation/inputs_true.dat
@@ -0,0 +1,35 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="11.35" units="g/cm3"/>
+      <nuclide name="Pb208" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 1000000000.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>10000</particles>
+    <batches>1</batches>
+    <source type="independent" strength="1.0" particle="photon">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <electron_treatment>led</electron_treatment>
+    <atomic_relaxation>false</atomic_relaxation>
+    <photon_transport>true</photon_transport>
+  </settings>
+  <tallies>
+    <filter id="1" type="particle">
+      <bins>photon electron</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux heating</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/atomic_relaxation/results_true.dat b/tests/regression_tests/atomic_relaxation/results_true.dat
new file mode 100644
index 000000000..6f100dac8
--- /dev/null
+++ b/tests/regression_tests/atomic_relaxation/results_true.dat
@@ -0,0 +1,9 @@
+tally 1:
+1.956204E+00
+3.826732E+00
+7.918768E+04
+6.270688E+09
+0.000000E+00
+0.000000E+00
+9.208123E+05
+8.478953E+11
diff --git a/tests/regression_tests/atomic_relaxation/test.py b/tests/regression_tests/atomic_relaxation/test.py
new file mode 100644
index 000000000..0d2413f59
--- /dev/null
+++ b/tests/regression_tests/atomic_relaxation/test.py
@@ -0,0 +1,41 @@
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def model():
+    mat = openmc.Material()
+    mat.add_nuclide('Pb208', 1.0)
+    mat.set_density('g/cm3', 11.35)
+
+    sphere = openmc.Sphere(r=1.0e9, boundary_type='reflective')
+    inside_sphere = openmc.Cell(fill=mat, region=-sphere)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([inside_sphere])
+
+    # Isotropic point source of 1 MeV photons at the origin
+    model.settings.source = openmc.IndependentSource(
+        particle='photon',
+        energy=openmc.stats.delta_function(1.0e6)
+    )
+
+    # Fixed-source photon transport with atomic relaxation disabled
+    model.settings.particles = 10000
+    model.settings.batches = 1
+    model.settings.photon_transport = True
+    model.settings.electron_treatment = 'led'
+    model.settings.atomic_relaxation = False
+    model.settings.run_mode = 'fixed source'
+
+    tally = openmc.Tally()
+    tally.filters = [openmc.ParticleFilter(['photon', 'electron'])]
+    tally.scores = ['flux', 'heating']
+    model.tallies = [tally]
+    return model
+
+
+def test_atomic_relaxation(model):
+    harness = PyAPITestHarness('statepoint.1.h5', model=model)
+    harness.main()
diff --git a/tests/regression_tests/cmfd_feed/results_true.dat b/tests/regression_tests/cmfd_feed/results_true.dat
index f5f22d9ac..1ef9624d4 100644
--- a/tests/regression_tests/cmfd_feed/results_true.dat
+++ b/tests/regression_tests/cmfd_feed/results_true.dat
@@ -1,117 +1,117 @@
 k-combined:
-1.164262E+00 9.207592E-03
+1.181723E+00 9.944883E-03
 tally 1:
-1.156972E+01
-1.339924E+01
-2.136306E+01
-4.567185E+01
-2.859527E+01
-8.195821E+01
-3.470754E+01
-1.207851E+02
-3.766403E+01
-1.422263E+02
-3.778821E+01
-1.432660E+02
-3.573197E+01
-1.278854E+02
-2.849979E+01
-8.135515E+01
-2.073803E+01
-4.303374E+01
-1.112117E+01
-1.242944E+01
+1.169899E+01
+1.373251E+01
+2.142380E+01
+4.605511E+01
+2.968085E+01
+8.838716E+01
+3.561418E+01
+1.271206E+02
+3.777783E+01
+1.428817E+02
+3.805832E+01
+1.450213E+02
+3.439836E+01
+1.184892E+02
+2.852438E+01
+8.161896E+01
+2.088423E+01
+4.376204E+01
+1.076670E+01
+1.168108E+01
 tally 2:
-2.388054E+01
-2.875255E+01
-1.667791E+01
-1.403426E+01
-4.224771E+01
-8.942109E+01
-2.993088E+01
-4.490335E+01
-5.689839E+01
-1.625557E+02
-4.043633E+01
-8.212299E+01
-6.764024E+01
-2.297126E+02
-4.807902E+01
-1.161468E+02
-7.314835E+01
-2.684645E+02
-5.203584E+01
-1.359261E+02
-7.375727E+01
-2.733105E+02
-5.252944E+01
-1.386205E+02
-6.909571E+01
-2.397721E+02
-4.922548E+01
-1.217465E+02
-5.685978E+01
-1.621746E+02
-4.051938E+01
-8.237277E+01
-4.185562E+01
-8.784067E+01
-2.983570E+01
-4.467414E+01
-2.238373E+01
-2.520356E+01
-1.566758E+01
-1.234103E+01
+2.321241E+01
+2.702156E+01
+1.620912E+01
+1.317752E+01
+4.197404E+01
+8.845008E+01
+2.982666E+01
+4.469221E+01
+5.810089E+01
+1.695857E+02
+4.134123E+01
+8.588866E+01
+6.982488E+01
+2.447068E+02
+4.966939E+01
+1.238763E+02
+7.428421E+01
+2.767613E+02
+5.287955E+01
+1.403163E+02
+7.447402E+01
+2.785012E+02
+5.324628E+01
+1.423393E+02
+6.895164E+01
+2.381937E+02
+4.916366E+01
+1.211701E+02
+5.679253E+01
+1.617881E+02
+4.043125E+01
+8.204061E+01
+4.218618E+01
+8.933666E+01
+2.978592E+01
+4.456592E+01
+2.196426E+01
+2.435867E+01
+1.525576E+01
+1.175879E+01
 tally 3:
-1.609520E+01
-1.307542E+01
-1.033429E+00
-5.510889E-02
-2.877073E+01
-4.149542E+01
-1.964219E+00
-1.954692E-01
-3.896816E+01
-7.629752E+01
-2.484053E+00
-3.103733E-01
-4.634285E+01
-1.079367E+02
-2.974750E+00
-4.468223E-01
-5.007964E+01
-1.259202E+02
-3.181802E+00
-5.103621E-01
-5.058915E+01
-1.286193E+02
-3.249442E+00
-5.337712E-01
-4.744464E+01
-1.131026E+02
-3.067644E+00
-4.736335E-01
-3.900632E+01
-7.634433E+01
-2.443552E+00
-3.028060E-01
-2.874166E+01
-4.146375E+01
-1.810421E+00
-1.671667E-01
-1.509222E+01
-1.145579E+01
-1.014919E+00
-5.391053E-02
+1.563788E+01
+1.226528E+01
+1.053289E+00
+5.666942E-02
+2.870755E+01
+4.139654E+01
+1.838017E+00
+1.710528E-01
+3.978616E+01
+7.955764E+01
+2.560657E+00
+3.334449E-01
+4.780385E+01
+1.147770E+02
+3.139243E+00
+4.967628E-01
+5.106650E+01
+1.308704E+02
+3.170056E+00
+5.078920E-01
+5.123992E+01
+1.318586E+02
+3.211706E+00
+5.205979E-01
+4.729862E+01
+1.121695E+02
+3.068662E+00
+4.749488E-01
+3.898816E+01
+7.630564E+01
+2.516911E+00
+3.199696E-01
+2.865357E+01
+4.125742E+01
+1.852314E+00
+1.741116E-01
+1.467340E+01
+1.088460E+01
+9.268633E-01
+4.450662E-02
 tally 4:
-3.148231E+00
-4.974555E-01
+3.029754E+00
+4.613561E-01
 0.000000E+00
 0.000000E+00
-2.805439E+00
-3.982239E-01
-5.574031E+00
-1.561105E+00
+2.832501E+00
+4.049252E-01
+5.517243E+00
+1.527794E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -128,14 +128,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.574031E+00
-1.561105E+00
-2.805439E+00
-3.982239E-01
-5.171038E+00
-1.344877E+00
-7.372031E+00
-2.725420E+00
+5.517243E+00
+1.527794E+00
+2.832501E+00
+4.049252E-01
+5.117178E+00
+1.316972E+00
+7.333303E+00
+2.701677E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -152,14 +152,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.372031E+00
-2.725420E+00
-5.171038E+00
-1.344877E+00
-6.946847E+00
-2.424850E+00
-8.496610E+00
-3.627542E+00
+7.333303E+00
+2.701677E+00
+5.117178E+00
+1.316972E+00
+7.248464E+00
+2.641591E+00
+8.817788E+00
+3.905530E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -176,14 +176,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.496610E+00
-3.627542E+00
-6.946847E+00
-2.424850E+00
-8.479501E+00
-3.607280E+00
-9.261869E+00
-4.305912E+00
+8.817788E+00
+3.905530E+00
+7.248464E+00
+2.641591E+00
+8.646465E+00
+3.749847E+00
+9.460948E+00
+4.495388E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -200,14 +200,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.261869E+00
-4.305912E+00
-8.479501E+00
-3.607280E+00
-9.232858E+00
-4.278432E+00
-9.306384E+00
-4.348594E+00
+9.460948E+00
+4.495388E+00
+8.646465E+00
+3.749847E+00
+9.379341E+00
+4.415049E+00
+9.278640E+00
+4.320720E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -224,14 +224,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.306384E+00
-4.348594E+00
-9.232858E+00
-4.278432E+00
-9.299764E+00
-4.347828E+00
-8.511976E+00
-3.639893E+00
+9.278640E+00
+4.320720E+00
+9.379341E+00
+4.415049E+00
+9.465746E+00
+4.498591E+00
+8.656146E+00
+3.760545E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -248,14 +248,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.511976E+00
-3.639893E+00
-9.299764E+00
-4.347828E+00
-8.726086E+00
-3.819567E+00
-7.147277E+00
-2.562747E+00
+8.656146E+00
+3.760545E+00
+9.465746E+00
+4.498591E+00
+8.589782E+00
+3.700308E+00
+6.996002E+00
+2.456935E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -272,14 +272,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.147277E+00
-2.562747E+00
-8.726086E+00
-3.819567E+00
-7.218790E+00
-2.612243E+00
-5.018287E+00
-1.263077E+00
+6.996002E+00
+2.456935E+00
+8.589782E+00
+3.700308E+00
+7.352050E+00
+2.714808E+00
+5.105164E+00
+1.312559E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -296,14 +296,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.018287E+00
-1.263077E+00
-7.218790E+00
-2.612243E+00
-5.443494E+00
-1.487018E+00
-2.732334E+00
-3.773047E-01
+5.105164E+00
+1.312559E+00
+7.352050E+00
+2.714808E+00
+5.442756E+00
+1.486776E+00
+2.697305E+00
+3.675580E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -320,12 +320,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.732334E+00
-3.773047E-01
-5.443494E+00
-1.487018E+00
-3.044773E+00
-4.655756E-01
+2.697305E+00
+3.675580E-01
+5.442756E+00
+1.486776E+00
+3.017025E+00
+4.571443E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -345,144 +345,144 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-1.609029E+01
-1.306718E+01
-2.230601E+00
-2.559496E-01
-2.876780E+01
-4.148686E+01
-3.835952E+00
-7.456562E-01
-3.895738E+01
-7.625344E+01
-4.841024E+00
-1.197335E+00
-4.633595E+01
-1.079043E+02
-6.236821E+00
-1.963311E+00
-5.007472E+01
-1.258967E+02
-6.749745E+00
-2.297130E+00
-5.058336E+01
-1.285894E+02
-6.727612E+00
-2.315656E+00
-4.743869E+01
-1.130735E+02
-6.338193E+00
-2.042159E+00
-3.899838E+01
-7.631289E+01
-5.187573E+00
-1.359733E+00
-2.873434E+01
-4.144233E+01
-3.815610E+00
-7.367619E-01
-1.509020E+01
-1.145268E+01
-2.125767E+00
-2.341894E-01
+1.563588E+01
+1.226217E+01
+2.209027E+00
+2.507131E-01
+2.870034E+01
+4.137550E+01
+3.726620E+00
+7.021948E-01
+3.977762E+01
+7.952285E+01
+5.304975E+00
+1.427333E+00
+4.779747E+01
+1.147456E+02
+6.528302E+00
+2.151715E+00
+5.105366E+01
+1.308037E+02
+6.986782E+00
+2.467487E+00
+5.123380E+01
+1.318264E+02
+6.845633E+00
+2.383542E+00
+4.729296E+01
+1.121433E+02
+6.252695E+00
+1.977351E+00
+3.898236E+01
+7.628315E+01
+5.461495E+00
+1.528579E+00
+2.864576E+01
+4.123538E+01
+3.857301E+00
+7.581323E-01
+1.467047E+01
+1.088031E+01
+2.277024E+00
+2.679801E-01
 cmfd indices
 1.000000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.129918E+00
-1.143848E+00
-1.147976E+00
-1.151534E+00
-1.152378E+00
-1.148219E+00
-1.150402E+00
-1.154647E+00
-1.156159E+00
-1.160048E+00
-1.167441E+00
-1.168163E+00
-1.168629E+00
-1.164120E+00
-1.165051E+00
-1.169177E+00
+1.169107E+00
+1.175079E+00
+1.173912E+00
+1.175368E+00
+1.174026E+00
+1.181745E+00
+1.182261E+00
+1.183559E+00
+1.178691E+00
+1.179222E+00
+1.179017E+00
+1.172979E+00
+1.175043E+00
+1.173458E+00
+1.174152E+00
+1.171451E+00
 cmfd entropy
-3.224769E+00
-3.225945E+00
-3.227421E+00
-3.226174E+00
-3.224429E+00
-3.227049E+00
-3.230710E+00
-3.230315E+00
-3.226825E+00
-3.226655E+00
-3.226588E+00
-3.224155E+00
-3.223246E+00
-3.222640E+00
-3.223920E+00
-3.222838E+00
+3.207640E+00
+3.210547E+00
+3.212218E+00
+3.209573E+00
+3.211619E+00
+3.212126E+00
+3.213163E+00
+3.214288E+00
+3.215737E+00
+3.213677E+00
+3.214925E+00
+3.215612E+00
+3.216708E+00
+3.221454E+00
+3.219048E+00
+3.218387E+00
 cmfd balance
-3.90454E-03
-4.08089E-03
-3.46511E-03
-4.09535E-03
-2.62009E-03
-2.23559E-03
-2.54033E-03
-2.12799E-03
-2.25864E-03
-1.85766E-03
-1.49916E-03
-1.63471E-03
-1.48377E-03
-1.59800E-03
-1.37354E-03
-1.32853E-03
+4.88208E-03
+4.75139E-03
+3.15783E-03
+3.67091E-03
+2.99797E-03
+2.91060E-03
+2.06576E-03
+1.83482E-03
+1.56292E-03
+1.58659E-03
+2.32986E-03
+1.47376E-03
+1.46673E-03
+1.22627E-03
+1.31963E-03
+1.26456E-03
 cmfd dominance ratio
-5.539E-01
-5.537E-01
-5.536E-01
-5.515E-01
-5.512E-01
-5.514E-01
-5.518E-01
-5.507E-01
-5.500E-01
-5.497E-01
-5.477E-01
-5.461E-01
-5.444E-01
-5.445E-01
-5.454E-01
+5.467E-01
+5.453E-01
+5.458E-01
+5.436E-01
+5.442E-01
+5.406E-01
+5.401E-01
+5.413E-01
+4.995E-01
+5.396E-01
+5.409E-01
+5.414E-01
+5.423E-01
+5.456E-01
+5.442E-01
 5.441E-01
 cmfd openmc source comparison
-9.875240E-03
-1.106163E-02
-9.847628E-03
-6.065921E-03
-5.772039E-03
-4.615656E-03
-4.244331E-03
-3.694299E-03
-3.545814E-03
-3.213063E-03
-3.467537E-03
-3.383489E-03
-3.697591E-03
-3.937358E-03
-3.369124E-03
-3.190359E-03
+9.587418E-03
+8.150978E-03
+6.677661E-03
+6.334727E-03
+5.153692E-03
+5.082964E-03
+4.633153E-03
+4.037383E-03
+3.528742E-03
+4.559089E-03
+3.517370E-03
+3.306117E-03
+2.913809E-03
+1.906045E-03
+1.932794E-03
+1.711341E-03
 cmfd source
-4.360494E-02
-8.397599E-02
-1.074181E-01
-1.294531E-01
-1.385611E-01
-1.407934E-01
-1.325191E-01
-1.044311E-01
-7.660359E-02
-4.263941E-02
+4.496492E-02
+7.869674E-02
+1.100280E-01
+1.354045E-01
+1.363339E-01
+1.380533E-01
+1.314512E-01
+1.077480E-01
+7.847306E-02
+3.884630E-02
diff --git a/tests/regression_tests/cmfd_feed_2g/results_true.dat b/tests/regression_tests/cmfd_feed_2g/results_true.dat
index 2b61d9f4e..e66eae13c 100644
--- a/tests/regression_tests/cmfd_feed_2g/results_true.dat
+++ b/tests/regression_tests/cmfd_feed_2g/results_true.dat
@@ -1,112 +1,112 @@
 k-combined:
-1.035567E+00 9.463160E-03
+1.027434E+00 6.509170E-03
 tally 1:
-1.146535E+02
-1.315267E+03
-1.157458E+02
-1.340166E+03
-1.140491E+02
-1.301364E+03
-1.146589E+02
-1.315433E+03
+1.162758E+02
+1.352562E+03
+1.138125E+02
+1.295815E+03
+1.143712E+02
+1.308316E+03
+1.150293E+02
+1.323834E+03
 tally 2:
-4.319968E+01
-9.360083E+01
-6.373035E+01
-2.038056E+02
-1.889646E+02
-1.812892E+03
-1.024866E+02
-5.254528E+02
-4.323262E+01
-9.360200E+01
-6.363111E+01
-2.028178E+02
-1.849746E+02
-1.711533E+03
-1.034532E+02
-5.352768E+02
-4.296541E+01
-9.249656E+01
-6.346919E+01
-2.018659E+02
-1.888697E+02
-1.812037E+03
-1.025707E+02
-5.262261E+02
-4.691085E+01
-1.269707E+02
-6.299377E+01
-1.990497E+02
-1.853864E+02
-1.719984E+03
-1.023015E+02
-5.235858E+02
+4.284580E+01
+9.207089E+01
+6.335165E+01
+2.014931E+02
+1.894187E+02
+1.818190E+03
+1.033212E+02
+5.340768E+02
+4.282771E+01
+9.186295E+01
+6.295029E+01
+1.983895E+02
+1.834276E+02
+1.684375E+03
+1.022482E+02
+5.228403E+02
+4.330690E+01
+9.402038E+01
+6.395965E+01
+2.053163E+02
+1.851113E+02
+1.714198E+03
+1.030809E+02
+5.314535E+02
+4.337097E+01
+9.426435E+01
+6.417590E+01
+2.063443E+02
+1.846817E+02
+1.706518E+03
+1.027233E+02
+5.279582E+02
 tally 3:
-6.034963E+01
-1.827718E+02
+5.992726E+01
+1.803120E+02
 0.000000E+00
 0.000000E+00
-1.865665E-02
-4.244195E-05
-4.170941E+00
-8.769372E-01
-3.453368E+00
-5.989168E-01
+2.172646E-02
+4.414237E-05
+4.181401E+00
+8.912796E-01
+3.536506E+00
+6.287425E-01
 0.000000E+00
 0.000000E+00
-9.743205E+01
-4.749420E+02
-8.570316E-01
-3.807993E-02
-6.005903E+01
-1.807233E+02
+9.824432E+01
+4.828691E+02
+9.116848E-01
+4.231247E-02
+5.955090E+01
+1.775522E+02
 0.000000E+00
 0.000000E+00
-1.885450E-02
-3.653402E-05
-4.323863E+00
-9.447512E-01
-3.465465E+00
-6.022861E-01
+1.893222E-02
+3.288000E-05
+4.048183E+00
+8.291130E-01
+3.384041E+00
+5.742363E-01
 0.000000E+00
 0.000000E+00
-9.843481E+01
-4.846158E+02
-9.048150E-01
-4.205551E-02
-5.996660E+01
-1.802150E+02
+9.734253E+01
+4.738861E+02
+9.157632E-01
+4.329280E-02
+6.045835E+01
+1.835255E+02
 0.000000E+00
 0.000000E+00
-1.221444E-02
-2.263445E-05
-4.301287E+00
-9.309882E-01
-3.456076E+00
-5.992144E-01
+1.501842E-02
+1.896931E-05
+4.289989E+00
+9.251538E-01
+3.481357E+00
+6.071667E-01
 0.000000E+00
 0.000000E+00
-9.761231E+01
-4.765834E+02
-8.434644E-01
-3.728180E-02
-5.961891E+01
-1.783106E+02
+9.799829E+01
+4.803591E+02
+8.899390E-01
+4.078750E-02
+6.064531E+01
+1.842746E+02
 0.000000E+00
 0.000000E+00
-1.500709E-02
-3.541280E-05
-4.155669E+00
-8.713442E-01
-3.455342E+00
-6.006426E-01
+1.495496E-02
+3.082640E-05
+4.321537E+00
+9.371611E-01
+3.453767E+00
+5.983727E-01
 0.000000E+00
 0.000000E+00
-9.726798E+01
-4.733393E+02
-9.202611E-01
-4.390503E-02
+9.771776E+01
+4.777781E+02
+8.975444E-01
+4.157471E-02
 tally 4:
 0.000000E+00
 0.000000E+00
@@ -116,14 +116,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.943264E+00
-4.008855E+00
-3.661063E+01
-6.704808E+01
-8.945553E+00
-4.011707E+00
-3.696832E+01
-6.835286E+01
+8.840487E+00
+3.915792E+00
+3.700362E+01
+6.851588E+01
+8.756789E+00
+3.844443E+00
+3.672366E+01
+6.747174E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -132,14 +132,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.844569E+00
-3.924591E+00
-3.666726E+01
-6.726522E+01
-8.769637E+00
-3.855006E+00
-3.654115E+01
-6.680777E+01
+8.860460E+00
+3.940908E+00
+3.704658E+01
+6.864069E+01
+8.832046E+00
+3.916611E+00
+3.736239E+01
+6.982147E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -156,14 +156,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.945553E+00
-4.011707E+00
-3.696832E+01
-6.835286E+01
-8.943264E+00
-4.008855E+00
-3.661063E+01
-6.704808E+01
+8.756789E+00
+3.844443E+00
+3.672366E+01
+6.747174E+01
+8.840487E+00
+3.915792E+00
+3.700362E+01
+6.851588E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -180,14 +180,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.648474E+00
-3.752219E+00
-3.689442E+01
-6.808997E+01
-8.757378E+00
-3.850408E+00
-3.716920E+01
-6.909715E+01
+8.892576E+00
+3.964978E+00
+3.703525E+01
+6.860645E+01
+8.824229E+00
+3.906925E+00
+3.685909E+01
+6.796123E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -212,22 +212,22 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.783669E+00
-3.870748E+00
-3.687358E+01
-6.802581E+01
-8.755250E+00
-3.846298E+00
-3.660278E+01
-6.704349E+01
-8.769637E+00
-3.855006E+00
-3.654115E+01
-6.680777E+01
-8.844569E+00
-3.924591E+00
-3.666726E+01
-6.726522E+01
+9.050876E+00
+4.111409E+00
+3.656082E+01
+6.687580E+01
+9.042402E+00
+4.105842E+00
+3.687247E+01
+6.801050E+01
+8.832046E+00
+3.916611E+00
+3.736239E+01
+6.982147E+01
+8.860460E+00
+3.940908E+00
+3.704658E+01
+6.864069E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -252,14 +252,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.755250E+00
-3.846298E+00
-3.660278E+01
-6.704349E+01
-8.783669E+00
-3.870748E+00
-3.687358E+01
-6.802581E+01
+9.042402E+00
+4.105842E+00
+3.687247E+01
+6.801050E+01
+9.050876E+00
+4.111409E+00
+3.656082E+01
+6.687580E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -268,14 +268,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.757378E+00
-3.850408E+00
-3.716920E+01
-6.909715E+01
-8.648474E+00
-3.752219E+00
-3.689442E+01
-6.808997E+01
+8.824229E+00
+3.906925E+00
+3.685909E+01
+6.796123E+01
+8.892576E+00
+3.964978E+00
+3.703525E+01
+6.860645E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -301,133 +301,133 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-6.036829E+01
-1.828885E+02
-1.008777E+02
-5.091033E+02
-1.353211E+01
-9.188155E+00
-4.618716E+01
-1.067410E+02
-6.007789E+01
-1.808371E+02
-1.018892E+02
-5.192269E+02
-1.357961E+01
-9.247052E+00
-4.618560E+01
-1.067058E+02
-5.997882E+01
-1.802895E+02
-1.010597E+02
-5.108478E+02
-1.374955E+01
-9.502494E+00
-4.619500E+01
-1.067547E+02
-5.963392E+01
-1.783999E+02
-1.007134E+02
-5.074700E+02
-1.319398E+01
-8.734106E+00
-4.586295E+01
-1.052870E+02
+5.994898E+01
+1.804403E+02
+1.017670E+02
+5.181130E+02
+1.354160E+01
+9.220935E+00
+4.648971E+01
+1.081636E+02
+5.956983E+01
+1.776668E+02
+1.007226E+02
+5.073592E+02
+1.347883E+01
+9.120344E+00
+4.609907E+01
+1.063127E+02
+6.047337E+01
+1.836168E+02
+1.014777E+02
+5.150557E+02
+1.390508E+01
+9.722899E+00
+4.629251E+01
+1.072027E+02
+6.066027E+01
+1.843661E+02
+1.011648E+02
+5.120629E+02
+1.365982E+01
+9.372236E+00
+4.602994E+01
+1.060579E+02
 cmfd indices
 2.000000E+00
 2.000000E+00
 1.000000E+00
 2.000000E+00
 k cmfd
-1.018115E+00
-1.022665E+00
-1.020323E+00
-1.020653E+00
-1.021036E+00
-1.020623E+00
-1.021482E+00
-1.025450E+00
-1.027292E+00
-1.028065E+00
-1.027065E+00
-1.024275E+00
-1.025309E+00
-1.026039E+00
-1.026700E+00
-1.023865E+00
+1.013488E+00
+1.024396E+00
+1.015533E+00
+1.009319E+00
+1.012726E+00
+1.014831E+00
+1.021757E+00
+1.022002E+00
+1.023619E+00
+1.020953E+00
+1.023910E+00
+1.027657E+00
+1.024501E+00
+1.023838E+00
+1.025464E+00
+1.022802E+00
 cmfd entropy
-1.998965E+00
-1.999214E+00
-1.999348E+00
-1.999366E+00
-1.999564E+00
-1.999453E+00
-1.999533E+00
-1.999630E+00
-1.999739E+00
-1.999588E+00
-1.999581E+00
-1.999719E+00
-1.999773E+00
-1.999764E+00
-1.999821E+00
-1.999843E+00
+1.998974E+00
+1.998742E+00
+1.999128E+00
+1.998952E+00
+1.998951E+00
+1.999439E+00
+1.999626E+00
+1.999826E+00
+1.999513E+00
+1.999451E+00
+1.999514E+00
+1.999590E+00
+1.999563E+00
+1.999604E+00
+1.999742E+00
+1.999736E+00
 cmfd balance
-5.73174E-04
-7.55398E-04
-1.46671E-03
-6.39625E-04
-8.19008E-04
-1.93449E-03
-1.15900E-03
-1.01690E-03
-5.62788E-04
-6.90450E-04
-6.01060E-04
-5.73418E-04
-4.37190E-04
-4.82966E-04
-4.09700E-04
-3.45096E-04
+9.79896E-04
+4.24873E-04
+8.05696E-04
+1.92071E-03
+3.70731E-04
+2.81424E-04
+8.28991E-04
+6.12217E-04
+5.29185E-04
+4.97799E-04
+3.09154E-04
+1.73703E-04
+2.56689E-04
+2.64938E-04
+1.96305E-04
+1.82702E-04
 cmfd dominance ratio
-6.264E-03
-6.142E-03
-5.987E-03
-6.082E-03
-5.895E-03
-5.939E-03
-5.910E-03
-5.948E-03
-6.013E-03
-6.017E-03
-6.024E-03
-6.008E-03
-5.976E-03
-5.987E-03
-5.967E-03
-5.929E-03
+6.304E-03
+6.246E-03
+6.159E-03
+6.249E-03
+6.101E-03
+6.155E-03
+6.010E-03
+6.177E-03
+6.349E-03
+6.241E-03
+6.244E-03
+6.249E-03
+6.270E-03
+6.272E-03
+6.278E-03
+6.290E-03
 cmfd openmc source comparison
-4.832872E-05
-6.552342E-05
-7.516800E-05
-7.916087E-05
-9.022260E-05
-8.574478E-05
-7.891622E-05
-7.281636E-05
-7.750571E-05
-6.565408E-05
-6.078665E-05
-5.834343E-05
-4.758176E-05
-5.723990E-05
-4.994116E-05
-4.116808E-05
+4.046094E-05
+5.979431E-05
+3.836521E-05
+4.577591E-05
+5.012911E-05
+2.114677E-05
+2.074571E-05
+3.042280E-05
+2.408163E-05
+2.434542E-05
+1.190699E-05
+9.499301E-06
+2.354221E-05
+2.937924E-05
+1.889875E-05
+1.913866E-05
 cmfd source
-2.455663E-01
-2.553511E-01
-2.512257E-01
-2.478570E-01
+2.489706E-01
+2.426801E-01
+2.532142E-01
+2.551351E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
diff --git a/tests/regression_tests/cmfd_feed_expanding_window/results_true.dat b/tests/regression_tests/cmfd_feed_expanding_window/results_true.dat
index 8fd8cdbb8..b39f82f96 100644
--- a/tests/regression_tests/cmfd_feed_expanding_window/results_true.dat
+++ b/tests/regression_tests/cmfd_feed_expanding_window/results_true.dat
@@ -1,117 +1,117 @@
 k-combined:
-1.167865E+00 7.492213E-03
+1.170835E+00 5.423480E-03
 tally 1:
-1.146860E+01
-1.318884E+01
-2.161527E+01
-4.685283E+01
-2.951158E+01
-8.733566E+01
-3.521610E+01
-1.242821E+02
-3.774236E+01
-1.426501E+02
-3.727918E+01
-1.391158E+02
-3.377176E+01
-1.143839E+02
-2.904497E+01
-8.452907E+01
-2.090871E+01
-4.384549E+01
-1.078642E+01
-1.168086E+01
+1.205100E+01
+1.456707E+01
+2.183882E+01
+4.781179E+01
+2.844010E+01
+8.102358E+01
+3.356334E+01
+1.130832E+02
+3.660829E+01
+1.344973E+02
+3.697740E+01
+1.371500E+02
+3.400119E+01
+1.160196E+02
+2.839868E+01
+8.083199E+01
+2.140398E+01
+4.615447E+01
+1.118179E+01
+1.262942E+01
 tally 2:
-1.136810E+00
-1.292338E+00
-7.987303E-01
-6.379700E-01
-2.266938E+00
-5.139009E+00
-1.613483E+00
-2.603328E+00
-3.046349E+00
-9.280239E+00
-2.182459E+00
-4.763126E+00
-3.568068E+00
-1.273111E+01
-2.532456E+00
-6.413333E+00
-3.989504E+00
-1.591614E+01
-2.848301E+00
-8.112818E+00
-3.853133E+00
-1.484663E+01
-2.718493E+00
-7.390202E+00
-3.478138E+00
-1.209745E+01
-2.467281E+00
-6.087476E+00
-2.952220E+00
-8.715605E+00
-2.103261E+00
-4.423706E+00
-1.917459E+00
-3.676649E+00
-1.378369E+00
-1.899902E+00
-1.048240E+00
-1.098807E+00
-7.511947E-01
-5.642934E-01
+1.218245E+00
+1.484121E+00
+8.387442E-01
+7.034918E-01
+2.142134E+00
+4.588738E+00
+1.526727E+00
+2.330895E+00
+2.736157E+00
+7.486556E+00
+1.973921E+00
+3.896363E+00
+3.606244E+00
+1.300500E+01
+2.537580E+00
+6.439313E+00
+3.668958E+00
+1.346126E+01
+2.599095E+00
+6.755294E+00
+3.647982E+00
+1.330777E+01
+2.539750E+00
+6.450332E+00
+3.118921E+00
+9.727669E+00
+2.186447E+00
+4.780549E+00
+2.881110E+00
+8.300795E+00
+2.042635E+00
+4.172360E+00
+2.045602E+00
+4.184486E+00
+1.458384E+00
+2.126884E+00
+1.022124E+00
+1.044738E+00
+7.112678E-01
+5.059018E-01
 tally 3:
-7.701233E-01
-5.930898E-01
-4.481585E-02
-2.008461E-03
-1.547307E+00
-2.394158E+00
-1.226539E-01
-1.504398E-02
-2.106373E+00
-4.436806E+00
-1.450618E-01
-2.104294E-02
-2.437654E+00
-5.942157E+00
-1.521380E-01
-2.314598E-02
-2.754639E+00
-7.588038E+00
-1.745460E-01
-3.046629E-02
-2.623852E+00
-6.884601E+00
-1.851602E-01
-3.428432E-02
-2.376886E+00
-5.649588E+00
-1.615729E-01
-2.610582E-02
-2.021856E+00
-4.087900E+00
-1.533174E-01
-2.350622E-02
-1.333190E+00
-1.777397E+00
-7.076188E-02
-5.007243E-03
-7.258527E-01
-5.268622E-01
-3.656030E-02
-1.336656E-03
+8.048428E-01
+6.477720E-01
+6.603741E-02
+4.360940E-03
+1.466886E+00
+2.151755E+00
+1.002354E-01
+1.004713E-02
+1.909238E+00
+3.645189E+00
+1.202824E-01
+1.446786E-02
+2.443130E+00
+5.968886E+00
+1.627351E-01
+2.648270E-02
+2.492602E+00
+6.213064E+00
+1.910368E-01
+3.649506E-02
+2.437262E+00
+5.940245E+00
+1.568389E-01
+2.459843E-02
+2.104091E+00
+4.427200E+00
+1.450465E-01
+2.103848E-02
+1.965900E+00
+3.864762E+00
+1.367918E-01
+1.871199E-02
+1.402871E+00
+1.968047E+00
+1.061316E-01
+1.126391E-02
+6.832689E-01
+4.668565E-01
+4.599034E-02
+2.115112E-03
 tally 4:
-1.667432E-01
-2.780328E-02
+1.497312E-01
+2.241943E-02
 0.000000E+00
 0.000000E+00
-1.292567E-01
-1.670730E-02
-2.813370E-01
-7.915052E-02
+1.535839E-01
+2.358801E-02
+2.882052E-01
+8.306225E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -128,14 +128,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.813370E-01
-7.915052E-02
-1.292567E-01
-1.670730E-02
-2.670549E-01
-7.131835E-02
-4.055324E-01
-1.644566E-01
+2.882052E-01
+8.306225E-02
+1.535839E-01
+2.358801E-02
+2.526805E-01
+6.384743E-02
+3.616220E-01
+1.307705E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -152,14 +152,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.055324E-01
-1.644566E-01
-2.670549E-01
-7.131835E-02
-3.848125E-01
-1.480807E-01
-4.809430E-01
-2.313062E-01
+3.616220E-01
+1.307705E-01
+2.526805E-01
+6.384743E-02
+3.594306E-01
+1.291904E-01
+4.229730E-01
+1.789062E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -176,14 +176,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.809430E-01
-2.313062E-01
-3.848125E-01
-1.480807E-01
-4.543918E-01
-2.064719E-01
-5.106133E-01
-2.607260E-01
+4.229730E-01
+1.789062E-01
+3.594306E-01
+1.291904E-01
+3.973299E-01
+1.578711E-01
+4.255879E-01
+1.811250E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -200,14 +200,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.106133E-01
-2.607260E-01
-4.543918E-01
-2.064719E-01
-4.543120E-01
-2.063994E-01
-4.626328E-01
-2.140291E-01
+4.255879E-01
+1.811250E-01
+3.973299E-01
+1.578711E-01
+4.633933E-01
+2.147333E-01
+4.672837E-01
+2.183540E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -224,14 +224,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.626328E-01
-2.140291E-01
-4.543120E-01
-2.063994E-01
-4.827759E-01
-2.330726E-01
-4.442622E-01
-1.973689E-01
+4.672837E-01
+2.183540E-01
+4.633933E-01
+2.147333E-01
+4.251073E-01
+1.807162E-01
+3.842922E-01
+1.476805E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -248,14 +248,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.442622E-01
-1.973689E-01
-4.827759E-01
-2.330726E-01
-4.630420E-01
-2.144079E-01
-3.886524E-01
-1.510507E-01
+3.842922E-01
+1.476805E-01
+4.251073E-01
+1.807162E-01
+4.045096E-01
+1.636280E-01
+3.192860E-01
+1.019436E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -272,14 +272,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.886524E-01
-1.510507E-01
-4.630420E-01
-2.144079E-01
-3.535870E-01
-1.250237E-01
-2.530312E-01
-6.402478E-02
+3.192860E-01
+1.019436E-01
+4.045096E-01
+1.636280E-01
+3.738326E-01
+1.397508E-01
+2.598153E-01
+6.750398E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -296,14 +296,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.530312E-01
-6.402478E-02
-3.535870E-01
-1.250237E-01
-2.465524E-01
-6.078808E-02
-1.197152E-01
-1.433173E-02
+2.598153E-01
+6.750398E-02
+3.738326E-01
+1.397508E-01
+2.453191E-01
+6.018146E-02
+1.098964E-01
+1.207721E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -320,12 +320,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.197152E-01
-1.433173E-02
-2.465524E-01
-6.078808E-02
-1.369631E-01
-1.875888E-02
+1.098964E-01
+1.207721E-02
+2.453191E-01
+6.018146E-02
+1.458094E-01
+2.126039E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -345,119 +345,119 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-7.701233E-01
-5.930898E-01
-1.386250E-01
-1.921688E-02
-1.547307E+00
-2.394158E+00
-2.630277E-01
-6.918357E-02
-2.106373E+00
-4.436806E+00
-2.807880E-01
-7.884187E-02
-2.435849E+00
-5.933361E+00
-3.322060E-01
-1.103608E-01
-2.753634E+00
-7.582501E+00
-3.825922E-01
-1.463768E-01
-2.623852E+00
-6.884601E+00
-3.888710E-01
-1.512206E-01
-2.376886E+00
-5.649588E+00
-3.196217E-01
-1.021581E-01
-2.021856E+00
-4.087900E+00
-2.897881E-01
-8.397715E-02
-1.333190E+00
-1.777397E+00
-1.627110E-01
-2.647486E-02
-7.258527E-01
-5.268622E-01
-9.348666E-02
-8.739755E-03
+8.048428E-01
+6.477720E-01
+1.018934E-01
+1.038226E-02
+1.466886E+00
+2.151755E+00
+1.414681E-01
+2.001322E-02
+1.909238E+00
+3.645189E+00
+2.450211E-01
+6.003535E-02
+2.443130E+00
+5.968886E+00
+3.360056E-01
+1.128997E-01
+2.492602E+00
+6.213064E+00
+3.266277E-01
+1.066856E-01
+2.437262E+00
+5.940245E+00
+2.878100E-01
+8.283461E-02
+2.104091E+00
+4.427200E+00
+3.440457E-01
+1.183675E-01
+1.965900E+00
+3.864762E+00
+2.880615E-01
+8.297945E-02
+1.401955E+00
+1.965478E+00
+1.646479E-01
+2.710892E-02
+6.832689E-01
+4.668565E-01
+1.147413E-01
+1.316557E-02
 cmfd indices
 1.000000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.149077E+00
-1.156751E+00
-1.158648E+00
-1.159506E+00
-1.156567E+00
-1.160259E+00
-1.150345E+00
-1.149846E+00
-1.151606E+00
-1.164544E+00
-1.174648E+00
+1.181376E+00
+1.176656E+00
+1.161939E+00
+1.163552E+00
+1.163035E+00
+1.170382E+00
+1.160597E+00
+1.154301E+00
+1.159007E+00
+1.148290E+00
+1.157088E+00
 cmfd entropy
-3.216173E+00
-3.228717E+00
-3.220402E+00
-3.214352E+00
-3.215636E+00
-3.213599E+00
-3.212854E+00
-3.213131E+00
-3.213196E+00
-3.205474E+00
-3.202869E+00
+3.246419E+00
+3.246511E+00
+3.252247E+00
+3.240919E+00
+3.237600E+00
+3.233990E+00
+3.234226E+00
+3.229356E+00
+3.224272E+00
+3.225381E+00
+3.226778E+00
 cmfd balance
-3.08825E-03
-1.42345E-03
-1.21253E-03
-1.17694E-03
-1.05901E-03
-9.29611E-04
-1.35587E-03
-1.13579E-03
-1.14964E-03
-1.29313E-03
-1.46566E-03
+4.18486E-03
+1.72126E-03
+1.10899E-03
+1.88170E-03
+1.31646E-03
+1.34128E-03
+1.57944E-03
+2.11251E-03
+1.79912E-03
+1.86000E-03
+1.47765E-03
 cmfd dominance ratio
 5.524E-01
-5.614E-01
-5.522E-01
-5.487E-01
-5.482E-01
-5.446E-01
-5.437E-01
-5.429E-01
-5.407E-01
-5.380E-01
-5.377E-01
+5.597E-01
+5.622E-01
+5.544E-01
+5.541E-01
+5.519E-01
+5.532E-01
+5.550E-01
+5.484E-01
+5.497E-01
+5.500E-01
 cmfd openmc source comparison
-1.586045E-02
-6.953134E-03
-6.860419E-03
-6.198467E-03
-5.142854E-03
-4.373354E-03
-5.564831E-03
-4.184765E-03
-1.867780E-03
-2.734784E-03
-2.523985E-03
+1.905464E-03
+4.145126E-03
+2.465876E-03
+2.346755E-03
+1.848120E-03
+3.263822E-03
+3.641639E-03
+4.031509E-03
+4.999010E-03
+6.640746E-03
+5.691414E-03
 cmfd source
-4.241440E-02
-8.226026E-02
-1.180811E-01
-1.328433E-01
-1.412410E-01
-1.424902E-01
-1.269340E-01
-1.096490E-01
-6.953251E-02
-3.455422E-02
+4.951338E-02
+8.478025E-02
+1.083132E-01
+1.301432E-01
+1.341190E-01
+1.445825E-01
+1.255119E-01
+1.063303E-01
+7.830158E-02
+3.840469E-02
diff --git a/tests/regression_tests/cmfd_feed_ng/results_true.dat b/tests/regression_tests/cmfd_feed_ng/results_true.dat
index 153238bfe..4ea1515f1 100644
--- a/tests/regression_tests/cmfd_feed_ng/results_true.dat
+++ b/tests/regression_tests/cmfd_feed_ng/results_true.dat
@@ -1,208 +1,208 @@
 k-combined:
-1.005987E+00 1.354263E-02
+1.008852E+00 9.028695E-03
 tally 1:
-1.140273E+02
-1.301245E+03
-1.147962E+02
-1.319049E+03
-1.151426E+02
-1.326442E+03
-1.149265E+02
-1.321518E+03
+1.151271E+02
+1.325871E+03
+1.143934E+02
+1.309051E+03
+1.142507E+02
+1.306616E+03
+1.140242E+02
+1.300786E+03
 tally 2:
-3.462748E+01
-7.542476E+01
-5.129219E+01
-1.658142E+02
-1.034704E+01
-6.730603E+00
-8.672967E+00
-4.715295E+00
-1.344669E+02
-1.132019E+03
-7.262519E+01
-3.300787E+02
-3.447358E+01
-7.459545E+01
-5.075836E+01
-1.619570E+02
-1.080516E+01
-7.366369E+00
-8.908065E+00
-4.991975E+00
-1.354224E+02
-1.146824E+03
-7.300078E+01
-3.332531E+02
-3.432298E+01
-7.388666E+01
-5.096378E+01
-1.627979E+02
-1.053664E+01
-7.029789E+00
-8.826624E+00
-4.904429E+00
-1.388389E+02
-1.207280E+03
-7.376623E+01
-3.403211E+02
-4.383841E+01
-1.943331E+02
-5.165881E+01
-1.675923E+02
-1.059646E+01
-7.048052E+00
-8.763673E+00
-4.823505E+00
-1.378179E+02
-1.188104E+03
-7.431708E+01
-3.453746E+02
+3.403617E+01
+7.260478E+01
+5.031678E+01
+1.588977E+02
+1.003700E+01
+6.373741E+00
+8.514575E+00
+4.571811E+00
+1.413036E+02
+1.264708E+03
+7.321799E+01
+3.353408E+02
+3.354839E+01
+7.052647E+01
+4.895930E+01
+1.501243E+02
+9.972495E+00
+6.271276E+00
+8.436263E+00
+4.481319E+00
+1.353506E+02
+1.146040E+03
+7.309382E+01
+3.341751E+02
+3.389861E+01
+7.205501E+01
+5.005946E+01
+1.571210E+02
+1.041650E+01
+6.810868E+00
+8.839753E+00
+4.897617E+00
+1.344145E+02
+1.130242E+03
+7.270373E+01
+3.307223E+02
+3.347928E+01
+7.040185E+01
+4.940585E+01
+1.535767E+02
+9.898649E+00
+6.175319E+00
+8.406032E+00
+4.442856E+00
+1.374544E+02
+1.182191E+03
+7.334301E+01
+3.365399E+02
 tally 3:
-4.858880E+01
-1.488705E+02
+4.755532E+01
+1.419592E+02
 0.000000E+00
 0.000000E+00
-8.148667E-03
-1.337050E-05
+1.628248E-02
+3.680742E-05
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.347376E+00
-7.045574E-01
-2.433484E+00
-3.727266E-01
+3.347160E+00
+7.104290E-01
+2.453669E+00
+3.797470E-01
 0.000000E+00
 0.000000E+00
-6.095478E+00
-2.332622E+00
+5.925795E+00
+2.220734E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.235421E-01
-1.205155E-03
-3.647699E-01
-8.953589E-03
+9.542527E-02
+1.118856E-03
+3.018759E-01
+6.371059E-03
 0.000000E+00
 0.000000E+00
-2.673965E+00
-4.517972E-01
+2.501316E+00
+3.938401E-01
 0.000000E+00
 0.000000E+00
-6.841105E+01
-2.929195E+02
-5.902473E-01
-2.307683E-02
-4.792291E+01
-1.444397E+02
+6.926265E+01
+3.001504E+02
+6.765221E-01
+2.991787E-02
+4.605523E+01
+1.328435E+02
 0.000000E+00
 0.000000E+00
-2.183275E-02
-8.061011E-05
+1.687782E-02
+3.921162E-05
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.435826E+00
-7.480244E-01
-2.450829E+00
-3.800327E-01
+3.481608E+00
+7.705602E-01
+2.439374E+00
+3.779705E-01
 0.000000E+00
 0.000000E+00
-6.331358E+00
-2.530492E+00
+5.855806E+00
+2.162361E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.012941E-01
-8.217580E-04
-2.981274E-01
-5.863186E-03
+1.016677E-01
+9.263706E-04
+3.264878E-01
+7.385168E-03
 0.000000E+00
 0.000000E+00
-2.535628E+00
-4.048310E-01
+2.519730E+00
+3.986182E-01
 0.000000E+00
 0.000000E+00
-6.912003E+01
-2.987684E+02
-5.984862E-01
-2.386115E-02
-4.822881E+01
-1.458309E+02
+6.920950E+01
+2.996184E+02
+5.985719E-01
+2.368062E-02
+4.730723E+01
+1.403550E+02
 0.000000E+00
 0.000000E+00
-1.525590E-02
-3.794082E-05
+6.800415E-03
+1.163614E-05
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.314494E+00
-6.944527E-01
-2.424209E+00
-3.691360E-01
+3.347607E+00
+7.085691E-01
+2.556997E+00
+4.109118E-01
 0.000000E+00
 0.000000E+00
-6.254897E+00
-2.474317E+00
+6.171063E+00
+2.391770E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.112542E-01
-1.051688E-03
-3.234880E-01
-7.156753E-03
+7.942016E-02
+5.628747E-04
+3.456912E-01
+7.940086E-03
 0.000000E+00
 0.000000E+00
-2.474142E+00
-3.837496E-01
+2.684459E+00
+4.546338E-01
 0.000000E+00
 0.000000E+00
-6.987095E+01
-3.053358E+02
-5.928782E-01
-2.368116E-02
-4.885415E+01
-1.499856E+02
+6.850588E+01
+2.936078E+02
+6.458806E-01
+2.672541E-02
+4.673301E+01
+1.374202E+02
 0.000000E+00
 0.000000E+00
-1.262416E-02
-2.486286E-05
+1.504681E-02
+4.202606E-05
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.426826E+00
-7.480396E-01
-2.487209E+00
-3.903882E-01
+3.139138E+00
+6.206027E-01
+2.341735E+00
+3.454035E-01
 0.000000E+00
 0.000000E+00
-6.127303E+00
-2.364605E+00
+5.923755E+00
+2.216341E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.439007E-02
-1.082564E-03
-2.963491E-01
-5.652952E-03
+7.030156E-02
+4.283974E-04
+3.247594E-01
+7.133772E-03
 0.000000E+00
 0.000000E+00
-2.620696E+00
-4.311792E-01
+2.559691E+00
+4.119674E-01
 0.000000E+00
 0.000000E+00
-7.030725E+01
-3.091241E+02
-5.986966E-01
-2.352487E-02
+6.938051E+01
+3.012078E+02
+5.159565E-01
+1.730381E-02
 tally 4:
 0.000000E+00
 0.000000E+00
@@ -216,18 +216,18 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.028624E+00
-3.105990E+00
-2.154648E+00
-2.948865E-01
-2.714077E+01
-4.607926E+01
-7.035506E+00
-3.118549E+00
-2.085916E+00
-2.730884E-01
-2.750091E+01
-4.731304E+01
+7.028166E+00
+3.103666E+00
+2.028371E+00
+2.606104E-01
+2.715466E+01
+4.614452E+01
+6.981028E+00
+3.059096E+00
+2.032450E+00
+2.610410E-01
+2.734281E+01
+4.675062E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -240,18 +240,18 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.170567E+00
-3.240715E+00
-2.131758E+00
-2.862355E-01
-2.747702E+01
-4.722977E+01
-7.068817E+00
-3.139275E+00
-2.095865E+00
-2.762179E-01
-2.737835E+01
-4.688689E+01
+6.969559E+00
+3.054867E+00
+2.042871E+00
+2.624815E-01
+2.766332E+01
+4.787778E+01
+7.022610E+00
+3.098329E+00
+2.109973E+00
+2.824233E-01
+2.733826E+01
+4.676309E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -276,18 +276,18 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.035506E+00
-3.118549E+00
-2.085916E+00
-2.730884E-01
-2.750091E+01
-4.731304E+01
-7.028624E+00
-3.105990E+00
-2.154648E+00
-2.948865E-01
-2.714077E+01
-4.607926E+01
+6.981028E+00
+3.059096E+00
+2.032450E+00
+2.610410E-01
+2.734281E+01
+4.675062E+01
+7.028166E+00
+3.103666E+00
+2.028371E+00
+2.606104E-01
+2.715466E+01
+4.614452E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -312,18 +312,18 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.103896E+00
-3.171147E+00
-2.095666E+00
-2.756673E-01
-2.711842E+01
-4.600196E+01
-7.197270E+00
-3.255501E+00
-2.046179E+00
-2.630301E-01
-2.729901E+01
-4.662030E+01
+6.782152E+00
+2.885448E+00
+1.951138E+00
+2.400912E-01
+2.739652E+01
+4.697696E+01
+6.873220E+00
+2.965563E+00
+1.999066E+00
+2.521586E-01
+2.732700E+01
+4.670990E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -360,30 +360,30 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.240906E+00
-3.296285E+00
-2.130816E+00
-2.850713E-01
-2.761433E+01
-4.771548E+01
-7.126427E+00
-3.199209E+00
-2.177254E+00
-2.998765E-01
-2.754936E+01
-4.748447E+01
-7.068817E+00
-3.139275E+00
-2.095865E+00
-2.762179E-01
-2.737835E+01
-4.688689E+01
-7.170567E+00
-3.240715E+00
-2.131758E+00
-2.862355E-01
-2.747702E+01
-4.722977E+01
+6.933695E+00
+3.028826E+00
+2.080336E+00
+2.719620E-01
+2.726925E+01
+4.650782E+01
+6.836291E+00
+2.935998E+00
+2.124868E+00
+2.841370E-01
+2.709685E+01
+4.591604E+01
+7.022610E+00
+3.098329E+00
+2.109973E+00
+2.824233E-01
+2.733826E+01
+4.676309E+01
+6.969559E+00
+3.054867E+00
+2.042871E+00
+2.624815E-01
+2.766332E+01
+4.787778E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -420,18 +420,18 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.126427E+00
-3.199209E+00
-2.177254E+00
-2.998765E-01
-2.754936E+01
-4.748447E+01
-7.240906E+00
-3.296285E+00
-2.130816E+00
-2.850713E-01
-2.761433E+01
-4.771548E+01
+6.836291E+00
+2.935998E+00
+2.124868E+00
+2.841370E-01
+2.709685E+01
+4.591604E+01
+6.933695E+00
+3.028826E+00
+2.080336E+00
+2.719620E-01
+2.726925E+01
+4.650782E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -444,18 +444,18 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.197270E+00
-3.255501E+00
-2.046179E+00
-2.630301E-01
-2.729901E+01
-4.662030E+01
-7.103896E+00
-3.171147E+00
-2.095666E+00
-2.756673E-01
-2.711842E+01
-4.600196E+01
+6.873220E+00
+2.965563E+00
+1.999066E+00
+2.521586E-01
+2.732700E+01
+4.670990E+01
+6.782152E+00
+2.885448E+00
+1.951138E+00
+2.400912E-01
+2.739652E+01
+4.697696E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -493,124 +493,124 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-4.859695E+01
-1.489217E+02
-8.528963E+00
-4.561881E+00
-7.144500E+01
-3.194467E+02
-1.120265E+01
-7.944038E+00
-4.009821E+00
-1.012575E+00
-3.235456E+01
-6.558658E+01
-4.794474E+01
-1.445671E+02
-8.782187E+00
-4.852477E+00
-7.195036E+01
-3.237389E+02
-1.075572E+01
-7.333822E+00
-4.084680E+00
-1.054714E+00
-3.267154E+01
-6.675266E+01
-4.824407E+01
-1.459220E+02
-8.679106E+00
-4.742342E+00
-7.266858E+01
-3.302641E+02
-1.094872E+01
-7.536318E+00
-3.935828E+00
-9.749543E-01
-3.319517E+01
-6.894791E+01
-4.886677E+01
-1.500624E+02
-8.614512E+00
-4.662618E+00
-7.321408E+01
-3.352172E+02
-1.095123E+01
-7.574359E+00
-3.885927E+00
-9.539315E-01
-3.334065E+01
-6.953215E+01
+4.757160E+01
+1.420561E+02
+8.379464E+00
+4.426892E+00
+7.205748E+01
+3.248178E+02
+1.047742E+01
+6.922945E+00
+3.879420E+00
+9.517577E-01
+3.297227E+01
+6.802001E+01
+4.607211E+01
+1.329402E+02
+8.295180E+00
+4.334618E+00
+7.205259E+01
+3.247324E+02
+1.059307E+01
+7.039382E+00
+3.783039E+00
+9.030658E-01
+3.288052E+01
+6.762048E+01
+4.731403E+01
+1.403940E+02
+8.728060E+00
+4.774750E+00
+7.152809E+01
+3.200993E+02
+1.074636E+01
+7.270116E+00
+4.007351E+00
+1.010562E+00
+3.249303E+01
+6.611829E+01
+4.674806E+01
+1.375015E+02
+8.265491E+00
+4.296947E+00
+7.226174E+01
+3.267127E+02
+1.079228E+01
+7.340408E+00
+3.843888E+00
+9.273339E-01
+3.300391E+01
+6.823122E+01
 cmfd indices
 2.000000E+00
 2.000000E+00
 1.000000E+00
 3.000000E+00
 k cmfd
-1.026473E+00
-1.024183E+00
-1.023151E+00
-1.025047E+00
-1.019801E+00
-1.020492E+00
-1.015249E+00
-1.016714E+00
-1.016047E+00
-1.019687E+00
-1.020955E+00
+1.011190E+00
+1.010705E+00
+1.014132E+00
+1.015900E+00
+1.019132E+00
+1.022616E+00
+1.023007E+00
+1.022300E+00
+1.014692E+00
+1.007628E+00
+1.006204E+00
 cmfd entropy
-1.999640E+00
-1.999556E+00
-1.999484E+00
-1.999718E+00
-1.999697E+00
-1.999657E+00
-1.999883E+00
-1.999902E+00
-1.999977E+00
-1.999977E+00
-1.999906E+00
+1.999167E+00
+1.999076E+00
+1.998507E+00
+1.997924E+00
+1.997814E+00
+1.997752E+00
+1.997882E+00
+1.998074E+00
+1.998109E+00
+1.998301E+00
+1.998581E+00
 cmfd balance
-8.10090E-04
-1.28103E-03
-7.97200E-04
-5.82188E-04
-7.20670E-04
-7.31475E-04
-5.71904E-04
-6.14057E-04
-6.00142E-04
-5.47870E-04
-3.53604E-04
+9.30124E-04
+2.56632E-04
+3.62598E-04
+4.17543E-04
+5.25720E-04
+4.90208E-04
+3.61304E-04
+2.24090E-04
+1.86602E-04
+1.78395E-04
+6.42497E-05
 cmfd dominance ratio
-3.977E-03
-4.018E-03
-3.950E-03
-3.866E-03
-3.840E-03
-3.888E-03
-3.867E-03
-3.896E-03
-3.924E-03
-3.885E-03
-3.913E-03
+4.194E-03
+4.234E-03
+4.149E-03
+4.209E-03
+4.185E-03
+4.197E-03
+4.175E-03
+4.105E-03
+4.056E-03
+4.122E-03
+4.113E-03
 cmfd openmc source comparison
-4.787501E-05
-4.450525E-05
-2.532345E-05
-3.844307E-05
-4.821504E-05
-4.840760E-05
-3.739246E-05
-3.957960E-05
-4.521480E-05
-4.072007E-05
-2.532636E-05
+2.078995E-05
+2.415218E-05
+3.311536E-05
+3.598613E-05
+3.156455E-05
+2.737017E-05
+2.468500E-05
+2.514215E-05
+1.601626E-05
+1.424027E-05
+4.201148E-06
 cmfd source
-2.486236E-01
-2.531628E-01
-2.460219E-01
-2.521918E-01
+2.558585E-01
+2.597562E-01
+2.529866E-01
+2.313986E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
diff --git a/tests/regression_tests/cmfd_feed_rectlin/results_true.dat b/tests/regression_tests/cmfd_feed_rectlin/results_true.dat
index 600b5d152..a98f3fbd3 100644
--- a/tests/regression_tests/cmfd_feed_rectlin/results_true.dat
+++ b/tests/regression_tests/cmfd_feed_rectlin/results_true.dat
@@ -1,149 +1,149 @@
 k-combined:
-1.160561E+00 1.029736E-02
+1.157362E+00 9.651846E-03
 tally 1:
-1.089904E+01
-1.193573E+01
-2.026534E+01
-4.113383E+01
-2.723584E+01
-7.440537E+01
-3.309956E+01
-1.101184E+02
-3.659327E+01
-1.341221E+02
-3.780158E+01
-1.430045E+02
-3.520883E+01
-1.241772E+02
-2.961801E+01
-8.784675E+01
-2.182029E+01
-4.781083E+01
-1.180347E+01
-1.399970E+01
+1.160989E+01
+1.351117E+01
+2.127132E+01
+4.540172E+01
+2.903242E+01
+8.450044E+01
+3.443549E+01
+1.188763E+02
+3.678332E+01
+1.355586E+02
+3.760088E+01
+1.418740E+02
+3.433077E+01
+1.181837E+02
+2.861986E+01
+8.231285E+01
+2.182277E+01
+4.792086E+01
+1.138713E+01
+1.304425E+01
 tally 2:
-8.794706E+00
-3.939413E+00
-6.131113E+00
-1.913116E+00
-3.265522E+01
-5.364042E+01
-2.304238E+01
-2.672742E+01
-2.250225E+01
-2.541491E+01
-1.601668E+01
-1.288405E+01
-5.525355E+01
-1.531915E+02
-3.929637E+01
-7.748545E+01
-3.222711E+01
-5.216630E+01
-2.285375E+01
-2.623641E+01
-7.051908E+01
-2.495413E+02
-5.010064E+01
-1.259701E+02
-3.728726E+01
-6.974440E+01
-2.652872E+01
-3.530919E+01
-3.747306E+01
-7.058235E+01
-2.681415E+01
-3.613314E+01
-7.296802E+01
-2.669214E+02
-5.202502E+01
-1.356733E+02
-3.333947E+01
-5.579355E+01
-2.361733E+01
-2.800800E+01
-5.785916E+01
-1.680561E+02
-4.093282E+01
-8.410711E+01
-2.377151E+01
-2.842464E+01
-1.681789E+01
-1.423646E+01
-3.422028E+01
-5.880493E+01
-2.415199E+01
-2.930240E+01
-8.890608E+00
-3.986356E+00
-6.140159E+00
-1.897764E+00
+8.861425E+00
+3.953963E+00
+6.090757E+00
+1.866235E+00
+3.309260E+01
+5.493482E+01
+2.330765E+01
+2.725350E+01
+2.295343E+01
+2.647375E+01
+1.629166E+01
+1.334081E+01
+5.714234E+01
+1.640293E+02
+4.054051E+01
+8.261123E+01
+3.331786E+01
+5.565082E+01
+2.366345E+01
+2.807637E+01
+7.034800E+01
+2.485209E+02
+5.001286E+01
+1.256087E+02
+3.651223E+01
+6.686058E+01
+2.606851E+01
+3.408881E+01
+3.729833E+01
+6.997492E+01
+2.657970E+01
+3.553107E+01
+7.212382E+01
+2.611253E+02
+5.124647E+01
+1.318756E+02
+3.304529E+01
+5.486643E+01
+2.347504E+01
+2.771353E+01
+5.711252E+01
+1.640619E+02
+4.060288E+01
+8.298231E+01
+2.384078E+01
+2.855909E+01
+1.684046E+01
+1.426180E+01
+3.329487E+01
+5.573544E+01
+2.349871E+01
+2.776487E+01
+8.676886E+00
+3.814132E+00
+5.980976E+00
+1.815143E+00
 tally 3:
-5.925339E+00
-1.788596E+00
-3.912632E-01
-8.061838E-03
-2.215544E+01
-2.471404E+01
-1.465191E+00
-1.092622E-01
-1.542988E+01
-1.195626E+01
-1.022876E+00
-5.356825E-02
-3.792029E+01
-7.218149E+01
-2.370476E+00
-2.839465E-01
-2.200154E+01
-2.432126E+01
-1.360836E+00
-9.402424E-02
-4.824980E+01
-1.168447E+02
-3.124366E+00
-4.907460E-01
-2.557420E+01
-3.282066E+01
-1.725855E+00
-1.519830E-01
-2.577963E+01
-3.341077E+01
-1.654042E+00
-1.386845E-01
-5.008220E+01
-1.257610E+02
-3.223857E+00
-5.237360E-01
-2.273380E+01
-2.595325E+01
-1.438369E+00
-1.050840E-01
-3.938822E+01
-7.789691E+01
-2.648324E+00
-3.559703E-01
-1.623604E+01
-1.327214E+01
-1.058882E+00
-5.680630E-02
-2.325730E+01
-2.717892E+01
-1.584276E+00
-1.275868E-01
-5.937929E+00
-1.774847E+00
-3.866918E-01
-7.994353E-03
+5.847135E+00
+1.719612E+00
+3.960040E-01
+8.297721E-03
+2.245534E+01
+2.530222E+01
+1.468678E+00
+1.094031E-01
+1.571194E+01
+1.240597E+01
+1.004169E+00
+5.156464E-02
+3.901605E+01
+7.652715E+01
+2.648696E+00
+3.571840E-01
+2.275978E+01
+2.597555E+01
+1.456067E+00
+1.075035E-01
+4.821184E+01
+1.167725E+02
+3.105774E+00
+4.859873E-01
+2.519281E+01
+3.183845E+01
+1.595498E+00
+1.292133E-01
+2.560583E+01
+3.297409E+01
+1.673871E+00
+1.429651E-01
+4.930025E+01
+1.220909E+02
+3.206604E+00
+5.220919E-01
+2.255825E+01
+2.560134E+01
+1.422870E+00
+1.027151E-01
+3.910818E+01
+7.698933E+01
+2.568903E+00
+3.333385E-01
+1.620292E+01
+1.321173E+01
+1.068678E+00
+5.798793E-02
+2.264343E+01
+2.578648E+01
+1.503553E+00
+1.158411E-01
+5.751110E+00
+1.676910E+00
+3.450582E-01
+6.411784E-03
 tally 4:
-3.063235E+00
-4.714106E-01
+3.051764E+00
+4.671879E-01
 0.000000E+00
 0.000000E+00
-1.425705E+00
-1.043616E-01
-4.355515E+00
-9.538346E-01
+1.407008E+00
+1.004485E-01
+4.354708E+00
+9.506434E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -160,14 +160,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.355515E+00
-9.538346E-01
-1.425705E+00
-1.043616E-01
-3.859174E+00
-7.519499E-01
-6.350310E+00
-2.024214E+00
+4.354708E+00
+9.506434E-01
+1.407008E+00
+1.004485E-01
+3.852730E+00
+7.498016E-01
+6.382605E+00
+2.043123E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -184,14 +184,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-6.350310E+00
-2.024214E+00
-3.859174E+00
-7.519499E-01
-4.993073E+00
-1.258264E+00
-7.194275E+00
-2.596886E+00
+6.382605E+00
+2.043123E+00
+3.852730E+00
+7.498016E-01
+5.061607E+00
+1.288306E+00
+7.281209E+00
+2.659444E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -208,14 +208,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.194275E+00
-2.596886E+00
-4.993073E+00
-1.258264E+00
-6.786617E+00
-2.312259E+00
-8.306978E+00
-3.459668E+00
+7.281209E+00
+2.659444E+00
+5.061607E+00
+1.288306E+00
+7.096602E+00
+2.527474E+00
+8.632232E+00
+3.736665E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -232,14 +232,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.306978E+00
-3.459668E+00
-6.786617E+00
-2.312259E+00
-7.603410E+00
-2.905228E+00
-8.791222E+00
-3.882935E+00
+8.632232E+00
+3.736665E+00
+7.096602E+00
+2.527474E+00
+7.759456E+00
+3.019026E+00
+8.968687E+00
+4.037738E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -256,14 +256,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.791222E+00
-3.882935E+00
-7.603410E+00
-2.905228E+00
-8.867113E+00
-3.938503E+00
-9.302808E+00
-4.340042E+00
+8.968687E+00
+4.037738E+00
+7.759456E+00
+3.019026E+00
+8.749025E+00
+3.839161E+00
+9.126289E+00
+4.176440E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -280,14 +280,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.302808E+00
-4.340042E+00
-8.867113E+00
-3.938503E+00
-9.270113E+00
-4.306513E+00
-9.263471E+00
-4.302184E+00
+9.126289E+00
+4.176440E+00
+8.749025E+00
+3.839161E+00
+9.259277E+00
+4.304020E+00
+9.165726E+00
+4.216173E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -304,14 +304,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.263471E+00
-4.302184E+00
-9.270113E+00
-4.306513E+00
-9.348712E+00
-4.388570E+00
-8.977713E+00
-4.047349E+00
+9.165726E+00
+4.216173E+00
+9.259277E+00
+4.304020E+00
+9.433925E+00
+4.473551E+00
+8.910566E+00
+3.991458E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -328,14 +328,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.977713E+00
-4.047349E+00
-9.348712E+00
-4.388570E+00
-9.234380E+00
-4.280666E+00
-8.036978E+00
-3.239853E+00
+8.910566E+00
+3.991458E+00
+9.433925E+00
+4.473551E+00
+9.099397E+00
+4.154062E+00
+7.770126E+00
+3.029928E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -352,14 +352,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.036978E+00
-3.239853E+00
-9.234380E+00
-4.280666E+00
-8.713633E+00
-3.808850E+00
-7.160878E+00
-2.572915E+00
+7.770126E+00
+3.029928E+00
+9.099397E+00
+4.154062E+00
+8.575842E+00
+3.694998E+00
+6.934243E+00
+2.418570E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -376,14 +376,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.160878E+00
-2.572915E+00
-8.713633E+00
-3.808850E+00
-7.378538E+00
-2.732122E+00
-5.145728E+00
-1.329190E+00
+6.934243E+00
+2.418570E+00
+8.575842E+00
+3.694998E+00
+7.437526E+00
+2.780629E+00
+5.136923E+00
+1.331553E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -400,14 +400,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.145728E+00
-1.329190E+00
-7.378538E+00
-2.732122E+00
-6.660125E+00
-2.228506E+00
-4.087925E+00
-8.435381E-01
+5.136923E+00
+1.331553E+00
+7.437526E+00
+2.780629E+00
+6.648582E+00
+2.216687E+00
+4.050691E+00
+8.279789E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -424,14 +424,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.087925E+00
-8.435381E-01
-6.660125E+00
-2.228506E+00
-4.466295E+00
-1.002320E+00
-1.468481E+00
-1.099604E-01
+4.050691E+00
+8.279789E-01
+6.648582E+00
+2.216687E+00
+4.390906E+00
+9.686686E-01
+1.391624E+00
+9.861955E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -448,12 +448,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.468481E+00
-1.099604E-01
-4.466295E+00
-1.002320E+00
-3.139355E+00
-4.955456E-01
+1.391624E+00
+9.861955E-02
+4.390906E+00
+9.686686E-01
+3.113477E+00
+4.874296E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -473,164 +473,164 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-5.925339E+00
-1.788596E+00
-8.954773E-01
-4.217679E-02
-2.215054E+01
-2.470273E+01
-2.944747E+00
-4.443186E-01
-1.542658E+01
-1.195085E+01
-1.950262E+00
-1.946340E-01
-3.791137E+01
-7.214779E+01
-4.955741E+00
-1.254830E+00
-2.200054E+01
-2.431894E+01
-3.031040E+00
-4.685971E-01
-4.823946E+01
-1.167941E+02
-6.226321E+00
-1.956308E+00
-2.556930E+01
-3.280828E+01
-3.582546E+00
-6.505962E-01
-2.577249E+01
-3.339204E+01
-3.316825E+00
-5.676842E-01
-5.007249E+01
-1.257124E+02
-6.462364E+00
-2.120157E+00
-2.273285E+01
-2.595106E+01
-2.960964E+00
-4.496277E-01
-3.937362E+01
-7.783849E+01
-5.339336E+00
-1.449257E+00
-1.623315E+01
-1.326746E+01
-2.289661E+00
-2.698618E-01
-2.325250E+01
-2.716827E+01
-3.253010E+00
-5.413837E-01
-5.937929E+00
-1.774847E+00
-9.208589E-01
-4.421403E-02
+5.846103E+00
+1.718959E+00
+8.952028E-01
+4.184063E-02
+2.245226E+01
+2.529524E+01
+3.092645E+00
+4.848878E-01
+1.571095E+01
+1.240426E+01
+2.178244E+00
+2.438479E-01
+3.900638E+01
+7.648853E+01
+5.265574E+00
+1.401505E+00
+2.275895E+01
+2.597348E+01
+3.067909E+00
+4.810969E-01
+4.820213E+01
+1.167232E+02
+6.403602E+00
+2.070034E+00
+2.519091E+01
+3.183360E+01
+3.463531E+00
+6.097568E-01
+2.560388E+01
+3.296889E+01
+3.456252E+00
+6.101655E-01
+4.929539E+01
+1.220666E+02
+6.629094E+00
+2.223366E+00
+2.255498E+01
+2.559363E+01
+2.833426E+00
+4.150907E-01
+3.909813E+01
+7.694976E+01
+5.582222E+00
+1.584757E+00
+1.620082E+01
+1.320814E+01
+2.282196E+00
+2.703156E-01
+2.263498E+01
+2.576758E+01
+3.162736E+00
+5.145038E-01
+5.750110E+00
+1.676357E+00
+9.181679E-01
+4.562885E-02
 cmfd indices
 1.400000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.125528E+00
-1.145509E+00
-1.158948E+00
-1.171983E+00
-1.180649E+00
-1.184072E+00
-1.188112E+00
-1.183095E+00
-1.182269E+00
-1.175467E+00
-1.175184E+00
-1.172637E+00
-1.171593E+00
-1.175439E+00
-1.174650E+00
-1.176474E+00
+1.166740E+00
+1.184008E+00
+1.166534E+00
+1.155559E+00
+1.164960E+00
+1.163229E+00
+1.165897E+00
+1.170104E+00
+1.170207E+00
+1.168091E+00
+1.170940E+00
+1.174589E+00
+1.174609E+00
+1.171505E+00
+1.174456E+00
+1.178370E+00
 cmfd entropy
-3.607059E+00
-3.604890E+00
-3.601329E+00
-3.597776E+00
-3.597360E+00
-3.597387E+00
-3.595379E+00
-3.596995E+00
-3.600901E+00
-3.601832E+00
-3.601426E+00
-3.604521E+00
-3.602848E+00
-3.603875E+00
-3.605213E+00
-3.606699E+00
+3.594757E+00
+3.587018E+00
+3.590385E+00
+3.595101E+00
+3.592151E+00
+3.600294E+00
+3.602102E+00
+3.604941E+00
+3.605897E+00
+3.604880E+00
+3.601658E+00
+3.602551E+00
+3.600160E+00
+3.604540E+00
+3.604094E+00
+3.602509E+00
 cmfd balance
-4.46212E-03
-4.66648E-03
-5.04274E-03
-5.21553E-03
-3.92498E-03
-2.97185E-03
-2.79785E-03
-2.66951E-03
-2.17472E-03
-1.98009E-03
-1.77035E-03
-1.51281E-03
-1.52807E-03
-1.33341E-03
-1.18155E-03
-1.07752E-03
+5.52960E-03
+5.42154E-03
+3.62152E-03
+2.92850E-03
+4.08642E-03
+2.07444E-03
+2.03704E-03
+2.06886E-03
+2.09646E-03
+1.94256E-03
+2.02728E-03
+1.89830E-03
+1.83910E-03
+1.48140E-03
+1.47034E-03
+1.64452E-03
 cmfd dominance ratio
-6.136E-01
-6.127E-01
-6.137E-01
-6.102E-01
-6.067E-01
-6.061E-01
-6.031E-01
 6.046E-01
-6.071E-01
-6.089E-01
-6.073E-01
-6.080E-01
-6.080E-01
+6.015E-01
+6.059E-01
+6.060E-01
+6.061E-01
+6.109E-01
+6.110E-01
+6.108E-01
+6.120E-01
+6.124E-01
+6.109E-01
 6.090E-01
-6.092E-01
-6.094E-01
+6.116E-01
+6.137E-01
+6.117E-01
+6.131E-01
 cmfd openmc source comparison
-1.043027E-02
-1.278226E-02
-1.184867E-02
-1.017186E-02
-1.099696E-02
-7.955341E-03
-8.360344E-03
-6.875508E-03
-4.824018E-03
-4.915363E-03
-5.371647E-03
-4.593100E-03
-4.894955E-03
-4.928253E-03
-4.292171E-03
-4.018545E-03
+1.035187E-02
+9.394886E-03
+6.879487E-03
+7.236029E-03
+6.543528E-03
+3.600620E-03
+2.859638E-03
+2.230047E-03
+2.180643E-03
+1.638534E-03
+1.764349E-03
+1.621487E-03
+1.221762E-03
+1.626297E-03
+1.951813E-03
+9.584126E-04
 cmfd source
-1.600876E-02
-6.000305E-02
-4.248071E-02
-9.935789E-02
-5.768092E-02
-1.338593E-01
-7.417398E-02
-7.102984E-02
-1.382563E-01
-6.181889E-02
-1.142473E-01
-4.571447E-02
-6.864655E-02
-1.672206E-02
+1.677059E-02
+6.229453E-02
+4.278394E-02
+1.134852E-01
+6.231020E-02
+1.327286E-01
+6.808362E-02
+7.130954E-02
+1.362127E-01
+6.048013E-02
+1.094460E-01
+4.546687E-02
+6.403310E-02
+1.459510E-02
diff --git a/tests/regression_tests/cmfd_feed_ref_d/results_true.dat b/tests/regression_tests/cmfd_feed_ref_d/results_true.dat
index 9cc26b8cf..34cde1a46 100644
--- a/tests/regression_tests/cmfd_feed_ref_d/results_true.dat
+++ b/tests/regression_tests/cmfd_feed_ref_d/results_true.dat
@@ -1,117 +1,117 @@
 k-combined:
-1.167869E+00 7.492916E-03
+1.162249E+00 5.812620E-03
 tally 1:
-1.146821E+01
-1.318787E+01
-2.161476E+01
-4.685066E+01
-2.951084E+01
-8.733116E+01
-3.521523E+01
-1.242762E+02
-3.774181E+01
-1.426460E+02
-3.727924E+01
-1.391162E+02
-3.377236E+01
-1.143877E+02
-2.904590E+01
-8.453427E+01
-2.090941E+01
-4.384824E+01
-1.078680E+01
-1.168172E+01
+1.153831E+01
+1.338142E+01
+2.155552E+01
+4.659071E+01
+2.813997E+01
+7.941672E+01
+3.270996E+01
+1.073664E+02
+3.639852E+01
+1.329148E+02
+3.729637E+01
+1.393474E+02
+3.443129E+01
+1.186461E+02
+2.832690E+01
+8.040998E+01
+2.177527E+01
+4.771147E+01
+1.146822E+01
+1.328252E+01
 tally 2:
-1.136805E+00
-1.292326E+00
-7.987282E-01
-6.379667E-01
-2.266961E+00
-5.139112E+00
-1.613498E+00
-2.603376E+00
-3.046379E+00
-9.280427E+00
-2.182480E+00
-4.763219E+00
-3.568101E+00
-1.273134E+01
-2.532478E+00
-6.413444E+00
-3.989532E+00
-1.591637E+01
-2.848319E+00
-8.112921E+00
-3.853139E+00
-1.484668E+01
-2.718497E+00
-7.390223E+00
-3.478134E+00
-1.209742E+01
-2.467279E+00
-6.087465E+00
-2.952214E+00
-8.715569E+00
-2.103257E+00
-4.423688E+00
-1.917446E+00
-3.676599E+00
-1.378361E+00
-1.899878E+00
-1.048230E+00
-1.098785E+00
-7.511876E-01
-5.642828E-01
+1.024353E+00
+1.049299E+00
+6.991057E-01
+4.887487E-01
+2.200432E+00
+4.841901E+00
+1.561655E+00
+2.438768E+00
+2.910400E+00
+8.470426E+00
+2.095155E+00
+4.389674E+00
+3.466006E+00
+1.201320E+01
+2.480456E+00
+6.152662E+00
+3.711781E+00
+1.377732E+01
+2.646019E+00
+7.001418E+00
+3.953648E+00
+1.563133E+01
+2.832759E+00
+8.024524E+00
+3.597870E+00
+1.294467E+01
+2.555396E+00
+6.530048E+00
+2.860871E+00
+8.184585E+00
+2.032282E+00
+4.130169E+00
+2.006740E+00
+4.027007E+00
+1.408150E+00
+1.982886E+00
+1.035163E+00
+1.071562E+00
+7.084068E-01
+5.018402E-01
 tally 3:
-7.701212E-01
-5.930866E-01
-4.481580E-02
-2.008456E-03
-1.547321E+00
-2.394203E+00
-1.226538E-01
-1.504395E-02
-2.106393E+00
-4.436893E+00
-1.450617E-01
-2.104289E-02
-2.437675E+00
-5.942260E+00
-1.521379E-01
-2.314593E-02
-2.754657E+00
-7.588135E+00
-1.745458E-01
-3.046622E-02
-2.623856E+00
-6.884619E+00
-1.851600E-01
-3.428423E-02
-2.376884E+00
-5.649579E+00
-1.615728E-01
-2.610576E-02
-2.021851E+00
-4.087882E+00
-1.533172E-01
-2.350617E-02
-1.333182E+00
-1.777374E+00
-7.076179E-02
-5.007231E-03
-7.258458E-01
-5.268521E-01
-3.656026E-02
-1.336653E-03
+6.713566E-01
+4.507196E-01
+5.581718E-02
+3.115557E-03
+1.508976E+00
+2.277009E+00
+1.139601E-01
+1.298690E-02
+2.035529E+00
+4.143377E+00
+1.221001E-01
+1.490843E-02
+2.385217E+00
+5.689262E+00
+1.500087E-01
+2.250260E-02
+2.546286E+00
+6.483574E+00
+1.546601E-01
+2.391975E-02
+2.726427E+00
+7.433407E+00
+1.686144E-01
+2.843081E-02
+2.466613E+00
+6.084179E+00
+1.558230E-01
+2.428079E-02
+1.951251E+00
+3.807379E+00
+1.197744E-01
+1.434590E-02
+1.347903E+00
+1.816842E+00
+9.419149E-02
+8.872037E-03
+6.840490E-01
+4.679231E-01
+5.000289E-02
+2.500289E-03
 tally 4:
-1.667426E-01
-2.780308E-02
+1.561665E-01
+2.438798E-02
 0.000000E+00
 0.000000E+00
-1.292556E-01
-1.670700E-02
-2.813401E-01
-7.915227E-02
+1.307011E-01
+1.708276E-02
+2.703168E-01
+7.307115E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -128,14 +128,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.813401E-01
-7.915227E-02
-1.292556E-01
-1.670700E-02
-2.670582E-01
-7.132006E-02
-4.055365E-01
-1.644599E-01
+2.703168E-01
+7.307115E-02
+1.307011E-01
+1.708276E-02
+2.637619E-01
+6.957033E-02
+3.685390E-01
+1.358210E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -152,14 +152,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.055365E-01
-1.644599E-01
-2.670582E-01
-7.132006E-02
-3.848164E-01
-1.480837E-01
-4.809472E-01
-2.313102E-01
+3.685390E-01
+1.358210E-01
+2.637619E-01
+6.957033E-02
+3.887017E-01
+1.510890E-01
+4.439407E-01
+1.970834E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -176,14 +176,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.809472E-01
-2.313102E-01
-3.848164E-01
-1.480837E-01
-4.543959E-01
-2.064756E-01
-5.106174E-01
-2.607301E-01
+4.439407E-01
+1.970834E-01
+3.887017E-01
+1.510890E-01
+4.456116E-01
+1.985697E-01
+4.737035E-01
+2.243950E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -200,14 +200,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.106174E-01
-2.607301E-01
-4.543959E-01
-2.064756E-01
-4.543155E-01
-2.064026E-01
-4.626331E-01
-2.140294E-01
+4.737035E-01
+2.243950E-01
+4.456116E-01
+1.985697E-01
+4.760577E-01
+2.266309E-01
+4.703245E-01
+2.212052E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -224,14 +224,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.626331E-01
-2.140294E-01
-4.543155E-01
-2.064026E-01
-4.827763E-01
-2.330729E-01
-4.442611E-01
-1.973679E-01
+4.703245E-01
+2.212052E-01
+4.760577E-01
+2.266309E-01
+4.878056E-01
+2.379543E-01
+4.373120E-01
+1.912418E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -248,14 +248,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.442611E-01
-1.973679E-01
-4.827763E-01
-2.330729E-01
-4.630415E-01
-2.144074E-01
-3.886521E-01
-1.510505E-01
+4.373120E-01
+1.912418E-01
+4.878056E-01
+2.379543E-01
+4.262194E-01
+1.816630E-01
+3.334152E-01
+1.111657E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -272,14 +272,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.886521E-01
-1.510505E-01
-4.630415E-01
-2.144074E-01
-3.535862E-01
-1.250232E-01
-2.530293E-01
-6.402384E-02
+3.334152E-01
+1.111657E-01
+4.262194E-01
+1.816630E-01
+3.560156E-01
+1.267471E-01
+2.409954E-01
+5.807879E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -296,14 +296,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.530293E-01
-6.402384E-02
-3.535862E-01
-1.250232E-01
-2.465508E-01
-6.078730E-02
-1.197139E-01
-1.433141E-02
+2.409954E-01
+5.807879E-02
+3.560156E-01
+1.267471E-01
+2.646501E-01
+7.003965E-02
+1.327244E-01
+1.761576E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -320,12 +320,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.197139E-01
-1.433141E-02
-2.465508E-01
-6.078730E-02
-1.369614E-01
-1.875841E-02
+1.327244E-01
+1.761576E-02
+2.646501E-01
+7.003965E-02
+1.480567E-01
+2.192079E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -345,119 +345,119 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-7.701212E-01
-5.930866E-01
-1.386254E-01
-1.921700E-02
-1.547321E+00
-2.394203E+00
-2.630318E-01
-6.918571E-02
-2.106393E+00
-4.436893E+00
-2.807911E-01
-7.884363E-02
-2.435870E+00
-5.933464E+00
-3.322093E-01
-1.103630E-01
-2.753652E+00
-7.582597E+00
-3.825961E-01
-1.463797E-01
-2.623856E+00
-6.884619E+00
-3.888719E-01
-1.512213E-01
-2.376884E+00
-5.649579E+00
-3.196211E-01
-1.021576E-01
-2.021851E+00
-4.087882E+00
-2.897873E-01
-8.397667E-02
-1.333182E+00
-1.777374E+00
-1.627096E-01
-2.647441E-02
-7.258458E-01
-5.268521E-01
-9.348575E-02
-8.739586E-03
+6.713566E-01
+4.507196E-01
+9.805793E-02
+9.615358E-03
+1.508976E+00
+2.277009E+00
+1.968348E-01
+3.874394E-02
+2.032573E+00
+4.131353E+00
+2.120922E-01
+4.498309E-02
+2.385217E+00
+5.689262E+00
+2.863031E-01
+8.196946E-02
+2.545200E+00
+6.478041E+00
+3.278920E-01
+1.075131E-01
+2.726427E+00
+7.433407E+00
+3.770758E-01
+1.421861E-01
+2.466613E+00
+6.084179E+00
+3.593230E-01
+1.291130E-01
+1.951251E+00
+3.807379E+00
+2.474112E-01
+6.121229E-02
+1.347903E+00
+1.816842E+00
+2.127109E-01
+4.524594E-02
+6.823620E-01
+4.656179E-01
+1.249863E-01
+1.562159E-02
 cmfd indices
 1.000000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.149087E+00
-1.156777E+00
-1.158641E+00
-1.159507E+00
-1.156564E+00
-1.160257E+00
-1.150344E+00
-1.149854E+00
-1.151616E+00
-1.164575E+00
-1.174683E+00
+1.181365E+00
+1.176693E+00
+1.161946E+00
+1.163565E+00
+1.163043E+00
+1.169908E+00
+1.149155E+00
+1.142379E+00
+1.152957E+00
+1.137602E+00
+1.141883E+00
 cmfd entropy
-3.216202E+00
-3.228703E+00
-3.220414E+00
-3.214361E+00
-3.215642E+00
-3.213607E+00
-3.212862E+00
-3.213128E+00
-3.213189E+00
-3.205465E+00
-3.202859E+00
+3.246422E+00
+3.246496E+00
+3.252238E+00
+3.240920E+00
+3.237606E+00
+3.234301E+00
+3.234103E+00
+3.229918E+00
+3.226983E+00
+3.221321E+00
+3.223622E+00
 cmfd balance
-3.08825E-03
-1.42554E-03
-1.21448E-03
-1.17859E-03
-1.06034E-03
-9.30949E-04
-1.35713E-03
-1.13694E-03
-1.14938E-03
-1.29296E-03
-1.46518E-03
+4.18486E-03
+1.72126E-03
+1.10906E-03
+1.88158E-03
+1.31626E-03
+1.30818E-03
+1.77315E-03
+2.16148E-03
+1.67789E-03
+2.31333E-03
+1.94932E-03
 cmfd dominance ratio
-5.503E-01
-5.596E-01
 5.505E-01
-5.471E-01
-5.468E-01
-5.427E-01
-5.421E-01
-5.412E-01
-5.389E-01
-5.371E-01
-5.329E-01
+5.580E-01
+5.610E-01
+5.526E-01
+5.519E-01
+5.499E-01
+5.504E-01
+5.504E-01
+5.475E-01
+5.465E-01
+5.477E-01
 cmfd openmc source comparison
-1.571006E-02
-6.945629E-03
-6.838511E-03
-6.183655E-03
-5.138825E-03
-4.362701E-03
-5.558586E-03
-4.188314E-03
-1.837101E-03
-2.737321E-03
-2.529244E-03
+1.902234E-03
+4.110960E-03
+2.452031E-03
+2.337951E-03
+1.838979E-03
+3.138637E-03
+2.684401E-03
+2.912891E-03
+2.823494E-03
+6.391584E-03
+5.904139E-03
 cmfd source
-4.240947E-02
-8.226746E-02
-1.180848E-01
-1.328470E-01
-1.412449E-01
-1.424870E-01
-1.269297E-01
-1.096476E-01
-6.953001E-02
-3.455212E-02
+4.488002E-02
+8.895136E-02
+1.085930E-01
+1.229651E-01
+1.330479E-01
+1.497140E-01
+1.309102E-01
+1.028556E-01
+7.738878E-02
+4.069397E-02
diff --git a/tests/regression_tests/cmfd_feed_rolling_window/results_true.dat b/tests/regression_tests/cmfd_feed_rolling_window/results_true.dat
index 5ff4a132f..2e31b35ba 100644
--- a/tests/regression_tests/cmfd_feed_rolling_window/results_true.dat
+++ b/tests/regression_tests/cmfd_feed_rolling_window/results_true.dat
@@ -1,117 +1,117 @@
 k-combined:
-1.173626E+00 1.098719E-02
+1.158333E+00 1.402684E-02
 tally 1:
-1.101892E+01
-1.218768E+01
-2.036233E+01
-4.152577E+01
-2.937587E+01
-8.637268E+01
-3.502389E+01
-1.231700E+02
-3.804803E+01
-1.453948E+02
-3.822561E+01
-1.465677E+02
-3.456290E+01
-1.198651E+02
-2.904088E+01
-8.470264E+01
-2.111529E+01
-4.463713E+01
-1.147633E+01
-1.326012E+01
+1.169478E+01
+1.373162E+01
+2.192038E+01
+4.844559E+01
+2.913292E+01
+8.542569E+01
+3.446069E+01
+1.201782E+02
+3.624088E+01
+1.320213E+02
+3.569791E+01
+1.278170E+02
+3.340601E+01
+1.119165E+02
+2.908648E+01
+8.514603E+01
+2.175458E+01
+4.767916E+01
+1.171268E+01
+1.378033E+01
 tally 2:
-1.010478E+00
-1.021066E+00
-6.902031E-01
-4.763804E-01
-1.899891E+00
-3.609584E+00
-1.322615E+00
-1.749312E+00
-2.756419E+00
-7.597845E+00
-1.955934E+00
-3.825676E+00
-3.818740E+00
-1.458278E+01
-2.704746E+00
-7.315652E+00
-3.920857E+00
-1.537312E+01
-2.843144E+00
-8.083470E+00
-3.835060E+00
-1.470768E+01
-2.728705E+00
-7.445831E+00
-3.510590E+00
-1.232424E+01
-2.497237E+00
-6.236191E+00
-2.717388E+00
-7.384198E+00
-1.903638E+00
-3.623837E+00
-2.207863E+00
-4.874659E+00
-1.563588E+00
-2.444808E+00
-1.289027E+00
-1.661591E+00
-9.022714E-01
-8.140937E-01
+1.132414E+00
+1.282361E+00
+7.822980E-01
+6.119901E-01
+2.124428E+00
+4.513196E+00
+1.490832E+00
+2.222581E+00
+3.158472E+00
+9.975946E+00
+2.221665E+00
+4.935795E+00
+3.994786E+00
+1.595831E+01
+2.828109E+00
+7.998199E+00
+3.491035E+00
+1.218732E+01
+2.461223E+00
+6.057617E+00
+3.461784E+00
+1.198395E+01
+2.473337E+00
+6.117398E+00
+3.027132E+00
+9.163527E+00
+2.150455E+00
+4.624458E+00
+2.471217E+00
+6.106911E+00
+1.737168E+00
+3.017752E+00
+1.880969E+00
+3.538044E+00
+1.316574E+00
+1.733367E+00
+1.233103E+00
+1.520543E+00
+8.543318E-01
+7.298828E-01
 tally 3:
-6.593197E-01
-4.347024E-01
-5.216387E-02
-2.721069E-03
-1.266446E+00
-1.603885E+00
-8.298797E-02
-6.887003E-03
-1.888709E+00
-3.567222E+00
-1.398940E-01
-1.957033E-02
-2.616078E+00
-6.843867E+00
-1.588627E-01
-2.523735E-02
-2.733300E+00
-7.470927E+00
-1.944290E-01
-3.780262E-02
-2.643656E+00
-6.988917E+00
-1.612338E-01
-2.599633E-02
-2.410643E+00
-5.811199E+00
-1.754603E-01
-3.078631E-02
-1.838012E+00
-3.378288E+00
-1.126265E-01
-1.268473E-02
-1.500033E+00
-2.250100E+00
-1.102554E-01
-1.215626E-02
-8.750096E-01
-7.656418E-01
-6.757592E-02
-4.566505E-03
+7.540113E-01
+5.685330E-01
+6.367610E-02
+4.054646E-03
+1.436984E+00
+2.064922E+00
+8.961822E-02
+8.031425E-03
+2.132240E+00
+4.546449E+00
+1.356065E-01
+1.838913E-02
+2.726356E+00
+7.433016E+00
+1.780572E-01
+3.170438E-02
+2.379700E+00
+5.662970E+00
+1.544735E-01
+2.386206E-02
+2.383471E+00
+5.680933E+00
+1.568319E-01
+2.459624E-02
+2.047271E+00
+4.191318E+00
+1.662654E-01
+2.764418E-02
+1.673107E+00
+2.799287E+00
+1.132020E-01
+1.281468E-02
+1.271576E+00
+1.616906E+00
+7.546797E-02
+5.695415E-03
+8.249906E-01
+6.806094E-01
+5.660098E-02
+3.203671E-03
 tally 4:
-1.490605E-01
-2.221904E-02
+1.551630E-01
+2.407556E-02
 0.000000E+00
 0.000000E+00
-1.139233E-01
-1.297851E-02
-2.549497E-01
-6.499934E-02
+1.487992E-01
+2.214121E-02
+2.878091E-01
+8.283409E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -128,14 +128,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.549497E-01
-6.499934E-02
-1.139233E-01
-1.297851E-02
-2.191337E-01
-4.801958E-02
-3.295187E-01
-1.085826E-01
+2.878091E-01
+8.283409E-02
+1.487992E-01
+2.214121E-02
+2.936596E-01
+8.623595E-02
+3.954149E-01
+1.563529E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -152,14 +152,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.295187E-01
-1.085826E-01
-2.191337E-01
-4.801958E-02
-3.872400E-01
-1.499548E-01
-4.595835E-01
-2.112170E-01
+3.954149E-01
+1.563529E-01
+2.936596E-01
+8.623595E-02
+3.991153E-01
+1.592930E-01
+4.758410E-01
+2.264247E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -176,14 +176,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.595835E-01
-2.112170E-01
-3.872400E-01
-1.499548E-01
-4.668106E-01
-2.179121E-01
-5.112307E-01
-2.613569E-01
+4.758410E-01
+2.264247E-01
+3.991153E-01
+1.592930E-01
+4.850882E-01
+2.353106E-01
+5.210840E-01
+2.715285E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -200,14 +200,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.112307E-01
-2.613569E-01
-4.668106E-01
-2.179121E-01
-4.716605E-01
-2.224636E-01
-4.916148E-01
-2.416851E-01
+5.210840E-01
+2.715285E-01
+4.850882E-01
+2.353106E-01
+4.790245E-01
+2.294645E-01
+4.570092E-01
+2.088574E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -224,14 +224,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.916148E-01
-2.416851E-01
-4.716605E-01
-2.224636E-01
-4.696777E-01
-2.205972E-01
-4.365150E-01
-1.905453E-01
+4.570092E-01
+2.088574E-01
+4.790245E-01
+2.294645E-01
+4.505886E-01
+2.030301E-01
+3.884038E-01
+1.508575E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -248,14 +248,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.365150E-01
-1.905453E-01
-4.696777E-01
-2.205972E-01
-4.179902E-01
-1.747158E-01
-3.350564E-01
-1.122628E-01
+3.884038E-01
+1.508575E-01
+4.505886E-01
+2.030301E-01
+3.986999E-01
+1.589616E-01
+3.220889E-01
+1.037412E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -272,14 +272,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.350564E-01
-1.122628E-01
-4.179902E-01
-1.747158E-01
-3.743487E-01
-1.401370E-01
-2.400661E-01
-5.763172E-02
+3.220889E-01
+1.037412E-01
+3.986999E-01
+1.589616E-01
+3.319083E-01
+1.101631E-01
+2.101261E-01
+4.415297E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -296,14 +296,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.400661E-01
-5.763172E-02
-3.743487E-01
-1.401370E-01
-3.063657E-01
-9.385994E-02
-1.605078E-01
-2.576275E-02
+2.101261E-01
+4.415297E-02
+3.319083E-01
+1.101631E-01
+2.795459E-01
+7.814588E-02
+1.306499E-01
+1.706938E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -320,12 +320,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.605078E-01
-2.576275E-02
-3.063657E-01
-9.385994E-02
-1.700639E-01
-2.892174E-02
+1.306499E-01
+1.706938E-02
+2.795459E-01
+7.814588E-02
+1.585507E-01
+2.513833E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -345,119 +345,119 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-6.593197E-01
-4.347024E-01
-1.314196E-01
-1.727112E-02
-1.266446E+00
-1.603885E+00
-2.002491E-01
-4.009970E-02
-1.888709E+00
-3.567222E+00
-2.444522E-01
-5.975686E-02
-2.616078E+00
-6.843867E+00
-3.234935E-01
-1.046481E-01
-2.733300E+00
-7.470927E+00
-3.309500E-01
-1.095279E-01
-2.643656E+00
-6.988917E+00
-4.025014E-01
-1.620074E-01
-2.410643E+00
-5.811199E+00
-3.050040E-01
-9.302747E-02
-1.838012E+00
-3.378288E+00
-2.800764E-01
-7.844279E-02
-1.500033E+00
-2.250100E+00
-2.324765E-01
-5.404531E-02
-8.750096E-01
-7.656418E-01
-1.256919E-01
-1.579844E-02
+7.530237E-01
+5.670447E-01
+1.178984E-01
+1.390004E-02
+1.436984E+00
+2.064922E+00
+1.458395E-01
+2.126916E-02
+2.132240E+00
+4.546449E+00
+2.950109E-01
+8.703141E-02
+2.726356E+00
+7.433016E+00
+3.397288E-01
+1.154157E-01
+2.379700E+00
+5.662970E+00
+3.113206E-01
+9.692053E-02
+2.383471E+00
+5.680933E+00
+3.455611E-01
+1.194125E-01
+2.047271E+00
+4.191318E+00
+2.910986E-01
+8.473841E-02
+1.673107E+00
+2.799287E+00
+2.381996E-01
+5.673907E-02
+1.270672E+00
+1.614606E+00
+1.795241E-01
+3.222889E-02
+8.249906E-01
+6.806094E-01
+1.201397E-01
+1.443354E-02
 cmfd indices
 1.000000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.166297E+00
-1.148237E+00
-1.162472E+00
-1.187078E+00
-1.188411E+00
-1.194879E+00
-1.216739E+00
-1.216829E+00
-1.196596E+00
-1.199223E+00
-1.211817E+00
+1.184474E+00
+1.188170E+00
+1.165127E+00
+1.135010E+00
+1.145439E+00
+1.158451E+00
+1.154420E+00
+1.179615E+00
+1.197843E+00
+1.181252E+00
+1.186316E+00
 cmfd entropy
-3.215349E+00
-3.225577E+00
-3.223357E+00
-3.208828E+00
-3.211072E+00
-3.206578E+00
-3.195799E+00
-3.198236E+00
-3.220074E+00
-3.230249E+00
-3.238015E+00
+3.243654E+00
+3.244091E+00
+3.249203E+00
+3.249952E+00
+3.245538E+00
+3.240838E+00
+3.238919E+00
+3.223131E+00
+3.216002E+00
+3.220324E+00
+3.224804E+00
 cmfd balance
-2.07468E-03
-2.39687E-03
-1.51020E-03
-2.07618E-03
-1.89367E-03
-2.00902E-03
-2.54856E-03
-2.43636E-03
-2.48527E-03
-3.07775E-03
-3.37967E-03
+4.21104E-03
+1.38052E-03
+1.34642E-03
+2.39255E-03
+2.07426E-03
+1.27927E-03
+2.26249E-03
+2.72103E-03
+2.71504E-03
+2.19156E-03
+1.91989E-03
 cmfd dominance ratio
-5.505E-01
-5.558E-01
-5.584E-01
-5.477E-01
-5.477E-01
-5.426E-01
-5.335E-01
-5.305E-01
-5.427E-01
+5.520E-01
+5.535E-01
+5.628E-01
+5.696E-01
+5.723E-01
+5.651E-01
+5.648E-01
+5.555E-01
+5.448E-01
+5.488E-01
 5.484E-01
-5.465E-01
 cmfd openmc source comparison
-5.598628E-03
-1.162952E-02
-1.083004E-02
-1.037470E-02
-5.649750E-03
-5.137914E-03
-3.868209E-03
-1.208756E-02
-5.398131E-03
-8.119598E-03
-4.573105E-03
+1.713810E-03
+2.429503E-03
+4.526209E-03
+7.978149E-03
+3.320012E-03
+3.880041E-03
+1.580215E-02
+1.663452E-02
+1.878103E-02
+7.436342E-03
+3.724478E-03
 cmfd source
-4.219187E-02
-8.692096E-02
-1.061389E-01
-1.199181E-01
-1.377328E-01
-1.326161E-01
-1.345872E-01
-1.112871E-01
-7.569533E-02
-5.291175E-02
+4.688167E-02
+9.066303E-02
+1.150679E-01
+1.430247E-01
+1.370466E-01
+1.267615E-01
+1.244218E-01
+1.048774E-01
+7.083441E-02
+4.042108E-02
diff --git a/tests/regression_tests/cmfd_nofeed/results_true.dat b/tests/regression_tests/cmfd_nofeed/results_true.dat
index 756aa4570..ea1a0230b 100644
--- a/tests/regression_tests/cmfd_nofeed/results_true.dat
+++ b/tests/regression_tests/cmfd_nofeed/results_true.dat
@@ -1,117 +1,117 @@
 k-combined:
-1.172893E+00 8.095197E-03
+1.169143E+00 7.248013E-03
 tally 1:
-1.156995E+01
-1.347019E+01
-2.120053E+01
-4.531200E+01
-2.995993E+01
-8.997889E+01
-3.498938E+01
-1.226414E+02
-3.794510E+01
-1.442188E+02
-3.798115E+01
-1.446436E+02
-3.415954E+01
-1.171635E+02
-2.960329E+01
-8.785532E+01
-2.182231E+01
-4.782353E+01
-1.147379E+01
-1.321150E+01
+1.115130E+01
+1.249933E+01
+2.147608E+01
+4.643964E+01
+2.923697E+01
+8.598273E+01
+3.439175E+01
+1.189653E+02
+3.729169E+01
+1.395456E+02
+3.709975E+01
+1.380839E+02
+3.415420E+01
+1.168226E+02
+2.895696E+01
+8.419764E+01
+2.140382E+01
+4.646784E+01
+1.125483E+01
+1.275812E+01
 tally 2:
-2.345900E+01
-2.783672E+01
-1.627300E+01
-1.339749E+01
-4.127267E+01
-8.563716E+01
-2.934800E+01
-4.334439E+01
-5.742644E+01
-1.658158E+02
-4.092600E+01
-8.423842E+01
-6.740126E+01
-2.279288E+02
-4.796500E+01
-1.154602E+02
-7.340327E+01
-2.701349E+02
-5.235900E+01
-1.374186E+02
-7.387392E+01
-2.740829E+02
-5.277400E+01
-1.398425E+02
-6.733428E+01
-2.274414E+02
-4.800100E+01
-1.156206E+02
-5.794970E+01
-1.685421E+02
-4.124600E+01
-8.540686E+01
-4.257013E+01
-9.092401E+01
-3.014500E+01
-4.566369E+01
-2.300274E+01
-2.659051E+01
-1.613400E+01
-1.307448E+01
+2.275147E+01
+2.604974E+01
+1.587800E+01
+1.271197E+01
+4.191959E+01
+8.846441E+01
+2.966500E+01
+4.430669E+01
+5.699815E+01
+1.637359E+02
+4.037600E+01
+8.219353E+01
+6.656851E+01
+2.228703E+02
+4.718000E+01
+1.119941E+02
+7.334215E+01
+2.701014E+02
+5.223500E+01
+1.370726E+02
+7.394394E+01
+2.748295E+02
+5.273400E+01
+1.397334E+02
+6.931604E+01
+2.406234E+02
+4.949000E+01
+1.226886E+02
+5.799672E+01
+1.687992E+02
+4.142300E+01
+8.612116E+01
+4.320102E+01
+9.402138E+01
+3.068300E+01
+4.749148E+01
+2.295257E+01
+2.657057E+01
+1.606200E+01
+1.301453E+01
 tally 3:
-1.564900E+01
-1.239852E+01
-1.086873E+00
-6.047264E-02
-2.821700E+01
-4.006730E+01
-1.855830E+00
-1.755984E-01
-3.946300E+01
-7.833997E+01
-2.523142E+00
-3.210725E-01
-4.622500E+01
-1.072588E+02
-2.919735E+00
-4.340292E-01
-5.033400E+01
-1.270010E+02
-3.243022E+00
-5.318534E-01
-5.082400E+01
-1.297502E+02
-3.313898E+00
-5.546608E-01
-4.623700E+01
-1.072944E+02
-2.887766E+00
-4.203084E-01
-3.975000E+01
-7.934279E+01
-2.567158E+00
-3.333121E-01
-2.903500E+01
-4.237270E+01
-1.852070E+00
-1.733821E-01
-1.557800E+01
-1.219084E+01
-9.951884E-01
-5.121758E-02
+1.528200E+01
+1.177982E+01
+1.040687E+00
+5.586386E-02
+2.857900E+01
+4.113603E+01
+1.871515E+00
+1.774689E-01
+3.888800E+01
+7.626262E+01
+2.534433E+00
+3.274088E-01
+4.541400E+01
+1.037867E+02
+2.926509E+00
+4.319780E-01
+5.034500E+01
+1.273517E+02
+3.215813E+00
+5.215716E-01
+5.076100E+01
+1.295218E+02
+3.194424E+00
+5.165993E-01
+4.768100E+01
+1.138949E+02
+3.058255E+00
+4.732131E-01
+3.995800E+01
+8.015691E+01
+2.454286E+00
+3.044948E-01
+2.957000E+01
+4.412743E+01
+1.938963E+00
+1.908501E-01
+1.544000E+01
+1.202869E+01
+1.038073E+00
+5.487827E-02
 tally 4:
-3.111000E+00
-4.872850E-01
+3.086000E+00
+4.780160E-01
 0.000000E+00
 0.000000E+00
-2.794000E+00
-3.972060E-01
-5.520000E+00
-1.535670E+00
+2.739000E+00
+3.790990E-01
+5.478000E+00
+1.505928E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -128,14 +128,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.520000E+00
-1.535670E+00
-2.794000E+00
-3.972060E-01
-5.071000E+00
-1.305697E+00
-7.303000E+00
-2.685365E+00
+5.478000E+00
+1.505928E+00
+2.739000E+00
+3.790990E-01
+5.094000E+00
+1.310476E+00
+7.282000E+00
+2.669810E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -152,14 +152,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.303000E+00
-2.685365E+00
-5.071000E+00
-1.305697E+00
-7.015000E+00
-2.471981E+00
-8.545000E+00
-3.670539E+00
+7.282000E+00
+2.669810E+00
+5.094000E+00
+1.310476E+00
+6.987000E+00
+2.461137E+00
+8.487000E+00
+3.624153E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -176,14 +176,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.545000E+00
-3.670539E+00
-7.015000E+00
-2.471981E+00
-8.431000E+00
-3.570057E+00
-9.224000E+00
-4.268004E+00
+8.487000E+00
+3.624153E+00
+6.987000E+00
+2.461137E+00
+8.250000E+00
+3.421824E+00
+9.022000E+00
+4.088536E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -200,14 +200,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.224000E+00
-4.268004E+00
-8.431000E+00
-3.570057E+00
-9.217000E+00
-4.259749E+00
-9.305000E+00
-4.340149E+00
+9.022000E+00
+4.088536E+00
+8.250000E+00
+3.421824E+00
+9.300000E+00
+4.344142E+00
+9.262000E+00
+4.308946E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -224,14 +224,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.305000E+00
-4.340149E+00
-9.217000E+00
-4.259749E+00
-9.374000E+00
-4.415290E+00
-8.611000E+00
-3.718227E+00
+9.262000E+00
+4.308946E+00
+9.300000E+00
+4.344142E+00
+9.267000E+00
+4.310941E+00
+8.487000E+00
+3.613257E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -248,14 +248,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.611000E+00
-3.718227E+00
-9.374000E+00
-4.415290E+00
-8.515000E+00
-3.639945E+00
-7.056000E+00
-2.501658E+00
+8.487000E+00
+3.613257E+00
+9.267000E+00
+4.310941E+00
+8.682000E+00
+3.778194E+00
+7.123000E+00
+2.544345E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -272,14 +272,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.056000E+00
-2.501658E+00
-8.515000E+00
-3.639945E+00
-7.385000E+00
-2.737677E+00
-5.194000E+00
-1.356022E+00
+7.123000E+00
+2.544345E+00
+8.682000E+00
+3.778194E+00
+7.421000E+00
+2.773897E+00
+5.198000E+00
+1.369216E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -296,14 +296,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.194000E+00
-1.356022E+00
-7.385000E+00
-2.737677E+00
-5.436000E+00
-1.481654E+00
-2.756000E+00
-3.843860E-01
+5.198000E+00
+1.369216E+00
+7.421000E+00
+2.773897E+00
+5.567000E+00
+1.561013E+00
+2.763000E+00
+3.882750E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -320,12 +320,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.756000E+00
-3.843860E-01
-5.436000E+00
-1.481654E+00
-3.030000E+00
-4.643920E-01
+2.763000E+00
+3.882750E-01
+5.567000E+00
+1.561013E+00
+3.106000E+00
+4.853380E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -345,144 +345,144 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-1.564300E+01
-1.238861E+01
-2.090227E+00
-2.262627E-01
-2.821400E+01
-4.005826E+01
-3.870834E+00
-7.614623E-01
-3.945400E+01
-7.830326E+01
-5.290557E+00
-1.422339E+00
-4.621500E+01
-1.072116E+02
-6.144745E+00
-1.903309E+00
-5.032900E+01
-1.269756E+02
-6.867524E+00
-2.373593E+00
-5.081000E+01
-1.296784E+02
-6.456283E+00
-2.130767E+00
-4.623000E+01
-1.072613E+02
-5.931623E+00
-1.776451E+00
-3.974500E+01
-7.932288E+01
-5.339603E+00
-1.456812E+00
-2.902800E+01
-4.235232E+01
-4.102240E+00
-8.519551E-01
-1.557800E+01
-1.219084E+01
-2.137954E+00
-2.347770E-01
+1.527700E+01
+1.177204E+01
+2.285003E+00
+2.661382E-01
+2.857200E+01
+4.111506E+01
+4.126099E+00
+8.737996E-01
+3.887800E+01
+7.622094E+01
+5.121343E+00
+1.333837E+00
+4.540600E+01
+1.037492E+02
+6.160114E+00
+1.913665E+00
+5.033400E+01
+1.272949E+02
+6.859861E+00
+2.384476E+00
+5.075800E+01
+1.295055E+02
+6.929393E+00
+2.443364E+00
+4.767500E+01
+1.138658E+02
+6.385463E+00
+2.080293E+00
+3.995300E+01
+8.013651E+01
+5.620641E+00
+1.603844E+00
+2.956200E+01
+4.410395E+01
+3.952701E+00
+7.877105E-01
+1.543500E+01
+1.202080E+01
+2.203583E+00
+2.512936E-01
 cmfd indices
 1.000000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.129918E+00
-1.148352E+00
-1.143137E+00
-1.145795E+00
-1.147285E+00
-1.148588E+00
-1.148151E+00
-1.162118E+00
-1.165380E+00
-1.162851E+00
-1.163379E+00
-1.166986E+00
-1.167838E+00
-1.171743E+00
-1.170398E+00
-1.169773E+00
+1.169107E+00
+1.173852E+00
+1.181921E+00
+1.187733E+00
+1.185766E+00
+1.177020E+00
+1.181200E+00
+1.180088E+00
+1.180676E+00
+1.174948E+00
+1.174167E+00
+1.174935E+00
+1.169912E+00
+1.169058E+00
+1.170366E+00
+1.169774E+00
 cmfd entropy
-3.224769E+00
-3.222795E+00
-3.221174E+00
-3.222164E+00
-3.221025E+00
-3.220967E+00
-3.223497E+00
-3.219213E+00
-3.221679E+00
-3.222084E+00
-3.222281E+00
-3.222540E+00
-3.224614E+00
-3.224193E+00
-3.224925E+00
-3.224367E+00
+3.207640E+00
+3.212075E+00
+3.215463E+00
+3.219545E+00
+3.225225E+00
+3.227103E+00
+3.229048E+00
+3.228263E+00
+3.229077E+00
+3.229932E+00
+3.229351E+00
+3.228195E+00
+3.228493E+00
+3.227823E+00
+3.225830E+00
+3.227270E+00
 cmfd balance
-3.90454E-03
-4.33180E-03
-3.77057E-03
-3.16391E-03
-3.11765E-03
-2.59886E-03
-2.81060E-03
-3.25473E-03
-2.68544E-03
-2.01716E-03
-1.89350E-03
-1.79159E-03
-1.51353E-03
-1.48514E-03
-1.50207E-03
-1.44045E-03
+4.88208E-03
+4.63702E-03
+3.41158E-03
+2.99755E-03
+2.78360E-03
+3.31542E-03
+2.64344E-03
+2.04609E-03
+1.84340E-03
+1.65450E-03
+1.70816E-03
+1.69952E-03
+1.51417E-03
+1.32738E-03
+1.41435E-03
+1.01462E-03
 cmfd dominance ratio
-5.539E-01
-5.522E-01
-5.491E-01
-5.511E-01
-5.506E-01
-5.523E-01
-5.523E-01
+5.467E-01
+5.468E-01
+5.448E-01
+5.457E-01
+5.457E-01
+5.485E-01
+5.500E-01
+5.497E-01
+5.495E-01
+5.499E-01
+5.502E-01
+5.485E-01
+5.492E-01
+5.483E-01
 5.473E-01
-5.478E-01
-5.461E-01
-5.462E-01
-5.459E-01
-5.475E-01
-5.463E-01
 5.466E-01
-5.469E-01
 cmfd openmc source comparison
-9.875240E-03
-1.119358E-02
-8.513903E-03
-7.728971E-03
-5.993771E-03
-5.837301E-03
-4.861789E-03
-5.624038E-03
-4.297229E-03
-4.029732E-03
-3.669197E-03
-3.598834E-03
-3.023310E-03
-3.347346E-03
-2.943658E-03
-2.764986E-03
+9.587418E-03
+7.785087E-03
+6.798967E-03
+5.947641E-03
+4.980801E-03
+4.272665E-03
+4.073759E-03
+4.305612E-03
+3.572759E-03
+3.785830E-03
+3.766828E-03
+3.495462E-03
+3.017281E-03
+2.857633E-03
+2.858606E-03
+2.449010E-03
 cmfd source
-4.561921E-02
-7.896381E-02
-1.084687E-01
-1.264057E-01
-1.408942E-01
-1.438180E-01
-1.247333E-01
-1.100896E-01
-7.897187E-02
-4.203556E-02
+4.390084E-02
+7.966902E-02
+1.087889E-01
+1.263915E-01
+1.394331E-01
+1.383156E-01
+1.319970E-01
+1.051339E-01
+8.248244E-02
+4.388774E-02
diff --git a/tests/regression_tests/cmfd_restart/results_true.dat b/tests/regression_tests/cmfd_restart/results_true.dat
index f5f22d9ac..1ef9624d4 100644
--- a/tests/regression_tests/cmfd_restart/results_true.dat
+++ b/tests/regression_tests/cmfd_restart/results_true.dat
@@ -1,117 +1,117 @@
 k-combined:
-1.164262E+00 9.207592E-03
+1.181723E+00 9.944883E-03
 tally 1:
-1.156972E+01
-1.339924E+01
-2.136306E+01
-4.567185E+01
-2.859527E+01
-8.195821E+01
-3.470754E+01
-1.207851E+02
-3.766403E+01
-1.422263E+02
-3.778821E+01
-1.432660E+02
-3.573197E+01
-1.278854E+02
-2.849979E+01
-8.135515E+01
-2.073803E+01
-4.303374E+01
-1.112117E+01
-1.242944E+01
+1.169899E+01
+1.373251E+01
+2.142380E+01
+4.605511E+01
+2.968085E+01
+8.838716E+01
+3.561418E+01
+1.271206E+02
+3.777783E+01
+1.428817E+02
+3.805832E+01
+1.450213E+02
+3.439836E+01
+1.184892E+02
+2.852438E+01
+8.161896E+01
+2.088423E+01
+4.376204E+01
+1.076670E+01
+1.168108E+01
 tally 2:
-2.388054E+01
-2.875255E+01
-1.667791E+01
-1.403426E+01
-4.224771E+01
-8.942109E+01
-2.993088E+01
-4.490335E+01
-5.689839E+01
-1.625557E+02
-4.043633E+01
-8.212299E+01
-6.764024E+01
-2.297126E+02
-4.807902E+01
-1.161468E+02
-7.314835E+01
-2.684645E+02
-5.203584E+01
-1.359261E+02
-7.375727E+01
-2.733105E+02
-5.252944E+01
-1.386205E+02
-6.909571E+01
-2.397721E+02
-4.922548E+01
-1.217465E+02
-5.685978E+01
-1.621746E+02
-4.051938E+01
-8.237277E+01
-4.185562E+01
-8.784067E+01
-2.983570E+01
-4.467414E+01
-2.238373E+01
-2.520356E+01
-1.566758E+01
-1.234103E+01
+2.321241E+01
+2.702156E+01
+1.620912E+01
+1.317752E+01
+4.197404E+01
+8.845008E+01
+2.982666E+01
+4.469221E+01
+5.810089E+01
+1.695857E+02
+4.134123E+01
+8.588866E+01
+6.982488E+01
+2.447068E+02
+4.966939E+01
+1.238763E+02
+7.428421E+01
+2.767613E+02
+5.287955E+01
+1.403163E+02
+7.447402E+01
+2.785012E+02
+5.324628E+01
+1.423393E+02
+6.895164E+01
+2.381937E+02
+4.916366E+01
+1.211701E+02
+5.679253E+01
+1.617881E+02
+4.043125E+01
+8.204061E+01
+4.218618E+01
+8.933666E+01
+2.978592E+01
+4.456592E+01
+2.196426E+01
+2.435867E+01
+1.525576E+01
+1.175879E+01
 tally 3:
-1.609520E+01
-1.307542E+01
-1.033429E+00
-5.510889E-02
-2.877073E+01
-4.149542E+01
-1.964219E+00
-1.954692E-01
-3.896816E+01
-7.629752E+01
-2.484053E+00
-3.103733E-01
-4.634285E+01
-1.079367E+02
-2.974750E+00
-4.468223E-01
-5.007964E+01
-1.259202E+02
-3.181802E+00
-5.103621E-01
-5.058915E+01
-1.286193E+02
-3.249442E+00
-5.337712E-01
-4.744464E+01
-1.131026E+02
-3.067644E+00
-4.736335E-01
-3.900632E+01
-7.634433E+01
-2.443552E+00
-3.028060E-01
-2.874166E+01
-4.146375E+01
-1.810421E+00
-1.671667E-01
-1.509222E+01
-1.145579E+01
-1.014919E+00
-5.391053E-02
+1.563788E+01
+1.226528E+01
+1.053289E+00
+5.666942E-02
+2.870755E+01
+4.139654E+01
+1.838017E+00
+1.710528E-01
+3.978616E+01
+7.955764E+01
+2.560657E+00
+3.334449E-01
+4.780385E+01
+1.147770E+02
+3.139243E+00
+4.967628E-01
+5.106650E+01
+1.308704E+02
+3.170056E+00
+5.078920E-01
+5.123992E+01
+1.318586E+02
+3.211706E+00
+5.205979E-01
+4.729862E+01
+1.121695E+02
+3.068662E+00
+4.749488E-01
+3.898816E+01
+7.630564E+01
+2.516911E+00
+3.199696E-01
+2.865357E+01
+4.125742E+01
+1.852314E+00
+1.741116E-01
+1.467340E+01
+1.088460E+01
+9.268633E-01
+4.450662E-02
 tally 4:
-3.148231E+00
-4.974555E-01
+3.029754E+00
+4.613561E-01
 0.000000E+00
 0.000000E+00
-2.805439E+00
-3.982239E-01
-5.574031E+00
-1.561105E+00
+2.832501E+00
+4.049252E-01
+5.517243E+00
+1.527794E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -128,14 +128,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.574031E+00
-1.561105E+00
-2.805439E+00
-3.982239E-01
-5.171038E+00
-1.344877E+00
-7.372031E+00
-2.725420E+00
+5.517243E+00
+1.527794E+00
+2.832501E+00
+4.049252E-01
+5.117178E+00
+1.316972E+00
+7.333303E+00
+2.701677E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -152,14 +152,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.372031E+00
-2.725420E+00
-5.171038E+00
-1.344877E+00
-6.946847E+00
-2.424850E+00
-8.496610E+00
-3.627542E+00
+7.333303E+00
+2.701677E+00
+5.117178E+00
+1.316972E+00
+7.248464E+00
+2.641591E+00
+8.817788E+00
+3.905530E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -176,14 +176,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.496610E+00
-3.627542E+00
-6.946847E+00
-2.424850E+00
-8.479501E+00
-3.607280E+00
-9.261869E+00
-4.305912E+00
+8.817788E+00
+3.905530E+00
+7.248464E+00
+2.641591E+00
+8.646465E+00
+3.749847E+00
+9.460948E+00
+4.495388E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -200,14 +200,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.261869E+00
-4.305912E+00
-8.479501E+00
-3.607280E+00
-9.232858E+00
-4.278432E+00
-9.306384E+00
-4.348594E+00
+9.460948E+00
+4.495388E+00
+8.646465E+00
+3.749847E+00
+9.379341E+00
+4.415049E+00
+9.278640E+00
+4.320720E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -224,14 +224,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-9.306384E+00
-4.348594E+00
-9.232858E+00
-4.278432E+00
-9.299764E+00
-4.347828E+00
-8.511976E+00
-3.639893E+00
+9.278640E+00
+4.320720E+00
+9.379341E+00
+4.415049E+00
+9.465746E+00
+4.498591E+00
+8.656146E+00
+3.760545E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -248,14 +248,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.511976E+00
-3.639893E+00
-9.299764E+00
-4.347828E+00
-8.726086E+00
-3.819567E+00
-7.147277E+00
-2.562747E+00
+8.656146E+00
+3.760545E+00
+9.465746E+00
+4.498591E+00
+8.589782E+00
+3.700308E+00
+6.996002E+00
+2.456935E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -272,14 +272,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.147277E+00
-2.562747E+00
-8.726086E+00
-3.819567E+00
-7.218790E+00
-2.612243E+00
-5.018287E+00
-1.263077E+00
+6.996002E+00
+2.456935E+00
+8.589782E+00
+3.700308E+00
+7.352050E+00
+2.714808E+00
+5.105164E+00
+1.312559E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -296,14 +296,14 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.018287E+00
-1.263077E+00
-7.218790E+00
-2.612243E+00
-5.443494E+00
-1.487018E+00
-2.732334E+00
-3.773047E-01
+5.105164E+00
+1.312559E+00
+7.352050E+00
+2.714808E+00
+5.442756E+00
+1.486776E+00
+2.697305E+00
+3.675580E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -320,12 +320,12 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.732334E+00
-3.773047E-01
-5.443494E+00
-1.487018E+00
-3.044773E+00
-4.655756E-01
+2.697305E+00
+3.675580E-01
+5.442756E+00
+1.486776E+00
+3.017025E+00
+4.571443E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -345,144 +345,144 @@ tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-1.609029E+01
-1.306718E+01
-2.230601E+00
-2.559496E-01
-2.876780E+01
-4.148686E+01
-3.835952E+00
-7.456562E-01
-3.895738E+01
-7.625344E+01
-4.841024E+00
-1.197335E+00
-4.633595E+01
-1.079043E+02
-6.236821E+00
-1.963311E+00
-5.007472E+01
-1.258967E+02
-6.749745E+00
-2.297130E+00
-5.058336E+01
-1.285894E+02
-6.727612E+00
-2.315656E+00
-4.743869E+01
-1.130735E+02
-6.338193E+00
-2.042159E+00
-3.899838E+01
-7.631289E+01
-5.187573E+00
-1.359733E+00
-2.873434E+01
-4.144233E+01
-3.815610E+00
-7.367619E-01
-1.509020E+01
-1.145268E+01
-2.125767E+00
-2.341894E-01
+1.563588E+01
+1.226217E+01
+2.209027E+00
+2.507131E-01
+2.870034E+01
+4.137550E+01
+3.726620E+00
+7.021948E-01
+3.977762E+01
+7.952285E+01
+5.304975E+00
+1.427333E+00
+4.779747E+01
+1.147456E+02
+6.528302E+00
+2.151715E+00
+5.105366E+01
+1.308037E+02
+6.986782E+00
+2.467487E+00
+5.123380E+01
+1.318264E+02
+6.845633E+00
+2.383542E+00
+4.729296E+01
+1.121433E+02
+6.252695E+00
+1.977351E+00
+3.898236E+01
+7.628315E+01
+5.461495E+00
+1.528579E+00
+2.864576E+01
+4.123538E+01
+3.857301E+00
+7.581323E-01
+1.467047E+01
+1.088031E+01
+2.277024E+00
+2.679801E-01
 cmfd indices
 1.000000E+01
 1.000000E+00
 1.000000E+00
 1.000000E+00
 k cmfd
-1.129918E+00
-1.143848E+00
-1.147976E+00
-1.151534E+00
-1.152378E+00
-1.148219E+00
-1.150402E+00
-1.154647E+00
-1.156159E+00
-1.160048E+00
-1.167441E+00
-1.168163E+00
-1.168629E+00
-1.164120E+00
-1.165051E+00
-1.169177E+00
+1.169107E+00
+1.175079E+00
+1.173912E+00
+1.175368E+00
+1.174026E+00
+1.181745E+00
+1.182261E+00
+1.183559E+00
+1.178691E+00
+1.179222E+00
+1.179017E+00
+1.172979E+00
+1.175043E+00
+1.173458E+00
+1.174152E+00
+1.171451E+00
 cmfd entropy
-3.224769E+00
-3.225945E+00
-3.227421E+00
-3.226174E+00
-3.224429E+00
-3.227049E+00
-3.230710E+00
-3.230315E+00
-3.226825E+00
-3.226655E+00
-3.226588E+00
-3.224155E+00
-3.223246E+00
-3.222640E+00
-3.223920E+00
-3.222838E+00
+3.207640E+00
+3.210547E+00
+3.212218E+00
+3.209573E+00
+3.211619E+00
+3.212126E+00
+3.213163E+00
+3.214288E+00
+3.215737E+00
+3.213677E+00
+3.214925E+00
+3.215612E+00
+3.216708E+00
+3.221454E+00
+3.219048E+00
+3.218387E+00
 cmfd balance
-3.90454E-03
-4.08089E-03
-3.46511E-03
-4.09535E-03
-2.62009E-03
-2.23559E-03
-2.54033E-03
-2.12799E-03
-2.25864E-03
-1.85766E-03
-1.49916E-03
-1.63471E-03
-1.48377E-03
-1.59800E-03
-1.37354E-03
-1.32853E-03
+4.88208E-03
+4.75139E-03
+3.15783E-03
+3.67091E-03
+2.99797E-03
+2.91060E-03
+2.06576E-03
+1.83482E-03
+1.56292E-03
+1.58659E-03
+2.32986E-03
+1.47376E-03
+1.46673E-03
+1.22627E-03
+1.31963E-03
+1.26456E-03
 cmfd dominance ratio
-5.539E-01
-5.537E-01
-5.536E-01
-5.515E-01
-5.512E-01
-5.514E-01
-5.518E-01
-5.507E-01
-5.500E-01
-5.497E-01
-5.477E-01
-5.461E-01
-5.444E-01
-5.445E-01
-5.454E-01
+5.467E-01
+5.453E-01
+5.458E-01
+5.436E-01
+5.442E-01
+5.406E-01
+5.401E-01
+5.413E-01
+4.995E-01
+5.396E-01
+5.409E-01
+5.414E-01
+5.423E-01
+5.456E-01
+5.442E-01
 5.441E-01
 cmfd openmc source comparison
-9.875240E-03
-1.106163E-02
-9.847628E-03
-6.065921E-03
-5.772039E-03
-4.615656E-03
-4.244331E-03
-3.694299E-03
-3.545814E-03
-3.213063E-03
-3.467537E-03
-3.383489E-03
-3.697591E-03
-3.937358E-03
-3.369124E-03
-3.190359E-03
+9.587418E-03
+8.150978E-03
+6.677661E-03
+6.334727E-03
+5.153692E-03
+5.082964E-03
+4.633153E-03
+4.037383E-03
+3.528742E-03
+4.559089E-03
+3.517370E-03
+3.306117E-03
+2.913809E-03
+1.906045E-03
+1.932794E-03
+1.711341E-03
 cmfd source
-4.360494E-02
-8.397599E-02
-1.074181E-01
-1.294531E-01
-1.385611E-01
-1.407934E-01
-1.325191E-01
-1.044311E-01
-7.660359E-02
-4.263941E-02
+4.496492E-02
+7.869674E-02
+1.100280E-01
+1.354045E-01
+1.363339E-01
+1.380533E-01
+1.314512E-01
+1.077480E-01
+7.847306E-02
+3.884630E-02
diff --git a/tests/regression_tests/collision_track/__init__.py b/tests/regression_tests/collision_track/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/collision_track/case_1_Reactions/collision_track_true.h5 b/tests/regression_tests/collision_track/case_1_Reactions/collision_track_true.h5
new file mode 100644
index 000000000..c31546213
Binary files /dev/null and b/tests/regression_tests/collision_track/case_1_Reactions/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_1_Reactions/inputs_true.dat b/tests/regression_tests/collision_track/case_1_Reactions/inputs_true.dat
new file mode 100644
index 000000000..005a56020
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_1_Reactions/inputs_true.dat
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <reactions>(n,fission) 101</reactions>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/case_2_Cell_ID/collision_track_true.h5 b/tests/regression_tests/collision_track/case_2_Cell_ID/collision_track_true.h5
new file mode 100644
index 000000000..850bba841
Binary files /dev/null and b/tests/regression_tests/collision_track/case_2_Cell_ID/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_2_Cell_ID/inputs_true.dat b/tests/regression_tests/collision_track/case_2_Cell_ID/inputs_true.dat
new file mode 100644
index 000000000..8d46c3d3b
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_2_Cell_ID/inputs_true.dat
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <cell_ids>22</cell_ids>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/case_3_Material_ID/collision_track_true.h5 b/tests/regression_tests/collision_track/case_3_Material_ID/collision_track_true.h5
new file mode 100644
index 000000000..777d91f08
Binary files /dev/null and b/tests/regression_tests/collision_track/case_3_Material_ID/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_3_Material_ID/inputs_true.dat b/tests/regression_tests/collision_track/case_3_Material_ID/inputs_true.dat
new file mode 100644
index 000000000..322e74f49
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_3_Material_ID/inputs_true.dat
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <material_ids>1</material_ids>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/case_4_Nuclide_ID/collision_track_true.h5 b/tests/regression_tests/collision_track/case_4_Nuclide_ID/collision_track_true.h5
new file mode 100644
index 000000000..527705297
Binary files /dev/null and b/tests/regression_tests/collision_track/case_4_Nuclide_ID/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_4_Nuclide_ID/inputs_true.dat b/tests/regression_tests/collision_track/case_4_Nuclide_ID/inputs_true.dat
new file mode 100644
index 000000000..6202bcacc
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_4_Nuclide_ID/inputs_true.dat
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <nuclides>O16 U235</nuclides>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/case_5_Universe_ID/collision_track_true.h5 b/tests/regression_tests/collision_track/case_5_Universe_ID/collision_track_true.h5
new file mode 100644
index 000000000..65419d5ea
Binary files /dev/null and b/tests/regression_tests/collision_track/case_5_Universe_ID/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_5_Universe_ID/inputs_true.dat b/tests/regression_tests/collision_track/case_5_Universe_ID/inputs_true.dat
new file mode 100644
index 000000000..c06061210
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_5_Universe_ID/inputs_true.dat
@@ -0,0 +1,59 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <cell_ids>22</cell_ids>
+      <universe_ids>77</universe_ids>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/case_6_deposited_energy_threshold/collision_track_true.h5 b/tests/regression_tests/collision_track/case_6_deposited_energy_threshold/collision_track_true.h5
new file mode 100644
index 000000000..507665a60
Binary files /dev/null and b/tests/regression_tests/collision_track/case_6_deposited_energy_threshold/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_6_deposited_energy_threshold/inputs_true.dat b/tests/regression_tests/collision_track/case_6_deposited_energy_threshold/inputs_true.dat
new file mode 100644
index 000000000..434f6b434
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_6_deposited_energy_threshold/inputs_true.dat
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <deposited_E_threshold>550000.0</deposited_E_threshold>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/case_7_all_parameters_used_together/collision_track_true.h5 b/tests/regression_tests/collision_track/case_7_all_parameters_used_together/collision_track_true.h5
new file mode 100644
index 000000000..b7ff67c48
Binary files /dev/null and b/tests/regression_tests/collision_track/case_7_all_parameters_used_together/collision_track_true.h5 differ
diff --git a/tests/regression_tests/collision_track/case_7_all_parameters_used_together/inputs_true.dat b/tests/regression_tests/collision_track/case_7_all_parameters_used_together/inputs_true.dat
new file mode 100644
index 000000000..1449c3596
--- /dev/null
+++ b/tests/regression_tests/collision_track/case_7_all_parameters_used_together/inputs_true.dat
@@ -0,0 +1,63 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="11">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="5" fill="77" region="-5 4 -7 6 -9 8" universe="1"/>
+    <cell id="8" material="11" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="1"/>
+    <cell id="22" material="1" region="-1 2 -3" universe="77"/>
+    <cell id="33" material="11" region="1 | -2 | 3" universe="77"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>80</particles>
+    <batches>5</batches>
+    <inactive>4</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <collision_track>
+      <cell_ids>22 33</cell_ids>
+      <reactions>elastic 18 (n,disappear)</reactions>
+      <universe_ids>77</universe_ids>
+      <material_ids>1 11</material_ids>
+      <nuclides>U238 U235 H1 U234</nuclides>
+      <deposited_E_threshold>100000.0</deposited_E_threshold>
+      <max_collisions>100</max_collisions>
+    </collision_track>
+    <seed>1</seed>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/collision_track/test.py b/tests/regression_tests/collision_track/test.py
new file mode 100644
index 000000000..2f9de9ac0
--- /dev/null
+++ b/tests/regression_tests/collision_track/test.py
@@ -0,0 +1,222 @@
+"""Test the 'collision_track' setting.
+
+Results
+-------
+
+All results are generated using only 1 MPI process.
+
+All results are generated using 1 thread except for "test_consistency_low_realization_number".
+This specific test verifies that when the number of realization (i.e., point being candidate
+to be stored) is lower than the capacity, results are reproducible even with multiple
+threads (i.e., there is no potential thread competition that would produce different
+results in that case).
+
+All results are generated using the history-based mode except for cases e01 to e03.
+
+All results are visually verified using the '_visualize.py' script in the regression test folder.
+
+OpenMC models
+-------------
+
+Four OpenMC models with CSG-only geometries are used to cover the transmission, vacuum,
+reflective and periodic Boundary Conditions (BC):
+
+- model_1: cylindrical core in 2 boxes (vacuum and transmission BC),
+
+# Test cases for simulation parameters using CSG-only geometries
+# ============================================================
+# Each test case is defined by a combination of folder name, model name, and specific parameters.
+# Below is a summary of the parameters used in the test cases:
+#
+# - max_collisions: Maximum number of particles to track in the simulation.
+# - reactions: List of MT numbers (reaction types- 2 for scattering, 18 for fission, 101 for absorbtion).
+# - cell_ids: IDs of specific cells in the model.
+# - mat_ids: Material IDs for filtering particles.
+# - nuclides: Nuclides for filtering particles.
+# - univ_ids: Universe IDs for filtering particles.
+# - E_threshold: Energy threshold for filtering particles (optional).
+#
+# The test cases are designed to validate the behavior of the simulation under various configurations.
+
+*: BC stands for Boundary Conditions, T for Transmission, R for Reflective, and V for Vacuum.
+
+An additional case, called 'case-a01', is used to check that the results are comparable when
+the number of threads is set to 2 if the number of realization is lower than the capacity.
+
+
+*: BC stands for Boundary Conditions, T for Transmission, and V for Vacuum.
+
+Notes:
+
+- The test cases list is non-exhaustive compared to the number of possible combinations.
+  Test cases have been selected based on use and internal code logic.
+
+
+
+TODO:
+
+- Test with a lattice.
+
+"""
+
+import openmc
+import pytest
+
+from tests.testing_harness import CollisionTrackTestHarness
+from tests.regression_tests import config
+
+
+@pytest.fixture(scope="module")
+def model_1():
+    """Cylindrical core contained in a first box which is contained in a larger box.
+    A lower universe is used to describe the interior of the first box which
+    contains the core and its surrounding space.
+
+    """
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+
+    # =============================================================================
+    # Materials
+    # =============================================================================
+
+    fuel = openmc.Material(material_id=1)
+    fuel.add_nuclide("U234", 0.0004524)
+    fuel.add_nuclide("U235", 0.0506068)
+    fuel.add_nuclide("U238", 0.9487090)
+    fuel.add_nuclide("U236", 0.0002318)
+    fuel.add_nuclide("O16", 2.0)
+    fuel.set_density("g/cm3", 11.0)
+
+    water = openmc.Material(material_id=11)
+    water.add_nuclide("H1", 2.0)
+    water.add_nuclide("O16", 1.0)
+    water.set_density("g/cm3", 1.0)
+
+    # =============================================================================
+    # Geometry
+    # =============================================================================
+
+    # -----------------------------------------------------------------------------
+    # Cylindrical core
+    # -----------------------------------------------------------------------------
+
+    # Parameters
+    core_radius = 2.0
+    core_height = 4.0
+
+    # Surfaces
+    core_cylinder = openmc.ZCylinder(r=core_radius)
+    core_lower_plane = openmc.ZPlane(-core_height / 2.0)
+    core_upper_plane = openmc.ZPlane(core_height / 2.0)
+
+    # Region
+    core_region = -core_cylinder & +core_lower_plane & -core_upper_plane
+
+    # Cells
+    core = openmc.Cell(fill=fuel, region=core_region, cell_id=22)
+    outside_core_region = +core_cylinder | -core_lower_plane | +core_upper_plane
+    outside_core = openmc.Cell(
+        fill=water, region=outside_core_region, cell_id=33)
+
+    # Universe
+    inside_box1_universe = openmc.Universe(
+        cells=[core, outside_core], universe_id=77)
+
+    # -----------------------------------------------------------------------------
+    # Box 1
+    # -----------------------------------------------------------------------------
+
+    # Parameters
+    box1_size = 6.0
+
+    # Surfaces
+    box1_rpp = openmc.model.RectangularParallelepiped(
+        -box1_size / 2.0, box1_size / 2.0,
+        -box1_size / 2.0, box1_size / 2.0,
+        -box1_size / 2.0, box1_size / 2.0,
+    )
+
+    # Cell
+    box1 = openmc.Cell(fill=inside_box1_universe, region=-box1_rpp, cell_id=5)
+
+    # -----------------------------------------------------------------------------
+    # Box 2
+    # -----------------------------------------------------------------------------
+
+    # Parameters
+    box2_size = 8
+
+    # Surfaces
+    box2_rpp = openmc.model.RectangularParallelepiped(
+        -box2_size / 2.0, box2_size / 2.0,
+        -box2_size / 2.0, box2_size / 2.0,
+        -box2_size / 2.0, box2_size / 2.0,
+        boundary_type="vacuum"
+    )
+
+    # Cell
+    box2 = openmc.Cell(fill=water, region=-box2_rpp & +box1_rpp, cell_id=8)
+
+    # Register geometry
+    model.geometry = openmc.Geometry([box1, box2])
+
+    # =============================================================================
+    # Settings
+    # =============================================================================
+
+    model.settings = openmc.Settings()
+    model.settings.particles = 80
+    model.settings.batches = 5
+    model.settings.inactive = 4
+    model.settings.seed = 1
+
+    bounds = [
+        -core_radius,
+        -core_radius,
+        -core_height / 2.0,
+        core_radius,
+        core_radius,
+        core_height / 2.0,
+    ]
+    distribution = openmc.stats.Box(bounds[:3], bounds[3:])
+    model.settings.source = openmc.IndependentSource(
+        space=distribution, constraints={'fissionable': True})
+
+    return model
+
+
+@pytest.mark.parametrize(
+    "folder, model_name, parameter",
+    [("case_1_Reactions", "model_1", {"max_collisions": 100, "reactions": ["(n,fission)", 101]}),
+        ("case_2_Cell_ID", "model_1", {
+         "max_collisions": 100, "cell_ids": [22]}),
+        ("case_3_Material_ID", "model_1", {
+         "max_collisions": 100, "material_ids": [1]}),
+        ("case_4_Nuclide_ID", "model_1", {
+         "max_collisions": 100, "nuclides": ["O16", "U235"]}),
+        ("case_5_Universe_ID", "model_1", {
+         "max_collisions": 100, "cell_ids": [22], "universe_ids": [77]}),
+        ("case_6_deposited_energy_threshold", "model_1", {
+         "max_collisions": 100, "deposited_E_threshold": 5.5e5}),
+        ("case_7_all_parameters_used_together", "model_1", {
+            "max_collisions": 100,
+            "reactions": ["elastic", 18, "(n,disappear)"],
+            "material_ids": [1, 11],
+            "universe_ids": [77],
+            "nuclides": ["U238", "U235", "H1", "U234"],
+            "cell_ids": [22, 33],
+            "deposited_E_threshold": 1e5})
+     ],
+)
+def test_collision_track_several_cases(
+    folder, model_name, parameter, request
+):
+    # Since for these tests the actual number of collisions recorded is < max_collisions,
+    # we can run them with 1 or 2 threads, and in history or event mode.
+    model = request.getfixturevalue(model_name)
+    model.settings.collision_track = parameter
+    harness = CollisionTrackTestHarness(
+        "statepoint.5.h5", model=model, workdir=folder
+    )
+    harness.main()
diff --git a/tests/regression_tests/complex_cell/results_true.dat b/tests/regression_tests/complex_cell/results_true.dat
index 3ce3d7cbc..ddedb07ff 100644
--- a/tests/regression_tests/complex_cell/results_true.dat
+++ b/tests/regression_tests/complex_cell/results_true.dat
@@ -1,11 +1,11 @@
 k-combined:
-2.564169E-01 4.095378E-03
+2.603220E-01 1.429366E-03
 tally 1:
-2.607144E+00
-1.360414E+00
-2.681079E+00
-1.439354E+00
-9.627534E-01
-1.855496E-01
-1.123751E-01
+2.624819E+00
+1.378200E+00
+2.730035E+00
+1.492361E+00
+1.013707E+00
+2.055807E-01
+1.123257E-01
 2.530233E-03
diff --git a/tests/regression_tests/confidence_intervals/results_true.dat b/tests/regression_tests/confidence_intervals/results_true.dat
index 6a906d07e..8ca256624 100644
--- a/tests/regression_tests/confidence_intervals/results_true.dat
+++ b/tests/regression_tests/confidence_intervals/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-2.759923E-01 6.988588E-03
+2.850178E-01 9.646334E-03
 tally 1:
-6.167984E+01
-4.772717E+02
+6.234169E+01
+4.884167E+02
diff --git a/tests/regression_tests/cpp_driver/driver.cpp b/tests/regression_tests/cpp_driver/driver.cpp
index 48ed7f317..a6c3e6510 100644
--- a/tests/regression_tests/cpp_driver/driver.cpp
+++ b/tests/regression_tests/cpp_driver/driver.cpp
@@ -2,6 +2,8 @@
 #include <mpi.h>
 #endif
 
+#include <cassert>
+
 #include "openmc/capi.h"
 #include "openmc/cell.h"
 #include "openmc/error.h"
@@ -15,14 +17,16 @@
 
 using namespace openmc;
 
-int main(int argc, char** argv) {
+int main(int argc, char** argv)
+{
 #ifdef OPENMC_MPI
   MPI_Comm world {MPI_COMM_WORLD};
   int err = openmc_init(argc, argv, &world);
 #else
   int err = openmc_init(argc, argv, nullptr);
 #endif
-  if (err) fatal_error(openmc_err_msg);
+  if (err)
+    fatal_error(openmc_err_msg);
 
   // create a new cell filter
   auto cell_filter = Filter::create<CellFilter>();
@@ -30,7 +34,7 @@ int main(int argc, char** argv) {
   // add all cells to the cell filter
   std::vector<int32_t> cell_indices;
   for (auto& entry : openmc::model::cell_map) {
-      cell_indices.push_back(entry.second);
+    cell_indices.push_back(entry.second);
   }
   // enable distribcells offsets for all cells
   prepare_distribcell(&cell_indices);
@@ -39,7 +43,6 @@ int main(int argc, char** argv) {
   std::sort(cell_indices.begin(), cell_indices.end());
   cell_filter->set_cells(cell_indices);
 
-
   // create a new tally
   auto tally = Tally::create();
   std::vector<Filter*> filters = {cell_filter};
@@ -60,14 +63,19 @@ int main(int argc, char** argv) {
     }
   }
 
-  // set a higher temperature for only one of the lattice cells (ID is 4 in the model)
+  // set a higher temperature for only one of the lattice cells (ID is 4 in the
+  // model)
   model::cells[model::cell_map[4]]->set_temperature(400.0, 3, true);
 
+  // set the density of another lattice cell to 2
+  model::cells[model::cell_map[4]]->set_density(2.0, 2, true);
+
   // the summary file will be used to check that
   // temperatures were set correctly so clear
   // error output can be provided
 #ifdef OPENMC_MPI
-  if (openmc::mpi::master) openmc::write_summary();
+  if (openmc::mpi::master)
+    openmc::write_summary();
 #else
   openmc::write_summary();
 #endif
diff --git a/tests/regression_tests/cpp_driver/inputs_true.dat b/tests/regression_tests/cpp_driver/inputs_true.dat
index 5d067cb68..fd450428a 100644
--- a/tests/regression_tests/cpp_driver/inputs_true.dat
+++ b/tests/regression_tests/cpp_driver/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="cladding">
-      <density units="g/cc" value="6.44"/>
-      <nuclide ao="1.0" name="Zr90"/>
+      <density value="6.44" units="g/cc"/>
+      <nuclide name="Zr90" ao="1.0"/>
     </material>
     <material id="3" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="2" region="1 -2" universe="1"/>
     <cell id="3" material="3" region="2" universe="1"/>
-    <cell fill="1" id="4" universe="2"/>
-    <cell fill="3" id="5" region="3 -4 5 -6" universe="4"/>
+    <cell id="4" fill="1" universe="2"/>
+    <cell id="5" fill="3" region="3 -4 5 -6" universe="4"/>
     <lattice id="3">
       <pitch>4.0 4.0</pitch>
       <dimension>2 2</dimension>
@@ -29,12 +29,12 @@
 2 2 
 2 2 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 1.5" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 1.7" id="2" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-4.0" id="3" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="4.0" id="4" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-4.0" id="5" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="4.0" id="6" name="maximum y" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 1.5"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 1.7"/>
+    <surface id="3" name="minimum x" type="x-plane" boundary="reflective" coeffs="-4.0"/>
+    <surface id="4" name="maximum x" type="x-plane" boundary="reflective" coeffs="4.0"/>
+    <surface id="5" name="minimum y" type="y-plane" boundary="reflective" coeffs="-4.0"/>
+    <surface id="6" name="maximum y" type="y-plane" boundary="reflective" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/cpp_driver/results_true.dat b/tests/regression_tests/cpp_driver/results_true.dat
index c2b0b8d6f..09f188db6 100644
--- a/tests/regression_tests/cpp_driver/results_true.dat
+++ b/tests/regression_tests/cpp_driver/results_true.dat
@@ -1,13 +1,13 @@
 k-combined:
-1.933305E+00 1.300360E-02
+1.874924E+00 2.180236E-02
 tally 1:
-9.552846E+01
-1.019358E+03
-2.887973E+01
-9.308509E+01
-9.732441E+01
-1.059022E+03
-2.217326E+02
-5.486892E+03
-2.217326E+02
-5.486892E+03
+9.484447E+01
+1.002269E+03
+2.746252E+01
+8.406603E+01
+9.833099E+01
+1.076376E+03
+2.206380E+02
+5.417609E+03
+2.206380E+02
+5.417609E+03
diff --git a/tests/regression_tests/create_fission_neutrons/inputs_true.dat b/tests/regression_tests/create_fission_neutrons/inputs_true.dat
index 9e44a0224..47e8c3830 100644
--- a/tests/regression_tests/create_fission_neutrons/inputs_true.dat
+++ b/tests/regression_tests/create_fission_neutrons/inputs_true.dat
@@ -1,30 +1,30 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="mat">
-      <density units="atom/b-cm" value="0.069335"/>
-      <nuclide ao="40.0" name="H1"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="mat" depletable="true">
+      <density value="0.069335" units="atom/b-cm"/>
+      <nuclide name="H1" ao="40.0"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="box" region="1 -2 3 -4 5 -6" universe="0"/>
-    <surface boundary="reflective" coeffs="-1" id="1" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1" id="3" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-1" id="5" type="z-plane"/>
-    <surface boundary="reflective" coeffs="1" id="6" type="z-plane"/>
+    <cell id="1" name="box" material="1" region="1 -2 3 -4 5 -6" universe="0"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="-1"/>
+    <surface id="2" type="x-plane" boundary="reflective" coeffs="1"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="-1"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="1"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="-1"/>
+    <surface id="6" type="z-plane" boundary="reflective" coeffs="1"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-1 -1 -1 1 1 1</parameters>
       </space>
-      <energy parameters="988000.0 2.249e-06" type="watt"/>
+      <energy type="watt" parameters="988000.0 2.249e-06"/>
     </source>
     <create_fission_neutrons>false</create_fission_neutrons>
   </settings>
diff --git a/tests/regression_tests/dagmc/external/inputs_true.dat b/tests/regression_tests/dagmc/external/inputs_true.dat
index 31f4c1f88..8a6c6fe74 100644
--- a/tests/regression_tests/dagmc/external/inputs_true.dat
+++ b/tests/regression_tests/dagmc/external/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/dagmc/external/main.cpp b/tests/regression_tests/dagmc/external/main.cpp
index 3765cf79a..e78ab03fa 100644
--- a/tests/regression_tests/dagmc/external/main.cpp
+++ b/tests/regression_tests/dagmc/external/main.cpp
@@ -100,6 +100,9 @@ int main(int argc, char* argv[])
     }
   }
 
+  // Finalize cell densities
+  openmc::finalize_cell_densities();
+
   // Run OpenMC
   openmc_err = openmc_run();
   if (openmc_err)
diff --git a/tests/regression_tests/dagmc/external/results_true.dat b/tests/regression_tests/dagmc/external/results_true.dat
index cda656937..9a6b481b7 100644
--- a/tests/regression_tests/dagmc/external/results_true.dat
+++ b/tests/regression_tests/dagmc/external/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-9.118190E-01 3.615552E-02
+1.083415E+00 5.991738E-02
 tally 1:
-8.430103E+00
-1.442878E+01
+8.862860E+00
+1.602117E+01
diff --git a/tests/regression_tests/dagmc/external/test.py b/tests/regression_tests/dagmc/external/test.py
index 57bc9ea7f..3580bfa11 100644
--- a/tests/regression_tests/dagmc/external/test.py
+++ b/tests/regression_tests/dagmc/external/test.py
@@ -32,7 +32,7 @@ def cpp_driver(request):
             target_link_libraries(main OpenMC::libopenmc)
             target_compile_features(main PUBLIC cxx_std_14)
             set(CMAKE_CXX_FLAGS "-pedantic-errors")
-            add_compile_definitions(DAGMC=1)
+            add_compile_definitions(OPENMC_DAGMC_ENABLED=1)
             """.format(openmc_dir)))
 
     # Create temporary build directory and change to there
diff --git a/tests/regression_tests/dagmc/legacy/inputs_true.dat b/tests/regression_tests/dagmc/legacy/inputs_true.dat
index b06516cb1..ad2f8e54d 100644
--- a/tests/regression_tests/dagmc/legacy/inputs_true.dat
+++ b/tests/regression_tests/dagmc/legacy/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/dagmc/legacy/results_true.dat b/tests/regression_tests/dagmc/legacy/results_true.dat
index cda656937..9a6b481b7 100644
--- a/tests/regression_tests/dagmc/legacy/results_true.dat
+++ b/tests/regression_tests/dagmc/legacy/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-9.118190E-01 3.615552E-02
+1.083415E+00 5.991738E-02
 tally 1:
-8.430103E+00
-1.442878E+01
+8.862860E+00
+1.602117E+01
diff --git a/tests/regression_tests/dagmc/refl/inputs_true.dat b/tests/regression_tests/dagmc/refl/inputs_true.dat
index 979eeb492..58cb9e66f 100644
--- a/tests/regression_tests/dagmc/refl/inputs_true.dat
+++ b/tests/regression_tests/dagmc/refl/inputs_true.dat
@@ -3,14 +3,14 @@
   <materials>
   </materials>
   <geometry>
-    <dagmc_universe auto_geom_ids="true" filename="dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" auto_geom_ids="true" filename="dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/dagmc/refl/results_true.dat b/tests/regression_tests/dagmc/refl/results_true.dat
index 533c62a90..b49a4a7a4 100644
--- a/tests/regression_tests/dagmc/refl/results_true.dat
+++ b/tests/regression_tests/dagmc/refl/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-2.035173E+00 3.967029E-02
+2.047107E+00 8.605767E-02
 tally 1:
-1.064492E+01
-2.301019E+01
+1.145034E+01
+2.636875E+01
diff --git a/tests/regression_tests/dagmc/universes/inputs_true.dat b/tests/regression_tests/dagmc/universes/inputs_true.dat
index 4b5be3612..be1a17383 100644
--- a/tests/regression_tests/dagmc/universes/inputs_true.dat
+++ b/tests/regression_tests/dagmc/universes/inputs_true.dat
@@ -1,33 +1,33 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="no-void fuel">
-      <density units="g/cc" value="10.29769"/>
-      <nuclide ao="0.93120485" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="no-void fuel" depletable="true">
+      <density value="10.29769" units="g/cc"/>
+      <nuclide name="U234" ao="0.93120485"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="clad">
-      <density units="g/cc" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="water">
-      <density units="g/cc" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cc"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell fill="4" id="1" region="1 -2 3 -4 5 -6" universe="5"/>
-    <dagmc_universe auto_geom_ids="true" filename="dagmc.h5m" id="1"/>
+    <cell id="1" fill="4" region="1 -2 3 -4 5 -6" universe="5"/>
+    <dagmc_universe id="1" auto_geom_ids="true" filename="dagmc.h5m"/>
     <lattice id="4">
       <pitch>24.0 24.0</pitch>
       <dimension>2 2</dimension>
@@ -36,18 +36,26 @@
 1 1 
 1 1 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="-24.0" id="1" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="24.0" id="2" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-24.0" id="3" name="front" type="y-plane"/>
-    <surface boundary="reflective" coeffs="24.0" id="4" name="back" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="5" name="bottom" type="z-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="6" name="top" type="z-plane"/>
+    <surface id="1" name="left" type="x-plane" boundary="reflective" coeffs="-24.0"/>
+    <surface id="2" name="right" type="x-plane" boundary="reflective" coeffs="24.0"/>
+    <surface id="3" name="front" type="y-plane" boundary="reflective" coeffs="-24.0"/>
+    <surface id="4" name="back" type="y-plane" boundary="reflective" coeffs="24.0"/>
+    <surface id="5" name="bottom" type="z-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="6" name="top" type="z-plane" boundary="reflective" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-10.0 -10.0 -24.0 10.0 10.0 24.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
     <output>
       <summary>false</summary>
     </output>
diff --git a/tests/regression_tests/dagmc/universes/results_true.dat b/tests/regression_tests/dagmc/universes/results_true.dat
index 76cbc9db0..aacb7d1ab 100644
--- a/tests/regression_tests/dagmc/universes/results_true.dat
+++ b/tests/regression_tests/dagmc/universes/results_true.dat
@@ -1,13 +1,13 @@
 k-combined:
-9.887663E-01 1.510336E-02
+9.719586E-01 3.630894E-02
 tally 1:
-4.340758E+00
-4.265459E+00
-4.712319E+00
-4.654778E+00
-4.151897E+00
-3.588090E+00
-2.965925E+00
-1.852746E+00
+4.463288E+00
+4.136647E+00
+4.769631E+00
+4.622840E+00
+4.315273E+00
+3.871129E+00
+4.091804E+00
+3.582192E+00
 0.000000E+00
 0.000000E+00
diff --git a/tests/regression_tests/dagmc/universes/test.py b/tests/regression_tests/dagmc/universes/test.py
index 057a7b0d4..d68c6b11c 100644
--- a/tests/regression_tests/dagmc/universes/test.py
+++ b/tests/regression_tests/dagmc/universes/test.py
@@ -11,81 +11,87 @@ pytestmark = pytest.mark.skipif(
     reason="DAGMC CAD geometry is not enabled.")
 
 
-class DAGMCUniverseTest(PyAPITestHarness):
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
+@pytest.fixture
+def pin_lattice_model():
+    ### MATERIALS ###
+    fuel = openmc.Material(name='no-void fuel')
+    fuel.set_density('g/cc', 10.29769)
+    fuel.add_nuclide('U234', 0.93120485)
+    fuel.add_nuclide('U235', 0.00055815)
+    fuel.add_nuclide('U238', 0.022408)
+    fuel.add_nuclide('O16', 0.045829)
 
-        ### MATERIALS ###
-        fuel = openmc.Material(name='no-void fuel')
-        fuel.set_density('g/cc', 10.29769)
-        fuel.add_nuclide('U234', 0.93120485)
-        fuel.add_nuclide('U235', 0.00055815)
-        fuel.add_nuclide('U238', 0.022408)
-        fuel.add_nuclide('O16', 0.045829)
+    cladding = openmc.Material(name='clad')
+    cladding.set_density('g/cc', 6.55)
+    cladding.add_nuclide('Zr90', 0.021827)
+    cladding.add_nuclide('Zr91', 0.00476)
+    cladding.add_nuclide('Zr92', 0.0072758)
+    cladding.add_nuclide('Zr94', 0.0073734)
+    cladding.add_nuclide('Zr96', 0.0011879)
 
-        cladding = openmc.Material(name='clad')
-        cladding.set_density('g/cc', 6.55)
-        cladding.add_nuclide('Zr90', 0.021827)
-        cladding.add_nuclide('Zr91', 0.00476)
-        cladding.add_nuclide('Zr92', 0.0072758)
-        cladding.add_nuclide('Zr94', 0.0073734)
-        cladding.add_nuclide('Zr96', 0.0011879)
+    water = openmc.Material(name='water')
+    water.set_density('g/cc', 0.740582)
+    water.add_nuclide('H1', 0.049457)
+    water.add_nuclide('O16', 0.024672)
+    water.add_nuclide('B10', 8.0042e-06)
+    water.add_nuclide('B11', 3.2218e-05)
+    water.add_s_alpha_beta('c_H_in_H2O')
 
-        water = openmc.Material(name='water')
-        water.set_density('g/cc', 0.740582)
-        water.add_nuclide('H1', 0.049457)
-        water.add_nuclide('O16', 0.024672)
-        water.add_nuclide('B10', 8.0042e-06)
-        water.add_nuclide('B11', 3.2218e-05)
-        water.add_s_alpha_beta('c_H_in_H2O')
+    model = openmc.Model()
+    model.materials = openmc.Materials([fuel, cladding, water])
 
-        self._model.materials = openmc.Materials([fuel, cladding, water])
+    ### GEOMETRY ###
+    # create the DAGMC universe
+    pincell_univ = openmc.DAGMCUniverse(filename='dagmc.h5m', auto_geom_ids=True)
 
-        ### GEOMETRY ###
-        # create the DAGMC universe
-        pincell_univ = openmc.DAGMCUniverse(filename='dagmc.h5m', auto_geom_ids=True)
+    # creates another DAGMC universe, this time with within a bounded cell
+    bound_pincell_universe = openmc.DAGMCUniverse(filename='dagmc.h5m').bounded_universe()
+    # uses the bound_dag_cell as the root argument to test the type checks in openmc.Geometry
+    bound_pincell_geometry = openmc.Geometry(root=bound_pincell_universe)
+    # assigns the bound_dag_geometry to the model to test the type checks in model.Geometry setter
+    model.geometry = bound_pincell_geometry
 
-        # creates another DAGMC universe, this time with within a bounded cell
-        bound_pincell_universe = openmc.DAGMCUniverse(filename='dagmc.h5m').bounded_universe()
-        # uses the bound_dag_cell as the root argument to test the type checks in openmc.Geometry
-        bound_pincell_geometry = openmc.Geometry(root=bound_pincell_universe)
-        # assigns the bound_dag_geometry to the model to test the type checks in model.Geometry setter
-        self._model.geometry = bound_pincell_geometry
+    # create a 2 x 2 lattice using the DAGMC pincell
+    pitch = np.asarray((24.0, 24.0))
+    lattice = openmc.RectLattice()
+    lattice.pitch = pitch
+    lattice.universes = [[pincell_univ] * 2] * 2
+    lattice.lower_left = -pitch
 
-        # create a 2 x 2 lattice using the DAGMC pincell
-        pitch = np.asarray((24.0, 24.0))
-        lattice = openmc.RectLattice()
-        lattice.pitch = pitch
-        lattice.universes = [[pincell_univ] * 2] * 2
-        lattice.lower_left = -pitch
+    left = openmc.XPlane(x0=-pitch[0], name='left', boundary_type='reflective')
+    right = openmc.XPlane(x0=pitch[0], name='right', boundary_type='reflective')
+    front = openmc.YPlane(y0=-pitch[1], name='front', boundary_type='reflective')
+    back = openmc.YPlane(y0=pitch[1], name='back', boundary_type='reflective')
+    # clip the DAGMC geometry at +/- 10 cm w/ CSG planes
+    bottom = openmc.ZPlane(z0=-10.0, name='bottom', boundary_type='reflective')
+    top = openmc.ZPlane(z0=10.0, name='top', boundary_type='reflective')
 
-        left = openmc.XPlane(x0=-pitch[0], name='left', boundary_type='reflective')
-        right = openmc.XPlane(x0=pitch[0], name='right', boundary_type='reflective')
-        front = openmc.YPlane(y0=-pitch[1], name='front', boundary_type='reflective')
-        back = openmc.YPlane(y0=pitch[1], name='back', boundary_type='reflective')
-        # clip the DAGMC geometry at +/- 10 cm w/ CSG planes
-        bottom = openmc.ZPlane(z0=-10.0, name='bottom', boundary_type='reflective')
-        top = openmc.ZPlane(z0=10.0, name='top', boundary_type='reflective')
+    bounding_region = +left & -right & +front & -back & +bottom & -top
+    bounding_cell = openmc.Cell(fill=lattice, region=bounding_region)
 
-        bounding_region = +left & -right & +front & -back & +bottom & -top
-        bounding_cell = openmc.Cell(fill=lattice, region=bounding_region)
+    model.geometry = openmc.Geometry([bounding_cell])
 
-        self._model.geometry = openmc.Geometry([bounding_cell])
+    # add a cell instance tally
+    tally = openmc.Tally(name='cell instance tally')
+    # using scattering
+    cell_instance_filter = openmc.CellInstanceFilter(((4, 0), (4, 1), (4, 2), (4, 3), (4, 4)))
+    tally.filters = [cell_instance_filter]
+    tally.scores = ['scatter']
+    model.tallies = [tally]
 
-        # add a cell instance tally
-        tally = openmc.Tally(name='cell instance tally')
-        # using scattering
-        cell_instance_filter = openmc.CellInstanceFilter(((4, 0), (4, 1), (4, 2), (4, 3), (4, 4)))
-        tally.filters = [cell_instance_filter]
-        tally.scores = ['scatter']
-        self._model.tallies = [tally]
+    # settings
+    model.settings.particles = 100
+    model.settings.batches = 10
+    model.settings.inactive = 5
+    model.settings.output = {'summary' : False}
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Box((-10., -10., -24.), (10., 10., 24.)),
+        constraints={'fissionable': True},
+    )
 
-        # settings
-        self._model.settings.particles = 100
-        self._model.settings.batches = 10
-        self._model.settings.inactive = 5
-        self._model.settings.output = {'summary' : False}
+    return model
 
-def test_univ():
-    harness = DAGMCUniverseTest('statepoint.10.h5', model=openmc.Model())
+
+def test_univ(pin_lattice_model):
+    harness = PyAPITestHarness('statepoint.10.h5', model=pin_lattice_model)
     harness.main()
diff --git a/tests/regression_tests/dagmc/uwuw/inputs_true.dat b/tests/regression_tests/dagmc/uwuw/inputs_true.dat
index d7e4a12aa..01082cb23 100644
--- a/tests/regression_tests/dagmc/uwuw/inputs_true.dat
+++ b/tests/regression_tests/dagmc/uwuw/inputs_true.dat
@@ -3,14 +3,14 @@
   <materials>
   </materials>
   <geometry>
-    <dagmc_universe filename="dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/density/results_true.dat b/tests/regression_tests/density/results_true.dat
index c8e3b1ede..42dc0c19f 100644
--- a/tests/regression_tests/density/results_true.dat
+++ b/tests/regression_tests/density/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.110057E+00 1.303260E-02
+1.082191E+00 3.064029E-02
diff --git a/tests/regression_tests/deplete_no_transport/test.py b/tests/regression_tests/deplete_no_transport/test.py
index 63ae584e1..5550ad048 100644
--- a/tests/regression_tests/deplete_no_transport/test.py
+++ b/tests/regression_tests/deplete_no_transport/test.py
@@ -76,6 +76,8 @@ def test_against_self(run_in_tmpdir,
     dt = [360]  # single step
 
     # Perform simulation using the predictor algorithm
+    if config['mpi'] and multiproc:
+        pytest.skip("Multiprocessing depletion is disabled when MPI is enabled.")
     openmc.deplete.pool.USE_MULTIPROCESSING = multiproc
     openmc.deplete.PredictorIntegrator(op,
                                        dt,
@@ -135,6 +137,8 @@ def test_against_coupled(run_in_tmpdir,
     dt = [dt]  # single step
 
     # Perform simulation using the predictor algorithm
+    if config['mpi'] and multiproc:
+        pytest.skip("Multiprocessing depletion is disabled when MPI is enabled.")
     openmc.deplete.pool.USE_MULTIPROCESSING = multiproc
     openmc.deplete.PredictorIntegrator(
         op, dt, power=174, timestep_units=time_units).integrate()
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_days.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_days.h5
index 5fdc656d9..9757c9791 100644
Binary files a/tests/regression_tests/deplete_no_transport/test_reference_coupled_days.h5 and b/tests/regression_tests/deplete_no_transport/test_reference_coupled_days.h5 differ
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_hours.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_hours.h5
index 65bfc19a9..08d4d3eed 100644
Binary files a/tests/regression_tests/deplete_no_transport/test_reference_coupled_hours.h5 and b/tests/regression_tests/deplete_no_transport/test_reference_coupled_hours.h5 differ
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_minutes.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_minutes.h5
index 294cb589f..41b5235bb 100644
Binary files a/tests/regression_tests/deplete_no_transport/test_reference_coupled_minutes.h5 and b/tests/regression_tests/deplete_no_transport/test_reference_coupled_minutes.h5 differ
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_coupled_months.h5 b/tests/regression_tests/deplete_no_transport/test_reference_coupled_months.h5
index ed62e4610..53d80aff0 100644
Binary files a/tests/regression_tests/deplete_no_transport/test_reference_coupled_months.h5 and b/tests/regression_tests/deplete_no_transport/test_reference_coupled_months.h5 differ
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_fission_q.h5 b/tests/regression_tests/deplete_no_transport/test_reference_fission_q.h5
index 9d32d89fe..fe695692d 100644
Binary files a/tests/regression_tests/deplete_no_transport/test_reference_fission_q.h5 and b/tests/regression_tests/deplete_no_transport/test_reference_fission_q.h5 differ
diff --git a/tests/regression_tests/deplete_no_transport/test_reference_source_rate.h5 b/tests/regression_tests/deplete_no_transport/test_reference_source_rate.h5
index 3f3b4aa2a..c9c7e9e36 100644
Binary files a/tests/regression_tests/deplete_no_transport/test_reference_source_rate.h5 and b/tests/regression_tests/deplete_no_transport/test_reference_source_rate.h5 differ
diff --git a/tests/regression_tests/deplete_with_keff_search_control/__init__.py b/tests/regression_tests/deplete_with_keff_search_control/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_refuel.h5 b/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_refuel.h5
new file mode 100644
index 000000000..a335e3b47
Binary files /dev/null and b/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_refuel.h5 differ
diff --git a/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_rotation.h5 b/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_rotation.h5
new file mode 100644
index 000000000..c8b4f67ff
Binary files /dev/null and b/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_rotation.h5 differ
diff --git a/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_translation.h5 b/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_translation.h5
new file mode 100644
index 000000000..35285321b
Binary files /dev/null and b/tests/regression_tests/deplete_with_keff_search_control/ref_depletion_with_translation.h5 differ
diff --git a/tests/regression_tests/deplete_with_keff_search_control/test.py b/tests/regression_tests/deplete_with_keff_search_control/test.py
new file mode 100644
index 000000000..82e601280
--- /dev/null
+++ b/tests/regression_tests/deplete_with_keff_search_control/test.py
@@ -0,0 +1,140 @@
+""" Tests for KeffSearchControl class """
+
+from hmac import new
+from pathlib import Path
+import shutil
+import sys
+
+import pytest
+import numpy as np
+
+import openmc
+import openmc.lib
+from openmc.deplete import CoupledOperator
+
+from tests.regression_tests import config
+
+@pytest.fixture
+def model():
+    f = openmc.Material(name='f')
+    f.set_density('g/cm3', 10.29769)
+    f.add_element('U', 1., enrichment=2.4)
+    f.add_element('O', 2.)
+
+    h = openmc.Material(name='h')
+    h.set_density('g/cm3', 0.001598)
+    h.add_element('He', 2.4044e-4)
+
+    w = openmc.Material(name='w')
+    w.set_density('g/cm3', 0.740582)
+    w.add_element('H', 2)
+    w.add_element('O', 1)
+
+    # Define overall material
+    materials = openmc.Materials([f, h, w])
+
+    # Define surfaces
+    radii = [0.5, 0.8, 1]
+    height = 80
+    surf_in = openmc.ZCylinder(r=radii[0])
+    surf_mid = openmc.ZCylinder(r=radii[1])
+    surf_out = openmc.ZCylinder(r=radii[2], boundary_type='reflective')
+    surf_top = openmc.ZPlane(z0=height/2, boundary_type='vacuum')
+    surf_bot = openmc.ZPlane(z0=-height/2, boundary_type='vacuum')
+
+    surf_trans = openmc.ZPlane(z0=0)
+    surf_rot1 = openmc.XPlane(x0=0)
+    surf_rot2 = openmc.YPlane(y0=0)
+
+    # Define cells
+    cell_f = openmc.Cell(name='fuel_cell', fill=f,
+            region=-surf_in  & -surf_top & +surf_bot)
+    cell_g = openmc.Cell(fill=h,
+            region = +surf_in & -surf_mid & -surf_top & +surf_bot & +surf_rot2)
+
+    # Define unbounded cells for rotation universe
+    cell_w = openmc.Cell(fill=w, region = -surf_rot1)
+    cell_h = openmc.Cell(fill=h, region = +surf_rot1)
+    universe_rot = openmc.Universe(cells=(cell_w, cell_h))
+    cell_rot = openmc.Cell(name="rot_cell", fill=universe_rot,
+            region = +surf_in & -surf_mid & -surf_top & +surf_bot & -surf_rot2)
+
+    # Define unbounded cells for translation universe
+    cell_w = openmc.Cell(fill=w, region=+surf_in  & -surf_trans )
+    cell_h = openmc.Cell(fill=h,  region=+surf_in & +surf_trans)
+    universe_trans = openmc.Universe(cells=(cell_w, cell_h))
+    cell_trans = openmc.Cell(name="trans_cell", fill=universe_trans,
+                          region=+surf_mid & -surf_out & -surf_top & +surf_bot)
+
+    # Define overall geometry
+    geometry = openmc.Geometry([cell_f, cell_g, cell_rot, cell_trans])
+
+    # Set material volume for depletion fuel.
+    f.volume = np.pi * radii[0]**2 * height
+
+    settings = openmc.Settings()
+    settings.particles = 1000
+    settings.inactive = 10
+    settings.batches = 50
+
+    return openmc.Model(geometry, materials, settings)
+
+
+def translate_cell(position):
+    cell_trans = [cell for cell in openmc.lib.cells.values() if cell.name == 'trans_cell'][0]
+    openmc.lib.cells[cell_trans.id].translation = [0, 0, position]
+
+
+def rotate_cell(angle):
+    cell_rot = [cell for cell in openmc.lib.cells.values() if cell.name == 'rot_cell'][0]
+    openmc.lib.cells[cell_rot.id].rotation = [0, 0, angle]
+
+
+def set_u235_density(u235_density):
+    fuel = [material for material in openmc.lib.materials.values()
+            if material.name == 'f'][0]
+    nuclides = openmc.lib.materials[fuel.id].nuclides
+    densities = openmc.lib.materials[fuel.id].densities
+    nuc_idx = nuclides.index('U235')
+    densities[nuc_idx] = u235_density
+    openmc.lib.materials[fuel.id].set_densities(nuclides, densities)
+
+
+@pytest.mark.parametrize("function, x0, x1, bracket, ref_result", [
+    (translate_cell, -11, -5, (-15, 0), 'depletion_with_translation'),
+    (rotate_cell, -80, -50, (-90, 0), 'depletion_with_rotation'),
+    (set_u235_density, 2e-4, 1e-3, (1e-4, 2e-3), 'depletion_with_refuel')
+])
+def test_keff_search_control(run_in_tmpdir, model, function, x0, x1, bracket, ref_result):
+    chain_file = Path(__file__).parents[2] / 'chain_simple.xml'
+    model.settings.verbosity = 1
+    op = CoupledOperator(model, chain_file)
+
+    integrator = openmc.deplete.PredictorIntegrator(
+        op, [1], 174., timestep_units = 'd')
+    integrator.add_keff_search_control(
+        function=function,
+        x0=x0,
+        x1=x1,
+        bracket=bracket,
+        output=True,
+        k_tol=0.1,
+        sigma_final=5e-2)
+
+    integrator.integrate()
+
+    # Get path to test and reference results
+    path_test = op.output_dir / 'depletion_results.h5'
+    path_reference = Path(__file__).with_name(f'ref_{ref_result}.h5')
+
+    # If updating results, do so and return
+    if config['update']:
+        shutil.copyfile(str(path_test), str(path_reference))
+        return
+
+    # Load the reference/test results
+    res_test = openmc.deplete.Results(path_test)
+    res_ref = openmc.deplete.Results(path_reference)
+
+    # Use high tolerance here
+    assert res_test[0].keff_search_root == pytest.approx(res_ref[0].keff_search_root, rel=2)
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_ext_source.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_ext_source.h5
new file mode 100644
index 000000000..2f9951a42
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_feed.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_feed.h5
index b9be76345..25ab31041 100644
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_redox.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_redox.h5
new file mode 100644
index 000000000..c7f002ba1
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal.h5
index 4b24bed52..c3121a953 100644
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal_and_redox.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_removal_and_redox.h5
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index 000000000..80ce771fc
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer.h5
index 51173f778..2a4c227bb 100644
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer_and_redox.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_depletion_with_transfer_and_redox.h5
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index 000000000..6847fd7df
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_feed.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_feed.h5
index d53adc7d8..af6733340 100644
Binary files a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_feed.h5 and b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_feed.h5 differ
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_removal.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_removal.h5
index 921755e15..59c65427c 100644
Binary files a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_removal.h5 and b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_only_removal.h5 differ
diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_ext_source.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_ext_source.h5
new file mode 100644
index 000000000..cceb7fc78
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diff --git a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_transfer.h5 b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_transfer.h5
index 469149f67..a1c0e5b41 100644
Binary files a/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_transfer.h5 and b/tests/regression_tests/deplete_with_transfer_rates/ref_no_depletion_with_transfer.h5 differ
diff --git a/tests/regression_tests/deplete_with_transfer_rates/test.py b/tests/regression_tests/deplete_with_transfer_rates/test.py
index 10d60866a..461287f6e 100644
--- a/tests/regression_tests/deplete_with_transfer_rates/test.py
+++ b/tests/regression_tests/deplete_with_transfer_rates/test.py
@@ -1,8 +1,7 @@
-""" TransferRates depletion test suite """
+""" ExternalRates depletion test suite """
 
 from pathlib import Path
 import shutil
-import sys
 
 import numpy as np
 import pytest
@@ -11,8 +10,7 @@ import openmc.deplete
 from openmc.deplete import CoupledOperator
 
 from tests.regression_tests import config, assert_reaction_rates_equal, \
-    assert_atoms_equal, assert_same_mats
-
+    assert_atoms_equal
 
 @pytest.fixture
 def model():
@@ -41,12 +39,13 @@ def model():
     geometry = openmc.Geometry([cell_f, cell_w])
 
     settings = openmc.Settings()
-    settings.particles = 100
+    settings.particles = 150
     settings.inactive = 0
     settings.batches = 10
 
     return openmc.Model(geometry, materials, settings)
 
+
 @pytest.mark.parametrize("rate, dest_mat, power, ref_result", [
     (1e-5, None, 0.0, 'no_depletion_only_removal'),
     (-1e-5, None, 0.0, 'no_depletion_only_feed'),
@@ -54,6 +53,9 @@ def model():
     (-1e-5, None, 174.0, 'depletion_with_feed'),
     (-1e-5, 'w', 0.0, 'no_depletion_with_transfer'),
     (1e-5, 'w', 174.0, 'depletion_with_transfer'),
+    (0.0, None, 174.0, 'depletion_with_redox'),
+    (1e-5, None, 174.0, 'depletion_with_removal_and_redox'),
+    (1e-5, 'w', 174.0, 'depletion_with_transfer_and_redox'),
     ])
 def test_transfer_rates(run_in_tmpdir, model, rate, dest_mat, power, ref_result):
     """Tests transfer_rates depletion class with transfer rates"""
@@ -61,13 +63,18 @@ def test_transfer_rates(run_in_tmpdir, model, rate, dest_mat, power, ref_result)
     chain_file = Path(__file__).parents[2] / 'chain_simple.xml'
 
     transfer_elements = ['Xe']
+    os = {'I': -1, 'Xe':0, 'Cs': 1, 'Gd': 3, 'U': 4}
 
     op = CoupledOperator(model, chain_file)
     op.round_number = True
     integrator = openmc.deplete.PredictorIntegrator(
         op, [1], power, timestep_units = 'd')
-    integrator.add_transfer_rate('f', transfer_elements, rate,
-                                 destination_material=dest_mat)
+    if rate != 0.0:
+        integrator.add_transfer_rate('f', transfer_elements, rate,
+                                    destination_material=dest_mat)
+    if 'redox' in ref_result.split('_'):
+        integrator.add_redox('f', {'Gd157':1}, os)
+
     integrator.integrate()
 
     # Get path to test and reference results
@@ -83,6 +90,39 @@ def test_transfer_rates(run_in_tmpdir, model, rate, dest_mat, power, ref_result)
     res_ref = openmc.deplete.Results(path_reference)
     res_test = openmc.deplete.Results(path_test)
 
-    assert_same_mats(res_ref, res_test)
-    assert_atoms_equal(res_ref, res_test, 1e-6)
-    assert_reaction_rates_equal(res_ref, res_test)
+    assert_atoms_equal(res_ref, res_test, tol=1e-3)
+    assert_reaction_rates_equal(res_ref, res_test, tol=1e-3)
+
+@pytest.mark.parametrize("rate, power, ref_result", [
+    (1e-1, 0.0, 'no_depletion_with_ext_source'),
+    (1e-1, 174., 'depletion_with_ext_source'),
+])
+def test_external_source_rates(run_in_tmpdir, model, rate, power, ref_result):
+    """Tests external_rates depletion class with external source rates"""
+
+    chain_file = Path(__file__).parents[2] / 'chain_simple.xml'
+
+    external_source_vector = {'U': 1}
+
+    op = CoupledOperator(model, chain_file)
+    op.round_number = True
+    integrator = openmc.deplete.PredictorIntegrator(
+        op, [1], power, timestep_units='d')
+    integrator.add_external_source_rate('f', external_source_vector, rate)
+    integrator.integrate()
+
+    # Get path to test and reference results
+    path_test = op.output_dir / 'depletion_results.h5'
+    path_reference = Path(__file__).with_name(f'ref_{ref_result}.h5')
+
+    # If updating results, do so and return
+    if config['update']:
+        shutil.copyfile(str(path_test), str(path_reference))
+        return
+
+    # Load the reference/test results
+    res_ref = openmc.deplete.Results(path_reference)
+    res_test = openmc.deplete.Results(path_test)
+
+    assert_atoms_equal(res_ref, res_test, tol=1e-3)
+    assert_reaction_rates_equal(res_ref, res_test, tol=1e-3)
diff --git a/tests/regression_tests/deplete_with_transport/last_step_reference_materials.xml b/tests/regression_tests/deplete_with_transport/last_step_reference_materials.xml
index f61595dd9..2dca55992 100644
--- a/tests/regression_tests/deplete_with_transport/last_step_reference_materials.xml
+++ b/tests/regression_tests/deplete_with_transport/last_step_reference_materials.xml
@@ -1,1049 +1,1049 @@
 <?xml version='1.0' encoding='utf-8'?>
 <materials>
-  <material depletable="true" id="1" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.4180489845847837e-07" name="Cs135"/>
-    <nuclide ao="7.2549793284842195e-09" name="Gd156"/>
-    <nuclide ao="4.6150717465438587e-10" name="Gd157"/>
-    <nuclide ao="8.610759149890541e-09" name="I135"/>
-    <nuclide ao="1.00447595096247e-05" name="U234"/>
-    <nuclide ao="0.001021319968105753" name="U235"/>
-    <nuclide ao="0.022134492490415457" name="U238"/>
-    <nuclide ao="4.898883540853392e-09" name="Xe135"/>
-    <nuclide ao="1.5016036288896978e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="2" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.7109992046231964e-07" name="Cs135"/>
-    <nuclide ao="7.042063874024541e-09" name="Gd156"/>
-    <nuclide ao="6.027071887061818e-10" name="Gd157"/>
-    <nuclide ao="7.95412956238769e-09" name="I135"/>
-    <nuclide ao="1.0044351820098593e-05" name="U234"/>
-    <nuclide ao="0.00102408632740032" name="U235"/>
-    <nuclide ao="0.02213423274784442" name="U238"/>
-    <nuclide ao="4.973037106302515e-09" name="Xe135"/>
-    <nuclide ao="1.3283930174680673e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="3" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.5196882278137964e-07" name="Cs135"/>
-    <nuclide ao="7.390520641508283e-09" name="Gd156"/>
-    <nuclide ao="6.1139624340991e-10" name="Gd157"/>
-    <nuclide ao="7.339426249127244e-09" name="I135"/>
-    <nuclide ao="1.0044458911815349e-05" name="U234"/>
-    <nuclide ao="0.0010213846058415266" name="U235"/>
-    <nuclide ao="0.02213428321817386" name="U238"/>
-    <nuclide ao="5.052047240880656e-09" name="Xe135"/>
-    <nuclide ao="1.492502282115296e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="4" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.610385562797368e-07" name="Cs135"/>
-    <nuclide ao="7.2542350999039044e-09" name="Gd156"/>
-    <nuclide ao="4.694384504892065e-10" name="Gd157"/>
-    <nuclide ao="8.99963252548058e-09" name="I135"/>
-    <nuclide ao="1.0044557857503748e-05" name="U234"/>
-    <nuclide ao="0.0010213889405514816" name="U235"/>
-    <nuclide ao="0.022134477440110358" name="U238"/>
-    <nuclide ao="5.198618342135029e-09" name="Xe135"/>
-    <nuclide ao="1.4782292628750817e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="5" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.0464035" name="O16"/>
-    <nuclide ao="4.6068906575995227e-07" name="Cs135"/>
-    <nuclide ao="4.45301183633431e-09" name="Gd156"/>
-    <nuclide ao="6.452882105032804e-05" name="Gd157"/>
-    <nuclide ao="5.350905347436304e-09" name="I135"/>
-    <nuclide ao="0.0010186933910628446" name="U235"/>
-    <nuclide ao="0.02169176973824294" name="U238"/>
-    <nuclide ao="6.032753962430652e-09" name="Xe135"/>
-    <nuclide ao="4.88334256170519e-07" name="Xe136"/>
-  </material>
-  <material depletable="true" id="6" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.5448917916965475e-07" name="Cs135"/>
-    <nuclide ao="7.987183377412963e-09" name="Gd156"/>
-    <nuclide ao="4.865379817976642e-10" name="Gd157"/>
-    <nuclide ao="8.523326262987082e-09" name="I135"/>
-    <nuclide ao="1.004449677662597e-05" name="U234"/>
-    <nuclide ao="0.0010175799065569445" name="U235"/>
-    <nuclide ao="0.02213426568393913" name="U238"/>
-    <nuclide ao="4.86982078246298e-09" name="Xe135"/>
-    <nuclide ao="1.6938385702297322e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="7" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.365851597102055e-07" name="Cs135"/>
-    <nuclide ao="7.885461639644807e-09" name="Gd156"/>
-    <nuclide ao="3.6063871000919685e-10" name="Gd157"/>
-    <nuclide ao="1.136967239656431e-08" name="I135"/>
-    <nuclide ao="1.004435753347176e-05" name="U234"/>
-    <nuclide ao="0.0010176281979544044" name="U235"/>
-    <nuclide ao="0.02213438660359147" name="U238"/>
-    <nuclide ao="5.3728946653715295e-09" name="Xe135"/>
-    <nuclide ao="1.7038388316063246e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="8" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.5625540310461215e-07" name="Cs135"/>
-    <nuclide ao="7.771490180621292e-09" name="Gd156"/>
-    <nuclide ao="4.146823023983641e-10" name="Gd157"/>
-    <nuclide ao="9.209488335030714e-09" name="I135"/>
-    <nuclide ao="1.0044402337263812e-05" name="U234"/>
-    <nuclide ao="0.0010186458655452861" name="U235"/>
-    <nuclide ao="0.022134339049317633" name="U238"/>
-    <nuclide ao="5.497659457987619e-09" name="Xe135"/>
-    <nuclide ao="1.6322553919462217e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="9" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.4849047261874304e-07" name="Cs135"/>
-    <nuclide ao="7.046983082489661e-09" name="Gd156"/>
-    <nuclide ao="4.957093622147675e-10" name="Gd157"/>
-    <nuclide ao="1.0283911483340312e-08" name="I135"/>
-    <nuclide ao="1.0044263707893839e-05" name="U234"/>
-    <nuclide ao="0.0010240145184075352" name="U235"/>
-    <nuclide ao="0.02213424387109948" name="U238"/>
-    <nuclide ao="5.102740934580265e-09" name="Xe135"/>
-    <nuclide ao="1.3522284050998152e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="10" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.54649330746641e-07" name="Cs135"/>
-    <nuclide ao="8.140359772312664e-09" name="Gd156"/>
-    <nuclide ao="5.697645603089687e-10" name="Gd157"/>
-    <nuclide ao="7.546282979584689e-09" name="I135"/>
-    <nuclide ao="1.0044621491322387e-05" name="U234"/>
-    <nuclide ao="0.0010168499066742598" name="U235"/>
-    <nuclide ao="0.022134202939750564" name="U238"/>
-    <nuclide ao="5.110958338411566e-09" name="Xe135"/>
-    <nuclide ao="1.7346785903470948e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="11" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.356328087111304e-07" name="Cs135"/>
-    <nuclide ao="7.482536561844464e-09" name="Gd156"/>
-    <nuclide ao="4.256220744995123e-10" name="Gd157"/>
-    <nuclide ao="9.633889473449001e-09" name="I135"/>
-    <nuclide ao="1.0044260492856067e-05" name="U234"/>
-    <nuclide ao="0.0010215775522793961" name="U235"/>
-    <nuclide ao="0.022134138392005908" name="U238"/>
-    <nuclide ao="4.875694297039386e-09" name="Xe135"/>
-    <nuclide ao="1.4987118043030461e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="12" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.5499172002525965e-07" name="Cs135"/>
-    <nuclide ao="7.90039271767826e-09" name="Gd156"/>
-    <nuclide ao="5.043357296023378e-10" name="Gd157"/>
-    <nuclide ao="9.717794257343092e-09" name="I135"/>
-    <nuclide ao="1.0044246640051479e-05" name="U234"/>
-    <nuclide ao="0.001018955480032775" name="U235"/>
-    <nuclide ao="0.022134093110617634" name="U238"/>
-    <nuclide ao="5.099520096684825e-09" name="Xe135"/>
-    <nuclide ao="1.6207687623132563e-06" name="Xe136"/>
-  </material>
-  <material depletable="true" id="13" temperature="293.6" volume="0.06674732823364922">
-    <density units="sum"/>
-    <nuclide ao="0.046295399999999994" name="O16"/>
-    <nuclide ao="0.00011276839999999999" name="O17"/>
-    <nuclide ao="4.2085912253520763e-07" name="Cs135"/>
-    <nuclide ao="7.387032997276892e-09" name="Gd156"/>
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+    <nuclide name="U235" ao="0.0010199260328263536"/>
+    <nuclide name="U238" ao="0.02213464791088917"/>
+    <nuclide name="Xe135" ao="5.0562409304992e-09"/>
+    <nuclide name="Xe136" ao="1.5820627773170167e-06"/>
+  </material>
+  <material id="59" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.0464035"/>
+    <nuclide name="Cs135" ao="4.5230881849700363e-07"/>
+    <nuclide name="Gd156" ao="4.850070823410614e-09"/>
+    <nuclide name="Gd157" ao="1.8483989393434286e-05"/>
+    <nuclide name="I135" ao="6.725814455601182e-09"/>
+    <nuclide name="U235" ao="0.0010144925922189567"/>
+    <nuclide name="U238" ao="0.021692066927375953"/>
+    <nuclide name="Xe135" ao="5.704892253586948e-09"/>
+    <nuclide name="Xe136" ao="7.167091844599147e-07"/>
+  </material>
+  <material id="60" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.454497767683894e-07"/>
+    <nuclide name="Gd156" ao="7.196390006044354e-09"/>
+    <nuclide name="Gd157" ao="4.2031817746046806e-10"/>
+    <nuclide name="I135" ao="9.201065366714289e-09"/>
+    <nuclide name="U234" ao="1.0045186817961686e-05"/>
+    <nuclide name="U235" ao="0.0010208696040929454"/>
+    <nuclide name="U238" ao="0.02213465839772633"/>
+    <nuclide name="Xe135" ao="5.089280209147232e-09"/>
+    <nuclide name="Xe136" ao="1.518671421481773e-06"/>
+  </material>
+  <material id="61" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.4189035801976633e-07"/>
+    <nuclide name="Gd156" ao="8.712564657203808e-09"/>
+    <nuclide name="Gd157" ao="3.811357562661636e-10"/>
+    <nuclide name="I135" ao="1.1620785273405822e-08"/>
+    <nuclide name="U234" ao="1.0044336288632069e-05"/>
+    <nuclide name="U235" ao="0.0010119886383078472"/>
+    <nuclide name="U238" ao="0.022134367811831677"/>
+    <nuclide name="Xe135" ao="5.348901930754654e-09"/>
+    <nuclide name="Xe136" ao="2.0018476265410578e-06"/>
+  </material>
+  <material id="62" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.495089867103096e-07"/>
+    <nuclide name="Gd156" ao="7.906963161591915e-09"/>
+    <nuclide name="Gd157" ao="3.6408897631430517e-10"/>
+    <nuclide name="I135" ao="1.1950713230937776e-08"/>
+    <nuclide name="U234" ao="1.0044642100307903e-05"/>
+    <nuclide name="U235" ao="0.0010172948034290832"/>
+    <nuclide name="U238" ao="0.022134397559942648"/>
+    <nuclide name="Xe135" ao="5.505591746019881e-09"/>
+    <nuclide name="Xe136" ao="1.7079328241951357e-06"/>
+  </material>
+  <material id="63" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.462711024188988e-07"/>
+    <nuclide name="Gd156" ao="7.739456100782371e-09"/>
+    <nuclide name="Gd157" ao="3.5314285741732257e-10"/>
+    <nuclide name="I135" ao="1.1695299577237885e-08"/>
+    <nuclide name="U234" ao="1.0044990649080159e-05"/>
+    <nuclide name="U235" ao="0.00101714384855181"/>
+    <nuclide name="U238" ao="0.022134662948265594"/>
+    <nuclide name="Xe135" ao="5.305471689749892e-09"/>
+    <nuclide name="Xe136" ao="1.715407896550972e-06"/>
+  </material>
+  <material id="64" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.4249695478290833e-07"/>
+    <nuclide name="Gd156" ao="7.906645616692851e-09"/>
+    <nuclide name="Gd157" ao="4.777104853554735e-10"/>
+    <nuclide name="I135" ao="8.818558147298291e-09"/>
+    <nuclide name="U234" ao="1.0044897730985513e-05"/>
+    <nuclide name="U235" ao="0.0010166286079598642"/>
+    <nuclide name="U238" ao="0.022134552806734164"/>
+    <nuclide name="Xe135" ao="5.075769477142743e-09"/>
+    <nuclide name="Xe136" ao="1.7517886490366798e-06"/>
+  </material>
+  <material id="65" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.27722871661117e-07"/>
+    <nuclide name="Gd156" ao="7.71901195619115e-09"/>
+    <nuclide name="Gd157" ao="4.5635218189574305e-10"/>
+    <nuclide name="I135" ao="8.439589024990906e-09"/>
+    <nuclide name="U234" ao="1.0044805721541428e-05"/>
+    <nuclide name="U235" ao="0.0010195626945820153"/>
+    <nuclide name="U238" ao="0.022134211944598508"/>
+    <nuclide name="Xe135" ao="4.495383369428082e-09"/>
+    <nuclide name="Xe136" ao="1.615305821872234e-06"/>
+  </material>
+  <material id="66" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.3610692191296446e-07"/>
+    <nuclide name="Gd156" ao="8.518503835547662e-09"/>
+    <nuclide name="Gd157" ao="4.5399900319492264e-10"/>
+    <nuclide name="I135" ao="9.09249537642733e-09"/>
+    <nuclide name="U234" ao="1.0044488112176157e-05"/>
+    <nuclide name="U235" ao="0.0010131432807183492"/>
+    <nuclide name="U238" ao="0.02213440970679264"/>
+    <nuclide name="Xe135" ao="5.399696603480261e-09"/>
+    <nuclide name="Xe136" ao="1.9473344000461746e-06"/>
+  </material>
+  <material id="67" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.4269733316848923e-07"/>
+    <nuclide name="Gd156" ao="6.8834912015221874e-09"/>
+    <nuclide name="Gd157" ao="5.268555293126039e-10"/>
+    <nuclide name="I135" ao="9.72792414512545e-09"/>
+    <nuclide name="U234" ao="1.0045216952994501e-05"/>
+    <nuclide name="U235" ao="0.0010221691167621608"/>
+    <nuclide name="U238" ao="0.02213481142663786"/>
+    <nuclide name="Xe135" ao="5.274398555953527e-09"/>
+    <nuclide name="Xe136" ao="1.4483484958518488e-06"/>
+  </material>
+  <material id="68" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.0464035"/>
+    <nuclide name="Cs135" ao="4.528731473512273e-07"/>
+    <nuclide name="Gd156" ao="5.276079336338315e-09"/>
+    <nuclide name="Gd157" ao="1.3671441071560035e-05"/>
+    <nuclide name="I135" ao="7.288078943337704e-09"/>
+    <nuclide name="U235" ao="0.0010136994033178508"/>
+    <nuclide name="U238" ao="0.021691660200183947"/>
+    <nuclide name="Xe135" ao="5.806302854653707e-09"/>
+    <nuclide name="Xe136" ao="7.647428117364966e-07"/>
+  </material>
+  <material id="69" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.52926912644081e-07"/>
+    <nuclide name="Gd156" ao="7.605960064126685e-09"/>
+    <nuclide name="Gd157" ao="4.3866976134467753e-10"/>
+    <nuclide name="I135" ao="1.0718373052984328e-08"/>
+    <nuclide name="U234" ao="1.004535540549945e-05"/>
+    <nuclide name="U235" ao="0.0010174852002284081"/>
+    <nuclide name="U238" ao="0.022134763845904754"/>
+    <nuclide name="Xe135" ao="5.519173025530596e-09"/>
+    <nuclide name="Xe136" ao="1.6894993848072207e-06"/>
+  </material>
+  <material id="70" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.448681938648973e-07"/>
+    <nuclide name="Gd156" ao="8.193263118004751e-09"/>
+    <nuclide name="Gd157" ao="3.6012099804909225e-10"/>
+    <nuclide name="I135" ao="1.1474071526782658e-08"/>
+    <nuclide name="U234" ao="1.0045002402839991e-05"/>
+    <nuclide name="U235" ao="0.0010131885250163563"/>
+    <nuclide name="U238" ao="0.022134816489828464"/>
+    <nuclide name="Xe135" ao="5.5082893868947435e-09"/>
+    <nuclide name="Xe136" ao="1.9270066654625705e-06"/>
+  </material>
+  <material id="71" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.430482643392291e-07"/>
+    <nuclide name="Gd156" ao="6.877045573690475e-09"/>
+    <nuclide name="Gd157" ao="4.989391562034294e-10"/>
+    <nuclide name="I135" ao="9.413699233026816e-09"/>
+    <nuclide name="U234" ao="1.0044755176343187e-05"/>
+    <nuclide name="U235" ao="0.0010220618298066796"/>
+    <nuclide name="U238" ao="0.02213485182592519"/>
+    <nuclide name="Xe135" ao="5.652564774411855e-09"/>
+    <nuclide name="Xe136" ao="1.4530725740390899e-06"/>
+  </material>
+  <material id="72" depletable="true" volume="0.06674732823364922" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="O16" ao="0.046295399999999994"/>
+    <nuclide name="O17" ao="0.00011276839999999999"/>
+    <nuclide name="Cs135" ao="4.5911326696160503e-07"/>
+    <nuclide name="Gd156" ao="8.189799197570779e-09"/>
+    <nuclide name="Gd157" ao="4.0611758795110294e-10"/>
+    <nuclide name="I135" ao="1.0680038521394817e-08"/>
+    <nuclide name="U234" ao="1.0044781496434218e-05"/>
+    <nuclide name="U235" ao="0.0010152415868637022"/>
+    <nuclide name="U238" ao="0.022134444076722617"/>
+    <nuclide name="Xe135" ao="5.540154017917435e-09"/>
+    <nuclide name="Xe136" ao="1.8091999503945539e-06"/>
+  </material>
+  <material id="73" volume="0.017528298008244847" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="N14" ao="3.3864599999999994e-05"/>
+    <nuclide name="N15" ao="1.23717e-07"/>
+    <nuclide name="O16" ao="7.86548e-06"/>
+    <nuclide name="O17" ao="2.9954799999999997e-09"/>
+  </material>
+  <material id="74" volume="0.16066450553933276" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="Cr50" ao="3.2962e-06"/>
+    <nuclide name="Cr52" ao="6.35639e-05"/>
+    <nuclide name="Cr53" ao="7.20763e-06"/>
+    <nuclide name="Cr54" ao="1.7941299999999999e-06"/>
+    <nuclide name="Fe54" ao="5.573499999999999e-06"/>
+    <nuclide name="Fe56" ao="8.749209999999999e-05"/>
+    <nuclide name="Fe57" ao="2.0205699999999998e-06"/>
+    <nuclide name="Fe58" ao="2.6890099999999995e-07"/>
+    <nuclide name="Ni58" ao="2.51631e-05"/>
+    <nuclide name="Ni60" ao="9.69278e-06"/>
+    <nuclide name="Ni61" ao="4.21338e-07"/>
+    <nuclide name="Ni62" ao="1.34341e-06"/>
+    <nuclide name="Ni64" ao="3.43127e-07"/>
+    <nuclide name="O16" ao="0.000307427"/>
+    <nuclide name="O17" ao="7.488679999999999e-07"/>
+    <nuclide name="Sn112" ao="4.67352e-06"/>
+    <nuclide name="Sn114" ao="3.17992e-06"/>
+    <nuclide name="Sn115" ao="1.6381399999999999e-06"/>
+    <nuclide name="Sn116" ao="7.005459999999999e-05"/>
+    <nuclide name="Sn117" ao="3.70027e-05"/>
+    <nuclide name="Sn118" ao="0.00011669399999999999"/>
+    <nuclide name="Sn119" ao="4.1387199999999996e-05"/>
+    <nuclide name="Sn120" ao="0.00015697299999999998"/>
+    <nuclide name="Sn122" ao="2.23076e-05"/>
+    <nuclide name="Sn124" ao="2.7896599999999996e-05"/>
+    <nuclide name="Zr90" ao="0.021831999999999997"/>
+    <nuclide name="Zr91" ao="0.004761039999999999"/>
+    <nuclide name="Zr92" ao="0.007277339999999999"/>
+    <nuclide name="Zr94" ao="0.007374939999999999"/>
+    <nuclide name="Zr96" ao="0.00118814"/>
+  </material>
+  <material id="75" volume="0.8803968514832288" temperature="293.6">
+    <density units="sum"/>
+    <nuclide name="H1" ao="0.046806299999999995"/>
+    <nuclide name="O16" ao="0.023342699999999997"/>
+    <nuclide name="O17" ao="8.890859999999999e-06"/>
     <sab name="c_H_in_H2O"/>
   </material>
-  <material id="76" temperature="293.6" volume="0.8803968514832288">
+  <material id="76" volume="0.8803968514832288" temperature="293.6">
     <density units="sum"/>
-    <nuclide ao="0.046806299999999995" name="H1"/>
-    <nuclide ao="0.023342699999999997" name="O16"/>
-    <nuclide ao="8.890859999999999e-06" name="O17"/>
+    <nuclide name="H1" ao="0.046806299999999995"/>
+    <nuclide name="O16" ao="0.023342699999999997"/>
+    <nuclide name="O17" ao="8.890859999999999e-06"/>
     <sab name="c_H_in_H2O"/>
   </material>
 </materials>
diff --git a/tests/regression_tests/deplete_with_transport/test.py b/tests/regression_tests/deplete_with_transport/test.py
index 0f7ebf00f..a2be22c56 100644
--- a/tests/regression_tests/deplete_with_transport/test.py
+++ b/tests/regression_tests/deplete_with_transport/test.py
@@ -65,6 +65,8 @@ def test_full(run_in_tmpdir, problem, multiproc):
     power = 2.337e15*4*JOULE_PER_EV*1e6  # MeV/second cm from CASMO
 
     # Perform simulation using the predictor algorithm
+    if config['mpi'] and multiproc:
+        pytest.skip("Multiprocessing depletion is disabled when MPI is enabled.")
     openmc.deplete.pool.USE_MULTIPROCESSING = multiproc
     openmc.deplete.PredictorIntegrator(op, dt, power).integrate()
 
diff --git a/tests/regression_tests/deplete_with_transport/test_reference.h5 b/tests/regression_tests/deplete_with_transport/test_reference.h5
index e8478250b..cc616e791 100644
Binary files a/tests/regression_tests/deplete_with_transport/test_reference.h5 and b/tests/regression_tests/deplete_with_transport/test_reference.h5 differ
diff --git a/tests/regression_tests/diff_tally/inputs_true.dat b/tests/regression_tests/diff_tally/inputs_true.dat
index 3ec4154e8..19356b64a 100644
--- a/tests/regression_tests/diff_tally/inputs_true.dat
+++ b/tests/regression_tests/diff_tally/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UOX fuel">
-      <density units="g/cm3" value="10.062"/>
-      <nuclide ao="4.9476e-06" name="U234"/>
-      <nuclide ao="0.00048218" name="U235"/>
-      <nuclide ao="0.021504" name="U238"/>
-      <nuclide ao="1.0801e-08" name="Xe135"/>
-      <nuclide ao="0.045737" name="O16"/>
+    <material id="1" name="UOX fuel" depletable="true">
+      <density value="10.062" units="g/cm3"/>
+      <nuclide name="U234" ao="4.9476e-06"/>
+      <nuclide name="U235" ao="0.00048218"/>
+      <nuclide name="U238" ao="0.021504"/>
+      <nuclide name="Xe135" ao="1.0801e-08"/>
+      <nuclide name="O16" ao="0.045737"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="Cold borated water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="4" name="Hot borated water">
-      <density units="atom/b-cm" value="0.06614"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.06614" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="5" name="Reactor pressure vessel steel">
-      <density units="g/cm3" value="7.9"/>
+      <density value="7.9" units="g/cm3"/>
       <nuclide name="Fe54" wo="0.05437098"/>
       <nuclide name="Fe56" wo="0.88500663"/>
       <nuclide name="Fe57" wo="0.0208008"/>
@@ -47,7 +47,7 @@
       <nuclide name="Cu63" wo="0.0013696"/>
     </material>
     <material id="6" name="Lower radial reflector">
-      <density units="g/cm3" value="4.32"/>
+      <density value="4.32" units="g/cm3"/>
       <nuclide name="H1" wo="0.0095661"/>
       <nuclide name="O16" wo="0.0759107"/>
       <nuclide name="B10" wo="3.08409e-05"/>
@@ -62,7 +62,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="7" name="Upper radial reflector / Top plate region">
-      <density units="g/cm3" value="4.28"/>
+      <density value="4.28" units="g/cm3"/>
       <nuclide name="H1" wo="0.0086117"/>
       <nuclide name="O16" wo="0.0683369"/>
       <nuclide name="B10" wo="2.77638e-05"/>
@@ -77,7 +77,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="8" name="Bottom plate region">
-      <density units="g/cm3" value="7.184"/>
+      <density value="7.184" units="g/cm3"/>
       <nuclide name="H1" wo="0.0011505"/>
       <nuclide name="O16" wo="0.0091296"/>
       <nuclide name="B10" wo="3.70915e-06"/>
@@ -92,7 +92,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="9" name="Bottom nozzle region">
-      <density units="g/cm3" value="2.53"/>
+      <density value="2.53" units="g/cm3"/>
       <nuclide name="H1" wo="0.0245014"/>
       <nuclide name="O16" wo="0.1944274"/>
       <nuclide name="B10" wo="7.89917e-05"/>
@@ -107,7 +107,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="10" name="Top nozzle region">
-      <density units="g/cm3" value="1.746"/>
+      <density value="1.746" units="g/cm3"/>
       <nuclide name="H1" wo="0.035887"/>
       <nuclide name="O16" wo="0.2847761"/>
       <nuclide name="B10" wo="0.000115699"/>
@@ -122,7 +122,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="11" name="Top of fuel assemblies">
-      <density units="g/cm3" value="3.044"/>
+      <density value="3.044" units="g/cm3"/>
       <nuclide name="H1" wo="0.0162913"/>
       <nuclide name="O16" wo="0.1292776"/>
       <nuclide name="B10" wo="5.25228e-05"/>
@@ -135,7 +135,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="12" name="Bottom of fuel assemblies">
-      <density units="g/cm3" value="1.762"/>
+      <density value="1.762" units="g/cm3"/>
       <nuclide name="H1" wo="0.0292856"/>
       <nuclide name="O16" wo="0.2323919"/>
       <nuclide name="B10" wo="9.44159e-05"/>
@@ -149,8 +149,8 @@
     </material>
   </materials>
   <geometry>
-    <cell fill="200" id="1" region="-6 34 -35" universe="0"/>
-    <cell fill="201" id="2" region="-6 35 -36" universe="0"/>
+    <cell id="1" fill="200" region="-6 34 -35" universe="0"/>
+    <cell id="2" fill="201" region="-6 35 -36" universe="0"/>
     <cell id="3" material="8" region="-7 31 -32" universe="0"/>
     <cell id="4" material="9" region="-5 32 -33" universe="0"/>
     <cell id="5" material="12" region="-5 33 -34" universe="0"/>
@@ -174,9 +174,9 @@
     <cell id="31" material="2" region="3 -4" universe="4"/>
     <cell id="32" material="4" region="4" universe="4"/>
     <cell id="50" material="3" region="34 -35" universe="5"/>
-    <cell fill="100" id="60" region="34 -35" universe="6"/>
+    <cell id="60" fill="100" region="34 -35" universe="6"/>
     <cell id="70" material="4" region="35 -36" universe="7"/>
-    <cell fill="101" id="80" region="35 -36" universe="8"/>
+    <cell id="80" fill="101" region="35 -36" universe="8"/>
     <lattice id="100" name="Fuel assembly (lower half)">
       <pitch>1.26 1.26</pitch>
       <dimension>17 17</dimension>
@@ -277,30 +277,30 @@
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.41" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.475" id="2" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.56" id="3" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.62" id="4" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 187.6" id="5" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 209.0" id="6" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 229.0" id="7" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 249.0" id="8" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="-229.0" id="31" type="z-plane"/>
-    <surface coeffs="-199.0" id="32" type="z-plane"/>
-    <surface coeffs="-193.0" id="33" type="z-plane"/>
-    <surface coeffs="-183.0" id="34" type="z-plane"/>
-    <surface coeffs="0.0" id="35" type="z-plane"/>
-    <surface coeffs="183.0" id="36" type="z-plane"/>
-    <surface coeffs="203.0" id="37" type="z-plane"/>
-    <surface coeffs="215.0" id="38" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="223.0" id="39" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.41"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.475"/>
+    <surface id="3" type="z-cylinder" coeffs="0.0 0.0 0.56"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.62"/>
+    <surface id="5" type="z-cylinder" coeffs="0.0 0.0 187.6"/>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 209.0"/>
+    <surface id="7" type="z-cylinder" coeffs="0.0 0.0 229.0"/>
+    <surface id="8" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 249.0"/>
+    <surface id="31" type="z-plane" boundary="vacuum" coeffs="-229.0"/>
+    <surface id="32" type="z-plane" coeffs="-199.0"/>
+    <surface id="33" type="z-plane" coeffs="-193.0"/>
+    <surface id="34" type="z-plane" coeffs="-183.0"/>
+    <surface id="35" type="z-plane" coeffs="0.0"/>
+    <surface id="36" type="z-plane" coeffs="183.0"/>
+    <surface id="37" type="z-plane" coeffs="203.0"/>
+    <surface id="38" type="z-plane" coeffs="215.0"/>
+    <surface id="39" type="z-plane" boundary="vacuum" coeffs="223.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>3</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-160 -160 -183 160 160 183</parameters>
       </space>
@@ -429,10 +429,10 @@
       <scores>nu-fission scatter</scores>
       <derivative>5</derivative>
     </tally>
-    <derivative id="1" material="3" variable="density"/>
-    <derivative id="2" material="1" variable="density"/>
-    <derivative id="3" material="1" nuclide="O16" variable="nuclide_density"/>
-    <derivative id="4" material="1" nuclide="U235" variable="nuclide_density"/>
-    <derivative id="5" material="1" variable="temperature"/>
+    <derivative id="1" variable="density" material="3"/>
+    <derivative id="2" variable="density" material="1"/>
+    <derivative id="3" variable="nuclide_density" material="1" nuclide="O16"/>
+    <derivative id="4" variable="nuclide_density" material="1" nuclide="U235"/>
+    <derivative id="5" variable="temperature" material="1"/>
   </tallies>
 </model>
diff --git a/tests/regression_tests/diff_tally/results_true.dat b/tests/regression_tests/diff_tally/results_true.dat
index 14729c821..a88081b7c 100644
--- a/tests/regression_tests/diff_tally/results_true.dat
+++ b/tests/regression_tests/diff_tally/results_true.dat
@@ -1,27 +1,27 @@
 d_material,d_nuclide,d_variable,score,mean,std. dev.
-3,,density,flux,-8.7368155e+00,1.5577812e+00
-3,,density,flux,-1.4842625e+01,2.2947682e+00
-1,,density,flux,-2.0922686e-01,4.9935069e-02
-1,,density,flux,-3.4582490e-01,1.7454386e-01
-1,O16,nuclide_density,flux,1.1301782e+01,2.2440389e+01
-1,O16,nuclide_density,flux,3.2481563e+00,3.2342885e+01
-1,U235,nuclide_density,flux,-1.5048665e+03,5.9045681e+02
-1,U235,nuclide_density,flux,-1.6193231e+03,9.7230073e+02
-1,,temperature,flux,-1.0891931e-04,2.3076849e-04
-1,,temperature,flux,-1.3563853e-04,2.0952841e-04
-3,,density,total,-3.9155374e+00,5.4862771e-01
-3,,density,absorption,-4.9210534e-01,3.4700776e-02
-3,,density,scatter,-3.4234320e+00,5.1443821e-01
-3,,density,fission,-3.1088949e-01,8.0540906e-02
-3,,density,nu-fission,-7.6137447e-01,1.9548876e-01
-3,,density,total,-3.8615156e-01,1.0175701e-01
-3,,density,absorption,-3.4458071e-01,9.3381192e-02
-3,,density,scatter,-4.1570854e-02,8.6942039e-03
-3,,density,fission,-2.9864832e-01,8.4711885e-02
-3,,density,nu-fission,-7.2797006e-01,2.0640455e-01
-3,,density,total,1.0491251e+01,9.1787723e+00
-3,,density,absorption,2.9819000e-01,3.0887165e-01
-3,,density,scatter,1.0193061e+01,8.8715930e+00
+3,,density,flux,-9.2822822e+00,1.6880315e+00
+3,,density,flux,-2.0591270e+01,3.0043477e+00
+1,,density,flux,-2.9765141e-01,5.2949290e-02
+1,,density,flux,-4.0095723e-01,1.1716168e-01
+1,O16,nuclide_density,flux,-1.4245069e+01,6.3028710e+00
+1,O16,nuclide_density,flux,-2.5098326e+01,5.7525657e+00
+1,U235,nuclide_density,flux,-2.1513560e+03,6.7234014e+02
+1,U235,nuclide_density,flux,-2.4222736e+03,7.7715656e+02
+1,,temperature,flux,-1.1242141e-04,8.1692839e-05
+1,,temperature,flux,6.1357500e-06,7.4812624e-05
+3,,density,total,-4.2880614e+00,7.0021512e-01
+3,,density,absorption,-5.0324695e-01,4.9079615e-02
+3,,density,scatter,-3.7848144e+00,6.5868388e-01
+3,,density,fission,-2.7732455e-01,6.4647487e-02
+3,,density,nu-fission,-6.7990282e-01,1.5692707e-01
+3,,density,total,-3.8043572e-01,9.6222860e-02
+3,,density,absorption,-3.3518262e-01,8.6810436e-02
+3,,density,scatter,-4.5253106e-02,9.5205809e-03
+3,,density,fission,-2.6400028e-01,6.8175997e-02
+3,,density,nu-fission,-6.4353897e-01,1.6613026e-01
+3,,density,total,-3.2070285e-01,2.8328990e+00
+3,,density,absorption,2.5921007e-02,2.2141808e-02
+3,,density,scatter,-3.4662385e-01,2.8171749e+00
 3,,density,fission,0.0000000e+00,0.0000000e+00
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
 3,,density,total,0.0000000e+00,0.0000000e+00
@@ -29,19 +29,19 @@ d_material,d_nuclide,d_variable,score,mean,std. dev.
 3,,density,scatter,0.0000000e+00,0.0000000e+00
 3,,density,fission,0.0000000e+00,0.0000000e+00
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
-1,,density,total,4.7128871e-01,5.7197095e-02
-1,,density,absorption,2.1981833e-02,1.7427232e-02
-1,,density,scatter,4.4930687e-01,4.0030120e-02
-1,,density,fission,6.0712328e-03,1.3719512e-02
-1,,density,nu-fission,1.5474650e-02,3.3437103e-02
-1,,density,total,1.2203418e-02,1.5798741e-02
-1,,density,absorption,7.0228085e-03,1.5294984e-02
-1,,density,scatter,5.1806097e-03,5.0807003e-04
-1,,density,fission,4.1074923e-03,1.3759608e-02
-1,,density,nu-fission,1.0046778e-02,3.3529881e-02
-1,,density,total,-5.2100220e-01,2.7618985e-01
-1,,density,absorption,-1.1039048e-02,6.2043549e-03
-1,,density,scatter,-5.0996315e-01,2.7027892e-01
+1,,density,total,4.0321800e-01,3.5365140e-02
+1,,density,absorption,4.8426976e-03,8.3784362e-03
+1,,density,scatter,3.9837530e-01,2.9076664e-02
+1,,density,fission,-5.4793062e-03,7.1720201e-03
+1,,density,nu-fission,-1.2481616e-02,1.7542625e-02
+1,,density,total,-3.6063021e-03,7.1950060e-03
+1,,density,absorption,-8.2666439e-03,6.9274342e-03
+1,,density,scatter,4.6603417e-03,4.1803496e-04
+1,,density,fission,-7.6315741e-03,7.1774359e-03
+1,,density,nu-fission,-1.8551295e-02,1.7493106e-02
+1,,density,total,-6.2852725e-01,1.9776006e-01
+1,,density,absorption,-1.4850233e-02,4.3086611e-03
+1,,density,scatter,-6.1367701e-01,1.9363188e-01
 1,,density,fission,0.0000000e+00,0.0000000e+00
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
 1,,density,total,0.0000000e+00,0.0000000e+00
@@ -49,19 +49,19 @@ d_material,d_nuclide,d_variable,score,mean,std. dev.
 1,,density,scatter,0.0000000e+00,0.0000000e+00
 1,,density,fission,0.0000000e+00,0.0000000e+00
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
-1,O16,nuclide_density,total,5.5744608e+01,1.3811361e+01
-1,O16,nuclide_density,absorption,2.1284059e+00,3.3353171e+00
-1,O16,nuclide_density,scatter,5.3616202e+01,1.0494347e+01
-1,O16,nuclide_density,fission,6.6058671e-01,1.9263932e+00
-1,O16,nuclide_density,nu-fission,1.5973305e+00,4.6977280e+00
-1,O16,nuclide_density,total,9.8433784e-01,2.4053581e+00
-1,O16,nuclide_density,absorption,9.1167507e-01,2.2565683e+00
-1,O16,nuclide_density,scatter,7.2662768e-02,1.5198625e-01
-1,O16,nuclide_density,fission,6.8845700e-01,1.9451444e+00
-1,O16,nuclide_density,nu-fission,1.6768628e+00,4.7395897e+00
-1,O16,nuclide_density,total,8.1604245e+00,3.7248338e+01
-1,O16,nuclide_density,absorption,2.4903834e-01,4.5609671e-01
-1,O16,nuclide_density,scatter,7.9113862e+00,3.6811434e+01
+1,O16,nuclide_density,total,4.2608989e+01,2.6322158e+00
+1,O16,nuclide_density,absorption,-5.9877963e-01,4.2139216e-01
+1,O16,nuclide_density,scatter,4.3207769e+01,2.5312922e+00
+1,O16,nuclide_density,fission,-9.2838585e-01,4.3505041e-01
+1,O16,nuclide_density,nu-fission,-2.2834786e+00,1.0634300e+00
+1,O16,nuclide_density,total,-1.1195412e+00,3.3033730e-01
+1,O16,nuclide_density,absorption,-1.0650267e+00,3.2780477e-01
+1,O16,nuclide_density,scatter,-5.4514540e-02,2.4911965e-02
+1,O16,nuclide_density,fission,-8.6787386e-01,4.4270048e-01
+1,O16,nuclide_density,nu-fission,-2.1159252e+00,1.0787495e+00
+1,O16,nuclide_density,total,-3.1004750e+01,8.0776339e+00
+1,O16,nuclide_density,absorption,-5.0354237e-01,2.6144390e-01
+1,O16,nuclide_density,scatter,-3.0501208e+01,7.8176703e+00
 1,O16,nuclide_density,fission,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,total,0.0000000e+00,0.0000000e+00
@@ -69,19 +69,19 @@ d_material,d_nuclide,d_variable,score,mean,std. dev.
 1,O16,nuclide_density,scatter,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,fission,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
-1,U235,nuclide_density,total,-2.8654443e+02,3.3721470e+02
-1,U235,nuclide_density,absorption,2.0329180e+02,1.0522263e+02
-1,U235,nuclide_density,scatter,-4.8983623e+02,2.3237977e+02
-1,U235,nuclide_density,fission,2.6089045e+02,7.2578344e+01
-1,U235,nuclide_density,nu-fission,6.3692044e+02,1.7710877e+02
-1,U235,nuclide_density,total,4.5611633e+02,9.1682367e+01
-1,U235,nuclide_density,absorption,3.3831545e+02,8.5035501e+01
-1,U235,nuclide_density,scatter,1.1780088e+02,7.4128809e+00
-1,U235,nuclide_density,fission,2.6094461e+02,7.2102445e+01
-1,U235,nuclide_density,nu-fission,6.3705705e+02,1.7578950e+02
-1,U235,nuclide_density,total,-4.1342174e+03,1.4666097e+03
-1,U235,nuclide_density,absorption,-1.1329170e+02,3.5066579e+01
-1,U235,nuclide_density,scatter,-4.0209257e+03,1.4320401e+03
+1,U235,nuclide_density,total,-5.3578869e+02,3.4044374e+02
+1,U235,nuclide_density,absorption,2.0769087e+02,1.0873543e+02
+1,U235,nuclide_density,scatter,-7.4347956e+02,2.5966153e+02
+1,U235,nuclide_density,fission,2.8982867e+02,8.5348425e+01
+1,U235,nuclide_density,nu-fission,7.0772332e+02,2.0847449e+02
+1,U235,nuclide_density,total,4.8356107e+02,1.0276621e+02
+1,U235,nuclide_density,absorption,3.6683548e+02,9.6924085e+01
+1,U235,nuclide_density,scatter,1.1672558e+02,6.1773010e+00
+1,U235,nuclide_density,fission,2.8941699e+02,8.4475937e+01
+1,U235,nuclide_density,nu-fission,7.0639136e+02,2.0594285e+02
+1,U235,nuclide_density,total,-4.6777522e+03,1.5037171e+03
+1,U235,nuclide_density,absorption,-1.1585081e+02,3.8299177e+01
+1,U235,nuclide_density,scatter,-4.5619014e+03,1.4660078e+03
 1,U235,nuclide_density,fission,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,total,0.0000000e+00,0.0000000e+00
@@ -89,19 +89,19 @@ d_material,d_nuclide,d_variable,score,mean,std. dev.
 1,U235,nuclide_density,scatter,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,fission,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
-1,,temperature,total,5.3733744e-05,1.1785516e-04
-1,,temperature,absorption,-1.3987780e-05,1.9819801e-05
-1,,temperature,scatter,6.7721524e-05,9.9934995e-05
-1,,temperature,fission,-1.4882696e-05,1.6930666e-05
-1,,temperature,nu-fission,-3.6272065e-05,4.1250892e-05
-1,,temperature,total,-1.8119796e-05,2.2651409e-05
-1,,temperature,absorption,-1.7632076e-05,2.1825006e-05
-1,,temperature,scatter,-4.8771972e-07,1.4270133e-06
-1,,temperature,fission,-1.4890811e-05,1.6935899e-05
-1,,temperature,nu-fission,-3.6292215e-05,4.1263951e-05
-1,,temperature,total,-2.0776498e-04,2.9286552e-04
-1,,temperature,absorption,-3.9068381e-06,3.4912438e-06
-1,,temperature,scatter,-2.0385814e-04,2.8941307e-04
+1,,temperature,total,8.2134941e-05,1.3941434e-05
+1,,temperature,absorption,4.1502002e-05,3.5967068e-05
+1,,temperature,scatter,4.0632939e-05,2.6595574e-05
+1,,temperature,fission,3.1768469e-06,2.1582455e-05
+1,,temperature,nu-fission,7.7368345e-06,5.2593375e-05
+1,,temperature,total,1.0834099e-06,2.6707333e-05
+1,,temperature,absorption,1.6240165e-06,2.6844372e-05
+1,,temperature,scatter,-5.4060665e-07,2.5905412e-07
+1,,temperature,fission,3.1674458e-06,2.1588836e-05
+1,,temperature,nu-fission,7.7134913e-06,5.2609321e-05
+1,,temperature,total,9.5175179e-05,1.0549922e-04
+1,,temperature,absorption,4.3553888e-06,2.9261613e-06
+1,,temperature,scatter,9.0819790e-05,1.0362538e-04
 1,,temperature,fission,0.0000000e+00,0.0000000e+00
 1,,temperature,nu-fission,0.0000000e+00,0.0000000e+00
 1,,temperature,total,0.0000000e+00,0.0000000e+00
@@ -109,68 +109,68 @@ d_material,d_nuclide,d_variable,score,mean,std. dev.
 1,,temperature,scatter,0.0000000e+00,0.0000000e+00
 1,,temperature,fission,0.0000000e+00,0.0000000e+00
 1,,temperature,nu-fission,0.0000000e+00,0.0000000e+00
-3,,density,absorption,-4.5579543e-01,5.5532252e-02
-3,,density,absorption,-1.3526251e-02,1.3917291e-01
-1,,density,absorption,3.8695654e-02,1.5823093e-02
-1,,density,absorption,-1.8171150e-02,1.2999773e-03
-1,O16,nuclide_density,absorption,2.0992238e+00,2.7230754e+00
-1,O16,nuclide_density,absorption,-6.8956953e-01,3.3064989e-01
-1,U235,nuclide_density,absorption,2.5000417e+02,7.3334295e+01
-1,U235,nuclide_density,absorption,-1.0832917e+02,1.1517513e+01
-1,,temperature,absorption,1.1819459e-05,4.2090670e-05
-1,,temperature,absorption,-7.2302974e-06,1.5200299e-05
+3,,density,absorption,-5.4897740e-01,8.6529144e-02
+3,,density,absorption,1.3410763e-01,5.6235628e-02
+1,,density,absorption,2.9551566e-02,1.5215007e-02
+1,,density,absorption,-8.6382033e-03,5.5181454e-03
+1,O16,nuclide_density,absorption,2.7111116e-01,9.6922067e-01
+1,O16,nuclide_density,absorption,-7.3785997e-01,7.7252792e-01
+1,U235,nuclide_density,absorption,2.7361252e+02,1.6168134e+02
+1,U235,nuclide_density,absorption,-9.6885408e+01,1.9390948e+01
+1,,temperature,absorption,1.0054736e-05,4.2876538e-05
+1,,temperature,absorption,-1.8280609e-06,1.0133065e-05
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
-3,,density,scatter,-7.6858714e-01,3.2467801e-01
+3,,density,scatter,-5.4273849e-01,1.8081153e-01
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
-3,,density,scatter,3.2513857e-04,3.2513857e-04
-3,,density,nu-fission,-6.4963717e-01,1.8008086e-01
-3,,density,scatter,-2.6911548e+00,2.6336008e-01
-3,,density,nu-fission,-6.3014524e-01,1.8056453e-01
-3,,density,scatter,-3.4247012e-02,2.2649385e-02
+3,,density,scatter,1.3451827e-02,7.2870304e-03
+3,,density,nu-fission,-8.4200508e-01,3.4267126e-01
+3,,density,scatter,-3.1963455e+00,5.0369564e-01
+3,,density,nu-fission,-8.2934702e-01,3.4526131e-01
+3,,density,scatter,-7.0221960e-02,6.8067882e-02
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
-3,,density,scatter,1.0254549e+01,1.1054242e+01
+3,,density,scatter,1.1368387e+00,1.2091778e+00
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
 3,,density,scatter,0.0000000e+00,0.0000000e+00
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
-3,,density,scatter,2.5022850e-01,2.1207285e+00
+3,,density,scatter,-1.5916491e+00,2.3551913e+00
 3,,density,nu-fission,0.0000000e+00,0.0000000e+00
 3,,density,scatter,0.0000000e+00,0.0000000e+00
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
-1,,density,scatter,8.1153611e-03,3.1226564e-02
+1,,density,scatter,-1.8655633e-03,2.4061808e-02
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
-1,,density,scatter,2.3728497e-04,9.7166041e-04
-1,,density,nu-fission,2.6248459e-02,3.9328080e-02
-1,,density,scatter,4.2447769e-01,1.2580873e-02
-1,,density,nu-fission,1.9600074e-02,3.8118776e-02
-1,,density,scatter,8.0978391e-03,2.6585488e-03
+1,,density,scatter,-1.7672807e-04,7.4702879e-04
+1,,density,nu-fission,4.4506944e-03,3.1478180e-02
+1,,density,scatter,3.7553200e-01,2.3420342e-02
+1,,density,nu-fission,-1.3567324e-03,3.0610901e-02
+1,,density,scatter,6.9330560e-03,4.6649873e-03
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
-1,,density,scatter,-3.8358302e-01,2.1525401e-01
+1,,density,scatter,-5.1560927e-01,1.4846253e-01
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
 1,,density,scatter,0.0000000e+00,0.0000000e+00
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
-1,,density,scatter,-1.1924803e-01,9.1254221e-02
+1,,density,scatter,-1.0427977e-01,6.6396271e-02
 1,,density,nu-fission,0.0000000e+00,0.0000000e+00
 1,,density,scatter,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
-1,O16,nuclide_density,scatter,-1.6577061e-01,1.0793192e-01
-1,O16,nuclide_density,nu-fission,2.4995074e+00,5.7946039e+00
-1,O16,nuclide_density,scatter,5.5800956e-01,5.7675606e-01
+1,O16,nuclide_density,scatter,4.6587151e-02,9.0232672e-02
+1,O16,nuclide_density,nu-fission,-2.1162226e+00,2.4773842e+00
+1,O16,nuclide_density,scatter,6.6533325e-01,2.4693113e-01
 1,O16,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,scatter,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
 1,O16,nuclide_density,scatter,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
-1,U235,nuclide_density,scatter,5.8641633e+00,2.9488422e+00
-1,U235,nuclide_density,nu-fission,6.7444160e+02,1.1308014e+02
-1,U235,nuclide_density,scatter,1.1592472e+02,2.8886339e+01
+1,U235,nuclide_density,scatter,2.8979849e+01,2.0235325e+01
+1,U235,nuclide_density,nu-fission,8.1210341e+02,2.9252040e+02
+1,U235,nuclide_density,scatter,1.0307302e+02,4.4919960e+01
 1,U235,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,scatter,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,nu-fission,0.0000000e+00,0.0000000e+00
 1,U235,nuclide_density,scatter,0.0000000e+00,0.0000000e+00
 1,,temperature,nu-fission,0.0000000e+00,0.0000000e+00
-1,,temperature,scatter,-7.7511866e-07,6.4761261e-07
-1,,temperature,nu-fission,-9.5466386e-05,3.1033330e-05
-1,,temperature,scatter,-2.7127084e-06,2.0699253e-07
+1,,temperature,scatter,-3.3782154e-06,3.8240314e-06
+1,,temperature,nu-fission,-6.8920903e-05,4.4384761e-05
+1,,temperature,scatter,1.6325729e-07,2.3270228e-06
 1,,temperature,nu-fission,0.0000000e+00,0.0000000e+00
 1,,temperature,scatter,0.0000000e+00,0.0000000e+00
 1,,temperature,nu-fission,0.0000000e+00,0.0000000e+00
diff --git a/tests/regression_tests/distribmat/inputs_true.dat b/tests/regression_tests/distribmat/inputs_true.dat
index ade86999d..8087d7564 100644
--- a/tests/regression_tests/distribmat/inputs_true.dat
+++ b/tests/regression_tests/distribmat/inputs_true.dat
@@ -2,24 +2,26 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
-    <material depletable="true" id="3">
-      <density units="g/cc" value="2.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="3" depletable="true">
+      <density value="2.0" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" universe="1"/>
-    <cell id="11" material="2 void 3 2" region="-1" universe="11"/>
+    <cell id="11" region="-1" universe="11">
+      <material>2 void 3 2</material>
+    </cell>
     <cell id="12" material="1" region="1" universe="11"/>
-    <cell fill="101" id="101" region="2 -3 4 -5" universe="0"/>
+    <cell id="101" fill="101" region="2 -3 4 -5" universe="0"/>
     <lattice id="101">
       <pitch>2.0 2.0</pitch>
       <outer>1</outer>
@@ -29,30 +31,30 @@
 11 11 
 11 11 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.3" id="1" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-3.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="3" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="5" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.3"/>
+    <surface id="2" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="3" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="5" type="y-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-1 -1 -1 1 1 1</parameters>
       </space>
     </source>
   </settings>
   <plots>
-    <plot basis="xy" color_by="cell" filename="cellplot" id="1" type="slice">
+    <plot id="1" filename="cellplot" color_by="cell" type="slice" basis="xy">
       <pixels>400 400</pixels>
       <origin>0 0 0</origin>
       <width>7 7</width>
     </plot>
-    <plot basis="xy" color_by="material" filename="matplot" id="2" type="slice">
+    <plot id="2" filename="matplot" color_by="material" type="slice" basis="xy">
       <pixels>400 400</pixels>
       <origin>0 0 0</origin>
       <width>7 7</width>
diff --git a/tests/regression_tests/distribmat/results_true.dat b/tests/regression_tests/distribmat/results_true.dat
index af7b27fe7..166fd4662 100644
--- a/tests/regression_tests/distribmat/results_true.dat
+++ b/tests/regression_tests/distribmat/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-1.257344E+00 6.246360E-04
+1.246391E+00 1.414798E-02
 Cell
 	ID             =	11
 	Name           =	
diff --git a/tests/regression_tests/distribmat/test.py b/tests/regression_tests/distribmat/test.py
index 02f7e773e..dd09eec36 100644
--- a/tests/regression_tests/distribmat/test.py
+++ b/tests/regression_tests/distribmat/test.py
@@ -73,7 +73,7 @@ class DistribmatTestHarness(PyAPITestHarness):
         # Plots
         ####################
 
-        plot1 = openmc.Plot(plot_id=1)
+        plot1 = openmc.SlicePlot(plot_id=1)
         plot1.basis = 'xy'
         plot1.color_by = 'cell'
         plot1.filename = 'cellplot'
@@ -81,7 +81,7 @@ class DistribmatTestHarness(PyAPITestHarness):
         plot1.width = (7, 7)
         plot1.pixels = (400, 400)
 
-        plot2 = openmc.Plot(plot_id=2)
+        plot2 = openmc.SlicePlot(plot_id=2)
         plot2.basis = 'xy'
         plot2.color_by = 'material'
         plot2.filename = 'matplot'
diff --git a/tests/regression_tests/eigenvalue_genperbatch/inputs_true.dat b/tests/regression_tests/eigenvalue_genperbatch/inputs_true.dat
index a3e67d62f..15b810a62 100644
--- a/tests/regression_tests/eigenvalue_genperbatch/inputs_true.dat
+++ b/tests/regression_tests/eigenvalue_genperbatch/inputs_true.dat
@@ -1,14 +1,14 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="4.5" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 10.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -16,7 +16,7 @@
     <batches>7</batches>
     <inactive>3</inactive>
     <generations_per_batch>3</generations_per_batch>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4.0 -4.0 -4.0 4.0 4.0 4.0</parameters>
       </space>
diff --git a/tests/regression_tests/eigenvalue_genperbatch/results_true.dat b/tests/regression_tests/eigenvalue_genperbatch/results_true.dat
index 3f7ea801a..d171c5e87 100644
--- a/tests/regression_tests/eigenvalue_genperbatch/results_true.dat
+++ b/tests/regression_tests/eigenvalue_genperbatch/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-3.037481E-01 1.247474E-04
+2.975937E-01 1.293390E-03
 tally 1:
-3.211129E+01
-2.578396E+02
+3.173222E+01
+2.517683E+02
diff --git a/tests/regression_tests/eigenvalue_no_inactive/results_true.dat b/tests/regression_tests/eigenvalue_no_inactive/results_true.dat
index 2b1021608..3ba7485a9 100644
--- a/tests/regression_tests/eigenvalue_no_inactive/results_true.dat
+++ b/tests/regression_tests/eigenvalue_no_inactive/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.998284E-01 7.587782E-03
+3.072780E-01 6.882841E-03
diff --git a/tests/regression_tests/electron_heating/__init__.py b/tests/regression_tests/electron_heating/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/electron_heating/inputs_true.dat b/tests/regression_tests/electron_heating/inputs_true.dat
new file mode 100644
index 000000000..ec8e5a837
--- /dev/null
+++ b/tests/regression_tests/electron_heating/inputs_true.dat
@@ -0,0 +1,32 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 1.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>10000</particles>
+    <batches>1</batches>
+    <source type="independent" strength="1.0" particle="electron">
+      <energy type="discrete">
+        <parameters>10000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <cutoff>
+      <energy_photon>1000.0</energy_photon>
+    </cutoff>
+  </settings>
+  <tallies>
+    <tally id="1">
+      <scores>heating</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/electron_heating/results_true.dat b/tests/regression_tests/electron_heating/results_true.dat
new file mode 100644
index 000000000..4f54ceaa4
--- /dev/null
+++ b/tests/regression_tests/electron_heating/results_true.dat
@@ -0,0 +1,3 @@
+tally 1:
+1.000000E+07
+1.000000E+14
diff --git a/tests/regression_tests/electron_heating/test.py b/tests/regression_tests/electron_heating/test.py
new file mode 100644
index 000000000..e7a58560c
--- /dev/null
+++ b/tests/regression_tests/electron_heating/test.py
@@ -0,0 +1,40 @@
+import pytest
+import openmc
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def water_model():
+    # Define materals and geometry
+    water = openmc.Material()
+    water.add_nuclide("H1", 2.0)
+    water.add_nuclide("O16", 1.0)
+    water.set_density("g/cc", 1.0)
+    sphere = openmc.Sphere(r=1.0, boundary_type="reflective")
+    sph = openmc.Cell(fill=water, region=-sphere)
+    geometry = openmc.Geometry([sph])
+    source = openmc.IndependentSource(
+        energy=openmc.stats.delta_function(10.0e6),
+        particle="electron"
+    )
+
+    # Define settings
+    settings = openmc.Settings()
+    settings.particles = 10000
+    settings.batches = 1
+    settings.cutoff = {"energy_photon": 1000.0}
+    settings.run_mode = "fixed source"
+    settings.source = source
+
+    # Define tallies
+    tally = openmc.Tally()
+    tally.scores = ["heating"]
+    tallies = openmc.Tallies([tally])
+
+    return openmc.Model(geometry=geometry, settings=settings, tallies=tallies)
+
+
+def test_electron_heating_calc(water_model):
+    harness = PyAPITestHarness("statepoint.1.h5", water_model)
+    harness.main()
diff --git a/tests/regression_tests/energy_cutoff/inputs_true.dat b/tests/regression_tests/energy_cutoff/inputs_true.dat
index af8f86b67..557cb7a3b 100644
--- a/tests/regression_tests/energy_cutoff/inputs_true.dat
+++ b/tests/regression_tests/energy_cutoff/inputs_true.dat
@@ -2,28 +2,28 @@
 <model>
   <materials>
     <material id="1" name="mat">
-      <density units="atom/b-cm" value="0.069335"/>
-      <nuclide ao="40.0" name="H1"/>
+      <density value="0.069335" units="atom/b-cm"/>
+      <nuclide name="H1" ao="40.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="box" region="1 -2 3 -4 5 -6" universe="0"/>
-    <surface boundary="reflective" coeffs="-1" id="1" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1" id="3" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-1" id="5" type="z-plane"/>
-    <surface boundary="reflective" coeffs="1" id="6" type="z-plane"/>
+    <cell id="1" name="box" material="1" region="1 -2 3 -4 5 -6" universe="0"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="-1"/>
+    <surface id="2" type="x-plane" boundary="reflective" coeffs="1"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="-1"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="1"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="-1"/>
+    <surface id="6" type="z-plane" boundary="reflective" coeffs="1"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-1 -1 -1 1 1 1</parameters>
       </space>
-      <energy parameters="988000.0 2.249e-06" type="watt"/>
+      <energy type="watt" parameters="988000.0 2.249e-06"/>
     </source>
     <cutoff>
       <energy_neutron>4.0</energy_neutron>
diff --git a/tests/regression_tests/energy_grid/results_true.dat b/tests/regression_tests/energy_grid/results_true.dat
index deab1e28b..3a042d882 100644
--- a/tests/regression_tests/energy_grid/results_true.dat
+++ b/tests/regression_tests/energy_grid/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.152586E-01 1.458068E-03
+3.218009E-01 4.687417E-03
diff --git a/tests/regression_tests/energy_laws/inputs_true.dat b/tests/regression_tests/energy_laws/inputs_true.dat
index 4d8e825d1..8c5191217 100644
--- a/tests/regression_tests/energy_laws/inputs_true.dat
+++ b/tests/regression_tests/energy_laws/inputs_true.dat
@@ -1,18 +1,18 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="20.0"/>
-      <nuclide ao="1.0" name="U233"/>
-      <nuclide ao="1.0" name="Am244"/>
-      <nuclide ao="1.0" name="H2"/>
-      <nuclide ao="1.0" name="Na23"/>
-      <nuclide ao="1.0" name="Ta181"/>
+    <material id="1" depletable="true">
+      <density value="20.0" units="g/cm3"/>
+      <nuclide name="U233" ao="1.0"/>
+      <nuclide name="Am244" ao="1.0"/>
+      <nuclide name="H2" ao="1.0"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="Ta181" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="reflective" coeffs="0.0 0.0 0.0 100.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 100.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/energy_laws/results_true.dat b/tests/regression_tests/energy_laws/results_true.dat
index 534a299b5..780278683 100644
--- a/tests/regression_tests/energy_laws/results_true.dat
+++ b/tests/regression_tests/energy_laws/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.444000E+00 1.044626E-02
+2.458770E+00 9.422203E-03
diff --git a/tests/regression_tests/entropy/results_true.dat b/tests/regression_tests/entropy/results_true.dat
index 1c8b2908b..92d7f091d 100644
--- a/tests/regression_tests/entropy/results_true.dat
+++ b/tests/regression_tests/entropy/results_true.dat
@@ -1,13 +1,13 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
 entropy:
 7.688862E+00
-8.237960E+00
-8.314772E+00
-8.285254E+00
-8.271486E+00
-8.291355E+00
-8.253349E+00
-8.336726E+00
-8.284741E+00
-8.328102E+00
+8.226316E+00
+8.308355E+00
+8.243413E+00
+8.369345E+00
+8.304865E+00
+8.230689E+00
+8.338304E+00
+8.270630E+00
+8.386598E+00
diff --git a/tests/regression_tests/external_moab/inputs_true.dat b/tests/regression_tests/external_moab/inputs_true.dat
index 23b656bce..ed035f897 100644
--- a/tests/regression_tests/external_moab/inputs_true.dat
+++ b/tests/regression_tests/external_moab/inputs_true.dat
@@ -1,63 +1,63 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
-      <angle reference_uvw="-1.0 0.0 0.0" type="monodirectional"/>
+      <angle type="monodirectional" reference_uvw="-1.0 0.0 0.0"/>
       <energy type="discrete">
         <parameters>15000000.0 1.0</parameters>
       </energy>
     </source>
   </settings>
   <tallies>
-    <mesh id="1" library="moab" type="unstructured">
+    <mesh id="1" type="unstructured" library="moab">
       <filename>test_mesh_tets.h5m</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/external_moab/test.py b/tests/regression_tests/external_moab/test.py
index ce4e78a2c..2d64eb14b 100644
--- a/tests/regression_tests/external_moab/test.py
+++ b/tests/regression_tests/external_moab/test.py
@@ -39,7 +39,7 @@ def cpp_driver(request):
             target_link_libraries(main OpenMC::libopenmc)
             target_compile_features(main PUBLIC cxx_std_14)
             set(CMAKE_CXX_FLAGS "-pedantic-errors")
-            add_compile_definitions(DAGMC=1)
+            add_compile_definitions(OPENMC_DAGMC_ENABLED=1)
             """.format(openmc_dir)))
 
     # Create temporary build directory and change to there
diff --git a/tests/regression_tests/filter_cellfrom/inputs_true.dat b/tests/regression_tests/filter_cellfrom/inputs_true.dat
index b85f63c6e..20d0d69d4 100644
--- a/tests/regression_tests/filter_cellfrom/inputs_true.dat
+++ b/tests/regression_tests/filter_cellfrom/inputs_true.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.97"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="10.97" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
     <material id="3">
-      <density units="g/cm3" value="0.001225"/>
-      <nuclide ao="0.1999242" name="O16"/>
-      <nuclide ao="7.58e-05" name="O17"/>
-      <nuclide ao="0.7970696" name="N14"/>
-      <nuclide ao="0.0029304" name="N15"/>
+      <density value="0.001225" units="g/cm3"/>
+      <nuclide name="O16" ao="0.1999242"/>
+      <nuclide name="O17" ao="7.58e-05"/>
+      <nuclide name="N14" ao="0.7970696"/>
+      <nuclide name="N15" ao="0.0029304"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="3" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-2.05" id="4" type="x-plane"/>
-    <surface coeffs="2.05" id="5" type="x-plane"/>
-    <surface coeffs="-2.05" id="6" type="y-plane"/>
-    <surface coeffs="2.05" id="7" type="y-plane"/>
-    <surface coeffs="-2.05" id="8" type="z-plane"/>
-    <surface coeffs="2.05" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-6.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="6.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-6.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="6.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-6.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="6.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-2.05"/>
+    <surface id="5" type="x-plane" coeffs="2.05"/>
+    <surface id="6" type="y-plane" coeffs="-2.05"/>
+    <surface id="7" type="y-plane" coeffs="2.05"/>
+    <surface id="8" type="z-plane" coeffs="-2.05"/>
+    <surface id="9" type="z-plane" coeffs="2.05"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-6.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="6.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-6.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="6.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-6.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="6.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>2000</particles>
     <batches>15</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/filter_cellfrom/results_true.dat b/tests/regression_tests/filter_cellfrom/results_true.dat
index 5dd43e1f3..408a4965b 100644
--- a/tests/regression_tests/filter_cellfrom/results_true.dat
+++ b/tests/regression_tests/filter_cellfrom/results_true.dat
@@ -1,53 +1,53 @@
 k-combined:
-9.640806E-02 2.655206E-03
+9.035025E-02 2.654309E-03
 tally 1:
-6.025999E+00
-3.635317E+00
+5.994069E+00
+3.594398E+00
 tally 2:
-4.523606E-04
-2.051522E-08
+4.559707E-04
+2.080739E-08
 tally 3:
-6.026452E+00
-3.635862E+00
+5.994525E+00
+3.594945E+00
 tally 4:
 0.000000E+00
 0.000000E+00
 tally 5:
-1.530903E+00
-2.357048E-01
+1.473892E+00
+2.187509E-01
 tally 6:
-4.992646E-05
-2.600391E-10
+5.223875E-05
+2.992861E-10
 tally 7:
-1.530953E+00
-2.357201E-01
+1.473945E+00
+2.187663E-01
 tally 8:
-1.889115E+01
-3.574727E+01
+1.885798E+01
+3.558423E+01
 tally 9:
-7.556902E+00
-5.717680E+00
+7.467961E+00
+5.580255E+00
 tally 10:
-5.022871E-04
-2.531352E-08
+5.082094E-04
+2.584432E-08
 tally 11:
-7.557405E+00
-5.718440E+00
+7.468470E+00
+5.581014E+00
 tally 12:
-1.889115E+01
-3.574727E+01
+1.885798E+01
+3.558423E+01
 tally 13:
 0.000000E+00
 0.000000E+00
 tally 14:
-2.663407E-04
-7.161725E-09
+2.739543E-04
+7.600983E-09
 tally 15:
-2.663407E-04
-7.161725E-09
+2.739543E-04
+7.600983E-09
 tally 16:
-8.025710E+01
-6.459305E+02
+7.881296E+01
+6.221087E+02
 tally 17:
-1.067059E+02
-1.140939E+03
+1.051397E+02
+1.106292E+03
diff --git a/tests/regression_tests/filter_cellinstance/inputs_true.dat b/tests/regression_tests/filter_cellinstance/inputs_true.dat
index b17cc0eb5..2677d7ad2 100644
--- a/tests/regression_tests/filter_cellinstance/inputs_true.dat
+++ b/tests/regression_tests/filter_cellinstance/inputs_true.dat
@@ -1,22 +1,22 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="1.0" name="H1"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="2"/>
     <cell id="2" material="2" region="1" universe="1"/>
-    <cell fill="1" id="3" universe="2"/>
+    <cell id="3" fill="1" universe="2"/>
     <cell id="4" material="1" region="-2" universe="3"/>
     <cell id="5" material="2" region="2" universe="3"/>
-    <cell fill="4" id="6" region="3 -4 5 -6" universe="5"/>
+    <cell id="6" fill="4" region="3 -4 5 -6" universe="5"/>
     <lattice id="4">
       <pitch>2 2</pitch>
       <dimension>4 4</dimension>
@@ -27,19 +27,19 @@
 3 3 2 3 
 3 3 3 2 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.7" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.5" id="2" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-4.0" id="3" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="4.0" id="4" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-4.0" id="5" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="4.0" id="6" name="maximum y" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.7"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.5"/>
+    <surface id="3" name="minimum x" type="x-plane" boundary="reflective" coeffs="-4.0"/>
+    <surface id="4" name="maximum x" type="x-plane" boundary="reflective" coeffs="4.0"/>
+    <surface id="5" name="minimum y" type="y-plane" boundary="reflective" coeffs="-4.0"/>
+    <surface id="6" name="maximum y" type="y-plane" boundary="reflective" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
diff --git a/tests/regression_tests/filter_cellinstance/results_true.dat b/tests/regression_tests/filter_cellinstance/results_true.dat
index 079b7d3d2..152986595 100644
--- a/tests/regression_tests/filter_cellinstance/results_true.dat
+++ b/tests/regression_tests/filter_cellinstance/results_true.dat
@@ -1,84 +1,84 @@
 k-combined:
-1.050139E+00 1.886614E-02
+1.097679E+00 8.074294E-03
 tally 1:
-9.724996E-02
-2.784804E-03
-1.146976E-01
-3.173782E-03
-1.698297E-01
-6.569993E-03
-1.158802E-01
-2.959280E-03
-2.533354E-01
-1.539905E-02
-1.908717E-01
-8.360058E-03
-1.738899E-01
-6.713494E-03
-3.172247E-01
-2.124184E-02
-1.363012E-01
-4.653519E-03
-1.698720E-01
-6.753819E-03
-9.934671E-02
-2.371660E-03
-6.038075E-02
-1.204656E-03
-1.038793E+01
-2.590382E+01
-2.841397E+01
-1.865709E+02
-2.786916E+01
-1.923869E+02
-1.085149E+01
-2.907194E+01
-1.038793E+01
-2.590382E+01
-2.841397E+01
-1.865709E+02
-2.786916E+01
-1.923869E+02
-1.085149E+01
-2.907194E+01
+7.125168E-02
+1.412152E-03
+1.254059E-01
+4.145686E-03
+1.609454E-01
+6.174116E-03
+1.444278E-01
+4.523981E-03
+3.363588E-01
+2.342988E-02
+1.751677E-01
+7.108034E-03
+1.384074E-01
+3.949804E-03
+2.856450E-01
+1.824185E-02
+9.680810E-02
+2.256338E-03
+1.691663E-01
+6.456530E-03
+1.160968E-01
+3.233815E-03
+7.334131E-02
+1.456688E-03
+1.168548E+01
+3.210924E+01
+2.860162E+01
+1.876786E+02
+2.857267E+01
+1.996553E+02
+1.050245E+01
+2.569953E+01
+1.168548E+01
+3.210924E+01
+2.860162E+01
+1.876786E+02
+2.857267E+01
+1.996553E+02
+1.050245E+01
+2.569953E+01
 tally 2:
-1.085149E+01
-2.907194E+01
-2.786916E+01
-1.923869E+02
-2.841397E+01
-1.865709E+02
-1.038793E+01
-2.590382E+01
-1.085149E+01
-2.907194E+01
-2.786916E+01
-1.923869E+02
-2.841397E+01
-1.865709E+02
-1.038793E+01
-2.590382E+01
-6.038075E-02
-1.204656E-03
-9.934671E-02
-2.371660E-03
-1.698720E-01
-6.753819E-03
-1.363012E-01
-4.653519E-03
-3.172247E-01
-2.124184E-02
-1.738899E-01
-6.713494E-03
-1.908717E-01
-8.360058E-03
-2.533354E-01
-1.539905E-02
-1.158802E-01
-2.959280E-03
-1.698297E-01
-6.569993E-03
-1.146976E-01
-3.173782E-03
-9.724996E-02
-2.784804E-03
+1.050245E+01
+2.569953E+01
+2.857267E+01
+1.996553E+02
+2.860162E+01
+1.876786E+02
+1.168548E+01
+3.210924E+01
+1.050245E+01
+2.569953E+01
+2.857267E+01
+1.996553E+02
+2.860162E+01
+1.876786E+02
+1.168548E+01
+3.210924E+01
+7.334131E-02
+1.456688E-03
+1.160968E-01
+3.233815E-03
+1.691663E-01
+6.456530E-03
+9.680810E-02
+2.256338E-03
+2.856450E-01
+1.824185E-02
+1.384074E-01
+3.949804E-03
+1.751677E-01
+7.108034E-03
+3.363588E-01
+2.342988E-02
+1.444278E-01
+4.523981E-03
+1.609454E-01
+6.174116E-03
+1.254059E-01
+4.145686E-03
+7.125168E-02
+1.412152E-03
diff --git a/tests/regression_tests/filter_distribcell/case-3/results_true.dat b/tests/regression_tests/filter_distribcell/case-3/results_true.dat
index f30e858f0..77f2a2a87 100644
--- a/tests/regression_tests/filter_distribcell/case-3/results_true.dat
+++ b/tests/regression_tests/filter_distribcell/case-3/results_true.dat
@@ -1 +1 @@
-1d09084dc41305687d53d1e800fb3d0ac3949aa4e1cb0bfbb327d0ec1ad4b74fc97ee73da84b910529296b858aab9b8a5d0924d17ca2cc373625e1a9e197aa94
\ No newline at end of file
+93c1efbc586874a715982d26609e9e79232de25a0b73093a00938d658440644f0ee6bb823902a6ef1b7a2a855e67b2da0a0e767ef550f9ffa4dea723e35af6f5
\ No newline at end of file
diff --git a/tests/regression_tests/filter_distribcell/case-4/results_true.dat b/tests/regression_tests/filter_distribcell/case-4/results_true.dat
index 228456c58..ed84838ee 100644
--- a/tests/regression_tests/filter_distribcell/case-4/results_true.dat
+++ b/tests/regression_tests/filter_distribcell/case-4/results_true.dat
@@ -1,12 +1,12 @@
 k-combined:
-1.068596E-01          INF
+1.069692E-01          INF
 tally 1:
 1.812612E-02
 3.285561E-04
 2.870442E-02
 8.239436E-04
-1.807689E-02
-3.267740E-04
+1.824058E-02
+3.327188E-04
 2.660796E-02
 7.079835E-04
 1.961572E-02
diff --git a/tests/regression_tests/filter_energyfun/inputs_true.dat b/tests/regression_tests/filter_energyfun/inputs_true.dat
index 4b7b74d28..b7a70290f 100644
--- a/tests/regression_tests/filter_energyfun/inputs_true.dat
+++ b/tests/regression_tests/filter_energyfun/inputs_true.dat
@@ -1,14 +1,14 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="Am241"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="Am241" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 100.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 100.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/filter_energyfun/results_true.dat b/tests/regression_tests/filter_energyfun/results_true.dat
index 0aadac82e..d64de58a6 100644
--- a/tests/regression_tests/filter_energyfun/results_true.dat
+++ b/tests/regression_tests/filter_energyfun/results_true.dat
@@ -1,10 +1,10 @@
    energyfunction nuclide                    score     mean  std. dev.
-0  448ee8dfd19c4f   Am241  ((n,gamma) / (n,gamma)) 1.74e-01   6.83e-03
+0  448ee8dfd19c4f   Am241  ((n,gamma) / (n,gamma)) 1.74e-01   5.44e-03
    energyfunction nuclide      score     mean  std. dev.
-0  37e006ae6b2e74   Am241  (n,gamma) 8.16e-02   2.24e-03
+0  37e006ae6b2e74   Am241  (n,gamma) 8.35e-02   1.83e-03
    energyfunction nuclide      score     mean  std. dev.
-0  b4e2ac84068d2d   Am241  (n,gamma) 8.19e-02   2.25e-03
+0  b4e2ac84068d2d   Am241  (n,gamma) 8.39e-02   1.84e-03
    energyfunction nuclide      score     mean  std. dev.
-0  dacf88242512ea   Am241  (n,gamma) 7.95e-02   2.19e-03
+0  dacf88242512ea   Am241  (n,gamma) 8.14e-02   1.78e-03
    energyfunction nuclide      score     mean  std. dev.
-0  fe168c70d9e078   Am241  (n,gamma) 1.06e-01   2.96e-03
+0  fe168c70d9e078   Am241  (n,gamma) 1.09e-01   2.41e-03
diff --git a/tests/regression_tests/filter_mesh/inputs_true.dat b/tests/regression_tests/filter_mesh/inputs_true.dat
index 0667c0341..10f70a720 100644
--- a/tests/regression_tests/filter_mesh/inputs_true.dat
+++ b/tests/regression_tests/filter_mesh/inputs_true.dat
@@ -1,28 +1,28 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="Zr90"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="Zr90" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2 3 -4 10 -9" universe="1"/>
     <cell id="2" material="2" region="(-1 | 2 | -3 | 4) (5 -6 7 -8) 10 -9" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="5" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="6" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="7" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="8" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="10" type="z-plane"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum x" type="x-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="6" name="maximum x" type="x-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="7" name="minimum y" type="y-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="8" name="maximum y" type="y-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="10" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -54,8 +54,8 @@
     <mesh id="5" type="cylindrical">
       <r_grid>0.0 0.4411764705882353 0.8823529411764706 1.3235294117647058 1.7647058823529411 2.2058823529411766 2.6470588235294117 3.0882352941176467 3.5294117647058822 3.9705882352941178 4.411764705882353 4.852941176470588 5.294117647058823 5.735294117647059 6.1764705882352935 6.617647058823529 7.0588235294117645 7.5</r_grid>
       <phi_grid>0.0 0.3490658503988659 0.6981317007977318 1.0471975511965976 1.3962634015954636 1.7453292519943295 2.0943951023931953 2.443460952792061 2.792526803190927 3.141592653589793 3.490658503988659 3.839724354387525 4.1887902047863905 4.537856055185257 4.886921905584122 5.235987755982989 5.585053606381854 5.93411945678072 6.283185307179586</phi_grid>
-      <z_grid>-7.5 -6.5625 -5.625 -4.6875 -3.75 -2.8125 -1.875 -0.9375 0.0 0.9375 1.875 2.8125 3.75 4.6875 5.625 6.5625 7.5</z_grid>
-      <origin>0.0 0.0 0.0</origin>
+      <z_grid>0.0 0.9375 1.875 2.8125 3.75 4.6875 5.625 6.5625 7.5 8.4375 9.375 10.3125 11.25 12.1875 13.125 14.0625 15.0</z_grid>
+      <origin>0.0 0.0 -7.5</origin>
     </mesh>
     <mesh id="6" type="spherical">
       <r_grid>0.0 0.4411764705882353 0.8823529411764706 1.3235294117647058 1.7647058823529411 2.2058823529411766 2.6470588235294117 3.0882352941176467 3.5294117647058822 3.9705882352941178 4.411764705882353 4.852941176470588 5.294117647058823 5.735294117647059 6.1764705882352935 6.617647058823529 7.0588235294117645 7.5</r_grid>
diff --git a/tests/regression_tests/filter_mesh/results_true.dat b/tests/regression_tests/filter_mesh/results_true.dat
index a91eadb35..34e99c17e 100644
--- a/tests/regression_tests/filter_mesh/results_true.dat
+++ b/tests/regression_tests/filter_mesh/results_true.dat
@@ -1 +1 @@
-bd55ee25094f9ad04fda4439f58ef0fed718632c88b9fde411a484dc624dedcd45dee643e14d8b107d8b92757737b8bb3d9a667de5482457d6d8346afffdf657
\ No newline at end of file
+e07ed2bc8893c69721abf61b123336f1f6128a3bee6ec63b84d1f549f31707a74a6ce885091ccc0eac6b7f16f7cab39ede4784584c08825829e108de878ea5fb
\ No newline at end of file
diff --git a/tests/regression_tests/filter_mesh/test.py b/tests/regression_tests/filter_mesh/test.py
index 6214e0486..165ba2a0c 100644
--- a/tests/regression_tests/filter_mesh/test.py
+++ b/tests/regression_tests/filter_mesh/test.py
@@ -61,9 +61,10 @@ def model():
     np.testing.assert_allclose(recti_mesh.volumes, recti_mesh_exp_vols)
 
     cyl_mesh = openmc.CylindricalMesh(
+        origin=(0, 0, -7.5),
         r_grid=np.linspace(0, 7.5, 18),
         phi_grid=np.linspace(0, 2*pi, 19),
-        z_grid=np.linspace(-7.5, 7.5, 17),
+        z_grid=np.linspace(0, 15, 17),
     )
     dr = 0.5 * np.diff(np.linspace(0, 7.5, 18)**2)
     dp = np.full(cyl_mesh.dimension[1], 2*pi / 18)
diff --git a/tests/regression_tests/filter_meshborn/inputs_true.dat b/tests/regression_tests/filter_meshborn/inputs_true.dat
index 3ba38d56e..a94646ec8 100644
--- a/tests/regression_tests/filter_meshborn/inputs_true.dat
+++ b/tests/regression_tests/filter_meshborn/inputs_true.dat
@@ -2,19 +2,19 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="H1"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="reflective" coeffs="0.0 0.0 0.0 10.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>2000</particles>
     <batches>8</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -10.0 -10.0 10.0 10.0 10.0</parameters>
       </space>
diff --git a/tests/regression_tests/filter_musurface/inputs_true.dat b/tests/regression_tests/filter_musurface/inputs_true.dat
index 031f62159..b457f5028 100644
--- a/tests/regression_tests/filter_musurface/inputs_true.dat
+++ b/tests/regression_tests/filter_musurface/inputs_true.dat
@@ -1,20 +1,20 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="Zr90"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="Zr90" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="2" region="1 -2" universe="1"/>
-    <surface coeffs="0.0 0.0 1.0" id="1" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 3.0" id="2" type="z-cylinder"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 1.0"/>
+    <surface id="2" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -31,7 +31,7 @@
     </filter>
     <tally id="1">
       <filters>1 2</filters>
-      <scores>current</scores>
+      <scores>current flux</scores>
     </tally>
   </tallies>
 </model>
diff --git a/tests/regression_tests/filter_musurface/results_true.dat b/tests/regression_tests/filter_musurface/results_true.dat
index 39c9f2b92..657c141a0 100644
--- a/tests/regression_tests/filter_musurface/results_true.dat
+++ b/tests/regression_tests/filter_musurface/results_true.dat
@@ -1,11 +1,19 @@
 k-combined:
-1.157005E-01 7.587090E-03
+1.202075E-01 1.113188E-02
 tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-8.770000E-01
-1.608710E-01
-3.909000E+00
-3.063035E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+9.230000E-01
+1.791510E-01
+3.294304E+00
+2.314403E+00
+3.869000E+00
+3.002523E+00
+5.154125E+00
+5.314334E+00
diff --git a/tests/regression_tests/filter_musurface/test.py b/tests/regression_tests/filter_musurface/test.py
index f2ec96b49..bfc12a47c 100644
--- a/tests/regression_tests/filter_musurface/test.py
+++ b/tests/regression_tests/filter_musurface/test.py
@@ -1,6 +1,3 @@
-import numpy as np
-from math import pi
-
 import openmc
 import pytest
 
@@ -32,7 +29,7 @@ def model():
     mu_filter = openmc.MuSurfaceFilter([-1.0, -0.5, 0.0, 0.5, 1.0])
     tally = openmc.Tally()
     tally.filters = [surf_filter, mu_filter]
-    tally.scores = ['current']
+    tally.scores = ['current', 'flux']
     model.tallies.append(tally)
 
     return model
diff --git a/tests/regression_tests/filter_reaction/__init__.py b/tests/regression_tests/filter_reaction/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/filter_reaction/inputs_true.dat b/tests/regression_tests/filter_reaction/inputs_true.dat
new file mode 100644
index 000000000..3ab9ab3fb
--- /dev/null
+++ b/tests/regression_tests/filter_reaction/inputs_true.dat
@@ -0,0 +1,27 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 100.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>5</batches>
+  </settings>
+  <tallies>
+    <filter id="1" type="reaction">
+      <bins>(n,elastic) (n,2n) (n,fission) (n,gamma) (n,total)</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/filter_reaction/results_true.dat b/tests/regression_tests/filter_reaction/results_true.dat
new file mode 100644
index 000000000..61ec1c65f
--- /dev/null
+++ b/tests/regression_tests/filter_reaction/results_true.dat
@@ -0,0 +1,13 @@
+k-combined:
+2.265297E+00 7.172807E-03
+tally 1:
+8.364316E+01
+1.400120E+03
+1.412312E-01
+4.277446E-03
+5.471503E+00
+7.015849E+00
+0.000000E+00
+0.000000E+00
+1.349840E+02
+3.644648E+03
diff --git a/tests/regression_tests/filter_reaction/test.py b/tests/regression_tests/filter_reaction/test.py
new file mode 100644
index 000000000..3c60fd517
--- /dev/null
+++ b/tests/regression_tests/filter_reaction/test.py
@@ -0,0 +1,31 @@
+import openmc
+
+from tests.testing_harness import PyAPITestHarness
+
+
+def test_filter_reaction():
+    model = openmc.Model()
+
+    m = openmc.Material()
+    m.set_density('g/cm3', 10.0)
+    m.add_nuclide('U235', 1.0)
+    model.materials.append(m)
+
+    s = openmc.Sphere(r=100.0, boundary_type='vacuum')
+    c = openmc.Cell(fill=m, region=-s)
+    model.geometry = openmc.Geometry([c])
+
+    # Create a tally with reaction filter
+    tally = openmc.Tally()
+    tally.filters = [openmc.ReactionFilter(
+        ['(n,elastic)', '(n,2n)', '(n,fission)', '(n,gamma)', 'total']
+    )]
+    tally.scores = ['flux']
+    model.tallies = openmc.Tallies([tally])
+
+    # Reduce particles for faster testing
+    model.settings.particles = 1000
+    model.settings.batches = 5
+
+    harness = PyAPITestHarness('statepoint.5.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/filter_rotations/__init__.py b/tests/regression_tests/filter_rotations/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/filter_rotations/inputs_true.dat b/tests/regression_tests/filter_rotations/inputs_true.dat
new file mode 100644
index 000000000..1ad2b9e86
--- /dev/null
+++ b/tests/regression_tests/filter_rotations/inputs_true.dat
@@ -0,0 +1,59 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+    <material id="2">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="Zr90" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="1 -2 3 -4 10 -9" universe="1"/>
+    <cell id="2" material="2" region="(-1 | 2 | -3 | 4) (5 -6 7 -8) 10 -9" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum x" type="x-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="6" name="maximum x" type="x-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="7" name="minimum y" type="y-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="8" name="maximum y" type="y-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="10" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>5</batches>
+    <inactive>0</inactive>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>3 4 5</dimension>
+      <lower_left>-9 -9 -9</lower_left>
+      <upper_right>9 9 9</upper_right>
+    </mesh>
+    <mesh id="2">
+      <dimension>3 4 5</dimension>
+      <lower_left>-9 -9 -9</lower_left>
+      <upper_right>9 9 9</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="mesh" rotation="0 0 10">
+      <bins>2</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>total</scores>
+    </tally>
+    <tally id="2">
+      <filters>2</filters>
+      <scores>total</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/filter_rotations/results_true.dat b/tests/regression_tests/filter_rotations/results_true.dat
new file mode 100644
index 000000000..5c8b83b76
--- /dev/null
+++ b/tests/regression_tests/filter_rotations/results_true.dat
@@ -0,0 +1,244 @@
+k-combined:
+7.729082E-01 3.775399E-02
+tally 1:
+5.296804E-02
+5.661701E-04
+8.356446E-02
+1.412139E-03
+5.041335E-02
+5.143568E-04
+1.299348E-01
+3.467618E-03
+3.929702E-01
+3.147038E-02
+1.379707E-01
+3.888484E-03
+1.405034E-01
+4.473799E-03
+3.785796E-01
+2.940585E-02
+1.422010E-01
+4.113723E-03
+5.647073E-02
+6.735251E-04
+7.911154E-02
+1.329137E-03
+5.160755E-02
+5.361448E-04
+6.669424E-02
+9.090832E-04
+1.008621E-01
+2.134534E-03
+6.808932E-02
+9.355993E-04
+1.873006E-01
+7.135961E-03
+6.221575E-01
+7.819842E-02
+1.856653E-01
+6.954762E-03
+2.014929E-01
+8.327845E-03
+5.853251E-01
+6.945708E-02
+1.709645E-01
+5.917124E-03
+7.214913E-02
+1.058962E-03
+1.027720E-01
+2.138475E-03
+6.099853E-02
+7.493941E-04
+6.892071E-02
+9.630680E-04
+1.035459E-01
+2.173883E-03
+6.973870E-02
+9.904237E-04
+2.125703E-01
+9.112659E-03
+9.012205E-01
+2.163546E-01
+2.066426E-01
+8.617414E-03
+2.258950E-01
+1.039607E-02
+9.476792E-01
+2.350708E-01
+2.225585E-01
+1.017898E-02
+7.111503E-02
+1.036847E-03
+1.117012E-01
+2.530040E-03
+6.870474E-02
+9.551035E-04
+5.738897E-02
+6.699030E-04
+9.522335E-02
+1.835769E-03
+6.570917E-02
+8.656870E-04
+1.945592E-01
+7.593336E-03
+5.514753E-01
+6.122981E-02
+2.144202E-01
+9.421739E-03
+1.971631E-01
+7.944046E-03
+6.088996E-01
+7.442954E-02
+1.965447E-01
+7.765628E-03
+7.005494E-02
+1.012891E-03
+1.010633E-01
+2.084095E-03
+6.145926E-02
+7.694351E-04
+4.999479E-02
+5.129164E-04
+7.238243E-02
+1.062921E-03
+4.902309E-02
+4.852193E-04
+1.324655E-01
+3.642431E-03
+3.305312E-01
+2.265726E-02
+1.332993E-01
+3.728385E-03
+1.547469E-01
+4.894837E-03
+3.625944E-01
+2.747313E-02
+1.435761E-01
+4.334405E-03
+5.789603E-02
+7.065383E-04
+7.589559E-02
+1.205386E-03
+5.210018E-02
+5.790843E-04
+tally 2:
+4.597932E-02
+4.246849E-04
+8.494738E-02
+1.467238E-03
+5.155072E-02
+5.373129E-04
+1.479395E-01
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+3.931843E-01
+3.141943E-02
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+5.382674E-02
+5.890600E-04
+1.116560E-01
+2.599817E-03
+6.648634E-02
+8.862677E-04
+1.996861E-01
+8.137198E-03
+6.218616E-01
+7.805532E-02
+1.672379E-01
+5.667797E-03
+1.841275E-01
+6.936896E-03
+5.887874E-01
+7.031665E-02
+1.872574E-01
+7.086584E-03
+7.574698E-02
+1.160992E-03
+1.100173E-01
+2.453972E-03
+5.427866E-02
+5.955556E-04
+5.644318E-02
+6.503634E-04
+1.058628E-01
+2.254338E-03
+8.012794E-02
+1.297032E-03
+2.292757E-01
+1.064490E-02
+9.001874E-01
+2.153097E-01
+1.921874E-01
+7.537343E-03
+2.058642E-01
+8.537136E-03
+9.442653E-01
+2.359241E-01
+2.419900E-01
+1.202201E-02
+7.563876E-02
+1.169874E-03
+1.165428E-01
+2.750405E-03
+5.646354E-02
+6.408683E-04
+4.963879E-02
+4.975915E-04
+1.005478E-01
+2.046400E-03
+7.041191E-02
+1.002590E-03
+1.959123E-01
+7.727902E-03
+5.625596E-01
+6.366248E-02
+1.987313E-01
+8.108832E-03
+1.922194E-01
+7.602752E-03
+6.062527E-01
+7.384124E-02
+2.039506E-01
+8.379010E-03
+7.019905E-02
+1.034239E-03
+1.081383E-01
+2.344040E-03
+5.125142E-02
+5.430397E-04
+4.230495E-02
+3.666730E-04
+7.722275E-02
+1.208856E-03
+4.998937E-02
+5.049355E-04
+1.461632E-01
+4.456332E-03
+3.315459E-01
+2.288314E-02
+1.238273E-01
+3.205844E-03
+1.355317E-01
+3.754923E-03
+3.646520E-01
+2.766067E-02
+1.543013E-01
+5.044487E-03
+6.082173E-02
+7.636985E-04
+8.490569E-02
+1.506309E-03
+4.046126E-02
+3.495832E-04
diff --git a/tests/regression_tests/filter_rotations/test.py b/tests/regression_tests/filter_rotations/test.py
new file mode 100644
index 000000000..f5d63d6b4
--- /dev/null
+++ b/tests/regression_tests/filter_rotations/test.py
@@ -0,0 +1,72 @@
+import numpy as np
+
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def model():
+
+    model = openmc.model.Model()
+
+    fuel = openmc.Material()
+    fuel.set_density('g/cm3', 10.0)
+    fuel.add_nuclide('U235', 1.0)
+    zr = openmc.Material()
+    zr.set_density('g/cm3', 1.0)
+    zr.add_nuclide('Zr90', 1.0)
+    model.materials.extend([fuel, zr])
+
+    box1 = openmc.model.RectangularPrism(10.0, 10.0)
+    box2 = openmc.model.RectangularPrism(20.0, 20.0, boundary_type='reflective')
+    top = openmc.ZPlane(z0=10.0, boundary_type='vacuum')
+    bottom = openmc.ZPlane(z0=-10.0, boundary_type='vacuum')
+    cell1 = openmc.Cell(fill=fuel, region=-box1 & +bottom & -top)
+    cell2 = openmc.Cell(fill=zr, region=+box1 & -box2 & +bottom & -top)
+    model.geometry = openmc.Geometry([cell1, cell2])
+
+    model.settings.batches = 5
+    model.settings.inactive = 0
+    model.settings.particles = 1000
+
+    rotation = np.array((0, 0, 10))
+
+    llc = np.array([-9, -9, -9])
+    urc = np.array([9, 9, 9])
+
+    mesh_dims = (3, 4, 5)
+
+    filters = []
+
+    # un-rotated meshes
+    reg_mesh = openmc.RegularMesh()
+    reg_mesh.dimension = mesh_dims
+    reg_mesh.lower_left = llc
+    reg_mesh.upper_right = urc
+
+    filters.append(openmc.MeshFilter(reg_mesh))
+
+    # rotated meshes
+    rotated_reg_mesh = openmc.RegularMesh()
+    rotated_reg_mesh.dimension = mesh_dims
+    rotated_reg_mesh.lower_left = llc
+    rotated_reg_mesh.upper_right = urc
+
+    filters.append(openmc.MeshFilter(rotated_reg_mesh))
+    filters[-1].rotation = rotation
+
+    # Create tallies
+    for f in filters:
+        tally = openmc.Tally()
+        tally.filters = [f]
+        tally.scores = ['total']
+        model.tallies.append(tally)
+
+    return model
+
+
+def test_filter_mesh_rotations(model):
+    harness = PyAPITestHarness('statepoint.5.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/filter_translations/inputs_true.dat b/tests/regression_tests/filter_translations/inputs_true.dat
index 5004c3217..41ae9b6dc 100644
--- a/tests/regression_tests/filter_translations/inputs_true.dat
+++ b/tests/regression_tests/filter_translations/inputs_true.dat
@@ -1,28 +1,28 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="Zr90"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="Zr90" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2 3 -4 10 -9" universe="1"/>
     <cell id="2" material="2" region="(-1 | 2 | -3 | 4) (5 -6 7 -8) 10 -9" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="5" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="6" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="7" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="8" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="10" type="z-plane"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum x" type="x-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="6" name="maximum x" type="x-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="7" name="minimum y" type="y-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="8" name="maximum y" type="y-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="10" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -57,10 +57,10 @@
     <filter id="2" type="mesh">
       <bins>2</bins>
     </filter>
-    <filter id="3" translation="10 -5 0" type="mesh">
+    <filter id="3" type="mesh" translation="10 -5 0">
       <bins>3</bins>
     </filter>
-    <filter id="4" translation="10 -5 0" type="mesh">
+    <filter id="4" type="mesh" translation="10 -5 0">
       <bins>4</bins>
     </filter>
     <tally id="1">
diff --git a/tests/regression_tests/filter_translations/results_true.dat b/tests/regression_tests/filter_translations/results_true.dat
index d257d71a6..a63586d2d 100644
--- a/tests/regression_tests/filter_translations/results_true.dat
+++ b/tests/regression_tests/filter_translations/results_true.dat
@@ -1,342 +1,342 @@
 k-combined:
-2.298294E-01 3.256961E-01
+7.729082E-01 3.775399E-02
 tally 1:
-4.880089E-02
-4.894233E-04
-8.221192E-02
-1.373712E-03
-5.205261E-02
-5.584673E-04
-1.272758E-01
-3.371623E-03
-3.599375E-01
-2.655645E-02
-1.377572E-01
-3.835778E-03
-1.646205E-01
-5.916595E-03
-3.806765E-01
-2.947576E-02
-1.470573E-01
-4.360385E-03
-5.568294E-02
-6.368003E-04
-6.775790E-02
-9.447074E-04
-5.580909E-02
-6.303584E-04
-6.674847E-02
-9.080853E-04
-9.747628E-02
-1.934325E-03
-6.824062E-02
-9.908844E-04
-1.891692E-01
-7.211874E-03
-5.976566E-01
-7.206211E-02
-1.805640E-01
-6.599680E-03
-2.114692E-01
-9.202185E-03
-6.514433E-01
-8.523087E-02
-1.905740E-01
-7.287607E-03
-7.633486E-02
-1.216488E-03
-1.117747E-01
-2.538705E-03
-7.438042E-02
-1.127109E-03
-7.356007E-02
-1.090617E-03
-1.161258E-01
-2.734941E-03
-6.745417E-02
-9.371031E-04
-2.153323E-01
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-9.414362E-01
-2.261845E-01
-2.016933E-01
-8.270477E-03
-2.370758E-01
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+2.084095E-03
+6.145926E-02
+7.694351E-04
+4.999479E-02
+5.129164E-04
+7.238243E-02
+1.062921E-03
+4.902309E-02
+4.852193E-04
+1.324655E-01
+3.642431E-03
+3.305312E-01
+2.265726E-02
+1.332993E-01
+3.728385E-03
+1.547469E-01
+4.894837E-03
+3.625944E-01
+2.747313E-02
+1.435761E-01
+4.334405E-03
+5.789603E-02
+7.065383E-04
+7.589559E-02
+1.205386E-03
+5.210018E-02
+5.790843E-04
 tally 2:
-2.379877E-01
-1.141258E-02
-2.376600E-01
-1.137350E-02
-6.546514E-01
-8.852321E-02
-5.823862E-01
-6.810779E-02
-2.692631E-01
-1.479426E-02
-2.360469E-01
-1.124950E-02
-3.534788E-01
-2.528363E-02
-3.247931E-01
-2.160130E-02
-1.181513E+00
-2.959140E-01
-1.078874E+00
-2.467321E-01
-3.743375E-01
-2.859905E-02
-3.468123E-01
-2.427809E-02
-3.203903E-01
-2.109786E-02
-3.315547E-01
-2.216053E-02
-1.039077E+00
-2.294722E-01
-1.091786E+00
-2.572834E-01
-3.615960E-01
-2.678347E-02
-3.323317E-01
-2.226424E-02
-2.519726E-01
-1.279098E-02
-2.332385E-01
-1.101251E-02
-5.587247E-01
-6.293355E-02
-5.426859E-01
-6.010041E-02
-2.332981E-01
-1.097940E-02
-2.239951E-01
-1.043440E-02
+2.572693E-01
+1.338921E-02
+2.567576E-01
+1.333558E-02
+6.127899E-01
+7.781914E-02
+6.163075E-01
+7.600273E-02
+2.566545E-01
+1.372553E-02
+2.509744E-01
+1.286275E-02
+3.366720E-01
+2.282560E-02
+3.216175E-01
+2.103222E-02
+1.118187E+00
+2.697988E-01
+1.050003E+00
+2.391098E-01
+3.407360E-01
+2.332214E-02
+3.150094E-01
+2.006040E-02
+3.130223E-01
+2.001074E-02
+3.238651E-01
+2.101351E-02
+1.061186E+00
+2.372828E-01
+1.099461E+00
+2.597134E-01
+3.445524E-01
+2.400393E-02
+3.429055E-01
+2.372392E-02
+2.299776E-01
+1.064851E-02
+2.208901E-01
+9.829109E-03
+6.015657E-01
+7.399276E-02
+5.851229E-01
+7.165716E-02
+2.583609E-01
+1.364046E-02
+2.456380E-01
+1.257735E-02
 tally 3:
-4.880089E-02
-4.894233E-04
-8.221192E-02
-1.373712E-03
-5.205261E-02
-5.584673E-04
-1.272758E-01
-3.371623E-03
-3.599375E-01
-2.655645E-02
-1.377572E-01
-3.835778E-03
-1.646205E-01
-5.916595E-03
-3.806765E-01
-2.947576E-02
-1.470573E-01
-4.360385E-03
-5.568294E-02
-6.368003E-04
-6.775790E-02
-9.447074E-04
-5.580909E-02
-6.303584E-04
-6.674847E-02
-9.080853E-04
-9.747628E-02
-1.934325E-03
-6.824062E-02
-9.908844E-04
-1.891692E-01
-7.211874E-03
-5.976566E-01
-7.206211E-02
-1.805640E-01
-6.599680E-03
-2.114692E-01
-9.202185E-03
-6.514433E-01
-8.523087E-02
-1.905740E-01
-7.287607E-03
-7.633486E-02
-1.216488E-03
-1.117747E-01
-2.538705E-03
-7.438042E-02
-1.127109E-03
-7.356007E-02
-1.090617E-03
-1.161258E-01
-2.734941E-03
-6.745417E-02
-9.371031E-04
-2.153323E-01
-9.293477E-03
-9.414362E-01
-2.261845E-01
-2.016933E-01
-8.270477E-03
-2.370758E-01
-1.150628E-02
-9.973982E-01
-2.489516E-01
-2.152505E-01
-9.424546E-03
-7.502684E-02
-1.208304E-03
-1.244515E-01
-3.168682E-03
-7.515812E-02
-1.151397E-03
-6.355178E-02
-8.423316E-04
-1.040915E-01
-2.224064E-03
-6.902234E-02
-9.843766E-04
-1.978591E-01
-7.894161E-03
-5.162186E-01
-5.357742E-02
-1.897326E-01
-7.231935E-03
-1.827950E-01
-6.839560E-03
-5.780661E-01
-6.799910E-02
-1.881215E-01
-7.093746E-03
-6.083700E-02
-7.861240E-04
-9.978991E-02
-2.022733E-03
-6.004653E-02
-7.428556E-04
-4.951220E-02
-4.952385E-04
-7.401227E-02
-1.124154E-03
-5.151090E-02
-5.339359E-04
-1.356118E-01
-3.711749E-03
-3.400140E-01
-2.367518E-02
-1.385305E-01
-4.060592E-03
-1.438711E-01
-4.191824E-03
-3.276170E-01
-2.210924E-02
-1.279501E-01
-3.435254E-03
-4.852204E-02
-4.919417E-04
-7.229090E-02
-1.062894E-03
-4.344414E-02
-3.825354E-04
+5.296804E-02
+5.661701E-04
+8.356446E-02
+1.412139E-03
+5.041335E-02
+5.143568E-04
+1.299348E-01
+3.467618E-03
+3.929702E-01
+3.147038E-02
+1.379707E-01
+3.888484E-03
+1.405034E-01
+4.473799E-03
+3.785796E-01
+2.940585E-02
+1.422010E-01
+4.113723E-03
+5.647073E-02
+6.735251E-04
+7.911154E-02
+1.329137E-03
+5.160755E-02
+5.361448E-04
+6.669424E-02
+9.090832E-04
+1.008621E-01
+2.134534E-03
+6.808932E-02
+9.355993E-04
+1.873006E-01
+7.135961E-03
+6.221575E-01
+7.819842E-02
+1.856653E-01
+6.954762E-03
+2.014929E-01
+8.327845E-03
+5.853251E-01
+6.945708E-02
+1.709645E-01
+5.917124E-03
+7.214913E-02
+1.058962E-03
+1.027720E-01
+2.138475E-03
+6.099853E-02
+7.493941E-04
+6.892071E-02
+9.630680E-04
+1.035459E-01
+2.173883E-03
+6.973870E-02
+9.904237E-04
+2.125703E-01
+9.112659E-03
+9.012205E-01
+2.163546E-01
+2.066426E-01
+8.617414E-03
+2.258950E-01
+1.039607E-02
+9.476792E-01
+2.350708E-01
+2.225585E-01
+1.017898E-02
+7.111503E-02
+1.036847E-03
+1.117012E-01
+2.530040E-03
+6.870474E-02
+9.551035E-04
+5.738897E-02
+6.699030E-04
+9.522335E-02
+1.835769E-03
+6.570917E-02
+8.656870E-04
+1.945592E-01
+7.593336E-03
+5.514753E-01
+6.122981E-02
+2.144202E-01
+9.421739E-03
+1.971631E-01
+7.944046E-03
+6.088996E-01
+7.442954E-02
+1.965447E-01
+7.765628E-03
+7.005494E-02
+1.012891E-03
+1.010633E-01
+2.084095E-03
+6.145926E-02
+7.694351E-04
+4.999479E-02
+5.129164E-04
+7.238243E-02
+1.062921E-03
+4.902309E-02
+4.852193E-04
+1.324655E-01
+3.642431E-03
+3.305312E-01
+2.265726E-02
+1.332993E-01
+3.728385E-03
+1.547469E-01
+4.894837E-03
+3.625944E-01
+2.747313E-02
+1.435761E-01
+4.334405E-03
+5.789603E-02
+7.065383E-04
+7.589559E-02
+1.205386E-03
+5.210018E-02
+5.790843E-04
 tally 4:
-2.379877E-01
-1.141258E-02
-2.376600E-01
-1.137350E-02
-6.546514E-01
-8.852321E-02
-5.823862E-01
-6.810779E-02
-2.692631E-01
-1.479426E-02
-2.360469E-01
-1.124950E-02
-3.534788E-01
-2.528363E-02
-3.247931E-01
-2.160130E-02
-1.181513E+00
-2.959140E-01
-1.078874E+00
-2.467321E-01
-3.743375E-01
-2.859905E-02
-3.468123E-01
-2.427809E-02
-3.203903E-01
-2.109786E-02
-3.315547E-01
-2.216053E-02
-1.039077E+00
-2.294722E-01
-1.091786E+00
-2.572834E-01
-3.615960E-01
-2.678347E-02
-3.323317E-01
-2.226424E-02
-2.519726E-01
-1.279098E-02
-2.332385E-01
-1.101251E-02
-5.587247E-01
-6.293355E-02
-5.426859E-01
-6.010041E-02
-2.332981E-01
-1.097940E-02
-2.239951E-01
-1.043440E-02
+2.572693E-01
+1.338921E-02
+2.567576E-01
+1.333558E-02
+6.127899E-01
+7.781914E-02
+6.163075E-01
+7.600273E-02
+2.566545E-01
+1.372553E-02
+2.509744E-01
+1.286275E-02
+3.366720E-01
+2.282560E-02
+3.216175E-01
+2.103222E-02
+1.118187E+00
+2.697988E-01
+1.050003E+00
+2.391098E-01
+3.407360E-01
+2.332214E-02
+3.150094E-01
+2.006040E-02
+3.130223E-01
+2.001074E-02
+3.238651E-01
+2.101351E-02
+1.061186E+00
+2.372828E-01
+1.099461E+00
+2.597134E-01
+3.445524E-01
+2.400393E-02
+3.429055E-01
+2.372392E-02
+2.299776E-01
+1.064851E-02
+2.208901E-01
+9.829109E-03
+6.015657E-01
+7.399276E-02
+5.851229E-01
+7.165716E-02
+2.583609E-01
+1.364046E-02
+2.456380E-01
+1.257735E-02
diff --git a/tests/regression_tests/fixed_source/inputs_true.dat b/tests/regression_tests/fixed_source/inputs_true.dat
index 0fdb1466e..a36741775 100644
--- a/tests/regression_tests/fixed_source/inputs_true.dat
+++ b/tests/regression_tests/fixed_source/inputs_true.dat
@@ -1,21 +1,21 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="7.5"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.0001" name="U238"/>
+    <material id="1" depletable="true">
+      <density value="7.5" units="g/cc"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="U238" ao="0.0001"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 10.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="10.0" type="independent">
+    <source type="independent" strength="10.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
diff --git a/tests/regression_tests/ifp/__init__.py b/tests/regression_tests/ifp/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/ifp/groupwise/__init__.py b/tests/regression_tests/ifp/groupwise/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/ifp/groupwise/inputs_true.dat b/tests/regression_tests/ifp/groupwise/inputs_true.dat
new file mode 100644
index 000000000..6d7e20717
--- /dev/null
+++ b/tests/regression_tests/ifp/groupwise/inputs_true.dat
@@ -0,0 +1,43 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="core" depletable="true">
+      <density value="16.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>20</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-10.0 -10.0 -10.0 10.0 10.0 10.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <ifp_n_generation>5</ifp_n_generation>
+  </settings>
+  <tallies>
+    <filter id="1" type="delayedgroup">
+      <bins>1 2 3 4 5 6</bins>
+    </filter>
+    <tally id="1" name="IFP time numerator">
+      <scores>ifp-time-numerator</scores>
+    </tally>
+    <tally id="2" name="IFP beta numerator">
+      <filters>1</filters>
+      <scores>ifp-beta-numerator</scores>
+    </tally>
+    <tally id="3" name="IFP denominator">
+      <scores>ifp-denominator</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/ifp/groupwise/results_true.dat b/tests/regression_tests/ifp/groupwise/results_true.dat
new file mode 100644
index 000000000..ea66a8de3
--- /dev/null
+++ b/tests/regression_tests/ifp/groupwise/results_true.dat
@@ -0,0 +1,21 @@
+k-combined:
+1.006559E+00 5.389391E-03
+tally 1:
+9.109384E-08
+5.667165E-16
+tally 2:
+3.000000E-03
+9.000000E-06
+0.000000E+00
+0.000000E+00
+2.100000E-02
+1.370000E-04
+2.800000E-02
+2.220000E-04
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+tally 3:
+1.489000E+01
+1.480036E+01
diff --git a/tests/regression_tests/ifp/groupwise/test.py b/tests/regression_tests/ifp/groupwise/test.py
new file mode 100644
index 000000000..a1a0ebefb
--- /dev/null
+++ b/tests/regression_tests/ifp/groupwise/test.py
@@ -0,0 +1,40 @@
+"""Test the Iterated Fission Probability (IFP) method to compute adjoint-weighted
+kinetics parameters using dedicated tallies."""
+
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+@pytest.fixture()
+def ifp_model():
+    # Material
+    material = openmc.Material(name="core")
+    material.add_nuclide("U235", 1.0)
+    material.set_density('g/cm3', 16.0)
+
+    # Geometry
+    radius = 10.0
+    sphere = openmc.Sphere(r=radius, boundary_type="vacuum")
+    cell = openmc.Cell(region=-sphere, fill=material)
+    geometry = openmc.Geometry([cell])
+
+    # Settings
+    settings = openmc.Settings()
+    settings.particles = 1000
+    settings.batches = 20
+    settings.inactive = 5
+    settings.ifp_n_generation = 5
+
+    model = openmc.Model(settings=settings, geometry=geometry)
+
+    space = openmc.stats.Box(*cell.bounding_box)
+    model.settings.source = openmc.IndependentSource(
+        space=space, constraints={'fissionable': True})
+    model.add_kinetics_parameters_tallies(num_groups=6)
+    return model
+
+
+def test_iterated_fission_probability(ifp_model):
+    harness = PyAPITestHarness("statepoint.20.h5", model=ifp_model)
+    harness.main()
diff --git a/tests/regression_tests/ifp/total/__init__.py b/tests/regression_tests/ifp/total/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/ifp/total/inputs_true.dat b/tests/regression_tests/ifp/total/inputs_true.dat
new file mode 100644
index 000000000..2d69b29ab
--- /dev/null
+++ b/tests/regression_tests/ifp/total/inputs_true.dat
@@ -0,0 +1,33 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="core" depletable="true">
+      <density value="16.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>20</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-10.0 -10.0 -10.0 10.0 10.0 10.0</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <ifp_n_generation>5</ifp_n_generation>
+  </settings>
+  <tallies>
+    <tally id="1" name="ifp-scores">
+      <scores>ifp-time-numerator ifp-beta-numerator ifp-denominator</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/ifp/total/results_true.dat b/tests/regression_tests/ifp/total/results_true.dat
new file mode 100644
index 000000000..466ca1f01
--- /dev/null
+++ b/tests/regression_tests/ifp/total/results_true.dat
@@ -0,0 +1,9 @@
+k-combined:
+1.006559E+00 5.389391E-03
+tally 1:
+9.109384E-08
+5.667165E-16
+5.200000E-02
+5.420000E-04
+1.489000E+01
+1.480036E+01
diff --git a/tests/regression_tests/ifp/total/test.py b/tests/regression_tests/ifp/total/test.py
new file mode 100644
index 000000000..18b89cfc0
--- /dev/null
+++ b/tests/regression_tests/ifp/total/test.py
@@ -0,0 +1,44 @@
+"""Test the Iterated Fission Probability (IFP) method to compute adjoint-weighted
+kinetics parameters using dedicated tallies."""
+
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+@pytest.fixture()
+def ifp_model():
+    model = openmc.Model()
+
+    # Material
+    material = openmc.Material(name="core")
+    material.add_nuclide("U235", 1.0)
+    material.set_density('g/cm3', 16.0)
+
+    # Geometry
+    radius = 10.0
+    sphere = openmc.Sphere(r=radius, boundary_type="vacuum")
+    cell = openmc.Cell(region=-sphere, fill=material)
+    model.geometry = openmc.Geometry([cell])
+
+    # Settings
+    model.settings.particles = 1000
+    model.settings.batches = 20
+    model.settings.inactive = 5
+    model.settings.ifp_n_generation = 5
+
+    space = openmc.stats.Box(*cell.bounding_box)
+    model.settings.source = openmc.IndependentSource(
+        space=space, constraints={'fissionable': True})
+
+    # Tally IFP scores
+    tally = openmc.Tally(name="ifp-scores")
+    tally.scores = ["ifp-time-numerator", "ifp-beta-numerator", "ifp-denominator"]
+    model.tallies = [tally]
+
+    return model
+
+
+def test_iterated_fission_probability(ifp_model):
+    harness = PyAPITestHarness("statepoint.20.h5", model=ifp_model)
+    harness.main()
diff --git a/tests/regression_tests/infinite_cell/results_true.dat b/tests/regression_tests/infinite_cell/results_true.dat
index 2c4ce082d..4cdbcaf88 100644
--- a/tests/regression_tests/infinite_cell/results_true.dat
+++ b/tests/regression_tests/infinite_cell/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.589951E-02 8.489144E-04
+9.603664E-02 1.050772E-03
diff --git a/tests/regression_tests/iso_in_lab/inputs_true.dat b/tests/regression_tests/iso_in_lab/inputs_true.dat
index bb6494ada..adfcf7e51 100644
--- a/tests/regression_tests/iso_in_lab/inputs_true.dat
+++ b/tests/regression_tests/iso_in_lab/inputs_true.dat
@@ -1,44 +1,44 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UOX fuel">
-      <density units="g/cm3" value="10.062"/>
-      <nuclide ao="4.9476e-06" name="U234"/>
-      <nuclide ao="0.00048218" name="U235"/>
-      <nuclide ao="0.021504" name="U238"/>
-      <nuclide ao="1.0801e-08" name="Xe135"/>
-      <nuclide ao="0.045737" name="O16"/>
+    <material id="1" name="UOX fuel" depletable="true">
+      <density value="10.062" units="g/cm3"/>
+      <nuclide name="U234" ao="4.9476e-06"/>
+      <nuclide name="U235" ao="0.00048218"/>
+      <nuclide name="U238" ao="0.021504"/>
+      <nuclide name="Xe135" ao="1.0801e-08"/>
+      <nuclide name="O16" ao="0.045737"/>
       <isotropic>U234 U235 U238 Xe135 O16</isotropic>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
       <isotropic>Zr90 Zr91 Zr92 Zr94 Zr96</isotropic>
     </material>
     <material id="3" name="Cold borated water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
       <isotropic>H1 O16 B10 B11</isotropic>
     </material>
     <material id="4" name="Hot borated water">
-      <density units="atom/b-cm" value="0.06614"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.06614" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
       <isotropic>H1 O16 B10 B11</isotropic>
     </material>
     <material id="5" name="Reactor pressure vessel steel">
-      <density units="g/cm3" value="7.9"/>
+      <density value="7.9" units="g/cm3"/>
       <nuclide name="Fe54" wo="0.05437098"/>
       <nuclide name="Fe56" wo="0.88500663"/>
       <nuclide name="Fe57" wo="0.0208008"/>
@@ -52,7 +52,7 @@
       <isotropic>Fe54 Fe56 Fe57 Fe58 Ni58 Ni60 Mn55 Cr52 C0 Cu63</isotropic>
     </material>
     <material id="6" name="Lower radial reflector">
-      <density units="g/cm3" value="4.32"/>
+      <density value="4.32" units="g/cm3"/>
       <nuclide name="H1" wo="0.0095661"/>
       <nuclide name="O16" wo="0.0759107"/>
       <nuclide name="B10" wo="3.08409e-05"/>
@@ -68,7 +68,7 @@
       <isotropic>H1 O16 B10 B11 Fe54 Fe56 Fe57 Fe58 Ni58 Mn55 Cr52</isotropic>
     </material>
     <material id="7" name="Upper radial reflector / Top plate region">
-      <density units="g/cm3" value="4.28"/>
+      <density value="4.28" units="g/cm3"/>
       <nuclide name="H1" wo="0.0086117"/>
       <nuclide name="O16" wo="0.0683369"/>
       <nuclide name="B10" wo="2.77638e-05"/>
@@ -84,7 +84,7 @@
       <isotropic>H1 O16 B10 B11 Fe54 Fe56 Fe57 Fe58 Ni58 Mn55 Cr52</isotropic>
     </material>
     <material id="8" name="Bottom plate region">
-      <density units="g/cm3" value="7.184"/>
+      <density value="7.184" units="g/cm3"/>
       <nuclide name="H1" wo="0.0011505"/>
       <nuclide name="O16" wo="0.0091296"/>
       <nuclide name="B10" wo="3.70915e-06"/>
@@ -100,7 +100,7 @@
       <isotropic>H1 O16 B10 B11 Fe54 Fe56 Fe57 Fe58 Ni58 Mn55 Cr52</isotropic>
     </material>
     <material id="9" name="Bottom nozzle region">
-      <density units="g/cm3" value="2.53"/>
+      <density value="2.53" units="g/cm3"/>
       <nuclide name="H1" wo="0.0245014"/>
       <nuclide name="O16" wo="0.1944274"/>
       <nuclide name="B10" wo="7.89917e-05"/>
@@ -116,7 +116,7 @@
       <isotropic>H1 O16 B10 B11 Fe54 Fe56 Fe57 Fe58 Ni58 Mn55 Cr52</isotropic>
     </material>
     <material id="10" name="Top nozzle region">
-      <density units="g/cm3" value="1.746"/>
+      <density value="1.746" units="g/cm3"/>
       <nuclide name="H1" wo="0.035887"/>
       <nuclide name="O16" wo="0.2847761"/>
       <nuclide name="B10" wo="0.000115699"/>
@@ -132,7 +132,7 @@
       <isotropic>H1 O16 B10 B11 Fe54 Fe56 Fe57 Fe58 Ni58 Mn55 Cr52</isotropic>
     </material>
     <material id="11" name="Top of fuel assemblies">
-      <density units="g/cm3" value="3.044"/>
+      <density value="3.044" units="g/cm3"/>
       <nuclide name="H1" wo="0.0162913"/>
       <nuclide name="O16" wo="0.1292776"/>
       <nuclide name="B10" wo="5.25228e-05"/>
@@ -146,7 +146,7 @@
       <isotropic>H1 O16 B10 B11 Zr90 Zr91 Zr92 Zr94 Zr96</isotropic>
     </material>
     <material id="12" name="Bottom of fuel assemblies">
-      <density units="g/cm3" value="1.762"/>
+      <density value="1.762" units="g/cm3"/>
       <nuclide name="H1" wo="0.0292856"/>
       <nuclide name="O16" wo="0.2323919"/>
       <nuclide name="B10" wo="9.44159e-05"/>
@@ -161,8 +161,8 @@
     </material>
   </materials>
   <geometry>
-    <cell fill="200" id="1" region="-6 34 -35" universe="0"/>
-    <cell fill="201" id="2" region="-6 35 -36" universe="0"/>
+    <cell id="1" fill="200" region="-6 34 -35" universe="0"/>
+    <cell id="2" fill="201" region="-6 35 -36" universe="0"/>
     <cell id="3" material="8" region="-7 31 -32" universe="0"/>
     <cell id="4" material="9" region="-5 32 -33" universe="0"/>
     <cell id="5" material="12" region="-5 33 -34" universe="0"/>
@@ -186,9 +186,9 @@
     <cell id="31" material="2" region="3 -4" universe="4"/>
     <cell id="32" material="4" region="4" universe="4"/>
     <cell id="50" material="3" region="34 -35" universe="5"/>
-    <cell fill="100" id="60" region="34 -35" universe="6"/>
+    <cell id="60" fill="100" region="34 -35" universe="6"/>
     <cell id="70" material="4" region="35 -36" universe="7"/>
-    <cell fill="101" id="80" region="35 -36" universe="8"/>
+    <cell id="80" fill="101" region="35 -36" universe="8"/>
     <lattice id="100" name="Fuel assembly (lower half)">
       <pitch>1.26 1.26</pitch>
       <dimension>17 17</dimension>
@@ -289,30 +289,30 @@
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.41" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.475" id="2" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.56" id="3" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.62" id="4" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 187.6" id="5" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 209.0" id="6" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 229.0" id="7" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 249.0" id="8" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="-229.0" id="31" type="z-plane"/>
-    <surface coeffs="-199.0" id="32" type="z-plane"/>
-    <surface coeffs="-193.0" id="33" type="z-plane"/>
-    <surface coeffs="-183.0" id="34" type="z-plane"/>
-    <surface coeffs="0.0" id="35" type="z-plane"/>
-    <surface coeffs="183.0" id="36" type="z-plane"/>
-    <surface coeffs="203.0" id="37" type="z-plane"/>
-    <surface coeffs="215.0" id="38" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="223.0" id="39" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.41"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.475"/>
+    <surface id="3" type="z-cylinder" coeffs="0.0 0.0 0.56"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.62"/>
+    <surface id="5" type="z-cylinder" coeffs="0.0 0.0 187.6"/>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 209.0"/>
+    <surface id="7" type="z-cylinder" coeffs="0.0 0.0 229.0"/>
+    <surface id="8" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 249.0"/>
+    <surface id="31" type="z-plane" boundary="vacuum" coeffs="-229.0"/>
+    <surface id="32" type="z-plane" coeffs="-199.0"/>
+    <surface id="33" type="z-plane" coeffs="-193.0"/>
+    <surface id="34" type="z-plane" coeffs="-183.0"/>
+    <surface id="35" type="z-plane" coeffs="0.0"/>
+    <surface id="36" type="z-plane" coeffs="183.0"/>
+    <surface id="37" type="z-plane" coeffs="203.0"/>
+    <surface id="38" type="z-plane" coeffs="215.0"/>
+    <surface id="39" type="z-plane" boundary="vacuum" coeffs="223.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-160 -160 -183 160 160 183</parameters>
       </space>
diff --git a/tests/regression_tests/iso_in_lab/results_true.dat b/tests/regression_tests/iso_in_lab/results_true.dat
index aaef22623..0378ec86d 100644
--- a/tests/regression_tests/iso_in_lab/results_true.dat
+++ b/tests/regression_tests/iso_in_lab/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.015537E-01 1.086850E-01
+9.365837E-01 5.366122E-02
diff --git a/tests/regression_tests/lattice/results_true.dat b/tests/regression_tests/lattice/results_true.dat
index f3e937b36..dca84bd6a 100644
--- a/tests/regression_tests/lattice/results_true.dat
+++ b/tests/regression_tests/lattice/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.610829E-01 2.522714E-02
+9.182679E-01 5.270201E-02
diff --git a/tests/regression_tests/lattice_corner_crossing/__init__.py b/tests/regression_tests/lattice_corner_crossing/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/lattice_corner_crossing/inputs_true.dat b/tests/regression_tests/lattice_corner_crossing/inputs_true.dat
new file mode 100644
index 000000000..4b49b1403
--- /dev/null
+++ b/tests/regression_tests/lattice_corner_crossing/inputs_true.dat
@@ -0,0 +1,58 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="0.001" units="g/cm3"/>
+      <nuclide name="N14" ao="1.0"/>
+    </material>
+    <material id="2">
+      <density value="7.0" units="g/cm3"/>
+      <nuclide name="Fe56" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="2" universe="1"/>
+    <cell id="2" material="1" universe="2"/>
+    <cell id="3" material="1" region="-5 (-1 | 2 | -3 | 4)" universe="4"/>
+    <cell id="4" fill="3" region="1 -2 3 -4" universe="4"/>
+    <lattice id="3">
+      <pitch>10.0 10.0</pitch>
+      <dimension>2 2</dimension>
+      <lower_left>-10.0 -10.0</lower_left>
+      <universes>
+1 2 
+2 1 </universes>
+    </lattice>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-10.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="10.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-10.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="10.0"/>
+    <surface id="5" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 20.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="point">
+        <parameters>-0.7071067811865476 -0.7071067811865475 0.0</parameters>
+      </space>
+      <angle type="monodirectional" reference_uvw="0.7071067811865476 0.7071067811865475 0.0"/>
+    </source>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>10 10</dimension>
+      <lower_left>-20.0 -20.0</lower_left>
+      <upper_right>20.0 20.0</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/lattice_corner_crossing/results_true.dat b/tests/regression_tests/lattice_corner_crossing/results_true.dat
new file mode 100644
index 000000000..bd81cf535
--- /dev/null
+++ b/tests/regression_tests/lattice_corner_crossing/results_true.dat
@@ -0,0 +1,201 @@
+tally 1:
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.648775E-03
+7.016009E-06
+0.000000E+00
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+0.000000E+00
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+0.000000E+00
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+0.000000E+00
+0.000000E+00
+2.981847E-03
+8.891414E-06
+3.331677E-03
+1.110007E-05
+6.742237E-03
+4.545776E-05
+0.000000E+00
+0.000000E+00
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+7.501461E-04
+5.627191E-07
+0.000000E+00
+0.000000E+00
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+0.000000E+00
+0.000000E+00
diff --git a/tests/regression_tests/lattice_corner_crossing/test.py b/tests/regression_tests/lattice_corner_crossing/test.py
new file mode 100644
index 000000000..4684d144e
--- /dev/null
+++ b/tests/regression_tests/lattice_corner_crossing/test.py
@@ -0,0 +1,83 @@
+"""
+This test is designed to ensure that we account for potential corner crossings
+in floating point precision.
+
+"""
+
+from math import pi, cos, sin
+
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def model():
+    model = openmc.Model()
+
+    # Length of lattice on each side
+    lat_size = 20.0
+
+    # Angle that we're crossing the corner at
+    phi = pi/4.0
+
+    air = openmc.Material()
+    air.set_density('g/cm3', 0.001)
+    air.add_nuclide('N14', 1.0)
+    metal = openmc.Material()
+    metal.set_density('g/cm3', 7.0)
+    metal.add_nuclide('Fe56', 1.0)
+
+    metal_cell = openmc.Cell(fill=metal)
+    metal_uni = openmc.Universe(cells=[metal_cell])
+
+    air_cell = openmc.Cell(fill=air)
+    air_uni = openmc.Universe(cells=[air_cell])
+
+    # Define a checkerboard lattice
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-lat_size/2.0, -lat_size/2.0)
+    lattice.pitch = (lat_size/2, lat_size/2)
+    lattice.universes = [
+        [metal_uni, air_uni],
+        [air_uni, metal_uni]
+    ]
+
+    box = openmc.model.RectangularPrism(lat_size, lat_size)
+    cyl = openmc.ZCylinder(r=lat_size, boundary_type='vacuum')
+    outside_lattice = openmc.Cell(region=-cyl & +box, fill=air)
+    inside_lattice = openmc.Cell(region=-box, fill=lattice)
+
+    model.geometry = openmc.Geometry([outside_lattice, inside_lattice])
+
+    # Set all runtime parameters
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 10
+    model.settings.particles = 1000
+
+    # Define a source located outside the lattice and pointing straight into its
+    # corner at 45 degrees
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Point((-cos(phi), -sin(phi), 0.0)),
+        angle=openmc.stats.Monodirectional((cos(phi), sin(phi), 0.0))
+    )
+
+    # Create a mesh tally
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (10, 10)
+    mesh.lower_left = (-lat_size, -lat_size)
+    mesh.upper_right = (lat_size, lat_size)
+    mesh_filter = openmc.MeshFilter(mesh)
+    tally = openmc.Tally(tally_id=1)
+    tally.filters = [mesh_filter]
+    tally.scores = ['flux']
+    tally.estimator = 'tracklength'
+    model.tallies = [tally]
+
+    return model
+
+
+def test_lattice_corner_crossing(model):
+    harness = PyAPITestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/lattice_distribmat/False/inputs_true.dat b/tests/regression_tests/lattice_distribmat/False/inputs_true.dat
new file mode 100644
index 000000000..783989057
--- /dev/null
+++ b/tests/regression_tests/lattice_distribmat/False/inputs_true.dat
@@ -0,0 +1,74 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="2" name="light water">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <cell id="2" material="2" region="1" universe="1"/>
+    <cell id="3" fill="2" region="2 -3 4 -5" universe="6"/>
+    <cell id="4" fill="3" region="2 -3 5 -9" universe="6"/>
+    <cell id="5" fill="4" region="3 -11 4 -5" universe="6"/>
+    <cell id="6" fill="5" region="3 -11 5 -9" universe="6"/>
+    <cell id="7" fill="6" region="18 -19 20 -21" universe="7"/>
+    <lattice id="2">
+      <pitch>1.0 1.0</pitch>
+      <outer>1</outer>
+      <dimension>1 1</dimension>
+      <lower_left>-1.0 -1.0</lower_left>
+      <universes>
+1 </universes>
+    </lattice>
+    <lattice id="3">
+      <pitch>1.0 1.0</pitch>
+      <outer>1</outer>
+      <dimension>1 1</dimension>
+      <lower_left>-1.0 0</lower_left>
+      <universes>
+1 </universes>
+    </lattice>
+    <lattice id="4">
+      <pitch>1.0 1.0</pitch>
+      <outer>1</outer>
+      <dimension>1 1</dimension>
+      <lower_left>0 -1.0</lower_left>
+      <universes>
+1 </universes>
+    </lattice>
+    <lattice id="5">
+      <pitch>1.0 1.0</pitch>
+      <outer>1</outer>
+      <dimension>1 1</dimension>
+      <lower_left>0 0</lower_left>
+      <universes>
+1 </universes>
+    </lattice>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.4"/>
+    <surface id="2" name="minimum x" type="x-plane" coeffs="-1.0"/>
+    <surface id="3" name="maximum x" type="x-plane" coeffs="0.0"/>
+    <surface id="4" name="minimum y" type="y-plane" coeffs="-1.0"/>
+    <surface id="5" name="maximum y" type="y-plane" coeffs="0.0"/>
+    <surface id="9" name="maximum y" type="y-plane" coeffs="1.0"/>
+    <surface id="11" name="maximum x" type="x-plane" coeffs="1.0"/>
+    <surface id="18" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.0"/>
+    <surface id="19" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.0"/>
+    <surface id="20" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.0"/>
+    <surface id="21" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/lattice_distribmat/False/results_true.dat b/tests/regression_tests/lattice_distribmat/False/results_true.dat
new file mode 100644
index 000000000..9ebbb43b8
--- /dev/null
+++ b/tests/regression_tests/lattice_distribmat/False/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.848895E+00 1.480242E-02
diff --git a/tests/regression_tests/lattice_distribmat/True/inputs_true.dat b/tests/regression_tests/lattice_distribmat/True/inputs_true.dat
new file mode 100644
index 000000000..ab827dfde
--- /dev/null
+++ b/tests/regression_tests/lattice_distribmat/True/inputs_true.dat
@@ -0,0 +1,111 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="2" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="3" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="4" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="5" name="light water">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="6" name="light water">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="7" name="light water">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="8" name="light water">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="8" region="-6" universe="8">
+      <material>1 2 3 4</material>
+    </cell>
+    <cell id="9" region="6" universe="8">
+      <material>5 6 7 8</material>
+    </cell>
+    <cell id="10" fill="9" region="7 -8 10 -12" universe="13"/>
+    <cell id="11" fill="10" region="7 -8 12 -16" universe="13"/>
+    <cell id="12" fill="11" region="8 -22 10 -12" universe="13"/>
+    <cell id="13" fill="12" region="8 -22 12 -16" universe="13"/>
+    <cell id="14" fill="13" region="29 -30 31 -32" universe="14"/>
+    <lattice id="9">
+      <pitch>1.0 1.0</pitch>
+      <outer>8</outer>
+      <dimension>1 1</dimension>
+      <lower_left>-1.0 -1.0</lower_left>
+      <universes>
+8 </universes>
+    </lattice>
+    <lattice id="10">
+      <pitch>1.0 1.0</pitch>
+      <outer>8</outer>
+      <dimension>1 1</dimension>
+      <lower_left>-1.0 0</lower_left>
+      <universes>
+8 </universes>
+    </lattice>
+    <lattice id="11">
+      <pitch>1.0 1.0</pitch>
+      <outer>8</outer>
+      <dimension>1 1</dimension>
+      <lower_left>0 -1.0</lower_left>
+      <universes>
+8 </universes>
+    </lattice>
+    <lattice id="12">
+      <pitch>1.0 1.0</pitch>
+      <outer>8</outer>
+      <dimension>1 1</dimension>
+      <lower_left>0 0</lower_left>
+      <universes>
+8 </universes>
+    </lattice>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 0.4"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-1.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="0.0"/>
+    <surface id="10" name="minimum y" type="y-plane" coeffs="-1.0"/>
+    <surface id="12" name="maximum y" type="y-plane" coeffs="0.0"/>
+    <surface id="16" name="maximum y" type="y-plane" coeffs="1.0"/>
+    <surface id="22" name="maximum x" type="x-plane" coeffs="1.0"/>
+    <surface id="29" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.0"/>
+    <surface id="30" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.0"/>
+    <surface id="31" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.0"/>
+    <surface id="32" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/lattice_distribmat/True/results_true.dat b/tests/regression_tests/lattice_distribmat/True/results_true.dat
new file mode 100644
index 000000000..9ebbb43b8
--- /dev/null
+++ b/tests/regression_tests/lattice_distribmat/True/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.848895E+00 1.480242E-02
diff --git a/tests/regression_tests/lattice_distribmat/__init__.py b/tests/regression_tests/lattice_distribmat/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/lattice_distribmat/test.py b/tests/regression_tests/lattice_distribmat/test.py
new file mode 100644
index 000000000..4d0b6e156
--- /dev/null
+++ b/tests/regression_tests/lattice_distribmat/test.py
@@ -0,0 +1,83 @@
+import numpy as np
+import openmc
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def model():
+    model = openmc.model.Model()
+
+    uo2 = openmc.Material(name='UO2')
+    uo2.set_density('g/cm3', 10.0)
+    uo2.add_nuclide('U235', 1.0)
+    uo2.add_nuclide('O16', 2.0)
+    water = openmc.Material(name='light water')
+    water.add_nuclide('H1', 2.0)
+    water.add_nuclide('O16', 1.0)
+    water.set_density('g/cm3', 1.0)
+    water.add_s_alpha_beta('c_H_in_H2O')
+    model.materials.extend([uo2, water])
+
+    cyl = openmc.ZCylinder(r=0.4)
+    pin = openmc.model.pin([cyl], [uo2, water])
+    d = 1.0
+    
+    lattice00 = openmc.RectLattice()
+    lattice00.lower_left = (-d, -d)
+    lattice00.pitch = (d, d)
+    lattice00.outer = pin
+    lattice00.universes = [[pin]]
+    box00 = openmc.model.RectangularPrism(d, d, origin=(-d/2,-d/2))
+    
+    lattice01 = openmc.RectLattice()
+    lattice01.lower_left = (-d, 0)
+    lattice01.pitch = (d, d)
+    lattice01.outer = pin
+    lattice01.universes = [[pin]]
+    box01 = openmc.model.RectangularPrism(d, d, origin=(-d/2,d/2))
+    
+    lattice10 = openmc.RectLattice()
+    lattice10.lower_left = (0, -d)
+    lattice10.pitch = (d, d)
+    lattice10.outer = pin
+    lattice10.universes = [[pin]]
+    box10 = openmc.model.RectangularPrism(d, d, origin=(d/2,-d/2))
+    
+    lattice11 = openmc.RectLattice()
+    lattice11.lower_left = (0, 0)
+    lattice11.pitch = (d, d)
+    lattice11.outer = pin
+    lattice11.universes = [[pin]]
+    box11 = openmc.model.RectangularPrism(d, d, origin=(d/2,d/2))
+    
+    
+    cell00 = openmc.Cell(fill=lattice00, region = -box00)
+    cell01 = openmc.Cell(fill=lattice01, region = -box01)
+    cell10 = openmc.Cell(fill=lattice10, region = -box10)
+    cell11 = openmc.Cell(fill=lattice11, region = -box11)
+    
+    univ = openmc.Universe(cells=[cell00, cell01, cell10, cell11])
+    
+    box = openmc.model.RectangularPrism(2*d, 2*d, boundary_type='reflective')
+
+    main_cell = openmc.Cell(fill=univ, region=-box)
+    model.geometry = openmc.Geometry([main_cell])
+    model.geometry.merge_surfaces = True
+   
+    model.settings.batches = 10
+    model.settings.inactive = 5
+    model.settings.particles = 1000
+    
+    return model
+
+@pytest.mark.parametrize("distribmat", [False, True])
+def test_lattice(model, distribmat):
+    with change_directory(str(distribmat)):
+        openmc.reset_auto_ids()
+        if distribmat:
+            model.differentiate_mats(depletable_only=False)
+        harness = PyAPITestHarness('statepoint.10.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/lattice_distribrho/__init__.py b/tests/regression_tests/lattice_distribrho/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/lattice_distribrho/inputs_true.dat b/tests/regression_tests/lattice_distribrho/inputs_true.dat
new file mode 100644
index 000000000..99994af3d
--- /dev/null
+++ b/tests/regression_tests/lattice_distribrho/inputs_true.dat
@@ -0,0 +1,42 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
+    </material>
+    <material id="2" name="light water">
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1">
+      <density>10.0 20.0 10.0 20.0</density>
+    </cell>
+    <cell id="2" material="2" region="1" universe="1"/>
+    <cell id="3" fill="2" region="2 -3 4 -5" universe="3"/>
+    <lattice id="2">
+      <pitch>1.0 1.0</pitch>
+      <dimension>2 2</dimension>
+      <lower_left>-1.0 -1.0</lower_left>
+      <universes>
+1 1 
+1 1 </universes>
+    </lattice>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.4"/>
+    <surface id="2" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.0"/>
+    <surface id="3" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.0"/>
+    <surface id="4" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.0"/>
+    <surface id="5" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/lattice_distribrho/results_true.dat b/tests/regression_tests/lattice_distribrho/results_true.dat
new file mode 100644
index 000000000..f7f3da8e6
--- /dev/null
+++ b/tests/regression_tests/lattice_distribrho/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.900249E+00 8.157834E-03
diff --git a/tests/regression_tests/lattice_distribrho/test.py b/tests/regression_tests/lattice_distribrho/test.py
new file mode 100644
index 000000000..ec94fe96b
--- /dev/null
+++ b/tests/regression_tests/lattice_distribrho/test.py
@@ -0,0 +1,51 @@
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def model():
+    model = openmc.Model()
+
+    uo2 = openmc.Material(name='UO2')
+    uo2.set_density('g/cm3', 10.0)
+    uo2.add_nuclide('U235', 1.0)
+    uo2.add_nuclide('O16', 2.0)
+    water = openmc.Material(name='light water')
+    water.add_nuclide('H1', 2.0)
+    water.add_nuclide('O16', 1.0)
+    water.set_density('g/cm3', 1.0)
+    water.add_s_alpha_beta('c_H_in_H2O')
+    model.materials.extend([uo2, water])
+
+    cyl = openmc.ZCylinder(r=0.4)
+    pin = openmc.model.pin([cyl], [uo2, water])
+    d = 1.0
+
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-d, -d)
+    lattice.pitch = (d, d)
+    lattice.universes = [[pin, pin],
+                         [pin, pin]]
+    box = openmc.model.RectangularPrism(
+        2.0 * d, 2.0 * d,
+        origin=(0.0, 0.0),
+        boundary_type='reflective'
+    )
+
+    pin.cells[1].density = [10.0, 20.0, 10.0, 20.0]
+
+    model.geometry = openmc.Geometry([openmc.Cell(fill=lattice, region=-box)])
+    model.geometry.merge_surfaces = True
+
+    model.settings.batches = 10
+    model.settings.inactive = 5
+    model.settings.particles = 1000
+
+    return model
+
+
+def test_lattice_checkerboard(model):
+    harness = PyAPITestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/lattice_hex/results_true.dat b/tests/regression_tests/lattice_hex/results_true.dat
index 789bacc40..46db641eb 100644
--- a/tests/regression_tests/lattice_hex/results_true.dat
+++ b/tests/regression_tests/lattice_hex/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.566607E-01 9.207770E-03
+2.595598E-01 9.089294E-03
diff --git a/tests/regression_tests/lattice_hex_coincident/inputs_true.dat b/tests/regression_tests/lattice_hex_coincident/inputs_true.dat
index ac82de0ad..fafc03da1 100644
--- a/tests/regression_tests/lattice_hex_coincident/inputs_true.dat
+++ b/tests/regression_tests/lattice_hex_coincident/inputs_true.dat
@@ -1,45 +1,45 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
+    <material id="1" depletable="true">
       <density units="sum"/>
-      <nuclide ao="0.0049817" name="U235"/>
+      <nuclide name="U235" ao="0.0049817"/>
     </material>
     <material id="2">
-      <density units="atom/b-cm" value="0.017742"/>
-      <nuclide ao="1.0" name="C0"/>
+      <density value="0.017742" units="atom/b-cm"/>
+      <nuclide name="C0" ao="1.0"/>
       <sab name="c_Graphite"/>
     </material>
     <material id="3" name="Lead">
-      <density units="g/cm3" value="10.32"/>
-      <nuclide ao="0.014" name="Pb204"/>
-      <nuclide ao="0.241" name="Pb206"/>
-      <nuclide ao="0.221" name="Pb207"/>
-      <nuclide ao="0.524" name="Pb208"/>
+      <density value="10.32" units="g/cm3"/>
+      <nuclide name="Pb204" ao="0.014"/>
+      <nuclide name="Pb206" ao="0.241"/>
+      <nuclide name="Pb207" ao="0.221"/>
+      <nuclide name="Pb208" ao="0.524"/>
     </material>
     <material id="4">
-      <density units="atom/b-cm" value="0.00054464"/>
-      <nuclide ao="1.0" name="He4"/>
+      <density value="0.00054464" units="atom/b-cm"/>
+      <nuclide name="He4" ao="1.0"/>
     </material>
     <material id="5" name="Zirc4">
       <density units="sum"/>
-      <nuclide ao="0.02217" name="Zr90"/>
-      <nuclide ao="0.004781" name="Zr91"/>
-      <nuclide ao="0.007228" name="Zr92"/>
-      <nuclide ao="0.007169" name="Zr94"/>
-      <nuclide ao="0.001131" name="Zr96"/>
+      <nuclide name="Zr90" ao="0.02217"/>
+      <nuclide name="Zr91" ao="0.004781"/>
+      <nuclide name="Zr92" ao="0.007228"/>
+      <nuclide name="Zr94" ao="0.007169"/>
+      <nuclide name="Zr96" ao="0.001131"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1" universe="1"/>
-    <cell id="3" material="4" name="coolant" region="-4 2 -3" universe="2"/>
-    <cell id="4" material="5" name="zirconium_shell" region="4 -5 2 -3" universe="2"/>
-    <cell id="5" material="3" name="lead_shell" region="5 -6 2 -3" universe="2"/>
-    <cell id="6" material="2" name="matrix coolant surround" region="6 2 -3" universe="2"/>
+    <cell id="3" name="coolant" material="4" region="-4 2 -3" universe="2"/>
+    <cell id="4" name="zirconium_shell" material="5" region="4 -5 2 -3" universe="2"/>
+    <cell id="5" name="lead_shell" material="3" region="5 -6 2 -3" universe="2"/>
+    <cell id="6" name="matrix coolant surround" material="2" region="6 2 -3" universe="2"/>
     <cell id="7" material="2" universe="3"/>
-    <cell fill="4" id="8" name="container cell" region="-7 8 -9 -12 10 11 2 -3" universe="5"/>
-    <hex_lattice id="4" n_rings="2" name="regular fuel assembly">
+    <cell id="8" name="container cell" fill="4" region="-7 8 -9 -12 10 11 2 -3" universe="5"/>
+    <hex_lattice id="4" name="regular fuel assembly" n_rings="2">
       <pitch>1.4</pitch>
       <outer>3</outer>
       <center>0.0 0.0</center>
@@ -50,25 +50,25 @@
 2 2
  2</universes>
     </hex_lattice>
-    <surface coeffs="0.0 0.0 0.7" id="1" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="0.0" id="2" type="z-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="3" type="z-plane"/>
-    <surface coeffs="0.0 0.0 0.293" id="4" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.35" id="5" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.352" id="6" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="1.4" id="7" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-1.4" id="8" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 2.8" id="9" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 -2.8" id="10" type="plane"/>
-    <surface boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 -2.8" id="11" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 2.8" id="12" type="plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.7"/>
+    <surface id="2" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="3" type="z-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.293"/>
+    <surface id="5" type="z-cylinder" coeffs="0.0 0.0 0.35"/>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 0.352"/>
+    <surface id="7" type="y-plane" boundary="reflective" coeffs="1.4"/>
+    <surface id="8" type="y-plane" boundary="reflective" coeffs="-1.4"/>
+    <surface id="9" type="plane" boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 2.8"/>
+    <surface id="10" type="plane" boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 -2.8"/>
+    <surface id="11" type="plane" boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 -2.8"/>
+    <surface id="12" type="plane" boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 2.8"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>5</batches>
     <inactive>2</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.9899494936611666 -0.9899494936611666 0.0 0.9899494936611666 0.9899494936611666 10.0</parameters>
       </space>
diff --git a/tests/regression_tests/lattice_hex_coincident/results_true.dat b/tests/regression_tests/lattice_hex_coincident/results_true.dat
index c134e123f..c798b66e4 100644
--- a/tests/regression_tests/lattice_hex_coincident/results_true.dat
+++ b/tests/regression_tests/lattice_hex_coincident/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.917792E+00 4.329425E-02
+1.931086E+00 5.968486E-02
diff --git a/tests/regression_tests/lattice_hex_x/inputs_true.dat b/tests/regression_tests/lattice_hex_x/inputs_true.dat
index 252fbfc0b..b6536c6a8 100644
--- a/tests/regression_tests/lattice_hex_x/inputs_true.dat
+++ b/tests/regression_tests/lattice_hex_x/inputs_true.dat
@@ -1,32 +1,32 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2">
+    <material id="1" name="UO2" depletable="true">
       <density units="sum"/>
-      <nuclide ao="0.0008737" name="U235"/>
-      <nuclide ao="0.018744" name="U238"/>
-      <nuclide ao="0.039235" name="O16"/>
+      <nuclide name="U235" ao="0.0008737"/>
+      <nuclide name="U238" ao="0.018744"/>
+      <nuclide name="O16" ao="0.039235"/>
     </material>
     <material id="2" name="borated H2O">
       <density units="sum"/>
-      <nuclide ao="0.06694" name="H1"/>
-      <nuclide ao="0.03347" name="O16"/>
-      <nuclide ao="6.6262e-06" name="B10"/>
-      <nuclide ao="2.6839e-05" name="B11"/>
+      <nuclide name="H1" ao="0.06694"/>
+      <nuclide name="O16" ao="0.03347"/>
+      <nuclide name="B10" ao="6.6262e-06"/>
+      <nuclide name="B11" ao="2.6839e-05"/>
     </material>
     <material id="3" name="pellet B4C">
       <density units="sum"/>
-      <nuclide ao="0.01966" name="C0"/>
-      <nuclide ao="4.7344e-06" name="B11"/>
-      <nuclide ao="1.9177e-05" name="B10"/>
+      <nuclide name="C0" ao="0.01966"/>
+      <nuclide name="B11" ao="4.7344e-06"/>
+      <nuclide name="B10" ao="1.9177e-05"/>
     </material>
     <material id="4" name="Zirc4">
       <density units="sum"/>
-      <nuclide ao="0.021763349999999997" name="Zr90"/>
-      <nuclide ao="0.00474606" name="Zr91"/>
-      <nuclide ao="0.00725445" name="Zr92"/>
-      <nuclide ao="0.00735174" name="Zr94"/>
-      <nuclide ao="0.0011844" name="Zr96"/>
+      <nuclide name="Zr90" ao="0.021763349999999997"/>
+      <nuclide name="Zr91" ao="0.00474606"/>
+      <nuclide name="Zr92" ao="0.00725445"/>
+      <nuclide name="Zr94" ao="0.00735174"/>
+      <nuclide name="Zr96" ao="0.0011844"/>
     </material>
   </materials>
   <geometry>
@@ -42,8 +42,8 @@
     <cell id="10" material="4" region="-8 7" universe="3"/>
     <cell id="11" material="2" region="8" universe="3"/>
     <cell id="12" material="2" universe="4"/>
-    <cell fill="6" id="13" name="container assembly cell" region="-11 12 -13 -16 14 15 9 -10" universe="5"/>
-    <hex_lattice id="6" n_axial="2" n_rings="11" name="regular fuel assembly" orientation="x">
+    <cell id="13" name="container assembly cell" fill="6" region="-11 12 -13 -16 14 15 9 -10" universe="5"/>
+    <hex_lattice id="6" name="regular fuel assembly" n_rings="11" orientation="x" n_axial="2">
       <pitch>1.235 5.0</pitch>
       <outer>4</outer>
       <center>0.0 0.0 5.0</center>
@@ -91,29 +91,29 @@
          1 1 1 1 1 1 1 1 1 1 1 1
           1 1 1 1 1 1 1 1 1 1 1</universes>
     </hex_lattice>
-    <surface coeffs="0.0 0.0 0.386" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.4582" id="2" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.5177" id="4" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.35" id="5" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.41" id="6" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.545" id="7" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6323" id="8" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="0.0" id="9" type="z-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="10" type="z-plane"/>
-    <surface boundary="reflective" coeffs="11.8" id="11" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-11.8" id="12" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 23.6" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 -23.6" id="14" type="plane"/>
-    <surface boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 -23.6" id="15" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 23.6" id="16" type="plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.386"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.4582"/>
+    <surface id="3" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.5177"/>
+    <surface id="5" type="z-cylinder" coeffs="0.0 0.0 0.35"/>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 0.41"/>
+    <surface id="7" type="z-cylinder" coeffs="0.0 0.0 0.545"/>
+    <surface id="8" type="z-cylinder" coeffs="0.0 0.0 0.6323"/>
+    <surface id="9" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="10" type="z-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="11" type="y-plane" boundary="reflective" coeffs="11.8"/>
+    <surface id="12" type="y-plane" boundary="reflective" coeffs="-11.8"/>
+    <surface id="13" type="plane" boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 23.6"/>
+    <surface id="14" type="plane" boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 -23.6"/>
+    <surface id="15" type="plane" boundary="reflective" coeffs="1.7320508075688772 1.0 0.0 -23.6"/>
+    <surface id="16" type="plane" boundary="reflective" coeffs="-1.7320508075688772 1.0 0.0 23.6"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-13.62546635287517 -13.62546635287517 0.0 13.62546635287517 13.62546635287517 10.0</parameters>
       </space>
diff --git a/tests/regression_tests/lattice_hex_x/results_true.dat b/tests/regression_tests/lattice_hex_x/results_true.dat
index 174dc70cd..44f947283 100644
--- a/tests/regression_tests/lattice_hex_x/results_true.dat
+++ b/tests/regression_tests/lattice_hex_x/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.367975E+00 2.264887E-02
+1.326294E+00 1.193578E-02
diff --git a/tests/regression_tests/lattice_large_pitch/__init__.py b/tests/regression_tests/lattice_large_pitch/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/lattice_large_pitch/inputs_true.dat b/tests/regression_tests/lattice_large_pitch/inputs_true.dat
new file mode 100644
index 000000000..4dd4f0bd7
--- /dev/null
+++ b/tests/regression_tests/lattice_large_pitch/inputs_true.dat
@@ -0,0 +1,51 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="0.001" units="g/cm3"/>
+      <nuclide name="N14" ao="1.0"/>
+    </material>
+    <material id="2">
+      <density value="7.0" units="g/cm3"/>
+      <nuclide name="Fe56" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="2" universe="1"/>
+    <cell id="2" material="1" universe="2"/>
+    <cell id="3" fill="3" region="1 -2 3 -4" universe="4"/>
+    <lattice id="3">
+      <pitch>100.0 100.0</pitch>
+      <outer>2</outer>
+      <dimension>3 3</dimension>
+      <lower_left>-150.0 -150.0</lower_left>
+      <universes>
+1 2 1 
+2 1 2 
+1 2 1 </universes>
+    </lattice>
+    <surface id="1" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-150.0"/>
+    <surface id="2" name="maximum x" type="x-plane" boundary="vacuum" coeffs="150.0"/>
+    <surface id="3" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-150.0"/>
+    <surface id="4" name="maximum y" type="y-plane" boundary="vacuum" coeffs="150.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>6 6</dimension>
+      <lower_left>-150.0 -150.0</lower_left>
+      <upper_right>150.0 150.0</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/lattice_large_pitch/results_true.dat b/tests/regression_tests/lattice_large_pitch/results_true.dat
new file mode 100644
index 000000000..58c4e306d
--- /dev/null
+++ b/tests/regression_tests/lattice_large_pitch/results_true.dat
@@ -0,0 +1,73 @@
+tally 1:
+1.749265E+01
+3.275925E+01
+4.159430E+01
+1.799568E+02
+7.027454E+01
+4.967568E+02
+6.789716E+01
+4.652258E+02
+4.272935E+01
+1.861129E+02
+2.073342E+01
+4.374279E+01
+4.028297E+01
+1.641642E+02
+6.734263E+01
+4.627171E+02
+1.037657E+02
+1.078682E+03
+1.108394E+02
+1.240927E+03
+7.236770E+01
+5.302034E+02
+4.356361E+01
+1.921990E+02
+6.731536E+01
+4.615977E+02
+9.850684E+01
+9.810272E+02
+5.164863E+02
+2.670336E+04
+5.097770E+02
+2.604770E+04
+1.064847E+02
+1.137536E+03
+7.052986E+01
+5.003220E+02
+7.065046E+01
+5.036723E+02
+1.044890E+02
+1.103250E+03
+5.121544E+02
+2.632389E+04
+5.065331E+02
+2.570752E+04
+1.044794E+02
+1.101249E+03
+6.569142E+01
+4.361120E+02
+4.739812E+01
+2.313289E+02
+7.402284E+01
+5.591689E+02
+1.066905E+02
+1.149521E+03
+1.038665E+02
+1.086813E+03
+7.160008E+01
+5.217744E+02
+4.219705E+01
+1.816100E+02
+2.089838E+01
+4.464899E+01
+4.154271E+01
+1.745866E+02
+7.081253E+01
+5.049997E+02
+6.673273E+01
+4.509038E+02
+4.184624E+01
+1.771546E+02
+1.853898E+01
+3.538608E+01
diff --git a/tests/regression_tests/lattice_large_pitch/test.py b/tests/regression_tests/lattice_large_pitch/test.py
new file mode 100644
index 000000000..0f409badd
--- /dev/null
+++ b/tests/regression_tests/lattice_large_pitch/test.py
@@ -0,0 +1,70 @@
+"""Regression test for rectangular lattices with large pitch values.
+
+Large pitches used to trigger a segmentation fault in ``RectLattice::distance``
+because the boundary-crossing check compared a reconstructed crossing position
+against an absolute tolerance that did not scale with the geometry (see #3852).
+This test transports particles across a lattice with a large pitch to ensure the
+crossing logic remains robust.
+
+"""
+
+import openmc
+import pytest
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def model():
+    model = openmc.Model()
+
+    # Large pitch that previously caused floating-point cancellation
+    pitch = 100.0
+    n = 3
+
+    air = openmc.Material()
+    air.set_density('g/cm3', 0.001)
+    air.add_nuclide('N14', 1.0)
+    metal = openmc.Material()
+    metal.set_density('g/cm3', 7.0)
+    metal.add_nuclide('Fe56', 1.0)
+
+    metal_cell = openmc.Cell(fill=metal)
+    metal_uni = openmc.Universe(cells=[metal_cell])
+    air_cell = openmc.Cell(fill=air)
+    air_uni = openmc.Universe(cells=[air_cell])
+
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-pitch*n/2, -pitch*n/2)
+    lattice.pitch = (pitch, pitch)
+    lattice.outer = air_uni
+    lattice.universes = [
+        [metal_uni, air_uni, metal_uni],
+        [air_uni, metal_uni, air_uni],
+        [metal_uni, air_uni, metal_uni],
+    ]
+
+    box = openmc.model.RectangularPrism(pitch*n, pitch*n, boundary_type='vacuum')
+    root_cell = openmc.Cell(region=-box, fill=lattice)
+    model.geometry = openmc.Geometry([root_cell])
+
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 10
+    model.settings.particles = 1000
+
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (6, 6)
+    mesh.lower_left = (-pitch*n/2, -pitch*n/2)
+    mesh.upper_right = (pitch*n/2, pitch*n/2)
+    mesh_filter = openmc.MeshFilter(mesh)
+    tally = openmc.Tally(tally_id=1)
+    tally.filters = [mesh_filter]
+    tally.scores = ['flux']
+    model.tallies = [tally]
+
+    return model
+
+
+def test_lattice_large_pitch(model):
+    harness = PyAPITestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/lattice_multiple/inputs_true.dat b/tests/regression_tests/lattice_multiple/inputs_true.dat
index b249d97c4..06abef582 100644
--- a/tests/regression_tests/lattice_multiple/inputs_true.dat
+++ b/tests/regression_tests/lattice_multiple/inputs_true.dat
@@ -1,15 +1,15 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2" name="light water">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
@@ -18,8 +18,8 @@
     <cell id="2" material="2" region="1" universe="1"/>
     <cell id="3" material="1" region="-2" universe="2"/>
     <cell id="4" material="2" region="2" universe="2"/>
-    <cell fill="3" id="5" universe="4"/>
-    <cell fill="5" id="6" region="3 -4 5 -6" universe="6"/>
+    <cell id="5" fill="3" universe="4"/>
+    <cell id="6" fill="5" region="3 -4 5 -6" universe="6"/>
     <lattice id="3">
       <pitch>1.2 1.2</pitch>
       <outer>1</outer>
@@ -37,12 +37,12 @@
 4 4 
 4 4 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.4" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.5" id="2" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-2.4" id="3" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="2.4" id="4" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-2.4" id="5" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="2.4" id="6" name="maximum y" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.4"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.5"/>
+    <surface id="3" name="minimum x" type="x-plane" boundary="reflective" coeffs="-2.4"/>
+    <surface id="4" name="maximum x" type="x-plane" boundary="reflective" coeffs="2.4"/>
+    <surface id="5" name="minimum y" type="y-plane" boundary="reflective" coeffs="-2.4"/>
+    <surface id="6" name="maximum y" type="y-plane" boundary="reflective" coeffs="2.4"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/lattice_multiple/results_true.dat b/tests/regression_tests/lattice_multiple/results_true.dat
index 7a06551bd..0866932d2 100644
--- a/tests/regression_tests/lattice_multiple/results_true.dat
+++ b/tests/regression_tests/lattice_multiple/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.859911E+00 8.282311E-03
+1.843982E+00 5.815875E-03
diff --git a/tests/regression_tests/lattice_rotated/inputs_true.dat b/tests/regression_tests/lattice_rotated/inputs_true.dat
index 1b7c74870..e53b93f93 100644
--- a/tests/regression_tests/lattice_rotated/inputs_true.dat
+++ b/tests/regression_tests/lattice_rotated/inputs_true.dat
@@ -1,18 +1,18 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U238"/>
+    <material id="2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U238" ao="1.0"/>
     </material>
     <material id="3">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
@@ -22,8 +22,8 @@
     <cell id="3" material="2" region="-2" universe="2"/>
     <cell id="4" material="3" region="2" universe="2"/>
     <cell id="5" material="3" universe="30"/>
-    <cell fill="4" id="6" region="-3" rotation="0.0 0.0 45.0" translation="-4.0 0.0 0.0" universe="5"/>
-    <cell fill="3" id="7" region="-4" rotation="0.0 0.0 30.0" translation="4.0 0.0 0.0" universe="5"/>
+    <cell id="6" fill="4" region="-3" translation="-4.0 0.0 0.0" rotation="0.0 0.0 45.0" universe="5"/>
+    <cell id="7" fill="3" region="-4" translation="4.0 0.0 0.0" rotation="0.0 0.0 30.0" universe="5"/>
     <cell id="8" material="3" region="-5 3 4" universe="5"/>
     <hex_lattice id="3" n_rings="3">
       <pitch>1.25</pitch>
@@ -51,18 +51,18 @@
 1 1 1 1 
 1 1 1 1 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.25" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.5" id="2" type="z-cylinder"/>
-    <surface coeffs="-4.0 0.0 4.0" id="3" type="z-cylinder"/>
-    <surface coeffs="4.0 0.0 4.0" id="4" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 8.0" id="5" type="z-cylinder"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.25"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.5"/>
+    <surface id="3" type="z-cylinder" coeffs="-4.0 0.0 4.0"/>
+    <surface id="4" type="z-cylinder" coeffs="4.0 0.0 4.0"/>
+    <surface id="5" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 8.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
diff --git a/tests/regression_tests/lattice_rotated/results_true.dat b/tests/regression_tests/lattice_rotated/results_true.dat
index eeaa268fb..9a96a9259 100644
--- a/tests/regression_tests/lattice_rotated/results_true.dat
+++ b/tests/regression_tests/lattice_rotated/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.426784E-01 6.627506E-03
+4.515246E-01 2.358354E-02
diff --git a/tests/regression_tests/mg_basic/inputs_true.dat b/tests/regression_tests/mg_basic/inputs_true.dat
index 8c6faaf07..aa2c2fa0a 100644
--- a/tests/regression_tests/mg_basic/inputs_true.dat
+++ b/tests/regression_tests/mg_basic/inputs_true.dat
@@ -3,29 +3,29 @@
   <materials>
     <cross_sections>2g.h5</cross_sections>
     <material id="1" name="base leg">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_1"/>
     </material>
     <material id="2" name="base tab">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_2"/>
     </material>
     <material id="3" name="base hist">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_3"/>
     </material>
     <material id="4" name="base matrix">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_4"/>
     </material>
     <material id="5" name="base ang">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_5"/>
     </material>
     <material id="6" name="micro">
       <density units="sum"/>
-      <nuclide ao="0.5" name="mat_1"/>
-      <nuclide ao="0.5" name="mat_6"/>
+      <nuclide name="mat_1" ao="0.5"/>
+      <nuclide name="mat_6" ao="0.5"/>
     </material>
   </materials>
   <geometry>
@@ -35,20 +35,20 @@
     <cell id="4" material="4" region="4 -5" universe="0"/>
     <cell id="5" material="5" region="5 -6" universe="0"/>
     <cell id="6" material="6" region="6 -7" universe="0"/>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface coeffs="154.90833333333333" id="2" type="x-plane"/>
-    <surface coeffs="309.81666666666666" id="3" type="x-plane"/>
-    <surface coeffs="464.725" id="4" type="x-plane"/>
-    <surface coeffs="619.6333333333333" id="5" type="x-plane"/>
-    <surface coeffs="774.5416666666666" id="6" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="929.45" id="7" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" coeffs="154.90833333333333"/>
+    <surface id="3" type="x-plane" coeffs="309.81666666666666"/>
+    <surface id="4" type="x-plane" coeffs="464.725"/>
+    <surface id="5" type="x-plane" coeffs="619.6333333333333"/>
+    <surface id="6" type="x-plane" coeffs="774.5416666666666"/>
+    <surface id="7" type="x-plane" boundary="vacuum" coeffs="929.45"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -1000.0 -1000.0 154.90833333333333 1000.0 1000.0</parameters>
       </space>
diff --git a/tests/regression_tests/mg_basic/results_true.dat b/tests/regression_tests/mg_basic/results_true.dat
index 15a9f2186..16980732d 100644
--- a/tests/regression_tests/mg_basic/results_true.dat
+++ b/tests/regression_tests/mg_basic/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.009864E+00 1.107115E-02
+1.004679E+00 1.329350E-02
diff --git a/tests/regression_tests/mg_basic_delayed/inputs_true.dat b/tests/regression_tests/mg_basic_delayed/inputs_true.dat
index e0bcad15c..cc3f2cfb2 100644
--- a/tests/regression_tests/mg_basic_delayed/inputs_true.dat
+++ b/tests/regression_tests/mg_basic_delayed/inputs_true.dat
@@ -3,27 +3,27 @@
   <materials>
     <cross_sections>2g.h5</cross_sections>
     <material id="1" name="vec beta">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_1"/>
     </material>
     <material id="2" name="vec no beta">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_2"/>
     </material>
     <material id="3" name="matrix beta">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_3"/>
     </material>
     <material id="4" name="matrix no beta">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_4"/>
     </material>
     <material id="5" name="vec group beta">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_5"/>
     </material>
     <material id="6" name="matrix group beta">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_6"/>
     </material>
   </materials>
@@ -34,20 +34,20 @@
     <cell id="4" material="4" region="4 -5" universe="0"/>
     <cell id="5" material="5" region="5 -6" universe="0"/>
     <cell id="6" material="6" region="6 -7" universe="0"/>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface coeffs="154.90833333333333" id="2" type="x-plane"/>
-    <surface coeffs="309.81666666666666" id="3" type="x-plane"/>
-    <surface coeffs="464.725" id="4" type="x-plane"/>
-    <surface coeffs="619.6333333333333" id="5" type="x-plane"/>
-    <surface coeffs="774.5416666666666" id="6" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="929.45" id="7" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" coeffs="154.90833333333333"/>
+    <surface id="3" type="x-plane" coeffs="309.81666666666666"/>
+    <surface id="4" type="x-plane" coeffs="464.725"/>
+    <surface id="5" type="x-plane" coeffs="619.6333333333333"/>
+    <surface id="6" type="x-plane" coeffs="774.5416666666666"/>
+    <surface id="7" type="x-plane" boundary="vacuum" coeffs="929.45"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -1000.0 -1000.0 154.90833333333333 1000.0 1000.0</parameters>
       </space>
diff --git a/tests/regression_tests/mg_basic_delayed/results_true.dat b/tests/regression_tests/mg_basic_delayed/results_true.dat
index 6f7c79c1b..f150030b9 100644
--- a/tests/regression_tests/mg_basic_delayed/results_true.dat
+++ b/tests/regression_tests/mg_basic_delayed/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.024610E+00 9.643746E-03
+1.017078E+00 1.181139E-02
diff --git a/tests/regression_tests/mg_convert/inputs_true.dat b/tests/regression_tests/mg_convert/inputs_true.dat
index 4e51ec80c..3b1f511e6 100644
--- a/tests/regression_tests/mg_convert/inputs_true.dat
+++ b/tests/regression_tests/mg_convert/inputs_true.dat
@@ -3,23 +3,23 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.1"/>
+      <density value="1.1" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="cell 1" region="4 -5 6 -7" universe="0"/>
-    <surface boundary="reflective" coeffs="-5.0" id="4" name="left" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="5.0" id="5" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-5.0" id="6" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="5.0" id="7" name="top" type="y-plane"/>
+    <cell id="1" name="cell 1" material="1" region="4 -5 6 -7" universe="0"/>
+    <surface id="4" name="left" type="x-plane" boundary="reflective" coeffs="-5.0"/>
+    <surface id="5" name="right" type="x-plane" boundary="vacuum" coeffs="5.0"/>
+    <surface id="6" name="bottom" type="y-plane" boundary="reflective" coeffs="-5.0"/>
+    <surface id="7" name="top" type="y-plane" boundary="reflective" coeffs="5.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-5 -5 -5 5 5 5</parameters>
       </space>
diff --git a/tests/regression_tests/mg_convert/results_true.dat b/tests/regression_tests/mg_convert/results_true.dat
index ff3d7bb91..f8f748cc7 100644
--- a/tests/regression_tests/mg_convert/results_true.dat
+++ b/tests/regression_tests/mg_convert/results_true.dat
@@ -1,24 +1,24 @@
 k-combined:
-9.984888E-01 1.558301E-03
+9.926427E-01 3.067527E-03
 k-combined:
-1.001035E+00 7.622447E-04
+9.932868E-01 2.780271E-03
 k-combined:
-9.984888E-01 1.558301E-03
+9.926427E-01 3.067527E-03
 k-combined:
-9.991101E-01 2.776191E-03
+1.000000E+00 0.000000E+00
 k-combined:
-9.965954E-01 5.185046E-03
+9.902969E-01 1.654717E-02
 k-combined:
-9.987613E-01 4.806845E-04
+9.882796E-01 1.929843E-03
 k-combined:
-9.991101E-01 2.776191E-03
+1.000000E+00 0.000000E+00
 k-combined:
-9.965954E-01 5.185315E-03
+9.902953E-01 1.654291E-02
 k-combined:
-9.987610E-01 4.791528E-04
+9.882814E-01 1.927488E-03
 k-combined:
-9.944808E-01 4.458524E-03
+9.893153E-01 7.576652E-03
 k-combined:
-9.984888E-01 1.558301E-03
+9.926427E-01 3.067527E-03
 k-combined:
-9.984888E-01 1.558301E-03
+9.926427E-01 3.067527E-03
diff --git a/tests/regression_tests/mg_convert/test.py b/tests/regression_tests/mg_convert/test.py
index 8099c89a2..0e50f3a74 100755
--- a/tests/regression_tests/mg_convert/test.py
+++ b/tests/regression_tests/mg_convert/test.py
@@ -1,3 +1,4 @@
+from math import isnan
 import os
 import hashlib
 
@@ -142,10 +143,13 @@ class MGXSTestHarness(PyAPITestHarness):
                 openmc.run(openmc_exec=config['exe'])
 
             with openmc.StatePoint('statepoint.{}.h5'.format(batches)) as sp:
+                # Sometimes NaN results are produced; convert these to 0.0
+                std_dev = 0.0 if isnan(sp.keff.s) else sp.keff.s
+
                 # Write out k-combined.
                 outstr += 'k-combined:\n'
                 form = '{:12.6E} {:12.6E}\n'
-                outstr += form.format(sp.keff.n, sp.keff.s)
+                outstr += form.format(sp.keff.n, std_dev)
 
         return outstr
 
diff --git a/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/__init__.py b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/inputs_true.dat b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/inputs_true.dat
new file mode 100644
index 000000000..318fdc7b9
--- /dev/null
+++ b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/inputs_true.dat
@@ -0,0 +1,65 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>2g.h5</cross_sections>
+    <material id="1" name="mat_1">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="mat_1"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="1 -2" universe="0"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="929.45"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>2</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0.0 -1000.0 -1000.0 929.45 1000.0 1000.0</parameters>
+      </space>
+    </source>
+    <output>
+      <summary>false</summary>
+    </output>
+    <energy_mode>multi-group</energy_mode>
+    <tabular_legendre>
+      <enable>false</enable>
+    </tabular_legendre>
+    <create_fission_neutrons>true</create_fission_neutrons>
+    <weight_windows id="1">
+      <mesh>1</mesh>
+      <particle_type>neutron</particle_type>
+      <energy_bounds>0.0 0.625 20000000.0</energy_bounds>
+      <lower_ww_bounds>0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5</lower_ww_bounds>
+      <upper_ww_bounds>2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5</upper_ww_bounds>
+      <survival_ratio>3.0</survival_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <mesh id="1">
+      <dimension>5 1 1</dimension>
+      <lower_left>0.0 -1000.0 -1000.0</lower_left>
+      <upper_right>929.45 1000.0 1000.0</upper_right>
+    </mesh>
+    <shared_secondary_bank>true</shared_secondary_bank>
+    <max_history_splits>100</max_history_splits>
+  </settings>
+  <tallies>
+    <mesh id="2">
+      <dimension>5 1 1</dimension>
+      <lower_left>0.0 -1000.0 -1000.0</lower_left>
+      <upper_right>929.45 1000.0 1000.0</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>2</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/results_true.dat b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/results_true.dat
new file mode 100644
index 000000000..38ff94417
--- /dev/null
+++ b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/results_true.dat
@@ -0,0 +1,11 @@
+tally 1:
+1.765369E+04
+3.087787E+08
+1.708316E+04
+2.842002E+08
+9.444106E+03
+7.488341E+07
+2.066528E+03
+2.142445E+06
+8.689619E+02
+5.652099E+05
diff --git a/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/test.py b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/test.py
new file mode 100644
index 000000000..336c8461f
--- /dev/null
+++ b/tests/regression_tests/mg_fixed_source_ww_fission_shared_secondary/test.py
@@ -0,0 +1,91 @@
+import os
+
+import numpy as np
+import openmc
+from openmc.examples import slab_mg
+
+from tests.testing_harness import PyAPITestHarness
+
+
+def create_library():
+    # Instantiate the energy group data and file object
+    groups = openmc.mgxs.EnergyGroups([0.0, 0.625, 20.0e6])
+
+    mg_cross_sections_file = openmc.MGXSLibrary(groups)
+
+    # Make the base, isotropic data
+    nu = [2.50, 2.50]
+    fiss = np.array([0.002817, 0.097])
+    capture = [0.008708, 0.02518]
+    absorption = np.add(capture, fiss)
+    scatter = np.array(
+        [[[0.31980, 0.06694], [0.004555, -0.0003972]],
+         [[0.00000, 0.00000], [0.424100, 0.05439000]]])
+    total = [0.33588, 0.54628]
+    chi = [1., 0.]
+
+    mat_1 = openmc.XSdata('mat_1', groups)
+    mat_1.order = 1
+    mat_1.set_nu_fission(np.multiply(nu, fiss))
+    mat_1.set_absorption(absorption)
+    mat_1.set_scatter_matrix(scatter)
+    mat_1.set_total(total)
+    mat_1.set_chi(chi)
+    mg_cross_sections_file.add_xsdata(mat_1)
+
+    # Write the file
+    mg_cross_sections_file.export_to_hdf5('2g.h5')
+
+
+class MGXSTestHarness(PyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = '2g.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_mg_fixed_source_ww_fission_shared_secondary():
+    create_library()
+    model = slab_mg()
+
+    # Override settings for fixed-source mode with shared secondary bank
+    model.settings.run_mode = 'fixed source'
+    model.settings.inactive = 0
+    model.settings.batches = 2
+    model.settings.particles = 100
+    model.settings.create_fission_neutrons = True
+    model.settings.shared_secondary_bank = True
+    model.settings.max_history_splits = 100
+
+    # Add weight windows on a simple 1D mesh
+    ww_mesh = openmc.RegularMesh()
+    ww_mesh.lower_left = (0.0, -1000.0, -1000.0)
+    ww_mesh.upper_right = (929.45, 1000.0, 1000.0)
+    ww_mesh.dimension = (5, 1, 1)
+
+    # Uniform lower bounds for 2 energy groups, 5 spatial bins
+    lower_bounds = np.full((2, 5, 1, 1), 0.5)
+    ww = openmc.WeightWindows(
+        ww_mesh,
+        lower_bounds.flatten(),
+        None,
+        5.0,
+        [0.0, 0.625, 20.0e6],
+        'neutron'
+    )
+    model.settings.weight_windows = [ww]
+
+    # Add a flux tally
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (0.0, -1000.0, -1000.0)
+    mesh.upper_right = (929.45, 1000.0, 1000.0)
+    mesh.dimension = (5, 1, 1)
+
+    tally = openmc.Tally()
+    tally.filters = [openmc.MeshFilter(mesh)]
+    tally.scores = ['flux']
+    model.tallies = [tally]
+
+    harness = MGXSTestHarness('statepoint.2.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/mg_legendre/inputs_true.dat b/tests/regression_tests/mg_legendre/inputs_true.dat
index 34ff7b630..81362f20a 100644
--- a/tests/regression_tests/mg_legendre/inputs_true.dat
+++ b/tests/regression_tests/mg_legendre/inputs_true.dat
@@ -3,21 +3,21 @@
   <materials>
     <cross_sections>2g.h5</cross_sections>
     <material id="1" name="mat_1">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2" universe="0"/>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="929.45" id="2" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="929.45"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -1000.0 -1000.0 929.45 1000.0 1000.0</parameters>
       </space>
diff --git a/tests/regression_tests/mg_legendre/results_true.dat b/tests/regression_tests/mg_legendre/results_true.dat
index e2989469c..04d9c9874 100644
--- a/tests/regression_tests/mg_legendre/results_true.dat
+++ b/tests/regression_tests/mg_legendre/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.003646E+00 9.134747E-03
+1.009220E+00 9.571832E-03
diff --git a/tests/regression_tests/mg_max_order/inputs_true.dat b/tests/regression_tests/mg_max_order/inputs_true.dat
index b6d70dc4c..c8f42d1a3 100644
--- a/tests/regression_tests/mg_max_order/inputs_true.dat
+++ b/tests/regression_tests/mg_max_order/inputs_true.dat
@@ -3,21 +3,21 @@
   <materials>
     <cross_sections>2g.h5</cross_sections>
     <material id="1" name="mat_1">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2" universe="0"/>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="929.45" id="2" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="929.45"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -1000.0 -1000.0 929.45 1000.0 1000.0</parameters>
       </space>
diff --git a/tests/regression_tests/mg_max_order/results_true.dat b/tests/regression_tests/mg_max_order/results_true.dat
index e2989469c..04d9c9874 100644
--- a/tests/regression_tests/mg_max_order/results_true.dat
+++ b/tests/regression_tests/mg_max_order/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.003646E+00 9.134747E-03
+1.009220E+00 9.571832E-03
diff --git a/tests/regression_tests/mg_survival_biasing/inputs_true.dat b/tests/regression_tests/mg_survival_biasing/inputs_true.dat
index 6529c60d8..bee69729d 100644
--- a/tests/regression_tests/mg_survival_biasing/inputs_true.dat
+++ b/tests/regression_tests/mg_survival_biasing/inputs_true.dat
@@ -3,21 +3,21 @@
   <materials>
     <cross_sections>2g.h5</cross_sections>
     <material id="1" name="mat_1">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2" universe="0"/>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="929.45" id="2" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="929.45"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -1000.0 -1000.0 929.45 1000.0 1000.0</parameters>
       </space>
diff --git a/tests/regression_tests/mg_survival_biasing/results_true.dat b/tests/regression_tests/mg_survival_biasing/results_true.dat
index cddbdaceb..4b26978ad 100644
--- a/tests/regression_tests/mg_survival_biasing/results_true.dat
+++ b/tests/regression_tests/mg_survival_biasing/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.878738E-01 8.326224E-03
+9.889968E-01 9.144186E-03
diff --git a/tests/regression_tests/mg_tallies/inputs_true.dat b/tests/regression_tests/mg_tallies/inputs_true.dat
index c526b65a2..f4f154244 100644
--- a/tests/regression_tests/mg_tallies/inputs_true.dat
+++ b/tests/regression_tests/mg_tallies/inputs_true.dat
@@ -3,21 +3,21 @@
   <materials>
     <cross_sections>2g.h5</cross_sections>
     <material id="1" name="mat_1">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="mat_1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2" universe="0"/>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="929.45" id="2" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="929.45"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0.0 -1000.0 -1000.0 929.45 1000.0 1000.0</parameters>
       </space>
diff --git a/tests/regression_tests/mg_tallies/results_true.dat b/tests/regression_tests/mg_tallies/results_true.dat
index 7484dd248..c1591cca2 100644
--- a/tests/regression_tests/mg_tallies/results_true.dat
+++ b/tests/regression_tests/mg_tallies/results_true.dat
@@ -1,1324 +1,1324 @@
 k-combined:
-1.003646E+00 9.134747E-03
+1.012390E+00 9.679132E-03
 tally 1:
-5.220000E-01
-5.995400E-02
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 0.000000E+00
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+5.839688E+06
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 tally 2:
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 tally 3:
-1.092010E+02
-2.386227E+03
-4.975000E+00
-4.950147E+00
-2.009472E+00
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 tally 4:
-1.092010E+02
-2.386227E+03
-4.979068E+00
-4.959690E+00
-2.016700E+00
-8.137138E-01
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 tally 5:
-1.097121E+02
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-5.009145E+00
-5.019563E+00
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 tally 6:
-1.042260E+02
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 tally 7:
-6.507000E+00
-8.478423E+00
-1.444000E+00
-4.172780E-01
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 tally 8:
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 tally 9:
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 tally 10:
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 tally 11:
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 tally 12:
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 tally 13:
-1.092010E+02
-2.386227E+03
-4.975000E+00
-4.950147E+00
-2.009472E+00
-8.076733E-01
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-4.939215E+00
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-1.092010E+02
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+1.048490E+02
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 tally 14:
-1.092010E+02
-2.386227E+03
-4.979068E+00
-4.959690E+00
-2.016700E+00
-8.137138E-01
-5.041749E+00
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-5.020635E-05
-5.047241E-10
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-5.021429E+00
-3.196471E-02
-2.044235E-04
-4.033399E+08
-3.254855E+16
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 tally 15:
-1.097121E+02
-2.408636E+03
-5.009145E+00
-5.019563E+00
-2.032191E+00
-8.264420E-01
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-2.076211E-04
-4.064382E+08
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 tally 16:
-1.042260E+02
-2.173864E+03
-1.042260E+02
-2.173864E+03
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 tally 17:
-6.507000E+00
-8.478423E+00
-1.444000E+00
-4.172780E-01
-1.146407E+00
-2.630079E-01
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 3.531000E+00
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 8.630652E-01
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 tally 18:
-6.507000E+00
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-4.241161E-01
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-8.580723E-03
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-8.612868E-01
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 tally 19:
-6.573230E+00
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 tally 20:
-5.063000E+00
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-5.063000E+00
-5.136983E+00
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 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
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 0.000000E+00
 0.000000E+00
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-9.771800E+01
-1.911083E+03
-2.133624E+00
-9.150435E-01
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diff --git a/tests/regression_tests/mg_temperature/build_2g.py b/tests/regression_tests/mg_temperature/build_2g.py
index 1fb723449..bca87364e 100644
--- a/tests/regression_tests/mg_temperature/build_2g.py
+++ b/tests/regression_tests/mg_temperature/build_2g.py
@@ -227,7 +227,7 @@ def analytical_solution_2g_therm(xsmin, xsmax=None, wgt=1.0):
     L = np.array([sa[0] + ss12, 0.0, -ss12, sa[1]]).reshape(2, 2)
     Q = np.array([nsf[0], nsf[1], 0.0, 0.0]).reshape(2, 2)
     arr = np.linalg.inv(L).dot(Q)
-    return np.amax(np.linalg.eigvals(arr))
+    return np.amax(np.linalg.eigvals(arr).real)
 
 
 def build_inf_model(xsnames, xslibname, temperature, tempmethod='nearest'):
diff --git a/tests/regression_tests/mg_temperature/results_true.dat b/tests/regression_tests/mg_temperature/results_true.dat
index 19364e1d9..6de4c6bb1 100644
--- a/tests/regression_tests/mg_temperature/results_true.dat
+++ b/tests/regression_tests/mg_temperature/results_true.dat
@@ -1,40 +1,40 @@
 micro, method: nearest, t: 300.0, k-combined:
-1.439563E+00 4.526076E-04
+1.439913E+00 4.285638E-04
 kanalyt
 1.440410E+00
 micro, method: nearest, t: 600.0, k-combined:
-1.409389E+00 4.684481E-04
+1.410750E+00 4.829834E-04
 kanalyt
 1.410164E+00
 micro, method: nearest, t: 900.0, k-combined:
-1.407593E+00 4.410387E-04
+1.408232E+00 4.946310E-04
 kanalyt
 1.407830E+00
 micro, method: interpolation, t: 520.0, k-combined:
-1.418259E+00 4.242856E-04
+1.418877E+00 4.651822E-04
 kanalyt
 1.418514E+00
 micro, method: interpolation, t: 600.0, k-combined:
-1.409389E+00 4.684481E-04
+1.410750E+00 4.829834E-04
 kanalyt
 1.410164E+00
 macro, method: nearest, t: 300.0, k-combined:
-1.439563E+00 4.526076E-04
+1.439913E+00 4.285638E-04
 kanalyt
 1.440410E+00
 macro, method: nearest, t: 600.0, k-combined:
-1.409389E+00 4.684481E-04
+1.410750E+00 4.829834E-04
 kanalyt
 1.410164E+00
 macro, method: nearest, t: 900.0, k-combined:
-1.407593E+00 4.410387E-04
+1.408232E+00 4.946310E-04
 kanalyt
 1.407830E+00
 macro, method: interpolation, t: 520.0, k-combined:
-1.418259E+00 4.242856E-04
+1.418877E+00 4.651822E-04
 kanalyt
 1.418514E+00
 macro, method: interpolation, t: 600, k-combined:
-1.409389E+00 4.684481E-04
+1.410750E+00 4.829834E-04
 kanalyt
 1.410164E+00
diff --git a/tests/regression_tests/mg_temperature_multi/inputs_true.dat b/tests/regression_tests/mg_temperature_multi/inputs_true.dat
index 0c452cd52..b84a6782b 100644
--- a/tests/regression_tests/mg_temperature_multi/inputs_true.dat
+++ b/tests/regression_tests/mg_temperature_multi/inputs_true.dat
@@ -3,31 +3,31 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner" region="-1" temperature="600.0" universe="1"/>
-    <cell id="2" material="1" name="fuel outer" region="1 -2" temperature="294.0" universe="1"/>
-    <cell id="3" material="2" name="moderator" region="2 3 -4 5 -6" universe="1"/>
-    <surface coeffs="0.0 0.0 0.25" id="1" name="Fuel IR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.54" id="2" name="Fuel OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="maximum y" type="y-plane"/>
+    <cell id="1" name="fuel inner" material="1" region="-1" temperature="600.0" universe="1"/>
+    <cell id="2" name="fuel outer" material="1" region="1 -2" temperature="294.0" universe="1"/>
+    <cell id="3" name="moderator" material="2" region="2 3 -4 5 -6" universe="1"/>
+    <surface id="1" name="Fuel IR" type="z-cylinder" coeffs="0.0 0.0 0.25"/>
+    <surface id="2" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="3" name="minimum x" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="maximum x" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="minimum y" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="maximum y" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mg_temperature_multi/results_true.dat b/tests/regression_tests/mg_temperature_multi/results_true.dat
index 80c2f5622..3e2f990c8 100644
--- a/tests/regression_tests/mg_temperature_multi/results_true.dat
+++ b/tests/regression_tests/mg_temperature_multi/results_true.dat
@@ -1,8 +1,8 @@
 k-combined:
-1.337115E+00 7.221249E-03
+1.309371E+00 6.765039E-03
 tally 1:
-2.549066E+01
-1.299987E+02
+2.532303E+01
+1.282689E+02
 tally 2:
-9.276778E+01
-1.721791E+03
+9.336894E+01
+1.743765E+03
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true.dat b/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true_multiplicity_matrix.dat
similarity index 80%
rename from tests/regression_tests/mgxs_library_ce_to_mg/inputs_true.dat
rename to tests/regression_tests/mgxs_library_ce_to_mg/inputs_true_multiplicity_matrix.dat
index 6c071263f..2f6dde1ab 100644
--- a/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true_multiplicity_matrix.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true_scatter_matrix.dat b/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true_scatter_matrix.dat
new file mode 100644
index 000000000..6e60f1b19
--- /dev/null
+++ b/tests/regression_tests/mgxs_library_ce_to_mg/inputs_true_scatter_matrix.dat
@@ -0,0 +1,217 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="7" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
+    </material>
+    <material id="8" name="Zircaloy">
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
+    </material>
+    <material id="9" name="Hot borated water">
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="4" name="Fuel" material="7" region="-7" universe="0"/>
+    <cell id="5" name="Cladding" material="8" region="7 -8" universe="0"/>
+    <cell id="6" name="Water" material="9" region="8 9 -10 11 -12" universe="0"/>
+    <surface id="7" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="8" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="9" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="10" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="11" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="12" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+  </settings>
+  <tallies>
+    <filter id="43" type="material">
+      <bins>7</bins>
+    </filter>
+    <filter id="44" type="energy">
+      <bins>0.0 0.625 20000000.0</bins>
+    </filter>
+    <filter id="49" type="energyout">
+      <bins>0.0 0.625 20000000.0</bins>
+    </filter>
+    <filter id="53" type="legendre">
+      <order>3</order>
+    </filter>
+    <filter id="54" type="material">
+      <bins>8</bins>
+    </filter>
+    <filter id="65" type="material">
+      <bins>9</bins>
+    </filter>
+    <tally id="76">
+      <filters>43 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="77">
+      <filters>43 44</filters>
+      <nuclides>total</nuclides>
+      <scores>total</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="78">
+      <filters>43 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="79">
+      <filters>43 44</filters>
+      <nuclides>total</nuclides>
+      <scores>absorption</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="80">
+      <filters>43 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="81">
+      <filters>43 44 49</filters>
+      <nuclides>total</nuclides>
+      <scores>nu-fission</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="82">
+      <filters>43 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="83">
+      <filters>43 44 49 53</filters>
+      <nuclides>total</nuclides>
+      <scores>scatter</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="84">
+      <filters>54 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="85">
+      <filters>54 44</filters>
+      <nuclides>total</nuclides>
+      <scores>total</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="86">
+      <filters>54 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="87">
+      <filters>54 44</filters>
+      <nuclides>total</nuclides>
+      <scores>absorption</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="88">
+      <filters>54 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="89">
+      <filters>54 44 49</filters>
+      <nuclides>total</nuclides>
+      <scores>nu-fission</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="90">
+      <filters>54 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="91">
+      <filters>54 44 49 53</filters>
+      <nuclides>total</nuclides>
+      <scores>scatter</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="92">
+      <filters>65 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="93">
+      <filters>65 44</filters>
+      <nuclides>total</nuclides>
+      <scores>total</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="94">
+      <filters>65 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="95">
+      <filters>65 44</filters>
+      <nuclides>total</nuclides>
+      <scores>absorption</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="96">
+      <filters>65 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="97">
+      <filters>65 44 49</filters>
+      <nuclides>total</nuclides>
+      <scores>nu-fission</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="98">
+      <filters>65 44</filters>
+      <nuclides>total</nuclides>
+      <scores>flux</scores>
+      <estimator>analog</estimator>
+    </tally>
+    <tally id="99">
+      <filters>65 44 49 53</filters>
+      <nuclides>total</nuclides>
+      <scores>scatter</scores>
+      <estimator>analog</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg/results_true.dat b/tests/regression_tests/mgxs_library_ce_to_mg/results_true.dat
index d95659534..7ce106314 100644
--- a/tests/regression_tests/mgxs_library_ce_to_mg/results_true.dat
+++ b/tests/regression_tests/mgxs_library_ce_to_mg/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.158563E+00 3.354681E-02
+1.152065E+00 2.768158E-02
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg/test.py b/tests/regression_tests/mgxs_library_ce_to_mg/test.py
index 4dff67323..b68fbd6c3 100644
--- a/tests/regression_tests/mgxs_library_ce_to_mg/test.py
+++ b/tests/regression_tests/mgxs_library_ce_to_mg/test.py
@@ -9,7 +9,7 @@ from tests.regression_tests import config
 
 
 class MGXSTestHarness(PyAPITestHarness):
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args, scatter_mgxs_type=None, **kwargs):
         # Generate inputs using parent class routine
         super().__init__(*args, **kwargs)
 
@@ -19,8 +19,8 @@ class MGXSTestHarness(PyAPITestHarness):
         # Initialize MGXS Library for a few cross section types
         self.mgxs_lib = openmc.mgxs.Library(self._model.geometry)
         self.mgxs_lib.by_nuclide = False
-        self.mgxs_lib.mgxs_types = ['total', 'absorption', 'nu-fission matrix',
-                                    'nu-scatter matrix', 'multiplicity matrix']
+        self.mgxs_lib.mgxs_types = ['total', 'absorption', 'nu-fission matrix']
+        self.mgxs_lib.mgxs_types += scatter_mgxs_type
         self.mgxs_lib.energy_groups = energy_groups
         self.mgxs_lib.correction = None
         self.mgxs_lib.legendre_order = 3
@@ -28,7 +28,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Initialize a tallies file
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _run_openmc(self):
         # Initial run
@@ -40,8 +40,8 @@ class MGXSTestHarness(PyAPITestHarness):
 
         # Build MG Inputs
         # Get data needed to execute Library calculations.
-        sp = openmc.StatePoint(self._sp_name)
-        self.mgxs_lib.load_from_statepoint(sp)
+        with openmc.StatePoint(self._sp_name) as sp:
+            self.mgxs_lib.load_from_statepoint(sp)
         self._model.mgxs_file, self._model.materials, \
             self._model.geometry = self.mgxs_lib.create_mg_mode()
 
@@ -55,11 +55,6 @@ class MGXSTestHarness(PyAPITestHarness):
         self._model.export_to_model_xml()
         self._model.mgxs_file.export_to_hdf5()
 
-        # Enforce closing statepoint and summary files so HDF5
-        # does not throw an error during the next OpenMC execution
-        sp._f.close()
-        sp._summary._f.close()
-
         # Re-run MG mode.
         if config['mpi']:
             mpi_args = [config['mpiexec'], '-n', config['mpi_np']]
@@ -74,9 +69,23 @@ class MGXSTestHarness(PyAPITestHarness):
             os.remove(f)
 
 
-def test_mgxs_library_ce_to_mg():
+def test_mgxs_library_ce_to_mg_multiplicity_matrix():
     # Set the input set to use the pincell model
     model = pwr_pin_cell()
 
-    harness = MGXSTestHarness('statepoint.10.h5', model)
+    harness = MGXSTestHarness(
+        'statepoint.10.h5', model,
+        inputs_true='inputs_true_multiplicity_matrix.dat',
+        scatter_mgxs_type=['nu-scatter matrix', 'multiplicity matrix']
+    )
+    harness.main()
+
+
+def test_mgxs_library_ce_to_mg_scatter_matrix():
+    # Set the input set to use the pincell model
+    model = pwr_pin_cell()
+
+    harness = MGXSTestHarness('statepoint.10.h5', model,
+                              inputs_true='inputs_true_scatter_matrix.dat',
+                              scatter_mgxs_type=['scatter matrix'])
     harness.main()
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/inputs_true.dat b/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/inputs_true.dat
index ad3d50974..e1f330cf9 100644
--- a/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/results_true.dat b/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/results_true.dat
index a4e9dafcc..50c8328c8 100644
--- a/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/results_true.dat
+++ b/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-6.025533E-01 1.157401E-02
+4.139942E-01 1.181308E-02
diff --git a/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/test.py b/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/test.py
index ee0cdf0d9..489105f8f 100644
--- a/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/test.py
+++ b/tests/regression_tests/mgxs_library_ce_to_mg_nuclides/test.py
@@ -28,7 +28,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Initialize a tallies file
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _run_openmc(self):
         # Initial run
@@ -40,8 +40,8 @@ class MGXSTestHarness(PyAPITestHarness):
 
         # Build MG Inputs
         # Get data needed to execute Library calculations.
-        sp = openmc.StatePoint(self._sp_name)
-        self.mgxs_lib.load_from_statepoint(sp)
+        with openmc.StatePoint(self._sp_name) as sp:
+            self.mgxs_lib.load_from_statepoint(sp)
         self._model.mgxs_file, self._model.materials, \
             self._model.geometry = self.mgxs_lib.create_mg_mode()
 
@@ -55,11 +55,6 @@ class MGXSTestHarness(PyAPITestHarness):
         self._model.export_to_model_xml()
         self._model.mgxs_file.export_to_hdf5()
 
-        # Enforce closing statepoint and summary files so HDF5
-        # does not throw an error during the next OpenMC execution
-        sp._f.close()
-        sp._summary._f.close()
-
         # Re-run MG mode.
         if config['mpi']:
             mpi_args = [config['mpiexec'], '-n', config['mpi_np']]
diff --git a/tests/regression_tests/mgxs_library_condense/inputs_true.dat b/tests/regression_tests/mgxs_library_condense/inputs_true.dat
index 8606e9fdd..4451d214c 100644
--- a/tests/regression_tests/mgxs_library_condense/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_condense/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_condense/results_true.dat b/tests/regression_tests/mgxs_library_condense/results_true.dat
index ae5bff6e3..023d5afa2 100644
--- a/tests/regression_tests/mgxs_library_condense/results_true.dat
+++ b/tests/regression_tests/mgxs_library_condense/results_true.dat
@@ -1,362 +1,362 @@
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.699306  0.028642
-2      1  2  1        1   total  0.687913  0.025966
-1      2  1  1        1   total  0.716035  0.018945
-3      2  2  1        1   total  0.702630  0.028574
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.443642  0.030984
-2      1  2  1        1   total  0.429968  0.028072
-1      2  1  1        1   total  0.461535  0.021345
-3      2  2  1        1   total  0.439454  0.031232
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.443642  0.030984
-2      1  2  1        1   total  0.429968  0.028072
-1      2  1  1        1   total  0.461535  0.021345
-3      2  2  1        1   total  0.439454  0.031232
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.021895  0.001041
-2      1  2  1        1   total  0.021903  0.001108
-1      2  1  1        1   total  0.024895  0.001401
-3      2  2  1        1   total  0.022105  0.001198
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.021883  0.001040
-2      1  2  1        1   total  0.021879  0.001108
-1      2  1  1        1   total  0.024875  0.001401
-3      2  2  1        1   total  0.022071  0.001197
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.011219  0.000958
-2      1  2  1        1   total  0.011437  0.001013
-1      2  1  1        1   total  0.012947  0.001437
-3      2  2  1        1   total  0.011756  0.001144
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.010676  0.000481
-2      1  2  1        1   total  0.010466  0.000446
-1      2  1  1        1   total  0.011948  0.000517
-3      2  2  1        1   total  0.010350  0.000547
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.026218  0.001180
-2      1  2  1        1   total  0.025724  0.001093
-1      2  1  1        1   total  0.029326  0.001262
-3      2  2  1        1   total  0.025451  0.001338
-  mesh 1       group in nuclide          mean      std. dev.
-       x  y  z                                              
-0      1  1  1        1   total  2.067337e+06   93152.452502
-2      1  2  1        1   total  2.026907e+06   86377.585629
-1      2  1  1        1   total  2.313358e+06   99980.823985
-3      2  2  1        1   total  2.004370e+06  105820.022073
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.677411  0.027731
-2      1  2  1        1   total  0.666009  0.025155
-1      2  1  1        1   total  0.691140  0.018119
-3      2  2  1        1   total  0.680525  0.027587
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.679832  0.030242
-2      1  2  1        1   total  0.663143  0.019259
-1      2  1  1        1   total  0.679503  0.027107
-3      2  2  1        1   total  0.681909  0.033506
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.679832  0.030426
-1       1  1  1        1         1       P1   total  0.255665  0.011704
-2       1  1  1        1         1       P2   total  0.087020  0.005603
-3       1  1  1        1         1       P3   total  0.006425  0.005919
-8       1  2  1        1         1       P0   total  0.663143  0.019088
-9       1  2  1        1         1       P1   total  0.257945  0.010887
-10      1  2  1        1         1       P2   total  0.095395  0.006988
-11      1  2  1        1         1       P3   total  0.009816  0.003950
-4       2  1  1        1         1       P0   total  0.679503  0.026695
-5       2  1  1        1         1       P1   total  0.254500  0.009901
-6       2  1  1        1         1       P2   total  0.093289  0.005481
-7       2  1  1        1         1       P3   total  0.004631  0.004081
-12      2  2  1        1         1       P0   total  0.681909  0.032740
-13      2  2  1        1         1       P1   total  0.263176  0.012659
-14      2  2  1        1         1       P2   total  0.093102  0.006603
-15      2  2  1        1         1       P3   total  0.007938  0.005673
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.679832  0.030426
-1       1  1  1        1         1       P1   total  0.255665  0.011704
-2       1  1  1        1         1       P2   total  0.087020  0.005603
-3       1  1  1        1         1       P3   total  0.006425  0.005919
-8       1  2  1        1         1       P0   total  0.663143  0.019088
-9       1  2  1        1         1       P1   total  0.257945  0.010887
-10      1  2  1        1         1       P2   total  0.095395  0.006988
-11      1  2  1        1         1       P3   total  0.009816  0.003950
-4       2  1  1        1         1       P0   total  0.679503  0.026695
-5       2  1  1        1         1       P1   total  0.254500  0.009901
-6       2  1  1        1         1       P2   total  0.093289  0.005481
-7       2  1  1        1         1       P3   total  0.004631  0.004081
-12      2  2  1        1         1       P0   total  0.681909  0.032740
-13      2  2  1        1         1       P1   total  0.263176  0.012659
-14      2  2  1        1         1       P2   total  0.093102  0.006603
-15      2  2  1        1         1       P3   total  0.007938  0.005673
-  mesh 1       group in group out nuclide mean std. dev.
-       x  y  z                                          
-0      1  1  1        1         1   total  1.0  0.041183
-2      1  2  1        1         1   total  1.0  0.021636
-1      2  1  1        1         1   total  1.0  0.046661
-3      2  2  1        1         1   total  1.0  0.041512
-  mesh 1       group in group out nuclide      mean std. dev.
-       x  y  z                                               
-0      1  1  1        1         1   total  0.027090  0.002808
-2      1  2  1        1         1   total  0.031522  0.004261
-1      2  1  1        1         1   total  0.021855  0.001457
-3      2  2  1        1         1   total  0.028262  0.002358
-  mesh 1       group in group out nuclide mean std. dev.
-       x  y  z                                          
-0      1  1  1        1         1   total  1.0  0.041183
-2      1  2  1        1         1   total  1.0  0.021636
-1      2  1  1        1         1   total  1.0  0.046661
-3      2  2  1        1         1   total  1.0  0.041512
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.677411  0.039336
-1       1  1  1        1         1       P1   total  0.254754  0.014995
-2       1  1  1        1         1       P2   total  0.086711  0.006439
-3       1  1  1        1         1       P3   total  0.006402  0.005903
-8       1  2  1        1         1       P0   total  0.666009  0.028990
-9       1  2  1        1         1       P1   total  0.259060  0.013824
-10      1  2  1        1         1       P2   total  0.095807  0.007684
-11      1  2  1        1         1       P3   total  0.009858  0.003980
-4       2  1  1        1         1       P0   total  0.691140  0.036991
-5       2  1  1        1         1       P1   total  0.258859  0.013782
-6       2  1  1        1         1       P2   total  0.094887  0.006555
-7       2  1  1        1         1       P3   total  0.004710  0.004154
-12      2  2  1        1         1       P0   total  0.680525  0.039486
-13      2  2  1        1         1       P1   total  0.262642  0.015259
-14      2  2  1        1         1       P2   total  0.092913  0.007251
-15      2  2  1        1         1       P3   total  0.007921  0.005668
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.677411  0.048224
-1       1  1  1        1         1       P1   total  0.254754  0.018301
-2       1  1  1        1         1       P2   total  0.086711  0.007363
-3       1  1  1        1         1       P3   total  0.006402  0.005909
-8       1  2  1        1         1       P0   total  0.666009  0.032374
-9       1  2  1        1         1       P1   total  0.259060  0.014917
-10      1  2  1        1         1       P2   total  0.095807  0.007958
-11      1  2  1        1         1       P3   total  0.009858  0.003985
-4       2  1  1        1         1       P0   total  0.691140  0.049075
-5       2  1  1        1         1       P1   total  0.258859  0.018326
-6       2  1  1        1         1       P2   total  0.094887  0.007910
-7       2  1  1        1         1       P3   total  0.004710  0.004160
-12      2  2  1        1         1       P0   total  0.680525  0.048551
-13      2  2  1        1         1       P1   total  0.262642  0.018754
-14      2  2  1        1         1       P2   total  0.092913  0.008213
-15      2  2  1        1         1       P3   total  0.007921  0.005677
-  mesh 1       group out nuclide mean std. dev.
-       x  y  z                                 
-0      1  1  1         1   total  1.0  0.103333
-2      1  2  1         1   total  1.0  0.118582
-1      2  1  1         1   total  1.0  0.112144
-3      2  2  1         1   total  1.0  0.130701
-  mesh 1       group out nuclide mean std. dev.
-       x  y  z                                 
-0      1  1  1         1   total  1.0  0.103333
-2      1  2  1         1   total  1.0  0.118582
-1      2  1  1         1   total  1.0  0.112144
-3      2  2  1         1   total  1.0  0.130701
-  mesh 1       group in nuclide          mean     std. dev.
-       x  y  z                                             
-0      1  1  1        1   total  5.304285e-07  2.560239e-08
-2      1  2  1        1   total  4.940321e-07  2.410417e-08
-1      2  1  1        1   total  5.587365e-07  3.382787e-08
-3      2  2  1        1   total  5.232210e-07  2.482800e-08
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.026032  0.001172
-2      1  2  1        1   total  0.025542  0.001086
-1      2  1  1        1   total  0.029121  0.001254
-3      2  2  1        1   total  0.025271  0.001329
-  mesh 1       group in group out nuclide      mean std. dev.
-       x  y  z                                               
-0      1  1  1        1         1   total  0.027090  0.002808
-2      1  2  1        1         1   total  0.031522  0.004261
-1      2  1  1        1         1   total  0.021855  0.001457
-3      2  2  1        1         1   total  0.028262  0.002358
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.702881  0.026175
+2      1  2        1   total  0.706921  0.029169
+1      2  1        1   total  0.707809  0.024766
+3      2  2        1   total  0.717967  0.024008
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.431023  0.028803
+2      1  2        1   total  0.451864  0.030748
+1      2  1        1   total  0.456990  0.026359
+3      2  2        1   total  0.450621  0.026744
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.431023  0.028803
+2      1  2        1   total  0.451864  0.030748
+1      2  1        1   total  0.456990  0.026359
+3      2  2        1   total  0.450621  0.026744
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    mesh 1               group in nuclide   mean std. dev.
         x  y       surf                                  
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-2       1  1  x-max out        1   total  4.250  0.107135
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-15      2  1   y-max in        1   total  4.402  0.182565
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-2      1  2  1        1   total  0.932532  0.086323
-1      2  1  1        1   total  0.885723  0.041074
-3      2  2  1        1   total  0.951783  0.101570
-  mesh 1       group in nuclide      mean std. dev.
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-0      1  1  1        1   total  0.950616  0.095874
-2      1  2  1        1   total  0.932532  0.086323
-1      2  1  1        1   total  0.885723  0.041074
-3      2  2  1        1   total  0.951783  0.101570
-   mesh 1       delayedgroup group in nuclide      mean     std. dev.
-        x  y  z                                                      
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-17      1  2  1            6        1   total  0.000013  5.903673e-07
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-22      2  2  1            5        1   total  0.000031  1.602620e-06
-23      2  2  1            6        1   total  0.000013  6.662067e-07
-   mesh 1       delayedgroup group out nuclide mean std. dev.
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-   mesh 1       delayedgroup group in nuclide      mean std. dev.
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-   mesh 1       delayedgroup nuclide      mean std. dev.
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-   mesh 1       delayedgroup group in group out nuclide mean std. dev.
-        x  y  z                                                       
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-3       1  1  1            4        1         1   total  0.0       0.0
-4       1  1  1            5        1         1   total  0.0       0.0
-5       1  1  1            6        1         1   total  0.0       0.0
-12      1  2  1            1        1         1   total  0.0       0.0
-13      1  2  1            2        1         1   total  0.0       0.0
-14      1  2  1            3        1         1   total  0.0       0.0
-15      1  2  1            4        1         1   total  0.0       0.0
-16      1  2  1            5        1         1   total  0.0       0.0
-17      1  2  1            6        1         1   total  0.0       0.0
-6       2  1  1            1        1         1   total  0.0       0.0
-7       2  1  1            2        1         1   total  0.0       0.0
-8       2  1  1            3        1         1   total  0.0       0.0
-9       2  1  1            4        1         1   total  0.0       0.0
-10      2  1  1            5        1         1   total  0.0       0.0
-11      2  1  1            6        1         1   total  0.0       0.0
-18      2  2  1            1        1         1   total  0.0       0.0
-19      2  2  1            2        1         1   total  0.0       0.0
-20      2  2  1            3        1         1   total  0.0       0.0
-21      2  2  1            4        1         1   total  0.0       0.0
-22      2  2  1            5        1         1   total  0.0       0.0
-23      2  2  1            6        1         1   total  0.0       0.0
+29      2  2   y-min in        1   total  4.416  0.093648
+28      2  2  y-min out        1   total  4.280  0.079070
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.943198  0.067915
+2      1  2        1   total  0.895741  0.058714
+1      2  1        1   total  0.911011  0.068461
+3      2  2        1   total  0.927611  0.066426
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.943198  0.067915
+2      1  2        1   total  0.895741  0.058714
+1      2  1        1   total  0.911011  0.068461
+3      2  2        1   total  0.927611  0.066426
+   mesh 1    delayedgroup group in nuclide      mean     std. dev.
+        x  y                                                      
+0       1  1            1        1   total  0.000006  4.453421e-07
+1       1  1            2        1   total  0.000032  2.303082e-06
+2       1  1            3        1   total  0.000031  2.202789e-06
+3       1  1            4        1   total  0.000072  4.961025e-06
+4       1  1            5        1   total  0.000032  2.071919e-06
+5       1  1            6        1   total  0.000013  8.662934e-07
+12      1  2            1        1   total  0.000006  3.282329e-07
+13      1  2            2        1   total  0.000031  1.701766e-06
+14      1  2            3        1   total  0.000030  1.629615e-06
+15      1  2            4        1   total  0.000070  3.675759e-06
+16      1  2            5        1   total  0.000031  1.536226e-06
+17      1  2            6        1   total  0.000013  6.423838e-07
+6       2  1            1        1   total  0.000006  2.308186e-07
+7       2  1            2        1   total  0.000032  1.211112e-06
+8       2  1            3        1   total  0.000031  1.169911e-06
+9       2  1            4        1   total  0.000072  2.685832e-06
+10      2  1            5        1   total  0.000032  1.187072e-06
+11      2  1            6        1   total  0.000013  4.939053e-07
+18      2  2            1        1   total  0.000006  3.819684e-07
+19      2  2            2        1   total  0.000032  1.976073e-06
+20      2  2            3        1   total  0.000031  1.889748e-06
+21      2  2            4        1   total  0.000072  4.252207e-06
+22      2  2            5        1   total  0.000032  1.765565e-06
+23      2  2            6        1   total  0.000013  7.386890e-07
+   mesh 1    delayedgroup group out nuclide mean std. dev.
+        x  y                                              
+0       1  1            1         1   total  0.0  0.000000
+1       1  1            2         1   total  0.0  0.000000
+2       1  1            3         1   total  0.0  0.000000
+3       1  1            4         1   total  1.0  1.414214
+4       1  1            5         1   total  0.0  0.000000
+5       1  1            6         1   total  0.0  0.000000
+12      1  2            1         1   total  0.0  0.000000
+13      1  2            2         1   total  0.0  0.000000
+14      1  2            3         1   total  0.0  0.000000
+15      1  2            4         1   total  1.0  0.869026
+16      1  2            5         1   total  0.0  0.000000
+17      1  2            6         1   total  0.0  0.000000
+6       2  1            1         1   total  0.0  0.000000
+7       2  1            2         1   total  0.0  0.000000
+8       2  1            3         1   total  0.0  0.000000
+9       2  1            4         1   total  1.0  1.414214
+10      2  1            5         1   total  0.0  0.000000
+11      2  1            6         1   total  0.0  0.000000
+18      2  2            1         1   total  0.0  0.000000
+19      2  2            2         1   total  0.0  0.000000
+20      2  2            3         1   total  0.0  0.000000
+21      2  2            4         1   total  1.0  1.414214
+22      2  2            5         1   total  0.0  0.000000
+23      2  2            6         1   total  0.0  0.000000
+   mesh 1    delayedgroup group in nuclide      mean std. dev.
+        x  y                                                  
+0       1  1            1        1   total  0.000227  0.000023
+1       1  1            2        1   total  0.001210  0.000119
+2       1  1            3        1   total  0.001176  0.000114
+3       1  1            4        1   total  0.002722  0.000261
+4       1  1            5        1   total  0.001204  0.000112
+5       1  1            6        1   total  0.000501  0.000047
+12      1  2            1        1   total  0.000227  0.000017
+13      1  2            2        1   total  0.001208  0.000087
+14      1  2            3        1   total  0.001174  0.000084
+15      1  2            4        1   total  0.002710  0.000192
+16      1  2            5        1   total  0.001194  0.000082
+17      1  2            6        1   total  0.000497  0.000034
+6       2  1            1        1   total  0.000227  0.000010
+7       2  1            2        1   total  0.001210  0.000053
+8       2  1            3        1   total  0.001177  0.000052
+9       2  1            4        1   total  0.002726  0.000119
+10      2  1            5        1   total  0.001208  0.000053
+11      2  1            6        1   total  0.000503  0.000022
+18      2  2            1        1   total  0.000227  0.000019
+19      2  2            2        1   total  0.001211  0.000102
+20      2  2            3        1   total  0.001178  0.000098
+21      2  2            4        1   total  0.002726  0.000223
+22      2  2            5        1   total  0.001207  0.000096
+23      2  2            6        1   total  0.000503  0.000040
+   mesh 1    delayedgroup nuclide      mean std. dev.
+        x  y                                         
+0       1  1            1   total  0.013353  0.001345
+1       1  1            2   total  0.032619  0.003120
+2       1  1            3   total  0.121042  0.011142
+3       1  1            4   total  0.305503  0.026418
+4       1  1            5   total  0.860433  0.064665
+5       1  1            6   total  2.889963  0.219527
+12      1  2            1   total  0.013352  0.001049
+13      1  2            2   total  0.032626  0.002465
+14      1  2            3   total  0.121027  0.008891
+15      1  2            4   total  0.305351  0.021426
+16      1  2            5   total  0.859866  0.054490
+17      1  2            6   total  2.888034  0.184436
+6       2  1            1   total  0.013353  0.000612
+7       2  1            2   total  0.032616  0.001412
+8       2  1            3   total  0.121047  0.005039
+9       2  1            4   total  0.305559  0.012002
+10      2  1            5   total  0.860640  0.030707
+11      2  1            6   total  2.890664  0.103692
+18      2  2            1   total  0.013353  0.001132
+19      2  2            2   total  0.032617  0.002633
+20      2  2            3   total  0.121046  0.009426
+21      2  2            4   total  0.305545  0.022417
+22      2  2            5   total  0.860588  0.055030
+23      2  2            6   total  2.890489  0.186821
+   mesh 1    delayedgroup group in group out nuclide      mean std. dev.
+        x  y                                                            
+0       1  1            1        1         1   total  0.000000  0.000000
+1       1  1            2        1         1   total  0.000000  0.000000
+2       1  1            3        1         1   total  0.000000  0.000000
+3       1  1            4        1         1   total  0.000219  0.000219
+4       1  1            5        1         1   total  0.000000  0.000000
+5       1  1            6        1         1   total  0.000000  0.000000
+12      1  2            1        1         1   total  0.000000  0.000000
+13      1  2            2        1         1   total  0.000000  0.000000
+14      1  2            3        1         1   total  0.000000  0.000000
+15      1  2            4        1         1   total  0.000467  0.000287
+16      1  2            5        1         1   total  0.000000  0.000000
+17      1  2            6        1         1   total  0.000000  0.000000
+6       2  1            1        1         1   total  0.000000  0.000000
+7       2  1            2        1         1   total  0.000000  0.000000
+8       2  1            3        1         1   total  0.000000  0.000000
+9       2  1            4        1         1   total  0.000211  0.000211
+10      2  1            5        1         1   total  0.000000  0.000000
+11      2  1            6        1         1   total  0.000000  0.000000
+18      2  2            1        1         1   total  0.000000  0.000000
+19      2  2            2        1         1   total  0.000000  0.000000
+20      2  2            3        1         1   total  0.000000  0.000000
+21      2  2            4        1         1   total  0.000219  0.000219
+22      2  2            5        1         1   total  0.000000  0.000000
+23      2  2            6        1         1   total  0.000000  0.000000
diff --git a/tests/regression_tests/mgxs_library_condense/test.py b/tests/regression_tests/mgxs_library_condense/test.py
index a7e60617f..bbc4c11bf 100644
--- a/tests/regression_tests/mgxs_library_condense/test.py
+++ b/tests/regression_tests/mgxs_library_condense/test.py
@@ -36,7 +36,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _get_results(self, hash_output=False):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/mgxs_library_correction/inputs_true.dat b/tests/regression_tests/mgxs_library_correction/inputs_true.dat
index ea2d1b714..bb463778f 100644
--- a/tests/regression_tests/mgxs_library_correction/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_correction/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_correction/results_true.dat b/tests/regression_tests/mgxs_library_correction/results_true.dat
index ff4bd9746..b6aaad044 100644
--- a/tests/regression_tests/mgxs_library_correction/results_true.dat
+++ b/tests/regression_tests/mgxs_library_correction/results_true.dat
@@ -1,60 +1,60 @@
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.342252   0.023795
-2         1         1          2   total  0.000695   0.000327
+3         1         1          1   total  0.353477   0.019952
+2         1         1          2   total  0.000522   0.000349
 1         1         2          1   total  0.000000   0.000000
-0         1         2          2   total  0.388426   0.020840
+0         1         2          2   total  0.414134   0.029955
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.342252   0.023795
-2         1         1          2   total  0.000695   0.000327
+3         1         1          1   total  0.353477   0.019952
+2         1         1          2   total  0.000522   0.000349
 1         1         2          1   total  0.000000   0.000000
-0         1         2          2   total  0.388426   0.020840
+0         1         2          2   total  0.414134   0.029955
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.343021   0.031016
-2         1         1          2   total  0.000697   0.000329
+3         1         1          1   total  0.356124   0.026110
+2         1         1          2   total  0.000526   0.000352
 1         1         2          1   total  0.000000   0.000000
-0         1         2          2   total  0.376544   0.025156
+0         1         2          2   total  0.413622   0.044755
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.343021   0.039761
-2         1         1          2   total  0.000697   0.000566
+3         1         1          1   total  0.356124   0.033873
+2         1         1          2   total  0.000526   0.000608
 1         1         2          1   total  0.000000   0.000000
-0         1         2          2   total  0.376544   0.032140
+0         1         2          2   total  0.413622   0.054899
    material  group in  group out nuclide      mean  std. dev.
-3         2         1          1   total  0.271174   0.022374
+3         2         1          1   total  0.261901   0.016456
 2         2         1          2   total  0.000000   0.000000
 1         2         2          1   total  0.000000   0.000000
-0         2         2          2   total  0.295401   0.032831
+0         2         2          2   total  0.322376   0.044195
    material  group in  group out nuclide      mean  std. dev.
-3         2         1          1   total  0.271174   0.022374
+3         2         1          1   total  0.261901   0.016456
 2         2         1          2   total  0.000000   0.000000
 1         2         2          1   total  0.000000   0.000000
-0         2         2          2   total  0.295401   0.032831
+0         2         2          2   total  0.322376   0.044195
    material  group in  group out nuclide      mean  std. dev.
-3         2         1          1   total  0.264654   0.028288
+3         2         1          1   total  0.266612   0.021169
 2         2         1          2   total  0.000000   0.000000
 1         2         2          1   total  0.000000   0.000000
-0         2         2          2   total  0.295178   0.032530
+0         2         2          2   total  0.321966   0.054140
    material  group in  group out nuclide      mean  std. dev.
-3         2         1          1   total  0.264654   0.036081
+3         2         1          1   total  0.266612   0.025384
 2         2         1          2   total  0.000000   0.000000
 1         2         2          1   total  0.000000   0.000000
-0         2         2          2   total  0.295178   0.044676
+0         2         2          2   total  0.321966   0.068014
    material  group in  group out nuclide      mean  std. dev.
-3         3         1          1   total  0.257831   0.014436
-2         3         1          2   total  0.030826   0.000973
-1         3         2          1   total  0.000467   0.000467
-0         3         2          2   total  1.443057   0.119909
+3         3         1          1   total  0.259426   0.012116
+2         3         1          2   total  0.031650   0.000613
+1         3         2          1   total  0.000474   0.000475
+0         3         2          2   total  1.416407   0.162435
    material  group in  group out nuclide      mean  std. dev.
-3         3         1          1   total  0.257831   0.014436
-2         3         1          2   total  0.030826   0.000973
-1         3         2          1   total  0.000467   0.000467
-0         3         2          2   total  1.443057   0.119909
+3         3         1          1   total  0.259426   0.012116
+2         3         1          2   total  0.031650   0.000613
+1         3         2          1   total  0.000474   0.000475
+0         3         2          2   total  1.416407   0.162435
    material  group in  group out nuclide      mean  std. dev.
-3         3         1          1   total  0.267025   0.029512
-2         3         1          2   total  0.031268   0.001416
-1         3         2          1   total  0.000466   0.000467
-0         3         2          2   total  1.440250   0.164573
+3         3         1          1   total  0.266083   0.024503
+2         3         1          2   total  0.031973   0.001157
+1         3         2          1   total  0.000477   0.000480
+0         3         2          2   total  1.427932   0.269812
    material  group in  group out nuclide      mean  std. dev.
-3         3         1          1   total  0.267025   0.032860
-2         3         1          2   total  0.031268   0.001768
-1         3         2          1   total  0.000466   0.000808
-0         3         2          2   total  1.440250   0.212183
+3         3         1          1   total  0.266083   0.027910
+2         3         1          2   total  0.031973   0.001391
+1         3         2          1   total  0.000477   0.000827
+0         3         2          2   total  1.427932   0.328026
diff --git a/tests/regression_tests/mgxs_library_correction/test.py b/tests/regression_tests/mgxs_library_correction/test.py
index 05eedfef8..64e638e44 100644
--- a/tests/regression_tests/mgxs_library_correction/test.py
+++ b/tests/regression_tests/mgxs_library_correction/test.py
@@ -29,7 +29,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _get_results(self, hash_output=False):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/mgxs_library_distribcell/inputs_true.dat b/tests/regression_tests/mgxs_library_distribcell/inputs_true.dat
index f1a605334..e3826cc18 100644
--- a/tests/regression_tests/mgxs_library_distribcell/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_distribcell/inputs_true.dat
@@ -1,38 +1,38 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="Fuel">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="Fuel" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Cladding">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="-1" universe="1"/>
-    <cell id="2" material="2" name="clad" region="1 -2" universe="1"/>
-    <cell id="3" material="3" name="hot water" region="2" universe="1"/>
-    <cell id="4" material="3" name="guide tube inner water" region="-1" universe="2"/>
-    <cell id="5" material="2" name="guide tube clad" region="1 -2" universe="2"/>
-    <cell id="6" material="3" name="guide tube outer water" region="2" universe="2"/>
-    <cell fill="3" id="7" name="root cell" region="3 -4 5 -6" universe="4"/>
+    <cell id="1" name="fuel" material="1" region="-1" universe="1"/>
+    <cell id="2" name="clad" material="2" region="1 -2" universe="1"/>
+    <cell id="3" name="hot water" material="3" region="2" universe="1"/>
+    <cell id="4" name="guide tube inner water" material="3" region="-1" universe="2"/>
+    <cell id="5" name="guide tube clad" material="2" region="1 -2" universe="2"/>
+    <cell id="6" name="guide tube outer water" material="3" region="2" universe="2"/>
+    <cell id="7" name="root cell" fill="3" region="3 -4 5 -6" universe="4"/>
     <lattice id="3" name="Fuel Assembly">
       <pitch>1.26 1.26</pitch>
       <dimension>17 17</dimension>
@@ -56,19 +56,19 @@
 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 
 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 </universes>
     </lattice>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-10.71" id="3" type="x-plane"/>
-    <surface boundary="reflective" coeffs="10.71" id="4" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-10.71" id="5" type="y-plane"/>
-    <surface boundary="reflective" coeffs="10.71" id="6" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" type="x-plane" boundary="reflective" coeffs="-10.71"/>
+    <surface id="4" type="x-plane" boundary="reflective" coeffs="10.71"/>
+    <surface id="5" type="y-plane" boundary="reflective" coeffs="-10.71"/>
+    <surface id="6" type="y-plane" boundary="reflective" coeffs="10.71"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-10.71 -10.71 -1 10.71 10.71 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_distribcell/results_true.dat b/tests/regression_tests/mgxs_library_distribcell/results_true.dat
index ef7cb2b96..127df75c1 100644
--- a/tests/regression_tests/mgxs_library_distribcell/results_true.dat
+++ b/tests/regression_tests/mgxs_library_distribcell/results_true.dat
@@ -1,97 +1,97 @@
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.455527   0.009851
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.459656   0.010039
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.409242   0.011118
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide     mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.40929   0.011119
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.416327    0.01121
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.066934   0.002424
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.416327    0.01121
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.066764   0.002423
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.070545   0.002486
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.028358   0.002669
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.070348   0.002485
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.038576   0.001526
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.029374   0.002719
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.094817   0.003725
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.041172   0.001562
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide      mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.101218   0.003812
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide          mean      std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  7.470225e+06  295170.385185
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  7.972654e+06  302079.851251
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide     mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.38911    0.00831
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.388593   0.008156
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.388874   0.013889
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.394876   0.014019
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in  group out legendre nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.388711   0.013887
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.046285   0.005155
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.023632   0.003772
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.006997   0.003207
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.394876   0.014019
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.043329   0.004988
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.027490   0.003974
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.016004   0.003232
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in  group out legendre nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.388874   0.013889
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.046237   0.005156
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.023571   0.003775
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.007058   0.003207
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total  1.000418   0.036246
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total  0.092139   0.005956
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.394876   0.014019
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.043329   0.004988
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.027490   0.003974
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.016004   0.003232
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in  group out nuclide  mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total   1.0   0.036242
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out legendre nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.388593   0.016275
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.046271   0.005251
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.023624   0.003806
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.006995   0.003209
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out legendre nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.388755   0.021528
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.046290   0.005515
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.023634   0.003903
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.006998   0.003221
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group out nuclide  mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)          1   total   1.0   0.084366
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group out nuclide  mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)          1   total   1.0   0.084331
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide          mean     std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  5.253873e-07  2.168462e-08
-                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.094147   0.003701
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total   1.0   0.036306
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in  group out nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total  0.091593   0.005919
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total  0.097856   0.006191
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out nuclide  mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total   1.0   0.036306
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out legendre nuclide      mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.389110   0.016390
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.042696   0.005009
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.027088   0.003964
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.015770   0.003204
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out legendre nuclide      mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P0   total  0.389110   0.021638
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P1   total  0.042696   0.005244
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P2   total  0.027088   0.004084
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1       P3   total  0.015770   0.003255
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group out nuclide  mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)          1   total   1.0   0.082469
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group out nuclide  mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)          1   total   1.0   0.082587
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide          mean     std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  5.626624e-07  2.235532e-08
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.814514   0.022129
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.100506   0.003787
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in  group out nuclide      mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1          1   total  0.097658   0.006185
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.814419   0.022125
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.800653   0.021558
+                                                                                                                                                                                                                                                                                                                                                                                                 sum(distribcell)  group in nuclide      mean  std. dev.
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)         1   total  0.800653   0.021558
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  delayedgroup  group in nuclide      mean     std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1         1   total  0.000021  8.473275e-07
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2         1   total  0.000115  4.405467e-06
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3         1   total  0.000112  4.225826e-06
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4         1   total  0.000259  9.559952e-06
-4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5         1   total  0.000115  4.025369e-06
-5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6         1   total  0.000048  1.682226e-06
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1         1   total  0.000023  8.667436e-07
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2         1   total  0.000122  4.499059e-06
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3         1   total  0.000119  4.311220e-06
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4         1   total  0.000275  9.735290e-06
+4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5         1   total  0.000122  4.078954e-06
+5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6         1   total  0.000051  1.705327e-06
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  delayedgroup  group out nuclide  mean  std. dev.
 0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1          1   total   0.0   0.000000
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2          1   total   0.0   0.000000
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3          1   total   1.0   1.000002
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4          1   total   1.0   1.414214
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2          1   total   1.0   1.414214
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3          1   total   0.0   0.000000
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4          1   total   0.0   0.000000
 4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5          1   total   0.0   0.000000
 5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6          1   total   0.0   0.000000
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  delayedgroup  group in nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1         1   total  0.000227   0.000012
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2         1   total  0.001210   0.000062
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3         1   total  0.001177   0.000060
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4         1   total  0.002728   0.000136
-4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5         1   total  0.001211   0.000059
-5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6         1   total  0.000504   0.000025
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1         1   total  0.000227   0.000011
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2         1   total  0.001208   0.000059
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3         1   total  0.001175   0.000056
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4         1   total  0.002721   0.000129
+4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5         1   total  0.001206   0.000055
+5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6         1   total  0.000502   0.000023
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  delayedgroup nuclide      mean  std. dev.
-0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1   total  0.013353   0.000692
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2   total  0.032613   0.001644
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3   total  0.121054   0.005983
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4   total  0.305630   0.014645
-4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5   total  0.860903   0.038693
-5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6   total  2.891558   0.130564
+0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1   total  0.013353   0.000658
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2   total  0.032616   0.001562
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3   total  0.121048   0.005678
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4   total  0.305568   0.013868
+4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5   total  0.860675   0.036434
+5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6   total  2.890786   0.122997
                                                                                                                                                                                                                                                                                                                                                                                                  sum(distribcell)  delayedgroup  group in  group out nuclide      mean  std. dev.
 0  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             1         1          1   total  0.000000   0.000000
-1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2         1          1   total  0.000000   0.000000
-2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3         1          1   total  0.000362   0.000256
-3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4         1          1   total  0.000185   0.000185
+1  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             2         1          1   total  0.000198   0.000198
+2  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             3         1          1   total  0.000000   0.000000
+3  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             4         1          1   total  0.000000   0.000000
 4  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             5         1          1   total  0.000000   0.000000
 5  ((0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...),)             6         1          1   total  0.000000   0.000000
diff --git a/tests/regression_tests/mgxs_library_distribcell/test.py b/tests/regression_tests/mgxs_library_distribcell/test.py
index fd6c8e938..464b309c0 100644
--- a/tests/regression_tests/mgxs_library_distribcell/test.py
+++ b/tests/regression_tests/mgxs_library_distribcell/test.py
@@ -36,7 +36,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
         self._model.tallies.export_to_xml()
 
     def _get_results(self, hash_output=False):
diff --git a/tests/regression_tests/mgxs_library_hdf5/inputs_true.dat b/tests/regression_tests/mgxs_library_hdf5/inputs_true.dat
index 8606e9fdd..4451d214c 100644
--- a/tests/regression_tests/mgxs_library_hdf5/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_hdf5/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_hdf5/results_true.dat b/tests/regression_tests/mgxs_library_hdf5/results_true.dat
index b2ef8eae9..14d7371fe 100644
--- a/tests/regression_tests/mgxs_library_hdf5/results_true.dat
+++ b/tests/regression_tests/mgxs_library_hdf5/results_true.dat
@@ -1,201 +1,201 @@
 domain=1 type=total
-[5.52629207e-01 1.46792426e+00]
-[2.73983948e-02 9.30468557e-02]
+[5.66580451e-01 1.44262943e+00]
+[1.96770003e-02 1.46112369e-01]
 domain=1 type=transport
-[3.09650231e-01 1.14528468e+00]
-[2.96126059e-02 1.01957125e-01]
+[3.14856281e-01 1.05346368e+00]
+[2.15203447e-02 1.60562179e-01]
 domain=1 type=nu-transport
-[3.09650231e-01 1.14528468e+00]
-[2.96126059e-02 1.01957125e-01]
+[3.14856281e-01 1.05346368e+00]
+[2.15203447e-02 1.60562179e-01]
 domain=1 type=absorption
-[7.90251362e-03 9.52195503e-02]
-[7.77996866e-04 5.32017792e-03]
+[8.63921263e-03 9.70718967e-02]
+[7.09849180e-04 9.96697703e-03]
 domain=1 type=reduced absorption
-[7.88836875e-03 9.52195503e-02]
-[7.77877367e-04 5.32017792e-03]
+[8.63459183e-03 9.70718967e-02]
+[7.09826160e-04 9.96697703e-03]
 domain=1 type=capture
-[5.66271991e-03 4.03380329e-02]
-[7.65494411e-04 4.41129849e-03]
+[6.29732743e-03 4.01344183e-02]
+[7.03181817e-04 9.43458504e-03]
 domain=1 type=fission
-[2.23979371e-03 5.48815174e-02]
-[1.44808204e-04 3.21965557e-03]
+[2.34188519e-03 5.69374784e-02]
+[1.04006997e-04 6.19904834e-03]
 domain=1 type=nu-fission
-[5.70078167e-03 1.33729794e-01]
-[3.71908758e-04 7.84533474e-03]
+[5.93985124e-03 1.38739554e-01]
+[2.57215008e-04 1.51052211e-02]
 domain=1 type=kappa-fission
-[4.36283087e+05 1.06143820e+07]
-[2.81939099e+04 6.22698786e+05]
+[4.55876276e+05 1.10120160e+07]
+[2.00450394e+04 1.19892945e+06]
 domain=1 type=scatter
-[5.44726693e-01 1.37270471e+00]
-[2.67443307e-02 8.86578418e-02]
+[5.57941239e-01 1.34555753e+00]
+[1.96103602e-02 1.38008873e-01]
 domain=1 type=nu-scatter
-[5.46058885e-01 1.38608802e+00]
-[2.72733439e-02 9.59957737e-02]
+[5.53883536e-01 1.40126963e+00]
+[1.89917740e-02 1.62647765e-01]
 domain=1 type=scatter matrix
-[[[5.25081070e-01 2.42978975e-01 9.65459393e-02 8.62265123e-03]
-  [2.09778151e-02 5.67750504e-03 -1.86093418e-03 -1.63034937e-03]]
+[[[5.35878034e-01 2.51724170e-01 1.01011269e-01 1.03439439e-02]
+  [1.80055019e-02 5.80562809e-03 -1.57470166e-03 -2.27320020e-03]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [1.38608802e+00 2.92665071e-01 4.65545679e-02 3.43129413e-03]]]
-[[[2.68584158e-02 1.12354079e-02 5.95151250e-03 5.08213314e-03]
-  [1.69810442e-03 1.03379654e-03 3.35798077e-04 8.28775769e-04]]
+  [1.40126963e+00 3.55339640e-01 7.06453615e-02 4.04065595e-02]]]
+[[[1.86010787e-02 8.71440749e-03 3.01351835e-03 5.22968320e-03]
+  [1.36795874e-03 6.67372701e-04 3.26122899e-04 8.68935369e-04]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [9.59957737e-02 3.98634629e-02 1.79245433e-02 2.53788407e-02]]]
+  [1.62647765e-01 6.23420543e-02 9.61788834e-03 8.80966610e-03]]]
 domain=1 type=nu-scatter matrix
-[[[5.25081070e-01 2.42978975e-01 9.65459393e-02 8.62265123e-03]
-  [2.09778151e-02 5.67750504e-03 -1.86093418e-03 -1.63034937e-03]]
+[[[5.35878034e-01 2.51724170e-01 1.01011269e-01 1.03439439e-02]
+  [1.80055019e-02 5.80562809e-03 -1.57470166e-03 -2.27320020e-03]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [1.38608802e+00 2.92665071e-01 4.65545679e-02 3.43129413e-03]]]
-[[[2.68584158e-02 1.12354079e-02 5.95151250e-03 5.08213314e-03]
-  [1.69810442e-03 1.03379654e-03 3.35798077e-04 8.28775769e-04]]
+  [1.40126963e+00 3.55339640e-01 7.06453615e-02 4.04065595e-02]]]
+[[[1.86010787e-02 8.71440749e-03 3.01351835e-03 5.22968320e-03]
+  [1.36795874e-03 6.67372701e-04 3.26122899e-04 8.68935369e-04]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [9.59957737e-02 3.98634629e-02 1.79245433e-02 2.53788407e-02]]]
+  [1.62647765e-01 6.23420543e-02 9.61788834e-03 8.80966610e-03]]]
 domain=1 type=multiplicity matrix
 [[1.00000000e+00 1.00000000e+00]
  [0.00000000e+00 1.00000000e+00]]
-[[4.49973217e-02 9.94834121e-02]
- [0.00000000e+00 8.90267353e-02]]
+[[3.27047397e-02 1.01015254e-01]
+ [0.00000000e+00 1.16966513e-01]]
 domain=1 type=nu-fission matrix
-[[6.79725552e-03 0.00000000e+00]
- [1.34226308e-01 0.00000000e+00]]
-[[1.39232734e-03 0.00000000e+00]
- [1.55126210e-02 0.00000000e+00]]
+[[7.11392182e-03 0.00000000e+00]
+ [1.52683850e-01 0.00000000e+00]]
+[[1.05605314e-03 0.00000000e+00]
+ [2.58713491e-02 0.00000000e+00]]
 domain=1 type=scatter probability matrix
-[[9.61583236e-01 3.84167637e-02]
+[[9.67492260e-01 3.25077399e-02]
  [0.00000000e+00 1.00000000e+00]]
-[[4.25251594e-02 2.94881301e-03]
- [0.00000000e+00 8.90267353e-02]]
+[[3.12124830e-02 2.43439942e-03]
+ [0.00000000e+00 1.16966513e-01]]
 domain=1 type=consistent scatter matrix
-[[[5.23800056e-01 2.42386192e-01 9.63104011e-02 8.60161499e-03]
-  [2.09266366e-02 5.66365392e-03 -1.85639416e-03 -1.62637189e-03]]
+[[[5.39803830e-01 2.53568280e-01 1.01751269e-01 1.04197228e-02]
+  [1.81374087e-02 5.84815962e-03 -1.58623779e-03 -2.28985347e-03]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [1.37270471e+00 2.89839256e-01 4.61050623e-02 3.39816342e-03]]]
-[[[3.46115177e-02 1.51137129e-02 7.17487890e-03 5.08248710e-03]
-  [1.90677851e-03 1.05813829e-03 3.43862086e-04 8.29548315e-04]]
+  [1.34555753e+00 3.41211940e-01 6.78366221e-02 3.88000634e-02]]]
+[[[2.57534994e-02 1.20804286e-02 4.50621873e-03 5.27902298e-03]
+  [1.50041314e-03 6.98980912e-04 3.32589287e-04 8.78503928e-04]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [1.50979688e-01 4.66032426e-02 1.81833349e-02 2.51354735e-02]]]
+  [2.09324012e-01 6.95175402e-02 1.16044952e-02 9.36549883e-03]]]
 domain=1 type=consistent nu-scatter matrix
-[[[5.23800056e-01 2.42386192e-01 9.63104011e-02 8.60161499e-03]
-  [2.09266366e-02 5.66365392e-03 -1.85639416e-03 -1.62637189e-03]]
+[[[5.39803830e-01 2.53568280e-01 1.01751269e-01 1.04197228e-02]
+  [1.81374087e-02 5.84815962e-03 -1.58623779e-03 -2.28985347e-03]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [1.37270471e+00 2.89839256e-01 4.61050623e-02 3.39816342e-03]]]
-[[[4.18746126e-02 1.86381616e-02 8.38211969e-03 5.09720340e-03]
-  [2.82310416e-03 1.19879975e-03 3.90317811e-04 8.45179676e-04]]
+  [1.34555753e+00 3.41211940e-01 6.78366221e-02 3.88000634e-02]]]
+[[[3.12235732e-02 1.46529414e-02 5.60177820e-03 5.29001047e-03]
+  [2.36812824e-03 9.15185125e-04 3.69175618e-04 9.08445666e-04]]
 
  [[0.00000000e+00 0.00000000e+00 0.00000000e+00 0.00000000e+00]
-  [1.94240879e-01 5.32698778e-02 1.86408495e-02 2.51372940e-02]]]
+  [2.61890501e-01 8.01593794e-02 1.40578233e-02 1.04071518e-02]]]
 domain=1 type=chi
 [1.00000000e+00 0.00000000e+00]
-[1.03333203e-01 0.00000000e+00]
+[1.42429813e-01 0.00000000e+00]
 domain=1 type=chi-prompt
 [1.00000000e+00 0.00000000e+00]
-[1.03333203e-01 0.00000000e+00]
+[1.43958515e-01 0.00000000e+00]
 domain=1 type=inverse-velocity
-[5.72461488e-08 3.00999757e-06]
-[2.80644407e-09 1.80993426e-07]
+[6.05275939e-08 2.92408191e-06]
+[4.98534008e-09 2.95326306e-07]
 domain=1 type=prompt-nu-fission
-[5.64594959e-03 1.32859920e-01]
-[3.68364598e-04 7.79430310e-03]
+[5.88433433e-03 1.37837093e-01]
+[2.56012352e-04 1.50069660e-02]
 domain=1 type=prompt-nu-fission matrix
-[[6.79725552e-03 0.00000000e+00]
- [1.34226308e-01 0.00000000e+00]]
-[[1.39232734e-03 0.00000000e+00]
- [1.55126210e-02 0.00000000e+00]]
+[[7.11392182e-03 0.00000000e+00]
+ [1.51190909e-01 0.00000000e+00]]
+[[1.05605314e-03 0.00000000e+00]
+ [2.57973847e-02 0.00000000e+00]]
 domain=1 type=current
-[[[0.00000000e+00 0.00000000e+00 3.54200000e+00 3.59000000e+00
-   0.00000000e+00 0.00000000e+00 3.73400000e+00 3.69000000e+00]
-  [0.00000000e+00 0.00000000e+00 7.08000000e-01 6.90000000e-01
-   0.00000000e+00 0.00000000e+00 6.78000000e-01 6.74000000e-01]]
+[[[0.00000000e+00 0.00000000e+00 3.71800000e+00 3.58600000e+00
+   0.00000000e+00 0.00000000e+00 3.62200000e+00 3.71800000e+00]
+  [0.00000000e+00 0.00000000e+00 6.60000000e-01 6.58000000e-01
+   0.00000000e+00 0.00000000e+00 6.62000000e-01 6.70000000e-01]]
 
- [[3.59000000e+00 3.54200000e+00 0.00000000e+00 0.00000000e+00
-   0.00000000e+00 0.00000000e+00 3.65200000e+00 3.73800000e+00]
-  [6.90000000e-01 7.08000000e-01 0.00000000e+00 0.00000000e+00
-   0.00000000e+00 0.00000000e+00 6.94000000e-01 6.64000000e-01]]
+ [[3.58600000e+00 3.71800000e+00 0.00000000e+00 0.00000000e+00
+   0.00000000e+00 0.00000000e+00 3.71200000e+00 3.60600000e+00]
+  [6.58000000e-01 6.60000000e-01 0.00000000e+00 0.00000000e+00
+   0.00000000e+00 0.00000000e+00 7.04000000e-01 6.74000000e-01]]
 
- [[0.00000000e+00 0.00000000e+00 3.76000000e+00 3.66600000e+00
-   3.69000000e+00 3.73400000e+00 0.00000000e+00 0.00000000e+00]
-  [0.00000000e+00 0.00000000e+00 6.68000000e-01 7.22000000e-01
-   6.74000000e-01 6.78000000e-01 0.00000000e+00 0.00000000e+00]]
+ [[0.00000000e+00 0.00000000e+00 3.48600000e+00 3.60600000e+00
+   3.71800000e+00 3.62200000e+00 0.00000000e+00 0.00000000e+00]
+  [0.00000000e+00 0.00000000e+00 6.66000000e-01 6.74000000e-01
+   6.70000000e-01 6.62000000e-01 0.00000000e+00 0.00000000e+00]]
 
- [[3.66600000e+00 3.76000000e+00 0.00000000e+00 0.00000000e+00
-   3.73800000e+00 3.65200000e+00 0.00000000e+00 0.00000000e+00]
-  [7.22000000e-01 6.68000000e-01 0.00000000e+00 0.00000000e+00
-   6.64000000e-01 6.94000000e-01 0.00000000e+00 0.00000000e+00]]]
-[[[0.00000000e+00 0.00000000e+00 1.04230514e-01 1.61183126e-01
-   0.00000000e+00 0.00000000e+00 1.50419414e-01 1.00498756e-01]
-  [0.00000000e+00 0.00000000e+00 2.47790234e-02 3.27108545e-02
-   0.00000000e+00 0.00000000e+00 4.84148737e-02 3.24961536e-02]]
+ [[3.60600000e+00 3.48600000e+00 0.00000000e+00 0.00000000e+00
+   3.60600000e+00 3.71200000e+00 0.00000000e+00 0.00000000e+00]
+  [6.74000000e-01 6.66000000e-01 0.00000000e+00 0.00000000e+00
+   6.74000000e-01 7.04000000e-01 0.00000000e+00 0.00000000e+00]]]
+[[[0.00000000e+00 0.00000000e+00 9.96192752e-02 8.73269718e-02
+   0.00000000e+00 0.00000000e+00 1.22531629e-01 1.08369737e-01]
+  [0.00000000e+00 0.00000000e+00 3.96232255e-02 4.06693988e-02
+   0.00000000e+00 0.00000000e+00 4.05462699e-02 4.27784993e-02]]
 
- [[1.61183126e-01 1.04230514e-01 0.00000000e+00 0.00000000e+00
-   0.00000000e+00 0.00000000e+00 1.44201248e-01 1.81229137e-01]
-  [3.27108545e-02 2.47790234e-02 0.00000000e+00 0.00000000e+00
-   0.00000000e+00 0.00000000e+00 3.41467422e-02 2.20454077e-02]]
+ [[8.73269718e-02 9.96192752e-02 0.00000000e+00 0.00000000e+00
+   0.00000000e+00 0.00000000e+00 7.09506871e-02 5.27825729e-02]
+  [4.06693988e-02 3.96232255e-02 0.00000000e+00 0.00000000e+00
+   0.00000000e+00 0.00000000e+00 6.11228272e-02 5.88727441e-02]]
 
- [[0.00000000e+00 0.00000000e+00 1.39355660e-01 9.70875893e-02
-   1.00498756e-01 1.50419414e-01 0.00000000e+00 0.00000000e+00]
-  [0.00000000e+00 0.00000000e+00 1.56204994e-02 3.27719392e-02
-   3.24961536e-02 4.84148737e-02 0.00000000e+00 0.00000000e+00]]
+ [[0.00000000e+00 0.00000000e+00 1.00279609e-01 1.31209756e-01
+   1.08369737e-01 1.22531629e-01 0.00000000e+00 0.00000000e+00]
+  [0.00000000e+00 0.00000000e+00 6.05475020e-02 4.87442304e-02
+   4.27784993e-02 4.05462699e-02 0.00000000e+00 0.00000000e+00]]
 
- [[9.70875893e-02 1.39355660e-01 0.00000000e+00 0.00000000e+00
-   1.81229137e-01 1.44201248e-01 0.00000000e+00 0.00000000e+00]
-  [3.27719392e-02 1.56204994e-02 0.00000000e+00 0.00000000e+00
-   2.20454077e-02 3.41467422e-02 0.00000000e+00 0.00000000e+00]]]
+ [[1.31209756e-01 1.00279609e-01 0.00000000e+00 0.00000000e+00
+   5.27825729e-02 7.09506871e-02 0.00000000e+00 0.00000000e+00]
+  [4.87442304e-02 6.05475020e-02 0.00000000e+00 0.00000000e+00
+   5.88727441e-02 6.11228272e-02 0.00000000e+00 0.00000000e+00]]]
 domain=1 type=diffusion-coefficient
-[1.07648340e+00 2.91048451e-01]
-[1.02946730e-01 2.59101197e-02]
+[1.05868408e+00 3.16416542e-01]
+[7.23607812e-02 4.82261806e-02]
 domain=1 type=nu-diffusion-coefficient
-[1.07648340e+00 2.91048451e-01]
-[1.02946730e-01 2.59101197e-02]
+[1.05868408e+00 3.16416542e-01]
+[7.23607812e-02 4.82261806e-02]
 domain=1 type=delayed-nu-fission
-[[1.27862358e-06 3.04521075e-05]
- [7.84219053e-06 1.57184471e-04]
- [8.19703020e-06 1.50062017e-04]
- [2.11585246e-05 3.36451683e-04]
- [1.15983232e-05 1.37940708e-04]
- [4.75823112e-06 5.77828684e-05]]
-[[8.24665595e-08 1.78649023e-06]
- [5.11270396e-07 9.22131636e-06]
- [5.43215007e-07 8.80347339e-06]
- [1.44919046e-06 1.97381285e-05]
- [8.56858673e-07 8.09236929e-06]
- [3.49663884e-07 3.38986452e-06]]
+[[1.33370452e-06 3.15928985e-05]
+ [8.06563789e-06 1.63072885e-04]
+ [8.37555675e-06 1.55683610e-04]
+ [2.14225848e-05 3.49055766e-04]
+ [1.15633419e-05 1.43108212e-04]
+ [4.74849054e-06 5.99475175e-05]]
+[[5.58190638e-08 3.43966581e-06]
+ [2.80924725e-07 1.77545031e-05]
+ [2.81900859e-07 1.69499982e-05]
+ [7.44207592e-07 3.80033227e-05]
+ [4.95821820e-07 1.55808550e-05]
+ [2.00315983e-07 6.52676436e-06]]
 domain=1 type=chi-delayed
 [[0.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]
- [0.00000000e+00 0.00000000e+00]
+ [1.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]]
 [[0.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]
- [0.00000000e+00 0.00000000e+00]
+ [1.41421356e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]
  [0.00000000e+00 0.00000000e+00]]
 domain=1 type=beta
-[[2.24289169e-04 2.27713711e-04]
- [1.37563425e-03 1.17538857e-03]
- [1.43787829e-03 1.12212853e-03]
- [3.71151288e-03 2.51590669e-03]
- [2.03451453e-03 1.03148823e-03]
- [8.34662928e-04 4.32086724e-04]]
-[[1.75760869e-05 1.28957660e-05]
- [1.08591736e-04 6.65640010e-05]
- [1.14785004e-04 6.35478047e-05]
- [3.03169937e-04 1.42479528e-04]
- [1.75476030e-04 5.84147049e-05]
- [7.17094876e-05 2.44697107e-05]]
+[[2.24535003e-04 2.27713710e-04]
+ [1.35788550e-03 1.17538857e-03]
+ [1.41006170e-03 1.12212852e-03]
+ [3.60658609e-03 2.51590665e-03]
+ [1.94673931e-03 1.03148820e-03]
+ [7.99429201e-04 4.32086711e-04]]
+[[1.15072629e-05 2.77827007e-05]
+ [6.20475481e-05 1.43405806e-04]
+ [6.31808205e-05 1.36907698e-04]
+ [1.64551758e-04 3.06958585e-04]
+ [1.01406915e-04 1.25848924e-04]
+ [4.11875799e-05 5.27176635e-05]]
 domain=1 type=decay-rate
-[1.33535692e-02 3.26115957e-02 1.21057117e-01 3.05655911e-01
- 8.60999995e-01 2.89188863e+00]
-[6.74373067e-04 1.71978136e-03 6.57917554e-03 1.73982053e-02
- 5.37683207e-02 1.79407115e-01]
+[1.33525569e-02 3.26187089e-02 1.21041926e-01 3.05503129e-01
+ 8.60433350e-01 2.88996258e+00]
+[1.34484408e-03 3.11955840e-03 1.11418058e-02 2.64184722e-02
+ 6.46651447e-02 2.19526509e-01]
 domain=1 type=delayed-nu-fission matrix
 [[[0.00000000e+00 0.00000000e+00]
   [0.00000000e+00 0.00000000e+00]]
@@ -207,7 +207,7 @@ domain=1 type=delayed-nu-fission matrix
   [0.00000000e+00 0.00000000e+00]]
 
  [[0.00000000e+00 0.00000000e+00]
-  [0.00000000e+00 0.00000000e+00]]
+  [1.49294023e-03 0.00000000e+00]]
 
  [[0.00000000e+00 0.00000000e+00]
   [0.00000000e+00 0.00000000e+00]]
@@ -224,7 +224,7 @@ domain=1 type=delayed-nu-fission matrix
   [0.00000000e+00 0.00000000e+00]]
 
  [[0.00000000e+00 0.00000000e+00]
-  [0.00000000e+00 0.00000000e+00]]
+  [1.49788268e-03 0.00000000e+00]]
 
  [[0.00000000e+00 0.00000000e+00]
   [0.00000000e+00 0.00000000e+00]]
diff --git a/tests/regression_tests/mgxs_library_hdf5/test.py b/tests/regression_tests/mgxs_library_hdf5/test.py
index 06625c25f..4fb4bf093 100644
--- a/tests/regression_tests/mgxs_library_hdf5/test.py
+++ b/tests/regression_tests/mgxs_library_hdf5/test.py
@@ -40,7 +40,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _get_results(self, hash_output=False):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/mgxs_library_histogram/inputs_true.dat b/tests/regression_tests/mgxs_library_histogram/inputs_true.dat
index 013fcd0fa..94c656418 100644
--- a/tests/regression_tests/mgxs_library_histogram/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_histogram/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_histogram/results_true.dat b/tests/regression_tests/mgxs_library_histogram/results_true.dat
index 2fa94cef5..f927d057e 100644
--- a/tests/regression_tests/mgxs_library_histogram/results_true.dat
+++ b/tests/regression_tests/mgxs_library_histogram/results_true.dat
@@ -1,18 +1,18 @@
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         1         1          1       1   total  0.032861   0.003703
-34         1         1          1       2   total  0.026428   0.003637
-35         1         1          1       3   total  0.029210   0.001648
-36         1         1          1       4   total  0.032513   0.004770
-37         1         1          1       5   total  0.030949   0.003129
-38         1         1          1       6   total  0.027124   0.004029
-39         1         1          1       7   total  0.030079   0.002186
-40         1         1          1       8   total  0.037034   0.002198
-41         1         1          1       9   total  0.038251   0.003825
-42         1         1          1      10   total  0.039294   0.003626
-43         1         1          1      11   total  0.062593   0.005934
-22         1         1          2       1   total  0.000000   0.000000
-23         1         1          2       2   total  0.000174   0.000174
-24         1         1          2       3   total  0.000348   0.000213
+33         1         1          1       1   total  0.029945   0.003043
+34         1         1          1       2   total  0.028378   0.003793
+35         1         1          1       3   total  0.033079   0.002866
+36         1         1          1       4   total  0.030119   0.002259
+37         1         1          1       5   total  0.033601   0.003739
+38         1         1          1       6   total  0.035516   0.001929
+39         1         1          1       7   total  0.032382   0.001744
+40         1         1          1       8   total  0.031860   0.002565
+41         1         1          1       9   total  0.038302   0.004757
+42         1         1          1      10   total  0.041784   0.003047
+43         1         1          1      11   total  0.057453   0.003686
+22         1         1          2       1   total  0.000174   0.000174
+23         1         1          2       2   total  0.000000   0.000000
+24         1         1          2       3   total  0.000174   0.000174
 25         1         1          2       4   total  0.000174   0.000174
 26         1         1          2       5   total  0.000000   0.000000
 27         1         1          2       6   total  0.000000   0.000000
@@ -32,32 +32,32 @@
 19         1         2          1       9   total  0.000000   0.000000
 20         1         2          1      10   total  0.000000   0.000000
 21         1         2          1      11   total  0.000000   0.000000
-0          1         2          2       1   total  0.032383   0.006826
-1          1         2          2       2   total  0.037146   0.001158
-2          1         2          2       3   total  0.036193   0.004463
-3          1         2          2       4   total  0.036193   0.007028
-4          1         2          2       5   total  0.038098   0.006769
-5          1         2          2       6   total  0.024764   0.004638
-6          1         2          2       7   total  0.034288   0.008049
-7          1         2          2       8   total  0.040003   0.007350
-8          1         2          2       9   total  0.032383   0.003211
-9          1         2          2      10   total  0.050480   0.005824
-10         1         2          2      11   total  0.045718   0.013291
+0          1         2          2       1   total  0.037212   0.004892
+1          1         2          2       2   total  0.039224   0.003682
+2          1         2          2       3   total  0.044253   0.005869
+3          1         2          2       4   total  0.043247   0.004970
+4          1         2          2       5   total  0.025144   0.006632
+5          1         2          2       6   total  0.037212   0.009199
+6          1         2          2       7   total  0.035201   0.008964
+7          1         2          2       8   total  0.041235   0.009733
+8          1         2          2       9   total  0.032184   0.002384
+9          1         2          2      10   total  0.026149   0.006714
+10         1         2          2      11   total  0.035201   0.008073
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         1         1          1       1   total  0.032861   0.003703
-34         1         1          1       2   total  0.026428   0.003637
-35         1         1          1       3   total  0.029210   0.001648
-36         1         1          1       4   total  0.032513   0.004770
-37         1         1          1       5   total  0.030949   0.003129
-38         1         1          1       6   total  0.027124   0.004029
-39         1         1          1       7   total  0.030079   0.002186
-40         1         1          1       8   total  0.037034   0.002198
-41         1         1          1       9   total  0.038251   0.003825
-42         1         1          1      10   total  0.039294   0.003626
-43         1         1          1      11   total  0.062593   0.005934
-22         1         1          2       1   total  0.000000   0.000000
-23         1         1          2       2   total  0.000174   0.000174
-24         1         1          2       3   total  0.000348   0.000213
+33         1         1          1       1   total  0.029945   0.003043
+34         1         1          1       2   total  0.028378   0.003793
+35         1         1          1       3   total  0.033079   0.002866
+36         1         1          1       4   total  0.030119   0.002259
+37         1         1          1       5   total  0.033601   0.003739
+38         1         1          1       6   total  0.035516   0.001929
+39         1         1          1       7   total  0.032382   0.001744
+40         1         1          1       8   total  0.031860   0.002565
+41         1         1          1       9   total  0.038302   0.004757
+42         1         1          1      10   total  0.041784   0.003047
+43         1         1          1      11   total  0.057453   0.003686
+22         1         1          2       1   total  0.000174   0.000174
+23         1         1          2       2   total  0.000000   0.000000
+24         1         1          2       3   total  0.000174   0.000174
 25         1         1          2       4   total  0.000174   0.000174
 26         1         1          2       5   total  0.000000   0.000000
 27         1         1          2       6   total  0.000000   0.000000
@@ -77,33 +77,33 @@
 19         1         2          1       9   total  0.000000   0.000000
 20         1         2          1      10   total  0.000000   0.000000
 21         1         2          1      11   total  0.000000   0.000000
-0          1         2          2       1   total  0.032383   0.006826
-1          1         2          2       2   total  0.037146   0.001158
-2          1         2          2       3   total  0.036193   0.004463
-3          1         2          2       4   total  0.036193   0.007028
-4          1         2          2       5   total  0.038098   0.006769
-5          1         2          2       6   total  0.024764   0.004638
-6          1         2          2       7   total  0.034288   0.008049
-7          1         2          2       8   total  0.040003   0.007350
-8          1         2          2       9   total  0.032383   0.003211
-9          1         2          2      10   total  0.050480   0.005824
-10         1         2          2      11   total  0.045718   0.013291
+0          1         2          2       1   total  0.037212   0.004892
+1          1         2          2       2   total  0.039224   0.003682
+2          1         2          2       3   total  0.044253   0.005869
+3          1         2          2       4   total  0.043247   0.004970
+4          1         2          2       5   total  0.025144   0.006632
+5          1         2          2       6   total  0.037212   0.009199
+6          1         2          2       7   total  0.035201   0.008964
+7          1         2          2       8   total  0.041235   0.009733
+8          1         2          2       9   total  0.032184   0.002384
+9          1         2          2      10   total  0.026149   0.006714
+10         1         2          2      11   total  0.035201   0.008073
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         1         1          1       1   total  0.032927   0.003848
-34         1         1          1       2   total  0.026481   0.003736
-35         1         1          1       3   total  0.029268   0.001884
-36         1         1          1       4   total  0.032578   0.004885
-37         1         1          1       5   total  0.031010   0.003280
-38         1         1          1       6   total  0.027177   0.004124
-39         1         1          1       7   total  0.030139   0.002382
-40         1         1          1       8   total  0.037108   0.002485
-41         1         1          1       9   total  0.038327   0.004013
-42         1         1          1      10   total  0.039372   0.003833
-43         1         1          1      11   total  0.062717   0.006256
-22         1         1          2       1   total  0.000000   0.000000
-23         1         1          2       2   total  0.000174   0.000174
-24         1         1          2       3   total  0.000348   0.000214
-25         1         1          2       4   total  0.000174   0.000174
+33         1         1          1       1   total  0.030147   0.003165
+34         1         1          1       2   total  0.028570   0.003893
+35         1         1          1       3   total  0.033302   0.003016
+36         1         1          1       4   total  0.030322   0.002411
+37         1         1          1       5   total  0.033828   0.003868
+38         1         1          1       6   total  0.035756   0.002159
+39         1         1          1       7   total  0.032601   0.001955
+40         1         1          1       8   total  0.032075   0.002718
+41         1         1          1       9   total  0.038560   0.004896
+42         1         1          1      10   total  0.042066   0.003262
+43         1         1          1      11   total  0.057840   0.004013
+22         1         1          2       1   total  0.000175   0.000175
+23         1         1          2       2   total  0.000000   0.000000
+24         1         1          2       3   total  0.000175   0.000175
+25         1         1          2       4   total  0.000175   0.000175
 26         1         1          2       5   total  0.000000   0.000000
 27         1         1          2       6   total  0.000000   0.000000
 28         1         1          2       7   total  0.000000   0.000000
@@ -122,33 +122,33 @@
 19         1         2          1       9   total  0.000000   0.000000
 20         1         2          1      10   total  0.000000   0.000000
 21         1         2          1      11   total  0.000000   0.000000
-0          1         2          2       1   total  0.031440   0.006860
-1          1         2          2       2   total  0.036063   0.002322
-2          1         2          2       3   total  0.035138   0.004763
-3          1         2          2       4   total  0.035138   0.007105
-4          1         2          2       5   total  0.036988   0.006894
-5          1         2          2       6   total  0.024042   0.004702
-6          1         2          2       7   total  0.033289   0.008036
-7          1         2          2       8   total  0.038837   0.007464
-8          1         2          2       9   total  0.031440   0.003585
-9          1         2          2      10   total  0.049009   0.006292
-10         1         2          2      11   total  0.044385   0.013144
+0          1         2          2       1   total  0.037164   0.005797
+1          1         2          2       2   total  0.039173   0.004933
+2          1         2          2       3   total  0.044195   0.006937
+3          1         2          2       4   total  0.043191   0.006147
+4          1         2          2       5   total  0.025111   0.006951
+5          1         2          2       6   total  0.037164   0.009702
+6          1         2          2       7   total  0.035155   0.009426
+7          1         2          2       8   total  0.041182   0.010317
+8          1         2          2       9   total  0.032142   0.003598
+9          1         2          2      10   total  0.026115   0.007055
+10         1         2          2      11   total  0.035155   0.008586
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         1         1          1       1   total  0.032927   0.004236
-34         1         1          1       2   total  0.026481   0.003998
-35         1         1          1       3   total  0.029268   0.002455
-36         1         1          1       4   total  0.032578   0.005190
-37         1         1          1       5   total  0.031010   0.003679
-38         1         1          1       6   total  0.027177   0.004376
-39         1         1          1       7   total  0.030139   0.002881
-40         1         1          1       8   total  0.037108   0.003187
-41         1         1          1       9   total  0.038327   0.004511
-42         1         1          1      10   total  0.039372   0.004379
-43         1         1          1      11   total  0.062717   0.007108
-22         1         1          2       1   total  0.000000   0.000000
-23         1         1          2       2   total  0.000174   0.000209
-24         1         1          2       3   total  0.000348   0.000315
-25         1         1          2       4   total  0.000174   0.000209
+33         1         1          1       1   total  0.030147   0.003454
+34         1         1          1       2   total  0.028570   0.004107
+35         1         1          1       3   total  0.033302   0.003381
+36         1         1          1       4   total  0.030322   0.002783
+37         1         1          1       5   total  0.033828   0.004168
+38         1         1          1       6   total  0.035756   0.002711
+39         1         1          1       7   total  0.032601   0.002461
+40         1         1          1       8   total  0.032075   0.003090
+41         1         1          1       9   total  0.038560   0.005205
+42         1         1          1      10   total  0.042066   0.003790
+43         1         1          1      11   total  0.057840   0.004811
+22         1         1          2       1   total  0.000175   0.000234
+23         1         1          2       2   total  0.000000   0.000000
+24         1         1          2       3   total  0.000175   0.000234
+25         1         1          2       4   total  0.000175   0.000234
 26         1         1          2       5   total  0.000000   0.000000
 27         1         1          2       6   total  0.000000   0.000000
 28         1         1          2       7   total  0.000000   0.000000
@@ -167,29 +167,29 @@
 19         1         2          1       9   total  0.000000   0.000000
 20         1         2          1      10   total  0.000000   0.000000
 21         1         2          1      11   total  0.000000   0.000000
-0          1         2          2       1   total  0.031440   0.007170
-1          1         2          2       2   total  0.036063   0.003334
-2          1         2          2       3   total  0.035138   0.005303
-3          1         2          2       4   total  0.035138   0.007478
-4          1         2          2       5   total  0.036988   0.007318
-5          1         2          2       6   total  0.024042   0.004965
-6          1         2          2       7   total  0.033289   0.008334
-7          1         2          2       8   total  0.038837   0.007896
-8          1         2          2       9   total  0.031440   0.004148
-9          1         2          2      10   total  0.049009   0.007083
-10         1         2          2      11   total  0.044385   0.013469
+0          1         2          2       1   total  0.037164   0.006511
+1          1         2          2       2   total  0.039173   0.005840
+2          1         2          2       3   total  0.044195   0.007782
+3          1         2          2       4   total  0.043191   0.007047
+4          1         2          2       5   total  0.025111   0.007234
+5          1         2          2       6   total  0.037164   0.010146
+6          1         2          2       7   total  0.035155   0.009835
+7          1         2          2       8   total  0.041182   0.010828
+8          1         2          2       9   total  0.032142   0.004419
+9          1         2          2      10   total  0.026115   0.007356
+10         1         2          2      11   total  0.035155   0.009032
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         2         1          1       1   total  0.026798   0.002892
-34         2         1          1       2   total  0.021339   0.003475
-35         2         1          1       3   total  0.021835   0.003963
-36         2         1          1       4   total  0.018361   0.006133
-37         2         1          1       5   total  0.023820   0.003932
-38         2         1          1       6   total  0.025805   0.003808
-39         2         1          1       7   total  0.026798   0.002299
-40         2         1          1       8   total  0.029279   0.003939
-41         2         1          1       9   total  0.034738   0.004323
-42         2         1          1      10   total  0.032753   0.006086
-43         2         1          1      11   total  0.057069   0.003798
+33         2         1          1       1   total  0.025373   0.004281
+34         2         1          1       2   total  0.023909   0.004395
+35         2         1          1       3   total  0.019518   0.003911
+36         2         1          1       4   total  0.019518   0.003424
+37         2         1          1       5   total  0.020006   0.002901
+38         2         1          1       6   total  0.024885   0.005801
+39         2         1          1       7   total  0.019030   0.002566
+40         2         1          1       8   total  0.030741   0.001410
+41         2         1          1       9   total  0.034156   0.002902
+42         2         1          1      10   total  0.042939   0.004498
+43         2         1          1      11   total  0.054650   0.003865
 22         2         1          2       1   total  0.000000   0.000000
 23         2         1          2       2   total  0.000000   0.000000
 24         2         1          2       3   total  0.000000   0.000000
@@ -212,29 +212,29 @@
 19         2         2          1       9   total  0.000000   0.000000
 20         2         2          1      10   total  0.000000   0.000000
 21         2         2          1      11   total  0.000000   0.000000
-0          2         2          2       1   total  0.032254   0.009449
-1          2         2          2       2   total  0.018815   0.005569
-2          2         2          2       3   total  0.032254   0.009449
-3          2         2          2       4   total  0.024191   0.006844
-4          2         2          2       5   total  0.013439   0.008563
-5          2         2          2       6   total  0.024191   0.005365
-6          2         2          2       7   total  0.043005   0.010420
-7          2         2          2       8   total  0.032254   0.012710
-8          2         2          2       9   total  0.034942   0.007365
-9          2         2          2      10   total  0.021503   0.007051
-10         2         2          2      11   total  0.018815   0.008193
+0          2         2          2       1   total  0.026892   0.012233
+1          2         2          2       2   total  0.034959   0.007449
+2          2         2          2       3   total  0.040337   0.009144
+3          2         2          2       4   total  0.021513   0.003750
+4          2         2          2       5   total  0.018824   0.005602
+5          2         2          2       6   total  0.026892   0.004806
+6          2         2          2       7   total  0.037648   0.012716
+7          2         2          2       8   total  0.026892   0.009768
+8          2         2          2       9   total  0.024202   0.005420
+9          2         2          2      10   total  0.018824   0.010183
+10         2         2          2      11   total  0.018824   0.007033
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         2         1          1       1   total  0.026798   0.002892
-34         2         1          1       2   total  0.021339   0.003475
-35         2         1          1       3   total  0.021835   0.003963
-36         2         1          1       4   total  0.018361   0.006133
-37         2         1          1       5   total  0.023820   0.003932
-38         2         1          1       6   total  0.025805   0.003808
-39         2         1          1       7   total  0.026798   0.002299
-40         2         1          1       8   total  0.029279   0.003939
-41         2         1          1       9   total  0.034738   0.004323
-42         2         1          1      10   total  0.032753   0.006086
-43         2         1          1      11   total  0.057069   0.003798
+33         2         1          1       1   total  0.025373   0.004281
+34         2         1          1       2   total  0.023909   0.004395
+35         2         1          1       3   total  0.019518   0.003911
+36         2         1          1       4   total  0.019518   0.003424
+37         2         1          1       5   total  0.020006   0.002901
+38         2         1          1       6   total  0.024885   0.005801
+39         2         1          1       7   total  0.019030   0.002566
+40         2         1          1       8   total  0.030741   0.001410
+41         2         1          1       9   total  0.034156   0.002902
+42         2         1          1      10   total  0.042939   0.004498
+43         2         1          1      11   total  0.054650   0.003865
 22         2         1          2       1   total  0.000000   0.000000
 23         2         1          2       2   total  0.000000   0.000000
 24         2         1          2       3   total  0.000000   0.000000
@@ -257,29 +257,29 @@
 19         2         2          1       9   total  0.000000   0.000000
 20         2         2          1      10   total  0.000000   0.000000
 21         2         2          1      11   total  0.000000   0.000000
-0          2         2          2       1   total  0.032254   0.009449
-1          2         2          2       2   total  0.018815   0.005569
-2          2         2          2       3   total  0.032254   0.009449
-3          2         2          2       4   total  0.024191   0.006844
-4          2         2          2       5   total  0.013439   0.008563
-5          2         2          2       6   total  0.024191   0.005365
-6          2         2          2       7   total  0.043005   0.010420
-7          2         2          2       8   total  0.032254   0.012710
-8          2         2          2       9   total  0.034942   0.007365
-9          2         2          2      10   total  0.021503   0.007051
-10         2         2          2      11   total  0.018815   0.008193
+0          2         2          2       1   total  0.026892   0.012233
+1          2         2          2       2   total  0.034959   0.007449
+2          2         2          2       3   total  0.040337   0.009144
+3          2         2          2       4   total  0.021513   0.003750
+4          2         2          2       5   total  0.018824   0.005602
+5          2         2          2       6   total  0.026892   0.004806
+6          2         2          2       7   total  0.037648   0.012716
+7          2         2          2       8   total  0.026892   0.009768
+8          2         2          2       9   total  0.024202   0.005420
+9          2         2          2      10   total  0.018824   0.010183
+10         2         2          2      11   total  0.018824   0.007033
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         2         1          1       1   total  0.026249   0.003012
-34         2         1          1       2   total  0.020902   0.003500
-35         2         1          1       3   total  0.021388   0.003971
-36         2         1          1       4   total  0.017986   0.006048
-37         2         1          1       5   total  0.023333   0.003958
-38         2         1          1       6   total  0.025277   0.003858
-39         2         1          1       7   total  0.026249   0.002473
-40         2         1          1       8   total  0.028680   0.004017
-41         2         1          1       9   total  0.034027   0.004437
-42         2         1          1      10   total  0.032082   0.006091
-43         2         1          1      11   total  0.055901   0.004311
+33         2         1          1       1   total  0.025753   0.004484
+34         2         1          1       2   total  0.024267   0.004581
+35         2         1          1       3   total  0.019810   0.004060
+36         2         1          1       4   total  0.019810   0.003579
+37         2         1          1       5   total  0.020305   0.003071
+38         2         1          1       6   total  0.025258   0.005987
+39         2         1          1       7   total  0.019315   0.002734
+40         2         1          1       8   total  0.031201   0.001961
+41         2         1          1       9   total  0.034668   0.003301
+42         2         1          1      10   total  0.043582   0.004935
+43         2         1          1      11   total  0.055468   0.004591
 22         2         1          2       1   total  0.000000   0.000000
 23         2         1          2       2   total  0.000000   0.000000
 24         2         1          2       3   total  0.000000   0.000000
@@ -302,29 +302,29 @@
 19         2         2          1       9   total  0.000000   0.000000
 20         2         2          1      10   total  0.000000   0.000000
 21         2         2          1      11   total  0.000000   0.000000
-0          2         2          2       1   total  0.032230   0.009604
-1          2         2          2       2   total  0.018801   0.005658
-2          2         2          2       3   total  0.032230   0.009604
-3          2         2          2       4   total  0.024172   0.006964
-4          2         2          2       5   total  0.013429   0.008588
-5          2         2          2       6   total  0.024172   0.005520
-6          2         2          2       7   total  0.042973   0.010671
-7          2         2          2       8   total  0.032230   0.012821
-8          2         2          2       9   total  0.034915   0.007601
-9          2         2          2      10   total  0.021486   0.007142
-10         2         2          2      11   total  0.018801   0.008251
+0          2         2          2       1   total  0.026854   0.012543
+1          2         2          2       2   total  0.034911   0.008307
+2          2         2          2       3   total  0.040281   0.010079
+3          2         2          2       4   total  0.021483   0.004382
+4          2         2          2       5   total  0.018798   0.005938
+5          2         2          2       6   total  0.026854   0.005579
+6          2         2          2       7   total  0.037596   0.013308
+7          2         2          2       8   total  0.026854   0.010161
+8          2         2          2       9   total  0.024169   0.005987
+9          2         2          2      10   total  0.018798   0.010362
+10         2         2          2      11   total  0.018798   0.007300
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         2         1          1       1   total  0.026249   0.003460
-34         2         1          1       2   total  0.020902   0.003754
-35         2         1          1       3   total  0.021388   0.004206
-36         2         1          1       4   total  0.017986   0.006160
-37         2         1          1       5   total  0.023333   0.004238
-38         2         1          1       6   total  0.025277   0.004192
-39         2         1          1       7   total  0.026249   0.003003
-40         2         1          1       8   total  0.028680   0.004427
-41         2         1          1       9   total  0.034027   0.004956
-42         2         1          1      10   total  0.032082   0.006437
-43         2         1          1      11   total  0.055901   0.005636
+33         2         1          1       1   total  0.025753   0.004661
+34         2         1          1       2   total  0.024267   0.004736
+35         2         1          1       3   total  0.019810   0.004177
+36         2         1          1       4   total  0.019810   0.003711
+37         2         1          1       5   total  0.020305   0.003231
+38         2         1          1       6   total  0.025258   0.006117
+39         2         1          1       7   total  0.019315   0.002897
+40         2         1          1       8   total  0.031201   0.002497
+41         2         1          1       9   total  0.034668   0.003721
+42         2         1          1      10   total  0.043582   0.005386
+43         2         1          1      11   total  0.055468   0.005350
 22         2         1          2       1   total  0.000000   0.000000
 23         2         1          2       2   total  0.000000   0.000000
 24         2         1          2       3   total  0.000000   0.000000
@@ -347,40 +347,40 @@
 19         2         2          1       9   total  0.000000   0.000000
 20         2         2          1      10   total  0.000000   0.000000
 21         2         2          1      11   total  0.000000   0.000000
-0          2         2          2       1   total  0.032230   0.010330
-1          2         2          2       2   total  0.018801   0.006077
-2          2         2          2       3   total  0.032230   0.010330
-3          2         2          2       4   total  0.024172   0.007526
-4          2         2          2       5   total  0.013429   0.008733
-5          2         2          2       6   total  0.024172   0.006213
-6          2         2          2       7   total  0.042973   0.011815
-7          2         2          2       8   total  0.032230   0.013373
-8          2         2          2       9   total  0.034915   0.008646
-9          2         2          2      10   total  0.021486   0.007579
-10         2         2          2      11   total  0.018801   0.008544
+0          2         2          2       1   total  0.026854   0.013054
+1          2         2          2       2   total  0.034911   0.009546
+2          2         2          2       3   total  0.040281   0.011446
+3          2         2          2       4   total  0.021483   0.005251
+4          2         2          2       5   total  0.018798   0.006456
+5          2         2          2       6   total  0.026854   0.006649
+6          2         2          2       7   total  0.037596   0.014239
+7          2         2          2       8   total  0.026854   0.010785
+8          2         2          2       9   total  0.024169   0.006815
+9          2         2          2      10   total  0.018798   0.010667
+10         2         2          2      11   total  0.018798   0.007727
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         3         1          1       1   total  0.007006   0.000692
-34         3         1          1       2   total  0.006819   0.000431
-35         3         1          1       3   total  0.005511   0.000591
-36         3         1          1       4   total  0.006165   0.000470
-37         3         1          1       5   total  0.007660   0.000919
-38         3         1          1       6   total  0.012237   0.000938
-39         3         1          1       7   total  0.040914   0.002235
-40         3         1          1       8   total  0.074356   0.001968
-41         3         1          1       9   total  0.121622   0.004662
-42         3         1          1      10   total  0.158707   0.004147
-43         3         1          1      11   total  0.200742   0.007791
-22         3         1          2       1   total  0.000187   0.000187
-23         3         1          2       2   total  0.001028   0.000096
-24         3         1          2       3   total  0.000841   0.000176
-25         3         1          2       4   total  0.000841   0.000310
-26         3         1          2       5   total  0.001308   0.000274
-27         3         1          2       6   total  0.003736   0.000538
-28         3         1          2       7   total  0.003830   0.000703
-29         3         1          2       8   total  0.006259   0.000452
-30         3         1          2       9   total  0.005231   0.000510
-31         3         1          2      10   total  0.005044   0.000440
-32         3         1          2      11   total  0.002522   0.000194
+33         3         1          1       1   total  0.007681   0.001043
+34         3         1          1       2   total  0.005645   0.000710
+35         3         1          1       3   total  0.007403   0.000552
+36         3         1          1       4   total  0.007866   0.000512
+37         3         1          1       5   total  0.007589   0.000524
+38         3         1          1       6   total  0.011198   0.001548
+39         3         1          1       7   total  0.039701   0.002533
+40         3         1          1       8   total  0.075978   0.001897
+41         3         1          1       9   total  0.112532   0.003677
+42         3         1          1      10   total  0.166670   0.005003
+43         3         1          1      11   total  0.210443   0.005656
+22         3         1          2       1   total  0.000463   0.000207
+23         3         1          2       2   total  0.000555   0.000227
+24         3         1          2       3   total  0.000740   0.000429
+25         3         1          2       4   total  0.001111   0.000236
+26         3         1          2       5   total  0.002128   0.000631
+27         3         1          2       6   total  0.003146   0.000645
+28         3         1          2       7   total  0.004905   0.000561
+29         3         1          2       8   total  0.005738   0.000742
+30         3         1          2       9   total  0.005090   0.000672
+31         3         1          2      10   total  0.005367   0.000561
+32         3         1          2      11   total  0.002406   0.000593
 11         3         2          1       1   total  0.000000   0.000000
 12         3         2          1       2   total  0.000000   0.000000
 13         3         2          1       3   total  0.000000   0.000000
@@ -390,42 +390,42 @@
 17         3         2          1       7   total  0.000000   0.000000
 18         3         2          1       8   total  0.000000   0.000000
 19         3         2          1       9   total  0.000000   0.000000
-20         3         2          1      10   total  0.000467   0.000467
-21         3         2          1      11   total  0.000000   0.000000
-0          3         2          2       1   total  0.084030   0.007630
-1          3         2          2       2   total  0.099902   0.010027
-2          3         2          2       3   total  0.112040   0.010928
-3          3         2          2       4   total  0.117642   0.008523
-4          3         2          2       5   total  0.139116   0.009002
-5          3         2          2       6   total  0.161057   0.013537
-6          3         2          2       7   total  0.180664   0.010135
-7          3         2          2       8   total  0.219411   0.014717
-8          3         2          2       9   total  0.239485   0.027005
-9          3         2          2      10   total  0.281033   0.021376
-10         3         2          2      11   total  0.369731   0.027200
+20         3         2          1      10   total  0.000000   0.000000
+21         3         2          1      11   total  0.000474   0.000475
+0          3         2          2       1   total  0.074402   0.007811
+1          3         2          2       2   total  0.103783   0.009042
+2          3         2          2       3   total  0.106153   0.012691
+3          3         2          2       4   total  0.115631   0.011475
+4          3         2          2       5   total  0.128900   0.014096
+5          3         2          2       6   total  0.169655   0.021010
+6          3         2          2       7   total  0.175816   0.016086
+7          3         2          2       8   total  0.217519   0.029631
+8          3         2          2       9   total  0.247374   0.021476
+9          3         2          2      10   total  0.299977   0.031756
+10         3         2          2      11   total  0.351157   0.027654
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         3         1          1       1   total  0.007006   0.000692
-34         3         1          1       2   total  0.006819   0.000431
-35         3         1          1       3   total  0.005511   0.000591
-36         3         1          1       4   total  0.006165   0.000470
-37         3         1          1       5   total  0.007660   0.000919
-38         3         1          1       6   total  0.012237   0.000938
-39         3         1          1       7   total  0.040914   0.002235
-40         3         1          1       8   total  0.074356   0.001968
-41         3         1          1       9   total  0.121622   0.004662
-42         3         1          1      10   total  0.158707   0.004147
-43         3         1          1      11   total  0.200742   0.007791
-22         3         1          2       1   total  0.000187   0.000187
-23         3         1          2       2   total  0.001028   0.000096
-24         3         1          2       3   total  0.000841   0.000176
-25         3         1          2       4   total  0.000841   0.000310
-26         3         1          2       5   total  0.001308   0.000274
-27         3         1          2       6   total  0.003736   0.000538
-28         3         1          2       7   total  0.003830   0.000703
-29         3         1          2       8   total  0.006259   0.000452
-30         3         1          2       9   total  0.005231   0.000510
-31         3         1          2      10   total  0.005044   0.000440
-32         3         1          2      11   total  0.002522   0.000194
+33         3         1          1       1   total  0.007681   0.001043
+34         3         1          1       2   total  0.005645   0.000710
+35         3         1          1       3   total  0.007403   0.000552
+36         3         1          1       4   total  0.007866   0.000512
+37         3         1          1       5   total  0.007589   0.000524
+38         3         1          1       6   total  0.011198   0.001548
+39         3         1          1       7   total  0.039701   0.002533
+40         3         1          1       8   total  0.075978   0.001897
+41         3         1          1       9   total  0.112532   0.003677
+42         3         1          1      10   total  0.166670   0.005003
+43         3         1          1      11   total  0.210443   0.005656
+22         3         1          2       1   total  0.000463   0.000207
+23         3         1          2       2   total  0.000555   0.000227
+24         3         1          2       3   total  0.000740   0.000429
+25         3         1          2       4   total  0.001111   0.000236
+26         3         1          2       5   total  0.002128   0.000631
+27         3         1          2       6   total  0.003146   0.000645
+28         3         1          2       7   total  0.004905   0.000561
+29         3         1          2       8   total  0.005738   0.000742
+30         3         1          2       9   total  0.005090   0.000672
+31         3         1          2      10   total  0.005367   0.000561
+32         3         1          2      11   total  0.002406   0.000593
 11         3         2          1       1   total  0.000000   0.000000
 12         3         2          1       2   total  0.000000   0.000000
 13         3         2          1       3   total  0.000000   0.000000
@@ -435,42 +435,42 @@
 17         3         2          1       7   total  0.000000   0.000000
 18         3         2          1       8   total  0.000000   0.000000
 19         3         2          1       9   total  0.000000   0.000000
-20         3         2          1      10   total  0.000467   0.000467
-21         3         2          1      11   total  0.000000   0.000000
-0          3         2          2       1   total  0.084030   0.007630
-1          3         2          2       2   total  0.099902   0.010027
-2          3         2          2       3   total  0.112040   0.010928
-3          3         2          2       4   total  0.117642   0.008523
-4          3         2          2       5   total  0.139116   0.009002
-5          3         2          2       6   total  0.161057   0.013537
-6          3         2          2       7   total  0.180664   0.010135
-7          3         2          2       8   total  0.219411   0.014717
-8          3         2          2       9   total  0.239485   0.027005
-9          3         2          2      10   total  0.281033   0.021376
-10         3         2          2      11   total  0.369731   0.027200
+20         3         2          1      10   total  0.000000   0.000000
+21         3         2          1      11   total  0.000474   0.000475
+0          3         2          2       1   total  0.074402   0.007811
+1          3         2          2       2   total  0.103783   0.009042
+2          3         2          2       3   total  0.106153   0.012691
+3          3         2          2       4   total  0.115631   0.011475
+4          3         2          2       5   total  0.128900   0.014096
+5          3         2          2       6   total  0.169655   0.021010
+6          3         2          2       7   total  0.175816   0.016086
+7          3         2          2       8   total  0.217519   0.029631
+8          3         2          2       9   total  0.247374   0.021476
+9          3         2          2      10   total  0.299977   0.031756
+10         3         2          2      11   total  0.351157   0.027654
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         3         1          1       1   total  0.007106   0.000737
-34         3         1          1       2   total  0.006917   0.000488
-35         3         1          1       3   total  0.005590   0.000624
-36         3         1          1       4   total  0.006254   0.000516
-37         3         1          1       5   total  0.007770   0.000964
-38         3         1          1       6   total  0.012412   0.001029
-39         3         1          1       7   total  0.041501   0.002618
-40         3         1          1       8   total  0.075421   0.003106
-41         3         1          1       9   total  0.123365   0.006125
-42         3         1          1      10   total  0.160981   0.006595
-43         3         1          1      11   total  0.203618   0.010185
-22         3         1          2       1   total  0.000190   0.000190
-23         3         1          2       2   total  0.001042   0.000103
-24         3         1          2       3   total  0.000853   0.000180
-25         3         1          2       4   total  0.000853   0.000316
-26         3         1          2       5   total  0.001327   0.000281
-27         3         1          2       6   total  0.003790   0.000559
-28         3         1          2       7   total  0.003885   0.000724
-29         3         1          2       8   total  0.006348   0.000500
-30         3         1          2       9   total  0.005306   0.000544
-31         3         1          2      10   total  0.005117   0.000475
-32         3         1          2      11   total  0.002558   0.000213
+33         3         1          1       1   total  0.007759   0.001080
+34         3         1          1       2   total  0.005703   0.000738
+35         3         1          1       3   total  0.007479   0.000602
+36         3         1          1       4   total  0.007946   0.000571
+37         3         1          1       5   total  0.007666   0.000578
+38         3         1          1       6   total  0.011312   0.001601
+39         3         1          1       7   total  0.040106   0.002833
+40         3         1          1       8   total  0.076753   0.003016
+41         3         1          1       9   total  0.113680   0.005068
+42         3         1          1      10   total  0.168370   0.007185
+43         3         1          1      11   total  0.212589   0.008616
+22         3         1          2       1   total  0.000467   0.000210
+23         3         1          2       2   total  0.000561   0.000230
+24         3         1          2       3   total  0.000748   0.000434
+25         3         1          2       4   total  0.001122   0.000241
+26         3         1          2       5   total  0.002150   0.000641
+27         3         1          2       6   total  0.003179   0.000659
+28         3         1          2       7   total  0.004955   0.000586
+29         3         1          2       8   total  0.005796   0.000770
+30         3         1          2       9   total  0.005142   0.000697
+31         3         1          2      10   total  0.005422   0.000590
+32         3         1          2      11   total  0.002431   0.000604
 11         3         2          1       1   total  0.000000   0.000000
 12         3         2          1       2   total  0.000000   0.000000
 13         3         2          1       3   total  0.000000   0.000000
@@ -480,42 +480,42 @@
 17         3         2          1       7   total  0.000000   0.000000
 18         3         2          1       8   total  0.000000   0.000000
 19         3         2          1       9   total  0.000000   0.000000
-20         3         2          1      10   total  0.000466   0.000467
-21         3         2          1      11   total  0.000000   0.000000
-0          3         2          2       1   total  0.083912   0.007782
-1          3         2          2       2   total  0.099762   0.010188
-2          3         2          2       3   total  0.111883   0.011115
-3          3         2          2       4   total  0.117477   0.008794
-4          3         2          2       5   total  0.138921   0.009363
-5          3         2          2       6   total  0.160831   0.013853
-6          3         2          2       7   total  0.180411   0.010676
-7          3         2          2       8   total  0.219104   0.015265
-8          3         2          2       9   total  0.239149   0.027341
-9          3         2          2      10   total  0.280639   0.021991
-10         3         2          2      11   total  0.369213   0.028038
+20         3         2          1      10   total  0.000000   0.000000
+21         3         2          1      11   total  0.000477   0.000479
+0          3         2          2       1   total  0.074833   0.009685
+1          3         2          2       2   total  0.104384   0.012046
+2          3         2          2       3   total  0.106768   0.015107
+3          3         2          2       4   total  0.116300   0.014515
+4          3         2          2       5   total  0.129646   0.017242
+5          3         2          2       6   total  0.170637   0.024765
+6          3         2          2       7   total  0.176834   0.020997
+7          3         2          2       8   total  0.218778   0.034093
+8          3         2          2       9   total  0.248807   0.028656
+9          3         2          2      10   total  0.301714   0.039263
+10         3         2          2      11   total  0.353191   0.038577
     material  group in  group out  mu bin nuclide      mean  std. dev.
-33         3         1          1       1   total  0.007106   0.000751
-34         3         1          1       2   total  0.006917   0.000509
-35         3         1          1       3   total  0.005590   0.000635
-36         3         1          1       4   total  0.006254   0.000532
-37         3         1          1       5   total  0.007770   0.000977
-38         3         1          1       6   total  0.012412   0.001060
-39         3         1          1       7   total  0.041501   0.002755
-40         3         1          1       8   total  0.075421   0.003475
-41         3         1          1       9   total  0.123365   0.006634
-42         3         1          1      10   total  0.160981   0.007387
-43         3         1          1      11   total  0.203618   0.011019
-22         3         1          2       1   total  0.000190   0.000190
-23         3         1          2       2   total  0.001042   0.000114
-24         3         1          2       3   total  0.000853   0.000185
-25         3         1          2       4   total  0.000853   0.000319
-26         3         1          2       5   total  0.001327   0.000288
-27         3         1          2       6   total  0.003790   0.000588
-28         3         1          2       7   total  0.003885   0.000748
-29         3         1          2       8   total  0.006348   0.000587
-30         3         1          2       9   total  0.005306   0.000601
-31         3         1          2      10   total  0.005117   0.000535
-32         3         1          2      11   total  0.002558   0.000246
+33         3         1          1       1   total  0.007759   0.001091
+34         3         1          1       2   total  0.005703   0.000746
+35         3         1          1       3   total  0.007479   0.000619
+36         3         1          1       4   total  0.007946   0.000592
+37         3         1          1       5   total  0.007666   0.000598
+38         3         1          1       6   total  0.011312   0.001616
+39         3         1          1       7   total  0.040106   0.002941
+40         3         1          1       8   total  0.076753   0.003373
+41         3         1          1       9   total  0.113680   0.005540
+42         3         1          1      10   total  0.168370   0.007913
+43         3         1          1      11   total  0.212589   0.009578
+22         3         1          2       1   total  0.000467   0.000211
+23         3         1          2       2   total  0.000561   0.000232
+24         3         1          2       3   total  0.000748   0.000436
+25         3         1          2       4   total  0.001122   0.000250
+26         3         1          2       5   total  0.002150   0.000653
+27         3         1          2       6   total  0.003179   0.000684
+28         3         1          2       7   total  0.004955   0.000654
+29         3         1          2       8   total  0.005796   0.000841
+30         3         1          2       9   total  0.005142   0.000759
+31         3         1          2      10   total  0.005422   0.000670
+32         3         1          2      11   total  0.002431   0.000620
 11         3         2          1       1   total  0.000000   0.000000
 12         3         2          1       2   total  0.000000   0.000000
 13         3         2          1       3   total  0.000000   0.000000
@@ -525,16 +525,16 @@
 17         3         2          1       7   total  0.000000   0.000000
 18         3         2          1       8   total  0.000000   0.000000
 19         3         2          1       9   total  0.000000   0.000000
-20         3         2          1      10   total  0.000466   0.000808
-21         3         2          1      11   total  0.000000   0.000000
-0          3         2          2       1   total  0.083912   0.008936
-1          3         2          2       2   total  0.099762   0.011448
-2          3         2          2       3   total  0.111883   0.012563
-3          3         2          2       4   total  0.117477   0.010731
-4          3         2          2       5   total  0.138921   0.011855
-5          3         2          2       6   total  0.160831   0.016211
-6          3         2          2       7   total  0.180411   0.014254
-7          3         2          2       8   total  0.219104   0.019093
-8          3         2          2       9   total  0.239149   0.030071
-9          3         2          2      10   total  0.280639   0.026447
-10         3         2          2      11   total  0.369213   0.034054
+20         3         2          1      10   total  0.000000   0.000000
+21         3         2          1      11   total  0.000477   0.000827
+0          3         2          2       1   total  0.074833   0.011051
+1          3         2          2       2   total  0.104384   0.014150
+2          3         2          2       3   total  0.106768   0.016908
+3          3         2          2       4   total  0.116300   0.016707
+4          3         2          2       5   total  0.129646   0.019553
+5          3         2          2       6   total  0.170637   0.027579
+6          3         2          2       7   total  0.176834   0.024476
+7          3         2          2       8   total  0.218778   0.037476
+8          3         2          2       9   total  0.248807   0.033680
+9          3         2          2      10   total  0.301714   0.044745
+10         3         2          2      11   total  0.353191   0.046035
diff --git a/tests/regression_tests/mgxs_library_histogram/test.py b/tests/regression_tests/mgxs_library_histogram/test.py
index b9905910a..42fc1957a 100644
--- a/tests/regression_tests/mgxs_library_histogram/test.py
+++ b/tests/regression_tests/mgxs_library_histogram/test.py
@@ -30,7 +30,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _get_results(self, hash_output=False):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/mgxs_library_mesh/inputs_true.dat b/tests/regression_tests/mgxs_library_mesh/inputs_true.dat
index 1ecb7a2d3..5a6e8a20a 100644
--- a/tests/regression_tests/mgxs_library_mesh/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_mesh/inputs_true.dat
@@ -1,28 +1,28 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="Zr90"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="Zr90" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2 3 -4 10 -9" universe="1"/>
     <cell id="2" material="2" region="(-1 | 2 | -3 | 4) (5 -6 7 -8) 10 -9" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="5" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="6" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="7" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="8" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="10" type="z-plane"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum x" type="x-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="6" name="maximum x" type="x-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="7" name="minimum y" type="y-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="8" name="maximum y" type="y-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="10" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/mgxs_library_mesh/results_true.dat b/tests/regression_tests/mgxs_library_mesh/results_true.dat
index b1e8ac003..ec7620c5c 100644
--- a/tests/regression_tests/mgxs_library_mesh/results_true.dat
+++ b/tests/regression_tests/mgxs_library_mesh/results_true.dat
@@ -1,362 +1,362 @@
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.102539  0.004894
-2      1  2  1        1   total  0.106476  0.007513
-1      2  1  1        1   total  0.101207  0.004814
-3      2  2  1        1   total  0.101733  0.004445
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.073423  0.005232
-2      1  2  1        1   total  0.077906  0.007749
-1      2  1  1        1   total  0.071315  0.005181
-3      2  2  1        1   total  0.071983  0.004856
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.073443  0.005235
-2      1  2  1        1   total  0.077909  0.007749
-1      2  1  1        1   total  0.071315  0.005181
-3      2  2  1        1   total  0.071928  0.004865
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.013089  0.000762
-2      1  2  1        1   total  0.013802  0.000950
-1      2  1  1        1   total  0.012818  0.000708
-3      2  2  1        1   total  0.012954  0.000699
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.013016  0.000761
-2      1  2  1        1   total  0.013715  0.000947
-1      2  1  1        1   total  0.012740  0.000707
-3      2  2  1        1   total  0.012886  0.000698
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.001244  0.000926
-2      1  2  1        1   total  0.001345  0.000851
-1      2  1  1        1   total  0.001196  0.000768
-3      2  2  1        1   total  0.001234  0.000843
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.011844  0.000690
-2      1  2  1        1   total  0.012457  0.000852
-1      2  1  1        1   total  0.011622  0.000636
-3      2  2  1        1   total  0.011719  0.000633
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.030919  0.001784
-2      1  2  1        1   total  0.032495  0.002202
-1      2  1  1        1   total  0.030363  0.001664
-3      2  2  1        1   total  0.030565  0.001671
-  mesh 1       group in nuclide          mean      std. dev.
-       x  y  z                                              
-0      1  1  1        1   total  2.290786e+06  133521.277651
-2      1  2  1        1   total  2.409292e+06  164752.430241
-1      2  1  1        1   total  2.247742e+06  123005.689035
-3      2  2  1        1   total  2.266603e+06  122428.448367
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.089450  0.004147
-2      1  2  1        1   total  0.092674  0.006571
-1      2  1  1        1   total  0.088389  0.004152
-3      2  2  1        1   total  0.088779  0.003763
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.089555  0.004061
-2      1  2  1        1   total  0.092658  0.006216
-1      2  1  1        1   total  0.088293  0.004423
-3      2  2  1        1   total  0.088717  0.004937
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.089523  0.004070
-1       1  1  1        1         1       P1   total  0.029116  0.001851
-2       1  1  1        1         1       P2   total  0.016529  0.000827
-3       1  1  1        1         1       P3   total  0.009975  0.000731
-8       1  2  1        1         1       P0   total  0.092626  0.006233
-9       1  2  1        1         1       P1   total  0.028570  0.001899
-10      1  2  1        1         1       P2   total  0.015737  0.001986
-11      1  2  1        1         1       P3   total  0.008492  0.000859
-4       2  1  1        1         1       P0   total  0.088293  0.004423
-5       2  1  1        1         1       P1   total  0.029892  0.001916
-6       2  1  1        1         1       P2   total  0.016736  0.000714
-7       2  1  1        1         1       P3   total  0.009129  0.000509
-12      2  2  1        1         1       P0   total  0.088617  0.004889
-13      2  2  1        1         1       P1   total  0.029750  0.001954
-14      2  2  1        1         1       P2   total  0.016440  0.001320
-15      2  2  1        1         1       P3   total  0.009758  0.000538
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.089555  0.004061
-1       1  1  1        1         1       P1   total  0.029096  0.001859
-2       1  1  1        1         1       P2   total  0.016532  0.000828
-3       1  1  1        1         1       P3   total  0.009986  0.000734
-8       1  2  1        1         1       P0   total  0.092658  0.006216
-9       1  2  1        1         1       P1   total  0.028567  0.001899
-10      1  2  1        1         1       P2   total  0.015721  0.001995
-11      1  2  1        1         1       P3   total  0.008496  0.000857
-4       2  1  1        1         1       P0   total  0.088293  0.004423
-5       2  1  1        1         1       P1   total  0.029892  0.001916
-6       2  1  1        1         1       P2   total  0.016736  0.000714
-7       2  1  1        1         1       P3   total  0.009129  0.000509
-12      2  2  1        1         1       P0   total  0.088717  0.004937
-13      2  2  1        1         1       P1   total  0.029805  0.001977
-14      2  2  1        1         1       P2   total  0.016448  0.001327
-15      2  2  1        1         1       P3   total  0.009752  0.000534
-  mesh 1       group in group out nuclide      mean std. dev.
-       x  y  z                                               
-0      1  1  1        1         1   total  1.000362  0.044694
-2      1  2  1        1         1   total  1.000346  0.059372
-1      2  1  1        1         1   total  1.000000  0.055277
-3      2  2  1        1         1   total  1.001132  0.057130
-  mesh 1       group in group out nuclide      mean std. dev.
-       x  y  z                                               
-0      1  1  1        1         1   total  0.031634  0.002236
-2      1  2  1        1         1   total  0.032561  0.002344
-1      2  1  1        1         1   total  0.030820  0.002579
-3      2  2  1        1         1   total  0.029308  0.002507
-  mesh 1       group in group out nuclide mean std. dev.
-       x  y  z                                          
-0      1  1  1        1         1   total  1.0  0.044796
-2      1  2  1        1         1   total  1.0  0.059579
-1      2  1  1        1         1   total  1.0  0.055277
-3      2  2  1        1         1   total  1.0  0.056598
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.089450  0.005766
-1       1  1  1        1         1       P1   total  0.029092  0.002277
-2       1  1  1        1         1       P2   total  0.016516  0.001119
-3       1  1  1        1         1       P3   total  0.009967  0.000861
-8       1  2  1        1         1       P0   total  0.092674  0.008583
-9       1  2  1        1         1       P1   total  0.028585  0.002630
-10      1  2  1        1         1       P2   total  0.015745  0.002226
-11      1  2  1        1         1       P3   total  0.008496  0.001015
-4       2  1  1        1         1       P0   total  0.088389  0.006412
-5       2  1  1        1         1       P1   total  0.029924  0.002479
-6       2  1  1        1         1       P2   total  0.016754  0.001133
-7       2  1  1        1         1       P3   total  0.009139  0.000699
-12      2  2  1        1         1       P0   total  0.088779  0.006278
-13      2  2  1        1         1       P1   total  0.029804  0.002360
-14      2  2  1        1         1       P2   total  0.016470  0.001510
-15      2  2  1        1         1       P3   total  0.009776  0.000691
-   mesh 1       group in group out legendre nuclide      mean std. dev.
-        x  y  z                                                        
-0       1  1  1        1         1       P0   total  0.089483  0.007018
-1       1  1  1        1         1       P1   total  0.029103  0.002623
-2       1  1  1        1         1       P2   total  0.016522  0.001341
-3       1  1  1        1         1       P3   total  0.009971  0.000970
-8       1  2  1        1         1       P0   total  0.092706  0.010197
-9       1  2  1        1         1       P1   total  0.028595  0.003131
-10      1  2  1        1         1       P2   total  0.015750  0.002415
-11      1  2  1        1         1       P3   total  0.008499  0.001134
-4       2  1  1        1         1       P0   total  0.088389  0.008061
-5       2  1  1        1         1       P1   total  0.029924  0.002980
-6       2  1  1        1         1       P2   total  0.016754  0.001463
-7       2  1  1        1         1       P3   total  0.009139  0.000863
-12      2  2  1        1         1       P0   total  0.088880  0.008076
-13      2  2  1        1         1       P1   total  0.029838  0.002912
-14      2  2  1        1         1       P2   total  0.016489  0.001781
-15      2  2  1        1         1       P3   total  0.009787  0.000889
-  mesh 1       group out nuclide mean std. dev.
-       x  y  z                                 
-0      1  1  1         1   total  1.0  0.088672
-2      1  2  1         1   total  1.0  0.069731
-1      2  1  1         1   total  1.0  0.109718
-3      2  2  1         1   total  1.0  0.108376
-  mesh 1       group out nuclide mean std. dev.
-       x  y  z                                 
-0      1  1  1         1   total  1.0  0.091154
-2      1  2  1         1   total  1.0  0.069438
-1      2  1  1         1   total  1.0  0.109233
-3      2  2  1         1   total  1.0  0.108119
-  mesh 1       group in nuclide          mean     std. dev.
-       x  y  z                                             
-0      1  1  1        1   total  8.424136e-10  3.612697e-11
-2      1  2  1        1   total  8.884166e-10  6.324091e-11
-1      2  1  1        1   total  8.415613e-10  4.520088e-11
-3      2  2  1        1   total  8.557929e-10  2.898037e-11
-  mesh 1       group in nuclide      mean std. dev.
-       x  y  z                                     
-0      1  1  1        1   total  0.030724  0.001773
-2      1  2  1        1   total  0.032290  0.002188
-1      2  1  1        1   total  0.030172  0.001654
-3      2  2  1        1   total  0.030372  0.001661
-  mesh 1       group in group out nuclide      mean std. dev.
-       x  y  z                                               
-0      1  1  1        1         1   total  0.031468  0.002273
-2      1  2  1        1         1   total  0.032418  0.002330
-1      2  1  1        1         1   total  0.030706  0.002559
-3      2  2  1        1         1   total  0.029134  0.002487
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.103374  0.004981
+2      1  2        1   total  0.103852  0.004752
+1      2  1        1   total  0.103322  0.003613
+3      2  2        1   total  0.102889  0.003387
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.072760  0.005235
+2      1  2        1   total  0.074856  0.004972
+1      2  1        1   total  0.074583  0.004000
+3      2  2        1   total  0.072925  0.003485
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.072768  0.005236
+2      1  2        1   total  0.074864  0.004972
+1      2  1        1   total  0.074603  0.004002
+3      2  2        1   total  0.072881  0.003491
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.013309  0.000729
+2      1  2        1   total  0.013223  0.000707
+1      2  1        1   total  0.013222  0.000596
+3      2  2        1   total  0.013282  0.000564
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.013241  0.000728
+2      1  2        1   total  0.013142  0.000705
+1      2  1        1   total  0.013137  0.000596
+3      2  2        1   total  0.013221  0.000564
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.001281  0.000874
+2      1  2        1   total  0.001297  0.000731
+1      2  1        1   total  0.001281  0.000744
+3      2  2        1   total  0.001288  0.000719
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.012029  0.000664
+2      1  2        1   total  0.011926  0.000639
+1      2  1        1   total  0.011941  0.000545
+3      2  2        1   total  0.011994  0.000515
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.031361  0.001735
+2      1  2        1   total  0.031091  0.001675
+1      2  1        1   total  0.031169  0.001418
+3      2  2        1   total  0.031255  0.001366
+  mesh 1    group in nuclide          mean      std. dev.
+       x  y                                              
+0      1  1        1   total  2.326447e+06  128504.886535
+2      1  2        1   total  2.306518e+06  123638.096130
+1      2  1        1   total  2.309435e+06  105395.059055
+3      2  2        1   total  2.319667e+06   99692.620001
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.090065  0.004262
+2      1  2        1   total  0.090629  0.004086
+1      2  1        1   total  0.090100  0.003045
+3      2  2        1   total  0.089607  0.002828
+  mesh 1    group in nuclide      mean std. dev.
+       x  y                                     
+0      1  1        1   total  0.090188  0.005151
+2      1  2        1   total  0.094339  0.004349
+1      2  1        1   total  0.088294  0.003953
+3      2  2        1   total  0.089633  0.002592
+   mesh 1    group in group out legendre nuclide      mean std. dev.
+        x  y                                                        
+0       1  1        1         1       P0   total  0.090122  0.005149
+1       1  1        1         1       P1   total  0.030614  0.001612
+2       1  1        1         1       P2   total  0.016490  0.000897
+3       1  1        1         1       P3   total  0.010163  0.000829
+8       1  2        1         1       P0   total  0.094307  0.004337
+9       1  2        1         1       P1   total  0.028996  0.001462
+10      1  2        1         1       P2   total  0.017486  0.000808
+11      1  2        1         1       P3   total  0.009753  0.000742
+4       2  1        1         1       P0   total  0.088198  0.003959
+5       2  1        1         1       P1   total  0.028739  0.001715
+6       2  1        1         1       P2   total  0.015608  0.000680
+7       2  1        1         1       P3   total  0.008232  0.000363
+12      2  2        1         1       P0   total  0.089536  0.002551
+13      2  2        1         1       P1   total  0.029964  0.000821
+14      2  2        1         1       P2   total  0.015742  0.001260
+15      2  2        1         1       P3   total  0.008944  0.000385
+   mesh 1    group in group out legendre nuclide      mean std. dev.
+        x  y                                                        
+0       1  1        1         1       P0   total  0.090188  0.005151
+1       1  1        1         1       P1   total  0.030606  0.001614
+2       1  1        1         1       P2   total  0.016462  0.000898
+3       1  1        1         1       P3   total  0.010173  0.000824
+8       1  2        1         1       P0   total  0.094339  0.004349
+9       1  2        1         1       P1   total  0.028988  0.001462
+10      1  2        1         1       P2   total  0.017473  0.000811
+11      1  2        1         1       P3   total  0.009763  0.000749
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    mesh 1               group in nuclide    mean std. dev.
         x  y       surf                                   
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-2       1  1  x-max out        1   total  0.1798  0.009646
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-2      1  2  1        1   total  4.278655  0.425575
-1      2  1  1        1   total  4.674097  0.339600
-3      2  2  1        1   total  4.630700  0.312366
-  mesh 1       group in nuclide      mean std. dev.
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-2      1  2  1        1   total  4.278494  0.425550
-1      2  1  1        1   total  4.674097  0.339600
-3      2  2  1        1   total  4.634259  0.313464
-   mesh 1       delayedgroup group in nuclide      mean     std. dev.
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-1       1  1  1            2        1   total  0.000035  2.063264e-06
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-23      2  2  1            6        1   total  0.000013  6.836821e-07
-   mesh 1       delayedgroup group out nuclide mean std. dev.
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-   mesh 1       delayedgroup group in nuclide      mean std. dev.
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-   mesh 1       delayedgroup group in group out nuclide      mean std. dev.
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+2       1  1            3         1   total  0.0  0.000000
+3       1  1            4         1   total  1.0  0.578922
+4       1  1            5         1   total  0.0  0.000000
+5       1  1            6         1   total  0.0  0.000000
+12      1  2            1         1   total  0.0  0.000000
+13      1  2            2         1   total  1.0  0.578922
+14      1  2            3         1   total  1.0  1.414214
+15      1  2            4         1   total  1.0  1.414214
+16      1  2            5         1   total  1.0  0.875472
+17      1  2            6         1   total  1.0  1.414214
+6       2  1            1         1   total  0.0  0.000000
+7       2  1            2         1   total  0.0  0.000000
+8       2  1            3         1   total  1.0  1.414214
+9       2  1            4         1   total  1.0  0.579392
+10      2  1            5         1   total  1.0  1.414214
+11      2  1            6         1   total  0.0  0.000000
+18      2  2            1         1   total  1.0  0.868163
+19      2  2            2         1   total  1.0  1.414214
+20      2  2            3         1   total  0.0  0.000000
+21      2  2            4         1   total  1.0  0.868969
+22      2  2            5         1   total  1.0  1.414214
+23      2  2            6         1   total  0.0  0.000000
+   mesh 1    delayedgroup group in nuclide      mean std. dev.
+        x  y                                                  
+0       1  1            1        1   total  0.000221  0.000015
+1       1  1            2        1   total  0.001140  0.000079
+2       1  1            3        1   total  0.001088  0.000075
+3       1  1            4        1   total  0.002440  0.000169
+4       1  1            5        1   total  0.001001  0.000069
+5       1  1            6        1   total  0.000419  0.000029
+12      1  2            1        1   total  0.000221  0.000013
+13      1  2            2        1   total  0.001139  0.000066
+14      1  2            3        1   total  0.001088  0.000063
+15      1  2            4        1   total  0.002439  0.000142
+16      1  2            5        1   total  0.001000  0.000058
+17      1  2            6        1   total  0.000419  0.000024
+6       2  1            1        1   total  0.000220  0.000013
+7       2  1            2        1   total  0.001138  0.000067
+8       2  1            3        1   total  0.001086  0.000064
+9       2  1            4        1   total  0.002435  0.000144
+10      2  1            5        1   total  0.000998  0.000059
+11      2  1            6        1   total  0.000418  0.000025
+18      2  2            1        1   total  0.000221  0.000013
+19      2  2            2        1   total  0.001141  0.000066
+20      2  2            3        1   total  0.001090  0.000063
+21      2  2            4        1   total  0.002443  0.000141
+22      2  2            5        1   total  0.001002  0.000058
+23      2  2            6        1   total  0.000420  0.000024
+   mesh 1    delayedgroup nuclide      mean std. dev.
+        x  y                                         
+0       1  1            1   total  0.013336  0.000919
+1       1  1            2   total  0.032739  0.002257
+2       1  1            3   total  0.120780  0.008325
+3       1  1            4   total  0.302780  0.020871
+4       1  1            5   total  0.849490  0.058555
+5       1  1            6   total  2.853000  0.196656
+12      1  2            1   total  0.013336  0.000770
+13      1  2            2   total  0.032739  0.001891
+14      1  2            3   total  0.120780  0.006977
+15      1  2            4   total  0.302780  0.017490
+16      1  2            5   total  0.849490  0.049071
+17      1  2            6   total  2.853000  0.164804
+6       2  1            1   total  0.013336  0.000790
+7       2  1            2   total  0.032739  0.001940
+8       2  1            3   total  0.120780  0.007157
+9       2  1            4   total  0.302780  0.017942
+10      2  1            5   total  0.849490  0.050338
+11      2  1            6   total  2.853000  0.169061
+18      2  2            1   total  0.013336  0.000757
+19      2  2            2   total  0.032739  0.001858
+20      2  2            3   total  0.120780  0.006855
+21      2  2            4   total  0.302780  0.017186
+22      2  2            5   total  0.849490  0.048217
+23      2  2            6   total  2.853000  0.161935
+   mesh 1    delayedgroup group in group out nuclide      mean std. dev.
+        x  y                                                            
+0       1  1            1        1         1   total  0.000026  0.000026
+1       1  1            2        1         1   total  0.000000  0.000000
+2       1  1            3        1         1   total  0.000000  0.000000
+3       1  1            4        1         1   total  0.000083  0.000034
+4       1  1            5        1         1   total  0.000000  0.000000
+5       1  1            6        1         1   total  0.000000  0.000000
+12      1  2            1        1         1   total  0.000000  0.000000
+13      1  2            2        1         1   total  0.000081  0.000033
+14      1  2            3        1         1   total  0.000026  0.000026
+15      1  2            4        1         1   total  0.000026  0.000026
+16      1  2            5        1         1   total  0.000059  0.000036
+17      1  2            6        1         1   total  0.000033  0.000033
+6       2  1            1        1         1   total  0.000000  0.000000
+7       2  1            2        1         1   total  0.000000  0.000000
+8       2  1            3        1         1   total  0.000032  0.000032
+9       2  1            4        1         1   total  0.000080  0.000033
+10      2  1            5        1         1   total  0.000028  0.000028
+11      2  1            6        1         1   total  0.000000  0.000000
+18      2  2            1        1         1   total  0.000054  0.000033
+19      2  2            2        1         1   total  0.000029  0.000029
+20      2  2            3        1         1   total  0.000000  0.000000
+21      2  2            4        1         1   total  0.000054  0.000033
+22      2  2            5        1         1   total  0.000032  0.000032
+23      2  2            6        1         1   total  0.000000  0.000000
diff --git a/tests/regression_tests/mgxs_library_mesh/test.py b/tests/regression_tests/mgxs_library_mesh/test.py
index 89c68a75a..c1a5980b5 100644
--- a/tests/regression_tests/mgxs_library_mesh/test.py
+++ b/tests/regression_tests/mgxs_library_mesh/test.py
@@ -57,7 +57,7 @@ def model():
     model.mgxs_lib.build_library()
 
     # Add tallies
-    model.mgxs_lib.add_to_tallies_file(model.tallies, merge=False)
+    model.mgxs_lib.add_to_tallies(model.tallies, merge=False)
 
     return model
 
diff --git a/tests/regression_tests/mgxs_library_no_nuclides/inputs_true.dat b/tests/regression_tests/mgxs_library_no_nuclides/inputs_true.dat
index 5cad36b7e..af09268cd 100644
--- a/tests/regression_tests/mgxs_library_no_nuclides/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_no_nuclides/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_no_nuclides/results_true.dat b/tests/regression_tests/mgxs_library_no_nuclides/results_true.dat
index c0dba7c0f..aec8e8bbc 100644
--- a/tests/regression_tests/mgxs_library_no_nuclides/results_true.dat
+++ b/tests/regression_tests/mgxs_library_no_nuclides/results_true.dat
@@ -1,227 +1,227 @@
 total
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.413814   0.018434
-0         1         2   total  0.663001   0.012535
+1         1         1   total  0.422772   0.014069
+0         1         2   total  0.661309   0.049458
 transport
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.369730   0.019494
-0         1         2   total  0.643777   0.018246
+1         1         1   total  0.383831   0.015331
+0         1         2   total  0.679181   0.050786
 nu-transport
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.369730   0.019494
-0         1         2   total  0.643777   0.018246
+1         1         1   total  0.383831   0.015331
+0         1         2   total  0.679181   0.050786
 absorption
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.026013   0.001851
-0         1         2   total  0.267233   0.007475
+1         1         1   total  0.027181   0.001539
+0         1         2   total  0.265559   0.020699
 reduced absorption
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.025932   0.001850
-0         1         2   total  0.267233   0.007475
+1         1         1   total  0.027162   0.001538
+0         1         2   total  0.265559   0.020699
 capture
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.018869   0.001758
-0         1         2   total  0.072358   0.008204
+1         1         1   total  0.019717   0.001492
+0         1         2   total  0.072029   0.019378
 fission
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.007144   0.000317
-0         1         2   total  0.194875   0.005528
+1         1         1   total  0.007464   0.000222
+0         1         2   total  0.193530   0.015119
 nu-fission
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.018243   0.000827
-0         1         2   total  0.474851   0.013471
+1         1         1   total  0.018925   0.000543
+0         1         2   total  0.471574   0.036840
 kappa-fission
    material  group in nuclide          mean     std. dev.
-1         1         1   total  1.391782e+06  6.192598e+04
-0         1         2   total  3.768981e+07  1.069211e+06
+1         1         1   total  1.453014e+06  4.287190e+04
+0         1         2   total  3.742974e+07  2.924022e+06
 scatter
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.387802   0.017462
-0         1         2   total  0.395768   0.007541
+1         1         1   total  0.395591   0.013417
+0         1         2   total  0.395750   0.029309
 nu-scatter
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.387032   0.024325
-0         1         2   total  0.407651   0.016266
+1         1         1   total  0.392941   0.019689
+0         1         2   total  0.396262   0.027471
 scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         1         1          1       P0   total  0.386336   0.024297
-13         1         1          1       P1   total  0.044084   0.006343
-14         1         1          1       P2   total  0.027336   0.006403
-15         1         1          1       P3   total  0.011672   0.004897
-8          1         1          2       P0   total  0.000695   0.000327
-9          1         1          2       P1   total -0.000359   0.000201
-10         1         1          2       P2   total -0.000047   0.000081
-11         1         1          2       P3   total  0.000239   0.000099
+12         1         1          1       P0   total  0.392419   0.019837
+13         1         1          1       P1   total  0.038941   0.006089
+14         1         1          1       P2   total  0.019512   0.003548
+15         1         1          1       P3   total  0.012951   0.002399
+8          1         1          2       P0   total  0.000522   0.000349
+9          1         1          2       P1   total -0.000301   0.000185
+10         1         1          2       P2   total  0.000045   0.000147
+11         1         1          2       P3   total  0.000069   0.000162
 4          1         2          1       P0   total  0.000000   0.000000
 5          1         2          1       P1   total  0.000000   0.000000
 6          1         2          1       P2   total  0.000000   0.000000
 7          1         2          1       P3   total  0.000000   0.000000
-0          1         2          2       P0   total  0.407651   0.016266
-1          1         2          2       P1   total  0.021192   0.013171
-2          1         2          2       P2   total  0.010327   0.017230
-3          1         2          2       P3   total  0.005148   0.013732
+0          1         2          2       P0   total  0.396262   0.027471
+1          1         2          2       P1   total -0.016133   0.010911
+2          1         2          2       P2   total -0.001147   0.010536
+3          1         2          2       P3   total  0.005359   0.008143
 nu-scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         1         1          1       P0   total  0.386336   0.024297
-13         1         1          1       P1   total  0.044084   0.006343
-14         1         1          1       P2   total  0.027336   0.006403
-15         1         1          1       P3   total  0.011672   0.004897
-8          1         1          2       P0   total  0.000695   0.000327
-9          1         1          2       P1   total -0.000359   0.000201
-10         1         1          2       P2   total -0.000047   0.000081
-11         1         1          2       P3   total  0.000239   0.000099
+12         1         1          1       P0   total  0.392419   0.019837
+13         1         1          1       P1   total  0.038941   0.006089
+14         1         1          1       P2   total  0.019512   0.003548
+15         1         1          1       P3   total  0.012951   0.002399
+8          1         1          2       P0   total  0.000522   0.000349
+9          1         1          2       P1   total -0.000301   0.000185
+10         1         1          2       P2   total  0.000045   0.000147
+11         1         1          2       P3   total  0.000069   0.000162
 4          1         2          1       P0   total  0.000000   0.000000
 5          1         2          1       P1   total  0.000000   0.000000
 6          1         2          1       P2   total  0.000000   0.000000
 7          1         2          1       P3   total  0.000000   0.000000
-0          1         2          2       P0   total  0.407651   0.016266
-1          1         2          2       P1   total  0.021192   0.013171
-2          1         2          2       P2   total  0.010327   0.017230
-3          1         2          2       P3   total  0.005148   0.013732
+0          1         2          2       P0   total  0.396262   0.027471
+1          1         2          2       P1   total -0.016133   0.010911
+2          1         2          2       P2   total -0.001147   0.010536
+3          1         2          2       P3   total  0.005359   0.008143
 multiplicity matrix
    material  group in  group out nuclide  mean  std. dev.
-3         1         1          1   total   1.0   0.064268
-2         1         1          2   total   1.0   0.661438
+3         1         1          1   total   1.0   0.054620
+2         1         1          2   total   1.0   0.942809
 1         1         2          1   total   0.0   0.000000
-0         1         2          2   total   1.0   0.050545
+0         1         2          2   total   1.0   0.080341
 nu-fission matrix
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.024633   0.002205
+3         1         1          1   total  0.019257   0.001831
 2         1         1          2   total  0.000000   0.000000
-1         1         2          1   total  0.435159   0.014743
+1         1         2          1   total  0.459401   0.022054
 0         1         2          2   total  0.000000   0.000000
 scatter probability matrix
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.998203   0.064101
-2         1         1          2   total  0.001797   0.000844
+3         1         1          1   total  0.998671   0.054518
+2         1         1          2   total  0.001329   0.000888
 1         1         2          1   total  0.000000   0.000000
-0         1         2          2   total  1.000000   0.050545
+0         1         2          2   total  1.000000   0.080341
 consistent scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         1         1          1       P0   total  0.387105   0.030361
-13         1         1          1       P1   total  0.044172   0.006684
-14         1         1          1       P2   total  0.027391   0.006543
-15         1         1          1       P3   total  0.011695   0.004937
-8          1         1          2       P0   total  0.000697   0.000329
-9          1         1          2       P1   total -0.000360   0.000202
-10         1         1          2       P2   total -0.000047   0.000082
-11         1         1          2       P3   total  0.000239   0.000100
+12         1         1          1       P0   total  0.395065   0.025390
+13         1         1          1       P1   total  0.039204   0.006325
+14         1         1          1       P2   total  0.019644   0.003656
+15         1         1          1       P3   total  0.013039   0.002470
+8          1         1          2       P0   total  0.000526   0.000352
+9          1         1          2       P1   total -0.000303   0.000186
+10         1         1          2       P2   total  0.000045   0.000148
+11         1         1          2       P3   total  0.000069   0.000163
 4          1         2          1       P0   total  0.000000   0.000000
 5          1         2          1       P1   total  0.000000   0.000000
 6          1         2          1       P2   total  0.000000   0.000000
 7          1         2          1       P3   total  0.000000   0.000000
-0          1         2          2       P0   total  0.395768   0.021378
-1          1         2          2       P1   total  0.020575   0.012809
-2          1         2          2       P2   total  0.010026   0.016732
-3          1         2          2       P3   total  0.004998   0.013333
+0          1         2          2       P0   total  0.395750   0.043243
+1          1         2          2       P1   total -0.016112   0.010982
+2          1         2          2       P2   total -0.001146   0.010523
+3          1         2          2       P3   total  0.005353   0.008145
 consistent nu-scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         1         1          1       P0   total  0.387105   0.039252
-13         1         1          1       P1   total  0.044172   0.007262
-14         1         1          1       P2   total  0.027391   0.006776
-15         1         1          1       P3   total  0.011695   0.004994
-8          1         1          2       P0   total  0.000697   0.000566
-9          1         1          2       P1   total -0.000360   0.000313
-10         1         1          2       P2   total -0.000047   0.000087
-11         1         1          2       P3   total  0.000239   0.000187
+12         1         1          1       P0   total  0.395065   0.033321
+13         1         1          1       P1   total  0.039204   0.006677
+14         1         1          1       P2   total  0.019644   0.003810
+15         1         1          1       P3   total  0.013039   0.002571
+8          1         1          2       P0   total  0.000526   0.000608
+9          1         1          2       P1   total -0.000303   0.000341
+10         1         1          2       P2   total  0.000045   0.000154
+11         1         1          2       P3   total  0.000069   0.000175
 4          1         2          1       P0   total  0.000000   0.000000
 5          1         2          1       P1   total  0.000000   0.000000
 6          1         2          1       P2   total  0.000000   0.000000
 7          1         2          1       P3   total  0.000000   0.000000
-0          1         2          2       P0   total  0.395768   0.029278
-1          1         2          2       P1   total  0.020575   0.012851
-2          1         2          2       P2   total  0.010026   0.016739
-3          1         2          2       P3   total  0.004998   0.013335
+0          1         2          2       P0   total  0.395750   0.053673
+1          1         2          2       P1   total -0.016112   0.011058
+2          1         2          2       P2   total -0.001146   0.010523
+3          1         2          2       P3   total  0.005353   0.008157
 chi
    material  group out nuclide  mean  std. dev.
-1         1          1   total   1.0   0.039887
+1         1          1   total   1.0   0.015644
 0         1          2   total   0.0   0.000000
 chi-prompt
    material  group out nuclide  mean  std. dev.
-1         1          1   total   1.0   0.039887
+1         1          1   total   1.0   0.017529
 0         1          2   total   0.0   0.000000
 inverse-velocity
    material  group in nuclide          mean     std. dev.
-1         1         1   total  5.956290e-08  2.255751e-09
-0         1         2   total  2.886630e-06  7.418452e-08
+1         1         1   total  6.047675e-08  4.367288e-09
+0         1         2   total  2.861927e-06  2.241423e-07
 prompt-nu-fission
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.018067   0.000817
-0         1         2   total  0.471762   0.013383
+1         1         1   total  0.018748    0.00054
+0         1         2   total  0.468507    0.03660
 prompt-nu-fission matrix
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.024633   0.002205
+3         1         1          1   total  0.019049   0.001675
 2         1         1          2   total  0.000000   0.000000
-1         1         2          1   total  0.435159   0.014743
+1         1         2          1   total  0.454399   0.022578
 0         1         2          2   total  0.000000   0.000000
 diffusion-coefficient
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.901559   0.047536
-0         1         2   total  0.517778   0.014675
+1         1         1   total  0.868438   0.034686
+0         1         2   total  0.490787   0.036698
 nu-diffusion-coefficient
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.901559   0.047536
-0         1         2   total  0.517778   0.014675
+1         1         1   total  0.868438   0.034686
+0         1         2   total  0.490787   0.036698
 (n,elastic)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.360179   0.016169
-0         1         2   total  0.395768   0.007541
+1         1         1   total  0.368341   0.013033
+0         1         2   total  0.395750   0.029309
 (n,level)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.000568   0.000028
+1         1         1   total  0.000518   0.000016
 0         1         2   total  0.000000   0.000000
 (n,2n)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.000079   0.000042
+1         1         1   total  0.000019   0.000012
 0         1         2   total  0.000000   0.000000
 (n,na)
-   material  group in nuclide          mean     std. dev.
-1         1         1   total  8.510945e-07  8.515320e-07
-0         1         2   total  0.000000e+00  0.000000e+00
+   material  group in nuclide  mean  std. dev.
+1         1         1   total   0.0        0.0
+0         1         2   total   0.0        0.0
 (n,nc)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.008258   0.000928
+1         1         1   total  0.007599   0.000559
 0         1         2   total  0.000000   0.000000
 (n,gamma)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.018749   0.001838
-0         1         2   total  0.072358   0.001951
+1         1         1   total  0.019608   0.001470
+0         1         2   total  0.072029   0.005584
 (n,a)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.000119   0.000023
-0         1         2   total  0.000000   0.000000
+1         1         1   total  0.000109    0.00002
+0         1         2   total  0.000000    0.00000
 (n,Xa)
-   material  group in nuclide     mean  std. dev.
-1         1         1   total  0.00012   0.000022
-0         1         2   total  0.00000   0.000000
+   material  group in nuclide      mean  std. dev.
+1         1         1   total  0.000109    0.00002
+0         1         2   total  0.000000    0.00000
 heating
-   material  group in nuclide          mean      std. dev.
-1         1         1   total  1.216089e+06   54827.397330
-0         1         2   total  3.262993e+07  904277.817101
+   material  group in nuclide          mean     std. dev.
+1         1         1   total  1.270644e+06  3.732629e+04
+0         1         2   total  3.241569e+07  2.530346e+06
 damage-energy
    material  group in nuclide         mean   std. dev.
-1         1         1   total  2317.176742  165.734097
-0         1         2   total  1371.933922   38.919660
+1         1         1   total  2405.735342   79.042000
+0         1         2   total  1362.470597  106.435803
 (n,n1)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.012055   0.000759
+1         1         1   total  0.011931   0.000455
 0         1         2   total  0.000000   0.000000
 (n,a0)
    material  group in nuclide      mean  std. dev.
-1         1         1   total  0.000109   0.000028
-0         1         2   total  0.000000   0.000000
+1         1         1   total  0.000108    0.00002
+0         1         2   total  0.000000    0.00000
 (n,nc) matrix
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.009563   0.001002
+3         1         1          1   total  0.005745   0.001068
 2         1         1          2   total  0.000000   0.000000
 1         1         2          1   total  0.000000   0.000000
 0         1         2          2   total  0.000000   0.000000
 (n,n1) matrix
    material  group in  group out nuclide      mean  std. dev.
-3         1         1          1   total  0.013909    0.00145
-2         1         1          2   total  0.000000    0.00000
-1         1         2          1   total  0.000000    0.00000
-0         1         2          2   total  0.000000    0.00000
+3         1         1          1   total  0.013232   0.001025
+2         1         1          2   total  0.000000   0.000000
+1         1         2          1   total  0.000000   0.000000
+0         1         2          2   total  0.000000   0.000000
 (n,2n) matrix
    material  group in  group out nuclide  mean  std. dev.
 3         1         1          1   total   0.0        0.0
@@ -230,104 +230,104 @@ damage-energy
 0         1         2          2   total   0.0        0.0
 delayed-nu-fission
     material  delayedgroup  group in nuclide      mean     std. dev.
-1          1             1         1   total  0.000004  1.857100e-07
-3          1             2         1   total  0.000025  1.293123e-06
-5          1             3         1   total  0.000026  1.448812e-06
-7          1             4         1   total  0.000068  4.132525e-06
-9          1             5         1   total  0.000037  2.670208e-06
-11         1             6         1   total  0.000015  1.084541e-06
-0          1             1         2   total  0.000108  3.067510e-06
-2          1             2         2   total  0.000558  1.583355e-05
-4          1             3         2   total  0.000533  1.511609e-05
-6          1             4         2   total  0.001195  3.389154e-05
-8          1             5         2   total  0.000490  1.389508e-05
-10         1             6         2   total  0.000205  5.820598e-06
+1          1             1         1   total  0.000004  1.226367e-07
+3          1             2         1   total  0.000026  6.725093e-07
+5          1             3         1   total  0.000027  7.028316e-07
+7          1             4         1   total  0.000068  1.919534e-06
+9          1             5         1   total  0.000037  1.268939e-06
+11         1             6         1   total  0.000015  5.136452e-07
+0          1             1         2   total  0.000107  8.388866e-06
+2          1             2         2   total  0.000554  4.330076e-05
+4          1             3         2   total  0.000529  4.133869e-05
+6          1             4         2   total  0.001186  9.268481e-05
+8          1             5         2   total  0.000486  3.799954e-05
+10         1             6         2   total  0.000204  1.591787e-05
 chi-delayed
     material  delayedgroup  group out nuclide  mean  std. dev.
-1          1             1          1   total   0.0        0.0
-3          1             2          1   total   0.0        0.0
-5          1             3          1   total   0.0        0.0
-7          1             4          1   total   0.0        0.0
-9          1             5          1   total   0.0        0.0
-11         1             6          1   total   0.0        0.0
-0          1             1          2   total   0.0        0.0
-2          1             2          2   total   0.0        0.0
-4          1             3          2   total   0.0        0.0
-6          1             4          2   total   0.0        0.0
-8          1             5          2   total   0.0        0.0
-10         1             6          2   total   0.0        0.0
+1          1             1          1   total   0.0   0.000000
+3          1             2          1   total   0.0   0.000000
+5          1             3          1   total   0.0   0.000000
+7          1             4          1   total   1.0   0.433956
+9          1             5          1   total   0.0   0.000000
+11         1             6          1   total   0.0   0.000000
+0          1             1          2   total   0.0   0.000000
+2          1             2          2   total   0.0   0.000000
+4          1             3          2   total   0.0   0.000000
+6          1             4          2   total   0.0   0.000000
+8          1             5          2   total   0.0   0.000000
+10         1             6          2   total   0.0   0.000000
 beta
     material  delayedgroup  group in nuclide      mean  std. dev.
-1          1             1         1   total  0.000223   0.000009
-3          1             2         1   total  0.001375   0.000067
-5          1             3         1   total  0.001439   0.000076
-7          1             4         1   total  0.003724   0.000217
-9          1             5         1   total  0.002049   0.000142
-11         1             6         1   total  0.000840   0.000058
-0          1             1         2   total  0.000228   0.000008
-2          1             2         2   total  0.001175   0.000041
-4          1             3         2   total  0.001122   0.000040
-6          1             4         2   total  0.002516   0.000089
-8          1             5         2   total  0.001031   0.000036
-10         1             6         2   total  0.000432   0.000015
+1          1             1         1   total  0.000225   0.000006
+3          1             2         1   total  0.001359   0.000033
+5          1             3         1   total  0.001412   0.000034
+7          1             4         1   total  0.003611   0.000094
+9          1             5         1   total  0.001950   0.000064
+11         1             6         1   total  0.000801   0.000026
+0          1             1         2   total  0.000228   0.000019
+2          1             2         2   total  0.001175   0.000096
+4          1             3         2   total  0.001122   0.000092
+6          1             4         2   total  0.002516   0.000206
+8          1             5         2   total  0.001031   0.000085
+10         1             6         2   total  0.000432   0.000035
 decay-rate
    material  delayedgroup nuclide      mean  std. dev.
-0         1             1   total  0.013353   0.000379
-1         1             2   total  0.032612   0.001002
-2         1             3   total  0.121056   0.003938
-3         1             4   total  0.305642   0.010892
-4         1             5   total  0.860947   0.036893
-5         1             6   total  2.891708   0.122310
+0         1             1   total  0.013352   0.000905
+1         1             2   total  0.032619   0.002094
+2         1             3   total  0.121041   0.007464
+3         1             4   total  0.305491   0.017639
+4         1             5   total  0.860388   0.042833
+5         1             6   total  2.889807   0.145503
 delayed-nu-fission matrix
-    material  delayedgroup  group in  group out nuclide  mean  std. dev.
-3          1             1         1          1   total   0.0        0.0
-7          1             2         1          1   total   0.0        0.0
-11         1             3         1          1   total   0.0        0.0
-15         1             4         1          1   total   0.0        0.0
-19         1             5         1          1   total   0.0        0.0
-23         1             6         1          1   total   0.0        0.0
-2          1             1         1          2   total   0.0        0.0
-6          1             2         1          2   total   0.0        0.0
-10         1             3         1          2   total   0.0        0.0
-14         1             4         1          2   total   0.0        0.0
-18         1             5         1          2   total   0.0        0.0
-22         1             6         1          2   total   0.0        0.0
-1          1             1         2          1   total   0.0        0.0
-5          1             2         2          1   total   0.0        0.0
-9          1             3         2          1   total   0.0        0.0
-13         1             4         2          1   total   0.0        0.0
-17         1             5         2          1   total   0.0        0.0
-21         1             6         2          1   total   0.0        0.0
-0          1             1         2          2   total   0.0        0.0
-4          1             2         2          2   total   0.0        0.0
-8          1             3         2          2   total   0.0        0.0
-12         1             4         2          2   total   0.0        0.0
-16         1             5         2          2   total   0.0        0.0
-20         1             6         2          2   total   0.0        0.0
+    material  delayedgroup  group in  group out nuclide      mean  std. dev.
+3          1             1         1          1   total  0.000000   0.000000
+7          1             2         1          1   total  0.000000   0.000000
+11         1             3         1          1   total  0.000000   0.000000
+15         1             4         1          1   total  0.000207   0.000207
+19         1             5         1          1   total  0.000000   0.000000
+23         1             6         1          1   total  0.000000   0.000000
+2          1             1         1          2   total  0.000000   0.000000
+6          1             2         1          2   total  0.000000   0.000000
+10         1             3         1          2   total  0.000000   0.000000
+14         1             4         1          2   total  0.000000   0.000000
+18         1             5         1          2   total  0.000000   0.000000
+22         1             6         1          2   total  0.000000   0.000000
+1          1             1         2          1   total  0.000000   0.000000
+5          1             2         2          1   total  0.000000   0.000000
+9          1             3         2          1   total  0.000000   0.000000
+13         1             4         2          1   total  0.005002   0.001278
+17         1             5         2          1   total  0.000000   0.000000
+21         1             6         2          1   total  0.000000   0.000000
+0          1             1         2          2   total  0.000000   0.000000
+4          1             2         2          2   total  0.000000   0.000000
+8          1             3         2          2   total  0.000000   0.000000
+12         1             4         2          2   total  0.000000   0.000000
+16         1             5         2          2   total  0.000000   0.000000
+20         1             6         2          2   total  0.000000   0.000000
 total
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.313574   0.016009
-0         2         2   total  0.300858   0.005911
+1         2         1   total  0.320478   0.012201
+0         2         2   total  0.300681   0.029685
 transport
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.266154   0.017348
-0         2         2   total  0.300597   0.011062
+1         2         1   total  0.267653   0.015887
+0         2         2   total  0.327250   0.035614
 nu-transport
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.266154   0.017348
-0         2         2   total  0.300597   0.011062
+1         2         1   total  0.267653   0.015887
+0         2         2   total  0.327250   0.035614
 absorption
-   material  group in nuclide     mean  std. dev.
-1         2         1   total  0.00150   0.000118
-0         2         2   total  0.00542   0.000201
+   material  group in nuclide      mean  std. dev.
+1         2         1   total  0.001040   0.000121
+0         2         2   total  0.005284   0.000548
 reduced absorption
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.001481   0.000119
-0         2         2   total  0.005420   0.000201
+1         2         1   total  0.001040   0.000121
+0         2         2   total  0.005284   0.000548
 capture
-   material  group in nuclide     mean  std. dev.
-1         2         1   total  0.00150   0.000118
-0         2         2   total  0.00542   0.000201
+   material  group in nuclide      mean  std. dev.
+1         2         1   total  0.001040   0.000121
+0         2         2   total  0.005284   0.000548
 fission
    material  group in nuclide  mean  std. dev.
 1         2         1   total   0.0        0.0
@@ -342,18 +342,18 @@ kappa-fission
 0         2         2   total   0.0        0.0
 scatter
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.312074   0.015934
-0         2         2   total  0.295438   0.005743
+1         2         1   total  0.319438   0.012181
+0         2         2   total  0.295397   0.029142
 nu-scatter
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.318594   0.023203
-0         2         2   total  0.295662   0.031486
+1         2         1   total  0.314727   0.014272
+0         2         2   total  0.295807   0.038170
 scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         2         1          1       P0   total  0.318594   0.023203
-13         2         1          1       P1   total  0.047420   0.006684
-14         2         1          1       P2   total  0.029426   0.008048
-15         2         1          1       P3   total  0.007335   0.004092
+12         2         1          1       P0   total  0.314727   0.014272
+13         2         1          1       P1   total  0.052826   0.010175
+14         2         1          1       P2   total  0.033176   0.002264
+15         2         1          1       P3   total  0.006129   0.004317
 8          2         1          2       P0   total  0.000000   0.000000
 9          2         1          2       P1   total  0.000000   0.000000
 10         2         1          2       P2   total  0.000000   0.000000
@@ -362,16 +362,16 @@ scatter matrix
 5          2         2          1       P1   total  0.000000   0.000000
 6          2         2          1       P2   total  0.000000   0.000000
 7          2         2          1       P3   total  0.000000   0.000000
-0          2         2          2       P0   total  0.295662   0.031486
-1          2         2          2       P1   total  0.000261   0.009350
-2          2         2          2       P2   total -0.004912   0.007866
-3          2         2          2       P3   total -0.018278   0.012339
+0          2         2          2       P0   total  0.295807   0.038170
+1          2         2          2       P1   total -0.026569   0.019676
+2          2         2          2       P2   total -0.005395   0.007578
+3          2         2          2       P3   total -0.005680   0.012737
 nu-scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         2         1          1       P0   total  0.318594   0.023203
-13         2         1          1       P1   total  0.047420   0.006684
-14         2         1          1       P2   total  0.029426   0.008048
-15         2         1          1       P3   total  0.007335   0.004092
+12         2         1          1       P0   total  0.314727   0.014272
+13         2         1          1       P1   total  0.052826   0.010175
+14         2         1          1       P2   total  0.033176   0.002264
+15         2         1          1       P3   total  0.006129   0.004317
 8          2         1          2       P0   total  0.000000   0.000000
 9          2         1          2       P1   total  0.000000   0.000000
 10         2         1          2       P2   total  0.000000   0.000000
@@ -380,16 +380,16 @@ nu-scatter matrix
 5          2         2          1       P1   total  0.000000   0.000000
 6          2         2          1       P2   total  0.000000   0.000000
 7          2         2          1       P3   total  0.000000   0.000000
-0          2         2          2       P0   total  0.295662   0.031486
-1          2         2          2       P1   total  0.000261   0.009350
-2          2         2          2       P2   total -0.004912   0.007866
-3          2         2          2       P3   total -0.018278   0.012339
+0          2         2          2       P0   total  0.295807   0.038170
+1          2         2          2       P1   total -0.026569   0.019676
+2          2         2          2       P2   total -0.005395   0.007578
+3          2         2          2       P3   total -0.005680   0.012737
 multiplicity matrix
    material  group in  group out nuclide  mean  std. dev.
-3         2         1          1   total   1.0   0.071770
+3         2         1          1   total   1.0   0.043852
 2         2         1          2   total   0.0   0.000000
 1         2         2          1   total   0.0   0.000000
-0         2         2          2   total   1.0   0.103652
+0         2         2          2   total   1.0   0.139361
 nu-fission matrix
    material  group in  group out nuclide  mean  std. dev.
 3         2         1          1   total   0.0        0.0
@@ -398,16 +398,16 @@ nu-fission matrix
 0         2         2          2   total   0.0        0.0
 scatter probability matrix
    material  group in  group out nuclide  mean  std. dev.
-3         2         1          1   total   1.0   0.071770
+3         2         1          1   total   1.0   0.043852
 2         2         1          2   total   0.0   0.000000
 1         2         2          1   total   0.0   0.000000
-0         2         2          2   total   1.0   0.103652
+0         2         2          2   total   1.0   0.139361
 consistent scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         2         1          1       P0   total  0.312074   0.027487
-13         2         1          1       P1   total  0.046450   0.006939
-14         2         1          1       P2   total  0.028824   0.008012
-15         2         1          1       P3   total  0.007185   0.004024
+12         2         1          1       P0   total  0.319438   0.018563
+13         2         1          1       P1   total  0.053616   0.010510
+14         2         1          1       P2   total  0.033673   0.002604
+15         2         1          1       P3   total  0.006221   0.004387
 8          2         1          2       P0   total  0.000000   0.000000
 9          2         1          2       P1   total  0.000000   0.000000
 10         2         1          2       P2   total  0.000000   0.000000
@@ -416,16 +416,16 @@ consistent scatter matrix
 5          2         2          1       P1   total  0.000000   0.000000
 6          2         2          1       P2   total  0.000000   0.000000
 7          2         2          1       P3   total  0.000000   0.000000
-0          2         2          2       P0   total  0.295438   0.031157
-1          2         2          2       P1   total  0.000261   0.009343
-2          2         2          2       P2   total -0.004909   0.007859
-3          2         2          2       P3   total -0.018264   0.012327
+0          2         2          2       P0   total  0.295397   0.050438
+1          2         2          2       P1   total -0.026532   0.019872
+2          2         2          2       P2   total -0.005388   0.007591
+3          2         2          2       P3   total -0.005672   0.012735
 consistent nu-scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         2         1          1       P0   total  0.312074   0.035456
-13         2         1          1       P1   total  0.046450   0.007699
-14         2         1          1       P2   total  0.028824   0.008274
-15         2         1          1       P3   total  0.007185   0.004057
+12         2         1          1       P0   total  0.319438   0.023255
+13         2         1          1       P1   total  0.053616   0.010769
+14         2         1          1       P2   total  0.033673   0.002993
+15         2         1          1       P3   total  0.006221   0.004396
 8          2         1          2       P0   total  0.000000   0.000000
 9          2         1          2       P1   total  0.000000   0.000000
 10         2         1          2       P2   total  0.000000   0.000000
@@ -434,10 +434,10 @@ consistent nu-scatter matrix
 5          2         2          1       P1   total  0.000000   0.000000
 6          2         2          1       P2   total  0.000000   0.000000
 7          2         2          1       P3   total  0.000000   0.000000
-0          2         2          2       P0   total  0.295438   0.043686
-1          2         2          2       P1   total  0.000261   0.009343
-2          2         2          2       P2   total -0.004909   0.007876
-3          2         2          2       P3   total -0.018264   0.012471
+0          2         2          2       P0   total  0.295397   0.065105
+1          2         2          2       P1   total -0.026532   0.020213
+2          2         2          2       P2   total -0.005388   0.007628
+3          2         2          2       P3   total -0.005672   0.012759
 chi
    material  group out nuclide  mean  std. dev.
 1         2          1   total   0.0        0.0
@@ -448,8 +448,8 @@ chi-prompt
 0         2          2   total   0.0        0.0
 inverse-velocity
    material  group in nuclide          mean     std. dev.
-1         2         1   total  6.076563e-08  2.727519e-09
-0         2         2   total  2.996176e-06  1.110139e-07
+1         2         1   total  6.068670e-08  4.861582e-09
+0         2         2   total  2.921021e-06  3.028269e-07
 prompt-nu-fission
    material  group in nuclide  mean  std. dev.
 1         2         1   total   0.0        0.0
@@ -462,72 +462,72 @@ prompt-nu-fission matrix
 0         2         2          2   total   0.0        0.0
 diffusion-coefficient
    material  group in nuclide      mean  std. dev.
-1         2         1   total  1.252406   0.081631
-0         2         2   total  1.108904   0.040809
+1         2         1   total  1.245396   0.073923
+0         2         2   total  1.018589   0.110853
 nu-diffusion-coefficient
    material  group in nuclide      mean  std. dev.
-1         2         1   total  1.252406   0.081631
-0         2         2   total  1.108904   0.040809
+1         2         1   total  1.245396   0.073923
+0         2         2   total  1.018589   0.110853
 (n,elastic)
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.302664   0.015531
-0         2         2   total  0.295438   0.005743
+1         2         1   total  0.310592   0.012188
+0         2         2   total  0.295397   0.029142
 (n,level)
    material  group in nuclide  mean  std. dev.
 1         2         1   total   0.0        0.0
 0         2         2   total   0.0        0.0
 (n,2n)
-   material  group in nuclide      mean  std. dev.
-1         2         1   total  0.000019   0.000017
-0         2         2   total  0.000000   0.000000
+   material  group in nuclide  mean  std. dev.
+1         2         1   total   0.0        0.0
+0         2         2   total   0.0        0.0
 (n,na)
    material  group in nuclide          mean     std. dev.
-1         2         1   total  4.892570e-09  4.436228e-09
+1         2         1   total  2.157338e-13  1.864239e-13
 0         2         2   total  0.000000e+00  0.000000e+00
 (n,nc)
-   material  group in nuclide      mean  std. dev.
-1         2         1   total  0.001998   0.000408
-0         2         2   total  0.000000   0.000000
+   material  group in nuclide     mean  std. dev.
+1         2         1   total  0.00187   0.000152
+0         2         2   total  0.00000   0.000000
 (n,gamma)
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.001497   0.000118
-0         2         2   total  0.005419   0.000201
+1         2         1   total  0.001038   0.000121
+0         2         2   total  0.005283   0.000548
 (n,a)
    material  group in nuclide          mean     std. dev.
-1         2         1   total  9.546292e-07  9.180854e-08
-0         2         2   total  2.736059e-07  5.318732e-09
+1         2         1   total  7.402079e-07  3.197538e-08
+0         2         2   total  2.735682e-07  2.698845e-08
 (n,Xa)
    material  group in nuclide          mean     std. dev.
-1         2         1   total  9.595218e-07  9.558005e-08
-0         2         2   total  2.736059e-07  5.318732e-09
+1         2         1   total  7.402081e-07  3.197534e-08
+0         2         2   total  2.735682e-07  2.698845e-08
 heating
    material  group in nuclide         mean   std. dev.
-1         2         1   total  2551.878627  212.765665
-0         2         2   total     2.332139    0.087384
+1         2         1   total  2479.710231  157.161879
+0         2         2   total     2.314706    0.265682
 damage-energy
-   material  group in nuclide         mean   std. dev.
-1         2         1   total  1585.677533  130.382718
-0         2         2   total     0.295108    0.010079
+   material  group in nuclide         mean  std. dev.
+1         2         1   total  1566.139383  61.602407
+0         2         2   total     0.288739   0.029698
 (n,n1)
    material  group in nuclide      mean  std. dev.
-1         2         1   total  0.002835   0.000246
+1         2         1   total  0.002897   0.000181
 0         2         2   total  0.000000   0.000000
 (n,a0)
    material  group in nuclide          mean     std. dev.
-1         2         1   total  7.169006e-07  6.251188e-08
-0         2         2   total  2.733303e-07  5.313375e-09
+1         2         1   total  6.646569e-07  2.959064e-08
+0         2         2   total  2.732927e-07  2.696126e-08
 (n,nc) matrix
    material  group in  group out nuclide      mean  std. dev.
-3         2         1          1   total  0.001489   0.001491
+3         2         1          1   total  0.000488   0.000488
 2         2         1          2   total  0.000000   0.000000
 1         2         2          1   total  0.000000   0.000000
 0         2         2          2   total  0.000000   0.000000
 (n,n1) matrix
-   material  group in  group out nuclide      mean  std. dev.
-3         2         1          1   total  0.001985   0.000507
-2         2         1          2   total  0.000000   0.000000
-1         2         2          1   total  0.000000   0.000000
-0         2         2          2   total  0.000000   0.000000
+   material  group in  group out nuclide     mean  std. dev.
+3         2         1          1   total  0.00244   0.001094
+2         2         1          2   total  0.00000   0.000000
+1         2         2          1   total  0.00000   0.000000
+0         2         2          2   total  0.00000   0.000000
 (n,2n) matrix
    material  group in  group out nuclide  mean  std. dev.
 3         2         1          1   total   0.0        0.0
@@ -612,28 +612,28 @@ delayed-nu-fission matrix
 20         2             6         2          2   total   0.0        0.0
 total
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.682896   0.024140
-0         3         2   total  2.032938   0.085675
+1         3         1   total  0.692034   0.019973
+0         3         2   total  2.033425   0.189463
 transport
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.298986   0.026905
-0         3         2   total  1.471887   0.096907
+1         3         1   total  0.298754   0.021602
+0         3         2   total  1.459465   0.197868
 nu-transport
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.298986   0.026905
-0         3         2   total  1.471887   0.096907
+1         3         1   total  0.298754   0.021602
+0         3         2   total  1.459465   0.197868
 absorption
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000693   0.000020
-0         3         2   total  0.031171   0.001403
+1         3         1   total  0.000699   0.000035
+0         3         2   total  0.031056   0.003017
 reduced absorption
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000693   0.000020
-0         3         2   total  0.031171   0.001403
+1         3         1   total  0.000699   0.000035
+0         3         2   total  0.031056   0.003017
 capture
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000693   0.000020
-0         3         2   total  0.031171   0.001403
+1         3         1   total  0.000699   0.000035
+0         3         2   total  0.031056   0.003017
 fission
    material  group in nuclide  mean  std. dev.
 1         3         1   total   0.0        0.0
@@ -648,54 +648,54 @@ kappa-fission
 0         3         2   total   0.0        0.0
 scatter
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.682202   0.024123
-0         3         2   total  2.001768   0.084281
+1         3         1   total  0.691336   0.019945
+0         3         2   total  2.002368   0.186454
 nu-scatter
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.672567   0.017434
-0         3         2   total  2.004576   0.129211
+1         3         1   total  0.684356   0.011639
+0         3         2   total  1.990840   0.178205
 scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         3         1          1       P0   total  0.641741   0.016600
-13         3         1          1       P1   total  0.383841   0.011878
-14         3         1          1       P2   total  0.149909   0.005512
-15         3         1          1       P3   total  0.004238   0.002485
-8          3         1          2       P0   total  0.030826   0.000973
-9          3         1          2       P1   total  0.009943   0.000415
-10         3         1          2       P2   total -0.003002   0.000628
-11         3         1          2       P3   total -0.004234   0.000393
-4          3         2          1       P0   total  0.000467   0.000467
-5          3         2          1       P1   total  0.000345   0.000345
-6          3         2          1       P2   total  0.000149   0.000149
-7          3         2          1       P3   total -0.000047   0.000047
-0          3         2          2       P0   total  2.004109   0.129225
-1          3         2          2       P1   total  0.511363   0.043448
-2          3         2          2       P2   total  0.108599   0.014070
-3          3         2          2       P3   total  0.035733   0.021921
+12         3         1          1       P0   total  0.652706   0.011111
+13         3         1          1       P1   total  0.393203   0.008164
+14         3         1          1       P2   total  0.162146   0.003332
+15         3         1          1       P3   total  0.014967   0.004773
+8          3         1          2       P0   total  0.031650   0.000613
+9          3         1          2       P1   total  0.009845   0.000647
+10         3         1          2       P2   total -0.003284   0.000758
+11         3         1          2       P3   total -0.003971   0.000187
+4          3         2          1       P0   total  0.000474   0.000475
+5          3         2          1       P1   total  0.000396   0.000397
+6          3         2          1       P2   total  0.000260   0.000261
+7          3         2          1       P3   total  0.000099   0.000099
+0          3         2          2       P0   total  1.990366   0.178042
+1          3         2          2       P1   total  0.523546   0.053239
+2          3         2          2       P2   total  0.101180   0.015832
+3          3         2          2       P3   total  0.017873   0.005685
 nu-scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         3         1          1       P0   total  0.641741   0.016600
-13         3         1          1       P1   total  0.383841   0.011878
-14         3         1          1       P2   total  0.149909   0.005512
-15         3         1          1       P3   total  0.004238   0.002485
-8          3         1          2       P0   total  0.030826   0.000973
-9          3         1          2       P1   total  0.009943   0.000415
-10         3         1          2       P2   total -0.003002   0.000628
-11         3         1          2       P3   total -0.004234   0.000393
-4          3         2          1       P0   total  0.000467   0.000467
-5          3         2          1       P1   total  0.000345   0.000345
-6          3         2          1       P2   total  0.000149   0.000149
-7          3         2          1       P3   total -0.000047   0.000047
-0          3         2          2       P0   total  2.004109   0.129225
-1          3         2          2       P1   total  0.511363   0.043448
-2          3         2          2       P2   total  0.108599   0.014070
-3          3         2          2       P3   total  0.035733   0.021921
+12         3         1          1       P0   total  0.652706   0.011111
+13         3         1          1       P1   total  0.393203   0.008164
+14         3         1          1       P2   total  0.162146   0.003332
+15         3         1          1       P3   total  0.014967   0.004773
+8          3         1          2       P0   total  0.031650   0.000613
+9          3         1          2       P1   total  0.009845   0.000647
+10         3         1          2       P2   total -0.003284   0.000758
+11         3         1          2       P3   total -0.003971   0.000187
+4          3         2          1       P0   total  0.000474   0.000475
+5          3         2          1       P1   total  0.000396   0.000397
+6          3         2          1       P2   total  0.000260   0.000261
+7          3         2          1       P3   total  0.000099   0.000099
+0          3         2          2       P0   total  1.990366   0.178042
+1          3         2          2       P1   total  0.523546   0.053239
+2          3         2          2       P2   total  0.101180   0.015832
+3          3         2          2       P3   total  0.017873   0.005685
 multiplicity matrix
    material  group in  group out nuclide  mean  std. dev.
-3         3         1          1   total   1.0   0.022200
-2         3         1          2   total   1.0   0.033880
+3         3         1          1   total   1.0   0.020267
+2         3         1          2   total   1.0   0.024112
 1         3         2          1   total   1.0   1.414214
-0         3         2          2   total   1.0   0.066922
+0         3         2          2   total   1.0   0.093189
 nu-fission matrix
    material  group in  group out nuclide  mean  std. dev.
 3         3         1          1   total   0.0        0.0
@@ -704,46 +704,46 @@ nu-fission matrix
 0         3         2          2   total   0.0        0.0
 scatter probability matrix
    material  group in  group out nuclide      mean  std. dev.
-3         3         1          1   total  0.954167   0.020729
-2         3         1          2   total  0.045833   0.001296
-1         3         2          1   total  0.000233   0.000233
-0         3         2          2   total  0.999767   0.066899
+3         3         1          1   total  0.953753   0.018903
+2         3         1          2   total  0.046247   0.001011
+1         3         2          1   total  0.000238   0.000239
+0         3         2          2   total  0.999762   0.093156
 consistent scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         3         1          1       P0   total  0.650935   0.027014
-13         3         1          1       P1   total  0.389340   0.017459
-14         3         1          1       P2   total  0.152056   0.007457
-15         3         1          1       P3   total  0.004299   0.002525
-8          3         1          2       P0   total  0.031268   0.001416
-9          3         1          2       P1   total  0.010085   0.000533
-10         3         1          2       P2   total -0.003045   0.000645
-11         3         1          2       P3   total -0.004295   0.000423
-4          3         2          1       P0   total  0.000466   0.000467
-5          3         2          1       P1   total  0.000344   0.000345
-6          3         2          1       P2   total  0.000148   0.000149
-7          3         2          1       P3   total -0.000047   0.000047
-0          3         2          2       P0   total  2.001301   0.158219
-1          3         2          2       P1   total  0.510647   0.049275
-2          3         2          2       P2   total  0.108446   0.014900
-3          3         2          2       P3   total  0.035683   0.021951
+12         3         1          1       P0   total  0.659363   0.023079
+13         3         1          1       P1   total  0.397213   0.014683
+14         3         1          1       P2   total  0.163800   0.006035
+15         3         1          1       P3   total  0.015120   0.004843
+8          3         1          2       P0   total  0.031973   0.001157
+9          3         1          2       P1   total  0.009945   0.000721
+10         3         1          2       P2   total -0.003318   0.000772
+11         3         1          2       P3   total -0.004011   0.000225
+4          3         2          1       P0   total  0.000477   0.000480
+5          3         2          1       P1   total  0.000399   0.000401
+6          3         2          1       P2   total  0.000262   0.000264
+7          3         2          1       P3   total  0.000099   0.000100
+0          3         2          2       P0   total  2.001892   0.263710
+1          3         2          2       P1   total  0.526577   0.073894
+2          3         2          2       P2   total  0.101766   0.018719
+3          3         2          2       P3   total  0.017976   0.005976
 consistent nu-scatter matrix
     material  group in  group out legendre nuclide      mean  std. dev.
-12         3         1          1       P0   total  0.650935   0.030637
-13         3         1          1       P1   total  0.389340   0.019481
-14         3         1          1       P2   total  0.152056   0.008185
-15         3         1          1       P3   total  0.004299   0.002526
-8          3         1          2       P0   total  0.031268   0.001768
-9          3         1          2       P1   total  0.010085   0.000633
-10         3         1          2       P2   total -0.003045   0.000653
-11         3         1          2       P3   total -0.004295   0.000447
-4          3         2          1       P0   total  0.000466   0.000808
-5          3         2          1       P1   total  0.000344   0.000597
-6          3         2          1       P2   total  0.000148   0.000257
-7          3         2          1       P3   total -0.000047   0.000081
-0          3         2          2       P0   total  2.001301   0.207294
-1          3         2          2       P1   total  0.510647   0.059966
-2          3         2          2       P2   total  0.108446   0.016574
-3          3         2          2       P3   total  0.035683   0.022080
+12         3         1          1       P0   total  0.659363   0.026669
+13         3         1          1       P1   total  0.397213   0.016746
+14         3         1          1       P2   total  0.163800   0.006888
+15         3         1          1       P3   total  0.015120   0.004853
+8          3         1          2       P0   total  0.031973   0.001391
+9          3         1          2       P1   total  0.009945   0.000759
+10         3         1          2       P2   total -0.003318   0.000776
+11         3         1          2       P3   total -0.004011   0.000245
+4          3         2          1       P0   total  0.000477   0.000827
+5          3         2          1       P1   total  0.000399   0.000692
+6          3         2          1       P2   total  0.000262   0.000455
+7          3         2          1       P3   total  0.000099   0.000172
+0          3         2          2       P0   total  2.001892   0.323026
+1          3         2          2       P1   total  0.526577   0.088703
+2          3         2          2       P2   total  0.101766   0.020985
+3          3         2          2       P3   total  0.017976   0.006207
 chi
    material  group out nuclide  mean  std. dev.
 1         3          1   total   0.0        0.0
@@ -754,8 +754,8 @@ chi-prompt
 0         3          2   total   0.0        0.0
 inverse-velocity
    material  group in nuclide          mean     std. dev.
-1         3         1   total  6.023693e-08  1.678511e-09
-0         3         2   total  2.995346e-06  1.347817e-07
+1         3         1   total  6.187551e-08  3.945367e-09
+0         3         2   total  2.984343e-06  2.898715e-07
 prompt-nu-fission
    material  group in nuclide  mean  std. dev.
 1         3         1   total   0.0        0.0
@@ -768,60 +768,60 @@ prompt-nu-fission matrix
 0         3         2          2   total   0.0        0.0
 diffusion-coefficient
    material  group in nuclide      mean  std. dev.
-1         3         1   total  1.114881   0.100326
-0         3         2   total  0.226467   0.014910
+1         3         1   total  1.115746   0.080676
+0         3         2   total  0.228394   0.030965
 nu-diffusion-coefficient
    material  group in nuclide      mean  std. dev.
-1         3         1   total  1.114881   0.100326
-0         3         2   total  0.226467   0.014910
+1         3         1   total  1.115746   0.080676
+0         3         2   total  0.228394   0.030965
 (n,elastic)
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.682168   0.024125
-0         3         2   total  2.001768   0.084281
+1         3         1   total  0.691333   0.019945
+0         3         2   total  2.002368   0.186454
 (n,level)
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000033   0.000019
+1         3         1   total  0.000003   0.000002
 0         3         2   total  0.000000   0.000000
 (n,2n)
    material  group in nuclide  mean  std. dev.
 1         3         1   total   0.0        0.0
 0         3         2   total   0.0        0.0
 (n,na)
-   material  group in nuclide          mean     std. dev.
-1         3         1   total  9.967562e-07  9.970492e-07
-0         3         2   total  0.000000e+00  0.000000e+00
+   material  group in nuclide  mean  std. dev.
+1         3         1   total   0.0        0.0
+0         3         2   total   0.0        0.0
 (n,nc)
-   material  group in nuclide          mean     std. dev.
-1         3         1   total  8.867935e-07  8.870902e-07
-0         3         2   total  0.000000e+00  0.000000e+00
+   material  group in nuclide  mean  std. dev.
+1         3         1   total   0.0        0.0
+0         3         2   total   0.0        0.0
 (n,gamma)
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000219   0.000006
-0         3         2   total  0.010855   0.000488
+1         3         1   total  0.000225   0.000014
+0         3         2   total  0.010815   0.001050
 (n,a)
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000474   0.000015
-0         3         2   total  0.020316   0.000914
+1         3         1   total  0.000474   0.000021
+0         3         2   total  0.020241   0.001966
 (n,Xa)
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000475   0.000015
-0         3         2   total  0.020316   0.000914
+1         3         1   total  0.000474   0.000021
+0         3         2   total  0.020242   0.001966
 heating
    material  group in nuclide          mean    std. dev.
-1         3         1   total  77986.174023  6103.108384
-0         3         2   total  59060.658662  3576.641806
+1         3         1   total  74978.139095  3970.725596
+0         3         2   total  58539.697122  6443.645451
 damage-energy
    material  group in nuclide         mean  std. dev.
-1         3         1   total  1107.847239  59.901669
-0         3         2   total   331.691439  14.926212
+1         3         1   total  1161.596397  41.187452
+0         3         2   total   330.473159  32.100259
 (n,n1)
-   material  group in nuclide      mean     std. dev.
-1         3         1   total  0.000001  3.988839e-07
-0         3         2   total  0.000000  0.000000e+00
+   material  group in nuclide          mean     std. dev.
+1         3         1   total  2.912377e-07  3.897408e-08
+0         3         2   total  0.000000e+00  0.000000e+00
 (n,a0)
    material  group in nuclide      mean  std. dev.
-1         3         1   total  0.000084   0.000016
-0         3         2   total  0.001278   0.000058
+1         3         1   total  0.000082   0.000009
+0         3         2   total  0.001273   0.000124
 (n,nc) matrix
    material  group in  group out nuclide  mean  std. dev.
 3         3         1          1   total   0.0        0.0
diff --git a/tests/regression_tests/mgxs_library_no_nuclides/test.py b/tests/regression_tests/mgxs_library_no_nuclides/test.py
index af14a5dc8..a02086af3 100644
--- a/tests/regression_tests/mgxs_library_no_nuclides/test.py
+++ b/tests/regression_tests/mgxs_library_no_nuclides/test.py
@@ -39,7 +39,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _get_results(self, hash_output=False):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/mgxs_library_nuclides/inputs_true.dat b/tests/regression_tests/mgxs_library_nuclides/inputs_true.dat
index 9b3a22510..c35e57f0e 100644
--- a/tests/regression_tests/mgxs_library_nuclides/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_nuclides/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_nuclides/results_true.dat b/tests/regression_tests/mgxs_library_nuclides/results_true.dat
index c2188997d..c05ec9b48 100644
--- a/tests/regression_tests/mgxs_library_nuclides/results_true.dat
+++ b/tests/regression_tests/mgxs_library_nuclides/results_true.dat
@@ -1 +1 @@
-4ae3b5a70ad72b1be261aee3ab19e0261d1c12f4d4ca50b712d1ba76041bd5387c69fa9fa326619ad588db206cd9aaf464b0025d71ea9b8b1137af0102bca87f
\ No newline at end of file
+d1e4ab2c0d85bb5da617db9c7a3731494114e2fd3ba75ae8eaa1f0a36648f73fcbcfb487390789b0124719cabdcbfa84af4bb118f11bdc548d1065e1b5af836a
\ No newline at end of file
diff --git a/tests/regression_tests/mgxs_library_nuclides/test.py b/tests/regression_tests/mgxs_library_nuclides/test.py
index 8a7673565..a10070358 100644
--- a/tests/regression_tests/mgxs_library_nuclides/test.py
+++ b/tests/regression_tests/mgxs_library_nuclides/test.py
@@ -36,7 +36,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=False)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=False)
 
     def _get_results(self, hash_output=True):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/mgxs_library_specific_nuclides/inputs_true.dat b/tests/regression_tests/mgxs_library_specific_nuclides/inputs_true.dat
index 1372741c9..dd9d1ceb0 100644
--- a/tests/regression_tests/mgxs_library_specific_nuclides/inputs_true.dat
+++ b/tests/regression_tests/mgxs_library_specific_nuclides/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 (2.4%)">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="6.55"/>
-      <nuclide ao="0.021827" name="Zr90"/>
-      <nuclide ao="0.00476" name="Zr91"/>
-      <nuclide ao="0.0072758" name="Zr92"/>
-      <nuclide ao="0.0073734" name="Zr94"/>
-      <nuclide ao="0.0011879" name="Zr96"/>
+      <density value="6.55" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.021827"/>
+      <nuclide name="Zr91" ao="0.00476"/>
+      <nuclide name="Zr92" ao="0.0072758"/>
+      <nuclide name="Zr94" ao="0.0073734"/>
+      <nuclide name="Zr96" ao="0.0011879"/>
     </material>
     <material id="3" name="Hot borated water">
-      <density units="g/cm3" value="0.740582"/>
-      <nuclide ao="0.049457" name="H1"/>
-      <nuclide ao="0.024672" name="O16"/>
-      <nuclide ao="8.0042e-06" name="B10"/>
-      <nuclide ao="3.2218e-05" name="B11"/>
+      <density value="0.740582" units="g/cm3"/>
+      <nuclide name="H1" ao="0.049457"/>
+      <nuclide name="O16" ao="0.024672"/>
+      <nuclide name="B10" ao="8.0042e-06"/>
+      <nuclide name="B11" ao="3.2218e-05"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="Fuel" region="-1" universe="0"/>
-    <cell id="2" material="2" name="Cladding" region="1 -2" universe="0"/>
-    <cell id="3" material="3" name="Water" region="2 3 -4 5 -6" universe="0"/>
-    <surface coeffs="0 0 0.39218" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0 0 0.4572" id="2" name="Clad OR" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-0.63" id="3" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="4" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-0.63" id="5" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.63" id="6" name="top" type="y-plane"/>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
       </space>
diff --git a/tests/regression_tests/mgxs_library_specific_nuclides/results_true.dat b/tests/regression_tests/mgxs_library_specific_nuclides/results_true.dat
index eaf56b966..0c44eb132 100644
--- a/tests/regression_tests/mgxs_library_specific_nuclides/results_true.dat
+++ b/tests/regression_tests/mgxs_library_specific_nuclides/results_true.dat
@@ -1 +1 @@
-08d5c199c51496f86fdd739bf7ee0e143a9a159da0f4d364ec970557e5c1fc92a202d906dcae91812e665fd2e88dd7db1e4913ef6b91f456f23b52093c83f483
\ No newline at end of file
+efcd9fd6be2ed6c98bbe5279cbacdb287597fcbbc4ed49e164b07e0861f28877b1bef2ad82d70fdac95965f7ed0d0990f77c8545718836b1b55a16b4243208d0
\ No newline at end of file
diff --git a/tests/regression_tests/mgxs_library_specific_nuclides/test.py b/tests/regression_tests/mgxs_library_specific_nuclides/test.py
index 61910e539..0ccbb83bd 100644
--- a/tests/regression_tests/mgxs_library_specific_nuclides/test.py
+++ b/tests/regression_tests/mgxs_library_specific_nuclides/test.py
@@ -37,7 +37,7 @@ class MGXSTestHarness(PyAPITestHarness):
         self.mgxs_lib.build_library()
 
         # Add tallies
-        self.mgxs_lib.add_to_tallies_file(self._model.tallies, merge=True)
+        self.mgxs_lib.add_to_tallies(self._model.tallies, merge=True)
 
     def _get_results(self, hash_output=True):
         """Digest info in the statepoint and return as a string."""
diff --git a/tests/regression_tests/microxs/test.py b/tests/regression_tests/microxs/test.py
index a35150a1b..781be3ef7 100644
--- a/tests/regression_tests/microxs/test.py
+++ b/tests/regression_tests/microxs/test.py
@@ -13,55 +13,66 @@ CHAIN_FILE = Path(__file__).parents[2] / "chain_simple.xml"
 @pytest.fixture(scope="module")
 def model():
     fuel = openmc.Material(name="uo2")
-    fuel.add_element("U", 1, percent_type="ao", enrichment=4.25)
-    fuel.add_element("O", 2)
+    fuel.add_nuclide("U235", 1.0)
+    fuel.add_nuclide("O16", 2.0)
     fuel.set_density("g/cc", 10.4)
 
-    clad = openmc.Material(name="clad")
-    clad.add_element("Zr", 1)
-    clad.set_density("g/cc", 6)
-
-    water = openmc.Material(name="water")
-    water.add_element("O", 1)
-    water.add_element("H", 2)
-    water.set_density("g/cc", 1.0)
-    water.add_s_alpha_beta("c_H_in_H2O")
-
-    radii = [0.42, 0.45]
-    fuel.volume = np.pi * radii[0] ** 2
-
-    materials = openmc.Materials([fuel, clad, water])
-
-    pin_surfaces = [openmc.ZCylinder(r=r) for r in radii]
-    pin_univ = openmc.model.pin(pin_surfaces, materials)
-    bound_box = openmc.model.RectangularPrism(1.24, 1.24, boundary_type="reflective")
-    root_cell = openmc.Cell(fill=pin_univ, region=-bound_box)
-    geometry = openmc.Geometry([root_cell])
+    sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere, fill=fuel)
+    geometry = openmc.Geometry([cell])
 
     settings = openmc.Settings()
     settings.particles = 1000
     settings.inactive = 5
     settings.batches = 10
 
-    return openmc.Model(geometry, materials, settings)
+    return openmc.Model(geometry, settings=settings)
 
 
-@pytest.mark.parametrize("domain_type", ["materials", "mesh"])
-def test_from_model(model, domain_type):
+@pytest.mark.parametrize(
+    "domain_type, rr_mode",
+    [
+        ("materials", "direct"),
+        ("materials", "flux"),
+        ("materials", "hybrid"),
+        ("mesh", "direct"),
+        ("mesh", "flux"),
+    ]
+)
+def test_from_model(model, domain_type, rr_mode):
     if domain_type == 'materials':
-        domains = model.materials[:1]
+        domains = list(model.geometry.get_all_materials().values())
     elif domain_type == 'mesh':
         mesh = openmc.RegularMesh()
-        mesh.lower_left = (-0.62, -0.62)
-        mesh.upper_right = (0.62, 0.62)
-        mesh.dimension = (3, 3)
+        mesh.lower_left = (-10., -10.)
+        mesh.upper_right = (10., 10.)
+        mesh.dimension = (1, 1)
         domains = mesh
-    nuclides = ['U234', 'U235', 'U238', 'U236', 'O16', 'O17', 'I135', 'Xe135',
-                'Xe136', 'Cs135', 'Gd157', 'Gd156']
-    _, test_xs = get_microxs_and_flux(model, domains, nuclides, chain_file=CHAIN_FILE)
+    nuclides = ['U235', 'O16', 'Xe135']
+    kwargs = {
+        'chain_file': CHAIN_FILE,
+        'path_statepoint': 'neutron_transport.h5',
+    }
+    if rr_mode == 'flux':
+        kwargs['reaction_rate_mode'] = 'flux'
+        kwargs['energies'] = 'CASMO-40'
+    elif rr_mode == 'hybrid':
+        kwargs['reaction_rate_mode'] = 'flux'
+        kwargs['energies'] = 'CASMO-40'
+        kwargs['reaction_rate_opts'] = {
+            'nuclides': ['U235'],
+            'reactions': ['fission'],
+        }
+    else:
+        kwargs['reaction_rate_mode'] = rr_mode
+    _, test_xs = get_microxs_and_flux(model, domains, nuclides, **kwargs)
     if config['update']:
-        test_xs[0].to_csv(f'test_reference_{domain_type}.csv')
-
-    ref_xs = MicroXS.from_csv(f'test_reference_{domain_type}.csv')
+        test_xs[0].to_csv(f'test_reference_{domain_type}_{rr_mode}.csv')
 
+    # Make sure results match reference results
+    ref_xs = MicroXS.from_csv(f'test_reference_{domain_type}_{rr_mode}.csv')
     np.testing.assert_allclose(test_xs[0].data, ref_xs.data, rtol=1e-11)
+
+    # Make sure statepoint file was saved
+    assert Path('neutron_transport.h5').exists()
+    Path('neutron_transport.h5').unlink()
diff --git a/tests/regression_tests/microxs/test_reference_materials.csv b/tests/regression_tests/microxs/test_reference_materials.csv
deleted file mode 100644
index 3e679f980..000000000
--- a/tests/regression_tests/microxs/test_reference_materials.csv
+++ /dev/null
@@ -1,25 +0,0 @@
-nuclides,reactions,groups,xs
-U234,"(n,gamma)",1,21.418670317831076
-U234,fission,1,0.5014588470882162
-U235,"(n,gamma)",1,10.343944102483215
-U235,fission,1,47.46718472611895
-U238,"(n,gamma)",1,0.8741166723597229
-U238,fission,1,0.10829568455139067
-U236,"(n,gamma)",1,9.08348678468935
-U236,fission,1,0.3325287927011424
-O16,"(n,gamma)",1,7.548646353912426e-05
-O16,fission,1,0.0
-O17,"(n,gamma)",1,0.00040184862213103105
-O17,fission,1,0.0
-I135,"(n,gamma)",1,6.691256508942912
-I135,fission,1,0.0
-Xe135,"(n,gamma)",1,223998.6418566729
-Xe135,fission,1,0.0
-Xe136,"(n,gamma)",1,0.022934362666193503
-Xe136,fission,1,0.0
-Cs135,"(n,gamma)",1,2.2845395222353204
-Cs135,fission,1,0.0
-Gd157,"(n,gamma)",1,12582.07962003624
-Gd157,fission,1,0.0
-Gd156,"(n,gamma)",1,2.942112751533234
-Gd156,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_materials_direct.csv b/tests/regression_tests/microxs/test_reference_materials_direct.csv
new file mode 100644
index 000000000..4a63fed85
--- /dev/null
+++ b/tests/regression_tests/microxs/test_reference_materials_direct.csv
@@ -0,0 +1,7 @@
+nuclides,reactions,groups,xs
+U235,"(n,gamma)",1,0.1475718536187164
+U235,fission,1,1.2504996049257149
+O16,"(n,gamma)",1,0.00010981236259441559
+O16,fission,1,0.0
+Xe135,"(n,gamma)",1,0.014570546772870611
+Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_materials_flux.csv b/tests/regression_tests/microxs/test_reference_materials_flux.csv
new file mode 100644
index 000000000..5eb29902e
--- /dev/null
+++ b/tests/regression_tests/microxs/test_reference_materials_flux.csv
@@ -0,0 +1,7 @@
+nuclides,reactions,groups,xs
+U235,"(n,gamma)",1,0.15003016703758473
+U235,fission,1,1.2646269005413537
+O16,"(n,gamma)",1,0.00012069778439640301
+O16,fission,1,0.0
+Xe135,"(n,gamma)",1,0.014820264774863562
+Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_materials_hybrid.csv b/tests/regression_tests/microxs/test_reference_materials_hybrid.csv
new file mode 100644
index 000000000..6d17a5e2b
--- /dev/null
+++ b/tests/regression_tests/microxs/test_reference_materials_hybrid.csv
@@ -0,0 +1,7 @@
+nuclides,reactions,groups,xs
+U235,"(n,gamma)",1,0.1500301670375847
+U235,fission,1,1.2578145727734291
+O16,"(n,gamma)",1,0.00012069778439640312
+O16,fission,1,0.0
+Xe135,"(n,gamma)",1,0.014820264774863558
+Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_mesh.csv b/tests/regression_tests/microxs/test_reference_mesh.csv
deleted file mode 100644
index 1882372b1..000000000
--- a/tests/regression_tests/microxs/test_reference_mesh.csv
+++ /dev/null
@@ -1,25 +0,0 @@
-nuclides,reactions,groups,xs
-U234,"(n,gamma)",1,27.027171724208227
-U234,fission,1,0.04333740860093498
-U235,"(n,gamma)",1,12.683875995776193
-U235,fission,1,4.2596665957162605
-U238,"(n,gamma)",1,4.479719141496804
-U238,fission,1,0.009460665924409056
-U236,"(n,gamma)",1,8.469286849810802
-U236,fission,1,0.027373590840715795
-O16,"(n,gamma)",1,7.478160204479271e-05
-O16,fission,1,0.0
-O17,"(n,gamma)",1,0.0004743959164164789
-O17,fission,1,0.0
-I135,"(n,gamma)",1,8.33959524761822
-I135,fission,1,0.0
-Xe135,"(n,gamma)",1,282068.447252079
-Xe135,fission,1,0.0
-Xe136,"(n,gamma)",1,0.02888928065916194
-Xe136,fission,1,0.0
-Cs135,"(n,gamma)",1,2.5863526577468408
-Cs135,fission,1,0.0
-Gd157,"(n,gamma)",1,16518.24083307153
-Gd157,fission,1,0.0
-Gd156,"(n,gamma)",1,2.838514589417232
-Gd156,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_mesh_direct.csv b/tests/regression_tests/microxs/test_reference_mesh_direct.csv
new file mode 100644
index 000000000..60160ee51
--- /dev/null
+++ b/tests/regression_tests/microxs/test_reference_mesh_direct.csv
@@ -0,0 +1,7 @@
+nuclides,reactions,groups,xs
+U235,"(n,gamma)",1,0.14757185361871633
+U235,fission,1,1.2504996049257142
+O16,"(n,gamma)",1,0.0001098123625944155
+O16,fission,1,0.0
+Xe135,"(n,gamma)",1,0.0145705467728706
+Xe135,fission,1,0.0
diff --git a/tests/regression_tests/microxs/test_reference_mesh_flux.csv b/tests/regression_tests/microxs/test_reference_mesh_flux.csv
new file mode 100644
index 000000000..5eb29902e
--- /dev/null
+++ b/tests/regression_tests/microxs/test_reference_mesh_flux.csv
@@ -0,0 +1,7 @@
+nuclides,reactions,groups,xs
+U235,"(n,gamma)",1,0.15003016703758473
+U235,fission,1,1.2646269005413537
+O16,"(n,gamma)",1,0.00012069778439640301
+O16,fission,1,0.0
+Xe135,"(n,gamma)",1,0.014820264774863562
+Xe135,fission,1,0.0
diff --git a/tests/regression_tests/model_xml/adj_cell_rotation_inputs_true.dat b/tests/regression_tests/model_xml/adj_cell_rotation_inputs_true.dat
index 7e2a0cf62..18c0552ce 100644
--- a/tests/regression_tests/model_xml/adj_cell_rotation_inputs_true.dat
+++ b/tests/regression_tests/model_xml/adj_cell_rotation_inputs_true.dat
@@ -1,35 +1,35 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cc" value="0.1"/>
-      <nuclide ao="0.1" name="H1"/>
+      <density value="0.1" units="g/cc"/>
+      <nuclide name="H1" ao="0.1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="2" region="1" universe="1"/>
-    <cell fill="1" id="3" region="2 -3 4 -5 6 -8" rotation="10 20 30" universe="2"/>
-    <cell fill="1" id="4" region="2 -3 4 -5 8 -7" translation="0 0 15" universe="2"/>
-    <surface coeffs="1.0 0.0 0.0 5.0" id="1" type="sphere"/>
-    <surface boundary="vacuum" coeffs="-7.5" id="2" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="7.5" id="3" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-7.5" id="4" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="7.5" id="5" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-7.5" id="6" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="22.5" id="7" type="z-plane"/>
-    <surface coeffs="7.5" id="8" type="z-plane"/>
+    <cell id="3" fill="1" region="2 -3 4 -5 6 -8" rotation="10 20 30" universe="2"/>
+    <cell id="4" fill="1" region="2 -3 4 -5 8 -7" translation="0 0 15" universe="2"/>
+    <surface id="1" type="sphere" coeffs="1.0 0.0 0.0 5.0"/>
+    <surface id="2" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-7.5"/>
+    <surface id="3" name="maximum x" type="x-plane" boundary="vacuum" coeffs="7.5"/>
+    <surface id="4" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-7.5"/>
+    <surface id="5" name="maximum y" type="y-plane" boundary="vacuum" coeffs="7.5"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="-7.5"/>
+    <surface id="7" type="z-plane" boundary="vacuum" coeffs="22.5"/>
+    <surface id="8" type="z-plane" coeffs="7.5"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>10000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4.0 -4.0 -4.0 4.0 4.0 4.0</parameters>
       </space>
diff --git a/tests/regression_tests/model_xml/energy_laws_inputs_true.dat b/tests/regression_tests/model_xml/energy_laws_inputs_true.dat
index 4d8e825d1..8c5191217 100644
--- a/tests/regression_tests/model_xml/energy_laws_inputs_true.dat
+++ b/tests/regression_tests/model_xml/energy_laws_inputs_true.dat
@@ -1,18 +1,18 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="20.0"/>
-      <nuclide ao="1.0" name="U233"/>
-      <nuclide ao="1.0" name="Am244"/>
-      <nuclide ao="1.0" name="H2"/>
-      <nuclide ao="1.0" name="Na23"/>
-      <nuclide ao="1.0" name="Ta181"/>
+    <material id="1" depletable="true">
+      <density value="20.0" units="g/cm3"/>
+      <nuclide name="U233" ao="1.0"/>
+      <nuclide name="Am244" ao="1.0"/>
+      <nuclide name="H2" ao="1.0"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="Ta181" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="reflective" coeffs="0.0 0.0 0.0 100.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 100.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/model_xml/lattice_multiple_inputs_true.dat b/tests/regression_tests/model_xml/lattice_multiple_inputs_true.dat
index b249d97c4..06abef582 100644
--- a/tests/regression_tests/model_xml/lattice_multiple_inputs_true.dat
+++ b/tests/regression_tests/model_xml/lattice_multiple_inputs_true.dat
@@ -1,15 +1,15 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" name="UO2" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2" name="light water">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
@@ -18,8 +18,8 @@
     <cell id="2" material="2" region="1" universe="1"/>
     <cell id="3" material="1" region="-2" universe="2"/>
     <cell id="4" material="2" region="2" universe="2"/>
-    <cell fill="3" id="5" universe="4"/>
-    <cell fill="5" id="6" region="3 -4 5 -6" universe="6"/>
+    <cell id="5" fill="3" universe="4"/>
+    <cell id="6" fill="5" region="3 -4 5 -6" universe="6"/>
     <lattice id="3">
       <pitch>1.2 1.2</pitch>
       <outer>1</outer>
@@ -37,12 +37,12 @@
 4 4 
 4 4 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.4" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.5" id="2" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-2.4" id="3" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="2.4" id="4" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-2.4" id="5" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="2.4" id="6" name="maximum y" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.4"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.5"/>
+    <surface id="3" name="minimum x" type="x-plane" boundary="reflective" coeffs="-2.4"/>
+    <surface id="4" name="maximum x" type="x-plane" boundary="reflective" coeffs="2.4"/>
+    <surface id="5" name="minimum y" type="y-plane" boundary="reflective" coeffs="-2.4"/>
+    <surface id="6" name="maximum y" type="y-plane" boundary="reflective" coeffs="2.4"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/model_xml/photon_production_inputs_true.dat b/tests/regression_tests/model_xml/photon_production_inputs_true.dat
index 10f0bad98..0ae0079f4 100644
--- a/tests/regression_tests/model_xml/photon_production_inputs_true.dat
+++ b/tests/regression_tests/model_xml/photon_production_inputs_true.dat
@@ -2,28 +2,28 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cm3" value="2.6989"/>
-      <nuclide ao="1.0" name="Al27"/>
+      <density value="2.6989" units="g/cm3"/>
+      <nuclide name="Al27" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="void" region="-1 2 -3" universe="1"/>
     <cell id="2" material="1" region="-1 3 -4" universe="1"/>
     <cell id="3" material="void" region="1 | -2 | 4" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 1.0" id="1" type="x-cylinder"/>
-    <surface boundary="vacuum" coeffs="-1.0" id="2" type="x-plane"/>
-    <surface coeffs="1.0" id="3" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="1000000000.0" id="4" type="x-plane"/>
+    <surface id="1" type="x-cylinder" boundary="vacuum" coeffs="0.0 0.0 1.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="-1.0"/>
+    <surface id="3" type="x-plane" coeffs="1.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="1000000000.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>10000</particles>
     <batches>1</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0 0 0</parameters>
       </space>
-      <angle reference_uvw="1.0 0.0 0.0" type="monodirectional"/>
+      <angle type="monodirectional" reference_uvw="1.0 0.0 0.0"/>
       <energy type="discrete">
         <parameters>14000000.0 1.0</parameters>
       </energy>
@@ -41,6 +41,16 @@
     <filter id="2" type="particle">
       <bins>neutron photon electron positron</bins>
     </filter>
+    <filter id="5" type="particle">
+      <bins>neutron</bins>
+    </filter>
+    <filter id="3" type="energy">
+      <bins>0.0 20000000.0</bins>
+    </filter>
+    <filter id="4" type="particleproduction">
+      <particles>neutron photon</particles>
+      <energies>0.0 100000.0 300000.0 500000.0 2000000.0 20000000.0</energies>
+    </filter>
     <tally id="1">
       <filters>1 2</filters>
       <scores>current</scores>
@@ -63,5 +73,10 @@
       <scores>total heating (n,gamma)</scores>
       <estimator>analog</estimator>
     </tally>
+    <tally id="5">
+      <filters>5 3 4</filters>
+      <scores>events</scores>
+      <estimator>analog</estimator>
+    </tally>
   </tallies>
 </model>
diff --git a/tests/regression_tests/multipole/inputs_true.dat b/tests/regression_tests/multipole/inputs_true.dat
index 7bea66930..3b78db77b 100644
--- a/tests/regression_tests/multipole/inputs_true.dat
+++ b/tests/regression_tests/multipole/inputs_true.dat
@@ -2,21 +2,23 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" universe="1"/>
-    <cell id="11" material="2" region="-1" temperature="500 700 0 800" universe="11"/>
+    <cell id="11" material="2" region="-1" universe="11">
+      <temperature>500 700 0 800</temperature>
+    </cell>
     <cell id="12" material="1" region="1" universe="11"/>
-    <cell fill="101" id="101" region="2 -3 4 -5" universe="0"/>
+    <cell id="101" fill="101" region="2 -3 4 -5" universe="0"/>
     <lattice id="101">
       <pitch>2.0 2.0</pitch>
       <outer>1</outer>
@@ -26,18 +28,18 @@
 11 11 
 11 11 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.3" id="1" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-3.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="3" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="5" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.3"/>
+    <surface id="2" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="3" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="5" type="y-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-1 -1 -1 1 1 1</parameters>
       </space>
diff --git a/tests/regression_tests/multipole/results_true.dat b/tests/regression_tests/multipole/results_true.dat
index 8e82655ce..40b42b1a0 100644
--- a/tests/regression_tests/multipole/results_true.dat
+++ b/tests/regression_tests/multipole/results_true.dat
@@ -1,36 +1,36 @@
 k-combined:
-1.354889E+00 8.437211E-03
+1.315804E+00 7.070811E-02
 tally 1:
-3.869969E+00
-3.000421E+00
-2.800393E+00
-1.570851E+00
-5.399091E-01
-5.842233E-02
-4.577147E-01
-4.203362E-02
+3.840178E+00
+2.953217E+00
+2.769369E+00
+1.535605E+00
+5.400560E-01
+5.840168E-02
+4.595308E-01
+4.237758E-02
 0.000000E+00
 0.000000E+00
-2.286465E+01
-1.045869E+02
+2.283789E+01
+1.044032E+02
 0.000000E+00
 0.000000E+00
-6.971793E-04
-9.724819E-08
-2.283955E+01
-1.043576E+02
-4.659256E-06
-2.170865E-11
-3.663230E+02
-2.685383E+04
-2.800393E+00
-1.570851E+00
-2.204999E+00
-9.728014E-01
-3.612213E+02
-2.611152E+04
-4.659256E-06
-2.170865E-11
+6.960971E-04
+9.704386E-08
+2.281620E+01
+1.042051E+02
+3.667073E-05
+1.048567E-09
+3.655557E+02
+2.676173E+04
+2.769369E+00
+1.535605E+00
+2.199058E+00
+9.681253E-01
+3.604950E+02
+2.602657E+04
+3.667073E-05
+1.048567E-09
 Cell
 	ID             =	11
 	Name           =	
@@ -38,5 +38,6 @@ Cell
 	Region         =	-1
 	Rotation       =	None
 	Temperature    =	[500. 700.   0. 800.]
+	Density        =	None
 	Translation    =	None
 	Volume         =	None
diff --git a/tests/regression_tests/ncrystal/inputs_true.dat b/tests/regression_tests/ncrystal/inputs_true.dat
index ecbc3c54f..81ee2e312 100644
--- a/tests/regression_tests/ncrystal/inputs_true.dat
+++ b/tests/regression_tests/ncrystal/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material cfg="Al_sg225.ncmat" id="1" temperature="293.15">
-      <density units="g/cm3" value="2.6986455176922477"/>
-      <nuclide ao="1.0" name="Al27"/>
+    <material id="1" cfg="Al_sg225.ncmat" temperature="293.15">
+      <density value="2.6986455176922477" units="g/cm3"/>
+      <nuclide name="Al27" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="void" region="1 -2" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 0.1" id="1" type="sphere"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 100" id="2" type="sphere"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 0.1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 100"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0 0 -20</parameters>
       </space>
-      <angle reference_uvw="0.0 0.0 1.0" type="monodirectional"/>
+      <angle type="monodirectional" reference_uvw="0.0 0.0 1.0"/>
       <energy type="discrete">
         <parameters>0.012 1.0</parameters>
       </energy>
diff --git a/tests/regression_tests/output/results_true.dat b/tests/regression_tests/output/results_true.dat
index bbf03de94..97b997ae6 100644
--- a/tests/regression_tests/output/results_true.dat
+++ b/tests/regression_tests/output/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
diff --git a/tests/regression_tests/particle_production_fission/__init__.py b/tests/regression_tests/particle_production_fission/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/particle_production_fission/local/inputs_true.dat b/tests/regression_tests/particle_production_fission/local/inputs_true.dat
new file mode 100644
index 000000000..e93dfc36e
--- /dev/null
+++ b/tests/regression_tests/particle_production_fission/local/inputs_true.dat
@@ -0,0 +1,38 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="18.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 5.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>2</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <create_fission_neutrons>true</create_fission_neutrons>
+    <shared_secondary_bank>false</shared_secondary_bank>
+  </settings>
+  <tallies>
+    <filter id="2" type="particle">
+      <bins>neutron</bins>
+    </filter>
+    <filter id="1" type="particleproduction">
+      <particles>neutron</particles>
+    </filter>
+    <tally id="1">
+      <filters>2 1</filters>
+      <scores>events</scores>
+      <estimator>analog</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/particle_production_fission/local/results_true.dat b/tests/regression_tests/particle_production_fission/local/results_true.dat
new file mode 100644
index 000000000..2f5288eba
--- /dev/null
+++ b/tests/regression_tests/particle_production_fission/local/results_true.dat
@@ -0,0 +1,3 @@
+tally 1:
+4.570000E+00
+1.047890E+01
diff --git a/tests/regression_tests/particle_production_fission/shared/inputs_true.dat b/tests/regression_tests/particle_production_fission/shared/inputs_true.dat
new file mode 100644
index 000000000..b50a39cd0
--- /dev/null
+++ b/tests/regression_tests/particle_production_fission/shared/inputs_true.dat
@@ -0,0 +1,38 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="18.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 5.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>2</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <create_fission_neutrons>true</create_fission_neutrons>
+    <shared_secondary_bank>true</shared_secondary_bank>
+  </settings>
+  <tallies>
+    <filter id="2" type="particle">
+      <bins>neutron</bins>
+    </filter>
+    <filter id="1" type="particleproduction">
+      <particles>neutron</particles>
+    </filter>
+    <tally id="1">
+      <filters>2 1</filters>
+      <scores>events</scores>
+      <estimator>analog</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/particle_production_fission/shared/results_true.dat b/tests/regression_tests/particle_production_fission/shared/results_true.dat
new file mode 100644
index 000000000..5a4dfc516
--- /dev/null
+++ b/tests/regression_tests/particle_production_fission/shared/results_true.dat
@@ -0,0 +1,3 @@
+tally 1:
+5.180000E+00
+1.355140E+01
diff --git a/tests/regression_tests/particle_production_fission/test.py b/tests/regression_tests/particle_production_fission/test.py
new file mode 100644
index 000000000..2ad6ca515
--- /dev/null
+++ b/tests/regression_tests/particle_production_fission/test.py
@@ -0,0 +1,49 @@
+import openmc
+import pytest
+from openmc.utility_funcs import change_directory
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.mark.parametrize("shared_secondary,subdir", [
+    (False, "local"),
+    (True, "shared"),
+])
+def test_particle_production_fission(shared_secondary, subdir):
+    """Fixed-source model with fissionable material to test that
+    ParticleProductionFilter correctly counts fission-born neutrons,
+    with both local and shared secondary bank modes."""
+    with change_directory(subdir):
+        openmc.reset_auto_ids()
+        model = openmc.Model()
+
+        mat = openmc.Material()
+        mat.set_density('g/cm3', 18.0)
+        mat.add_nuclide('U235', 1.0)
+        model.materials.append(mat)
+
+        sph = openmc.Sphere(r=5.0, boundary_type='vacuum')
+        cell = openmc.Cell(fill=mat, region=-sph)
+        model.geometry = openmc.Geometry([cell])
+
+        source = openmc.IndependentSource()
+        source.energy = openmc.stats.delta_function(1.0e6)
+
+        model.settings.particles = 100
+        model.settings.run_mode = 'fixed source'
+        model.settings.batches = 2
+        model.settings.source = source
+        model.settings.create_fission_neutrons = True
+        model.settings.shared_secondary_bank = shared_secondary
+
+        # ParticleProductionFilter tracking fission neutron production
+        ppf = openmc.ParticleProductionFilter(['neutron'])
+        neutron_filter = openmc.ParticleFilter(['neutron'])
+        tally = openmc.Tally()
+        tally.filters = [neutron_filter, ppf]
+        tally.scores = ['events']
+        tally.estimator = 'analog'
+        model.tallies = [tally]
+
+        harness = PyAPITestHarness('statepoint.2.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/particle_restart_eigval/results_true.dat b/tests/regression_tests/particle_restart_eigval/results_true.dat
index bd81920ca..3feeb5e74 100644
--- a/tests/regression_tests/particle_restart_eigval/results_true.dat
+++ b/tests/regression_tests/particle_restart_eigval/results_true.dat
@@ -1,16 +1,16 @@
 current batch:
-1.100000E+01
+8.000000E+00
 current generation:
 1.000000E+00
 particle id:
-2.540000E+02
+6.000000E+01
 run mode:
 eigenvalue
 particle weight:
 1.000000E+00
 particle energy:
-2.220048E+06
+2.028153E+06
 particle xyz:
--4.820850E+01 2.432936E+01 4.397668E+01
+-3.678172E+01 -6.073321E+01 2.756488E+01
 particle uvw:
--3.148565E-01 9.184190E-01 2.395245E-01
+-3.284774E-01 -8.920284E-01 3.104639E-01
diff --git a/tests/regression_tests/particle_restart_eigval/settings.xml b/tests/regression_tests/particle_restart_eigval/settings.xml
index 64b1a4dd4..de580e4c4 100644
--- a/tests/regression_tests/particle_restart_eigval/settings.xml
+++ b/tests/regression_tests/particle_restart_eigval/settings.xml
@@ -6,7 +6,7 @@
   <batches>12</batches>
   <inactive>5</inactive>
   <particles>1200</particles>
-
+  <max_lost_particles>1000000</max_lost_particles>
   <source>
     <space type="box">
       <parameters>-10 -10 -5  10  10  5</parameters>
diff --git a/tests/regression_tests/particle_restart_eigval/test.py b/tests/regression_tests/particle_restart_eigval/test.py
index f9d22edb8..bad3f158c 100644
--- a/tests/regression_tests/particle_restart_eigval/test.py
+++ b/tests/regression_tests/particle_restart_eigval/test.py
@@ -2,5 +2,5 @@ from tests.testing_harness import ParticleRestartTestHarness
 
 
 def test_particle_restart_eigval():
-    harness = ParticleRestartTestHarness('particle_11_254.h5')
+    harness = ParticleRestartTestHarness('particle_8_60.h5')
     harness.main()
diff --git a/tests/regression_tests/particle_restart_fixed_shared_secondary/__init__.py b/tests/regression_tests/particle_restart_fixed_shared_secondary/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/particle_restart_fixed_shared_secondary/geometry.xml b/tests/regression_tests/particle_restart_fixed_shared_secondary/geometry.xml
new file mode 100644
index 000000000..c86e016c6
--- /dev/null
+++ b/tests/regression_tests/particle_restart_fixed_shared_secondary/geometry.xml
@@ -0,0 +1,9 @@
+<?xml version="1.0"?>
+<geometry>
+
+  <!-- Sphere with no boundary condition -->
+  <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 80.0" />
+  <surface id="2" type="sphere" coeffs="0.0 0.0 0.0 10000.0" boundary="vacuum" />
+  <cell id="1" material="1" region="-1" />
+
+</geometry>
diff --git a/tests/regression_tests/statepoint_batch/materials.xml b/tests/regression_tests/particle_restart_fixed_shared_secondary/materials.xml
similarity index 51%
rename from tests/regression_tests/statepoint_batch/materials.xml
rename to tests/regression_tests/particle_restart_fixed_shared_secondary/materials.xml
index 2472a7471..f3851d7ef 100644
--- a/tests/regression_tests/statepoint_batch/materials.xml
+++ b/tests/regression_tests/particle_restart_fixed_shared_secondary/materials.xml
@@ -2,8 +2,8 @@
 <materials>
 
   <material id="1">
-    <density value="4.5" units="g/cc" />
-    <nuclide name="U235" ao="1.0" />
+    <density value="1.4" units="g/cc" />
+    <nuclide name="B10" ao="1.0" />
   </material>
 
 </materials>
diff --git a/tests/regression_tests/particle_restart_fixed_shared_secondary/results_true.dat b/tests/regression_tests/particle_restart_fixed_shared_secondary/results_true.dat
new file mode 100644
index 000000000..0c84541f9
--- /dev/null
+++ b/tests/regression_tests/particle_restart_fixed_shared_secondary/results_true.dat
@@ -0,0 +1,16 @@
+current batch:
+4.000000E+00
+current generation:
+1.000000E+00
+particle id:
+3.241000E+03
+run mode:
+fixed source
+particle weight:
+1.000000E+00
+particle energy:
+3.896365E+06
+particle xyz:
+8.710681E-01 3.698823E+00 -2.286229E+00
+particle uvw:
+-5.882735E-01 4.665422E-01 -6.605093E-01
diff --git a/tests/regression_tests/particle_restart_fixed_shared_secondary/settings.xml b/tests/regression_tests/particle_restart_fixed_shared_secondary/settings.xml
new file mode 100644
index 000000000..3a80fb17b
--- /dev/null
+++ b/tests/regression_tests/particle_restart_fixed_shared_secondary/settings.xml
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<settings>
+
+  <run_mode>fixed source</run_mode>
+  <batches>12</batches>
+  <particles>1000</particles>
+  <shared_secondary_bank>true</shared_secondary_bank>
+
+  <source>
+    <space type="box">
+      <parameters>-10 -10 -5  10  10  5</parameters>
+    </space>
+  </source>
+
+</settings>
diff --git a/tests/regression_tests/particle_restart_fixed_shared_secondary/test.py b/tests/regression_tests/particle_restart_fixed_shared_secondary/test.py
new file mode 100644
index 000000000..770010900
--- /dev/null
+++ b/tests/regression_tests/particle_restart_fixed_shared_secondary/test.py
@@ -0,0 +1,6 @@
+from tests.testing_harness import ParticleRestartTestHarness
+
+
+def test_particle_restart_fixed_shared_secondary():
+    harness = ParticleRestartTestHarness('particle_4_3241.h5')
+    harness.main()
diff --git a/tests/regression_tests/periodic/inputs_true.dat b/tests/regression_tests/periodic/inputs_true.dat
index 65e1b5ce3..9183db3c4 100644
--- a/tests/regression_tests/periodic/inputs_true.dat
+++ b/tests/regression_tests/periodic/inputs_true.dat
@@ -2,33 +2,33 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2 3 -4 5 -6 7" universe="0"/>
     <cell id="2" material="2" region="3 5 -6 -7" universe="0"/>
-    <surface boundary="periodic" coeffs="0.0" id="1" periodic_surface_id="3" type="x-plane"/>
-    <surface boundary="reflective" coeffs="5.0" id="2" type="x-plane"/>
-    <surface boundary="periodic" coeffs="0.0" id="3" periodic_surface_id="1" type="y-plane"/>
-    <surface boundary="reflective" coeffs="5.0" id="4" type="y-plane"/>
-    <surface boundary="periodic" coeffs="-5.0" id="5" type="z-plane"/>
-    <surface boundary="periodic" coeffs="0 0 1 5.0" id="6" type="plane"/>
-    <surface coeffs="2.5 0.0 2.0" id="7" type="z-cylinder"/>
+    <surface id="1" type="x-plane" boundary="periodic" coeffs="0.0" periodic_surface_id="3"/>
+    <surface id="2" type="x-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="3" type="y-plane" boundary="periodic" coeffs="0.0" periodic_surface_id="1"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="5" type="z-plane" boundary="periodic" coeffs="-5.0"/>
+    <surface id="6" type="plane" boundary="periodic" coeffs="0 0 1 5.0"/>
+    <surface id="7" type="z-cylinder" coeffs="2.5 0.0 2.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>4</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>0 0 0 5 5 0</parameters>
       </space>
diff --git a/tests/regression_tests/periodic/results_true.dat b/tests/regression_tests/periodic/results_true.dat
index 2189597e1..626004b16 100644
--- a/tests/regression_tests/periodic/results_true.dat
+++ b/tests/regression_tests/periodic/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.623300E+00 2.572983E-02
+1.624889E+00 1.108153E-02
diff --git a/tests/regression_tests/periodic_6fold/False-False/inputs_true.dat b/tests/regression_tests/periodic_6fold/False-False/inputs_true.dat
new file mode 100644
index 000000000..075cffc12
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/False-False/inputs_true.dat
@@ -0,0 +1,34 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="1 2 -3 4" universe="0"/>
+    <cell id="2" material="2" region="1 2 -4" universe="0"/>
+    <surface id="1" type="plane" boundary="periodic" coeffs="0.4999999999999999 0.8660254037844387 0.0 0.0"/>
+    <surface id="2" type="plane" boundary="periodic" coeffs="0.4999999999999999 -0.8660254037844387 0.0 0.0"/>
+    <surface id="3" type="x-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="4" type="z-cylinder" coeffs="2.598076211353316 1.4999999999999998 2.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>4</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0 0 0 5 5 0</parameters>
+      </space>
+    </source>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/periodic_6fold/False-False/results_true.dat b/tests/regression_tests/periodic_6fold/False-False/results_true.dat
new file mode 100644
index 000000000..04aa30878
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/False-False/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.848492E+00 2.933785E-03
diff --git a/tests/regression_tests/periodic_6fold/False-True/inputs_true.dat b/tests/regression_tests/periodic_6fold/False-True/inputs_true.dat
new file mode 100644
index 000000000..cbc1e9100
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/False-True/inputs_true.dat
@@ -0,0 +1,34 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="5">
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="6" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="5" region="9 -10 -11 12" universe="0"/>
+    <cell id="2" material="6" region="9 -10 -12" universe="0"/>
+    <surface id="9" type="plane" boundary="periodic" coeffs="0.4999999999999999 0.8660254037844387 0.0 0.0"/>
+    <surface id="10" type="plane" boundary="periodic" coeffs="-0.4999999999999999 0.8660254037844387 0.0 0.0"/>
+    <surface id="11" type="x-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="12" type="z-cylinder" coeffs="2.598076211353316 1.4999999999999998 2.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>4</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0 0 0 5 5 0</parameters>
+      </space>
+    </source>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/periodic_6fold/False-True/results_true.dat b/tests/regression_tests/periodic_6fold/False-True/results_true.dat
new file mode 100644
index 000000000..04aa30878
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/False-True/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.848492E+00 2.933785E-03
diff --git a/tests/regression_tests/periodic_6fold/True-False/inputs_true.dat b/tests/regression_tests/periodic_6fold/True-False/inputs_true.dat
new file mode 100644
index 000000000..675a8b300
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/True-False/inputs_true.dat
@@ -0,0 +1,34 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="3">
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="4" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="3" region="-5 6 -7 8" universe="0"/>
+    <cell id="2" material="4" region="-5 6 -8" universe="0"/>
+    <surface id="5" type="plane" boundary="periodic" coeffs="-0.4999999999999999 -0.8660254037844387 0.0 0.0"/>
+    <surface id="6" type="plane" boundary="periodic" coeffs="0.4999999999999999 -0.8660254037844387 0.0 0.0"/>
+    <surface id="7" type="x-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="8" type="z-cylinder" coeffs="2.598076211353316 1.4999999999999998 2.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>4</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0 0 0 5 5 0</parameters>
+      </space>
+    </source>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/periodic_6fold/True-False/results_true.dat b/tests/regression_tests/periodic_6fold/True-False/results_true.dat
new file mode 100644
index 000000000..04aa30878
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/True-False/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.848492E+00 2.933785E-03
diff --git a/tests/regression_tests/periodic_6fold/True-True/inputs_true.dat b/tests/regression_tests/periodic_6fold/True-True/inputs_true.dat
new file mode 100644
index 000000000..e5ac03b48
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/True-True/inputs_true.dat
@@ -0,0 +1,34 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="7">
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <sab name="c_H_in_H2O"/>
+    </material>
+    <material id="8" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="7" region="-13 -14 -15 16" universe="0"/>
+    <cell id="2" material="8" region="-13 -14 -16" universe="0"/>
+    <surface id="13" type="plane" boundary="periodic" coeffs="-0.4999999999999999 -0.8660254037844387 0.0 0.0"/>
+    <surface id="14" type="plane" boundary="periodic" coeffs="-0.4999999999999999 0.8660254037844387 0.0 0.0"/>
+    <surface id="15" type="x-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="16" type="z-cylinder" coeffs="2.598076211353316 1.4999999999999998 2.0"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>4</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0 0 0 5 5 0</parameters>
+      </space>
+    </source>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/periodic_6fold/True-True/results_true.dat b/tests/regression_tests/periodic_6fold/True-True/results_true.dat
new file mode 100644
index 000000000..04aa30878
--- /dev/null
+++ b/tests/regression_tests/periodic_6fold/True-True/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.848492E+00 2.933785E-03
diff --git a/tests/regression_tests/periodic_6fold/inputs_true.dat b/tests/regression_tests/periodic_6fold/inputs_true.dat
deleted file mode 100644
index f7f413d09..000000000
--- a/tests/regression_tests/periodic_6fold/inputs_true.dat
+++ /dev/null
@@ -1,34 +0,0 @@
-<?xml version='1.0' encoding='utf-8'?>
-<model>
-  <materials>
-    <material id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <sab name="c_H_in_H2O"/>
-    </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
-    </material>
-  </materials>
-  <geometry>
-    <cell id="1" material="1" region="1 2 -3 4" universe="0"/>
-    <cell id="2" material="2" region="1 2 -4" universe="0"/>
-    <surface boundary="periodic" coeffs="0.4999999999999999 0.8660254037844387 0.0 0.0" id="1" type="plane"/>
-    <surface boundary="periodic" coeffs="0.4999999999999999 -0.8660254037844387 0.0 0.0" id="2" type="plane"/>
-    <surface boundary="reflective" coeffs="5.0" id="3" type="x-plane"/>
-    <surface coeffs="2.598076211353316 1.4999999999999998 2.0" id="4" type="z-cylinder"/>
-  </geometry>
-  <settings>
-    <run_mode>eigenvalue</run_mode>
-    <particles>1000</particles>
-    <batches>4</batches>
-    <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space type="box">
-        <parameters>0 0 0 5 5 0</parameters>
-      </space>
-    </source>
-  </settings>
-  </model>
diff --git a/tests/regression_tests/periodic_6fold/results_true.dat b/tests/regression_tests/periodic_6fold/results_true.dat
deleted file mode 100644
index e60182809..000000000
--- a/tests/regression_tests/periodic_6fold/results_true.dat
+++ /dev/null
@@ -1,2 +0,0 @@
-k-combined:
-1.858730E+00 6.824588E-03
diff --git a/tests/regression_tests/periodic_6fold/test.py b/tests/regression_tests/periodic_6fold/test.py
index 272c65df5..bded1c465 100644
--- a/tests/regression_tests/periodic_6fold/test.py
+++ b/tests/regression_tests/periodic_6fold/test.py
@@ -1,14 +1,16 @@
 from math import sin, cos, pi
 
 import openmc
+from openmc.utility_funcs import change_directory
 import pytest
 
 from tests.testing_harness import PyAPITestHarness
 
 
-@pytest.fixture
-def model():
-    model = openmc.model.Model()
+@pytest.mark.parametrize("flip1", [False, True])
+@pytest.mark.parametrize("flip2", [False, True])
+def test_periodic(flip1, flip2):
+    model = openmc.Model()
 
     # Define materials
     water = openmc.Material()
@@ -27,31 +29,52 @@ def model():
     # answers.
     theta1 = (-1/6 + 1/2) * pi
     theta2 = (1/6 - 1/2) * pi
-    plane1 = openmc.Plane(a=cos(theta1), b=sin(theta1), boundary_type='periodic')
-    plane2 = openmc.Plane(a=cos(theta2), b=sin(theta2), boundary_type='periodic')
+    if flip1:
+        plane1 = openmc.Plane(a=-cos(theta1), b=-sin(theta1), boundary_type='periodic')
+    else:
+        plane1 = openmc.Plane(a=cos(theta1), b=sin(theta1), boundary_type='periodic')
+    if flip2:
+        plane2 = openmc.Plane(a=-cos(theta2), b=-sin(theta2), boundary_type='periodic')
+    else:
+        plane2 = openmc.Plane(a=cos(theta2), b=sin(theta2), boundary_type='periodic')
 
     x_max = openmc.XPlane(5., boundary_type='reflective')
 
     z_cyl = openmc.ZCylinder(x0=3*cos(pi/6), y0=3*sin(pi/6), r=2.0)
 
-    outside_cyl = openmc.Cell(1, fill=water, region=(
-        +plane1 & +plane2 & -x_max & +z_cyl))
-    inside_cyl = openmc.Cell(2, fill=fuel, region=(
-        +plane1 & +plane2 & -z_cyl))
+    match (flip1, flip2):
+        case (False, False):
+            outside_cyl = openmc.Cell(1, fill=water, region=(
+                          +plane1 & +plane2 & -x_max & +z_cyl))
+            inside_cyl = openmc.Cell(2, fill=fuel, region=(
+                          +plane1 & +plane2 & -z_cyl))
+        case (False, True):
+            outside_cyl = openmc.Cell(1, fill=water, region=(
+                          +plane1 & -plane2 & -x_max & +z_cyl))
+            inside_cyl = openmc.Cell(2, fill=fuel, region=(
+                          +plane1 & -plane2 & -z_cyl))
+        case (True, False):
+            outside_cyl = openmc.Cell(1, fill=water, region=(
+                          -plane1 & +plane2 & -x_max & +z_cyl))
+            inside_cyl = openmc.Cell(2, fill=fuel, region=(
+                          -plane1 & +plane2 & -z_cyl))
+        case (True, True):
+            outside_cyl = openmc.Cell(1, fill=water, region=(
+                          -plane1 & -plane2 & -x_max & +z_cyl))
+            inside_cyl = openmc.Cell(2, fill=fuel, region=(
+                          -plane1 & -plane2 & -z_cyl))
     root_universe = openmc.Universe(0, cells=(outside_cyl, inside_cyl))
     model.geometry = openmc.Geometry(root_universe)
 
     # Define settings
-    model.settings = openmc.Settings()
     model.settings.particles = 1000
     model.settings.batches = 4
     model.settings.inactive = 0
-    model.settings.source = openmc.IndependentSource(space=openmc.stats.Box(
-        (0, 0, 0), (5, 5, 0))
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Box((0, 0, 0), (5, 5, 0))
     )
-    return model
 
+    with change_directory(f'{flip1}-{flip2}'):
+        harness = PyAPITestHarness('statepoint.4.h5', model)
+        harness.main()
 
-def test_periodic(model):
-    harness = PyAPITestHarness('statepoint.4.h5', model)
-    harness.main()
diff --git a/tests/regression_tests/periodic_cyls/__init__.py b/tests/regression_tests/periodic_cyls/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/periodic_cyls/test.py b/tests/regression_tests/periodic_cyls/test.py
new file mode 100644
index 000000000..a341e3799
--- /dev/null
+++ b/tests/regression_tests/periodic_cyls/test.py
@@ -0,0 +1,91 @@
+import openmc
+import numpy as np
+import pytest
+from openmc.utility_funcs import change_directory
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.fixture
+def xcyl_model():
+    model = openmc.Model()
+    # Define materials
+    fuel = openmc.Material()
+    fuel.add_nuclide('U235', 0.2)
+    fuel.add_nuclide('U238', 0.8)
+    fuel.set_density('g/cc', 19.1)
+    model.materials = openmc.Materials([fuel])
+
+    # Define geometry
+    # finite cylinder
+    x_min = openmc.XPlane(x0=0.0, boundary_type='reflective')
+    x_max = openmc.XPlane(x0=20.0, boundary_type='reflective')
+    x_cyl = openmc.XCylinder(r=20.0,boundary_type='vacuum')
+    # slice cylinder for periodic BC
+    periodic_bounding_yplane = openmc.YPlane(y0=0, boundary_type='periodic')
+    periodic_bounding_plane = openmc.Plane(
+        a=0.0, b=-np.sqrt(3) / 3, c=1, boundary_type='periodic',
+    )
+    sixth_cyl_cell = openmc.Cell(1, fill=fuel, region = 
+        +x_min &- x_max & -x_cyl & +periodic_bounding_yplane & +periodic_bounding_plane)
+    periodic_bounding_yplane.periodic_surface = periodic_bounding_plane
+    periodic_bounding_plane.periodic_surface = periodic_bounding_yplane
+
+    model.geometry = openmc.Geometry([sixth_cyl_cell])
+
+
+    # Define settings
+    model.settings.particles = 1000
+    model.settings.batches = 4
+    model.settings.inactive = 0
+    model.settings.source = openmc.IndependentSource(space=openmc.stats.Box(
+        (0, 0, 0), (20, 20, 20))
+    )
+    return model
+
+@pytest.fixture
+def ycyl_model():
+    model = openmc.Model()
+    # Define materials
+    fuel = openmc.Material()
+    fuel.add_nuclide('U235', 0.2)
+    fuel.add_nuclide('U238', 0.8)
+    fuel.set_density('g/cc', 19.1)
+    model.materials = openmc.Materials([fuel])
+
+    # Define geometry
+    # finite cylinder
+    y_min = openmc.YPlane(y0=0.0, boundary_type='reflective')
+    y_max = openmc.YPlane(y0=20.0, boundary_type='reflective')
+    y_cyl = openmc.YCylinder(r=20.0,boundary_type='vacuum')
+    # slice cylinder for periodic BC
+    periodic_bounding_xplane = openmc.XPlane(x0=0, boundary_type='periodic')
+    periodic_bounding_plane = openmc.Plane(
+        a=-np.sqrt(3) / 3, b=0.0, c=1, boundary_type='periodic',
+    )
+    sixth_cyl_cell = openmc.Cell(1, fill=fuel, region = 
+        +y_min &- y_max & -y_cyl & +periodic_bounding_xplane & +periodic_bounding_plane)
+    periodic_bounding_xplane.periodic_surface = periodic_bounding_plane
+    periodic_bounding_plane.periodic_surface = periodic_bounding_xplane
+    model.geometry = openmc.Geometry([sixth_cyl_cell])
+
+
+    # Define settings
+    model.settings.particles = 1000
+    model.settings.batches = 4
+    model.settings.inactive = 0
+    model.settings.source = openmc.IndependentSource(space=openmc.stats.Box(
+        (0, 0, 0), (20, 20, 20))
+    )
+    return model
+
+def test_xcyl(xcyl_model):
+    with change_directory("xcyl_model"):
+        openmc.reset_auto_ids()
+        harness = PyAPITestHarness('statepoint.4.h5', xcyl_model)
+        harness.main()
+
+def test_ycyl(ycyl_model):
+    with change_directory("ycyl_model"):
+        openmc.reset_auto_ids()
+        harness = PyAPITestHarness('statepoint.4.h5', ycyl_model)
+        harness.main()
\ No newline at end of file
diff --git a/tests/regression_tests/periodic_cyls/xcyl_model/inputs_true.dat b/tests/regression_tests/periodic_cyls/xcyl_model/inputs_true.dat
new file mode 100644
index 000000000..7d1ecf426
--- /dev/null
+++ b/tests/regression_tests/periodic_cyls/xcyl_model/inputs_true.dat
@@ -0,0 +1,29 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" depletable="true">
+      <density value="19.1" units="g/cc"/>
+      <nuclide name="U235" ao="0.2"/>
+      <nuclide name="U238" ao="0.8"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="1 -2 -3 4 5" universe="1"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="reflective" coeffs="20.0"/>
+    <surface id="3" type="x-cylinder" boundary="vacuum" coeffs="0.0 0.0 20.0"/>
+    <surface id="4" type="y-plane" boundary="periodic" coeffs="0" periodic_surface_id="5"/>
+    <surface id="5" type="plane" boundary="periodic" coeffs="0.0 -0.5773502691896257 1 0.0" periodic_surface_id="4"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>4</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0 0 0 20 20 20</parameters>
+      </space>
+    </source>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/periodic_cyls/xcyl_model/results_true.dat b/tests/regression_tests/periodic_cyls/xcyl_model/results_true.dat
new file mode 100644
index 000000000..c7e3eb670
--- /dev/null
+++ b/tests/regression_tests/periodic_cyls/xcyl_model/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.082283E+00 6.676373E-02
diff --git a/tests/regression_tests/periodic_cyls/ycyl_model/inputs_true.dat b/tests/regression_tests/periodic_cyls/ycyl_model/inputs_true.dat
new file mode 100644
index 000000000..f3ca7e0f4
--- /dev/null
+++ b/tests/regression_tests/periodic_cyls/ycyl_model/inputs_true.dat
@@ -0,0 +1,29 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="2" depletable="true">
+      <density value="19.1" units="g/cc"/>
+      <nuclide name="U235" ao="0.2"/>
+      <nuclide name="U238" ao="0.8"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="2" region="6 -7 -8 9 10" universe="2"/>
+    <surface id="6" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="7" type="y-plane" boundary="reflective" coeffs="20.0"/>
+    <surface id="8" type="y-cylinder" boundary="vacuum" coeffs="0.0 0.0 20.0"/>
+    <surface id="9" type="x-plane" boundary="periodic" coeffs="0" periodic_surface_id="10"/>
+    <surface id="10" type="plane" boundary="periodic" coeffs="-0.5773502691896257 0.0 1 0.0" periodic_surface_id="9"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>1000</particles>
+    <batches>4</batches>
+    <inactive>0</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0 0 0 20 20 20</parameters>
+      </space>
+    </source>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/periodic_cyls/ycyl_model/results_true.dat b/tests/regression_tests/periodic_cyls/ycyl_model/results_true.dat
new file mode 100644
index 000000000..562467f2c
--- /dev/null
+++ b/tests/regression_tests/periodic_cyls/ycyl_model/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+1.082652E+00 3.316031E-02
diff --git a/tests/regression_tests/periodic_hex/inputs_true.dat b/tests/regression_tests/periodic_hex/inputs_true.dat
index 188c8dfa9..e65af3d94 100644
--- a/tests/regression_tests/periodic_hex/inputs_true.dat
+++ b/tests/regression_tests/periodic_hex/inputs_true.dat
@@ -1,19 +1,19 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3 -4 5 6" universe="1"/>
-    <surface boundary="periodic" coeffs="8.660254037844386" id="1" periodic_surface_id="2" type="x-plane"/>
-    <surface boundary="periodic" coeffs="-8.660254037844386" id="2" periodic_surface_id="1" type="x-plane"/>
-    <surface boundary="periodic" coeffs="0.5773502691896257 1.0 0.0 10.0" id="3" periodic_surface_id="6" type="plane"/>
-    <surface boundary="periodic" coeffs="-0.5773502691896257 1.0 0.0 10.0" id="4" periodic_surface_id="5" type="plane"/>
-    <surface boundary="periodic" coeffs="-0.5773502691896257 1.0 0.0 -10.0" id="5" periodic_surface_id="4" type="plane"/>
-    <surface boundary="periodic" coeffs="0.5773502691896257 1.0 0.0 -10.0" id="6" periodic_surface_id="3" type="plane"/>
+    <surface id="1" type="x-plane" boundary="periodic" coeffs="8.660254037844386" periodic_surface_id="2"/>
+    <surface id="2" type="x-plane" boundary="periodic" coeffs="-8.660254037844386" periodic_surface_id="1"/>
+    <surface id="3" type="plane" boundary="periodic" coeffs="0.5773502691896257 1.0 0.0 10.0" periodic_surface_id="6"/>
+    <surface id="4" type="plane" boundary="periodic" coeffs="-0.5773502691896257 1.0 0.0 10.0" periodic_surface_id="5"/>
+    <surface id="5" type="plane" boundary="periodic" coeffs="-0.5773502691896257 1.0 0.0 -10.0" periodic_surface_id="4"/>
+    <surface id="6" type="plane" boundary="periodic" coeffs="0.5773502691896257 1.0 0.0 -10.0" periodic_surface_id="3"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/periodic_hex/results_true.dat b/tests/regression_tests/periodic_hex/results_true.dat
index 584089a15..eff00b6e1 100644
--- a/tests/regression_tests/periodic_hex/results_true.dat
+++ b/tests/regression_tests/periodic_hex/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.284468E+00 2.781588E-02
+2.285622E+00 2.576768E-03
diff --git a/tests/regression_tests/photon_production/inputs_true.dat b/tests/regression_tests/photon_production/inputs_true.dat
index 10f0bad98..0ae0079f4 100644
--- a/tests/regression_tests/photon_production/inputs_true.dat
+++ b/tests/regression_tests/photon_production/inputs_true.dat
@@ -2,28 +2,28 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cm3" value="2.6989"/>
-      <nuclide ao="1.0" name="Al27"/>
+      <density value="2.6989" units="g/cm3"/>
+      <nuclide name="Al27" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="void" region="-1 2 -3" universe="1"/>
     <cell id="2" material="1" region="-1 3 -4" universe="1"/>
     <cell id="3" material="void" region="1 | -2 | 4" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 1.0" id="1" type="x-cylinder"/>
-    <surface boundary="vacuum" coeffs="-1.0" id="2" type="x-plane"/>
-    <surface coeffs="1.0" id="3" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="1000000000.0" id="4" type="x-plane"/>
+    <surface id="1" type="x-cylinder" boundary="vacuum" coeffs="0.0 0.0 1.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="-1.0"/>
+    <surface id="3" type="x-plane" coeffs="1.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="1000000000.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>10000</particles>
     <batches>1</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0 0 0</parameters>
       </space>
-      <angle reference_uvw="1.0 0.0 0.0" type="monodirectional"/>
+      <angle type="monodirectional" reference_uvw="1.0 0.0 0.0"/>
       <energy type="discrete">
         <parameters>14000000.0 1.0</parameters>
       </energy>
@@ -41,6 +41,16 @@
     <filter id="2" type="particle">
       <bins>neutron photon electron positron</bins>
     </filter>
+    <filter id="5" type="particle">
+      <bins>neutron</bins>
+    </filter>
+    <filter id="3" type="energy">
+      <bins>0.0 20000000.0</bins>
+    </filter>
+    <filter id="4" type="particleproduction">
+      <particles>neutron photon</particles>
+      <energies>0.0 100000.0 300000.0 500000.0 2000000.0 20000000.0</energies>
+    </filter>
     <tally id="1">
       <filters>1 2</filters>
       <scores>current</scores>
@@ -63,5 +73,10 @@
       <scores>total heating (n,gamma)</scores>
       <estimator>analog</estimator>
     </tally>
+    <tally id="5">
+      <filters>5 3 4</filters>
+      <scores>events</scores>
+      <estimator>analog</estimator>
+    </tally>
   </tallies>
 </model>
diff --git a/tests/regression_tests/photon_production/results_true.dat b/tests/regression_tests/photon_production/results_true.dat
index 398deeed8..c4f5fafa2 100644
--- a/tests/regression_tests/photon_production/results_true.dat
+++ b/tests/regression_tests/photon_production/results_true.dat
@@ -1,8 +1,8 @@
 tally 1:
 8.610000E-01
 7.413210E-01
-9.493000E-01
-9.011705E-01
+9.491000E-01
+9.007908E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -16,12 +16,12 @@ tally 2:
 1.573004E+00
 4.296434E-04
 1.845934E-07
-2.337049E-01
-5.461796E-02
+2.350047E-01
+5.522722E-02
 0.000000E+00
 0.000000E+00
-2.337049E-01
-5.461796E-02
+2.350047E-01
+5.522722E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -53,40 +53,40 @@ tally 3:
 4.104374E+12
 4.296582E-04
 1.846062E-07
-2.286000E-01
-5.225796E-02
-7.054033E+03
-4.975938E+07
+2.297000E-01
+5.276209E-02
+4.196651E+00
+1.761188E+01
 0.000000E+00
 0.000000E+00
-2.286000E-01
-5.225796E-02
-7.054033E+03
-4.975938E+07
+2.297000E-01
+5.276209E-02
+4.196651E+00
+1.761188E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
+1.774484E+05
+3.148794E+10
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-0.000000E+00
-0.000000E+00
-1.764573E+05
-3.113718E+10
+1.774484E+05
+3.148794E+10
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
+1.474427E+04
+2.173936E+08
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-0.000000E+00
-0.000000E+00
-7.691658E+03
-5.916160E+07
+1.474427E+04
+2.173936E+08
 0.000000E+00
 0.000000E+00
 tally 4:
@@ -102,16 +102,16 @@ tally 4:
 4.104374E+12
 0.000000E+00
 0.000000E+00
-2.286000E-01
-5.225796E-02
-7.054033E+03
-4.975938E+07
+2.297000E-01
+5.276209E-02
+4.196651E+00
+1.761188E+01
 0.000000E+00
 0.000000E+00
-2.286000E-01
-5.225796E-02
-7.054033E+03
-4.975938E+07
+2.297000E-01
+5.276209E-02
+4.196651E+00
+1.761188E+01
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -122,8 +122,8 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.764573E+05
-3.113718E+10
+1.774484E+05
+3.148794E+10
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -134,7 +134,28 @@ tally 4:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.691658E+03
-5.916160E+07
+1.474427E+04
+2.173936E+08
 0.000000E+00
 0.000000E+00
+tally 5:
+5.000000E-04
+2.500000E-07
+1.300000E-03
+1.690000E-06
+8.000000E-04
+6.400000E-07
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.000000E-02
+4.000000E-04
+8.200000E-03
+6.724000E-05
+5.280000E-02
+2.787840E-03
+3.546000E-01
+1.257412E-01
+5.063000E-01
+2.563397E-01
diff --git a/tests/regression_tests/photon_production/test.py b/tests/regression_tests/photon_production/test.py
index 150448a12..5ed5c5ecf 100644
--- a/tests/regression_tests/photon_production/test.py
+++ b/tests/regression_tests/photon_production/test.py
@@ -25,7 +25,8 @@ def model():
     inner_cyl_right.region = -cyl & +x_plane_center & -x_plane_right
     outer_cyl.region = ~(-cyl & +x_plane_left & -x_plane_right)
     inner_cyl_right.fill = mat
-    model.geometry = openmc.Geometry([inner_cyl_left, inner_cyl_right, outer_cyl])
+    model.geometry = openmc.Geometry(
+        [inner_cyl_left, inner_cyl_right, outer_cyl])
 
     source = openmc.IndependentSource()
     source.space = openmc.stats.Point((0, 0, 0))
@@ -38,17 +39,19 @@ def model():
     model.settings.batches = 1
     model.settings.photon_transport = True
     model.settings.electron_treatment = 'ttb'
-    model.settings.cutoff = {'energy_photon' : 1000.0}
+    model.settings.cutoff = {'energy_photon': 1000.0}
     model.settings.source = source
 
     surface_filter = openmc.SurfaceFilter(cyl)
-    particle_filter = openmc.ParticleFilter(['neutron', 'photon', 'electron', 'positron'])
+    particle_filter = openmc.ParticleFilter(
+        ['neutron', 'photon', 'electron', 'positron'])
     current_tally = openmc.Tally()
     current_tally.filters = [surface_filter, particle_filter]
     current_tally.scores = ['current']
     tally_tracklength = openmc.Tally()
     tally_tracklength.filters = [particle_filter]
-    tally_tracklength.scores = ['total', '(n,gamma)']  # heating doesn't work with tracklength
+    # heating doesn't work with tracklength
+    tally_tracklength.scores = ['total', '(n,gamma)']
     tally_tracklength.nuclides = ['Al27', 'total']
     tally_tracklength.estimator = 'tracklength'
     tally_collision = openmc.Tally()
@@ -61,8 +64,25 @@ def model():
     tally_analog.scores = ['total', 'heating', '(n,gamma)']
     tally_analog.nuclides = ['Al27', 'total']
     tally_analog.estimator = 'analog'
+
+    # This is an analog tally tracking the energy distribution of photons
+    # generated by neutrons. The sum of the tally should give the total
+    # number of photons generated per source neutron.
+    ene_filter = openmc.EnergyFilter([0.0, 20e6])  # incident neutron energy
+
+    # Track source energies of secondary gammas
+    ene2_filter = openmc.ParticleProductionFilter(
+        ['neutron', 'photon'], [0.0, 100e3, 300e3, 500e3, 2e6, 20e6])
+
+    neutron_only = openmc.ParticleFilter(['neutron'])
+    tally_gam_ene = openmc.Tally()
+    tally_gam_ene.filters = [neutron_only, ene_filter, ene2_filter]
+    tally_gam_ene.scores = ['events']
+    tally_gam_ene.estimator = 'analog'
+
     model.tallies.extend([current_tally, tally_tracklength,
-                          tally_collision, tally_analog])
+                          tally_collision, tally_analog,
+                          tally_gam_ene])
 
     return model
 
diff --git a/tests/regression_tests/photon_production_fission/inputs_true.dat b/tests/regression_tests/photon_production_fission/inputs_true.dat
index fa379b39e..11a194e3c 100644
--- a/tests/regression_tests/photon_production_fission/inputs_true.dat
+++ b/tests/regression_tests/photon_production_fission/inputs_true.dat
@@ -1,21 +1,21 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="reflective" coeffs="0.0 0.0 0.0 100.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 100.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>5</batches>
     <inactive>2</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0 0 0</parameters>
       </space>
diff --git a/tests/regression_tests/photon_production_fission/results_true.dat b/tests/regression_tests/photon_production_fission/results_true.dat
index 1aa08b147..af325d4b0 100644
--- a/tests/regression_tests/photon_production_fission/results_true.dat
+++ b/tests/regression_tests/photon_production_fission/results_true.dat
@@ -1,12 +1,12 @@
 k-combined:
-2.272421E+00 4.418825E-02
+2.297165E+00 1.955494E-02
 tally 1:
-2.665301E+00
-2.369839E+00
+2.696393E+00
+2.423937E+00
 0.000000E+00
 0.000000E+00
-2.665301E+00
-2.369839E+00
+2.696393E+00
+2.423937E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -18,52 +18,52 @@ tally 1:
 0.000000E+00
 0.000000E+00
 tally 2:
-2.637532E+00
-2.320051E+00
-4.231377E+08
-5.971498E+16
+2.672029E+00
+2.380251E+00
+4.288244E+08
+6.130569E+16
 0.000000E+00
 0.000000E+00
-2.637532E+00
-2.320051E+00
-4.231377E+08
-5.971498E+16
+2.672029E+00
+2.380251E+00
+4.288244E+08
+6.130569E+16
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.448745E+06
-2.005297E+12
+1.711908E+05
+9.769096E+09
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.448745E+06
-2.005297E+12
+1.711908E+05
+9.769096E+09
 0.000000E+00
 0.000000E+00
 tally 3:
-2.660000E+00
-2.358558E+00
-4.231377E+08
-5.971498E+16
+2.649127E+00
+2.339294E+00
+4.288244E+08
+6.130569E+16
 0.000000E+00
 0.000000E+00
-2.660000E+00
-2.358558E+00
-4.231377E+08
-5.971498E+16
+2.649127E+00
+2.339294E+00
+4.288244E+08
+6.130569E+16
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.448745E+06
-2.005297E+12
+1.711908E+05
+9.769096E+09
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.448745E+06
-2.005297E+12
+1.711908E+05
+9.769096E+09
 0.000000E+00
 0.000000E+00
diff --git a/tests/regression_tests/photon_source/inputs_true.dat b/tests/regression_tests/photon_source/inputs_true.dat
index 3d2e05243..adaa5fb42 100644
--- a/tests/regression_tests/photon_source/inputs_true.dat
+++ b/tests/regression_tests/photon_source/inputs_true.dat
@@ -2,22 +2,22 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cm3" value="0.998207"/>
-      <nuclide ao="0.11187657362844" name="H1"/>
-      <nuclide ao="1.7426371559999997e-05" name="H2"/>
-      <nuclide ao="0.8877694078259999" name="O16"/>
-      <nuclide ao="0.000336592174" name="O17"/>
+      <density value="0.998207" units="g/cm3"/>
+      <nuclide name="H1" ao="0.11187657362844"/>
+      <nuclide name="H2" ao="1.7426371559999997e-05"/>
+      <nuclide name="O16" ao="0.8877694078259999"/>
+      <nuclide name="O17" ao="0.000336592174"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="reflective" coeffs="0.0 0.0 0.0 1000000000.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="reflective" coeffs="0.0 0.0 0.0 1000000000.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>10000</particles>
     <batches>1</batches>
-    <source particle="photon" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="photon">
       <space type="point">
         <parameters>0 0 0</parameters>
       </space>
diff --git a/tests/regression_tests/photon_source/results_true.dat b/tests/regression_tests/photon_source/results_true.dat
index fcc75cbca..8d934afc6 100644
--- a/tests/regression_tests/photon_source/results_true.dat
+++ b/tests/regression_tests/photon_source/results_true.dat
@@ -1,5 +1,5 @@
 tally 1:
-2.263938E+02
-5.125417E+04
+2.263761E+02
+5.124615E+04
 0.000000E+00
 0.000000E+00
diff --git a/tests/regression_tests/ptables_off/results_true.dat b/tests/regression_tests/ptables_off/results_true.dat
index 1652088ce..374274862 100644
--- a/tests/regression_tests/ptables_off/results_true.dat
+++ b/tests/regression_tests/ptables_off/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.978318E-01 1.560055E-03
+2.968228E-01 1.770320E-03
diff --git a/tests/regression_tests/pulse_height/local_neutron/inputs_true.dat b/tests/regression_tests/pulse_height/local_neutron/inputs_true.dat
new file mode 100644
index 000000000..e25de58b0
--- /dev/null
+++ b/tests/regression_tests/pulse_height/local_neutron/inputs_true.dat
@@ -0,0 +1,40 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="3.7" units="g/cm3"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="I127" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="inner sphere" material="1" region="-1" universe="1"/>
+    <cell id="2" name="outer sphere" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 2"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <photon_transport>true</photon_transport>
+    <shared_secondary_bank>false</shared_secondary_bank>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0 100000.0 110000.0 120000.0 130000.0 140000.0 150000.0 160000.0 170000.0 180000.0 190000.0 200000.0 210000.0 220000.0 230000.0 240000.0 250000.0 260000.0 270000.0 280000.0 290000.0 300000.0 310000.0 320000.0 330000.0 340000.0 350000.0 360000.0 370000.0 380000.0 390000.0 400000.0 410000.0 420000.0 430000.0 440000.0 450000.0 460000.0 470000.0 480000.0 490000.0 500000.0 510000.0 520000.0 530000.0 540000.0 550000.0 560000.0 570000.0 580000.0 590000.0 600000.0 610000.0 620000.0 630000.0 640000.0 650000.0 660000.0 670000.0 680000.0 690000.0 700000.0 710000.0 720000.0 730000.0 740000.0 750000.0 760000.0 770000.0 780000.0 790000.0 800000.0 810000.0 820000.0 830000.0 840000.0 850000.0 860000.0 870000.0 880000.0 890000.0 900000.0 910000.0 920000.0 930000.0 940000.0 950000.0 960000.0 970000.0 980000.0 990000.0 1000000.0</bins>
+    </filter>
+    <tally id="1" name="pht tally">
+      <filters>1 2</filters>
+      <scores>pulse-height</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/pulse_height/local_neutron/results_true.dat b/tests/regression_tests/pulse_height/local_neutron/results_true.dat
new file mode 100644
index 000000000..ffa284d92
--- /dev/null
+++ b/tests/regression_tests/pulse_height/local_neutron/results_true.dat
@@ -0,0 +1,201 @@
+tally 1:
+4.890000E+00
+4.784900E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.000000E-02
+1.000000E-04
+6.000000E-02
+1.800000E-03
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.000000E-02
+1.000000E-04
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.000000E-02
+1.000000E-04
+0.000000E+00
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diff --git a/tests/regression_tests/pulse_height/inputs_true.dat b/tests/regression_tests/pulse_height/local_photon/inputs_true.dat
similarity index 75%
rename from tests/regression_tests/pulse_height/inputs_true.dat
rename to tests/regression_tests/pulse_height/local_photon/inputs_true.dat
index 544ddb097..84d033d85 100644
--- a/tests/regression_tests/pulse_height/inputs_true.dat
+++ b/tests/regression_tests/pulse_height/local_photon/inputs_true.dat
@@ -2,30 +2,28 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cc" value="3.7"/>
-      <nuclide ao="1.0" name="Na23"/>
-      <nuclide ao="1.0" name="I127"/>
+      <density value="3.7" units="g/cm3"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="I127" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="inner sphere" region="-1" universe="1"/>
-    <cell id="2" material="void" name="outer sphere" region="1 -2" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 1" id="1" type="sphere"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 2" id="2" type="sphere"/>
+    <cell id="1" name="inner sphere" material="1" region="-1" universe="1"/>
+    <cell id="2" name="outer sphere" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 2"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100</particles>
     <batches>5</batches>
-    <source particle="photon" strength="1.0" type="independent">
-      <space type="point">
-        <parameters>0.0 0.0 0.0</parameters>
-      </space>
+    <source type="independent" strength="1.0" particle="photon">
       <energy type="discrete">
         <parameters>1000000.0 1.0</parameters>
       </energy>
     </source>
     <photon_transport>true</photon_transport>
+    <shared_secondary_bank>false</shared_secondary_bank>
   </settings>
   <tallies>
     <filter id="1" type="cell">
diff --git a/tests/regression_tests/pulse_height/results_true.dat b/tests/regression_tests/pulse_height/local_photon/results_true.dat
similarity index 100%
rename from tests/regression_tests/pulse_height/results_true.dat
rename to tests/regression_tests/pulse_height/local_photon/results_true.dat
diff --git a/tests/regression_tests/pulse_height/shared_neutron/inputs_true.dat b/tests/regression_tests/pulse_height/shared_neutron/inputs_true.dat
new file mode 100644
index 000000000..b13c53ef8
--- /dev/null
+++ b/tests/regression_tests/pulse_height/shared_neutron/inputs_true.dat
@@ -0,0 +1,40 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="3.7" units="g/cm3"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="I127" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="inner sphere" material="1" region="-1" universe="1"/>
+    <cell id="2" name="outer sphere" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 2"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <photon_transport>true</photon_transport>
+    <shared_secondary_bank>true</shared_secondary_bank>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0 100000.0 110000.0 120000.0 130000.0 140000.0 150000.0 160000.0 170000.0 180000.0 190000.0 200000.0 210000.0 220000.0 230000.0 240000.0 250000.0 260000.0 270000.0 280000.0 290000.0 300000.0 310000.0 320000.0 330000.0 340000.0 350000.0 360000.0 370000.0 380000.0 390000.0 400000.0 410000.0 420000.0 430000.0 440000.0 450000.0 460000.0 470000.0 480000.0 490000.0 500000.0 510000.0 520000.0 530000.0 540000.0 550000.0 560000.0 570000.0 580000.0 590000.0 600000.0 610000.0 620000.0 630000.0 640000.0 650000.0 660000.0 670000.0 680000.0 690000.0 700000.0 710000.0 720000.0 730000.0 740000.0 750000.0 760000.0 770000.0 780000.0 790000.0 800000.0 810000.0 820000.0 830000.0 840000.0 850000.0 860000.0 870000.0 880000.0 890000.0 900000.0 910000.0 920000.0 930000.0 940000.0 950000.0 960000.0 970000.0 980000.0 990000.0 1000000.0</bins>
+    </filter>
+    <tally id="1" name="pht tally">
+      <filters>1 2</filters>
+      <scores>pulse-height</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/pulse_height/shared_neutron/results_true.dat b/tests/regression_tests/pulse_height/shared_neutron/results_true.dat
new file mode 100644
index 000000000..ffa284d92
--- /dev/null
+++ b/tests/regression_tests/pulse_height/shared_neutron/results_true.dat
@@ -0,0 +1,201 @@
+tally 1:
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diff --git a/tests/regression_tests/pulse_height/shared_photon/inputs_true.dat b/tests/regression_tests/pulse_height/shared_photon/inputs_true.dat
new file mode 100644
index 000000000..608410c50
--- /dev/null
+++ b/tests/regression_tests/pulse_height/shared_photon/inputs_true.dat
@@ -0,0 +1,40 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="3.7" units="g/cm3"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="I127" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="inner sphere" material="1" region="-1" universe="1"/>
+    <cell id="2" name="outer sphere" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 2"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="photon">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <photon_transport>true</photon_transport>
+    <shared_secondary_bank>true</shared_secondary_bank>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0 100000.0 110000.0 120000.0 130000.0 140000.0 150000.0 160000.0 170000.0 180000.0 190000.0 200000.0 210000.0 220000.0 230000.0 240000.0 250000.0 260000.0 270000.0 280000.0 290000.0 300000.0 310000.0 320000.0 330000.0 340000.0 350000.0 360000.0 370000.0 380000.0 390000.0 400000.0 410000.0 420000.0 430000.0 440000.0 450000.0 460000.0 470000.0 480000.0 490000.0 500000.0 510000.0 520000.0 530000.0 540000.0 550000.0 560000.0 570000.0 580000.0 590000.0 600000.0 610000.0 620000.0 630000.0 640000.0 650000.0 660000.0 670000.0 680000.0 690000.0 700000.0 710000.0 720000.0 730000.0 740000.0 750000.0 760000.0 770000.0 780000.0 790000.0 800000.0 810000.0 820000.0 830000.0 840000.0 850000.0 860000.0 870000.0 880000.0 890000.0 900000.0 910000.0 920000.0 930000.0 940000.0 950000.0 960000.0 970000.0 980000.0 990000.0 1000000.0</bins>
+    </filter>
+    <tally id="1" name="pht tally">
+      <filters>1 2</filters>
+      <scores>pulse-height</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/pulse_height/shared_photon/results_true.dat b/tests/regression_tests/pulse_height/shared_photon/results_true.dat
new file mode 100644
index 000000000..c57e8ff1c
--- /dev/null
+++ b/tests/regression_tests/pulse_height/shared_photon/results_true.dat
@@ -0,0 +1,201 @@
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+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.000000E-01
+8.600000E-03
diff --git a/tests/regression_tests/pulse_height/test.py b/tests/regression_tests/pulse_height/test.py
index 90d960f66..371fcbd0f 100644
--- a/tests/regression_tests/pulse_height/test.py
+++ b/tests/regression_tests/pulse_height/test.py
@@ -1,53 +1,54 @@
 import numpy as np
 import openmc
 import pytest
+from openmc.utility_funcs import change_directory
 
 from tests.testing_harness import PyAPITestHarness
 
 
-@pytest.fixture
-def sphere_model():
-    
-    model = openmc.model.Model()
+@pytest.mark.parametrize("shared_secondary,particle", [
+    (False, "photon"),
+    (False, "neutron"),
+    (True, "photon"),
+    (True, "neutron")
+])
+def test_pulse_height(shared_secondary, particle):
+    subdir = f"shared_{particle}" if shared_secondary else f"local_{particle}"
+    with change_directory(subdir):
+        openmc.reset_auto_ids()
+        model = openmc.Model()
 
-    # Define materials
-    NaI = openmc.Material()
-    NaI.set_density('g/cc', 3.7)
-    NaI.add_element('Na', 1.0)
-    NaI.add_element('I', 1.0)
+        # Define materials
+        NaI = openmc.Material()
+        NaI.set_density('g/cm3', 3.7)
+        NaI.add_element('Na', 1.0)
+        NaI.add_element('I', 1.0)
 
-    model.materials = openmc.Materials([NaI])
+        # Define geometry: two spheres in each other
+        s1 = openmc.Sphere(r=1)
+        s2 = openmc.Sphere(r=2, boundary_type='vacuum')
+        inner_sphere = openmc.Cell(name='inner sphere', fill=NaI, region=-s1)
+        outer_sphere = openmc.Cell(name='outer sphere', region=+s1 & -s2)
+        model.geometry = openmc.Geometry([inner_sphere, outer_sphere])
 
-    # Define geometry: two spheres in each other
-    s1 = openmc.Sphere(r=1)
-    s2 = openmc.Sphere(r=2, boundary_type='vacuum')
-    inner_sphere = openmc.Cell(name='inner sphere', fill=NaI, region=-s1)
-    outer_sphere = openmc.Cell(name='outer sphere', region=+s1 & -s2)
-    model.geometry = openmc.Geometry([inner_sphere, outer_sphere])
+        # Define settings
+        model.settings.run_mode = 'fixed source'
+        model.settings.batches = 5
+        model.settings.particles = 100
+        model.settings.photon_transport = True
+        model.settings.shared_secondary_bank = shared_secondary
+        model.settings.source = openmc.IndependentSource(
+            energy=openmc.stats.delta_function(1e6),
+            particle=particle
+        )
 
-    # Define settings
-    model.settings.run_mode = 'fixed source'
-    model.settings.batches = 5
-    model.settings.particles = 100
-    model.settings.photon_transport = True
-    model.settings.source = openmc.IndependentSource(
-        space=openmc.stats.Point(),
-        energy=openmc.stats.Discrete([1e6], [1]),
-        particle='photon'
-    )
+        # Define tallies
+        tally = openmc.Tally(name="pht tally")
+        tally.scores = ['pulse-height']
+        cell_filter = openmc.CellFilter(inner_sphere)
+        energy_filter = openmc.EnergyFilter(np.linspace(0, 1e6, 101))
+        tally.filters = [cell_filter, energy_filter]
+        model.tallies = [tally]
 
-    # Define tallies
-    tally = openmc.Tally(name="pht tally")
-    tally.scores = ['pulse-height']
-    cell_filter = openmc.CellFilter(inner_sphere)
-    energy_filter = openmc.EnergyFilter(np.linspace(0, 1_000_000, 101))
-    tally.filters = [cell_filter, energy_filter]
-    model.tallies = [tally]
-
-    return model
-
-
-
-def test_pulse_height(sphere_model):
-    harness = PyAPITestHarness('statepoint.5.h5', sphere_model)
-    harness.main()
+        harness = PyAPITestHarness('statepoint.5.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/quadric_surfaces/results_true.dat b/tests/regression_tests/quadric_surfaces/results_true.dat
index 382c4241e..6fb569f52 100644
--- a/tests/regression_tests/quadric_surfaces/results_true.dat
+++ b/tests/regression_tests/quadric_surfaces/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.193830E+00 1.824184E-02
+1.216213E+00 2.789559E-02
diff --git a/tests/regression_tests/random_ray_adjoint_fixed_source/inputs_true.dat b/tests/regression_tests/random_ray_adjoint_fixed_source/inputs_true.dat
index 65ffd5af7..94e270976 100644
--- a/tests/regression_tests/random_ray_adjoint_fixed_source/inputs_true.dat
+++ b/tests/regression_tests/random_ray_adjoint_fixed_source/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>500</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,13 +207,15 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
-      <adjoint>True</adjoint>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+      <adjoint>true</adjoint>
       <volume_estimator>naive</volume_estimator>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_adjoint_fixed_source/results_true.dat b/tests/regression_tests/random_ray_adjoint_fixed_source/results_true.dat
index 7178e2f11..e9aa9015b 100644
--- a/tests/regression_tests/random_ray_adjoint_fixed_source/results_true.dat
+++ b/tests/regression_tests/random_ray_adjoint_fixed_source/results_true.dat
@@ -6,4 +6,4 @@ tally 2:
 9.482551E+08
 tally 3:
 1.956327E+05
-7.654469E+09
+7.654468E+09
diff --git a/tests/regression_tests/random_ray_adjoint_k_eff/inputs_true.dat b/tests/regression_tests/random_ray_adjoint_k_eff/inputs_true.dat
index 725702a49..755afd6c4 100644
--- a/tests/regression_tests/random_ray_adjoint_k_eff/inputs_true.dat
+++ b/tests/regression_tests/random_ray_adjoint_k_eff/inputs_true.dat
@@ -3,32 +3,32 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="15" material="2" name="moderator infinite" universe="3"/>
-    <cell fill="4" id="16" universe="5"/>
-    <cell fill="6" id="17" name="assembly" region="14 -15 16 -17" universe="7"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
     <lattice id="4">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -53,23 +53,23 @@
 2 2 
 2 5 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="14" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="15" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="16" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="17" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -80,13 +80,15 @@
     <random_ray>
       <distance_active>100.0</distance_active>
       <distance_inactive>20.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
-      <adjoint>True</adjoint>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+      <adjoint>true</adjoint>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_adjoint_k_eff/results_true.dat b/tests/regression_tests/random_ray_adjoint_k_eff/results_true.dat
index 657c841b5..dfef53cd2 100644
--- a/tests/regression_tests/random_ray_adjoint_k_eff/results_true.dat
+++ b/tests/regression_tests/random_ray_adjoint_k_eff/results_true.dat
@@ -1,171 +1,171 @@
 k-combined:
 1.006640E+00 1.812969E-03
 tally 1:
-6.684129E+00
-8.939821E+00
-2.685967E+00
-1.443592E+00
+1.208044E+00
+2.920182E-01
+4.854426E-01
+4.715453E-02
 0.000000E+00
 0.000000E+00
-6.358774E+00
-8.091444E+00
-9.687217E-01
-1.878029E-01
+1.149242E+00
+2.643067E-01
+1.750801E-01
+6.134563E-03
 0.000000E+00
 0.000000E+00
-5.963160E+00
-7.117108E+00
-1.932332E-01
-7.473914E-03
+1.077743E+00
+2.324814E-01
+3.492371E-02
+2.441363E-04
 0.000000E+00
 0.000000E+00
-5.137593E+00
-5.283310E+00
-1.714616E-01
-5.884834E-03
-1.086218E-06
-2.361752E-13
-4.857253E+00
-4.719856E+00
-5.689580E-02
-6.476286E-04
-2.989356E-03
-1.787808E-06
-4.830516E+00
-4.666801E+00
-7.203015E-03
-1.037676E-05
-3.620020E+00
-2.620927E+00
-5.161382E+00
-5.328124E+00
-6.786255E-02
-9.210763E-04
-5.531943E+00
-6.120553E+00
-5.414034E+00
-5.864661E+00
+9.285362E-01
+1.725808E-01
+3.098889E-02
+1.922297E-04
+1.963161E-07
+7.714727E-15
+8.778641E-01
+1.541719E-01
+1.028293E-02
+2.115448E-05
+5.402741E-04
+5.839789E-08
+8.730274E-01
+1.524358E-01
+1.301813E-03
+3.389450E-07
+6.542525E-01
+8.560964E-02
+9.328247E-01
+1.740366E-01
+1.226491E-02
+3.008584E-05
+9.997969E-01
+1.999204E-01
+9.784958E-01
+1.915688E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.632338E+00
-6.347626E+00
+1.017952E+00
+2.073461E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.682608E+00
-6.462382E+00
+1.027039E+00
+2.110955E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.310716E+00
-5.645180E+00
+9.598240E-01
+1.844004E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.945409E+00
-4.893171E+00
+8.937969E-01
+1.598332E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.842688E+00
-4.690352E+00
+8.752275E-01
+1.532052E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.117198E+00
-5.237280E+00
+9.248400E-01
+1.710699E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-6.938711E+00
-9.633345E+00
-2.835258E+00
-1.608212E+00
+1.254054E+00
+3.146708E-01
+5.124223E-01
+5.253093E-02
 0.000000E+00
 0.000000E+00
-6.549505E+00
-8.584036E+00
-1.015138E+00
-2.061993E-01
+1.183712E+00
+2.803961E-01
+1.834683E-01
+6.735381E-03
 0.000000E+00
 0.000000E+00
-6.050651E+00
-7.327711E+00
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diff --git a/tests/regression_tests/random_ray_adjoint_local/__init__.py b/tests/regression_tests/random_ray_adjoint_local/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_adjoint_local/inputs_true.dat b/tests/regression_tests/random_ray_adjoint_local/inputs_true.dat
new file mode 100644
index 000000000..9021d1675
--- /dev/null
+++ b/tests/regression_tests/random_ray_adjoint_local/inputs_true.dat
@@ -0,0 +1,293 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="source">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="source"/>
+    </material>
+    <material id="2" name="cavity">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="cavity"/>
+    </material>
+    <material id="3" name="absorber">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="absorber"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="cube cavity region" material="2" universe="2"/>
+    <cell id="3" name="absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="6" name="detector 1" material="3" region="1 -10 4 -8 5 -11" universe="6"/>
+    <cell id="7" name="detector 2" material="3" region="2 -7 4 -8 6 -9" universe="6"/>
+    <cell id="8" name="outside cube" material="2" region="(2 3 5 -7 ((-8 -6) | (-4 6 -9))) | (4 -8 ((10 -2 5 -9) | (-10 1 11 -9) | (2 -7 5 -6))) | (1 3 6 -9 ((-8 -2) | (-4 2 -7)))" universe="6"/>
+    <lattice id="4">
+      <pitch>2.5 2.5 2.5</pitch>
+      <dimension>12 12 12</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" coeffs="30.0"/>
+    <surface id="7" type="x-plane" boundary="vacuum" coeffs="35.0"/>
+    <surface id="8" type="y-plane" boundary="vacuum" coeffs="35.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="35.0"/>
+    <surface id="10" type="x-plane" coeffs="5.0"/>
+    <surface id="11" type="z-plane" coeffs="5.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>500</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="3.14" particle="neutron">
+      <energy type="discrete">
+        <parameters>100.0 1.0</parameters>
+      </energy>
+      <constraints>
+        <domain_type>universe</domain_type>
+        <domain_ids>1</domain_ids>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <distance_active>800.0</distance_active>
+      <distance_inactive>100.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 35.0 35.0 35.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="6" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <adjoint>true</adjoint>
+      <adjoint_source>
+        <source type="independent" strength="3.14" particle="neutron">
+          <energy type="discrete">
+            <parameters>100.0 1.0</parameters>
+          </energy>
+          <constraints>
+            <domain_type>cell</domain_type>
+            <domain_ids>6 7</domain_ids>
+          </constraints>
+        </source>
+      </adjoint_source>
+      <volume_estimator>naive</volume_estimator>
+    </random_ray>
+    <mesh id="1">
+      <dimension>14 14 14</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <upper_right>35.0 35.0 35.0</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>6</bins>
+    </filter>
+    <filter id="2" type="cell">
+      <bins>7</bins>
+    </filter>
+    <filter id="3" type="material">
+      <bins>3</bins>
+    </filter>
+    <filter id="4" type="material">
+      <bins>2</bins>
+    </filter>
+    <filter id="5" type="material">
+      <bins>1</bins>
+    </filter>
+    <tally id="1" name="Detector 1 Tally">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="2" name="Detector 2 Tally">
+      <filters>2</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="3" name="Absorber Tally">
+      <filters>3</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="4" name="Cavity Tally">
+      <filters>4</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="5" name="Source Tally">
+      <filters>5</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_adjoint_local/results_true.dat b/tests/regression_tests/random_ray_adjoint_local/results_true.dat
new file mode 100644
index 000000000..daa948565
--- /dev/null
+++ b/tests/regression_tests/random_ray_adjoint_local/results_true.dat
@@ -0,0 +1,15 @@
+tally 1:
+2.215273E+01
+9.815738E+01
+tally 2:
+1.873933E+01
+7.023420E+01
+tally 3:
+4.802282E-01
+4.612707E-02
+tally 4:
+2.516720E-01
+1.271063E-02
+tally 5:
+1.169938E-02
+3.277334E-05
diff --git a/tests/regression_tests/random_ray_adjoint_local/test.py b/tests/regression_tests/random_ray_adjoint_local/test.py
new file mode 100644
index 000000000..c11b8e847
--- /dev/null
+++ b/tests/regression_tests/random_ray_adjoint_local/test.py
@@ -0,0 +1,35 @@
+import os
+import openmc
+
+from openmc.examples import random_ray_three_region_cube_with_detectors
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_random_ray_adjoint_local():
+    model = random_ray_three_region_cube_with_detectors()
+
+    detector1_cells = model.geometry.get_cells_by_name("detector 1")
+    detector2_cells = model.geometry.get_cells_by_name("detector 2")
+    detector_cells = detector1_cells + detector2_cells
+
+    strengths = [1.0]
+    midpoints = [100.0]
+    energy_distribution = openmc.stats.Discrete(x=midpoints, p=strengths)
+
+    adj_source = openmc.IndependentSource(energy=energy_distribution, constraints={
+                                          'domains': detector_cells}, strength=3.14)
+    
+    model.settings.random_ray['adjoint'] = True
+    model.settings.random_ray['adjoint_source'] = adj_source
+    model.settings.random_ray['volume_estimator'] = 'naive'
+    harness = MGXSTestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/random_ray_auto_convert/__init__.py b/tests/regression_tests/random_ray_auto_convert/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_auto_convert/infinite_medium/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert/infinite_medium/inputs_true.dat
new file mode 100644
index 000000000..81f8c98ca
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/infinite_medium/inputs_true.dat
@@ -0,0 +1,66 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert/infinite_medium/results_true.dat b/tests/regression_tests/random_ray_auto_convert/infinite_medium/results_true.dat
new file mode 100644
index 000000000..f984f3718
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/infinite_medium/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.479770E-01 1.624548E-02
diff --git a/tests/regression_tests/random_ray_auto_convert/material_wise/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert/material_wise/inputs_true.dat
new file mode 100644
index 000000000..81f8c98ca
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/material_wise/inputs_true.dat
@@ -0,0 +1,66 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert/material_wise/results_true.dat b/tests/regression_tests/random_ray_auto_convert/material_wise/results_true.dat
new file mode 100644
index 000000000..3bade01e0
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/material_wise/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.372542E-01 6.967831E-03
diff --git a/tests/regression_tests/random_ray_auto_convert/stochastic_slab/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert/stochastic_slab/inputs_true.dat
new file mode 100644
index 000000000..81f8c98ca
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/stochastic_slab/inputs_true.dat
@@ -0,0 +1,66 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert/stochastic_slab/results_true.dat b/tests/regression_tests/random_ray_auto_convert/stochastic_slab/results_true.dat
new file mode 100644
index 000000000..674dee4aa
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/stochastic_slab/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+6.413334E-01 2.083132E-02
diff --git a/tests/regression_tests/random_ray_auto_convert/test.py b/tests/regression_tests/random_ray_auto_convert/test.py
new file mode 100644
index 000000000..99a931dce
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert/test.py
@@ -0,0 +1,54 @@
+import os
+
+import openmc
+from openmc.examples import pwr_pin_cell
+from openmc import RegularMesh
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+@pytest.mark.parametrize("method", ["material_wise", "stochastic_slab", "infinite_medium"])
+def test_random_ray_auto_convert(method):
+    with change_directory(method):
+        openmc.reset_auto_ids()
+
+        # Start with a normal continuous energy model
+        model = pwr_pin_cell()
+
+        # Convert to a multi-group model
+        model.convert_to_multigroup(
+            method=method, groups='CASMO-2', nparticles=100,
+            overwrite_mgxs_library=False, mgxs_path="mgxs.h5"
+        )
+
+        # Convert to a random ray model
+        model.convert_to_random_ray()
+
+        # Set the number of particles
+        model.settings.particles = 100
+
+        # Overlay a basic 2x2 mesh
+        n = 2
+        mesh = RegularMesh()
+        mesh.dimension = (n, n)
+        bbox = model.geometry.bounding_box
+        mesh.lower_left = (bbox.lower_left[0], bbox.lower_left[1])
+        mesh.upper_right = (bbox.upper_right[0], bbox.upper_right[1])
+        model.settings.random_ray['source_region_meshes'] = [
+            (mesh, [model.geometry.root_universe])]
+
+        # Set the source shape to linear
+        model.settings.random_ray['source_shape'] = 'linear'
+
+        harness = MGXSTestHarness('statepoint.10.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/__init__.py b/tests/regression_tests/random_ray_auto_convert_kappa_fission/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/infinite_medium/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_kappa_fission/infinite_medium/inputs_true.dat
new file mode 100644
index 000000000..48b0e8256
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/infinite_medium/inputs_true.dat
@@ -0,0 +1,75 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="61" type="material">
+      <bins>1</bins>
+    </filter>
+    <tally id="193" name="KF Tally">
+      <filters>61</filters>
+      <scores>kappa-fission</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/infinite_medium/results_true.dat b/tests/regression_tests/random_ray_auto_convert_kappa_fission/infinite_medium/results_true.dat
new file mode 100644
index 000000000..27b32d787
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/infinite_medium/results_true.dat
@@ -0,0 +1,5 @@
+k-combined:
+7.479770E-01 1.624548E-02
+tally 1:
+2.965503E+08
+1.762157E+16
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/material_wise/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_kappa_fission/material_wise/inputs_true.dat
new file mode 100644
index 000000000..be738c3e0
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/material_wise/inputs_true.dat
@@ -0,0 +1,75 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="97" type="material">
+      <bins>1</bins>
+    </filter>
+    <tally id="203" name="KF Tally">
+      <filters>97</filters>
+      <scores>kappa-fission</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/material_wise/results_true.dat b/tests/regression_tests/random_ray_auto_convert_kappa_fission/material_wise/results_true.dat
new file mode 100644
index 000000000..5a379db7e
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/material_wise/results_true.dat
@@ -0,0 +1,5 @@
+k-combined:
+7.372542E-01 6.967831E-03
+tally 1:
+2.909255E+08
+1.693395E+16
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/stochastic_slab/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_kappa_fission/stochastic_slab/inputs_true.dat
new file mode 100644
index 000000000..be738c3e0
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/stochastic_slab/inputs_true.dat
@@ -0,0 +1,75 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="97" type="material">
+      <bins>1</bins>
+    </filter>
+    <tally id="203" name="KF Tally">
+      <filters>97</filters>
+      <scores>kappa-fission</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/stochastic_slab/results_true.dat b/tests/regression_tests/random_ray_auto_convert_kappa_fission/stochastic_slab/results_true.dat
new file mode 100644
index 000000000..a60e5aa3c
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/stochastic_slab/results_true.dat
@@ -0,0 +1,5 @@
+k-combined:
+6.413334E-01 2.083132E-02
+tally 1:
+2.541463E+08
+1.297259E+16
diff --git a/tests/regression_tests/random_ray_auto_convert_kappa_fission/test.py b/tests/regression_tests/random_ray_auto_convert_kappa_fission/test.py
new file mode 100644
index 000000000..6decf165a
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_kappa_fission/test.py
@@ -0,0 +1,60 @@
+import os
+
+import openmc
+from openmc.examples import pwr_pin_cell
+from openmc import RegularMesh
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+@pytest.mark.parametrize("method", ["material_wise", "stochastic_slab", "infinite_medium"])
+def test_random_ray_auto_convert(method):
+    with change_directory(method):
+        openmc.reset_auto_ids()
+
+        # Start with a normal continuous energy model
+        model = pwr_pin_cell()
+
+        # Convert to a multi-group model
+        model.convert_to_multigroup(
+            method=method, groups='CASMO-2', nparticles=100,
+            overwrite_mgxs_library=False, mgxs_path="mgxs.h5"
+        )
+
+        # Convert to a random ray model
+        model.convert_to_random_ray()
+
+        # Set the number of particles
+        model.settings.particles = 100
+
+        # Overlay a basic 2x2 mesh
+        n = 2
+        mesh = RegularMesh()
+        mesh.dimension = (n, n)
+        bbox = model.geometry.bounding_box
+        mesh.lower_left = (bbox.lower_left[0], bbox.lower_left[1])
+        mesh.upper_right = (bbox.upper_right[0], bbox.upper_right[1])
+        model.settings.random_ray['source_region_meshes'] = [
+            (mesh, [model.geometry.root_universe])]
+
+        # Set the source shape to linear
+        model.settings.random_ray['source_shape'] = 'linear'
+
+        # Set a material tally
+        t = openmc.Tally(name = 'KF Tally')
+        t.filters = [openmc.MaterialFilter(bins=1)]
+        t.scores = ['kappa-fission']
+        model.tallies.append(t)
+
+        harness = MGXSTestHarness('statepoint.10.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/__init__.py b/tests/regression_tests/random_ray_auto_convert_source_energy/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/model/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/model/inputs_true.dat
new file mode 100644
index 000000000..d2d8289ff
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/model/inputs_true.dat
@@ -0,0 +1,63 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <energy type="discrete">
+        <parameters>7000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/model/results_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/model/results_true.dat
new file mode 100644
index 000000000..1fb09fd68
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/model/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.657815E-01 2.317564E-02
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/user/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/user/inputs_true.dat
new file mode 100644
index 000000000..81f8c98ca
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/user/inputs_true.dat
@@ -0,0 +1,66 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/user/results_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/user/results_true.dat
new file mode 100644
index 000000000..073c5c99f
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/infinite_medium/user/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.827784E-01 2.062954E-02
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/model/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/model/inputs_true.dat
new file mode 100644
index 000000000..d2d8289ff
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/model/inputs_true.dat
@@ -0,0 +1,63 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <energy type="discrete">
+        <parameters>7000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/model/results_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/model/results_true.dat
new file mode 100644
index 000000000..c5cdf8e29
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/model/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.479571E-01 2.398563E-02
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/user/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/user/inputs_true.dat
new file mode 100644
index 000000000..81f8c98ca
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/user/inputs_true.dat
@@ -0,0 +1,66 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/user/results_true.dat b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/user/results_true.dat
new file mode 100644
index 000000000..c6cce2e39
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/stochastic_slab/user/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.620306E-01 2.175179E-02
diff --git a/tests/regression_tests/random_ray_auto_convert_source_energy/test.py b/tests/regression_tests/random_ray_auto_convert_source_energy/test.py
new file mode 100644
index 000000000..bb9119d89
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_source_energy/test.py
@@ -0,0 +1,66 @@
+import os
+
+import openmc
+from openmc.examples import pwr_pin_cell
+from openmc import RegularMesh
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+@pytest.mark.parametrize("source_type", ["model", "user"])
+@pytest.mark.parametrize("method", ["stochastic_slab", "infinite_medium"])
+def test_random_ray_auto_convert_source_energy(method, source_type):
+    dirname = f"{method}/{source_type}"
+    with change_directory(dirname):
+        openmc.reset_auto_ids()
+
+        # Start with a normal continuous energy model
+        model = pwr_pin_cell()
+
+        # Define the source energy distribution, using different methods
+        source_energy = None
+        if source_type == "model":
+            model.settings.source = openmc.IndependentSource(
+                energy=openmc.stats.delta_function(7.0e6)
+            )
+        elif source_type == "user":
+            source_energy = openmc.stats.delta_function(1.0e4)
+
+        # Convert to a multi-group model
+        model.convert_to_multigroup(
+            method=method, groups='CASMO-8', nparticles=100,
+            overwrite_mgxs_library=False, mgxs_path="mgxs.h5",
+            source_energy=source_energy
+        )
+
+        # Convert to a random ray model
+        model.convert_to_random_ray()
+
+        # Set the number of particles
+        model.settings.particles = 100
+
+        # Overlay a basic 2x2 mesh
+        n = 2
+        mesh = RegularMesh()
+        mesh.dimension = (n, n)
+        bbox = model.geometry.bounding_box
+        mesh.lower_left = (bbox.lower_left[0], bbox.lower_left[1])
+        mesh.upper_right = (bbox.upper_right[0], bbox.upper_right[1])
+        model.settings.random_ray['source_region_meshes'] = [
+            (mesh, [model.geometry.root_universe])]
+
+        # Set the source shape to linear
+        model.settings.random_ray['source_shape'] = 'linear'
+
+        harness = MGXSTestHarness('statepoint.10.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/__init__.py b/tests/regression_tests/random_ray_auto_convert_temperature/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/infinite_medium/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_temperature/infinite_medium/inputs_true.dat
new file mode 100644
index 000000000..c49020d55
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/infinite_medium/inputs_true.dat
@@ -0,0 +1,72 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0">
+      <temperature>395.0</temperature>
+    </cell>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <temperature_method>nearest</temperature_method>
+    <temperature_multipole>true</temperature_multipole>
+    <temperature_range>200.0 400.0</temperature_range>
+    <temperature_tolerance>200.0</temperature_tolerance>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/infinite_medium/results_true.dat b/tests/regression_tests/random_ray_auto_convert_temperature/infinite_medium/results_true.dat
new file mode 100644
index 000000000..fee8bf670
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/infinite_medium/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.499800E-01 1.615317E-02
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/material_wise/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_temperature/material_wise/inputs_true.dat
new file mode 100644
index 000000000..c49020d55
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/material_wise/inputs_true.dat
@@ -0,0 +1,72 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0">
+      <temperature>395.0</temperature>
+    </cell>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <temperature_method>nearest</temperature_method>
+    <temperature_multipole>true</temperature_multipole>
+    <temperature_range>200.0 400.0</temperature_range>
+    <temperature_tolerance>200.0</temperature_tolerance>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/material_wise/results_true.dat b/tests/regression_tests/random_ray_auto_convert_temperature/material_wise/results_true.dat
new file mode 100644
index 000000000..ef0a4b87a
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/material_wise/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.367927E-01 6.850805E-03
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/stochastic_slab/inputs_true.dat b/tests/regression_tests/random_ray_auto_convert_temperature/stochastic_slab/inputs_true.dat
new file mode 100644
index 000000000..c49020d55
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/stochastic_slab/inputs_true.dat
@@ -0,0 +1,72 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water" temperature="294.0">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0">
+      <temperature>395.0</temperature>
+    </cell>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <temperature_method>nearest</temperature_method>
+    <temperature_multipole>true</temperature_multipole>
+    <temperature_range>200.0 400.0</temperature_range>
+    <temperature_tolerance>200.0</temperature_tolerance>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/stochastic_slab/results_true.dat b/tests/regression_tests/random_ray_auto_convert_temperature/stochastic_slab/results_true.dat
new file mode 100644
index 000000000..a702ec851
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/stochastic_slab/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+6.431774E-01 2.076589E-02
diff --git a/tests/regression_tests/random_ray_auto_convert_temperature/test.py b/tests/regression_tests/random_ray_auto_convert_temperature/test.py
new file mode 100644
index 000000000..99c99e614
--- /dev/null
+++ b/tests/regression_tests/random_ray_auto_convert_temperature/test.py
@@ -0,0 +1,73 @@
+import os
+
+import openmc
+from openmc.examples import pwr_pin_cell
+from openmc import RegularMesh
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+@pytest.mark.parametrize("method", ["material_wise", "stochastic_slab", "infinite_medium"])
+def test_random_ray_auto_convert(method):
+    with change_directory(method):
+        openmc.reset_auto_ids()
+
+        # Start with a normal continuous energy model
+        model = pwr_pin_cell()
+
+        temp_settings = {
+          'method' : 'nearest',
+          'tolerance' : 200.0,
+          'range' : (200.0, 400.0),
+          'multipole' : True
+        }
+
+        # Convert to a multi-group model
+        model.convert_to_multigroup(
+            method=method, groups='CASMO-2', nparticles=100,
+            overwrite_mgxs_library=False, mgxs_path="mgxs.h5",
+            temperatures=[294.0, 394.0], temperature_settings=temp_settings
+        )
+
+        # Convert to a random ray model
+        model.convert_to_random_ray()
+        model.settings.temperature = temp_settings
+
+        # Set all material temperatures to room temperature
+        for mat in model.geometry.get_all_materials().values():
+            mat.temperature = 294.0
+
+        # Set the cell temperature of the fuel such that it moves up to the next
+        # temperature bin.
+        for cell in model.geometry.get_all_cells().values():
+            if cell.name == "Fuel":
+                cell.temperature = [395.0]
+
+        # Set the number of particles
+        model.settings.particles = 100
+
+        # Overlay a basic 2x2 mesh
+        n = 2
+        mesh = RegularMesh()
+        mesh.dimension = (n, n)
+        bbox = model.geometry.bounding_box
+        mesh.lower_left = (bbox.lower_left[0], bbox.lower_left[1])
+        mesh.upper_right = (bbox.upper_right[0], bbox.upper_right[1])
+        model.settings.random_ray['source_region_meshes'] = [
+            (mesh, [model.geometry.root_universe])]
+
+        # Set the source shape to linear
+        model.settings.random_ray['source_shape'] = 'linear'
+
+        harness = MGXSTestHarness('statepoint.10.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/random_ray_cell_density/__init__.py b/tests/regression_tests/random_ray_cell_density/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_cell_density/eigen/inputs_true.dat b/tests/regression_tests/random_ray_cell_density/eigen/inputs_true.dat
new file mode 100644
index 000000000..eacd54f83
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_density/eigen/inputs_true.dat
@@ -0,0 +1,117 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 fuel">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2"/>
+    </material>
+    <material id="2" name="Water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="LWTR"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1">
+      <density>2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0</density>
+    </cell>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1">
+      <density>2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0</density>
+    </cell>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1">
+      <density>2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0</density>
+    </cell>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
+    <lattice id="4">
+      <pitch>0.126 0.126</pitch>
+      <dimension>10 10</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <lattice id="6">
+      <pitch>1.26 1.26</pitch>
+      <dimension>2 2</dimension>
+      <lower_left>-1.26 -1.26</lower_left>
+      <universes>
+2 2 
+2 5 </universes>
+    </lattice>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <distance_active>100.0</distance_active>
+      <distance_inactive>20.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+    </random_ray>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-1.26 -1.26</lower_left>
+      <upper_right>1.26 1.26</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>1e-05 0.0635 10.0 100.0 1000.0 500000.0 1000000.0 20000000.0</bins>
+    </filter>
+    <tally id="1" name="Mesh tally">
+      <filters>1 2</filters>
+      <scores>flux fission nu-fission</scores>
+      <estimator>analog</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_cell_density/eigen/results_true.dat b/tests/regression_tests/random_ray_cell_density/eigen/results_true.dat
new file mode 100644
index 000000000..3e6ce4039
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_density/eigen/results_true.dat
@@ -0,0 +1,171 @@
+k-combined:
+7.606488E-01 9.025978E-03
+tally 1:
+6.209503E-01
+7.732732E-02
+4.335729E-01
+3.769026E-02
+1.055230E+00
+2.232538E-01
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+1.164050E+00
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+3.370803E-02
+1.358439E-02
+3.774180E-05
+3.306168E-02
+2.235591E-04
+1.279744E+00
+3.345476E-01
+1.512005E-02
+4.668112E-05
+3.679962E-02
+2.765169E-04
+3.917888E+00
+3.090981E+00
+6.082371E-03
+7.449625E-06
+1.505040E-02
+4.561259E-05
+2.520906E+00
+1.271106E+00
+3.477796E-02
+2.419730E-04
+9.673314E-02
+1.872015E-03
diff --git a/tests/regression_tests/random_ray_cell_density/fs/inputs_true.dat b/tests/regression_tests/random_ray_cell_density/fs/inputs_true.dat
new file mode 100644
index 000000000..f369bae89
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_density/fs/inputs_true.dat
@@ -0,0 +1,246 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="source">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="source"/>
+    </material>
+    <material id="2" name="void">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="void"/>
+    </material>
+    <material id="3" name="absorber">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="absorber"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="infinite source region" material="1" density="1000.0" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
+    <lattice id="4">
+      <pitch>2.5 2.5 2.5</pitch>
+      <dimension>12 12 12</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>90</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="3.14" particle="neutron">
+      <energy type="discrete">
+        <parameters>100.0 1.0</parameters>
+      </energy>
+      <constraints>
+        <domain_type>universe</domain_type>
+        <domain_ids>1</domain_ids>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <distance_active>500.0</distance_active>
+      <distance_inactive>100.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+    </random_ray>
+  </settings>
+  <tallies>
+    <filter id="3" type="material">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="material">
+      <bins>2</bins>
+    </filter>
+    <filter id="1" type="material">
+      <bins>3</bins>
+    </filter>
+    <tally id="3" name="Source Tally">
+      <filters>3</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="2" name="Void Tally">
+      <filters>2</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="1" name="Absorber Tally">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_cell_density/fs/results_true.dat b/tests/regression_tests/random_ray_cell_density/fs/results_true.dat
new file mode 100644
index 000000000..e12f476e0
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_density/fs/results_true.dat
@@ -0,0 +1,9 @@
+tally 1:
+6.203077E-01
+7.706659E-02
+tally 2:
+3.203448E-02
+2.058679E-04
+tally 3:
+2.091970E-03
+8.765168E-07
diff --git a/tests/regression_tests/random_ray_cell_density/test.py b/tests/regression_tests/random_ray_cell_density/test.py
new file mode 100644
index 000000000..48ebe0baa
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_density/test.py
@@ -0,0 +1,43 @@
+import os
+
+import openmc
+from openmc.examples import random_ray_lattice, random_ray_three_region_cube
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+@pytest.mark.parametrize("run_mode", ["eigen", "fs"])
+def test_random_ray_basic(run_mode):
+    with change_directory(run_mode):
+        if run_mode == "eigen":
+            openmc.reset_auto_ids()
+            model = random_ray_lattice()
+            # Double the densities of the lower-left fuel pin -> cell instances [0, 8).
+            for id, cell in model.geometry.get_all_cells().items():
+                if cell.fill.name == "UO2 fuel":
+                    cell.density = [((i < 8) + 1.0) for i in range(24)]
+
+            # Gold file was generated with manually scaled fuel cross sections.
+            harness = MGXSTestHarness('statepoint.10.h5', model)
+            harness.main()
+        else:
+            openmc.reset_auto_ids()
+            model = random_ray_three_region_cube()
+            # Increase the density in the source region.
+            for id, cell in model.geometry.get_all_cells().items():
+                if cell.fill.name == "source":
+                    cell.density = 1e3
+
+            # Gold file was generated with manually scaled source cross sections.
+            harness = MGXSTestHarness('statepoint.10.h5', model)
+            harness.main()
diff --git a/tests/regression_tests/random_ray_cell_temperature/__init__.py b/tests/regression_tests/random_ray_cell_temperature/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_cell_temperature/inputs_true.dat b/tests/regression_tests/random_ray_cell_temperature/inputs_true.dat
new file mode 100644
index 000000000..99363ed87
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_temperature/inputs_true.dat
@@ -0,0 +1,120 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 fuel">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2"/>
+    </material>
+    <material id="2" name="Water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="LWTR"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1">
+      <temperature>395.0 395.0 395.0 395.0 395.0 395.0 395.0 395.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0</temperature>
+    </cell>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1">
+      <temperature>395.0 395.0 395.0 395.0 395.0 395.0 395.0 395.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0</temperature>
+    </cell>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1">
+      <temperature>395.0 395.0 395.0 395.0 395.0 395.0 395.0 395.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0 295.0</temperature>
+    </cell>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
+    <lattice id="4">
+      <pitch>0.126 0.126</pitch>
+      <dimension>10 10</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <lattice id="6">
+      <pitch>1.26 1.26</pitch>
+      <dimension>2 2</dimension>
+      <lower_left>-1.26 -1.26</lower_left>
+      <universes>
+2 2 
+2 5 </universes>
+    </lattice>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <energy_mode>multi-group</energy_mode>
+    <temperature_method>nearest</temperature_method>
+    <temperature_range>200.0 400.0</temperature_range>
+    <temperature_tolerance>10.0</temperature_tolerance>
+    <random_ray>
+      <distance_active>100.0</distance_active>
+      <distance_inactive>20.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+    </random_ray>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-1.26 -1.26</lower_left>
+      <upper_right>1.26 1.26</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>1e-05 0.0635 10.0 100.0 1000.0 500000.0 1000000.0 20000000.0</bins>
+    </filter>
+    <tally id="1" name="Mesh tally">
+      <filters>1 2</filters>
+      <scores>flux fission nu-fission</scores>
+      <estimator>analog</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_cell_temperature/results_true.dat b/tests/regression_tests/random_ray_cell_temperature/results_true.dat
new file mode 100644
index 000000000..50476beb6
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_temperature/results_true.dat
@@ -0,0 +1,171 @@
+k-combined:
+8.721099E-01 6.686066E-03
+tally 1:
+1.530055E+00
+4.684582E-01
+2.918032E-01
+1.703772E-02
+7.101906E-01
+1.009209E-01
+7.732582E-01
+1.197247E-01
+5.787213E-02
+6.705930E-04
+1.408492E-01
+3.972179E-03
+4.223488E-01
+3.605283E-02
+6.818063E-03
+9.396342E-06
+1.659380E-02
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+9.932163E-03
+2.004773E-05
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+1.187504E-04
+1.685106E+00
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+9.834826E-03
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+1.161151E-04
+4.400457E+00
+3.886563E+00
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+8.211544E-03
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+2.814971E+00
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+7.040316E-05
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+1.970406E+00
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+8.643086E-01
+1.494764E-01
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+0.000000E+00
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+0.000000E+00
+4.421562E-01
+3.961223E-02
+0.000000E+00
+0.000000E+00
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+0.000000E+00
+6.421077E-01
+8.385255E-02
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.717683E+00
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+0.000000E+00
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+4.024715E+00
+3.251629E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.507885E+00
+1.258277E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.237600E+00
+3.066006E-01
+4.632318E-01
+4.294997E-02
+1.127413E+00
+2.544090E-01
+7.085300E-01
+1.005838E-01
+1.066845E-01
+2.280854E-03
+2.596486E-01
+1.351037E-02
+4.080999E-01
+3.359223E-02
+1.334606E-02
+3.590848E-05
+3.248163E-02
+2.126996E-04
+5.587686E-01
+6.339264E-02
+1.868162E-02
+7.084921E-05
+4.546734E-02
+4.196670E-04
+1.647852E+00
+5.503007E-01
+1.940765E-02
+7.635704E-05
+4.723492E-02
+4.523030E-04
+4.672431E+00
+4.381400E+00
+7.228567E-03
+1.048470E-05
+1.788658E-02
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+3.054158E+00
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+1.168957E-01
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+3.454374E-01
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+1.208158E+00
+2.922789E-01
+7.298176E-01
+1.067017E-01
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+2.452869E-03
+2.692559E-01
+1.452928E-02
+4.129038E-01
+3.441066E-02
+1.357107E-02
+3.713729E-05
+3.302926E-02
+2.199783E-04
+5.677464E-01
+6.543807E-02
+1.908992E-02
+7.390190E-05
+4.646105E-02
+4.377492E-04
+1.651067E+00
+5.522453E-01
+1.956802E-02
+7.754346E-05
+4.762523E-02
+4.593308E-04
+4.583305E+00
+4.217589E+00
+7.135836E-03
+1.022408E-05
+1.765713E-02
+6.260005E-05
+2.988394E+00
+1.786305E+00
+4.141550E-02
+3.431964E-04
+1.151951E-01
+2.655125E-03
diff --git a/tests/regression_tests/random_ray_cell_temperature/test.py b/tests/regression_tests/random_ray_cell_temperature/test.py
new file mode 100644
index 000000000..ef36d9238
--- /dev/null
+++ b/tests/regression_tests/random_ray_cell_temperature/test.py
@@ -0,0 +1,36 @@
+import os
+
+import openmc
+from openmc.examples import random_ray_lattice, random_ray_three_region_cube
+from openmc.utility_funcs import change_directory
+import pytest
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_random_ray_basic():
+    openmc.reset_auto_ids()
+    model = random_ray_lattice(second_temp=True)
+    # Set the temperature of the lower-left pin to 395 K -> cell instances [0, 8).
+    # All other pins are set to 295.
+    for id, cell in model.geometry.get_all_cells().items():
+        if cell.fill.name == "UO2 fuel":
+            cell.temperature = [(100.0 * (i < 8) + 295.0) for i in range(24)]
+
+    model.settings.temperature = {
+        'method' : 'nearest',
+        'tolerance' : 10.0,
+        'range' : (200.0, 400.0)
+    }
+
+    # Gold file was generated with manually scaled fuel cross sections.
+    harness = MGXSTestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/random_ray_diagonal_stabilization/__init__.py b/tests/regression_tests/random_ray_diagonal_stabilization/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_diagonal_stabilization/inputs_true.dat b/tests/regression_tests/random_ray_diagonal_stabilization/inputs_true.dat
new file mode 100644
index 000000000..0ea8c0177
--- /dev/null
+++ b/tests/regression_tests/random_ray_diagonal_stabilization/inputs_true.dat
@@ -0,0 +1,67 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="UO2 (2.4%)" depletable="true">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="UO2__2_4___1"/>
+    </material>
+    <material id="2" name="Zircaloy">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Zircaloy_2"/>
+    </material>
+    <material id="3" name="Hot borated water">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="Hot_borated_water_3"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="Fuel" material="1" region="-1" universe="0"/>
+    <cell id="2" name="Cladding" material="2" region="1 -2" universe="0"/>
+    <cell id="3" name="Water" material="3" region="2 3 -4 5 -6" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 0.39218"/>
+    <surface id="2" name="Clad OR" type="z-cylinder" coeffs="0 0 0.4572"/>
+    <surface id="3" name="left" type="x-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="4" name="right" type="x-plane" boundary="reflective" coeffs="0.63"/>
+    <surface id="5" name="bottom" type="y-plane" boundary="reflective" coeffs="-0.63"/>
+    <surface id="6" name="top" type="y-plane" boundary="reflective" coeffs="0.63"/>
+  </geometry>
+  <settings>
+    <run_mode>eigenvalue</run_mode>
+    <particles>100</particles>
+    <batches>20</batches>
+    <inactive>15</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>-0.63 -0.63 -1 0.63 0.63 1</parameters>
+      </space>
+      <constraints>
+        <fissionable>true</fissionable>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-0.63 -0.63 -1.0 0.63 0.63 1.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <distance_inactive>30.0</distance_inactive>
+      <distance_active>150.0</distance_active>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="0" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <source_shape>linear</source_shape>
+      <diagonal_stabilization_rho>0.5</diagonal_stabilization_rho>
+    </random_ray>
+    <mesh id="1">
+      <dimension>2 2</dimension>
+      <lower_left>-0.63 -0.63</lower_left>
+      <upper_right>0.63 0.63</upper_right>
+    </mesh>
+  </settings>
+  </model>
diff --git a/tests/regression_tests/random_ray_diagonal_stabilization/results_true.dat b/tests/regression_tests/random_ray_diagonal_stabilization/results_true.dat
new file mode 100644
index 000000000..034d7f7c6
--- /dev/null
+++ b/tests/regression_tests/random_ray_diagonal_stabilization/results_true.dat
@@ -0,0 +1,2 @@
+k-combined:
+7.134473E-01 1.422763E-02
diff --git a/tests/regression_tests/random_ray_diagonal_stabilization/test.py b/tests/regression_tests/random_ray_diagonal_stabilization/test.py
new file mode 100644
index 000000000..8d36e1d25
--- /dev/null
+++ b/tests/regression_tests/random_ray_diagonal_stabilization/test.py
@@ -0,0 +1,61 @@
+import os
+
+from openmc.examples import pwr_pin_cell
+from openmc import RegularMesh
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_random_ray_diagonal_stabilization():
+    # Start with a normal continuous energy model
+    model = pwr_pin_cell()
+
+    # Convert to a multi-group model, with 70 group XS
+    # and transport correction enabled. This will generate
+    # MGXS data with some negatives on the diagonal, in order
+    # to trigger diagonal correction.
+    model.convert_to_multigroup(
+        method='material_wise', groups='CASMO-70', nparticles=13,
+        overwrite_mgxs_library=True, mgxs_path="mgxs.h5", correction='P0'
+    )
+
+    # Convert to a random ray model
+    model.convert_to_random_ray()
+
+    # Set the number of particles
+    model.settings.particles = 100
+
+    # Overlay a basic 2x2 mesh
+    n = 2
+    mesh = RegularMesh()
+    mesh.dimension = (n, n)
+    bbox = model.geometry.bounding_box
+    mesh.lower_left = (bbox.lower_left[0], bbox.lower_left[1])
+    mesh.upper_right = (bbox.upper_right[0], bbox.upper_right[1])
+    model.settings.random_ray['source_region_meshes'] = [
+        (mesh, [model.geometry.root_universe])]
+
+    # Set the source shape to linear
+    model.settings.random_ray['source_shape'] = 'linear'
+
+    # Explicitly set the diagonal stabilization rho (default is otherwise 1.0).
+    # Note that if we set this to 0.0 (thus distabling stabilization), the
+    # problem should fail due to instability, so this is actually a good test
+    # problem.
+    model.settings.random_ray['diagonal_stabilization_rho'] = 0.5
+
+    # If rho was 0.0, the instability would cause failure after iteration 14,
+    # so we go a little past that.
+    model.settings.inactive = 15
+    model.settings.batches = 20
+
+    harness = MGXSTestHarness('statepoint.20.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/random_ray_entropy/settings.xml b/tests/regression_tests/random_ray_entropy/settings.xml
index 81deaa775..0d830417b 100644
--- a/tests/regression_tests/random_ray_entropy/settings.xml
+++ b/tests/regression_tests/random_ray_entropy/settings.xml
@@ -6,11 +6,13 @@
   <inactive>5</inactive>
   <energy_mode>multi-group</energy_mode>
   <random_ray>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space type="box">
-        <parameters>0.0 0.0 0.0 100.0 100.0 100.0</parameters>
-      </space>
-    </source>
+    <ray_source>
+      <source particle="neutron" strength="1.0" type="independent">
+        <space type="box">
+          <parameters>0.0 0.0 0.0 100.0 100.0 100.0</parameters>
+        </space>
+      </source>
+    </ray_source>
     <distance_inactive>40.0</distance_inactive>
     <distance_active>400.0</distance_active>
   </random_ray>
diff --git a/tests/regression_tests/random_ray_fixed_source_domain/cell/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_domain/cell/inputs_true.dat
index d6ca8918c..d650bbaf9 100644
--- a/tests/regression_tests/random_ray_fixed_source_domain/cell/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_domain/cell/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_fixed_source_domain/material/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_domain/material/inputs_true.dat
index 7fae491ae..98a51add1 100644
--- a/tests/regression_tests/random_ray_fixed_source_domain/material/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_domain/material/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_fixed_source_domain/universe/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_domain/universe/inputs_true.dat
index 75d84efe9..20deba664 100644
--- a/tests/regression_tests/random_ray_fixed_source_domain/universe/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_domain/universe/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_fixed_source_linear/linear/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_linear/linear/inputs_true.dat
index 6ef3f0871..2268d82c3 100644
--- a/tests/regression_tests/random_ray_fixed_source_linear/linear/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_linear/linear/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_fixed_source_linear/linear/results_true.dat b/tests/regression_tests/random_ray_fixed_source_linear/linear/results_true.dat
index 2fc08d0a4..e90d6bfdc 100644
--- a/tests/regression_tests/random_ray_fixed_source_linear/linear/results_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_linear/linear/results_true.dat
@@ -1,6 +1,6 @@
 tally 1:
-2.339085E+00
-2.747304E-01
+2.339086E+00
+2.747305E-01
 tally 2:
 1.089827E-01
 6.069324E-04
diff --git a/tests/regression_tests/random_ray_fixed_source_linear/linear_xy/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_linear/linear_xy/inputs_true.dat
index 805c53fe6..fe95baa7b 100644
--- a/tests/regression_tests/random_ray_fixed_source_linear/linear_xy/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_linear/linear_xy/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear_xy</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_fixed_source_mesh/flat/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_mesh/flat/inputs_true.dat
index 67c2ffc15..a5632ece9 100644
--- a/tests/regression_tests/random_ray_fixed_source_mesh/flat/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_mesh/flat/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>30</batches>
     <inactive>15</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_region_meshes>
         <mesh id="1">
           <domain id="1" type="universe"/>
diff --git a/tests/regression_tests/random_ray_fixed_source_mesh/linear/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_mesh/linear/inputs_true.dat
index 10b7c74f1..9d22603c6 100644
--- a/tests/regression_tests/random_ray_fixed_source_mesh/linear/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_mesh/linear/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>30</batches>
     <inactive>15</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_region_meshes>
         <mesh id="1">
           <domain id="1" type="universe"/>
diff --git a/tests/regression_tests/random_ray_fixed_source_normalization/False/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_normalization/False/inputs_true.dat
index 559cff0f0..de941f10f 100644
--- a/tests/regression_tests/random_ray_fixed_source_normalization/False/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_normalization/False/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>False</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>false</volume_normalized_flux_tallies>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_fixed_source_normalization/True/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_normalization/True/inputs_true.dat
index 75d84efe9..20deba664 100644
--- a/tests/regression_tests/random_ray_fixed_source_normalization/True/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_normalization/True/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_fixed_source_subcritical/flat/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_subcritical/flat/inputs_true.dat
index 0bf4cfdc9..943468a10 100644
--- a/tests/regression_tests/random_ray_fixed_source_subcritical/flat/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_subcritical/flat/inputs_true.dat
@@ -3,34 +3,34 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="18" material="2" name="moderator infinite" universe="8"/>
-    <cell id="19" material="2" name="moderator infinite 2" universe="9"/>
-    <cell fill="10" id="20" universe="12"/>
-    <cell fill="11" id="21" universe="13"/>
-    <cell fill="14" id="22" name="assembly" region="18 -19 20 -21" universe="15"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="18" name="moderator infinite" material="2" universe="8"/>
+    <cell id="19" name="moderator infinite 2" material="2" universe="9"/>
+    <cell id="20" fill="10" universe="12"/>
+    <cell id="21" fill="11" universe="13"/>
+    <cell id="22" name="assembly" fill="14" region="18 -19 20 -21" universe="15"/>
     <lattice id="10">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -71,30 +71,30 @@
 2 12 
 12 13 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="18" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="19" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="20" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="21" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="18" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="19" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="20" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="21" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>30</particles>
     <batches>125</batches>
     <inactive>100</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>1.134 -1.26 -1.0 1.26 -1.134 1.0</parameters>
       </space>
@@ -110,12 +110,14 @@
     <random_ray>
       <distance_active>40.0</distance_active>
       <distance_inactive>40.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>False</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>false</volume_normalized_flux_tallies>
       <source_shape>flat</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_fixed_source_subcritical/flat/results_true.dat b/tests/regression_tests/random_ray_fixed_source_subcritical/flat/results_true.dat
index 831eac501..4d2fb6c57 100644
--- a/tests/regression_tests/random_ray_fixed_source_subcritical/flat/results_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_subcritical/flat/results_true.dat
@@ -83,7 +83,7 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.073356E+02
+1.073355E+02
 4.630895E+02
 1.263975E+01
 6.427678E+00
@@ -107,7 +107,7 @@ tally 1:
 2.388612E-03
 5.941898E-01
 1.414866E-02
-5.319555E+01
+5.319554E+01
 1.134034E+02
 1.967517E-01
 1.551313E-03
@@ -122,7 +122,7 @@ tally 1:
 8.044764E+01
 2.602455E+02
 3.474828E-01
-4.908567E-03
+4.908566E-03
 9.665057E-01
 3.797493E-02
 1.561720E+02
diff --git a/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/inputs_true.dat b/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/inputs_true.dat
index f7572a072..650953c4b 100644
--- a/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/inputs_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/inputs_true.dat
@@ -3,34 +3,34 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="18" material="2" name="moderator infinite" universe="8"/>
-    <cell id="19" material="2" name="moderator infinite 2" universe="9"/>
-    <cell fill="10" id="20" universe="12"/>
-    <cell fill="11" id="21" universe="13"/>
-    <cell fill="14" id="22" name="assembly" region="18 -19 20 -21" universe="15"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="18" name="moderator infinite" material="2" universe="8"/>
+    <cell id="19" name="moderator infinite 2" material="2" universe="9"/>
+    <cell id="20" fill="10" universe="12"/>
+    <cell id="21" fill="11" universe="13"/>
+    <cell id="22" name="assembly" fill="14" region="18 -19 20 -21" universe="15"/>
     <lattice id="10">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -71,30 +71,30 @@
 2 12 
 12 13 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="18" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="19" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="20" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="21" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="18" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="19" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="20" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="21" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>30</particles>
     <batches>125</batches>
     <inactive>100</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>1.134 -1.26 -1.0 1.26 -1.134 1.0</parameters>
       </space>
@@ -110,12 +110,14 @@
     <random_ray>
       <distance_active>40.0</distance_active>
       <distance_inactive>40.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>False</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>false</volume_normalized_flux_tallies>
       <source_shape>linear_xy</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/results_true.dat b/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/results_true.dat
index 49813144d..bf602d3e2 100644
--- a/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/results_true.dat
+++ b/tests/regression_tests/random_ray_fixed_source_subcritical/linear_xy/results_true.dat
@@ -23,7 +23,7 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.235812E+01
+5.235813E+01
 1.099243E+02
 0.000000E+00
 0.000000E+00
@@ -72,12 +72,12 @@ tally 1:
 0.000000E+00
 0.000000E+00
 9.495275E+01
-3.613372E+02
+3.613373E+02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-5.799250E+01
+5.799251E+01
 1.356178E+02
 0.000000E+00
 0.000000E+00
@@ -86,7 +86,7 @@ tally 1:
 1.066414E+02
 4.570291E+02
 1.254014E+01
-6.325640E+00
+6.325641E+00
 3.052020E+01
 3.746918E+01
 4.977302E+01
@@ -107,7 +107,7 @@ tally 1:
 2.371074E-03
 5.920173E-01
 1.404478E-02
-5.299862E+01
+5.299863E+01
 1.125576E+02
 1.959914E-01
 1.539191E-03
@@ -118,11 +118,11 @@ tally 1:
 5.781230E-02
 1.341759E-04
 1.430525E-01
-8.215326E-04
+8.215327E-04
 7.995734E+01
 2.570099E+02
 3.452934E-01
-4.844058E-03
+4.844059E-03
 9.604162E-01
 3.747587E-02
 1.556795E+02
diff --git a/tests/regression_tests/random_ray_halton_samples/inputs_true.dat b/tests/regression_tests/random_ray_halton_samples/inputs_true.dat
index 624ab495f..1b86d2dae 100644
--- a/tests/regression_tests/random_ray_halton_samples/inputs_true.dat
+++ b/tests/regression_tests/random_ray_halton_samples/inputs_true.dat
@@ -3,32 +3,32 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="15" material="2" name="moderator infinite" universe="3"/>
-    <cell fill="4" id="16" universe="5"/>
-    <cell fill="6" id="17" name="assembly" region="14 -15 16 -17" universe="7"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
     <lattice id="4">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -53,23 +53,23 @@
 2 2 
 2 5 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="14" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="15" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="16" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="17" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -80,12 +80,14 @@
     <random_ray>
       <distance_active>100.0</distance_active>
       <distance_inactive>20.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <sample_method>halton</sample_method>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_halton_samples/results_true.dat b/tests/regression_tests/random_ray_halton_samples/results_true.dat
index 7eb307da0..256f8a744 100644
--- a/tests/regression_tests/random_ray_halton_samples/results_true.dat
+++ b/tests/regression_tests/random_ray_halton_samples/results_true.dat
@@ -1,171 +1,171 @@
 k-combined:
 8.388051E-01 7.383265E-03
 tally 1:
-5.033308E+00
-5.072162E+00
-1.917335E+00
-7.360725E-01
-4.666410E+00
-4.360038E+00
-2.851812E+00
-1.629362E+00
-4.365590E-01
-3.818884E-02
-1.062497E+00
-2.262071E-01
-1.697621E+00
-5.829333E-01
-5.639912E-02
-6.427568E-04
-1.372642E-01
-3.807294E-03
-2.376683E+00
-1.151027E+00
-8.060902E-02
-1.323179E-03
-1.961862E-01
-7.837693E-03
-7.145452E+00
-1.037540E+01
-8.551803E-02
-1.486269E-03
-2.081363E-01
-8.803955E-03
-2.053205E+01
-8.469498E+01
-3.235618E-02
-2.102891E-04
-8.006311E-02
-1.287559E-03
-1.326545E+01
-3.519484E+01
-1.867471E-01
-6.975133E-03
-5.194275E-01
-5.396284E-02
-7.558115E+00
-1.142535E+01
+1.065839E+00
+2.274384E-01
+4.060094E-01
+3.300592E-02
+9.881457E-01
+1.955066E-01
+6.038893E-01
+7.306038E-02
+9.244411E-02
+1.712380E-03
+2.249905E-01
+1.014308E-02
+3.594916E-01
+2.614145E-02
+1.194318E-02
+2.882412E-05
+2.906731E-02
+1.707363E-04
+5.032954E-01
+5.161897E-02
+1.707007E-02
+5.933921E-05
+4.154514E-02
+3.514889E-04
+1.513147E+00
+4.652938E-01
+1.810962E-02
+6.665306E-05
+4.407572E-02
+3.948212E-04
+4.347893E+00
+3.798064E+00
+6.851785E-03
+9.430195E-06
+1.695426E-02
+5.773923E-05
+2.809071E+00
+1.578195E+00
+3.954523E-02
+3.127754E-04
+1.099931E-01
+2.419775E-03
+1.600493E+00
+5.123291E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.386211E+00
-2.294414E+00
+7.170555E-01
+1.028835E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.827274E+00
-6.782305E-01
+3.869491E-01
+3.041553E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.702858E+00
-1.489752E+00
+5.723683E-01
+6.680970E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.475537E+00
-1.133971E+01
+1.583046E+00
+5.085372E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.828685E+01
-6.719606E+01
+3.872447E+00
+3.013344E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.143600E+01
-2.615734E+01
+2.421670E+00
+1.172938E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.590713E+00
-4.224107E+00
-1.705847E+00
-5.831967E-01
-4.151691E+00
-3.454497E+00
-2.730853E+00
-1.495413E+00
-4.072633E-01
-3.325944E-02
-9.911973E-01
-1.970084E-01
-1.664732E+00
-5.598668E-01
-5.385645E-02
-5.858353E-04
-1.310758E-01
-3.470126E-03
-2.312239E+00
-1.088485E+00
-7.662778E-02
-1.195422E-03
-1.864967E-01
-7.080943E-03
-7.105765E+00
-1.025960E+01
-8.287512E-02
-1.396474E-03
-2.017039E-01
-8.272053E-03
-2.099024E+01
-8.854251E+01
-3.191885E-02
-2.048368E-04
-7.898095E-02
-1.254175E-03
-1.355820E+01
-3.676862E+01
-1.815102E-01
-6.590727E-03
-5.048614E-01
-5.098890E-02
-5.093659E+00
-5.192360E+00
-1.874632E+00
-7.031793E-01
-4.562478E+00
-4.165199E+00
-2.870213E+00
-1.650126E+00
-4.244068E-01
-3.608745E-02
-1.032921E+00
-2.137598E-01
-1.703400E+00
-5.873029E-01
-5.464557E-02
-6.042855E-04
-1.329964E-01
-3.579413E-03
-2.389118E+00
-1.163674E+00
-7.832237E-02
-1.251254E-03
-1.906210E-01
-7.411654E-03
-7.162706E+00
-1.042515E+01
-8.273831E-02
-1.391799E-03
-2.013709E-01
-8.244359E-03
-2.043145E+01
-8.383557E+01
-3.096158E-02
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-1.314148E+01
-3.454143E+01
-1.771732E-01
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+2.816358E-02
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+5.611375E-05
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+3.323832E-04
+1.516801E+00
+4.675245E-01
+1.752096E-02
+6.241641E-05
+4.264304E-02
+3.697253E-04
+4.326588E+00
+3.759517E+00
+6.556454E-03
+8.628460E-06
+1.622349E-02
+5.283037E-05
+2.782815E+00
+1.548888E+00
+3.751781E-02
+2.815972E-04
+1.043539E-01
+2.178566E-03
diff --git a/tests/regression_tests/random_ray_k_eff/inputs_true.dat b/tests/regression_tests/random_ray_k_eff/inputs_true.dat
index a2b058f2b..72b783344 100644
--- a/tests/regression_tests/random_ray_k_eff/inputs_true.dat
+++ b/tests/regression_tests/random_ray_k_eff/inputs_true.dat
@@ -3,32 +3,32 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="15" material="2" name="moderator infinite" universe="3"/>
-    <cell fill="4" id="16" universe="5"/>
-    <cell fill="6" id="17" name="assembly" region="14 -15 16 -17" universe="7"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
     <lattice id="4">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -53,23 +53,23 @@
 2 2 
 2 5 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="14" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="15" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="16" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="17" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -80,12 +80,14 @@
     <random_ray>
       <distance_active>100.0</distance_active>
       <distance_inactive>20.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
     </random_ray>
   </settings>
   <tallies>
diff --git a/tests/regression_tests/random_ray_k_eff/results_true.dat b/tests/regression_tests/random_ray_k_eff/results_true.dat
index a21929d53..ace18df8c 100644
--- a/tests/regression_tests/random_ray_k_eff/results_true.dat
+++ b/tests/regression_tests/random_ray_k_eff/results_true.dat
@@ -1,171 +1,171 @@
 k-combined:
-8.400321E-01 8.023358E-03
+8.400321E-01 8.023357E-03
 tally 1:
-5.086559E+00
-5.180935E+00
-1.885166E+00
-7.115503E-01
-4.588116E+00
-4.214784E+00
-2.860400E+00
-1.639328E+00
-4.245221E-01
-3.610929E-02
-1.033202E+00
-2.138892E-01
-1.692631E+00
-5.793966E-01
-5.445818E-02
-5.996625E-04
-1.325403E-01
-3.552030E-03
-2.372248E+00
-1.146944E+00
-7.808142E-02
-1.242278E-03
-1.900346E-01
-7.358490E-03
-7.134948E+00
-1.034824E+01
-8.272647E-02
-1.391871E-03
-2.013421E-01
-8.244788E-03
-2.043539E+01
-8.389902E+01
-3.099367E-02
-1.930673E-04
-7.669167E-02
-1.182113E-03
-1.313212E+01
-3.449537E+01
-1.764293E-01
-6.225586E-03
-4.907292E-01
-4.816400E-02
-7.567715E+00
-1.145439E+01
+1.075769E+00
+2.317354E-01
+3.986991E-01
+3.182682E-02
+9.703538E-01
+1.885224E-01
+6.049423E-01
+7.331941E-02
+8.978161E-02
+1.614997E-03
+2.185105E-01
+9.566247E-03
+3.579852E-01
+2.591721E-02
+1.151770E-02
+2.682363E-05
+2.803176E-02
+1.588866E-04
+5.017326E-01
+5.130808E-02
+1.651428E-02
+5.557274E-05
+4.019245E-02
+3.291786E-04
+1.509054E+00
+4.629325E-01
+1.749679E-02
+6.226587E-05
+4.258421E-02
+3.688336E-04
+4.322054E+00
+3.753085E+00
+6.555113E-03
+8.636537E-06
+1.622017E-02
+5.287982E-05
+2.777351E+00
+1.542942E+00
+3.731363E-02
+2.784662E-04
+1.037860E-01
+2.154343E-03
+1.600522E+00
+5.123477E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.383194E+00
-2.290468E+00
+7.155116E-01
+1.024445E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.819672E+00
-6.726158E-01
+3.848568E-01
+3.008769E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.693683E+00
-1.480961E+00
+5.697160E-01
+6.624996E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.453758E+00
-1.128171E+01
+1.576480E+00
+5.046890E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.823561E+01
-6.681652E+01
+3.856827E+00
+2.989000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.137517E+01
-2.588512E+01
+2.405807E+00
+1.157889E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.601916E+00
-4.242624E+00
-1.719723E+00
-5.923465E-01
-4.185462E+00
-3.508695E+00
-2.730305E+00
-1.494324E+00
-4.108214E-01
-3.383938E-02
-9.998572E-01
-2.004436E-01
-1.660852E+00
-5.570432E-01
-5.428709E-02
-5.947848E-04
-1.321239E-01
-3.523137E-03
-2.306069E+00
-1.082855E+00
-7.697031E-02
-1.206036E-03
-1.873303E-01
-7.143815E-03
-7.075194E+00
-1.017519E+01
-8.322052E-02
-1.408294E-03
-2.025446E-01
-8.342070E-03
-2.094832E+01
-8.816715E+01
-3.234739E-02
-2.101889E-04
-8.004135E-02
-1.286945E-03
-1.357413E+01
-3.685983E+01
-1.861680E-01
-6.934827E-03
-5.178169E-01
-5.365102E-02
-5.072149E+00
-5.151429E+00
-1.916643E+00
-7.358710E-01
-4.664726E+00
-4.358845E+00
-2.859464E+00
-1.638250E+00
-4.332944E-01
-3.763170E-02
-1.054552E+00
-2.229070E-01
-1.693008E+00
-5.796671E-01
-5.561096E-02
-6.247543E-04
-1.353459E-01
-3.700658E-03
-2.368860E+00
-1.143296E+00
-7.951819E-02
-1.286690E-03
-1.935314E-01
-7.621556E-03
-7.119586E+00
-1.030023E+01
-8.428175E-02
-1.442931E-03
-2.051274E-01
-8.547243E-03
-2.046758E+01
-8.418767E+01
-3.181946E-02
-2.034766E-04
-7.873502E-02
-1.245847E-03
-1.325834E+01
-3.515919E+01
-1.832838E-01
-6.720555E-03
-5.097947E-01
-5.199331E-02
+9.732717E-01
+1.897670E-01
+3.637108E-01
+2.649544E-02
+8.851992E-01
+1.569426E-01
+5.774286E-01
+6.683395E-02
+8.688408E-02
+1.513473E-03
+2.114585E-01
+8.964883E-03
+3.512640E-01
+2.491729E-02
+1.148149E-02
+2.660532E-05
+2.794365E-02
+1.575935E-04
+4.877362E-01
+4.844138E-02
+1.627932E-02
+5.395212E-05
+3.962062E-02
+3.195790E-04
+1.496416E+00
+4.551920E-01
+1.760130E-02
+6.300084E-05
+4.283856E-02
+3.731872E-04
+4.430519E+00
+3.943947E+00
+6.841363E-03
+9.402132E-06
+1.692847E-02
+5.756741E-05
+2.870816E+00
+1.648662E+00
+3.937267E-02
+3.101707E-04
+1.095131E-01
+2.399624E-03
+1.072714E+00
+2.304093E-01
+4.053505E-01
+3.291277E-02
+9.865420E-01
+1.949549E-01
+6.047420E-01
+7.327008E-02
+9.163611E-02
+1.683034E-03
+2.230240E-01
+9.969255E-03
+3.580639E-01
+2.592902E-02
+1.176143E-02
+2.794541E-05
+2.862497E-02
+1.655313E-04
+5.010143E-01
+5.114429E-02
+1.681803E-02
+5.755790E-05
+4.093173E-02
+3.409375E-04
+1.505803E+00
+4.607828E-01
+1.782566E-02
+6.454911E-05
+4.338462E-02
+3.823584E-04
+4.328879E+00
+3.766055E+00
+6.729800E-03
+9.102372E-06
+1.665242E-02
+5.573204E-05
+2.804070E+00
+1.572690E+00
+3.876396E-02
+3.006259E-04
+1.078200E-01
+2.325780E-03
diff --git a/tests/regression_tests/random_ray_k_eff_mesh/inputs_true.dat b/tests/regression_tests/random_ray_k_eff_mesh/inputs_true.dat
index cdc717198..f6e9c8e3e 100644
--- a/tests/regression_tests/random_ray_k_eff_mesh/inputs_true.dat
+++ b/tests/regression_tests/random_ray_k_eff_mesh/inputs_true.dat
@@ -3,32 +3,32 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="15" material="2" name="moderator infinite" universe="3"/>
-    <cell fill="4" id="16" universe="5"/>
-    <cell fill="6" id="17" name="assembly" region="14 -15 16 -17" universe="7"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
     <lattice id="4">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -53,23 +53,23 @@
 2 2 
 2 5 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="14" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="15" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="16" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="17" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -80,12 +80,14 @@
     <random_ray>
       <distance_active>100.0</distance_active>
       <distance_inactive>20.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_region_meshes>
         <mesh id="2">
           <domain id="7" type="universe"/>
diff --git a/tests/regression_tests/random_ray_k_eff_mesh/results_true.dat b/tests/regression_tests/random_ray_k_eff_mesh/results_true.dat
index 535db3b55..2ae8fad85 100644
--- a/tests/regression_tests/random_ray_k_eff_mesh/results_true.dat
+++ b/tests/regression_tests/random_ray_k_eff_mesh/results_true.dat
@@ -1,171 +1,171 @@
 k-combined:
 8.379203E-01 8.057199E-03
 tally 1:
-5.080171E+00
-5.167984E+00
-1.880341E+00
-7.079266E-01
-4.576373E+00
-4.193319E+00
-2.859914E+00
-1.638769E+00
-4.243332E-01
-3.607732E-02
-1.032742E+00
-2.136998E-01
-1.692643E+00
-5.794069E-01
-5.445214E-02
-5.995212E-04
-1.325256E-01
-3.551193E-03
-2.372336E+00
-1.147031E+00
-7.807378E-02
-1.242019E-03
-1.900160E-01
-7.356955E-03
-7.135636E+00
-1.035026E+01
-8.273225E-02
-1.392069E-03
-2.013562E-01
-8.245961E-03
-2.044034E+01
-8.394042E+01
-3.100485E-02
-1.932097E-04
-7.671933E-02
-1.182985E-03
-1.313652E+01
-3.451847E+01
-1.764978E-01
-6.230420E-03
-4.909196E-01
-4.820140E-02
-7.585874E+00
-1.150936E+01
+1.073897E+00
+2.309328E-01
+3.974859E-01
+3.163415E-02
+9.674011E-01
+1.873811E-01
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+8.969819E-02
+1.612011E-03
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+9.548557E-03
+3.578121E-01
+2.589198E-02
+1.151076E-02
+2.679073E-05
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+1.586917E-04
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+5.126076E-02
+1.650453E-02
+5.550572E-05
+4.016872E-02
+3.287816E-04
+1.508456E+00
+4.625615E-01
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+6.221267E-05
+4.256620E-02
+3.685184E-04
+4.320984E+00
+3.751237E+00
+6.554265E-03
+8.634389E-06
+1.621807E-02
+5.286667E-05
+2.776930E+00
+1.542475E+00
+3.730995E-02
+2.784113E-04
+1.037757E-01
+2.153918E-03
+1.603583E+00
+5.143065E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.386790E+00
-2.295327E+00
+7.159242E-01
+1.025623E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.820058E+00
-6.729238E-01
+3.847488E-01
+3.007151E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.694013E+00
-1.481363E+00
+5.695050E-01
+6.620177E-02
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-7.453818E+00
-1.128202E+01
+1.575716E+00
+5.042003E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.823642E+01
-6.682239E+01
+3.855109E+00
+2.986316E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.137903E+01
-2.590265E+01
+2.405453E+00
+1.157547E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-4.589529E+00
-4.219985E+00
-1.712446E+00
-5.873675E-01
-4.167750E+00
-3.479202E+00
-2.728285E+00
-1.492132E+00
-4.104063E-01
-3.377132E-02
-9.988468E-01
-2.000404E-01
-1.660506E+00
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-5.427391E-02
-5.944708E-04
-1.320918E-01
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-2.305889E+00
-1.082691E+00
-7.695793E-02
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-1.873002E-01
-7.141490E-03
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-1.017946E+01
-8.323139E-02
-1.408687E-03
-2.025710E-01
-8.344398E-03
-2.095897E+01
-8.825741E+01
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-2.104220E-04
-8.008525E-02
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-1.358006E+01
-3.689205E+01
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-6.941428E-03
-5.180621E-01
-5.370209E-02
-5.067386E+00
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-1.912704E+00
-7.328484E-01
-4.655140E+00
-4.340941E+00
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-1.637705E+00
-4.331050E-01
-3.759875E-02
-1.054091E+00
-2.227118E-01
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-5.796396E-01
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-2.368737E+00
-1.143184E+00
-7.950085E-02
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-7.618268E-03
-7.119767E+00
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-8.427627E-02
-1.442764E-03
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-8.546249E-03
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+1.495984E+00
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+6.295564E-05
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+1.505100E+00
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+6.447733E-05
+4.336044E-02
+3.819332E-04
+4.328647E+00
+3.765700E+00
+6.730396E-03
+9.104085E-06
+1.665389E-02
+5.574253E-05
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+1.572540E+00
+3.876306E-02
+3.006136E-04
+1.078175E-01
+2.325685E-03
diff --git a/tests/regression_tests/random_ray_linear/linear/inputs_true.dat b/tests/regression_tests/random_ray_linear/linear/inputs_true.dat
index 4df51bad5..269d9892e 100644
--- a/tests/regression_tests/random_ray_linear/linear/inputs_true.dat
+++ b/tests/regression_tests/random_ray_linear/linear/inputs_true.dat
@@ -3,32 +3,32 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="15" material="2" name="moderator infinite" universe="3"/>
-    <cell fill="4" id="16" universe="5"/>
-    <cell fill="6" id="17" name="assembly" region="14 -15 16 -17" universe="7"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
     <lattice id="4">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -53,23 +53,23 @@
 2 2 
 2 5 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="14" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="15" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="16" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="17" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -80,12 +80,14 @@
     <random_ray>
       <distance_active>100.0</distance_active>
       <distance_inactive>20.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_linear/linear/results_true.dat b/tests/regression_tests/random_ray_linear/linear/results_true.dat
index 6617c7821..77d41f373 100644
--- a/tests/regression_tests/random_ray_linear/linear/results_true.dat
+++ b/tests/regression_tests/random_ray_linear/linear/results_true.dat
@@ -1,171 +1,171 @@
 k-combined:
 1.095967E+00 1.543581E-02
 tally 1:
-2.548108E+01
-3.269093E+01
-9.271804E+00
-4.327275E+00
-2.256572E+01
-2.563210E+01
-1.816107E+01
-1.653421E+01
-2.659203E+00
-3.544951E-01
-6.471969E+00
-2.099810E+00
-1.364193E+01
-9.308675E+00
-4.362828E-01
-9.521133E-03
-1.061825E+00
-5.639730E-02
-1.746102E+01
-1.524680E+01
-5.733016E-01
-1.643671E-02
-1.395301E+00
-9.736091E-02
-4.539598E+01
-1.030472E+02
-5.263055E-01
-1.385088E-02
-1.280938E+00
-8.204609E-02
-9.945736E+01
-4.946716E+02
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 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.224363E+01
-2.481131E+01
+4.736166E+00
+1.124858E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.436097E+01
-1.031496E+01
+3.057755E+00
+4.676344E-01
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
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diff --git a/tests/regression_tests/random_ray_linear/linear_xy/inputs_true.dat b/tests/regression_tests/random_ray_linear/linear_xy/inputs_true.dat
index 113771438..217e95516 100644
--- a/tests/regression_tests/random_ray_linear/linear_xy/inputs_true.dat
+++ b/tests/regression_tests/random_ray_linear/linear_xy/inputs_true.dat
@@ -3,32 +3,32 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="UO2 fuel">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="UO2"/>
     </material>
     <material id="2" name="Water">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="LWTR"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel inner a" region="-2" universe="1"/>
-    <cell id="2" material="1" name="fuel inner b" region="2 -3" universe="1"/>
-    <cell id="3" material="1" name="fuel inner c" region="3 -1" universe="1"/>
-    <cell id="4" material="2" name="moderator inner a" region="1 -4" universe="1"/>
-    <cell id="5" material="2" name="moderator outer b" region="4 -5" universe="1"/>
-    <cell id="6" material="2" name="moderator outer c" region="5" universe="1"/>
-    <cell fill="1" id="7" name="azimuthal_cell_0" region="6 -7" universe="2"/>
-    <cell fill="1" id="8" name="azimuthal_cell_1" region="7 -8" universe="2"/>
-    <cell fill="1" id="9" name="azimuthal_cell_2" region="8 -9" universe="2"/>
-    <cell fill="1" id="10" name="azimuthal_cell_3" region="9 -10" universe="2"/>
-    <cell fill="1" id="11" name="azimuthal_cell_4" region="10 -11" universe="2"/>
-    <cell fill="1" id="12" name="azimuthal_cell_5" region="11 -12" universe="2"/>
-    <cell fill="1" id="13" name="azimuthal_cell_6" region="12 -13" universe="2"/>
-    <cell fill="1" id="14" name="azimuthal_cell_7" region="13 -6" universe="2"/>
-    <cell id="15" material="2" name="moderator infinite" universe="3"/>
-    <cell fill="4" id="16" universe="5"/>
-    <cell fill="6" id="17" name="assembly" region="14 -15 16 -17" universe="7"/>
+    <cell id="1" name="fuel inner a" material="1" region="-2" universe="1"/>
+    <cell id="2" name="fuel inner b" material="1" region="2 -3" universe="1"/>
+    <cell id="3" name="fuel inner c" material="1" region="3 -1" universe="1"/>
+    <cell id="4" name="moderator inner a" material="2" region="1 -4" universe="1"/>
+    <cell id="5" name="moderator outer b" material="2" region="4 -5" universe="1"/>
+    <cell id="6" name="moderator outer c" material="2" region="5" universe="1"/>
+    <cell id="7" name="azimuthal_cell_0" fill="1" region="6 -7" universe="2"/>
+    <cell id="8" name="azimuthal_cell_1" fill="1" region="7 -8" universe="2"/>
+    <cell id="9" name="azimuthal_cell_2" fill="1" region="8 -9" universe="2"/>
+    <cell id="10" name="azimuthal_cell_3" fill="1" region="9 -10" universe="2"/>
+    <cell id="11" name="azimuthal_cell_4" fill="1" region="10 -11" universe="2"/>
+    <cell id="12" name="azimuthal_cell_5" fill="1" region="11 -12" universe="2"/>
+    <cell id="13" name="azimuthal_cell_6" fill="1" region="12 -13" universe="2"/>
+    <cell id="14" name="azimuthal_cell_7" fill="1" region="13 -6" universe="2"/>
+    <cell id="15" name="moderator infinite" material="2" universe="3"/>
+    <cell id="16" fill="4" universe="5"/>
+    <cell id="17" name="assembly" fill="6" region="14 -15 16 -17" universe="7"/>
     <lattice id="4">
       <pitch>0.126 0.126</pitch>
       <dimension>10 10</dimension>
@@ -53,23 +53,23 @@
 2 2 
 2 5 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.54" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.33" id="2" name="inner ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.45" id="3" name="inner ring b" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.6" id="4" name="outer ring a" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.69" id="5" name="outer ring b" type="z-cylinder"/>
-    <surface coeffs="-0.0 1.0 0 0" id="6" type="plane"/>
-    <surface coeffs="-0.7071067811865475 0.7071067811865476 0 0" id="7" type="plane"/>
-    <surface coeffs="-1.0 6.123233995736766e-17 0 0" id="8" type="plane"/>
-    <surface coeffs="-0.7071067811865476 -0.7071067811865475 0 0" id="9" type="plane"/>
-    <surface coeffs="-1.2246467991473532e-16 -1.0 0 0" id="10" type="plane"/>
-    <surface coeffs="0.7071067811865475 -0.7071067811865477 0 0" id="11" type="plane"/>
-    <surface coeffs="1.0 -1.8369701987210297e-16 0 0" id="12" type="plane"/>
-    <surface coeffs="0.7071067811865477 0.7071067811865474 0 0" id="13" type="plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="14" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="15" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.26" id="16" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.26" id="17" name="maximum y" type="y-plane"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0.0 0.0 0.54"/>
+    <surface id="2" name="inner ring a" type="z-cylinder" coeffs="0.0 0.0 0.33"/>
+    <surface id="3" name="inner ring b" type="z-cylinder" coeffs="0.0 0.0 0.45"/>
+    <surface id="4" name="outer ring a" type="z-cylinder" coeffs="0.0 0.0 0.6"/>
+    <surface id="5" name="outer ring b" type="z-cylinder" coeffs="0.0 0.0 0.69"/>
+    <surface id="6" type="plane" coeffs="-0.0 1.0 0 0"/>
+    <surface id="7" type="plane" coeffs="-0.7071067811865475 0.7071067811865476 0 0"/>
+    <surface id="8" type="plane" coeffs="-1.0 6.123233995736766e-17 0 0"/>
+    <surface id="9" type="plane" coeffs="-0.7071067811865476 -0.7071067811865475 0 0"/>
+    <surface id="10" type="plane" coeffs="-1.2246467991473532e-16 -1.0 0 0"/>
+    <surface id="11" type="plane" coeffs="0.7071067811865475 -0.7071067811865477 0 0"/>
+    <surface id="12" type="plane" coeffs="1.0 -1.8369701987210297e-16 0 0"/>
+    <surface id="13" type="plane" coeffs="0.7071067811865477 0.7071067811865474 0 0"/>
+    <surface id="14" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="15" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.26"/>
+    <surface id="16" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.26"/>
+    <surface id="17" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.26"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -80,12 +80,14 @@
     <random_ray>
       <distance_active>100.0</distance_active>
       <distance_inactive>20.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>-1.26 -1.26 -1 1.26 1.26 1</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear_xy</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_linear/linear_xy/results_true.dat b/tests/regression_tests/random_ray_linear/linear_xy/results_true.dat
index abfd03c06..052608b42 100644
--- a/tests/regression_tests/random_ray_linear/linear_xy/results_true.dat
+++ b/tests/regression_tests/random_ray_linear/linear_xy/results_true.dat
@@ -1,171 +1,171 @@
 k-combined:
 1.104727E+00 1.593303E-02
 tally 1:
-2.566934E+01
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-9.417202E+00
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+6.899411E-01
+1.249118E-01
+7.802521E-04
+3.040104E-01
+4.621732E-03
+9.663183E+00
+4.669215E+00
+1.146768E-01
+6.575767E-04
+2.791038E-01
+3.895173E-03
+2.112461E+01
+2.231346E+01
+3.262864E-02
+5.323508E-05
+8.073730E-02
+3.259479E-04
+1.128817E+01
+6.372070E+00
+1.551272E-01
+1.203670E-03
+4.314785E-01
+9.312148E-03
diff --git a/tests/regression_tests/random_ray_low_density/__init__.py b/tests/regression_tests/random_ray_low_density/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_low_density/inputs_true.dat b/tests/regression_tests/random_ray_low_density/inputs_true.dat
new file mode 100644
index 000000000..20deba664
--- /dev/null
+++ b/tests/regression_tests/random_ray_low_density/inputs_true.dat
@@ -0,0 +1,246 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="source">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="source"/>
+    </material>
+    <material id="2" name="void">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="void"/>
+    </material>
+    <material id="3" name="absorber">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="absorber"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
+    <lattice id="4">
+      <pitch>2.5 2.5 2.5</pitch>
+      <dimension>12 12 12</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>90</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="3.14" particle="neutron">
+      <energy type="discrete">
+        <parameters>100.0 1.0</parameters>
+      </energy>
+      <constraints>
+        <domain_type>universe</domain_type>
+        <domain_ids>1</domain_ids>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <distance_active>500.0</distance_active>
+      <distance_inactive>100.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+    </random_ray>
+  </settings>
+  <tallies>
+    <filter id="3" type="material">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="material">
+      <bins>2</bins>
+    </filter>
+    <filter id="1" type="material">
+      <bins>3</bins>
+    </filter>
+    <tally id="3" name="Source Tally">
+      <filters>3</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="2" name="Void Tally">
+      <filters>2</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="1" name="Absorber Tally">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_low_density/results_true.dat b/tests/regression_tests/random_ray_low_density/results_true.dat
new file mode 100644
index 000000000..a4b3ee1bc
--- /dev/null
+++ b/tests/regression_tests/random_ray_low_density/results_true.dat
@@ -0,0 +1,9 @@
+tally 1:
+5.973607E-01
+7.155477E-02
+tally 2:
+3.206216E-02
+2.063375E-04
+tally 3:
+2.096415E-03
+8.804963E-07
diff --git a/tests/regression_tests/random_ray_low_density/test.py b/tests/regression_tests/random_ray_low_density/test.py
new file mode 100644
index 000000000..1b4ffb781
--- /dev/null
+++ b/tests/regression_tests/random_ray_low_density/test.py
@@ -0,0 +1,60 @@
+import os
+
+import numpy as np
+import openmc
+from openmc.examples import random_ray_three_region_cube
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_random_ray_low_density():
+    model = random_ray_three_region_cube()
+
+    # Rebuild the MGXS library to have a material with very
+    # low macroscopic cross sections
+    ebins = [1e-5, 20.0e6]
+    groups = openmc.mgxs.EnergyGroups(group_edges=ebins)
+
+    void_sigma_a = 4.0e-6
+    void_sigma_s = 3.0e-4
+    void_mat_data = openmc.XSdata('void', groups)
+    void_mat_data.order = 0
+    void_mat_data.set_total([void_sigma_a + void_sigma_s])
+    void_mat_data.set_absorption([void_sigma_a])
+    void_mat_data.set_scatter_matrix(
+        np.rollaxis(np.array([[[void_sigma_s]]]), 0, 3))
+
+    absorber_sigma_a = 0.75
+    absorber_sigma_s = 0.25
+    absorber_mat_data = openmc.XSdata('absorber', groups)
+    absorber_mat_data.order = 0
+    absorber_mat_data.set_total([absorber_sigma_a + absorber_sigma_s])
+    absorber_mat_data.set_absorption([absorber_sigma_a])
+    absorber_mat_data.set_scatter_matrix(
+        np.rollaxis(np.array([[[absorber_sigma_s]]]), 0, 3))
+
+    multiplier = 0.0000001
+    source_sigma_a = void_sigma_a * multiplier
+    source_sigma_s = void_sigma_s * multiplier
+    source_mat_data = openmc.XSdata('source', groups)
+    source_mat_data.order = 0
+    source_mat_data.set_total([source_sigma_a + source_sigma_s])
+    source_mat_data.set_absorption([source_sigma_a])
+    source_mat_data.set_scatter_matrix(
+        np.rollaxis(np.array([[[source_sigma_s]]]), 0, 3))
+
+    mg_cross_sections_file = openmc.MGXSLibrary(groups)
+    mg_cross_sections_file.add_xsdatas(
+        [source_mat_data, void_mat_data, absorber_mat_data])
+    mg_cross_sections_file.export_to_hdf5()
+
+    harness = MGXSTestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/random_ray_point_source_locator/__init__.py b/tests/regression_tests/random_ray_point_source_locator/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_point_source_locator/inputs_true.dat b/tests/regression_tests/random_ray_point_source_locator/inputs_true.dat
new file mode 100644
index 000000000..b4bd263f5
--- /dev/null
+++ b/tests/regression_tests/random_ray_point_source_locator/inputs_true.dat
@@ -0,0 +1,255 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="source">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="source"/>
+    </material>
+    <material id="2" name="void">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="void"/>
+    </material>
+    <material id="3" name="absorber">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="absorber"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
+    <lattice id="4">
+      <pitch>2.5 2.5 2.5</pitch>
+      <dimension>12 12 12</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+1 1 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>90</particles>
+    <batches>30</batches>
+    <inactive>15</inactive>
+    <source type="independent" strength="3.14" particle="neutron">
+      <space type="point">
+        <parameters>2.5 2.5 2.5</parameters>
+      </space>
+      <energy type="discrete">
+        <parameters>100.0 1.0</parameters>
+      </energy>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <distance_active>500.0</distance_active>
+      <distance_inactive>100.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="6" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+    </random_ray>
+    <mesh id="1">
+      <dimension>30 30 30</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <upper_right>30.0 30.0 30.0</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="3" type="material">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="material">
+      <bins>2</bins>
+    </filter>
+    <filter id="1" type="material">
+      <bins>3</bins>
+    </filter>
+    <tally id="3" name="Source Tally">
+      <filters>3</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="2" name="Void Tally">
+      <filters>2</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="1" name="Absorber Tally">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_point_source_locator/results_true.dat b/tests/regression_tests/random_ray_point_source_locator/results_true.dat
new file mode 100644
index 000000000..8c6f358dd
--- /dev/null
+++ b/tests/regression_tests/random_ray_point_source_locator/results_true.dat
@@ -0,0 +1,9 @@
+tally 1:
+2.633923E+00
+2.948228E+00
+tally 2:
+1.440456E-01
+3.293984E-03
+tally 3:
+9.425207E-03
+1.089748E-05
diff --git a/tests/regression_tests/random_ray_point_source_locator/test.py b/tests/regression_tests/random_ray_point_source_locator/test.py
new file mode 100644
index 000000000..fd3d8a18f
--- /dev/null
+++ b/tests/regression_tests/random_ray_point_source_locator/test.py
@@ -0,0 +1,44 @@
+import os
+
+import openmc
+from openmc.examples import random_ray_three_region_cube
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_random_ray_point_source_locator():
+    model = random_ray_three_region_cube()
+
+    # Overlay subdivided SR mesh to reduce resolution from 2.5cm -> 1cm
+    width = 30.0
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (30, 30, 30)
+    mesh.lower_left = (0.0, 0.0, 0.0)
+    mesh.upper_right = (width, width, width)
+    model.settings.random_ray['source_region_meshes'] = [
+        (mesh, [model.geometry.root_universe]),
+    ]
+
+    # Define a point source
+    strengths = [1.0]
+    midpoints = [100.0]
+    energy_distribution = openmc.stats.Discrete(x=midpoints, p=strengths)
+    spatial_distribution = openmc.stats.Point([2.5, 2.5, 2.5])
+    source = openmc.IndependentSource(
+        energy=energy_distribution, space=spatial_distribution, strength=3.14)
+    model.settings.source = [source]
+
+    # Settings
+    model.settings.inactive = 15
+    model.settings.batches = 30
+
+    harness = MGXSTestHarness('statepoint.30.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/random_ray_s2/__init__.py b/tests/regression_tests/random_ray_s2/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/random_ray_s2/inputs_true.dat b/tests/regression_tests/random_ray_s2/inputs_true.dat
new file mode 100644
index 000000000..c0dc6292f
--- /dev/null
+++ b/tests/regression_tests/random_ray_s2/inputs_true.dat
@@ -0,0 +1,71 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="domain">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="domain"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="5 -6 1 -2 3 -4" universe="1"/>
+    <surface id="1" name="minimum y" type="y-plane" boundary="reflective" coeffs="-5.0"/>
+    <surface id="2" name="maximum y" type="y-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="3" name="minimum z" type="z-plane" boundary="reflective" coeffs="-5.0"/>
+    <surface id="4" name="maximum z" type="z-plane" boundary="reflective" coeffs="5.0"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="0.0"/>
+    <surface id="6" type="x-plane" boundary="vacuum" coeffs="40.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>30</batches>
+    <inactive>10</inactive>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="box">
+        <parameters>0.0 -5.0 -5.0 40.0 5.0 5.0</parameters>
+      </space>
+      <energy type="discrete">
+        <parameters>1000.0 1.0</parameters>
+      </energy>
+      <constraints>
+        <domain_type>material</domain_type>
+        <domain_ids>1</domain_ids>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <random_ray>
+      <distance_inactive>100.0</distance_inactive>
+      <distance_active>400.0</distance_active>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 -5.0 -5.0 40.0 5.0 5.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <source_shape>flat</source_shape>
+      <sample_method>s2</sample_method>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="1" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+    </random_ray>
+    <mesh id="1">
+      <dimension>10 1 1</dimension>
+      <lower_left>0.0 -5.0 -5.0</lower_left>
+      <upper_right>40.0 5.0 5.0</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <tally id="1" name="LR">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/random_ray_s2/results_true.dat b/tests/regression_tests/random_ray_s2/results_true.dat
new file mode 100644
index 000000000..5fb6f7904
--- /dev/null
+++ b/tests/regression_tests/random_ray_s2/results_true.dat
@@ -0,0 +1,21 @@
+tally 1:
+1.153280E+01
+6.650271E+00
+1.413926E+01
+9.995935E+00
+1.579543E+01
+1.247479E+01
+1.676986E+01
+1.406141E+01
+1.722054E+01
+1.482734E+01
+1.722054E+01
+1.482734E+01
+1.676986E+01
+1.406141E+01
+1.579543E+01
+1.247479E+01
+1.413926E+01
+9.995935E+00
+1.153280E+01
+6.650271E+00
diff --git a/tests/regression_tests/random_ray_s2/test.py b/tests/regression_tests/random_ray_s2/test.py
new file mode 100644
index 000000000..712d9c124
--- /dev/null
+++ b/tests/regression_tests/random_ray_s2/test.py
@@ -0,0 +1,82 @@
+import os
+
+import openmc
+import openmc.model
+import numpy as np
+
+from tests.testing_harness import TolerantPyAPITestHarness
+
+
+class MGXSTestHarness(TolerantPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_random_ray_s2():
+    NUM_SOURCE_REGIONS = 10
+    L = 40.0
+
+    # Make the simple MGXS library and material.
+    groups = openmc.mgxs.EnergyGroups(group_edges=[1e-5, 20.0e6])
+
+    domain_mat_data = openmc.XSdata('domain', groups)
+    domain_mat_data.order = 0
+    domain_mat_data.set_total([0.1])
+    domain_mat_data.set_absorption([0.1])
+    domain_mat_data.set_scatter_matrix(np.rollaxis(np.array([[[0.0]]]), 0, 3))
+    mg_cross_sections_file = openmc.MGXSLibrary(groups)
+    mg_cross_sections_file.add_xsdatas([domain_mat_data])
+    mg_cross_sections_file.export_to_hdf5()
+
+    domain_data = openmc.Macroscopic('domain')
+    domain_mat = openmc.Material(name='domain')
+    domain_mat.set_density('macro', 1.0)
+    domain_mat.add_macroscopic(domain_data)
+
+    container = openmc.model.RectangularPrism(width=10.0, height=10.0, axis='x',
+                                              origin=(0.0, 0.0), boundary_type='reflective')
+    left = openmc.XPlane(x0 = 0.0, boundary_type='vacuum')
+    right = openmc.XPlane(x0 = L, boundary_type='vacuum')
+    cell = [openmc.Cell(region = +left & -right & -container, fill = domain_mat)]
+
+    model = openmc.model.Model()
+    model.geometry = openmc.Geometry(root=openmc.Universe(cells=cell))
+    model.materials = openmc.Materials([domain_mat])
+    model.materials.cross_sections = './mgxs.h5'
+
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (NUM_SOURCE_REGIONS, 1, 1)
+    mesh.lower_left = (0.0, -5.0, -5.0)
+    mesh.upper_right = (L, 5.0, 5.0)
+
+    tally = openmc.Tally(name="LR")
+    tally.filters = [openmc.MeshFilter(mesh)]
+    tally.scores = ['flux']
+    tally.estimator = 'tracklength'
+    model.tallies.append(tally)
+
+    uniform_dist = openmc.stats.Box((0.0, -5.0, -5.0), (L, 5.0, 5.0))
+    model.settings.source = [
+      openmc.IndependentSource(space=uniform_dist,
+                              energy=openmc.stats.Discrete(x = 1e3, p = 1.0),
+                              constraints={'domains' : [domain_mat]})
+    ]
+    model.settings.energy_mode = "multi-group"
+    model.settings.batches = 30
+    model.settings.inactive = 10
+    model.settings.particles = 100
+    model.settings.run_mode = 'fixed source'
+    model.settings.random_ray['distance_inactive'] = 100.0
+    model.settings.random_ray['distance_active'] = 400.0
+    model.settings.random_ray['ray_source'] = openmc.IndependentSource(space=uniform_dist)
+    model.settings.random_ray['source_shape'] = 'flat'
+    model.settings.random_ray['sample_method'] = 's2'
+    model.settings.random_ray['source_region_meshes'] = [(mesh, [model.geometry.root_universe])]
+
+    model.export_to_model_xml()
+
+    harness = MGXSTestHarness('statepoint.30.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/random_ray_void/flat/inputs_true.dat b/tests/regression_tests/random_ray_void/flat/inputs_true.dat
index 617a66053..66390c766 100644
--- a/tests/regression_tests/random_ray_void/flat/inputs_true.dat
+++ b/tests/regression_tests/random_ray_void/flat/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="void" name="infinite source region" universe="1"/>
-    <cell id="2" material="void" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="void" universe="1"/>
+    <cell id="2" name="infinite void region" material="void" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>flat</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_void/linear/inputs_true.dat b/tests/regression_tests/random_ray_void/linear/inputs_true.dat
index 4b6a682d9..45228a039 100644
--- a/tests/regression_tests/random_ray_void/linear/inputs_true.dat
+++ b/tests/regression_tests/random_ray_void/linear/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="void" name="infinite source region" universe="1"/>
-    <cell id="2" material="void" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="void" universe="1"/>
+    <cell id="2" name="infinite void region" material="void" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear</source_shape>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_volume_estimator/hybrid/inputs_true.dat b/tests/regression_tests/random_ray_volume_estimator/hybrid/inputs_true.dat
index 9c15ec97d..4d1af46b1 100644
--- a/tests/regression_tests/random_ray_volume_estimator/hybrid/inputs_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator/hybrid/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <volume_estimator>hybrid</volume_estimator>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_volume_estimator/naive/inputs_true.dat b/tests/regression_tests/random_ray_volume_estimator/naive/inputs_true.dat
index 7d05f0978..a268d55d0 100644
--- a/tests/regression_tests/random_ray_volume_estimator/naive/inputs_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator/naive/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <volume_estimator>naive</volume_estimator>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_volume_estimator/simulation_averaged/inputs_true.dat b/tests/regression_tests/random_ray_volume_estimator/simulation_averaged/inputs_true.dat
index 301671e6d..777ccaea5 100644
--- a/tests/regression_tests/random_ray_volume_estimator/simulation_averaged/inputs_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator/simulation_averaged/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <volume_estimator>simulation_averaged</volume_estimator>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/inputs_true.dat b/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/inputs_true.dat
index 6440cca04..dd11567f6 100644
--- a/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/inputs_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear</source_shape>
       <volume_estimator>hybrid</volume_estimator>
     </random_ray>
diff --git a/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/results_true.dat b/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/results_true.dat
index 2fc08d0a4..e90d6bfdc 100644
--- a/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/results_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator_linear/hybrid/results_true.dat
@@ -1,6 +1,6 @@
 tally 1:
-2.339085E+00
-2.747304E-01
+2.339086E+00
+2.747305E-01
 tally 2:
 1.089827E-01
 6.069324E-04
diff --git a/tests/regression_tests/random_ray_volume_estimator_linear/naive/inputs_true.dat b/tests/regression_tests/random_ray_volume_estimator_linear/naive/inputs_true.dat
index fcf93b046..6933fba43 100644
--- a/tests/regression_tests/random_ray_volume_estimator_linear/naive/inputs_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator_linear/naive/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear</source_shape>
       <volume_estimator>naive</volume_estimator>
     </random_ray>
diff --git a/tests/regression_tests/random_ray_volume_estimator_linear/simulation_averaged/inputs_true.dat b/tests/regression_tests/random_ray_volume_estimator_linear/simulation_averaged/inputs_true.dat
index 1df3204a3..3ccab1d21 100644
--- a/tests/regression_tests/random_ray_volume_estimator_linear/simulation_averaged/inputs_true.dat
+++ b/tests/regression_tests/random_ray_volume_estimator_linear/simulation_averaged/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>40</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -207,12 +207,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_shape>linear</source_shape>
       <volume_estimator>simulation_averaged</volume_estimator>
     </random_ray>
diff --git a/tests/regression_tests/reflective_plane/results_true.dat b/tests/regression_tests/reflective_plane/results_true.dat
index a1acfaad7..a4d6edb67 100644
--- a/tests/regression_tests/reflective_plane/results_true.dat
+++ b/tests/regression_tests/reflective_plane/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.276857E+00 8.776678E-03
+2.279066E+00 4.793565E-03
diff --git a/tests/regression_tests/resonance_scattering/inputs_true.dat b/tests/regression_tests/resonance_scattering/inputs_true.dat
index dba4535c2..ebe6a5dbe 100644
--- a/tests/regression_tests/resonance_scattering/inputs_true.dat
+++ b/tests/regression_tests/resonance_scattering/inputs_true.dat
@@ -1,24 +1,24 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="1.0" name="U238"/>
-      <nuclide ao="0.02" name="U235"/>
-      <nuclide ao="0.02" name="Pu239"/>
-      <nuclide ao="20.0" name="H1"/>
+    <material id="1" depletable="true">
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="U238" ao="1.0"/>
+      <nuclide name="U235" ao="0.02"/>
+      <nuclide name="Pu239" ao="0.02"/>
+      <nuclide name="H1" ao="20.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="0"/>
-    <surface boundary="reflective" coeffs="100" id="1" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="100"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/resonance_scattering/results_true.dat b/tests/regression_tests/resonance_scattering/results_true.dat
index 8316e720e..72f0933b8 100644
--- a/tests/regression_tests/resonance_scattering/results_true.dat
+++ b/tests/regression_tests/resonance_scattering/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.462509E+00 2.205413E-02
+1.462428E+00 1.828903E-02
diff --git a/tests/regression_tests/rotation/results_true.dat b/tests/regression_tests/rotation/results_true.dat
index 0fb8ba981..6db3d329a 100644
--- a/tests/regression_tests/rotation/results_true.dat
+++ b/tests/regression_tests/rotation/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.175600E-01 1.117465E-02
+4.459219E-01 1.899168E-02
diff --git a/tests/regression_tests/salphabeta/inputs_true.dat b/tests/regression_tests/salphabeta/inputs_true.dat
index ef0e843c2..56f4be375 100644
--- a/tests/regression_tests/salphabeta/inputs_true.dat
+++ b/tests/regression_tests/salphabeta/inputs_true.dat
@@ -1,36 +1,36 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="1.0" name="H1"/>
-      <sab fraction="0.5" name="c_H_in_H2O"/>
+    <material id="1" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="H1" ao="1.0"/>
+      <sab name="c_H_in_H2O" fraction="0.5"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="1.0" name="C0"/>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="C0" ao="1.0"/>
       <sab name="c_Graphite"/>
     </material>
-    <material depletable="true" id="3">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="1.0" name="Be9"/>
-      <nuclide ao="1.0" name="O16"/>
+    <material id="3" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="Be9" ao="1.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_Be_in_BeO"/>
       <sab name="c_O_in_BeO"/>
     </material>
-    <material depletable="true" id="4">
-      <density units="g/cm3" value="5.90168"/>
-      <nuclide ao="0.3" name="H1"/>
-      <nuclide ao="0.15" name="Zr90"/>
-      <nuclide ao="0.1" name="Zr91"/>
-      <nuclide ao="0.1" name="Zr92"/>
-      <nuclide ao="0.05" name="Zr94"/>
-      <nuclide ao="0.05" name="Zr96"/>
-      <nuclide ao="0.1" name="U235"/>
-      <nuclide ao="0.15" name="U238"/>
+    <material id="4" depletable="true">
+      <density value="5.90168" units="g/cm3"/>
+      <nuclide name="H1" ao="0.3"/>
+      <nuclide name="Zr90" ao="0.15"/>
+      <nuclide name="Zr91" ao="0.1"/>
+      <nuclide name="Zr92" ao="0.1"/>
+      <nuclide name="Zr94" ao="0.05"/>
+      <nuclide name="Zr96" ao="0.05"/>
+      <nuclide name="U235" ao="0.1"/>
+      <nuclide name="U238" ao="0.15"/>
       <sab name="c_Zr_in_ZrH"/>
       <sab name="c_H_in_ZrH"/>
     </material>
@@ -40,18 +40,18 @@
     <cell id="2" material="2" region="2 -3" universe="1"/>
     <cell id="3" material="3" region="3 -4" universe="1"/>
     <cell id="4" material="4" region="4 -5" universe="1"/>
-    <surface boundary="vacuum" coeffs="-10" id="1" type="x-plane"/>
-    <surface coeffs="-5" id="2" type="x-plane"/>
-    <surface coeffs="0" id="3" type="x-plane"/>
-    <surface coeffs="5" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="5" type="x-plane"/>
+    <surface id="1" type="x-plane" boundary="vacuum" coeffs="-10"/>
+    <surface id="2" type="x-plane" coeffs="-5"/>
+    <surface id="3" type="x-plane" coeffs="0"/>
+    <surface id="4" type="x-plane" coeffs="5"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="10"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>400</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/salphabeta/results_true.dat b/tests/regression_tests/salphabeta/results_true.dat
index d5c7f66e9..75a81075e 100644
--- a/tests/regression_tests/salphabeta/results_true.dat
+++ b/tests/regression_tests/salphabeta/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-8.694866E-01 2.033328E-02
+8.628529E-01 3.120924E-02
diff --git a/tests/regression_tests/score_current/inputs_true.dat b/tests/regression_tests/score_current/inputs_true.dat
index ddbd6c24b..42c2d3df2 100644
--- a/tests/regression_tests/score_current/inputs_true.dat
+++ b/tests/regression_tests/score_current/inputs_true.dat
@@ -1,28 +1,28 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="Zr90"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="Zr90" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="1 -2 3 -4 10 -9" universe="1"/>
     <cell id="2" material="2" region="(-1 | 2 | -3 | 4) (5 -6 7 -8) 10 -9" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="5" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="6" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-10.0" id="7" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="10.0" id="8" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="10" type="z-plane"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum x" type="x-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="6" name="maximum x" type="x-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="7" name="minimum y" type="y-plane" boundary="reflective" coeffs="-10.0"/>
+    <surface id="8" name="maximum y" type="y-plane" boundary="reflective" coeffs="10.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="10" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/score_current/results_true.dat b/tests/regression_tests/score_current/results_true.dat
index ff939301d..6bb74445c 100644
--- a/tests/regression_tests/score_current/results_true.dat
+++ b/tests/regression_tests/score_current/results_true.dat
@@ -1,274 +1,490 @@
 k-combined:
-2.298294E-01 3.256961E-01
+7.729082E-01 3.775399E-02
 tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.150000E-01
-2.753000E-03
-1.820000E-01
-6.756000E-03
+1.200000E-01
+2.924000E-03
+1.740000E-01
+6.270000E-03
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.110000E-01
-2.601000E-03
-1.530000E-01
-4.865000E-03
-1.910000E-01
-7.535000E-03
+1.200000E-01
+3.044000E-03
+1.510000E-01
+4.615000E-03
+1.710000E-01
+6.067000E-03
 0.000000E+00
 0.000000E+00
-7.100000E-02
-1.079000E-03
-1.500000E-01
-4.606000E-03
-1.820000E-01
-6.756000E-03
-1.150000E-01
-2.753000E-03
-1.610000E-01
-5.383000E-03
-1.230000E-01
-3.141000E-03
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-1.370000E-01
-3.951000E-03
-2.600000E-01
-1.366400E-02
-1.930000E-01
-7.573000E-03
-0.000000E+00
-0.000000E+00
-8.500000E-02
-1.533000E-03
-2.040000E-01
-8.426000E-03
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@@ -1668,232 +1668,248 @@ tally 2:
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@@ -1901,11 +1917,11 @@ tally 2:
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@@ -1916,32 +1932,16 @@ tally 2:
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diff --git a/tests/regression_tests/seed/results_true.dat b/tests/regression_tests/seed/results_true.dat
index bcd711825..ff34071c1 100644
--- a/tests/regression_tests/seed/results_true.dat
+++ b/tests/regression_tests/seed/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.069730E-01 2.632099E-03
+3.015003E-01 5.094212E-03
diff --git a/tests/regression_tests/source/inputs_true.dat b/tests/regression_tests/source/inputs_true.dat
index e787f24d4..0c3764ba6 100644
--- a/tests/regression_tests/source/inputs_true.dat
+++ b/tests/regression_tests/source/inputs_true.dat
@@ -1,150 +1,150 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" temperature="294">
-      <density units="g/cm3" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true" temperature="294">
+      <density value="4.5" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="0"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 10.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="0.3" type="independent">
+    <source type="independent" strength="0.3" particle="neutron">
       <space type="cartesian">
-        <x parameters="-3.0 3.0" type="uniform"/>
+        <x type="uniform" parameters="-3.0 3.0"/>
         <y type="discrete">
           <parameters>-4.0 -1.0 3.0 0.2 0.3 0.5</parameters>
         </y>
-        <z interpolation="linear-linear" type="tabular">
+        <z type="tabular" interpolation="linear-linear">
           <parameters>-2.0 0.0 2.0 0.2 0.3 0.2</parameters>
         </z>
       </space>
-      <angle reference_uvw="0.0 0.0 1.0" type="mu-phi">
+      <angle type="mu-phi" reference_uvw="0.0 0.0 1.0" reference_vwu="1.0 0.0 0.0">
         <mu type="discrete">
           <parameters>-1.0 0.0 1.0 0.5 0.25 0.25</parameters>
         </mu>
-        <phi parameters="0.0 6.28318530718" type="uniform"/>
+        <phi type="uniform" parameters="0.0 6.28318530718"/>
       </angle>
-      <energy parameters="1289500.0" type="maxwell"/>
+      <energy type="maxwell" parameters="1289500.0"/>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
+    <source type="independent" strength="0.1" particle="neutron">
       <space type="box">
         <parameters>-4.0 -4.0 -4.0 4.0 4.0 4.0</parameters>
       </space>
-      <angle reference_uvw="0.0 1.0 0.0" type="monodirectional"/>
-      <energy parameters="988000.0 2.249e-06" type="watt"/>
+      <angle type="monodirectional" reference_uvw="0.0 1.0 0.0"/>
+      <energy type="watt" parameters="988000.0 2.249e-06"/>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
+    <source type="independent" strength="0.1" particle="neutron">
       <space type="point">
         <parameters>1.2 -2.3 0.781</parameters>
       </space>
       <angle type="isotropic"/>
-      <energy interpolation="histogram" type="tabular">
+      <energy type="tabular" interpolation="histogram">
         <parameters>1.0 1.3894954943731377 1.93069772888325 2.6826957952797255 3.72759372031494 5.17947467923121 7.196856730011519 10.0 13.894954943731374 19.306977288832496 26.826957952797247 37.2759372031494 51.7947467923121 71.96856730011518 100.0 138.94954943731375 193.06977288832496 268.26957952797244 372.7593720314938 517.9474679231207 719.6856730011514 1000.0 1389.4954943731375 1930.6977288832495 2682.6957952797247 3727.593720314938 5179.474679231207 7196.856730011514 10000.0 13894.95494373136 19306.977288832495 26826.95795279722 37275.93720314938 51794.74679231213 71968.56730011514 100000.0 138949.5494373136 193069.77288832495 268269.5795279722 372759.3720314938 517947.4679231202 719685.6730011514 1000000.0 1389495.494373136 1930697.7288832497 2682695.7952797217 3727593.720314938 5179474.679231202 7196856.730011513 10000000.0 0.0 2.9086439299358713e-08 5.80533561806147e-08 8.67817193689187e-08 1.1515347785771536e-07 1.4305204600565115e-07 1.7036278261198208e-07 1.9697346200185813e-07 2.227747351856934e-07 2.4766057919761985e-07 2.715287327665956e-07 2.9428111652990295e-07 3.1582423606228735e-07 3.360695660646056e-07 3.549339141332686e-07 3.723397626156721e-07 3.882155871468592e-07 4.024961505584776e-07 4.151227709522976e-07 4.260435628367196e-07 4.3521365033538783e-07 4.4259535159179273e-07 4.4815833361210174e-07 4.5187973690993757e-07 4.5374426944091084e-07 4.5374426944091084e-07 4.5187973690993757e-07 4.4815833361210174e-07 4.4259535159179273e-07 4.352136503353879e-07 4.2604356283671966e-07 4.1512277095229767e-07 4.0249615055847764e-07 3.8821558714685926e-07 3.723397626156722e-07 3.5493391413326864e-07 3.360695660646057e-07 3.158242360622874e-07 2.942811165299031e-07 2.715287327665957e-07 2.4766057919762e-07 2.2277473518569352e-07 1.9697346200185819e-07 1.7036278261198226e-07 1.4305204600565126e-07 1.1515347785771556e-07 8.678171936891881e-08 5.805335618061493e-08 2.9086439299358858e-08 5.559621115282002e-23</parameters>
       </energy>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
-      <space origin="1.0 1.0 0.0" type="spherical">
-        <r parameters="2.0 3.0" type="uniform"/>
+    <source type="independent" strength="0.1" particle="neutron">
+      <space type="spherical" origin="1.0 1.0 0.0">
+        <r type="uniform" parameters="2.0 3.0"/>
         <cos_theta type="discrete">
           <parameters>0.7071067811865476 0.0 -0.7071067811865475 0.3 0.4 0.3</parameters>
         </cos_theta>
-        <phi parameters="0.0 6.283185307179586" type="uniform"/>
+        <phi type="uniform" parameters="0.0 6.283185307179586"/>
       </space>
       <angle type="isotropic"/>
-      <energy interpolation="histogram" type="tabular">
+      <energy type="tabular" interpolation="histogram">
         <parameters>1.0 1.3894954943731377 1.93069772888325 2.6826957952797255 3.72759372031494 5.17947467923121 7.196856730011519 10.0 13.894954943731374 19.306977288832496 26.826957952797247 37.2759372031494 51.7947467923121 71.96856730011518 100.0 138.94954943731375 193.06977288832496 268.26957952797244 372.7593720314938 517.9474679231207 719.6856730011514 1000.0 1389.4954943731375 1930.6977288832495 2682.6957952797247 3727.593720314938 5179.474679231207 7196.856730011514 10000.0 13894.95494373136 19306.977288832495 26826.95795279722 37275.93720314938 51794.74679231213 71968.56730011514 100000.0 138949.5494373136 193069.77288832495 268269.5795279722 372759.3720314938 517947.4679231202 719685.6730011514 1000000.0 1389495.494373136 1930697.7288832497 2682695.7952797217 3727593.720314938 5179474.679231202 7196856.730011513 10000000.0 0.0 2.9086439299358713e-08 5.80533561806147e-08 8.67817193689187e-08 1.1515347785771536e-07 1.4305204600565115e-07 1.7036278261198208e-07 1.9697346200185813e-07 2.227747351856934e-07 2.4766057919761985e-07 2.715287327665956e-07 2.9428111652990295e-07 3.1582423606228735e-07 3.360695660646056e-07 3.549339141332686e-07 3.723397626156721e-07 3.882155871468592e-07 4.024961505584776e-07 4.151227709522976e-07 4.260435628367196e-07 4.3521365033538783e-07 4.4259535159179273e-07 4.4815833361210174e-07 4.5187973690993757e-07 4.5374426944091084e-07 4.5374426944091084e-07 4.5187973690993757e-07 4.4815833361210174e-07 4.4259535159179273e-07 4.352136503353879e-07 4.2604356283671966e-07 4.1512277095229767e-07 4.0249615055847764e-07 3.8821558714685926e-07 3.723397626156722e-07 3.5493391413326864e-07 3.360695660646057e-07 3.158242360622874e-07 2.942811165299031e-07 2.715287327665957e-07 2.4766057919762e-07 2.2277473518569352e-07 1.9697346200185819e-07 1.7036278261198226e-07 1.4305204600565126e-07 1.1515347785771556e-07 8.678171936891881e-08 5.805335618061493e-08 2.9086439299358858e-08 5.559621115282002e-23</parameters>
       </energy>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
-      <space origin="1.0 1.0 0.0" type="cylindrical">
-        <r parameters="2.0 3.0" type="uniform"/>
-        <phi parameters="0.0 6.283185307179586" type="uniform"/>
-        <z interpolation="linear-linear" type="tabular">
+    <source type="independent" strength="0.1" particle="neutron">
+      <space type="cylindrical" origin="1.0 1.0 0.0">
+        <r type="uniform" parameters="2.0 3.0"/>
+        <phi type="uniform" parameters="0.0 6.283185307179586"/>
+        <z type="tabular" interpolation="linear-linear">
           <parameters>-2.0 0.0 2.0 0.2 0.3 0.2</parameters>
         </z>
       </space>
       <angle type="isotropic"/>
-      <energy interpolation="histogram" type="tabular">
+      <energy type="tabular" interpolation="histogram">
         <parameters>1.0 1.3894954943731377 1.93069772888325 2.6826957952797255 3.72759372031494 5.17947467923121 7.196856730011519 10.0 13.894954943731374 19.306977288832496 26.826957952797247 37.2759372031494 51.7947467923121 71.96856730011518 100.0 138.94954943731375 193.06977288832496 268.26957952797244 372.7593720314938 517.9474679231207 719.6856730011514 1000.0 1389.4954943731375 1930.6977288832495 2682.6957952797247 3727.593720314938 5179.474679231207 7196.856730011514 10000.0 13894.95494373136 19306.977288832495 26826.95795279722 37275.93720314938 51794.74679231213 71968.56730011514 100000.0 138949.5494373136 193069.77288832495 268269.5795279722 372759.3720314938 517947.4679231202 719685.6730011514 1000000.0 1389495.494373136 1930697.7288832497 2682695.7952797217 3727593.720314938 5179474.679231202 7196856.730011513 10000000.0 0.0 2.9086439299358713e-08 5.80533561806147e-08 8.67817193689187e-08 1.1515347785771536e-07 1.4305204600565115e-07 1.7036278261198208e-07 1.9697346200185813e-07 2.227747351856934e-07 2.4766057919761985e-07 2.715287327665956e-07 2.9428111652990295e-07 3.1582423606228735e-07 3.360695660646056e-07 3.549339141332686e-07 3.723397626156721e-07 3.882155871468592e-07 4.024961505584776e-07 4.151227709522976e-07 4.260435628367196e-07 4.3521365033538783e-07 4.4259535159179273e-07 4.4815833361210174e-07 4.5187973690993757e-07 4.5374426944091084e-07 4.5374426944091084e-07 4.5187973690993757e-07 4.4815833361210174e-07 4.4259535159179273e-07 4.352136503353879e-07 4.2604356283671966e-07 4.1512277095229767e-07 4.0249615055847764e-07 3.8821558714685926e-07 3.723397626156722e-07 3.5493391413326864e-07 3.360695660646057e-07 3.158242360622874e-07 2.942811165299031e-07 2.715287327665957e-07 2.4766057919762e-07 2.2277473518569352e-07 1.9697346200185819e-07 1.7036278261198226e-07 1.4305204600565126e-07 1.1515347785771556e-07 8.678171936891881e-08 5.805335618061493e-08 2.9086439299358858e-08 5.559621115282002e-23</parameters>
       </energy>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
-      <space origin="1.0 1.0 0.0" type="cylindrical">
-        <r parameters="2.0 3.0" type="uniform"/>
-        <phi parameters="0.0 6.283185307179586" type="uniform"/>
-        <z interpolation="linear-linear" type="tabular">
+    <source type="independent" strength="0.1" particle="neutron">
+      <space type="cylindrical" origin="1.0 1.0 0.0">
+        <r type="uniform" parameters="2.0 3.0"/>
+        <phi type="uniform" parameters="0.0 6.283185307179586"/>
+        <z type="tabular" interpolation="linear-linear">
           <parameters>-2.0 0.0 2.0 0.2 0.3 0.2</parameters>
         </z>
       </space>
       <angle type="isotropic"/>
       <energy type="mixture">
         <pair probability="1.0">
-          <dist parameters="1289500.0" type="maxwell"/>
+          <dist type="maxwell" parameters="1289500.0"/>
         </pair>
         <pair probability="2.0">
-          <dist parameters="988000.0 2.249e-06" type="watt"/>
+          <dist type="watt" parameters="988000.0 2.249e-06"/>
         </pair>
         <pair probability="3.0">
-          <dist interpolation="histogram" type="tabular">
+          <dist type="tabular" interpolation="histogram">
             <parameters>1.0 1.3894954943731377 1.93069772888325 2.6826957952797255 3.72759372031494 5.17947467923121 7.196856730011519 10.0 13.894954943731374 19.306977288832496 26.826957952797247 37.2759372031494 51.7947467923121 71.96856730011518 100.0 138.94954943731375 193.06977288832496 268.26957952797244 372.7593720314938 517.9474679231207 719.6856730011514 1000.0 1389.4954943731375 1930.6977288832495 2682.6957952797247 3727.593720314938 5179.474679231207 7196.856730011514 10000.0 13894.95494373136 19306.977288832495 26826.95795279722 37275.93720314938 51794.74679231213 71968.56730011514 100000.0 138949.5494373136 193069.77288832495 268269.5795279722 372759.3720314938 517947.4679231202 719685.6730011514 1000000.0 1389495.494373136 1930697.7288832497 2682695.7952797217 3727593.720314938 5179474.679231202 7196856.730011513 10000000.0 0.0 2.9086439299358713e-08 5.80533561806147e-08 8.67817193689187e-08 1.1515347785771536e-07 1.4305204600565115e-07 1.7036278261198208e-07 1.9697346200185813e-07 2.227747351856934e-07 2.4766057919761985e-07 2.715287327665956e-07 2.9428111652990295e-07 3.1582423606228735e-07 3.360695660646056e-07 3.549339141332686e-07 3.723397626156721e-07 3.882155871468592e-07 4.024961505584776e-07 4.151227709522976e-07 4.260435628367196e-07 4.3521365033538783e-07 4.4259535159179273e-07 4.4815833361210174e-07 4.5187973690993757e-07 4.5374426944091084e-07 4.5374426944091084e-07 4.5187973690993757e-07 4.4815833361210174e-07 4.4259535159179273e-07 4.352136503353879e-07 4.2604356283671966e-07 4.1512277095229767e-07 4.0249615055847764e-07 3.8821558714685926e-07 3.723397626156722e-07 3.5493391413326864e-07 3.360695660646057e-07 3.158242360622874e-07 2.942811165299031e-07 2.715287327665957e-07 2.4766057919762e-07 2.2277473518569352e-07 1.9697346200185819e-07 1.7036278261198226e-07 1.4305204600565126e-07 1.1515347785771556e-07 8.678171936891881e-08 5.805335618061493e-08 2.9086439299358858e-08 5.559621115282002e-23</parameters>
           </dist>
         </pair>
       </energy>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
-      <space origin="1.0 1.0 0.0" type="spherical">
-        <r parameters="2.0 3.0 2.0" type="powerlaw"/>
+    <source type="independent" strength="0.1" particle="neutron">
+      <space type="spherical" origin="1.0 1.0 0.0">
+        <r type="powerlaw" parameters="2.0 3.0 2.0"/>
         <cos_theta type="discrete">
           <parameters>0.7071067811865476 0.0 -0.7071067811865475 0.3 0.4 0.3</parameters>
         </cos_theta>
-        <phi parameters="0.0 6.283185307179586" type="uniform"/>
+        <phi type="uniform" parameters="0.0 6.283185307179586"/>
       </space>
       <angle type="isotropic"/>
       <energy type="mixture">
         <pair probability="1.0">
-          <dist parameters="1289500.0" type="maxwell"/>
+          <dist type="maxwell" parameters="1289500.0"/>
         </pair>
         <pair probability="2.0">
-          <dist parameters="988000.0 2.249e-06" type="watt"/>
+          <dist type="watt" parameters="988000.0 2.249e-06"/>
         </pair>
         <pair probability="3.0">
-          <dist interpolation="histogram" type="tabular">
+          <dist type="tabular" interpolation="histogram">
             <parameters>1.0 1.3894954943731377 1.93069772888325 2.6826957952797255 3.72759372031494 5.17947467923121 7.196856730011519 10.0 13.894954943731374 19.306977288832496 26.826957952797247 37.2759372031494 51.7947467923121 71.96856730011518 100.0 138.94954943731375 193.06977288832496 268.26957952797244 372.7593720314938 517.9474679231207 719.6856730011514 1000.0 1389.4954943731375 1930.6977288832495 2682.6957952797247 3727.593720314938 5179.474679231207 7196.856730011514 10000.0 13894.95494373136 19306.977288832495 26826.95795279722 37275.93720314938 51794.74679231213 71968.56730011514 100000.0 138949.5494373136 193069.77288832495 268269.5795279722 372759.3720314938 517947.4679231202 719685.6730011514 1000000.0 1389495.494373136 1930697.7288832497 2682695.7952797217 3727593.720314938 5179474.679231202 7196856.730011513 10000000.0 0.0 2.9086439299358713e-08 5.80533561806147e-08 8.67817193689187e-08 1.1515347785771536e-07 1.4305204600565115e-07 1.7036278261198208e-07 1.9697346200185813e-07 2.227747351856934e-07 2.4766057919761985e-07 2.715287327665956e-07 2.9428111652990295e-07 3.1582423606228735e-07 3.360695660646056e-07 3.549339141332686e-07 3.723397626156721e-07 3.882155871468592e-07 4.024961505584776e-07 4.151227709522976e-07 4.260435628367196e-07 4.3521365033538783e-07 4.4259535159179273e-07 4.4815833361210174e-07 4.5187973690993757e-07 4.5374426944091084e-07 4.5374426944091084e-07 4.5187973690993757e-07 4.4815833361210174e-07 4.4259535159179273e-07 4.352136503353879e-07 4.2604356283671966e-07 4.1512277095229767e-07 4.0249615055847764e-07 3.8821558714685926e-07 3.723397626156722e-07 3.5493391413326864e-07 3.360695660646057e-07 3.158242360622874e-07 2.942811165299031e-07 2.715287327665957e-07 2.4766057919762e-07 2.2277473518569352e-07 1.9697346200185819e-07 1.7036278261198226e-07 1.4305204600565126e-07 1.1515347785771556e-07 8.678171936891881e-08 5.805335618061493e-08 2.9086439299358858e-08 5.559621115282002e-23</parameters>
           </dist>
         </pair>
       </energy>
-      <time parameters="2 5" type="uniform"/>
+      <time type="uniform" parameters="2 5"/>
     </source>
-    <source particle="neutron" strength="0.1" type="independent">
-      <space origin="1.0 1.0 0.0" type="cylindrical">
-        <r parameters="2.0 3.0 1.0" type="powerlaw"/>
-        <phi parameters="0.0 6.283185307179586" type="uniform"/>
-        <z interpolation="linear-linear" type="tabular">
+    <source type="independent" strength="0.1" particle="neutron">
+      <space type="cylindrical" origin="1.0 1.0 0.0">
+        <r type="powerlaw" parameters="2.0 3.0 1.0"/>
+        <phi type="uniform" parameters="0.0 6.283185307179586"/>
+        <z type="tabular" interpolation="linear-linear">
           <parameters>-2.0 0.0 2.0 0.2 0.3 0.2</parameters>
         </z>
       </space>
       <angle type="isotropic"/>
       <energy type="mixture">
         <pair probability="1.0">
-          <dist parameters="1289500.0" type="maxwell"/>
+          <dist type="maxwell" parameters="1289500.0"/>
         </pair>
         <pair probability="2.0">
-          <dist parameters="988000.0 2.249e-06" type="watt"/>
+          <dist type="watt" parameters="988000.0 2.249e-06"/>
         </pair>
         <pair probability="3.0">
-          <dist interpolation="histogram" type="tabular">
+          <dist type="tabular" interpolation="histogram">
             <parameters>1.0 1.3894954943731377 1.93069772888325 2.6826957952797255 3.72759372031494 5.17947467923121 7.196856730011519 10.0 13.894954943731374 19.306977288832496 26.826957952797247 37.2759372031494 51.7947467923121 71.96856730011518 100.0 138.94954943731375 193.06977288832496 268.26957952797244 372.7593720314938 517.9474679231207 719.6856730011514 1000.0 1389.4954943731375 1930.6977288832495 2682.6957952797247 3727.593720314938 5179.474679231207 7196.856730011514 10000.0 13894.95494373136 19306.977288832495 26826.95795279722 37275.93720314938 51794.74679231213 71968.56730011514 100000.0 138949.5494373136 193069.77288832495 268269.5795279722 372759.3720314938 517947.4679231202 719685.6730011514 1000000.0 1389495.494373136 1930697.7288832497 2682695.7952797217 3727593.720314938 5179474.679231202 7196856.730011513 10000000.0 0.0 2.9086439299358713e-08 5.80533561806147e-08 8.67817193689187e-08 1.1515347785771536e-07 1.4305204600565115e-07 1.7036278261198208e-07 1.9697346200185813e-07 2.227747351856934e-07 2.4766057919761985e-07 2.715287327665956e-07 2.9428111652990295e-07 3.1582423606228735e-07 3.360695660646056e-07 3.549339141332686e-07 3.723397626156721e-07 3.882155871468592e-07 4.024961505584776e-07 4.151227709522976e-07 4.260435628367196e-07 4.3521365033538783e-07 4.4259535159179273e-07 4.4815833361210174e-07 4.5187973690993757e-07 4.5374426944091084e-07 4.5374426944091084e-07 4.5187973690993757e-07 4.4815833361210174e-07 4.4259535159179273e-07 4.352136503353879e-07 4.2604356283671966e-07 4.1512277095229767e-07 4.0249615055847764e-07 3.8821558714685926e-07 3.723397626156722e-07 3.5493391413326864e-07 3.360695660646057e-07 3.158242360622874e-07 2.942811165299031e-07 2.715287327665957e-07 2.4766057919762e-07 2.2277473518569352e-07 1.9697346200185819e-07 1.7036278261198226e-07 1.4305204600565126e-07 1.1515347785771556e-07 8.678171936891881e-08 5.805335618061493e-08 2.9086439299358858e-08 5.559621115282002e-23</parameters>
           </dist>
         </pair>
       </energy>
-      <time parameters="2 5" type="uniform"/>
+      <time type="uniform" parameters="2 5"/>
     </source>
   </settings>
   </model>
diff --git a/tests/regression_tests/source/results_true.dat b/tests/regression_tests/source/results_true.dat
index 673d27c8a..c671463be 100644
--- a/tests/regression_tests/source/results_true.dat
+++ b/tests/regression_tests/source/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.959436E-01 2.782384E-03
+2.942254E-01 2.571435E-03
diff --git a/tests/regression_tests/source_dlopen/inputs_true.dat b/tests/regression_tests/source_dlopen/inputs_true.dat
index 28d1e06e2..9b4601d9f 100644
--- a/tests/regression_tests/source_dlopen/inputs_true.dat
+++ b/tests/regression_tests/source_dlopen/inputs_true.dat
@@ -2,23 +2,23 @@
 <model>
   <materials>
     <material id="1" name="natural_lead">
-      <density units="g/cm3" value="11.34"/>
-      <nuclide ao="0.014" name="Pb204"/>
-      <nuclide ao="0.241" name="Pb206"/>
-      <nuclide ao="0.221" name="Pb207"/>
-      <nuclide ao="0.524" name="Pb208"/>
+      <density value="11.34" units="g/cm3"/>
+      <nuclide name="Pb204" ao="0.014"/>
+      <nuclide name="Pb206" ao="0.241"/>
+      <nuclide name="Pb207" ao="0.221"/>
+      <nuclide name="Pb208" ao="0.524"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 100" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 100"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>0</inactive>
-    <source library="build/libsource.so" strength="1.0" type="compiled"/>
+    <source type="compiled" strength="1.0" library="build/libsource.so"/>
   </settings>
   <tallies>
     <filter id="1" type="material">
diff --git a/tests/regression_tests/source_dlopen/source_sampling.cpp b/tests/regression_tests/source_dlopen/source_sampling.cpp
index fc61ef1fd..678eea4be 100644
--- a/tests/regression_tests/source_dlopen/source_sampling.cpp
+++ b/tests/regression_tests/source_dlopen/source_sampling.cpp
@@ -10,7 +10,7 @@ class CustomSource : public openmc::Source {
   {
     openmc::SourceSite particle;
     // wgt
-    particle.particle = openmc::ParticleType::neutron;
+    particle.particle = openmc::ParticleType::neutron();
     particle.wgt = 1.0;
     // position
 
diff --git a/tests/regression_tests/source_file/results_true.dat b/tests/regression_tests/source_file/results_true.dat
index 264e3d580..359e0526e 100644
--- a/tests/regression_tests/source_file/results_true.dat
+++ b/tests/regression_tests/source_file/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.983135E-01 5.116978E-03
+2.827397E-01 1.150437E-03
diff --git a/tests/regression_tests/source_mcpl_file/results_true.dat b/tests/regression_tests/source_mcpl_file/results_true.dat
index 264e3d580..3ba1a4520 100644
--- a/tests/regression_tests/source_mcpl_file/results_true.dat
+++ b/tests/regression_tests/source_mcpl_file/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.983135E-01 5.116978E-03
+2.827397E-01 1.150438E-03
diff --git a/tests/regression_tests/source_mcpl_file/settings.xml b/tests/regression_tests/source_mcpl_file/settings.xml
index 47b010bff..bb08e6fd7 100644
--- a/tests/regression_tests/source_mcpl_file/settings.xml
+++ b/tests/regression_tests/source_mcpl_file/settings.xml
@@ -1,12 +1,11 @@
 <?xml version="1.0"?>
 <settings>
+  <run_mode>eigenvalue</run_mode>
   <state_point batches="10" />
   <source_point mcpl="true" separate="true" />
-  <eigenvalue>
-    <batches>10</batches>
-    <inactive>5</inactive>
-    <particles>1000</particles>
-  </eigenvalue>
+  <batches>10</batches>
+  <inactive>5</inactive>
+  <particles>1000</particles>
   <source>
     <space type="box">
       <parameters>-4 -4 -4  4  4  4</parameters>
diff --git a/tests/regression_tests/source_mcpl_file/test.py b/tests/regression_tests/source_mcpl_file/test.py
index 84ec005ae..6e668ef19 100644
--- a/tests/regression_tests/source_mcpl_file/test.py
+++ b/tests/regression_tests/source_mcpl_file/test.py
@@ -1,23 +1,23 @@
 #!/usr/bin/env python
-import openmc.lib
 import pytest
 import glob
 import os
-
+import shutil
 from tests.testing_harness import *
+
 pytestmark = pytest.mark.skipif(
-    not openmc.lib._mcpl_enabled(),
-    reason="MCPL is not enabled.")
+    shutil.which("mcpl-config") is None,
+    reason="mcpl-config command not found in PATH; MCPL is likely not available."
+)
 
 settings1="""<?xml version="1.0"?>
 <settings>
+  <run_mode>eigenvalue</run_mode>
   <state_point batches="10" />
   <source_point mcpl="true" separate="true" />
-  <eigenvalue>
-    <batches>10</batches>
-    <inactive>5</inactive>
-    <particles>1000</particles>
-  </eigenvalue>
+  <batches>10</batches>
+  <inactive>5</inactive>
+  <particles>1000</particles>
   <source>
     <space type="box">
       <parameters>-4 -4 -4  4  4  4</parameters>
@@ -28,11 +28,10 @@ settings1="""<?xml version="1.0"?>
 
 settings2 = """<?xml version="1.0"?>
 <settings>
-  <eigenvalue>
-    <batches>10</batches>
-    <inactive>5</inactive>
-    <particles>1000</particles>
-  </eigenvalue>
+  <run_mode>eigenvalue</run_mode>
+  <batches>10</batches>
+  <inactive>5</inactive>
+  <particles>1000</particles>
   <source>
     <file>source.10.{}</file>
   </source>
diff --git a/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat b/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat
index 6cf3473f9..088d65ada 100644
--- a/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat
+++ b/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat
@@ -2,23 +2,23 @@
 <model>
   <materials>
     <material id="1" name="natural_lead">
-      <density units="g/cm3" value="11.34"/>
-      <nuclide ao="0.014" name="Pb204"/>
-      <nuclide ao="0.241" name="Pb206"/>
-      <nuclide ao="0.221" name="Pb207"/>
-      <nuclide ao="0.524" name="Pb208"/>
+      <density value="11.34" units="g/cm3"/>
+      <nuclide name="Pb204" ao="0.014"/>
+      <nuclide name="Pb206" ao="0.241"/>
+      <nuclide name="Pb207" ao="0.221"/>
+      <nuclide name="Pb208" ao="0.524"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 100" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 100"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>0</inactive>
-    <source library="build/libsource.so" parameters="1e3" strength="1.0" type="compiled"/>
+    <source type="compiled" strength="1.0" library="build/libsource.so" parameters="1e3"/>
   </settings>
   <tallies>
     <filter id="1" type="material">
diff --git a/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp b/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp
index bf49af4c3..81f3770c6 100644
--- a/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp
+++ b/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp
@@ -2,36 +2,37 @@
 #include "openmc/source.h"
 
 class CustomSource : public openmc::Source {
-  public:
-    CustomSource(double energy) : energy_(energy) { }
+public:
+  CustomSource(double energy) : energy_(energy) {}
 
-    // Samples from an instance of this class.
-    openmc::SourceSite sample(uint64_t* seed) const
-    {
-      openmc::SourceSite particle;
-      // wgt
-      particle.particle = openmc::ParticleType::neutron;
-      particle.wgt = 1.0;
-      // position
-      particle.r.x = 0.0;
-      particle.r.y = 0.0;
-      particle.r.z = 0.0;
-      // angle
-      particle.u = {1.0, 0.0, 0.0};
-      particle.E = this->energy_;
-      particle.delayed_group = 0;
+  // Samples from an instance of this class.
+  openmc::SourceSite sample(uint64_t* seed) const
+  {
+    openmc::SourceSite particle;
+    // wgt
+    particle.particle = openmc::ParticleType::neutron();
+    particle.wgt = 1.0;
+    // position
+    particle.r.x = 0.0;
+    particle.r.y = 0.0;
+    particle.r.z = 0.0;
+    // angle
+    particle.u = {1.0, 0.0, 0.0};
+    particle.E = this->energy_;
+    particle.delayed_group = 0;
 
-      return particle;
-    }
+    return particle;
+  }
 
-  private:
-    double energy_;
+private:
+  double energy_;
 };
 
-// A function to create a unique pointer to an instance of this class when generated
-// via a plugin call using dlopen/dlsym.
-// You must have external C linkage here otherwise dlopen will not find the file
-extern "C" std::unique_ptr<CustomSource> openmc_create_source(std::string parameter)
+// A function to create a unique pointer to an instance of this class when
+// generated via a plugin call using dlopen/dlsym. You must have external C
+// linkage here otherwise dlopen will not find the file
+extern "C" std::unique_ptr<CustomSource> openmc_create_source(
+  std::string parameter)
 {
   double energy = std::stod(parameter);
   return std::make_unique<CustomSource>(energy);
diff --git a/tests/regression_tests/sourcepoint_batch/results_true.dat b/tests/regression_tests/sourcepoint_batch/results_true.dat
index 2770f25ed..3665bdd08 100644
--- a/tests/regression_tests/sourcepoint_batch/results_true.dat
+++ b/tests/regression_tests/sourcepoint_batch/results_true.dat
@@ -1,3 +1,3 @@
 k-combined:
-3.074376E-01 3.049465E-03
-3.667754E+00 -7.701697E+00 2.213664E+00
+2.920435E-01 9.109227E-04
+1.101997E+00 -8.197502E+00 4.294606E+00
diff --git a/tests/regression_tests/sourcepoint_latest/results_true.dat b/tests/regression_tests/sourcepoint_latest/results_true.dat
index bbf03de94..97b997ae6 100644
--- a/tests/regression_tests/sourcepoint_latest/results_true.dat
+++ b/tests/regression_tests/sourcepoint_latest/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
diff --git a/tests/regression_tests/sourcepoint_restart/results_true.dat b/tests/regression_tests/sourcepoint_restart/results_true.dat
index b6d85872d..c20b5f2a0 100644
--- a/tests/regression_tests/sourcepoint_restart/results_true.dat
+++ b/tests/regression_tests/sourcepoint_restart/results_true.dat
@@ -1,46 +1,14 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
 tally 1:
 1.300000E-02
-4.700000E-05
-5.741381E-03
-9.030024E-06
+3.900000E-05
+5.833114E-03
+7.476880E-06
 0.000000E+00
 0.000000E+00
-3.067112E-04
-9.407179E-08
-1.000000E-03
-1.000000E-06
-3.015814E-04
-9.095135E-08
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-2.600000E-02
-1.420000E-04
-1.268989E-02
-3.464490E-05
-0.000000E+00
-0.000000E+00
-2.948908E-04
-8.696057E-08
-1.000000E-03
-1.000000E-06
-1.535009E-03
-1.224718E-06
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-3.200000E-02
-2.760000E-04
-1.248507E-02
-3.714039E-05
-0.000000E+00
-0.000000E+00
-9.208601E-04
-4.708050E-07
+1.164000E-03
+5.072152E-07
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -49,602 +17,26 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.900000E-02
-7.500000E-05
-8.218811E-03
-1.462963E-05
+2.100000E-02
+1.110000E-04
+1.108363E-02
+2.889480E-05
 0.000000E+00
 0.000000E+00
-9.070399E-04
-2.743247E-07
-0.000000E+00
-0.000000E+00
-9.047443E-04
-8.185622E-07
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-1.200000E-02
-5.000000E-05
-3.938144E-03
-5.245641E-06
-0.000000E+00
-0.000000E+00
-3.067112E-04
-9.407179E-08
-1.000000E-03
-1.000000E-06
-5.897816E-04
-3.478423E-07
-0.000000E+00
-0.000000E+00
-2.948908E-04
-8.696057E-08
-2.000000E-02
-1.260000E-04
-9.079852E-03
-2.657486E-05
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-3.100000E-02
-2.010000E-04
-1.336346E-02
-3.645780E-05
-0.000000E+00
-0.000000E+00
-2.102710E-03
-1.524522E-06
-1.000000E-03
-1.000000E-06
-9.083133E-04
-4.632477E-07
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-2.300000E-02
-1.090000E-04
-1.334578E-02
-3.590708E-05
-0.000000E+00
-0.000000E+00
-6.031628E-04
-3.638054E-07
-1.000000E-03
-1.000000E-06
-1.530605E-03
-1.035684E-06
+5.861433E-04
+3.435640E-07
 2.000000E-03
 2.000000E-06
-6.082927E-04
-1.850231E-07
-1.400000E-02
-4.200000E-05
-7.257468E-03
-1.221578E-05
-0.000000E+00
-0.000000E+00
-2.948908E-04
-8.696057E-08
-0.000000E+00
-0.000000E+00
-8.980536E-04
-4.507660E-07
+2.043490E-03
+1.793389E-06
 2.000000E-03
-4.000000E-06
-0.000000E+00
-0.000000E+00
-9.000000E-03
-2.700000E-05
-5.193169E-03
-1.187321E-05
-0.000000E+00
-0.000000E+00
-3.054379E-04
-9.329231E-08
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-1.000000E-02
-2.400000E-05
-5.170002E-03
-7.361546E-06
-0.000000E+00
-0.000000E+00
-6.003287E-04
-1.802529E-07
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-2.800000E-02
-1.720000E-04
-1.699195E-02
-6.267008E-05
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-2.800000E-02
-1.980000E-04
-1.338750E-02
-4.565234E-05
-0.000000E+00
-0.000000E+00
-9.183134E-04
-4.676871E-07
-1.000000E-03
-1.000000E-06
-1.537188E-03
-2.362946E-06
-2.000000E-03
-4.000000E-06
-0.000000E+00
-0.000000E+00
-2.900000E-02
-1.950000E-04
-1.178615E-02
-3.130999E-05
-0.000000E+00
-0.000000E+00
-3.015814E-04
-9.095135E-08
-0.000000E+00
-0.000000E+00
-1.179563E-03
-1.391369E-06
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-1.500000E-02
-6.500000E-05
-5.706064E-03
-9.729293E-06
-0.000000E+00
-0.000000E+00
-2.948908E-04
-8.696057E-08
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-3.000000E-03
-9.000000E-06
-1.516642E-03
-1.579637E-06
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-5.897816E-04
-3.478423E-07
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-2.900000E-02
-1.970000E-04
-1.274851E-02
-3.511219E-05
-0.000000E+00
-0.000000E+00
-9.057666E-04
-4.601298E-07
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-1.800000E-02
-7.600000E-05
-8.803642E-03
-1.730938E-05
-0.000000E+00
-0.000000E+00
-3.074376E-04
-9.451786E-08
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-2.900000E-02
-1.830000E-04
-1.245316E-02
-3.378176E-05
-0.000000E+00
-0.000000E+00
-6.016020E-04
-1.810324E-07
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-1.000000E-03
-1.000000E-06
-6.090190E-04
-1.854692E-07
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-6.031628E-04
-3.638054E-07
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-1.900000E-02
-8.700000E-05
-8.781239E-03
-1.676750E-05
-0.000000E+00
-0.000000E+00
-6.128755E-04
-1.878102E-07
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-3.500000E-02
-2.930000E-04
-1.372494E-02
-4.750881E-05
-0.000000E+00
-0.000000E+00
-9.090396E-04
-2.755502E-07
-0.000000E+00
-0.000000E+00
-5.897816E-04
-3.478423E-07
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-3.700000E-02
-3.270000E-04
-1.492073E-02
-5.134793E-05
-0.000000E+00
-0.000000E+00
-6.134225E-04
-3.762872E-07
-1.000000E-03
-1.000000E-06
-3.074376E-04
-9.451786E-08
-1.000000E-03
-1.000000E-06
-6.148751E-04
-3.780714E-07
-3.500000E-02
-2.810000E-04
-1.728232E-02
-6.621096E-05
-0.000000E+00
-0.000000E+00
-1.214477E-03
-3.688425E-07
-0.000000E+00
-0.000000E+00
-1.481145E-03
-1.482321E-06
-1.000000E-03
-1.000000E-06
-3.015814E-04
-9.095135E-08
-8.000000E-03
-1.800000E-05
-3.359923E-03
-3.081200E-06
-0.000000E+00
-0.000000E+00
-3.067112E-04
-9.407179E-08
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-7.000000E-03
-1.500000E-05
-3.660087E-03
-3.561493E-06
-0.000000E+00
-0.000000E+00
-9.203130E-04
-4.713637E-07
-1.000000E-03
-1.000000E-06
-3.015814E-04
-9.095135E-08
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-3.500000E-02
-2.630000E-04
-1.425810E-02
-4.231681E-05
-0.000000E+00
-0.000000E+00
-1.516059E-03
-4.597939E-07
-0.000000E+00
-0.000000E+00
-1.221752E-03
-1.492677E-06
-1.000000E-03
-1.000000E-06
-0.000000E+00
-0.000000E+00
-2.300000E-02
-1.110000E-04
-1.184871E-02
-2.918189E-05
-0.000000E+00
-0.000000E+00
-6.016020E-04
-1.810324E-07
-0.000000E+00
-0.000000E+00
-0.000000E+00
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@@ -818,9 +626,9 @@ tally 1:
 0.000000E+00
 0.000000E+00
 7.000000E-03
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 0.000000E+00
 0.000000E+00
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@@ -833,46 +641,10 @@ tally 1:
 0.000000E+00
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 0.000000E+00
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@@ -885,6 +657,42 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
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 0.000000E+00
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@@ -897,14 +705,110 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
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 0.000000E+00
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@@ -914,13 +818,9 @@ tally 1:
 0.000000E+00
 0.000000E+00
 8.000000E-03
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 0.000000E+00
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@@ -929,20 +829,120 @@ tally 1:
 0.000000E+00
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 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -962,11 +962,11 @@ tally 1:
 0.000000E+00
 0.000000E+00
 tally 2:
-5.818100E-01
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diff --git a/tests/regression_tests/statepoint_batch/geometry.xml b/tests/regression_tests/statepoint_batch/geometry.xml
deleted file mode 100644
index bc56030e1..000000000
--- a/tests/regression_tests/statepoint_batch/geometry.xml
+++ /dev/null
@@ -1,8 +0,0 @@
-<?xml version="1.0"?>
-<geometry>
-
-  <!-- Sphere with radius 10 -->
-  <surface id="1" type="sphere" coeffs="0 0 0 10" boundary="vacuum"/>
-  <cell id="1" material="1" region="-1" />
-    
-</geometry>
diff --git a/tests/regression_tests/statepoint_batch/results_true.dat b/tests/regression_tests/statepoint_batch/results_true.dat
deleted file mode 100644
index f5c855d77..000000000
--- a/tests/regression_tests/statepoint_batch/results_true.dat
+++ /dev/null
@@ -1,2 +0,0 @@
-k-combined:
-3.048864E-01 1.689118E-03
diff --git a/tests/regression_tests/statepoint_batch/settings.xml b/tests/regression_tests/statepoint_batch/settings.xml
deleted file mode 100644
index e2f8dad47..000000000
--- a/tests/regression_tests/statepoint_batch/settings.xml
+++ /dev/null
@@ -1,17 +0,0 @@
-<?xml version="1.0"?>
-<settings>
-
-  <state_point batches="3 6 9" />
-
-  <run_mode>eigenvalue</run_mode>
-  <batches>10</batches>
-  <inactive>5</inactive>
-  <particles>1000</particles>
-
-  <source>
-    <space type="box">
-      <parameters>-4 -4 -4  4  4  4</parameters>
-    </space>
-  </source>
-
-</settings>
diff --git a/tests/regression_tests/statepoint_batch/test.py b/tests/regression_tests/statepoint_batch/test.py
deleted file mode 100644
index 323b28fc6..000000000
--- a/tests/regression_tests/statepoint_batch/test.py
+++ /dev/null
@@ -1,18 +0,0 @@
-from tests.testing_harness import TestHarness
-
-
-class StatepointTestHarness(TestHarness):
-    def __init__(self):
-        super().__init__(None)
-
-    def _test_output_created(self):
-        """Make sure statepoint files have been created."""
-        sps = ('statepoint.03.h5', 'statepoint.06.h5', 'statepoint.09.h5')
-        for sp in sps:
-            self._sp_name = sp
-            TestHarness._test_output_created(self)
-
-
-def test_statepoint_batch():
-    harness = StatepointTestHarness()
-    harness.main()
diff --git a/tests/regression_tests/statepoint_restart/results_true.dat b/tests/regression_tests/statepoint_restart/results_true.dat
index aebc971f4..b919b3000 100644
--- a/tests/regression_tests/statepoint_restart/results_true.dat
+++ b/tests/regression_tests/statepoint_restart/results_true.dat
@@ -1,66 +1,18 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
 tally 1:
 1.300000E-02
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@@ -73,900 +25,36 @@ tally 1:
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 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -1335,28 +1057,162 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
+9.000000E-03
+1.900000E-05
+9.000000E-03
+1.900000E-05
+2.915422E-03
+2.203274E-06
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
+8.775182E-04
+4.280701E-07
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.300000E-02
-4.700000E-05
-1.300000E-02
-4.700000E-05
-5.486562E-03
-7.965488E-06
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.054379E-04
-9.329231E-08
+0.000000E+00
+0.000000E+00
+1.000000E-03
+1.000000E-06
+1.000000E-03
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+2.920435E-04
+8.528942E-08
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+9.000000E-06
+5.000000E-03
+9.000000E-06
+2.044244E-03
+1.796182E-06
+0.000000E+00
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+9.000000E-05
+1.800000E-02
+9.000000E-05
+6.718632E-03
+1.203946E-05
+0.000000E+00
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+0.000000E+00
+0.000000E+00
+2.927374E-04
+8.569516E-08
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+1.100000E-02
+2.700000E-05
+1.100000E-02
+2.700000E-05
+4.675703E-03
+5.128811E-06
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.914595E-04
+8.494867E-08
+0.000000E+00
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+1.000000E-03
+1.000000E-06
+1.000000E-03
+1.000000E-06
+2.914595E-04
+8.494867E-08
+0.000000E+00
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+0.000000E+00
+0.000000E+00
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+0.000000E+00
+1.000000E-03
+1.000000E-06
+1.000000E-03
+1.000000E-06
+2.920435E-04
+8.528942E-08
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -1370,17 +1226,11 @@ tally 1:
 0.000000E+00
 0.000000E+00
 8.000000E-03
-1.800000E-05
+2.600000E-05
 8.000000E-03
-1.800000E-05
-3.334317E-03
-3.609712E-06
-0.000000E+00
-0.000000E+00
-0.000000E+00
-0.000000E+00
-2.948908E-04
-8.696057E-08
+2.600000E-05
+4.670349E-03
+6.133014E-06
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -1393,28 +1243,178 @@ tally 1:
 0.000000E+00
 0.000000E+00
 0.000000E+00
-2.000000E-02
-1.080000E-04
-2.000000E-02
-1.080000E-04
-9.741552E-03
-2.552400E-05
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-3.015814E-04
-9.095135E-08
+0.000000E+00
+0.000000E+00
+1.500000E-02
+6.100000E-05
+1.500000E-02
+6.100000E-05
+8.751735E-03
+1.922421E-05
+0.000000E+00
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+0.000000E+00
+1.400000E-02
+5.000000E-05
+1.400000E-02
+5.000000E-05
+6.126605E-03
+9.620966E-06
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+5.835031E-04
+1.702381E-07
+0.000000E+00
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+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.000000E-03
+2.000000E-06
+2.000000E-03
+2.000000E-06
+5.847809E-04
+1.709846E-07
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
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+0.000000E+00
+0.000000E+00
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+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.000000E-03
+2.000000E-06
+2.000000E-03
+2.000000E-06
+8.719569E-04
+4.219258E-07
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
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+0.000000E+00
+0.000000E+00
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+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+9.000000E-03
+2.700000E-05
+9.000000E-03
+2.700000E-05
+4.085939E-03
+4.936434E-06
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.930717E-04
+8.589100E-08
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
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+0.000000E+00
+9.000000E-03
+2.500000E-05
+9.000000E-03
+2.500000E-05
+5.846700E-03
+9.739696E-06
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
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+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.300000E-02
+3.900000E-05
+1.300000E-02
+3.900000E-05
+6.128455E-03
+1.013669E-05
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+5.851152E-04
+1.711804E-07
+0.000000E+00
+0.000000E+00
+0.000000E+00
+0.000000E+00
+1.172287E-03
+1.374256E-06
 0.000000E+00
 0.000000E+00
 0.000000E+00
 0.000000E+00
-1.800986E-03
-1.622276E-06
-1.000000E-03
-1.000000E-06
-1.000000E-03
-1.000000E-06
 0.000000E+00
 0.000000E+00
 0.000000E+00
@@ -1442,11 +1442,11 @@ tally 1:
 0.000000E+00
 0.000000E+00
 tally 2:
-5.818100E-01
-6.772558E-02
-6.344749E-01
-8.054355E-02
-3.690180E+00
-2.724752E+00
-4.102635E+01
-3.367908E+02
+5.554367E-01
+6.170503E-02
+6.055520E-01
+7.334109E-02
+3.526894E+00
+2.488065E+00
+3.925114E+01
+3.081807E+02
diff --git a/tests/regression_tests/statepoint_sourcesep/results_true.dat b/tests/regression_tests/statepoint_sourcesep/results_true.dat
index bbf03de94..97b997ae6 100644
--- a/tests/regression_tests/statepoint_sourcesep/results_true.dat
+++ b/tests/regression_tests/statepoint_sourcesep/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
diff --git a/tests/regression_tests/stride/inputs_true.dat b/tests/regression_tests/stride/inputs_true.dat
index f93ec33d1..ebae53c05 100644
--- a/tests/regression_tests/stride/inputs_true.dat
+++ b/tests/regression_tests/stride/inputs_true.dat
@@ -1,21 +1,21 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="4.5" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 10.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -4 4 4 4</parameters>
       </space>
diff --git a/tests/regression_tests/stride/results_true.dat b/tests/regression_tests/stride/results_true.dat
index a65411150..825de3766 100644
--- a/tests/regression_tests/stride/results_true.dat
+++ b/tests/regression_tests/stride/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.978080E-01 6.106774E-03
+2.953207E-01 2.874356E-03
diff --git a/tests/regression_tests/surface_source/inputs_true_read.dat b/tests/regression_tests/surface_source/inputs_true_read_h5.dat
similarity index 74%
rename from tests/regression_tests/surface_source/inputs_true_read.dat
rename to tests/regression_tests/surface_source/inputs_true_read_h5.dat
index 14a55f0bf..321c11d42 100644
--- a/tests/regression_tests/surface_source/inputs_true_read.dat
+++ b/tests/regression_tests/surface_source/inputs_true_read_h5.dat
@@ -7,10 +7,10 @@
     <cell id="2" material="void" region="1 -2" universe="1"/>
     <cell id="3" material="void" region="2 -3" universe="1"/>
     <cell id="4" material="void" region="3 -4" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 1.0" id="1" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 2.0" id="2" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 2.5" id="3" type="sphere"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 4.0" id="4" type="sphere"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" coeffs="0.0 0.0 0.0 2.0"/>
+    <surface id="3" type="sphere" coeffs="0.0 0.0 0.0 2.5"/>
+    <surface id="4" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 4.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
diff --git a/tests/regression_tests/surface_source/inputs_true_read_mcpl.dat b/tests/regression_tests/surface_source/inputs_true_read_mcpl.dat
new file mode 100644
index 000000000..2ee76f618
--- /dev/null
+++ b/tests/regression_tests/surface_source/inputs_true_read_mcpl.dat
@@ -0,0 +1,33 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+  </materials>
+  <geometry>
+    <cell id="1" material="void" region="-1" universe="1"/>
+    <cell id="2" material="void" region="1 -2" universe="1"/>
+    <cell id="3" material="void" region="2 -3" universe="1"/>
+    <cell id="4" material="void" region="3 -4" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" coeffs="0.0 0.0 0.0 2.0"/>
+    <surface id="3" type="sphere" coeffs="0.0 0.0 0.0 2.5"/>
+    <surface id="4" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+    <surf_source_read>
+      <path>surface_source_true.mcpl</path>
+    </surf_source_read>
+    <seed>1</seed>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>3</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/surface_source/inputs_true_write.dat b/tests/regression_tests/surface_source/inputs_true_write_h5.dat
similarity index 73%
rename from tests/regression_tests/surface_source/inputs_true_write.dat
rename to tests/regression_tests/surface_source/inputs_true_write_h5.dat
index 5c85de631..10e3af0a7 100644
--- a/tests/regression_tests/surface_source/inputs_true_write.dat
+++ b/tests/regression_tests/surface_source/inputs_true_write_h5.dat
@@ -7,16 +7,16 @@
     <cell id="2" material="void" region="1 -2" universe="1"/>
     <cell id="3" material="void" region="2 -3" universe="1"/>
     <cell id="4" material="void" region="3 -4" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 1.0" id="1" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 2.0" id="2" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 2.5" id="3" type="sphere"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 4.0" id="4" type="sphere"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" coeffs="0.0 0.0 0.0 2.0"/>
+    <surface id="3" type="sphere" coeffs="0.0 0.0 0.0 2.5"/>
+    <surface id="4" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 4.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0 0 0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source/inputs_true_write_mcpl.dat b/tests/regression_tests/surface_source/inputs_true_write_mcpl.dat
new file mode 100644
index 000000000..e9758144d
--- /dev/null
+++ b/tests/regression_tests/surface_source/inputs_true_write_mcpl.dat
@@ -0,0 +1,40 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+  </materials>
+  <geometry>
+    <cell id="1" material="void" region="-1" universe="1"/>
+    <cell id="2" material="void" region="1 -2" universe="1"/>
+    <cell id="3" material="void" region="2 -3" universe="1"/>
+    <cell id="4" material="void" region="3 -4" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" coeffs="0.0 0.0 0.0 2.0"/>
+    <surface id="3" type="sphere" coeffs="0.0 0.0 0.0 2.5"/>
+    <surface id="4" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 4.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>1000</particles>
+    <batches>10</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="point">
+        <parameters>0 0 0</parameters>
+      </space>
+    </source>
+    <surf_source_write>
+      <surface_ids>1</surface_ids>
+      <mcpl>true</mcpl>
+      <max_particles>1000</max_particles>
+    </surf_source_write>
+    <seed>1</seed>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>3</bins>
+    </filter>
+    <tally id="1">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/surface_source/surface_source_true.h5 b/tests/regression_tests/surface_source/surface_source_true.h5
index f2055d09d..02e94c90b 100644
Binary files a/tests/regression_tests/surface_source/surface_source_true.h5 and b/tests/regression_tests/surface_source/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source/surface_source_true.mcpl b/tests/regression_tests/surface_source/surface_source_true.mcpl
new file mode 100644
index 000000000..ca4b7c2c4
Binary files /dev/null and b/tests/regression_tests/surface_source/surface_source_true.mcpl differ
diff --git a/tests/regression_tests/surface_source/test.py b/tests/regression_tests/surface_source/test.py
index 83b78e61d..13966b26d 100644
--- a/tests/regression_tests/surface_source/test.py
+++ b/tests/regression_tests/surface_source/test.py
@@ -10,6 +10,25 @@ from tests.testing_harness import PyAPITestHarness
 from tests.regression_tests import config
 
 
+def mcpl_to_array(filepath):
+    import mcpl
+
+    source = []
+    with mcpl.MCPLFile(filepath) as f:
+        for p in f.particles:
+            source.append(
+                [
+                    *tuple(p.position),
+                    *tuple(p.direction),
+                    1.0e6 * p.ekin,
+                    1.0e-3 * p.time,
+                    p.weight,
+                    p.pdgcode,
+                ]
+            )
+    return np.sort(np.array(source), axis=0)
+
+
 def assert_structured_arrays_close(arr1, arr2, rtol=1e-5, atol=1e-8):
     assert arr1.dtype == arr2.dtype
 
@@ -24,8 +43,7 @@ def assert_structured_arrays_close(arr1, arr2, rtol=1e-5, atol=1e-8):
 @pytest.fixture
 def model(request):
     openmc.reset_auto_ids()
-    marker = request.node.get_closest_marker("surf_source_op")
-    surf_source_op = marker.args[0]
+    operation, file_format = request.node.get_closest_marker("params").args
 
     openmc_model = openmc.model.Model()
 
@@ -54,15 +72,19 @@ def model(request):
     openmc_model.settings.batches = 10
     openmc_model.settings.seed = 1
 
-    if surf_source_op == 'write':
+    if operation == 'write':
         point = openmc.stats.Point((0, 0, 0))
         pt_src = openmc.IndependentSource(space=point)
         openmc_model.settings.source = pt_src
 
-        openmc_model.settings.surf_source_write = {'surface_ids': [1],
-                                                   'max_particles': 1000}
-    elif surf_source_op == 'read':
-        openmc_model.settings.surf_source_read = {'path': 'surface_source_true.h5'}
+        surf_source_write_settings = {'surface_ids': [1],
+                                      'max_particles': 1000}
+        if file_format == "mcpl":
+            surf_source_write_settings["mcpl"] = True
+
+        openmc_model.settings.surf_source_write = surf_source_write_settings
+    elif operation == 'read':
+        openmc_model.settings.surf_source_read = {'path': f"surface_source_true.{file_format}"}
 
     # Tallies
     tal = openmc.Tally()
@@ -75,22 +97,30 @@ def model(request):
 
 
 class SurfaceSourceTestHarness(PyAPITestHarness):
+    def __init__(self, statepoint_name, model=None, inputs_true=None, file_format="h5"):
+        super().__init__(statepoint_name, model, inputs_true)
+        self.file_format = file_format
+
     def _test_output_created(self):
-        """Make sure surface_source.h5 has also been created."""
+        """Make sure the surface_source file has also been created."""
         super()._test_output_created()
-        # Check if 'surface_source.h5' has been created.
         if self._model.settings.surf_source_write:
-            assert os.path.exists('surface_source.h5'), \
+            assert os.path.exists(f"surface_source.{self.file_format}"), \
                 'Surface source file does not exist.'
 
     def _compare_output(self):
         """Make sure the current surface_source.h5 agree with the reference."""
         if self._model.settings.surf_source_write:
-            with h5py.File("surface_source_true.h5", 'r') as f:
-                source_true = np.sort(f['source_bank'][()])
-            with h5py.File("surface_source.h5", 'r') as f:
-                source_test = np.sort(f['source_bank'][()])
-            assert_structured_arrays_close(source_true, source_test, atol=1e-07)
+            if self.file_format == "h5":
+                with h5py.File("surface_source_true.h5", 'r') as f:
+                    source_true = np.sort(f['source_bank'][()])
+                with h5py.File("surface_source.h5", 'r') as f:
+                    source_test = np.sort(f['source_bank'][()])
+                assert_structured_arrays_close(source_true, source_test, atol=1e-07)
+            elif self.file_format == "mcpl":
+                source_true = mcpl_to_array("surface_source_true.mcpl")
+                source_test = mcpl_to_array("surface_source.mcpl")
+                np.testing.assert_allclose(source_true, source_test, rtol=1e-5, atol=1e-7)
 
     def execute_test(self):
         """Build input XMLs, run OpenMC, check output and results."""
@@ -111,30 +141,56 @@ class SurfaceSourceTestHarness(PyAPITestHarness):
     def _overwrite_results(self):
         """Overwrite the results_true with the results_test."""
         shutil.copyfile('results_test.dat', 'results_true.dat')
-        if os.path.exists('surface_source.h5'):
-            shutil.copyfile('surface_source.h5', 'surface_source_true.h5')
+        if os.path.exists(f"surface_source.{self.file_format}"):
+            shutil.copyfile(f"surface_source.{self.file_format}", f"surface_source_true.{self.file_format}")
 
     def _cleanup(self):
         """Delete statepoints, tally, and test files."""
         super()._cleanup()
-        fs = 'surface_source.h5'
+        fs = f"surface_source.{self.file_format}"
         if os.path.exists(fs):
             os.remove(fs)
 
 
-@pytest.mark.surf_source_op('write')
-def test_surface_source_write(model, monkeypatch):
-    # Test result is based on 1 MPI process
-    monkeypatch.setitem(config, "mpi_np", "1")
-    harness = SurfaceSourceTestHarness('statepoint.10.h5',
-                                       model,
-                                       'inputs_true_write.dat')
+@pytest.mark.params('write', 'h5')
+def test_surface_source_write(model, monkeypatch, request):
+    monkeypatch.setitem(config, "mpi_np", "1")  # Results generated with 1 MPI process
+    operation, file_format = request.node.get_closest_marker("params").args
+    harness = SurfaceSourceTestHarness(
+        "statepoint.10.h5", model, f"inputs_true_{operation}_{file_format}.dat",
+        file_format=file_format
+    )
     harness.main()
 
 
-@pytest.mark.surf_source_op('read')
-def test_surface_source_read(model):
-    harness = SurfaceSourceTestHarness('statepoint.10.h5',
-                                       model,
-                                       'inputs_true_read.dat')
+@pytest.mark.params('read', 'h5')
+def test_surface_source_read(model, request):
+    operation, file_format = request.node.get_closest_marker("params").args
+    harness = SurfaceSourceTestHarness(
+        "statepoint.10.h5", model, f"inputs_true_{operation}_{file_format}.dat",
+        file_format=file_format
+    )
+    harness.main()
+
+
+@pytest.mark.skipif(shutil.which("mcpl-config") is None, reason="MCPL is not available.")
+@pytest.mark.params('write', 'mcpl')
+def test_surface_source_write_mcpl(model, monkeypatch, request):
+    monkeypatch.setitem(config, "mpi_np", "1")  # Results generated with 1 MPI process
+    operation, file_format = request.node.get_closest_marker("params").args
+    harness = SurfaceSourceTestHarness(
+        "statepoint.10.h5", model, f"inputs_true_{operation}_{file_format}.dat",
+        file_format=file_format
+    )
+    harness.main()
+
+
+@pytest.mark.skipif(shutil.which("mcpl-config") is None, reason="MCPL is not available.")
+@pytest.mark.params('read', 'mcpl')
+def test_surface_source_read_mcpl(model, request):
+    operation, file_format = request.node.get_closest_marker("params").args
+    harness = SurfaceSourceTestHarness(
+        "statepoint.10.h5", model, f"inputs_true_{operation}_{file_format}.dat",
+        file_format=file_format
+    )
     harness.main()
diff --git a/tests/regression_tests/surface_source_write/case-01/inputs_true.dat b/tests/regression_tests/surface_source_write/case-01/inputs_true.dat
index 7368debd5..2a67b03dd 100644
--- a/tests/regression_tests/surface_source_write/case-01/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-01/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-01/results_true.dat b/tests/regression_tests/surface_source_write/case-01/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-01/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-01/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-01/surface_source_true.h5
index 82dca4d03..3a2315acc 100644
Binary files a/tests/regression_tests/surface_source_write/case-01/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-01/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-02/inputs_true.dat b/tests/regression_tests/surface_source_write/case-02/inputs_true.dat
index 12f15499f..527f076b9 100644
--- a/tests/regression_tests/surface_source_write/case-02/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-02/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-02/results_true.dat b/tests/regression_tests/surface_source_write/case-02/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-02/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-02/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-02/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-02/surface_source_true.h5
index 872efa070..2c2d5bc00 100644
Binary files a/tests/regression_tests/surface_source_write/case-02/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-02/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-03/inputs_true.dat b/tests/regression_tests/surface_source_write/case-03/inputs_true.dat
index cd0f7deda..58c4e0a24 100644
--- a/tests/regression_tests/surface_source_write/case-03/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-03/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-03/results_true.dat b/tests/regression_tests/surface_source_write/case-03/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-03/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-03/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-03/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-03/surface_source_true.h5
index 04a6ac9a3..d14771f5a 100644
Binary files a/tests/regression_tests/surface_source_write/case-03/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-03/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-04/inputs_true.dat b/tests/regression_tests/surface_source_write/case-04/inputs_true.dat
index ac3c03e37..46701aea7 100644
--- a/tests/regression_tests/surface_source_write/case-04/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-04/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-04/results_true.dat b/tests/regression_tests/surface_source_write/case-04/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-04/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-04/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-04/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-04/surface_source_true.h5
index 75c309eb4..d2acc6991 100644
Binary files a/tests/regression_tests/surface_source_write/case-04/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-04/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-05/inputs_true.dat b/tests/regression_tests/surface_source_write/case-05/inputs_true.dat
index 143e85200..c420d797c 100644
--- a/tests/regression_tests/surface_source_write/case-05/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-05/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-05/results_true.dat b/tests/regression_tests/surface_source_write/case-05/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-05/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-05/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-05/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-05/surface_source_true.h5
index 1a2f54d76..553090b19 100644
Binary files a/tests/regression_tests/surface_source_write/case-05/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-05/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-06/inputs_true.dat b/tests/regression_tests/surface_source_write/case-06/inputs_true.dat
index f2dc7ea35..e02d5e90c 100644
--- a/tests/regression_tests/surface_source_write/case-06/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-06/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-06/results_true.dat b/tests/regression_tests/surface_source_write/case-06/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-06/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-06/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-06/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-06/surface_source_true.h5
index bef135e5a..60a9f4f82 100644
Binary files a/tests/regression_tests/surface_source_write/case-06/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-06/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-07/inputs_true.dat b/tests/regression_tests/surface_source_write/case-07/inputs_true.dat
index a6107d16f..a4588c8d0 100644
--- a/tests/regression_tests/surface_source_write/case-07/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-07/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-07/results_true.dat b/tests/regression_tests/surface_source_write/case-07/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-07/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-07/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-07/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-07/surface_source_true.h5
index f84a5c4c5..cb3079251 100644
Binary files a/tests/regression_tests/surface_source_write/case-07/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-07/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-08/inputs_true.dat b/tests/regression_tests/surface_source_write/case-08/inputs_true.dat
index b87e2b207..ecf3a6a2e 100644
--- a/tests/regression_tests/surface_source_write/case-08/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-08/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-08/results_true.dat b/tests/regression_tests/surface_source_write/case-08/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-08/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-08/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-08/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-08/surface_source_true.h5
index 1fd302a7c..ed3aba907 100644
Binary files a/tests/regression_tests/surface_source_write/case-08/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-08/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-09/inputs_true.dat b/tests/regression_tests/surface_source_write/case-09/inputs_true.dat
index f4f6ebd87..5d60f9dbe 100644
--- a/tests/regression_tests/surface_source_write/case-09/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-09/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-09/results_true.dat b/tests/regression_tests/surface_source_write/case-09/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-09/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-09/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-09/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-09/surface_source_true.h5
index af832f060..d62363f97 100644
Binary files a/tests/regression_tests/surface_source_write/case-09/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-09/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-10/inputs_true.dat b/tests/regression_tests/surface_source_write/case-10/inputs_true.dat
index 372e5c011..1940826c2 100644
--- a/tests/regression_tests/surface_source_write/case-10/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-10/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-10/results_true.dat b/tests/regression_tests/surface_source_write/case-10/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-10/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-10/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-10/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-10/surface_source_true.h5
index 1512d8692..acaf39cb4 100644
Binary files a/tests/regression_tests/surface_source_write/case-10/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-10/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-11/inputs_true.dat b/tests/regression_tests/surface_source_write/case-11/inputs_true.dat
index d5401dc87..a4feff1b1 100644
--- a/tests/regression_tests/surface_source_write/case-11/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-11/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-11/results_true.dat b/tests/regression_tests/surface_source_write/case-11/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-11/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-11/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-11/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-11/surface_source_true.h5
index 0edb06c88..962f95fdd 100644
Binary files a/tests/regression_tests/surface_source_write/case-11/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-11/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-12/inputs_true.dat b/tests/regression_tests/surface_source_write/case-12/inputs_true.dat
index cb8f87741..c069f425c 100644
--- a/tests/regression_tests/surface_source_write/case-12/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-12/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-12/results_true.dat b/tests/regression_tests/surface_source_write/case-12/results_true.dat
index d793a7e42..ad927bdf3 100644
--- a/tests/regression_tests/surface_source_write/case-12/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-12/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.752377E-02 6.773395E-03
+4.929000E-02 8.212396E-03
diff --git a/tests/regression_tests/surface_source_write/case-12/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-12/surface_source_true.h5
index ea81a8250..4b830d057 100644
Binary files a/tests/regression_tests/surface_source_write/case-12/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-12/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-13/inputs_true.dat b/tests/regression_tests/surface_source_write/case-13/inputs_true.dat
index 23bbf0455..2a93fdb4d 100644
--- a/tests/regression_tests/surface_source_write/case-13/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-13/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-13/results_true.dat b/tests/regression_tests/surface_source_write/case-13/results_true.dat
index d793a7e42..ad927bdf3 100644
--- a/tests/regression_tests/surface_source_write/case-13/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-13/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.752377E-02 6.773395E-03
+4.929000E-02 8.212396E-03
diff --git a/tests/regression_tests/surface_source_write/case-13/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-13/surface_source_true.h5
index 08bd80985..f53e2c3a2 100644
Binary files a/tests/regression_tests/surface_source_write/case-13/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-13/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-14/inputs_true.dat b/tests/regression_tests/surface_source_write/case-14/inputs_true.dat
index 29acd94f2..893f8ddc1 100644
--- a/tests/regression_tests/surface_source_write/case-14/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-14/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-14/results_true.dat b/tests/regression_tests/surface_source_write/case-14/results_true.dat
index d793a7e42..ad927bdf3 100644
--- a/tests/regression_tests/surface_source_write/case-14/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-14/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.752377E-02 6.773395E-03
+4.929000E-02 8.212396E-03
diff --git a/tests/regression_tests/surface_source_write/case-14/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-14/surface_source_true.h5
index 50cddcc70..ec049fec4 100644
Binary files a/tests/regression_tests/surface_source_write/case-14/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-14/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-15/inputs_true.dat b/tests/regression_tests/surface_source_write/case-15/inputs_true.dat
index c775f2f52..875a2fb0b 100644
--- a/tests/regression_tests/surface_source_write/case-15/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-15/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-15/results_true.dat b/tests/regression_tests/surface_source_write/case-15/results_true.dat
index d793a7e42..ad927bdf3 100644
--- a/tests/regression_tests/surface_source_write/case-15/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-15/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.752377E-02 6.773395E-03
+4.929000E-02 8.212396E-03
diff --git a/tests/regression_tests/surface_source_write/case-15/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-15/surface_source_true.h5
index 3e15017a1..fd5892096 100644
Binary files a/tests/regression_tests/surface_source_write/case-15/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-15/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-16/inputs_true.dat b/tests/regression_tests/surface_source_write/case-16/inputs_true.dat
index 969a8f31e..347855e90 100644
--- a/tests/regression_tests/surface_source_write/case-16/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-16/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-16/results_true.dat b/tests/regression_tests/surface_source_write/case-16/results_true.dat
index a97be70ca..cd0619c1f 100644
--- a/tests/regression_tests/surface_source_write/case-16/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-16/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.496403E+00 3.891283E-02
+1.403371E+00 1.456192E-02
diff --git a/tests/regression_tests/surface_source_write/case-16/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-16/surface_source_true.h5
index 49f67e820..147ce0358 100644
Binary files a/tests/regression_tests/surface_source_write/case-16/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-16/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-17/inputs_true.dat b/tests/regression_tests/surface_source_write/case-17/inputs_true.dat
index 0a65db510..95d0a6712 100644
--- a/tests/regression_tests/surface_source_write/case-17/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-17/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-17/results_true.dat b/tests/regression_tests/surface_source_write/case-17/results_true.dat
index a97be70ca..cd0619c1f 100644
--- a/tests/regression_tests/surface_source_write/case-17/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-17/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.496403E+00 3.891283E-02
+1.403371E+00 1.456192E-02
diff --git a/tests/regression_tests/surface_source_write/case-17/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-17/surface_source_true.h5
index 35878eeca..436063aee 100644
Binary files a/tests/regression_tests/surface_source_write/case-17/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-17/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-18/inputs_true.dat b/tests/regression_tests/surface_source_write/case-18/inputs_true.dat
index 89df27b47..807c72ae6 100644
--- a/tests/regression_tests/surface_source_write/case-18/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-18/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-18/results_true.dat b/tests/regression_tests/surface_source_write/case-18/results_true.dat
index a97be70ca..cd0619c1f 100644
--- a/tests/regression_tests/surface_source_write/case-18/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-18/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.496403E+00 3.891283E-02
+1.403371E+00 1.456192E-02
diff --git a/tests/regression_tests/surface_source_write/case-18/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-18/surface_source_true.h5
index 36da9e7a7..436063aee 100644
Binary files a/tests/regression_tests/surface_source_write/case-18/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-18/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-19/inputs_true.dat b/tests/regression_tests/surface_source_write/case-19/inputs_true.dat
index 3ea7dbffe..42aa78a09 100644
--- a/tests/regression_tests/surface_source_write/case-19/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-19/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-19/results_true.dat b/tests/regression_tests/surface_source_write/case-19/results_true.dat
index a97be70ca..cd0619c1f 100644
--- a/tests/regression_tests/surface_source_write/case-19/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-19/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.496403E+00 3.891283E-02
+1.403371E+00 1.456192E-02
diff --git a/tests/regression_tests/surface_source_write/case-19/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-19/surface_source_true.h5
index f4261451d..eda3e030b 100644
Binary files a/tests/regression_tests/surface_source_write/case-19/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-19/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-20/inputs_true.dat b/tests/regression_tests/surface_source_write/case-20/inputs_true.dat
index c3c768a3d..4c7a3f11d 100644
--- a/tests/regression_tests/surface_source_write/case-20/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-20/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="4 -5 6 -7 8 -9" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="periodic" coeffs="-3.0" id="4" type="z-plane"/>
-    <surface boundary="periodic" coeffs="3.0" id="5" type="z-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="6" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="7" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="8" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="9" type="y-plane"/>
+    <cell id="3" fill="1" region="4 -5 6 -7 8 -9" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="z-plane" boundary="periodic" coeffs="-3.0"/>
+    <surface id="5" type="z-plane" boundary="periodic" coeffs="3.0"/>
+    <surface id="6" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="7" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="8" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="9" type="y-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-20/results_true.dat b/tests/regression_tests/surface_source_write/case-20/results_true.dat
index 7ccce7cc3..44293cf09 100644
--- a/tests/regression_tests/surface_source_write/case-20/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-20/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.149925E+00 2.542255E-01
+1.266853E+00 4.552028E-02
diff --git a/tests/regression_tests/surface_source_write/case-20/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-20/surface_source_true.h5
index fa8a4e628..083ce90c8 100644
Binary files a/tests/regression_tests/surface_source_write/case-20/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-20/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-21/inputs_true.dat b/tests/regression_tests/surface_source_write/case-21/inputs_true.dat
index 5dae8374d..71f9aae6a 100644
--- a/tests/regression_tests/surface_source_write/case-21/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-21/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="4 -5 6 -7 8 -9" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="periodic" coeffs="-3.0" id="4" type="z-plane"/>
-    <surface boundary="periodic" coeffs="3.0" id="5" type="z-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="6" type="x-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="7" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-3.0" id="8" type="y-plane"/>
-    <surface boundary="reflective" coeffs="3.0" id="9" type="y-plane"/>
+    <cell id="3" fill="1" region="4 -5 6 -7 8 -9" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="z-plane" boundary="periodic" coeffs="-3.0"/>
+    <surface id="5" type="z-plane" boundary="periodic" coeffs="3.0"/>
+    <surface id="6" type="x-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="7" type="x-plane" boundary="reflective" coeffs="3.0"/>
+    <surface id="8" type="y-plane" boundary="reflective" coeffs="-3.0"/>
+    <surface id="9" type="y-plane" boundary="reflective" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-21/results_true.dat b/tests/regression_tests/surface_source_write/case-21/results_true.dat
index 7ccce7cc3..44293cf09 100644
--- a/tests/regression_tests/surface_source_write/case-21/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-21/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.149925E+00 2.542255E-01
+1.266853E+00 4.552028E-02
diff --git a/tests/regression_tests/surface_source_write/case-21/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-21/surface_source_true.h5
index ed87ce849..f7e886db4 100644
Binary files a/tests/regression_tests/surface_source_write/case-21/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-21/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-a01/inputs_true.dat b/tests/regression_tests/surface_source_write/case-a01/inputs_true.dat
index 5cec1b76f..e9840be87 100644
--- a/tests/regression_tests/surface_source_write/case-a01/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-a01/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-a01/results_true.dat b/tests/regression_tests/surface_source_write/case-a01/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-a01/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-a01/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-a01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-a01/surface_source_true.h5
index 4a8623567..807211e3a 100644
Binary files a/tests/regression_tests/surface_source_write/case-a01/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-a01/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d01/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d01/inputs_true.dat
index b5452080c..04703acfe 100644
--- a/tests/regression_tests/surface_source_write/case-d01/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d01/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d01/results_true.dat b/tests/regression_tests/surface_source_write/case-d01/results_true.dat
index 7fb415cb1..26b9e30a3 100644
--- a/tests/regression_tests/surface_source_write/case-d01/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d01/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.263843E-01 3.193920E-02
+9.288719E-01 6.877101E-03
diff --git a/tests/regression_tests/surface_source_write/case-d01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d01/surface_source_true.h5
index 8f8cd604d..9a66315ff 100644
Binary files a/tests/regression_tests/surface_source_write/case-d01/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d01/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d02/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d02/inputs_true.dat
index d6ca4143c..d8b4e6803 100644
--- a/tests/regression_tests/surface_source_write/case-d02/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d02/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d02/results_true.dat b/tests/regression_tests/surface_source_write/case-d02/results_true.dat
index 7fb415cb1..26b9e30a3 100644
--- a/tests/regression_tests/surface_source_write/case-d02/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d02/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.263843E-01 3.193920E-02
+9.288719E-01 6.877101E-03
diff --git a/tests/regression_tests/surface_source_write/case-d02/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d02/surface_source_true.h5
index 3abc471e0..291062942 100644
Binary files a/tests/regression_tests/surface_source_write/case-d02/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d02/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d03/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d03/inputs_true.dat
index ba326c978..d769184aa 100644
--- a/tests/regression_tests/surface_source_write/case-d03/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d03/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d03/results_true.dat b/tests/regression_tests/surface_source_write/case-d03/results_true.dat
index 7fb415cb1..26b9e30a3 100644
--- a/tests/regression_tests/surface_source_write/case-d03/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d03/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.263843E-01 3.193920E-02
+9.288719E-01 6.877101E-03
diff --git a/tests/regression_tests/surface_source_write/case-d03/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d03/surface_source_true.h5
index 8435e8f17..0af7160d4 100644
Binary files a/tests/regression_tests/surface_source_write/case-d03/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d03/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d04/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d04/inputs_true.dat
index c309dd155..abefc2692 100644
--- a/tests/regression_tests/surface_source_write/case-d04/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d04/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d04/results_true.dat b/tests/regression_tests/surface_source_write/case-d04/results_true.dat
index 7fb415cb1..26b9e30a3 100644
--- a/tests/regression_tests/surface_source_write/case-d04/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d04/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.263843E-01 3.193920E-02
+9.288719E-01 6.877101E-03
diff --git a/tests/regression_tests/surface_source_write/case-d04/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d04/surface_source_true.h5
index 6c45d92e6..90ab00748 100644
Binary files a/tests/regression_tests/surface_source_write/case-d04/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d04/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d05/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d05/inputs_true.dat
index c49703f83..8b46b2b4e 100644
--- a/tests/regression_tests/surface_source_write/case-d05/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d05/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d05/results_true.dat b/tests/regression_tests/surface_source_write/case-d05/results_true.dat
index 7fb415cb1..26b9e30a3 100644
--- a/tests/regression_tests/surface_source_write/case-d05/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d05/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.263843E-01 3.193920E-02
+9.288719E-01 6.877101E-03
diff --git a/tests/regression_tests/surface_source_write/case-d05/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d05/surface_source_true.h5
index 24b04a862..2f5521679 100644
Binary files a/tests/regression_tests/surface_source_write/case-d05/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d05/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d06/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d06/inputs_true.dat
index 48db2c30d..ca52cf455 100644
--- a/tests/regression_tests/surface_source_write/case-d06/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d06/inputs_true.dat
@@ -1,26 +1,26 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d06/results_true.dat b/tests/regression_tests/surface_source_write/case-d06/results_true.dat
index 7fb415cb1..26b9e30a3 100644
--- a/tests/regression_tests/surface_source_write/case-d06/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d06/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.263843E-01 3.193920E-02
+9.288719E-01 6.877101E-03
diff --git a/tests/regression_tests/surface_source_write/case-d06/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d06/surface_source_true.h5
index 3eb95bef9..27227ad49 100644
Binary files a/tests/regression_tests/surface_source_write/case-d06/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d06/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d07/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d07/inputs_true.dat
index e2897125b..40c439fe6 100644
--- a/tests/regression_tests/surface_source_write/case-d07/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d07/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell fill="1" id="8" region="101 -102 103 -104 105 -106" universe="2"/>
+    <cell id="8" fill="1" region="101 -102 103 -104 105 -106" universe="2"/>
     <cell id="9" material="41" region="107 -108 109 -110 111 -112 (-101 | 102 | -103 | 104 | -105 | 106)" universe="2"/>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
-    <surface coeffs="-22.0" id="101" type="z-plane"/>
-    <surface coeffs="22.0" id="102" type="z-plane"/>
-    <surface coeffs="-22.0" id="103" type="x-plane"/>
-    <surface coeffs="22.0" id="104" type="x-plane"/>
-    <surface coeffs="-22.0" id="105" type="y-plane"/>
-    <surface coeffs="22.0" id="106" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-24.0" id="107" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="24.0" id="108" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-24.0" id="109" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="24.0" id="110" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-24.0" id="111" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="24.0" id="112" type="y-plane"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
+    <surface id="101" type="z-plane" coeffs="-22.0"/>
+    <surface id="102" type="z-plane" coeffs="22.0"/>
+    <surface id="103" type="x-plane" coeffs="-22.0"/>
+    <surface id="104" type="x-plane" coeffs="22.0"/>
+    <surface id="105" type="y-plane" coeffs="-22.0"/>
+    <surface id="106" type="y-plane" coeffs="22.0"/>
+    <surface id="107" type="z-plane" boundary="vacuum" coeffs="-24.0"/>
+    <surface id="108" type="z-plane" boundary="vacuum" coeffs="24.0"/>
+    <surface id="109" type="x-plane" boundary="vacuum" coeffs="-24.0"/>
+    <surface id="110" type="x-plane" boundary="vacuum" coeffs="24.0"/>
+    <surface id="111" type="y-plane" boundary="vacuum" coeffs="-24.0"/>
+    <surface id="112" type="y-plane" boundary="vacuum" coeffs="24.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d07/results_true.dat b/tests/regression_tests/surface_source_write/case-d07/results_true.dat
index 63ea1a64d..5a4ea6689 100644
--- a/tests/regression_tests/surface_source_write/case-d07/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d07/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.756086E-01 4.639209E-02
+9.947197E-01 3.711779E-02
diff --git a/tests/regression_tests/surface_source_write/case-d07/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d07/surface_source_true.h5
index 0332f2dcf..fd9f0dc57 100644
Binary files a/tests/regression_tests/surface_source_write/case-d07/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d07/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-d08/inputs_true.dat b/tests/regression_tests/surface_source_write/case-d08/inputs_true.dat
index 185b8629b..c81d21b97 100644
--- a/tests/regression_tests/surface_source_write/case-d08/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d08/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="40" name="no-void fuel">
-      <density units="g/cc" value="11"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="40" name="no-void fuel" depletable="true">
+      <density value="11" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="41" name="water">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
-    <cell fill="1" id="8" region="101 -102 103 -104 105 -106" universe="2"/>
+    <cell id="8" fill="1" region="101 -102 103 -104 105 -106" universe="2"/>
     <cell id="9" material="41" region="107 -108 109 -110 111 -112 (-101 | 102 | -103 | 104 | -105 | 106)" universe="2"/>
-    <dagmc_universe filename="../../dagmc/legacy/dagmc.h5m" id="1"/>
-    <surface coeffs="-22.0" id="101" type="z-plane"/>
-    <surface coeffs="22.0" id="102" type="z-plane"/>
-    <surface coeffs="-22.0" id="103" type="x-plane"/>
-    <surface coeffs="22.0" id="104" type="x-plane"/>
-    <surface coeffs="-22.0" id="105" type="y-plane"/>
-    <surface coeffs="22.0" id="106" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-24.0" id="107" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="24.0" id="108" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-24.0" id="109" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="24.0" id="110" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-24.0" id="111" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="24.0" id="112" type="y-plane"/>
+    <dagmc_universe id="1" filename="../../dagmc/legacy/dagmc.h5m"/>
+    <surface id="101" type="z-plane" coeffs="-22.0"/>
+    <surface id="102" type="z-plane" coeffs="22.0"/>
+    <surface id="103" type="x-plane" coeffs="-22.0"/>
+    <surface id="104" type="x-plane" coeffs="22.0"/>
+    <surface id="105" type="y-plane" coeffs="-22.0"/>
+    <surface id="106" type="y-plane" coeffs="22.0"/>
+    <surface id="107" type="z-plane" boundary="vacuum" coeffs="-24.0"/>
+    <surface id="108" type="z-plane" boundary="vacuum" coeffs="24.0"/>
+    <surface id="109" type="x-plane" boundary="vacuum" coeffs="-24.0"/>
+    <surface id="110" type="x-plane" boundary="vacuum" coeffs="24.0"/>
+    <surface id="111" type="y-plane" boundary="vacuum" coeffs="-24.0"/>
+    <surface id="112" type="y-plane" boundary="vacuum" coeffs="24.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-4 -4 -20 4 4 20</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-d08/results_true.dat b/tests/regression_tests/surface_source_write/case-d08/results_true.dat
index 63ea1a64d..5a4ea6689 100644
--- a/tests/regression_tests/surface_source_write/case-d08/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-d08/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.756086E-01 4.639209E-02
+9.947197E-01 3.711779E-02
diff --git a/tests/regression_tests/surface_source_write/case-d08/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-d08/surface_source_true.h5
index 252559204..fd9f0dc57 100644
Binary files a/tests/regression_tests/surface_source_write/case-d08/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-d08/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-e01/inputs_true.dat b/tests/regression_tests/surface_source_write/case-e01/inputs_true.dat
index ac3c03e37..46701aea7 100644
--- a/tests/regression_tests/surface_source_write/case-e01/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-e01/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-e01/results_true.dat b/tests/regression_tests/surface_source_write/case-e01/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-e01/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-e01/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-e01/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-e01/surface_source_true.h5
index 3047519e8..bbfbd152b 100644
Binary files a/tests/regression_tests/surface_source_write/case-e01/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-e01/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-e02/inputs_true.dat b/tests/regression_tests/surface_source_write/case-e02/inputs_true.dat
index a6107d16f..a4588c8d0 100644
--- a/tests/regression_tests/surface_source_write/case-e02/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-e02/inputs_true.dat
@@ -1,47 +1,47 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
     <cell id="4" material="2" region="-11 10 -13 12 -15 14 (5 | -4 | 7 | -6 | 9 | -8)" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface coeffs="-3.0" id="4" type="x-plane"/>
-    <surface coeffs="3.0" id="5" type="x-plane"/>
-    <surface coeffs="-3.0" id="6" type="y-plane"/>
-    <surface coeffs="3.0" id="7" type="y-plane"/>
-    <surface coeffs="-3.0" id="8" type="z-plane"/>
-    <surface coeffs="3.0" id="9" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="10" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="11" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="12" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="13" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-4.0" id="14" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="4.0" id="15" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" coeffs="3.0"/>
+    <surface id="6" type="y-plane" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" coeffs="3.0"/>
+    <surface id="8" type="z-plane" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" coeffs="3.0"/>
+    <surface id="10" type="x-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="11" type="x-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="12" type="y-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="13" type="y-plane" boundary="vacuum" coeffs="4.0"/>
+    <surface id="14" type="z-plane" boundary="vacuum" coeffs="-4.0"/>
+    <surface id="15" type="z-plane" boundary="vacuum" coeffs="4.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-e02/results_true.dat b/tests/regression_tests/surface_source_write/case-e02/results_true.dat
index 8979eb554..d4d1d1e5a 100644
--- a/tests/regression_tests/surface_source_write/case-e02/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-e02/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-5.169123E-02 9.144814E-03
+5.642735E-02 1.494035E-02
diff --git a/tests/regression_tests/surface_source_write/case-e02/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-e02/surface_source_true.h5
index 806152853..bbfbd152b 100644
Binary files a/tests/regression_tests/surface_source_write/case-e02/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-e02/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/case-e03/inputs_true.dat b/tests/regression_tests/surface_source_write/case-e03/inputs_true.dat
index 23bbf0455..2a93fdb4d 100644
--- a/tests/regression_tests/surface_source_write/case-e03/inputs_true.dat
+++ b/tests/regression_tests/surface_source_write/case-e03/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="11.0"/>
-      <nuclide ao="0.0004524" name="U234"/>
-      <nuclide ao="0.0506068" name="U235"/>
-      <nuclide ao="0.948709" name="U238"/>
-      <nuclide ao="0.0002318" name="U236"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="11.0" units="g/cm3"/>
+      <nuclide name="U234" ao="0.0004524"/>
+      <nuclide name="U235" ao="0.0506068"/>
+      <nuclide name="U238" ao="0.948709"/>
+      <nuclide name="U236" ao="0.0002318"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1 2 -3" universe="1"/>
     <cell id="2" material="2" region="1 | -2 | 3" universe="1"/>
-    <cell fill="1" id="3" region="-5 4 -7 6 -9 8" universe="2"/>
-    <surface coeffs="0.0 0.0 2.0" id="1" type="z-cylinder"/>
-    <surface coeffs="-2.0" id="2" type="z-plane"/>
-    <surface coeffs="2.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="5" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="6" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="7" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-3.0" id="8" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="3.0" id="9" type="z-plane"/>
+    <cell id="3" fill="1" region="-5 4 -7 6 -9 8" universe="2"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 2.0"/>
+    <surface id="2" type="z-plane" coeffs="-2.0"/>
+    <surface id="3" type="z-plane" coeffs="2.0"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="6" type="y-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="7" type="y-plane" boundary="vacuum" coeffs="3.0"/>
+    <surface id="8" type="z-plane" boundary="vacuum" coeffs="-3.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="3.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>5</batches>
     <inactive>1</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-2.0 -2.0 -2.0 2.0 2.0 2.0</parameters>
       </space>
diff --git a/tests/regression_tests/surface_source_write/case-e03/results_true.dat b/tests/regression_tests/surface_source_write/case-e03/results_true.dat
index d793a7e42..ad927bdf3 100644
--- a/tests/regression_tests/surface_source_write/case-e03/results_true.dat
+++ b/tests/regression_tests/surface_source_write/case-e03/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.752377E-02 6.773395E-03
+4.929000E-02 8.212396E-03
diff --git a/tests/regression_tests/surface_source_write/case-e03/surface_source_true.h5 b/tests/regression_tests/surface_source_write/case-e03/surface_source_true.h5
index becc9254a..22e108745 100644
Binary files a/tests/regression_tests/surface_source_write/case-e03/surface_source_true.h5 and b/tests/regression_tests/surface_source_write/case-e03/surface_source_true.h5 differ
diff --git a/tests/regression_tests/surface_source_write/test.py b/tests/regression_tests/surface_source_write/test.py
index f144eb82a..094df1f8b 100644
--- a/tests/regression_tests/surface_source_write/test.py
+++ b/tests/regression_tests/surface_source_write/test.py
@@ -634,7 +634,7 @@ def return_surface_source_data(filepath):
         wgt = point.wgt
         delayed_group = point.delayed_group
         surf_id = point.surf_id
-        particle = point.particle
+        particle = point.particle.pdg_number
         key = (
             f"{r[0]:.10e} {r[1]:.10e} {r[2]:.10e} {u[0]:.10e} {u[1]:.10e} {u[2]:.10e}"
             f"{e:.10e} {time:.10e} {wgt:.10e} {delayed_group} {surf_id} {particle}"
diff --git a/tests/regression_tests/surface_tally/inputs_true.dat b/tests/regression_tests/surface_tally/inputs_true.dat
index 192c2e89f..2b070c5a3 100644
--- a/tests/regression_tests/surface_tally/inputs_true.dat
+++ b/tests/regression_tests/surface_tally/inputs_true.dat
@@ -1,35 +1,35 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2 fuel at 2.4% wt enrichment">
-      <density units="g/cc" value="10.0"/>
-      <nuclide ao="1.0" name="U238"/>
-      <nuclide ao="0.02" name="U235"/>
-      <nuclide ao="2.0" name="O16"/>
+    <material id="1" name="UO2 fuel at 2.4% wt enrichment" depletable="true">
+      <density value="10.0" units="g/cc"/>
+      <nuclide name="U238" ao="1.0"/>
+      <nuclide name="U235" ao="0.02"/>
+      <nuclide name="O16" ao="2.0"/>
     </material>
     <material id="2" name="Borated water">
-      <density units="g/cm3" value="1"/>
-      <nuclide ao="0.0001" name="B10"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1" units="g/cm3"/>
+      <nuclide name="B10" ao="0.0001"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="-1" universe="1"/>
-    <cell id="2" material="2" name="water" region="1 -3 4 -5" universe="1"/>
-    <cell fill="1" id="3" name="root cell" region="2 -3 4 -5" universe="0"/>
-    <surface coeffs="0 0 1" id="1" name="Fuel OR" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="-2" id="2" name="left" type="x-plane"/>
-    <surface boundary="reflective" coeffs="2" id="3" name="right" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-2" id="4" name="bottom" type="y-plane"/>
-    <surface boundary="reflective" coeffs="2" id="5" name="top" type="y-plane"/>
+    <cell id="1" name="fuel" material="1" region="-1" universe="1"/>
+    <cell id="2" name="water" material="2" region="1 -3 4 -5" universe="1"/>
+    <cell id="3" name="root cell" fill="1" region="2 -3 4 -5" universe="0"/>
+    <surface id="1" name="Fuel OR" type="z-cylinder" coeffs="0 0 1"/>
+    <surface id="2" name="left" type="x-plane" boundary="vacuum" coeffs="-2"/>
+    <surface id="3" name="right" type="x-plane" boundary="reflective" coeffs="2"/>
+    <surface id="4" name="bottom" type="y-plane" boundary="reflective" coeffs="-2"/>
+    <surface id="5" name="top" type="y-plane" boundary="reflective" coeffs="2"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-0.62992 -0.62992 -1 0.62992 0.62992 1</parameters>
       </space>
diff --git a/tests/regression_tests/surface_tally/results_true.dat b/tests/regression_tests/surface_tally/results_true.dat
index 8b27fa192..e50102a88 100644
--- a/tests/regression_tests/surface_tally/results_true.dat
+++ b/tests/regression_tests/surface_tally/results_true.dat
@@ -1,52 +1,52 @@
 mean,std. dev.
-2.1200000e-02,1.8366636e-03
-7.4900000e-02,2.2083176e-03
-1.6220000e-01,2.8079253e-03
-6.4010000e-01,8.6838931e-03
-2.1000000e-03,3.7859389e-04
-5.6000000e-03,6.8637534e-04
-1.1700000e-02,1.4456832e-03
-4.1700000e-02,2.7041121e-03
-2.1200000e-02,1.8366636e-03
-7.4900000e-02,2.2083176e-03
-1.6220000e-01,2.8079253e-03
-6.4010000e-01,8.6838931e-03
-2.1000000e-03,3.7859389e-04
-5.6000000e-03,6.8637534e-04
-1.1700000e-02,1.4456832e-03
-4.1700000e-02,2.7041121e-03
-5.2000000e-03,5.9254629e-04
-3.9200000e-02,2.5508169e-03
-4.0200000e-02,1.9310331e-03
-4.1360000e-01,8.0072190e-03
-0.0000000e+00,0.0000000e+00
-1.9000000e-03,3.7859389e-04
-1.0000000e-04,1.0000000e-04
-1.6400000e-02,1.2840907e-03
--5.2000000e-03,5.9254629e-04
--3.9200000e-02,2.5508169e-03
--4.0200000e-02,1.9310331e-03
--4.1360000e-01,8.0072190e-03
-0.0000000e+00,0.0000000e+00
--1.9000000e-03,3.7859389e-04
+2.4100000e-02,2.1052844e-03
+6.7900000e-02,2.3211587e-03
+1.6860000e-01,4.4800794e-03
+6.4710000e-01,9.1826527e-03
+2.2000000e-03,3.8873013e-04
+5.5000000e-03,1.0979779e-03
+1.2500000e-02,1.5438048e-03
+4.4700000e-02,1.9035055e-03
+2.4100000e-02,2.1052844e-03
+6.7900000e-02,2.3211587e-03
+1.6860000e-01,4.4800794e-03
+6.4710000e-01,9.1826527e-03
+2.2000000e-03,3.8873013e-04
+5.5000000e-03,1.0979779e-03
+1.2500000e-02,1.5438048e-03
+4.4700000e-02,1.9035055e-03
+-7.3000000e-03,9.8938814e-04
+-3.6600000e-02,2.5086517e-03
+-4.1600000e-02,2.4864075e-03
+-4.2380000e-01,9.6087923e-03
 -1.0000000e-04,1.0000000e-04
--1.6400000e-02,1.2840907e-03
-1.6000000e-02,2.1602469e-03
-3.5700000e-02,2.9441090e-03
-1.2200000e-01,3.5932035e-03
-2.2650000e-01,9.2463206e-03
+-1.5000000e-03,4.5338235e-04
+-3.0000000e-04,1.5275252e-04
+-1.7300000e-02,1.4609738e-03
+-7.3000000e-03,9.8938814e-04
+-3.6600000e-02,2.5086517e-03
+-4.1600000e-02,2.4864075e-03
+-4.2380000e-01,9.6087923e-03
+-1.0000000e-04,1.0000000e-04
+-1.5000000e-03,4.5338235e-04
+-3.0000000e-04,1.5275252e-04
+-1.7300000e-02,1.4609738e-03
+1.6800000e-02,1.5902481e-03
+3.1300000e-02,3.8094911e-03
+1.2700000e-01,5.0990195e-03
+2.2330000e-01,9.3631073e-03
 2.1000000e-03,3.7859389e-04
-3.7000000e-03,8.1717671e-04
-1.1600000e-02,1.4772347e-03
-2.5300000e-02,1.9723083e-03
+4.0000000e-03,1.0540926e-03
+1.2200000e-02,1.6110728e-03
+2.7400000e-02,1.6613248e-03
 0.0000000e+00,0.0000000e+00
--2.9700000e-02,2.4587711e-03
+-3.2000000e-02,1.5634719e-03
 0.0000000e+00,0.0000000e+00
--3.5090000e-01,3.7161808e-03
+-3.5400000e-01,7.5938572e-03
 0.0000000e+00,0.0000000e+00
--2.1000000e-03,4.8189441e-04
+-3.0000000e-03,6.4978629e-04
 0.0000000e+00,0.0000000e+00
--2.1400000e-02,1.7397318e-03
+-2.1800000e-02,1.4892205e-03
 0.0000000e+00,0.0000000e+00
 0.0000000e+00,0.0000000e+00
 0.0000000e+00,0.0000000e+00
diff --git a/tests/regression_tests/surface_tally/test.py b/tests/regression_tests/surface_tally/test.py
index e496ac0f6..aa03a8aa2 100644
--- a/tests/regression_tests/surface_tally/test.py
+++ b/tests/regression_tests/surface_tally/test.py
@@ -169,7 +169,6 @@ class SurfaceTallyTestHarness(PyAPITestHarness):
         # Extract the relevant data as a CSV string.
         cols = ('mean', 'std. dev.')
         return df.to_csv(None, columns=cols, index=False, float_format='%.7e')
-        return outstr
 
 
 def test_surface_tally():
diff --git a/tests/regression_tests/survival_biasing/results_true.dat b/tests/regression_tests/survival_biasing/results_true.dat
index 740272201..932414e98 100644
--- a/tests/regression_tests/survival_biasing/results_true.dat
+++ b/tests/regression_tests/survival_biasing/results_true.dat
@@ -1,20 +1,20 @@
 k-combined:
-9.879232E-01 8.097582E-03
+9.517646E-01 1.303111E-02
 tally 1:
-4.295331E+01
-3.691096E+02
-1.802724E+01
-6.501934E+01
-2.193500E+00
-9.627609E-01
-1.893529E+00
-7.174104E-01
-4.902380E+00
-4.808570E+00
-3.418014E-02
-2.337353E-04
-3.667128E+08
-2.690758E+16
+4.164635E+01
+3.470110E+02
+1.724300E+01
+5.949580E+01
+2.124917E+00
+9.034789E-01
+1.844790E+00
+6.809026E-01
+4.784396E+00
+4.579552E+00
+3.348849E-02
+2.243613E-04
+3.573090E+08
+2.554338E+16
 tally 2:
-1.802724E+01
-6.501934E+01
+1.724300E+01
+5.949580E+01
diff --git a/tests/regression_tests/tallies/inputs_true.dat b/tests/regression_tests/tallies/inputs_true.dat
index 2ce36c5df..40829f865 100644
--- a/tests/regression_tests/tallies/inputs_true.dat
+++ b/tests/regression_tests/tallies/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UOX fuel">
-      <density units="g/cm3" value="10.062"/>
-      <nuclide ao="4.9476e-06" name="U234"/>
-      <nuclide ao="0.00048218" name="U235"/>
-      <nuclide ao="0.021504" name="U238"/>
-      <nuclide ao="1.0801e-08" name="Xe135"/>
-      <nuclide ao="0.045737" name="O16"/>
+    <material id="1" name="UOX fuel" depletable="true">
+      <density value="10.062" units="g/cm3"/>
+      <nuclide name="U234" ao="4.9476e-06"/>
+      <nuclide name="U235" ao="0.00048218"/>
+      <nuclide name="U238" ao="0.021504"/>
+      <nuclide name="Xe135" ao="1.0801e-08"/>
+      <nuclide name="O16" ao="0.045737"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="Cold borated water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="4" name="Hot borated water">
-      <density units="atom/b-cm" value="0.06614"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.06614" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="5" name="Reactor pressure vessel steel">
-      <density units="g/cm3" value="7.9"/>
+      <density value="7.9" units="g/cm3"/>
       <nuclide name="Fe54" wo="0.05437098"/>
       <nuclide name="Fe56" wo="0.88500663"/>
       <nuclide name="Fe57" wo="0.0208008"/>
@@ -47,7 +47,7 @@
       <nuclide name="Cu63" wo="0.0013696"/>
     </material>
     <material id="6" name="Lower radial reflector">
-      <density units="g/cm3" value="4.32"/>
+      <density value="4.32" units="g/cm3"/>
       <nuclide name="H1" wo="0.0095661"/>
       <nuclide name="O16" wo="0.0759107"/>
       <nuclide name="B10" wo="3.08409e-05"/>
@@ -62,7 +62,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="7" name="Upper radial reflector / Top plate region">
-      <density units="g/cm3" value="4.28"/>
+      <density value="4.28" units="g/cm3"/>
       <nuclide name="H1" wo="0.0086117"/>
       <nuclide name="O16" wo="0.0683369"/>
       <nuclide name="B10" wo="2.77638e-05"/>
@@ -77,7 +77,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="8" name="Bottom plate region">
-      <density units="g/cm3" value="7.184"/>
+      <density value="7.184" units="g/cm3"/>
       <nuclide name="H1" wo="0.0011505"/>
       <nuclide name="O16" wo="0.0091296"/>
       <nuclide name="B10" wo="3.70915e-06"/>
@@ -92,7 +92,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="9" name="Bottom nozzle region">
-      <density units="g/cm3" value="2.53"/>
+      <density value="2.53" units="g/cm3"/>
       <nuclide name="H1" wo="0.0245014"/>
       <nuclide name="O16" wo="0.1944274"/>
       <nuclide name="B10" wo="7.89917e-05"/>
@@ -107,7 +107,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="10" name="Top nozzle region">
-      <density units="g/cm3" value="1.746"/>
+      <density value="1.746" units="g/cm3"/>
       <nuclide name="H1" wo="0.035887"/>
       <nuclide name="O16" wo="0.2847761"/>
       <nuclide name="B10" wo="0.000115699"/>
@@ -122,7 +122,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="11" name="Top of fuel assemblies">
-      <density units="g/cm3" value="3.044"/>
+      <density value="3.044" units="g/cm3"/>
       <nuclide name="H1" wo="0.0162913"/>
       <nuclide name="O16" wo="0.1292776"/>
       <nuclide name="B10" wo="5.25228e-05"/>
@@ -135,7 +135,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="12" name="Bottom of fuel assemblies">
-      <density units="g/cm3" value="1.762"/>
+      <density value="1.762" units="g/cm3"/>
       <nuclide name="H1" wo="0.0292856"/>
       <nuclide name="O16" wo="0.2323919"/>
       <nuclide name="B10" wo="9.44159e-05"/>
@@ -149,8 +149,8 @@
     </material>
   </materials>
   <geometry>
-    <cell fill="200" id="1" region="-6 34 -35" universe="0"/>
-    <cell fill="201" id="2" region="-6 35 -36" universe="0"/>
+    <cell id="1" fill="200" region="-6 34 -35" universe="0"/>
+    <cell id="2" fill="201" region="-6 35 -36" universe="0"/>
     <cell id="3" material="8" region="-7 31 -32" universe="0"/>
     <cell id="4" material="9" region="-5 32 -33" universe="0"/>
     <cell id="5" material="12" region="-5 33 -34" universe="0"/>
@@ -174,9 +174,9 @@
     <cell id="31" material="2" region="3 -4" universe="4"/>
     <cell id="32" material="4" region="4" universe="4"/>
     <cell id="50" material="3" region="34 -35" universe="5"/>
-    <cell fill="100" id="60" region="34 -35" universe="6"/>
+    <cell id="60" fill="100" region="34 -35" universe="6"/>
     <cell id="70" material="4" region="35 -36" universe="7"/>
-    <cell fill="101" id="80" region="35 -36" universe="8"/>
+    <cell id="80" fill="101" region="35 -36" universe="8"/>
     <lattice id="100" name="Fuel assembly (lower half)">
       <pitch>1.26 1.26</pitch>
       <dimension>17 17</dimension>
@@ -277,30 +277,30 @@
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.41" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.475" id="2" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.56" id="3" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.62" id="4" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 187.6" id="5" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 209.0" id="6" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 229.0" id="7" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 249.0" id="8" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="-229.0" id="31" type="z-plane"/>
-    <surface coeffs="-199.0" id="32" type="z-plane"/>
-    <surface coeffs="-193.0" id="33" type="z-plane"/>
-    <surface coeffs="-183.0" id="34" type="z-plane"/>
-    <surface coeffs="0.0" id="35" type="z-plane"/>
-    <surface coeffs="183.0" id="36" type="z-plane"/>
-    <surface coeffs="203.0" id="37" type="z-plane"/>
-    <surface coeffs="215.0" id="38" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="223.0" id="39" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.41"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.475"/>
+    <surface id="3" type="z-cylinder" coeffs="0.0 0.0 0.56"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.62"/>
+    <surface id="5" type="z-cylinder" coeffs="0.0 0.0 187.6"/>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 209.0"/>
+    <surface id="7" type="z-cylinder" coeffs="0.0 0.0 229.0"/>
+    <surface id="8" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 249.0"/>
+    <surface id="31" type="z-plane" boundary="vacuum" coeffs="-229.0"/>
+    <surface id="32" type="z-plane" coeffs="-199.0"/>
+    <surface id="33" type="z-plane" coeffs="-193.0"/>
+    <surface id="34" type="z-plane" coeffs="-183.0"/>
+    <surface id="35" type="z-plane" coeffs="0.0"/>
+    <surface id="36" type="z-plane" coeffs="183.0"/>
+    <surface id="37" type="z-plane" coeffs="203.0"/>
+    <surface id="38" type="z-plane" coeffs="215.0"/>
+    <surface id="39" type="z-plane" boundary="vacuum" coeffs="223.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>400</particles>
     <batches>5</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-160 -160 -183 160 160 183</parameters>
       </space>
@@ -342,7 +342,7 @@
     <filter id="10" type="legendre">
       <order>4</order>
     </filter>
-    <filter cosine="particle" id="11" type="sphericalharmonics">
+    <filter id="11" type="sphericalharmonics" cosine="particle">
       <order>4</order>
     </filter>
     <filter id="12" type="universe">
diff --git a/tests/regression_tests/tallies/results_true.dat b/tests/regression_tests/tallies/results_true.dat
index 3dda0f9b2..1d3aca4b0 100644
--- a/tests/regression_tests/tallies/results_true.dat
+++ b/tests/regression_tests/tallies/results_true.dat
@@ -1 +1 @@
-ddfbb0a6f5498eb8ff33bb10beb64e244b015861919eb4837bd82855e5fd87c3ff97dfa382d3d60afa81c48d9f857fba8e0b99263e7b35587f8adb75be1fc0ec
\ No newline at end of file
+d01c3accd5b4de2aa166a77df28cfe42f5738a44c2480752fcfae7564a507362fff006b6dffb7b1dfe248e14bacef0070cabacea5d75c5996653e5605f7c7384
\ No newline at end of file
diff --git a/tests/regression_tests/tally_aggregation/inputs_true.dat b/tests/regression_tests/tally_aggregation/inputs_true.dat
index 9f4f95655..7351b230c 100644
--- a/tests/regression_tests/tally_aggregation/inputs_true.dat
+++ b/tests/regression_tests/tally_aggregation/inputs_true.dat
@@ -1,24 +1,24 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UO2">
-      <density units="g/cm3" value="10.29769"/>
-      <nuclide ao="4.4843e-06" name="U234"/>
-      <nuclide ao="0.00055815" name="U235"/>
-      <nuclide ao="0.022408" name="U238"/>
-      <nuclide ao="0.045829" name="O16"/>
+    <material id="1" name="UO2" depletable="true">
+      <density value="10.29769" units="g/cm3"/>
+      <nuclide name="U234" ao="4.4843e-06"/>
+      <nuclide name="U235" ao="0.00055815"/>
+      <nuclide name="U238" ao="0.022408"/>
+      <nuclide name="O16" ao="0.045829"/>
     </material>
     <material id="2" name="light water">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="2" region="1" universe="1"/>
-    <cell fill="2" id="3" region="2 -3 4 -5" universe="3"/>
+    <cell id="3" fill="2" region="2 -3 4 -5" universe="3"/>
     <lattice id="2">
       <pitch>1.2 1.2</pitch>
       <outer>1</outer>
@@ -28,11 +28,11 @@
 1 1 
 1 1 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.4" id="1" type="z-cylinder"/>
-    <surface boundary="reflective" coeffs="-1.2" id="2" name="minimum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="1.2" id="3" name="maximum x" type="x-plane"/>
-    <surface boundary="reflective" coeffs="-1.2" id="4" name="minimum y" type="y-plane"/>
-    <surface boundary="reflective" coeffs="1.2" id="5" name="maximum y" type="y-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.4"/>
+    <surface id="2" name="minimum x" type="x-plane" boundary="reflective" coeffs="-1.2"/>
+    <surface id="3" name="maximum x" type="x-plane" boundary="reflective" coeffs="1.2"/>
+    <surface id="4" name="minimum y" type="y-plane" boundary="reflective" coeffs="-1.2"/>
+    <surface id="5" name="maximum y" type="y-plane" boundary="reflective" coeffs="1.2"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/tally_aggregation/results_true.dat b/tests/regression_tests/tally_aggregation/results_true.dat
index 172adbfe1..ee6263373 100644
--- a/tests/regression_tests/tally_aggregation/results_true.dat
+++ b/tests/regression_tests/tally_aggregation/results_true.dat
@@ -1,97 +1,97 @@
-[[1.6001087e-05 5.4190949e-04]
- [3.2669968e-01 1.7730523e-01]
- [1.8149266e-02 7.1525113e-01]], [[1.6239097e-05 5.7499676e-04]
- [3.1268633e-01 1.7004165e-01]
- [1.8873778e-02 7.0217885e-01]], [[1.6693071e-05 5.4106379e-04]
- [3.3370208e-01 1.8051505e-01]
- [1.9081306e-02 7.3647547e-01]], [[1.6725399e-05 5.1680146e-04]
- [3.2854628e-01 1.7757185e-01]
- [1.9529646e-02 7.1005198e-01]][[2.4751834e-07 4.3304565e-05]
- [8.6840718e-03 4.3442535e-03]
- [3.7054051e-04 6.5678133e-03]], [[4.0830852e-07 4.9612204e-05]
- [1.2104241e-02 5.9708675e-03]
- [7.1398189e-04 8.2408482e-03]], [[2.6546344e-07 2.5234256e-05]
- [6.5083211e-03 3.2185071e-03]
- [4.2935150e-04 5.1707774e-03]], [[2.7845203e-07 3.4453402e-05]
- [3.3125427e-03 1.7749508e-03]
- [6.0407731e-04 7.5353872e-03]][[1.0455251e-06 8.1938339e-04]
- [1.1392765e+00 5.6434761e-01]
- [1.4142831e-06 5.1213654e-01]], [[2.2009815e-06 1.0418935e-03]
- [1.4422554e-01 1.0070647e-01]
- [3.9488382e-05 7.9236390e-01]], [[1.4606612e-05 2.2374470e-04]
- [1.2962842e-02 2.9268594e-02]
- [3.1339824e-04 1.1056439e+00]], [[4.7805535e-05 8.9749934e-05]
- [5.1694597e-03 1.1111105e-02]
- [7.5279695e-02 4.5381305e-01]][[1.4955183e-08 1.1461548e-05]
- [1.6454649e-02 8.1236707e-03]
- [2.0028007e-08 6.8264819e-03]], [[1.6334486e-07 7.7617340e-05]
- [2.1161584e-03 1.4078445e-03]
- [1.4742097e-05 6.9350862e-03]], [[1.7444757e-07 1.9026095e-06]
- [1.4084092e-04 2.0345990e-04]
- [5.9546175e-06 8.6038812e-03]], [[5.6449127e-07 1.0110585e-06]
- [5.9158873e-05 1.2485325e-04]
- [1.0936497e-03 5.0836702e-03]][[0.2866239 0.2725595]], [[0.2729156 0.258831 ]], [[0.2920724 0.2755922]], [[0.287667  0.2703207]], [[0.035617  0.2232707]], [[0.0353082 0.2224423]], [[0.0370072 0.2288263]], [[0.0363348 0.219573 ]], [[0.0033013 0.2850034]], [[0.0032726 0.2771099]], [[0.0034234 0.2951295]], [[0.0032935 0.2778935]], [[0.0193227 0.1122647]], [[0.0200799 0.1144123]], [[0.0202971 0.1179836]], [[0.0207973 0.1203534]][[0.0086351 0.0061876]], [[0.0120688 0.0074831]], [[0.0064221 0.0035246]], [[0.0030483 0.0024267]], [[0.0009185 0.0033896]], [[0.0009223 0.0039584]], [[0.0010541 0.0038599]], [[0.0012934 0.0028331]], [[6.5126963e-05 2.8486593e-03]], [[7.1110242e-05 4.3424340e-03]], [[5.8911409e-05 2.4296178e-03]], [[8.4278765e-05 6.4182190e-03]], [[0.0003712 0.0020298]], [[0.0007152 0.0036115]], [[0.0004298 0.0019677]], [[0.0006046 0.0021965]][[2.0694650e-04]
- [4.2868191e-01]
- [1.3029457e-01]], [[1.9694048e-04]
- [4.0809445e-01]
- [1.2345525e-01]], [[2.1012645e-04]
- [4.3670571e-01]
- [1.3074883e-01]], [[2.0641548e-04]
- [4.3014207e-01]
- [1.2763930e-01]], [[0.0002584]
- [0.0608867]
- [0.1977426]], [[0.0003026]
- [0.0602368]
- [0.1972111]], [[0.0002509]
- [0.0624699]
- [0.2031126]], [[0.0002322]
- [0.0613385]
- [0.1943371]], [[5.9349736e-05]
- [1.0506741e-02]
- [2.7773865e-01]], [[5.7855564e-05]
- [1.0389781e-02]
- [2.6993483e-01]], [[6.1819700e-05]
- [1.0910874e-02]
- [2.8758020e-01]], [[5.9326317e-05]
- [1.0424040e-02]
- [2.7070367e-01]], [[3.3192167e-05]
- [3.9295320e-03]
- [1.2762462e-01]], [[3.3882691e-05]
- [4.0069157e-03]
- [1.3045143e-01]], [[3.4882103e-05]
- [4.1306182e-03]
- [1.3411514e-01]], [[3.5598508e-05]
- [4.2134987e-03]
- [1.3690156e-01]][[6.4505980e-06]
- [9.6369034e-03]
- [4.4700460e-03]], [[8.2880772e-06]
- [1.3456171e-02]
- [4.5368708e-03]], [[3.9384462e-06]
- [7.1570638e-03]
- [1.5629142e-03]], [[2.3565894e-06]
- [3.4040401e-03]
- [1.8956916e-03]], [[4.2806047e-05]
- [1.1841015e-03]
- [3.3058802e-03]], [[4.8905696e-05]
- [1.0360139e-03]
- [3.9298937e-03]], [[2.4911390e-05]
- [1.2173305e-03]
- [3.8114473e-03]], [[3.4347869e-05]
- [1.5820360e-03]
- [2.6824612e-03]], [[1.0809230e-06]
- [1.0316180e-04]
- [2.8475354e-03]], [[6.4150814e-07]
- [1.1179425e-04]
- [4.3415770e-03]], [[7.3848139e-07]
- [9.3111666e-05]
- [2.4285475e-03]], [[1.2349384e-06]
- [1.7152843e-04]
- [6.4164799e-03]], [[4.5871493e-07]
- [5.4732075e-05]
- [2.0627309e-03]], [[8.1649994e-07]
- [9.7699188e-05]
- [3.6803256e-03]], [[4.5174850e-07]
- [5.3913784e-05]
- [2.0133571e-03]], [[5.0962865e-07]
- [6.0338032e-05]
- [2.2773911e-03]]
\ No newline at end of file
+[[1.6242805e-05 6.2367673e-04]
+ [3.2895972e-01 1.7786452e-01]
+ [1.8044266e-02 7.0451122e-01]], [[1.6113947e-05 5.3572864e-04]
+ [3.1517504e-01 1.7132833e-01]
+ [1.8305682e-02 7.0072832e-01]], [[1.6472052e-05 5.5758006e-04]
+ [3.2362364e-01 1.7597163e-01]
+ [1.9107080e-02 7.2600440e-01]], [[1.6693277e-05 4.9204218e-04]
+ [3.2429262e-01 1.7600573e-01]
+ [1.9042489e-02 7.3053854e-01]][[2.9719061e-07 8.0925438e-05]
+ [8.9432259e-03 4.4057583e-03]
+ [5.2078407e-04 6.4387688e-03]], [[1.8572081e-07 2.5667235e-05]
+ [1.1641603e-02 5.7444311e-03]
+ [2.6983611e-04 6.9783224e-03]], [[2.4994113e-07 5.4993390e-05]
+ [6.0464041e-03 3.0764811e-03]
+ [2.6559118e-04 7.7279780e-03]], [[2.5965232e-07 3.9618775e-05]
+ [1.0409564e-02 5.1850104e-03]
+ [2.7831231e-04 7.4480576e-03]][[1.0329435e-06 8.0861229e-04]
+ [1.1265142e+00 5.5796079e-01]
+ [1.3965860e-06 5.0338828e-01]], [[2.2264172e-06 1.0828687e-03]
+ [1.4724979e-01 1.0226107e-01]
+ [2.2656045e-05 7.7618945e-01]], [[1.4789782e-05 2.2786630e-04]
+ [1.3149968e-02 2.9847169e-02]
+ [3.2638111e-04 1.1287326e+00]], [[4.7472938e-05 8.9680338e-05]
+ [5.1371041e-03 1.1101179e-02]
+ [7.4149084e-02 4.5347217e-01]][[1.7024771e-08 1.2878244e-05]
+ [1.8795566e-02 9.2705008e-03]
+ [2.2676749e-08 7.0878353e-03]], [[2.2018925e-07 1.0785388e-04]
+ [2.6715019e-03 1.6657928e-03]
+ [1.2479083e-05 8.6814018e-03]], [[2.0928485e-07 1.6600875e-06]
+ [1.1859535e-04 1.7969083e-04]
+ [1.5981387e-05 8.0489308e-03]], [[4.0016381e-07 7.6799157e-07]
+ [4.4157583e-05 9.4462925e-05]
+ [7.0115110e-04 3.8678806e-03]][[0.287433  0.2698042]], [[0.2751228 0.2598525]], [[0.280516  0.2648274]], [[0.2834448 0.2676736]], [[0.0370558 0.2148509]], [[0.0355217 0.2130189]], [[0.0385317 0.2292804]], [[0.0361655 0.2223832]], [[0.0033268 0.2855078]], [[0.0033498 0.2849403]], [[0.003359  0.2901157]], [[0.0034555 0.2982438]], [[0.0192046 0.1128364]], [[0.0195026 0.1147807]], [[0.0203405 0.1183102]], [[0.0202861 0.1187358]][[0.00889   0.0051934]], [[0.0115249 0.0072767]], [[0.0058841 0.0040289]], [[0.0103341 0.0063266]], [[0.0009734 0.0048865]], [[0.0016425 0.0038133]], [[0.0013902 0.0048875]], [[0.0012487 0.0039808]], [[1.8994140e-05 2.0410145e-03]], [[6.8277082e-05 3.4865782e-03]], [[6.1643568e-05 4.9220817e-03]], [[7.4147413e-05 4.9263295e-03]], [[0.0005213 0.0024208]], [[0.0002702 0.0014313]], [[0.0002665 0.0022006]], [[0.0002788 0.0014892]][[2.0601081e-04]
+ [4.2976789e-01]
+ [1.2726338e-01]], [[1.9802070e-04]
+ [4.1138025e-01]
+ [1.2339705e-01]], [[2.0174854e-04]
+ [4.1947542e-01]
+ [1.2566615e-01]], [[2.0386518e-04]
+ [4.2385140e-01]
+ [1.2706310e-01]], [[0.0003403]
+ [0.0624955]
+ [0.189071 ]], [[0.0002601]
+ [0.060449 ]
+ [0.1878314]], [[0.0002765]
+ [0.0652412]
+ [0.2022944]], [[0.0002082]
+ [0.0613252]
+ [0.1970153]], [[6.0336874e-05]
+ [1.0617692e-02]
+ [2.7815664e-01]], [[5.9823997e-05]
+ [1.0664059e-02]
+ [2.7756620e-01]], [[6.0873689e-05]
+ [1.0744209e-02]
+ [2.8266963e-01]], [[6.1621521e-05]
+ [1.0971177e-02]
+ [2.9066650e-01]], [[3.3297391e-05]
+ [3.9432037e-03]
+ [1.2806450e-01]], [[3.3863464e-05]
+ [4.0100180e-03]
+ [1.3023932e-01]], [[3.4930617e-05]
+ [4.1344613e-03]
+ [1.3448129e-01]], [[3.5061805e-05]
+ [4.1505999e-03]
+ [1.3483614e-01]][[5.8259835e-06]
+ [9.9096571e-03]
+ [2.7933778e-03]], [[8.0858909e-06]
+ [1.2843377e-02]
+ [4.5631248e-03]], [[4.3418585e-06]
+ [6.5638646e-03]
+ [2.7874612e-03]], [[6.9046376e-06]
+ [1.1531588e-02]
+ [3.7205377e-03]], [[8.0710599e-05]
+ [1.0889301e-03]
+ [4.8613465e-03]], [[2.4337121e-05]
+ [1.8861117e-03]
+ [3.6987509e-03]], [[5.4812439e-05]
+ [1.7085353e-03]
+ [4.7852139e-03]], [[3.9007791e-05]
+ [1.4998363e-03]
+ [3.8929646e-03]], [[7.5757577e-07]
+ [2.5845755e-05]
+ [2.0409391e-03]], [[1.0302346e-06]
+ [1.1907557e-04]
+ [3.4852130e-03]], [[9.1936614e-07]
+ [1.2974020e-04]
+ [4.9207575e-03]], [[5.6493555e-07]
+ [1.2113685e-04]
+ [4.9253980e-03]], [[5.4602210e-07]
+ [6.5233025e-05]
+ [2.4754550e-03]], [[3.2091909e-07]
+ [3.8472906e-05]
+ [1.4560974e-03]], [[4.8475651e-07]
+ [5.9108262e-05]
+ [2.2158781e-03]], [[3.3737822e-07]
+ [4.0544269e-05]
+ [1.5145628e-03]]
\ No newline at end of file
diff --git a/tests/regression_tests/tally_arithmetic/inputs_true.dat b/tests/regression_tests/tally_arithmetic/inputs_true.dat
index 7669b4d03..2b5234c34 100644
--- a/tests/regression_tests/tally_arithmetic/inputs_true.dat
+++ b/tests/regression_tests/tally_arithmetic/inputs_true.dat
@@ -1,24 +1,24 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.0"/>
-      <nuclide ao="1.0" name="U234"/>
-      <nuclide ao="4.0" name="U235"/>
-      <nuclide ao="95.0" name="U238"/>
+    <material id="1" depletable="true">
+      <density value="10.0" units="g/cm3"/>
+      <nuclide name="U234" ao="1.0"/>
+      <nuclide name="U235" ao="4.0"/>
+      <nuclide name="U238" ao="95.0"/>
     </material>
     <material id="2" name="light water">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
       <sab name="c_H_in_H2O"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
     <cell id="2" material="2" region="1 -2" universe="1"/>
-    <surface coeffs="0.0 0.0 5.0" id="1" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 10.0" id="2" type="z-cylinder"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 5.0"/>
+    <surface id="2" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/tally_arithmetic/results_true.dat b/tests/regression_tests/tally_arithmetic/results_true.dat
index 143930dd4..baa380587 100644
--- a/tests/regression_tests/tally_arithmetic/results_true.dat
+++ b/tests/regression_tests/tally_arithmetic/results_true.dat
@@ -1,49 +1,49 @@
-[2.04229e-07 1.28343e-13 1.79333e-04 1.12698e-10 1.89952e-07 1.51210e-07
- 1.66796e-04 1.32777e-04 6.13215e-03 3.85362e-09 3.14746e-03 1.97795e-09
- 5.70345e-03 4.54021e-03 2.92742e-03 2.33037e-03 2.10575e-07 1.28419e-13
- 1.84905e-04 1.12764e-10 1.89188e-07 1.50221e-07 1.66125e-04 1.31908e-04
- 6.32269e-03 3.85587e-09 3.24526e-03 1.97911e-09 5.68054e-03 4.51051e-03
- 2.91566e-03 2.31512e-03 1.96696e-07 1.26388e-13 1.72718e-04 1.10981e-10
- 1.91283e-07 1.53449e-07 1.67964e-04 1.34743e-04 5.90596e-03 3.79491e-09
- 3.03136e-03 1.94782e-09 5.74342e-03 4.60742e-03 2.94794e-03 2.36486e-03
- 2.06016e-07 1.35264e-13 1.80901e-04 1.18775e-10 2.05873e-07 1.65814e-07
- 1.80776e-04 1.45600e-04 6.18579e-03 4.06142e-09 3.17499e-03 2.08461e-09
- 6.18150e-03 4.97869e-03 3.17279e-03 2.55542e-03 2.36089e-03 4.46651e-05
- 1.02263e-02 1.93469e-04 1.18179e-04 2.62375e-05 5.11897e-04 1.13649e-04
- 2.56040e-02 4.84394e-04 4.55595e-02 8.61927e-04 1.28165e-03 2.84546e-04
- 2.28056e-03 5.06320e-04 2.29877e-03 4.53056e-05 9.95724e-03 1.96244e-04
- 1.14803e-04 2.57898e-05 4.97276e-04 1.11710e-04 2.49302e-02 4.91340e-04
- 4.43606e-02 8.74289e-04 1.24504e-03 2.79691e-04 2.21542e-03 4.97680e-04
- 2.31940e-03 4.34238e-05 1.00466e-02 1.88093e-04 1.17620e-04 2.69280e-05
- 5.09479e-04 1.16640e-04 2.51540e-02 4.70932e-04 4.47588e-02 8.37974e-04
- 1.27560e-03 2.92034e-04 2.26979e-03 5.19644e-04 2.19212e-03 4.53914e-05
- 9.49530e-03 1.96615e-04 1.09609e-04 2.41615e-05 4.74778e-04 1.04657e-04
- 2.37736e-02 4.92271e-04 4.23026e-02 8.75944e-04 1.18871e-03 2.62032e-04
- 2.11519e-03 4.66257e-04][2.04229e-07 1.28343e-13 1.79333e-04 1.12698e-10 1.89952e-07 1.51210e-07
- 1.66796e-04 1.32777e-04 6.13215e-03 3.85362e-09 3.14746e-03 1.97795e-09
- 5.70345e-03 4.54021e-03 2.92742e-03 2.33037e-03 2.10575e-07 1.28419e-13
- 1.84905e-04 1.12764e-10 1.89188e-07 1.50221e-07 1.66125e-04 1.31908e-04
- 6.32269e-03 3.85587e-09 3.24526e-03 1.97911e-09 5.68054e-03 4.51051e-03
- 2.91566e-03 2.31512e-03 1.96696e-07 1.26388e-13 1.72718e-04 1.10981e-10
- 1.91283e-07 1.53449e-07 1.67964e-04 1.34743e-04 5.90596e-03 3.79491e-09
- 3.03136e-03 1.94782e-09 5.74342e-03 4.60742e-03 2.94794e-03 2.36486e-03
- 2.06016e-07 1.35264e-13 1.80901e-04 1.18775e-10 2.05873e-07 1.65814e-07
- 1.80776e-04 1.45600e-04 6.18579e-03 4.06142e-09 3.17499e-03 2.08461e-09
- 6.18150e-03 4.97869e-03 3.17279e-03 2.55542e-03 2.36089e-03 4.46651e-05
- 1.02263e-02 1.93469e-04 1.18179e-04 2.62375e-05 5.11897e-04 1.13649e-04
- 2.56040e-02 4.84394e-04 4.55595e-02 8.61927e-04 1.28165e-03 2.84546e-04
- 2.28056e-03 5.06320e-04 2.29877e-03 4.53056e-05 9.95724e-03 1.96244e-04
- 1.14803e-04 2.57898e-05 4.97276e-04 1.11710e-04 2.49302e-02 4.91340e-04
- 4.43606e-02 8.74289e-04 1.24504e-03 2.79691e-04 2.21542e-03 4.97680e-04
- 2.31940e-03 4.34238e-05 1.00466e-02 1.88093e-04 1.17620e-04 2.69280e-05
- 5.09479e-04 1.16640e-04 2.51540e-02 4.70932e-04 4.47588e-02 8.37974e-04
- 1.27560e-03 2.92034e-04 2.26979e-03 5.19644e-04 2.19212e-03 4.53914e-05
- 9.49530e-03 1.96615e-04 1.09609e-04 2.41615e-05 4.74778e-04 1.04657e-04
- 2.37736e-02 4.92271e-04 4.23026e-02 8.75944e-04 1.18871e-03 2.62032e-04
- 2.11519e-03 4.66257e-04][0.0057  0.00454 0.00293 0.00233 0.00568 0.00451 0.00292 0.00232 0.00574
- 0.00461 0.00295 0.00236 0.00618 0.00498 0.00317 0.00256 0.00128 0.00028
- 0.00228 0.00051 0.00125 0.00028 0.00222 0.0005  0.00128 0.00029 0.00227
- 0.00052 0.00119 0.00026 0.00212 0.00047][0.00018 0.00017 0.00315 0.00293 0.00018 0.00017 0.00325 0.00292 0.00017
- 0.00017 0.00303 0.00295 0.00018 0.00018 0.00317 0.00317 0.01023 0.00051
- 0.04556 0.00228 0.00996 0.0005  0.04436 0.00222 0.01005 0.00051 0.04476
- 0.00227 0.0095  0.00047 0.0423  0.00212][0.00293 0.00292 0.00295 0.00317 0.00228 0.00222 0.00227 0.00212]
\ No newline at end of file
+[2.29467e-07 1.47622e-13 2.02646e-04 1.30367e-10 2.20704e-07 1.76624e-07
+ 1.94907e-04 1.55980e-04 6.32267e-03 4.06754e-09 3.26873e-03 2.10286e-09
+ 6.08121e-03 4.86666e-03 3.14390e-03 2.51599e-03 1.89223e-07 1.06256e-13
+ 1.67106e-04 9.38364e-11 1.53300e-07 1.20220e-07 1.35382e-04 1.06168e-04
+ 5.21380e-03 2.92775e-09 2.69546e-03 1.51361e-09 4.22399e-03 3.31252e-03
+ 2.18374e-03 1.71253e-03 2.21146e-07 1.41349e-13 1.95297e-04 1.24828e-10
+ 2.11844e-07 1.69398e-07 1.87083e-04 1.49598e-04 6.09339e-03 3.89470e-09
+ 3.15020e-03 2.01351e-09 5.83710e-03 4.66754e-03 3.01770e-03 2.41305e-03
+ 1.92872e-07 1.05908e-13 1.70328e-04 9.35293e-11 1.59466e-07 1.25399e-07
+ 1.40827e-04 1.10741e-04 5.31433e-03 2.91817e-09 2.74744e-03 1.50865e-09
+ 4.39388e-03 3.45520e-03 2.27158e-03 1.78629e-03 2.44601e-03 4.71663e-05
+ 1.09430e-02 2.11013e-04 1.23573e-04 2.76181e-05 5.52841e-04 1.23558e-04
+ 2.71755e-02 5.24023e-04 4.76200e-02 9.18253e-04 1.37291e-03 3.06841e-04
+ 2.40577e-03 5.37681e-04 2.29014e-03 4.44674e-05 1.02456e-02 1.98938e-04
+ 1.12524e-04 2.48126e-05 5.03411e-04 1.11007e-04 2.54438e-02 4.94038e-04
+ 4.45855e-02 8.65710e-04 1.25016e-03 2.75671e-04 2.19067e-03 4.83061e-04
+ 2.38500e-03 4.58053e-05 1.06700e-02 2.04924e-04 1.22899e-04 2.95267e-05
+ 5.49826e-04 1.32097e-04 2.64977e-02 5.08903e-04 4.64323e-02 8.91758e-04
+ 1.36542e-03 3.28045e-04 2.39265e-03 5.74839e-04 2.17094e-03 4.21772e-05
+ 9.71236e-03 1.88693e-04 1.09358e-04 2.46984e-05 4.89245e-04 1.10496e-04
+ 2.41194e-02 4.68594e-04 4.22648e-02 8.21124e-04 1.21498e-03 2.74402e-04
+ 2.12902e-03 4.80839e-04][2.29467e-07 1.47622e-13 2.02646e-04 1.30367e-10 2.20704e-07 1.76624e-07
+ 1.94907e-04 1.55980e-04 6.32267e-03 4.06754e-09 3.26873e-03 2.10286e-09
+ 6.08121e-03 4.86666e-03 3.14390e-03 2.51599e-03 1.89223e-07 1.06256e-13
+ 1.67106e-04 9.38364e-11 1.53300e-07 1.20220e-07 1.35382e-04 1.06168e-04
+ 5.21380e-03 2.92775e-09 2.69546e-03 1.51361e-09 4.22399e-03 3.31252e-03
+ 2.18374e-03 1.71253e-03 2.21146e-07 1.41349e-13 1.95297e-04 1.24828e-10
+ 2.11844e-07 1.69398e-07 1.87083e-04 1.49598e-04 6.09339e-03 3.89470e-09
+ 3.15020e-03 2.01351e-09 5.83710e-03 4.66754e-03 3.01770e-03 2.41305e-03
+ 1.92872e-07 1.05908e-13 1.70328e-04 9.35293e-11 1.59466e-07 1.25399e-07
+ 1.40827e-04 1.10741e-04 5.31433e-03 2.91817e-09 2.74744e-03 1.50865e-09
+ 4.39388e-03 3.45520e-03 2.27158e-03 1.78629e-03 2.44601e-03 4.71663e-05
+ 1.09430e-02 2.11013e-04 1.23573e-04 2.76181e-05 5.52841e-04 1.23558e-04
+ 2.71755e-02 5.24023e-04 4.76200e-02 9.18253e-04 1.37291e-03 3.06841e-04
+ 2.40577e-03 5.37681e-04 2.29014e-03 4.44674e-05 1.02456e-02 1.98938e-04
+ 1.12524e-04 2.48126e-05 5.03411e-04 1.11007e-04 2.54438e-02 4.94038e-04
+ 4.45855e-02 8.65710e-04 1.25016e-03 2.75671e-04 2.19067e-03 4.83061e-04
+ 2.38500e-03 4.58053e-05 1.06700e-02 2.04924e-04 1.22899e-04 2.95267e-05
+ 5.49826e-04 1.32097e-04 2.64977e-02 5.08903e-04 4.64323e-02 8.91758e-04
+ 1.36542e-03 3.28045e-04 2.39265e-03 5.74839e-04 2.17094e-03 4.21772e-05
+ 9.71236e-03 1.88693e-04 1.09358e-04 2.46984e-05 4.89245e-04 1.10496e-04
+ 2.41194e-02 4.68594e-04 4.22648e-02 8.21124e-04 1.21498e-03 2.74402e-04
+ 2.12902e-03 4.80839e-04][0.00608 0.00487 0.00314 0.00252 0.00422 0.00331 0.00218 0.00171 0.00584
+ 0.00467 0.00302 0.00241 0.00439 0.00346 0.00227 0.00179 0.00137 0.00031
+ 0.00241 0.00054 0.00125 0.00028 0.00219 0.00048 0.00137 0.00033 0.00239
+ 0.00057 0.00121 0.00027 0.00213 0.00048][0.0002  0.00019 0.00327 0.00314 0.00017 0.00014 0.0027  0.00218 0.0002
+ 0.00019 0.00315 0.00302 0.00017 0.00014 0.00275 0.00227 0.01094 0.00055
+ 0.04762 0.00241 0.01025 0.0005  0.04459 0.00219 0.01067 0.00055 0.04643
+ 0.00239 0.00971 0.00049 0.04226 0.00213][0.00314 0.00218 0.00302 0.00227 0.00241 0.00219 0.00239 0.00213]
\ No newline at end of file
diff --git a/tests/regression_tests/tally_assumesep/results_true.dat b/tests/regression_tests/tally_assumesep/results_true.dat
index 9d7cbf9c3..c9ccf0928 100644
--- a/tests/regression_tests/tally_assumesep/results_true.dat
+++ b/tests/regression_tests/tally_assumesep/results_true.dat
@@ -1,11 +1,11 @@
 k-combined:
-5.683578E-01 1.129170E-02
+6.268465E-01 1.154810E-02
 tally 1:
-6.753950E+00
-9.188305E+00
+7.828708E+00
+1.230478E+01
 tally 2:
-2.278558E-01
-1.052944E-02
+2.582239E-01
+1.360117E-02
 tally 3:
-1.179091E+01
-2.795539E+01
+1.339335E+01
+3.663458E+01
diff --git a/tests/regression_tests/tally_nuclides/results_true.dat b/tests/regression_tests/tally_nuclides/results_true.dat
index 7f9641fc8..93a0e03fb 100644
--- a/tests/regression_tests/tally_nuclides/results_true.dat
+++ b/tests/regression_tests/tally_nuclides/results_true.dat
@@ -1,28 +1,28 @@
 k-combined:
-1.132463E+00 5.721067E-02
+9.732610E-01 1.400780E-02
 tally 1:
-7.310528E+00
-1.080411E+01
-1.664215E+00
-5.567756E-01
-1.609340E+00
-5.205634E-01
-5.646313E+00
-6.459724E+00
-7.310528E+00
-1.080411E+01
-1.664215E+00
-5.567756E-01
-1.609340E+00
-5.205634E-01
-5.646313E+00
-6.459724E+00
+7.123025E+00
+1.021136E+01
+1.619905E+00
+5.258592E-01
+1.561322E+00
+4.881841E-01
+5.503120E+00
+6.105683E+00
+7.123025E+00
+1.021136E+01
+1.619905E+00
+5.258592E-01
+1.561322E+00
+4.881841E-01
+5.503120E+00
+6.105683E+00
 tally 2:
-7.310528E+00
-1.080411E+01
-1.664215E+00
-5.567756E-01
-1.609340E+00
-5.205634E-01
-5.646313E+00
-6.459724E+00
+7.123025E+00
+1.021136E+01
+1.619905E+00
+5.258592E-01
+1.561322E+00
+4.881841E-01
+5.503120E+00
+6.105683E+00
diff --git a/tests/regression_tests/tally_slice_merge/inputs_true.dat b/tests/regression_tests/tally_slice_merge/inputs_true.dat
index 7159d833a..7a992cc39 100644
--- a/tests/regression_tests/tally_slice_merge/inputs_true.dat
+++ b/tests/regression_tests/tally_slice_merge/inputs_true.dat
@@ -1,40 +1,40 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="UOX fuel">
-      <density units="g/cm3" value="10.062"/>
-      <nuclide ao="4.9476e-06" name="U234"/>
-      <nuclide ao="0.00048218" name="U235"/>
-      <nuclide ao="0.021504" name="U238"/>
-      <nuclide ao="1.0801e-08" name="Xe135"/>
-      <nuclide ao="0.045737" name="O16"/>
+    <material id="1" name="UOX fuel" depletable="true">
+      <density value="10.062" units="g/cm3"/>
+      <nuclide name="U234" ao="4.9476e-06"/>
+      <nuclide name="U235" ao="0.00048218"/>
+      <nuclide name="U238" ao="0.021504"/>
+      <nuclide name="Xe135" ao="1.0801e-08"/>
+      <nuclide name="O16" ao="0.045737"/>
     </material>
     <material id="2" name="Zircaloy">
-      <density units="g/cm3" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cm3"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="Cold borated water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="4" name="Hot borated water">
-      <density units="atom/b-cm" value="0.06614"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.000649" name="B10"/>
-      <nuclide ao="0.002689" name="B11"/>
+      <density value="0.06614" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.000649"/>
+      <nuclide name="B11" ao="0.002689"/>
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="5" name="Reactor pressure vessel steel">
-      <density units="g/cm3" value="7.9"/>
+      <density value="7.9" units="g/cm3"/>
       <nuclide name="Fe54" wo="0.05437098"/>
       <nuclide name="Fe56" wo="0.88500663"/>
       <nuclide name="Fe57" wo="0.0208008"/>
@@ -47,7 +47,7 @@
       <nuclide name="Cu63" wo="0.0013696"/>
     </material>
     <material id="6" name="Lower radial reflector">
-      <density units="g/cm3" value="4.32"/>
+      <density value="4.32" units="g/cm3"/>
       <nuclide name="H1" wo="0.0095661"/>
       <nuclide name="O16" wo="0.0759107"/>
       <nuclide name="B10" wo="3.08409e-05"/>
@@ -62,7 +62,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="7" name="Upper radial reflector / Top plate region">
-      <density units="g/cm3" value="4.28"/>
+      <density value="4.28" units="g/cm3"/>
       <nuclide name="H1" wo="0.0086117"/>
       <nuclide name="O16" wo="0.0683369"/>
       <nuclide name="B10" wo="2.77638e-05"/>
@@ -77,7 +77,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="8" name="Bottom plate region">
-      <density units="g/cm3" value="7.184"/>
+      <density value="7.184" units="g/cm3"/>
       <nuclide name="H1" wo="0.0011505"/>
       <nuclide name="O16" wo="0.0091296"/>
       <nuclide name="B10" wo="3.70915e-06"/>
@@ -92,7 +92,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="9" name="Bottom nozzle region">
-      <density units="g/cm3" value="2.53"/>
+      <density value="2.53" units="g/cm3"/>
       <nuclide name="H1" wo="0.0245014"/>
       <nuclide name="O16" wo="0.1944274"/>
       <nuclide name="B10" wo="7.89917e-05"/>
@@ -107,7 +107,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="10" name="Top nozzle region">
-      <density units="g/cm3" value="1.746"/>
+      <density value="1.746" units="g/cm3"/>
       <nuclide name="H1" wo="0.035887"/>
       <nuclide name="O16" wo="0.2847761"/>
       <nuclide name="B10" wo="0.000115699"/>
@@ -122,7 +122,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="11" name="Top of fuel assemblies">
-      <density units="g/cm3" value="3.044"/>
+      <density value="3.044" units="g/cm3"/>
       <nuclide name="H1" wo="0.0162913"/>
       <nuclide name="O16" wo="0.1292776"/>
       <nuclide name="B10" wo="5.25228e-05"/>
@@ -135,7 +135,7 @@
       <sab name="c_H_in_H2O"/>
     </material>
     <material id="12" name="Bottom of fuel assemblies">
-      <density units="g/cm3" value="1.762"/>
+      <density value="1.762" units="g/cm3"/>
       <nuclide name="H1" wo="0.0292856"/>
       <nuclide name="O16" wo="0.2323919"/>
       <nuclide name="B10" wo="9.44159e-05"/>
@@ -149,8 +149,8 @@
     </material>
   </materials>
   <geometry>
-    <cell fill="200" id="1" region="-6 34 -35" universe="0"/>
-    <cell fill="201" id="2" region="-6 35 -36" universe="0"/>
+    <cell id="1" fill="200" region="-6 34 -35" universe="0"/>
+    <cell id="2" fill="201" region="-6 35 -36" universe="0"/>
     <cell id="3" material="8" region="-7 31 -32" universe="0"/>
     <cell id="4" material="9" region="-5 32 -33" universe="0"/>
     <cell id="5" material="12" region="-5 33 -34" universe="0"/>
@@ -174,9 +174,9 @@
     <cell id="31" material="2" region="3 -4" universe="4"/>
     <cell id="32" material="4" region="4" universe="4"/>
     <cell id="50" material="3" region="34 -35" universe="5"/>
-    <cell fill="100" id="60" region="34 -35" universe="6"/>
+    <cell id="60" fill="100" region="34 -35" universe="6"/>
     <cell id="70" material="4" region="35 -36" universe="7"/>
-    <cell fill="101" id="80" region="35 -36" universe="8"/>
+    <cell id="80" fill="101" region="35 -36" universe="8"/>
     <lattice id="100" name="Fuel assembly (lower half)">
       <pitch>1.26 1.26</pitch>
       <dimension>17 17</dimension>
@@ -277,30 +277,30 @@
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 
 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 </universes>
     </lattice>
-    <surface coeffs="0.0 0.0 0.41" id="1" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.475" id="2" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.56" id="3" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 0.62" id="4" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 187.6" id="5" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 209.0" id="6" type="z-cylinder"/>
-    <surface coeffs="0.0 0.0 229.0" id="7" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 249.0" id="8" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="-229.0" id="31" type="z-plane"/>
-    <surface coeffs="-199.0" id="32" type="z-plane"/>
-    <surface coeffs="-193.0" id="33" type="z-plane"/>
-    <surface coeffs="-183.0" id="34" type="z-plane"/>
-    <surface coeffs="0.0" id="35" type="z-plane"/>
-    <surface coeffs="183.0" id="36" type="z-plane"/>
-    <surface coeffs="203.0" id="37" type="z-plane"/>
-    <surface coeffs="215.0" id="38" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="223.0" id="39" type="z-plane"/>
+    <surface id="1" type="z-cylinder" coeffs="0.0 0.0 0.41"/>
+    <surface id="2" type="z-cylinder" coeffs="0.0 0.0 0.475"/>
+    <surface id="3" type="z-cylinder" coeffs="0.0 0.0 0.56"/>
+    <surface id="4" type="z-cylinder" coeffs="0.0 0.0 0.62"/>
+    <surface id="5" type="z-cylinder" coeffs="0.0 0.0 187.6"/>
+    <surface id="6" type="z-cylinder" coeffs="0.0 0.0 209.0"/>
+    <surface id="7" type="z-cylinder" coeffs="0.0 0.0 229.0"/>
+    <surface id="8" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 249.0"/>
+    <surface id="31" type="z-plane" boundary="vacuum" coeffs="-229.0"/>
+    <surface id="32" type="z-plane" coeffs="-199.0"/>
+    <surface id="33" type="z-plane" coeffs="-193.0"/>
+    <surface id="34" type="z-plane" coeffs="-183.0"/>
+    <surface id="35" type="z-plane" coeffs="0.0"/>
+    <surface id="36" type="z-plane" coeffs="183.0"/>
+    <surface id="37" type="z-plane" coeffs="203.0"/>
+    <surface id="38" type="z-plane" coeffs="215.0"/>
+    <surface id="39" type="z-plane" boundary="vacuum" coeffs="223.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="box">
         <parameters>-160 -160 -183 160 160 183</parameters>
       </space>
diff --git a/tests/regression_tests/tally_slice_merge/results_true.dat b/tests/regression_tests/tally_slice_merge/results_true.dat
index 58b15fc18..4b2cbdf1a 100644
--- a/tests/regression_tests/tally_slice_merge/results_true.dat
+++ b/tests/regression_tests/tally_slice_merge/results_true.dat
@@ -1,45 +1,45 @@
    cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    21         0.00e+00          6.25e-01    U235  fission 1.75e-01   1.20e-02   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    21         0.00e+00          6.25e-01    U235  nu-fission 4.26e-01   2.92e-02   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    21         0.00e+00          6.25e-01    U238  fission 2.41e-07   1.61e-08   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    21         0.00e+00          6.25e-01    U238  nu-fission 6.00e-07   4.01e-08   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    21         6.25e-01          2.00e+07    U235  fission 2.89e-02   2.07e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    21         6.25e-01          2.00e+07    U235  nu-fission 7.07e-02   5.06e-03   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    21         6.25e-01          2.00e+07    U238  fission 1.41e-02   5.76e-04   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    21         6.25e-01          2.00e+07    U238  nu-fission 3.95e-02   1.90e-03   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    27         0.00e+00          6.25e-01    U235  fission 1.51e-01   1.17e-02   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    27         0.00e+00          6.25e-01    U235  nu-fission 3.69e-01   2.86e-02   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    27         0.00e+00          6.25e-01    U238  fission 2.07e-07   1.47e-08   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    27         0.00e+00          6.25e-01    U238  nu-fission 5.17e-07   3.66e-08   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    27         6.25e-01          2.00e+07    U235  fission 2.57e-02   2.43e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    27         6.25e-01          2.00e+07    U235  nu-fission 6.30e-02   5.92e-03   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
-0    27         6.25e-01          2.00e+07    U238  fission 1.47e-02   1.41e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0    27         6.25e-01          2.00e+07    U238  nu-fission 4.12e-02   4.02e-03    cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0     21         0.00e+00          6.25e-01    U235     fission 1.75e-01   1.20e-02
-1     21         0.00e+00          6.25e-01    U235  nu-fission 4.26e-01   2.92e-02
-2     21         0.00e+00          6.25e-01    U238     fission 2.41e-07   1.61e-08
-3     21         0.00e+00          6.25e-01    U238  nu-fission 6.00e-07   4.01e-08
-4     21         6.25e-01          2.00e+07    U235     fission 2.89e-02   2.07e-03
-5     21         6.25e-01          2.00e+07    U235  nu-fission 7.07e-02   5.06e-03
-6     21         6.25e-01          2.00e+07    U238     fission 1.41e-02   5.76e-04
-7     21         6.25e-01          2.00e+07    U238  nu-fission 3.95e-02   1.90e-03
-8     27         0.00e+00          6.25e-01    U235     fission 1.51e-01   1.17e-02
-9     27         0.00e+00          6.25e-01    U235  nu-fission 3.69e-01   2.86e-02
-10    27         0.00e+00          6.25e-01    U238     fission 2.07e-07   1.47e-08
-11    27         0.00e+00          6.25e-01    U238  nu-fission 5.17e-07   3.66e-08
-12    27         6.25e-01          2.00e+07    U235     fission 2.57e-02   2.43e-03
-13    27         6.25e-01          2.00e+07    U235  nu-fission 6.30e-02   5.92e-03
-14    27         6.25e-01          2.00e+07    U238     fission 1.47e-02   1.41e-03
-15    27         6.25e-01          2.00e+07    U238  nu-fission 4.12e-02   4.02e-03
+0    21         0.00e+00          6.25e-01    U235  fission 1.93e-01   1.92e-02   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    21         0.00e+00          6.25e-01    U235  nu-fission 4.70e-01   4.67e-02   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    21         0.00e+00          6.25e-01    U238  fission 2.65e-07   2.60e-08   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    21         0.00e+00          6.25e-01    U238  nu-fission 6.60e-07   6.47e-08   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    21         6.25e-01          2.00e+07    U235  fission 3.39e-02   1.53e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    21         6.25e-01          2.00e+07    U235  nu-fission 8.30e-02   3.75e-03   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    21         6.25e-01          2.00e+07    U238  fission 1.70e-02   2.01e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    21         6.25e-01          2.00e+07    U238  nu-fission 4.76e-02   6.12e-03   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    27         0.00e+00          6.25e-01    U235  fission 7.61e-02   5.84e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    27         0.00e+00          6.25e-01    U235  nu-fission 1.85e-01   1.42e-02   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    27         0.00e+00          6.25e-01    U238  fission 1.06e-07   7.62e-09   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    27         0.00e+00          6.25e-01    U238  nu-fission 2.64e-07   1.90e-08   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    27         6.25e-01          2.00e+07    U235  fission 1.69e-02   1.34e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    27         6.25e-01          2.00e+07    U235  nu-fission 4.15e-02   3.29e-03   cell  energy low [eV]  energy high [eV] nuclide    score     mean  std. dev.
+0    27         6.25e-01          2.00e+07    U238  fission 1.10e-02   1.69e-03   cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0    27         6.25e-01          2.00e+07    U238  nu-fission 3.13e-02   5.35e-03    cell  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
+0     21         0.00e+00          6.25e-01    U235     fission 1.93e-01   1.92e-02
+1     21         0.00e+00          6.25e-01    U235  nu-fission 4.70e-01   4.67e-02
+2     21         0.00e+00          6.25e-01    U238     fission 2.65e-07   2.60e-08
+3     21         0.00e+00          6.25e-01    U238  nu-fission 6.60e-07   6.47e-08
+4     21         6.25e-01          2.00e+07    U235     fission 3.39e-02   1.53e-03
+5     21         6.25e-01          2.00e+07    U235  nu-fission 8.30e-02   3.75e-03
+6     21         6.25e-01          2.00e+07    U238     fission 1.70e-02   2.01e-03
+7     21         6.25e-01          2.00e+07    U238  nu-fission 4.76e-02   6.12e-03
+8     27         0.00e+00          6.25e-01    U235     fission 7.61e-02   5.84e-03
+9     27         0.00e+00          6.25e-01    U235  nu-fission 1.85e-01   1.42e-02
+10    27         0.00e+00          6.25e-01    U238     fission 1.06e-07   7.62e-09
+11    27         0.00e+00          6.25e-01    U238  nu-fission 2.64e-07   1.90e-08
+12    27         6.25e-01          2.00e+07    U235     fission 1.69e-02   1.34e-03
+13    27         6.25e-01          2.00e+07    U235  nu-fission 4.15e-02   3.29e-03
+14    27         6.25e-01          2.00e+07    U238     fission 1.10e-02   1.69e-03
+15    27         6.25e-01          2.00e+07    U238  nu-fission 3.13e-02   5.35e-03
          sum(distribcell)  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
 0   (0, 100, 2000, 30000)         0.00e+00          6.25e-01    U235     fission 0.00e+00   0.00e+00
 1   (0, 100, 2000, 30000)         0.00e+00          6.25e-01    U235  nu-fission 0.00e+00   0.00e+00
 2   (0, 100, 2000, 30000)         0.00e+00          6.25e-01    U238     fission 0.00e+00   0.00e+00
 3   (0, 100, 2000, 30000)         0.00e+00          6.25e-01    U238  nu-fission 0.00e+00   0.00e+00
-4   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U235     fission 9.53e-07   9.53e-07
-5   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U235  nu-fission 2.37e-06   2.37e-06
-6   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U238     fission 1.25e-08   1.25e-08
-7   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U238  nu-fission 3.15e-08   3.15e-08
+4   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U235     fission 0.00e+00   0.00e+00
+5   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U235  nu-fission 0.00e+00   0.00e+00
+6   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U238     fission 0.00e+00   0.00e+00
+7   (0, 100, 2000, 30000)         6.25e-01          2.00e+07    U238  nu-fission 0.00e+00   0.00e+00
 8      (500, 5000, 50000)         0.00e+00          6.25e-01    U235     fission 0.00e+00   0.00e+00
 9      (500, 5000, 50000)         0.00e+00          6.25e-01    U235  nu-fission 0.00e+00   0.00e+00
 10     (500, 5000, 50000)         0.00e+00          6.25e-01    U238     fission 0.00e+00   0.00e+00
@@ -49,19 +49,19 @@
 14     (500, 5000, 50000)         6.25e-01          2.00e+07    U238     fission 0.00e+00   0.00e+00
 15     (500, 5000, 50000)         6.25e-01          2.00e+07    U238  nu-fission 0.00e+00   0.00e+00
            sum(mesh)  energy low [eV]  energy high [eV] nuclide       score     mean  std. dev.
-0   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U235     fission 6.73e-03   3.04e-03
-1   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U235  nu-fission 1.64e-02   7.40e-03
-2   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U238     fission 8.80e-09   3.84e-09
-3   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U238  nu-fission 2.19e-08   9.56e-09
-4   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U235     fission 9.89e-04   3.53e-04
-5   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U235  nu-fission 2.42e-03   8.60e-04
-6   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U238     fission 5.01e-04   2.13e-04
-7   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U238  nu-fission 1.36e-03   5.86e-04
-8   ((2, 1), (2, 2))         0.00e+00          6.25e-01    U235     fission 1.63e-02   4.21e-03
-9   ((2, 1), (2, 2))         0.00e+00          6.25e-01    U235  nu-fission 3.98e-02   1.02e-02
-10  ((2, 1), (2, 2))         0.00e+00          6.25e-01    U238     fission 2.28e-08   5.90e-09
-11  ((2, 1), (2, 2))         0.00e+00          6.25e-01    U238  nu-fission 5.67e-08   1.47e-08
-12  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U235     fission 1.79e-03   4.31e-04
-13  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U235  nu-fission 4.37e-03   1.05e-03
-14  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U238     fission 7.51e-04   2.51e-04
-15  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U238  nu-fission 2.02e-03   6.71e-04
+0   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U235     fission 1.94e-03   1.03e-03
+1   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U235  nu-fission 4.74e-03   2.50e-03
+2   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U238     fission 2.74e-09   1.42e-09
+3   ((1, 1), (1, 2))         0.00e+00          6.25e-01    U238  nu-fission 6.83e-09   3.53e-09
+4   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U235     fission 9.02e-04   3.69e-04
+5   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U235  nu-fission 2.24e-03   9.12e-04
+6   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U238     fission 1.44e-03   8.42e-04
+7   ((1, 1), (1, 2))         6.25e-01          2.00e+07    U238  nu-fission 4.37e-03   2.64e-03
+8   ((2, 1), (2, 2))         0.00e+00          6.25e-01    U235     fission 1.27e-02   2.76e-03
+9   ((2, 1), (2, 2))         0.00e+00          6.25e-01    U235  nu-fission 3.09e-02   6.72e-03
+10  ((2, 1), (2, 2))         0.00e+00          6.25e-01    U238     fission 1.70e-08   3.57e-09
+11  ((2, 1), (2, 2))         0.00e+00          6.25e-01    U238  nu-fission 4.25e-08   8.90e-09
+12  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U235     fission 1.43e-03   1.69e-04
+13  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U235  nu-fission 3.52e-03   4.20e-04
+14  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U238     fission 1.37e-03   2.98e-04
+15  ((2, 1), (2, 2))         6.25e-01          2.00e+07    U238  nu-fission 4.16e-03   1.08e-03
diff --git a/tests/regression_tests/time_cutoff/inputs_true.dat b/tests/regression_tests/time_cutoff/inputs_true.dat
index 7d3e94f50..e1102d475 100644
--- a/tests/regression_tests/time_cutoff/inputs_true.dat
+++ b/tests/regression_tests/time_cutoff/inputs_true.dat
@@ -4,13 +4,13 @@
   </materials>
   <geometry>
     <cell id="1" material="void" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 200" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 200"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>100</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
diff --git a/tests/regression_tests/time_cutoff/results_true.dat b/tests/regression_tests/time_cutoff/results_true.dat
index 1cafceb77..d3d5e1b3c 100644
--- a/tests/regression_tests/time_cutoff/results_true.dat
+++ b/tests/regression_tests/time_cutoff/results_true.dat
@@ -1,5 +1,5 @@
 tally 1:
-2.000000E+03
-4.000000E+05
+1.383148E+02
+1.913099E+03
 0.000000E+00
 0.000000E+00
diff --git a/tests/regression_tests/torus/inputs_true.dat b/tests/regression_tests/torus/inputs_true.dat
index a5c7a8d7a..df4af1443 100644
--- a/tests/regression_tests/torus/inputs_true.dat
+++ b/tests/regression_tests/torus/inputs_true.dat
@@ -1,13 +1,13 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="12.0"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="12.0" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="H1"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="H1" ao="1.0"/>
     </material>
   </materials>
   <geometry>
@@ -15,15 +15,15 @@
     <cell id="2" material="1" region="-3" universe="1"/>
     <cell id="3" material="1" region="-1" universe="1"/>
     <cell id="4" material="2" region="-5 4 -7 6 -9 8 2 3 1" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 3 1.5 1" id="1" type="z-torus"/>
-    <surface coeffs="6 0.0 0.0 3 1.5 1" id="2" type="x-torus"/>
-    <surface coeffs="6 0.0 0.0 6 1 0.75" id="3" type="y-torus"/>
-    <surface boundary="vacuum" coeffs="-5" id="4" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="14" id="5" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-5" id="6" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="5" id="7" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-8" id="8" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="8" id="9" type="z-plane"/>
+    <surface id="1" type="z-torus" coeffs="0.0 0.0 0.0 3 1.5 1"/>
+    <surface id="2" type="x-torus" coeffs="6 0.0 0.0 3 1.5 1"/>
+    <surface id="3" type="y-torus" coeffs="6 0.0 0.0 6 1 0.75"/>
+    <surface id="4" type="x-plane" boundary="vacuum" coeffs="-5"/>
+    <surface id="5" type="x-plane" boundary="vacuum" coeffs="14"/>
+    <surface id="6" type="y-plane" boundary="vacuum" coeffs="-5"/>
+    <surface id="7" type="y-plane" boundary="vacuum" coeffs="5"/>
+    <surface id="8" type="z-plane" boundary="vacuum" coeffs="-8"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="8"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
diff --git a/tests/regression_tests/torus/large_major/inputs_true.dat b/tests/regression_tests/torus/large_major/inputs_true.dat
index fa7deb29a..513a8c67e 100644
--- a/tests/regression_tests/torus/large_major/inputs_true.dat
+++ b/tests/regression_tests/torus/large_major/inputs_true.dat
@@ -2,27 +2,27 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="W184"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="W184" ao="1.0"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="5.0"/>
-      <nuclide ao="1.0" name="Fe56"/>
+      <density value="5.0" units="g/cm3"/>
+      <nuclide name="Fe56" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="void" region="-1" universe="1"/>
     <cell id="2" material="1" region="1 -2" universe="1"/>
     <cell id="3" material="2" region="2 -3" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 1000.0 30.0 30.0" id="1" type="z-torus"/>
-    <surface coeffs="0.0 0.0 0.0 1000.0 35.0 35.0" id="2" type="z-torus"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 1000.0 40.0 40.0" id="3" type="z-torus"/>
+    <surface id="1" type="z-torus" coeffs="0.0 0.0 0.0 1000.0 30.0 30.0"/>
+    <surface id="2" type="z-torus" coeffs="0.0 0.0 0.0 1000.0 35.0 35.0"/>
+    <surface id="3" type="z-torus" boundary="vacuum" coeffs="0.0 0.0 0.0 1000.0 40.0 40.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>-1000.0 0 0</parameters>
       </space>
diff --git a/tests/regression_tests/torus/results_true.dat b/tests/regression_tests/torus/results_true.dat
index 42fb209cc..84cd3c7a4 100644
--- a/tests/regression_tests/torus/results_true.dat
+++ b/tests/regression_tests/torus/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-7.667201E-01 1.136882E-02
+7.666453E-01 1.478848E-02
diff --git a/tests/regression_tests/trace/results_true.dat b/tests/regression_tests/trace/results_true.dat
index bbf03de94..97b997ae6 100644
--- a/tests/regression_tests/trace/results_true.dat
+++ b/tests/regression_tests/trace/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
diff --git a/tests/regression_tests/track_output/results_true.dat b/tests/regression_tests/track_output/results_true.dat
index 148b54a30..205ac3eec 100644
--- a/tests/regression_tests/track_output/results_true.dat
+++ b/tests/regression_tests/track_output/results_true.dat
@@ -143,76 +143,74 @@ neutron [((-9.469716e-01, -2.580266e-01, 1.414357e-01), (1.438873e-01, 2.365819e
  ((-3.920090e+00, -9.211794e-01, 2.908406e+00), (9.094673e-01, -3.659267e-01, -1.974002e-01), 9.547085e+00, 1.260840e-07, 1.000000e+00, 23, 2387, 1)
  ((-3.729948e+00, -9.976834e-01, 2.867135e+00), (-2.916959e-01, -6.494657e-01, -7.022164e-01), 8.590591e+00, 1.750036e-07, 1.000000e+00, 23, 2387, 1)
  ((-3.744933e+00, -1.031047e+00, 2.831062e+00), (-2.916959e-01, -6.494657e-01, -7.022164e-01), 8.590591e+00, 1.876753e-07, 0.000000e+00, 23, 2387, 1)]
-neutron [((6.474155e+00, -5.192870e+00, 5.413003e+00), (-9.112559e-01, 3.950037e-01, 1.165536e-01), 2.052226e+06, 4.450859e-05, 1.000000e+00, 21, 2367, 2)
- ((6.037250e+00, -5.003484e+00, 5.468885e+00), (-9.112559e-01, 3.950037e-01, 1.165536e-01), 2.052226e+06, 4.450883e-05, 1.000000e+00, 22, 2367, 3)
- ((5.942573e+00, -4.962444e+00, 5.480995e+00), (-9.112559e-01, 3.950037e-01, 1.165536e-01), 2.052226e+06, 4.450889e-05, 1.000000e+00, 23, 2367, 1)
- ((5.861800e+00, -4.927431e+00, 5.491326e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.450893e-05, 1.000000e+00, 23, 2367, 1)
- ((5.725160e+00, -4.971424e+00, 5.491002e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.450903e-05, 1.000000e+00, 23, 2366, 1)
- ((5.494150e+00, -5.045802e+00, 5.490454e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.450919e-05, 1.000000e+00, 22, 2366, 3)
- ((5.392865e+00, -5.078412e+00, 5.490213e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.450927e-05, 1.000000e+00, 21, 2366, 2)
- ((4.612759e+00, -5.329579e+00, 5.488362e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.450982e-05, 1.000000e+00, 22, 2366, 3)
- ((4.511474e+00, -5.362189e+00, 5.488121e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.450989e-05, 1.000000e+00, 23, 2366, 1)
- ((4.089400e+00, -5.498082e+00, 5.487119e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.451019e-05, 1.000000e+00, 23, 2365, 1)
- ((3.384113e+00, -5.725160e+00, 5.485445e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.451069e-05, 1.000000e+00, 23, 2351, 1)
- ((2.453640e+00, -6.024740e+00, 5.483237e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.451136e-05, 1.000000e+00, 23, 2350, 1)
- ((2.086813e+00, -6.142846e+00, 5.482366e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.451162e-05, 1.000000e+00, 22, 2350, 3)
- ((1.993594e+00, -6.172859e+00, 5.482145e+00), (-9.518772e-01, -3.064714e-01, -2.259335e-03), 1.141417e+06, 4.451168e-05, 1.000000e+00, 21, 2350, 2)
- ((1.325704e+00, -6.387896e+00, 5.480560e+00), (8.986086e-01, -4.380574e-01, -2.466265e-02), 9.775839e+05, 4.451216e-05, 1.000000e+00, 21, 2350, 2)
- ((2.061403e+00, -6.746538e+00, 5.460368e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451276e-05, 1.000000e+00, 21, 2350, 2)
- ((1.122794e+00, -6.498940e+00, 4.743664e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451375e-05, 1.000000e+00, 22, 2350, 3)
- ((1.036483e+00, -6.476172e+00, 4.677758e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451384e-05, 1.000000e+00, 23, 2350, 1)
- ((8.178800e-01, -6.418507e+00, 4.510837e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451407e-05, 1.000000e+00, 23, 2349, 1)
- ((5.725264e-01, -6.353784e+00, 4.323490e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451432e-05, 1.000000e+00, 22, 2349, 3)
- ((4.668297e-01, -6.325902e+00, 4.242782e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451444e-05, 1.000000e+00, 21, 2349, 2)
- ((-2.989816e-01, -6.123888e+00, 3.658023e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451524e-05, 1.000000e+00, 22, 2349, 3)
- ((-4.046782e-01, -6.096006e+00, 3.577315e+00), (-7.778764e-01, 2.051975e-01, -5.939716e-01), 7.813219e+05, 4.451535e-05, 1.000000e+00, 23, 2349, 1)
- ((-8.040445e-01, -5.990656e+00, 3.272367e+00), (-7.989001e-01, -1.890553e-01, -5.709787e-01), 6.637114e+05, 4.451577e-05, 1.000000e+00, 23, 2349, 1)
- ((-8.178800e-01, -5.993930e+00, 3.262478e+00), (-7.989001e-01, -1.890553e-01, -5.709787e-01), 6.637114e+05, 4.451579e-05, 1.000000e+00, 23, 2348, 1)
- ((-1.077910e+00, -6.055465e+00, 3.076633e+00), (-4.487118e-01, 1.670254e-01, -8.779295e-01), 4.550608e+05, 4.451608e-05, 1.000000e+00, 23, 2348, 1)
- ((-1.215611e+00, -6.004208e+00, 2.807214e+00), (7.872329e-01, 4.481433e-01, -4.235941e-01), 3.544207e+03, 4.451641e-05, 1.000000e+00, 23, 2348, 1)
- ((-1.100296e+00, -5.938564e+00, 2.745166e+00), (8.468456e-01, 5.310954e-01, 2.811230e-02), 2.812308e+03, 4.451819e-05, 1.000000e+00, 23, 2348, 1)
- ((-8.178800e-01, -5.761448e+00, 2.754541e+00), (8.468456e-01, 5.310954e-01, 2.811230e-02), 2.812308e+03, 4.452273e-05, 1.000000e+00, 23, 2349, 1)
- ((-7.600184e-01, -5.725160e+00, 2.756462e+00), (8.468456e-01, 5.310954e-01, 2.811230e-02), 2.812308e+03, 4.452366e-05, 1.000000e+00, 23, 2363, 1)
- ((-2.947156e-01, -5.433347e+00, 2.771908e+00), (8.468456e-01, 5.310954e-01, 2.811230e-02), 2.812308e+03, 4.453115e-05, 1.000000e+00, 22, 2363, 3)
- ((-2.073333e-01, -5.378546e+00, 2.774809e+00), (8.468456e-01, 5.310954e-01, 2.811230e-02), 2.812308e+03, 4.453256e-05, 1.000000e+00, 21, 2363, 2)
- ((-1.746705e-01, -5.358062e+00, 2.775893e+00), (5.278774e-01, 5.459205e-01, 6.506276e-01), 2.806481e+03, 4.453309e-05, 1.000000e+00, 21, 2363, 2)
- ((-8.681718e-02, -5.267205e+00, 2.884176e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.453536e-05, 1.000000e+00, 21, 2363, 2)
- ((-3.684221e-01, -5.266924e+00, 2.745840e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.453967e-05, 1.000000e+00, 22, 2363, 3)
- ((-4.840903e-01, -5.266809e+00, 2.689019e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.454144e-05, 1.000000e+00, 23, 2363, 1)
- ((-8.178800e-01, -5.266475e+00, 2.525049e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.454656e-05, 1.000000e+00, 23, 2362, 1)
- ((-1.151175e+00, -5.266142e+00, 2.361321e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.455166e-05, 1.000000e+00, 22, 2362, 3)
- ((-1.266464e+00, -5.266027e+00, 2.304687e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.455343e-05, 1.000000e+00, 21, 2362, 2)
- ((-2.005772e+00, -5.265288e+00, 1.941509e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.456475e-05, 1.000000e+00, 22, 2362, 3)
- ((-2.121061e+00, -5.265173e+00, 1.884875e+00), (-8.975500e-01, 8.968076e-04, -4.409119e-01), 2.764929e+03, 4.456652e-05, 1.000000e+00, 23, 2362, 1)
- ((-2.393759e+00, -5.264900e+00, 1.750915e+00), (-9.607392e-01, -1.749373e-01, 2.153533e-01), 1.619469e+03, 4.457070e-05, 1.000000e+00, 23, 2362, 1)
- ((-2.453640e+00, -5.275804e+00, 1.764337e+00), (-9.607392e-01, -1.749373e-01, 2.153533e-01), 1.619469e+03, 4.457182e-05, 1.000000e+00, 23, 2361, 1)
- ((-2.470658e+00, -5.278903e+00, 1.768152e+00), (-6.728606e-01, -2.916408e-01, -6.798561e-01), 4.873672e+02, 4.457214e-05, 1.000000e+00, 23, 2361, 1)
- ((-3.463692e+00, -5.709318e+00, 7.647939e-01), (-2.249302e-01, -9.330150e-01, -2.808728e-01), 1.790901e+02, 4.462047e-05, 1.000000e+00, 23, 2361, 1)
- ((-3.467511e+00, -5.725160e+00, 7.600247e-01), (-2.249302e-01, -9.330150e-01, -2.808728e-01), 1.790901e+02, 4.462139e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.533785e+00, -6.000066e+00, 6.772677e-01), (-2.249302e-01, -9.330150e-01, -2.808728e-01), 1.790901e+02, 4.463730e-05, 1.000000e+00, 22, 2347, 3)
- ((-3.562245e+00, -6.118119e+00, 6.417291e-01), (-2.249302e-01, -9.330150e-01, -2.808728e-01), 1.790901e+02, 4.464414e-05, 1.000000e+00, 21, 2347, 2)
- ((-3.723934e+00, -6.788806e+00, 4.398272e-01), (-2.249302e-01, -9.330150e-01, -2.808728e-01), 1.790901e+02, 4.468297e-05, 1.000000e+00, 22, 2347, 3)
- ((-3.752394e+00, -6.906859e+00, 4.042885e-01), (-2.249302e-01, -9.330150e-01, -2.808728e-01), 1.790901e+02, 4.468981e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.848515e+00, -7.305571e+00, 2.842613e-01), (6.498448e-01, -3.243413e-01, -6.873896e-01), 2.328979e+01, 4.471290e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.737619e+00, -7.360920e+00, 1.669579e-01), (6.498448e-01, -3.243413e-01, -6.873896e-01), 2.328979e+01, 4.473846e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.574308e+00, -7.442429e+00, -5.788041e-03), (6.794505e-01, 3.157106e-01, -6.623246e-01), 1.590416e+01, 4.477611e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.398889e+00, -7.360920e+00, -1.767852e-01), (6.794505e-01, 3.157106e-01, -6.623246e-01), 1.590416e+01, 4.482292e-05, 1.000000e+00, 23, 2347, 1)
- ((-2.904712e+00, -7.131298e+00, -6.585068e-01), (-4.836985e-02, -2.819627e-01, -9.582053e-01), 2.610202e+00, 4.495477e-05, 1.000000e+00, 23, 2347, 1)
- ((-2.923579e+00, -7.241285e+00, -1.032280e+00), (-6.089807e-01, -2.347428e-01, -7.576532e-01), 1.671268e+00, 4.512933e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.012516e+00, -7.275567e+00, -1.142930e+00), (3.221931e-01, -5.406104e-01, -7.771306e-01), 1.275709e-01, 4.521100e-05, 1.000000e+00, 23, 2347, 1)
- ((-2.984032e+00, -7.323360e+00, -1.211633e+00), (-8.629564e-01, -1.719529e-01, -4.751195e-01), 1.897424e-02, 4.538995e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.172528e+00, -7.360920e+00, -1.315413e+00), (-8.629564e-01, -1.719529e-01, -4.751195e-01), 1.897424e-02, 4.653641e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.602328e+00, -7.446562e+00, -1.552049e+00), (-1.530190e-01, -7.092235e-01, 6.881767e-01), 1.306951e-02, 4.915052e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.625236e+00, -7.552741e+00, -1.449021e+00), (-1.976867e-01, 2.826475e-01, 9.386322e-01), 2.145548e-02, 5.009730e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.636290e+00, -7.536936e+00, -1.396537e+00), (1.266676e-01, 2.265353e-01, -9.657314e-01), 1.815564e-02, 5.037329e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.621792e+00, -7.511008e+00, -1.507069e+00), (-4.275550e-01, 8.441443e-01, 3.234457e-01), 1.848049e-02, 5.098741e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.697811e+00, -7.360920e+00, -1.449560e+00), (-4.275550e-01, 8.441443e-01, 3.234457e-01), 1.848049e-02, 5.193300e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.703436e+00, -7.349814e+00, -1.445305e+00), (-7.832167e-01, 5.713670e-01, 2.451762e-01), 1.861705e-02, 5.200297e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.823243e+00, -7.262414e+00, -1.407801e+00), (-9.917106e-01, 5.069164e-02, 1.180695e-01), 4.703334e-02, 5.281350e-05, 1.000000e+00, 23, 2347, 1)
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- ((-4.131081e+00, -7.246679e+00, -1.371151e+00), (9.108632e-01, -4.018526e-01, 9.403556e-02), 8.057393e-02, 5.384831e-05, 1.000000e+00, 23, 2346, 1)
- ((-4.089400e+00, -7.265067e+00, -1.366848e+00), (9.108632e-01, -4.018526e-01, 9.403556e-02), 8.057393e-02, 5.396486e-05, 1.000000e+00, 23, 2347, 1)
- ((-3.872134e+00, -7.360920e+00, -1.344418e+00), (9.108632e-01, -4.018526e-01, 9.403556e-02), 8.057393e-02, 5.457240e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.673062e+00, -7.448746e+00, -1.323866e+00), (2.976906e-01, 8.174443e-01, -4.931178e-01), 5.538060e-02, 5.512905e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.658580e+00, -7.408978e+00, -1.347855e+00), (-5.409120e-01, 8.345378e-01, -1.046943e-01), 8.122576e-02, 5.527851e-05, 1.000000e+00, 11, 1750, 1)
- ((-3.682981e+00, -7.371331e+00, -1.352578e+00), (-5.409120e-01, 8.345378e-01, -1.046943e-01), 8.122576e-02, 5.539295e-05, 0.000000e+00, 11, 1750, 1)]
+neutron [((6.474155e+00, -5.192870e+00, 5.413003e+00), (-9.597810e-01, -1.081529e-01, -2.590820e-01), 9.900333e+05, 4.450859e-05, 1.000000e+00, 21, 2367, 2)
+ ((6.142109e+00, -5.230286e+00, 5.323371e+00), (-9.597810e-01, -1.081529e-01, -2.590820e-01), 9.900333e+05, 4.450884e-05, 1.000000e+00, 22, 2367, 3)
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+ ((5.599064e+00, -5.725160e+00, 5.102979e+00), (-6.195407e-01, -7.332723e-01, -2.801446e-01), 5.512347e+05, 4.450958e-05, 1.000000e+00, 23, 2352, 1)
+ ((5.296886e+00, -6.082810e+00, 4.966340e+00), (-6.195407e-01, -7.332723e-01, -2.801446e-01), 5.512347e+05, 4.451006e-05, 1.000000e+00, 22, 2352, 3)
+ ((5.240003e+00, -6.150135e+00, 4.940619e+00), (-6.195407e-01, -7.332723e-01, -2.801446e-01), 5.512347e+05, 4.451015e-05, 1.000000e+00, 21, 2352, 2)
+ ((4.752072e+00, -6.727638e+00, 4.719986e+00), (3.721510e-02, -9.647240e-01, 2.606197e-01), 5.045933e+05, 4.451092e-05, 1.000000e+00, 21, 2352, 2)
+ ((4.764028e+00, -7.037568e+00, 4.803713e+00), (3.721510e-02, -9.647240e-01, 2.606197e-01), 5.045933e+05, 4.451124e-05, 1.000000e+00, 22, 2352, 3)
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+ ((4.776502e+00, -7.360920e+00, 4.891066e+00), (3.721510e-02, -9.647240e-01, 2.606197e-01), 5.045933e+05, 4.451158e-05, 1.000000e+00, 23, 2338, 1)
+ ((4.778286e+00, -7.407162e+00, 4.903559e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451163e-05, 1.000000e+00, 23, 2338, 1)
+ ((4.981926e+00, -7.580442e+00, 5.259893e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451239e-05, 1.000000e+00, 22, 2338, 3)
+ ((5.154144e+00, -7.726985e+00, 5.561246e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451303e-05, 1.000000e+00, 21, 2338, 2)
+ ((5.313757e+00, -7.862801e+00, 5.840540e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451362e-05, 1.000000e+00, 22, 2338, 3)
+ ((5.485976e+00, -8.009344e+00, 6.141893e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451426e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.725160e+00, -8.212869e+00, 6.560424e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451515e-05, 1.000000e+00, 23, 2339, 1)
+ ((6.004950e+00, -8.450945e+00, 7.050007e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451618e-05, 1.000000e+00, 22, 2339, 3)
+ ((6.360530e+00, -8.753512e+00, 7.672210e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451750e-05, 1.000000e+00, 23, 2339, 1)
+ ((6.646303e+00, -8.996680e+00, 8.172263e+00), (4.571054e-01, -3.889563e-01, 7.998548e-01), 1.818775e+05, 4.451856e-05, 1.000000e+00, 23, 2326, 1)
+ ((6.702490e+00, -9.044491e+00, 8.270582e+00), (2.028534e-01, -9.607965e-01, -1.889990e-01), 1.805295e+04, 4.451877e-05, 1.000000e+00, 23, 2326, 1)
+ ((6.745127e+00, -9.246436e+00, 8.230857e+00), (2.028534e-01, -9.607965e-01, -1.889990e-01), 1.805295e+04, 4.451990e-05, 1.000000e+00, 22, 2326, 3)
+ ((6.767219e+00, -9.351070e+00, 8.210274e+00), (2.028534e-01, -9.607965e-01, -1.889990e-01), 1.805295e+04, 4.452049e-05, 1.000000e+00, 21, 2326, 2)
+ ((6.885344e+00, -9.910562e+00, 8.100216e+00), (-3.857824e-01, -3.006310e-01, -8.722344e-01), 1.795765e+04, 4.452362e-05, 1.000000e+00, 21, 2326, 2)
+ ((6.803615e+00, -9.974251e+00, 7.915431e+00), (-7.096135e-01, 6.778415e-01, 1.923009e-01), 1.779137e+04, 4.452476e-05, 1.000000e+00, 21, 2326, 2)
+ ((6.334815e+00, -9.526441e+00, 8.042473e+00), (-1.748210e-01, 9.821705e-01, -6.912779e-02), 1.729669e+04, 4.452835e-05, 1.000000e+00, 21, 2326, 2)
+ ((6.304853e+00, -9.358111e+00, 8.030625e+00), (-1.748210e-01, 9.821705e-01, -6.912779e-02), 1.729669e+04, 4.452929e-05, 1.000000e+00, 22, 2326, 3)
+ ((6.288777e+00, -9.267793e+00, 8.024268e+00), (-1.748210e-01, 9.821705e-01, -6.912779e-02), 1.729669e+04, 4.452979e-05, 1.000000e+00, 23, 2326, 1)
+ ((6.275105e+00, -9.190979e+00, 8.018862e+00), (-7.174952e-01, 5.897531e-01, -3.706642e-01), 9.207900e+03, 4.453022e-05, 1.000000e+00, 23, 2326, 1)
+ ((6.038719e+00, -8.996680e+00, 7.896743e+00), (-7.174952e-01, 5.897531e-01, -3.706642e-01), 9.207900e+03, 4.453271e-05, 1.000000e+00, 23, 2339, 1)
+ ((6.028805e+00, -8.988531e+00, 7.891621e+00), (-2.989803e-01, 4.300779e-01, -8.518473e-01), 5.653774e+03, 4.453281e-05, 1.000000e+00, 23, 2339, 1)
+ ((5.725160e+00, -8.551743e+00, 7.026484e+00), (-2.989803e-01, 4.300779e-01, -8.518473e-01), 5.653774e+03, 4.454257e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.507324e+00, -8.238390e+00, 6.405832e+00), (-2.989803e-01, 4.300779e-01, -8.518473e-01), 5.653774e+03, 4.454958e-05, 1.000000e+00, 22, 2338, 3)
+ ((5.417387e+00, -8.109017e+00, 6.149584e+00), (-2.989803e-01, 4.300779e-01, -8.518473e-01), 5.653774e+03, 4.455247e-05, 1.000000e+00, 21, 2338, 2)
+ ((5.162017e+00, -7.741671e+00, 5.421991e+00), (7.522612e-01, -1.036919e-01, -6.506543e-01), 5.619608e+03, 4.456069e-05, 1.000000e+00, 21, 2338, 2)
+ ((5.184146e+00, -7.744722e+00, 5.402851e+00), (7.522612e-01, -1.036919e-01, -6.506543e-01), 5.619608e+03, 4.456097e-05, 1.000000e+00, 22, 2338, 3)
+ ((5.348057e+00, -7.767315e+00, 5.261079e+00), (7.522612e-01, -1.036919e-01, -6.506543e-01), 5.619608e+03, 4.456307e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.581568e+00, -7.799502e+00, 5.059108e+00), (2.029690e-01, -4.131185e-01, -8.877706e-01), 3.359662e+03, 4.456606e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.707007e+00, -8.054818e+00, 4.510447e+00), (3.894183e-01, -7.459178e-01, -5.403333e-01), 3.303714e+03, 4.457377e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.725160e+00, -8.089590e+00, 4.485259e+00), (3.894183e-01, -7.459178e-01, -5.403333e-01), 3.303714e+03, 4.457436e-05, 1.000000e+00, 23, 2339, 1)
+ ((5.750893e+00, -8.138881e+00, 4.449554e+00), (-2.879829e-01, -5.545287e-01, -7.807456e-01), 1.744626e+03, 4.457519e-05, 1.000000e+00, 23, 2339, 1)
+ ((5.725160e+00, -8.188431e+00, 4.379790e+00), (-2.879829e-01, -5.545287e-01, -7.807456e-01), 1.744626e+03, 4.457674e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.532143e+00, -8.560097e+00, 3.856504e+00), (-6.185172e-01, -7.766669e-01, -1.192687e-01), 8.559363e+02, 4.458834e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.522333e+00, -8.572415e+00, 3.854613e+00), (-3.452053e-01, -2.657159e-02, -9.381510e-01), 9.272133e+01, 4.458873e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.366948e+00, -8.584375e+00, 3.432328e+00), (4.761677e-01, -8.187551e-01, -3.207871e-01), 6.522002e+00, 4.462253e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.516634e+00, -8.841755e+00, 3.331487e+00), (4.237115e-01, -6.022578e-01, 6.765752e-01), 9.968491e-01, 4.471152e-05, 1.000000e+00, 23, 2338, 1)
+ ((5.625629e+00, -8.996680e+00, 3.505530e+00), (4.237115e-01, -6.022578e-01, 6.765752e-01), 9.968491e-01, 4.489779e-05, 1.000000e+00, 23, 2325, 1)
+ ((5.723424e+00, -9.135684e+00, 3.661687e+00), (7.706137e-01, -6.358415e-01, 4.312932e-02), 5.695813e-01, 4.506493e-05, 1.000000e+00, 23, 2325, 1)
+ ((5.725160e+00, -9.137116e+00, 3.661784e+00), (7.706137e-01, -6.358415e-01, 4.312932e-02), 5.695813e-01, 4.506708e-05, 1.000000e+00, 23, 2326, 1)
+ ((6.079210e+00, -9.429247e+00, 3.681599e+00), (7.706137e-01, -6.358415e-01, 4.312932e-02), 5.695813e-01, 4.550721e-05, 1.000000e+00, 22, 2326, 3)
+ ((6.147194e+00, -9.485341e+00, 3.685404e+00), (7.706137e-01, -6.358415e-01, 4.312932e-02), 5.695813e-01, 4.559172e-05, 1.000000e+00, 21, 2326, 2)
+ ((6.864744e+00, -1.007740e+01, 3.725564e+00), (3.888690e-01, -8.895503e-01, -2.397522e-01), 5.691123e-01, 4.648372e-05, 1.000000e+00, 21, 2326, 2)
+ ((6.908415e+00, -1.017730e+01, 3.698639e+00), (3.888690e-01, -8.895503e-01, -2.397522e-01), 5.691123e-01, 4.659135e-05, 1.000000e+00, 22, 2326, 3)
+ ((6.945959e+00, -1.026318e+01, 3.675492e+00), (3.888690e-01, -8.895503e-01, -2.397522e-01), 5.691123e-01, 4.668388e-05, 1.000000e+00, 23, 2326, 1)
+ ((6.966122e+00, -1.030930e+01, 3.663060e+00), (7.493672e-01, -6.303754e-01, 2.026713e-01), 2.849108e-01, 4.673357e-05, 1.000000e+00, 23, 2326, 1)
+ ((7.350254e+00, -1.063244e+01, 3.766951e+00), (7.493672e-01, -6.303754e-01, 2.026713e-01), 2.849108e-01, 4.742789e-05, 1.000000e+00, 23, 2311, 1)
+ ((7.360920e+00, -1.064141e+01, 3.769836e+00), (7.493672e-01, -6.303754e-01, 2.026713e-01), 2.849108e-01, 4.744717e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.449168e+00, -1.071565e+01, 3.793703e+00), (1.626881e-01, -1.480918e-01, 9.755006e-01), 2.025207e-01, 4.760667e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.495754e+00, -1.075805e+01, 4.073039e+00), (-1.185723e-01, -6.787603e-01, 7.247241e-01), 2.121260e-01, 4.806671e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.451135e+00, -1.101347e+01, 4.345753e+00), (-4.978877e-01, -3.750598e-02, 8.664301e-01), 2.609768e-02, 4.865741e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.426401e+00, -1.101533e+01, 4.388795e+00), (9.221835e-01, -6.407530e-02, 3.814078e-01), 2.031765e-02, 4.887973e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.438938e+00, -1.101621e+01, 4.393980e+00), (8.832139e-01, 3.563144e-01, 3.049151e-01), 2.038019e-02, 4.894869e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.578355e+00, -1.095996e+01, 4.442112e+00), (5.564936e-01, 8.135970e-01, 1.684482e-01), 4.869241e-02, 4.974811e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.802376e+00, -1.063244e+01, 4.509922e+00), (5.564936e-01, 8.135970e-01, 1.684482e-01), 4.869241e-02, 5.106705e-05, 1.000000e+00, 23, 2327, 1)
+ ((7.826035e+00, -1.059785e+01, 4.517083e+00), (-8.227572e-01, -5.662268e-01, 4.957651e-02), 8.202195e-02, 5.120634e-05, 1.000000e+00, 23, 2327, 1)
+ ((7.775776e+00, -1.063244e+01, 4.520112e+00), (-8.227572e-01, -5.662268e-01, 4.957651e-02), 8.202195e-02, 5.136055e-05, 1.000000e+00, 23, 2312, 1)
+ ((7.360920e+00, -1.091795e+01, 4.545109e+00), (-8.227572e-01, -5.662268e-01, 4.957651e-02), 8.202195e-02, 5.263343e-05, 1.000000e+00, 23, 2311, 1)
+ ((7.180267e+00, -1.104227e+01, 4.555995e+00), (5.568446e-01, -6.706436e-01, -4.900625e-01), 5.560680e-02, 5.318772e-05, 1.000000e+00, 23, 2311, 1)
+ ((7.207392e+00, -1.107494e+01, 4.532123e+00), (9.936512e-01, 4.529367e-02, -1.029847e-01), 8.142074e-02, 5.333706e-05, 1.000000e+00, 23, 2311, 1)
+ ((7.252245e+00, -1.107290e+01, 4.527474e+00), (9.936512e-01, 4.529367e-02, -1.029847e-01), 8.142074e-02, 5.345144e-05, 0.000000e+00, 23, 2311, 1)]
diff --git a/tests/regression_tests/translation/results_true.dat b/tests/regression_tests/translation/results_true.dat
index f832aa296..6e03d2224 100644
--- a/tests/regression_tests/translation/results_true.dat
+++ b/tests/regression_tests/translation/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-4.087580E-01 4.466279E-03
+4.076610E-01 6.454244E-03
diff --git a/tests/regression_tests/trigger_batch_interval/results_true.dat b/tests/regression_tests/trigger_batch_interval/results_true.dat
index 0571d6c4f..92fa99d87 100644
--- a/tests/regression_tests/trigger_batch_interval/results_true.dat
+++ b/tests/regression_tests/trigger_batch_interval/results_true.dat
@@ -1,28 +1,28 @@
 k-combined:
-9.767929E-01 5.327552E-03
+9.863217E-01 6.499354E-03
 tally 1:
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
 tally 2:
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
diff --git a/tests/regression_tests/trigger_no_batch_interval/results_true.dat b/tests/regression_tests/trigger_no_batch_interval/results_true.dat
index 0571d6c4f..92fa99d87 100644
--- a/tests/regression_tests/trigger_no_batch_interval/results_true.dat
+++ b/tests/regression_tests/trigger_no_batch_interval/results_true.dat
@@ -1,28 +1,28 @@
 k-combined:
-9.767929E-01 5.327552E-03
+9.863217E-01 6.499354E-03
 tally 1:
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
 tally 2:
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
diff --git a/tests/regression_tests/trigger_no_status/results_true.dat b/tests/regression_tests/trigger_no_status/results_true.dat
index fb6c0086a..a62e1b3fe 100644
--- a/tests/regression_tests/trigger_no_status/results_true.dat
+++ b/tests/regression_tests/trigger_no_status/results_true.dat
@@ -1,28 +1,28 @@
 k-combined:
-9.682315E-01 3.302924E-03
+9.858966E-01 1.500542E-02
 tally 1:
-7.046510E+00
-9.932168E+00
-1.591675E+00
-5.067777E-01
-1.541572E+00
-4.753743E-01
-5.454835E+00
-5.951990E+00
-7.046510E+00
-9.932168E+00
-1.591675E+00
-5.067777E-01
-1.541572E+00
-4.753743E-01
-5.454835E+00
-5.951990E+00
+7.081828E+00
+1.003709E+01
+1.607382E+00
+5.171386E-01
+1.559242E+00
+4.866413E-01
+5.474447E+00
+5.997737E+00
+7.081828E+00
+1.003709E+01
+1.607382E+00
+5.171386E-01
+1.559242E+00
+4.866413E-01
+5.474447E+00
+5.997737E+00
 tally 2:
-7.046510E+00
-9.932168E+00
-1.591675E+00
-5.067777E-01
-1.541572E+00
-4.753743E-01
-5.454835E+00
-5.951990E+00
+7.081828E+00
+1.003709E+01
+1.607382E+00
+5.171386E-01
+1.559242E+00
+4.866413E-01
+5.474447E+00
+5.997737E+00
diff --git a/tests/regression_tests/trigger_statepoint_restart/inputs_true.dat b/tests/regression_tests/trigger_statepoint_restart/inputs_true.dat
index 6f0b629eb..59a829700 100644
--- a/tests/regression_tests/trigger_statepoint_restart/inputs_true.dat
+++ b/tests/regression_tests/trigger_statepoint_restart/inputs_true.dat
@@ -1,14 +1,14 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" depletable="true">
+      <density value="4.5" units="g/cm3"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="1" region="-1" universe="1"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 10.0" id="1" type="sphere"/>
+    <surface id="1" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 10.0"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
@@ -16,7 +16,7 @@
     <batches>15</batches>
     <inactive>10</inactive>
     <keff_trigger>
-      <threshold>0.003</threshold>
+      <threshold>0.002</threshold>
       <type>std_dev</type>
     </keff_trigger>
     <trigger>
diff --git a/tests/regression_tests/trigger_statepoint_restart/results_true.dat b/tests/regression_tests/trigger_statepoint_restart/results_true.dat
index 3e6619d74..3cb1e230d 100644
--- a/tests/regression_tests/trigger_statepoint_restart/results_true.dat
+++ b/tests/regression_tests/trigger_statepoint_restart/results_true.dat
@@ -1,5 +1,5 @@
 k-combined:
-3.014717E-01 2.864764E-03
+2.948661E-01 1.949846E-03
 tally 1:
-8.946107E+01
-7.281263E+02
+5.515170E+01
+4.349007E+02
diff --git a/tests/regression_tests/trigger_statepoint_restart/test.py b/tests/regression_tests/trigger_statepoint_restart/test.py
index 8144c1d0b..b242f7f1c 100644
--- a/tests/regression_tests/trigger_statepoint_restart/test.py
+++ b/tests/regression_tests/trigger_statepoint_restart/test.py
@@ -29,7 +29,7 @@ def model():
     settings.inactive = 10
     settings.particles = 400
     # Choose a sufficiently low threshold to enable use of trigger
-    settings.keff_trigger = {'type': 'std_dev', 'threshold': 0.003}
+    settings.keff_trigger = {'type': 'std_dev', 'threshold': 0.002}
     settings.trigger_max_batches = 1000
     settings.trigger_batch_interval = 1
     settings.trigger_active = True
@@ -41,10 +41,10 @@ def model():
     tallies = openmc.Tallies([t])
 
     # Put it all together
-    model = openmc.model.Model(materials=materials,
-                               geometry=geometry,
-                               settings=settings,
-                               tallies=tallies)
+    model = openmc.Model(materials=materials,
+                         geometry=geometry,
+                         settings=settings,
+                         tallies=tallies)
     return model
 
 
diff --git a/tests/regression_tests/trigger_tallies/results_true.dat b/tests/regression_tests/trigger_tallies/results_true.dat
index 0571d6c4f..92fa99d87 100644
--- a/tests/regression_tests/trigger_tallies/results_true.dat
+++ b/tests/regression_tests/trigger_tallies/results_true.dat
@@ -1,28 +1,28 @@
 k-combined:
-9.767929E-01 5.327552E-03
+9.863217E-01 6.499354E-03
 tally 1:
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
 tally 2:
-1.405471E+01
-1.976444E+01
-3.186139E+00
-1.015515E+00
-3.088643E+00
-9.542994E-01
-1.086857E+01
-1.182011E+01
+1.423436E+01
+2.027258E+01
+3.223740E+00
+1.039872E+00
+3.125016E+00
+9.771709E-01
+1.101062E+01
+1.212977E+01
diff --git a/tests/regression_tests/triso/inputs_true.dat b/tests/regression_tests/triso/inputs_true.dat
index 37f37af78..c96684568 100644
--- a/tests/regression_tests/triso/inputs_true.dat
+++ b/tests/regression_tests/triso/inputs_true.dat
@@ -1,38 +1,38 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1">
-      <density units="g/cm3" value="10.5"/>
-      <nuclide ao="0.14154" name="U235"/>
-      <nuclide ao="0.85846" name="U238"/>
-      <nuclide ao="0.5" name="C0"/>
-      <nuclide ao="1.5" name="O16"/>
+    <material id="1" depletable="true">
+      <density value="10.5" units="g/cm3"/>
+      <nuclide name="U235" ao="0.14154"/>
+      <nuclide name="U238" ao="0.85846"/>
+      <nuclide name="C0" ao="0.5"/>
+      <nuclide name="O16" ao="1.5"/>
     </material>
     <material id="2">
-      <density units="g/cm3" value="1.0"/>
-      <nuclide ao="1.0" name="C0"/>
+      <density value="1.0" units="g/cm3"/>
+      <nuclide name="C0" ao="1.0"/>
       <sab name="c_Graphite"/>
     </material>
     <material id="3">
-      <density units="g/cm3" value="1.9"/>
-      <nuclide ao="1.0" name="C0"/>
+      <density value="1.9" units="g/cm3"/>
+      <nuclide name="C0" ao="1.0"/>
       <sab name="c_Graphite"/>
     </material>
     <material id="4">
-      <density units="g/cm3" value="3.2"/>
-      <nuclide ao="1.0" name="C0"/>
-      <nuclide ao="0.9222968" name="Si28"/>
-      <nuclide ao="0.0468316" name="Si29"/>
-      <nuclide ao="0.0308716" name="Si30"/>
+      <density value="3.2" units="g/cm3"/>
+      <nuclide name="C0" ao="1.0"/>
+      <nuclide name="Si28" ao="0.9222968"/>
+      <nuclide name="Si29" ao="0.0468316"/>
+      <nuclide name="Si30" ao="0.0308716"/>
     </material>
     <material id="5">
-      <density units="g/cm3" value="1.87"/>
-      <nuclide ao="1.0" name="C0"/>
+      <density value="1.87" units="g/cm3"/>
+      <nuclide name="C0" ao="1.0"/>
       <sab name="c_Graphite"/>
     </material>
     <material id="6">
-      <density units="g/cm3" value="1.1995"/>
-      <nuclide ao="1.0" name="C0"/>
+      <density value="1.1995" units="g/cm3"/>
+      <nuclide name="C0" ao="1.0"/>
       <sab name="c_Graphite"/>
     </material>
   </materials>
@@ -42,171 +42,171 @@
     <cell id="3" material="3" region="2 -3" universe="1"/>
     <cell id="4" material="4" region="3 -4" universe="1"/>
     <cell id="5" material="5" region="4" universe="1"/>
-    <cell fill="2" id="6" region="5 -6 7 -8 9 -10" universe="0"/>
-    <cell fill="1" id="107" region="-111" translation="-0.0014062011817633224 -0.015257747167296776 -0.09184766966024316" universe="27"/>
-    <cell fill="1" id="108" region="-112" translation="0.10909902639777394 -0.004179177767328068 -0.046240930185910245" universe="15"/>
-    <cell fill="1" id="109" region="-113" translation="0.13878336652768952 -0.06900710536877097 -0.03848821497485455" universe="10"/>
-    <cell fill="1" id="110" region="-114" translation="-0.19454996680564374 -0.06900710536877097 -0.03848821497485455" universe="11"/>
-    <cell fill="1" id="111" region="-115" translation="-0.09846455191076986 -0.02594038069469251 -0.06155174933292362" universe="18"/>
-    <cell fill="1" id="112" region="-116" translation="-0.09321591784308736 -0.12248857482372477 -0.04999802384282931" universe="14"/>
-    <cell fill="1" id="113" region="-117" translation="0.2028198996079864 0.08501514690643278 0.07431198844628262" universe="22"/>
-    <cell fill="1" id="114" region="-118" translation="-0.13051343372534685 0.08501514690643278 0.07431198844628262" universe="23"/>
-    <cell fill="1" id="115" region="-119" translation="0.03417350410997999 -0.09641153719944834 -0.10112098092715327" universe="5"/>
-    <cell fill="1" id="116" region="-120" translation="0.037913118842446136 0.06870772519039314 -0.10875752039644154" universe="19"/>
-    <cell fill="1" id="117" region="-121" translation="0.07388008140640989 -0.07130227505402231 -0.09782982564187503" universe="6"/>
-    <cell fill="1" id="118" region="-122" translation="0.08897214754414112 0.08694165211410448 0.07587737301598729" universe="3"/>
-    <cell fill="1" id="119" region="-123" translation="-0.03698302711710488 0.1408784014909229 -0.10474284149572705" universe="4"/>
-    <cell fill="1" id="120" region="-124" translation="-0.03698302711710488 -0.19245493184241047 -0.10474284149572705" universe="7"/>
-    <cell fill="1" id="121" region="-125" translation="-0.02428069661424781 0.05168393238830282 0.1302704892646539" universe="17"/>
-    <cell fill="1" id="122" region="-126" translation="-0.02428069661424781 0.05168393238830282 -0.20306284406867936" universe="26"/>
-    <cell fill="1" id="123" region="-127" translation="0.04578051975994746 0.1175901605138544 -0.0038022862212453434" universe="27"/>
-    <cell fill="1" id="124" region="-128" translation="-0.013741908464450836 0.18016577553053154 0.20276897514248216" universe="12"/>
-    <cell fill="1" id="125" region="-129" translation="-0.013741908464450836 -0.15316755780280183 0.20276897514248216" universe="15"/>
-    <cell fill="1" id="126" region="-130" translation="-0.013741908464450836 0.18016577553053154 -0.1305643581908511" universe="21"/>
-    <cell fill="1" id="127" region="-131" translation="-0.013741908464450836 -0.15316755780280183 -0.1305643581908511" universe="24"/>
-    <cell fill="1" id="128" region="-132" translation="0.19334606515602848 0.066228023881572 -0.07131104153532436" universe="19"/>
-    <cell fill="1" id="129" region="-133" translation="-0.13998726817730478 0.066228023881572 -0.07131104153532436" universe="20"/>
-    <cell fill="1" id="130" region="-134" translation="-0.031185656140686913 0.155914746052082 0.15331720509241947" universe="8"/>
-    <cell fill="1" id="131" region="-135" translation="-0.031185656140686913 -0.17741858728125126 0.15331720509241947" universe="11"/>
-    <cell fill="1" id="132" region="-136" translation="-0.031185656140686913 0.155914746052082 -0.1800161282409139" universe="17"/>
-    <cell fill="1" id="133" region="-137" translation="-0.031185656140686913 -0.17741858728125126 -0.1800161282409139" universe="20"/>
-    <cell fill="1" id="134" region="-138" translation="0.08018004492442865 0.016826191928232426 -0.016389596762212966" universe="28"/>
-    <cell fill="1" id="135" region="-139" translation="0.00483733774062417 0.08148156718302602 -0.09280826916311341" universe="7"/>
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+    <surface id="242" type="sphere" coeffs="0.045127058382085194 0.0967807216119615 0.0967653665331849 0.04225"/>
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+    <surface id="262" type="sphere" coeffs="0.152805274648522 0.18166556620660584 0.040488366683789634 0.04225"/>
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+    <surface id="264" type="sphere" coeffs="0.152805274648522 -0.15166776712672753 0.040488366683789634 0.04225"/>
+    <surface id="265" type="sphere" coeffs="-0.18052805868481137 -0.15166776712672753 0.040488366683789634 0.04225"/>
+    <surface id="266" type="sphere" coeffs="0.07231116694829032 0.08785370537091952 0.09997055589943346 0.04225"/>
+    <surface id="267" type="sphere" coeffs="-0.10576424362412257 0.09874497232579027 -0.05820080940738992 0.04225"/>
+    <surface id="268" type="sphere" coeffs="0.046877953125145755 -0.059492732184222796 -0.055635341857650844 0.04225"/>
+    <surface id="269" type="sphere" coeffs="0.1239423066569788 0.14183046274229005 -0.06583818749209458 0.04225"/>
+    <surface id="270" type="sphere" coeffs="0.1239423066569788 -0.19150287059104332 -0.06583818749209458 0.04225"/>
+    <surface id="271" type="sphere" coeffs="0.0013087859341976982 -0.06327594480884835 -0.05382522074205143 0.04225"/>
+    <surface id="272" type="sphere" coeffs="0.11058883669906516 -0.038954175820645864 0.16834558789033005 0.04225"/>
+    <surface id="273" type="sphere" coeffs="0.11058883669906516 -0.038954175820645864 -0.16498774544300332 0.04225"/>
+    <surface id="274" type="sphere" coeffs="-0.02657010065724863 0.013144978841080679 0.050587944514051575 0.04225"/>
   </geometry>
   <settings>
     <run_mode>eigenvalue</run_mode>
     <particles>100</particles>
     <batches>4</batches>
     <inactive>0</inactive>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
diff --git a/tests/regression_tests/triso/results_true.dat b/tests/regression_tests/triso/results_true.dat
index fa1842e54..d9850e410 100644
--- a/tests/regression_tests/triso/results_true.dat
+++ b/tests/regression_tests/triso/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-1.716873E+00 5.266107E-02
+1.604832E+00 2.031393E-03
diff --git a/tests/regression_tests/uniform_fs/results_true.dat b/tests/regression_tests/uniform_fs/results_true.dat
index 46654b49b..f7ceecfb7 100644
--- a/tests/regression_tests/uniform_fs/results_true.dat
+++ b/tests/regression_tests/uniform_fs/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.685309E-01 1.861720E-03
+3.675645E-01 4.342970E-03
diff --git a/tests/regression_tests/universe/results_true.dat b/tests/regression_tests/universe/results_true.dat
index bbf03de94..97b997ae6 100644
--- a/tests/regression_tests/universe/results_true.dat
+++ b/tests/regression_tests/universe/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-3.070134E-01 3.900396E-03
+2.940336E-01 7.338463E-04
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true.dat b/tests/regression_tests/unstructured_mesh/inputs_true.dat
index 07486d3e0..e7a485b4d 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="libmesh" type="unstructured">
+    <mesh id="2" type="unstructured" library="libmesh">
       <filename>test_mesh_hexes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true0.dat b/tests/regression_tests/unstructured_mesh/inputs_true0.dat
index c6b28d890..2508c25b3 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true0.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true0.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="libmesh" type="unstructured">
+    <mesh id="2" type="unstructured" library="libmesh">
       <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true1.dat b/tests/regression_tests/unstructured_mesh/inputs_true1.dat
index 40183c89b..04bbdb4b2 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true1.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true1.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="libmesh" type="unstructured">
+    <mesh id="2" type="unstructured" library="libmesh">
       <filename>test_mesh_tets.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true10.dat b/tests/regression_tests/unstructured_mesh/inputs_true10.dat
index 81dfbdc52..169f0a6b4 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true10.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true10.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true11.dat b/tests/regression_tests/unstructured_mesh/inputs_true11.dat
index b224a3ebc..8163750ca 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true11.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true11.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true12.dat b/tests/regression_tests/unstructured_mesh/inputs_true12.dat
index a18aee627..be0655e98 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true12.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true12.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true13.dat b/tests/regression_tests/unstructured_mesh/inputs_true13.dat
index 0e9123c90..42c0cc122 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true13.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true13.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true14.dat b/tests/regression_tests/unstructured_mesh/inputs_true14.dat
index 9d4db4904..a27c7445d 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true14.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true14.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true15.dat b/tests/regression_tests/unstructured_mesh/inputs_true15.dat
index eab97b952..c69bd15d0 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true15.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true15.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true2.dat b/tests/regression_tests/unstructured_mesh/inputs_true2.dat
index 3521ac7b8..ba9d477b5 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true2.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true2.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="libmesh" type="unstructured">
+    <mesh id="2" type="unstructured" library="libmesh">
       <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true3.dat b/tests/regression_tests/unstructured_mesh/inputs_true3.dat
index a376b401d..d840bfa09 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true3.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true3.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="libmesh" type="unstructured">
+    <mesh id="2" type="unstructured" library="libmesh">
       <filename>test_mesh_tets.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true4.dat b/tests/regression_tests/unstructured_mesh/inputs_true4.dat
index 843632fdd..db8e37177 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true4.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true4.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="13" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
-    <material id="14" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+    <material id="2" name="zircaloy">
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
-    <material id="15" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+    <material id="3" name="water">
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="13" material="13" name="fuel" region="73 -74 75 -76 77 -78" universe="5"/>
-    <cell id="14" material="14" name="clad" region="(-73 | 74 | -75 | 76 | -77 | 78) (79 -80 81 -82 83 -84)" universe="5"/>
-    <cell id="15" material="15" name="water" region="(-79 | 80 | -81 | 82 | -83 | 84) (85 -86 87 -88 89 -90)" universe="5"/>
-    <surface coeffs="-5.0" id="73" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="74" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="75" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="76" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="77" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="78" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="79" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="80" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="81" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="82" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="83" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="84" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15" id="85" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15" id="86" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15" id="87" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15" id="88" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15" id="89" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15" id="90" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
-    <particles>100</particles>
+    <particles>1000</particles>
     <batches>10</batches>
-    <source strength="1.0">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -56,34 +56,33 @@
           <parameters>0.0 1.0</parameters>
         </phi>
       </space>
-      <angle reference_uvw="-1.0 0.0 0.0" type="monodirectional"/>
       <energy type="discrete">
         <parameters>15000000.0 1.0</parameters>
       </energy>
     </source>
   </settings>
   <tallies>
-    <mesh id="9">
+    <mesh id="1">
       <dimension>10 10 10</dimension>
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="10" library="moab" type="unstructured">
-      <filename>test_mesh_tets_w_holes.exo</filename>
+    <mesh id="2" type="unstructured" library="libmesh">
+      <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
-    <filter id="9" type="mesh">
-      <bins>9</bins>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
     </filter>
-    <filter id="10" type="mesh">
-      <bins>10</bins>
+    <filter id="2" type="mesh">
+      <bins>2</bins>
     </filter>
-    <tally id="9" name="regular mesh tally">
-      <filters>9</filters>
+    <tally id="1" name="regular mesh tally">
+      <filters>1</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
-    <tally id="10" name="unstructured mesh tally">
-      <filters>10</filters>
+    <tally id="2" name="unstructured mesh tally">
+      <filters>2</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true5.dat b/tests/regression_tests/unstructured_mesh/inputs_true5.dat
index dc9129d98..b2bbc7e16 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true5.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true5.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="16" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
-    <material id="17" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+    <material id="2" name="zircaloy">
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
-    <material id="18" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+    <material id="3" name="water">
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="16" material="16" name="fuel" region="91 -92 93 -94 95 -96" universe="6"/>
-    <cell id="17" material="17" name="clad" region="(-91 | 92 | -93 | 94 | -95 | 96) (97 -98 99 -100 101 -102)" universe="6"/>
-    <cell id="18" material="18" name="water" region="(-97 | 98 | -99 | 100 | -101 | 102) (103 -104 105 -106 107 -108)" universe="6"/>
-    <surface coeffs="-5.0" id="91" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="92" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="93" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="94" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="95" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="96" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="97" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="98" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="99" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="100" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="101" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="102" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15" id="103" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15" id="104" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15" id="105" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15" id="106" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15" id="107" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15" id="108" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
-    <particles>100</particles>
+    <particles>1000</particles>
     <batches>10</batches>
-    <source strength="1.0">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -56,34 +56,33 @@
           <parameters>0.0 1.0</parameters>
         </phi>
       </space>
-      <angle reference_uvw="-1.0 0.0 0.0" type="monodirectional"/>
       <energy type="discrete">
         <parameters>15000000.0 1.0</parameters>
       </energy>
     </source>
   </settings>
   <tallies>
-    <mesh id="11">
+    <mesh id="1">
       <dimension>10 10 10</dimension>
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="12" library="moab" type="unstructured">
-      <filename>test_mesh_tets.exo</filename>
+    <mesh id="2" type="unstructured" library="libmesh">
+      <filename>test_mesh_tets.e</filename>
     </mesh>
-    <filter id="11" type="mesh">
-      <bins>11</bins>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
     </filter>
-    <filter id="12" type="mesh">
-      <bins>12</bins>
+    <filter id="2" type="mesh">
+      <bins>2</bins>
     </filter>
-    <tally id="11" name="regular mesh tally">
-      <filters>11</filters>
+    <tally id="1" name="regular mesh tally">
+      <filters>1</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
-    <tally id="12" name="unstructured mesh tally">
-      <filters>12</filters>
+    <tally id="2" name="unstructured mesh tally">
+      <filters>2</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true6.dat b/tests/regression_tests/unstructured_mesh/inputs_true6.dat
index 82bc30703..d6ad0d98d 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true6.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true6.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="19" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
-    <material id="20" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+    <material id="2" name="zircaloy">
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
-    <material id="21" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+    <material id="3" name="water">
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="19" material="19" name="fuel" region="109 -110 111 -112 113 -114" universe="7"/>
-    <cell id="20" material="20" name="clad" region="(-109 | 110 | -111 | 112 | -113 | 114) (115 -116 117 -118 119 -120)" universe="7"/>
-    <cell id="21" material="21" name="water" region="(-115 | 116 | -117 | 118 | -119 | 120) (121 -122 123 -124 125 -126)" universe="7"/>
-    <surface coeffs="-5.0" id="109" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="110" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="111" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="112" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="113" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="114" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="115" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="116" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="117" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="118" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="119" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="120" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="121" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="122" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="123" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="124" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="125" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="126" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
-    <particles>100</particles>
+    <particles>1000</particles>
     <batches>10</batches>
-    <source strength="1.0">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -56,34 +56,33 @@
           <parameters>0.0 1.0</parameters>
         </phi>
       </space>
-      <angle reference_uvw="-1.0 0.0 0.0" type="monodirectional"/>
       <energy type="discrete">
         <parameters>15000000.0 1.0</parameters>
       </energy>
     </source>
   </settings>
   <tallies>
-    <mesh id="13">
+    <mesh id="1">
       <dimension>10 10 10</dimension>
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="14" library="moab" type="unstructured">
-      <filename>test_mesh_tets_w_holes.exo</filename>
+    <mesh id="2" type="unstructured" library="libmesh">
+      <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
-    <filter id="13" type="mesh">
-      <bins>13</bins>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
     </filter>
-    <filter id="14" type="mesh">
-      <bins>14</bins>
+    <filter id="2" type="mesh">
+      <bins>2</bins>
     </filter>
-    <tally id="13" name="regular mesh tally">
-      <filters>13</filters>
+    <tally id="1" name="regular mesh tally">
+      <filters>1</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
-    <tally id="14" name="unstructured mesh tally">
-      <filters>14</filters>
+    <tally id="2" name="unstructured mesh tally">
+      <filters>2</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true7.dat b/tests/regression_tests/unstructured_mesh/inputs_true7.dat
index b982c40d0..924790a8c 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true7.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true7.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="22" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
-    <material id="23" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+    <material id="2" name="zircaloy">
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
-    <material id="24" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+    <material id="3" name="water">
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="22" material="22" name="fuel" region="127 -128 129 -130 131 -132" universe="8"/>
-    <cell id="23" material="23" name="clad" region="(-127 | 128 | -129 | 130 | -131 | 132) (133 -134 135 -136 137 -138)" universe="8"/>
-    <cell id="24" material="24" name="water" region="(-133 | 134 | -135 | 136 | -137 | 138) (139 -140 141 -142 143 -144)" universe="8"/>
-    <surface coeffs="-5.0" id="127" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="128" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="129" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="130" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="131" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="132" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="133" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="134" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="135" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="136" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="137" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="138" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="139" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="140" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="141" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="142" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-10" id="143" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="10" id="144" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="10.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-10.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="10.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
-    <particles>100</particles>
+    <particles>1000</particles>
     <batches>10</batches>
-    <source strength="1.0">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -56,34 +56,33 @@
           <parameters>0.0 1.0</parameters>
         </phi>
       </space>
-      <angle reference_uvw="-1.0 0.0 0.0" type="monodirectional"/>
       <energy type="discrete">
         <parameters>15000000.0 1.0</parameters>
       </energy>
     </source>
   </settings>
   <tallies>
-    <mesh id="15">
+    <mesh id="1">
       <dimension>10 10 10</dimension>
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="16" library="moab" type="unstructured">
-      <filename>test_mesh_tets.exo</filename>
+    <mesh id="2" type="unstructured" library="libmesh">
+      <filename>test_mesh_tets.e</filename>
     </mesh>
-    <filter id="15" type="mesh">
-      <bins>15</bins>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
     </filter>
-    <filter id="16" type="mesh">
-      <bins>16</bins>
+    <filter id="2" type="mesh">
+      <bins>2</bins>
     </filter>
-    <tally id="15" name="regular mesh tally">
-      <filters>15</filters>
+    <tally id="1" name="regular mesh tally">
+      <filters>1</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
-    <tally id="16" name="unstructured mesh tally">
-      <filters>16</filters>
+    <tally id="2" name="unstructured mesh tally">
+      <filters>2</filters>
       <scores>flux</scores>
       <estimator>tracklength</estimator>
     </tally>
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true8.dat b/tests/regression_tests/unstructured_mesh/inputs_true8.dat
index 7994e1add..c04beb211 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true8.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true8.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets_w_holes.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/inputs_true9.dat b/tests/regression_tests/unstructured_mesh/inputs_true9.dat
index 4fff123d7..111898ff2 100644
--- a/tests/regression_tests/unstructured_mesh/inputs_true9.dat
+++ b/tests/regression_tests/unstructured_mesh/inputs_true9.dat
@@ -1,54 +1,54 @@
 <?xml version='1.0' encoding='utf-8'?>
 <model>
   <materials>
-    <material depletable="true" id="1" name="fuel">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
+    <material id="1" name="fuel" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
     </material>
     <material id="2" name="zircaloy">
-      <density units="g/cc" value="5.77"/>
-      <nuclide ao="0.5145" name="Zr90"/>
-      <nuclide ao="0.1122" name="Zr91"/>
-      <nuclide ao="0.1715" name="Zr92"/>
-      <nuclide ao="0.1738" name="Zr94"/>
-      <nuclide ao="0.028" name="Zr96"/>
+      <density value="5.77" units="g/cc"/>
+      <nuclide name="Zr90" ao="0.5145"/>
+      <nuclide name="Zr91" ao="0.1122"/>
+      <nuclide name="Zr92" ao="0.1715"/>
+      <nuclide name="Zr94" ao="0.1738"/>
+      <nuclide name="Zr96" ao="0.028"/>
     </material>
     <material id="3" name="water">
-      <density units="atom/b-cm" value="0.07416"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
+      <density value="0.07416" units="atom/b-cm"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="fuel" region="1 -2 3 -4 5 -6" universe="1"/>
-    <cell id="2" material="2" name="clad" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
-    <cell id="3" material="3" name="water" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
-    <surface coeffs="-5.0" id="1" name="minimum x" type="x-plane"/>
-    <surface coeffs="5.0" id="2" name="maximum x" type="x-plane"/>
-    <surface coeffs="-5.0" id="3" name="minimum y" type="y-plane"/>
-    <surface coeffs="5.0" id="4" name="maximum y" type="y-plane"/>
-    <surface coeffs="-5.0" id="5" name="minimum z" type="z-plane"/>
-    <surface coeffs="5.0" id="6" name="maximum z" type="z-plane"/>
-    <surface coeffs="-6.0" id="7" name="minimum x" type="x-plane"/>
-    <surface coeffs="6.0" id="8" name="maximum x" type="x-plane"/>
-    <surface coeffs="-6.0" id="9" name="minimum y" type="y-plane"/>
-    <surface coeffs="6.0" id="10" name="maximum y" type="y-plane"/>
-    <surface coeffs="-6.0" id="11" name="minimum z" type="z-plane"/>
-    <surface coeffs="6.0" id="12" name="maximum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="13" name="minimum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="14" name="maximum x" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="15" name="minimum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="16" name="maximum y" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="-15.0" id="17" name="minimum z" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="15.0" id="18" name="maximum z" type="z-plane"/>
+    <cell id="1" name="fuel" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <cell id="2" name="clad" material="2" region="(-1 | 2 | -3 | 4 | -5 | 6) (7 -8 9 -10 11 -12)" universe="1"/>
+    <cell id="3" name="water" material="3" region="(-7 | 8 | -9 | 10 | -11 | 12) (13 -14 15 -16 17 -18)" universe="1"/>
+    <surface id="1" name="minimum x" type="x-plane" coeffs="-5.0"/>
+    <surface id="2" name="maximum x" type="x-plane" coeffs="5.0"/>
+    <surface id="3" name="minimum y" type="y-plane" coeffs="-5.0"/>
+    <surface id="4" name="maximum y" type="y-plane" coeffs="5.0"/>
+    <surface id="5" name="minimum z" type="z-plane" coeffs="-5.0"/>
+    <surface id="6" name="maximum z" type="z-plane" coeffs="5.0"/>
+    <surface id="7" name="minimum x" type="x-plane" coeffs="-6.0"/>
+    <surface id="8" name="maximum x" type="x-plane" coeffs="6.0"/>
+    <surface id="9" name="minimum y" type="y-plane" coeffs="-6.0"/>
+    <surface id="10" name="maximum y" type="y-plane" coeffs="6.0"/>
+    <surface id="11" name="minimum z" type="z-plane" coeffs="-6.0"/>
+    <surface id="12" name="maximum z" type="z-plane" coeffs="6.0"/>
+    <surface id="13" name="minimum x" type="x-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="14" name="maximum x" type="x-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="15" name="minimum y" type="y-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="16" name="maximum y" type="y-plane" boundary="vacuum" coeffs="15.0"/>
+    <surface id="17" name="minimum z" type="z-plane" boundary="vacuum" coeffs="-15.0"/>
+    <surface id="18" name="maximum z" type="z-plane" boundary="vacuum" coeffs="15.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>10</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space origin="0.0 0.0 0.0" type="spherical">
-        <r parameters="0.0 0.0" type="uniform"/>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="spherical" origin="0.0 0.0 0.0">
+        <r type="uniform" parameters="0.0 0.0"/>
         <cos_theta type="discrete">
           <parameters>1.0 1.0</parameters>
         </cos_theta>
@@ -67,7 +67,7 @@
       <lower_left>-10.0 -10.0 -10.0</lower_left>
       <upper_right>10.0 10.0 10.0</upper_right>
     </mesh>
-    <mesh id="2" library="moab" options="MAX_DEPTH=15;PLANE_SET=2" type="unstructured">
+    <mesh id="2" type="unstructured" library="moab" options="MAX_DEPTH=15;PLANE_SET=2">
       <filename>test_mesh_tets.e</filename>
     </mesh>
     <filter id="1" type="mesh">
diff --git a/tests/regression_tests/unstructured_mesh/test.py b/tests/regression_tests/unstructured_mesh/test.py
index 0082198dd..7607531d8 100644
--- a/tests/regression_tests/unstructured_mesh/test.py
+++ b/tests/regression_tests/unstructured_mesh/test.py
@@ -256,7 +256,7 @@ for i, (lib, estimator, ext_geom, holes) in enumerate(product(*param_values)):
 def test_unstructured_mesh_tets(model, test_opts):
     # skip the test if the library is not enabled
     if test_opts['library'] == 'moab' and not openmc.lib._dagmc_enabled():
-        pytest.skip("DAGMC (and MOAB) mesh not enbaled in this build.")
+        pytest.skip("DAGMC (and MOAB) mesh not enabled in this build.")
 
     if test_opts['library'] == 'libmesh' and not openmc.lib._libmesh_enabled():
         pytest.skip("LibMesh is not enabled in this build.")
diff --git a/tests/regression_tests/unstructured_mesh/test_mesh_dagmc_tets.vtk b/tests/regression_tests/unstructured_mesh/test_mesh_dagmc_tets.vtk
new file mode 100644
index 000000000..2ddd228cf
--- /dev/null
+++ b/tests/regression_tests/unstructured_mesh/test_mesh_dagmc_tets.vtk
@@ -0,0 +1,159 @@
+# vtk DataFile Version 2.0
+made_with_cad_to_dagmc_package, Created by Gmsh 4.12.1 
+ASCII
+DATASET UNSTRUCTURED_GRID
+POINTS 14 double
+-0.5 -0.5 0.5
+-0.5 -0.5 -0.5
+-0.5 0.5 0.5
+-0.5 0.5 -0.5
+0.5 -0.5 0.5
+0.5 -0.5 -0.5
+0.5 0.5 0.5
+0.5 0.5 -0.5
+-0.5 0 0
+0.5 0 0
+0 -0.5 0
+0 0.5 0
+0 0 -0.5
+0 0 0.5
+
+CELLS 68 268
+1 0
+1 1
+1 2
+1 3
+1 4
+1 5
+1 6
+1 7
+2 1 0
+2 0 2
+2 3 2
+2 1 3
+2 5 4
+2 4 6
+2 7 6
+2 5 7
+2 1 5
+2 0 4
+2 3 7
+2 2 6
+3 1 0 8
+3 0 2 8
+3 3 1 8
+3 2 3 8
+3 5 9 4
+3 4 9 6
+3 7 9 5
+3 6 9 7
+3 0 1 10
+3 4 0 10
+3 1 5 10
+3 5 4 10
+3 2 11 3
+3 6 11 2
+3 3 11 7
+3 7 11 6
+3 1 3 12
+3 5 1 12
+3 3 7 12
+3 7 5 12
+3 0 13 2
+3 4 13 0
+3 2 13 6
+3 6 13 4
+4 13 8 12 10
+4 11 8 12 13
+4 12 11 13 9
+4 10 12 13 9
+4 12 3 11 7
+4 13 2 8 0
+4 8 12 1 3
+4 11 3 8 2
+4 10 8 1 0
+4 0 10 13 4
+4 1 12 10 5
+4 11 2 13 6
+4 4 9 13 6
+4 6 9 11 7
+4 10 9 4 5
+4 7 9 12 5
+4 3 8 12 11
+4 1 12 8 10
+4 8 11 2 13
+4 10 13 8 0
+4 13 10 9 4
+4 11 13 9 6
+4 12 11 9 7
+4 9 10 12 5
+
+CELL_TYPES 68
+1
+1
+1
+1
+1
+1
+1
+1
+3
+3
+3
+3
+3
+3
+3
+3
+3
+3
+3
+3
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+5
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
diff --git a/tests/regression_tests/void/inputs_true.dat b/tests/regression_tests/void/inputs_true.dat
index 7fee0779e..cd2d14206 100644
--- a/tests/regression_tests/void/inputs_true.dat
+++ b/tests/regression_tests/void/inputs_true.dat
@@ -2,8 +2,8 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cm3" value="7.14"/>
-      <nuclide ao="1.0" name="Zn64"/>
+      <density value="7.14" units="g/cm3"/>
+      <nuclide name="Zn64" ao="1.0"/>
     </material>
   </materials>
   <geometry>
@@ -58,62 +58,62 @@
     <cell id="49" material="1" region="48 -49" universe="1"/>
     <cell id="50" material="void" region="49 -50" universe="1"/>
     <cell id="51" material="1" region="50" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 1.0" id="1" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 3.020408163265306" id="2" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 5.040816326530612" id="3" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 7.061224489795918" id="4" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 9.081632653061224" id="5" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 11.102040816326529" id="6" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 13.122448979591836" id="7" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 15.142857142857142" id="8" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 17.163265306122447" id="9" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 19.183673469387752" id="10" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 21.204081632653057" id="11" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 23.224489795918366" id="12" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 25.24489795918367" id="13" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 27.265306122448976" id="14" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 29.285714285714285" id="15" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 31.30612244897959" id="16" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 33.326530612244895" id="17" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 35.3469387755102" id="18" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 37.367346938775505" id="19" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 39.38775510204081" id="20" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 41.408163265306115" id="21" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 43.42857142857142" id="22" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 45.44897959183673" id="23" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 47.469387755102034" id="24" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 49.48979591836734" id="25" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 51.51020408163265" id="26" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 53.53061224489795" id="27" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 55.55102040816326" id="28" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 57.57142857142857" id="29" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 59.59183673469387" id="30" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 61.61224489795918" id="31" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 63.63265306122448" id="32" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 65.65306122448979" id="33" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 67.67346938775509" id="34" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 69.6938775510204" id="35" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 71.71428571428571" id="36" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 73.73469387755101" id="37" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 75.75510204081633" id="38" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 77.77551020408163" id="39" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 79.79591836734693" id="40" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 81.81632653061223" id="41" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 83.83673469387755" id="42" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 85.85714285714285" id="43" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 87.87755102040815" id="44" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 89.89795918367346" id="45" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 91.91836734693877" id="46" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 93.93877551020407" id="47" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 95.95918367346938" id="48" type="sphere"/>
-    <surface coeffs="0.0 0.0 0.0 97.97959183673468" id="49" type="sphere"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 100.0" id="50" type="sphere"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" coeffs="0.0 0.0 0.0 3.020408163265306"/>
+    <surface id="3" type="sphere" coeffs="0.0 0.0 0.0 5.040816326530612"/>
+    <surface id="4" type="sphere" coeffs="0.0 0.0 0.0 7.061224489795918"/>
+    <surface id="5" type="sphere" coeffs="0.0 0.0 0.0 9.081632653061224"/>
+    <surface id="6" type="sphere" coeffs="0.0 0.0 0.0 11.102040816326529"/>
+    <surface id="7" type="sphere" coeffs="0.0 0.0 0.0 13.122448979591836"/>
+    <surface id="8" type="sphere" coeffs="0.0 0.0 0.0 15.142857142857142"/>
+    <surface id="9" type="sphere" coeffs="0.0 0.0 0.0 17.163265306122447"/>
+    <surface id="10" type="sphere" coeffs="0.0 0.0 0.0 19.183673469387752"/>
+    <surface id="11" type="sphere" coeffs="0.0 0.0 0.0 21.204081632653057"/>
+    <surface id="12" type="sphere" coeffs="0.0 0.0 0.0 23.224489795918366"/>
+    <surface id="13" type="sphere" coeffs="0.0 0.0 0.0 25.24489795918367"/>
+    <surface id="14" type="sphere" coeffs="0.0 0.0 0.0 27.265306122448976"/>
+    <surface id="15" type="sphere" coeffs="0.0 0.0 0.0 29.285714285714285"/>
+    <surface id="16" type="sphere" coeffs="0.0 0.0 0.0 31.30612244897959"/>
+    <surface id="17" type="sphere" coeffs="0.0 0.0 0.0 33.326530612244895"/>
+    <surface id="18" type="sphere" coeffs="0.0 0.0 0.0 35.3469387755102"/>
+    <surface id="19" type="sphere" coeffs="0.0 0.0 0.0 37.367346938775505"/>
+    <surface id="20" type="sphere" coeffs="0.0 0.0 0.0 39.38775510204081"/>
+    <surface id="21" type="sphere" coeffs="0.0 0.0 0.0 41.408163265306115"/>
+    <surface id="22" type="sphere" coeffs="0.0 0.0 0.0 43.42857142857142"/>
+    <surface id="23" type="sphere" coeffs="0.0 0.0 0.0 45.44897959183673"/>
+    <surface id="24" type="sphere" coeffs="0.0 0.0 0.0 47.469387755102034"/>
+    <surface id="25" type="sphere" coeffs="0.0 0.0 0.0 49.48979591836734"/>
+    <surface id="26" type="sphere" coeffs="0.0 0.0 0.0 51.51020408163265"/>
+    <surface id="27" type="sphere" coeffs="0.0 0.0 0.0 53.53061224489795"/>
+    <surface id="28" type="sphere" coeffs="0.0 0.0 0.0 55.55102040816326"/>
+    <surface id="29" type="sphere" coeffs="0.0 0.0 0.0 57.57142857142857"/>
+    <surface id="30" type="sphere" coeffs="0.0 0.0 0.0 59.59183673469387"/>
+    <surface id="31" type="sphere" coeffs="0.0 0.0 0.0 61.61224489795918"/>
+    <surface id="32" type="sphere" coeffs="0.0 0.0 0.0 63.63265306122448"/>
+    <surface id="33" type="sphere" coeffs="0.0 0.0 0.0 65.65306122448979"/>
+    <surface id="34" type="sphere" coeffs="0.0 0.0 0.0 67.67346938775509"/>
+    <surface id="35" type="sphere" coeffs="0.0 0.0 0.0 69.6938775510204"/>
+    <surface id="36" type="sphere" coeffs="0.0 0.0 0.0 71.71428571428571"/>
+    <surface id="37" type="sphere" coeffs="0.0 0.0 0.0 73.73469387755101"/>
+    <surface id="38" type="sphere" coeffs="0.0 0.0 0.0 75.75510204081633"/>
+    <surface id="39" type="sphere" coeffs="0.0 0.0 0.0 77.77551020408163"/>
+    <surface id="40" type="sphere" coeffs="0.0 0.0 0.0 79.79591836734693"/>
+    <surface id="41" type="sphere" coeffs="0.0 0.0 0.0 81.81632653061223"/>
+    <surface id="42" type="sphere" coeffs="0.0 0.0 0.0 83.83673469387755"/>
+    <surface id="43" type="sphere" coeffs="0.0 0.0 0.0 85.85714285714285"/>
+    <surface id="44" type="sphere" coeffs="0.0 0.0 0.0 87.87755102040815"/>
+    <surface id="45" type="sphere" coeffs="0.0 0.0 0.0 89.89795918367346"/>
+    <surface id="46" type="sphere" coeffs="0.0 0.0 0.0 91.91836734693877"/>
+    <surface id="47" type="sphere" coeffs="0.0 0.0 0.0 93.93877551020407"/>
+    <surface id="48" type="sphere" coeffs="0.0 0.0 0.0 95.95918367346938"/>
+    <surface id="49" type="sphere" coeffs="0.0 0.0 0.0 97.97959183673468"/>
+    <surface id="50" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 100.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>1000</particles>
     <batches>3</batches>
-    <source particle="neutron" strength="1.0" type="independent">
+    <source type="independent" strength="1.0" particle="neutron">
       <space type="point">
         <parameters>0.0 0.0 0.0</parameters>
       </space>
diff --git a/tests/regression_tests/volume_calc/inputs_true.dat b/tests/regression_tests/volume_calc/inputs_true.dat
index e3a8162e3..ed7024c57 100644
--- a/tests/regression_tests/volume_calc/inputs_true.dat
+++ b/tests/regression_tests/volume_calc/inputs_true.dat
@@ -2,27 +2,27 @@
 <model>
   <materials>
     <material id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.0001" name="B10"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.0001"/>
       <sab name="c_H_in_H2O"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="0.1" name="Mo99"/>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="Mo99" ao="0.1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="2" region="-1 -3 5" universe="0"/>
     <cell id="2" material="1" region="-2 3" universe="0"/>
     <cell id="3" material="1" region="-4 -3" universe="0"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 1.0" id="1" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 5.0 1.0" id="2" type="sphere"/>
-    <surface coeffs="5.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 -5.0 1.0" id="4" type="sphere"/>
-    <surface coeffs="-5.0" id="5" type="z-plane"/>
+    <surface id="1" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 5.0 1.0"/>
+    <surface id="3" type="z-plane" coeffs="5.0"/>
+    <surface id="4" type="sphere" boundary="vacuum" coeffs="0.0 0.0 -5.0 1.0"/>
+    <surface id="5" type="z-plane" coeffs="-5.0"/>
   </geometry>
   <settings>
     <run_mode>volume</run_mode>
@@ -53,7 +53,7 @@
       <samples>100</samples>
       <lower_left>-1.0 -1.0 -6.0</lower_left>
       <upper_right>1.0 1.0 6.0</upper_right>
-      <threshold threshold="0.1" type="std_dev"/>
+      <threshold type="std_dev" threshold="0.1"/>
     </volume_calc>
     <volume_calc>
       <domain_type>material</domain_type>
@@ -61,7 +61,7 @@
       <samples>100</samples>
       <lower_left>-1.0 -1.0 -6.0</lower_left>
       <upper_right>1.0 1.0 6.0</upper_right>
-      <threshold threshold="0.1" type="rel_err"/>
+      <threshold type="rel_err" threshold="0.1"/>
     </volume_calc>
     <volume_calc>
       <domain_type>cell</domain_type>
@@ -69,7 +69,7 @@
       <samples>100</samples>
       <lower_left>-1.0 -1.0 -6.0</lower_left>
       <upper_right>1.0 1.0 6.0</upper_right>
-      <threshold threshold="0.05" type="variance"/>
+      <threshold type="variance" threshold="0.05"/>
     </volume_calc>
   </settings>
   </model>
diff --git a/tests/regression_tests/volume_calc/inputs_true_mg.dat b/tests/regression_tests/volume_calc/inputs_true_mg.dat
index 2ace5cd3c..127566084 100644
--- a/tests/regression_tests/volume_calc/inputs_true_mg.dat
+++ b/tests/regression_tests/volume_calc/inputs_true_mg.dat
@@ -3,26 +3,26 @@
   <materials>
     <cross_sections>mg_lib.h5</cross_sections>
     <material id="1">
-      <density units="g/cc" value="1.0"/>
-      <nuclide ao="2.0" name="H1"/>
-      <nuclide ao="1.0" name="O16"/>
-      <nuclide ao="0.0001" name="B10"/>
+      <density value="1.0" units="g/cc"/>
+      <nuclide name="H1" ao="2.0"/>
+      <nuclide name="O16" ao="1.0"/>
+      <nuclide name="B10" ao="0.0001"/>
     </material>
-    <material depletable="true" id="2">
-      <density units="g/cc" value="4.5"/>
-      <nuclide ao="1.0" name="U235"/>
-      <nuclide ao="0.1" name="Mo99"/>
+    <material id="2" depletable="true">
+      <density value="4.5" units="g/cc"/>
+      <nuclide name="U235" ao="1.0"/>
+      <nuclide name="Mo99" ao="0.1"/>
     </material>
   </materials>
   <geometry>
     <cell id="1" material="2" region="-1 -3 5" universe="0"/>
     <cell id="2" material="1" region="-2 3" universe="0"/>
     <cell id="3" material="1" region="-4 -3" universe="0"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 1.0" id="1" type="z-cylinder"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 5.0 1.0" id="2" type="sphere"/>
-    <surface coeffs="5.0" id="3" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 -5.0 1.0" id="4" type="sphere"/>
-    <surface coeffs="-5.0" id="5" type="z-plane"/>
+    <surface id="1" type="z-cylinder" boundary="vacuum" coeffs="0.0 0.0 1.0"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 5.0 1.0"/>
+    <surface id="3" type="z-plane" coeffs="5.0"/>
+    <surface id="4" type="sphere" boundary="vacuum" coeffs="0.0 0.0 -5.0 1.0"/>
+    <surface id="5" type="z-plane" coeffs="-5.0"/>
   </geometry>
   <settings>
     <run_mode>volume</run_mode>
@@ -54,7 +54,7 @@
       <samples>100</samples>
       <lower_left>-1.0 -1.0 -6.0</lower_left>
       <upper_right>1.0 1.0 6.0</upper_right>
-      <threshold threshold="0.1" type="std_dev"/>
+      <threshold type="std_dev" threshold="0.1"/>
     </volume_calc>
     <volume_calc>
       <domain_type>material</domain_type>
@@ -62,7 +62,7 @@
       <samples>100</samples>
       <lower_left>-1.0 -1.0 -6.0</lower_left>
       <upper_right>1.0 1.0 6.0</upper_right>
-      <threshold threshold="0.1" type="rel_err"/>
+      <threshold type="rel_err" threshold="0.1"/>
     </volume_calc>
     <volume_calc>
       <domain_type>cell</domain_type>
@@ -70,7 +70,7 @@
       <samples>100</samples>
       <lower_left>-1.0 -1.0 -6.0</lower_left>
       <upper_right>1.0 1.0 6.0</upper_right>
-      <threshold threshold="0.05" type="variance"/>
+      <threshold type="variance" threshold="0.05"/>
     </volume_calc>
   </settings>
   </model>
diff --git a/tests/regression_tests/weightwindows/generators/test.py b/tests/regression_tests/weightwindows/generators/test.py
index d6a40b44f..4dc0ab80d 100644
--- a/tests/regression_tests/weightwindows/generators/test.py
+++ b/tests/regression_tests/weightwindows/generators/test.py
@@ -46,14 +46,14 @@ def test_ww_generator(run_in_tmpdir):
     # just test that the generation happens successfully here
     assert os.path.exists('weight_windows.h5')
 
-    wws_mean = openmc.hdf5_to_wws()
+    wws_mean = openmc.WeightWindowsList.from_hdf5()
     assert len(wws_mean) == 1
 
     # check that generation using the relative error works too
     wwg.update_parameters['value'] = 'rel_err'
     model.run()
 
-    wws_rel_err = openmc.hdf5_to_wws()
+    wws_rel_err = openmc.WeightWindowsList.from_hdf5()
     assert len(wws_rel_err) == 1
 
     # we should not get the same set of weight windows when switching to use of
diff --git a/tests/regression_tests/weightwindows/inputs_true.dat b/tests/regression_tests/weightwindows/inputs_true.dat
deleted file mode 100644
index dcc41ae84..000000000
--- a/tests/regression_tests/weightwindows/inputs_true.dat
+++ /dev/null
@@ -1,95 +0,0 @@
-<?xml version='1.0' encoding='utf-8'?>
-<model>
-  <materials>
-    <material id="1">
-      <density units="g/cc" value="2.3"/>
-      <nuclide ao="0.168018676" name="H1"/>
-      <nuclide ao="1.93244e-05" name="H2"/>
-      <nuclide ao="0.561814465" name="O16"/>
-      <nuclide ao="0.00021401" name="O17"/>
-      <nuclide ao="0.021365" name="Na23"/>
-      <nuclide ao="0.021343" name="Al27"/>
-      <nuclide ao="0.187439342" name="Si28"/>
-      <nuclide ao="0.009517714" name="Si29"/>
-      <nuclide ao="0.006273944" name="Si30"/>
-      <nuclide ao="0.018026179" name="Ca40"/>
-      <nuclide ao="0.00012031" name="Ca42"/>
-      <nuclide ao="2.51033e-05" name="Ca43"/>
-      <nuclide ao="0.000387892" name="Ca44"/>
-      <nuclide ao="7.438e-07" name="Ca46"/>
-      <nuclide ao="3.47727e-05" name="Ca48"/>
-      <nuclide ao="0.000248179" name="Fe54"/>
-      <nuclide ao="0.003895875" name="Fe56"/>
-      <nuclide ao="8.99727e-05" name="Fe57"/>
-      <nuclide ao="1.19737e-05" name="Fe58"/>
-    </material>
-  </materials>
-  <geometry>
-    <cell id="1" material="1" region="-1" universe="1"/>
-    <cell id="2" material="void" region="1 -2" universe="1"/>
-    <surface coeffs="0.0 0.0 0.0 240" id="1" type="sphere"/>
-    <surface boundary="vacuum" coeffs="0.0 0.0 0.0 250" id="2" type="sphere"/>
-  </geometry>
-  <settings>
-    <run_mode>fixed source</run_mode>
-    <particles>200</particles>
-    <batches>2</batches>
-    <source particle="neutron" strength="1.0" type="independent">
-      <space type="point">
-        <parameters>0.001 0.001 0.001</parameters>
-      </space>
-      <energy type="discrete">
-        <parameters>14000000.0 1.0</parameters>
-      </energy>
-    </source>
-    <photon_transport>true</photon_transport>
-    <weight_windows id="1">
-      <mesh>2</mesh>
-      <particle_type>neutron</particle_type>
-      <energy_bounds>0.0 0.5 20000000.0</energy_bounds>
-      <lower_ww_bounds>-1.0 0.007673145137236567 6.542700644645627e-07 0.0017541380096893788 0.0007245451610090619 7.476545089278482e-06 2.253382683081525e-06 2.1040609012865134e-05 0.08285899874014539 -1.0 1.0262431550731535e-13 3.3164825349503764e-16 0.0064803314120165335 3.5981031766416143e-06 -1.0 1.6205064883026195e-11 1.7770136411255912e-05 0.0001670700632870335 4.323138781337963e-05 -1.0 -1.0 6.922231352729155e-05 0.06550426960512012 -1.0 2.5639656866250453e-06 1.4508262073256659e-13 -1.0 -1.0 0.0009127050763987511 3.8523247550378815e-12 -1.0 0.0008175033526488423 -1.0 0.015148978428271613 2.5157355790201773e-07 2.1612150425221703e-06 1.3181562352445092e-10 2.9282097947816224e-05 2.762955748645507e-05 2.936268747135548e-14 1.6859219614058024e-19 3.937160538176268e-07 2.6011787393916806e-06 0.001570829313580841 -1.0 1.7041669224797384e-09 1.6421491993751715e-11 0.4899999304038166 6.948292975998466e-07 -1.0 -1.0 0.08657573566388353 0.006639379668946672 1.681305446488884e-11 2.2487617828938326e-05 4.109066205029878e-05 0.08288806458368345 -1.0 0.007385384405891666 -1.0 -1.0 1.008394964750947e-06 6.655926357459275e-08 0.0016279794552427574 1.4825219199133353e-12 -1.0 1.93847081892941e-05 1.284759166582202e-05 5.1161740516205715e-05 2.5447281241730366e-07 3.853097455417877e-06 -1.0 8.887234042637684e-05 0.01621821782442112 -1.0 1.8747824667478637e-14 5.97653814110781e-06 -1.0 -1.0 1.2296747260635405e-06 -1.0 -1.0 7.754700849337902e-06 7.091263906557291e-05 3.2799045883372075e-06 0.00020573181450240786 5.6949597066784976e-05 1.271128669411295e-07 0.0010019432401508087 -1.0 -1.0 6.848642863465585e-07 5.783974359937857e-06 2.380955631371226e-06 4.756872959630257e-05 2.1360401784344975e-18 0.00046029617115127556 1.0092905083158804e-11 0.08095280568305474 3.89841667138598e-08 4.354946981329799e-05 -1.0 0.5 0.0015447823995470042 -1.0 -1.0 0.001959411999677113 0.48573842213878143 -1.0 7.962899789920809e-06 5.619835883512353e-14 0.0835648710074336 2.7207433165796702e-11 0.00027464244062887683 4.076762579823379e-17 2.124999182004655e-05 8.037131813528501e-07 8.846493399850663e-06 1.080711295768551e-05 -1.0 -1.0 0.0010737081832502356 2.679340942769232e-06 2.1648918040467363e-05 0.0002668579837809081 1.3455341306162382e-05 2.5629068330446215e-05 1.3228013765525015e-05 -1.0 -1.0 7.002861620919232e-08 1.5172912057312398e-14 -1.0 7.989579285134734e-06 2.639268952045579e-10 -1.0 0.013330619853379314 0.00021534568699157805 2.4469456008493614e-09 8.211982683649991e-09 1.951356547084204e-15 0.00016419031150495913 5.985108617124989e-06 1.28584557092134e-05 1.5476769237571295e-14 5.5379154134462336e-08 0.0016637126543321873 2.499931357628383e-07 8.020767115181574e-07 2.7697233171399416e-13 -1.0 0.007197662162700777 -1.0 0.0833931797381589 2.8226276944532696e-05 3.353295403842037e-05 5.504813693036933e-12 0.007136750029575816 0.08636569925236791 -1.0 -1.0 6.315154082213375e-07 0.4877856021170283 -1.0 3.7051518220083886e-06 -1.0 0.0017950613169359904 7.84289689494925e-06 9.041149893307806e-07 -1.0 8.48176145671274e-10 9.03108551826469e-05 3.634644995026692e-05 2.0295213621716706e-09 1.0305672698745657e-11 1.5550455959178486e-06 0.01664109232689093 8.533030345699353e-17 0.0009252532821720597 -1.0 7.695031155172816e-14 0.0007151459162818186 -1.0 -1.0 5.182269672663266e-10 3.0160758454323657e-07 -1.0 0.06539005235506369 3.452537179033895e-05 -1.0 -1.0 3.855884234344845e-06 0.00017699858357744463 2.594951085377588e-06 1.1333627604823673e-09 -1.0 2.4306375693722205e-06 0.0063790654507599135 2.2060992580622125e-11 -1.0 -1.0 0.0848872967381878 3.932389299316285e-06 1.2911827198500372e-08 1.957092814065688e-05 0.0007213722022489493 0.0018327593437608 1.6699254277734109e-06 0.007188410694198128 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0</lower_ww_bounds>
-      <upper_ww_bounds>-10.0 0.07673145137236567 6.5427006446456266e-06 0.017541380096893787 0.0072454516100906195 7.476545089278483e-05 2.253382683081525e-05 0.00021040609012865135 0.828589987401454 -10.0 1.0262431550731535e-12 3.3164825349503764e-15 0.06480331412016534 3.5981031766416144e-05 -10.0 1.6205064883026195e-10 0.0001777013641125591 0.001670700632870335 0.0004323138781337963 -10.0 -10.0 0.0006922231352729155 0.6550426960512012 -10.0 2.5639656866250452e-05 1.4508262073256658e-12 -10.0 -10.0 0.009127050763987512 3.852324755037882e-11 -10.0 0.008175033526488424 -10.0 0.15148978428271614 2.515735579020177e-06 2.1612150425221703e-05 1.3181562352445092e-09 0.00029282097947816227 0.0002762955748645507 2.9362687471355483e-13 1.6859219614058023e-18 3.937160538176268e-06 2.6011787393916804e-05 0.015708293135808408 -10.0 1.7041669224797384e-08 1.6421491993751715e-10 4.899999304038166 6.948292975998466e-06 -10.0 -10.0 0.8657573566388354 0.06639379668946672 1.681305446488884e-10 0.00022487617828938325 0.0004109066205029878 0.8288806458368345 -10.0 0.07385384405891665 -10.0 -10.0 1.0083949647509469e-05 6.655926357459275e-07 0.016279794552427576 1.4825219199133352e-11 -10.0 0.000193847081892941 0.0001284759166582202 0.0005116174051620571 2.5447281241730365e-06 3.853097455417877e-05 -10.0 0.0008887234042637684 0.16218217824421122 -10.0 1.8747824667478637e-13 5.97653814110781e-05 -10.0 -10.0 1.2296747260635405e-05 -10.0 -10.0 7.754700849337902e-05 0.0007091263906557291 3.2799045883372076e-05 0.0020573181450240785 0.0005694959706678497 1.271128669411295e-06 0.010019432401508087 -10.0 -10.0 6.848642863465585e-06 5.7839743599378564e-05 2.380955631371226e-05 0.0004756872959630257 2.1360401784344976e-17 0.004602961711512756 1.0092905083158804e-10 0.8095280568305474 3.89841667138598e-07 0.00043549469813297995 -10.0 5.0 0.015447823995470043 -10.0 -10.0 0.01959411999677113 4.8573842213878144 -10.0 7.96289978992081e-05 5.619835883512353e-13 0.8356487100743359 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diff --git a/tests/regression_tests/weightwindows/local/inputs_true.dat b/tests/regression_tests/weightwindows/local/inputs_true.dat
new file mode 100644
index 000000000..57dc42ca4
--- /dev/null
+++ b/tests/regression_tests/weightwindows/local/inputs_true.dat
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+      <upper_ww_bounds>-10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.00020445691096074745 -10.0 -10.0 -10.0 0.0013281328509288383 0.006267128082339883 -10.0 -10.0 -10.0 -10.0 0.004209808495056742 0.0005340221214300681 -10.0 -10.0 -10.0 0.00015536934920423439 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.00037096847855980485 -10.0 -10.0 0.000541969614042385 0.007918090215756443 0.0009763572147337743 -10.0 -10.0 0.026398319229115234 0.4039871468258457 0.02806654735003014 -10.0 -10.0 0.027608366482897245 0.40190978087723006 0.025084416103979976 -10.0 -10.0 0.0009774868433298411 0.0061264049168796506 0.0014841730774317252 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 9.753424087171071e-05 0.00017610849405640073 -10.0 -10.0 0.001160370359598608 0.02822592083222468 0.003897177301239376 -10.0 -10.0 0.14965179733172532 5.0 0.11723491704290401 -10.0 0.0002260493623875525 0.15157364795884923 4.937699395340239 0.13111419473204966 -10.0 -10.0 0.0014919915366377564 0.02197964829133137 0.003209711829219673 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.00014806422867046437 -10.0 -10.0 -10.0 0.0009207268175745281 0.007066033561638983 -10.0 -10.0 -10.0 0.03246064903858183 0.39054930280659084 0.025380574501073606 -10.0 -10.0 0.03249926021191793 0.4335567738779479 0.03157721455701706 0.0004521352809838806 -10.0 0.0018268090756001904 0.004353654810741932 0.001127325957309237 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 9.650194311662424e-05 0.0029403373970835075 0.0013506203419326437 -10.0 -10.0 4.5065302725431816e-05 0.002772532363874424 0.0003547290864255937 -10.0 -10.0 -10.0 0.0008786172732576295 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0</upper_ww_bounds>
+      <survival_ratio>3.0</survival_ratio>
+      <max_lower_bound_ratio>1.5</max_lower_bound_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <shared_secondary_bank>false</shared_secondary_bank>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+    <max_history_splits>200</max_history_splits>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>5 10 15</dimension>
+      <lower_left>-240 -240 -240</lower_left>
+      <upper_right>240 240 240</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 0.5 20000000.0</bins>
+    </filter>
+    <filter id="3" type="particle">
+      <bins>neutron photon</bins>
+    </filter>
+    <tally id="1">
+      <filters>1 2 3</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows/local/results_true.dat b/tests/regression_tests/weightwindows/local/results_true.dat
new file mode 100644
index 000000000..d3dfa119d
--- /dev/null
+++ b/tests/regression_tests/weightwindows/local/results_true.dat
@@ -0,0 +1 @@
+a78972fe3c0dadfc256cfb139c5ca8c7b634738e1895ad2d6286ed5e2567c6dc518e499363908e9058432684148a706a179a39110f703d5322491184a1d0c3e4
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/results_true.dat b/tests/regression_tests/weightwindows/results_true.dat
deleted file mode 100644
index 6c05af31a..000000000
--- a/tests/regression_tests/weightwindows/results_true.dat
+++ /dev/null
@@ -1 +0,0 @@
-ebc761815175b25fc95a226174928c226a3ab5dbf3b2a2abf09e079a0b87dee1dee74aa9b9eaec35acd58b8c481d264be7e9b3f052905bcc14ccd76f36b01549
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/shared/inputs_true.dat b/tests/regression_tests/weightwindows/shared/inputs_true.dat
new file mode 100644
index 000000000..6ec7d12e3
--- /dev/null
+++ b/tests/regression_tests/weightwindows/shared/inputs_true.dat
@@ -0,0 +1,94 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="2.3" units="g/cc"/>
+      <nuclide name="H1" ao="0.168018676"/>
+      <nuclide name="O16" ao="0.561814465"/>
+      <nuclide name="O17" ao="0.00021401"/>
+      <nuclide name="Na23" ao="0.021365"/>
+      <nuclide name="Al27" ao="0.021343"/>
+      <nuclide name="Si28" ao="0.187439342"/>
+      <nuclide name="Si29" ao="0.009517714"/>
+      <nuclide name="Si30" ao="0.006273944"/>
+      <nuclide name="Ca40" ao="0.018026179"/>
+      <nuclide name="Ca42" ao="0.00012031"/>
+      <nuclide name="Ca44" ao="0.000387892"/>
+      <nuclide name="Fe54" ao="0.000248179"/>
+      <nuclide name="Fe56" ao="0.003895875"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="-1" universe="1"/>
+    <cell id="2" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 240"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 250"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>500</particles>
+    <batches>2</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="point">
+        <parameters>0.001 0.001 0.001</parameters>
+      </space>
+      <energy type="discrete">
+        <parameters>14000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <photon_transport>true</photon_transport>
+    <weight_windows id="1">
+      <mesh>2</mesh>
+      <particle_type>neutron</particle_type>
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+      <survival_ratio>3.0</survival_ratio>
+      <max_lower_bound_ratio>1.5</max_lower_bound_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <shared_secondary_bank>true</shared_secondary_bank>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+    <max_history_splits>200</max_history_splits>
+  </settings>
+  <tallies>
+    <mesh id="1">
+      <dimension>5 10 15</dimension>
+      <lower_left>-240 -240 -240</lower_left>
+      <upper_right>240 240 240</upper_right>
+    </mesh>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 0.5 20000000.0</bins>
+    </filter>
+    <filter id="3" type="particle">
+      <bins>neutron photon</bins>
+    </filter>
+    <tally id="1">
+      <filters>1 2 3</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows/shared/results_true.dat b/tests/regression_tests/weightwindows/shared/results_true.dat
new file mode 100644
index 000000000..01381db32
--- /dev/null
+++ b/tests/regression_tests/weightwindows/shared/results_true.dat
@@ -0,0 +1 @@
+27c2d218ecf46161088003fbb39ccd63495f80d6e0b27dbc3b3e845b49b282dabf0eaba4fb619be15c6116fc963ec5ce1fe4197efbe4084972ec761e02e70eb5
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/survival_biasing/__init__.py b/tests/regression_tests/weightwindows/survival_biasing/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/weightwindows/survival_biasing/local/inputs_true.dat b/tests/regression_tests/weightwindows/survival_biasing/local/inputs_true.dat
new file mode 100644
index 000000000..fe144dc62
--- /dev/null
+++ b/tests/regression_tests/weightwindows/survival_biasing/local/inputs_true.dat
@@ -0,0 +1,69 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="Tungsten">
+      <density value="19.25" units="g/cm3"/>
+      <nuclide name="W180" ao="0.0012"/>
+      <nuclide name="W182" ao="0.265"/>
+      <nuclide name="W183" ao="0.1431"/>
+      <nuclide name="W184" ao="0.3064"/>
+      <nuclide name="W186" ao="0.2843"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="160.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="160.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="reflective" coeffs="160.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>50</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="cartesian">
+        <x type="discrete">
+          <parameters>0.01 1.0</parameters>
+        </x>
+        <y type="uniform" parameters="0.0 160.0"/>
+        <z type="uniform" parameters="0.0 160.0"/>
+      </space>
+      <angle type="monodirectional" reference_uvw="1.0 0.0 0.0"/>
+      <energy type="discrete">
+        <parameters>14100000.0 1.0</parameters>
+      </energy>
+    </source>
+    <survival_biasing>true</survival_biasing>
+    <weight_windows id="1">
+      <mesh>1</mesh>
+      <particle_type>neutron</particle_type>
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+      <survival_ratio>3.0</survival_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <mesh id="1">
+      <dimension>20 20 1</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <upper_right>160.0 160.0 160.0</upper_right>
+    </mesh>
+    <shared_secondary_bank>false</shared_secondary_bank>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+  </settings>
+  <tallies>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <tally id="1" name="flux">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat b/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat
new file mode 100644
index 000000000..b457a245c
--- /dev/null
+++ b/tests/regression_tests/weightwindows/survival_biasing/local/results_true.dat
@@ -0,0 +1 @@
+386e507008ed3c72c6e1a101aafc01cfaaff2c0b10555558d1eab6332441f7b17264b55002d721151bac2f3e7c1a8aac4c5ed243f5270533d171850f985206ed
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/survival_biasing/shared/inputs_true.dat b/tests/regression_tests/weightwindows/survival_biasing/shared/inputs_true.dat
new file mode 100644
index 000000000..bafff89c1
--- /dev/null
+++ b/tests/regression_tests/weightwindows/survival_biasing/shared/inputs_true.dat
@@ -0,0 +1,69 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1" name="Tungsten">
+      <density value="19.25" units="g/cm3"/>
+      <nuclide name="W180" ao="0.0012"/>
+      <nuclide name="W182" ao="0.265"/>
+      <nuclide name="W183" ao="0.1431"/>
+      <nuclide name="W184" ao="0.3064"/>
+      <nuclide name="W186" ao="0.2843"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" material="1" region="1 -2 3 -4 5 -6" universe="1"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="160.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="reflective" coeffs="160.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="reflective" coeffs="160.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>50</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="neutron">
+      <space type="cartesian">
+        <x type="discrete">
+          <parameters>0.01 1.0</parameters>
+        </x>
+        <y type="uniform" parameters="0.0 160.0"/>
+        <z type="uniform" parameters="0.0 160.0"/>
+      </space>
+      <angle type="monodirectional" reference_uvw="1.0 0.0 0.0"/>
+      <energy type="discrete">
+        <parameters>14100000.0 1.0</parameters>
+      </energy>
+    </source>
+    <survival_biasing>true</survival_biasing>
+    <weight_windows id="1">
+      <mesh>1</mesh>
+      <particle_type>neutron</particle_type>
+      <lower_ww_bounds>0.46135961568957096 0.2874485390202603 0.13256013950494605 0.052765209609807295 0.020958440832685638 0.006317250035749876 0.002627037175682506 0.00030032343592437284 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4611178493390162 0.28624862560842024 0.13999870622621136 0.05508479408959756 0.018281241958254993 0.0055577764872361555 0.0015265432226293397 0.00024298772352636966 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.46294957913063317 0.2857734367546051 0.13365623190336945 0.05034742166227103 0.017525851812913266 0.005096725451851077 0.0026297640951531403 0.00033732424392026144 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.45772598750472554 0.281494921471495 0.13604397962762246 0.054792881472295364 0.019802376898755525 0.0073351745579128495 0.002067392946066426 9.529474085926143e-05 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4739762983490318 0.28419061537046764 0.12757509901507888 0.0516920293019733 0.01919167874787235 0.007593035964707848 0.0017488176314107277 9.678267909681988e-05 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.47560199098558276 0.27728389146956994 0.12760802572535623 0.05251965811713552 0.022498960644951632 0.01063372459325151 0.004242071054914633 0.00045603112202665183 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.47659199471934693 0.28886666944252215 0.1258637573398161 0.05818862375955657 0.020476313720744762 0.007143193244018263 0.0022364083022515273 0.0003953123781408474 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4693487369606823 0.27180862139490125 0.12833455570423483 0.055146743559938344 0.020667403529470333 0.007891508723137146 0.001643551705407358 0.0006501416781353278 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4538724931023337 0.2824491421084296 0.1275341706351566 0.055027501141893705 0.02305114640508087 0.008599303158607198 0.0025082611194161804 0.000518530311231696 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.45258444753220123 0.26839102466484255 0.12991633918797083 0.05442263709947767 0.019695895223471486 0.005353204961887518 0.0015074698293840157 0.0001598680898180058 8.872047880811405e-05 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4482467985982804 0.2813276470183359 0.13449860790768647 0.056103278190533436 0.026274320334196962 0.006884071908429303 0.0033099390738735458 0.0005337454397674922 0.0001386380399465575 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4875748950139487 0.2908586705316248 0.14212060658392278 0.06068108009637272 0.019467970468789477 0.00637102427946399 0.0028510425266191687 0.0012184421778115488 0.00029404541567146187 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.5 0.2852101551297824 0.13622307172321643 0.058302519884339946 0.023124734915152625 0.007401527839638796 0.002527635652743992 0.0008509612315162482 0.00018213334574554768 7.815977984795461e-06 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.47706397688984176 0.28313436859495966 0.13170624744221976 0.0525453512766549 0.021111753158319105 0.0067451713955609645 0.003204270539718037 0.0010308107242263192 0.0002759210144794204 0.00013727805365387318 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.48641988659284513 0.2816173231471242 0.131541850061199 0.054793200248327956 0.016517701691156576 0.006757591781856257 0.0025425350750908644 0.0006383278085839443 2.279421641064226e-05 0.0003597432472224371 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4307755159245372 0.2652713784080849 0.12753857957535653 0.05470396852881577 0.02049817309420563 0.00566559741247352 0.0007213846765465147 6.848024591311009e-05 -1.0 8.609308814924014e-05 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.43631324024956025 0.2639077825530567 0.13368737962742414 0.05213531603720763 0.020628860469743833 0.008268384844678654 0.0028599221013934375 0.00013059757129982714 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.45926287238219 0.2635824148883888 0.13104280996799478 0.055379812186041905 0.019255042561941074 0.007252095690246872 0.0018026593488140996 0.0003215836458542421 9.14840176000331e-06 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4574378998400604 0.28714971179927723 0.1363507911051337 0.05010315954654347 0.017963278989571074 0.0058998176297971605 0.0021393135666168 0.00015677575033083482 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 0.4821442782978812 0.27704132233083506 0.14149074035662307 0.05546042815834048 0.016800649382269998 0.004096804603054233 0.0019161108398578026 0.0005289600253155307 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0</lower_ww_bounds>
+      <upper_ww_bounds>2.306798078447855 1.4372426951013015 0.6628006975247303 0.2638260480490365 0.10479220416342819 0.03158625017874938 0.013135185878412529 0.0015016171796218643 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.3055892466950807 1.4312431280421012 0.6999935311310568 0.2754239704479878 0.09140620979127496 0.027788882436180776 0.007632716113146698 0.0012149386176318483 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.314747895653166 1.4288671837730256 0.6682811595168472 0.25173710831135515 0.08762925906456634 0.025483627259255386 0.013148820475765701 0.0016866212196013073 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.288629937523628 1.407474607357475 0.6802198981381123 0.2739644073614768 0.09901188449377762 0.036675872789564246 0.01033696473033213 0.00047647370429630714 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.369881491745159 1.4209530768523382 0.6378754950753944 0.2584601465098665 0.09595839373936176 0.03796517982353924 0.008744088157053638 0.00048391339548409945 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.3780099549279137 1.3864194573478497 0.6380401286267812 0.26259829058567763 0.11249480322475816 0.053168622966257545 0.021210355274573163 0.0022801556101332593 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.382959973596735 1.4443333472126108 0.6293187866990806 0.29094311879778284 0.10238156860372381 0.035715966220091315 0.011182041511257637 0.001976561890704237 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.3467436848034113 1.3590431069745064 0.6416727785211741 0.2757337177996917 0.10333701764735166 0.03945754361568573 0.00821775852703679 0.0032507083906766388 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.2693624655116684 1.412245710542148 0.637670853175783 0.27513750570946854 0.11525573202540434 0.04299651579303599 0.012541305597080901 0.00259265155615848 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.262922237661006 1.3419551233242126 0.6495816959398542 0.2721131854973884 0.09847947611735743 0.02676602480943759 0.007537349146920078 0.000799340449090029 0.0004436023940405703 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.2412339929914022 1.4066382350916795 0.6724930395384323 0.28051639095266717 0.1313716016709848 0.03442035954214652 0.016549695369367727 0.0026687271988374613 0.0006931901997327875 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.4378744750697434 1.4542933526581239 0.7106030329196139 0.3034054004818636 0.09733985234394739 0.03185512139731995 0.014255212633095843 0.006092210889057744 0.0014702270783573093 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.5 1.4260507756489118 0.6811153586160822 0.2915125994216997 0.11562367457576313 0.03700763919819398 0.012638178263719959 0.004254806157581241 0.0009106667287277384 3.9079889923977307e-05 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.385319884449209 1.4156718429747983 0.6585312372110987 0.2627267563832745 0.10555876579159552 0.03372585697780482 0.016021352698590185 0.005154053621131596 0.001379605072397102 0.000686390268269366 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.432099432964226 1.4080866157356209 0.657709250305995 0.2739660012416398 0.08258850845578287 0.03378795890928128 0.012712675375454322 0.0031916390429197216 0.0001139710820532113 0.0017987162361121855 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.1538775796226863 1.3263568920404245 0.6376928978767826 0.27351984264407886 0.10249086547102815 0.028327987062367603 0.0036069233827325737 0.0003424012295655504 -5.0 0.0004304654407462007 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.181566201247801 1.3195389127652835 0.6684368981371207 0.2606765801860381 0.10314430234871916 0.041341924223393264 0.014299610506967188 0.0006529878564991358 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.29631436191095 1.317912074441944 0.6552140498399739 0.27689906093020955 0.09627521280970537 0.03626047845123436 0.009013296744070498 0.0016079182292712106 4.574200880001655e-05 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.287189499200302 1.435748558996386 0.6817539555256685 0.25051579773271737 0.08981639494785537 0.029499088148985803 0.010696567833083998 0.0007838787516541741 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 2.410721391489406 1.3852066116541752 0.7074537017831153 0.2773021407917024 0.08400324691134999 0.020484023015271167 0.009580554199289014 0.0026448001265776534 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0</upper_ww_bounds>
+      <survival_ratio>3.0</survival_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <mesh id="1">
+      <dimension>20 20 1</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <upper_right>160.0 160.0 160.0</upper_right>
+    </mesh>
+    <shared_secondary_bank>true</shared_secondary_bank>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+  </settings>
+  <tallies>
+    <filter id="1" type="mesh">
+      <bins>1</bins>
+    </filter>
+    <tally id="1" name="flux">
+      <filters>1</filters>
+      <scores>flux</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat b/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat
new file mode 100644
index 000000000..11e5ffa77
--- /dev/null
+++ b/tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat
@@ -0,0 +1 @@
+d17a437262d3316985fba4b48e21a7fcd5f32ac2d96ef0e66849f567deaeadc1e0f18d5d0bf5e9cab4246dbf823e7f43f249a1f67e928b27ae8c70b89f3cefbf
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/survival_biasing/test.py b/tests/regression_tests/weightwindows/survival_biasing/test.py
new file mode 100644
index 000000000..c3a3e91a2
--- /dev/null
+++ b/tests/regression_tests/weightwindows/survival_biasing/test.py
@@ -0,0 +1,91 @@
+from pathlib import Path
+
+import pytest
+import numpy as np
+import openmc
+from openmc.utility_funcs import change_directory
+
+from tests.testing_harness import HashedPyAPITestHarness
+
+
+def build_model(shared_secondary):
+    openmc.reset_auto_ids()
+
+    # Material
+    w = openmc.Material(name='Tungsten')
+    w.add_element('W', 1.0)
+    w.set_density('g/cm3', 19.25)
+
+    materials = openmc.Materials([w])
+
+    # Geometry surfaces
+    x0 = openmc.XPlane(x0=0.0, boundary_type='reflective')
+    x1 = openmc.XPlane(x0=160.0, boundary_type='vacuum')
+    y0 = openmc.YPlane(y0=0.0, boundary_type='reflective')
+    y1 = openmc.YPlane(y0=160.0, boundary_type='reflective')
+    z0 = openmc.ZPlane(z0=0.0, boundary_type='reflective')
+    z1 = openmc.ZPlane(z0=160.0, boundary_type='reflective')
+
+    region = +x0 & -x1 & +y0 & -y1 & +z0 & -z1
+    cell = openmc.Cell(region=region, fill=w)
+    root = openmc.Universe(cells=[cell])
+    geometry = openmc.Geometry(root)
+
+    # Source: planar on x=0, mono-directional along +x, 14.1 MeV neutrons
+    space = openmc.stats.CartesianIndependent(
+        openmc.stats.Discrete([0.01], [1.0]),
+        openmc.stats.Uniform(0.0, 160.0),
+        openmc.stats.Uniform(0.0, 160.0),
+    )
+    angle = openmc.stats.Monodirectional((1.0, 0.0, 0.0))
+    energy = openmc.stats.Discrete([14.1e6], [1.0])
+
+    source = openmc.IndependentSource(space=space, angle=angle, energy=energy)
+
+    settings = openmc.Settings()
+    settings.run_mode = 'fixed source'
+    settings.batches = 5
+    settings.particles = 50
+    settings.source = source
+    settings.shared_secondary_bank = shared_secondary
+
+    model = openmc.Model(geometry=geometry, materials=materials, settings=settings)
+
+    # Mesh tally: 1 cm voxels, flux only
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (20, 20, 1)
+    mesh.lower_left = (0.0, 0.0, 0.0)
+    mesh.upper_right = (160.0, 160.0, 160.0)
+
+    mesh_filter = openmc.MeshFilter(mesh)
+    flux_tally = openmc.Tally(name='flux')
+    flux_tally.filters = [mesh_filter]
+    flux_tally.scores = ['flux']
+    tallies = openmc.Tallies([flux_tally])
+    model.tallies = tallies
+
+    parent_dir = Path(__file__).parent
+    lower_ww_bounds = np.loadtxt(parent_dir / 'ww_n.txt')
+
+    weight_windows = openmc.WeightWindows(mesh,
+                                          lower_ww_bounds,
+                                          upper_bound_ratio=5.0,
+                                          particle_type='neutron')
+
+    model.settings.weight_windows = weight_windows
+    model.settings.weight_window_checkpoints = {'surface': True,
+                                                'collision': True}
+    model.settings.survival_biasing = True
+
+    return model
+
+
+@pytest.mark.parametrize("shared_secondary,subdir", [
+    (False, "local"),
+    (True, "shared"),
+])
+def test_weight_windows_with_survival_biasing(shared_secondary, subdir):
+    with change_directory(subdir):
+        model = build_model(shared_secondary)
+        harness = HashedPyAPITestHarness('statepoint.5.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/weightwindows/survival_biasing/ww_n.txt b/tests/regression_tests/weightwindows/survival_biasing/ww_n.txt
new file mode 100644
index 000000000..aea1ff132
--- /dev/null
+++ b/tests/regression_tests/weightwindows/survival_biasing/ww_n.txt
@@ -0,0 +1,400 @@
+4.613596156895709566e-01
+4.611178493390161726e-01
+4.629495791306331709e-01
+4.577259875047255400e-01
+4.739762983490318216e-01
+4.756019909855827565e-01
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+4.693487369606823001e-01
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+4.525844475322012284e-01
+4.482467985982804271e-01
+4.875748950139486837e-01
+5.000000000000000000e-01
+4.770639768898417565e-01
+4.864198865928451299e-01
+4.307755159245372223e-01
+4.363132402495602524e-01
+4.592628723821899905e-01
+4.574378998400603913e-01
+4.821442782978812014e-01
+2.874485390202602964e-01
+2.862486256084202374e-01
+2.857734367546050924e-01
+2.814949214714950187e-01
+2.841906153704676363e-01
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+2.888666694425221504e-01
+2.718086213949012508e-01
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diff --git a/tests/regression_tests/weightwindows/test.py b/tests/regression_tests/weightwindows/test.py
index 864f4f062..c5b6bd889 100644
--- a/tests/regression_tests/weightwindows/test.py
+++ b/tests/regression_tests/weightwindows/test.py
@@ -1,22 +1,23 @@
-from copy import deepcopy
+from pathlib import Path
+
 import pytest
 import numpy as np
 
 import openmc
 from openmc.stats import Discrete, Point
+from openmc.utility_funcs import change_directory
 
 from tests.testing_harness import HashedPyAPITestHarness
 
 
-@pytest.fixture
-def model():
+def build_model(shared_secondary):
+    openmc.reset_auto_ids()
     model = openmc.Model()
 
     # materials (M4 steel alloy)
     m4 = openmc.Material()
     m4.set_density('g/cc', 2.3)
     m4.add_nuclide('H1', 0.168018676)
-    m4.add_nuclide("H2", 1.93244e-05)
     m4.add_nuclide("O16", 0.561814465)
     m4.add_nuclide("O17", 0.00021401)
     m4.add_nuclide("Na23", 0.021365)
@@ -26,14 +27,9 @@ def model():
     m4.add_nuclide("Si30", 0.006273944)
     m4.add_nuclide("Ca40", 0.018026179)
     m4.add_nuclide("Ca42", 0.00012031)
-    m4.add_nuclide("Ca43", 2.51033e-05)
     m4.add_nuclide("Ca44", 0.000387892)
-    m4.add_nuclide("Ca46", 7.438e-07)
-    m4.add_nuclide("Ca48", 3.47727e-05)
     m4.add_nuclide("Fe54", 0.000248179)
     m4.add_nuclide("Fe56", 0.003895875)
-    m4.add_nuclide("Fe57", 8.99727e-05)
-    m4.add_nuclide("Fe58", 1.19737e-05)
 
     s0 = openmc.Sphere(r=240)
     s1 = openmc.Sphere(r=250, boundary_type='vacuum')
@@ -46,10 +42,13 @@ def model():
     # settings
     settings = model.settings
     settings.run_mode = 'fixed source'
-    settings.particles = 200
+    settings.particles = 500
     settings.batches = 2
     settings.max_history_splits = 200
     settings.photon_transport = True
+    settings.shared_secondary_bank = shared_secondary
+    settings.weight_window_checkpoints = {'surface': True,
+                                          'collision': True}
     space = Point((0.001, 0.001, 0.001))
     energy = Discrete([14E6], [1.0])
 
@@ -76,10 +75,10 @@ def model():
 
     # weight windows
 
-    # load pre-generated weight windows
-    # (created using the same tally as above)
-    ww_n_lower_bnds = np.loadtxt('ww_n.txt')
-    ww_p_lower_bnds = np.loadtxt('ww_p.txt')
+    # load pre-generated weight windows from parent directory
+    parent_dir = Path(__file__).parent
+    ww_n_lower_bnds = np.loadtxt(parent_dir / 'ww_n.txt')
+    ww_p_lower_bnds = np.loadtxt(parent_dir / 'ww_p.txt')
 
     # create a mesh matching the one used
     # to generate the weight windows
@@ -93,6 +92,7 @@ def model():
                                 None,
                                 10.0,
                                 e_bnds,
+                                'neutron',
                                 max_lower_bound_ratio=1.5)
 
     ww_p = openmc.WeightWindows(ww_mesh,
@@ -100,6 +100,7 @@ def model():
                                 None,
                                 10.0,
                                 e_bnds,
+                                'photon',
                                 max_lower_bound_ratio=1.5)
 
     model.settings.weight_windows = [ww_n, ww_p]
@@ -107,14 +108,75 @@ def model():
     return model
 
 
-def test_weightwindows(model):
-    test = HashedPyAPITestHarness('statepoint.2.h5', model)
-    test.main()
+@pytest.mark.parametrize("shared_secondary,subdir", [
+    (False, "local"),
+    (True, "shared"),
+])
+def test_weightwindows(shared_secondary, subdir):
+    with change_directory(subdir):
+        model = build_model(shared_secondary)
+        test = HashedPyAPITestHarness('statepoint.2.h5', model)
+        test.main()
+
+
+def test_zero_bound_windows_play_no_game(tmp_path):
+    # A weight window lower bound of zero means no weight window information
+    # exists there (MCNP wwinp files use zero to turn the game off in a cell),
+    # so transport must proceed as if weight windows were disabled. Previously,
+    # zero-bound windows demanded a split at every checkpoint (weight/0 ->
+    # max_split), multiplying the particle population until terminated by the
+    # split or weight cutoff limits.
+    model = build_model(False)
+    for ww in model.settings.weight_windows:
+        ww.lower_ww_bounds = np.zeros_like(ww.lower_ww_bounds)
+        ww.upper_ww_bounds = np.zeros_like(ww.upper_ww_bounds)
+    sp_zero = model.run(cwd=tmp_path / 'zero_windows')
+
+    model.settings.weight_windows_on = False
+    sp_off = model.run(cwd=tmp_path / 'windows_off')
+
+    with openmc.StatePoint(sp_zero) as sp:
+        flux_zero = list(sp.tallies.values())[0].mean
+    with openmc.StatePoint(sp_off) as sp:
+        flux_off = list(sp.tallies.values())[0].mean
+
+    np.testing.assert_allclose(flux_zero, flux_off, rtol=1e-12)
+
+
+def test_zero_and_negative_bounds_equivalent(tmp_path):
+    # Zero and negative lower bounds both mean that no weight window
+    # information exists in a cell (generators mark such cells with -1, and
+    # MCNP wwinp files use zero), so they must produce identical transport.
+    # Unlike the all-zero case above, here particles are born under valid
+    # windows and encounter the no-information region in flight; previously a
+    # zero lower bound in that situation demanded a split at every checkpoint
+    # in the cell (weight/0 -> max_split), multiplying the particle population,
+    # while -1 played no game.
+    def run_with(bound_value, subdir):
+        model = build_model(False)
+        for ww in model.settings.weight_windows:
+            lb = np.array(ww.lower_ww_bounds, copy=True)
+            ub = np.array(ww.upper_ww_bounds, copy=True)
+            lb[3:, :, :, :] = bound_value
+            ub[3:, :, :, :] = bound_value
+            ww.lower_ww_bounds = lb
+            ww.upper_ww_bounds = ub
+        return model.run(cwd=tmp_path / subdir)
+
+    sp_zero = run_with(0.0, 'zero_region')
+    sp_negative = run_with(-1.0, 'negative_region')
+
+    with openmc.StatePoint(sp_zero) as sp:
+        flux_zero = list(sp.tallies.values())[0].mean
+    with openmc.StatePoint(sp_negative) as sp:
+        flux_negative = list(sp.tallies.values())[0].mean
+
+    np.testing.assert_allclose(flux_zero, flux_negative, rtol=1e-12)
 
 
 def test_wwinp_cylindrical():
 
-    ww = openmc.wwinp_to_wws('ww_n_cyl.txt')[0]
+    ww = openmc.WeightWindowsList.from_wwinp('ww_n_cyl.txt')[0]
 
     mesh = ww.mesh
 
@@ -144,7 +206,7 @@ def test_wwinp_cylindrical():
 
 def test_wwinp_spherical():
 
-    ww = openmc.wwinp_to_wws('ww_n_sph.txt')[0]
+    ww = openmc.WeightWindowsList.from_wwinp('ww_n_sph.txt')[0]
 
     mesh = ww.mesh
 
diff --git a/tests/regression_tests/weightwindows/ww_n.txt b/tests/regression_tests/weightwindows/ww_n.txt
index dbb49537b..36d7673c6 100644
--- a/tests/regression_tests/weightwindows/ww_n.txt
+++ b/tests/regression_tests/weightwindows/ww_n.txt
@@ -10,43 +10,30 @@
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@@ -60,301 +47,329 @@
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diff --git a/tests/regression_tests/weightwindows_fw_cadis/inputs_true.dat b/tests/regression_tests/weightwindows_fw_cadis/inputs_true.dat
index 586bbb120..6bdabfbee 100644
--- a/tests/regression_tests/weightwindows_fw_cadis/inputs_true.dat
+++ b/tests/regression_tests/weightwindows_fw_cadis/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>90</particles>
     <batches>10</batches>
     <inactive>5</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -222,12 +222,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <volume_estimator>naive</volume_estimator>
     </random_ray>
   </settings>
diff --git a/tests/regression_tests/weightwindows_fw_cadis_local/__init__.py b/tests/regression_tests/weightwindows_fw_cadis_local/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/weightwindows_fw_cadis_local/inputs_true.dat b/tests/regression_tests/weightwindows_fw_cadis_local/inputs_true.dat
new file mode 100644
index 000000000..ffdd977ec
--- /dev/null
+++ b/tests/regression_tests/weightwindows_fw_cadis_local/inputs_true.dat
@@ -0,0 +1,273 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <cross_sections>mgxs.h5</cross_sections>
+    <material id="1" name="source">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="source"/>
+    </material>
+    <material id="2" name="cavity">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="cavity"/>
+    </material>
+    <material id="3" name="absorber">
+      <density value="1.0" units="macro"/>
+      <macroscopic name="absorber"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="cube cavity region" material="2" universe="2"/>
+    <cell id="3" name="absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="6" name="detector 1" material="3" region="1 -10 4 -8 5 -11" universe="6"/>
+    <cell id="7" name="detector 2" material="3" region="2 -7 4 -8 6 -9" universe="6"/>
+    <cell id="8" name="outside cube" material="2" region="(2 3 5 -7 ((-8 -6) | (-4 6 -9))) | (4 -8 ((10 -2 5 -9) | (-10 1 11 -9) | (2 -7 5 -6))) | (1 3 6 -9 ((-8 -2) | (-4 2 -7)))" universe="6"/>
+    <lattice id="4">
+      <pitch>2.5 2.5 2.5</pitch>
+      <dimension>12 12 12</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <universes>
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
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+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
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+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
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+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+2 2 2 2 2 2 2 2 2 2 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
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+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 
+3 3 3 3 3 3 3 3 3 3 3 3 </universes>
+    </lattice>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" coeffs="30.0"/>
+    <surface id="7" type="x-plane" boundary="vacuum" coeffs="35.0"/>
+    <surface id="8" type="y-plane" boundary="vacuum" coeffs="35.0"/>
+    <surface id="9" type="z-plane" boundary="vacuum" coeffs="35.0"/>
+    <surface id="10" type="x-plane" coeffs="5.0"/>
+    <surface id="11" type="z-plane" coeffs="5.0"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>500</particles>
+    <batches>10</batches>
+    <inactive>5</inactive>
+    <source type="independent" strength="3.14" particle="neutron">
+      <energy type="discrete">
+        <parameters>100.0 1.0</parameters>
+      </energy>
+      <constraints>
+        <domain_type>universe</domain_type>
+        <domain_ids>1</domain_ids>
+      </constraints>
+    </source>
+    <energy_mode>multi-group</energy_mode>
+    <weight_window_generators>
+      <weight_windows_generator>
+        <mesh>2</mesh>
+        <particle_type>neutron</particle_type>
+        <max_realizations>10</max_realizations>
+        <update_interval>1</update_interval>
+        <on_the_fly>true</on_the_fly>
+        <method>fw_cadis</method>
+        <targets>1 2</targets>
+      </weight_windows_generator>
+    </weight_window_generators>
+    <mesh id="2">
+      <dimension>7 7 7</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <upper_right>35.0 35.0 35.0</upper_right>
+    </mesh>
+    <random_ray>
+      <distance_active>800.0</distance_active>
+      <distance_inactive>100.0</distance_inactive>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 35.0 35.0 35.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
+      <source_region_meshes>
+        <mesh id="1">
+          <domain id="6" type="universe"/>
+        </mesh>
+      </source_region_meshes>
+      <volume_estimator>naive</volume_estimator>
+    </random_ray>
+    <mesh id="1">
+      <dimension>14 14 14</dimension>
+      <lower_left>0.0 0.0 0.0</lower_left>
+      <upper_right>35.0 35.0 35.0</upper_right>
+    </mesh>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>6</bins>
+    </filter>
+    <filter id="2" type="cell">
+      <bins>7</bins>
+    </filter>
+    <tally id="1" name="Detector 1 Tally">
+      <filters>1</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+    <tally id="2" name="Detector 2 Tally">
+      <filters>2</filters>
+      <scores>flux</scores>
+      <estimator>tracklength</estimator>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows_fw_cadis_local/results_true.dat b/tests/regression_tests/weightwindows_fw_cadis_local/results_true.dat
new file mode 100644
index 000000000..5991fc621
--- /dev/null
+++ b/tests/regression_tests/weightwindows_fw_cadis_local/results_true.dat
@@ -0,0 +1,696 @@
+RegularMesh
+	ID             =	2
+	Name           =	
+	Dimensions     =	3
+	Voxels         =	[7 7 7]
+	Lower left     =	[0. 0. 0.]
+	Upper Right    =	[np.float64(35.0), np.float64(35.0), np.float64(35.0)]
+	Width          =	[5. 5. 5.]
+Lower Bounds
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+1.98e-02
+6.30e-03
+8.90e-03
+6.49e-03
+5.28e-03
+8.94e-03
+7.00e-03
+1.59e-02
+9.64e-03
+8.62e-03
+8.88e-03
+4.21e-03
+4.33e-03
+4.00e-03
+5.04e-03
+7.71e-03
+6.51e-03
+5.56e-03
+4.90e-03
+2.51e-03
+1.55e-03
+1.75e-03
+6.19e-03
+8.31e-03
+4.10e-03
+2.65e-03
+1.81e-03
+6.34e-04
+3.01e-04
+3.14e-03
+3.10e-03
+2.51e-03
+2.14e-03
+1.47e-03
+2.69e-04
+3.12e-05
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows_fw_cadis_local/test.py b/tests/regression_tests/weightwindows_fw_cadis_local/test.py
new file mode 100644
index 000000000..abd3f2098
--- /dev/null
+++ b/tests/regression_tests/weightwindows_fw_cadis_local/test.py
@@ -0,0 +1,42 @@
+import os
+
+import openmc
+from openmc.examples import random_ray_three_region_cube_with_detectors
+
+from tests.testing_harness import WeightWindowPyAPITestHarness
+
+
+class MGXSTestHarness(WeightWindowPyAPITestHarness):
+    def _cleanup(self):
+        super()._cleanup()
+        f = 'mgxs.h5'
+        if os.path.exists(f):
+            os.remove(f)
+
+
+def test_weight_windows_fw_cadis_local():
+    model = random_ray_three_region_cube_with_detectors()
+
+    for tally in list(model.tallies):
+        if tally.name in {"Source Tally", "Absorber Tally", "Cavity Tally"}:
+            # leave only the tallies of interest
+            model.tallies.remove(tally)
+
+    ww_mesh = openmc.RegularMesh()
+    n = 7
+    width = 35.0
+    ww_mesh.dimension = (n, n, n)
+    ww_mesh.lower_left = (0.0, 0.0, 0.0)
+    ww_mesh.upper_right = (width, width, width)
+
+    wwg = openmc.WeightWindowGenerator(
+        method="fw_cadis", 
+        targets=model.tallies,
+        mesh=ww_mesh, 
+        max_realizations=model.settings.batches
+    )
+    model.settings.weight_window_generators = wwg
+    model.settings.random_ray['volume_estimator'] = 'naive'
+
+    harness = MGXSTestHarness('statepoint.10.h5', model)
+    harness.main()
diff --git a/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/inputs_true.dat b/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/inputs_true.dat
index ffb2ac112..a0d84257a 100644
--- a/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/inputs_true.dat
+++ b/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>750</particles>
     <batches>30</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -222,12 +222,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_region_meshes>
         <mesh id="1">
           <domain id="6" type="universe"/>
diff --git a/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/results_true.dat b/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/results_true.dat
index e14b7e199..0354501d1 100644
--- a/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/results_true.dat
+++ b/tests/regression_tests/weightwindows_fw_cadis_mesh/flat/results_true.dat
@@ -36,15 +36,15 @@ Lower Bounds
 1.73e-01
 2.44e-01
 3.63e-01
-1.23e-01
+1.22e-01
 1.71e-01
 1.66e-01
 1.69e-01
-1.71e-01
+1.70e-01
 1.67e-01
 1.71e-01
 1.64e-01
-1.56e-01
+1.55e-01
 1.67e-01
 1.64e-01
 1.68e-01
@@ -96,7 +96,7 @@ Lower Bounds
 1.80e-01
 2.59e-01
 2.97e-01
-1.07e-01
+1.06e-01
 1.68e-01
 1.60e-01
 1.58e-01
@@ -156,7 +156,7 @@ Lower Bounds
 1.49e-01
 2.14e-01
 1.76e-01
-6.71e-02
+6.70e-02
 1.72e-01
 1.66e-01
 1.74e-01
@@ -192,7 +192,7 @@ Lower Bounds
 2.37e-01
 2.51e-01
 2.64e-01
-2.43e-01
+2.42e-01
 2.39e-01
 2.32e-01
 2.32e-01
@@ -209,12 +209,12 @@ Lower Bounds
 2.86e-01
 3.17e-01
 2.60e-01
-2.39e-01
+2.38e-01
 1.88e-01
 1.97e-01
 1.84e-01
 1.50e-01
-9.99e-02
+9.98e-02
 3.88e-02
 1.90e-02
 2.18e-01
@@ -340,7 +340,7 @@ Lower Bounds
 1.65e-01
 1.63e-01
 1.59e-01
-1.58e-01
+1.57e-01
 1.59e-01
 1.54e-01
 1.54e-01
@@ -416,7 +416,7 @@ Lower Bounds
 2.65e-01
 2.54e-01
 2.58e-01
-2.81e-01
+2.80e-01
 2.53e-01
 2.33e-01
 2.40e-01
@@ -503,7 +503,7 @@ Lower Bounds
 3.36e-01
 1.29e-01
 1.57e-01
-1.68e-01
+1.67e-01
 1.66e-01
 1.63e-01
 1.63e-01
@@ -513,7 +513,7 @@ Lower Bounds
 1.56e-01
 1.59e-01
 1.65e-01
-1.60e-01
+1.59e-01
 2.43e-01
 3.30e-01
 1.08e-01
@@ -543,7 +543,7 @@ Lower Bounds
 1.54e-01
 1.56e-01
 1.67e-01
-1.70e-01
+1.69e-01
 2.42e-01
 3.48e-01
 1.32e-01
@@ -661,7 +661,7 @@ Lower Bounds
 2.34e-01
 2.20e-01
 1.88e-01
-1.78e-01
+1.77e-01
 1.34e-01
 1.25e-01
 1.07e-01
@@ -676,7 +676,7 @@ Lower Bounds
 8.09e-02
 8.37e-02
 5.69e-02
-6.46e-02
+6.45e-02
 3.15e-02
 3.86e-02
 3.51e-02
@@ -770,14 +770,14 @@ Lower Bounds
 1.53e-01
 1.64e-01
 2.46e-01
-3.03e-01
+3.02e-01
 1.11e-01
 1.63e-01
 1.59e-01
 1.68e-01
-1.56e-01
+1.59e-01
 1.52e-01
-1.55e-01
+1.54e-01
 1.56e-01
 1.49e-01
 1.49e-01
@@ -850,7 +850,7 @@ Lower Bounds
 1.73e-01
 1.65e-01
 1.70e-01
-7.83e-02
+1.66e-01
 1.73e-01
 1.78e-01
 1.65e-01
@@ -877,7 +877,7 @@ Lower Bounds
 2.14e-01
 8.26e-02
 2.65e-02
-3.07e-01
+3.06e-01
 2.51e-01
 2.10e-01
 2.95e-01
@@ -1091,7 +1091,7 @@ Lower Bounds
 2.30e-01
 2.33e-01
 2.43e-01
-2.46e-01
+2.45e-01
 2.48e-01
 2.46e-01
 2.21e-01
@@ -1124,7 +1124,7 @@ Lower Bounds
 1.07e-01
 8.75e-02
 1.17e-01
-5.68e-02
+5.67e-02
 7.16e-02
 5.22e-02
 3.31e-02
@@ -1317,7 +1317,7 @@ Lower Bounds
 2.20e-01
 2.27e-01
 2.41e-01
-2.35e-01
+2.34e-01
 2.13e-01
 2.04e-01
 2.13e-01
@@ -1359,14 +1359,14 @@ Lower Bounds
 8.92e-03
 1.61e-01
 1.59e-01
-1.57e-01
+1.56e-01
 1.59e-01
 1.51e-01
 1.49e-01
 1.54e-01
 1.51e-01
 1.53e-01
-1.49e-01
+1.48e-01
 1.52e-01
 1.48e-01
 2.27e-01
@@ -1478,12 +1478,12 @@ Lower Bounds
 1.68e-01
 5.02e-02
 1.53e-01
-7.09e-02
+1.48e-01
 1.47e-01
 1.48e-01
 1.43e-01
 1.46e-01
-6.84e-02
+1.42e-01
 1.27e-01
 1.24e-01
 1.31e-01
@@ -1523,7 +1523,7 @@ Lower Bounds
 7.50e-02
 2.64e-02
 1.58e-01
-1.56e-01
+1.55e-01
 1.50e-01
 1.45e-01
 1.50e-01
@@ -1575,7 +1575,7 @@ Lower Bounds
 6.95e-02
 7.10e-02
 4.92e-02
-4.74e-02
+4.73e-02
 3.52e-02
 4.69e-02
 4.03e-02
@@ -1669,7 +1669,7 @@ Lower Bounds
 1.34e-01
 1.36e-01
 1.50e-01
-1.96e-01
+1.95e-01
 1.29e-01
 4.48e-02
 1.49e-01
@@ -1784,7 +1784,7 @@ Lower Bounds
 1.53e-01
 1.76e-01
 1.13e-01
-9.77e-02
+9.76e-02
 8.62e-02
 1.49e-01
 1.29e-01
@@ -1898,7 +1898,7 @@ Lower Bounds
 1.53e-01
 5.25e-02
 1.45e-01
-1.41e-01
+1.44e-01
 1.44e-01
 1.36e-01
 1.36e-01
@@ -1928,13 +1928,13 @@ Lower Bounds
 7.71e-02
 3.01e-02
 1.38e-01
-1.12e-01
+1.35e-01
 1.38e-01
 1.41e-01
 1.44e-01
 1.46e-01
 1.34e-01
-1.11e-01
+1.28e-01
 1.24e-01
 1.20e-01
 1.22e-01
@@ -1955,7 +1955,7 @@ Lower Bounds
 1.21e-01
 1.25e-01
 1.84e-01
-9.19e-02
+9.18e-02
 2.31e-02
 1.48e-01
 1.32e-01
@@ -2039,7 +2039,7 @@ Lower Bounds
 1.55e-01
 1.46e-01
 1.44e-01
-1.48e-01
+1.47e-01
 1.42e-01
 1.44e-01
 1.40e-01
@@ -2067,7 +2067,7 @@ Lower Bounds
 1.43e-01
 1.44e-01
 1.46e-01
-1.54e-01
+1.53e-01
 1.50e-01
 1.51e-01
 1.54e-01
@@ -2214,7 +2214,7 @@ Lower Bounds
 1.83e-02
 2.39e-01
 1.91e-01
-1.79e-01
+1.80e-01
 2.07e-01
 1.97e-01
 2.03e-01
@@ -2238,7 +2238,7 @@ Lower Bounds
 1.30e-01
 1.06e-01
 9.60e-02
-7.07e-02
+7.06e-02
 4.00e-02
 1.71e-02
 7.45e-03
@@ -2343,7 +2343,7 @@ Lower Bounds
 1.32e-01
 1.41e-01
 1.41e-01
-1.40e-01
+1.39e-01
 2.04e-01
 1.15e-01
 3.10e-02
@@ -2471,7 +2471,7 @@ Lower Bounds
 3.06e-02
 3.68e-02
 4.09e-02
-5.27e-02
+5.26e-02
 4.71e-02
 3.30e-02
 4.67e-02
@@ -2493,7 +2493,7 @@ Lower Bounds
 1.53e-01
 1.51e-01
 1.54e-01
-1.45e-01
+1.44e-01
 1.43e-01
 3.50e-02
 1.38e-02
@@ -2533,7 +2533,7 @@ Lower Bounds
 1.54e-01
 1.59e-01
 1.65e-01
-5.07e-02
+1.63e-01
 1.43e-01
 1.52e-01
 1.45e-01
@@ -2548,7 +2548,7 @@ Lower Bounds
 1.55e-01
 1.59e-01
 1.59e-01
-1.53e-01
+1.54e-01
 1.47e-01
 1.53e-01
 1.51e-01
@@ -2648,7 +2648,7 @@ Lower Bounds
 5.02e-02
 1.48e-02
 1.33e-01
-5.34e-02
+1.31e-01
 1.34e-01
 1.36e-01
 1.35e-01
@@ -2678,7 +2678,7 @@ Lower Bounds
 2.21e-02
 7.42e-03
 1.30e-01
-7.03e-02
+7.02e-02
 8.47e-02
 1.05e-01
 8.11e-02
@@ -2726,7 +2726,7 @@ Lower Bounds
 2.10e-01
 2.47e-01
 2.20e-01
-2.21e-01
+2.20e-01
 2.29e-01
 2.22e-01
 2.06e-01
@@ -2776,7 +2776,7 @@ Lower Bounds
 2.54e-01
 2.18e-01
 2.16e-01
-2.24e-01
+2.23e-01
 1.99e-01
 1.77e-01
 1.57e-01
@@ -3026,7 +3026,7 @@ Lower Bounds
 1.80e-01
 2.09e-01
 2.09e-01
-1.83e-01
+1.82e-01
 2.00e-01
 1.82e-01
 1.57e-01
@@ -3120,7 +3120,7 @@ Lower Bounds
 8.59e-02
 7.51e-02
 7.34e-02
-6.11e-02
+6.10e-02
 6.19e-02
 4.42e-02
 3.60e-02
@@ -3164,7 +3164,7 @@ Lower Bounds
 1.09e-01
 8.20e-02
 7.49e-02
-7.94e-02
+7.93e-02
 7.32e-02
 5.71e-02
 3.93e-02
@@ -3188,7 +3188,7 @@ Lower Bounds
 1.79e-02
 8.68e-03
 8.76e-02
-6.94e-02
+6.93e-02
 1.01e-01
 9.79e-02
 5.28e-02
@@ -3236,7 +3236,7 @@ Lower Bounds
 5.48e-02
 7.05e-02
 7.64e-02
-6.19e-02
+6.18e-02
 5.28e-02
 6.84e-02
 5.17e-02
@@ -3404,7 +3404,7 @@ Upper Bounds
 7.61e-01
 8.32e-01
 8.60e-01
-8.42e-01
+8.41e-01
 8.42e-01
 8.32e-01
 8.34e-01
@@ -3412,15 +3412,15 @@ Upper Bounds
 8.65e-01
 1.22e+00
 1.82e+00
-6.13e-01
+6.12e-01
 8.56e-01
-8.32e-01
+8.31e-01
 8.43e-01
-8.53e-01
+8.52e-01
 8.33e-01
 8.57e-01
 8.19e-01
-7.78e-01
+7.77e-01
 8.33e-01
 8.19e-01
 8.41e-01
@@ -3472,8 +3472,8 @@ Upper Bounds
 8.98e-01
 1.29e+00
 1.49e+00
-5.33e-01
-8.40e-01
+5.32e-01
+8.39e-01
 7.98e-01
 7.91e-01
 8.03e-01
@@ -3526,7 +3526,7 @@ Upper Bounds
 7.71e-01
 7.75e-01
 7.58e-01
-7.64e-01
+7.63e-01
 7.06e-01
 7.64e-01
 7.43e-01
@@ -3556,7 +3556,7 @@ Upper Bounds
 8.72e-01
 7.91e-01
 8.39e-01
-7.95e-01
+7.94e-01
 7.87e-01
 7.88e-01
 7.38e-01
@@ -3589,7 +3589,7 @@ Upper Bounds
 9.40e-01
 9.83e-01
 9.19e-01
-7.52e-01
+7.51e-01
 4.99e-01
 1.94e-01
 9.48e-02
@@ -3615,7 +3615,7 @@ Upper Bounds
 8.13e-01
 7.97e-01
 8.76e-01
-8.45e-01
+8.44e-01
 7.95e-01
 8.18e-01
 8.84e-01
@@ -3649,7 +3649,7 @@ Upper Bounds
 8.01e-01
 8.48e-01
 8.52e-01
-8.71e-01
+8.70e-01
 1.31e+00
 1.65e+00
 5.73e-01
@@ -3713,10 +3713,10 @@ Upper Bounds
 1.17e+00
 1.50e+00
 5.74e-01
-8.24e-01
+8.23e-01
 8.15e-01
 7.95e-01
-7.88e-01
+7.87e-01
 7.97e-01
 7.70e-01
 7.69e-01
@@ -3761,7 +3761,7 @@ Upper Bounds
 8.45e-01
 8.73e-01
 8.56e-01
-8.16e-01
+8.15e-01
 8.44e-01
 8.36e-01
 7.99e-01
@@ -3774,11 +3774,11 @@ Upper Bounds
 6.73e-01
 2.04e-01
 8.93e-01
-8.59e-01
+8.60e-01
 8.64e-01
 8.51e-01
 8.70e-01
-8.91e-01
+8.90e-01
 8.41e-01
 7.94e-01
 7.85e-01
@@ -3856,10 +3856,10 @@ Upper Bounds
 8.14e-01
 7.91e-01
 8.00e-01
-7.95e-01
+7.94e-01
 8.24e-01
 8.16e-01
-8.87e-01
+8.86e-01
 1.22e+00
 1.96e+00
 8.18e-01
@@ -3879,7 +3879,7 @@ Upper Bounds
 1.68e+00
 6.47e-01
 7.87e-01
-8.38e-01
+8.37e-01
 8.29e-01
 8.15e-01
 8.16e-01
@@ -3889,13 +3889,13 @@ Upper Bounds
 7.82e-01
 7.95e-01
 8.24e-01
-7.98e-01
+7.97e-01
 1.22e+00
 1.65e+00
 5.42e-01
 8.09e-01
 8.09e-01
-8.05e-01
+8.04e-01
 7.72e-01
 8.14e-01
 8.20e-01
@@ -3919,7 +3919,7 @@ Upper Bounds
 7.72e-01
 7.80e-01
 8.33e-01
-8.48e-01
+8.47e-01
 1.21e+00
 1.74e+00
 6.61e-01
@@ -3934,7 +3934,7 @@ Upper Bounds
 7.56e-01
 7.70e-01
 7.83e-01
-7.97e-01
+7.96e-01
 1.13e+00
 1.26e+00
 4.66e-01
@@ -3943,7 +3943,7 @@ Upper Bounds
 7.98e-01
 7.75e-01
 7.87e-01
-7.92e-01
+7.91e-01
 7.70e-01
 7.35e-01
 7.31e-01
@@ -3959,7 +3959,7 @@ Upper Bounds
 8.01e-01
 7.99e-01
 7.90e-01
-7.89e-01
+7.88e-01
 7.83e-01
 7.33e-01
 7.48e-01
@@ -3975,7 +3975,7 @@ Upper Bounds
 8.32e-01
 7.85e-01
 7.86e-01
-7.51e-01
+7.50e-01
 7.01e-01
 7.38e-01
 7.30e-01
@@ -4037,13 +4037,13 @@ Upper Bounds
 1.17e+00
 1.10e+00
 9.39e-01
-8.88e-01
+8.87e-01
 6.70e-01
 6.23e-01
 5.35e-01
 2.34e-01
 6.47e-02
-6.45e-01
+6.44e-01
 4.31e-01
 7.11e-01
 6.80e-01
@@ -4063,7 +4063,7 @@ Upper Bounds
 8.31e-01
 8.52e-01
 8.43e-01
-8.06e-01
+8.05e-01
 7.88e-01
 7.57e-01
 7.58e-01
@@ -4083,7 +4083,7 @@ Upper Bounds
 7.76e-01
 7.95e-01
 8.30e-01
-7.96e-01
+7.95e-01
 8.37e-01
 1.18e+00
 1.62e+00
@@ -4105,7 +4105,7 @@ Upper Bounds
 5.36e-01
 8.08e-01
 8.15e-01
-8.00e-01
+7.99e-01
 8.03e-01
 7.68e-01
 7.86e-01
@@ -4122,7 +4122,7 @@ Upper Bounds
 7.72e-01
 8.04e-01
 8.28e-01
-7.75e-01
+7.74e-01
 7.64e-01
 7.80e-01
 7.54e-01
@@ -4151,12 +4151,12 @@ Upper Bounds
 8.13e-01
 7.95e-01
 8.40e-01
-7.80e-01
+7.97e-01
 7.59e-01
-7.73e-01
+7.72e-01
 7.80e-01
 7.46e-01
-7.45e-01
+7.44e-01
 7.55e-01
 7.27e-01
 7.44e-01
@@ -4226,8 +4226,8 @@ Upper Bounds
 8.65e-01
 8.27e-01
 8.48e-01
-3.91e-01
-8.66e-01
+8.31e-01
+8.65e-01
 8.92e-01
 8.23e-01
 7.50e-01
@@ -4265,7 +4265,7 @@ Upper Bounds
 9.09e-01
 6.43e-01
 6.18e-01
-5.19e-01
+5.18e-01
 2.26e-01
 8.71e-02
 5.78e-01
@@ -4367,7 +4367,7 @@ Upper Bounds
 7.08e-01
 7.78e-01
 7.45e-01
-7.62e-01
+7.61e-01
 7.55e-01
 7.92e-01
 1.16e+00
@@ -4376,7 +4376,7 @@ Upper Bounds
 7.40e-01
 7.79e-01
 7.94e-01
-7.60e-01
+7.59e-01
 7.99e-01
 7.16e-01
 7.14e-01
@@ -4420,7 +4420,7 @@ Upper Bounds
 2.76e-01
 8.13e-01
 8.09e-01
-7.87e-01
+7.86e-01
 7.66e-01
 7.26e-01
 7.62e-01
@@ -4531,7 +4531,7 @@ Upper Bounds
 7.86e-01
 7.98e-01
 7.43e-01
-7.69e-01
+7.68e-01
 7.51e-01
 7.70e-01
 8.08e-01
@@ -4583,7 +4583,7 @@ Upper Bounds
 1.19e+00
 1.33e+00
 5.12e-01
-7.76e-01
+7.75e-01
 7.68e-01
 7.84e-01
 7.62e-01
@@ -4631,14 +4631,14 @@ Upper Bounds
 7.78e-01
 7.65e-01
 7.86e-01
-7.10e-01
+7.09e-01
 7.60e-01
 7.43e-01
 7.27e-01
 6.62e-01
 6.51e-01
 6.35e-01
-7.04e-01
+7.03e-01
 7.07e-01
 1.06e+00
 7.73e-01
@@ -4648,7 +4648,7 @@ Upper Bounds
 7.63e-01
 7.80e-01
 7.34e-01
-7.23e-01
+7.24e-01
 7.02e-01
 6.87e-01
 7.13e-01
@@ -4724,7 +4724,7 @@ Upper Bounds
 4.81e-01
 4.96e-01
 3.78e-01
-5.26e-01
+5.25e-01
 3.10e-01
 2.83e-01
 1.99e-01
@@ -4735,14 +4735,14 @@ Upper Bounds
 4.46e-02
 8.04e-01
 7.95e-01
-7.83e-01
+7.82e-01
 7.95e-01
 7.54e-01
 7.47e-01
 7.72e-01
 7.56e-01
 7.67e-01
-7.43e-01
+7.42e-01
 7.58e-01
 7.40e-01
 1.14e+00
@@ -4779,7 +4779,7 @@ Upper Bounds
 1.24e+00
 5.06e-01
 7.87e-01
-7.41e-01
+7.40e-01
 7.55e-01
 7.36e-01
 7.30e-01
@@ -4821,7 +4821,7 @@ Upper Bounds
 7.08e-01
 7.57e-01
 1.07e+00
-7.06e-01
+7.05e-01
 2.16e-01
 7.82e-01
 7.85e-01
@@ -4854,12 +4854,12 @@ Upper Bounds
 8.40e-01
 2.51e-01
 7.67e-01
-3.55e-01
+7.41e-01
 7.36e-01
 7.41e-01
 7.16e-01
-7.29e-01
-3.42e-01
+7.28e-01
+7.11e-01
 6.33e-01
 6.18e-01
 6.57e-01
@@ -4874,7 +4874,7 @@ Upper Bounds
 7.36e-01
 7.09e-01
 6.77e-01
-6.93e-01
+6.94e-01
 6.91e-01
 6.86e-01
 6.77e-01
@@ -4899,8 +4899,8 @@ Upper Bounds
 3.75e-01
 1.32e-01
 7.90e-01
-7.78e-01
-7.50e-01
+7.77e-01
+7.49e-01
 7.25e-01
 7.49e-01
 7.50e-01
@@ -4964,7 +4964,7 @@ Upper Bounds
 7.66e-01
 7.81e-01
 7.83e-01
-7.80e-01
+7.79e-01
 7.68e-01
 7.20e-01
 7.09e-01
@@ -5045,7 +5045,7 @@ Upper Bounds
 6.71e-01
 6.81e-01
 7.49e-01
-9.78e-01
+9.77e-01
 6.44e-01
 2.24e-01
 7.44e-01
@@ -5148,7 +5148,7 @@ Upper Bounds
 8.35e-01
 8.39e-01
 9.59e-01
-9.14e-01
+9.15e-01
 7.74e-01
 5.71e-01
 1.72e-01
@@ -5191,7 +5191,7 @@ Upper Bounds
 7.60e-01
 7.46e-01
 7.71e-01
-7.12e-01
+7.11e-01
 6.80e-01
 6.82e-01
 7.51e-01
@@ -5229,7 +5229,7 @@ Upper Bounds
 8.27e-01
 2.75e-01
 7.59e-01
-7.20e-01
+7.19e-01
 7.47e-01
 7.32e-01
 7.40e-01
@@ -5256,7 +5256,7 @@ Upper Bounds
 7.13e-01
 7.47e-01
 9.68e-01
-6.86e-01
+6.85e-01
 2.05e-01
 7.59e-01
 7.40e-01
@@ -5266,22 +5266,22 @@ Upper Bounds
 6.98e-01
 7.04e-01
 6.61e-01
-6.80e-01
-6.80e-01
+6.79e-01
+6.79e-01
 6.86e-01
 7.42e-01
 1.01e+00
 7.65e-01
 2.63e-01
 7.27e-01
-7.07e-01
+7.22e-01
 7.21e-01
 6.82e-01
 6.82e-01
 6.79e-01
 6.66e-01
 6.58e-01
-6.32e-01
+6.31e-01
 6.61e-01
 6.87e-01
 6.88e-01
@@ -5304,13 +5304,13 @@ Upper Bounds
 3.86e-01
 1.50e-01
 6.91e-01
-5.58e-01
+6.77e-01
 6.91e-01
 7.07e-01
 7.18e-01
 7.31e-01
 6.70e-01
-5.57e-01
+6.39e-01
 6.18e-01
 6.02e-01
 6.11e-01
@@ -5349,7 +5349,7 @@ Upper Bounds
 4.84e-01
 1.41e-01
 7.40e-01
-6.74e-01
+6.73e-01
 6.74e-01
 7.15e-01
 7.22e-01
@@ -5415,16 +5415,16 @@ Upper Bounds
 7.76e-01
 7.30e-01
 7.18e-01
-7.38e-01
+7.37e-01
 7.09e-01
-7.21e-01
+7.20e-01
 7.00e-01
 7.72e-01
 9.68e-01
 6.90e-01
 2.52e-01
 7.57e-01
-7.65e-01
+7.64e-01
 7.45e-01
 7.47e-01
 7.46e-01
@@ -5433,9 +5433,9 @@ Upper Bounds
 7.52e-01
 7.26e-01
 6.82e-01
-7.10e-01
+7.09e-01
 7.86e-01
-1.05e+00
+1.04e+00
 7.77e-01
 2.28e-01
 7.74e-01
@@ -5443,7 +5443,7 @@ Upper Bounds
 7.17e-01
 7.21e-01
 7.29e-01
-7.68e-01
+7.67e-01
 7.52e-01
 7.57e-01
 7.68e-01
@@ -5504,7 +5504,7 @@ Upper Bounds
 7.08e-01
 7.09e-01
 7.14e-01
-6.80e-01
+6.79e-01
 6.85e-01
 6.58e-01
 6.83e-01
@@ -5519,7 +5519,7 @@ Upper Bounds
 7.21e-01
 7.00e-01
 6.83e-01
-6.44e-01
+6.43e-01
 6.62e-01
 6.42e-01
 6.10e-01
@@ -5566,7 +5566,7 @@ Upper Bounds
 6.70e-01
 6.87e-01
 6.53e-01
-6.04e-01
+6.03e-01
 5.84e-01
 5.54e-01
 5.65e-01
@@ -5574,7 +5574,7 @@ Upper Bounds
 4.40e-01
 1.48e-01
 7.67e-01
-6.96e-01
+6.97e-01
 7.26e-01
 7.26e-01
 6.91e-01
@@ -5590,7 +5590,7 @@ Upper Bounds
 9.16e-02
 1.20e+00
 9.57e-01
-8.97e-01
+8.98e-01
 1.03e+00
 9.83e-01
 1.01e+00
@@ -5655,7 +5655,7 @@ Upper Bounds
 7.63e-01
 7.61e-01
 7.53e-01
-7.70e-01
+7.69e-01
 7.59e-01
 7.54e-01
 7.39e-01
@@ -5667,18 +5667,18 @@ Upper Bounds
 7.39e-01
 7.32e-01
 7.64e-01
-7.36e-01
+7.35e-01
 7.69e-01
 7.88e-01
 7.27e-01
 7.35e-01
 7.32e-01
 7.27e-01
-7.50e-01
+7.49e-01
 9.97e-01
 7.08e-01
 2.42e-01
-7.31e-01
+7.30e-01
 7.41e-01
 7.80e-01
 7.20e-01
@@ -5719,7 +5719,7 @@ Upper Bounds
 6.58e-01
 7.03e-01
 7.03e-01
-6.98e-01
+6.97e-01
 1.02e+00
 5.77e-01
 1.55e-01
@@ -5766,12 +5766,12 @@ Upper Bounds
 6.04e-01
 6.27e-01
 8.72e-01
-4.59e-01
+4.58e-01
 1.30e-01
 6.94e-01
 6.76e-01
 6.98e-01
-7.17e-01
+7.16e-01
 7.24e-01
 6.61e-01
 6.58e-01
@@ -5815,7 +5815,7 @@ Upper Bounds
 7.13e-02
 9.78e-01
 8.94e-01
-9.01e-01
+9.02e-01
 9.75e-01
 9.93e-01
 9.72e-01
@@ -5869,7 +5869,7 @@ Upper Bounds
 7.65e-01
 7.57e-01
 7.69e-01
-7.23e-01
+7.22e-01
 7.14e-01
 1.75e-01
 6.92e-02
@@ -5901,7 +5901,7 @@ Upper Bounds
 7.12e-01
 8.00e-01
 1.03e+00
-5.17e-01
+5.16e-01
 1.38e-01
 7.80e-01
 7.93e-01
@@ -5909,7 +5909,7 @@ Upper Bounds
 7.70e-01
 7.94e-01
 8.27e-01
-2.53e-01
+8.15e-01
 7.14e-01
 7.62e-01
 7.24e-01
@@ -5924,7 +5924,7 @@ Upper Bounds
 7.75e-01
 7.94e-01
 7.94e-01
-7.67e-01
+7.68e-01
 7.35e-01
 7.64e-01
 7.55e-01
@@ -5940,7 +5940,7 @@ Upper Bounds
 7.62e-01
 7.55e-01
 7.49e-01
-7.14e-01
+7.15e-01
 7.33e-01
 7.29e-01
 6.88e-01
@@ -5953,10 +5953,10 @@ Upper Bounds
 7.63e-01
 7.78e-01
 7.67e-01
-7.39e-01
+7.38e-01
 7.39e-01
 7.04e-01
-6.89e-01
+6.90e-01
 7.02e-01
 7.00e-01
 6.50e-01
@@ -5967,7 +5967,7 @@ Upper Bounds
 7.31e-01
 7.02e-01
 7.48e-01
-7.26e-01
+7.25e-01
 7.23e-01
 7.48e-01
 7.26e-01
@@ -6005,7 +6005,7 @@ Upper Bounds
 5.90e-01
 6.17e-01
 5.70e-01
-8.00e-01
+7.99e-01
 3.80e-01
 1.34e-01
 6.68e-01
@@ -6024,7 +6024,7 @@ Upper Bounds
 2.51e-01
 7.41e-02
 6.65e-01
-2.67e-01
+6.54e-01
 6.72e-01
 6.79e-01
 6.77e-01
@@ -6036,11 +6036,11 @@ Upper Bounds
 5.40e-01
 5.07e-01
 5.77e-01
-2.23e-01
+2.24e-01
 6.42e-02
 8.66e-01
 8.00e-01
-8.00e-01
+8.01e-01
 8.48e-01
 8.27e-01
 8.32e-01
@@ -6048,13 +6048,13 @@ Upper Bounds
 9.02e-01
 9.05e-01
 7.29e-01
-6.76e-01
+6.75e-01
 5.88e-01
 3.48e-01
 1.10e-01
 3.71e-02
 6.49e-01
-3.52e-01
+3.51e-01
 4.24e-01
 5.24e-01
 4.05e-01
@@ -6103,7 +6103,7 @@ Upper Bounds
 1.23e+00
 1.10e+00
 1.10e+00
-1.15e+00
+1.14e+00
 1.11e+00
 1.03e+00
 9.91e-01
@@ -6147,7 +6147,7 @@ Upper Bounds
 1.03e+00
 1.06e+00
 1.10e+00
-1.04e+00
+1.05e+00
 1.13e+00
 1.27e+00
 1.09e+00
@@ -6184,7 +6184,7 @@ Upper Bounds
 8.41e-01
 9.51e-01
 9.08e-01
-7.46e-01
+7.45e-01
 7.77e-01
 3.05e-01
 9.78e-02
@@ -6218,7 +6218,7 @@ Upper Bounds
 8.06e-01
 2.24e-01
 7.49e-02
-9.70e-01
+9.69e-01
 1.03e+00
 1.10e+00
 1.01e+00
@@ -6229,7 +6229,7 @@ Upper Bounds
 8.30e-01
 8.02e-01
 8.09e-01
-8.10e-01
+8.11e-01
 7.80e-01
 2.46e-01
 7.63e-02
@@ -6250,7 +6250,7 @@ Upper Bounds
 7.72e-02
 7.36e-01
 7.62e-01
-8.87e-01
+8.86e-01
 8.71e-01
 9.21e-01
 9.57e-01
@@ -6287,7 +6287,7 @@ Upper Bounds
 2.20e-01
 2.69e-01
 4.39e-01
-2.89e-01
+2.88e-01
 1.35e-01
 1.30e-01
 7.47e-02
@@ -6335,7 +6335,7 @@ Upper Bounds
 7.01e-01
 6.46e-01
 5.40e-01
-4.50e-01
+4.49e-01
 2.04e-01
 7.38e-02
 1.18e+00
@@ -6345,7 +6345,7 @@ Upper Bounds
 9.38e-01
 1.19e+00
 9.74e-01
-1.02e+00
+1.01e+00
 9.23e-01
 8.23e-01
 8.89e-01
@@ -6362,7 +6362,7 @@ Upper Bounds
 1.36e+00
 1.18e+00
 1.09e+00
-8.67e-01
+8.66e-01
 6.32e-01
 4.57e-01
 3.13e-01
@@ -6402,7 +6402,7 @@ Upper Bounds
 9.01e-01
 1.04e+00
 1.05e+00
-9.13e-01
+9.12e-01
 1.00e+00
 9.12e-01
 7.83e-01
@@ -6515,7 +6515,7 @@ Upper Bounds
 1.43e-01
 8.60e-02
 5.91e-02
-4.19e-02
+4.18e-02
 2.09e-02
 1.01e-02
 6.92e-02
@@ -6639,7 +6639,7 @@ Upper Bounds
 3.97e-02
 3.67e-02
 1.54e-01
-2.67e-01
+2.66e-01
 2.58e-01
 2.66e-01
 2.93e-01
diff --git a/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/inputs_true.dat b/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/inputs_true.dat
index 6aa91ca79..62f847858 100644
--- a/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/inputs_true.dat
+++ b/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/inputs_true.dat
@@ -3,24 +3,24 @@
   <materials>
     <cross_sections>mgxs.h5</cross_sections>
     <material id="1" name="source">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="source"/>
     </material>
     <material id="2" name="void">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="void"/>
     </material>
     <material id="3" name="absorber">
-      <density units="macro" value="1.0"/>
+      <density value="1.0" units="macro"/>
       <macroscopic name="absorber"/>
     </material>
   </materials>
   <geometry>
-    <cell id="1" material="1" name="infinite source region" universe="1"/>
-    <cell id="2" material="2" name="infinite void region" universe="2"/>
-    <cell id="3" material="3" name="infinite absorber region" universe="3"/>
-    <cell fill="4" id="4" universe="5"/>
-    <cell fill="5" id="5" name="full domain" region="1 -2 3 -4 5 -6" universe="6"/>
+    <cell id="1" name="infinite source region" material="1" universe="1"/>
+    <cell id="2" name="infinite void region" material="2" universe="2"/>
+    <cell id="3" name="infinite absorber region" material="3" universe="3"/>
+    <cell id="4" fill="4" universe="5"/>
+    <cell id="5" name="full domain" fill="5" region="1 -2 3 -4 5 -6" universe="6"/>
     <lattice id="4">
       <pitch>2.5 2.5 2.5</pitch>
       <dimension>12 12 12</dimension>
@@ -182,19 +182,19 @@
 3 3 3 3 3 3 3 3 3 3 3 3 
 3 3 3 3 3 3 3 3 3 3 3 3 </universes>
     </lattice>
-    <surface boundary="reflective" coeffs="0.0" id="1" type="x-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="2" type="x-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="3" type="y-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="4" type="y-plane"/>
-    <surface boundary="reflective" coeffs="0.0" id="5" type="z-plane"/>
-    <surface boundary="vacuum" coeffs="30.0" id="6" type="z-plane"/>
+    <surface id="1" type="x-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="3" type="y-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="30.0"/>
+    <surface id="5" type="z-plane" boundary="reflective" coeffs="0.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="30.0"/>
   </geometry>
   <settings>
     <run_mode>fixed source</run_mode>
     <particles>750</particles>
     <batches>30</batches>
     <inactive>20</inactive>
-    <source particle="neutron" strength="3.14" type="independent">
+    <source type="independent" strength="3.14" particle="neutron">
       <energy type="discrete">
         <parameters>100.0 1.0</parameters>
       </energy>
@@ -222,12 +222,14 @@
     <random_ray>
       <distance_active>500.0</distance_active>
       <distance_inactive>100.0</distance_inactive>
-      <source particle="neutron" strength="1.0" type="independent">
-        <space type="box">
-          <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
-        </space>
-      </source>
-      <volume_normalized_flux_tallies>True</volume_normalized_flux_tallies>
+      <ray_source>
+        <source type="independent" strength="1.0" particle="neutron">
+          <space type="box">
+            <parameters>0.0 0.0 0.0 30.0 30.0 30.0</parameters>
+          </space>
+        </source>
+      </ray_source>
+      <volume_normalized_flux_tallies>true</volume_normalized_flux_tallies>
       <source_region_meshes>
         <mesh id="1">
           <domain id="6" type="universe"/>
diff --git a/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/results_true.dat b/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/results_true.dat
index 3afdc56ca..c1066ae82 100644
--- a/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/results_true.dat
+++ b/tests/regression_tests/weightwindows_fw_cadis_mesh/linear/results_true.dat
@@ -60,7 +60,7 @@ Lower Bounds
 1.68e-01
 1.70e-01
 1.75e-01
-1.73e-01
+1.72e-01
 1.73e-01
 1.65e-01
 1.84e-01
@@ -281,7 +281,7 @@ Lower Bounds
 1.86e-01
 1.81e-01
 1.83e-01
-1.73e-01
+1.74e-01
 1.67e-01
 1.72e-01
 1.75e-01
@@ -722,7 +722,7 @@ Lower Bounds
 1.63e-01
 1.58e-01
 1.70e-01
-1.70e-01
+1.69e-01
 1.84e-01
 2.44e-01
 2.71e-01
@@ -775,7 +775,7 @@ Lower Bounds
 1.69e-01
 1.67e-01
 1.76e-01
-1.59e-01
+1.64e-01
 1.59e-01
 1.58e-01
 1.64e-01
@@ -835,7 +835,7 @@ Lower Bounds
 1.76e-01
 1.78e-01
 1.68e-01
-1.78e-01
+1.79e-01
 1.73e-01
 1.69e-01
 1.62e-01
@@ -850,11 +850,11 @@ Lower Bounds
 1.88e-01
 1.77e-01
 1.78e-01
-8.59e-02
+1.76e-01
 1.82e-01
 1.90e-01
 1.76e-01
-1.63e-01
+1.62e-01
 1.69e-01
 1.69e-01
 1.69e-01
@@ -1201,7 +1201,7 @@ Lower Bounds
 1.57e-01
 1.58e-01
 1.58e-01
-1.57e-01
+1.56e-01
 1.58e-01
 1.66e-01
 2.50e-01
@@ -1223,7 +1223,7 @@ Lower Bounds
 2.18e-01
 9.03e-02
 1.68e-01
-1.70e-01
+1.69e-01
 1.64e-01
 1.56e-01
 1.58e-01
@@ -1382,7 +1382,7 @@ Lower Bounds
 1.52e-01
 1.59e-01
 1.52e-01
-1.49e-01
+1.48e-01
 1.55e-01
 2.25e-01
 2.01e-01
@@ -1478,12 +1478,12 @@ Lower Bounds
 1.63e-01
 4.89e-02
 1.64e-01
-6.61e-02
+1.55e-01
 1.57e-01
 1.53e-01
 1.48e-01
 1.53e-01
-7.58e-02
+1.52e-01
 1.40e-01
 1.38e-01
 1.40e-01
@@ -1539,7 +1539,7 @@ Lower Bounds
 2.68e-02
 2.40e-01
 2.25e-01
-2.21e-01
+2.22e-01
 2.27e-01
 2.31e-01
 2.20e-01
@@ -1572,7 +1572,7 @@ Lower Bounds
 8.97e-02
 4.80e-02
 4.87e-02
-6.44e-02
+6.43e-02
 6.38e-02
 4.70e-02
 4.40e-02
@@ -1581,7 +1581,7 @@ Lower Bounds
 4.03e-02
 1.52e-02
 8.66e-03
-6.15e-03
+6.14e-03
 1.56e-01
 1.57e-01
 1.54e-01
@@ -1637,7 +1637,7 @@ Lower Bounds
 1.53e-01
 1.47e-01
 1.42e-01
-1.57e-01
+1.56e-01
 1.66e-01
 2.10e-01
 1.46e-01
@@ -1792,7 +1792,7 @@ Lower Bounds
 4.29e-02
 1.08e-02
 4.27e-03
-7.46e-02
+7.45e-02
 7.65e-02
 5.07e-02
 4.76e-02
@@ -1898,7 +1898,7 @@ Lower Bounds
 1.50e-01
 5.21e-02
 1.53e-01
-1.49e-01
+1.53e-01
 1.52e-01
 1.45e-01
 1.45e-01
@@ -1928,13 +1928,13 @@ Lower Bounds
 7.19e-02
 2.90e-02
 1.48e-01
-1.19e-01
+1.44e-01
 1.48e-01
 1.48e-01
 1.49e-01
 1.54e-01
 1.45e-01
-1.21e-01
+1.38e-01
 1.29e-01
 1.30e-01
 1.31e-01
@@ -1970,7 +1970,7 @@ Lower Bounds
 1.18e-01
 1.31e-01
 1.83e-01
-9.56e-02
+9.55e-02
 2.75e-02
 1.63e-01
 1.47e-01
@@ -2058,7 +2058,7 @@ Lower Bounds
 1.58e-01
 1.43e-01
 1.49e-01
-1.60e-01
+1.59e-01
 2.21e-01
 1.56e-01
 4.57e-02
@@ -2208,7 +2208,7 @@ Lower Bounds
 1.36e-01
 1.33e-01
 1.26e-01
-1.10e-01
+1.11e-01
 1.31e-01
 5.72e-02
 1.80e-02
@@ -2245,7 +2245,7 @@ Lower Bounds
 7.21e-02
 6.08e-02
 6.40e-02
-5.54e-02
+5.53e-02
 6.28e-02
 5.59e-02
 4.39e-02
@@ -2405,7 +2405,7 @@ Lower Bounds
 1.23e-01
 1.24e-01
 1.55e-01
-8.14e-02
+8.15e-02
 2.87e-02
 1.44e-01
 1.47e-01
@@ -2416,7 +2416,7 @@ Lower Bounds
 1.42e-01
 1.37e-01
 1.27e-01
-1.28e-01
+1.29e-01
 1.17e-01
 1.21e-01
 1.63e-01
@@ -2479,12 +2479,12 @@ Lower Bounds
 2.61e-02
 2.32e-02
 1.25e-02
-9.63e-03
+9.62e-03
 6.12e-03
 3.33e-03
 1.57e-01
 1.68e-01
-1.77e-01
+1.76e-01
 1.71e-01
 1.70e-01
 1.76e-01
@@ -2533,7 +2533,7 @@ Lower Bounds
 1.65e-01
 1.69e-01
 1.74e-01
-6.32e-02
+1.78e-01
 1.61e-01
 1.64e-01
 1.57e-01
@@ -2547,8 +2547,8 @@ Lower Bounds
 1.61e-01
 1.66e-01
 1.68e-01
-1.70e-01
-1.63e-01
+1.71e-01
+1.64e-01
 1.61e-01
 1.62e-01
 1.60e-01
@@ -2647,8 +2647,8 @@ Lower Bounds
 1.45e-01
 4.24e-02
 1.27e-02
-1.42e-01
-6.20e-02
+1.43e-01
+1.40e-01
 1.44e-01
 1.45e-01
 1.42e-01
@@ -2660,7 +2660,7 @@ Lower Bounds
 1.15e-01
 1.07e-01
 1.24e-01
-3.83e-02
+3.91e-02
 1.19e-02
 1.85e-01
 1.70e-01
@@ -2678,7 +2678,7 @@ Lower Bounds
 2.05e-02
 7.17e-03
 1.27e-01
-6.47e-02
+6.46e-02
 8.27e-02
 1.04e-01
 7.86e-02
@@ -3043,7 +3043,7 @@ Lower Bounds
 1.57e-01
 1.80e-01
 1.80e-01
-1.24e-01
+1.23e-01
 1.22e-01
 8.18e-02
 7.30e-02
@@ -3055,7 +3055,7 @@ Lower Bounds
 1.13e-01
 1.37e-01
 1.61e-01
-2.21e-01
+2.20e-01
 1.71e-01
 1.51e-01
 1.16e-01
@@ -3187,14 +3187,14 @@ Lower Bounds
 2.64e-02
 1.76e-02
 8.34e-03
-8.72e-02
+8.71e-02
 6.82e-02
 1.03e-01
 9.93e-02
 4.68e-02
 8.00e-02
 6.54e-02
-5.74e-02
+5.73e-02
 5.88e-02
 4.17e-02
 6.05e-02
@@ -3252,7 +3252,7 @@ Lower Bounds
 7.34e-02
 7.91e-02
 4.60e-02
-5.73e-02
+5.72e-02
 6.01e-02
 5.26e-02
 5.01e-02
@@ -3399,7 +3399,7 @@ Upper Bounds
 2.14e+00
 7.89e-01
 8.70e-01
-9.12e-01
+9.11e-01
 9.06e-01
 8.15e-01
 8.92e-01
@@ -3436,7 +3436,7 @@ Upper Bounds
 8.38e-01
 8.52e-01
 8.74e-01
-8.63e-01
+8.62e-01
 8.63e-01
 8.27e-01
 9.20e-01
@@ -3509,8 +3509,8 @@ Upper Bounds
 9.09e-01
 8.56e-01
 8.36e-01
-8.27e-01
-8.09e-01
+8.26e-01
+8.10e-01
 7.69e-01
 7.73e-01
 7.71e-01
@@ -3642,14 +3642,14 @@ Upper Bounds
 8.80e-01
 8.68e-01
 8.83e-01
-8.81e-01
+8.82e-01
 8.83e-01
 8.68e-01
 8.89e-01
 8.61e-01
 9.00e-01
 9.04e-01
-9.07e-01
+9.06e-01
 1.33e+00
 1.66e+00
 5.75e-01
@@ -3657,7 +3657,7 @@ Upper Bounds
 9.28e-01
 9.04e-01
 9.15e-01
-8.67e-01
+8.68e-01
 8.37e-01
 8.58e-01
 8.74e-01
@@ -3673,7 +3673,7 @@ Upper Bounds
 8.97e-01
 8.66e-01
 8.29e-01
-8.49e-01
+8.48e-01
 8.85e-01
 8.47e-01
 8.39e-01
@@ -3734,7 +3734,7 @@ Upper Bounds
 8.72e-01
 8.86e-01
 8.25e-01
-8.06e-01
+8.05e-01
 7.70e-01
 7.66e-01
 7.77e-01
@@ -4084,7 +4084,7 @@ Upper Bounds
 8.23e-01
 8.70e-01
 8.45e-01
-8.81e-01
+8.80e-01
 1.24e+00
 1.66e+00
 6.51e-01
@@ -4098,7 +4098,7 @@ Upper Bounds
 8.14e-01
 7.91e-01
 8.49e-01
-8.48e-01
+8.47e-01
 9.21e-01
 1.22e+00
 1.35e+00
@@ -4124,7 +4124,7 @@ Upper Bounds
 8.56e-01
 8.12e-01
 7.90e-01
-8.10e-01
+8.09e-01
 7.88e-01
 8.08e-01
 8.07e-01
@@ -4141,7 +4141,7 @@ Upper Bounds
 7.98e-01
 8.23e-01
 8.40e-01
-8.12e-01
+8.11e-01
 8.01e-01
 8.15e-01
 8.78e-01
@@ -4149,14 +4149,14 @@ Upper Bounds
 1.49e+00
 5.48e-01
 8.43e-01
-8.37e-01
+8.36e-01
 8.81e-01
-7.97e-01
+8.18e-01
 7.95e-01
 7.90e-01
 8.20e-01
 7.94e-01
-7.75e-01
+7.74e-01
 7.96e-01
 7.56e-01
 8.00e-01
@@ -4176,10 +4176,10 @@ Upper Bounds
 7.46e-01
 7.89e-01
 1.17e+00
-9.25e-01
+9.24e-01
 3.31e-01
 8.47e-01
-8.26e-01
+8.25e-01
 8.14e-01
 8.34e-01
 8.09e-01
@@ -4211,7 +4211,7 @@ Upper Bounds
 8.80e-01
 8.88e-01
 8.39e-01
-8.92e-01
+8.93e-01
 8.67e-01
 8.43e-01
 8.12e-01
@@ -4226,11 +4226,11 @@ Upper Bounds
 9.42e-01
 8.84e-01
 8.89e-01
-4.30e-01
+8.81e-01
 9.12e-01
 9.52e-01
 8.80e-01
-8.13e-01
+8.12e-01
 8.44e-01
 8.47e-01
 8.47e-01
@@ -4284,7 +4284,7 @@ Upper Bounds
 1.28e-01
 6.06e-02
 8.53e-01
-8.42e-01
+8.41e-01
 8.16e-01
 8.39e-01
 8.62e-01
@@ -4293,7 +4293,7 @@ Upper Bounds
 7.57e-01
 7.84e-01
 8.17e-01
-7.86e-01
+7.85e-01
 8.91e-01
 1.29e+00
 1.58e+00
@@ -4384,7 +4384,7 @@ Upper Bounds
 7.51e-01
 7.69e-01
 7.83e-01
-7.95e-01
+7.94e-01
 1.09e+00
 1.03e+00
 3.79e-01
@@ -4439,7 +4439,7 @@ Upper Bounds
 8.29e-01
 8.34e-01
 8.10e-01
-8.16e-01
+8.15e-01
 8.13e-01
 7.91e-01
 7.58e-01
@@ -4500,7 +4500,7 @@ Upper Bounds
 5.38e-01
 4.35e-01
 5.87e-01
-2.74e-01
+2.73e-01
 3.52e-01
 2.53e-01
 1.54e-01
@@ -4577,7 +4577,7 @@ Upper Bounds
 7.84e-01
 7.91e-01
 7.91e-01
-7.83e-01
+7.82e-01
 7.90e-01
 8.30e-01
 1.25e+00
@@ -4590,7 +4590,7 @@ Upper Bounds
 7.98e-01
 7.82e-01
 7.55e-01
-7.52e-01
+7.51e-01
 8.14e-01
 8.19e-01
 7.83e-01
@@ -4599,7 +4599,7 @@ Upper Bounds
 1.09e+00
 4.51e-01
 8.40e-01
-8.48e-01
+8.47e-01
 8.18e-01
 7.81e-01
 7.89e-01
@@ -4623,7 +4623,7 @@ Upper Bounds
 7.25e-01
 7.18e-01
 7.55e-01
-7.87e-01
+7.86e-01
 8.22e-01
 1.16e+00
 5.88e-01
@@ -4658,7 +4658,7 @@ Upper Bounds
 1.05e+00
 6.00e-01
 1.91e-01
-8.27e-01
+8.26e-01
 7.94e-01
 8.07e-01
 8.05e-01
@@ -4758,7 +4758,7 @@ Upper Bounds
 7.61e-01
 7.94e-01
 7.58e-01
-7.43e-01
+7.42e-01
 7.76e-01
 1.12e+00
 1.01e+00
@@ -4784,12 +4784,12 @@ Upper Bounds
 7.60e-01
 7.73e-01
 8.04e-01
-7.87e-01
+7.86e-01
 7.68e-01
 7.69e-01
 7.72e-01
 7.79e-01
-8.20e-01
+8.19e-01
 1.12e+00
 7.80e-01
 3.07e-01
@@ -4817,7 +4817,7 @@ Upper Bounds
 7.52e-01
 7.32e-01
 7.17e-01
-7.61e-01
+7.60e-01
 7.74e-01
 8.04e-01
 1.13e+00
@@ -4854,12 +4854,12 @@ Upper Bounds
 8.15e-01
 2.44e-01
 8.21e-01
-3.30e-01
+7.75e-01
 7.87e-01
 7.66e-01
 7.39e-01
 7.65e-01
-3.79e-01
+7.62e-01
 6.99e-01
 6.88e-01
 6.98e-01
@@ -4962,10 +4962,10 @@ Upper Bounds
 7.87e-01
 7.72e-01
 8.00e-01
-8.25e-01
-8.31e-01
+8.24e-01
+8.30e-01
 8.27e-01
-7.89e-01
+7.88e-01
 7.35e-01
 7.02e-01
 6.79e-01
@@ -4991,13 +4991,13 @@ Upper Bounds
 7.91e-01
 7.84e-01
 8.12e-01
-8.14e-01
+8.13e-01
 7.93e-01
 7.86e-01
 7.80e-01
 7.88e-01
 7.62e-01
-7.26e-01
+7.25e-01
 7.48e-01
 8.00e-01
 1.08e+00
@@ -5013,7 +5013,7 @@ Upper Bounds
 7.67e-01
 7.34e-01
 7.08e-01
-7.83e-01
+7.82e-01
 8.28e-01
 1.05e+00
 7.28e-01
@@ -5043,7 +5043,7 @@ Upper Bounds
 7.26e-01
 7.36e-01
 7.31e-01
-7.41e-01
+7.40e-01
 7.92e-01
 1.07e+00
 6.18e-01
@@ -5089,7 +5089,7 @@ Upper Bounds
 6.75e-01
 6.66e-01
 7.38e-01
-7.32e-01
+7.31e-01
 9.88e-01
 5.41e-01
 1.68e-01
@@ -5219,7 +5219,7 @@ Upper Bounds
 7.96e-01
 8.02e-01
 7.57e-01
-7.80e-01
+7.79e-01
 7.99e-01
 7.43e-01
 7.26e-01
@@ -5251,7 +5251,7 @@ Upper Bounds
 7.54e-01
 7.53e-01
 7.66e-01
-7.41e-01
+7.40e-01
 7.01e-01
 7.50e-01
 8.07e-01
@@ -5274,9 +5274,9 @@ Upper Bounds
 7.50e-01
 2.61e-01
 7.64e-01
-7.45e-01
+7.65e-01
 7.60e-01
-7.26e-01
+7.25e-01
 7.24e-01
 7.17e-01
 6.99e-01
@@ -5294,7 +5294,7 @@ Upper Bounds
 7.14e-01
 7.38e-01
 7.21e-01
-6.81e-01
+6.80e-01
 6.69e-01
 6.84e-01
 6.84e-01
@@ -5304,13 +5304,13 @@ Upper Bounds
 3.59e-01
 1.45e-01
 7.38e-01
-5.93e-01
+7.18e-01
 7.38e-01
 7.39e-01
 7.46e-01
 7.70e-01
 7.25e-01
-6.07e-01
+6.90e-01
 6.46e-01
 6.49e-01
 6.56e-01
@@ -5318,7 +5318,7 @@ Upper Bounds
 9.65e-01
 3.98e-01
 1.02e-01
-7.63e-01
+7.64e-01
 7.31e-01
 7.32e-01
 7.05e-01
@@ -5369,7 +5369,7 @@ Upper Bounds
 1.11e+00
 1.11e+00
 1.08e+00
-9.96e-01
+9.95e-01
 1.02e+00
 9.59e-01
 1.01e+00
@@ -5434,7 +5434,7 @@ Upper Bounds
 7.89e-01
 7.16e-01
 7.47e-01
-7.98e-01
+7.97e-01
 1.11e+00
 7.82e-01
 2.29e-01
@@ -5447,7 +5447,7 @@ Upper Bounds
 7.75e-01
 7.99e-01
 8.00e-01
-7.27e-01
+7.26e-01
 7.00e-01
 7.95e-01
 1.12e+00
@@ -5584,7 +5584,7 @@ Upper Bounds
 6.82e-01
 6.66e-01
 6.30e-01
-5.52e-01
+5.53e-01
 6.56e-01
 2.86e-01
 9.02e-02
@@ -5603,7 +5603,7 @@ Upper Bounds
 5.90e-01
 1.70e-01
 6.04e-02
-9.35e-01
+9.34e-01
 7.67e-01
 7.48e-01
 8.36e-01
@@ -5688,7 +5688,7 @@ Upper Bounds
 7.74e-01
 7.69e-01
 7.48e-01
-7.27e-01
+7.26e-01
 7.70e-01
 1.04e+00
 5.38e-01
@@ -5698,7 +5698,7 @@ Upper Bounds
 7.98e-01
 7.72e-01
 8.10e-01
-7.95e-01
+7.96e-01
 7.80e-01
 7.69e-01
 7.86e-01
@@ -5776,7 +5776,7 @@ Upper Bounds
 7.07e-01
 6.87e-01
 6.74e-01
-6.34e-01
+6.35e-01
 6.34e-01
 6.14e-01
 6.22e-01
@@ -5792,10 +5792,10 @@ Upper Bounds
 7.08e-01
 6.86e-01
 6.37e-01
-6.42e-01
+6.43e-01
 5.84e-01
 6.05e-01
-8.13e-01
+8.14e-01
 2.50e-01
 7.65e-02
 7.35e-01
@@ -5805,7 +5805,7 @@ Upper Bounds
 7.24e-01
 7.52e-01
 7.60e-01
-7.36e-01
+7.35e-01
 6.72e-01
 6.44e-01
 6.12e-01
@@ -5860,7 +5860,7 @@ Upper Bounds
 1.67e-02
 7.86e-01
 8.39e-01
-8.83e-01
+8.82e-01
 8.55e-01
 8.50e-01
 8.79e-01
@@ -5901,7 +5901,7 @@ Upper Bounds
 7.49e-01
 8.31e-01
 1.07e+00
-4.89e-01
+4.88e-01
 1.30e-01
 8.26e-01
 8.49e-01
@@ -5909,7 +5909,7 @@ Upper Bounds
 8.27e-01
 8.44e-01
 8.70e-01
-3.16e-01
+8.88e-01
 8.04e-01
 8.19e-01
 7.85e-01
@@ -5923,8 +5923,8 @@ Upper Bounds
 8.03e-01
 8.28e-01
 8.39e-01
-8.52e-01
-8.17e-01
+8.53e-01
+8.18e-01
 8.07e-01
 8.12e-01
 8.02e-01
@@ -6023,8 +6023,8 @@ Upper Bounds
 7.24e-01
 2.12e-01
 6.37e-02
-7.12e-01
-3.10e-01
+7.13e-01
+6.99e-01
 7.21e-01
 7.24e-01
 7.11e-01
@@ -6036,7 +6036,7 @@ Upper Bounds
 5.74e-01
 5.37e-01
 6.18e-01
-1.92e-01
+1.95e-01
 5.93e-02
 9.23e-01
 8.49e-01
@@ -6053,8 +6053,8 @@ Upper Bounds
 3.58e-01
 1.02e-01
 3.58e-02
-6.34e-01
-3.24e-01
+6.33e-01
+3.23e-01
 4.14e-01
 5.18e-01
 3.93e-01
@@ -6067,7 +6067,7 @@ Upper Bounds
 2.08e-01
 1.27e-01
 3.00e-02
-9.84e-03
+9.83e-03
 2.53e-01
 1.24e-01
 1.72e-01
@@ -6211,7 +6211,7 @@ Upper Bounds
 1.03e+00
 1.15e+00
 1.11e+00
-9.64e-01
+9.63e-01
 1.01e+00
 9.05e-01
 8.67e-01
@@ -6229,8 +6229,8 @@ Upper Bounds
 8.93e-01
 8.70e-01
 8.58e-01
-8.56e-01
-8.00e-01
+8.57e-01
+8.01e-01
 2.31e-01
 7.13e-02
 9.12e-01
@@ -6249,7 +6249,7 @@ Upper Bounds
 2.01e-01
 7.22e-02
 7.80e-01
-8.00e-01
+7.99e-01
 9.64e-01
 9.33e-01
 9.70e-01
@@ -6277,7 +6277,7 @@ Upper Bounds
 5.08e-01
 3.32e-01
 9.24e-02
-4.15e-02
+4.14e-02
 3.42e-01
 2.04e-01
 2.56e-01
@@ -6414,12 +6414,12 @@ Upper Bounds
 1.17e-01
 7.24e-02
 5.31e-01
-7.17e-01
+7.16e-01
 8.03e-01
 7.87e-01
 9.01e-01
 9.00e-01
-6.18e-01
+6.17e-01
 6.08e-01
 4.09e-01
 3.65e-01
@@ -6545,7 +6545,7 @@ Upper Bounds
 2.87e-01
 1.88e-01
 1.85e-01
-1.80e-01
+1.79e-01
 1.07e-01
 3.77e-02
 5.65e-01
@@ -6578,13 +6578,13 @@ Upper Bounds
 1.41e-01
 5.88e-02
 6.26e-02
-4.47e-01
+4.46e-01
 3.47e-01
 5.27e-01
 6.11e-01
 3.91e-01
 4.22e-01
-4.77e-01
+4.76e-01
 4.54e-01
 4.31e-01
 3.19e-01
@@ -6727,14 +6727,14 @@ Upper Bounds
 5.73e-02
 5.81e-02
 1.92e-02
-6.10e-03
+6.09e-03
 2.72e-02
 4.77e-02
 4.74e-02
 3.34e-02
 5.41e-02
 7.93e-02
-6.70e-02
+6.69e-02
 7.67e-02
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 5.17e-02
diff --git a/tests/regression_tests/weightwindows_pulse_height/__init__.py b/tests/regression_tests/weightwindows_pulse_height/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/regression_tests/weightwindows_pulse_height/local/inputs_true.dat b/tests/regression_tests/weightwindows_pulse_height/local/inputs_true.dat
new file mode 100644
index 000000000..4cd24d529
--- /dev/null
+++ b/tests/regression_tests/weightwindows_pulse_height/local/inputs_true.dat
@@ -0,0 +1,60 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="3.7" units="g/cc"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="I127" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="inner sphere" material="1" region="-1" universe="1"/>
+    <cell id="2" name="outer sphere" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 2"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="photon">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <photon_transport>true</photon_transport>
+    <weight_windows id="1">
+      <mesh>1</mesh>
+      <particle_type>photon</particle_type>
+      <energy_bounds>0.0 2000000.0</energy_bounds>
+      <lower_ww_bounds>0.01</lower_ww_bounds>
+      <upper_ww_bounds>0.05</upper_ww_bounds>
+      <survival_ratio>3.0</survival_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <mesh id="1">
+      <dimension>1 1 1</dimension>
+      <lower_left>-2 -2 -2</lower_left>
+      <upper_right>2 2 2</upper_right>
+    </mesh>
+    <shared_secondary_bank>false</shared_secondary_bank>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+    <max_history_splits>50</max_history_splits>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0 100000.0 110000.0 120000.0 130000.0 140000.0 150000.0 160000.0 170000.0 180000.0 190000.0 200000.0 210000.0 220000.0 230000.0 240000.0 250000.0 260000.0 270000.0 280000.0 290000.0 300000.0 310000.0 320000.0 330000.0 340000.0 350000.0 360000.0 370000.0 380000.0 390000.0 400000.0 410000.0 420000.0 430000.0 440000.0 450000.0 460000.0 470000.0 480000.0 490000.0 500000.0 510000.0 520000.0 530000.0 540000.0 550000.0 560000.0 570000.0 580000.0 590000.0 600000.0 610000.0 620000.0 630000.0 640000.0 650000.0 660000.0 670000.0 680000.0 690000.0 700000.0 710000.0 720000.0 730000.0 740000.0 750000.0 760000.0 770000.0 780000.0 790000.0 800000.0 810000.0 820000.0 830000.0 840000.0 850000.0 860000.0 870000.0 880000.0 890000.0 900000.0 910000.0 920000.0 930000.0 940000.0 950000.0 960000.0 970000.0 980000.0 990000.0 1000000.0</bins>
+    </filter>
+    <tally id="1" name="pht tally">
+      <filters>1 2</filters>
+      <scores>pulse-height</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows_pulse_height/local/results_true.dat b/tests/regression_tests/weightwindows_pulse_height/local/results_true.dat
new file mode 100644
index 000000000..c57e8ff1c
--- /dev/null
+++ b/tests/regression_tests/weightwindows_pulse_height/local/results_true.dat
@@ -0,0 +1,201 @@
+tally 1:
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diff --git a/tests/regression_tests/weightwindows_pulse_height/shared/inputs_true.dat b/tests/regression_tests/weightwindows_pulse_height/shared/inputs_true.dat
new file mode 100644
index 000000000..8bb499a73
--- /dev/null
+++ b/tests/regression_tests/weightwindows_pulse_height/shared/inputs_true.dat
@@ -0,0 +1,60 @@
+<?xml version='1.0' encoding='utf-8'?>
+<model>
+  <materials>
+    <material id="1">
+      <density value="3.7" units="g/cc"/>
+      <nuclide name="Na23" ao="1.0"/>
+      <nuclide name="I127" ao="1.0"/>
+    </material>
+  </materials>
+  <geometry>
+    <cell id="1" name="inner sphere" material="1" region="-1" universe="1"/>
+    <cell id="2" name="outer sphere" material="void" region="1 -2" universe="1"/>
+    <surface id="1" type="sphere" coeffs="0.0 0.0 0.0 1"/>
+    <surface id="2" type="sphere" boundary="vacuum" coeffs="0.0 0.0 0.0 2"/>
+  </geometry>
+  <settings>
+    <run_mode>fixed source</run_mode>
+    <particles>100</particles>
+    <batches>5</batches>
+    <source type="independent" strength="1.0" particle="photon">
+      <energy type="discrete">
+        <parameters>1000000.0 1.0</parameters>
+      </energy>
+    </source>
+    <photon_transport>true</photon_transport>
+    <weight_windows id="1">
+      <mesh>1</mesh>
+      <particle_type>photon</particle_type>
+      <energy_bounds>0.0 2000000.0</energy_bounds>
+      <lower_ww_bounds>0.01</lower_ww_bounds>
+      <upper_ww_bounds>0.05</upper_ww_bounds>
+      <survival_ratio>3.0</survival_ratio>
+      <max_split>10</max_split>
+      <weight_cutoff>1e-38</weight_cutoff>
+    </weight_windows>
+    <mesh id="1">
+      <dimension>1 1 1</dimension>
+      <lower_left>-2 -2 -2</lower_left>
+      <upper_right>2 2 2</upper_right>
+    </mesh>
+    <shared_secondary_bank>true</shared_secondary_bank>
+    <weight_window_checkpoints>
+      <collision>true</collision>
+      <surface>true</surface>
+    </weight_window_checkpoints>
+    <max_history_splits>50</max_history_splits>
+  </settings>
+  <tallies>
+    <filter id="1" type="cell">
+      <bins>1</bins>
+    </filter>
+    <filter id="2" type="energy">
+      <bins>0.0 10000.0 20000.0 30000.0 40000.0 50000.0 60000.0 70000.0 80000.0 90000.0 100000.0 110000.0 120000.0 130000.0 140000.0 150000.0 160000.0 170000.0 180000.0 190000.0 200000.0 210000.0 220000.0 230000.0 240000.0 250000.0 260000.0 270000.0 280000.0 290000.0 300000.0 310000.0 320000.0 330000.0 340000.0 350000.0 360000.0 370000.0 380000.0 390000.0 400000.0 410000.0 420000.0 430000.0 440000.0 450000.0 460000.0 470000.0 480000.0 490000.0 500000.0 510000.0 520000.0 530000.0 540000.0 550000.0 560000.0 570000.0 580000.0 590000.0 600000.0 610000.0 620000.0 630000.0 640000.0 650000.0 660000.0 670000.0 680000.0 690000.0 700000.0 710000.0 720000.0 730000.0 740000.0 750000.0 760000.0 770000.0 780000.0 790000.0 800000.0 810000.0 820000.0 830000.0 840000.0 850000.0 860000.0 870000.0 880000.0 890000.0 900000.0 910000.0 920000.0 930000.0 940000.0 950000.0 960000.0 970000.0 980000.0 990000.0 1000000.0</bins>
+    </filter>
+    <tally id="1" name="pht tally">
+      <filters>1 2</filters>
+      <scores>pulse-height</scores>
+    </tally>
+  </tallies>
+</model>
diff --git a/tests/regression_tests/weightwindows_pulse_height/shared/results_true.dat b/tests/regression_tests/weightwindows_pulse_height/shared/results_true.dat
new file mode 100644
index 000000000..c57e8ff1c
--- /dev/null
+++ b/tests/regression_tests/weightwindows_pulse_height/shared/results_true.dat
@@ -0,0 +1,201 @@
+tally 1:
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+0.000000E+00
+0.000000E+00
+0.000000E+00
+2.000000E-01
+8.600000E-03
diff --git a/tests/regression_tests/weightwindows_pulse_height/test.py b/tests/regression_tests/weightwindows_pulse_height/test.py
new file mode 100644
index 000000000..b6b49be26
--- /dev/null
+++ b/tests/regression_tests/weightwindows_pulse_height/test.py
@@ -0,0 +1,73 @@
+import numpy as np
+import openmc
+import pytest
+from openmc.utility_funcs import change_directory
+
+from tests.testing_harness import PyAPITestHarness
+
+
+@pytest.mark.parametrize("shared_secondary,subdir", [
+    (False, "local"),
+    (True, "shared"),
+])
+def test_weightwindows_pulse_height(shared_secondary, subdir):
+    with change_directory(subdir):
+        openmc.reset_auto_ids()
+        model = openmc.Model()
+
+        # Define materials (NaI scintillator)
+        NaI = openmc.Material()
+        NaI.set_density('g/cc', 3.7)
+        NaI.add_element('Na', 1.0)
+        NaI.add_element('I', 1.0)
+
+        model.materials = openmc.Materials([NaI])
+
+        # Define geometry: NaI sphere inside vacuum sphere
+        s1 = openmc.Sphere(r=1)
+        s2 = openmc.Sphere(r=2, boundary_type='vacuum')
+        inner_sphere = openmc.Cell(name='inner sphere', fill=NaI, region=-s1)
+        outer_sphere = openmc.Cell(name='outer sphere', region=+s1 & -s2)
+        model.geometry = openmc.Geometry([inner_sphere, outer_sphere])
+
+        # Define settings
+        model.settings.run_mode = 'fixed source'
+        model.settings.batches = 5
+        model.settings.particles = 100
+        model.settings.photon_transport = True
+        model.settings.shared_secondary_bank = shared_secondary
+        model.settings.max_history_splits = 50
+        model.settings.weight_window_checkpoints = {
+            'surface': True,
+            'collision': True,
+        }
+        model.settings.source = openmc.IndependentSource(
+            energy=openmc.stats.delta_function(1e6),
+            particle='photon'
+        )
+
+        # Define pulse-height tally
+        tally = openmc.Tally(name="pht tally")
+        tally.scores = ['pulse-height']
+        cell_filter = openmc.CellFilter(inner_sphere)
+        energy_filter = openmc.EnergyFilter(np.linspace(0, 1_000_000, 101))
+        tally.filters = [cell_filter, energy_filter]
+        model.tallies = [tally]
+
+        # Define weight windows on a simple mesh covering the geometry
+        ww_mesh = openmc.RegularMesh()
+        ww_mesh.lower_left = (-2, -2, -2)
+        ww_mesh.upper_right = (2, 2, 2)
+        ww_mesh.dimension = (1, 1, 1)
+
+        # Single energy bin for photons
+        e_bnds = [0.0, 2e6]
+
+        # Uniform weight window bounds (low enough to trigger some splitting)
+        lower_bounds = np.array([0.01])
+
+        ww = openmc.WeightWindows(ww_mesh, lower_bounds, None, 5.0, e_bnds, 'photon')
+        model.settings.weight_windows = [ww]
+
+        harness = PyAPITestHarness('statepoint.5.h5', model)
+        harness.main()
diff --git a/tests/regression_tests/white_plane/results_true.dat b/tests/regression_tests/white_plane/results_true.dat
index 6f1d064eb..ffb19491d 100644
--- a/tests/regression_tests/white_plane/results_true.dat
+++ b/tests/regression_tests/white_plane/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-2.279719E+00 5.380792E-03
+2.274312E+00 4.223342E-03
diff --git a/tests/testing_harness.py b/tests/testing_harness.py
index 6de475e6e..c1977556d 100644
--- a/tests/testing_harness.py
+++ b/tests/testing_harness.py
@@ -68,7 +68,7 @@ class TestHarness:
         if config['mpi']:
             mpi_args = [config['mpiexec'], '-n', config['mpi_np']]
             openmc.run(openmc_exec=config['exe'], mpi_args=mpi_args,
-              event_based=config['event'])
+                       event_based=config['event'])
         else:
             openmc.run(openmc_exec=config['exe'], event_based=config['event'])
 
@@ -143,7 +143,8 @@ class TestHarness:
     def _cleanup(self):
         """Delete statepoints, tally, and test files."""
         output = glob.glob('statepoint.*.h5')
-        output += ['tallies.out', 'results_test.dat', 'summary.h5']
+        output += ['tallies.out', 'tallies.forward.out']
+        output += ['results_test.dat', 'summary.h5']
         output += glob.glob('volume_*.h5')
         for f in output:
             if os.path.exists(f):
@@ -305,9 +306,12 @@ class PyAPITestHarness(TestHarness):
         else:
             self.execute_test()
 
-    def execute_test(self):
+    def execute_test(self, change_dir=False):
         """Build input XMLs, run OpenMC, and verify correct results."""
+        base_dir = os.getcwd() if change_dir else None
         try:
+            if change_dir:
+                os.chdir(self.workdir)
             self._build_inputs()
             inputs = self._get_inputs()
             self._write_inputs(inputs)
@@ -319,10 +323,15 @@ class PyAPITestHarness(TestHarness):
             self._compare_results()
         finally:
             self._cleanup()
+            if base_dir:
+                os.chdir(base_dir)
 
-    def update_results(self):
+    def update_results(self, change_dir=False):
         """Update results_true.dat and inputs_true.dat"""
+        base_dir = os.getcwd() if change_dir else None
         try:
+            if change_dir:
+                os.chdir(self.workdir)
             self._build_inputs()
             inputs = self._get_inputs()
             self._write_inputs(inputs)
@@ -334,6 +343,8 @@ class PyAPITestHarness(TestHarness):
             self._overwrite_results()
         finally:
             self._cleanup()
+            if base_dir:
+                os.chdir(base_dir)
 
     def _build_inputs(self):
         """Write input XML files."""
@@ -371,7 +382,8 @@ class PyAPITestHarness(TestHarness):
         """Delete XMLs, statepoints, tally, and test files."""
         super()._cleanup()
         output = ['materials.xml', 'geometry.xml', 'settings.xml',
-                  'tallies.xml', 'plots.xml', 'inputs_test.dat', 'model.xml']
+                  'tallies.xml', 'plots.xml', 'inputs_test.dat', 'model.xml',
+                  'collision_track.h5', 'collision_track.mcpl']
         for f in output:
             if os.path.exists(f):
                 os.remove(f)
@@ -389,6 +401,7 @@ class TolerantPyAPITestHarness(PyAPITestHarness):
     due to single precision usage (e.g., as in the random ray solver).
 
     """
+
     def _are_files_equal(self, actual_path, expected_path, tolerance):
         def isfloat(value):
             try:
@@ -428,7 +441,8 @@ class TolerantPyAPITestHarness(PyAPITestHarness):
 
     def _compare_results(self):
         """Make sure the current results agree with the reference."""
-        compare = self._are_files_equal('results_test.dat', 'results_true.dat', 1e-6)
+        compare = self._are_files_equal(
+            'results_test.dat', 'results_true.dat', 1e-6)
         if not compare:
             expected = open('results_true.dat').readlines()
             actual = open('results_test.dat').readlines()
@@ -443,7 +457,7 @@ class TolerantPyAPITestHarness(PyAPITestHarness):
 class WeightWindowPyAPITestHarness(PyAPITestHarness):
     def _get_results(self):
         """Digest info in the weight window file and return as a string."""
-        ww = openmc.hdf5_to_wws()[0]
+        ww = openmc.WeightWindowsList.from_hdf5()[0]
 
         # Access the weight window bounds
         lower_bound = ww.lower_ww_bounds
@@ -476,6 +490,7 @@ class WeightWindowPyAPITestHarness(PyAPITestHarness):
 
 class PlotTestHarness(TestHarness):
     """Specialized TestHarness for running OpenMC plotting tests."""
+
     def __init__(self, plot_names, voxel_convert_checks=[]):
         super().__init__(None)
         self._plot_names = plot_names
@@ -523,3 +538,124 @@ class PlotTestHarness(TestHarness):
         outstr = sha512.hexdigest()
 
         return outstr
+
+
+class CollisionTrackTestHarness(PyAPITestHarness):
+    def __init__(self, statepoint_name, model=None, inputs_true=None, workdir=None):
+        super().__init__(statepoint_name, model, inputs_true)
+        self.workdir = workdir
+
+    def _test_output_created(self):
+        """Make sure collision_track.h5 has also been created."""
+        if self._model.settings.collision_track:
+            assert os.path.exists(
+                "collision_track.h5"
+            ), "collision_track file has not been created."
+
+    def _compare_results(self):
+        """Compare collision_track.h5 files."""
+        if self._model.settings.collision_track:
+            collision_track_true = self._return_collision_track_data(
+                "collision_track_true.h5")
+            collision_track_test = self._return_collision_track_data(
+                "collision_track.h5")
+            assert collision_track_true.shape == collision_track_test.shape
+            np.testing.assert_allclose(
+                collision_track_true, collision_track_test, rtol=1e-07)
+
+    def main(self):
+        """Accept commandline arguments and either run or update tests."""
+        if config["build_inputs"]:
+            self.build_inputs()
+        elif config["update"]:
+            self.update_results(change_dir=True)
+        else:
+            self.execute_test(change_dir=True)
+
+    def build_inputs(self):
+        """Build inputs."""
+        base_dir = os.getcwd()
+        try:
+            os.chdir(self.workdir)
+            self._build_inputs()
+        finally:
+            os.chdir(base_dir)
+
+    def _overwrite_results(self):
+        """Also add the 'collision_track.h5' file during overwriting."""
+        if os.path.exists("collision_track.h5"):
+            shutil.copyfile("collision_track.h5", "collision_track_true.h5")
+
+    def _write_results(self, results_string):
+        # The result file for this test are written by the OpenMC executable itself
+        pass
+
+    @staticmethod
+    def _return_collision_track_data(filepath):
+        """
+        Read a collision_track file and return a sorted array composed of
+        flattened collision records.
+
+        Parameters
+        ----------
+        filepath : str
+            Path to the collision_track file
+
+        Returns
+        -------
+        data : np.array
+            Sorted array composed of flattened collision-track records.
+        """
+        source = openmc.read_collision_track_file(filepath)
+        columns = [
+            source['r']['x'],
+            source['r']['y'],
+            source['r']['z'],
+            source['u']['x'],
+            source['u']['y'],
+            source['u']['z'],
+            source['E'],
+            source['dE'],
+            source['time'],
+            source['wgt'],
+            source['event_mt'],
+            source['delayed_group'],
+            source['cell_id'],
+            source['nuclide_id'],
+            source['material_id'],
+            source['universe_id'],
+            source['n_collision'],
+            source['particle'],
+            source['parent_id'],
+            source['progeny_id'],
+        ]
+        data = np.column_stack(columns)
+
+        # Sort by the complete record, prioritizing stable integer identifiers
+        # before floating-point fields. This removes dependence on the order in
+        # which threads append otherwise reproducible collision records.
+        sort_columns = [
+            source['parent_id'],
+            source['progeny_id'],
+            source['n_collision'],
+            source['particle'],
+            source['cell_id'],
+            source['material_id'],
+            source['universe_id'],
+            source['nuclide_id'],
+            source['event_mt'],
+            source['delayed_group'],
+            source['r']['x'],
+            source['r']['y'],
+            source['r']['z'],
+            source['u']['x'],
+            source['u']['y'],
+            source['u']['z'],
+            source['E'],
+            source['dE'],
+            source['time'],
+            source['wgt'],
+        ]
+        sorted_idx = np.lexsort(tuple(reversed(sort_columns)))
+
+        return data[sorted_idx]
diff --git a/tests/unit_tests/__init__.py b/tests/unit_tests/__init__.py
index e97ab13e2..44b5a5018 100644
--- a/tests/unit_tests/__init__.py
+++ b/tests/unit_tests/__init__.py
@@ -14,3 +14,12 @@ def assert_unbounded(obj):
     ll, ur = obj.bounding_box
     assert ll == pytest.approx((-np.inf, -np.inf, -np.inf))
     assert ur == pytest.approx((np.inf, np.inf, np.inf))
+
+
+def assert_sample_mean(samples, expected_mean):
+    # Calculate sample standard deviation
+    std_dev = samples.std() / np.sqrt(samples.size - 1)
+
+    # Means should agree within 4 sigma 99.993% of the time. Note that this is
+    # expected to fail about 1 out of 16,000 times
+    assert np.abs(expected_mean - samples.mean()) < 4*std_dev
diff --git a/tests/unit_tests/dagmc/dagmc_tetrahedral_no_graveyard.h5m b/tests/unit_tests/dagmc/dagmc_tetrahedral_no_graveyard.h5m
new file mode 100644
index 000000000..2aa72956b
Binary files /dev/null and b/tests/unit_tests/dagmc/dagmc_tetrahedral_no_graveyard.h5m differ
diff --git a/tests/unit_tests/dagmc/test_convert_to_multigroup.py b/tests/unit_tests/dagmc/test_convert_to_multigroup.py
new file mode 100644
index 000000000..069dc658e
--- /dev/null
+++ b/tests/unit_tests/dagmc/test_convert_to_multigroup.py
@@ -0,0 +1,53 @@
+"""Test that convert_to_multigroup works with DAGMC models without requiring
+particles/batches to be set beforehand.
+"""
+
+from pathlib import Path
+import pytest
+import openmc
+import openmc.lib
+
+pytestmark = pytest.mark.skipif(
+    not openmc.lib._dagmc_enabled(),
+    reason="DAGMC CAD geometry is not enabled.")
+
+
+def test_convert_to_multigroup_without_particles_batches(run_in_tmpdir):
+    """Test that convert_to_multigroup works with DAGMC model without
+    setting particles/batches beforehand."""
+    openmc.reset_auto_ids()
+    
+    mat = openmc.Material(name="mat")
+    mat.add_nuclide("Fe56", 1.0)
+    mat.set_density("g/cm3", 7.0)
+
+    # Use minimal tetrahedral DAGMC file
+    dagmc_file = Path(__file__).parent / "dagmc_tetrahedral_no_graveyard.h5m"
+    dagmc_univ = openmc.DAGMCUniverse(dagmc_file, auto_geom_ids=True)
+    bound_dagmc_univ = dagmc_univ.bounded_universe(padding_distance=1)
+
+    # Create model WITHOUT setting particles or batches
+    model = openmc.Model()
+    model.materials = openmc.Materials([mat])
+    model.geometry = openmc.Geometry(bound_dagmc_univ)
+    model.settings = openmc.Settings()  # Note: no particles or batches set!
+
+    model.settings.run_mode = 'fixed source'
+    
+    # Create a point source
+    my_source = openmc.IndependentSource()
+    my_source.space = openmc.stats.Point((0.25, 0.25, 0.25))
+    my_source.energy = openmc.stats.delta_function(14e6)
+    model.settings.source = my_source
+
+    # This should work without requiring particles/batches to be set
+    # convert_to_multigroup handles initialization internally using non-transport mode
+    model.convert_to_multigroup(
+        method='material_wise',
+        groups='CASMO-2',
+        nparticles=10,
+        overwrite_mgxs_library=True
+    )
+
+    # Verify the model was converted successfully
+    assert model.settings.energy_mode == 'multi-group'
diff --git a/tests/unit_tests/dagmc/test_lost_particles.py b/tests/unit_tests/dagmc/test_lost_particles.py
index 502bd795e..48b6cf165 100644
--- a/tests/unit_tests/dagmc/test_lost_particles.py
+++ b/tests/unit_tests/dagmc/test_lost_particles.py
@@ -57,11 +57,16 @@ def broken_dagmc_model(request):
     model.settings.inactive = 2
     model.settings.output = {'summary': False}
 
+    # Limit max particle events to prevent particles from getting stuck in the
+    # implicit complement of the non-root DAGMC universe.
+    model.settings.max_particle_events = 100
+
     model.export_to_xml()
 
     return model
 
 
+@pytest.mark.skip(reason="Test causes CI to hang intermittently")
 def test_lost_particles(run_in_tmpdir, broken_dagmc_model):
     broken_dagmc_model.export_to_xml()
     # ensure that particles will be lost when cell intersections can't be found
@@ -70,12 +75,12 @@ def test_lost_particles(run_in_tmpdir, broken_dagmc_model):
         openmc.run()
 
     # run this again, but with the dagmc universe as the root unvierse
+    # to ensure that lost particles are still caught in this case
     for univ in broken_dagmc_model.geometry.get_all_universes().values():
         if isinstance(univ, openmc.DAGMCUniverse):
-            broken_dagmc_model.geometry.root_unvierse = univ
+            broken_dagmc_model.geometry.root_universe = univ
             break
 
     broken_dagmc_model.export_to_xml()
     with pytest.raises(RuntimeError, match='Maximum number of lost particles has been reached.'):
         openmc.run()
-
diff --git a/tests/unit_tests/dagmc/test_model.py b/tests/unit_tests/dagmc/test_model.py
index 9fdfbcebc..5889fa542 100644
--- a/tests/unit_tests/dagmc/test_model.py
+++ b/tests/unit_tests/dagmc/test_model.py
@@ -4,6 +4,7 @@ import lxml.etree as ET
 import numpy as np
 import pytest
 import openmc
+import openmc.lib
 from openmc.utility_funcs import change_directory
 
 pytestmark = pytest.mark.skipif(
@@ -30,7 +31,7 @@ def model(request):
 
     p = Path(request.fspath).parent / "dagmc.h5m"
 
-    daguniv = openmc.DAGMCUniverse(p, auto_geom_ids=True)
+    daguniv = openmc.DAGMCUniverse(p, name='simple-dagmc', auto_geom_ids=True)
 
     lattice = openmc.RectLattice()
     lattice.dimension = [2, 2]
@@ -69,28 +70,20 @@ def model(request):
             openmc.reset_auto_ids()
 
 
-def test_dagmc_replace_material_assignment(model):
-    mats = {}
-
-    mats["foo"] = openmc.Material(name="foo")
-    mats["foo"].add_nuclide("H1", 2.0)
-    mats["foo"].add_element("O", 1.0)
-    mats["foo"].set_density("g/cm3", 1.0)
-    mats["foo"].add_s_alpha_beta("c_H_in_H2O")
-
+def test_dagmc_sync_cell_names(model):
+    dag_univ = None
     for univ in model.geometry.get_all_universes().values():
-        if not isinstance(univ, openmc.DAGMCUniverse):
+        if isinstance(univ, openmc.DAGMCUniverse):
+            dag_univ = univ
             break
 
-        cells_with_41 = []
-        for cell in univ.cells.values():
-            if cell.fill is None:
-                continue
-            if cell.fill.name == "41":
-                cells_with_41.append(cell.id)
-        univ.replace_material_assignment("41", mats["foo"])
-        for cell_id in cells_with_41:
-            assert univ.cells[cell_id] == mats["foo"]
+    assert dag_univ is not None
+
+    for cell_id, cell in dag_univ.cells.items():
+        assert cell.name == openmc.lib.cells[cell_id].name
+
+    assert any(cell.name == "implicit complement"
+               for cell in dag_univ.cells.values())
 
 
 def test_dagmc_add_material_override_with_id(model):
@@ -113,7 +106,7 @@ def test_dagmc_add_material_override_with_id(model):
                 cells_with_41.append(cell.id)
                 univ.add_material_override(cell.id, mats["foo"])
         for cell_id in cells_with_41:
-            assert univ.cells[cell_id] == mats["foo"]
+            assert univ.cells[cell_id].fill == mats["foo"]
 
 
 def test_dagmc_add_material_override_with_cell(model):
@@ -136,7 +129,7 @@ def test_dagmc_add_material_override_with_cell(model):
                 cells_with_41.append(cell.id)
                 univ.add_material_override(cell, mats["foo"])
         for cell_id in cells_with_41:
-            assert univ.cells[cell_id] == mats["foo"]
+            assert univ.cells[cell_id].fill == mats["foo"]
 
 
 def test_model_differentiate_depletable_with_dagmc(model, run_in_tmpdir):
@@ -173,57 +166,20 @@ def test_model_differentiate_with_dagmc(model):
     assert len(model.materials) == 4*2 + 4
 
 
-def test_bad_override_cell_id(model):
-    for univ in model.geometry.get_all_universes().values():
-        if isinstance(univ, openmc.DAGMCUniverse):
-            break
-    with pytest.raises(ValueError, match="Cell ID '1' not found in DAGMC universe"):
-        univ.material_overrides = {1: model.materials[0]}
-
-
-def test_bad_override_type(model):
-    not_a_dag_cell = openmc.Cell()
-    for univ in model.geometry.get_all_universes().values():
-        if isinstance(univ, openmc.DAGMCUniverse):
-            break
-    with pytest.raises(ValueError, match="Unrecognized key type. Must be an integer or openmc.DAGMCCell object"):
-        univ.material_overrides = {not_a_dag_cell: model.materials[0]}
-
-
-def test_bad_replacement_mat_name(model):
-    for univ in model.geometry.get_all_universes().values():
-        if isinstance(univ, openmc.DAGMCUniverse):
-            break
-    with pytest.raises(ValueError, match="No material with name 'not_a_mat' found in the DAGMC universe"):
-        univ.replace_material_assignment("not_a_mat", model.materials[0])
-
-
 def test_dagmc_xml(model):
-    # Set the environment
-    mats = {}
-    mats["no-void fuel"] = openmc.Material(1, name="no-void fuel")
-    mats["no-void fuel"].add_nuclide("U235", 0.03)
-    mats["no-void fuel"].add_nuclide("U238", 0.97)
-    mats["no-void fuel"].add_nuclide("O16", 2.0)
-    mats["no-void fuel"].set_density("g/cm3", 10.0)
-
-    mats[5] = openmc.Material(name="41")
-    mats[5].add_nuclide("H1", 2.0)
-    mats[5].add_element("O", 1.0)
-    mats[5].set_density("g/cm3", 1.0)
-    mats[5].add_s_alpha_beta("c_H_in_H2O")
+    override_mat = openmc.Material(name="41")
+    override_mat.add_nuclide("H1", 2.0)
+    override_mat.add_element("O", 1.0)
+    override_mat.set_density("g/cm3", 1.0)
+    override_mat.add_s_alpha_beta("c_H_in_H2O")
+    model.materials.append(override_mat)
 
     for univ in model.geometry.get_all_universes().values():
         if isinstance(univ, openmc.DAGMCUniverse):
             dag_univ = univ
             break
 
-    for k, v in mats.items():
-        if isinstance(k, int):
-            dag_univ.add_material_override(k, v)
-            model.materials.append(v)
-        elif isinstance(k, str):
-            dag_univ.replace_material_assignment(k, v)
+    dag_univ.add_material_override(5, override_mat)
 
     # Tesing the XML subelement generation
     root = ET.Element('dagmc_universe')
@@ -231,15 +187,28 @@ def test_dagmc_xml(model):
     dagmc_ele = root.find('dagmc_universe')
 
     assert dagmc_ele.get('id') == str(dag_univ.id)
+    assert dagmc_ele.get('name') == str(dag_univ.name)
     assert dagmc_ele.get('filename') == str(dag_univ.filename)
     assert dagmc_ele.get('auto_geom_ids') == str(dag_univ.auto_geom_ids).lower()
 
-    override_eles = dagmc_ele.find('material_overrides').findall('cell_override')
-    assert len(override_eles) == 4
+    assert dagmc_ele.find('material_overrides') is None
 
-    for i, override_ele in enumerate(override_eles):
-        cell_id = override_ele.get('id')
-        assert dag_univ.material_overrides[int(cell_id)][0].id == int(override_ele.find('material_ids').text)
+    override_elements = dagmc_ele.findall('cell')
+    assert len(override_elements) == len(dag_univ.cells)
+    xml_cells = {int(elem.get('id')): elem for elem in override_elements}
+    for cell_id, cell in dag_univ.cells.items():
+        assert cell_id in xml_cells
+        xml_cell = xml_cells[cell_id]
+        if cell.fill_type == 'void':
+            assert xml_cell.get('material') == 'void'
+        elif cell.fill_type == 'material':
+            assert xml_cell.get('material') == str(cell.fill.id)
+        elif cell.fill_type == 'distribmat':
+            mat_list = xml_cell.find('material').text.split()
+            expected = ["void" if m is None else str(m.id) for m in cell.fill]
+            assert mat_list == expected
+        else:
+            pytest.fail(f"Unexpected DAGMC cell fill type: {cell.fill_type}")
 
     model.export_to_model_xml()
 
@@ -250,7 +219,147 @@ def test_dagmc_xml(model):
             xml_dagmc_univ = univ
             break
 
-    assert xml_dagmc_univ._material_overrides.keys() == dag_univ._material_overrides.keys()
+    assert xml_dagmc_univ.cells.keys() == dag_univ.cells.keys()
 
-    for xml_mats, model_mats in zip(xml_dagmc_univ._material_overrides.values(), dag_univ._material_overrides.values()):
-        assert all([xml_mat.id == orig_mat.id for xml_mat, orig_mat in zip(xml_mats, model_mats)])
+    for cell_id, cell in dag_univ.cells.items():
+        xml_cell = xml_dagmc_univ.cells[cell_id]
+        assert xml_cell.fill_type == cell.fill_type
+        if cell.fill_type == 'void':
+            assert xml_cell.fill is None
+        elif cell.fill_type == 'material':
+            assert xml_cell.fill.id == cell.fill.id
+        elif cell.fill_type == 'distribmat':
+            xml_ids = [m.id if m is not None else None for m in xml_cell.fill]
+            model_ids = [m.id if m is not None else None for m in cell.fill]
+            assert xml_ids == model_ids
+        else:
+            pytest.fail(f"Unexpected DAGMC cell fill type: {cell.fill_type}")
+
+
+def test_dagmc_xml_reject_fill_override():
+    mats = {'1': openmc.Material(1), 'void': None}
+    elem = ET.fromstring(
+        '<dagmc_universe id="1" filename="dagmc.h5m">'
+        '<cell id="1" fill="2"/>'
+        '</dagmc_universe>'
+    )
+    with pytest.raises(ValueError, match="cannot specify 'fill'"):
+        openmc.DAGMCUniverse.from_xml_element(elem, mats)
+
+
+def test_dagmc_xml_reject_region_override():
+    mats = {'1': openmc.Material(1), 'void': None}
+    elem = ET.fromstring(
+        '<dagmc_universe id="1" filename="dagmc.h5m">'
+        '<cell id="1" material="1" region="-1"/>'
+        '</dagmc_universe>'
+    )
+    with pytest.raises(ValueError, match="cannot specify 'region'"):
+        openmc.DAGMCUniverse.from_xml_element(elem, mats)
+
+
+def _legacy_xml(cell_overrides):
+    """Helper to build a <dagmc_universe> with old-format <material_overrides>."""
+    inner = ''.join(
+        f'<cell_override id="{cid}"><material_ids>{mids}</material_ids></cell_override>'
+        for cid, mids in cell_overrides.items()
+    )
+    return ET.fromstring(
+        f'<dagmc_universe id="1" filename="dagmc.h5m">'
+        f'<material_overrides>{inner}</material_overrides>'
+        f'</dagmc_universe>'
+    )
+
+
+def test_dagmc_xml_legacy_single_material_compat():
+    mat = openmc.Material(1)
+    mats = {'1': mat, 'void': None}
+    elem = _legacy_xml({3: '1'})
+    with pytest.warns(DeprecationWarning, match="deprecated"):
+        univ = openmc.DAGMCUniverse.from_xml_element(elem, mats)
+    assert 3 in univ.cells
+    assert univ.cells[3].fill is mat
+
+
+def test_dagmc_xml_legacy_distribmat_compat():
+    mat1, mat2 = openmc.Material(2), openmc.Material(3)
+    mats = {'2': mat1, '3': mat2, 'void': None}
+    elem = _legacy_xml({5: '2 3'})
+    with pytest.warns(DeprecationWarning):
+        univ = openmc.DAGMCUniverse.from_xml_element(elem, mats)
+    assert univ.cells[5].fill_type == 'distribmat'
+    assert list(univ.cells[5].fill) == [mat1, mat2]
+
+
+def test_dagmc_xml_legacy_void_compat():
+    mats = {'void': None}
+    elem = _legacy_xml({7: 'void'})
+    with pytest.warns(DeprecationWarning):
+        univ = openmc.DAGMCUniverse.from_xml_element(elem, mats)
+    assert univ.cells[7].fill_type == 'void'
+
+
+def test_dagmc_xml_legacy_both_raises():
+    mat = openmc.Material(1)
+    mats = {'1': mat, 'void': None}
+    elem = ET.fromstring(
+        '<dagmc_universe id="1" filename="dagmc.h5m">'
+        '<material_overrides>'
+        '<cell_override id="3"><material_ids>1</material_ids></cell_override>'
+        '</material_overrides>'
+        '<cell id="5" material="1"/>'
+        '</dagmc_universe>'
+    )
+    with pytest.raises(ValueError, match="both"):
+        openmc.DAGMCUniverse.from_xml_element(elem, mats)
+
+
+def test_dagmc_xml_legacy_deprecation_warning():
+    mats = {'1': openmc.Material(1), 'void': None}
+    elem = _legacy_xml({3: '1'})
+    with pytest.warns(DeprecationWarning):
+        openmc.DAGMCUniverse.from_xml_element(elem, mats)
+
+
+def test_dagmc_xml_legacy_roundtrip():
+    """Old-format XML loads correctly and re-exports using the new <cell> format."""
+    mat = openmc.Material(1)
+    mats = {'1': mat, 'void': None}
+    elem = _legacy_xml({3: '1'})
+    with pytest.warns(DeprecationWarning):
+        univ = openmc.DAGMCUniverse.from_xml_element(elem, mats)
+
+    root = ET.Element('geometry')
+    univ.create_xml_subelement(root)
+    dagmc_elem = root.find('dagmc_universe')
+
+    assert dagmc_elem.find('material_overrides') is None
+    cell_elems = dagmc_elem.findall('cell')
+    assert len(cell_elems) == 1
+    assert int(cell_elems[0].get('id')) == 3
+    assert cell_elems[0].get('material') == '1'
+
+
+def test_dagmc_xml_temperature_roundtrip():
+    mat = openmc.Material(1)
+    mats = {'1': mat, 'void': None}
+
+    elem = ET.fromstring(
+        '<dagmc_universe id="10" filename="dagmc.h5m">'
+        '<cell id="7" material="1" temperature="825.0"/>'
+        '</dagmc_universe>'
+    )
+
+    dag_univ = openmc.DAGMCUniverse.from_xml_element(elem, mats)
+    assert dag_univ.cells[7].fill.id == 1
+    assert dag_univ.cells[7].temperature == pytest.approx(825.0)
+
+    root = ET.Element('geometry')
+    dag_univ.create_xml_subelement(root)
+    dagmc_elem = root.find('dagmc_universe')
+    xml_cell = dagmc_elem.find('cell')
+    assert xml_cell.get('temperature') == '825.0'
+
+    dag_univ_roundtrip = openmc.DAGMCUniverse.from_xml_element(dagmc_elem, mats)
+    assert dag_univ_roundtrip.cells[7].fill.id == 1
+    assert dag_univ_roundtrip.cells[7].temperature == pytest.approx(825.0)
diff --git a/tests/unit_tests/dagmc/test_plot.py b/tests/unit_tests/dagmc/test_plot.py
new file mode 100644
index 000000000..6ce1d79a2
--- /dev/null
+++ b/tests/unit_tests/dagmc/test_plot.py
@@ -0,0 +1,71 @@
+import pytest
+import openmc
+import openmc.lib
+
+
+pytestmark = pytest.mark.skipif(
+    not openmc.lib._dagmc_enabled(), reason="DAGMC CAD geometry is not enabled."
+)
+
+def test_plotting_dagmc_model(request):
+    """Test plotting a DAGMC model with OpenMC. This is different to CSG
+    model plotting as the path to the DAGMC file needs handling."""
+
+    dag_universe = openmc.DAGMCUniverse(request.path.parent / 'dagmc.h5m')
+    csg_with_dag_inside = dag_universe.bounded_universe()
+    model = openmc.Model()
+    model.geometry = openmc.Geometry(csg_with_dag_inside)
+
+    for mat_name in dag_universe.material_names:
+        material = openmc.Material(name=mat_name)
+        material.add_nuclide("Fe56", 1.0)
+        material.set_density("g/cm3", 7.0)
+        model.materials.append(material)
+
+    # putting the source at the center of the bounding box of the DAGMC
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Point(dag_universe.bounding_box.center)
+    )
+    model.settings.batches = 10
+    model.settings.particles = 50
+
+    model.plot()
+
+
+def test_plotting_dagmc_universe(request):
+    """Test plotting a DAGMCUniverse with OpenMC. This is different to plotting
+    UniverseBase as the materials are not defined withing the DAGMCUniverse."""
+
+    dag_universe = openmc.DAGMCUniverse(request.path.parent / 'dagmc.h5m')
+    dag_universe.plot()
+
+
+def test_plotting_geometry_filled_with_dagmc_universe(request):
+    """Test plotting a geometry with OpenMC. This is an edge case when plotting
+    geometry as often geometry objects don't include a DAGMCUniverse. The
+    inclusion of a DAGMCUniverse requires special handling for the materials."""
+
+    dag_universe = openmc.DAGMCUniverse(request.path.parent / 'dagmc.h5m', auto_geom_ids=True)
+
+    sphere1 = openmc.Sphere(r=50.0)
+    sphere2 = openmc.Sphere(r=60.0)
+    sphere2 = openmc.Sphere(r=70.0, boundary_type='vacuum')
+
+    # Adding a material to the CSG Universe to check universe materials are accounted for
+    csg_material = openmc.Material(name='csg_material')
+    csg_material.add_nuclide("H1", 1.0)
+
+    # Adding a material with the same name as a dagmc material to check that
+    # the plot can handel two materials with the same name from different universes
+    csg_material = openmc.Material(name=dag_universe.material_names[0])
+    csg_material.add_nuclide("H1", 1.0)
+
+    cell1 = openmc.Cell(fill=dag_universe, region=-sphere1)
+    cell2 = openmc.Cell(fill=csg_material, region=+sphere1 & -sphere2)
+
+    geometry = openmc.Geometry([cell1, cell2])
+    geometry.plot()
+
+    # Close plot to avoid warning
+    import matplotlib.pyplot as plt
+    plt.close()
diff --git a/tests/unit_tests/mesh_to_vtk/test.py b/tests/unit_tests/mesh_to_vtk/test.py
index 3ee6c0298..f00aa4626 100644
--- a/tests/unit_tests/mesh_to_vtk/test.py
+++ b/tests/unit_tests/mesh_to_vtk/test.py
@@ -49,7 +49,10 @@ sphere_mesh = openmc.SphericalMesh(
 
 def mesh_data(mesh_dims):
     data = 100 * np.arange(np.prod(mesh_dims), dtype=float)
-    return data.reshape(*mesh_dims)
+    # data is returned reshaped with order 'F' to ensure that
+    # the resulting data is interpreted correctly by the
+    # write_data_to_vtk method
+    return data.reshape(*mesh_dims, order='F')
 
 test_data = ((reg_mesh, False, 'regular'),
              (rect_mesh, False, 'rectilinear'),
@@ -83,7 +86,6 @@ def test_mesh_write_vtk(mesh_params, run_in_tmpdir):
 
 # check data writing
 def test_mesh_write_vtk_data(run_in_tmpdir):
-
     data = {'ascending_data': mesh_data(cyl_mesh.dimension)}
     filename_expected = full_path('cyl-data.vtk')
     filename_actual = full_path('cyl-data-actual.vtk')
diff --git a/tests/unit_tests/mesh_to_vtk/test_vtk_dims.py b/tests/unit_tests/mesh_to_vtk/test_vtk_dims.py
index 0b6acf5aa..366ce18ef 100644
--- a/tests/unit_tests/mesh_to_vtk/test_vtk_dims.py
+++ b/tests/unit_tests/mesh_to_vtk/test_vtk_dims.py
@@ -90,7 +90,7 @@ def test_write_data_to_vtk(mesh, tmpdir):
     # kji (i changing fastest) orering is expected for input data
     # by using the volumes transposed as the data here, we can ensure the
     # normalization is happening correctly
-    data = mesh.volumes.T
+    data = mesh.volumes
 
     # RUN
     mesh.write_data_to_vtk(filename=filename, datasets={"label1": data, "label2": data})
@@ -131,7 +131,7 @@ def test_write_data_to_vtk_size_mismatch(mesh):
     mesh : openmc.StructuredMesh
         The mesh to test
     """
-    right_size = mesh.num_mesh_cells
+    right_size = mesh.n_elements
     data = np.random.random(right_size + 1)
 
     # Error message has \ in to escape characters that are otherwise recognized
@@ -139,7 +139,7 @@ def test_write_data_to_vtk_size_mismatch(mesh):
     # string when using the match argument as that uses regular expression
     expected_error_msg = (
         fr"The size of the dataset 'label' \({len(data)}\) should be equal to "
-        fr"the number of mesh cells \({mesh.num_mesh_cells}\)"
+        fr"the number of mesh cells \({mesh.n_elements}\)"
     )
     with pytest.raises(ValueError, match=expected_error_msg):
         mesh.write_data_to_vtk(filename="out.vtk", datasets={"label": data})
diff --git a/tests/unit_tests/test_cell.py b/tests/unit_tests/test_cell.py
index 95c8249bb..8d39c071b 100644
--- a/tests/unit_tests/test_cell.py
+++ b/tests/unit_tests/test_cell.py
@@ -20,6 +20,12 @@ def test_contains():
     assert (10.0, -4., 2.0) in c
 
 
+def test_id():
+    openmc.Cell(cell_id=0)
+    with pytest.raises(ValueError):
+        openmc.Cell(cell_id=-1)
+
+
 def test_repr(cell_with_lattice):
     cells, mats, univ, lattice = cell_with_lattice
     repr(cells[0])  # cell with distributed materials
@@ -126,6 +132,29 @@ def test_temperature(cell_with_lattice):
     c.temperature = (300., 600., 900.)
 
 
+def test_densities(cell_with_lattice):
+    # Make sure density propagates through universes
+    m = openmc.Material()
+    s = openmc.XPlane()
+    c1 = openmc.Cell(fill=m, region=+s)
+    c2 = openmc.Cell(fill=m, region=-s)
+    u1 = openmc.Universe(cells=[c1, c2])
+    c = openmc.Cell(fill=u1)
+
+    c.density = 1.
+    assert c1.density == 1.
+    assert c2.density == 1.
+    with pytest.raises(ValueError):
+        c.density = -1.
+    c.density = None
+    assert c1.density == None
+    assert c2.density == None
+
+    # distributed density
+    cells, _, _, _ = cell_with_lattice
+    c = cells[0]
+    c.density = (1., 2., 3.)
+
 def test_rotation():
     u = openmc.Universe()
     c = openmc.Cell(fill=u)
@@ -345,6 +374,49 @@ def test_rotation_from_xml(rotation):
     np.testing.assert_allclose(new_cell.rotation, cell.rotation)
 
 
+def test_dagmccell_from_xml_element():
+    """DAGMCCell.from_xml_element parses material, temperature, density,
+    and volume; rejects unsupported attributes."""
+    mat = openmc.Material(1)
+    mat.add_nuclide('U235', 1.0)
+    mats = {'1': mat}
+    # In practice, from_xml_element is always called from DAGMCUniverse
+    # during XML parsing. A placeholder universe is used here so the test
+    # can exercise the method directly without a real DAGMC model file.
+    placeholder_univ = openmc.DAGMCUniverse('model.h5m')
+
+    # material + temperature + density round-trip
+    xml = '<cell id="5" name="fuel" material="1" temperature="900.0" density="10.5"/>'
+    cell = openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
+    assert cell.id == 5
+    assert cell.name == 'fuel'
+    assert cell.fill is mat
+    assert cell.density == 10.5
+    assert cell.temperature == 900.0
+
+    # volume round-trip
+    xml = '<cell id="6" material="1" volume="42.0"/>'
+    cell = openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
+    assert cell.volume == 42.0
+
+    # forbidden: region, fill, universe
+    for tag, val in [('region', '-1'), ('fill', '2'), ('universe', '0')]:
+        xml = f'<cell id="7" material="1" {tag}="{val}"/>'
+        with pytest.raises(ValueError, match=tag):
+            openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
+
+    # forbidden: translation, rotation
+    for tag, val in [('translation', '1 0 0'), ('rotation', '0 0 90')]:
+        xml = f'<cell id="7" material="1" {tag}="{val}"/>'
+        with pytest.raises(ValueError, match=tag):
+            openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
+
+    # missing material raises
+    xml = '<cell id="8"/>'
+    with pytest.raises(ValueError, match='material'):
+        openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
+
+
 def test_plot(run_in_tmpdir):
     zcyl = openmc.ZCylinder()
     c = openmc.Cell(region=-zcyl)
diff --git a/tests/unit_tests/test_collision_track.py b/tests/unit_tests/test_collision_track.py
new file mode 100644
index 000000000..96676e2cd
--- /dev/null
+++ b/tests/unit_tests/test_collision_track.py
@@ -0,0 +1,170 @@
+"""Test the 'collision_track' setting used to store particle information
+during specified collision conditions in a file for a given simulation."""
+
+import openmc
+import pytest
+import h5py
+import numpy as np
+import shutil
+
+from tests.testing_harness import CollisionTrackTestHarness as ctt
+
+
+@pytest.fixture(scope="module")
+def geometry():
+    """Simple hydrogen sphere geometry"""
+    openmc.reset_auto_ids()
+    material = openmc.Material(name="H1")
+    material.add_element("H", 1.0)
+    sphere = openmc.Sphere(r=1.0, boundary_type="vacuum")
+    cell = openmc.Cell(region=-sphere, fill=material)
+    return openmc.Geometry([cell])
+
+
+@pytest.mark.parametrize(
+    "parameter",
+    [
+        {"max_collisions": 200},
+        {"max_collisions": 200, "reactions": ["(n,disappear)"]},
+        {"max_collisions": 200, "cell_ids": [1]},
+        {"max_collisions": 200, "material_ids": [1]},
+        {"max_collisions": 200, "universe_ids": [1]},
+        {"max_collisions": 200, "nuclides": ["H1"]},
+        {"max_collisions": 200, "deposited_E_threshold": 200000.0},
+        {"max_collisions": 200, "mcpl": True}
+
+    ],
+    ids=str
+)
+def test_xml_serialization(parameter, run_in_tmpdir):
+    """Check that the different use cases can be written and read in XML."""
+    settings = openmc.Settings()
+    settings.collision_track = parameter
+    settings.export_to_xml()
+
+    read_settings = openmc.Settings.from_xml()
+    assert read_settings.collision_track == parameter
+
+
+@pytest.fixture
+def model():
+    """Simple hydrogen sphere divided in two hemispheres
+    by a z-plane to form 2 cells."""
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+
+    # Material
+    material = openmc.Material(name="H1")
+    material.add_element("H", 1.0)
+
+    # Geometry
+    radius = 1.0
+    sphere = openmc.Sphere(r=radius, boundary_type="reflective")
+    plane = openmc.ZPlane(0.0)
+    cell_1 = openmc.Cell(region=-sphere & -plane, fill=material, cell_id=1)
+    cell_2 = openmc.Cell(region=-sphere & +plane, fill=material, cell_id=2)
+    root = openmc.Universe(cells=[cell_1, cell_2])
+    model.geometry = openmc.Geometry(root)
+
+    # Settings
+    model.settings = openmc.Settings()
+    model.settings.run_mode = "fixed source"
+    model.settings.particles = 1
+    model.settings.batches = 1
+    model.settings.seed = 2
+
+    bounds = [-radius, -radius, -radius, radius, radius, radius]
+    distribution = openmc.stats.Box(bounds[:3], bounds[3:])
+    model.settings.source = openmc.IndependentSource(space=distribution)
+
+    return model
+
+
+def test_particle_location(run_in_tmpdir, model):
+    """Test the location of particles with respected to the "cell_ids"
+    and the location x, y, z of the particle itself. the upper sphere will
+    have positive z component and the bottom sphere a negative z compnent.
+
+    """
+    model.settings.collision_track = {
+        "max_collisions": 200,
+        "reactions": ["elastic"],
+        "cell_ids": [1, 2]
+    }
+    model.run()
+
+    with h5py.File("collision_track.h5", "r") as f:
+        source = f["collision_track_bank"]
+
+        assert len(source) == 60
+
+        # We want to verify that the collisions happenening are in the right cells
+        # and the position of the particle is either positive or negative relative
+        # to the z plane. In this case, we track the position of the particle
+        # relative to the cell_id already set.
+        for point in source:
+            if point['cell_id'] == 1:
+                assert point['r'][2] < 0.0  # z component negative
+            elif point['cell_id'] == 2:
+                assert point["r"][2] > 0.0  # z component positive
+            else:
+                assert False
+
+
+@pytest.mark.skipif(shutil.which("mcpl-config") is None, reason="MCPL is not available.")
+def test_format_similarity(run_in_tmpdir, model):
+    model.settings.collision_track = {"max_collisions": 200, "reactions": ['elastic'],
+                                      "cell_ids": [1, 2], "mcpl": False}
+    model.run()
+    data_h5 = ctt._return_collision_track_data('collision_track.h5')
+
+    model.settings.collision_track["mcpl"] = True
+    model.run()
+    data_mcpl = ctt._return_collision_track_data('collision_track.mcpl')
+
+    assert len(data_h5) == 60
+    assert len(data_mcpl) == 60
+
+    np.testing.assert_allclose(data_h5, data_mcpl, rtol=1e-05)
+    # tolerance not that low due to the strings that is saved in MCPL,
+    # not enough precision!
+
+
+def test_photon_particles(run_in_tmpdir, model):
+    """Test that the collision track can be used to track photon particles."""
+    model.settings.collision_track = {"max_collisions": 200, "cell_ids": [1, 2]}
+
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Box(*model.geometry.bounding_box),
+        energy=openmc.stats.delta_function(1e5),
+        particle='photon'
+    )
+    model.run()
+
+    with h5py.File("collision_track.h5", "r") as f:
+        source = f["collision_track_bank"]
+
+        assert len(source) < 200
+
+        allowed_particles = (openmc.ParticleType.PHOTON, openmc.ParticleType.ELECTRON)
+
+        for point in source:
+            particle_type = openmc.ParticleType(point['particle'])
+            assert particle_type in allowed_particles
+
+            if particle_type == openmc.ParticleType.ELECTRON:
+                assert point['nuclide_id'] == 0
+
+
+def test_collision_track_two_threads(model, run_in_tmpdir):
+    # This test checks that the `max_collisions` setting is honored:
+    # no collisions beyond the specified limit should be recorded.
+    #
+    # The exact set of events in the capped bank is not reproducible with
+    # multiple threads because the bank stores whichever thread appends first
+    # until capacity is reached.
+    model.settings.collision_track = {"max_collisions": 200}
+    model.run(threads=2, particles=500)
+
+    collision_track = openmc.read_collision_track_hdf5("collision_track.h5")
+    assert len(collision_track) == 200
diff --git a/tests/unit_tests/test_config.py b/tests/unit_tests/test_config.py
index 9d3f53a74..45e6a7e5a 100644
--- a/tests/unit_tests/test_config.py
+++ b/tests/unit_tests/test_config.py
@@ -1,53 +1,104 @@
 from collections.abc import Mapping
 import os
+from pathlib import Path
 
 import openmc
+from openmc.config import _default_config
+from openmc.data import decay
 import pytest
 
 
-@pytest.fixture(autouse=True, scope='module')
-def reset_config():
-    config = dict(openmc.config)
+@pytest.fixture(autouse=True, scope='function')
+def reset_config_and_env():
+    """A fixture to ensure each test has a clean config, env, and CWD."""
+    original_env = dict(os.environ)
+    original_cwd = os.getcwd()
+    original_resolve_paths = openmc.config["resolve_paths"]
+
+    # Reset environment variables that affect config
+    for key in ['OPENMC_CROSS_SECTIONS', 'OPENMC_MG_CROSS_SECTIONS', 'OPENMC_CHAIN_FILE']:
+        if key in os.environ:
+            del os.environ[key]
+
+    # Re-initialize the global config object
+    openmc.config = _default_config()
+
     try:
         yield
     finally:
-        openmc.config.clear()
-        openmc.config.update(config)
+        # Restore environment, CWD and resolve_paths
+        os.environ.clear()
+        os.environ.update(original_env)
+        os.chdir(original_cwd)
+
+        # Restore config one last time for safety between modules
+        openmc.config = _default_config(resolve_paths=original_resolve_paths)
 
 
 def test_config_basics():
     assert isinstance(openmc.config, Mapping)
-    for key, value in openmc.config.items():
-        assert isinstance(key, str)
-        if key == 'resolve_paths':
-            assert isinstance(value, bool)
-        else:
-            assert isinstance(value, os.PathLike)
-
-    # Set and delete
-    openmc.config['cross_sections'] = '/path/to/cross_sections.xml'
+    with pytest.warns(UserWarning):
+        openmc.config['cross_sections'] = '/path/to/cross_sections.xml'
     del openmc.config['cross_sections']
     assert 'cross_sections' not in openmc.config
     assert 'OPENMC_CROSS_SECTIONS' not in os.environ
-
-    # Can't use any key
-    with pytest.raises(KeyError):
-        openmc.config['🐖'] = '/like/to/eat/bacon'
+    with pytest.raises(KeyError, match="Unrecognized config key: nuke"):
+        openmc.config['nuke'] = '/like/to/eat/bacon'
+    with pytest.raises(TypeError):
+        openmc.config['resolve_paths'] = 'not a bool'
 
 
-def test_config_patch():
-    openmc.config['cross_sections'] = '/path/to/cross_sections.xml'
-    with openmc.config.patch('cross_sections', '/path/to/other.xml'):
-        assert str(openmc.config['cross_sections']) == '/path/to/other.xml'
-    assert str(openmc.config['cross_sections']) == '/path/to/cross_sections.xml'
+def test_config_path_resolution(tmp_path):
+    """Test path resolution logic."""
+    os.chdir(tmp_path)
+    relative_path = Path("some/file.xml")
+    absolute_path = relative_path.resolve()
+
+    # Test with resolve_paths = True (default)
+    with pytest.warns(UserWarning):
+        openmc.config['cross_sections'] = relative_path
+    assert openmc.config['cross_sections'] == absolute_path
+    assert openmc.config['cross_sections'].is_absolute()
+
+    # Test with resolve_paths = False
+    with openmc.config.patch('resolve_paths', False):
+        with pytest.warns(UserWarning):
+            openmc.config['chain_file'] = relative_path
+        assert openmc.config['chain_file'] == relative_path
+        assert not openmc.config['chain_file'].is_absolute()
+
+    assert openmc.config['resolve_paths'] is True
 
 
-def test_config_set_envvar():
-    openmc.config['cross_sections'] = '/path/to/cross_sections.xml'
-    assert os.environ['OPENMC_CROSS_SECTIONS'] == '/path/to/cross_sections.xml'
+def test_config_patch(tmp_path):
+    file_a = tmp_path / "a.xml"; file_a.touch()
+    file_b = tmp_path / "b.xml"; file_b.touch()
+    openmc.config['cross_sections'] = file_a
+    with openmc.config.patch('cross_sections', file_b):
+        assert openmc.config['cross_sections'] == file_b.resolve()
+    assert openmc.config['cross_sections'] == file_a.resolve()
 
-    openmc.config['mg_cross_sections'] = '/path/to/mg_cross_sections.h5'
-    assert os.environ['OPENMC_MG_CROSS_SECTIONS'] == '/path/to/mg_cross_sections.h5'
+def test_config_set_envvar(tmp_path):
+    """Test that setting config also sets environment variables correctly."""
+    os.chdir(tmp_path)
+    relative_path = Path("relative.xml")
+    with pytest.warns(UserWarning):
+        openmc.config['cross_sections'] = relative_path
+    expected_path = str(relative_path.resolve())
+    assert os.environ['OPENMC_CROSS_SECTIONS'] == expected_path
 
-    openmc.config['chain_file'] = '/path/to/chain_file.xml'
-    assert os.environ['OPENMC_CHAIN_FILE'] == '/path/to/chain_file.xml'
+
+def test_config_warning_nonexistent_path(tmp_path):
+    """Test that a warning is issued for a path that does not exist."""
+    bad_path = tmp_path / "a/path/that/does/not/exist.xml"
+    with pytest.warns(UserWarning, match=f"Path '{bad_path}' does not exist."):
+        openmc.config['chain_file'] = bad_path
+
+
+def test_config_chain_side_effect(tmp_path):
+    """Test that modifying chain_file clears decay data caches."""
+    chain_file = tmp_path / "chain.xml"; chain_file.touch()
+    decay._DECAY_ENERGY['U235'] = (1.0, 2.0)
+    decay._DECAY_PHOTON_ENERGY['PU239'] = {}
+    openmc.config['chain_file'] = chain_file
+    assert not decay._DECAY_ENERGY and not decay._DECAY_PHOTON_ENERGY
diff --git a/tests/unit_tests/test_cylindrical_mesh.py b/tests/unit_tests/test_cylindrical_mesh.py
index b408bc0e9..645269825 100644
--- a/tests/unit_tests/test_cylindrical_mesh.py
+++ b/tests/unit_tests/test_cylindrical_mesh.py
@@ -49,6 +49,7 @@ def model():
 
     return openmc.Model(geometry=geom, settings=settings, tallies=tallies)
 
+
 def test_origin_read_write_to_xml(run_in_tmpdir, model):
     """Tests that the origin attribute can be written and read back to XML
     """
@@ -63,8 +64,9 @@ def test_origin_read_write_to_xml(run_in_tmpdir, model):
     np.testing.assert_equal(new_mesh.origin, mesh.origin)
 
 estimators = ('tracklength', 'collision')
-origins = set(permutations((-geom_size, 0, 0)))
-origins |= set(permutations((geom_size, 0, 0)))
+offset = geom_size + 0.001
+origins = set(permutations((-offset , 0, 0)))
+origins |= set(permutations((offset, 0, 0)))
 
 test_cases = product(estimators, origins)
 
@@ -74,8 +76,9 @@ def label(p):
     if isinstance(p, str):
         return f'estimator:{p}'
 
+
 @pytest.mark.parametrize('estimator,origin', test_cases, ids=label)
-def test_offset_mesh(run_in_tmpdir, model, estimator, origin):
+def test_offset_mesh(model, estimator, origin):
     """Tests that the mesh has been moved based on tally results
     """
     mesh = model.tallies[0].filters[0].mesh
@@ -103,6 +106,7 @@ def test_offset_mesh(run_in_tmpdir, model, estimator, origin):
             else:
                 mean[i, j, k] != 0.0
 
+
 @pytest.fixture()
 def void_coincident_geom_model():
     """A model with many geometric boundaries coincident with mesh boundaries
@@ -155,6 +159,7 @@ def _check_void_cylindrical_tally(statepoint_filename):
         d_r = mesh.r_grid[1] - mesh.r_grid[0]
         assert neutron_flux == pytest.approx(d_r)
 
+
 def test_void_geom_pnt_src(run_in_tmpdir, void_coincident_geom_model):
     src = openmc.IndependentSource()
     src.space = openmc.stats.Point()
diff --git a/tests/unit_tests/test_d1s.py b/tests/unit_tests/test_d1s.py
index 7c92dd6d9..49c1b3049 100644
--- a/tests/unit_tests/test_d1s.py
+++ b/tests/unit_tests/test_d1s.py
@@ -22,7 +22,8 @@ def model():
 
 def test_get_radionuclides(model):
     # Check that radionuclides are correct and are unstable
-    nuclides = d1s.get_radionuclides(model, CHAIN_PATH)
+    chain = openmc.deplete.Chain.from_xml(CHAIN_PATH)
+    nuclides = d1s.get_radionuclides(model, chain)
     assert sorted(nuclides) == [
         'Co58', 'Co60', 'Co61', 'Co62', 'Co64',
         'Fe55', 'Fe59', 'Fe61', 'Ni57', 'Ni59', 'Ni63', 'Ni65'
@@ -83,6 +84,11 @@ def test_prepare_tallies(model):
     assert tally.contains_filter(openmc.ParentNuclideFilter)
     assert sorted(tally.filters[-1].bins) == sorted(radionuclides)
 
+    assert len(tally.filters) == 2
+    # calling prepare_tallies twice should not add another ParentNuclideFilter
+    d1s.prepare_tallies(model, chain_file=CHAIN_PATH)
+    assert len(tally.filters) == 2
+
 
 def test_apply_time_correction(run_in_tmpdir):
     # Make simple sphere model with elemental Ni
@@ -114,6 +120,13 @@ def test_apply_time_correction(run_in_tmpdir):
         tally = sp.tallies[tally.id]
         flux = tally.mean.flatten()
 
+    # Copy attributes from original tally
+    tally_filters = list(tally.filters)
+    tally_sum = tally.sum.copy()
+    tally_sum_sq = tally.sum_sq.copy()
+    tally_mean = tally.mean.copy()
+    tally_std_dev = tally.std_dev.copy()
+
     # Apply TCF and make sure results are consistent
     result = d1s.apply_time_correction(tally, factors, sum_nuclides=False)
     tcf = np.array([factors[nuc][-1] for nuc in nuclides])
@@ -123,6 +136,13 @@ def test_apply_time_correction(run_in_tmpdir):
     result_summed = d1s.apply_time_correction(tally, factors)
     assert result_summed.mean.flatten()[0] == pytest.approx(result.mean.sum())
 
+    # Make sure original tally is unchanged
+    assert tally.filters == tally_filters
+    assert np.all(tally.sum == tally_sum)
+    assert np.all(tally.sum_sq == tally_sum_sq)
+    assert np.all(tally.mean == tally_mean)
+    assert np.all(tally.std_dev == tally_std_dev)
+
     # Make sure various tally methods work
     result.get_values()
     result_summed.get_values()
@@ -130,3 +150,7 @@ def test_apply_time_correction(run_in_tmpdir):
     result_summed.get_reshaped_data()
     result.get_pandas_dataframe()
     result_summed.get_pandas_dataframe()
+
+    # The summed tally is derived, so sum/sum_sq are None
+    assert result_summed.sum is None
+    assert result_summed.sum_sq is None
diff --git a/tests/unit_tests/test_data_decay.py b/tests/unit_tests/test_data_decay.py
index 8900e7eac..f81f857e0 100644
--- a/tests/unit_tests/test_data_decay.py
+++ b/tests/unit_tests/test_data_decay.py
@@ -17,25 +17,22 @@ def ufloat_close(a, b):
 
 
 @pytest.fixture(scope='module')
-def nb90():
+def nb90(endf_data):
     """Nb90 decay data."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'decay', 'dec-041_Nb_090.endf')
     return openmc.data.Decay.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def ba137m():
+def ba137m(endf_data):
     """Ba137_m1 decay data."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'decay', 'dec-056_Ba_137m1.endf')
     return openmc.data.Decay.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def u235_yields():
+def u235_yields(endf_data):
     """U235 fission product yield data."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'nfy', 'nfy-092_U_235.endf')
     return openmc.data.FissionProductYields.from_endf(filename)
 
@@ -103,6 +100,28 @@ def test_fpy(u235_yields):
     ufloat_close(thermal['I135'], ufloat(0.0292737, 0.000819663))
 
 
+def test_decay_from_endf_material(endf_data):
+    filename = os.path.join(endf_data, 'decay', 'dec-041_Nb_090.endf')
+    material = openmc.data.endf.get_evaluations(filename)[0]
+
+    data = openmc.data.Decay.from_endf(material)
+
+    assert data.nuclide['name'] == 'Nb90'
+    assert not data.nuclide['stable']
+    assert len(data.modes) == 2
+
+
+def test_fpy_from_endf_material(endf_data):
+    filename = os.path.join(endf_data, 'nfy', 'nfy-092_U_235.endf')
+    material = openmc.data.endf.get_evaluations(filename)[0]
+
+    data = openmc.data.FissionProductYields.from_endf(material)
+
+    assert data.nuclide['name'] == 'U235'
+    assert data.energies == pytest.approx([0.0253, 500.e3, 1.4e7])
+    assert 'I135' in data.cumulative[0]
+
+
 def test_sources(ba137m, nb90):
     # Running .sources twice should give same objects
     sources = ba137m.sources
@@ -136,16 +155,16 @@ def test_decay_photon_energy():
     with pytest.raises(DataError):
         openmc.data.decay_photon_energy('I135')
 
-    # Set chain file to simple chain
-    openmc.config['chain_file'] = Path(__file__).parents[1] / "chain_simple.xml"
+    # Temporarily Set chain file to simple chain
+    with openmc.config.patch('chain_file', Path(__file__).parents[1] / 'chain_simple.xml'):
 
-    # Check strength of I135 source and presence of specific spectral line
-    src = openmc.data.decay_photon_energy('I135')
-    assert isinstance(src, openmc.stats.Discrete)
-    assert src.integral() == pytest.approx(3.920996223799345e-05)
-    assert 1260409. in src.x
+        # Check strength of I135 source and presence of specific spectral line
+        src = openmc.data.decay_photon_energy('I135')
+        assert isinstance(src, openmc.stats.Discrete)
+        assert src.integral() == pytest.approx(3.920996223799345e-05)
+        assert 1260409. in src.x
 
-    # Check Xe135 source, which should be tabular
-    src = openmc.data.decay_photon_energy('Xe135')
-    assert isinstance(src, openmc.stats.Tabular)
-    assert src.integral() == pytest.approx(2.076506258964966e-05)
+        # Check Xe135 source, which should be tabular
+        src = openmc.data.decay_photon_energy('Xe135')
+        assert isinstance(src, openmc.stats.Tabular)
+        assert src.integral() == pytest.approx(2.076506258964966e-05)
diff --git a/tests/unit_tests/test_data_dose.py b/tests/unit_tests/test_data_dose.py
index 2d80cf838..32be5eb89 100644
--- a/tests/unit_tests/test_data_dose.py
+++ b/tests/unit_tests/test_data_dose.py
@@ -34,6 +34,20 @@ def test_dose_coefficients():
     assert energy[-1] == approx(20.0e6)
     assert dose[-1] == approx(338.0)
 
+    energy, dose = dose_coefficients(
+        'neutron', data_source='icrp74', dose_quantity='ambient')
+    assert energy[0] == approx(1e-3)
+    assert dose[0] == approx(6.60)
+    assert energy[-1] == approx(20.0e6)
+    assert dose[-1] == approx(600)
+
+    energy, dose = dose_coefficients(
+        'photon', data_source='icrp74', dose_quantity='ambient')
+    assert energy[0] == approx(0.01e6)
+    assert dose[0] == approx(0.061)
+    assert energy[-1] == approx(10e6)
+    assert dose[-1] == approx(25.6)
+
     # Invalid particle/geometry should raise an exception
     with raises(ValueError):
         dose_coefficients('slime', 'LAT')
@@ -41,3 +55,32 @@ def test_dose_coefficients():
         dose_coefficients('neutron', 'ZZ')
     with raises(ValueError):
         dose_coefficients('neutron', data_source='icrp7000')
+    with raises(ValueError):
+        dose_coefficients('neutron', dose_quantity='banana')
+    with raises(ValueError):
+        dose_coefficients(
+            'neutron', data_source='icrp116', dose_quantity='ambient')
+    with raises(ValueError):
+        dose_coefficients(
+            'neutron', 'ISO', data_source='icrp74', dose_quantity='ambient')
+    with raises(ValueError) as excinfo:
+        dose_coefficients("photons", data_source="icrp116")
+    expected_particles = [
+        "electron",
+        "helium",
+        "mu+",
+        "mu-",
+        "neutron",
+        "photon",
+        "photon kerma",
+        "pi+",
+        "pi-",
+        "positron",
+        "proton",
+    ]
+    expected_msg = (
+        "'photons' has no effective dose data in data source icrp116. "
+        "Available particles for icrp116 with dose quantity effective are: "
+        f"{expected_particles}"
+    )
+    assert str(excinfo.value) == expected_msg
diff --git a/tests/unit_tests/test_data_kalbach_mann.py b/tests/unit_tests/test_data_kalbach_mann.py
index 3b837af7d..5d06669f7 100644
--- a/tests/unit_tests/test_data_kalbach_mann.py
+++ b/tests/unit_tests/test_data_kalbach_mann.py
@@ -7,6 +7,7 @@ import pytest
 
 import numpy as np
 
+import openmc
 from openmc.data import IncidentNeutron
 from openmc.data.kalbach_mann import _separation_energy, _AtomicRepresentation
 from openmc.data import kalbach_slope
@@ -129,7 +130,7 @@ def test_kalbach_slope():
         ('Hg204.h5', 'n-080_Hg_204.endf')
     ]
 )
-def test_comparison_slope_hdf5(hdf5_filename, endf_filename):
+def test_comparison_slope_hdf5(hdf5_filename, endf_filename, endf_data):
     """Test the calculation of the Kalbach-Mann slope done by OpenMC
     by comparing it to HDF5 data. The test is based on the first product
     of MT=5 (neutron). The isotopes tested have been selected because the
@@ -147,13 +148,13 @@ def test_comparison_slope_hdf5(hdf5_filename, endf_filename):
 
     """
     # HDF5 data
-    hdf5_directory = Path(os.environ['OPENMC_CROSS_SECTIONS']).parent
+    hdf5_directory = Path(openmc.config.get('cross_sections')).parent
     hdf5_data = IncidentNeutron.from_hdf5(hdf5_directory / hdf5_filename)
     hdf5_product = hdf5_data[5].products[0]
     hdf5_distribution = hdf5_product.distribution[0]
 
     # ENDF data
-    endf_directory = Path(os.environ['OPENMC_ENDF_DATA'])
+    endf_directory = Path(endf_data)
     endf_path = endf_directory / 'neutrons' / endf_filename
     endf_data = IncidentNeutron.from_endf(endf_path)
     endf_product = endf_data[5].products[0]
diff --git a/tests/unit_tests/test_data_mass_attenuation.py b/tests/unit_tests/test_data_mass_attenuation.py
new file mode 100644
index 000000000..0fe12a7db
--- /dev/null
+++ b/tests/unit_tests/test_data_mass_attenuation.py
@@ -0,0 +1,53 @@
+from pytest import approx, raises
+
+from openmc.data import mass_energy_absorption_coefficient, mass_attenuation_coefficient
+from openmc.data.function import Tabulated1D
+
+
+def test_mass_attenuation_type():
+    mu = mass_attenuation_coefficient(26)  # Fe
+    assert isinstance(mu, Tabulated1D)
+
+
+def test_mass_attenuation_spot_values():
+    # Spot checks for Fe (Z=26) against NIST data: first/last tabulated points
+    # and a mid-range value at 1 MeV
+    mu = mass_attenuation_coefficient(26)
+    assert mu(1e3) == approx(9085.0)
+    assert mu(1e6) == approx(0.05995)
+    assert mu(2e7) == approx(0.03224)
+
+
+def test_mass_attenuation_caching():
+    # Repeated calls with the same Z should return the identical object
+    mu1 = mass_attenuation_coefficient(26)
+    mu2 = mass_attenuation_coefficient('Fe')
+    assert mu1 is mu2
+
+
+def test_mass_attenuation_invalid_z():
+    with raises(ValueError, match="Z=0"):
+        mass_attenuation_coefficient(0)
+    with raises(ValueError, match="Z=200"):
+        mass_attenuation_coefficient(200)
+
+
+def test_mass_energy_absorption_type():
+    # Spot checks on values from NIST tables
+    mu_en = mass_energy_absorption_coefficient("air")
+    assert isinstance(mu_en, Tabulated1D)
+
+
+def test_mass_energy_absorption_spot_values():
+    mu_en = mass_energy_absorption_coefficient("air")
+    assert mu_en(1e3) == approx(3.599e3)
+    assert mu_en(10.e3) == approx(4.742)
+    assert mu_en(2e7) == approx(1.311e-2)
+
+
+def test_mass_energy_absorption_invalid():
+    # Invalid material/data_source should raise an exception
+    with raises(ValueError):
+        mass_energy_absorption_coefficient("pasta")
+    with raises(ValueError):
+        mass_energy_absorption_coefficient("air", data_source="nist000")
diff --git a/tests/unit_tests/test_data_misc.py b/tests/unit_tests/test_data_misc.py
index 226d848a5..320d09eda 100644
--- a/tests/unit_tests/test_data_misc.py
+++ b/tests/unit_tests/test_data_misc.py
@@ -7,6 +7,7 @@ from pathlib import Path
 import numpy as np
 import pytest
 import openmc.data
+from openmc.deplete import Chain, Nuclide
 
 
 def test_data_library(tmpdir):
@@ -30,7 +31,7 @@ def test_data_library(tmpdir):
     assert os.path.exists(filename)
 
     new_lib = openmc.data.DataLibrary()
-    directory = os.path.dirname(os.environ['OPENMC_CROSS_SECTIONS'])
+    directory = os.path.dirname(openmc.config.get('cross_sections'))
     new_lib.register_file(os.path.join(directory, 'H1.h5'))
     assert new_lib[-1]['type'] == 'neutron'
     new_lib.register_file(os.path.join(directory, 'c_Zr_in_ZrH.h5'))
@@ -131,9 +132,11 @@ def test_zam():
     assert openmc.data.zam('Am242_m10') == (95, 242, 10)
     with pytest.raises(ValueError):
         openmc.data.zam('garbage')
+    with pytest.raises(ValueError):
+        openmc.data.zam('Am242-m1')
 
 
-def test_half_life():
+def test_half_life(tmp_path):
     assert openmc.data.half_life('H2') is None
     assert openmc.data.half_life('U235') == pytest.approx(2.22102e16)
     assert openmc.data.half_life('Am242') == pytest.approx(57672.0)
@@ -142,3 +145,32 @@ def test_half_life():
     assert openmc.data.decay_constant('U235') == pytest.approx(log(2.0)/2.22102e16)
     assert openmc.data.decay_constant('Am242') == pytest.approx(log(2.0)/57672.0)
     assert openmc.data.decay_constant('Am242_m1') == pytest.approx(log(2.0)/4449622000.0)
+
+    # Create minimal chain with H3 and Am242 to test half-life and decay
+    # constant retrieval from chain file
+    chain = Chain()
+    h3 = Nuclide("H3")
+    h3.half_life = 1.0
+    chain.add_nuclide(h3)
+    am242 = Nuclide("Am242")
+    chain.add_nuclide(am242)
+
+    assert openmc.data.half_life('H3', chain_file=chain) == 1.0
+    assert openmc.data.decay_constant('H3', chain_file=chain) == pytest.approx(log(2.0))
+
+    # Nuclides that are present but stable in the chain should not fall back to
+    # ENDF/B-VIII.0 data.
+    assert openmc.data.half_life('Am242', chain_file=chain) is None
+    assert openmc.data.decay_constant('Am242', chain_file=chain) == 0.0
+
+    # Nuclides missing from the chain fall back to ENDF/B-VIII.0 data.
+    assert openmc.data.half_life('U235', chain_file=chain) == pytest.approx(2.22102e16)
+
+    chain_path = tmp_path / "chain.xml"
+    chain.export_to_xml(chain_path)
+    assert openmc.data.half_life('H3', chain_file=chain_path) == 1.0
+
+    endf_h3 = openmc.data.half_life('H3')
+    with openmc.config.patch('chain_file', chain_path):
+        assert openmc.data.half_life('H3', chain_file=None) == 1.0
+        assert openmc.data.half_life('H3', chain_file=False) == endf_h3
diff --git a/tests/unit_tests/test_data_multipole.py b/tests/unit_tests/test_data_multipole.py
index 4c2ba96d6..a58ab4865 100644
--- a/tests/unit_tests/test_data_multipole.py
+++ b/tests/unit_tests/test_data_multipole.py
@@ -8,14 +8,14 @@ import openmc.data
 
 @pytest.fixture(scope='module')
 def u235():
-    directory = pathlib.Path(os.environ['OPENMC_CROSS_SECTIONS']).parent
+    directory = pathlib.Path(openmc.config.get('cross_sections')).parent
     u235 = directory / 'wmp' / '092235.h5'
     return openmc.data.WindowedMultipole.from_hdf5(u235)
 
 
 @pytest.fixture(scope='module')
 def b10():
-    directory = pathlib.Path(os.environ['OPENMC_CROSS_SECTIONS']).parent
+    directory = pathlib.Path(openmc.config.get('cross_sections')).parent
     b10 = directory / 'wmp' / '005010.h5'
     return openmc.data.WindowedMultipole.from_hdf5(b10)
 
@@ -48,17 +48,41 @@ def test_export_to_hdf5(tmpdir, u235):
     assert os.path.exists(filename)
 
 
-def test_from_endf():
-    pytest.importorskip('vectfit')
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def test_from_endf(endf_data):
     endf_file = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
-    return openmc.data.WindowedMultipole.from_endf(
-            endf_file, log=True, wmp_options={"n_win": 400, "n_cf": 3})
+    assert openmc.data.WindowedMultipole.from_endf(
+            endf_file,
+            log=True,
+            # Keep the test lightweight
+            vf_options={
+                "njoy_error": 5e-3,
+                "vf_pieces": 1,
+                "rtol": 5e-2,
+                "atol": 1e-3,
+                "orders": [8, 12],
+                "n_vf_iter": 6,
+            },
+            wmp_options={"n_win": 50, "n_cf": 3, "rtol": 5e-2, "atol": 1e-3},
+        )
 
 
-def test_from_endf_search():
-    pytest.importorskip('vectfit')
-    endf_data = os.environ['OPENMC_ENDF_DATA']
-    endf_file = os.path.join(endf_data, 'neutrons', 'n-095_Am_244.endf')
-    return openmc.data.WindowedMultipole.from_endf(
-            endf_file, log=True, wmp_options={"search": True, 'rtol':1e-2})
+def test_from_endf_search(endf_data):
+    endf_file = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
+    assert openmc.data.WindowedMultipole.from_endf(
+            endf_file,
+            log=True,
+            vf_options={
+                "njoy_error": 5e-3,
+                "vf_pieces": 1,
+                "rtol": 5e-2,
+                "atol": 1e-3,
+                "orders": [8, 12],
+                "n_vf_iter": 6,
+            },
+            wmp_options={
+                "search": True,
+                "rtol": 5e-2,
+                "search_n_win": 3,
+                "search_cf_orders": [5, 3],
+            },
+        )
diff --git a/tests/unit_tests/test_data_neutron.py b/tests/unit_tests/test_data_neutron.py
index db6ae1eb8..c767f0171 100644
--- a/tests/unit_tests/test_data_neutron.py
+++ b/tests/unit_tests/test_data_neutron.py
@@ -14,128 +14,113 @@ _TEMPERATURES = [300., 600., 900.]
 @pytest.fixture(scope='module')
 def pu239():
     """Pu239 HDF5 data."""
-    directory = os.path.dirname(os.environ['OPENMC_CROSS_SECTIONS'])
+    directory = os.path.dirname(openmc.config.get('cross_sections'))
     filename = os.path.join(directory, 'Pu239.h5')
     return openmc.data.IncidentNeutron.from_hdf5(filename)
 
 
 @pytest.fixture(scope='module')
-def xe135():
+def xe135(endf_data):
     """Xe135 ENDF data (contains SLBW resonance range)"""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-054_Xe_135.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def sm150():
+def sm150(endf_data):
     """Sm150 ENDF data (contains MLBW resonance range)"""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-062_Sm_150.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def gd154():
+def gd154(endf_data):
     """Gd154 ENDF data (contains Reich Moore resonance range and reosnance
     covariance with LCOMP=1)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-064_Gd_154.endf')
     return openmc.data.IncidentNeutron.from_endf(filename, covariance=True)
 
 
 @pytest.fixture(scope='module')
-def cl35():
+def cl35(endf_data):
     """Cl35 ENDF data (contains RML resonance range)"""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-017_Cl_035.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def am241():
+def am241(endf_data):
     """Am241 ENDF data (contains Madland-Nix fission energy distribution)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-095_Am_241.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def u233():
+def u233(endf_data):
     """U233 ENDF data (contains Watt fission energy distribution)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-092_U_233.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def u236():
+def u236(endf_data):
     """U236 ENDF data (contains Watt fission energy distribution)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-092_U_236.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def na22():
+def na22(endf_data):
     """Na22 ENDF data (contains evaporation spectrum)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-011_Na_022.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def na23():
+def na23(endf_data):
     """Na23 ENDF data (contains MLBW resonance covariance with LCOMP=0)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-011_Na_023.endf')
     return openmc.data.IncidentNeutron.from_endf(filename, covariance=True)
 
 
 @pytest.fixture(scope='module')
-def be9():
+def be9(endf_data):
     """Be9 ENDF data (contains laboratory angle-energy distribution)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-004_Be_009.endf')
     return openmc.data.IncidentNeutron.from_endf(filename)
 
 
 @pytest.fixture(scope='module')
-def h2():
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def h2(endf_data):
     endf_file = os.path.join(endf_data, 'neutrons', 'n-001_H_002.endf')
     return openmc.data.IncidentNeutron.from_njoy(
         endf_file, temperatures=_TEMPERATURES)
 
 
 @pytest.fixture(scope='module')
-def am244():
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def am244(endf_data):
     endf_file = os.path.join(endf_data, 'neutrons', 'n-095_Am_244.endf')
     return openmc.data.IncidentNeutron.from_njoy(endf_file)
 
 
 @pytest.fixture(scope='module')
-def ti50():
+def ti50(endf_data):
     """Ti50 ENDF data (contains Multi-level Breit-Wigner resonance range and
        resonance covariance with LCOMP=1)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-022_Ti_050.endf')
     return openmc.data.IncidentNeutron.from_endf(filename, covariance=True)
 
 
 @pytest.fixture(scope='module')
-def cf252():
+def cf252(endf_data):
     """Cf252 ENDF data (contains RM resonance covariance with LCOMP=0)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-098_Cf_252.endf')
     return openmc.data.IncidentNeutron.from_endf(filename, covariance=True)
 
 
 @pytest.fixture(scope='module')
-def th232():
+def th232(endf_data):
     """Th232 ENDF data (contains RM resonance covariance with LCOMP=2)."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'neutrons', 'n-090_Th_232.endf')
     return openmc.data.IncidentNeutron.from_endf(filename, covariance=True)
 
@@ -148,6 +133,28 @@ def test_attributes(pu239):
     assert pu239.atomic_weight_ratio == pytest.approx(236.9986)
 
 
+def test_from_endf_material(endf_data):
+    filename = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
+    material = openmc.data.endf.get_evaluations(filename)[0]
+
+    data = openmc.data.IncidentNeutron.from_endf(material)
+
+    assert data.name == 'H1'
+    assert data.atomic_number == 1
+    assert data.mass_number == 1
+    assert 2 in data.reactions
+
+
+def test_fission_energy_from_endf_material(endf_data):
+    filename = os.path.join(endf_data, 'neutrons', 'n-092_U_235.endf')
+    material = openmc.data.endf.get_evaluations(filename)[0]
+    neutron = openmc.data.IncidentNeutron.from_endf(material)
+
+    data = openmc.data.FissionEnergyRelease.from_endf(material, neutron)
+
+    assert data.fragments(0.0) > 0.0
+
+
 def test_fission_energy(pu239):
     fer = pu239.fission_energy
     assert isinstance(fer, openmc.data.FissionEnergyRelease)
@@ -453,8 +460,7 @@ def test_laboratory(be9):
 
 
 @needs_njoy
-def test_correlated(tmpdir):
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def test_correlated(tmpdir, endf_data):
     endf_file = os.path.join(endf_data, 'neutrons', 'n-014_Si_030.endf')
     si30 = openmc.data.IncidentNeutron.from_njoy(endf_file, heatr=False)
 
@@ -480,8 +486,7 @@ def test_nbody(tmpdir, h2):
 
 
 @needs_njoy
-def test_ace_convert(run_in_tmpdir):
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def test_ace_convert(run_in_tmpdir, endf_data):
     filename = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
     ace_ascii = 'ace_ascii'
     ace_binary = 'ace_binary'
@@ -515,8 +520,7 @@ def test_ace_table_types():
 
 
 @needs_njoy
-def test_high_temperature():
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def test_high_temperature(endf_data):
     endf_file = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
 
     # Ensure that from_njoy works when given a high temperature
diff --git a/tests/unit_tests/test_data_photon.py b/tests/unit_tests/test_data_photon.py
index 010ac1f35..aceb82c3a 100644
--- a/tests/unit_tests/test_data_photon.py
+++ b/tests/unit_tests/test_data_photon.py
@@ -9,9 +9,8 @@ import openmc.data
 
 
 @pytest.fixture(scope='module')
-def elements_endf():
+def elements_endf(endf_data):
     """Dictionary of element ENDF data indexed by atomic symbol."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     elements = {'H': 1, 'O': 8, 'Al': 13, 'Cu': 29, 'Ag': 47, 'U': 92, 'Pu': 94}
     data = {}
     for symbol, Z in elements.items():
@@ -145,8 +144,32 @@ def test_export_to_hdf5(tmpdir, element):
     element2.export_to_hdf5(filename, 'w')
 
 
-def test_photodat_only(run_in_tmpdir):
-    endf_dir = Path(os.environ['OPENMC_ENDF_DATA'])
+def test_photodat_only(run_in_tmpdir, endf_data):
+    endf_dir = Path(endf_data)
     photoatomic_file = endf_dir / 'photoat' / 'photoat-001_H_000.endf'
     data = openmc.data.IncidentPhoton.from_endf(photoatomic_file)
     data.export_to_hdf5('tmp.h5', 'w')
+
+
+def test_from_endf_material(endf_data):
+    endf_dir = Path(endf_data)
+    photoatomic_file = endf_dir / 'photoat' / 'photoat-001_H_000.endf'
+    relaxation_file = endf_dir / 'atomic_relax' / 'atom-001_H_000.endf'
+    photoatomic = openmc.data.endf.get_evaluations(photoatomic_file)[0]
+    relaxation = openmc.data.endf.get_evaluations(relaxation_file)[0]
+
+    data = openmc.data.IncidentPhoton.from_endf(photoatomic, relaxation)
+
+    assert data.atomic_number == 1
+    assert 502 in data.reactions
+    assert data.atomic_relaxation.binding_energy['K'] == pytest.approx(13.61)
+
+
+def test_atomic_relaxation_from_endf_material(endf_data):
+    filename = Path(endf_data) / 'atomic_relax' / 'atom-001_H_000.endf'
+    material = openmc.data.endf.get_evaluations(filename)[0]
+
+    data = openmc.data.AtomicRelaxation.from_endf(material)
+
+    assert data.binding_energy['K'] == pytest.approx(13.61)
+    assert data.num_electrons['K'] == pytest.approx(1.0)
diff --git a/tests/unit_tests/test_data_thermal.py b/tests/unit_tests/test_data_thermal.py
index c9fd9045b..81ed42b41 100644
--- a/tests/unit_tests/test_data_thermal.py
+++ b/tests/unit_tests/test_data_thermal.py
@@ -6,6 +6,8 @@ import random
 import numpy as np
 import pytest
 import openmc.data
+from openmc.data.thermal import _THERMAL_NAMES
+from openmc.data.njoy import _THERMAL_DATA
 
 from . import needs_njoy
 
@@ -13,7 +15,7 @@ from . import needs_njoy
 @pytest.fixture(scope='module')
 def h2o():
     """H in H2O thermal scattering data."""
-    directory = os.path.dirname(os.environ['OPENMC_CROSS_SECTIONS'])
+    directory = os.path.dirname(openmc.config.get('cross_sections'))
     filename = os.path.join(directory, 'c_H_in_H2O.h5')
     return openmc.data.ThermalScattering.from_hdf5(filename)
 
@@ -21,15 +23,14 @@ def h2o():
 @pytest.fixture(scope='module')
 def graphite():
     """Graphite thermal scattering data."""
-    directory = os.path.dirname(os.environ['OPENMC_CROSS_SECTIONS'])
+    directory = os.path.dirname(openmc.config.get('cross_sections'))
     filename = os.path.join(directory, 'c_Graphite.h5')
     return openmc.data.ThermalScattering.from_hdf5(filename)
 
 
 @pytest.fixture(scope='module')
-def h2o_njoy():
+def h2o_njoy(endf_data):
     """H in H2O generated using NJOY."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     path_h1 = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
     path_h2o = os.path.join(endf_data, 'thermal_scatt', 'tsl-HinH2O.endf')
     return openmc.data.ThermalScattering.from_njoy(
@@ -37,17 +38,15 @@ def h2o_njoy():
 
 
 @pytest.fixture(scope='module')
-def hzrh():
+def hzrh(endf_data):
     """H in ZrH thermal scattering data."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'thermal_scatt', 'tsl-HinZrH.endf')
     return openmc.data.ThermalScattering.from_endf(filename, divide_incoherent_elastic=True)
 
 
 @pytest.fixture(scope='module')
-def hzrh_njoy():
+def hzrh_njoy(endf_data):
     """H in ZrH generated using NJOY."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     path_h1 = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
     path_hzrh = os.path.join(endf_data, 'thermal_scatt', 'tsl-HinZrH.endf')
     with_endf_data = openmc.data.ThermalScattering.from_njoy(
@@ -60,9 +59,8 @@ def hzrh_njoy():
 
 
 @pytest.fixture(scope='module')
-def sio2():
+def sio2(endf_data):
     """SiO2 thermal scattering data."""
-    endf_data = os.environ['OPENMC_ENDF_DATA']
     filename = os.path.join(endf_data, 'thermal_scatt', 'tsl-SiO2.endf')
     return openmc.data.ThermalScattering.from_endf(filename, divide_incoherent_elastic=True)
 
@@ -102,8 +100,7 @@ def test_graphite_xs(graphite):
     assert elastic([1e-3, 1.0]) == pytest.approx([0.0, 0.62586153])
 
 @needs_njoy
-def test_graphite_njoy():
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def test_graphite_njoy(endf_data):
     path_c0 = os.path.join(endf_data, 'neutrons', 'n-006_C_000.endf')
     path_gr = os.path.join(endf_data, 'thermal_scatt', 'tsl-graphite.endf')
     graphite = openmc.data.ThermalScattering.from_njoy(
@@ -141,8 +138,7 @@ def test_continuous_dist(h2o_njoy):
         assert isinstance(dist, openmc.data.IncoherentInelasticAE)
 
 
-def test_h2o_endf():
-    endf_data = os.environ['OPENMC_ENDF_DATA']
+def test_h2o_endf(endf_data):
     filename = os.path.join(endf_data, 'thermal_scatt', 'tsl-HinH2O.endf')
     h2o = openmc.data.ThermalScattering.from_endf(filename, divide_incoherent_elastic=True)
     assert not h2o.elastic
@@ -152,6 +148,18 @@ def test_h2o_endf():
                                 '600K', '650K', '800K']
 
 
+def test_from_endf_material(endf_data):
+    filename = os.path.join(endf_data, 'thermal_scatt', 'tsl-HinH2O.endf')
+    material = openmc.data.endf.get_evaluations(filename)[0]
+
+    h2o = openmc.data.ThermalScattering.from_endf(
+        material, divide_incoherent_elastic=True)
+
+    assert not h2o.elastic
+    assert h2o.atomic_weight_ratio == pytest.approx(0.99917)
+    assert h2o.temperatures[0] == '294K'
+
+
 def test_hzrh_attributes(hzrh):
     assert hzrh.atomic_weight_ratio == pytest.approx(0.99917)
     assert hzrh.energy_max == pytest.approx(1.9734)
@@ -264,6 +272,27 @@ def test_get_thermal_name():
         assert f('boogie_monster') == 'c_boogie_monster'
 
 
+def test_thermal_names_data_consistency():
+    # Check that keys in _THERMAL_NAMES are also in _THERMAL_DATA
+    names_only = set(_THERMAL_NAMES.keys()) - set(_THERMAL_DATA.keys())
+    assert not names_only, f"Keys in _THERMAL_NAMES but not in _THERMAL_DATA: {names_only}"
+
+    # Check that keys in _THERMAL_DATA are also in _THERMAL_NAMES
+    data_only = set(_THERMAL_DATA.keys()) - set(_THERMAL_NAMES.keys())
+    assert not data_only, f"Keys in _THERMAL_DATA but not in _THERMAL_NAMES: {data_only}"
+
+    # Check that the name from each ThermalTuple in _THERMAL_DATA appears as
+    # a recognized alias in _THERMAL_NAMES for the same key
+    missing_aliases = []
+    for key, thermal_tuple in _THERMAL_DATA.items():
+        name = thermal_tuple.name
+        if name not in _THERMAL_NAMES[key]:
+            missing_aliases.append((key, name, _THERMAL_NAMES[key]))
+    assert not missing_aliases, (
+        f"ThermalTuple names not in _THERMAL_NAMES aliases: {missing_aliases}"
+    )
+
+
 @pytest.fixture
 def fake_mixed_elastic():
     fake_tsl = openmc.data.ThermalScattering("c_D_in_7LiD", 1.9968, 4.9, [0.0253])
diff --git a/tests/unit_tests/test_deplete_activation.py b/tests/unit_tests/test_deplete_activation.py
index 6afea8c9f..2f1713880 100644
--- a/tests/unit_tests/test_deplete_activation.py
+++ b/tests/unit_tests/test_deplete_activation.py
@@ -45,10 +45,11 @@ ENERGIES = np.logspace(log10(1e-5), log10(2e7), 100)
 
 @pytest.mark.parametrize("reaction_rate_mode,reaction_rate_opts,tolerance", [
     ("direct", {}, 1e-5),
-    ("flux", {'energies': ENERGIES}, 0.01),
+    ("flux", {'energies': ENERGIES}, 0.1),
     ("flux", {'energies': ENERGIES, 'reactions': ['(n,gamma)']}, 1e-5),
     ("flux", {'energies': ENERGIES, 'reactions': ['(n,gamma)'], 'nuclides': ['W186', 'H3']}, 1e-2),
 ])
+@pytest.mark.flaky(reruns=1)
 def test_activation(run_in_tmpdir, model, reaction_rate_mode, reaction_rate_opts, tolerance):
     # Determine (n.gamma) reaction rate using initial run
     sp = model.run()
@@ -61,11 +62,10 @@ def test_activation(run_in_tmpdir, model, reaction_rate_mode, reaction_rate_opts
     w186 = openmc.deplete.Nuclide('W186')
     w186.add_reaction('(n,gamma)', None, 0.0, 1.0)
     chain.add_nuclide(w186)
-    chain.export_to_xml('test_chain.xml')
 
     # Create transport operator
     op = openmc.deplete.CoupledOperator(
-        model, 'test_chain.xml',
+        model, chain,
         normalization_mode="source-rate",
         reaction_rate_mode=reaction_rate_mode,
         reaction_rate_opts=reaction_rate_opts,
@@ -113,6 +113,11 @@ def test_activation(run_in_tmpdir, model, reaction_rate_mode, reaction_rate_opts
     assert atoms[0] == pytest.approx(n0)
     assert atoms[1] / atoms[0] == pytest.approx(0.5, rel=tolerance)
 
+    # Check that material name is preserved in depletion results
+    step_result = results[0]
+    mat_from_results = step_result.get_material(f"{w.id}")
+    assert mat_from_results.name == 'tungsten'
+
 
 def test_decay(run_in_tmpdir):
     """Test decay-only timesteps where no transport solve is performed"""
diff --git a/tests/unit_tests/test_deplete_chain.py b/tests/unit_tests/test_deplete_chain.py
index 13267cb05..ef8cb9ef2 100644
--- a/tests/unit_tests/test_deplete_chain.py
+++ b/tests/unit_tests/test_deplete_chain.py
@@ -54,11 +54,11 @@ def simple_chain():
 
 
 @pytest.fixture(scope='module')
-def endf_chain():
-    endf_data = Path(os.environ['OPENMC_ENDF_DATA'])
-    decay_data = (endf_data / 'decay').glob('*.endf')
-    fpy_data = (endf_data / 'nfy').glob('*.endf')
-    neutron_data = (endf_data / 'neutrons').glob('*.endf')
+def endf_chain(endf_data):
+    endf_dir = Path(endf_data)
+    decay_data = (endf_dir / 'decay').glob('*.endf')
+    fpy_data = (endf_dir / 'nfy').glob('*.endf')
+    neutron_data = (endf_dir / 'neutrons').glob('*.endf')
     return Chain.from_endf(decay_data, fpy_data, neutron_data)
 
 
@@ -246,6 +246,29 @@ def test_form_matrix(simple_chain):
         assert new_mat[r, c] == mat[r, c]
 
 
+def test_decay_matrix(simple_chain):
+    """Test that decay_matrix contains only radioactive decay terms."""
+    # Nuclide order: H1(0), A(1), B(2), C(3)
+    decay_A = log(2) / 2.36520E+04
+    decay_B = log(2) / 3.29040E+04
+
+    expected = np.zeros((4, 4))
+    expected[1, 1] = -decay_A           # Loss: A decays
+    expected[2, 1] = decay_A * 0.6      # A -> B (branching ratio 0.6)
+    expected[3, 1] = decay_A * 0.4      # A -> C (branching ratio 0.4)
+    expected[1, 2] = decay_B            # B -> A (branching ratio 1.0)
+    expected[2, 2] = -decay_B           # Loss: B decays
+
+    assert np.allclose(expected, simple_chain.decay_matrix.toarray())
+
+
+def test_decay_matrix_cached(simple_chain):
+    """Test that decay_matrix is lazily computed and returns the same object."""
+    m1 = simple_chain.decay_matrix
+    m2 = simple_chain.decay_matrix
+    assert m1 is m2
+
+
 def test_getitem():
     """Test nuc_by_ind converter function."""
     chain = Chain()
@@ -310,8 +333,7 @@ def test_capture_branch_infer_ground():
     # Create nuclide to be added into the chain
     xe136m = nuclide.Nuclide("Xe136_m1")
 
-    chain.nuclides.append(xe136m)
-    chain.nuclide_dict[xe136m.name] = len(chain.nuclides) - 1
+    chain.add_nuclide(xe136m)
 
     chain.set_branch_ratios(infer_br, "(n,gamma)")
 
@@ -327,8 +349,7 @@ def test_capture_branch_no_rxn():
 
     u5m = nuclide.Nuclide("U235_m1")
 
-    chain.nuclides.append(u5m)
-    chain.nuclide_dict[u5m.name] = len(chain.nuclides) - 1
+    chain.add_nuclide(u5m)
 
     with pytest.raises(AttributeError, match="U234"):
         chain.set_branch_ratios(u4br)
diff --git a/tests/unit_tests/test_deplete_continue.py b/tests/unit_tests/test_deplete_continue.py
index 53c4c56d2..1b6eac238 100644
--- a/tests/unit_tests/test_deplete_continue.py
+++ b/tests/unit_tests/test_deplete_continue.py
@@ -4,6 +4,7 @@ These tests run in two steps: first a normal run and then a continue run using t
 """
 
 import pytest
+import numpy as np
 import openmc.deplete
 
 from tests import dummy_operator
@@ -16,7 +17,7 @@ def test_continue(run_in_tmpdir):
     operator = dummy_operator.DummyOperator()
 
     # initial depletion
-    bundle.solver(operator, [1.0, 2.0], [1.0, 2.0]).integrate()
+    bundle.solver(operator, [1.0, 2.0], [1.0, 2.0]).integrate(write_rates=True)
 
     # set up continue run
     prev_res = openmc.deplete.Results(operator.output_dir / "depletion_results.h5")
@@ -24,8 +25,45 @@ def test_continue(run_in_tmpdir):
 
     # if continue run happens, test passes
     bundle.solver(operator, [1.0, 2.0, 3.0, 4.0], [1.0, 2.0, 3.0, 4.0],
+                  continue_timesteps=True).integrate(write_rates=True)
+
+    final_res = openmc.deplete.Results(operator.output_dir / "depletion_results.h5")
+
+    assert np.array_equal(
+        np.diff(final_res.get_times(time_units="s")),
+        [1.0, 2.0, 3.0, 4.0]
+    )
+
+
+def test_continue_continue(run_in_tmpdir):
+    """Test to ensure that a continue run can be continued"""
+    # set up the problem
+    bundle = dummy_operator.SCHEMES['predictor']
+    operator = dummy_operator.DummyOperator()
+
+    # initial depletion
+    bundle.solver(operator, [1.0, 2.0], [1.0, 2.0]).integrate(write_rates=True)
+
+    # set up continue run
+    prev_res = openmc.deplete.Results(operator.output_dir / "depletion_results.h5")
+    operator = dummy_operator.DummyOperator(prev_res)
+
+    # first continue run
+    bundle.solver(operator, [1.0, 2.0, 3.0, 4.0], [1.0, 2.0, 3.0, 4.0],
+                  continue_timesteps=True).integrate(write_rates=True)
+
+    prev_res = openmc.deplete.Results(operator.output_dir / "depletion_results.h5")
+    # second continue run
+    bundle.solver(operator, [1.0, 2.0, 3.0, 4.0, 5.0, 6.0], [1.0, 2.0, 3.0, 4.0, 5.0, 6.0],
                   continue_timesteps=True).integrate()
 
+    final_res = openmc.deplete.Results(operator.output_dir / "depletion_results.h5")
+
+    assert np.array_equal(
+        np.diff(final_res.get_times(time_units="s")),
+        [1.0, 2.0, 3.0, 4.0, 5.0, 6.0]
+    )
+
 
 def test_mismatched_initial_times(run_in_tmpdir):
     """Test to ensure that a continue run with different initial steps is properly caught"""
diff --git a/tests/unit_tests/test_deplete_coupled_operator.py b/tests/unit_tests/test_deplete_coupled_operator.py
index ba291e07d..119a15923 100644
--- a/tests/unit_tests/test_deplete_coupled_operator.py
+++ b/tests/unit_tests/test_deplete_coupled_operator.py
@@ -5,7 +5,7 @@
 from pathlib import Path
 
 import pytest
-from openmc.deplete import CoupledOperator
+from openmc.deplete import CoupledOperator, Chain
 import openmc
 import numpy as np
 
@@ -106,11 +106,13 @@ def test_diff_volume_method_match_cell(model_with_volumes):
 def test_diff_volume_method_divide_equally(model_with_volumes):
     """Tests the volumes assigned to the materials are divided equally"""
 
+    chain = Chain.from_xml(CHAIN_PATH)
+
     operator = openmc.deplete.CoupledOperator(
         model=model_with_volumes,
         diff_burnable_mats=True,
         diff_volume_method='divide equally',
-        chain_file=CHAIN_PATH
+        chain_file=chain
     )
 
     all_cells = list(operator.model.geometry.get_all_cells().values())
diff --git a/tests/unit_tests/test_deplete_cram.py b/tests/unit_tests/test_deplete_cram.py
index 8987fbd7a..64cff3a8b 100644
--- a/tests/unit_tests/test_deplete_cram.py
+++ b/tests/unit_tests/test_deplete_cram.py
@@ -1,12 +1,15 @@
-""" Tests for cram.py
+"""Tests for cram.py.
 
 Compares a few Mathematica matrix exponentials to CRAM16/CRAM48.
+Tests substep accuracy against self-converged reference solutions.
 """
 
-from pytest import approx
 import numpy as np
+import pytest
 import scipy.sparse as sp
-from openmc.deplete.cram import CRAM16, CRAM48
+from pytest import approx
+from openmc.deplete.cram import (CRAM16, CRAM48, Cram16Solver, Cram48Solver,
+                                  IPFCramSolver)
 
 
 def test_CRAM16():
@@ -35,3 +38,63 @@ def test_CRAM48():
     z0 = np.array((0.904837418035960, 0.576799023327476))
 
     assert z == approx(z0)
+
+
+def test_substeps1_matches_original():
+    """substeps=1 must be bitwise identical to original spsolve path."""
+    x = np.array([1.0, 1.0])
+    mat = sp.csr_matrix([[-1.0, 0.0], [-2.0, -3.0]])
+    dt = 0.1
+
+    z_orig = CRAM48(mat, x, dt)
+    z_sub1 = CRAM48(mat, x, dt, substeps=1)
+
+    np.testing.assert_array_equal(z_sub1, z_orig)
+
+
+def test_substeps2_matches_two_half_steps():
+    """substeps=2 must match two independent CRAM calls with dt/2."""
+    x = np.array([1.0, 1.0])
+    mat = sp.csr_matrix([[-1.0, 0.0], [-2.0, -3.0]])
+    dt = 1.0
+
+    # Two manual half-steps using original spsolve path
+    z_half = CRAM48(mat, x, dt / 2)
+    z_two = CRAM48(mat, z_half, dt / 2)
+
+    # Single call with substeps=2
+    z_sub2 = CRAM48(mat, x, dt, substeps=2)
+
+    assert z_sub2 == approx(z_two, rel=1e-12)
+
+
+@pytest.mark.parametrize("substeps", [0, -1])
+def test_invalid_substeps(substeps):
+    """substeps must be a positive integer at call time."""
+    x = np.array([1.0, 1.0])
+    mat = sp.csr_matrix([[-1.0, 0.0], [-2.0, -3.0]])
+
+    with pytest.raises(ValueError, match="substeps"):
+        CRAM48(mat, x, 0.1, substeps=substeps)
+
+
+def test_substeps_self_convergence():
+    """Increasing substeps converges toward reference solution.
+
+    Uses CRAM16 (alpha0 ~ 2e-16) where substep convergence is visible.
+    CRAM48 (alpha0 ~ 2e-47) is already near machine precision for small
+    systems; its correctness is verified by the other substep tests.
+    """
+    mat = sp.csr_matrix([[-1.0, 0.0], [-2.0, -3.0]])
+    x = np.array([1.0, 1.0])
+    dt = 50  # lambda*dt = 50 and 150, stresses CRAM16
+
+    n_ref = CRAM16(mat, x, dt, substeps=128)
+
+    prev_err = np.inf
+    for s in [1, 2, 4, 8, 16]:
+        n_s = CRAM16(mat, x, dt, substeps=s)
+        err = np.linalg.norm(n_s - n_ref) / np.linalg.norm(n_ref)
+        assert err < prev_err, \
+            f"substeps={s} error {err:.2e} not less than previous {prev_err:.2e}"
+        prev_err = err
diff --git a/tests/unit_tests/test_deplete_decay.py b/tests/unit_tests/test_deplete_decay.py
index 6e7b0b101..db96fcbe2 100644
--- a/tests/unit_tests/test_deplete_decay.py
+++ b/tests/unit_tests/test_deplete_decay.py
@@ -82,3 +82,8 @@ def test_deplete_decay_step_fissionable(run_in_tmpdir):
     _, u238 = results.get_atoms(f"{mat.id}", "U238")
 
     assert u238[1] == pytest.approx(original_atoms)
+
+    # Check that material name is preserved in depletion results
+    step_result = results[0]
+    mat_from_results = step_result.get_material(f"{mat.id}")
+    assert mat_from_results.name == 'I do not decay.'
diff --git a/tests/unit_tests/test_deplete_external_source_rates.py b/tests/unit_tests/test_deplete_external_source_rates.py
new file mode 100644
index 000000000..a8cf9dde5
--- /dev/null
+++ b/tests/unit_tests/test_deplete_external_source_rates.py
@@ -0,0 +1,170 @@
+""" Tests for ExternalSourceRates class """
+
+from pathlib import Path
+
+import pytest
+import numpy as np
+import re
+
+import openmc
+from openmc.data import AVOGADRO, atomic_mass
+from openmc.deplete import CoupledOperator
+from openmc.deplete.transfer_rates import ExternalSourceRates
+from openmc.deplete.abc import (_SECONDS_PER_MINUTE, _SECONDS_PER_HOUR,
+                                _SECONDS_PER_DAY, _SECONDS_PER_JULIAN_YEAR)
+
+CHAIN_PATH = Path(__file__).parents[1] / "chain_simple.xml"
+
+@pytest.fixture
+def model():
+    openmc.reset_auto_ids()
+    f = openmc.Material(name="f")
+    f.add_element("U", 1, enrichment=4.25)
+    f.add_element("O", 2)
+    f.set_density("g/cm3", 10.4)
+
+    w = openmc.Material(name="w")
+    w.add_element("O", 1)
+    w.add_element("H", 2)
+    w.set_density("g/cm3", 1.0)
+    w.depletable = True
+
+    # material just to test multiple destination material
+    h = openmc.Material(name="h")
+    h.add_element("He", 1)
+    h.set_density("g/cm3", 1.78e-4)
+
+    radii = [0.42, 0.45]
+    f.volume = np.pi * radii[0] ** 2
+    w.volume = np.pi * (radii[1]**2 - radii[0]**2)
+    h.volume = 1
+    materials = openmc.Materials([f, w, h])
+
+    surf_f = openmc.Sphere(r=radii[0])
+    surf_w = openmc.Sphere(r=radii[1], boundary_type='vacuum')
+    surf_h = openmc.Sphere(x0=10, r=1, boundary_type='vacuum')
+    cell_f = openmc.Cell(fill=f, region=-surf_f)
+    cell_w = openmc.Cell(fill=w, region=+surf_f & -surf_w)
+    cell_h = openmc.Cell(fill=h, region=-surf_h)
+    geometry = openmc.Geometry([cell_f, cell_w, cell_h])
+
+    settings = openmc.Settings()
+    settings.particles = 1000
+    settings.inactive = 10
+    settings.batches = 50
+
+    return openmc.Model(geometry, materials, settings)
+
+
+@pytest.mark.parametrize(
+"case_name, external_source_vectors, external_source_rate, timesteps", [
+    ('elements', [{'U': 0.9, 'Xe': 0.1}], 1, None),
+    ('nuclides', [{'I135': 0.1, 'Gd156': 0.3, 'Gd157': 0.6}], 1, None),
+    ('nuclides_elements', [{'I135': 0.01, 'Gd156': 0.1, 'Gd157': 0.01, 'U': 0.8,
+                           'Xe': 0.08}], 1, None),
+    ('elements_nuclides', [{'U': 0.78, 'Xe': 0.1, 'I135': 0.01, 'Gd156': 0.1,
+                           'Gd157': 0.01}], 1, None),
+    ('multiple_vectors', [{'U': 1.}, {'Xe': 1}], 1, None),
+    ('timesteps', [{'U': 0.9, 'Xe': 0.1}], 1, [1]),
+    ('rates_invalid_1', [{'Gb': 1.}], 1, None),
+    ('rates_invalid_2', [{'Pu': 1.}], 1, None)
+    ])
+def test_get_set(model, case_name, external_source_vectors, external_source_rate,
+                 timesteps):
+    """Tests the get/set methods"""
+
+    op = CoupledOperator(model, CHAIN_PATH)
+    number_of_timesteps = 2
+    transfer = ExternalSourceRates(op, model.materials, number_of_timesteps)
+
+    if timesteps is None:
+        timesteps = np.arange(number_of_timesteps)
+
+    # Test by Openmc material, material name and material id
+    material= [m for m in model.materials if m.depletable][0]
+
+    for material_input in [material, material.name, material.id]:
+        for external_source_vector in external_source_vectors:
+            if case_name == 'rates_invalid_1':
+                with pytest.raises(ValueError, match='Gb is not a valid '
+                                   'nuclide or element.'):
+                    transfer.set_external_source_rate(material_input,
+                                               external_source_vector,
+                                               external_source_rate)
+            elif case_name == 'rates_invalid_2':
+                with pytest.raises(ValueError, match='Cannot add element Pu'):
+                    transfer.set_external_source_rate(material_input,
+                                               external_source_vector,
+                                               external_source_rate)
+            else:
+                transfer.set_external_source_rate(material_input,
+                                               external_source_vector,
+                                               external_source_rate,
+                                               timesteps=timesteps)
+                for component, percent in external_source_vector.items():
+                    split_component = re.split(r'\d+', component)
+                    if len(split_component) == 1:
+                        for nuc, frac in openmc.data.isotopes(component):
+                            val = external_source_rate * percent * frac * \
+                                AVOGADRO / atomic_mass(nuc)
+                            assert transfer.get_external_rate(
+                                material_input, nuc, timesteps)[0] == pytest.approx(val)
+                    else:
+                        val = external_source_rate * percent * AVOGADRO / atomic_mass(component)
+                        assert transfer.get_external_rate(
+                            material_input, component, timesteps)[0] == pytest.approx(val)
+
+                assert np.all(transfer.external_timesteps == timesteps)
+
+
+@pytest.mark.parametrize("units, unit_conv", [
+    ('g/s', 1),
+    ('g/sec', 1),
+    ('g/min', _SECONDS_PER_MINUTE),
+    ('g/minute', _SECONDS_PER_MINUTE),
+    ('g/h', _SECONDS_PER_HOUR),
+    ('g/hr', _SECONDS_PER_HOUR),
+    ('g/hour', _SECONDS_PER_HOUR),
+    ('g/d', _SECONDS_PER_DAY),
+    ('g/day', _SECONDS_PER_DAY),
+    ('g/a', _SECONDS_PER_JULIAN_YEAR),
+    ('g/year', _SECONDS_PER_JULIAN_YEAR),
+    ])
+def test_units(units, unit_conv, model):
+    """ Units testing"""
+    # create external rate Xe
+    components = ['Xe135', 'U235']
+    external_source_rate = 1.0
+    number_of_timesteps = 2
+    op = CoupledOperator(model, CHAIN_PATH)
+    transfer = ExternalSourceRates(op, model.materials, number_of_timesteps)
+    timesteps = np.arange(number_of_timesteps)
+
+    for component in components:
+        rate = external_source_rate * unit_conv * atomic_mass(component) / AVOGADRO
+        transfer.set_external_source_rate('f', {component: 1}, rate, rate_units=units)
+        assert transfer.get_external_rate(
+            'f', component, timesteps)[0] == pytest.approx(external_source_rate)
+
+
+def test_external_source(run_in_tmpdir, model):
+    """Tests external source depletion class without neither reaction rates nor
+    decay but only external source rates"""
+    # create transfer rate for U
+    vector = {'U235': 1}
+    external_source = 10  # grams
+    op = CoupledOperator(model, CHAIN_PATH)
+    integrator = openmc.deplete.PredictorIntegrator(
+        op, [1, 1], 0.0, timestep_units = 'd')
+    integrator.add_external_source_rate('f', vector, external_source/(24*3600))
+    integrator.integrate()
+
+    # Get number of U238 atoms from results
+    results = openmc.deplete.Results('depletion_results.h5')
+    _, atoms = results.get_atoms(model.materials[0], "U235")
+
+    # Ensure number of atoms equal external source
+    assert atoms[1] - atoms[0] == pytest.approx(
+        external_source * AVOGADRO / atomic_mass('U235'))
+    assert atoms[2] - atoms[1] == pytest.approx(
+        external_source * AVOGADRO / atomic_mass('U235'))
diff --git a/tests/unit_tests/test_deplete_independent_operator.py b/tests/unit_tests/test_deplete_independent_operator.py
index c765d0650..aca83399a 100644
--- a/tests/unit_tests/test_deplete_independent_operator.py
+++ b/tests/unit_tests/test_deplete_independent_operator.py
@@ -7,7 +7,7 @@ from pathlib import Path
 import pytest
 
 from openmc import Material
-from openmc.deplete import IndependentOperator, MicroXS
+from openmc.deplete import IndependentOperator, MicroXS, Chain
 
 CHAIN_PATH = Path(__file__).parents[1] / "chain_simple.xml"
 ONE_GROUP_XS = Path(__file__).parents[1] / "micro_xs_simple.csv"
@@ -25,8 +25,9 @@ def test_operator_init():
                 'O17': 1.7588724018066158e+19}
     flux = 1.0
     micro_xs = MicroXS.from_csv(ONE_GROUP_XS)
+    chain = Chain.from_xml(CHAIN_PATH)
     IndependentOperator.from_nuclides(
-        volume, nuclides, flux, micro_xs, CHAIN_PATH, nuc_units='atom/cm3')
+        volume, nuclides, flux, micro_xs, chain, nuc_units='atom/cm3')
 
     fuel = Material(name="uo2")
     fuel.add_element("U", 1, percent_type="ao", enrichment=4.25)
diff --git a/tests/unit_tests/test_deplete_integrator.py b/tests/unit_tests/test_deplete_integrator.py
index b1d2cb950..ec886e707 100644
--- a/tests/unit_tests/test_deplete_integrator.py
+++ b/tests/unit_tests/test_deplete_integrator.py
@@ -10,6 +10,7 @@ import copy
 from random import uniform
 from unittest.mock import MagicMock
 
+import h5py
 import numpy as np
 from uncertainties import ufloat
 import pytest
@@ -18,7 +19,7 @@ from openmc.mpi import comm
 from openmc.deplete import (
     ReactionRates, StepResult, Results, OperatorResult, PredictorIntegrator,
     CECMIntegrator, CF4Integrator, CELIIntegrator, EPCRK4Integrator,
-    LEQIIntegrator, SICELIIntegrator, SILEQIIntegrator, cram)
+    LEQIIntegrator, SICELIIntegrator, SILEQIIntegrator, cram, pool)
 
 from tests import dummy_operator
 
@@ -38,8 +39,6 @@ INTEGRATORS = [
 def test_results_save(run_in_tmpdir):
     """Test data save module"""
 
-    stages = 3
-
     rng = np.random.RandomState(comm.rank)
 
     # Mock geometry
@@ -60,34 +59,26 @@ def test_results_save(run_in_tmpdir):
     burn_list = full_burn_list[2*comm.rank: 2*comm.rank + 2]
     nuc_list = ["na", "nb"]
 
+    name_list = {mat: "" for mat in full_burn_list}
     op.get_results_info.return_value = (
-        vol_dict, nuc_list, burn_list, full_burn_list)
+        vol_dict, nuc_list, burn_list, full_burn_list, name_list)
 
-    # Construct x
-    x1 = []
-    x2 = []
+    # Construct end-of-step concentrations
+    x1 = [rng.random(2), rng.random(2)]
+    x2 = [rng.random(2), rng.random(2)]
 
-    for i in range(stages):
-        x1.append([rng.random(2), rng.random(2)])
-        x2.append([rng.random(2), rng.random(2)])
-
-    # Construct r
+    # Construct reaction rates
     r1 = ReactionRates(burn_list, ["na", "nb"], ["ra", "rb"])
     r1[:] = rng.random((2, 2, 2))
+    rate1 = copy.deepcopy(r1)
 
-    rate1 = []
-    rate2 = []
+    r2 = ReactionRates(burn_list, ["na", "nb"], ["ra", "rb"])
+    r2[:] = rng.random((2, 2, 2))
+    rate2 = copy.deepcopy(r2)
 
-    for i in range(stages):
-        rate1.append(copy.deepcopy(r1))
-        r1[:] = rng.random((2, 2, 2))
-        rate2.append(copy.deepcopy(r1))
-        r1[:] = rng.random((2, 2, 2))
-
-    # Create global terms
-    # Col 0: eig, Col 1: uncertainty
-    eigvl1 = rng.random((stages, 2))
-    eigvl2 = rng.random((stages, 2))
+    # Create global terms (eigenvalue and uncertainty)
+    eigvl1 = rng.random(2)
+    eigvl2 = rng.random(2)
 
     eigvl1 = comm.bcast(eigvl1, root=0)
     eigvl2 = comm.bcast(eigvl2, root=0)
@@ -95,29 +86,35 @@ def test_results_save(run_in_tmpdir):
     t1 = [0.0, 1.0]
     t2 = [1.0, 2.0]
 
-    op_result1 = [OperatorResult(ufloat(*k), rates)
-                  for k, rates in zip(eigvl1, rate1)]
-    op_result2 = [OperatorResult(ufloat(*k), rates)
-                  for k, rates in zip(eigvl2, rate2)]
+    op_result1 = OperatorResult(ufloat(*eigvl1), rate1)
+    op_result2 = OperatorResult(ufloat(*eigvl2), rate2)
 
     # saves within a subdirectory
-    StepResult.save(op, x1, op_result1, t1, 0, 0, path='out/put/depletion.h5')
+    StepResult.save(
+        op,
+        x1,
+        op_result1,
+        t1,
+        0,
+        0,
+        write_rates=True,
+        path='out/put/depletion.h5'
+    )
     res = Results('out/put/depletion.h5')
 
     # saves with default filename
-    StepResult.save(op, x1, op_result1, t1, 0, 0)
-    StepResult.save(op, x2, op_result2, t2, 0, 1)
+    StepResult.save(op, x1, op_result1, t1, 0, 0, write_rates=True)
+    StepResult.save(op, x2, op_result2, t2, 0, 1, write_rates=True)
 
     # Load the files
     res = Results("depletion_results.h5")
 
-    for i in range(stages):
-        for mat_i, mat in enumerate(burn_list):
-            for nuc_i, nuc in enumerate(nuc_list):
-                assert res[0][i, mat, nuc] == x1[i][mat_i][nuc_i]
-                assert res[1][i, mat, nuc] == x2[i][mat_i][nuc_i]
-        np.testing.assert_array_equal(res[0].rates[i], rate1[i])
-        np.testing.assert_array_equal(res[1].rates[i], rate2[i])
+    for mat_i, mat in enumerate(burn_list):
+        for nuc_i, nuc in enumerate(nuc_list):
+            assert res[0][mat, nuc] == x1[mat_i][nuc_i]
+            assert res[1][mat, nuc] == x2[mat_i][nuc_i]
+    np.testing.assert_array_equal(res[0].rates, rate1)
+    np.testing.assert_array_equal(res[1].rates, rate2)
 
     np.testing.assert_array_equal(res[0].k, eigvl1)
     np.testing.assert_array_equal(res[0].time, t1)
@@ -126,6 +123,32 @@ def test_results_save(run_in_tmpdir):
     np.testing.assert_array_equal(res[1].time, t2)
 
 
+def test_results_save_without_rates(run_in_tmpdir):
+    """StepResult.save skips reaction-rate datasets by default"""
+
+    op = MagicMock()
+    op.prev_res = None
+    vol_dict = {"0": 1.0}
+    nuc_list = ["na"]
+    burn_list = ["0"]
+    name_list = {mat: "" for mat in burn_list}
+    op.get_results_info.return_value = (vol_dict, nuc_list, burn_list, burn_list, name_list)
+
+    x = [np.array([1.0])]
+    rates = ReactionRates(burn_list, nuc_list, ["ra"])
+    rates[:] = np.array([[[2.0]]])
+    op_result = OperatorResult(ufloat(1.0, 0.1), rates)
+
+    StepResult.save(op, x, op_result, [0.0, 1.0], 0.0, 0)
+
+    with h5py.File('depletion_results.h5', 'r') as handle:
+        assert 'reaction rates' not in handle
+        assert 'reactions' not in handle
+
+    res = Results('depletion_results.h5')
+    assert res[0].rates.size == 0
+
+
 def test_bad_integrator_inputs():
     """Test failure modes for Integrator inputs"""
 
@@ -160,18 +183,42 @@ def test_bad_integrator_inputs():
     with pytest.raises(TypeError, match=".*callable.*NoneType"):
         PredictorIntegrator(op, timesteps, power=1, solver=None)
 
-    with pytest.raises(ValueError, match=".*arguments"):
+    with pytest.raises(ValueError, match="four arguments"):
         PredictorIntegrator(op, timesteps, power=1, solver=mock_bad_solver_nargs)
 
+    with pytest.raises(ValueError, match="default to 1"):
+        PredictorIntegrator(op, timesteps, power=1,
+                            solver=mock_bad_solver_fourth_required)
 
-def mock_good_solver(A, n, t):
-    pass
+    with pytest.raises(ValueError, match="substeps"):
+        PredictorIntegrator(op, timesteps, power=1, substeps=0)
+
+    with pytest.raises(ValueError, match="substeps"):
+        PredictorIntegrator(op, timesteps, power=1, substeps=-1)
+
+
+def mock_good_solver(A, n, t, substeps=1):
+    return n.copy()
+
+
+def mock_good_solver_substeps(A, n, t, substeps=1):
+    return n + substeps
+
+
+def mock_unsupported_substeps_solver(A, n, t, substeps=1):
+    if substeps > 1:
+        raise NotImplementedError("substeps > 1 not supported")
+    return n.copy()
 
 
 def mock_bad_solver_nargs(A, n):
     pass
 
 
+def mock_bad_solver_fourth_required(A, n, t, substeps):
+    pass
+
+
 @pytest.mark.parametrize("scheme", dummy_operator.SCHEMES)
 def test_integrator(run_in_tmpdir, scheme):
     """Test the integrators against their expected values"""
@@ -203,13 +250,71 @@ def test_integrator(run_in_tmpdir, scheme):
     integrator = bundle.solver(operator, [0.75], 1, solver="cram16")
     assert integrator.solver is cram.CRAM16
 
+    integrator = bundle.solver(operator, [0.75], 1, solver=cram.Cram48Solver,
+                               substeps=2)
+    assert integrator.solver is cram.Cram48Solver
+    assert integrator.substeps == 2
+
     integrator.solver = mock_good_solver
     assert integrator.solver is mock_good_solver
 
-    lfunc = lambda A, n, t: mock_good_solver(A, n, t)
+    lfunc = lambda A, n, t, substeps=1: mock_good_solver(A, n, t, substeps)
     integrator.solver = lfunc
     assert integrator.solver is lfunc
 
+    integrator.solver = mock_good_solver_substeps
+    assert integrator.solver is mock_good_solver_substeps
+
+
+def test_custom_solver_with_default_substeps(monkeypatch):
+    operator = dummy_operator.DummyOperator()
+    n = operator.initial_condition()
+    rates = operator(n, 1.0).rates
+    integrator = PredictorIntegrator(
+        operator, [0.75], power=1.0, solver=mock_good_solver)
+    monkeypatch.setattr(pool, "USE_MULTIPROCESSING", False)
+
+    _, result = integrator._timed_deplete(n, rates, 0.75)
+
+    np.testing.assert_array_equal(result[0], n[0])
+
+
+def test_substep_aware_custom_solver_receives_substeps(monkeypatch):
+    operator = dummy_operator.DummyOperator()
+    n = operator.initial_condition()
+    rates = operator(n, 1.0).rates
+    integrator = PredictorIntegrator(
+        operator, [0.75], power=1.0, solver=mock_good_solver_substeps,
+        substeps=3)
+    monkeypatch.setattr(pool, "USE_MULTIPROCESSING", False)
+
+    _, result = integrator._timed_deplete(n, rates, 0.75)
+
+    np.testing.assert_array_equal(result[0], n[0] + 3)
+
+
+def test_custom_solver_propagates_substeps_error(monkeypatch):
+    operator = dummy_operator.DummyOperator()
+    n = operator.initial_condition()
+    rates = operator(n, 1.0).rates
+    integrator = PredictorIntegrator(
+        operator, [0.75], power=1.0,
+        solver=mock_unsupported_substeps_solver, substeps=2)
+    monkeypatch.setattr(pool, "USE_MULTIPROCESSING", False)
+
+    with pytest.raises(NotImplementedError, match="not supported"):
+        integrator._timed_deplete(n, rates, 0.75)
+
+
+def test_custom_solver_requires_four_args():
+    op = MagicMock()
+    op.prev_res = None
+    op.chain = None
+    op.heavy_metal = 1.0
+
+    with pytest.raises(ValueError, match="four arguments"):
+        PredictorIntegrator(op, [1], power=1, solver=mock_bad_solver_nargs)
+
 
 @pytest.mark.parametrize("integrator", INTEGRATORS)
 def test_timesteps(integrator):
diff --git a/tests/unit_tests/test_deplete_keff_search_control.py b/tests/unit_tests/test_deplete_keff_search_control.py
new file mode 100644
index 000000000..425b8f840
--- /dev/null
+++ b/tests/unit_tests/test_deplete_keff_search_control.py
@@ -0,0 +1,116 @@
+""" Tests for KeffSearchControl class """
+
+from pathlib import Path
+
+import pytest
+import numpy as np
+
+import openmc
+import openmc.lib
+from openmc.deplete import CoupledOperator
+
+CHAIN_PATH = Path(__file__).parents[1] / "chain_simple.xml"
+
+
+def make_model():
+    f = openmc.Material(name="fuel")
+    f.add_element("U", 1, percent_type="ao", enrichment=4.25)
+    f.add_element("O", 2)
+    f.set_density("g/cc", 10.4)
+    f.temperature = 293.15
+
+    w = openmc.Material(name="water")
+    w.add_element("O", 1)
+    w.add_element("H", 2)
+    w.set_density("g/cc", 1.0)
+    w.temperature = 293.15
+    w.depletable = True
+
+    h = openmc.Material(name='helium')
+    h.add_element('He', 1)
+    h.set_density('g/cm3', 0.001598)
+
+    radii = [0.42, 0.45]
+    height = 0.5
+
+    f.volume = np.pi * radii[0] ** 2 * height
+    w.volume = np.pi * (radii[1]**2 - radii[0]**2) * height/2
+
+    materials = openmc.Materials([f, w, h])
+
+    surf_interface = openmc.ZPlane(z0=0)
+    surf_top = openmc.ZPlane(z0=height/2)
+    surf_bot = openmc.ZPlane(z0=-height/2)
+    surf_in = openmc.Sphere(r=radii[0])
+    surf_out = openmc.Sphere(r=radii[1], boundary_type='vacuum')
+
+    cell_water = openmc.Cell(fill=w, region=-surf_interface)
+    cell_helium = openmc.Cell(fill=h, region=+surf_interface)
+    universe = openmc.Universe(cells=(cell_water, cell_helium))
+    cell_fuel = openmc.Cell(name='fuel_cell', fill=f,
+                            region=-surf_in & -surf_top & +surf_bot)
+    cell_universe = openmc.Cell(name='universe_cell',fill=universe,
+                            region=+surf_in & -surf_out & -surf_top & +surf_bot)
+    geometry = openmc.Geometry([cell_fuel, cell_universe])
+
+    settings = openmc.Settings()
+    settings.particles = 1000
+    settings.inactive = 10
+    settings.batches = 50
+
+    return openmc.Model(geometry, materials, settings)
+
+
+def translate_cell(position):
+    """Helper function to translate a cell"""
+    cell = [c for c in openmc.lib.cells.values() if c.name == 'universe_cell'][0]
+    openmc.lib.cells[cell.id].translation = [0, 0, position]
+    return position
+
+
+def rotate_cell(angle):
+    """Helper function to rotate a cell"""
+    cell = [c for c in openmc.lib.cells.values() if c.name == 'universe_cell'][0]
+    openmc.lib.cells[cell.id].rotation = [0, 0, angle]
+    return angle
+
+
+def set_u235_density(u235_density):
+    """Helper function to set the U235 density directly"""
+    fuel = [m for m in openmc.lib.materials.values() if m.name == 'fuel'][0]
+    nuclides = openmc.lib.materials[fuel.id].nuclides
+    densities = openmc.lib.materials[fuel.id].densities
+    u235_idx = nuclides.index('U235')
+    densities[u235_idx] = u235_density
+    openmc.lib.materials[fuel.id].set_densities(nuclides, densities)
+    return u235_density
+
+
+@pytest.mark.parametrize("function, x0, x1, bracket", [
+    (translate_cell, -1.0, 1.0, (-5.0, 5.0)),
+    (rotate_cell, -45.0, 45.0, (-90.0, 90.0)),
+    (set_u235_density, 0.8, 1.2, (0.5, 1.5))
+])
+def test_integrator_add_keff_search_control(run_in_tmpdir, function, x0, x1, bracket):
+    """Test adding add_keff_search_control to integrator"""
+    model = make_model()
+    operator = CoupledOperator(model, CHAIN_PATH)
+    integrator = openmc.deplete.PredictorIntegrator(
+        operator, [1, 1], 0.0, timestep_units='d')
+
+    integrator.add_keff_search_control(
+        function=function,
+        x0=x0,
+        x1=x1,
+        bracket=bracket,
+        k_tol=0.1,
+        output=False,
+    )
+
+    assert integrator._keff_search_control.x0 == x0
+    assert integrator._keff_search_control.x1 == x1
+    assert integrator._keff_search_control.function == function
+    assert integrator._keff_search_control.search_kwargs['x_min'] == bracket[0]
+    assert integrator._keff_search_control.search_kwargs['x_max'] == bracket[1]
+    assert integrator._keff_search_control.search_kwargs['k_tol'] == 0.1
+    assert not integrator._keff_search_control.search_kwargs['output']
diff --git a/tests/unit_tests/test_deplete_microxs.py b/tests/unit_tests/test_deplete_microxs.py
index 073b3f162..0b1937fac 100644
--- a/tests/unit_tests/test_deplete_microxs.py
+++ b/tests/unit_tests/test_deplete_microxs.py
@@ -6,12 +6,15 @@ to a custom file with new depletion_chain node
 
 from os import remove
 from pathlib import Path
+from unittest.mock import patch
 
 import pytest
-from openmc.deplete import MicroXS
+import openmc
+from openmc.deplete import MicroXS, get_microxs_and_flux
 import numpy as np
 
 ONE_GROUP_XS = Path(__file__).parents[1] / "micro_xs_simple.csv"
+CHAIN_FILE = Path(__file__).parents[1] / "chain_simple.xml"
 
 
 def test_from_array():
@@ -111,3 +114,200 @@ def test_multigroup_flux_same():
         energies=energies, multigroup_flux=flux, chain_file=chain_file)
 
     assert microxs_4g.data == pytest.approx(microxs_2g.data)
+
+
+def test_microxs_zero_flux():
+    chain_file = Path(__file__).parents[1] / 'chain_simple.xml'
+
+    # Generate micro XS based on zero flux
+    energies = [0., 6.25e-1, 5.53e3, 8.21e5, 2.e7]
+    flux = [0.0, 0.0, 0.0, 0.0]
+    microxs = MicroXS.from_multigroup_flux(
+        energies=energies, multigroup_flux=flux, chain_file=chain_file)
+
+    # All microscopic cross sections should be zero
+    assert np.all(microxs.data == 0.0)
+
+
+def test_hybrid_tally_setup():
+    """In hybrid mode a 1-group RR tally is added alongside the flux tally."""
+    # Create a simple model with one material and a few nuclides for testing
+    model = openmc.Model()
+    mat = openmc.Material(components={'U235': 1.0, 'O16': 2.0})
+    sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere, fill=mat)
+    model.geometry = openmc.Geometry([cell])
+    model.settings.batches = 2
+    model.settings.particles = 10
+
+    # Define 2-group energy structure for the test
+    energies = [0., 0.625, 2.0e7]
+
+    # Function to replace Model.run and capture the tallies that were created
+    captured = {}
+    def capture_run(**kwargs):
+        captured['tallies'] = list(model.tallies)
+        raise StopIteration
+
+    # Call get_microxs_and_flux but replace Model.run with a function that
+    # captures the tallies and raises StopIteration to exit early
+    with patch.object(model, 'run', side_effect=capture_run):
+        with pytest.raises(StopIteration):
+            get_microxs_and_flux(
+                model, [mat],
+                nuclides=['U235', 'O16'],
+                reactions=['fission', '(n,gamma)'],
+                energies=energies,
+                reaction_rate_mode='flux',
+                reaction_rate_opts={'nuclides': ['U235'], 'reactions': ['fission']},
+                chain_file=CHAIN_FILE,
+            )
+
+    # Check that both tallies were created with the expected properties
+    tally_names = [t.name for t in captured['tallies']]
+    assert 'MicroXS flux 0' in tally_names
+    assert 'MicroXS RR 0' in tally_names
+
+    # Check that the RR tally has the expected nuclides and reactions
+    rr = next(t for t in captured['tallies'] if t.name == 'MicroXS RR 0')
+    assert rr.nuclides == ['U235']
+    assert rr.scores == ['fission']
+
+    # RR tally must use a 1-group energy filter spanning the full energy range
+    ef = next(f for f in rr.filters if isinstance(f, openmc.EnergyFilter))
+    assert len(ef.values) == 2
+    assert ef.values[0] == pytest.approx(energies[0])
+    assert ef.values[-1] == pytest.approx(energies[-1])
+
+
+def _simple_model():
+    model = openmc.Model()
+    mat = openmc.Material(components={'H1': 1.0, 'H2': 1.0},
+                          density=5.0, density_units='g/cm3')
+    sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere, fill=mat)
+    model.geometry = openmc.Geometry([cell])
+    model.settings.particles = 100
+    model.settings.batches = 5
+    model.settings.run_mode = 'fixed source'
+    return model, mat
+
+
+def test_hybrid_tally_defaults_to_all_nuclides(run_in_tmpdir):
+    energies = [0., 0.625, 2.0e7]
+    kwargs = {
+        'nuclides': ['H1', 'H2'],
+        'reactions': ['(n,2n)', '(n,gamma)'],
+        'energies': energies,
+        'reaction_rate_mode': 'flux',
+        'chain_file': CHAIN_FILE,
+    }
+
+    model, mat = _simple_model()
+    default_fluxes, default_micros = get_microxs_and_flux(
+        model, [mat], reaction_rate_opts={'reactions': ['(n,2n)']}, **kwargs
+    )
+
+    model, mat = _simple_model()
+    explicit_fluxes, explicit_micros = get_microxs_and_flux(
+        model, [mat],
+        reaction_rate_opts={
+            'nuclides': ['H1', 'H2'],
+            'reactions': ['(n,2n)']
+        },
+        **kwargs
+    )
+
+    np.testing.assert_allclose(default_fluxes[0], explicit_fluxes[0])
+    np.testing.assert_allclose(default_micros[0].data, explicit_micros[0].data)
+    assert default_micros[0].nuclides == explicit_micros[0].nuclides
+    assert default_micros[0].reactions == explicit_micros[0].reactions
+
+
+def test_flux_mode_returns_one_group_flux(run_in_tmpdir):
+    model, mat = _simple_model()
+    fluxes, micros = get_microxs_and_flux(
+        model, [mat],
+        nuclides=['H1'],
+        reactions=['(n,2n)'],
+        energies=[0., 0.625, 2.0e7],
+        reaction_rate_mode='flux',
+        chain_file=CHAIN_FILE,
+    )
+
+    assert fluxes[0].shape == (1,)
+    assert micros[0].data.shape == (1, 1, 1)
+    assert fluxes[0][0] > 0.0
+
+# ---------------------------------------------------------------------------
+# Tests for MicroXS.merge()
+# ---------------------------------------------------------------------------
+
+def _make_microxs(nuclides, reactions, values, groups=1):
+    """Helper: build a MicroXS from a flat list of values (nuclide-major order)."""
+    data = np.array(values, dtype=float).reshape(
+        len(nuclides), len(reactions), groups)
+    return MicroXS(data, nuclides, reactions)
+
+
+def test_merge_disjoint():
+    """Merging two MicroXS with no overlapping nuclides or reactions."""
+    m1 = _make_microxs(['U235', 'U238'], ['fission', '(n,gamma)'],
+                        [1., 2., 3., 4.])
+    m2 = _make_microxs(['Pu239'], ['(n,2n)'], [5.])
+
+    merged = m1.merge(m2)
+
+    assert merged.nuclides == ['U235', 'U238', 'Pu239']
+    assert merged.reactions == ['fission', '(n,gamma)', '(n,2n)']
+    assert merged.data.shape == (3, 3, 1)
+
+    # Self data preserved
+    assert merged['U235', 'fission'] == pytest.approx([1.])
+    assert merged['U238', '(n,gamma)'] == pytest.approx([4.])
+    # New data from other
+    assert merged['Pu239', '(n,2n)'] == pytest.approx([5.])
+    # Cross-terms that had no data should be zero
+    assert merged['U235', '(n,2n)'] == pytest.approx([0.])
+    assert merged['Pu239', 'fission'] == pytest.approx([0.])
+
+
+def test_merge_prefer_other():
+    """prefer='other': other overwrites shared entries, adds new ones."""
+    # m1: U235 and U238, reactions fission and (n,gamma)
+    m1 = _make_microxs(['U235', 'U238'], ['fission', '(n,gamma)'],
+                        [1., 2., 3., 4.])
+    # m2: only U235, reactions fission (overlap) and (n,2n) (new)
+    m2 = _make_microxs(['U235'], ['fission', '(n,2n)'], [9., 5.])
+
+    merged = m1.merge(m2)
+
+    # Nuclide/reaction sets
+    assert set(merged.nuclides) == {'U235', 'U238'}
+    assert set(merged.reactions) == {'fission', '(n,gamma)', '(n,2n)'}
+
+    # U235/fission overwritten by m2
+    assert merged['U235', 'fission'] == pytest.approx([9.])
+    # U235/(n,gamma) untouched
+    assert merged['U235', '(n,gamma)'] == pytest.approx([2.])
+    # New reaction added from m2
+    assert merged['U235', '(n,2n)'] == pytest.approx([5.])
+    # U238 data preserved
+    assert merged['U238', 'fission'] == pytest.approx([3.])
+    assert merged['U238', '(n,gamma)'] == pytest.approx([4.])
+    # U238/(n,2n) not in either → zero
+    assert merged['U238', '(n,2n)'] == pytest.approx([0.])
+
+
+def test_merge_prefer_self():
+    """prefer='self': shared pairs keep self's value; new entries use other's value."""
+    m1 = _make_microxs(['U235', 'U238'], ['fission', '(n,gamma)'],
+                        [1., 2., 3., 4.])
+    m2 = _make_microxs(['U235'], ['fission', '(n,2n)'], [9., 5.])
+
+    merged = m1.merge(m2, prefer='self')
+
+    # U235/fission: self wins
+    assert merged['U235', 'fission'] == pytest.approx([1.])
+    # U235/(n,2n): other used
+    assert merged['U235', '(n,2n)'] == pytest.approx([5.])
diff --git a/tests/unit_tests/test_deplete_restart.py b/tests/unit_tests/test_deplete_restart.py
index e8bfc062a..1cbff30a5 100644
--- a/tests/unit_tests/test_deplete_restart.py
+++ b/tests/unit_tests/test_deplete_restart.py
@@ -21,7 +21,7 @@ def test_restart_predictor_cecm(run_in_tmpdir):
     # Perform simulation using the predictor algorithm
     dt = [0.75]
     power = 1.0
-    openmc.deplete.PredictorIntegrator(op, dt, power).integrate()
+    openmc.deplete.PredictorIntegrator(op, dt, power).integrate(write_rates=True)
 
     # Load the files
     prev_res = openmc.deplete.Results(op.output_dir / "depletion_results.h5")
@@ -30,10 +30,6 @@ def test_restart_predictor_cecm(run_in_tmpdir):
     op = dummy_operator.DummyOperator(prev_res)
     op.output_dir = output_dir
 
-    # check ValueError is raised, indicating previous and current stages
-    with pytest.raises(ValueError, match="incompatible.* 1.*2"):
-        openmc.deplete.CECMIntegrator(op, dt, power)
-
 
 def test_restart_cecm_predictor(run_in_tmpdir):
     """Integral regression test of integrator algorithm using CE/CM for the
@@ -47,7 +43,7 @@ def test_restart_cecm_predictor(run_in_tmpdir):
     dt = [0.75]
     power = 1.0
     cecm = openmc.deplete.CECMIntegrator(op, dt, power)
-    cecm.integrate()
+    cecm.integrate(write_rates=True)
 
     # Load the files
     prev_res = openmc.deplete.Results(op.output_dir / "depletion_results.h5")
@@ -56,10 +52,6 @@ def test_restart_cecm_predictor(run_in_tmpdir):
     op = dummy_operator.DummyOperator(prev_res)
     op.output_dir = output_dir
 
-    # check ValueError is raised, indicating previous and current stages
-    with pytest.raises(ValueError, match="incompatible.* 2.*1"):
-        openmc.deplete.PredictorIntegrator(op, dt, power)
-
 
 @pytest.mark.parametrize("scheme", dummy_operator.SCHEMES)
 def test_restart(run_in_tmpdir, scheme):
@@ -70,7 +62,7 @@ def test_restart(run_in_tmpdir, scheme):
     operator = dummy_operator.DummyOperator()
 
     # take first step
-    bundle.solver(operator, [0.75], 1.0).integrate()
+    bundle.solver(operator, [0.75], 1.0).integrate(write_rates=True)
 
     # restart
     prev_res = openmc.deplete.Results(
diff --git a/tests/unit_tests/test_deplete_resultslist.py b/tests/unit_tests/test_deplete_resultslist.py
index b22a78650..bc1a078b0 100644
--- a/tests/unit_tests/test_deplete_resultslist.py
+++ b/tests/unit_tests/test_deplete_resultslist.py
@@ -1,10 +1,12 @@
 """Tests the Results class"""
 
-from pathlib import Path
 from math import inf
+from pathlib import Path
 
 import numpy as np
 import pytest
+
+import openmc
 import openmc.deplete
 
 
@@ -21,14 +23,14 @@ def test_get_activity(res):
 
     t_ref = np.array([0.0, 1296000.0, 2592000.0, 3888000.0])
     a_ref = np.array(
-        [1.25167956e+06, 3.71938527e+11, 4.43264300e+11, 3.55547176e+11])
+        [1.25167956e+06, 3.69842310e+11, 3.70099291e+11, 3.53629755e+11])
 
     np.testing.assert_allclose(t, t_ref)
     np.testing.assert_allclose(a, a_ref)
 
     # Check by_nuclide
     a_xe135_ref = np.array(
-        [2.106574218e+05, 1.227519888e+11, 1.177491828e+11, 1.031986176e+11])
+        [2.10657422e+05, 1.12825236e+11, 1.09055177e+11, 1.07491257e+11])
     t_nuc, a_nuc = res.get_activity("1", by_nuclide=True)
 
     a_xe135 = np.array([a_nuc_i["Xe135"] for a_nuc_i in a_nuc])
@@ -37,13 +39,43 @@ def test_get_activity(res):
     np.testing.assert_allclose(a_xe135, a_xe135_ref)
 
 
+def test_get_activity_chain_file(res, tmp_path):
+    """Tests evaluating activity with chain half-life data"""
+    _, a_endf = res.get_activity("1", by_nuclide=True, chain_file=False)
+    xe135_endf = np.array([a["Xe135"] for a in a_endf])
+
+    chain = openmc.deplete.Chain()
+    xe135 = openmc.deplete.Nuclide("Xe135")
+    xe135.half_life = openmc.data.half_life("Xe135") / 2.0
+    chain.add_nuclide(xe135)
+
+    t_chain, a_chain = res.get_activity("1", by_nuclide=True, chain_file=chain)
+    xe135_chain = np.array([a["Xe135"] for a in a_chain])
+
+    t_ref = np.array([0.0, 1296000.0, 2592000.0, 3888000.0])
+    np.testing.assert_allclose(t_chain, t_ref)
+    np.testing.assert_allclose(xe135_chain, 2.0 * xe135_endf)
+
+    chain_path = tmp_path / "chain.xml"
+    chain.export_to_xml(chain_path)
+    with openmc.config.patch('chain_file', chain_path):
+        _, a_config = res.get_activity("1", by_nuclide=True)
+    xe135_config = np.array([a["Xe135"] for a in a_config])
+    np.testing.assert_allclose(xe135_config, xe135_chain)
+
+    stable_chain = openmc.deplete.Chain()
+    stable_chain.add_nuclide(openmc.deplete.Nuclide("Xe135"))
+    _, a_stable = res.get_activity("1", by_nuclide=True, chain_file=stable_chain)
+    assert all(a["Xe135"] == 0.0 for a in a_stable)
+
+
 def test_get_atoms(res):
     """Tests evaluating single nuclide concentration."""
     t, n = res.get_atoms("1", "Xe135")
 
     t_ref = np.array([0.0, 1296000.0, 2592000.0, 3888000.0])
     n_ref = np.array(
-        [6.67473282e+08, 3.88942731e+14, 3.73091215e+14, 3.26987387e+14])
+        [6.67473282e+08, 3.57489567e+14, 3.45544042e+14, 3.40588723e+14])
 
     np.testing.assert_allclose(t, t_ref)
     np.testing.assert_allclose(n, n_ref)
@@ -71,7 +103,7 @@ def test_get_decay_heat(res):
 
     t_ref = np.array([0.0, 1296000.0, 2592000.0, 3888000.0])
     dh_ref = np.array(
-        [1.27933813e-09, 5.85347232e-03, 7.38773010e-03, 5.79954067e-03])
+        [1.27933813e-09, 5.95370258e-03, 6.01335600e-03, 5.69831173e-03])
 
     t, dh = res.get_decay_heat("1")
 
@@ -80,7 +112,7 @@ def test_get_decay_heat(res):
 
     # Check by nuclide
     dh_xe135_ref = np.array(
-        [1.27933813e-09, 7.45481920e-04, 7.15099509e-04, 6.26732849e-04])
+        [1.27933813e-09, 6.85196014e-04, 6.62300168e-04, 6.52802366e-04])
     t_nuc, dh_nuc = res.get_decay_heat("1", by_nuclide=True)
 
     dh_nuc_xe135 = np.array([dh_nuc_i["Xe135"] for dh_nuc_i in dh_nuc])
@@ -95,7 +127,7 @@ def test_get_mass(res):
 
     t_ref = np.array([0.0, 1296000.0, 2592000.0, 3888000.0])
     n_ref = np.array(
-        [6.67473282e+08, 3.88942731e+14, 3.73091215e+14, 3.26987387e+14])
+        [6.67473282e+08, 3.57489567e+14, 3.45544042e+14, 3.40588723e+14])
 
     # Get g
     n_ref *= openmc.data.atomic_mass('Xe135') / openmc.data.AVOGADRO
@@ -123,8 +155,8 @@ def test_get_reaction_rate(res):
     t, r = res.get_reaction_rate("1", "Xe135", "(n,gamma)")
 
     t_ref = [0.0, 1296000.0, 2592000.0, 3888000.0]
-    n_ref = [6.67473282e+08, 3.88942731e+14, 3.73091215e+14, 3.26987387e+14]
-    xs_ref = [2.53336104e-05, 4.21747011e-05, 3.48616127e-05, 3.61775563e-05]
+    n_ref = [6.67473282e+08, 3.57489567e+14, 3.45544042e+14, 3.40588723e+14]
+    xs_ref = [3.10220818e-05, 3.36754072e-05, 3.12740350e-05, 3.86717693e-05]
 
     np.testing.assert_allclose(t, t_ref)
     np.testing.assert_allclose(r, np.array(n_ref) * xs_ref)
@@ -136,8 +168,8 @@ def test_get_keff(res):
     t_min, k = res.get_keff(time_units='min')
 
     t_ref = [0.0, 1296000.0, 2592000.0, 3888000.0]
-    k_ref = [1.1596402556, 1.1914183335, 1.2292570871, 1.1797030302]
-    u_ref = [0.0270680649, 0.0219163444, 0.024268508 , 0.0221401194]
+    k_ref = [1.1773089172, 1.2231748584, 1.1611455694, 1.1714783649]
+    u_ref = [0.0384666252, 0.0311915665, 0.0226370102, 0.0315964732]
 
     np.testing.assert_allclose(t, t_ref)
     np.testing.assert_allclose(t_min * 60, t_ref)
@@ -220,5 +252,12 @@ def test_stepresult_get_material(res):
     # Spot check number densities
     densities = mat1.get_nuclide_atom_densities()
     assert densities['Xe135'] == pytest.approx(1e-14)
-    assert densities['I135'] == pytest.approx(1e-21)
     assert densities['U234'] == pytest.approx(1.00506e-05)
+
+
+def test_stepresult_get_material_mat_id_as_int(res):
+    # Get material at first timestep using int mat_id
+    step_result = res[0]
+    mat1 = step_result.get_material(1)
+    assert mat1.id == 1
+    assert mat1.volume == step_result.volume["1"]
diff --git a/tests/unit_tests/test_deplete_transfer_rates.py b/tests/unit_tests/test_deplete_transfer_rates.py
index 140777cd6..a6dcb2b27 100644
--- a/tests/unit_tests/test_deplete_transfer_rates.py
+++ b/tests/unit_tests/test_deplete_transfer_rates.py
@@ -16,6 +16,7 @@ CHAIN_PATH = Path(__file__).parents[1] / "chain_simple.xml"
 
 @pytest.fixture
 def model():
+    openmc.reset_auto_ids()
     f = openmc.Material(name="f")
     f.add_element("U", 1, percent_type="ao", enrichment=4.25)
     f.add_element("O", 2)
@@ -54,32 +55,35 @@ def model():
 
     return openmc.Model(geometry, materials, settings)
 
-@pytest.mark.parametrize("case_name, transfer_rates", [
-    ('elements', {'U': 0.01, 'Xe': 0.1}),
-    ('nuclides', {'I135': 0.01, 'Gd156': 0.1, 'Gd157': 0.01}),
+@pytest.mark.parametrize("case_name, transfer_rates, timesteps", [
+    ('elements', {'U': 0.01, 'Xe': 0.1}, None),
+    ('nuclides', {'I135': 0.01, 'Gd156': 0.1, 'Gd157': 0.01}, None),
     ('nuclides_elements', {'I135': 0.01, 'Gd156': 0.1, 'Gd157': 0.01, 'U': 0.01,
-                           'Xe': 0.1}),
+                           'Xe': 0.1}, None),
     ('elements_nuclides', {'U': 0.01, 'Xe': 0.1, 'I135': 0.01, 'Gd156': 0.1,
-                           'Gd157': 0.01}),
+                           'Gd157': 0.01}, None),
     ('multiple_transfer', {'U': 0.01, 'Xe': 0.1, 'I135': 0.01, 'Gd156': 0.1,
-                           'Gd157': 0.01}),
-    ('rates_invalid_1', {'Gd': 0.01, 'Gd157': 0.01, 'Gd156': 0.01}),
-    ('rates_invalid_2', {'Gd156': 0.01, 'Gd157': 0.01, 'Gd': 0.01}),
-    ('rates_invalid_3', {'Gb156': 0.01}),
-    ('rates_invalid_4', {'Gb': 0.01})
+                           'Gd157': 0.01}, None),
+    ('timesteps', {'U': 0.01, 'Xe': 0.1}, [1]),
+    ('rates_invalid_1', {'Gd': 0.01, 'Gd157': 0.01, 'Gd156': 0.01}, None),
+    ('rates_invalid_2', {'Gd156': 0.01, 'Gd157': 0.01, 'Gd': 0.01}, None),
+    ('rates_invalid_3', {'Gb156': 0.01}, None),
+    ('rates_invalid_4', {'Gb': 0.01}, None)
     ])
-def test_get_set(model, case_name, transfer_rates):
+def test_get_set(model, case_name, transfer_rates, timesteps):
     """Tests the get/set methods"""
-
-    openmc.reset_auto_ids()
     op = CoupledOperator(model, CHAIN_PATH)
-    transfer = TransferRates(op, model)
+    number_of_timesteps = 2
+    transfer = TransferRates(op, model.materials, number_of_timesteps)
+
+    if timesteps is None:
+        timesteps = np.arange(number_of_timesteps)
 
     # Test by Openmc material, material name and material id
     material, dest_material, dest_material2 = [m for m in model.materials
                                                if m.depletable]
     for material_input in [material, material.name, material.id]:
-        for dest_material_input in [dest_material, dest_material.name,
+        for dest_material_input in [None, dest_material, dest_material.name,
                                     dest_material.id]:
             if case_name == 'rates_invalid_1':
                 with pytest.raises(ValueError, match='Cannot add transfer '
@@ -117,14 +121,21 @@ def test_get_set(model, case_name, transfer_rates):
                 for component, transfer_rate in transfer_rates.items():
                     transfer.set_transfer_rate(material_input, [component],
                                                transfer_rate,
+                                               timesteps=timesteps,
                                                destination_material=\
                                                dest_material_input)
-                    assert transfer.get_transfer_rate(
-                        material_input, component)[0] == transfer_rate
-                    assert transfer.get_destination_material(
-                        material_input, component)[0] == str(dest_material.id)
-                assert transfer.get_components(material_input) == \
-                    transfer_rates.keys()
+                    assert transfer.get_external_rate(
+                        material_input, component, timesteps,
+                        dest_material_input)[0] == transfer_rate
+                    assert np.all(transfer.external_timesteps == timesteps)
+
+                if timesteps is not None:
+                    for timestep in timesteps:
+                        assert transfer.get_components(material_input, timestep,
+                            dest_material_input) == list(transfer_rates.keys())
+                else:
+                    assert transfer.get_components(material_input, timesteps,
+                            dest_material_input) == list(transfer_rates.keys())
 
             if case_name == 'multiple_transfer':
                 for dest2_material_input in [dest_material2, dest_material2.name,
@@ -135,10 +146,8 @@ def test_get_set(model, case_name, transfer_rates):
                                                destination_material=\
                                                dest2_material_input)
                         for id, dest_mat in zip([0,1],[dest_material,dest_material2]):
-                            assert transfer.get_transfer_rate(
-                                material_input, component)[id] == transfer_rate
-                            assert transfer.get_destination_material(
-                                material_input, component)[id] == str(dest_mat.id)
+                            assert transfer.get_external_rate(
+                                material_input, component, timesteps)[0] == transfer_rate
 
 @pytest.mark.parametrize("transfer_rate_units, unit_conv", [
     ('1/s', 1),
@@ -158,13 +167,15 @@ def test_units(transfer_rate_units, unit_conv, model):
     # create transfer rate Xe
     components = ['Xe', 'U235']
     transfer_rate = 1e-5
+    number_of_timesteps = 2
     op = CoupledOperator(model, CHAIN_PATH)
-    transfer = TransferRates(op, model)
+    transfer = TransferRates(op, model.materials, number_of_timesteps)
 
     for component in components:
         transfer.set_transfer_rate('f', [component], transfer_rate * unit_conv,
                                    transfer_rate_units=transfer_rate_units)
-        assert transfer.get_transfer_rate('f', component)[0] == transfer_rate
+        for timestep in range(transfer.number_of_timesteps):
+            assert transfer.get_external_rate('f', component, timestep)[0] == transfer_rate
 
 
 def test_transfer(run_in_tmpdir, model):
@@ -187,3 +198,35 @@ def test_transfer(run_in_tmpdir, model):
     # Ensure number of atoms equal transfer decay
     assert atoms[1] == pytest.approx(atoms[0]*exp(-transfer_rate*3600*24))
     assert atoms[2] == pytest.approx(atoms[1]*exp(-transfer_rate*3600*24))
+
+@pytest.mark.parametrize("case_name, buffer, ox", [
+    ('redox', {'Gd157':1}, {'Gd': 3, 'U': 4}),
+    ('buffer_invalid', {'Gd158':1}, {'Gd': 3, 'U': 4}),
+    ('elm_invalid', {'Gd157':1}, {'Gb': 3, 'U': 4}),
+    ])
+def test_redox(case_name, buffer, ox, model):
+    op = CoupledOperator(model, CHAIN_PATH)
+    number_of_timesteps = 2
+    transfer = TransferRates(op, model.materials, number_of_timesteps)
+
+    # Test by Openmc material, material name and material id
+    material, dest_material, dest_material2 = [m for m in model.materials
+                                               if m.depletable]
+    for material_input in [material, material.name, material.id]:
+        for dest_material_input in [dest_material, dest_material.name,
+                                    dest_material.id]:
+
+            if case_name == 'buffer_invalid':
+                with pytest.raises(ValueError, match='Gd158 is not a valid '
+                                   'nuclide.'):
+                    transfer.set_redox(material_input, buffer, ox)
+
+            elif case_name == 'elm_invalid':
+                with pytest.raises(ValueError, match='Gb is not a valid '
+                                   'element.'):
+                    transfer.set_redox(material_input, buffer, ox)
+            else:
+                transfer.set_redox(material_input, buffer, ox)
+                mat_id = transfer._get_material_id(material_input)
+                assert transfer.redox[mat_id][0] == buffer
+                assert transfer.redox[mat_id][1] == ox
diff --git a/tests/unit_tests/test_element.py b/tests/unit_tests/test_element.py
index d3555701e..82e526ec3 100644
--- a/tests/unit_tests/test_element.py
+++ b/tests/unit_tests/test_element.py
@@ -44,6 +44,13 @@ def test_expand_no_isotopes():
         element.expand(100.0, 'ao')
 
 
+def test_expand_ta():
+    ref = {'Ta181': 100.0}
+    element = openmc.Element('Ta')
+    for isotope in element.expand(100.0, 'ao'):
+        assert isotope[1] == approx(ref[isotope[0]])
+
+
 def test_expand_exceptions():
     """ Test that correct exceptions are raised for invalid input """
 
diff --git a/tests/unit_tests/test_endf.py b/tests/unit_tests/test_endf.py
index 1d4982054..cdd21d7a6 100644
--- a/tests/unit_tests/test_endf.py
+++ b/tests/unit_tests/test_endf.py
@@ -2,6 +2,17 @@ from openmc.data import endf
 from pytest import approx
 
 
+def test_evaluation_from_material(endf_data):
+    filename = f'{endf_data}/neutrons/n-001_H_001.endf'
+    material = endf.get_evaluations(filename)[0]
+    evaluation = endf.Evaluation(material)
+
+    assert evaluation.material == material.MAT
+    assert evaluation.gnds_name == 'H1'
+    assert evaluation.section == material.section_text
+    assert evaluation.reaction_list == material[1, 451]['section_list']
+
+
 def test_float_endf():
     assert endf.float_endf('+3.2146') == approx(3.2146)
     assert endf.float_endf('.12345') == approx(0.12345)
diff --git a/tests/unit_tests/test_filter_distribcell.py b/tests/unit_tests/test_filter_distribcell.py
new file mode 100644
index 000000000..d5734a2c0
--- /dev/null
+++ b/tests/unit_tests/test_filter_distribcell.py
@@ -0,0 +1,53 @@
+import openmc
+import pandas as pd
+
+
+def test_distribcell_filter_apply_tally_results(run_in_tmpdir):
+    # Reset IDs to ensure consistent paths
+    openmc.reset_auto_ids()
+
+    mat = openmc.Material()
+    mat.add_nuclide("U235", 1.0)
+    mat.set_density("g/cm3", 1.0)
+
+    # Define 2x2 lattice with a cylinder in each universe
+    cyl = openmc.ZCylinder(r=1.0)
+    cell1 = openmc.Cell(fill=mat, region=-cyl)
+    cell2 = openmc.Cell(fill=None, region=+cyl)
+    univ = openmc.Universe(cells=[cell1, cell2])
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-3.0, -3.0)
+    lattice.pitch = (3.0, 3.0)
+    lattice.universes = [[univ, univ], [univ, univ]]
+    box = openmc.model.RectangularPrism(6., 6., boundary_type='reflective')
+    root_cell = openmc.Cell(region=-box, fill=lattice)
+    geometry = openmc.Geometry([root_cell])
+
+    # Create model and add tally with distribcell filter
+    model = openmc.Model(geometry)
+    model.settings.batches = 10
+    model.settings.particles = 1000
+    tally = openmc.Tally()
+    distribcell_filter = openmc.DistribcellFilter(cell1)
+    tally.filters = [distribcell_filter]
+    tally.scores = ['flux']
+    model.tallies = [tally]
+
+    # Run OpenMC and apply tally results
+    model.run(apply_tally_results=True)
+
+    # Check that mean and standard deviation are available on tally
+    assert tally.mean.shape == (4, 1, 1)
+    assert tally.std_dev.shape == (4, 1, 1)
+
+    # Make sure paths attribute on filter is correct
+    assert distribcell_filter.paths == [
+        'u3->c3->l2(0,0)->u1->c1',
+        'u3->c3->l2(1,0)->u1->c1',
+        'u3->c3->l2(0,1)->u1->c1',
+        'u3->c3->l2(1,1)->u1->c1',
+    ]
+
+    # Check that we can get a DataFrame from the tally
+    df = tally.get_pandas_dataframe()
+    assert isinstance(df, pd.DataFrame)
diff --git a/tests/unit_tests/test_filter_mesh.py b/tests/unit_tests/test_filter_mesh.py
index a8bd4996d..e26bea337 100644
--- a/tests/unit_tests/test_filter_mesh.py
+++ b/tests/unit_tests/test_filter_mesh.py
@@ -259,3 +259,154 @@ def test_get_reshaped_data(run_in_tmpdir):
 
     assert data1.shape == (2, 19*3*2, 1, 1)
     assert data2.shape == (2, 19, 3, 2, 1, 1)
+
+def test_mesh_filter_rotation_roundtrip(run_in_tmpdir):
+    """Test that MeshFilter rotation works as expected"""
+
+
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = [-10, -10, -10]
+    mesh.upper_right = [10, 10, 10]
+    mesh.dimension = [2, 3, 4]
+
+    # check that rotatoin is round-tripped correctly for a set of angles
+    mesh_filter = openmc.MeshFilter(mesh)
+    mesh_filter.rotation = [0, 0, 90]  # Rotate around z-axis by 90 degrees
+
+    elem = mesh_filter.to_xml_element()
+    mesh_filter_xml = openmc.MeshFilter.from_xml_element(elem, meshes={mesh.id: mesh})
+    assert all(mesh_filter_xml.rotation == mesh_filter.rotation)
+
+    # check that rotation matrix is round-tripped correctly for a rotation matrix
+    mesh_filter.rotation = np.array([[0.7071, 0, 0.7071],
+                                     [0, 1, 0],
+                                     [-0.7071, 0, 0.7071]])
+
+    elem = mesh_filter.to_xml_element()
+    mesh_filter_xml = openmc.MeshFilter.from_xml_element(elem, meshes={mesh.id: mesh})
+    assert np.allclose(mesh_filter_xml.rotation, mesh_filter.rotation)
+
+
+def test_mesh_filter_dataframe_regular_3d():
+    """Test MeshFilter.get_pandas_dataframe with a 3D RegularMesh."""
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = [0, 0, 0]
+    mesh.upper_right = [3, 4, 5]
+    mesh.dimension = [3, 4, 5]
+
+    f = openmc.MeshFilter(mesh)
+    data_size = 3 * 4 * 5
+    df = f.get_pandas_dataframe(data_size, stride=1)
+
+    mesh_key = f'mesh {mesh.id}'
+    assert (mesh_key, 'x') in df.columns
+    assert (mesh_key, 'y') in df.columns
+    assert (mesh_key, 'z') in df.columns
+    assert len(df) == data_size
+    # x varies fastest (stride=1), then y (stride=3), then z (stride=12)
+    assert list(df[(mesh_key, 'x')][:3]) == [1, 2, 3]
+    assert df[(mesh_key, 'y')].iloc[0] == 1
+    assert df[(mesh_key, 'y')].iloc[3] == 2
+
+
+def test_mesh_filter_dataframe_regular_2d():
+    """Test MeshFilter.get_pandas_dataframe with a 2D RegularMesh."""
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = [0, 0]
+    mesh.upper_right = [2, 3]
+    mesh.dimension = [2, 3]
+
+    f = openmc.MeshFilter(mesh)
+    data_size = 2 * 3
+    df = f.get_pandas_dataframe(data_size, stride=1)
+
+    mesh_key = f'mesh {mesh.id}'
+    assert (mesh_key, 'x') in df.columns
+    assert (mesh_key, 'y') in df.columns
+    # Should NOT have a z column for a 2D mesh
+    assert (mesh_key, 'z') not in df.columns
+    assert len(df) == data_size
+
+
+def test_mesh_filter_dataframe_regular_1d():
+    """Test MeshFilter.get_pandas_dataframe with a 1D RegularMesh."""
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = [0]
+    mesh.upper_right = [5]
+    mesh.dimension = [5]
+
+    f = openmc.MeshFilter(mesh)
+    data_size = 5
+    df = f.get_pandas_dataframe(data_size, stride=1)
+
+    mesh_key = f'mesh {mesh.id}'
+    assert (mesh_key, 'x') in df.columns
+    assert (mesh_key, 'y') not in df.columns
+    assert (mesh_key, 'z') not in df.columns
+    assert len(df) == data_size
+    assert list(df[(mesh_key, 'x')]) == [1, 2, 3, 4, 5]
+
+
+def test_mesh_filter_dataframe_cylindrical():
+    """Test MeshFilter.get_pandas_dataframe with a CylindricalMesh."""
+    mesh = openmc.CylindricalMesh(
+        r_grid=[0.0, 1.0, 2.0],
+        phi_grid=[0.0, math.pi],
+        z_grid=[0.0, 5.0, 10.0]
+    )
+
+    f = openmc.MeshFilter(mesh)
+    nr, nphi, nz = mesh.dimension  # 2, 1, 2
+    data_size = nr * nphi * nz
+    df = f.get_pandas_dataframe(data_size, stride=1)
+
+    mesh_key = f'mesh {mesh.id}'
+    assert (mesh_key, 'r') in df.columns
+    assert (mesh_key, 'phi') in df.columns
+    assert (mesh_key, 'z') in df.columns
+    # Should NOT have x, y columns
+    assert (mesh_key, 'x') not in df.columns
+    assert (mesh_key, 'y') not in df.columns
+    assert len(df) == data_size
+    # r varies fastest
+    assert list(df[(mesh_key, 'r')][:nr]) == [1, 2]
+
+
+def test_mesh_filter_dataframe_spherical():
+    """Test MeshFilter.get_pandas_dataframe with a SphericalMesh."""
+    mesh = openmc.SphericalMesh(
+        r_grid=[0.0, 1.0, 2.0, 3.0],
+        theta_grid=[0, math.pi],
+        phi_grid=[0, 2 * math.pi]
+    )
+
+    f = openmc.MeshFilter(mesh)
+    nr, ntheta, nphi = mesh.dimension  # 3, 1, 1
+    data_size = nr * ntheta * nphi
+    df = f.get_pandas_dataframe(data_size, stride=1)
+
+    mesh_key = f'mesh {mesh.id}'
+    assert (mesh_key, 'r') in df.columns
+    assert (mesh_key, 'theta') in df.columns
+    assert (mesh_key, 'phi') in df.columns
+    assert (mesh_key, 'x') not in df.columns
+    assert len(df) == data_size
+
+
+def test_mesh_filter_dataframe_rectilinear():
+    """Test MeshFilter.get_pandas_dataframe with a RectilinearMesh."""
+    mesh = openmc.RectilinearMesh()
+    mesh.x_grid = [0.0, 1.0, 2.0]
+    mesh.y_grid = [0.0, 1.0, 2.0, 3.0]
+    mesh.z_grid = [0.0, 5.0]
+
+    f = openmc.MeshFilter(mesh)
+    nx, ny, nz = mesh.dimension  # 2, 3, 1
+    data_size = nx * ny * nz
+    df = f.get_pandas_dataframe(data_size, stride=1)
+
+    mesh_key = f'mesh {mesh.id}'
+    assert (mesh_key, 'x') in df.columns
+    assert (mesh_key, 'y') in df.columns
+    assert (mesh_key, 'z') in df.columns
+    assert len(df) == data_size
diff --git a/tests/unit_tests/test_filter_meshmaterial.py b/tests/unit_tests/test_filter_meshmaterial.py
new file mode 100644
index 000000000..a05ed5a12
--- /dev/null
+++ b/tests/unit_tests/test_filter_meshmaterial.py
@@ -0,0 +1,66 @@
+import numpy as np
+import openmc
+from pytest import approx
+
+
+def test_filter_mesh_material(run_in_tmpdir):
+    # Create four identical materials
+    openmc.reset_auto_ids()
+    materials = []
+    for i in range(4):
+        mat = openmc.Material()
+        mat.id = 10*(i+1)
+        mat.add_nuclide('Fe56', 1.0)
+        materials.append(mat)
+
+    # Create a slab model with four cells
+    z_values = [-10., -5., 0., 5., 10.]
+    planes = [openmc.ZPlane(z) for z in z_values]
+    planes[0].boundary_type = 'vacuum'
+    planes[-1].boundary_type = 'vacuum'
+    regions = [+left & -right for left, right in zip(planes[:-1], planes[1:])]
+    cells = [openmc.Cell(fill=m, region=r) for r, m in zip(regions, materials)]
+    model = openmc.Model()
+    model.geometry = openmc.Geometry(cells)
+    model.settings.particles = 1_000
+    model.settings.batches = 5
+    model.settings.run_mode = 'fixed source'
+
+    # Create a mesh that does not align with all planar surfaces
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-1., -1., -10.)
+    mesh.upper_right = (1., 1., 10.)
+    mesh.dimension = (1, 1, 5)
+
+    # Determine material volumes in each mesh element and use result to create a
+    # MeshMaterialFilter with corresponding bins
+    vols = mesh.material_volumes(model)
+    mmf = openmc.MeshMaterialFilter.from_volumes(mesh, vols)
+    expected_bins = [(0, 10), (1, 10), (1, 20), (2, 20), (2, 30), (3, 40), (3, 30), (4, 40)]
+    np.testing.assert_equal(mmf.bins, expected_bins)
+
+    # Create two tallies, one with a mesh filter and one with mesh-material
+    mesh_tally = openmc.Tally()
+    mesh_tally.filters = [openmc.MeshFilter(mesh)]
+    mesh_tally.scores = ['flux']
+    mesh_material_tally = openmc.Tally()
+    mesh_material_tally.filters = [mmf]
+    mesh_material_tally.scores = ['flux']
+    model.tallies = [mesh_tally, mesh_material_tally]
+
+    # Run model to get results on the two tallies
+    model.run(apply_tally_results=True)
+
+    # The sum of the flux in each mesh-material combination within a single mesh
+    # element should be equal to the flux in that mesh element
+    mesh_mean = mesh_tally.mean.ravel()
+    meshmat_mean = mesh_material_tally.mean.ravel()
+    assert mesh_mean[0] == approx(meshmat_mean[0])
+    assert mesh_mean[1] == approx(meshmat_mean[1] + meshmat_mean[2])
+    assert mesh_mean[2] == approx(meshmat_mean[3] + meshmat_mean[4])
+    assert mesh_mean[3] == approx(meshmat_mean[5] + meshmat_mean[6])
+    assert mesh_mean[4] == approx(meshmat_mean[7])
+    assert mesh_tally.mean.sum() == approx(mesh_material_tally.mean.sum())
+
+    # Make sure get_pandas_dataframe method works
+    mesh_material_tally.get_pandas_dataframe()
diff --git a/tests/unit_tests/test_filter_reaction.py b/tests/unit_tests/test_filter_reaction.py
new file mode 100644
index 000000000..df8fc9d7b
--- /dev/null
+++ b/tests/unit_tests/test_filter_reaction.py
@@ -0,0 +1,87 @@
+import pytest
+import openmc
+
+
+def test_reaction_filter_construction_with_strings():
+    f = openmc.ReactionFilter(['(n,elastic)', '(n,gamma)'])
+    assert len(f.bins) == 2
+    assert f.bins[0] == '(n,elastic)'
+    assert f.bins[1] == '(n,gamma)'
+
+
+def test_reaction_filter_construction_with_mt():
+    f = openmc.ReactionFilter([2, 102])
+    assert len(f.bins) == 2
+    assert f.bins[0] == '(n,elastic)'
+    assert f.bins[1] == '(n,gamma)'
+
+
+def test_reaction_filter_mixed():
+    f = openmc.ReactionFilter([2, '(n,gamma)'])
+    assert f.bins[0] == '(n,elastic)'
+    assert f.bins[1] == '(n,gamma)'
+
+
+def test_reaction_filter_single_bin_string():
+    f = openmc.ReactionFilter('(n,elastic)')
+    assert len(f.bins) == 1
+    assert f.bins[0] == '(n,elastic)'
+
+
+def test_reaction_filter_single_bin_mt():
+    f = openmc.ReactionFilter(2)
+    assert len(f.bins) == 1
+    assert f.bins[0] == '(n,elastic)'
+
+
+def test_reaction_filter_single_bin_naming():
+    f = openmc.ReactionFilter('total')
+    assert len(f.bins) == 1
+    assert f.bins[0] == '(n,total)'
+
+
+def test_reaction_filter_invalid_mt():
+    with pytest.raises(ValueError, match="No known reaction"):
+        openmc.ReactionFilter([999999])
+
+
+def test_reaction_filter_invalid_string():
+    with pytest.raises(ValueError, match="Unknown reaction name"):
+        openmc.ReactionFilter(['not-a-reaction'])
+
+
+def test_reaction_filter_invalid_type():
+    with pytest.raises(TypeError, match="Expected str or int"):
+        openmc.ReactionFilter([3.14])
+
+
+def test_reaction_filter_xml_roundtrip():
+    f = openmc.ReactionFilter([2, 102], filter_id=42)
+    elem = f.to_xml_element()
+    f2 = openmc.ReactionFilter.from_xml_element(elem)
+    assert f2.id == 42
+    assert len(f2.bins) == 2
+    assert f2.bins[0] == '(n,elastic)'
+    assert f2.bins[1] == '(n,gamma)'
+
+
+def test_reaction_filter_num_bins():
+    f = openmc.ReactionFilter(['(n,elastic)', '(n,fission)', '(n,gamma)'])
+    assert f.num_bins == 3
+
+
+def test_reaction_filter_repr():
+    f = openmc.ReactionFilter([2, 102])
+    r = repr(f)
+    assert 'ReactionFilter' in r
+
+
+def test_reaction_filter_short_name():
+    assert openmc.ReactionFilter.short_name == 'Reaction'
+
+
+def test_reaction_filter_total_warning():
+    """Test that using 'total' emits a warning about ambiguity."""
+    with pytest.warns(UserWarning, match="ambiguous"):
+        f = openmc.ReactionFilter(['total'])
+    assert f.bins[0] == '(n,total)'
diff --git a/tests/unit_tests/test_filter_weight.py b/tests/unit_tests/test_filter_weight.py
new file mode 100644
index 000000000..878929ee0
--- /dev/null
+++ b/tests/unit_tests/test_filter_weight.py
@@ -0,0 +1,44 @@
+import openmc
+import numpy as np
+
+
+def test_weightfilter(run_in_tmpdir):
+    steel = openmc.Material(name='Stainless Steel')
+    steel.set_density('g/cm3', 8.00)
+    steel.add_nuclide('Fe56', 1.0)
+
+    sphere = openmc.Sphere(r=50.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere, fill=steel)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell])
+    model.settings.particles = 100
+    model.settings.batches = 10
+
+    model.settings.source = openmc.IndependentSource(
+        energy=openmc.stats.delta_function(14e6),
+    )
+    model.settings.run_mode = "fixed source"
+
+    radius = list(range(1, 50))
+    sphere_mesh = openmc.SphericalMesh(radius)
+    mesh_filter = openmc.MeshFilter(sphere_mesh)
+    weight_filter = openmc.WeightFilter(
+        [0.999, 0.9999, 0.99999, 0.999999, 1.0, 1.000001 ,1.00001, 1.0001, 1.001]
+    )
+
+    tally = openmc.Tally()
+    tally.filters = [mesh_filter, weight_filter]
+    tally.estimator = 'analog'
+    tally.scores = ['flux']
+    model.tallies = openmc.Tallies([tally])
+
+    # Run OpenMC
+    model.run(apply_tally_results=True)
+
+    # Get current binned by mu
+    neutron_flux = tally.mean.reshape(48, 8)
+
+    # All contributions should show up in the fourth bin
+    assert np.all(neutron_flux[:, 3] != 0.0)
+    neutron_flux[:, 3] = 0.0
+    assert np.all(neutron_flux == 0.0)
diff --git a/tests/unit_tests/test_filters.py b/tests/unit_tests/test_filters.py
index 55bb62075..2e7481c87 100644
--- a/tests/unit_tests/test_filters.py
+++ b/tests/unit_tests/test_filters.py
@@ -17,14 +17,16 @@ def box_model():
     model.settings.particles = 100
     model.settings.batches = 10
     model.settings.inactive = 0
-    model.settings.source = openmc.IndependentSource(space=openmc.stats.Point())
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Point())
     return model
 
 
 def test_cell_instance():
     c1 = openmc.Cell()
     c2 = openmc.Cell()
-    f = openmc.CellInstanceFilter([(c1, 0), (c1, 1), (c1, 2), (c2, 0), (c2, 1)])
+    f = openmc.CellInstanceFilter(
+        [(c1, 0), (c1, 1), (c1, 2), (c2, 0), (c2, 1)])
 
     # Make sure __repr__ works
     repr(f)
@@ -234,7 +236,8 @@ def test_first_moment(run_in_tmpdir, box_model):
         flux, scatter = sp.tallies[plain_tally.id].mean.ravel()
 
         # Check that first moment matches
-        first_score = lambda t: sp.tallies[t.id].mean.ravel()[0]
+        def first_score(t):
+            return sp.tallies[t.id].mean.ravel()[0]
         assert first_score(leg_tally) == scatter
         assert first_score(leg_sptl_tally) == scatter
         assert first_score(sph_scat_tally) == scatter
@@ -258,7 +261,8 @@ def test_lethargy_bin_width():
     assert len(f.lethargy_bin_width) == 175
     energy_bins = openmc.mgxs.GROUP_STRUCTURES['VITAMIN-J-175']
     assert f.lethargy_bin_width[0] == np.log10(energy_bins[1]/energy_bins[0])
-    assert f.lethargy_bin_width[-1] == np.log10(energy_bins[-1]/energy_bins[-2])
+    assert f.lethargy_bin_width[-1] == np.log10(
+        energy_bins[-1]/energy_bins[-2])
 
 
 def test_energyfunc():
@@ -292,5 +296,172 @@ def test_tabular_from_energyfilter():
     # 'histogram' is the default
     assert tab.interpolation == 'histogram'
 
-    tab = efilter.get_tabular(values=np.array([10, 10, 5]), interpolation='linear-linear')
+    tab = efilter.get_tabular(values=np.array(
+        [10, 10, 5]), interpolation='linear-linear')
     assert tab.interpolation == 'linear-linear'
+
+
+def test_energy_filter():
+
+    # testing that bins descending value raises error
+    msg = "Values 1.0 and 0.5 appear to be out of order"
+    with raises(ValueError, match=msg):
+        openmc.EnergyFilter([0.0, 1.0, 0.5])
+
+    # testing that bins with same value raises error
+    msg = "Values 0.25 and 0.25 appear to be out of order"
+    with raises(ValueError, match=msg):
+        openmc.EnergyFilter([0.0, 0.25, 0.25])
+
+    # testing that negative bins values raises error
+    msg = 'Unable to set "filter value" to "-1.2" since it is less than "0.0"'
+    with raises(ValueError, match=msg):
+        openmc.EnergyFilter([-1.2, 0.25, 0.5])
+
+
+def test_particle_production_filter():
+    energy_bins = [1e3, 1e4, 1e5, 1e6]
+
+    # --- Single particle with energy bins ---
+    f = openmc.ParticleProductionFilter('photon', energy_bins)
+
+    # particles getter always returns a list
+    assert isinstance(f.particles, list)
+    assert len(f.particles) == 1
+    assert f.particles[0] == openmc.ParticleType.PHOTON
+
+    assert f.num_energy_bins == 3
+    assert f.num_bins == 3
+    assert f.shape == (1, 3)
+    assert len(f.bins) == 3
+    # Each bin is (particle_name, e_low, e_high)
+    assert f.bins[0] == ('photon', 1e3, 1e4)
+    assert f.bins[2] == ('photon', 1e5, 1e6)
+
+    # __repr__ check
+    repr(f)
+
+    # to_xml_element()
+    elem = f.to_xml_element()
+    assert elem.tag == 'filter'
+    assert elem.attrib['type'] == 'particleproduction'
+    assert elem.find('particles').text == 'photon'
+    assert elem.find('energies').text.split()[0] == str(energy_bins[0])
+
+    # from_xml_element()
+    new_f = openmc.Filter.from_xml_element(elem)
+    assert new_f.id == f.id
+    assert new_f.particles == f.particles
+    assert np.allclose(new_f.energies, f.energies)
+
+    # pandas output (with energy bins -> 3 MultiIndex columns)
+    df = f.get_pandas_dataframe(data_size=3, stride=1)
+    assert df.shape[0] == 3
+    assert ('particleproduction', 'particle') in df.columns
+    assert ('particleproduction', 'energy low [eV]') in df.columns
+    assert ('particleproduction', 'energy high [eV]') in df.columns
+
+    # --- Multiple particles with energy bins ---
+    f2 = openmc.ParticleProductionFilter(['photon', 'neutron'], energy_bins)
+    assert len(f2.particles) == 2
+    assert f2.num_bins == 6  # 2 particles * 3 energy bins
+    assert f2.shape == (2, 3)
+    assert len(f2.bins) == 6
+    # First 3 bins are photon, next 3 are neutron
+    assert f2.bins[0] == ('photon', 1e3, 1e4)
+    assert f2.bins[3] == ('neutron', 1e3, 1e4)
+
+    df2 = f2.get_pandas_dataframe(data_size=6, stride=1)
+    assert df2.shape[0] == 6
+    assert list(df2[('particleproduction', 'particle')]) == \
+        ['photon'] * 3 + ['neutron'] * 3
+
+    # XML round-trip
+    elem2 = f2.to_xml_element()
+    new_f2 = openmc.Filter.from_xml_element(elem2)
+    assert len(new_f2.particles) == 2
+    assert np.allclose(new_f2.energies, energy_bins)
+
+    # --- Multiple particles without energy bins ---
+    f3 = openmc.ParticleProductionFilter(['photon', 'neutron', 'electron'])
+    assert f3.energies is None
+    assert f3.num_bins == 3
+    assert f3.num_energy_bins == 1
+    assert f3.shape == (3, 1)
+    assert f3.bins == ['photon', 'neutron', 'electron']
+
+    repr(f3)
+
+    df3 = f3.get_pandas_dataframe(data_size=3, stride=1)
+    assert df3.shape[0] == 3
+    assert ('particleproduction', 'particle') in df3.columns
+    # Should not have energy columns
+    assert ('particleproduction', 'energy low [eV]') not in df3.columns
+
+    # XML round-trip without energies
+    elem3 = f3.to_xml_element()
+    assert elem3.find('energies') is None
+    new_f3 = openmc.Filter.from_xml_element(elem3)
+    assert new_f3.energies is None
+    assert len(new_f3.particles) == 3
+
+    # --- Energies from group structure name ---
+    f4 = openmc.ParticleProductionFilter('photon', energies='CCFE-709')
+    expected = openmc.mgxs.GROUP_STRUCTURES['CCFE-709']
+    assert np.allclose(f4.energies, expected)
+    assert f4.num_energy_bins == len(expected) - 1
+    assert f4.num_bins == len(expected) - 1
+
+
+def test_weight():
+    f = openmc.WeightFilter([0.01, 0.1, 1.0, 10.0])
+    expected_bins = [[0.01, 0.1], [0.1, 1.0], [1.0, 10.0]]
+
+    assert np.allclose(f.bins, expected_bins)
+    assert len(f.bins) == 3
+
+    # Make sure __repr__ works
+    repr(f)
+
+    # to_xml_element()
+    elem = f.to_xml_element()
+    assert elem.tag == 'filter'
+    assert elem.attrib['type'] == 'weight'
+
+    # from_xml_element()
+    new_f = openmc.Filter.from_xml_element(elem)
+    assert new_f.id == f.id
+    assert np.allclose(new_f.bins, f.bins)
+
+
+def test_mesh_material():
+    mat1 = openmc.Material()
+    mat2 = openmc.Material()
+
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-1., -1., -1.)
+    mesh.upper_right = (1., 1., 1.)
+    mesh.dimension = (2, 4, 1)
+    bins = [(0, mat1), (0, mat2), (6, mat1), (7, mat2)]
+    f = openmc.MeshMaterialFilter(mesh, bins)
+
+    expected_bins = [(0, mat1.id), (0, mat2.id), (6, mat1.id), (7, mat2.id)]
+    assert np.allclose(f.bins, expected_bins)
+    assert f.mesh == mesh
+    assert f.shape == (4,)
+
+    # to_xml_element()
+    elem = f.to_xml_element()
+    assert elem.tag == 'filter'
+    assert elem.attrib['type'] == 'meshmaterial'
+
+    # from_xml_element()
+    new_f = openmc.Filter.from_xml_element(elem, meshes={mesh.id: mesh})
+    assert isinstance(new_f, openmc.MeshMaterialFilter)
+    assert new_f.id == f.id
+    assert new_f.mesh == f.mesh
+    assert np.allclose(new_f.bins, expected_bins)
+
+    # Test hash and str
+    hash(f)
+    str(f)
diff --git a/tests/unit_tests/test_ifp.py b/tests/unit_tests/test_ifp.py
new file mode 100644
index 000000000..e527f1624
--- /dev/null
+++ b/tests/unit_tests/test_ifp.py
@@ -0,0 +1,88 @@
+"""Test the Iterated Fission Probability (IFP) method to compute adjoint-weighted
+kinetics parameters using dedicated tallies."""
+
+import pytest
+import openmc
+
+
+def test_xml_serialization(run_in_tmpdir):
+    """Check that a simple use case can be written and read in XML."""
+    parameter = 5
+    settings = openmc.Settings()
+    settings.ifp_n_generation = parameter
+    settings.export_to_xml()
+
+    read_settings = openmc.Settings.from_xml()
+    assert read_settings.ifp_n_generation == parameter
+
+
+@pytest.fixture(scope="module")
+def geometry():
+    openmc.reset_auto_ids()
+    material = openmc.Material()
+    material.add_nuclide("U235", 1.0)
+    sphere = openmc.Sphere(r=1.0, boundary_type="vacuum")
+    cell = openmc.Cell(region=-sphere, fill=material)
+    return openmc.Geometry([cell])
+
+
+@pytest.mark.parametrize(
+    "options, error",
+    [
+        ({"ifp_n_generation": 0}, ValueError),
+        ({"ifp_n_generation": -1}, ValueError),
+        ({"run_mode": "fixed source"}, RuntimeError),
+        ({"inactive": 5, "ifp_n_generation": 6}, RuntimeError),
+        ({"inactive": 9}, RuntimeError)
+    ],
+)
+def test_exceptions(options, error, run_in_tmpdir, geometry):
+    """Test settings configuration that should return an error."""
+    with pytest.raises(error):
+        settings = openmc.Settings(**options)
+        settings.particles = 100
+        settings.batches = 15
+        tally = openmc.Tally(name="ifp-scores")
+        tally.scores = ["ifp-time-numerator", "ifp-beta-numerator", "ifp-denominator"]
+        tallies = openmc.Tallies([tally])
+        model = openmc.Model(geometry=geometry, settings=settings, tallies=tallies)
+        model.run()
+
+
+@pytest.mark.parametrize(
+    "num_groups, use_auto_tallies",
+    [
+        (None, True),
+        (None, False),
+        (6, True),
+        (6, False),
+    ],
+)
+def test_get_kinetics_parameters(run_in_tmpdir, geometry, num_groups, use_auto_tallies):
+    # Create basic model
+    model = openmc.Model(geometry=geometry)
+    model.settings.particles = 1000
+    model.settings.batches = 20
+    model.settings.inactive = 5
+    model.settings.ifp_n_generation = 5
+
+    # Add IFP tallies either via the convenience method or manually
+    if use_auto_tallies:
+        model.add_kinetics_parameters_tallies(num_groups=num_groups)
+    else:
+        for score in ["ifp-time-numerator", "ifp-beta-numerator", "ifp-denominator"]:
+            tally = openmc.Tally()
+            tally.scores = [score]
+            if score == "ifp-beta-numerator" and num_groups is not None:
+                tally.filters = [openmc.DelayedGroupFilter(list(range(1, num_groups + 1)))]
+            model.tallies.append(tally)
+
+    # Run and get kinetics parameters
+    sp_file = model.run()
+    with openmc.StatePoint(sp_file) as sp:
+        params = sp.get_kinetics_parameters()
+    assert isinstance(params, openmc.KineticsParameters)
+    assert params.generation_time is not None
+    assert params.beta_effective is not None
+    if num_groups is not None:
+        assert len(params.beta_effective) == num_groups
diff --git a/tests/unit_tests/test_lib.py b/tests/unit_tests/test_lib.py
index 8ab35335f..4cfae0df2 100644
--- a/tests/unit_tests/test_lib.py
+++ b/tests/unit_tests/test_lib.py
@@ -5,6 +5,7 @@ import os
 import numpy as np
 import pytest
 import openmc
+from openmc.examples import random_ray_pin_cell
 import openmc.exceptions as exc
 import openmc.lib
 
@@ -83,6 +84,19 @@ def uo2_trigger_model():
         yield
 
 
+@pytest.fixture(scope='module')
+def random_ray_pincell_model():
+    """Set up a random ray model to test with and delete files when done"""
+    openmc.reset_auto_ids()
+    # Write XML and MGXS files in tmpdir
+    with cdtemp():
+        model = random_ray_pin_cell()
+        model.settings.batches = 200
+        model.settings.inactive = 50
+        model.settings.particles = 50
+        model.export_to_xml()
+        yield
+
 @pytest.fixture(scope='module')
 def lib_init(pincell_model, mpi_intracomm):
     openmc.lib.init(intracomm=mpi_intracomm)
@@ -159,6 +173,34 @@ def test_properties_temperature(lib_init):
     assert cell.get_temperature() == pytest.approx(200.0)
 
 
+def test_cell_density(lib_init):
+    cell = openmc.lib.cells[1]
+    print('density', cell.get_density())
+    orig_density = cell.get_density()
+    try:
+        cell.set_density(1.5, 0)
+        assert cell.get_density(0) == pytest.approx(1.5)
+        cell.set_density(2.0)
+        assert cell.get_density() == pytest.approx(2.0)
+    finally:
+        cell.set_density(orig_density)
+
+
+def test_properties_cell_density(lib_init):
+    # Cell density should be 2.0 from above test
+    cell = openmc.lib.cells[1]
+    orig_density = cell.get_density()
+
+    # Export properties and change density
+    openmc.lib.export_properties('properties.h5')
+    cell.set_density(3.0)
+    assert cell.get_density() == pytest.approx(3.0)
+
+    # Import properties and check that density is restored
+    openmc.lib.import_properties('properties.h5')
+    assert cell.get_density() == pytest.approx(orig_density)
+
+
 def test_new_cell(lib_init):
     with pytest.raises(exc.AllocationError):
         openmc.lib.Cell(1)
@@ -209,9 +251,9 @@ def test_material(lib_init):
     m.name = "Not hot borated water"
     assert m.name == "Not hot borated water"
 
-    assert m.depletable == False
+    assert not m.depletable
     m.depletable = True
-    assert m.depletable == True
+    assert m.depletable
 
 
 def test_properties_density(lib_init):
@@ -468,9 +510,6 @@ def test_set_n_batches(lib_run):
 
     for i in range(7):
         openmc.lib.next_batch()
-    # Setting n_batches less than current_batch should raise error
-    with pytest.raises(exc.InvalidArgumentError):
-        settings.set_batches(6)
     # n_batches should stay the same
     assert settings.get_batches() == 10
 
@@ -583,6 +622,13 @@ def test_regular_mesh(lib_init):
         assert isinstance(mesh, openmc.lib.RegularMesh)
         assert mesh_id == mesh.id
 
+    rotation = (180.0, 0.0, 0.0)
+
+    mf = openmc.lib.MeshFilter(mesh)
+    assert mf.mesh == mesh
+    mf.rotation = rotation
+    assert np.allclose(mf.rotation, rotation)
+
     translation = (1.0, 2.0, 3.0)
 
     mf = openmc.lib.MeshFilter(mesh)
@@ -849,22 +895,23 @@ def test_load_nuclide(lib_init):
         openmc.lib.load_nuclide('Pu3')
 
 
+class LegacySlicePlot:
+    origin = (0.0, 0.0, 0.0)
+    width = 1.26
+    height = 1.26
+    basis = 'xy'
+    h_res = 3
+    v_res = 3
+    level = -1
+
+
 def test_id_map(lib_init):
     expected_ids = np.array([[(3, 0, 3), (2, 0, 2), (3, 0, 3)],
                              [(2, 0, 2), (1, 0, 1), (2, 0, 2)],
                              [(3, 0, 3), (2, 0, 2), (3, 0, 3)]], dtype='int32')
 
-    # create a plot object
-    s = openmc.lib.plot._PlotBase()
-    s.width = 1.26
-    s.height = 1.26
-    s.v_res = 3
-    s.h_res = 3
-    s.origin = (0.0, 0.0, 0.0)
-    s.basis = 'xy'
-    s.level = -1
-
-    ids = openmc.lib.plot.id_map(s)
+    with pytest.warns(FutureWarning, match="deprecated"):
+        ids = openmc.lib.id_map(LegacySlicePlot())
     assert np.array_equal(expected_ids, ids)
 
 
@@ -874,20 +921,81 @@ def test_property_map(lib_init):
          [ (293.6, 6.55), (293.6, 10.29769),  (293.6, 6.55)],
          [(293.6, 0.740582), (293.6, 6.55), (293.6, 0.740582)]], dtype='float')
 
-    # create a plot object
-    s = openmc.lib.plot._PlotBase()
-    s.width = 1.26
-    s.height = 1.26
-    s.v_res = 3
-    s.h_res = 3
-    s.origin = (0.0, 0.0, 0.0)
-    s.basis = 'xy'
-    s.level = -1
-
-    properties = openmc.lib.plot.property_map(s)
+    with pytest.warns(FutureWarning, match="deprecated"):
+        properties = openmc.lib.property_map(LegacySlicePlot())
     assert np.allclose(expected_properties, properties, atol=1e-04)
 
 
+def test_slice_data(lib_init):
+    expected_properties = np.array(
+        [[(293.6, 0.740582), (293.6, 6.55), (293.6, 0.740582)],
+         [ (293.6, 6.55), (293.6, 10.29769),  (293.6, 6.55)],
+         [(293.6, 0.740582), (293.6, 6.55), (293.6, 0.740582)]], dtype='float')
+    origin = (0.0, 0.0, 0.0)
+    _, properties = openmc.lib.slice_data(
+        origin,
+        width=(1.26, 1.26),
+        basis='xy',
+        pixels=(3, 3),
+        include_properties=True
+    )
+    assert np.allclose(expected_properties, properties, atol=1e-04)
+
+
+def test_solid_raytrace_plot(lib_init, pincell_model):
+    # Ensure plot mapping can be accessed and grows after allocation
+    n0 = len(openmc.lib.plots)
+    plot = openmc.lib.SolidRayTracePlot()
+    assert len(openmc.lib.plots) == n0 + 1
+    assert plot.id in openmc.lib.plots
+    assert openmc.lib.plots[plot.id] is plot
+
+    # Exercise plot property getters/setters
+    plot.pixels = (8, 6)
+    assert plot.pixels == (8, 6)
+
+    plot.color_by = openmc.lib.SolidRayTracePlot.COLOR_BY_MATERIAL
+    assert plot.color_by == openmc.lib.SolidRayTracePlot.COLOR_BY_MATERIAL
+
+    plot.camera_position = (2.0, 0.0, 1.0)
+    plot.look_at = (0.0, 0.0, 0.0)
+    plot.up = (0.0, 0.0, 1.0)
+    plot.light_position = (3.0, 2.0, 4.0)
+    plot.fov = 60.0
+    plot.diffuse_fraction = 0.4
+    assert plot.camera_position == pytest.approx((2.0, 0.0, 1.0))
+    assert plot.look_at == pytest.approx((0.0, 0.0, 0.0))
+    assert plot.up == pytest.approx((0.0, 0.0, 1.0))
+    assert plot.light_position == pytest.approx((3.0, 2.0, 4.0))
+    assert plot.fov == pytest.approx(60.0)
+    assert plot.diffuse_fraction == pytest.approx(0.4)
+
+    # Exercise color/visibility CAPI wrappers
+    plot.set_default_colors()
+    plot.set_color(1, (12, 34, 56))
+    assert plot.get_color(1) == (12, 34, 56)
+    plot.set_visibility(1, False)
+    plot.set_visibility(1, True)
+
+    # Confirm image creation path works and dimensions match pixels
+    plot.update_view()
+    image = plot.create_image()
+    assert image.shape == (6, 8, 3)
+    assert image.dtype == np.uint8
+
+    # Change some properties and confirm image changes
+    plot.set_color(1, (255, 0, 0))
+    plot.update_view()
+    image2 = plot.create_image()
+    assert not np.array_equal(image, image2)
+
+    # Solid raytrace uses Phong/diffuse shading, so rendered RGB values are
+    # generally modulated and need not exactly match the assigned palette.
+    changed = np.any(image != image2, axis=2)
+    assert np.any(changed)
+    assert np.mean(image2[..., 0][changed]) > np.mean(image[..., 0][changed])
+
+
 def test_position(lib_init):
 
     pos = openmc.lib.plot._Position(1.0, 2.0, 3.0)
@@ -1014,9 +1122,29 @@ def test_sample_external_source(run_in_tmpdir, mpi_intracomm):
         assert p1.time == p2.time
         assert p1.wgt == p2.wgt
 
+    # as_array should return a numpy structured array with matching values
+    arr = openmc.lib.sample_external_source(10, prn_seed=3, as_array=True)
+    assert isinstance(arr, np.ndarray)
+    assert len(arr) == 10
+    for p, row in zip(particles, arr):
+        assert p.r == pytest.approx(row['r'])
+        assert p.E == pytest.approx(row['E'])
+
     openmc.lib.finalize()
 
     # Make sure sampling works in volume calculation mode
     openmc.lib.init(["-c"])
     openmc.lib.sample_external_source(100)
     openmc.lib.finalize()
+
+
+def test_random_ray(random_ray_pincell_model, mpi_intracomm):
+    openmc.lib.finalize()
+    openmc.lib.init(intracomm=mpi_intracomm)
+    openmc.lib.simulation_init()
+    openmc.lib.run_random_ray()
+    keff = openmc.lib.keff()
+
+    assert keff[0]==pytest.approx(1.3236826574065745)
+
+    openmc.lib.finalize()
diff --git a/tests/unit_tests/test_material.py b/tests/unit_tests/test_material.py
index cb71cd09b..12d363376 100644
--- a/tests/unit_tests/test_material.py
+++ b/tests/unit_tests/test_material.py
@@ -3,8 +3,11 @@ from pathlib import Path
 
 import pytest
 
+import numpy as np
+
 import openmc
 from openmc.data import decay_photon_energy
+from openmc.deplete import Chain, Nuclide
 import openmc.examples
 import openmc.model
 import openmc.stats
@@ -74,6 +77,13 @@ def test_add_components():
     with pytest.raises(ValueError):
         m.add_components({'H1': 1.0}, percent_type = 'oa')
 
+
+def test_id():
+    openmc.Material(material_id=0)
+    with pytest.raises(ValueError):
+        openmc.Material(material_id=-1)
+
+
 def test_nuclides_to_ignore(run_in_tmpdir):
     """Test nuclides_to_ignore when exporting a material to XML"""
     m = openmc.Material()
@@ -478,6 +488,7 @@ def test_borated_water():
     # Test the density override
     m = openmc.model.borated_water(975, 566.5, 15.51, density=0.9)
     assert m.density == pytest.approx(0.9, 1e-3)
+    assert m.temperature == pytest.approx(566.5)
 
 
 def test_from_xml(run_in_tmpdir):
@@ -550,6 +561,10 @@ def test_get_activity():
     m1.add_element("Fe", 0.7)
     m1.add_element("Li", 0.3)
     m1.set_density('g/cm3', 1.5)
+    with pytest.raises(ValueError, match="Volume must be set"):
+        m1.get_activity(units='Bq')
+    with pytest.raises(ValueError, match="Volume must be set"):
+        m1.get_activity(units='Ci')
     # activity in Bq/cc and Bq/g should not require volume setting
     assert m1.get_activity(units='Bq/cm3') == 0
     assert m1.get_activity(units='Bq/g') == 0
@@ -583,6 +598,8 @@ def test_get_activity():
     assert pytest.approx(m4.get_activity(units='Bq/g', by_nuclide=True)["H3"]) == 355978108155965.94  # [Bq/g]
     assert pytest.approx(m4.get_activity(units='Bq/cm3')) == 355978108155965.94*3/2 # [Bq/cc]
     assert pytest.approx(m4.get_activity(units='Bq/cm3', by_nuclide=True)["H3"]) == 355978108155965.94*3/2 # [Bq/cc]
+    assert pytest.approx(m4.get_activity(units='Bq/m3')) == 355978108155965.94*3/2*1e6 # [Bq/m3]
+    assert pytest.approx(m4.get_activity(units='Bq/m3', by_nuclide=True)["H3"]) == 355978108155965.94*3/2*1e6 # [Bq/m3]
     # volume is required to calculate total activity
     m4.volume = 10.
     assert pytest.approx(m4.get_activity(units='Bq')) == 355978108155965.94*3/2*10 # [Bq]
@@ -590,6 +607,41 @@ def test_get_activity():
     # Test with volume specified as argument
     assert pytest.approx(m4.get_activity(units='Bq', volume=1.0)) == 355978108155965.94*3/2
 
+    # Test units based on Ci
+    bq = m4.get_activity(units='Bq')
+    m3 = m4.volume * 1e-6
+    assert (ci := m4.get_activity(units='Ci')) == pytest.approx(bq/3.7e10)
+    assert m4.get_activity(units='Ci/m3') == pytest.approx(ci/m3)
+
+
+def test_get_activity_chain_file(tmp_path):
+    m = openmc.Material()
+    m.add_nuclide("H3", 1.0)
+    m.set_density('g/cm3', 1.0)
+
+    chain = Chain()
+    h3 = Nuclide("H3")
+    h3.half_life = 1.0
+    chain.add_nuclide(h3)
+
+    atoms_per_bcm = m.get_nuclide_atom_densities()["H3"]
+    expected = np.log(2.0) * 1e24 * atoms_per_bcm
+
+    assert m.get_activity(chain_file=chain) == pytest.approx(expected)
+
+    chain_path = tmp_path / "chain.xml"
+    chain.export_to_xml(chain_path)
+    assert m.get_activity(chain_file=chain_path) == pytest.approx(expected)
+
+    endf_activity = m.get_activity(chain_file=False)
+    with openmc.config.patch('chain_file', chain_path):
+        assert m.get_activity() == pytest.approx(expected)
+        assert m.get_activity(chain_file=False) == pytest.approx(endf_activity)
+
+    stable_chain = Chain()
+    stable_chain.add_nuclide(Nuclide("H3"))
+    assert m.get_activity(chain_file=stable_chain) == 0.0
+
 
 def test_get_decay_heat():
     # Set chain file for testing
@@ -600,6 +652,8 @@ def test_get_decay_heat():
     m1.add_nuclide("U235", 0.2)
     m1.add_nuclide("U238", 0.8)
     m1.set_density('g/cm3', 10.5)
+    with pytest.raises(ValueError, match="Volume must be set"):
+        m1.get_decay_heat(units='W')
     # decay heat in W/cc and W/g should not require volume setting
     assert m1.get_decay_heat(units='W/cm3') == 0
     assert m1.get_decay_heat(units='W/g') == 0
@@ -633,6 +687,8 @@ def test_get_decay_heat():
     assert pytest.approx(m4.get_decay_heat(units='W/g', by_nuclide=True)["I135"]) == 40175.15720273193 # [W/g]
     assert pytest.approx(m4.get_decay_heat(units='W/cm3')) == 40175.15720273193*3/2 # [W/cc]
     assert pytest.approx(m4.get_decay_heat(units='W/cm3', by_nuclide=True)["I135"]) == 40175.15720273193*3/2 #[W/cc]
+    assert pytest.approx(m4.get_decay_heat(units='W/m3')) == 40175.15720273193*3/2*1e6 # [W/m3]
+    assert pytest.approx(m4.get_decay_heat(units='W/m3', by_nuclide=True)["I135"]) == 40175.15720273193*3/2*1e6 # [W/m3]
     # volume is required to calculate total decay heat
     m4.volume = 10.
     assert pytest.approx(m4.get_decay_heat(units='W')) == 40175.15720273193*3/2*10 # [W]
@@ -663,6 +719,8 @@ def test_decay_photon_energy():
     src_per_bqg = m.get_decay_photon_energy(units='Bq/g')
     src_per_bqkg = m.get_decay_photon_energy(units='Bq/kg')
     assert pytest.approx(src_per_bqg.integral()) == src_per_bqkg.integral() / 1000.
+    src_per_bqm3 = m.get_decay_photon_energy(units='Bq/m3')
+    assert pytest.approx(src_per_bqm3.integral()) == src_per_cm3.integral() * 1e6
 
     # If we add Xe135 (which has a tabular distribution), the photon source
     # should be a mixture distribution
@@ -704,3 +762,163 @@ def test_avoid_subnormal(run_in_tmpdir):
     # When read back in, the density should be zero
     mats = openmc.Materials.from_xml()
     assert mats[0].get_nuclide_atom_densities()['H2'] == 0.0
+
+
+def test_material_deplete():
+    pristine_material = openmc.Material()
+    pristine_material.add_nuclide("Ni58", 1.0)
+    pristine_material.set_density("g/cm3", 7.87)
+    pristine_material.depletable = True
+    pristine_material.temperature = 293.6
+    pristine_material.volume = 1.
+
+    mg_flux = [0.5e11] * 42
+
+    chain = Chain.from_xml(
+        Path(__file__).parents[1] / "chain_ni.xml"
+    )
+
+    depleted_material = pristine_material.deplete(
+        multigroup_flux=mg_flux,
+        energy_group_structure="VITAMIN-J-42",
+        timesteps=[10, 70.86],
+        source_rates=[1e19, 0.0],
+        timestep_units="d",
+        chain_file=chain,
+    )
+
+    for i_step, material in enumerate(depleted_material):
+        assert isinstance(material, openmc.Material)
+        if i_step > 0:
+            assert len(material.get_nuclides()) > len(pristine_material.get_nuclides())
+
+    Co58_mat_1_step_0 = depleted_material[0].get_nuclide_atom_densities("Co58").get("Co58", 0.0)
+    Co58_mat_1_step_1 = depleted_material[1].get_nuclide_atom_densities("Co58")["Co58"]
+    Co58_mat_1_step_2 = depleted_material[2].get_nuclide_atom_densities("Co58")["Co58"]
+
+    assert Co58_mat_1_step_0 == 0.0
+
+    # Check that Co58 is produced in the first step
+    assert Co58_mat_1_step_1 > 0.0
+
+    # Check that Co58 is halved in the second step which is one halflife later
+    assert np.allclose(Co58_mat_1_step_1 * 0.5, Co58_mat_1_step_2)
+
+
+def test_mean_free_path():
+
+    mat1 = openmc.Material()
+    mat1.add_nuclide('Si28', 1.0)
+    mat1.set_density('g/cm3', 2.32)
+    assert mat1.mean_free_path(energy=14e6) == pytest.approx(11.41, abs=1e-2)
+
+    mat2 = openmc.Material()
+    mat2.add_nuclide('Pb208', 1.0)
+    mat2.set_density('g/cm3', 11.34)
+    assert mat2.mean_free_path(energy=14e6) == pytest.approx(5.65, abs=1e-2)
+
+
+def test_material_from_constructor():
+    # Test that components and percent_type work in the constructor
+    components = {
+        'Li': {'percent': 0.5, 'enrichment': 60.0, 'enrichment_target': 'Li7'},
+        'O16': 1.0,
+        'Be': 0.5
+    }
+    mat = openmc.Material(
+        material_id=123,
+        name="test-mat",
+        components=components,
+        percent_type="ao"
+    )
+    # Check that nuclides were added
+    nuclide_names = [nuc.name for nuc in mat.nuclides]
+    assert 'O16' in nuclide_names
+    assert 'Be9' in nuclide_names
+    assert 'Li7' in nuclide_names
+    assert 'Li6' in nuclide_names
+    assert mat.id == 123
+    assert mat.name == "test-mat"
+
+    mat1 = openmc.Material(
+        **{
+            "material_id": 1,
+            "name": "neutron_star",
+            "density": 1e17,
+            "density_units": "kg/m3",
+        }
+    )
+    assert mat1.id == 1
+    assert mat1.name == "neutron_star"
+    assert mat1._density == 1e17
+    assert mat1._density_units == "kg/m3"
+    assert mat1.nuclides == []
+
+    mat2 = openmc.Material(
+        material_id=42,
+        name="plasma",
+        temperature=None,
+        density=1e-7,
+        density_units="g/cm3",
+    )
+    assert mat2.id == 42
+    assert mat2.name == "plasma"
+    assert mat2.temperature is None
+    assert mat2.density == 1e-7
+    assert mat2.density_units == "g/cm3"
+    assert mat2.nuclides == []
+
+
+def test_get_photon_contact_dose_rate():
+    # Set chain file for testing
+    openmc.config['chain_file'] = Path(__file__).parents[1] / 'chain_simple.xml'
+
+    # A purely stable material (Fe) should give zero dose
+    m_stable = openmc.Material()
+    m_stable.add_element('Fe', 1.0)
+    m_stable.set_density('g/cm3', 7.87)
+    assert m_stable.get_photon_contact_dose_rate('absorbed-air') == 0.0
+    assert m_stable.get_photon_contact_dose_rate('effective') == 0.0
+
+    # I135 has a Discrete photon source (lines)
+    m_i135 = openmc.Material()
+    m_i135.add_nuclide('I135', 1.0)
+    m_i135.set_density('atom/b-cm', 1.0)
+
+    cdr_abs = m_i135.get_photon_contact_dose_rate('absorbed-air')
+    cdr_eff = m_i135.get_photon_contact_dose_rate('effective')
+    assert cdr_abs == pytest.approx(6.091547e10, rel=1e-4)   # [Gy/h]
+    assert cdr_eff == pytest.approx(6.102167e10, rel=1e-4)   # [Sv/h]
+
+    # Xe135 has a Tabular photon source (continuous distribution)
+    m_xe135 = openmc.Material()
+    m_xe135.add_nuclide('Xe135', 1.0)
+    m_xe135.set_density('atom/b-cm', 1.0)
+
+    cdr_xe_abs = m_xe135.get_photon_contact_dose_rate('absorbed-air')
+    cdr_xe_eff = m_xe135.get_photon_contact_dose_rate('effective')
+    assert cdr_xe_abs == pytest.approx(7.886077e8, rel=1e-4)  # [Gy/h]
+    assert cdr_xe_eff == pytest.approx(9.488298e8, rel=1e-4)  # [Sv/h]
+
+    # by_nuclide=True should return a dict whose values sum to the total
+    cdr_by_nuc = m_i135.get_photon_contact_dose_rate('absorbed-air', by_nuclide=True)
+    assert isinstance(cdr_by_nuc, dict)
+    assert 'I135' in cdr_by_nuc
+    assert sum(cdr_by_nuc.values()) == pytest.approx(cdr_abs)
+
+    # For a mixed material the sum over nuclides must equal the total
+    m_mix = openmc.Material()
+    m_mix.add_nuclide('I135', 0.5)
+    m_mix.add_nuclide('Xe135', 0.5)
+    m_mix.set_density('atom/b-cm', 1.0)
+    cdr_mix_total = m_mix.get_photon_contact_dose_rate('absorbed-air')
+    cdr_mix_nuc = m_mix.get_photon_contact_dose_rate('absorbed-air', by_nuclide=True)
+    assert sum(cdr_mix_nuc.values()) == pytest.approx(cdr_mix_total)
+
+    # Input validation
+    with pytest.raises(ValueError):
+        m_i135.get_photon_contact_dose_rate('invalid-quantity')
+    with pytest.raises(TypeError):
+        m_i135.get_photon_contact_dose_rate('absorbed-air', build_up='two')
+    with pytest.raises(ValueError):
+        m_i135.get_photon_contact_dose_rate('absorbed-air', build_up=-1.0)
diff --git a/tests/unit_tests/test_materials.py b/tests/unit_tests/test_materials.py
new file mode 100644
index 000000000..6620402fd
--- /dev/null
+++ b/tests/unit_tests/test_materials.py
@@ -0,0 +1,129 @@
+from pathlib import Path
+
+import pytest
+
+import openmc
+from openmc.deplete import Chain
+
+
+def test_materials_deplete():
+    pristine_material_1 = openmc.Material()
+    pristine_material_1.add_nuclide("Ni58", 1.)
+    pristine_material_1.set_density("g/cm3", 7.87)
+    pristine_material_1.depletable = True
+    pristine_material_1.temperature = 293.6
+    pristine_material_1.volume = 1.
+
+    pristine_material_2 = openmc.Material()
+    pristine_material_2.add_nuclide("Ni60", 1.)
+    pristine_material_2.set_density("g/cm3", 7.87)
+    pristine_material_2.depletable = True
+    pristine_material_2.temperature = 293.6
+    pristine_material_2.volume = 1.
+
+    pristine_materials = openmc.Materials([pristine_material_1, pristine_material_2])
+
+    mg_flux = [0.5e11] * 42
+
+    chain = Chain.from_xml(
+        Path(__file__).parents[1] / "chain_ni.xml"
+    )
+
+    depleted_material = pristine_materials.deplete(
+        multigroup_fluxes=[mg_flux, mg_flux],
+        energy_group_structures=["VITAMIN-J-42", "VITAMIN-J-42"],
+        timesteps=[100, 100],
+        source_rates=[1e19, 0.0],
+        timestep_units="d",
+        chain_file=chain,
+    )
+
+    assert list(depleted_material.keys()) == [pristine_material_1.id, pristine_material_2.id]
+    for mat_id, materials in depleted_material.items():
+        for i_step, material in enumerate(materials):
+            assert isinstance(material, openmc.Material)
+            if i_step > 0:
+                assert len(material.get_nuclides()) > 1
+            assert mat_id == material.id
+
+    mats = depleted_material[pristine_material_1.id]
+    Co58_mat_1_step_0 = mats[0].get_nuclide_atom_densities("Co58").get("Co58", 0.0)
+    Co58_mat_1_step_1 = mats[1].get_nuclide_atom_densities("Co58")["Co58"]
+    Co58_mat_1_step_2 = mats[2].get_nuclide_atom_densities("Co58")["Co58"]
+
+    assert Co58_mat_1_step_0 == 0.0
+    # Co58 is the main activation product of Ni58 in the first irradiation step.
+    # It then decays in the second cooling step (flux = 0)
+    assert Co58_mat_1_step_1 > 0.0 and Co58_mat_1_step_1 > Co58_mat_1_step_2
+
+    Ni59_mat_1_step_0 = mats[0].get_nuclide_atom_densities("Ni59").get("Ni59", 0.0)
+    Ni59_mat_1_step_1 = mats[1].get_nuclide_atom_densities("Ni59")["Ni59"]
+    Ni59_mat_1_step_2 = mats[2].get_nuclide_atom_densities("Ni59")["Ni59"]
+
+    assert Ni59_mat_1_step_0 == 0.0
+    # Ni59 is one of the main activation product of Ni60 in the first irradiation
+    # step. It then decays in the second cooling step (flux = 0)
+    assert Ni59_mat_1_step_1 > 0.0 and Ni59_mat_1_step_1 > Ni59_mat_1_step_2
+
+
+def test_export_duplicate_materials_to_xml(run_in_tmpdir):
+    """
+    Test exporting Materials to xml with a duplicate and checking that only
+    unique entities are exported.
+    """
+    my_mat = openmc.Material(name="my_mat")
+    my_mat2 = openmc.Material(name="my_mat2")
+
+    materials = openmc.Materials([my_mat, my_mat2, my_mat])
+
+    materials.export_to_xml("materials.xml")
+
+    materials_in = openmc.Materials.from_xml("materials.xml")
+    assert len(materials_in) == 2
+
+
+def test_materials_deplete_length_mismatch():
+    mats = openmc.Materials([openmc.Material()])
+
+    with pytest.raises(ValueError, match="multigroup_fluxes length"):
+        mats.deplete(
+            multigroup_fluxes=[],
+            energy_group_structures=["VITAMIN-J-42"],
+            timesteps=[1.0],
+            source_rates=1.0,
+        )
+
+    with pytest.raises(ValueError, match="energy_group_structures length"):
+        mats.deplete(
+            multigroup_fluxes=[[1.0]],
+            energy_group_structures=[],
+            timesteps=[1.0],
+            source_rates=1.0,
+        )
+
+
+def test_materials_deplete_missing_volume(monkeypatch):
+    mat = openmc.Material()
+    mat.add_nuclide("Ni58", 1.0)
+    mat.set_density("g/cm3", 7.87)
+
+    mats = openmc.Materials([mat])
+
+    class DummySession:
+        def __enter__(self):
+            return self
+
+        def __exit__(self, exc_type, exc, tb):
+            return False
+
+    monkeypatch.setattr(openmc.lib, "TemporarySession", DummySession)
+
+    chain = Path(__file__).parents[1] / "chain_ni.xml"
+    with pytest.raises(ValueError, match="has no volume"):
+        mats.deplete(
+            multigroup_fluxes=[[1.0]],
+            energy_group_structures=["VITAMIN-J-42"],
+            timesteps=[1.0],
+            source_rates=1.0,
+            chain_file=chain,
+        )
diff --git a/tests/unit_tests/test_mcpl_stat_sum.py b/tests/unit_tests/test_mcpl_stat_sum.py
new file mode 100644
index 000000000..b69192956
--- /dev/null
+++ b/tests/unit_tests/test_mcpl_stat_sum.py
@@ -0,0 +1,69 @@
+"""Test for MCPL stat:sum functionality"""
+
+from pathlib import Path
+import shutil
+
+import pytest
+import openmc
+
+
+@pytest.mark.skipif(shutil.which("mcpl-config") is None, reason="MCPL is not available.")
+def test_mcpl_stat_sum_field(run_in_tmpdir):
+    """Test that MCPL files contain proper stat:sum field with particle count.
+
+    This test verifies that when OpenMC creates MCPL source files, they contain
+    the stat:sum field. Since MCPL functions are not exposed in the Python API,
+    this test creates an actual OpenMC simulation to generate MCPL files and
+    then checks their content.
+    """
+
+    mcpl = pytest.importorskip("mcpl")
+
+    # Create a minimal working model that will generate MCPL files
+    model = openmc.examples.pwr_pin_cell()
+    model.settings.batches = 5
+    model.settings.inactive = 2
+    model.settings.particles = 1000
+    model.settings.sourcepoint = {'mcpl': True, 'separate': True}
+
+    # Run a short simulation to generate MCPL files
+    model.run(output=False)
+
+    # Find the generated MCPL file (from the last batch)
+    mcpl_file = Path('source.5.mcpl')
+    assert mcpl_file.exists(), "No MCPL files were generated"
+
+    # Open and verify the stat:sum field exists
+    with mcpl.MCPLFile(mcpl_file) as f:
+        # Check if stat:sum field exists using convenience property
+        if hasattr(f, 'stat_sum'):
+            # Use the convenience .stat_sum property directly
+            stat_sum_dict = f.stat_sum
+            assert 'openmc_np1' in stat_sum_dict, "openmc_np1 key not found in stat_sum"
+            stat_sum_value = int(stat_sum_dict['openmc_np1'])
+        else:
+            # Fallback to checking comments for older MCPL versions
+            comments = f.comments
+
+            # Check for stat:sum in comments (MCPL stores these as comments)
+            stat_sum_value = None
+
+            for comment in comments:
+                if 'stat:sum:openmc_np1' in comment:
+                    # Extract the value
+                    parts = comment.split(':')
+                    if len(parts) >= 4:
+                        stat_sum_value = int(parts[3].strip())
+                    break
+            else:
+                pytest.skip("stat:sum field not found - may be running with MCPL < 2.1.0")
+
+    # Verify the stat:sum value is reasonable
+    assert stat_sum_value != -1, "stat:sum was not updated from initial -1 value"
+
+    # In eigenvalue mode, active batches generate source particles
+    active_batches = model.settings.batches - model.settings.inactive  # 3 active batches
+    expected_particles = active_batches * model.settings.particles     # 3000 total
+
+    assert stat_sum_value == expected_particles, \
+        f"stat:sum value {stat_sum_value} doesn't match expected {expected_particles}"
diff --git a/tests/unit_tests/test_mesh.py b/tests/unit_tests/test_mesh.py
index f1f5e9f67..9b1469fc5 100644
--- a/tests/unit_tests/test_mesh.py
+++ b/tests/unit_tests/test_mesh.py
@@ -1,12 +1,17 @@
-from math import pi
+from math import pi, sqrt
 from tempfile import TemporaryDirectory
 from pathlib import Path
+import itertools
+import random
 
+import h5py
 import numpy as np
+from scipy.stats import chi2
 import pytest
 import openmc
 import openmc.lib
 from openmc.utility_funcs import change_directory
+from uncertainties.unumpy import uarray, nominal_values, std_devs
 
 
 @pytest.mark.parametrize("val_left,val_right", [(0, 0), (-1., -1.), (2.0, 2)])
@@ -480,6 +485,114 @@ def test_umesh(run_in_tmpdir, simple_umesh, export_type):
     mean = np.array([arr.GetTuple1(i) for i in range(ref_data.size)])
     np.testing.assert_almost_equal(mean, ref_data)
 
+    # attempt to apply a dataset with an improper size to a VTK write
+    with pytest.raises(ValueError, match='Cannot apply dataset "mean"'):
+        simple_umesh.write_data_to_vtk(datasets={'mean': ref_data[:-2]}, filename=filename)
+
+
+vtkhdf_tests = [
+    (
+        Path("test_mesh_dagmc_tets.vtk"),
+        "moab"
+    ),
+    (
+        Path("test_mesh_hexes.exo"),
+        "libmesh"
+    )
+]
+@pytest.mark.parametrize('mesh_file, mesh_library', vtkhdf_tests)
+def test_write_vtkhdf(mesh_file, mesh_library, request, run_in_tmpdir):
+    """Performs a minimal UnstructuredMesh simulation, reads in the resulting
+    statepoint file and writes the mesh data to vtk and vtkhdf files. It is
+    necessary to read in the unstructured mesh from a statepoint file to ensure
+    it has all the required attributes
+    """
+    if mesh_library == 'moab' and not openmc.lib._dagmc_enabled():
+        pytest.skip("DAGMC not enabled.")
+    if mesh_library == 'libmesh' and not openmc.lib._libmesh_enabled():
+        pytest.skip("LibMesh not enabled.")
+
+    model = openmc.Model()
+
+    surf1 = openmc.Sphere(r=1000.0, boundary_type="vacuum")
+    cell1 = openmc.Cell(region=-surf1)
+    model.geometry = openmc.Geometry([cell1])
+
+    umesh = openmc.UnstructuredMesh(
+        request.path.parent / mesh_file,
+        mesh_library,
+        mesh_id = 1
+    )
+    mesh_filter = openmc.MeshFilter(umesh)
+
+    # Create flux mesh tally to score alpha production
+    mesh_tally = openmc.Tally(name="test_tally")
+    mesh_tally.filters = [mesh_filter]
+    mesh_tally.scores = ["flux"]
+    if mesh_library == "libmesh":
+        mesh_tally.estimator = "collision"
+
+    model.tallies = [mesh_tally]
+
+    model.settings.run_mode = "fixed source"
+    model.settings.batches = 2
+    model.settings.particles = 10
+
+    statepoint_file = model.run()
+
+    with openmc.StatePoint(statepoint_file) as statepoint:
+        my_tally = statepoint.get_tally(name="test_tally")
+
+    umesh_from_sp = statepoint.meshes[umesh.id]
+
+    datasets={
+        "mean": my_tally.mean.flatten(),
+        "std_dev": my_tally.std_dev.flatten()
+    }
+
+    umesh_from_sp.write_data_to_vtk(datasets=datasets, filename="test_mesh.vtkhdf")
+    umesh_from_sp.write_data_to_vtk(datasets=datasets, filename="test_mesh.vtk")
+
+    with pytest.raises(ValueError, match="Unsupported file extension"):
+        # Supported file extensions are vtk or vtkhdf, not hdf5, so this should raise an error
+        umesh_from_sp.write_data_to_vtk(
+            datasets=datasets,
+            filename="test_mesh.hdf5",
+        )
+    with pytest.raises(ValueError, match="Cannot apply dataset"):
+        # The shape of the data should match the shape of the mesh, so this should raise an error
+        umesh_from_sp.write_data_to_vtk(
+            datasets={'incorrectly_shaped_data': np.array(([1,2,3]))},
+            filename="test_mesh_incorrect_shape.vtkhdf",
+        )
+
+    assert Path("test_mesh.vtk").exists()
+    assert Path("test_mesh.vtkhdf").exists()
+
+    # just ensure we can open the file without error
+    with h5py.File("test_mesh.vtkhdf", "r"):
+        ...
+
+    import vtk
+    reader = vtk.vtkHDFReader()
+    reader.SetFileName("test_mesh.vtkhdf")
+    reader.Update()
+
+    # Get mean from file and make sure it matches original data
+    num_elements = reader.GetOutput().GetNumberOfCells()
+    assert num_elements == umesh_from_sp.n_elements
+
+    num_vertices = reader.GetOutput().GetNumberOfPoints()
+    assert num_vertices == umesh_from_sp.vertices.shape[0]
+
+    arr = reader.GetOutput().GetCellData().GetArray("mean")
+    mean = np.array([arr.GetTuple1(i) for i in range(my_tally.mean.size)])
+    np.testing.assert_almost_equal(mean, my_tally.mean.flatten()/umesh_from_sp.volumes)
+
+    arr = reader.GetOutput().GetCellData().GetArray("std_dev")
+    std_dev = np.array([arr.GetTuple1(i) for i in range(my_tally.std_dev.size)])
+    np.testing.assert_almost_equal(std_dev, my_tally.std_dev.flatten()/umesh_from_sp.volumes)
+
 
 def test_mesh_get_homogenized_materials():
     """Test the get_homogenized_materials method"""
@@ -536,6 +649,7 @@ def test_mesh_get_homogenized_materials():
 
 @pytest.fixture
 def sphere_model():
+    openmc.reset_auto_ids()
     # Model with three materials separated by planes x=0 and z=0
     mats = []
     for i in range(3):
@@ -615,3 +729,428 @@ def test_mesh_material_volumes_serialize():
     assert new_volumes.by_element(1) == [(None, 1.0)]
     assert new_volumes.by_element(2) == [(2, 0.5), (1, 0.5)]
     assert new_volumes.by_element(3) == [(2, 1.0)]
+
+
+def test_mesh_material_volumes_serialize_with_bboxes():
+    materials = np.array([
+        [1, -1, -2],
+        [-1, -2, -2],
+        [2, 1, -2],
+        [2, -2, -2]
+    ])
+    volumes = np.array([
+        [0.5, 0.5, 0.0],
+        [1.0, 0.0, 0.0],
+        [0.5, 0.5, 0.0],
+        [1.0, 0.0, 0.0]
+    ])
+
+    # (xmin, ymin, zmin, xmax, ymax, zmax)
+    bboxes = np.empty((4, 3, 6))
+    bboxes[..., 0:3] = np.inf
+    bboxes[..., 3:6] = -np.inf
+    bboxes[0, 0] = [-1.0, -2.0, -3.0, 1.0, 2.0, 3.0]    # material 1
+    bboxes[0, 1] = [-5.0, -6.0, -7.0, 5.0, 6.0, 7.0]    # void
+    bboxes[1, 0] = [0.0, 0.0, 0.0, 10.0, 1.0, 2.0]      # void
+    bboxes[2, 0] = [-1.0, -1.0, -1.0, 0.0, 0.0, 0.0]    # material 2
+    bboxes[2, 1] = [0.0, 0.0, 0.0, 1.0, 1.0, 1.0]       # material 1
+    bboxes[3, 0] = [-2.0, -2.0, -2.0, 2.0, 2.0, 2.0]    # material 2
+
+    mmv = openmc.MeshMaterialVolumes(materials, volumes, bboxes)
+    with TemporaryDirectory() as tmpdir:
+        path = f'{tmpdir}/volumes_bboxes.npz'
+        mmv.save(path)
+        loaded = openmc.MeshMaterialVolumes.from_npz(path)
+
+    assert loaded.has_bounding_boxes
+    first = loaded.by_element(0, include_bboxes=True)[0][2]
+    assert isinstance(first, openmc.BoundingBox)
+    np.testing.assert_array_equal(first.lower_left, (-1.0, -2.0, -3.0))
+    np.testing.assert_array_equal(first.upper_right, (1.0, 2.0, 3.0))
+
+    second = loaded.by_element(0, include_bboxes=True)[1][2]
+    assert isinstance(second, openmc.BoundingBox)
+    np.testing.assert_array_equal(second.lower_left, (-5.0, -6.0, -7.0))
+    np.testing.assert_array_equal(second.upper_right, (5.0, 6.0, 7.0))
+
+
+def test_mesh_material_volumes_boundary_conditions(sphere_model):
+    """Test the material volumes method using a regular mesh
+    that overlaps with a vacuum boundary condition."""
+
+    mesh = openmc.SphericalMesh.from_domain(sphere_model.geometry, dimension=(1, 1, 1))
+    # extend mesh beyond the outer sphere surface to test rays crossing the boundary condition
+    mesh.r_grid[-1] += 5.0
+
+    # add a new cell to the modelthat occupies the outside of the sphere
+    sphere_surfaces = list(filter(lambda s: isinstance(s, openmc.Sphere),
+                            sphere_model.geometry.get_all_surfaces().values()))
+    outer_cell = openmc.Cell(region=+sphere_surfaces[0])
+    sphere_model.geometry.root_universe.add_cell(outer_cell)
+
+    volumes = mesh.material_volumes(sphere_model, (0, 100, 100))
+    sphere_volume = 4/3*np.pi*25**3
+    mats = sphere_model.materials
+    expected_volumes = [(mats[0].id, 0.25*sphere_volume),
+                        (mats[1].id, 0.25*sphere_volume),
+                        (mats[2].id, 0.5*sphere_volume),
+                        (None, 4/3*np.pi*mesh.r_grid[-1]**3 - sphere_volume)]
+
+    for evaluated, expected in zip(volumes.by_element(0), expected_volumes):
+        assert evaluated[0] == expected[0]
+        assert evaluated[1] == pytest.approx(expected[1], rel=1e-2)
+
+
+def test_mesh_material_volumes_bounding_boxes():
+    # Create a model with 8 spherical cells at known locations with random radii
+    box = openmc.model.RectangularParallelepiped(
+        -10, 10, -10, 10, -10, 10, boundary_type='vacuum')
+
+    mat = openmc.Material()
+    mat.add_nuclide('H1', 1.0)
+
+    sph_cells = []
+    for x, y, z in itertools.product((-5., 5.), repeat=3):
+        mat_i = mat.clone()
+        sph = openmc.Sphere(x, y, z, r=random.uniform(0.5, 1.5))
+        sph_cells.append(openmc.Cell(region=-sph, fill=mat_i))
+    background = openmc.Cell(region=-box & openmc.Intersection([~c.region for c in sph_cells]))
+
+    model = openmc.Model()
+    model.geometry = openmc.Geometry(sph_cells + [background])
+    model.settings.particles = 1000
+    model.settings.batches = 10
+
+    # Create a one-element mesh that encompasses the entire geometry
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-10., -10., -10.)
+    mesh.upper_right = (10., 10., 10.)
+    mesh.dimension = (1, 1, 1)
+
+    # Run material volume calculation with bounding boxes
+    n_samples = (400, 400, 400)
+    mmv = mesh.material_volumes(model, n_samples, max_materials=10, bounding_boxes=True)
+    assert mmv.has_bounding_boxes
+
+    # Create a mapping of material ID to bounding box
+    bbox_by_mat = {
+        mat_id: bbox
+        for mat_id, vol, bbox in mmv.by_element(0, include_bboxes=True)
+        if mat_id is not None and vol > 0.0
+    }
+
+    # Match the mesh ray spacing used for the bounding box estimator.
+    tol = 0.5 * mesh.bounding_box.width[0] / n_samples[0]
+    for cell in sph_cells:
+        bbox = bbox_by_mat[cell.fill.id]
+        cell_bbox = cell.bounding_box
+        np.testing.assert_allclose(bbox.lower_left, cell_bbox.lower_left, atol=tol)
+        np.testing.assert_allclose(bbox.upper_right, cell_bbox.upper_right, atol=tol)
+
+
+def test_raytrace_mesh_infinite_loop(run_in_tmpdir):
+    # Create a model with one large spherical cell
+    sphere = openmc.Sphere(r=100, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell])
+
+    # Create a regular mesh and associated tally
+    mesh_surface = openmc.RegularMesh()
+    mesh_surface.lower_left = (-30, -30, 30)
+    mesh_surface.upper_right = (30, 30, 60)
+    mesh_surface.dimension = (1, 1, 1)
+    reg_filter = openmc.MeshSurfaceFilter(mesh_surface)
+    mesh_surface_tally = openmc.Tally()
+    mesh_surface_tally.filters = [reg_filter]
+    mesh_surface_tally.scores = ['current']
+    model.tallies = [mesh_surface_tally]
+
+    # Define a source such that the z position is on a mesh boundary with a very
+    # small directional cosine in the z direction
+    polar = openmc.stats.delta_function(1.75e-7)
+    azimuthal = openmc.stats.Uniform(0.0, 2.0*pi)
+    model.settings.source = openmc.IndependentSource(
+        angle=openmc.stats.PolarAzimuthal(polar, azimuthal)
+    )
+    model.settings.run_mode = 'fixed source'
+    model.settings.particles = 10
+    model.settings.batches =  1
+
+    # Run the model; this should not cause an infinite loop
+    model.run()
+
+
+def test_filter_time_mesh(run_in_tmpdir):
+    """Test combination of TimeFilter and MeshFilter"""
+
+    # Define material
+    mat = openmc.Material()
+    mat.add_nuclide('Fe56', 1.0)
+    mat.set_density('g/cm3', 7.8)
+
+    # Define geometry
+    surf_Z1 = openmc.XPlane(x0=-1e10, boundary_type="reflective")
+    surf_Z2 = openmc.XPlane(x0=1e10, boundary_type="reflective")
+    cell_F = openmc.Cell(fill=mat, region=+surf_Z1 & -surf_Z2)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell_F])
+
+    # Define settings
+    model.settings.run_mode = "fixed source"
+    model.settings.particles = 1000
+    model.settings.batches = 20
+    model.settings.output = {"tallies": False}
+    model.settings.cutoff = {"time_neutron": 1e-7}
+
+    # Define tallies
+
+    # Create a mesh filter that can be used in a tally
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (21, 1, 1)
+    mesh.lower_left = (-20.5, -1e10, -1e10)
+    mesh.upper_right = (20.5, 1e10, 1e10)
+    time_grid = np.linspace(0.0, 1e-7, 21)
+
+    mesh_filter = openmc.MeshFilter(mesh)
+    time_filter = openmc.TimeFilter(time_grid)
+
+    # Now use the mesh filter in a tally and indicate what scores are desired
+    tally1 = openmc.Tally(name="collision")
+    tally1.estimator = "collision"
+    tally1.filters = [time_filter, mesh_filter]
+    tally1.scores = ["flux"]
+    tally2 = openmc.Tally(name="tracklength")
+    tally2.estimator = "tracklength"
+    tally2.filters = [time_filter, mesh_filter]
+    tally2.scores = ["flux"]
+    model.tallies = openmc.Tallies([tally1, tally2])
+
+    # Run and post-process
+    model.run(apply_tally_results=True)
+
+    # Get radial flux distribution
+    flux_collision = tally1.mean.ravel()
+    flux_collision_unc = tally1.std_dev.ravel()
+    flux_tracklength = tally2.mean.ravel()
+    flux_tracklength_unc = tally2.std_dev.ravel()
+
+    # Construct arrays with uncertainties
+    collision = uarray(flux_collision, flux_collision_unc)
+    tracklength = uarray(flux_tracklength, flux_tracklength_unc)
+    delta = collision - tracklength
+
+    # Compute differences and standard deviations
+    diff = nominal_values(delta)
+    std_dev = std_devs(delta)
+
+    # Exclude zero-uncertainty bins
+    mask = std_dev > 0.0
+    dof = int(np.sum(mask))
+
+    # Global chi-square consistency test between collision and tracklength
+    # estimators. Target false positive rate ~1e-4 (1 in 10,000)
+    z = diff[mask] / std_dev[mask]
+    chi2_stat = np.sum(z * z)
+    alpha = 1.0e-4
+    crit = chi2.ppf(1 - alpha, dof)
+    assert chi2_stat < crit, (
+        f"Collision vs tracklength tallies disagree: chi2={chi2_stat:.2f} "
+        f">= {crit=:.2f} ({dof=}, {alpha=})"
+    )
+
+
+def test_regular_mesh_get_indices_at_coords():
+    """Test get_indices_at_coords method for RegularMesh"""
+    # Create a 10x10x10 mesh from (0,0,0) to (1,1,1)
+    # Each voxel is 0.1 x 0.1 x 0.1
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (0, 0, 0)
+    mesh.upper_right = (1, 1, 1)
+    mesh.dimension = [10, 10, 10]
+
+    # Test lower-left corner maps to first voxel (0, 0, 0)
+    assert mesh.get_indices_at_coords([0.0, 0.0, 0.0]) == (0, 0, 0)
+
+    # Test centroid of first voxel
+    # Voxel 0 spans [0.0, 0.1], so centroid is at 0.05
+    assert mesh.get_indices_at_coords([0.05, 0.05, 0.05]) == (0, 0, 0)
+
+    # Test centroid of last voxel maps correctly
+    # Voxel 9 spans [0.9, 1.0], so centroid is at 0.95
+    assert mesh.get_indices_at_coords([0.95, 0.95, 0.95]) == (9, 9, 9)
+
+    # Test a middle voxel
+    # Voxel 4 spans [0.4, 0.5], so 0.45 should map to it
+    assert mesh.get_indices_at_coords([0.45, 0.45, 0.45]) == (4, 4, 4)
+
+    # Test mixed indices
+    assert mesh.get_indices_at_coords([0.05, 0.45, 0.95]) == (0, 4, 9)
+    assert mesh.get_indices_at_coords([0.95, 0.05, 0.45]) == (9, 0, 4)
+
+    # Test coordinates outside mesh bounds raise ValueError
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([-0.5, 0.5, 0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([1.5, 0.5, 0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([0.5, -0.5, 0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([0.5, 1.5, 0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([0.5, 0.5, -0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([0.5, 0.5, 1.5])
+
+    # Test that results match expected dimensionality (3D mesh returns 3-tuple)
+    result = mesh.get_indices_at_coords([0.5, 0.5, 0.5])
+    assert isinstance(result, tuple)
+    assert len(result) == 3
+
+    # Test that indices can be used directly with centroids array
+    idx = mesh.get_indices_at_coords([0.95, 0.95, 0.95])
+    centroid = mesh.centroids[idx]
+    np.testing.assert_array_almost_equal(centroid, [0.95, 0.95, 0.95])
+
+    # Test with a 2D mesh
+    mesh_2d = openmc.RegularMesh()
+    mesh_2d.lower_left = (0, 0)
+    mesh_2d.upper_right = (1, 1)
+    mesh_2d.dimension = [10, 10]
+    result_2d = mesh_2d.get_indices_at_coords([0.5, 0.5, 999.0])
+    assert isinstance(result_2d, tuple)
+    assert len(result_2d) == 2
+    assert result_2d == (5, 5)
+
+    # Test with a 1D mesh
+    mesh_1d = openmc.RegularMesh()
+    mesh_1d.lower_left = [0]
+    mesh_1d.upper_right = [1]
+    mesh_1d.dimension = [10]
+    result_1d = mesh_1d.get_indices_at_coords([0.5, 999.0, 999.0])
+    assert isinstance(result_1d, tuple)
+    assert len(result_1d) == 1
+    assert result_1d == (5,)
+
+
+def test_rectilinear_mesh_get_indices_at_coords():
+    """Test get_indices_at_coords method for RectilinearMesh"""
+    # Create a 3x2x2 rectilinear mesh with non-uniform spacing
+    mesh = openmc.RectilinearMesh()
+    mesh.x_grid = [0., 1., 5., 10.]
+    mesh.y_grid = [-10., -5., 0.]
+    mesh.z_grid = [-100., 0., 100.]
+
+    # Test lower-left corner maps to first voxel (0, 0, 0)
+    assert mesh.get_indices_at_coords([0.0, -10., -100.]) == (0, 0, 0)
+
+    # Test centroid of first voxel
+    assert mesh.get_indices_at_coords([0.5, -7.5, -50.]) == (0, 0, 0)
+
+    # Test centroid of last voxel maps correctly
+    assert mesh.get_indices_at_coords([7.5, -2.5, 50.]) == (2, 1, 1)
+
+    # Test upper_right corner maps to last voxel
+    assert mesh.get_indices_at_coords([10., 0., 100.]) == (2, 1, 1)
+
+    # Test a middle voxel
+    assert mesh.get_indices_at_coords([2., -5., 0.]) == (1, 1, 1)
+
+    # Test coordinates outside mesh bounds raise ValueError
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([-0.5, 0.5, 0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([1.5, 0.5, 0.5])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([0.5, -0.5, 110.])
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([0.5, -20., 110.])
+
+
+def test_SphericalMesh_get_indices_at_coords():
+    """Test get_indices_at_coords method for SphericalMesh"""
+
+    # Basic mesh with default phi and theta grids (single angular bin)
+    mesh = openmc.SphericalMesh(r_grid=(0, 5, 10))
+
+    assert mesh.get_indices_at_coords([3, 0, 0]) == (0, 0, 0)
+    assert mesh.get_indices_at_coords([0, 0, 3]) == (0, 0, 0)
+    assert mesh.get_indices_at_coords([0, 0, -3]) == (0, 0, 0)
+    assert mesh.get_indices_at_coords([7, 0, 0]) == (1, 0, 0)
+    assert mesh.get_indices_at_coords([10, 0, 0]) == (1, 0, 0)
+
+    # Out-of-bounds r
+    with pytest.raises(ValueError):
+        mesh.get_indices_at_coords([11, 0, 0])
+
+    mesh2 = openmc.SphericalMesh(r_grid=(2, 5, 10))
+    with pytest.raises(ValueError):
+        mesh2.get_indices_at_coords([1, 0, 0])
+
+    # Multi-bin angular grids: use points clearly inside bins
+    mesh3 = openmc.SphericalMesh(
+        r_grid=(0, 5, 10),
+        theta_grid=(0, pi/4, pi/2, pi),
+        phi_grid=(0, pi/2, pi, 3*pi/2, 2*pi)
+    )
+
+    # Near z-axis: theta~0 -> bin 0
+    assert mesh3.get_indices_at_coords([0.01, 0, 3]) == (0, 0, 0)
+
+    # theta in (0, pi/4) -> bin 0: [1, 0, 2] theta=arccos(2/sqrt(5))~0.46
+    assert mesh3.get_indices_at_coords([1, 0, 2]) == (0, 0, 0)
+
+    # theta in (pi/4, pi/2) -> bin 1: [2, 0, 1] theta=arccos(1/sqrt(5))~1.107
+    assert mesh3.get_indices_at_coords([2, 0, 1]) == (0, 1, 0)
+
+    # theta in (pi/2, pi) -> bin 2: [1, 0, -2] theta=arccos(-2/sqrt(5))~2.034
+    assert mesh3.get_indices_at_coords([1, 0, -2]) == (0, 2, 0)
+
+    # phi in (pi/2, pi) -> bin 1: [-1, 1, 0.5]
+    assert mesh3.get_indices_at_coords([-1, 1, 0.5]) == (0, 1, 1)
+
+    # phi in (pi, 3*pi/2) -> bin 2: [-1, -1, 0.5]
+    assert mesh3.get_indices_at_coords([-1, -1, 0.5]) == (0, 1, 2)
+
+    # phi in (3*pi/2, 2*pi) -> bin 3: [1, -1, 0.5]
+    assert mesh3.get_indices_at_coords([1, -1, 0.5]) == (0, 1, 3)
+
+    # Non-default origin
+    mesh4 = openmc.SphericalMesh(
+        r_grid=(0, 5, 10),
+        origin=(100, 200, 300)
+    )
+
+    assert mesh4.get_indices_at_coords([103, 200, 300]) == (0, 0, 0)
+    assert mesh4.get_indices_at_coords([100, 200, 307]) == (1, 0, 0)
+
+    with pytest.raises(ValueError):
+        mesh4.get_indices_at_coords([111, 200, 300])
+
+    # Degenerate case: point at origin with r_grid starting at 0
+    mesh5 = openmc.SphericalMesh(r_grid=(0, 5))
+    assert mesh5.get_indices_at_coords([0, 0, 0]) == (0, 0, 0)
+
+    # Out-of-bounds theta: restricted theta grid
+    mesh6 = openmc.SphericalMesh(
+        r_grid=(0, 10),
+        theta_grid=(0, pi/4)
+    )
+    with pytest.raises(ValueError):
+        mesh6.get_indices_at_coords([5, 0, 0])  # theta=pi/2 > pi/4
+
+    # Out-of-bounds phi: restricted phi grid
+    mesh7 = openmc.SphericalMesh(
+        r_grid=(0, 10),
+        phi_grid=(0, pi/2)
+    )
+    with pytest.raises(ValueError):
+        mesh7.get_indices_at_coords([-5, 0, 0])  # phi=pi > pi/2
+
+    # Diagonal point: verify r, theta, phi all computed correctly
+    r = 6.0
+    val = r / sqrt(3)
+    result = mesh3.get_indices_at_coords([val, val, val])
+    assert result[0] == 1  # r=6 in second bin [5, 10]
+    assert result[1] == 1  # theta=arccos(1/sqrt(3))~0.955, in (pi/4, pi/2)
+    assert result[2] == 0  # phi=pi/4, in [0, pi/2)
diff --git a/tests/unit_tests/test_mesh_dagmc_tets.vtk b/tests/unit_tests/test_mesh_dagmc_tets.vtk
new file mode 120000
index 000000000..9f7000175
--- /dev/null
+++ b/tests/unit_tests/test_mesh_dagmc_tets.vtk
@@ -0,0 +1 @@
+../regression_tests/unstructured_mesh/test_mesh_dagmc_tets.vtk
\ No newline at end of file
diff --git a/tests/unit_tests/test_mesh_from_domain.py b/tests/unit_tests/test_mesh_from_domain.py
index 0cbe413e8..46d22f0d2 100644
--- a/tests/unit_tests/test_mesh_from_domain.py
+++ b/tests/unit_tests/test_mesh_from_domain.py
@@ -16,6 +16,31 @@ def test_reg_mesh_from_cell():
     assert np.array_equal(mesh.upper_right, cell.bounding_box[1])
 
 
+def test_reg_mesh_from_bounding_box():
+    """Tests a RegularMesh can be made from a BoundingBox directly."""
+    bb = openmc.BoundingBox([-8, -7, -5], [12, 13, 15])
+
+    mesh = openmc.RegularMesh.from_domain(domain=bb, dimension=[7, 11, 13])
+    assert isinstance(mesh, openmc.RegularMesh)
+    assert np.array_equal(mesh.dimension, (7, 11, 13))
+    assert np.array_equal(mesh.lower_left, bb[0])
+    assert np.array_equal(mesh.upper_right, bb[1])
+
+
+def test_rectilinear_mesh_from_bounding_box():
+    """Tests a RectilinearMesh can be made from a BoundingBox directly."""
+    bb = openmc.BoundingBox([-8, -7, -5], [12, 13, 15])
+
+    mesh = openmc.RectilinearMesh.from_bounding_box(bb, dimension=[2, 4, 5])
+    assert isinstance(mesh, openmc.RectilinearMesh)
+    assert np.array_equal(mesh.dimension, (2, 4, 5))
+    assert np.array_equal(mesh.lower_left, bb[0])
+    assert np.array_equal(mesh.upper_right, bb[1])
+    assert np.array_equal(mesh.x_grid, [-8., 2., 12.])
+    assert np.array_equal(mesh.y_grid, [-7., -2., 3., 8., 13.])
+    assert np.array_equal(mesh.z_grid, [-5., -1., 3., 7., 11., 15.])
+
+
 def test_cylindrical_mesh_from_cell():
     """Tests a CylindricalMesh can be made from a Cell and the specified
     dimensions are propagated through."""
@@ -40,7 +65,7 @@ def test_cylindrical_mesh_from_cell():
 
     assert isinstance(mesh, openmc.CylindricalMesh)
     assert np.array_equal(mesh.dimension, (1, 1, 1))
-    assert np.array_equal(mesh.r_grid, [0., 150.])
+    assert np.array_equal(mesh.r_grid, [0., 50.])
     assert np.array_equal(mesh.origin, [100., 0., 10.])
 
     # Cell is not centralized on Z, X or Y axis
@@ -49,7 +74,7 @@ def test_cylindrical_mesh_from_cell():
     mesh = openmc.CylindricalMesh.from_domain(domain=cell, dimension=[1, 1, 1])
 
     assert isinstance(mesh, openmc.CylindricalMesh)
-    assert np.array_equal(mesh.r_grid, [0., 220.])
+    assert np.array_equal(mesh.r_grid, [0., 50.])
     assert np.array_equal(mesh.origin, [100., 170., 10.])
 
 
@@ -87,6 +112,34 @@ def test_cylindrical_mesh_from_region():
     assert np.array_equal(mesh.origin, (0.0, 0.0, -30.))
 
 
+def test_spherical_mesh_from_domain():
+    """Tests a SphericalMesh can be made from a Region and the specified
+    dimensions are propagated through. Cell is not centralized"""
+    sphere = openmc.Sphere(r=5, x0=2, y0=3, z0=4)
+    region = -sphere
+
+    geometry = openmc.Geometry(openmc.Universe(cells=[openmc.Cell(region=region)]))
+
+    region_mesh = openmc.SphericalMesh.from_domain(
+        domain=region, dimension=(4, 3, 4))
+    universe_mesh = openmc.SphericalMesh.from_domain(
+        domain=geometry.root_universe, dimension=(4, 3, 4))
+    geometry_mesh = openmc.SphericalMesh.from_domain(
+        domain=geometry, dimension=(4, 3, 4))
+
+
+    for mesh in (region_mesh, universe_mesh, geometry_mesh):
+        assert isinstance(mesh, openmc.SphericalMesh)
+        assert np.array_equal(mesh.dimension, (4, 3, 4))
+        assert np.array_equal(mesh.r_grid, [0., 1.25, 2.5, 3.75, 5.0])
+        assert np.array_equal(mesh.theta_grid, [0., np.pi/3., 2*np.pi/3., np.pi])
+        assert np.array_equal(mesh.phi_grid, [0., np.pi/2., np.pi, 3*np.pi/2., 2*np.pi])
+        assert np.array_equal(mesh.origin, (2.0, 3.0, 4.0))
+
+        for p in mesh.centroids.reshape(-1, 3):
+            assert p in mesh.bounding_box
+
+
 def test_reg_mesh_from_universe():
     """Tests a RegularMesh can be made from a Universe and the default
     dimensions are propagated through. Universe is centralized"""
@@ -119,3 +172,27 @@ def test_reg_mesh_from_geometry():
 def test_error_from_unsupported_object():
     with pytest.raises(TypeError):
         openmc.RegularMesh.from_domain("vacuum energy")
+
+
+def test_regularmesh_from_domain_error_from_small_dimensions():
+    surface = openmc.Sphere(r=20)
+    cell = openmc.Cell(region=-surface)
+    with pytest.raises(
+        ValueError, match='Unable to set "dimension" to "-2" since it is less than "1"'
+    ):
+        openmc.RegularMesh.from_domain(domain=cell, dimension=-2)
+
+
+def test_dimensions_from_domain_dimensions_from_int():
+    region = openmc.model.RectangularParallelepiped(
+        xmin=-100,
+        xmax=150,
+        ymin=-50,
+        ymax=200,
+        zmin=300,
+        zmax=400,
+        boundary_type="vacuum",
+    )
+    cell = openmc.Cell(region=-region)
+    mesh = openmc.RegularMesh.from_domain(domain=cell, dimension=1000)
+    assert mesh.dimension == (14, 14, 5)
diff --git a/tests/unit_tests/test_mesh_hexes.exo b/tests/unit_tests/test_mesh_hexes.exo
new file mode 120000
index 000000000..272da92a4
--- /dev/null
+++ b/tests/unit_tests/test_mesh_hexes.exo
@@ -0,0 +1 @@
+../regression_tests/unstructured_mesh/test_mesh_hexes.exo
\ No newline at end of file
diff --git a/tests/unit_tests/test_mg_inverse_velocity.py b/tests/unit_tests/test_mg_inverse_velocity.py
new file mode 100644
index 000000000..b520e48d1
--- /dev/null
+++ b/tests/unit_tests/test_mg_inverse_velocity.py
@@ -0,0 +1,59 @@
+import openmc
+import numpy as np
+import pytest
+
+@pytest.fixture
+def one_group_lib():
+    groups = openmc.mgxs.EnergyGroups([0.0, 20.0e6])
+    xsdata = openmc.XSdata('slab_mat', groups)
+    xsdata.order = 0
+    xsdata.set_total([0.0])               
+    xsdata.set_absorption([0.0])
+    xsdata.set_scatter_matrix([[[0.0]]])
+    
+    mg_library = openmc.MGXSLibrary(groups)
+    mg_library.add_xsdata(xsdata)
+    name = 'mgxs.h5'
+    mg_library.export_to_hdf5(name)
+    yield name
+
+@pytest.fixture
+def slab_model(one_group_lib):
+    model = openmc.Model()
+    mat = openmc.Material(name='slab_material')
+    mat.set_density('macro', 1.0)
+    mat.add_macroscopic('slab_mat')
+
+    model.materials = openmc.Materials([mat])
+    model.materials.cross_sections = one_group_lib
+    
+    x_min = openmc.XPlane(x0=0.0, boundary_type='vacuum')
+    x_max = openmc.XPlane(x0=10.0, boundary_type='vacuum')
+
+    y_min = openmc.YPlane(y0=-10.0, boundary_type='vacuum')
+    y_max = openmc.YPlane(y0=10.0, boundary_type='vacuum')
+    z_min = openmc.ZPlane(z0=-10.0, boundary_type='vacuum')
+    z_max = openmc.ZPlane(z0=19.0, boundary_type='vacuum')
+
+    cell = openmc.Cell(fill=mat, region=+z_min & -x_max & +y_min & -y_max & +z_min & -z_max)
+    model.geometry = openmc.Geometry([cell])
+
+    model.settings = openmc.Settings()
+    model.settings.energy_mode = 'multi-group'
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 3
+    model.settings.particles = 10
+
+    source = openmc.IndependentSource()
+    source.space = openmc.stats.Point((5.0, 0.0, 0.0))
+    model.settings.source = source
+    return model
+
+def test_inverse_velocity(run_in_tmpdir, slab_model):
+    tally = openmc.Tally()
+    tally.scores = ['flux','inverse-velocity']
+    slab_model.tallies = [tally]
+    slab_model.run(apply_tally_results=True)
+    inverse_velocity = tally.mean.squeeze()[1]/tally.mean.squeeze()[0]
+
+    assert inverse_velocity == pytest.approx(1.6144e-5, rel=1e-4)
diff --git a/tests/unit_tests/test_mgxs_convert_flux.py b/tests/unit_tests/test_mgxs_convert_flux.py
new file mode 100644
index 000000000..e235f88e0
--- /dev/null
+++ b/tests/unit_tests/test_mgxs_convert_flux.py
@@ -0,0 +1,62 @@
+"""Tests for openmc.mgxs.convert_flux_groups function."""
+
+import numpy as np
+import pytest
+from pytest import approx
+
+import openmc.mgxs
+
+
+def test_coarse_to_fine():
+    """Test coarse to fine conversion with flux conservation."""
+    source = openmc.mgxs.EnergyGroups([1.0, 10.0, 100.0])
+    target = openmc.mgxs.EnergyGroups([1.0, 5.0, 10.0, 50.0, 100.0])
+    flux_source = np.array([1e8, 2e8])
+
+    flux_target = openmc.mgxs.convert_flux_groups(flux_source, source, target)
+
+    # Check conservation
+    assert np.sum(flux_target) == approx(np.sum(flux_source))
+    assert len(flux_target) == 4
+    assert np.all(flux_target >= 0)
+
+
+def test_fine_to_coarse():
+    """Test fine to coarse conversion (reverse direction)."""
+    source = openmc.mgxs.EnergyGroups([1.0, 5.0, 10.0, 50.0, 100.0])
+    target = openmc.mgxs.EnergyGroups([1.0, 10.0, 100.0])
+    flux_source = np.array([1e7, 2e7, 3e7, 4e7])
+
+    flux_target = openmc.mgxs.convert_flux_groups(flux_source, source, target)
+
+    assert np.sum(flux_target) == approx(np.sum(flux_source))
+    assert len(flux_target) == 2
+
+
+def test_lethargy_distribution():
+    """Test that flux is distributed by lethargy, not linear energy."""
+    # Single group from 1 to 100 eV
+    source = openmc.mgxs.EnergyGroups([1.0, 100.0])
+    # Split into two groups: 1-10 eV and 10-100 eV
+    target = openmc.mgxs.EnergyGroups([1.0, 10.0, 100.0])
+    flux_source = np.array([1e8])
+
+    flux_target = openmc.mgxs.convert_flux_groups(flux_source, source, target)
+
+    # Each target group spans one decade (ln(10) lethargy each)
+    # So flux should be split 50/50 by lethargy
+    assert flux_target[0] == approx(5e7)
+    assert flux_target[1] == approx(5e7)
+
+
+def test_fns_ccfe709_to_ukaea1102():
+    """Test CCFE-709 to UKAEA-1102 conversion with real FNS flux spectrum."""
+    from pathlib import Path
+    flux_file = Path(__file__).parent.parent / 'fns_flux_709.npy'
+    fns_flux_709 = np.load(flux_file)
+
+    flux_1102 = openmc.mgxs.convert_flux_groups(fns_flux_709, 'CCFE-709', 'UKAEA-1102')
+
+    assert len(flux_1102) == 1102
+    assert np.sum(flux_1102) == approx(np.sum(fns_flux_709), rel=1e-10)
+    assert np.all(flux_1102 >= 0)
diff --git a/tests/unit_tests/test_model.py b/tests/unit_tests/test_model.py
index ee5d8895c..f60c458b6 100644
--- a/tests/unit_tests/test_model.py
+++ b/tests/unit_tests/test_model.py
@@ -7,6 +7,7 @@ import pytest
 
 import openmc
 import openmc.lib
+from openmc.plots import id_map_to_rgb
 
 
 @pytest.fixture(scope='function')
@@ -73,13 +74,13 @@ def pin_model_attributes():
     tal.scores = ['flux', 'fission']
     tals.append(tal)
 
-    plot1 = openmc.Plot(plot_id=1)
+    plot1 = openmc.SlicePlot(plot_id=1)
     plot1.origin = (0., 0., 0.)
     plot1.width = (pitch, pitch)
     plot1.pixels = (300, 300)
     plot1.color_by = 'material'
     plot1.filename = 'test'
-    plot2 = openmc.Plot(plot_id=2)
+    plot2 = openmc.SlicePlot(plot_id=2)
     plot2.origin = (0., 0., 0.)
     plot2.width = (pitch, pitch)
     plot2.pixels = (300, 300)
@@ -251,10 +252,11 @@ def test_import_properties(run_in_tmpdir, mpi_intracomm):
     model = openmc.examples.pwr_pin_cell()
     model.init_lib(output=False, intracomm=mpi_intracomm)
 
-    # Change fuel temperature and density and export properties
+    # Change cell fuel temperature, density, material density and export properties
     cell = openmc.lib.cells[1]
     cell.set_temperature(600.0)
     cell.fill.set_density(5.0, 'g/cm3')
+    cell.set_density(10.0)
     openmc.lib.export_properties(output=False)
 
     # Import properties to existing model
@@ -263,10 +265,12 @@ def test_import_properties(run_in_tmpdir, mpi_intracomm):
     # Check to see that values are assigned to the C and python representations
     # First python
     cell = model.geometry.get_all_cells()[1]
-    assert cell.temperature == [600.0]
+    assert cell.temperature == 600.0
+    assert cell.density == pytest.approx(10.0, 1e-5)
     assert cell.fill.get_mass_density() == pytest.approx(5.0)
     # Now C
     assert openmc.lib.cells[1].get_temperature() == 600.
+    assert openmc.lib.cells[1].get_density() == pytest.approx(10.0, 1e-5)
     assert openmc.lib.materials[1].get_density('g/cm3') == pytest.approx(5.0)
 
     # Clear the C API
@@ -282,7 +286,8 @@ def test_import_properties(run_in_tmpdir, mpi_intracomm):
         'with_properties/settings.xml'
     )
     cell = model_with_properties.geometry.get_all_cells()[1]
-    assert cell.temperature == [600.0]
+    assert cell.temperature == 600.0
+    assert cell.density == pytest.approx(10.0, 1e-5)
     assert cell.fill.get_mass_density() == pytest.approx(5.0)
 
 
@@ -448,7 +453,7 @@ def test_deplete(run_in_tmpdir, pin_model_attributes, mpi_intracomm):
     # In this test we first run without pre-initializing the shared library
     # data and then compare. Then we repeat with the C API already initialized
     # and make sure we get the same answer
-    test_model.deplete([1e6], 'predictor', final_step=False,
+    test_model.deplete(timesteps=[1e6], method='predictor', final_step=False,
                        operator_kwargs=op_kwargs,
                        power=1., output=False)
     # Get the new Xe136 and U235 atom densities
@@ -482,7 +487,7 @@ def test_deplete(run_in_tmpdir, pin_model_attributes, mpi_intracomm):
 
     # Now we can re-run with the pre-initialized API
     test_model.init_lib(output=False, intracomm=mpi_intracomm)
-    test_model.deplete([1e6], 'predictor', final_step=False,
+    test_model.deplete(timesteps=[1e6], method='predictor', final_step=False,
                        operator_kwargs=op_kwargs,
                        power=1., output=False)
     # Get the new Xe136 and U235 atom densities
@@ -649,3 +654,439 @@ def test_model_plot():
     # ensure that all of the data in the image data is either white or red
     test_mask = (image_data == white) | (image_data == red)
     assert np.all(test_mask), "Colors other than white or red found in overlap plot image"
+
+    # Close plots to avoid warning
+    import matplotlib.pyplot as plt
+    plt.close('all')
+
+
+def test_model_plot_invalid_inputs():
+    surface = openmc.Sphere(r=10.0, boundary_type="vacuum")
+    cell = openmc.Cell(region=-surface)
+    model = openmc.Model(openmc.Geometry([cell]))
+
+    with pytest.raises(ValueError):
+        model.plot(n_samples=-1)
+    with pytest.raises(TypeError):
+        model.plot(n_samples=1.5)
+    with pytest.raises(ValueError):
+        model.plot(plane_tolerance=0.0)
+    with pytest.raises(TypeError):
+        model.plot(plane_tolerance='1')
+    with pytest.raises(ValueError):
+        model.plot(pixels=-1)
+    with pytest.raises(ValueError):
+        model.plot(pixels=(0, 100))
+    with pytest.raises(ValueError):
+        model.plot(pixels=(100,))
+    with pytest.raises(ValueError):
+        model.slice_data(u_span=(2, 0, 0), v_span=(0, 2, 0), pixels=-1)
+
+
+def test_model_id_map_initialization(run_in_tmpdir):
+    model = openmc.examples.pwr_assembly()
+    model.init_lib(output=False)
+
+    id_map = model.id_map(
+        pixels=(100, 100),
+        basis='xy',
+        origin=(0, 0, 0),
+        width=(10, 10),
+    )
+
+    assert id_map.shape == (100, 100, 3)
+    assert id_map.dtype == np.int32
+
+    max_cell_id = max(model.geometry.get_all_cells().keys())
+    max_material_id = max(model.geometry.get_all_materials().keys())
+
+    # add some spot checks for the id_map
+    # Check that the array contains valid cell/material IDs (not all -2)
+    # The -2 values indicate outside the geometry
+    assert not np.all(id_map == -2), "All values are -2, indicating no valid geometry found"
+
+    # Check that we have valid cell IDs (first dimension)
+    valid_cell_ids = id_map[:, :, 0]
+    assert np.any(valid_cell_ids >= 0), "No valid cell IDs found in the id_map"
+
+    # Check that we have valid material IDs (third dimension)
+    valid_material_ids = id_map[:, :, 2]
+    assert np.any(valid_material_ids >= 0), "No valid material IDs found in the id_map"
+
+    # Check that the middle dimension (cell instances) is consistent
+    # Cell instances should be >= 0 when cell IDs are valid
+    cell_instances = id_map[:, :, 1]
+    valid_cells = valid_cell_ids >= 0
+    if np.any(valid_cells):
+        assert np.all(cell_instances[valid_cells] >= 0), "Invalid cell instances found for valid cells"
+
+    # Check that the array contains reasonable ranges of values
+    # Cell IDs should be within the expected range for the assembly
+    if np.any(valid_cell_ids >= 0):
+        max_map_cell_id = np.max(valid_cell_ids)
+        assert max_map_cell_id <= max_cell_id, \
+            f"Cell ID {max_map_cell_id} in the map is greater than the maximum cell ID {max_cell_id}"
+
+    # Material IDs should be within the expected range
+    if np.any(valid_material_ids >= 0):
+        max_map_material_id = np.max(valid_material_ids)
+        assert max_map_material_id <= max_material_id, \
+            f"Material ID {max_map_material_id} in the map is greater than the maximum material ID {max_material_id}"
+
+    # Test id_map with pixels outside the model geometry
+    # Use a plot that's far from the model center to ensure we get -2 values
+    outside_id_map = model.id_map(
+        pixels=(50, 50),
+        basis='xy',
+        origin=(1000, 1000, 0),  # Far from the model center
+        width=(10, 10),
+    )
+
+    assert outside_id_map.shape == (50, 50, 3)
+    assert outside_id_map.dtype == np.int32
+
+    # All values should be -2 (outside geometry) for this plot
+    assert np.all(outside_id_map == -2), "Expected all values to be -2 for plot outside model geometry"
+
+    # Verify that the outside plot has the correct structure
+    assert np.all(outside_id_map[:, :, 0] == -2), "Cell IDs should all be -2 outside geometry"
+    assert np.all(outside_id_map[:, :, 1] == -2), "Cell instances should all be -2 outside geometry"
+    assert np.all(outside_id_map[:, :, 2] == -2), "Material IDs should all be -2 outside geometry"
+
+    # if the model is already initialized, it should not be finalized
+    # after calling this method
+    model.id_map(
+        pixels=(100, 100),
+        basis='xy',
+        origin=(0, 0, 0),
+        width=(10, 10),
+    )
+    assert model.is_initialized
+
+    # if the model is not initialized, it should be finalized
+    # before exiting this method
+    model.finalize_lib()
+    model.id_map(
+        pixels=(100, 100),
+        basis='xy',
+        origin=(0, 0, 0),
+        width=(10, 10),
+    )
+    assert not model.is_initialized
+
+
+def test_id_map_aligned_model():
+    """Test id_map with a 2x2 lattice where pixel boundaries align to cell boundaries"""
+    # Create materials -- identical compositions, different IDs
+    mat1 = openmc.Material(material_id=1, name='Material 1')
+    mat1.set_density('g/cm3', 1.0)
+    mat1.add_element('H', 1.0)
+
+    mat2 = openmc.Material(material_id=2, name='Material 2')
+    mat2.set_density('g/cm3', 1.0)
+    mat2.add_element('H', 1.0)
+
+    mat3 = openmc.Material(material_id=3, name='Material 3')
+    mat3.set_density('g/cm3', 1.0)
+    mat3.add_element('H', 1.0)
+
+    mat4 = openmc.Material(material_id=4, name='Material 4')
+    mat4.set_density('g/cm3', 1.0)
+    mat4.add_element('H', 1.0)
+
+    outer_mat = openmc.Material(material_id=5, name='Material 5')
+    outer_mat.set_density('g/cm3', 1.0)
+    outer_mat.add_element('H', 1.0)
+
+    inner_materials = [mat1, mat2, mat3, mat4]
+
+    # Create square surface that fits inside the lattice cell
+    # Lattice cell is 1 cm x 1 cm, so square will be 0.6 cm x 0.6 cm centered on the origin
+    square = openmc.model.RectangularPrism(0.6, 0.6, boundary_type='transmission')
+
+    # Create cells for this universe
+    inner_cell = openmc.Cell(cell_id=10, region=-square, name='inner_cell')
+    inner_cell.fill = inner_materials
+
+    outer_cell = openmc.Cell(cell_id=20, region=+square, name='outer_cell')
+    outer_cell.fill = outer_mat
+
+    # Create universe
+    universe = openmc.Universe(universe_id=100, cells=[inner_cell, outer_cell])
+
+    # Create 2x2 lattice
+    lattice = openmc.RectLattice(lattice_id=1)
+    lattice.lower_left = [-1.0, -1.0]
+    lattice.pitch = [1.0, 1.0]
+    lattice.universes = [[universe, universe], [universe, universe]]
+
+    # Create outer boundary
+    outer_boundary = openmc.model.RectangularPrism(2.0, 2.0, boundary_type='vacuum')
+
+    # Create root cell
+    root_cell = openmc.Cell(cell_id=1, name='root', fill=lattice, region=-outer_boundary)
+
+    # Create geometry
+    geometry = openmc.Geometry([root_cell])
+
+    # Create settings
+    settings = openmc.Settings()
+    settings.particles = 1000
+    settings.batches = 10
+
+    # Create model
+    model = openmc.Model(settings=settings, geometry=geometry)
+
+    # Generate id_map with pixel boundaries aligned to cell boundaries
+    # The model is 2 cm x 2 cm, so we'll use 200x200 pixels to get 0.01 cm resolution
+    # This allows us to align pixels with the squares inside each lattice cell
+    id_map = model.id_map(
+        pixels=(200, 200),
+        basis='xy',
+        origin=(0.0, 0.0, 0.0),  # Align with lattice lower_left
+        width=(2.0, 2.0),     # Align with lattice size
+    )
+
+    # Verify id_map properties
+    assert id_map.shape == (200, 200, 3)
+    assert id_map.dtype == np.int32
+
+    cell_id_map = id_map[:, :, 0]
+    material_ids_map = id_map[:, :, 2]
+
+    # Check that we have valid cell IDs (not all -2)
+    assert np.any(cell_id_map >= 0), "No valid cell IDs found in the id_map"
+
+    # Check that we have valid material IDs
+    assert np.any(material_ids_map >= 0), "No valid material IDs found in the id_map"
+
+    # Check that the expected cell IDs are present
+    expected_cell_ids = [10, 20]  # Root cell, inner cell, outer cell
+    found_cell_ids = np.unique(cell_id_map[cell_id_map >= 0])
+    for cell_id in expected_cell_ids:
+        assert cell_id in found_cell_ids, f"Expected cell ID {cell_id} not found in id_map"
+
+    # Check that the expected material IDs are present
+    expected_material_ids = [1, 2, 3, 4, 5]  # All materials defined above
+    found_material_ids = np.unique(material_ids_map[material_ids_map >= 0])
+    for mat_id in expected_material_ids:
+        assert mat_id in found_material_ids, f"Expected material ID {mat_id} not found in id_map"
+
+    # Test specific regions to verify lattice structure
+    # Check center of each lattice cell (should be inner cells)
+    # Lattice cell centers are at (-0.5, -0.5), (0.5, -0.5), (-0.5, 0.5), (0.5, 0.5)
+    # With 200x200 pixels over 2x2 units, each pixel is 0.01 units
+
+    # Bottom-left lattice cell center (should be inner cell 10)
+    bl_cell, bl_instance, bl_material = id_map[-50, 50]
+    assert bl_cell == 10, f"Expected cell ID 10 at bottom-left center, got {bl_cell}"
+    assert bl_instance == 0, f"Expected cell instance 0 at bottom-left center, got {bl_instance}"
+    assert bl_material == 1, f"Expected material ID 1 at bottom-left center, got {bl_material}"
+
+    # Bottom-right lattice cell center (should be inner cell 10)
+    br_cell, br_instance, br_material = id_map[-50, 150]
+    assert br_cell == 10, f"Expected cell ID 10 at bottom-right center, got {br_cell}"
+    assert br_instance == 1, f"Expected cell instance 1 at bottom-right center, got {br_instance}"
+    assert br_material == 2, f"Expected material ID 2 at bottom-right center, got {br_material}"
+
+    # Top-left lattice cell center (should be inner cell 10)
+    tl_cell, tl_instance, tl_material = id_map[-150, 50]
+    assert tl_cell == 10, f"Expected cell ID 10 at top-left center, got {tl_cell}"
+    assert tl_instance == 2, f"Expected cell instance 2 at top-left center, got {tl_instance}"
+    assert tl_material == 3, f"Expected material ID 3 at top-left center, got {tl_material}"
+
+    # Top-right lattice cell center (should be inner cell 10)
+    tr_cell, tr_instance, tr_material = id_map[-150, 150]
+    assert tr_cell == 10, f"Expected cell ID 10 at top-right center, got {tr_cell}"
+    assert tr_instance == 3, f"Expected cell instance 3 at top-right center, got {tr_instance}"
+    assert tr_material == 4, f"Expected material ID 4 at top-right center, got {tr_material}"
+
+    # Check that the model is properly finalized after id_map call
+    assert not model.is_initialized, "Model should be finalized after id_map call"
+
+    # Check that the values at the corners are correctly set as the outer cell and material
+    bl_cell, bl_instance, bl_material = id_map[-1, 0]
+    assert bl_cell == 20, f"Expected cell ID 20 at bottom-left corner, got {bl_cell}"
+    assert bl_instance == 0, f"Expected cell instance 0 at bottom-left corner, got {bl_instance}"
+    assert bl_material == 5, f"Expected material ID 5 at bottom-left corner, got {bl_material}"
+
+    br_cell, br_instance, br_material = id_map[-1, -1]
+    assert br_cell == 20, f"Expected cell ID 20 at bottom-right corner, got {br_cell}"
+    assert br_instance == 1, f"Expected cell instance 1 at bottom-right corner, got {br_instance}"
+    assert br_material == 5, f"Expected material ID 5 at bottom-right corner, got {br_material}"
+
+    tl_cell, tl_instance, tl_material = id_map[0, 0]
+    assert tl_cell == 20, f"Expected cell ID 20 at top-left corner, got {tl_cell}"
+    assert tl_instance == 2, f"Expected cell instance 2 at top-left corner, got {tl_instance}"
+    assert tl_material == 5, f"Expected material ID 5 at top-left corner, got {tl_material}"
+
+    tr_cell, tr_instance, tr_material = id_map[0, -1]
+    assert tr_cell == 20, f"Expected cell ID 20 at top-right corner, got {tr_cell}"
+    assert tr_instance == 3, f"Expected cell instance 3 at top-right corner, got {tr_instance}"
+    assert tr_material == 5, f"Expected material ID 5 at top-right corner, got {tr_material}"
+
+
+def test_id_map_model_with_overlaps():
+    """Test id_map with a model that has overlaps and color_overlaps option"""
+    surface1 = openmc.Sphere(r=50, boundary_type="vacuum")
+    surface2 = openmc.Sphere(r=30)
+    cell1 = openmc.Cell(region=-surface1)
+    cell2 = openmc.Cell(region=-surface2)
+    geometry = openmc.Geometry([cell1, cell2])
+    settings = openmc.Settings()
+    model = openmc.Model(geometry=geometry, settings=settings)
+    id_slice = model.id_map(
+        pixels=(10, 10),
+        basis='xy',
+        origin=(0, 0, 0),
+        width=(100, 100),
+    )
+    assert -3 not in id_slice  # -3 indicates overlap region
+    id_slice = model.id_map(
+        pixels=(10, 10),
+        basis='xy',
+        origin=(0, 0, 0),
+        width=(100, 100),
+        color_overlaps=True,  # enables id_map to return -3 for overlaps
+    )
+    assert -3 in id_slice
+
+
+def test_setter_from_list():
+    mat = openmc.Material()
+    model = openmc.Model(materials=[mat])
+    assert isinstance(model.materials, openmc.Materials)
+
+    tally = openmc.Tally()
+    model = openmc.Model(tallies=[tally])
+    assert isinstance(model.tallies, openmc.Tallies)
+
+    plot = openmc.SlicePlot()
+    model = openmc.Model(plots=[plot])
+    assert isinstance(model.plots, openmc.Plots)
+
+
+def test_keff_search(run_in_tmpdir):
+    """Test the Model.keff_search method"""
+
+    # Create model of a sphere of U235
+    mat = openmc.Material()
+    mat.set_density('g/cm3', 18.9)
+    mat.add_nuclide('U235', 1.0)
+    sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(fill=mat, region=-sphere)
+    geometry = openmc.Geometry([cell])
+    settings = openmc.Settings(particles=1000, inactive=10, batches=30)
+    model = openmc.Model(geometry=geometry, settings=settings)
+
+    # Define function to modify sphere radius
+    def modify_radius(radius):
+        sphere.r = radius
+
+    # Perform keff search
+    k_tol = 4e-3
+    sigma_final = 2e-3
+    result = model.keff_search(
+        func=modify_radius,
+        x0=6.0,
+        x1=9.0,
+        k_tol=k_tol,
+        sigma_final=sigma_final,
+        output=True,
+    )
+
+    final_keff = result.means[-1] + 1.0  # Add back target since means are (keff - target)
+    final_sigma = result.stdevs[-1]
+
+    # Check for convergence and that tolerances are met
+    assert result.converged, "keff_search did not converge"
+    assert abs(final_keff - 1.0) <= k_tol, \
+        f"Final keff {final_keff:.5f} not within k_tol {k_tol}"
+    assert final_sigma <= sigma_final, \
+        f"Final uncertainty {final_sigma:.5f} exceeds sigma_final {sigma_final}"
+
+    # Check type of result
+    assert isinstance(result, openmc.model.SearchResult)
+
+    # Check that we have function evaluation history
+    assert len(result.parameters) >= 2
+    assert len(result.means) == len(result.parameters)
+    assert len(result.stdevs) == len(result.parameters)
+    assert len(result.batches) == len(result.parameters)
+
+    # Check that function_calls property works
+    assert result.function_calls == len(result.parameters)
+
+    # Check that total_batches property works
+    assert result.total_batches == sum(result.batches)
+    assert result.total_batches > 0
+
+
+def test_id_map_to_rgb():
+    """Test conversion of ID map to RGB image array."""
+    # Create a simple model
+    mat = openmc.Material()
+    mat.set_density('g/cm3', 1.0)
+    mat.add_nuclide('Li7', 1.0)
+
+    sphere = openmc.Sphere(r=5.0, boundary_type='vacuum')
+    cell = openmc.Cell(fill=mat, region=-sphere)
+    geometry = openmc.Geometry([cell])
+    settings = openmc.Settings(
+        batches=10, particles=100, run_mode='fixed source'
+    )
+    model = openmc.Model(geometry, settings=settings)
+
+    id_data = np.zeros((10, 10, 3), dtype=np.int32)
+    id_data[:, :, 0] = cell.id  # Cell IDs
+    id_data[:, :, 2] = mat.id   # Material IDs
+
+    # Test color_by with default colors
+    for color_by in ['cell', 'material']:
+        rgb = id_map_to_rgb(id_data, color_by=color_by)
+        assert rgb.shape == (10, 10, 3)
+        assert rgb.dtype == float
+        assert np.all((rgb >= 0) & (rgb <= 1))  # RGB values in [0, 1]
+
+    # Test with custom colors
+    colors = {cell.id: (255, 0, 0)}  # Red
+    rgb_custom = id_map_to_rgb(id_data, color_by='cell', colors=colors)
+    assert np.allclose(rgb_custom, [1.0, 0.0, 0.0])  # All pixels should be red
+
+    # Test with overlaps
+    id_data_overlap = id_data.copy()
+    id_data_overlap[5:, 5:, 0] = -3  # Mark some pixels as overlaps
+    rgb_overlap = id_map_to_rgb(
+        id_data_overlap, overlap_color=(0, 255, 0)
+    )
+    # Check that overlap region is green
+    assert np.allclose(rgb_overlap[5:, 5:], [0.0, 1.0, 0.0])
+
+
+def test_convert_to_multigroup_preserves_material_names(run_in_tmpdir):
+    """convert_to_multigroup leaves the user's material names unchanged and keys
+    the MGXS library by a unique sanitised name + id, so distinct materials that
+    share a name do not collapse to a single cross section."""
+    a = openmc.Material(name="Steel Plate #1")
+    a.add_element("Fe", 1.0)
+    a.set_density("g/cm3", 7.9)
+    b = openmc.Material(name="Steel Plate #1")  # same name, distinct material
+    b.add_element("Fe", 1.0)
+    b.set_density("g/cm3", 7.9)
+
+    s1 = openmc.Sphere(r=1.0)
+    s2 = openmc.Sphere(r=2.0, boundary_type="vacuum")
+    c1 = openmc.Cell(fill=a, region=-s1)
+    c2 = openmc.Cell(fill=b, region=+s1 & -s2)
+    model = openmc.Model(openmc.Geometry([c1, c2]), openmc.Materials([a, b]))
+
+    # Pre-create the library so MGXS generation (and transport) is skipped.
+    Path("mgxs.h5").touch()
+    model.convert_to_multigroup(method="material_wise", mgxs_path="mgxs.h5")
+
+    # User names are preserved, not sanitised or de-duplicated.
+    assert [m.name for m in model.materials] == ["Steel Plate #1", "Steel Plate #1"]
+    # Each material reads a unique, sanitised library entry (name + id).
+    macro = [m._macroscopic for m in model.materials]
+    assert macro == [f"Steel_Plate__1_{a.id}", f"Steel_Plate__1_{b.id}"]
+    assert len(set(macro)) == 2
diff --git a/tests/unit_tests/test_no_reduce.py b/tests/unit_tests/test_no_reduce.py
new file mode 100644
index 000000000..00ddb5a95
--- /dev/null
+++ b/tests/unit_tests/test_no_reduce.py
@@ -0,0 +1,40 @@
+"""Test the settings.no_reduce feature to ensure tallies are correctly
+reduced across MPI processes."""
+
+import openmc
+import pytest
+
+from tests.testing_harness import config
+
+
+@pytest.mark.parametrize('no_reduce', [True, False])
+def test_no_reduce(no_reduce, run_in_tmpdir):
+    """Test that tally results are correct with and without no_reduce."""
+
+    # Create simple sphere model with vacuum
+    model = openmc.Model()
+    sphere = openmc.Sphere(r=1.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere)
+    model.geometry = openmc.Geometry([cell])
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 10
+    model.settings.particles = 100
+    model.settings.source = openmc.IndependentSource(space=openmc.stats.Point())
+    model.settings.no_reduce = no_reduce
+
+    # Tally: surface current on vacuum boundary
+    surf_filter = openmc.SurfaceFilter(sphere)
+    tally = openmc.Tally()
+    tally.filters = [surf_filter]
+    tally.scores = ['current']
+    model.tallies = [tally]
+
+    # Run OpenMC with proper MPI arguments if needed
+    kwargs = {'apply_tally_results': True, 'openmc_exec': config['exe']}
+    if config['mpi']:
+        kwargs['mpi_args'] = [config['mpiexec'], '-n', config['mpi_np']]
+    model.run(**kwargs)
+
+    # The tally should be ~1.0 (every particle crosses the surface once)
+    tally_mean = tally.mean.flatten()[0]
+    assert tally_mean == pytest.approx(1.0)
diff --git a/tests/unit_tests/test_nuclide_heating.py b/tests/unit_tests/test_nuclide_heating.py
new file mode 100644
index 000000000..e00d57b4f
--- /dev/null
+++ b/tests/unit_tests/test_nuclide_heating.py
@@ -0,0 +1,39 @@
+import openmc
+from pytest import approx
+
+
+def test_nuclide_heating(run_in_tmpdir):
+    mat = openmc.Material()
+    mat.add_nuclide("Li6", 0.5)
+    mat.add_nuclide("Li7", 0.5)
+    mat.set_density("g/cm3", 1.0)
+
+    sphere = openmc.Sphere(r=20, boundary_type="reflective")
+    inside_sphere = openmc.Cell(fill=mat, region=-sphere)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([inside_sphere])
+
+    model.settings.particles = 1000
+    model.settings.batches = 1
+    model.settings.photon_transport = True
+    model.settings.electron_treatment = "ttb"
+    model.settings.cutoff = {"energy_photon": 1000}
+    model.settings.run_mode = "fixed source"
+    model.settings.source = openmc.IndependentSource(
+        energy=openmc.stats.delta_function(10.0e6),
+        particle="photon"
+    )
+
+    # Create two tallies, one with heating by nuclide and one with total heating
+    tally1 = openmc.Tally()
+    tally1.scores = ["heating"]
+    tally1.nuclides = mat.get_nuclides()
+    tally2 = openmc.Tally()
+    tally2.scores = ["heating"]
+    model.tallies = [tally1, tally2]
+
+    # Run the model
+    model.run(apply_tally_results=True)
+
+    # Make sure the heating results are consistent
+    assert tally1.mean.sum() == approx(tally2.mean.sum())
diff --git a/tests/unit_tests/test_particle_type.py b/tests/unit_tests/test_particle_type.py
new file mode 100644
index 000000000..67e385ea6
--- /dev/null
+++ b/tests/unit_tests/test_particle_type.py
@@ -0,0 +1,298 @@
+"""Unit tests for ParticleType class."""
+
+import pytest
+from openmc import ParticleType
+
+
+# Tests for creating ParticleType instances
+
+def test_create_from_int():
+    """Test creation from PDG number."""
+    p = ParticleType(2112)
+    assert p.pdg_number == 2112
+    assert int(p) == 2112
+
+
+def test_create_from_string_name():
+    """Test creation from particle name."""
+    p = ParticleType('neutron')
+    assert p.pdg_number == 2112
+
+    p = ParticleType('photon')
+    assert p.pdg_number == 22
+
+
+def test_create_from_string_aliases():
+    """Test creation from particle aliases."""
+    assert ParticleType('n').pdg_number == 2112
+    assert ParticleType('gamma').pdg_number == 22
+    assert ParticleType('p').pdg_number == 2212
+    assert ParticleType('proton').pdg_number == 2212
+    assert ParticleType('d').pdg_number == 1000010020
+    assert ParticleType('t').pdg_number == 1000010030
+
+
+def test_create_from_string_nuclide():
+    """Test creation from GNDS nuclide name."""
+    p = ParticleType('He4')
+    assert p.pdg_number == 1000020040
+
+    p = ParticleType('U235')
+    assert p.pdg_number == 1000922350
+
+    p = ParticleType('Am242_m1')
+    assert p.pdg_number == 1000952421
+
+
+def test_create_from_string_pdg_prefix():
+    """Test creation with pdg: prefix."""
+    p = ParticleType('pdg:2112')
+    assert p.pdg_number == 2112
+
+    p = ParticleType('PDG:22')
+    assert p.pdg_number == 22
+
+
+def test_create_from_particle_type():
+    """Test creation from existing ParticleType."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(p1)
+    assert p1 == p2
+    assert p1 is not p2  # Different instances
+
+
+def test_legacy_particle_indices():
+    """Test backward compatibility with legacy indices 0-3."""
+    assert ParticleType(0) == ParticleType.NEUTRON
+    assert ParticleType(1) == ParticleType.PHOTON
+    assert ParticleType(2) == ParticleType.ELECTRON
+    assert ParticleType(3) == ParticleType.POSITRON
+
+
+def test_create_invalid_type():
+    """Test creation with invalid type raises TypeError."""
+    with pytest.raises(TypeError):
+        ParticleType([2112])
+
+    with pytest.raises(TypeError):
+        ParticleType({'pdg': 2112})
+
+
+def test_create_invalid_string():
+    """Test creation with invalid string raises ValueError."""
+    with pytest.raises(ValueError):
+        ParticleType('')
+
+    with pytest.raises(ValueError):
+        ParticleType('pdg:invalid')
+
+
+def test_create_case_insensitive():
+    """Test that string parsing is case insensitive for aliases."""
+    assert ParticleType('NEUTRON').pdg_number == 2112
+    assert ParticleType('Neutron').pdg_number == 2112
+    assert ParticleType('PHOTON').pdg_number == 22
+
+
+# Tests for equality and comparison
+
+def test_equality_same_pdg():
+    """Test that instances with same PDG are equal."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(2112)
+    assert p1 == p2
+
+
+def test_equality_int():
+    """Test equality comparison with int."""
+    p = ParticleType(2112)
+    assert p == 2112
+    assert 2112 == p
+
+
+def test_equality_string():
+    """Test equality comparison with string."""
+    p = ParticleType(2112)
+    assert p == 'neutron'
+    assert p == 'pdg:2112'
+    assert p == 'n'
+
+
+def test_inequality():
+    """Test inequality comparisons."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(22)
+    assert p1 != p2
+    assert p1 != 22
+    assert p1 != 'photon'
+
+
+def test_equality_with_constants():
+    """Test equality with class constants."""
+    p = ParticleType(2112)
+    assert p == ParticleType.NEUTRON
+    assert ParticleType.NEUTRON == p
+
+
+def test_equality_invalid_string():
+    """Test equality with invalid string returns False."""
+    p = ParticleType(2112)
+    assert not (p == 'invalid_particle')
+    assert p != 'invalid_particle'
+
+
+# Tests for hashing behavior
+
+def test_hash_consistency():
+    """Test that equal instances hash to the same value."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(2112)
+    assert hash(p1) == hash(p2)
+
+
+def test_set_deduplication():
+    """Test that equal instances deduplicate in sets."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(2112)
+    s = {p1, p2}
+    assert len(s) == 1
+
+
+def test_dict_key():
+    """Test use as dictionary key."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(2112)
+    d = {p1: 'neutron'}
+    assert d[p2] == 'neutron'
+
+
+def test_hash_different_particles():
+    """Test that different particles have different hashes (usually)."""
+    p1 = ParticleType(2112)
+    p2 = ParticleType(22)
+    # Different PDG numbers should (almost always) have different hashes
+    assert hash(p1) != hash(p2)
+
+
+# Tests for properties and computed attributes
+
+def test_pdg_number_property():
+    """Test pdg_number property."""
+    p = ParticleType(2112)
+    assert p.pdg_number == 2112
+    assert isinstance(p.pdg_number, int)
+
+
+def test_int_conversion():
+    """Test __int__ conversion."""
+    p = ParticleType(1000020040)
+    assert int(p) == 1000020040
+
+
+def test_zam_elementary():
+    """Test zam property for elementary particles."""
+    assert ParticleType.NEUTRON.zam is None
+    assert ParticleType.PHOTON.zam is None
+    assert ParticleType.ELECTRON.zam is None
+    assert ParticleType.POSITRON.zam is None
+
+
+def test_zam_nucleus():
+    """Test zam property for nuclear particles."""
+    he4 = ParticleType('He4')
+    assert he4.zam == (2, 4, 0)
+
+    u235 = ParticleType('U235')
+    Z, A, m = u235.zam
+    assert Z == 92
+    assert A == 235
+    assert m == 0
+
+
+def test_zam_metastable():
+    """Test zam property for metastable nuclei."""
+    # Am242m has m=1
+    am242m = ParticleType('Am242_m1')
+    Z, A, m = am242m.zam
+    assert Z == 95
+    assert A == 242
+    assert m == 1
+
+
+def test_is_nucleus_false():
+    """Test is_nucleus for elementary particles."""
+    assert not ParticleType.NEUTRON.is_nucleus
+    assert not ParticleType.PHOTON.is_nucleus
+    assert not ParticleType.ELECTRON.is_nucleus
+    assert not ParticleType.POSITRON.is_nucleus
+
+
+def test_is_nucleus_true():
+    """Test is_nucleus for nuclear particles."""
+    assert ParticleType.ALPHA.is_nucleus
+    assert ParticleType('He4').is_nucleus
+    assert ParticleType('U235').is_nucleus
+    assert ParticleType.DEUTERON.is_nucleus
+    assert ParticleType.TRITON.is_nucleus
+
+
+# Tests for __str__ and __repr__
+
+def test_str_elementary():
+    """Test string representation of elementary particles."""
+    assert str(ParticleType.NEUTRON) == 'neutron'
+    assert str(ParticleType.PHOTON) == 'photon'
+    assert str(ParticleType.ELECTRON) == 'electron'
+    assert str(ParticleType.POSITRON) == 'positron'
+    assert str(ParticleType.PROTON) == 'H1'
+
+
+def test_str_nucleus():
+    """Test string representation of nuclei."""
+    assert str(ParticleType.ALPHA) == 'He4'
+    assert str(ParticleType('U235')) == 'U235'
+    assert str(ParticleType.DEUTERON) == 'H2'
+    assert str(ParticleType.TRITON) == 'H3'
+
+
+def test_str_arbitrary_pdg():
+    """Test string representation of arbitrary PDG number."""
+    # PDG number that doesn't match any known particle
+    p = ParticleType(12345)
+    assert str(p) == 'pdg:12345'
+
+
+def test_repr():
+    """Test repr includes PDG number."""
+    p = ParticleType.NEUTRON
+    repr_str = repr(p)
+    assert 'ParticleType' in repr_str
+    assert 'PDG=2112' in repr_str
+    assert 'neutron' in repr_str
+
+
+def test_repr_nucleus():
+    """Test repr for nuclear particles."""
+    p = ParticleType.ALPHA
+    repr_str = repr(p)
+    assert 'ParticleType' in repr_str
+    assert 'PDG=1000020040' in repr_str
+    assert 'He4' in repr_str
+
+
+def test_create_from_numpy_int():
+    """Test creation from numpy integer types."""
+    import numpy as np
+    p = ParticleType(np.int32(2112))
+    assert p.pdg_number == 2112
+
+    p = ParticleType(np.int64(22))
+    assert p.pdg_number == 22
+
+
+def test_equality_numpy_int():
+    """Test equality comparison with numpy integer types."""
+    import numpy as np
+    p = ParticleType(2112)
+    assert p == np.int32(2112)
+    assert p == np.int64(2112)
diff --git a/tests/unit_tests/test_pathlike_simple.py b/tests/unit_tests/test_pathlike_simple.py
new file mode 100644
index 000000000..e0116bf01
--- /dev/null
+++ b/tests/unit_tests/test_pathlike_simple.py
@@ -0,0 +1,46 @@
+"""Simple test for PathLike filename support"""
+
+from pathlib import Path
+
+import pytest
+import openmc
+from openmc.checkvalue import check_type, PathLike
+
+
+def test_pathlike_type_checking():
+    """Test that PathLike type checking works correctly"""
+
+    # Test with string (should work)
+    check_type('filename', 'test.txt', PathLike)
+
+    # Test with Path object (should work)
+    path_obj = Path('test.txt')
+    check_type('filename', path_obj, PathLike)
+
+    # Test with Path object containing subdirectories (should work)
+    path_with_subdir = Path('subdir') / 'test.txt'
+    check_type('filename', path_with_subdir, PathLike)
+
+    # Test with invalid type (should raise TypeError)
+    with pytest.raises(TypeError):
+        check_type('filename', 123, PathLike)
+
+
+def test_plot_filename_pathlike():
+    """Test that plot filename accepts Path objects"""
+
+    plot = openmc.Plot()
+
+    # Test with string (should still work)
+    plot.filename = "test_plot"
+    assert plot.filename == "test_plot"
+
+    # Test with Path object
+    path_obj = Path("test_plot_path")
+    plot.filename = path_obj
+    assert plot.filename == path_obj
+
+    # Test with Path object containing subdirectories
+    path_with_subdir = Path("subdir") / "test_plot"
+    plot.filename = path_with_subdir
+    assert plot.filename == path_with_subdir
diff --git a/tests/unit_tests/test_periodic_bc.py b/tests/unit_tests/test_periodic_bc.py
new file mode 100644
index 000000000..4cc126abc
--- /dev/null
+++ b/tests/unit_tests/test_periodic_bc.py
@@ -0,0 +1,47 @@
+from math import cos, sin, radians
+import random
+
+import openmc
+import pytest
+
+
+@pytest.mark.parametrize("angle", [30., 45., 60., 90., 120.])
+def test_rotational_periodic_bc(angle):
+    # Pick random starting angle
+    start = random.uniform(0., 360.)
+    degrees = angle
+    ang1 = radians(start)
+    ang2 = radians(start + degrees)
+
+    # Define three points on each plane and then randomly shuffle them
+    p1_points = [(0., 0., 0.), (cos(ang1), sin(ang1), 0.), (0., 0., 1.)]
+    p2_points = [(0., 0., 0.), (cos(ang2), sin(ang2), 0.), (0., 0., 1.)]
+    random.shuffle(p1_points)
+    random.shuffle(p2_points)
+
+    # Create periodic planes and a cylinder
+    p1 = openmc.Plane.from_points(*p1_points, boundary_type='periodic')
+    p2 = openmc.Plane.from_points(*p2_points, boundary_type='periodic')
+    p1.periodic_surface = p2
+    zcyl = openmc.ZCylinder(r=5., boundary_type='vacuum')
+
+    # Figure out which side of planes to use based on a point in the middle
+    ang_mid = radians(start + degrees/2.)
+    mid_point = (cos(ang_mid), sin(ang_mid), 0.)
+    r1 = -p1 if mid_point in -p1 else +p1
+    r2 = -p2 if mid_point in -p2 else +p2
+
+    # Create one cell bounded by the two planes and the cylinder
+    mat = openmc.Material(density=1.0, density_units='g/cm3', components={'U235': 1.0})
+    cell = openmc.Cell(fill=mat, region=r1 & r2 & -zcyl)
+
+    # Make the model complete
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell])
+    model.settings.source = openmc.IndependentSource(space=openmc.stats.Point(mid_point))
+    model.settings.particles = 1000
+    model.settings.batches = 10
+    model.settings.inactive = 5
+
+    # Run the model
+    model.run()
diff --git a/tests/unit_tests/test_photon_heating.py b/tests/unit_tests/test_photon_heating.py
new file mode 100644
index 000000000..05473f5c6
--- /dev/null
+++ b/tests/unit_tests/test_photon_heating.py
@@ -0,0 +1,30 @@
+import openmc
+
+
+def test_negative_positron_heating():
+    m = openmc.Material()
+    m.add_element('Li', 1.0)
+    m.set_density('g/cm3', 10.0)
+
+    surf = openmc.Sphere(r=100.0, boundary_type='reflective')
+    cell = openmc.Cell(fill=m, region=-surf)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell])
+    model.settings.run_mode = 'fixed source'
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Point(),
+        energy=openmc.stats.Discrete([5.0e6], [1.0]),
+        particle='photon',
+    )
+    model.settings.particles = 7
+    model.settings.batches = 1
+    model.settings.electron_treatment = 'led'
+    model.settings.seed = 513836
+
+    tally = openmc.Tally()
+    tally.filters = [openmc.ParticleFilter(['photon', 'electron', 'positron'])]
+    tally.scores = ['heating']
+    model.tallies = openmc.Tallies([tally])
+    model.run(apply_tally_results=True)
+
+    assert (tally.mean >= 0.0).all(), "Negative heating detected"
diff --git a/tests/unit_tests/test_pin.py b/tests/unit_tests/test_pin.py
index 5496a3b73..53665b1a6 100644
--- a/tests/unit_tests/test_pin.py
+++ b/tests/unit_tests/test_pin.py
@@ -46,11 +46,6 @@ def test_failure(pin_mats, good_radii):
     with pytest.raises(ValueError, match="length"):
         pin(good_surfaces[:len(pin_mats) - 2], pin_mats)
 
-    # Non-positive radii
-    rad = [openmc.ZCylinder(r=-0.1)] + good_surfaces[1:]
-    with pytest.raises(ValueError, match="index 0"):
-        pin(rad, pin_mats)
-
     # Non-increasing radii
     surfs = tuple(reversed(good_surfaces))
     with pytest.raises(ValueError, match="index 1"):
diff --git a/tests/unit_tests/test_plots.py b/tests/unit_tests/test_plots.py
index fad574ee6..98a93e44b 100644
--- a/tests/unit_tests/test_plots.py
+++ b/tests/unit_tests/test_plots.py
@@ -9,12 +9,11 @@ from openmc.plots import _SVG_COLORS
 
 @pytest.fixture(scope='module')
 def myplot():
-    plot = openmc.Plot(name='myplot')
+    plot = openmc.SlicePlot(name='myplot')
     plot.width = (100., 100.)
     plot.origin = (2., 3., -10.)
     plot.pixels = (500, 500)
     plot.filename = './not-a-dir/myplot'
-    plot.type = 'slice'
     plot.basis = 'yz'
     plot.background = 'black'
     plot.background = (0, 0, 0)
@@ -80,8 +79,7 @@ def test_voxel_plot(run_in_tmpdir):
     geometry.export_to_xml()
     materials = openmc.Materials()
     materials.export_to_xml()
-    vox_plot = openmc.Plot()
-    vox_plot.type = 'voxel'
+    vox_plot = openmc.VoxelPlot()
     vox_plot.id = 12
     vox_plot.width = (1500., 1500., 1500.)
     vox_plot.pixels = (200, 200, 200)
@@ -97,8 +95,9 @@ def test_voxel_plot(run_in_tmpdir):
     assert Path('h5_voxel_plot.h5').is_file()
     assert Path('another_test_voxel_plot.vti').is_file()
 
-    slice_plot = openmc.Plot()
-    with pytest.raises(ValueError):
+    # SlicePlot should not have to_vtk method
+    slice_plot = openmc.SlicePlot()
+    with pytest.raises(AttributeError):
         slice_plot.to_vtk('shimmy.vti')
 
 
@@ -153,14 +152,14 @@ def test_from_geometry():
     geom = openmc.Geometry(univ)
 
     for basis in ('xy', 'yz', 'xz'):
-        plot = openmc.Plot.from_geometry(geom, basis)
+        plot = openmc.SlicePlot.from_geometry(geom, basis)
         assert plot.origin == pytest.approx((0., 0., 0.))
         assert plot.width == pytest.approx((width, width))
         assert plot.basis == basis
 
 
 def test_highlight_domains():
-    plot = openmc.Plot()
+    plot = openmc.SlicePlot()
     plot.color_by = 'material'
     plots = openmc.Plots([plot])
 
@@ -179,8 +178,8 @@ def test_xml_element(myplot):
     assert elem.find('pixels') is not None
     assert elem.find('background').text == '0 0 0'
 
-    newplot = openmc.Plot.from_xml_element(elem)
-    attributes = ('id', 'color_by', 'filename', 'type', 'basis', 'level',
+    newplot = openmc.SlicePlot.from_xml_element(elem)
+    attributes = ('id', 'color_by', 'filename', 'basis', 'level',
                   'meshlines', 'show_overlaps', 'origin', 'width', 'pixels',
                   'background', 'mask_background')
     for attr in attributes:
@@ -200,11 +199,11 @@ def test_to_xml_element_proj(myprojectionplot):
 
 
 def test_plots(run_in_tmpdir):
-    p1 = openmc.Plot(name='plot1')
+    p1 = openmc.SlicePlot(name='plot1')
     p1.origin = (5., 5., 5.)
     p1.colors = {10: (255, 100, 0)}
     p1.mask_components = [2, 4, 6]
-    p2 = openmc.Plot(name='plot2')
+    p2 = openmc.SlicePlot(name='plot2')
     p2.origin = (-3., -3., -3.)
     plots = openmc.Plots([p1, p2])
     assert len(plots) == 2
@@ -213,7 +212,7 @@ def test_plots(run_in_tmpdir):
     plots = openmc.Plots([p1, p2, p3])
     assert len(plots) == 3
 
-    p4 = openmc.Plot(name='plot4')
+    p4 = openmc.VoxelPlot(name='plot4')
     plots.append(p4)
     assert len(plots) == 4
 
@@ -230,8 +229,7 @@ def test_plots(run_in_tmpdir):
 
 def test_voxel_plot_roundtrip():
     # Define a voxel plot and create XML element
-    plot = openmc.Plot(name='my voxel plot')
-    plot.type = 'voxel'
+    plot = openmc.VoxelPlot(name='my voxel plot')
     plot.filename = 'voxel1'
     plot.pixels = (50, 50, 50)
     plot.origin = (0., 0., 0.)
@@ -243,7 +241,6 @@ def test_voxel_plot_roundtrip():
     new_plot = plot.from_xml_element(elem)
     assert new_plot.name == plot.name
     assert new_plot.filename == plot.filename
-    assert new_plot.type == plot.type
     assert new_plot.pixels == plot.pixels
     assert new_plot.origin == plot.origin
     assert new_plot.width == plot.width
@@ -288,10 +285,9 @@ def test_phong_plot_roundtrip():
 def test_plot_directory(run_in_tmpdir):
     pwr_pin = openmc.examples.pwr_pin_cell()
 
-    # create a standard plot, expected to work
-    plot = openmc.Plot()
+    # create a standard slice plot, expected to work
+    plot = openmc.SlicePlot()
     plot.filename = 'plot_1'
-    plot.type = 'slice'
     plot.pixels = (10, 10)
     plot.color_by = 'material'
     plot.width = (100., 100.)
diff --git a/tests/unit_tests/test_pulse_height.py b/tests/unit_tests/test_pulse_height.py
new file mode 100644
index 000000000..1f27cc6f2
--- /dev/null
+++ b/tests/unit_tests/test_pulse_height.py
@@ -0,0 +1,59 @@
+import numpy as np
+import pytest
+import openmc
+
+
+@pytest.fixture
+def model():
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+
+    # Define materials
+    NaI = openmc.Material()
+    NaI.set_density('g/cm3', 3.7)
+    NaI.add_element('Na', 1.0)
+    NaI.add_element('I', 1.0)
+
+    # Define geometry: two spheres in each other
+    s1 = openmc.Sphere(r=1)
+    s2 = openmc.Sphere(r=2, boundary_type='vacuum')
+    inner_sphere = openmc.Cell(name='inner sphere', fill=NaI, region=-s1)
+    outer_sphere = openmc.Cell(name='outer sphere', fill=NaI, region=+s1 & -s2)
+    model.geometry = openmc.Geometry([inner_sphere, outer_sphere])
+
+    # Define settings
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 1
+    model.settings.particles = 10000
+    model.settings.photon_transport = True
+    model.settings.source = openmc.IndependentSource(
+        energy=openmc.stats.delta_function(1e6),
+        particle='photon'
+    )
+
+    # Define tallies
+    energy_filter = openmc.EnergyFilter([1e3, 1e7])    
+
+    tally1 = openmc.Tally()
+    tally1.scores = ['pulse-height']
+    cell_filter1 = openmc.CellFilter([inner_sphere, outer_sphere])
+    tally1.filters = [cell_filter1, energy_filter]
+
+    tally2 = openmc.Tally()
+    tally2.scores = ['pulse-height']
+    cell_filter2 = openmc.CellFilter([outer_sphere, inner_sphere])
+    tally2.filters = [cell_filter2, energy_filter]
+
+    model.tallies = [tally1, tally2]
+    return model
+
+
+def test_pulse_height(model, run_in_tmpdir):
+    sp_path = model.run()
+    sp = openmc.StatePoint(sp_path)
+    t1 = sp.tallies[1].mean.squeeze()
+    t2 = sp.tallies[2].mean.squeeze()
+    
+    np.testing.assert_array_equal(t1, t2[::-1])
+
+
diff --git a/tests/unit_tests/test_r2s.py b/tests/unit_tests/test_r2s.py
new file mode 100644
index 000000000..5e617919d
--- /dev/null
+++ b/tests/unit_tests/test_r2s.py
@@ -0,0 +1,325 @@
+from pathlib import Path
+
+import pytest
+import openmc
+from openmc.deplete import Chain, R2SManager
+
+
+@pytest.fixture
+def simple_model_and_mesh():
+    # Define two materials: water and Ni
+    h2o = openmc.Material()
+    h2o.add_nuclide("H1", 2.0)
+    h2o.add_nuclide("O16", 1.0)
+    h2o.set_density("g/cm3", 1.0)
+    nickel = openmc.Material()
+    nickel.add_element("Ni", 1.0)
+    nickel.set_density("g/cm3", 4.0)
+
+    # Geometry: two half-spaces split by x=0 plane
+    left = openmc.XPlane(0.0)
+    x_min = openmc.XPlane(-10.0, boundary_type='vacuum')
+    x_max = openmc.XPlane(10.0, boundary_type='vacuum')
+    y_min = openmc.YPlane(-10.0, boundary_type='vacuum')
+    y_max = openmc.YPlane(10.0, boundary_type='vacuum')
+    z_min = openmc.ZPlane(-10.0, boundary_type='vacuum')
+    z_max = openmc.ZPlane(10.0, boundary_type='vacuum')
+
+    c1 = openmc.Cell(fill=h2o, region=+x_min & -left & +y_min & -y_max & +z_min & -z_max)
+    c2 = openmc.Cell(fill=nickel, region=+left & -x_max & +y_min & -y_max & +z_min & -z_max)
+    c1.volume = 4000.0
+    c2.volume = 4000.0
+    geometry = openmc.Geometry([c1, c2])
+
+    # Simple settings with a point source
+    settings = openmc.Settings()
+    settings.batches = 2
+    settings.particles = 250
+    settings.run_mode = 'fixed source'
+    settings.source = openmc.IndependentSource()
+    model = openmc.Model(geometry, settings=settings)
+
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-10.0, -10.0, -10.0)
+    mesh.upper_right = (10.0, 10.0, 10.0)
+    mesh.dimension = (1, 1, 1)
+    return model, (c1, c2), mesh
+
+
+@pytest.fixture
+def source_stage_manager(simple_model_and_mesh):
+    model, (c1, c2), _ = simple_model_and_mesh
+    r2s = R2SManager(model, [c1, c2])
+    r2s.results['depletion_results'] = [None, None]
+    r2s.results['activation_materials'] = [c1.fill, c2.fill]
+    bounding_boxes = {c1.id: c1.bounding_box, c2.id: c2.bounding_box}
+    return r2s, bounding_boxes
+
+
+def test_r2s_mesh_expected_output(simple_model_and_mesh, tmp_path):
+    model, (c1, c2), mesh = simple_model_and_mesh
+
+    # Use mesh-based domains
+    r2s = R2SManager(model, mesh)
+
+    # Use custom reduced chain file for Ni
+    chain = Chain.from_xml(Path(__file__).parents[1] / "chain_ni.xml")
+
+    # Run R2S calculation
+    outdir = r2s.run(
+        timesteps=[(1.0, 'd')],
+        source_rates=[1.0],
+        output_dir=tmp_path,
+        chain_file=chain,
+        micro_kwargs={'nuclides': ['Ni58'], 'reactions': ['(n,p)']},
+    )
+
+    # Check directories and files exist
+    nt = Path(outdir) / 'neutron_transport'
+    assert (nt / 'fluxes.npy').exists()
+    assert (nt / 'micros.h5').exists()
+    assert (nt / 'mesh_material_volumes_0.npz').exists()
+    act = Path(outdir) / 'activation'
+    assert (act / 'depletion_results.h5').exists()
+    pt = Path(outdir) / 'photon_transport'
+    assert (pt / 'tally_ids.json').exists()
+    assert not (pt / 'time_0').exists()
+    assert (pt / 'time_1' / 'statepoint.2.h5').exists()
+
+    # Basic results structure checks
+    assert len(r2s.results['fluxes']) == 2
+    assert len(r2s.results['micros']) == 2
+    assert len(r2s.results['mesh_material_volumes']) == 1
+    assert len(r2s.results['mesh_material_volumes'][0]) == 2
+    assert len(r2s.results['activation_materials']) == 2
+    assert len(r2s.results['depletion_results']) == 2
+    assert list(r2s.results['photon_sources']) == [1]
+    assert r2s.results['photon_sources'][1]
+
+    # Check activation materials
+    amats = r2s.results['activation_materials']
+    assert all(m.depletable for m in amats)
+    # Volumes preserved
+    assert {m.volume for m in amats} == {c1.volume, c2.volume}
+
+    # Check loading results
+    r2s_loaded = R2SManager(model, mesh)
+    r2s_loaded.load_results(outdir)
+    assert len(r2s_loaded.results['fluxes']) == 2
+    assert len(r2s_loaded.results['micros']) == 2
+    assert len(r2s_loaded.results['mesh_material_volumes']) == 1
+    assert len(r2s_loaded.results['mesh_material_volumes'][0]) == 2
+    assert len(r2s_loaded.results['activation_materials']) == 2
+    assert len(r2s_loaded.results['depletion_results']) == 2
+
+
+def test_r2s_multi_mesh(simple_model_and_mesh, tmp_path):
+    model, _, _ = simple_model_and_mesh
+
+    # Two 1x1x1 meshes that together cover the full domain, split along y.
+    # Each mesh element spans the full x range [-10, 10], crossing the x=0
+    # material boundary, so both meshes contain both materials within their
+    # single element.
+    mesh1 = openmc.RegularMesh()
+    mesh1.lower_left = (-10.0, -10.0, -10.0)
+    mesh1.upper_right = (10.0, 0.0, 10.0)
+    mesh1.dimension = (1, 1, 1)
+    mesh2 = openmc.RegularMesh()
+    mesh2.lower_left = (-10.0, 0.0, -10.0)
+    mesh2.upper_right = (10.0, 10.0, 10.0)
+    mesh2.dimension = (1, 1, 1)
+
+    r2s = R2SManager(model, [mesh1, mesh2])
+    chain = Chain.from_xml(Path(__file__).parents[1] / "chain_ni.xml")
+
+    outdir = r2s.run(
+        timesteps=[(1.0, 'd')],
+        source_rates=[1.0],
+        photon_time_indices=[1],
+        output_dir=tmp_path,
+        chain_file=chain,
+        micro_kwargs={'nuclides': ['Ni58'], 'reactions': ['(n,p)']},
+    )
+
+    # Check that per-mesh MMV files were written
+    nt = Path(outdir) / 'neutron_transport'
+    assert (nt / 'fluxes.npy').exists()
+    assert (nt / 'micros.h5').exists()
+    assert (nt / 'mesh_material_volumes_0.npz').exists()
+    assert (nt / 'mesh_material_volumes_1.npz').exists()
+    act = Path(outdir) / 'activation'
+    assert (act / 'depletion_results.h5').exists()
+    pt = Path(outdir) / 'photon_transport'
+    assert (pt / 'tally_ids.json').exists()
+    assert (pt / 'time_1' / 'statepoint.2.h5').exists()
+
+    # Two meshes, each with 1 element containing both materials →
+    # 2 element-material combinations per mesh, 4 total
+    assert len(r2s.results['mesh_material_volumes']) == 2
+    assert len(r2s.results['mesh_material_volumes'][0]) == 2
+    assert len(r2s.results['mesh_material_volumes'][1]) == 2
+    assert len(r2s.results['fluxes']) == 4
+    assert len(r2s.results['micros']) == 4
+    assert len(r2s.results['activation_materials']) == 4
+    assert len(r2s.results['depletion_results']) == 2
+
+    # Activation material names encode mesh index
+    amats = r2s.results['activation_materials']
+    assert all(m.depletable for m in amats)
+    assert any('Mesh 0' in m.name for m in amats)
+    assert any('Mesh 1' in m.name for m in amats)
+
+    # Check loading results
+    r2s_loaded = R2SManager(model, [mesh1, mesh2])
+    r2s_loaded.load_results(outdir)
+    assert len(r2s_loaded.results['mesh_material_volumes']) == 2
+    assert len(r2s_loaded.results['mesh_material_volumes'][0]) == 2
+    assert len(r2s_loaded.results['mesh_material_volumes'][1]) == 2
+    assert len(r2s_loaded.results['activation_materials']) == 4
+    assert len(r2s_loaded.results['depletion_results']) == 2
+
+
+def test_r2s_cell_expected_output(simple_model_and_mesh, tmp_path):
+    model, (c1, c2), _ = simple_model_and_mesh
+    tally = openmc.Tally()
+    tally.scores = ['flux']
+    model.tallies = [tally]
+
+    # Use cell-based domains
+    r2s = R2SManager(model, [c1, c2])
+
+    # Use custom reduced chain file for Ni
+    chain = Chain.from_xml(Path(__file__).parents[1] / "chain_ni.xml")
+
+    # Run R2S calculation
+    bounding_boxes = {c1.id: c1.bounding_box, c2.id: c2.bounding_box}
+    outdir = r2s.run(
+        timesteps=[(1.0, 'd')],
+        source_rates=[1.0],
+        photon_time_indices=[1],
+        by_parent_nuclide=True,
+        output_dir=tmp_path,
+        bounding_boxes=bounding_boxes,
+        chain_file=chain,
+        micro_kwargs={'nuclides': ['Ni58'], 'reactions': ['(n,p)']},
+    )
+
+    # Check directories and files exist
+    nt = Path(outdir) / 'neutron_transport'
+    assert (nt / 'fluxes.npy').exists()
+    assert (nt / 'micros.h5').exists()
+    act = Path(outdir) / 'activation'
+    assert (act / 'depletion_results.h5').exists()
+    pt = Path(outdir) / 'photon_transport'
+    assert (pt / 'tally_ids.json').exists()
+    assert (pt / 'time_1' / 'statepoint.2.h5').exists()
+
+    # Basic results structure checks
+    assert len(r2s.results['fluxes']) == 2
+    assert len(r2s.results['micros']) == 2
+    assert len(r2s.results['activation_materials']) == 2
+    assert len(r2s.results['depletion_results']) == 2
+    assert r2s.photon_model.tallies[0].contains_filter(
+        openmc.ParentNuclideFilter)
+
+    # Check activation materials
+    amats = r2s.results['activation_materials']
+    assert all(m.depletable for m in amats)
+    # Names include cell IDs
+    assert any(f"Cell {c1.id}" in m.name for m in amats)
+    assert any(f"Cell {c2.id}" in m.name for m in amats)
+    # Volumes preserved
+    assert {m.volume for m in amats} == {c1.volume, c2.volume}
+
+    # Check loading results
+    r2s_loaded = R2SManager(model, [c1, c2])
+    r2s_loaded.load_results(outdir)
+    assert len(r2s_loaded.results['fluxes']) == 2
+    assert len(r2s_loaded.results['micros']) == 2
+    assert len(r2s_loaded.results['activation_materials']) == 2
+    assert len(r2s_loaded.results['depletion_results']) == 2
+
+
+def test_step4_requires_photon_sources(simple_model_and_mesh, tmp_path):
+    model, (c1, c2), _ = simple_model_and_mesh
+    r2s = R2SManager(model, [c1, c2])
+    output_dir = tmp_path / 'photon'
+
+    with pytest.raises(RuntimeError, match='step3_photon_source'):
+        r2s.step4_photon_transport(output_dir)
+
+    r2s.results['photon_sources'] = {}
+    with pytest.raises(RuntimeError, match='No decay photon sources'):
+        r2s.step4_photon_transport(output_dir)
+
+    assert not output_dir.exists()
+
+
+def test_default_photon_times_skip_empty_sources(
+    source_stage_manager, tmp_path, monkeypatch
+):
+    r2s, bounding_boxes = source_stage_manager
+    source = object()
+    sources_by_time = {0: [], 1: [source]}
+    monkeypatch.setattr(
+        r2s, '_create_photon_sources',
+        lambda time_index, work_items: sources_by_time[time_index])
+
+    r2s.step3_photon_source(
+        bounding_boxes=bounding_boxes, output_dir=tmp_path)
+
+    assert r2s.results['photon_sources'] == {1: [source]}
+
+
+def test_explicit_empty_photon_source_fails(
+    source_stage_manager, tmp_path, monkeypatch
+):
+    r2s, bounding_boxes = source_stage_manager
+    source = object()
+    sources_by_time = {0: [], 1: [source]}
+    monkeypatch.setattr(
+        r2s, '_create_photon_sources',
+        lambda time_index, work_items: sources_by_time[time_index])
+    r2s.results['photon_sources'] = {99: [source]}
+
+    with pytest.raises(RuntimeError, match='requested time indices: 0'):
+        r2s.step3_photon_source(
+            [0, 1], bounding_boxes, output_dir=tmp_path)
+
+    assert 'photon_sources' not in r2s.results
+
+
+def test_default_photon_times_require_a_source(
+    source_stage_manager, tmp_path, monkeypatch
+):
+    r2s, bounding_boxes = source_stage_manager
+    monkeypatch.setattr(
+        r2s, '_create_photon_sources',
+        lambda time_index, work_items: [])
+
+    with pytest.raises(RuntimeError, match='at any depletion time'):
+        r2s.step3_photon_source(
+            bounding_boxes=bounding_boxes, output_dir=tmp_path)
+
+    assert 'photon_sources' not in r2s.results
+
+
+@pytest.mark.parametrize(
+    ('time_indices', 'exception'),
+    [
+        ([], ValueError),
+        ([2], IndexError),
+        ([-3], IndexError),
+        ([1.0], TypeError),
+    ],
+)
+def test_photon_time_index_validation(
+    source_stage_manager, tmp_path, time_indices, exception
+):
+    r2s, bounding_boxes = source_stage_manager
+
+    with pytest.raises(exception):
+        r2s.step3_photon_source(
+            time_indices, bounding_boxes, output_dir=tmp_path)
+
+    assert 'photon_sources' not in r2s.results
diff --git a/tests/unit_tests/test_region.py b/tests/unit_tests/test_region.py
index cbcd19831..cb9fa171b 100644
--- a/tests/unit_tests/test_region.py
+++ b/tests/unit_tests/test_region.py
@@ -248,6 +248,10 @@ def test_plot():
     c_before = openmc.Cell()
     region.plot()
 
+    # Close plot to avoid warning
+    import matplotlib.pyplot as plt
+    plt.close()
+
     # Ensure that calling plot doesn't affect cell ID space
     c_after = openmc.Cell()
     assert c_after.id - 1 == c_before.id
diff --git a/tests/unit_tests/test_settings.py b/tests/unit_tests/test_settings.py
index 397d70434..bdb3ea8fe 100644
--- a/tests/unit_tests/test_settings.py
+++ b/tests/unit_tests/test_settings.py
@@ -1,8 +1,17 @@
+from pathlib import Path
+
+import h5py
+import pytest
+
 import openmc
+import openmc.lib
 import openmc.stats
 
 
 def test_export_to_xml(run_in_tmpdir):
+
+    tmp_properties_file = 'properties_test.h5'
+
     s = openmc.Settings(run_mode='fixed source', batches=1000, seed=17)
     s.generations_per_batch = 10
     s.inactive = 100
@@ -21,6 +30,8 @@ def test_export_to_xml(run_in_tmpdir):
     s.statepoint = {'batches': [50, 150, 500, 1000]}
     s.surf_source_read = {'path': 'surface_source_1.h5'}
     s.surf_source_write = {'surface_ids': [2], 'max_particles': 200}
+    s.surface_grazing_ratio = 0.7
+    s.surface_grazing_cutoff = 0.1
     s.confidence_intervals = True
     s.ptables = True
     s.plot_seed = 100
@@ -43,6 +54,7 @@ def test_export_to_xml(run_in_tmpdir):
     s.tabular_legendre = {'enable': True, 'num_points': 50}
     s.temperature = {'default': 293.6, 'method': 'interpolation',
                      'multipole': True, 'range': (200., 1000.)}
+    s.properties_file = tmp_properties_file
     s.trace = (10, 1, 20)
     s.track = [(1, 1, 1), (2, 1, 1)]
     s.ufs_mesh = mesh
@@ -57,21 +69,36 @@ def test_export_to_xml(run_in_tmpdir):
     s.log_grid_bins = 2000
     s.photon_transport = False
     s.electron_treatment = 'led'
+    s.atomic_relaxation = False
     s.write_initial_source = True
     s.weight_window_checkpoints = {'surface': True, 'collision': False}
+    source_region_mesh = openmc.RegularMesh()
+    source_region_mesh.dimension = [2, 2, 2]
+    source_region_mesh.lower_left = [-2, -2, -2]
+    source_region_mesh.upper_right = [2, 2, 2]
+    root_universe = openmc.Universe()
     s.random_ray = {
         'distance_inactive': 10.0,
         'distance_active': 100.0,
         'ray_source': openmc.IndependentSource(
             space=openmc.stats.Box((-1., -1., -1.), (1., 1., 1.))
-        )
+        ),
+        'source_region_meshes': [(source_region_mesh, [root_universe])],
+        'volume_estimator': 'hybrid',
+        'source_shape': 'linear',
+        'volume_normalized_flux_tallies': True,
+        'adjoint': False,
+        'sample_method': 'halton'
     }
-
     s.max_particle_events = 100
+    s.max_secondaries = 1_000_000
+    s.source_rejection_fraction = 0.01
+    s.free_gas_threshold = 800.0
 
     # Make sure exporting XML works
     s.export_to_xml()
 
+
     # Generate settings from XML
     s = openmc.Settings.from_xml()
     assert s.run_mode == 'fixed source'
@@ -94,6 +121,8 @@ def test_export_to_xml(run_in_tmpdir):
     assert s.statepoint == {'batches': [50, 150, 500, 1000]}
     assert s.surf_source_read['path'].name == 'surface_source_1.h5'
     assert s.surf_source_write == {'surface_ids': [2], 'max_particles': 200}
+    assert s.surface_grazing_ratio == 0.7
+    assert s.surface_grazing_cutoff == 0.1
     assert s.confidence_intervals
     assert s.ptables
     assert s.plot_seed == 100
@@ -116,6 +145,7 @@ def test_export_to_xml(run_in_tmpdir):
     assert s.tabular_legendre == {'enable': True, 'num_points': 50}
     assert s.temperature == {'default': 293.6, 'method': 'interpolation',
                              'multipole': True, 'range': [200., 1000.]}
+    assert s.properties_file == Path(tmp_properties_file)
     assert s.trace == [10, 1, 20]
     assert s.track == [(1, 1, 1), (2, 1, 1)]
     assert isinstance(s.ufs_mesh, openmc.RegularMesh)
@@ -130,7 +160,8 @@ def test_export_to_xml(run_in_tmpdir):
     assert s.log_grid_bins == 2000
     assert not s.photon_transport
     assert s.electron_treatment == 'led'
-    assert s.write_initial_source == True
+    assert not s.atomic_relaxation
+    assert s.write_initial_source
     assert len(s.volume_calculations) == 1
     vol = s.volume_calculations[0]
     assert vol.domain_type == 'cell'
@@ -144,3 +175,74 @@ def test_export_to_xml(run_in_tmpdir):
     assert s.random_ray['distance_active'] == 100.0
     assert s.random_ray['ray_source'].space.lower_left == [-1., -1., -1.]
     assert s.random_ray['ray_source'].space.upper_right == [1., 1., 1.]
+    assert 'source_region_meshes' in s.random_ray
+    assert len(s.random_ray['source_region_meshes']) == 1
+    mesh_and_domains = s.random_ray['source_region_meshes'][0]
+    recovered_mesh = mesh_and_domains[0]
+    assert recovered_mesh.dimension == (2, 2, 2)
+    assert recovered_mesh.lower_left == [-2., -2., -2.]
+    assert recovered_mesh.upper_right == [2., 2., 2.]
+    assert s.random_ray['volume_estimator'] == 'hybrid'
+    assert s.random_ray['source_shape'] == 'linear'
+    assert s.random_ray['volume_normalized_flux_tallies']
+    assert not s.random_ray['adjoint']
+    assert s.random_ray['sample_method'] == 'halton'
+    assert s.max_secondaries == 1_000_000
+    assert s.source_rejection_fraction == 0.01
+    assert s.free_gas_threshold == 800.0
+
+
+def test_properties_file_load(tmp_path, mpi_intracomm):
+    model = openmc.examples.pwr_assembly()
+
+    # Session 1: export a structurally valid properties file via the C++ API,
+    # then collect the cell/material structure so we can patch it with h5py.
+    cell_instances = {}   # {cell_id: n_instances} — material cells only
+    mat_densities = {}    # {mat_id: original atom/b-cm density}
+
+    props_path = tmp_path / 'properties.h5'
+    with openmc.lib.TemporarySession(model, intracomm=mpi_intracomm):
+        openmc.lib.export_properties(str(props_path))
+        for cell_id, cell in openmc.lib.cells.items():
+            try:
+                cell.fill  # raises NotImplementedError for non-material cells
+                cell_instances[cell_id] = cell.num_instances
+            except NotImplementedError:
+                pass
+        for mat_id, mat in openmc.lib.materials.items():
+            mat_densities[mat_id] = mat.get_density('atom/b-cm')
+
+    assert any(n > 1 for n in cell_instances.values())
+
+    # Patch the exported properties file overwriting temperatures
+    # with per-instance values and scale material atom densities.
+    density_factor = 0.75
+    with h5py.File(props_path, 'r+') as f:
+        cells_grp = f['geometry/cells']
+        for cell_id, n in cell_instances.items():
+            cell_grp = cells_grp[f'cell {cell_id}']
+            del cell_grp['temperature']
+            cell_grp.create_dataset(
+                'temperature', data=[500.0 + 5.0 * i for i in range(n)]
+            )
+
+        for mat_id, orig_density in mat_densities.items():
+            f['materials'][f'material {mat_id}'].attrs['atom_density'] = \
+                orig_density * density_factor
+
+    # now apply the newly patched properties file using the settings
+    # and load the model again, checking that the new temperature and
+    # density values match those in the new file
+    model.settings.properties_file = props_path
+
+    with openmc.lib.TemporarySession(model, intracomm=mpi_intracomm):
+        for cell_id, n in cell_instances.items():
+            cell = openmc.lib.cells[cell_id]
+            for i in range(n):
+                assert cell.get_temperature(i) == pytest.approx(500.0 + 5.0 * i)
+
+        for mat_id, orig_density in mat_densities.items():
+            mat = openmc.lib.materials[mat_id]
+            assert mat.get_density('atom/b-cm') == pytest.approx(
+                orig_density * density_factor, rel=1e-5
+            )
diff --git a/tests/unit_tests/test_slice_data.py b/tests/unit_tests/test_slice_data.py
new file mode 100644
index 000000000..cc5fb0474
--- /dev/null
+++ b/tests/unit_tests/test_slice_data.py
@@ -0,0 +1,168 @@
+import numpy as np
+import openmc
+from openmc.examples import pwr_pin_cell
+
+
+def test_slice_data_basic(run_in_tmpdir):
+    """Test basic slice_data functionality."""
+    model = pwr_pin_cell()
+    geom_data, prop_data = model.slice_data(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(100, 100),
+        basis='xy'
+    )
+
+    # Without filter, should have 3 fields
+    assert geom_data.shape == (100, 100, 3)
+    assert geom_data.dtype == np.int32
+    assert prop_data.shape == (100, 100, 2)
+    assert prop_data.dtype == np.float64
+
+    # Check we have valid geometry
+    assert np.any(geom_data[:, :, 0] >= 0)  # Valid cell IDs
+    assert np.any(prop_data[:, :, 0] > 0)   # Valid temperatures
+
+
+def test_slice_data_no_properties(run_in_tmpdir):
+    """Test slice_data without property data."""
+    model = pwr_pin_cell()
+    geom_data, prop_data = model.slice_data(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(50, 50),
+        include_properties=False
+    )
+
+    # Without filter, should have 3 fields
+    assert geom_data.shape == (50, 50, 3)
+    assert prop_data is None
+
+
+def test_slice_data_with_filter(run_in_tmpdir):
+    """Test slice_data with a cell filter."""
+    model = pwr_pin_cell()
+    cell_ids = [c.id for c in model.geometry.get_all_cells().values()]
+    cell_filter = openmc.CellFilter(cell_ids)
+
+    geom_data, _ = model.slice_data(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(50, 50),
+        filter=cell_filter,
+        include_properties=False
+    )
+
+    # With filter, should have 4 fields
+    assert geom_data.shape == (50, 50, 4)
+
+    # Filter bin index should be populated where cells exist
+    filter_bins = geom_data[:, :, 3]
+    valid_cells = geom_data[:, :, 0] >= 0
+    assert np.any(filter_bins[valid_cells] >= 0)
+
+
+def test_slice_data_overlaps(run_in_tmpdir):
+    """Test slice_data with overlap detection."""
+    model = pwr_pin_cell()
+    geom_data, _ = model.slice_data(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(50, 50),
+        show_overlaps=True,
+        include_properties=False
+    )
+
+    # Without filter, should have 3 fields
+    assert geom_data.shape == (50, 50, 3)
+    # Check for overlap markers (-3) if any exist
+    # Note: This test may pass without finding overlaps if geometry is correct
+
+
+def test_slice_data_overlaps_with_filter(run_in_tmpdir):
+    """Test that overlaps don't overwrite filter bin data."""
+    model = pwr_pin_cell()
+    cell_ids = [c.id for c in model.geometry.get_all_cells().values()]
+    cell_filter = openmc.CellFilter(cell_ids)
+
+    geom_data, _ = model.slice_data(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(50, 50),
+        filter=cell_filter,
+        show_overlaps=True,
+        include_properties=False
+    )
+
+    assert geom_data.shape == (50, 50, 4)
+
+    # If any overlaps exist, verify filter bin is still valid (not -3)
+    overlap_pixels = geom_data[:, :, 0] == -3
+    if np.any(overlap_pixels):
+        # Filter bins at overlap locations should NOT be -3
+        filter_bins_at_overlaps = geom_data[overlap_pixels, 3]
+        assert not np.all(filter_bins_at_overlaps == -3), \
+            "Filter bins should be preserved even where overlaps are detected"
+
+
+def test_slice_data_different_bases(run_in_tmpdir):
+    """Test slice_data with different basis planes."""
+    model = pwr_pin_cell()
+
+    for basis in ['xy', 'xz', 'yz']:
+        geom_data, prop_data = model.slice_data(
+            origin=(0, 0, 0),
+            width=(1.0, 1.0),
+            pixels=(25, 25),
+            basis=basis
+        )
+
+        assert geom_data.shape == (25, 25, 3)
+        assert prop_data.shape == (25, 25, 2)
+
+
+def test_slice_data_oriented_spans(run_in_tmpdir):
+    """Test slice_data with oriented span vectors."""
+    model = pwr_pin_cell()
+
+    geom_data, prop_data = model.slice_data(
+        origin=(0, 0, 0),
+        u_span=(1.0, 0.0, 0.0),
+        v_span=(0.0, 0.0, 1.0),
+        pixels=(25, 25)
+    )
+
+    assert geom_data.shape == (25, 25, 3)
+    assert prop_data.shape == (25, 25, 2)
+
+
+def test_slice_data_level(run_in_tmpdir):
+    """Test slice_data with specific universe level."""
+    model = pwr_pin_cell()
+    geom_data, _ = model.slice_data(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(50, 50),
+        level=0,  # Root universe only
+        include_properties=False
+    )
+
+    assert geom_data.shape == (50, 50, 3)
+
+
+def test_id_map_reverted(run_in_tmpdir):
+    """Test that id_map returns 3D array without filter support."""
+    model = pwr_pin_cell()
+    id_data = model.id_map(
+        origin=(0, 0, 0),
+        width=(1.0, 1.0),
+        pixels=(50, 50),
+        basis='xy'
+    )
+
+    # Should have 3 fields (cell_id, cell_instance, material_id)
+    assert id_data.shape == (50, 50, 3)
+    assert id_data.dtype == np.int32
+
+    # Check valid data
+    assert np.any(id_data[:, :, 0] >= 0)  # Valid cell IDs
diff --git a/tests/unit_tests/test_slice_data_overlap_info.py b/tests/unit_tests/test_slice_data_overlap_info.py
new file mode 100644
index 000000000..e5f08245d
--- /dev/null
+++ b/tests/unit_tests/test_slice_data_overlap_info.py
@@ -0,0 +1,89 @@
+import pytest
+import numpy as np
+import openmc
+import openmc.lib
+
+# Sentinel value matching _OVERLAP in plotmodel.py and OVERLAP in plot.cpp
+_OVERLAP = -3
+
+
+@pytest.fixture(scope='module')
+def overlap_model():
+    openmc.reset_auto_ids()
+
+    # Three cylinders: cyl1 and cyl2 overlap near x=0, cyl2 and cyl3 overlap
+    # near x=4. This gives us two spatially distinct overlap regions in one model.
+    mat1 = openmc.Material(components={'H1': 1.0})
+    mat2 = openmc.Material(components={'H1': 1.0})
+    mat3 = openmc.Material(components={'H1': 1.0})
+
+    # cyl1 and cyl2 overlap on the left, cyl2 and cyl3 overlap on the right
+    cyl1 = openmc.ZCylinder(x0=-2.0, r=2.5)
+    cyl2 = openmc.ZCylinder(x0=0.0, r=2.5)
+    cyl3 = openmc.ZCylinder(x0=2.0, r=2.5)
+    boundary = openmc.Sphere(r=20.0, boundary_type='vacuum')
+    cell1 = openmc.Cell(region=-cyl1, fill=mat1)
+    cell2 = openmc.Cell(region=-cyl2, fill=mat2)
+    cell3 = openmc.Cell(region=-cyl3, fill=mat3)
+    cell_outside = openmc.Cell(region=+cyl1 & +cyl2 & +cyl3 & -boundary)
+    geometry = openmc.Geometry([cell1, cell2, cell3, cell_outside])
+
+    settings = openmc.Settings()
+    settings.run_mode = 'fixed source'
+    settings.particles = 100
+    settings.batches = 1
+    model = openmc.Model(geometry=geometry, settings=settings)
+
+    with openmc.lib.TemporarySession(model, args=['-s', '1']):
+        yield
+
+
+def run_slice(origin=(0.0, 0.0, 0.0), width=(10.0, 6.0), show_overlaps=True):
+    # Helper that runs a slice over a region covering both overlap zones
+    geom_data, _ = openmc.lib.slice_data(
+        origin=origin,
+        width=width,
+        basis='xy',
+        pixels=(100, 60),
+        show_overlaps=show_overlaps,
+        include_properties=False,
+    )
+    return geom_data
+
+
+def test_overlaps_enabled(overlap_model):
+    # Run a single slice with overlap detection enabled and check all
+    # expected properties in one pass.
+    geom_data = run_slice()
+    overlap_info = openmc.lib.slice_data_overlap_info()
+    n = overlap_info.shape[0]
+    cell_ids = geom_data[:, :, 0]
+
+    # cell_ids should contain values more negative than _OVERLAP; RasterData
+    # encodes each unique overlap as OVERLAP - overlap_idx - 1 into slot 2.
+    assert np.any(cell_ids < _OVERLAP)
+
+    # overlap_keys should have 2 entries for the two distinct overlapping
+    # cylinder pairs in this model.
+    assert n == 2, f"Expected exactly 2 overlap entries, got {n}"
+
+    # Each entry is a (universe_id, cell1_id, cell2_id) triple; verify values.
+    for i in range(n):
+        universe_id = int(overlap_info[i, 0])
+        cell1_id = int(overlap_info[i, 1])
+        cell2_id = int(overlap_info[i, 2])
+        assert universe_id == 1
+        assert cell1_id in {1, 2, 3}
+        assert cell2_id in {1, 2, 3}
+        assert cell1_id != cell2_id
+
+
+def test_overlaps_disabled(overlap_model):
+    # With show_overlaps=False, set_overlap is never called and overlap_keys
+    # is never written to, so the image and map should both be clean.
+    geom_data = run_slice(show_overlaps=False)
+    overlap_info = openmc.lib.slice_data_overlap_info()
+    n = overlap_info.shape[0]
+
+    assert not np.any(geom_data[:, :, 2] < _OVERLAP)
+    assert n == 0
diff --git a/tests/unit_tests/test_slice_voxel_plots.py b/tests/unit_tests/test_slice_voxel_plots.py
new file mode 100644
index 000000000..48ca31b7a
--- /dev/null
+++ b/tests/unit_tests/test_slice_voxel_plots.py
@@ -0,0 +1,273 @@
+"""Tests for SlicePlot and VoxelPlot classes
+
+This module tests the functionality of the new SlicePlot and VoxelPlot
+classes that replace the legacy Plot class.
+"""
+import warnings
+
+import pytest
+import openmc
+
+
+def test_slice_plot_initialization():
+    """Test SlicePlot initialization with defaults"""
+    plot = openmc.SlicePlot()
+    assert plot.width == [4.0, 4.0]
+    assert plot.pixels == [400, 400]
+    assert plot.basis == 'xy'
+    assert plot.origin == [0., 0., 0.]
+
+
+def test_slice_plot_width_validation():
+    """Test that SlicePlot only accepts 2 values for width"""
+    plot = openmc.SlicePlot()
+
+    # Should accept 2 values
+    plot.width = [10.0, 20.0]
+    assert plot.width == [10.0, 20.0]
+
+    # Should reject 1 value
+    with pytest.raises(ValueError, match='must be of length "2"'):
+        plot.width = [10.0]
+
+    # Should reject 3 values
+    with pytest.raises(ValueError, match='must be of length "2"'):
+        plot.width = [10.0, 20.0, 30.0]
+
+
+def test_slice_plot_pixels_validation():
+    """Test that SlicePlot only accepts 2 values for pixels"""
+    plot = openmc.SlicePlot()
+
+    # Should accept 2 values
+    plot.pixels = [100, 200]
+    assert plot.pixels == [100, 200]
+
+    # Should reject 1 value
+    with pytest.raises(ValueError, match='must be of length "2"'):
+        plot.pixels = [100]
+
+    # Should reject 3 values
+    with pytest.raises(ValueError, match='must be of length "2"'):
+        plot.pixels = [100, 200, 300]
+
+
+def test_slice_plot_basis():
+    """Test that SlicePlot has basis attribute"""
+    plot = openmc.SlicePlot()
+
+    # Test all valid basis values
+    for basis in ['xy', 'xz', 'yz']:
+        plot.basis = basis
+        assert plot.basis == basis
+
+    # Test invalid basis
+    with pytest.raises(ValueError):
+        plot.basis = 'invalid'
+
+
+def test_slice_plot_meshlines():
+    """Test that SlicePlot has meshlines attribute"""
+    plot = openmc.SlicePlot()
+
+    meshlines = {
+        'type': 'tally',
+        'id': 1,
+        'linewidth': 2,
+        'color': (255, 0, 0)
+    }
+    plot.meshlines = meshlines
+    assert plot.meshlines == meshlines
+
+
+def test_slice_plot_xml_roundtrip():
+    """Test SlicePlot XML serialization and deserialization"""
+    plot = openmc.SlicePlot(name='test_slice')
+    plot.width = [15.0, 25.0]
+    plot.pixels = [150, 250]
+    plot.basis = 'xz'
+    plot.origin = [1.0, 2.0, 3.0]
+    plot.color_by = 'material'
+    plot.filename = 'test_plot'
+
+    # Convert to XML and back
+    elem = plot.to_xml_element()
+    new_plot = openmc.SlicePlot.from_xml_element(elem)
+
+    # Check all attributes preserved
+    assert new_plot.name == plot.name
+    assert new_plot.width == pytest.approx(plot.width)
+    assert new_plot.pixels == tuple(plot.pixels)
+    assert new_plot.basis == plot.basis
+    assert new_plot.origin == pytest.approx(plot.origin)
+    assert new_plot.color_by == plot.color_by
+    assert new_plot.filename == plot.filename
+
+
+def test_slice_plot_from_geometry():
+    """Test creating SlicePlot from geometry"""
+    # Create simple geometry
+    s = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    c = openmc.Cell(region=-s)
+    univ = openmc.Universe(cells=[c])
+    geom = openmc.Geometry(univ)
+
+    # Test all basis options
+    for basis in ['xy', 'xz', 'yz']:
+        plot = openmc.SlicePlot.from_geometry(geom, basis=basis)
+        assert plot.basis == basis
+        assert plot.width == pytest.approx([20.0, 20.0])
+        assert plot.origin == pytest.approx([0.0, 0.0, 0.0])
+
+
+def test_voxel_plot_initialization():
+    """Test VoxelPlot initialization with defaults"""
+    plot = openmc.VoxelPlot()
+    assert plot.width == [4.0, 4.0, 4.0]
+    assert plot.pixels == [400, 400, 400]
+    assert plot.origin == [0., 0., 0.]
+
+
+def test_voxel_plot_width_validation():
+    """Test that VoxelPlot only accepts 3 values for width"""
+    plot = openmc.VoxelPlot()
+
+    # Should accept 3 values
+    plot.width = [10.0, 20.0, 30.0]
+    assert plot.width == [10.0, 20.0, 30.0]
+
+    # Should reject 2 values
+    with pytest.raises(ValueError, match='must be of length "3"'):
+        plot.width = [10.0, 20.0]
+
+    # Should reject 1 value
+    with pytest.raises(ValueError, match='must be of length "3"'):
+        plot.width = [10.0]
+
+
+def test_voxel_plot_pixels_validation():
+    """Test that VoxelPlot only accepts 3 values for pixels"""
+    plot = openmc.VoxelPlot()
+
+    # Should accept 3 values
+    plot.pixels = [100, 200, 300]
+    assert plot.pixels == [100, 200, 300]
+
+    # Should reject 2 values
+    with pytest.raises(ValueError, match='must be of length "3"'):
+        plot.pixels = [100, 200]
+
+    # Should reject 1 value
+    with pytest.raises(ValueError, match='must be of length "3"'):
+        plot.pixels = [100]
+
+
+def test_voxel_plot_xml_roundtrip():
+    """Test VoxelPlot XML serialization and deserialization"""
+    plot = openmc.VoxelPlot(name='test_voxel')
+    plot.width = [10.0, 20.0, 30.0]
+    plot.pixels = [100, 200, 300]
+    plot.origin = [1.0, 2.0, 3.0]
+    plot.color_by = 'cell'
+    plot.filename = 'voxel_plot'
+
+    # Convert to XML and back
+    elem = plot.to_xml_element()
+    new_plot = openmc.VoxelPlot.from_xml_element(elem)
+
+    # Check all attributes preserved
+    assert new_plot.name == plot.name
+    assert new_plot.width == pytest.approx(plot.width)
+    assert new_plot.pixels == tuple(plot.pixels)
+    assert new_plot.origin == pytest.approx(plot.origin)
+    assert new_plot.color_by == plot.color_by
+    assert new_plot.filename == plot.filename
+
+
+def test_plot_deprecation_warning():
+    """Test that Plot class raises deprecation warning"""
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        openmc.Plot()
+
+        assert len(w) == 1
+        assert issubclass(w[0].category, FutureWarning)
+        assert "deprecated" in str(w[0].message).lower()
+
+
+def test_plot_returns_slice_plot():
+    """Test that Plot() returns a SlicePlot instance"""
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        plot = openmc.Plot()
+
+        # Should be an actual SlicePlot instance
+        assert isinstance(plot, openmc.SlicePlot)
+
+
+def test_plot_type_setter_raises_error():
+    """Test that setting plot.type raises a helpful error"""
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        plot = openmc.Plot()
+
+        with pytest.raises(TypeError, match="no longer supported"):
+            plot.type = 'voxel'
+
+        with pytest.raises(TypeError, match="no longer supported"):
+            plot.type = 'slice'
+
+
+def test_plot_type_getter_warns():
+    """Test that getting plot.type raises a deprecation warning"""
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        plot = openmc.Plot()
+
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        plot_type = plot.type
+
+        assert plot_type == 'slice'
+        assert len(w) == 1
+        assert issubclass(w[0].category, FutureWarning)
+        assert "deprecated" in str(w[0].message).lower()
+
+
+def test_plots_collection_mixed_types():
+    """Test Plots collection with different plot types"""
+    slice_plot = openmc.SlicePlot(name='slice')
+    voxel_plot = openmc.VoxelPlot(name='voxel')
+    wireframe_plot = openmc.WireframeRayTracePlot(name='wireframe')
+
+    plots = openmc.Plots([slice_plot, voxel_plot, wireframe_plot])
+
+    assert len(plots) == 3
+    assert isinstance(plots[0], openmc.SlicePlot)
+    assert isinstance(plots[1], openmc.VoxelPlot)
+    assert isinstance(plots[2], openmc.WireframeRayTracePlot)
+
+
+def test_plots_collection_xml_roundtrip(run_in_tmpdir):
+    """Test XML export and import with new plot types"""
+    s1 = openmc.SlicePlot(name='slice1')
+    s1.width = [10.0, 20.0]
+    s1.basis = 'xz'
+
+    v1 = openmc.VoxelPlot(name='voxel1')
+    v1.width = [10.0, 20.0, 30.0]
+
+    plots = openmc.Plots([s1, v1])
+    plots.export_to_xml()
+
+    # Read back
+    new_plots = openmc.Plots.from_xml()
+
+    assert len(new_plots) == 2
+    assert isinstance(new_plots[0], openmc.SlicePlot)
+    assert isinstance(new_plots[1], openmc.VoxelPlot)
+    assert new_plots[0].name == 'slice1'
+    assert new_plots[1].name == 'voxel1'
+    assert new_plots[0].basis == 'xz'
+    assert new_plots[0].width == pytest.approx([10.0, 20.0])
+    assert new_plots[1].width == pytest.approx([10.0, 20.0, 30.0])
diff --git a/tests/unit_tests/test_source.py b/tests/unit_tests/test_source.py
index c88fbcbe6..394c09e73 100644
--- a/tests/unit_tests/test_source.py
+++ b/tests/unit_tests/test_source.py
@@ -1,5 +1,6 @@
 from collections import Counter
 from math import pi
+from pathlib import Path
 
 import openmc
 import openmc.lib
@@ -35,21 +36,6 @@ def test_source():
     assert src.strength == 1.0
 
 
-def test_spherical_uniform():
-    r_outer = 2.0
-    r_inner = 1.0
-    thetas = (0.0, pi/2)
-    phis = (0.0, pi)
-    origin = (0.0, 1.0, 2.0)
-
-    sph_indep_function = openmc.stats.spherical_uniform(r_outer,
-                                                        r_inner,
-                                                        thetas,
-                                                        phis,
-                                                        origin)
-
-    assert isinstance(sph_indep_function, openmc.stats.SphericalIndependent)
-
 def test_point_cloud():
     positions = [(1, 0, 2), (0, 1, 0), (0, 0, 3), (4, 9, 2)]
     strengths = [1, 2, 3, 4]
@@ -106,6 +92,74 @@ def test_point_cloud_strengths(run_in_tmpdir, sphere_box_model):
         assert sampled_strength == expected_strength, f'Strength incorrect for {positions[i]}'
 
 
+def test_decay_spectrum_parent_nuclide(run_in_tmpdir):
+    chain_file = Path('chain_decay_spectrum_parent.xml')
+    chain_file.write_text("""<?xml version="1.0"?>
+<depletion_chain>
+  <nuclide name="ParentA" decay_modes="0" reactions="0" half_life="1.0">
+    <source type="discrete" particle="photon">
+      <parameters>1000000.0 1.0</parameters>
+    </source>
+  </nuclide>
+  <nuclide name="ParentB" decay_modes="0" reactions="0" half_life="1.0">
+    <source type="discrete" particle="photon">
+      <parameters>2000000.0 1.0</parameters>
+    </source>
+  </nuclide>
+</depletion_chain>
+""")
+
+    inner_sphere = openmc.Sphere(r=10.0)
+    outer_sphere = openmc.Sphere(r=20.0, boundary_type='vacuum')
+
+    shell_mat = openmc.Material()
+    shell_mat.add_nuclide('H1', 1.0)
+    shell_mat.set_density('atom/b-cm', 1.0e-12)
+
+    void_cell = openmc.Cell(region=-inner_sphere)
+    shell_cell = openmc.Cell(fill=shell_mat, region=+inner_sphere & -outer_sphere)
+
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([void_cell, shell_cell])
+    model.materials = [shell_mat]
+    model.settings.run_mode = 'fixed source'
+    model.settings.photon_transport = True
+    model.settings.particles = 1000
+    model.settings.batches = 5
+    model.settings.source = openmc.IndependentSource(
+        particle='photon',
+        space=openmc.stats.Point((0.0, 0.0, 0.0)),
+        energy=openmc.stats.DecaySpectrum(
+            {'ParentA': 1.0, 'ParentB': 1.0},
+            volume=1.0
+        )
+    )
+
+    tally = openmc.Tally()
+    tally.filters = [
+        openmc.CellFilter([void_cell]),
+        openmc.ParticleFilter(['photon']),
+        openmc.EnergyFilter([0.0, 1.5e6, 2.5e6]),
+        openmc.ParentNuclideFilter(['ParentA', 'ParentB'])
+    ]
+    tally.scores = ['flux']
+    model.tallies = [tally]
+
+    with openmc.config.patch('chain_file', chain_file):
+        sp_filename = model.run()
+
+    with openmc.StatePoint(sp_filename) as sp:
+        tally_out = sp.tallies[tally.id]
+        mean = tally_out.get_reshaped_data('mean').squeeze()
+
+    assert mean.shape == (2, 2)
+    assert mean[0, 0] > 0.0
+    assert mean[1, 1] > 0.0
+    assert mean[0, 1] == 0.0
+    assert mean[1, 0] == 0.0
+    assert np.count_nonzero(mean) == 2
+
+
 def test_source_file():
     filename = 'source.h5'
     src = openmc.FileSource(path=filename)
@@ -264,21 +318,36 @@ def test_constraints_file(sphere_box_model, run_in_tmpdir):
 
 
 @pytest.mark.skipif(config['mpi'], reason='Not compatible with MPI')
-def test_rejection_limit(sphere_box_model, run_in_tmpdir):
-    model, cell1 = sphere_box_model[:2]
+def test_rejection_fraction(run_in_tmpdir):
+    mat = openmc.Material()
+    mat.add_nuclide('H1', 1.0)
+    w = 0.25
+    rpp1 = openmc.model.RectangularParallelepiped(
+        -w/2, w/2, -w/2, w/2, -w/2, w/2)
+    rpp2 = openmc.model.RectangularParallelepiped(
+        -0.5, 0.5, -0.5, 0.5, -0.5, 0.5, boundary_type='vacuum')
+    cell1 = openmc.Cell(fill=mat, region=-rpp1)
+    cell2 = openmc.Cell(region=+rpp1 & -rpp2)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell1, cell2])
 
-    # Define a point source that will get rejected 100% of the time
+    # Create a box source over a 1 cm³ volume that is constrained to the source
+    # cell of volume (0.25 cm)³ = 0.0125 cm³, which means the default rejection
+    # fraction of 0.05 won't work
+    model.settings.particles = 1000
+    model.settings.batches = 1
+    model.settings.run_mode = 'fixed source'
     model.settings.source = openmc.IndependentSource(
-        space=openmc.stats.Point((-3., 0., 0.)),
+        space=openmc.stats.Box(*(-rpp2).bounding_box),
         constraints={'domains': [cell1]}
     )
-
-    # Confirm that OpenMC doesn't run in an infinite loop. Note that this may
-    # work when running with MPI since it won't necessarily capture the error
-    # message correctly
-    with pytest.raises(RuntimeError, match="rejected"):
+    with pytest.raises(RuntimeError, match='Too few source sites'):
         model.run(openmc_exec=config['exe'])
 
+    # With a source rejection fraction below 0.0125, the simulation should run
+    model.settings.source_rejection_fraction = 0.005
+    model.run(openmc_exec=config['exe'])
+
 
 def test_exceptions():
 
diff --git a/tests/unit_tests/test_source_biasing.py b/tests/unit_tests/test_source_biasing.py
new file mode 100644
index 000000000..12588594c
--- /dev/null
+++ b/tests/unit_tests/test_source_biasing.py
@@ -0,0 +1,276 @@
+"""Tests for source biasing using C++ sampling routines via openmc.lib
+
+This test module validates that the C++ distribution sampling implementations
+correctly handle both unbiased and biased sampling when used in source
+definitions. Each test:
+
+1. Creates a minimal model with a source using a specific energy distribution
+2. Uses model.sample_external_source() to generate samples via openmc.lib
+3. Extracts energies from the returned particle list
+4. Validates that:
+   - Unbiased sampling produces the expected mean
+   - Biased sampling with importance weighting produces the expected mean
+   - Weights are correctly applied (non-unity for biased case)
+
+These tests complement the Python-level tests in test_stats.py by exercising
+the full C++ sampling codepath that is used during actual simulations.
+"""
+
+import numpy as np
+import pytest
+import openmc
+
+from tests.unit_tests import assert_sample_mean
+
+
+@pytest.fixture
+def model():
+    """Create a minimal model for source sampling tests."""
+    sphere = openmc.Sphere(r=100.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere)
+    geometry = openmc.Geometry([cell])
+    settings = openmc.Settings(particles=100, batches=1)
+    space = openmc.stats.Point()
+    angle = openmc.stats.Monodirectional((1.0, 0.0, 0.0))
+    settings.source = openmc.IndependentSource(space=space, angle=angle)
+    return openmc.Model(geometry=geometry, settings=settings)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_discrete(run_in_tmpdir, model):
+    """Test Discrete distribution sampling via C++ routines."""
+    vals = np.array([1.0, 2.0, 3.0])
+    probs = np.array([0.1, 0.7, 0.2])
+    exp_mean = (vals * probs).sum()
+
+    # Create source with discrete energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.Discrete(vals, probs)
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample from biased distribution
+    energy_dist.bias = np.array([0.2, 0.1, 0.7])
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_uniform(run_in_tmpdir, model):
+    """Test Uniform distribution sampling via C++ routines."""
+    a, b = 5.0, 10.0
+    exp_mean = 0.5 * (a + b)
+
+    # Create source with uniform energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.Uniform(a, b)
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample from biased distribution
+    energy_dist.bias = openmc.stats.PowerLaw(a, b, 2)
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_powerlaw(run_in_tmpdir, model):
+    """Test PowerLaw distribution sampling via C++ routines."""
+    a, b, n = 1.0, 20.0, 2.0
+
+    # Determine mean of distribution
+    exp_mean = (n+1)*(b**(n+2) - a**(n+2))/((n+2)*(b**(n+1) - a**(n+1)))
+
+    # Create source with powerlaw energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.PowerLaw(a, b, n)
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample from biased distribution
+    energy_dist.bias = openmc.stats.Uniform(a, b)
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_maxwell(run_in_tmpdir, model):
+    """Test Maxwell distribution sampling via C++ routines."""
+    theta = 1.2895e6
+    exp_mean = 3/2 * theta
+
+    # Create source with Maxwell energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.Maxwell(theta)
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample from biased distribution
+    energy_dist.bias = openmc.stats.Maxwell(theta * 1.1)
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_watt(run_in_tmpdir, model):
+    """Test Watt distribution sampling via C++ routines."""
+    a, b = 0.965e6, 2.29e-6
+    exp_mean = 3/2 * a + a**2 * b / 4
+
+    # Create source with Watt energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.Watt(a, b)
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample from biased distribution
+    energy_dist.bias = openmc.stats.Watt(a*1.05, b)
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_tabular(run_in_tmpdir, model):
+    """Test Tabular distribution sampling via C++ routines."""
+    # Test linear-linear sampling
+    x = np.array([0.0, 5.0, 7.0, 10.0])
+    p = np.array([10.0, 20.0, 5.0, 6.0])
+
+    # Create tabular distribution and normalize to get expected mean
+    model.settings.source[0].energy = energy_dist = openmc.stats.Tabular(x, p, 'linear-linear')
+    energy_dist.normalize()
+    exp_mean = energy_dist.mean()
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample from biased distribution
+    energy_dist.bias = openmc.stats.Uniform(x[0], x[-1])
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_mixture(run_in_tmpdir, model):
+    """Test Mixture distribution sampling via C++ routines."""
+    d1 = openmc.stats.Uniform(0, 5)
+    d2 = openmc.stats.Uniform(3, 7)
+    p = [0.5, 0.5]
+
+    # Create mixture energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.Mixture(p, [d1, d2])
+    exp_mean = (2.5 + 5.0) / 2
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, exp_mean)
+
+    # Sample using biased sub-distribution
+    energy_dist.distribution[0].bias = openmc.stats.PowerLaw(0, 5, 2)
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, exp_mean)
+    assert np.any(weights != 1.0)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_normal(run_in_tmpdir, model):
+    """Test Normal distribution sampling via C++ routines."""
+    mean_val = 25.0
+    std_dev = 2.0
+
+    # Create source with normal energy distribution
+    model.settings.source[0].energy = energy_dist = openmc.stats.Normal(mean_val, std_dev)
+
+    # Sample using C++ routines and extract energies
+    n_samples = 10_000
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+
+    # Check unbiased mean
+    assert_sample_mean(energies, mean_val)
+
+    # Sample from biased distribution
+    energy_dist.bias = openmc.stats.Normal(mean_val * 1.1, std_dev)
+    particles = model.sample_external_source(n_samples)
+    energies = np.array([p.E for p in particles])
+    weights = np.array([p.wgt for p in particles])
+
+    # Check biased weighted mean
+    weighted_energies = energies * weights
+    assert_sample_mean(weighted_energies, mean_val)
+    assert np.any(weights != 1.0)
diff --git a/tests/unit_tests/test_source_file.py b/tests/unit_tests/test_source_file.py
index 41906c80f..f19ea74a6 100644
--- a/tests/unit_tests/test_source_file.py
+++ b/tests/unit_tests/test_source_file.py
@@ -42,7 +42,7 @@ def test_source_file(run_in_tmpdir):
     assert np.all(arr['E'] == n - np.arange(n))
     assert np.all(arr['wgt'] == 1.0)
     assert np.all(arr['delayed_group'] == 0)
-    assert np.all(arr['particle'] == 0)
+    assert np.all(arr['particle'] == 2112)  # PDG number for neutron
 
     # Ensure sites read in are consistent
     sites = openmc.ParticleList.from_hdf5('test_source.h5')
@@ -64,7 +64,7 @@ def test_source_file(run_in_tmpdir):
     dgs = np.array([s.delayed_group for s in sites])
     assert np.all(dgs == 0)
     p_types = np.array([s.particle for s in sites])
-    assert np.all(p_types == 0)
+    assert np.all(p_types == 2112)  # PDG number for neutron
 
     # Ensure a ParticleList item is a SourceParticle
     site = sites[0]
@@ -145,3 +145,27 @@ def test_source_file_transport(run_in_tmpdir):
 
     # Try running OpenMC
     model.run()
+
+
+def test_source_file_photon_transport(run_in_tmpdir):
+    # Create a source file containing a photon. Note that photon_transport is
+    # not explicitly enabled in the settings -- it should be turned on
+    # automatically because the source file contains a photon.
+    particle = openmc.SourceParticle(E=1.0e6, particle='photon')
+    openmc.write_source_file([particle], 'photon_source.h5')
+
+    # Create simple model to use the photon source file
+    model = openmc.Model()
+    al = openmc.Material()
+    al.add_element('Al', 1.0)
+    al.set_density('g/cm3', 2.7)
+    sph = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(fill=al, region=-sph)
+    model.geometry = openmc.Geometry([cell])
+    model.settings.source = openmc.FileSource(path='photon_source.h5')
+    model.settings.particles = 10
+    model.settings.batches = 3
+    model.settings.run_mode = 'fixed source'
+
+    # Running OpenMC should succeed
+    model.run()
diff --git a/tests/unit_tests/test_source_mesh.py b/tests/unit_tests/test_source_mesh.py
index 43bb1678c..2550cb87e 100644
--- a/tests/unit_tests/test_source_mesh.py
+++ b/tests/unit_tests/test_source_mesh.py
@@ -1,5 +1,7 @@
 from itertools import product
 from pathlib import Path
+from math import sqrt
+import random
 
 import pytest
 import numpy as np
@@ -334,13 +336,10 @@ def test_umesh_source_independent(run_in_tmpdir, request, void_model, library):
     n_elements = 12_000
     model.settings.source = openmc.MeshSource(uscd_mesh, n_elements*[ind_source])
     model.export_to_model_xml()
-    try:
-        openmc.lib.init()
+    with openmc.lib.run_in_memory():
         openmc.lib.simulation_init()
         sites = openmc.lib.sample_external_source(10)
         openmc.lib.statepoint_write('statepoint.h5')
-    finally:
-        openmc.lib.finalize()
 
     with openmc.StatePoint('statepoint.h5') as sp:
         uscd_mesh = sp.meshes[uscd_mesh.id]
@@ -351,32 +350,86 @@ def test_umesh_source_independent(run_in_tmpdir, request, void_model, library):
         assert site.r in bounding_box
 
 
-def test_mesh_source_file(run_in_tmpdir):
-    # Creating a source file with a single particle
-    source_particle = openmc.SourceParticle(time=10.0)
-    openmc.write_source_file([source_particle], 'source.h5')
-    file_source = openmc.FileSource('source.h5')
+def test_mesh_source_constraints(run_in_tmpdir):
+    """Test application of constraints to underlying mesh element sources"""
 
+    # Create simple model with two cells
+    m1 = openmc.Material()
+    m1.add_nuclide('H1', 1.0)
+    m2 = m1.clone()
+    sph = openmc.Sphere(r=100, boundary_type='vacuum')
+    box1 = openmc.model.RectangularParallelepiped(-1, 0, -1, 1, -1, 1)
+    box2 = openmc.model.RectangularParallelepiped(0, 2, -1, 1, -1, 1)
+    cell1 = openmc.Cell(fill=m1, region=-box1)
+    cell2 = openmc.Cell(fill=m2, region=-box2)
+    outer = openmc.Cell(region=-sph & (+box1 | +box2))
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell1, cell2, outer])
+
+    # Define a mesh covering the two cells: the first mesh element contains
+    # cell1 (-1 < x < 0) and the second element contains cells2 (0 < x < 2)
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-3., -1., -1.)
+    mesh.upper_right = (3., 1., 1.)
+    mesh.dimension = (2, 1, 1)
+
+    # Define a mesh source with a randomly chosen probability
+    p = random.random()
+    src1 = openmc.IndependentSource(strength=p, constraints={'domains': [cell1]})
+    src2 = openmc.IndependentSource(strength=1 - p, constraints={'domains': [cell2]})
+    model.settings.source = openmc.MeshSource(mesh, [src1, src2])
+
+    # Finish settings and export
+    model.settings.particles = 100
+    model.settings.batches = 1
+    model.export_to_model_xml()
+
+    with openmc.lib.run_in_memory():
+        # Sample sites from the source
+        sites = openmc.lib.sample_external_source(N := 1000)
+
+        # Check that all sites are either in cell1 or cell2
+        xs = np.array([s.r[0] for s in sites])
+        assert (xs >= -1.0).all()
+        assert (xs <= 2.0).all()
+
+        # Check that the correct percentage of the sites are in cell1
+        sigma = sqrt(p*(1- p)/N)
+        frac = xs[(-1.0 <= xs) & (xs <= 0.0)].size / N
+        assert frac == pytest.approx(p, abs=5*sigma)
+
+
+@pytest.mark.parametrize("mesh_type", ('rectangular', 'cylindrical', 'spherical'))
+def test_mesh_spatial(run_in_tmpdir, mesh_type):
+    """Test that a spherical mesh source works as expected."""
     model = openmc.Model()
 
-    rect_prism = openmc.model.RectangularParallelepiped(
-        -5.0, 5.0, -5.0, 5.0, -5.0, 5.0, boundary_type='vacuum')
-
+    # Set up geometry, a box that is shifted in x, y, and z
+    box = openmc.model.RectangularParallelepiped(5.0, 25.0, -20.0, 20.0, -30.0, 30.0, boundary_type='vacuum')
     mat = openmc.Material()
     mat.add_nuclide('H1', 1.0)
+    model.geometry = openmc.Geometry([openmc.Cell(fill=mat, region=-box)])
 
-    model.geometry = openmc.Geometry([openmc.Cell(fill=mat, region=-rect_prism)])
-    model.settings.particles = 1000
+    # Create a mesh of each type in turn
+    if mesh_type == 'rectangular':
+        mesh = openmc.RegularMesh.from_domain(model.geometry, (10, 2, 2))
+    elif mesh_type == 'cylindrical':
+        mesh = openmc.CylindricalMesh.from_domain(model.geometry, (10, 2, 2))
+        assert max(mesh.r_grid) == 10.0, "Cylindrical mesh radius exceeds geometry bounds"
+        assert mesh.origin[0] == 15.0, "Cylindrical mesh origin x-coordinate is incorrect"
+    elif mesh_type == 'spherical':
+        mesh = openmc.SphericalMesh.from_domain(model.geometry, (10, 2, 2))
+        assert max(mesh.r_grid) == 10.0, "Spherical mesh radius exceeds geometry bounds"
+        assert mesh.origin[0] == 15.0, "Spherical mesh origin x-coordinate is incorrect"
+
+    # Create a mesh source with a single particle
+    ind_source = openmc.IndependentSource(space=openmc.stats.MeshSpatial(mesh, np.prod(mesh.dimension)*[1.0]))
+    model.settings.source = ind_source
+
+    model.settings.particles = 100
     model.settings.batches = 10
     model.settings.run_mode = 'fixed source'
 
-    mesh = openmc.RegularMesh()
-    mesh.lower_left = (-1, -2, -3)
-    mesh.upper_right = (2, 3, 4)
-    mesh.dimension = (1, 1, 1)
-
-    model.settings.source = openmc.MeshSource(mesh, [file_source])
-
     model.export_to_model_xml()
 
     openmc.lib.init()
@@ -385,14 +438,7 @@ def test_mesh_source_file(run_in_tmpdir):
     openmc.lib.simulation_finalize()
     openmc.lib.finalize()
 
-    # The mesh bounds do not contain the point of the lone source site in the
-    # file source, so it should not appear in the set of source sites produced
-    # from the mesh source. Additionally, the source should be located within
-    # the mesh
+    # Check that the sites are within the spherical mesh bounds
     bbox = mesh.bounding_box
     for site in sites:
-        assert site.r != (0, 0, 0)
-        assert site.E == source_particle.E
-        assert site.u == source_particle.u
-        assert site.time == source_particle.time
         assert site.r in bbox
diff --git a/tests/unit_tests/test_source_tokamak.py b/tests/unit_tests/test_source_tokamak.py
new file mode 100644
index 000000000..f926c2007
--- /dev/null
+++ b/tests/unit_tests/test_source_tokamak.py
@@ -0,0 +1,263 @@
+import numpy as np
+import pytest
+
+import openmc
+import openmc.stats
+
+from tests.unit_tests import assert_sample_mean
+
+
+def make_source(**kwargs):
+    """Build a valid TokamakSource, overriding defaults via kwargs."""
+    r_over_a = np.linspace(0.0, 1.0, 10)
+    params = dict(
+        major_radius=620.0,
+        minor_radius=200.0,
+        elongation=1.8,
+        triangularity=0.45,
+        shafranov_shift=10.0,
+        r_over_a=r_over_a,
+        emission_density=(1.0 - r_over_a**2),
+        energy=openmc.stats.muir(e0=14.08e6, m_rat=5.0, kt=2.0e4),
+    )
+    params.update(kwargs)
+    return openmc.TokamakSource(**params)
+
+
+def test_tokamak_source_roundtrip():
+    src = make_source(
+        phi_start=0.1, phi_extent=np.pi, n_alpha=51, vertical_shift=5.0,
+        strength=2.0, time=openmc.stats.Uniform(0.0, 1e-6))
+
+    elem = src.to_xml_element()
+    assert elem.get('type') == 'tokamak'
+
+    new = openmc.SourceBase.from_xml_element(elem)
+    assert isinstance(new, openmc.TokamakSource)
+    assert new.major_radius == src.major_radius
+    assert new.minor_radius == src.minor_radius
+    assert new.elongation == src.elongation
+    assert new.triangularity == src.triangularity
+    assert new.shafranov_shift == src.shafranov_shift
+    assert new.phi_start == src.phi_start
+    assert new.phi_extent == src.phi_extent
+    assert new.n_alpha == src.n_alpha
+    assert new.vertical_shift == src.vertical_shift
+    assert new.strength == src.strength
+    np.testing.assert_allclose(new.r_over_a, src.r_over_a)
+    np.testing.assert_allclose(new.emission_density, src.emission_density)
+    assert len(new.energy) == 1
+    assert isinstance(new.time, openmc.stats.Uniform)
+    assert new.time.a == src.time.a
+    assert new.time.b == src.time.b
+
+
+def test_tokamak_source_default_time():
+    src = make_source()
+    assert src.time is None
+
+    new = openmc.SourceBase.from_xml_element(src.to_xml_element())
+    assert new.time is None
+
+    with pytest.raises(TypeError):
+        make_source(time=1.0)
+
+
+def test_tokamak_source_multiple_energies():
+    r_over_a = np.linspace(0.0, 1.0, 5)
+    energies = [openmc.stats.muir(e0=14.08e6, m_rat=5.0, kt=kt)
+                for kt in (1.0e4, 1.5e4, 2.0e4, 2.5e4, 3.0e4)]
+    src = make_source(r_over_a=r_over_a,
+                      emission_density=np.ones_like(r_over_a),
+                      energy=energies)
+    assert len(src.energy) == len(r_over_a)
+
+    new = openmc.SourceBase.from_xml_element(src.to_xml_element())
+    assert len(new.energy) == len(r_over_a)
+
+
+@pytest.mark.parametrize("kwargs, match", [
+    (dict(minor_radius=700.0), "smaller than major_radius"),
+    (dict(shafranov_shift=150.0), "half the minor_radius"),
+    (dict(emission_density=np.ones(5)), "same length as r_over_a"),
+    (dict(energy=[openmc.stats.muir(14.08e6, 5.0, 2.0e4)] * 2),
+     "Number of energy distributions"),
+    (dict(r_over_a=np.linspace(0.1, 1.0, 10)), "must start at 0"),
+    (dict(r_over_a=np.linspace(0.0, 0.9, 10)), "must end at 1"),
+    (dict(emission_density=-np.linspace(0.0, 1.0, 10)), "cannot be negative"),
+    (dict(emission_density=np.zeros(10)), "must contain a positive value"),
+])
+def test_tokamak_source_invalid(kwargs, match):
+    with pytest.raises(ValueError, match=match):
+        make_source(**kwargs)
+
+
+@pytest.mark.parametrize("value", [-1.5, 1.5])
+def test_tokamak_source_invalid_triangularity(value):
+    with pytest.raises(ValueError):
+        make_source(triangularity=value)
+
+
+def test_tokamak_source_invalid_n_alpha():
+    with pytest.raises(ValueError):
+        make_source(n_alpha=2)
+
+
+@pytest.mark.parametrize(("attribute", "value", "match"), [
+    ("minor_radius", 700.0, "smaller than major_radius"),
+    ("shafranov_shift", 150.0, "half the minor_radius"),
+    ("emission_density", np.ones(5), "same length as r_over_a"),
+    ("energy", [openmc.stats.delta_function(1.0)] * 2,
+     "Number of energy distributions"),
+])
+def test_tokamak_source_mutation_validation(attribute, value, match):
+    src = make_source()
+    setattr(src, attribute, value)
+    with pytest.raises(ValueError, match=match):
+        src.to_xml_element()
+
+
+@pytest.mark.parametrize(("emission_density", "expected_mean"), [
+    ([1.0, 1.0], 2.0 / 3.0),
+    ([1.0, 0.0], 0.5),
+])
+def test_tokamak_source_radial_sampling(
+    run_in_tmpdir, emission_density, expected_mean
+):
+    """Check radial sampling for profiles on the coarsest valid grid."""
+    major_radius = 620.0
+    minor_radius = 200.0
+    src = make_source(
+        major_radius=major_radius,
+        minor_radius=minor_radius,
+        elongation=1.0,
+        triangularity=0.0,
+        shafranov_shift=0.0,
+        r_over_a=[0.0, 1.0],
+        emission_density=emission_density,
+        energy=openmc.stats.delta_function(14.07e6),
+    )
+
+    sphere = openmc.Sphere(r=2000.0, boundary_type='vacuum')
+    model = openmc.Model(
+        geometry=openmc.Geometry([openmc.Cell(region=-sphere)]),
+        settings=openmc.Settings(
+            particles=100, batches=1, run_mode='fixed source', source=src),
+    )
+
+    sites = model.sample_external_source(20_000)
+    xyz = np.array([site.r for site in sites])
+    major_r = np.hypot(xyz[:, 0], xyz[:, 1])
+    r_over_a = np.hypot(major_r - major_radius, xyz[:, 2]) / minor_radius
+    assert_sample_mean(r_over_a, expected_mean)
+
+
+def test_tokamak_source_poloidal_sampling(run_in_tmpdir):
+    """Check linear-linear poloidal sampling on a coarse internal grid."""
+    major_radius = 620.0
+    minor_radius = 200.0
+    with pytest.warns(UserWarning, match="below 51"):
+        src = make_source(
+            major_radius=major_radius,
+            minor_radius=minor_radius,
+            elongation=1.0,
+            triangularity=0.0,
+            shafranov_shift=0.0,
+            emission_density=np.ones(10),
+            n_alpha=3,
+            energy=openmc.stats.delta_function(14.07e6),
+        )
+
+    sphere = openmc.Sphere(r=2000.0, boundary_type='vacuum')
+    model = openmc.Model(
+        geometry=openmc.Geometry([openmc.Cell(region=-sphere)]),
+        settings=openmc.Settings(
+            particles=100, batches=1, run_mode='fixed source', source=src),
+    )
+
+    sites = model.sample_external_source(100_000)
+    xyz = np.array([site.r for site in sites])
+    major_r = np.hypot(xyz[:, 0], xyz[:, 1])
+
+    # With three alpha points, linear interpolation of cos(alpha) on [0, pi]
+    # gives 1 - 2*alpha/pi. Integrating the resulting density gives this mean.
+    expected_R = (
+        major_radius
+        + 2.0 * minor_radius**2 / (np.pi**2 * major_radius)
+    )
+    assert_sample_mean(major_r, expected_R)
+
+
+@pytest.mark.flaky(reruns=1)
+def test_tokamak_source_sampling(run_in_tmpdir):
+    """Exercise the compiled C++ sampling path and check invariants.
+
+    Sampled moments are compared against direct numerical quadrature of the
+    exact source density S(r)*R*|J|, where the Jacobian J of the flux-surface
+    map is computed from analytic partial derivatives. This check is
+    independent of the Bernstein-mixture factorization used by the
+    implementation.
+    """
+    R0, a, kappa, delta = 620.0, 200.0, 1.8, -0.5
+    shafranov, zshift = 40.0, 25.0
+    phi_start, phi_extent = 0.5, np.pi / 2
+
+    # Fine grids to make discretization error negligible relative to
+    # statistical uncertainty
+    r_over_a = np.linspace(0.0, 1.0, 200)
+    src = make_source(
+        major_radius=R0, minor_radius=a, elongation=kappa,
+        triangularity=delta, shafranov_shift=shafranov, vertical_shift=zshift,
+        r_over_a=r_over_a, emission_density=1.0 - r_over_a**2,
+        phi_start=phi_start, phi_extent=phi_extent, n_alpha=201,
+        energy=openmc.stats.delta_function(14.07e6))
+
+    sphere = openmc.Sphere(r=2000.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere)
+    settings = openmc.Settings(
+        particles=100, batches=1, run_mode='fixed source', source=src)
+    model = openmc.Model(geometry=openmc.Geometry([cell]), settings=settings)
+
+    n_samples = 20_000
+    sites = model.sample_external_source(n_samples)
+
+    xyz = np.array([s.r for s in sites])
+    R = np.hypot(xyz[:, 0], xyz[:, 1])
+    z = xyz[:, 2]
+
+    # Energy, weight, and time invariants
+    assert np.all([s.E == 14.07e6 for s in sites])
+    assert np.all([s.wgt == 1.0 for s in sites])
+    assert np.all([s.time == 0.0 for s in sites])
+
+    # Toroidal angle within the requested sector
+    phi = np.arctan2(xyz[:, 1], xyz[:, 0])
+    assert phi.min() >= phi_start
+    assert phi.max() <= phi_start + phi_extent
+
+    # Positions bounded by the last closed flux surface
+    assert R.min() >= R0 - a
+    assert R.max() <= R0 + a + shafranov
+    assert np.abs(z - zshift).max() <= kappa * a
+
+    # Up-down symmetry about the vertical shift
+    assert_sample_mean(z, zshift)
+
+    # Reference moments by 2D quadrature of the exact density
+    r = np.linspace(0.0, 1.0, 1001)[:, np.newaxis]  # r/a
+    alpha = np.linspace(0.0, 2 * np.pi, 2001)[np.newaxis, :]
+    psi = alpha + delta * np.sin(alpha)
+    R_map = R0 + a * r * np.cos(psi) + shafranov * (1.0 - r**2)
+    Z_map = kappa * a * r * np.sin(alpha)
+    dR_dr = a * np.cos(psi) - 2.0 * shafranov * r
+    dR_da = -a * r * np.sin(psi) * (1.0 + delta * np.cos(alpha))
+    dZ_dr = kappa * a * np.sin(alpha) * np.ones_like(psi)
+    dZ_da = kappa * a * r * np.cos(alpha)
+    jac = np.abs(dR_dr * dZ_da - dR_da * dZ_dr)
+    dens = (1.0 - r**2) * R_map * jac
+    norm = dens.sum()
+    expected_R = (R_map * dens).sum() / norm
+    expected_z2 = (Z_map**2 * dens).sum() / norm
+
+    assert_sample_mean(R, expected_R)
+    assert_sample_mean((z - zshift)**2, expected_z2)
diff --git a/tests/unit_tests/test_spherical_mesh.py b/tests/unit_tests/test_spherical_mesh.py
index 0b579be35..a95a4151a 100644
--- a/tests/unit_tests/test_spherical_mesh.py
+++ b/tests/unit_tests/test_spherical_mesh.py
@@ -63,8 +63,6 @@ def test_origin_read_write_to_xml(run_in_tmpdir, model):
     np.testing.assert_equal(new_mesh.origin, mesh.origin)
 
 estimators = ('tracklength', 'collision')
-# TODO: determine why this is needed for spherical mesh
-# but not cylindrical mesh
 offset = geom_size + 0.001
 
 origins = set(permutations((-offset, 0, 0)))
diff --git a/tests/unit_tests/test_statepoint.py b/tests/unit_tests/test_statepoint.py
new file mode 100644
index 000000000..7ffaf7ec2
--- /dev/null
+++ b/tests/unit_tests/test_statepoint.py
@@ -0,0 +1,65 @@
+import openmc
+
+
+def test_get_tally_filter_type(run_in_tmpdir):
+    """Test various ways of retrieving tallies from a StatePoint object."""
+
+    mat = openmc.Material()
+    mat.add_nuclide("H1", 1.0)
+    mat.set_density("g/cm3", 10.0)
+
+    sphere = openmc.Sphere(r=10.0, boundary_type="vacuum")
+    cell = openmc.Cell(fill=mat, region=-sphere)
+    geometry = openmc.Geometry([cell])
+
+    settings = openmc.Settings()
+    settings.particles = 10
+    settings.batches = 2
+    settings.run_mode = "fixed source"
+
+    reg_mesh = openmc.RegularMesh().from_domain(cell)
+    tally1 = openmc.Tally(tally_id=1)
+    mesh_filter = openmc.MeshFilter(reg_mesh)
+    tally1.filters = [mesh_filter]
+    tally1.scores = ["flux"]
+
+    tally2 = openmc.Tally(tally_id=2, name="heating tally")
+    cell_filter = openmc.CellFilter(cell)
+    tally2.filters = [cell_filter]
+    tally2.scores = ["heating"]
+
+    tallies = openmc.Tallies([tally1, tally2])
+    model = openmc.Model(
+        geometry=geometry, materials=[mat], settings=settings, tallies=tallies
+    )
+
+    sp_filename = model.run()
+
+    sp = openmc.StatePoint(sp_filename)
+
+    tally_found = sp.get_tally(filter_type=openmc.MeshFilter)
+    assert tally_found.id == 1
+
+    tally_found = sp.get_tally(filter_type=openmc.CellFilter)
+    assert tally_found.id == 2
+
+    tally_found = sp.get_tally(filters=[mesh_filter])
+    assert tally_found.id == 1
+
+    tally_found = sp.get_tally(filters=[cell_filter])
+    assert tally_found.id == 2
+
+    tally_found = sp.get_tally(scores=["heating"])
+    assert tally_found.id == 2
+
+    tally_found = sp.get_tally(name="heating tally")
+    assert tally_found.id == 2
+
+    tally_found = sp.get_tally(name=None)
+    assert tally_found.id == 1
+
+    tally_found = sp.get_tally(id=1)
+    assert tally_found.id == 1
+
+    tally_found = sp.get_tally(id=2)
+    assert tally_found.id == 2
diff --git a/tests/unit_tests/test_statepoint_batches.py b/tests/unit_tests/test_statepoint_batches.py
new file mode 100644
index 000000000..bf54e1878
--- /dev/null
+++ b/tests/unit_tests/test_statepoint_batches.py
@@ -0,0 +1,26 @@
+from pathlib import Path
+
+import openmc
+
+
+def test_statepoint_batches(run_in_tmpdir):
+    # Create a minimal model
+    mat = openmc.Material()
+    mat.add_nuclide('U235', 1.0)
+    mat.set_density('g/cm3', 4.5)
+    sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(fill=mat, region=-sphere)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell])
+    model.settings.batches = 10
+    model.settings.inactive = 5
+    model.settings.particles = 100
+
+    # Specify when statepoints should be written
+    model.settings.statepoint = {'batches': [3, 6, 9]}
+
+    # Run model and ensure that statepoints are created
+    model.run()
+    sp_files = ['statepoint.03.h5', 'statepoint.06.h5', 'statepoint.09.h5']
+    for f in sp_files:
+        assert Path(f).is_file()
diff --git a/tests/unit_tests/test_stats.py b/tests/unit_tests/test_stats.py
index 386181f34..2754cf5d0 100644
--- a/tests/unit_tests/test_stats.py
+++ b/tests/unit_tests/test_stats.py
@@ -1,21 +1,17 @@
 from math import pi
+from pathlib import Path
 
 import numpy as np
 import pytest
 import openmc
 import openmc.stats
+from openmc.stats.univariate import _INTERPOLATION_SCHEMES, DecaySpectrum
 from scipy.integrate import trapezoid
 
-
-def assert_sample_mean(samples, expected_mean):
-    # Calculate sample standard deviation
-    std_dev = samples.std() / np.sqrt(samples.size - 1)
-
-    # Means should agree within 4 sigma 99.993% of the time. Note that this is
-    # expected to fail about 1 out of 16,000 times
-    assert np.abs(expected_mean - samples.mean()) < 4*std_dev
+from tests.unit_tests import assert_sample_mean
 
 
+@pytest.mark.flaky(reruns=1)
 def test_discrete():
     x = [0.0, 1.0, 10.0]
     p = [0.3, 0.2, 0.5]
@@ -46,8 +42,19 @@ def test_discrete():
     # sample discrete distribution and check that the mean of the samples is
     # within 4 std. dev. of the expected mean
     n_samples = 1_000_000
-    samples = d3.sample(n_samples)
+    samples, weights = d3.sample(n_samples)
     assert_sample_mean(samples, exp_mean)
+    assert np.all(weights == 1.0)
+
+    # Test biased distribution
+    d3.bias = np.array([0.2, 0.1, 0.7])
+    bias_elem = d3.to_xml_element('distribution')
+    d4 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    np.testing.assert_array_equal(d4.bias, [0.2, 0.1, 0.7])
+    samples, weights = d4.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, exp_mean)
+    assert np.all(weights != 1.0)
 
 
 def test_delta_function():
@@ -82,6 +89,28 @@ def test_merge_discrete():
     assert triple.integral() == pytest.approx(6.0)
 
 
+def test_merge_discrete_with_bias():
+    # Two discrete distributions with different biases
+    d1 = openmc.stats.Discrete([1.0, 2.0], [0.5, 0.5])
+    d2 = openmc.stats.Discrete([2.0, 3.0], [0.3, 0.7], bias=[0.1, 0.9])
+
+    merged = openmc.stats.Discrete.merge([d1, d2], [0.6, 0.4])
+    exp_mean = 0.6 * d1.mean() + 0.4 * d2.mean()
+
+    # Verify merged distribution has correct x values
+    assert set(merged.x) == {1.0, 2.0, 3.0}
+
+    # Bias should not be changed in original distributions
+    assert d1.bias is None
+    assert np.all(d2.bias == [0.1, 0.9])
+
+    # Sample and verify bias is applied correctly
+    samples, weights = merged.sample(10_000)
+
+    # Verify weighted mean matches expected unbiased mean
+    assert_sample_mean(samples*weights, exp_mean)
+
+
 def test_clip_discrete():
     # Create discrete distribution with two points that are not important, one
     # because the x value is very small, and one because the p value is very
@@ -104,6 +133,7 @@ def test_clip_discrete():
         d.clip(5)
 
 
+@pytest.mark.flaky(reruns=1)
 def test_uniform():
     a, b = 10.0, 20.0
     d = openmc.stats.Uniform(a, b)
@@ -123,10 +153,22 @@ def test_uniform():
     # std. dev. of the expected mean
     exp_mean = 0.5 * (a + b)
     n_samples = 1_000_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, exp_mean)
+    assert np.all(weights == 1.0)
+
+    # Test biased distribution
+    d.bias = openmc.stats.PowerLaw(a, b, 2)
+    bias_elem = d.to_xml_element('distribution')
+    d2 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d2.bias, openmc.stats.PowerLaw)
+    samples, weights = d2.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, exp_mean)
+    assert np.all(weights != 1.0)
 
 
+@pytest.mark.flaky(reruns=1)
 def test_powerlaw():
     a, b, n = 10.0, 100.0, 2.0
     d = openmc.stats.PowerLaw(a, b, n)
@@ -144,10 +186,22 @@ def test_powerlaw():
     # sample power law distribution and check that the mean of the samples is
     # within 4 std. dev. of the expected mean
     n_samples = 1_000_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, exp_mean)
+    assert np.all(weights == 1.0)
+
+    # Test biased distribution
+    d.bias = openmc.stats.Uniform(a, b)
+    bias_elem = d.to_xml_element('distribution')
+    d2 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d2.bias, openmc.stats.Uniform)
+    samples, weights = d2.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, exp_mean)
+    assert np.all(weights != 1.0)
 
 
+@pytest.mark.flaky(reruns=1)
 def test_maxwell():
     theta = 1.2895e6
     d = openmc.stats.Maxwell(theta)
@@ -162,15 +216,28 @@ def test_maxwell():
     # sample maxwell distribution and check that the mean of the samples is
     # within 4 std. dev. of the expected mean
     n_samples = 1_000_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, exp_mean)
+    assert np.all(weights == 1.0)
 
     # A second sample starting from a different seed
-    samples_2 = d.sample(n_samples)
+    samples_2, weights_2 = d.sample(n_samples)
     assert_sample_mean(samples_2, exp_mean)
     assert samples_2.mean() != samples.mean()
+    assert np.all(weights_2 == 1)
+
+    # Test biased distribution
+    d.bias = openmc.stats.Maxwell((theta * 1.1))
+    bias_elem = d.to_xml_element('distribution')
+    d2 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d2.bias, openmc.stats.Maxwell)
+    samples, weights = d2.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, exp_mean)
+    assert np.all(weights != 1.0)
 
 
+@pytest.mark.flaky(reruns=1)
 def test_watt():
     a, b = 0.965e6, 2.29e-6
     d = openmc.stats.Watt(a, b)
@@ -190,28 +257,51 @@ def test_watt():
     # sample Watt distribution and check that the mean of the samples is within
     # 4 std. dev. of the expected mean
     n_samples = 1_000_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, exp_mean)
+    assert np.all(weights == 1.0)
+
+    # Test biased distribution with 5 percent higher T_e
+    d.bias = openmc.stats.Watt(a*1.05, b)
+    bias_elem = d.to_xml_element('distribution')
+    d2 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d2.bias, openmc.stats.Watt)
+    samples, weights = d2.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, exp_mean)
+    assert np.all(weights != 1.0)
 
 
+@pytest.mark.flaky(reruns=1)
 def test_tabular():
     # test linear-linear sampling
-    x = np.array([0.0, 5.0, 7.0, 10.0])
+    x = np.array([0.001, 5.0, 7.0, 10.0])
     p = np.array([10.0, 20.0, 5.0, 6.0])
     d = openmc.stats.Tabular(x, p, 'linear-linear')
     n_samples = 100_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, d.mean())
+    assert np.all(weights == 1.0)
 
-    # test linear-linear normalization
-    d.normalize()
-    assert d.integral() == pytest.approx(1.0)
+    for scheme in _INTERPOLATION_SCHEMES:
+        # test sampling
+        d = openmc.stats.Tabular(x, p, scheme)
+        n_samples = 100_000
+        samples = d.sample(n_samples)[0]
+        assert_sample_mean(samples, d.mean())
 
     # test histogram sampling
     d = openmc.stats.Tabular(x, p, interpolation='histogram')
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, d.mean())
+    assert np.all(weights == 1.0)
 
+    # Multiplying the probabilities should preserve the mean but change the integral
+    d2 = openmc.stats.Tabular(x, p*2, interpolation='histogram')
+    assert d2.mean() == pytest.approx(d.mean())
+    assert d2.integral() == pytest.approx(2.0*d.integral())
+
+    # Normalizing should result in an integral of 1
     d.normalize()
     assert d.integral() == pytest.approx(1.0)
 
@@ -220,7 +310,7 @@ def test_tabular():
     d = openmc.stats.Tabular(x, p[:-1], interpolation='histogram')
     d.cdf()
     d.mean()
-    assert_sample_mean(d.sample(n_samples), d.mean())
+    assert_sample_mean(d.sample(n_samples)[0], d.mean())
 
     # passing a shorter probability set should raise an error for linear-linear
     with pytest.raises(ValueError):
@@ -232,6 +322,16 @@ def test_tabular():
     d = openmc.stats.Tabular(x, p, interpolation='linear-linear')
     d.cdf()
 
+    # Test biased distribution
+    d.bias = openmc.stats.Uniform(x[0], x[-1])
+    bias_elem = d.to_xml_element('distribution')
+    d2 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d2.bias, openmc.stats.Uniform)
+    samples, weights = d2.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, d2.mean())
+    assert np.all(weights != 1.0)
+
 
 def test_tabular_from_xml():
     x = np.array([0.0, 5.0, 7.0, 10.0])
@@ -270,6 +370,7 @@ def test_legendre():
         d.to_xml_element('distribution')
 
 
+@pytest.mark.flaky(reruns=1)
 def test_mixture():
     d1 = openmc.stats.Uniform(0, 5)
     d2 = openmc.stats.Uniform(3, 7)
@@ -281,8 +382,9 @@ def test_mixture():
 
     # Sample and make sure sample mean is close to expected mean
     n_samples = 1_000_000
-    samples = mix.sample(n_samples)
+    samples, weights = mix.sample(n_samples)
     assert_sample_mean(samples, (2.5 + 5.0)/2)
+    assert np.all(weights == 1.0)
 
     elem = mix.to_xml_element('distribution')
 
@@ -291,6 +393,26 @@ def test_mixture():
     assert d.distribution == [d1, d2]
     assert len(d) == 4
 
+    # Test biased sub-distribution
+    d.distribution[0].bias = openmc.stats.PowerLaw(0, 5, 2)
+    bias_elem = d.to_xml_element('distribution')
+    d3 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d3.distribution[0].bias, openmc.stats.PowerLaw)
+    samples, weights = d3.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, (2.5 + 5.0)/2)
+
+    # Test biased meta-probability
+    d.distribution[0].bias = None
+    d.bias = [0.25, 0.75]
+    bias_elem_2 = d.to_xml_element('distribution')
+    d4 = openmc.stats.Univariate.from_xml_element(bias_elem_2)
+    assert isinstance (d4.bias, np.ndarray)
+    samples, weights = d4.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, (2.5 + 5.0)/2)
+    assert np.all(weights != 1.0)
+
 
 def test_mixture_clip():
     # Create mixture distribution containing a discrete distribution with two
@@ -323,6 +445,13 @@ def test_mixture_clip():
     with pytest.warns(UserWarning):
         mix_clip = mix.clip(1e-6)
 
+    # Make sure warning is raised if a biased Discrete is clipped
+    d3 = openmc.stats.Discrete([1.0, 1.001], [1.0, 0.7e-8])
+    d3.bias = [0.9, 0.1]
+    mix = openmc.stats.Mixture([1.0, 1.0], [d3, d2])
+    with pytest.raises(RuntimeError):
+        mix_clip = mix.clip(1e-6)
+
 
 def test_polar_azimuthal():
     # default polar-azimuthal should be uniform in mu and phi
@@ -358,12 +487,17 @@ def test_polar_azimuthal():
 
 def test_isotropic():
     d = openmc.stats.Isotropic()
+    mu = openmc.stats.Uniform(-1.0, 1.0)
+    phi = openmc.stats.PowerLaw(0., 2*np.pi, 2)
+    d2 = openmc.stats.PolarAzimuthal(mu, phi)
+    d.bias = d2
     elem = d.to_xml_element()
     assert elem.tag == 'angle'
     assert elem.attrib['type'] == 'isotropic'
 
     d = openmc.stats.Isotropic.from_xml_element(elem)
     assert isinstance(d, openmc.stats.Isotropic)
+    assert isinstance(d.bias, openmc.stats.PolarAzimuthal)
 
 
 def test_monodirectional():
@@ -380,6 +514,7 @@ def test_cartesian():
     x = openmc.stats.Uniform(-10., 10.)
     y = openmc.stats.Uniform(-10., 10.)
     z = openmc.stats.Uniform(0., 20.)
+    z.bias = openmc.stats.PowerLaw(0., 20., 3)
     d = openmc.stats.CartesianIndependent(x, y, z)
 
     elem = d.to_xml_element()
@@ -395,6 +530,7 @@ def test_cartesian():
 
     d = openmc.stats.Spatial.from_xml_element(elem)
     assert isinstance(d, openmc.stats.CartesianIndependent)
+    assert isinstance (d.z.bias, openmc.stats.PowerLaw)
 
 
 def test_box():
@@ -425,6 +561,114 @@ def test_point():
     assert d.xyz == pytest.approx(p)
 
 
+def test_spherical_uniform():
+    r_outer = 2.0
+    r_inner = 1.0
+    thetas = (0.0, pi/2)
+    phis = (0.0, pi)
+    origin = (0.0, 1.0, 2.0)
+
+    sph_indep_function = openmc.stats.spherical_uniform(r_outer,
+                                                        r_inner,
+                                                        thetas,
+                                                        phis,
+                                                        origin)
+
+    assert isinstance(sph_indep_function, openmc.stats.SphericalIndependent)
+
+
+def test_cylindrical_uniform():
+    r_outer = 2.0
+    r_inner = 1.0
+    height = 1.0
+    phis = (0.0, pi)
+    origin = (0.0, 1.0, 2.0)
+
+    dist = openmc.stats.cylindrical_uniform(r_outer, height, r_inner, phis,
+                                            origin=origin)
+
+    assert isinstance(dist, openmc.stats.CylindricalIndependent)
+
+    # Check r distribution (PowerLaw with exponent 1 for uniform area sampling)
+    assert isinstance(dist.r, openmc.stats.PowerLaw)
+    assert dist.r.a == pytest.approx(r_inner)
+    assert dist.r.b == pytest.approx(r_outer)
+    assert dist.r.n == pytest.approx(1.0)
+
+    # Check phi distribution
+    assert isinstance(dist.phi, openmc.stats.Uniform)
+    assert dist.phi.a == pytest.approx(phis[0])
+    assert dist.phi.b == pytest.approx(phis[1])
+
+    # Check z distribution (centered on origin along z_dir)
+    assert isinstance(dist.z, openmc.stats.Uniform)
+    assert dist.z.a == pytest.approx(-height / 2)
+    assert dist.z.b == pytest.approx(height / 2)
+
+    # Check origin and default directions
+    np.testing.assert_allclose(dist.origin, origin)
+    np.testing.assert_allclose(dist.r_dir, [1., 0., 0.])
+    np.testing.assert_allclose(dist.z_dir, [0., 0., 1.])
+
+    # XML round-trip preserves all parameters
+    elem = dist.to_xml_element()
+    dist2 = openmc.stats.CylindricalIndependent.from_xml_element(elem)
+    np.testing.assert_allclose(dist2.origin, origin)
+    np.testing.assert_allclose(dist2.r_dir, dist.r_dir)
+    np.testing.assert_allclose(dist2.z_dir, dist.z_dir)
+
+
+def test_cylindrical_uniform_tilted():
+    # Test with non-default axis orientation (y-axis as cylinder axis)
+    dist = openmc.stats.cylindrical_uniform(
+        r_outer=3.0, height=2.0, r_dir=(1., 0., 0.), z_dir=(0., 1., 0.)
+    )
+    np.testing.assert_allclose(dist.z_dir, [0., 1., 0.])
+    np.testing.assert_allclose(dist.r_dir, [1., 0., 0.])
+
+    # XML round-trip preserves tilted directions
+    elem = dist.to_xml_element()
+    dist2 = openmc.stats.CylindricalIndependent.from_xml_element(elem)
+    np.testing.assert_allclose(dist2.z_dir, dist.z_dir)
+    np.testing.assert_allclose(dist2.r_dir, dist.r_dir)
+
+
+def test_cylindrical_uniform_ring():
+    # height=0 should produce a flat ring (delta function at z=0)
+    r_outer = 2.0
+    r_inner = 1.0
+    phis = (0.0, pi)
+    origin = (0.0, 1.0, 2.0)
+
+    dist = openmc.stats.cylindrical_uniform(r_outer, 0.0, r_inner, phis,
+                                            origin=origin)
+
+    assert isinstance(dist, openmc.stats.CylindricalIndependent)
+
+    # Check r distribution
+    assert isinstance(dist.r, openmc.stats.PowerLaw)
+    assert dist.r.a == pytest.approx(r_inner)
+    assert dist.r.b == pytest.approx(r_outer)
+    assert dist.r.n == pytest.approx(1.0)
+
+    # Check phi distribution
+    assert isinstance(dist.phi, openmc.stats.Uniform)
+    assert dist.phi.a == pytest.approx(phis[0])
+    assert dist.phi.b == pytest.approx(phis[1])
+
+    # z distribution must be a delta function at 0.0 (local frame)
+    assert isinstance(dist.z, openmc.stats.Discrete)
+    assert dist.z.x[0] == pytest.approx(0.0)
+
+    # XML round-trip
+    elem = dist.to_xml_element()
+    dist2 = openmc.stats.CylindricalIndependent.from_xml_element(elem)
+    np.testing.assert_allclose(dist2.origin, origin)
+    np.testing.assert_allclose(dist2.r_dir, dist.r_dir)
+    np.testing.assert_allclose(dist2.z_dir, dist.z_dir)
+
+
+@pytest.mark.flaky(reruns=1)
 def test_normal():
     mean = 10.0
     std_dev = 2.0
@@ -440,10 +684,108 @@ def test_normal():
 
     # sample normal distribution
     n_samples = 100_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, mean)
+    assert np.all(weights == 1.0)
+
+    # Test biased distribution
+    d.bias = openmc.stats.Normal(10.0, 4.0)
+    bias_elem = d.to_xml_element('distribution')
+    d2 = openmc.stats.Univariate.from_xml_element(bias_elem)
+    assert isinstance (d2.bias, openmc.stats.Normal)
+    samples, weights = d2.sample(n_samples)
+    weighted_sample = samples * weights
+    assert_sample_mean(weighted_sample, mean)
+    assert np.all(weights != 1.0)
 
 
+@pytest.mark.flaky(reruns=1)
+def test_normal_truncated():
+    mean = 10.0
+    std_dev = 2.0
+    lower = 6.0
+    upper = 14.0
+
+    d = openmc.stats.Normal(mean, std_dev, lower, upper)
+
+    # Check attributes
+    assert d.mean_value == pytest.approx(mean)
+    assert d.std_dev == pytest.approx(std_dev)
+    assert d.lower == pytest.approx(lower)
+    assert d.upper == pytest.approx(upper)
+    assert len(d) == 4
+    assert d.support == (lower, upper)
+
+    # Test XML round-trip
+    elem = d.to_xml_element('distribution')
+    assert elem.attrib['type'] == 'normal'
+    params = elem.attrib['parameters'].split()
+    assert len(params) == 4
+
+    d2 = openmc.stats.Normal.from_xml_element(elem)
+    assert d2.mean_value == pytest.approx(mean)
+    assert d2.std_dev == pytest.approx(std_dev)
+    assert d2.lower == pytest.approx(lower)
+    assert d2.upper == pytest.approx(upper)
+
+    # Test PDF evaluation
+    # PDF should be zero outside bounds
+    assert d.evaluate(lower - 1.0) == 0.0
+    assert d.evaluate(upper + 1.0) == 0.0
+
+    # PDF should be positive inside bounds
+    assert d.evaluate(mean) > 0.0
+
+    # PDF should be higher than untruncated at the mean (due to renormalization)
+    d_unbounded = openmc.stats.Normal(mean, std_dev)
+    assert d.evaluate(mean) > d_unbounded.evaluate(mean)
+
+    # Verify that PDF integrates to approximately 1
+    x = np.linspace(lower, upper, 1000)
+    integral = trapezoid(d.evaluate(x), x)
+    assert integral == pytest.approx(1.0, rel=0.01)
+
+    # Sample truncated distribution
+    n_samples = 10_000
+    samples, weights = d.sample(n_samples)
+
+    # All samples should be within bounds
+    assert np.all(samples >= lower)
+    assert np.all(samples <= upper)
+
+    # Weights should all be 1 (no biasing)
+    assert np.all(weights == 1.0)
+
+
+def test_normal_truncated_one_sided():
+    # Test lower-bounded only (positive half-normal centered at 0)
+    d_lower = openmc.stats.Normal(0.0, 1.0, lower=0.0)
+    assert d_lower.lower == 0.0
+    assert d_lower.upper == np.inf
+    assert d_lower.evaluate(-1.0) == 0.0
+    assert d_lower.evaluate(1.0) > 0.0
+
+    # PDF at 0 should be approximately 2 * 0.3989 ≈ 0.798 (half-normal)
+    assert d_lower.evaluate(0.0) == pytest.approx(0.798, rel=0.01)
+
+    # Test upper-bounded only
+    d_upper = openmc.stats.Normal(0.0, 1.0, upper=0.0)
+    assert d_upper.lower == -np.inf
+    assert d_upper.upper == 0.0
+    assert d_upper.evaluate(1.0) == 0.0
+    assert d_upper.evaluate(-1.0) > 0.0
+
+
+def test_normal_truncated_errors():
+    # Invalid bounds (lower >= upper)
+    with pytest.raises(ValueError):
+        openmc.stats.Normal(0.0, 1.0, lower=1.0, upper=0.0)
+
+    with pytest.raises(ValueError):
+        openmc.stats.Normal(0.0, 1.0, lower=1.0, upper=1.0)
+
+
+@pytest.mark.flaky(reruns=1)
 def test_muir():
     mean = 10.0
     mass = 5.0
@@ -459,10 +801,12 @@ def test_muir():
 
     # sample muir distribution
     n_samples = 100_000
-    samples = d.sample(n_samples)
+    samples, weights = d.sample(n_samples)
     assert_sample_mean(samples, mean)
+    assert np.all(weights == 1.0)
 
 
+@pytest.mark.flaky(reruns=1)
 def test_combine_distributions():
     # Combine two discrete (same data as in test_merge_discrete)
     x1 = [0.0, 1.0, 10.0]
@@ -492,17 +836,313 @@ def test_combine_distributions():
     assert isinstance(mixed, openmc.stats.Mixture)
     assert len(mixed.distribution) == 2
     assert len(mixed.probability) == 2
+    assert mixed == openmc.stats.combine_distributions([mixed], [1.0])
+
+    # Mixture combined with another distribution: probabilities should be
+    # correctly scaled when the Mixture is flattened
+    d_a = openmc.stats.delta_function(1.0)
+    d_b = openmc.stats.delta_function(2.0)
+    m = openmc.stats.Mixture([0.3, 0.7], [d_a, d_b])
+    extra = openmc.stats.delta_function(3.0)
+    result = openmc.stats.combine_distributions([m, extra], [0.5, 0.5])
+    assert isinstance(result, openmc.stats.Discrete)
+    assert result.x == pytest.approx([1.0, 2.0, 3.0])
+    assert result.p == pytest.approx([0.5*0.3, 0.5*0.7, 0.5])
+
+    # Passing a Mixture with a bias should warn that the bias is dropped
+    biased_m = openmc.stats.Mixture([0.5, 0.5], [d_a, d_b], bias=[0.8, 0.2])
+    with pytest.warns(UserWarning, match='bias'):
+        openmc.stats.combine_distributions([biased_m], [1.0])
+
+    # Single tabular returns a tabular distribution with scaled probabilities
+    t_single = openmc.stats.Tabular([0.0, 1.0], [2.0, 0.0])
+    scaled = openmc.stats.combine_distributions([t_single], [0.25])
+    assert isinstance(scaled, openmc.stats.Tabular)
+    assert scaled.p == pytest.approx([0.5, 0.0])
+
+    # Mixture with biased tabular should preserve unbiased mean via weights
+    bias = openmc.stats.Tabular([0.0, 1.0], [2.0, 0.0])
+    t_biased = openmc.stats.Tabular([0.0, 1.0], [1.0, 1.0], bias=bias)
+    d1 = openmc.stats.delta_function(0.0)
+    mixed = openmc.stats.combine_distributions([t_biased, d1], [0.5, 0.5])
+    assert isinstance(mixed, openmc.stats.Mixture)
+    samples, weights = mixed.sample(10_000)
+    assert_sample_mean(samples*weights, 0.25)
 
     # Combine 1 discrete and 2 tabular -- the tabular distributions should
     # combine to produce a uniform distribution with mean 0.5. The combined
     # distribution should have a mean of 0.25.
     t1 = openmc.stats.Tabular([0., 1.], [2.0, 0.0])
     t2 = openmc.stats.Tabular([0., 1.], [0.0, 2.0])
-    d1 = openmc.stats.Discrete([0.0], [1.0])
+    d1 = openmc.stats.delta_function(0.0)
     combined = openmc.stats.combine_distributions([t1, t2, d1], [0.25, 0.25, 0.5])
     assert combined.integral() == pytest.approx(1.0)
 
     # Sample the combined distribution and make sure the sample mean is within
     # uncertainty of the expected value
-    samples = combined.sample(10_000)
+    samples, weights = combined.sample(10_000)
     assert_sample_mean(samples, 0.25)
+    assert np.all(weights == 1.0)
+
+    # If biased/unbiased Discrete distributions are combined, unbiased probability
+    # should be conserved and points from both original distributions should be
+    # assigned bias probabilities.
+    x1 = [0.0, 1.0, 10.0]
+    p1 = [0.3, 0.2, 0.5]
+    b1 = [0.2, 0.5, 0.3]
+    d1 = openmc.stats.Discrete(x1, p1, b1)
+    x2 = [0.5, 1.0, 5.0]
+    p2 = [0.4, 0.5, 0.1]
+    d2 = openmc.stats.Discrete(x2, p2)
+    combined = openmc.stats.combine_distributions([d1, d2, t1], [0.25, 0.25, 0.5])
+
+    p3 = [0.075, 0.1, 0.175, 0.025, 0.125]
+    b3 = [0.05, 0.1, 0.25, 0.025, 0.075]
+    assert all(combined.distribution[-1].p == p3)
+    assert all(combined.distribution[-1].bias == b3)
+
+
+def test_reference_vwu_projection():
+    """When a non-orthogonal vector is provided, the setter should project out
+    any component along reference_uvw so the stored vector is orthogonal.
+    """
+    pa = openmc.stats.PolarAzimuthal()  # default reference_uvw == (0, 0, 1)
+
+    # Provide a vector that is not orthogonal to (0,0,1)
+    pa.reference_vwu = (2.0, 0.5, 0.3)
+
+    reference_v = np.asarray(pa.reference_vwu)
+    reference_u = np.asarray(pa.reference_uvw)
+
+    # reference_v should be orthogonal to reference_u
+    assert abs(np.dot(reference_v, reference_u)) < 1e-6
+
+
+def test_reference_vwu_normalization():
+    """When a non-normalized vector is provided, the setter should normalize
+    the projected vector to unit length.
+    """
+    pa = openmc.stats.PolarAzimuthal()  # default reference_uvw == (0, 0, 1)
+
+    # Provide a vector that is neither orthogonal to (0,0,1) nor unit-length
+    pa.reference_vwu = (2.0, 0.5, 0.3)
+
+    reference_v = np.asarray(pa.reference_vwu)
+
+    # reference_v should be unit length
+    assert np.isclose(np.linalg.norm(reference_v), 1.0, atol=1e-12)
+
+
+def test_fusion_spectrum_dd():
+    d = openmc.stats.fusion_neutron_spectrum(10e3, 'DD')
+    assert isinstance(d, openmc.stats.Normal)
+
+    # E_0 for D(d,n)3He is ~2.45 MeV; thermal shift at 10 keV should be
+    # several tens of keV, so mean should be noticeably above E_0
+    assert d.mean_value > 2.45e6
+    assert d.mean_value < 2.6e6
+
+    # Standard deviation should be positive and on order of ~50-100 keV
+    assert d.std_dev > 30e3
+    assert d.std_dev < 200e3
+
+
+def test_fusion_spectrum_dt():
+    d = openmc.stats.fusion_neutron_spectrum(10e3, 'DT')
+    assert isinstance(d, openmc.stats.Normal)
+
+    # E_0 for T(d,n)alpha is ~14.02 MeV; with thermal shift mean should be
+    # above E_0 by several tens of keV
+    assert d.mean_value > 14.02e6
+    assert d.mean_value < 14.2e6
+
+    # Standard deviation should be on order of ~200-400 keV
+    assert d.std_dev > 100e3
+    assert d.std_dev < 500e3
+
+
+def test_fusion_spectrum_temp_continuity():
+    # Verify the low-T and high-T formulas produce nearly identical results
+    # at the 40 keV switchover point
+    d_lo = openmc.stats.fusion_neutron_spectrum(39.99e3, 'DT')
+    d_hi = openmc.stats.fusion_neutron_spectrum(40.01e3, 'DT')
+
+    assert d_lo.mean_value == pytest.approx(d_hi.mean_value, rel=1e-3)
+    assert d_lo.std_dev == pytest.approx(d_hi.std_dev, rel=1e-3)
+
+    # Same check for DD
+    d_lo = openmc.stats.fusion_neutron_spectrum(39.99e3, 'DD')
+    d_hi = openmc.stats.fusion_neutron_spectrum(40.01e3, 'DD')
+
+    assert d_lo.mean_value == pytest.approx(d_hi.mean_value, rel=1e-3)
+    assert d_lo.std_dev == pytest.approx(d_hi.std_dev, rel=1e-3)
+
+
+def test_fusion_spectrum_high_temp():
+    # At T_i = 80 keV (high-T regime), ensure the function still produces
+    # reasonable results using Table IV formulas
+    for reactants in ('DD', 'DT'):
+        d = openmc.stats.fusion_neutron_spectrum(80e3, reactants)
+        assert isinstance(d, openmc.stats.Normal)
+        assert d.mean_value > 0
+        assert d.std_dev > 0
+
+    # DT mean at 80 keV should be higher than at 10 keV
+    d_10 = openmc.stats.fusion_neutron_spectrum(10e3, 'DT')
+    d_80 = openmc.stats.fusion_neutron_spectrum(80e3, 'DT')
+    assert d_80.mean_value > d_10.mean_value
+    assert d_80.std_dev > d_10.std_dev
+
+
+def test_fusion_spectrum_zero_temp():
+    # At very low temperature, mean should approach E_0 and width should
+    # approach zero
+    d = openmc.stats.fusion_neutron_spectrum(1.0, 'DT')
+    assert d.mean_value == pytest.approx(14.049e6, rel=1e-3)
+    assert d.std_dev < 5e3  # width approaches zero at low temperature
+
+
+def test_fusion_spectrum_invalid():
+    # Invalid reactant string should raise an error
+    with pytest.raises(ValueError):
+        openmc.stats.fusion_neutron_spectrum(10e3, '🐔🧇')
+
+    # Negative temperature should raise an error
+    with pytest.raises(ValueError):
+        openmc.stats.fusion_neutron_spectrum(-10e3, 'DT')
+
+    # Temperature above 100 keV should raise an error
+    with pytest.raises(ValueError):
+        openmc.stats.fusion_neutron_spectrum(101e3, 'DT')
+
+
+@pytest.fixture(autouse=False)
+def decay_spectrum_chain():
+    """Set chain_file for the duration of a test and clear the _photon_integral
+    cache so results from a different chain don't bleed across tests."""
+    CHAIN_FILE = (Path(__file__).parents[1] / 'chain_simple.xml').resolve()
+    DecaySpectrum._photon_integral.cache_clear()
+    with openmc.config.patch('chain_file', CHAIN_FILE):
+        yield
+    DecaySpectrum._photon_integral.cache_clear()
+
+
+def test_decay_spectrum_construction():
+    nuclides = {'I135': 1.5e-3, 'Xe135': 8.2e-4}
+    d = openmc.stats.DecaySpectrum(nuclides, volume=100.0)
+    assert d.nuclides == nuclides
+    assert d.volume == pytest.approx(100.0)
+    assert len(d) == 2
+
+
+def test_decay_spectrum_validation():
+    # nuclides must be a dict
+    with pytest.raises(TypeError):
+        openmc.stats.DecaySpectrum(['I135'], volume=1.0)
+
+    # densities must be > 0
+    with pytest.raises(ValueError):
+        openmc.stats.DecaySpectrum({'I135': -1.0}, volume=1.0)
+
+    # volume must be > 0
+    with pytest.raises(ValueError):
+        openmc.stats.DecaySpectrum({'I135': 1e-3}, volume=-1.0)
+
+    with pytest.raises(ValueError):
+        openmc.stats.DecaySpectrum({'I135': 1e-3}, volume=0.0)
+
+
+def test_decay_spectrum_xml_roundtrip():
+    nuclides = {'I135': 1.5e-3, 'Xe135': 8.2e-4}
+    d = openmc.stats.DecaySpectrum(nuclides, volume=100.0)
+
+    elem = d.to_xml_element('energy')
+    assert elem.get('type') == 'decay_spectrum'
+    assert float(elem.get('volume')) == pytest.approx(100.0)
+    assert elem.findtext('nuclides').split() == list(nuclides)
+    assert [float(x) for x in elem.findtext('parameters').split()] == pytest.approx(
+        list(nuclides.values()))
+
+    # Round-trip via DecaySpectrum.from_xml_element
+    d2 = openmc.stats.DecaySpectrum.from_xml_element(elem)
+    assert d2.nuclides == nuclides
+    assert d2.volume == pytest.approx(100.0)
+
+    # Round-trip via the Univariate dispatcher
+    d3 = openmc.stats.Univariate.from_xml_element(elem)
+    assert isinstance(d3, openmc.stats.DecaySpectrum)
+    assert d3 == d
+
+
+def test_decay_spectrum_to_distribution(decay_spectrum_chain):
+    # Single emitting nuclide -> concrete distribution, not None
+    d = openmc.stats.DecaySpectrum({'I135': 1e-3}, volume=10.0)
+    dist = d.to_distribution()
+    assert dist is not None
+
+    # Result is cached on second call
+    dist2 = d.to_distribution()
+    assert dist2 is dist
+
+    # Nuclide with no photon source -> None
+    d_stable = openmc.stats.DecaySpectrum({'Xe136': 1e-3}, volume=10.0)
+    assert d_stable.to_distribution() is None
+
+    # Mixture of emitters -> non-None combined distribution
+    d_mix = openmc.stats.DecaySpectrum(
+        {'I135': 1e-3, 'Xe135': 5e-4}, volume=10.0
+    )
+    dist_mix = d_mix.to_distribution()
+    assert dist_mix is not None
+
+
+def test_decay_spectrum_integral(decay_spectrum_chain):
+    # For an emitting nuclide, integral should be > 0
+    d = openmc.stats.DecaySpectrum({'I135': 1e-3}, volume=10.0)
+    assert d.integral() > 0.0
+
+    # Proportional to density: doubling density doubles integral
+    d2 = openmc.stats.DecaySpectrum({'I135': 2e-3}, volume=10.0)
+    assert d2.integral() == pytest.approx(2.0 * d.integral())
+
+    # Proportional to volume
+    d3 = openmc.stats.DecaySpectrum({'I135': 1e-3}, volume=20.0)
+    assert d3.integral() == pytest.approx(2.0 * d.integral())
+
+    # Pure non-emitter -> 0.0
+    d_stable = openmc.stats.DecaySpectrum({'Xe136': 1e-3}, volume=10.0)
+    assert d_stable.integral() == pytest.approx(0.0)
+
+
+def test_decay_spectrum_clip(decay_spectrum_chain):
+    # Stable / non-emitting nuclides are removed unconditionally
+    d = openmc.stats.DecaySpectrum(
+        {'I135': 1e-3, 'Xe135': 5e-4, 'Xe136': 1.0, 'Cs135': 1.0},
+        volume=10.0,
+    )
+    d_clip = d.clip()
+    assert 'Xe136' not in d_clip.nuclides
+    assert 'Cs135' not in d_clip.nuclides
+    assert 'I135' in d_clip.nuclides
+    assert 'Xe135' in d_clip.nuclides
+    # Original is unchanged
+    assert 'Xe136' in d.nuclides
+
+    # inplace=True modifies and returns the same object
+    d_same = d.clip(inplace=True)
+    assert d_same is d
+    assert 'Xe136' not in d.nuclides
+
+    # A nuclide with negligible emission rate is removed by tolerance clipping.
+    # U235 has a very small integral (~4e-17 Bq/atom) compared with I135 (~4e-5)
+    d_tight = openmc.stats.DecaySpectrum(
+        {'I135': 1e-3, 'U235': 1e-3}, volume=10.0
+    )
+    d_tight_clip = d_tight.clip(tolerance=1e-9)
+    assert 'U235' not in d_tight_clip.nuclides
+    assert 'I135' in d_tight_clip.nuclides
+
+    # All non-emitters -> empty nuclides dict
+    d_empty = openmc.stats.DecaySpectrum({'Xe136': 1e-3}, volume=10.0)
+    d_empty.clip(inplace=True)
+    assert d_empty.nuclides == {}
diff --git a/tests/unit_tests/test_summary.py b/tests/unit_tests/test_summary.py
new file mode 100644
index 000000000..315ac0ba5
--- /dev/null
+++ b/tests/unit_tests/test_summary.py
@@ -0,0 +1,41 @@
+import openmc
+
+
+def test_periodic_surface_roundtrip(run_in_tmpdir):
+    # Create a simple model with periodic surfaces
+    mat = openmc.Material()
+    mat.add_nuclide('H1', 1.0)
+    mat.set_density('g/cm3', 1.0)
+    cyl = openmc.ZCylinder(r=1.0)
+    x0 = openmc.XPlane(-5.0, boundary_type='periodic')
+    y0 = openmc.YPlane(-5.0, boundary_type='periodic')
+    z0 = openmc.ZPlane(-5.0, boundary_type='periodic')
+    x1 = openmc.XPlane(5.0, boundary_type='periodic')
+    y1 = openmc.YPlane(5.0, boundary_type='periodic')
+    z1 = openmc.ZPlane(5.0, boundary_type='periodic')
+    x0.periodic_surface = x1
+    y0.periodic_surface = y1
+    z0.periodic_surface = z1
+    cell1 = openmc.Cell(fill=mat, region=-cyl)
+    cell2 = openmc.Cell(fill=mat, region=+cyl & +x0 & -x1 & +y0 & -y1 & +z0 & -z1)
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell1, cell2])
+    model.settings.particles = 100
+    model.settings.batches = 1
+    model.settings.run_mode = 'fixed source'
+    model.settings.source = openmc.IndependentSource(
+        energy=openmc.stats.delta_function(1.0e4)
+    )
+
+    # Run model
+    model.run()
+
+    # Load summary data and check periodic surfaces
+    summary = openmc.Summary('summary.h5')
+    surfs = summary.geometry.get_all_surfaces()
+    for s in [x0, y0, z0, x1, y1, z1]:
+        assert surfs[s.id].boundary_type == 'periodic'
+    pairs = [(x0, x1), (y0, y1), (z0, z1)]
+    for s0, s1 in pairs:
+        assert surfs[s0.id].periodic_surface == surfs[s1.id]
+        assert surfs[s1.id].periodic_surface == surfs[s0.id]
diff --git a/tests/unit_tests/test_surface.py b/tests/unit_tests/test_surface.py
index e9560223d..f5c0b8f8b 100644
--- a/tests/unit_tests/test_surface.py
+++ b/tests/unit_tests/test_surface.py
@@ -7,6 +7,12 @@ import openmc
 import pytest
 
 
+def test_id():
+    for i in range(-10, 1):
+        with pytest.raises(ValueError):
+            openmc.Plane(a=1, b=2, c=-1, d=3, surface_id=i)
+
+
 def assert_infinite_bb(s):
     ll, ur = (-s).bounding_box
     assert np.all(np.isinf(ll))
@@ -189,6 +195,12 @@ def test_cylinder():
     assert s.dy == -1
     assert s.dz == 1
     assert s.r == 2
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        openmc.Cylinder(x0=x0, y0=y0, z0=z0, dx=dx, dy=dy, dz=dz, r=0.0)
+    with pytest.raises(ValueError):
+        openmc.Cylinder(x0=x0, y0=y0, z0=z0, dx=dx, dy=dy, dz=dz, r=-1.0)        
 
     # Check bounding box
     assert_infinite_bb(s)
@@ -238,6 +250,12 @@ def test_xcylinder():
     assert s.y0 == y
     assert s.z0 == z
     assert s.r == r
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        openmc.XCylinder(y0=y, z0=z, r=0.0)
+    with pytest.raises(ValueError):
+        openmc.XCylinder(y0=y, z0=z, r=-1.0)
 
     # Check bounding box
     ll, ur = (+s).bounding_box
@@ -284,6 +302,12 @@ def test_ycylinder():
     assert s.x0 == x
     assert s.z0 == z
     assert s.r == r
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        openmc.YCylinder(x0=x, z0=z, r=0.0)
+    with pytest.raises(ValueError):
+        openmc.YCylinder(x0=x, z0=z, r=-1.0)
 
     # Check bounding box
     ll, ur = (+s).bounding_box
@@ -321,6 +345,12 @@ def test_zcylinder():
     assert s.x0 == x
     assert s.y0 == y
     assert s.r == r
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        openmc.ZCylinder(x0=x, y0=y, r=0.0)
+    with pytest.raises(ValueError):
+        openmc.ZCylinder(x0=x, y0=y, r=-1.0)
 
     # Check bounding box
     ll, ur = (+s).bounding_box
@@ -359,6 +389,12 @@ def test_sphere():
     assert s.y0 == y
     assert s.z0 == z
     assert s.r == r
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        openmc.Sphere(x0=x, y0=y, z0=z, r=0.0)
+    with pytest.raises(ValueError):
+        openmc.Sphere(x0=x, y0=y, z0=z, r=-1.0)
 
     # Check bounding box
     ll, ur = (+s).bounding_box
@@ -398,6 +434,12 @@ def cone_common(apex, r2, cls):
     assert s.y0 == y
     assert s.z0 == z
     assert s.r2 == r2
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        cls(x0=x, y0=y, z0=z, r2=0.0)
+    with pytest.raises(ValueError):
+        cls(x0=x, y0=y, z0=z, r2=-1.0)
 
     # Check bounding box
     assert_infinite_bb(s)
@@ -436,6 +478,12 @@ def test_cone():
     assert s.dy == -1
     assert s.dz == 1
     assert s.r2 == 4
+    
+    # Check radius must be positive
+    with pytest.raises(ValueError):
+        openmc.Cone(x0=x0, y0=y0, z0=z0, dx=dx, dy=dy, dz=dz, r2=0.0)
+    with pytest.raises(ValueError):
+        openmc.Cone(x0=x0, y0=y0, z0=z0, dx=dx, dy=dy, dz=dz, r2=-1.0)   
 
     # Check bounding box
     assert_infinite_bb(s)
@@ -616,6 +664,13 @@ def torus_common(center, R, r1, r2, cls):
     assert s.a == R
     assert s.b == r1
     assert s.c == r2
+    
+    # Check radius must be positive
+    params = [(0.0, r1, r2), (R, 0.0, r2), (R, r1, 0.0), 
+              (-1.0, r1, r2), (R, -1.0, r2), (R, r1, -1.0)]
+    for a,b,c in params:
+        with pytest.raises(ValueError):
+            cls(x0=x, y0=y, z0=z, a=a, b=b, c=c)
 
     # evaluate method
     assert s.evaluate((x, y, z)) > 0.0
diff --git a/tests/unit_tests/test_surface_flux.py b/tests/unit_tests/test_surface_flux.py
new file mode 100644
index 000000000..e4419252e
--- /dev/null
+++ b/tests/unit_tests/test_surface_flux.py
@@ -0,0 +1,164 @@
+"""Tests for surface flux tallying via flux score + SurfaceFilter."""
+
+import math
+import pytest
+
+import openmc
+
+
+@pytest.fixture
+def two_cell_model():
+    """Simple two-cell slab model with a monodirectional fixed source.
+
+    Cell1 occupies x in [-10, 0], cell2 x in [0, 10].  The source fires all
+    particles from (-5, 0, 0) in the +x direction with weight 1.  Every
+    particle therefore crosses the surface at x=0 from cell1 into cell2 at
+    mu = 1 (normal incidence).
+    """
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+
+    xmin = openmc.XPlane(-10.0, boundary_type="vacuum")
+    xmid = openmc.XPlane(0.0)
+    xmax = openmc.XPlane(10.0, boundary_type="vacuum")
+    ymin = openmc.YPlane(-10.0, boundary_type="vacuum")
+    ymax = openmc.YPlane(10.0, boundary_type="vacuum")
+    zmin = openmc.ZPlane(-10.0, boundary_type="vacuum")
+    zmax = openmc.ZPlane(10.0, boundary_type="vacuum")
+
+    cell1 = openmc.Cell(region=+xmin & -xmid & +ymin & -ymax & +zmin & -zmax)
+    cell2 = openmc.Cell(region=+xmid & -xmax & +ymin & -ymax & +zmin & -zmax)
+    model.geometry = openmc.Geometry([cell1, cell2])
+
+    src = openmc.IndependentSource()
+    src.space = openmc.stats.Point((-5.0, 0.0, 0.0))
+    src.angle = openmc.stats.Monodirectional((1.0, 0.0, 0.0))
+
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 1
+    model.settings.particles = 100
+    model.settings.source = src
+
+    return model, xmid, cell1, cell2
+
+
+def test_surface_filter_flux_normal_incidence(two_cell_model, run_in_tmpdir):
+    """SurfaceFilter + flux at mu=1 gives w/|mu| = 1.0 per source particle."""
+    model, xmid, *_ = two_cell_model
+
+    surf_filter = openmc.SurfaceFilter([xmid])
+    flux_tally = openmc.Tally()
+    flux_tally.filters = [surf_filter]
+    flux_tally.scores = ['flux']
+    model.tallies = [flux_tally]
+
+    model.run(apply_tally_results=True)
+    flux_mean = flux_tally.mean.flat[0]
+
+    # Every particle crosses at mu=1 with weight 1, so flux = 1.0
+    assert flux_mean == pytest.approx(1.0, rel=1e-8)
+
+
+def test_surface_filter_current_outward(two_cell_model, run_in_tmpdir):
+    """SurfaceFilter + current gives +1.0 for purely outward crossings."""
+    model, xmid, *_ = two_cell_model
+
+    surf_filter = openmc.SurfaceFilter([xmid])
+    current_tally = openmc.Tally()
+    current_tally.filters = [surf_filter]
+    current_tally.scores = ['current']
+    model.tallies = [current_tally]
+
+    model.run(apply_tally_results=True)
+    current_mean = current_tally.mean.flat[0]
+
+    # All crossings are outward → net current = +1.0
+    assert current_mean == pytest.approx(1.0)
+
+
+def test_surface_filter_flux_angled(two_cell_model, run_in_tmpdir):
+    """Surface flux at 60-degree incidence gives w/|mu| = 2.0."""
+    model, xmid, *_ = two_cell_model
+
+    # Modify source to use 60-degree angle from normal: mu = cos(60°) = 0.5
+    mu = ux = 0.5
+    uy = math.sqrt(1.0 - ux**2)
+    model.settings.source[0].angle = openmc.stats.Monodirectional((ux, uy, 0.0))
+
+    surf_filter = openmc.SurfaceFilter([xmid])
+    flux_tally = openmc.Tally()
+    flux_tally.filters = [surf_filter]
+    flux_tally.scores = ['flux']
+    model.tallies = [flux_tally]
+
+    model.run(apply_tally_results=True)
+    flux_mean = flux_tally.mean.flat[0]
+
+    # flux = w/|mu| = 1/0.5 = 2.0
+    assert flux_mean == pytest.approx(1.0 / mu)
+
+
+def test_surface_tally_during_lattice_crossing(run_in_tmpdir):
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+
+    xmin = openmc.XPlane(-1.0, boundary_type="vacuum")
+    xmax = openmc.XPlane(1.0, boundary_type="vacuum")
+    ymin = openmc.YPlane(-1.0, boundary_type="vacuum")
+    ymax = openmc.YPlane(1.0, boundary_type="vacuum")
+    zmin = openmc.ZPlane(-1.0, boundary_type="vacuum")
+    zmax = openmc.ZPlane(1.0, boundary_type="vacuum")
+    
+    inner_cell = openmc.Cell()
+    inner_univ = openmc.Universe(cells=[inner_cell])
+
+    tile_cell = openmc.Cell(fill=inner_univ)
+    tile_univ = openmc.Universe(cells=[tile_cell])
+
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-1.0, -1.0)
+    lattice.pitch = (1.0, 2.0)
+    lattice.universes = [[tile_univ, tile_univ]]
+
+    root_cell = openmc.Cell(
+        fill=lattice, region=+xmin & -xmax & +ymin & -ymax & +zmin & -zmax)
+    model.geometry = openmc.Geometry([root_cell])
+
+    src = openmc.IndependentSource()
+    src.space = openmc.stats.Point((-0.5, 0.0, 0.0))
+    src.angle = openmc.stats.Monodirectional((1.0, 0.0, 0.0))
+
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 1
+    model.settings.particles = 5
+    model.settings.source = src
+
+    current_tally = openmc.Tally()
+    current_tally.filters = [openmc.SurfaceFilter(xmax)]
+    current_tally.scores = ['current']
+    model.tallies = [current_tally]
+
+    model.run(apply_tally_results=True)
+    assert current_tally.mean.flat[0] == pytest.approx(1.0)
+
+
+def test_cellfrom_filter_flux_directional(two_cell_model, run_in_tmpdir):
+    """SurfaceFilter + CellFromFilter + flux scores only the correct direction."""
+    model, xmid, cell1, cell2 = two_cell_model
+
+    surf_filter = openmc.SurfaceFilter([xmid])
+    cellfrom_filter = openmc.CellFromFilter([cell1, cell2])
+
+    tally = openmc.Tally()
+    tally.filters = [surf_filter, cellfrom_filter]
+    tally.scores = ['flux']
+
+    model.tallies = [tally]
+    model.run(apply_tally_results=True)
+    mean_from1 = tally.mean.flat[0]
+    mean_from2 = tally.mean.flat[1]
+
+    # All particles cross xmid from cell1 at mu=1 → flux = 1.0
+    assert mean_from1 == pytest.approx(1.0)
+    # No particles cross xmid from cell2 → flux = 0
+    assert mean_from2 == pytest.approx(0.0)
diff --git a/tests/unit_tests/test_tallies.py b/tests/unit_tests/test_tallies.py
index ccc6ff343..7b1bf0a2f 100644
--- a/tests/unit_tests/test_tallies.py
+++ b/tests/unit_tests/test_tallies.py
@@ -1,6 +1,8 @@
+from math import sqrt
 import numpy as np
-
+import pytest
 import openmc
+import scipy.stats as sps
 
 
 def test_xml_roundtrip(run_in_tmpdir):
@@ -96,6 +98,22 @@ def test_tally_equivalence():
     assert tally_a == tally_b
 
 
+def test_figure_of_merit(sphere_model, run_in_tmpdir):
+    # Run model with a few simple tally scores
+    tally = openmc.Tally()
+    tally.scores = ['total', 'absorption', 'scatter']
+    sphere_model.tallies = [tally]
+    sp_path = sphere_model.run(apply_tally_results=True)
+
+    # Get execution time and relative error
+    with openmc.StatePoint(sp_path) as sp:
+        time = sp.runtime['simulation']
+    rel_err = tally.std_dev / tally.mean
+
+    # Check that figure of merit is calculated correctly
+    assert tally.figure_of_merit == pytest.approx(1 / (rel_err**2 * time))
+
+
 def test_tally_application(sphere_model, run_in_tmpdir):
     # Create a tally with most possible gizmos
     tally = openmc.Tally(name='test tally')
@@ -147,3 +165,214 @@ def test_tally_application(sphere_model, run_in_tmpdir):
     assert (sp_tally.std_dev == tally.std_dev).all()
     assert (sp_tally.mean == tally.mean).all()
     assert sp_tally.nuclides == tally.nuclides
+
+def _tally_from_data(x, *, higher_moments=True, normality=True):
+    t = openmc.Tally()
+    t.scores = ["flux"]  # 1 score
+    t.nuclides = [openmc.Nuclide("H1")]  # 1 nuclide
+    t._sp_filename = "dummy.h5"  # mark "results available"
+    t._results_read = True  # don't try to read from disk
+    t._num_realizations = int(len(x))  # n
+    t.higher_moments = bool(higher_moments)
+
+    x = np.asarray(x, dtype=float)
+    # (num_filter_bins=1, num_nuclides=1, num_scores=1) -> (1,1,1) arrays
+    t._sum = np.array([[[np.sum(x)]]], dtype=float)
+    t._sum_sq = np.array([[[np.sum(x**2)]]], dtype=float)
+    if higher_moments:
+        t._sum_third = np.array([[[np.sum(x**3)]]], dtype=float)
+        t._sum_fourth = np.array([[[np.sum(x**4)]]], dtype=float)
+    return t
+
+@pytest.mark.parametrize(
+    "x, skew_true, kurt_true",
+    [   # Rademacher distribution
+        (np.array([1.0, -1.0] * 200), 0.0, 1.0),
+        # Two-point {0,3} with p(0)=3/4, p(3)=1/4
+        (np.concatenate([np.zeros(600), np.full(200, 3.0)]), 2.0 / sqrt(3.0), 7.0 / 3.0),
+        # Bernoulli distribution
+        (np.concatenate([np.ones(300), np.zeros(700)]), (1 - 2 * 0.3) / sqrt(0.3 * 0.7), (1 - 3 * 0.3 + 3 * 0.3**2) / (0.3 * 0.7)),
+    ],
+)
+def test_b1_b2_analytical_against_tally(x, skew_true, kurt_true):
+    t = _tally_from_data(x, higher_moments=True, normality=False)
+
+    g1 = t.skew(bias=True)[0, 0, 0]
+    b2 = t.kurtosis(bias=True, fisher=False)[0, 0, 0]
+
+    assert np.isclose(g1, skew_true, rtol=0, atol=1e-12)
+    assert np.isclose(b2, kurt_true, rtol=0, atol=1e-12)
+
+@pytest.mark.parametrize(
+    "draw, skew_true, kurt_true",
+    [(lambda rng, n: rng.normal(0, 1, n), 0.0, 3.0),  # Normal
+     (lambda rng, n: rng.random(n), 0.0, 1.8),  # Uniform(0,1)
+     (lambda rng, n: rng.exponential(1.0, n), 2.0, 9.0),  # Exp(1)
+     (lambda rng, n: (rng.random(n) < 0.3).astype(float),
+            (1 - 2 * 0.3) / sqrt(0.3 * 0.7),
+            (1 - 3 * 0.3 + 3 * 0.3**2) / (0.3 * 0.7),),],)
+
+def test_b1_b2_scipy_and_theory(draw, skew_true, kurt_true):
+    rng = np.random.default_rng(12345)
+    N = 200_000
+    x = draw(rng, N)
+
+    # Tally outputs
+    t = _tally_from_data(x, higher_moments=True, normality=False)
+    g1_t = t.skew(bias=True)[0, 0, 0]
+    b2_t = t.kurtosis(bias=True, fisher=False)[0, 0, 0]
+
+    # SciPy (population, bias=True to match population-moment style)
+    skew_sp = sps.skew(x, bias=True)
+    kurt_sp = sps.kurtosis(x, fisher=False, bias=True)
+
+    # Compare to SciPy numerically
+    assert np.isclose(g1_t, skew_sp, rtol=0, atol=5e-3)
+    assert np.isclose(b2_t, kurt_sp, rtol=0, atol=5e-3)
+
+    # Compare to analytical targets with size-dependent tolerances
+    tol_skew = 0.02 if abs(skew_true) < 0.5 else 0.05
+    tol_kurt = 0.03 if kurt_true < 4 else 0.1
+    assert abs(g1_t - skew_true) < tol_skew
+    assert abs(b2_t - kurt_true) < tol_kurt
+
+
+def test_kurtosis_bias_fisher_combinations():
+    """Test that all combinations of bias and fisher match scipy.stats.kurtosis"""
+    rng = np.random.default_rng(42)
+    x = rng.normal(0, 1, 10000)
+
+    t = _tally_from_data(x, higher_moments=True, normality=False)    # Test all four combinations
+    # 1. bias=True, fisher=False (Pearson's kurtosis, b2)
+    b2_tally = t.kurtosis(bias=True, fisher=False)[0, 0, 0]
+    b2_scipy = sps.kurtosis(x, fisher=False, bias=True)
+    assert np.isclose(b2_tally, b2_scipy, rtol=0, atol=1e-10)
+    assert np.isclose(b2_tally, 3.0, rtol=0.05, atol=0.1)  # Should be ~3 for normal
+
+    # 2. bias=True, fisher=True (excess kurtosis, g2)
+    g2_tally = t.kurtosis(bias=True, fisher=True)[0, 0, 0]
+    g2_scipy = sps.kurtosis(x, fisher=True, bias=True)
+    assert np.isclose(g2_tally, g2_scipy, rtol=0, atol=1e-10)
+    assert np.isclose(g2_tally, 0.0, rtol=0, atol=0.1)  # Should be ~0 for normal
+    assert np.isclose(g2_tally, b2_tally - 3.0, rtol=0, atol=1e-10)  # g2 = b2 - 3
+
+    # 3. bias=False, fisher=True (adjusted excess kurtosis, G2)
+    G2_tally = t.kurtosis(bias=False, fisher=True)[0, 0, 0]
+    G2_tally_default = t.kurtosis()[0, 0, 0]  # Should be same as default
+    G2_scipy = sps.kurtosis(x, fisher=True, bias=False)
+    assert np.isclose(G2_tally, G2_tally_default, rtol=0, atol=1e-10)
+    assert np.isclose(G2_tally, G2_scipy, rtol=0, atol=1e-10)
+    assert np.isclose(G2_tally, 0.0, rtol=0, atol=0.1)  # Should be ~0 for normal
+
+    # 4. bias=False, fisher=False (adjusted Pearson's kurtosis)
+    adj_b2_tally = t.kurtosis(bias=False, fisher=False)[0, 0, 0]
+    adj_b2_scipy = sps.kurtosis(x, fisher=False, bias=False)
+    assert np.isclose(adj_b2_tally, adj_b2_scipy, rtol=0, atol=1e-10)
+    assert np.isclose(adj_b2_tally, 3.0, rtol=0.05, atol=0.1)  # Should be ~3 for normal
+    assert np.isclose(adj_b2_tally, G2_tally + 3.0, rtol=0, atol=1e-10)  # adj_b2 = G2 + 3
+
+
+def test_ztests_scipy_comparison():
+    rng = np.random.default_rng(987)
+    x_norm = rng.normal(size=50_000)
+    x_exp = rng.exponential(size=50_000)
+
+    # -------- Normal dataset (should not reject) --------
+    t0 = _tally_from_data(x_norm, higher_moments=True, normality=True)
+    Zb1_0, p_skew_0 = t0.skewtest(alternative="two-sided")
+    Zb2_0, p_kurt_0 = t0.kurtosistest(alternative="two-sided")
+    K2_0, p_omni_0 = t0.normaltest(alternative="two-sided")
+
+    Zb1_0 = Zb1_0.ravel()[0]
+    p_skew_0 = p_skew_0.ravel()[0]
+    Zb2_0 = Zb2_0.ravel()[0]
+    p_kurt_0 = p_kurt_0.ravel()[0]
+    K2_0 = K2_0.ravel()[0]
+    p_omni_0 = p_omni_0.ravel()[0]
+
+    z_skew_sp0, p_skew_sp0 = sps.skewtest(x_norm)
+    z_kurt_sp0, p_kurt_sp0 = sps.kurtosistest(x_norm)
+    k2_sp0, p_omni_sp0 = sps.normaltest(x_norm)
+
+    assert np.isclose(Zb1_0, z_skew_sp0, atol=0.15)
+    assert np.isclose(Zb2_0, z_kurt_sp0, atol=0.15)
+    assert np.isclose(K2_0, k2_sp0, atol=0.30)
+    assert np.isclose(p_skew_0, p_skew_sp0, atol=5e-3)
+    assert np.isclose(p_kurt_0, p_kurt_sp0, atol=5e-3)
+    assert np.isclose(p_omni_0, p_omni_sp0, atol=5e-3)
+
+    # -------- Exponential dataset (should strongly reject) --------
+    t1 = _tally_from_data(x_exp, higher_moments=True, normality=True)
+
+    Zb1_1, p_skew_1 = t1.skewtest(alternative="two-sided")
+    Zb2_1, p_kurt_1 = t1.kurtosistest(alternative="two-sided")
+    K2_1, p_omni_1 = t1.normaltest(alternative="two-sided")
+
+    Zb1_1 = Zb1_1.ravel()[0]
+    p_skew_1 = p_skew_1.ravel()[0]
+    Zb2_1 = Zb2_1.ravel()[0]
+    p_kurt_1 = p_kurt_1.ravel()[0]
+    K2_1 = K2_1.ravel()[0]
+    p_omni_1 = p_omni_1.ravel()[0]
+
+    z_skew_sp1, p_skew_sp1 = sps.skewtest(x_exp)
+    z_kurt_sp1, p_kurt_sp1 = sps.kurtosistest(x_exp)
+    k2_sp1, p_omni_sp1 = sps.normaltest(x_exp)
+
+    # Both pipelines should reject very strongly
+    assert p_skew_1 < 1e-6 and p_skew_sp1 < 1e-6
+    assert p_kurt_1 < 1e-6 and p_kurt_sp1 < 1e-6
+    assert p_omni_1 < 1e-6 and p_omni_sp1 < 1e-6
+
+    # Right-skewed and heavy-tailed → large positive Z-statistics
+    assert Zb1_1 > 30 and z_skew_sp1 > 30
+    assert Zb2_1 > 30 and z_kurt_sp1 > 30
+    assert K2_1 > 2000 and k2_sp1 > 2000
+
+def test_vov_stochastic(sphere_model, run_in_tmpdir):
+    tally = openmc.Tally(name="test tally")
+    ef = openmc.EnergyFilter([0.0, 0.1, 1.0, 10.0e6])
+    mesh = openmc.RegularMesh.from_domain(sphere_model.geometry, (2, 2, 2))
+    mf = openmc.MeshFilter(mesh)
+    tally.filters = [ef, mf]
+    tally.scores = ["flux", "absorption", "fission", "scatter"]
+    tally.higher_moments = True
+    sphere_model.tallies = [tally]
+
+    sp_file = sphere_model.run(apply_tally_results=True)
+
+    assert tally._mean is None
+    assert tally._std_dev is None
+    assert tally._sum is None
+    assert tally._sum_sq is None
+    assert tally._sum_third is None
+    assert tally._sum_fourth is None
+    assert tally._num_realizations == 0
+    assert tally._sp_filename == sp_file
+
+    with openmc.StatePoint(sp_file) as sp:
+        assert tally in sp.tallies.values()
+        sp_tally = sp.tallies[tally.id]
+
+    assert np.all(sp_tally.std_dev == tally.std_dev)
+    assert np.all(sp_tally.mean == tally.mean)
+    assert np.all(sp_tally.vov == tally.vov)
+    assert sp_tally.nuclides == tally.nuclides
+
+    n = sp_tally.num_realizations
+    mean = sp_tally.mean
+    sum_ = sp_tally._sum
+    sum_sq = sp_tally._sum_sq
+    sum_third = sp_tally._sum_third
+    sum_fourth = sp_tally._sum_fourth
+
+    expected_vov = np.zeros_like(mean)
+    nonzero = np.abs(mean) > 0
+
+    num = (sum_fourth - (4.0*sum_third*sum_)/n + (6.0*sum_sq*sum_**2)/(n**2)
+           - (3.0*sum_**4)/(n**3))
+    den = (sum_sq - (1.0/n)*sum_**2)**2
+
+    expected_vov[nonzero] = num[nonzero]/den[nonzero] - 1.0/n
+
+    assert np.allclose(expected_vov, sp_tally.vov, rtol=1e-7, atol=0.0)
diff --git a/tests/unit_tests/test_tally.py b/tests/unit_tests/test_tally.py
new file mode 100644
index 000000000..7136a621b
--- /dev/null
+++ b/tests/unit_tests/test_tally.py
@@ -0,0 +1,63 @@
+import openmc
+
+
+def test_tally_init_args():
+    """Test that Tally constructor kwargs are applied correctly."""
+    filter = openmc.EnergyFilter([0.0, 1.0, 20.0e6])
+    tally = openmc.Tally(
+        name='my tally',
+        scores=['flux', 'fission'],
+        filters=[filter],
+        nuclides=['U235'],
+        estimator='tracklength',
+    )
+
+    assert tally.name == 'my tally'
+    assert tally.scores == ['flux', 'fission']
+    assert tally.filters == [filter]
+    assert tally.nuclides == ['U235']
+    assert tally.estimator == 'tracklength'
+
+
+def test_heating_estimator_with_photon_transport(run_in_tmpdir):
+    """Test that a neutron-only heating tally uses the tracklength estimator
+    even when photon transport is enabled.
+
+    Without a neutron particle filter, photon heating requires the collision
+    estimator (analog energy balance). But neutron heating has kerma
+    coefficients that support tracklength scoring, so a neutron-only tally
+    should keep the tracklength estimator.
+    """
+    mat = openmc.Material()
+    mat.add_nuclide('Si28', 1.0)
+    mat.set_density('g/cm3', 2.3)
+    sph = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    cell = openmc.Cell(fill=mat, region=-sph)
+
+    model = openmc.Model()
+    model.geometry = openmc.Geometry([cell])
+    model.settings.run_mode = 'fixed source'
+    model.settings.particles = 100
+    model.settings.batches = 1
+    model.settings.photon_transport = True
+    model.settings.source = openmc.IndependentSource(
+        space=openmc.stats.Point()
+    )
+
+    neutron_filter = openmc.ParticleFilter(['neutron'])
+
+    # Neutron-only heating — should get tracklength estimator
+    t_neutron = openmc.Tally(name='heating_neutron')
+    t_neutron.filters = [neutron_filter]
+    t_neutron.scores = ['heating']
+
+    # No particle filter — should get collision estimator
+    t_all = openmc.Tally(name='heating_all')
+    t_all.scores = ['heating']
+
+    model.tallies = [t_neutron, t_all]
+
+    sp_filename = model.run()
+    with openmc.StatePoint(sp_filename) as sp:
+        assert sp.tallies[t_neutron.id].estimator == 'tracklength'
+        assert sp.tallies[t_all.id].estimator == 'collision'
diff --git a/tests/unit_tests/test_tracks.py b/tests/unit_tests/test_tracks.py
index fa8664159..74da83552 100644
--- a/tests/unit_tests/test_tracks.py
+++ b/tests/unit_tests/test_tracks.py
@@ -70,7 +70,7 @@ def test_tracks(sphere_model, particle, run_in_tmpdir):
         particle_track = track.particle_tracks[0]
         assert isinstance(particle_track, openmc.ParticleTrack)
 
-        assert particle_track.particle.name.lower() == particle
+        assert str(particle_track.particle).lower() == particle
         assert isinstance(particle_track.states, np.ndarray)
 
         # Sanity checks on actual data
@@ -140,8 +140,10 @@ def test_filter(sphere_model, run_in_tmpdir):
     assert matches == tracks
     matches = tracks.filter(state_filter=lambda s: s['E'] > 0.0)
     assert matches == tracks
-    matches = tracks.filter(particle='bunnytron')
+    matches = tracks.filter(particle='proton')
     assert matches == []
+    with pytest.raises(ValueError):
+        tracks.filter(particle='bunnytron')
 
 
 def test_write_to_vtk(sphere_model):
diff --git a/tests/unit_tests/test_uniform_source_sampling.py b/tests/unit_tests/test_uniform_source_sampling.py
index 7f805e37d..0d1930328 100644
--- a/tests/unit_tests/test_uniform_source_sampling.py
+++ b/tests/unit_tests/test_uniform_source_sampling.py
@@ -14,10 +14,15 @@ def sphere_model():
 
     model.settings.particles = 100
     model.settings.batches = 1
-    model.settings.source = openmc.IndependentSource(
+    src1 = openmc.IndependentSource(
         energy=openmc.stats.delta_function(1.0e3),
-        strength=100.0
+        strength=75.0
     )
+    src2 = openmc.IndependentSource(
+        energy=openmc.stats.delta_function(1.0e3),
+        strength=25.0
+    )
+    model.settings.source = [src1, src2]
     model.settings.run_mode = "fixed source"
     model.settings.surf_source_write = {
         "max_particles": 100,
@@ -42,11 +47,13 @@ def test_source_weight(run_in_tmpdir, sphere_model):
     sphere_model.settings.uniform_source_sampling = True
     sphere_model.run()
     particles = openmc.ParticleList.from_hdf5('surface_source.h5')
-    strength = sphere_model.settings.source[0].strength
-    assert set(p.wgt for p in particles) == {strength}
+    assert set(p.wgt for p in particles) == {0.5, 1.5}
 
 
 def test_tally_mean(run_in_tmpdir, sphere_model):
+    # Use only one source
+    sphere_model.settings.source.pop()
+
     # Run without uniform source sampling
     sphere_model.settings.uniform_source_sampling = False
     sp_file = sphere_model.run()
diff --git a/tests/unit_tests/test_universe.py b/tests/unit_tests/test_universe.py
index 46d4ec3f7..0955347f7 100644
--- a/tests/unit_tests/test_universe.py
+++ b/tests/unit_tests/test_universe.py
@@ -1,3 +1,5 @@
+from pathlib import Path
+
 import lxml.etree as ET
 import numpy as np
 import openmc
@@ -47,6 +49,12 @@ def test_bounding_box():
     assert_unbounded(u)
 
 
+def test_id():
+    openmc.Universe(universe_id=0)
+    with pytest.raises(ValueError):
+        openmc.Universe(universe_id=-1)
+
+
 def test_plot(run_in_tmpdir, sphere_model):
 
     # model with -inf and inf in the bounding box
@@ -99,6 +107,47 @@ def test_plot(run_in_tmpdir, sphere_model):
             pixels=100,
         )
 
+    # Close plots to avoid warning
+    import matplotlib.pyplot as plt
+    plt.close('all')
+
+
+def test_mg_plot(run_in_tmpdir):
+    # Create a simple universe with macroscopic data
+    h2o_data = openmc.Macroscopic('LWTR')
+    water = openmc.Material(name='Water')
+    water.set_density('macro', 1.0)
+    water.add_macroscopic(h2o_data)
+    sph = openmc.Sphere(r=10, boundary_type="vacuum")
+    cell = openmc.Cell(region=-sph, fill=water)
+    univ = openmc.Universe(cells=[cell])
+
+    # Create MGXS library and export to HDF5
+    groups = openmc.mgxs.EnergyGroups([1e-5, 20.0e6])
+    h2o_xsdata = openmc.XSdata('LWTR', groups)
+    h2o_xsdata.order = 0
+    h2o_xsdata.set_total([1.0])
+    h2o_xsdata.set_absorption([0.5])
+    scatter_matrix = np.array([[[0.5]]])
+    scatter_matrix = np.rollaxis(scatter_matrix, 0, 3)
+    h2o_xsdata.set_scatter_matrix(scatter_matrix)
+    mg_library = openmc.MGXSLibrary(groups)
+    mg_library.add_xsdatas([h2o_xsdata])
+    mgxs_path = Path.cwd() / 'mgxs.h5'
+    mg_library.export_to_hdf5(mgxs_path)
+
+    # Set MG cross sections in config and plot
+    with openmc.config.patch('mg_cross_sections', mgxs_path):
+        univ.plot(width=(200, 200), basis='yz', color_by='cell')
+        univ.plot(width=(200, 200), basis='yz', color_by='material')
+
+    with pytest.raises(RuntimeError):
+        univ.plot(width=(200, 200), basis='yz', color_by='cell')
+
+    # Close plots to avoid warning
+    import matplotlib.pyplot as plt
+    plt.close('all')
+
 
 def test_get_nuclides(uo2):
     c = openmc.Cell(fill=uo2)
diff --git a/tests/unit_tests/test_vectfit.py b/tests/unit_tests/test_vectfit.py
new file mode 100644
index 000000000..f7b2e905d
--- /dev/null
+++ b/tests/unit_tests/test_vectfit.py
@@ -0,0 +1,224 @@
+"""
+Initially from Jingang Liang: https://github.com/mit-crpg/vectfit.git
+"""
+
+import numpy as np
+import pytest
+from openmc.data.vectfit import evaluate, vectfit
+
+
+@pytest.fixture
+def ref_poles():
+    """Reference poles for real-pole test."""
+    return np.array(
+        [
+            9.709261771920490e02 + 0.0j,
+            -1.120960794075339e03 + 0.0j,
+            1.923889557426567e00 + 7.543700246109742e01j,
+            1.923889557426567e00 - 7.543700246109742e01j,
+            1.159741300380281e02 + 3.595650922556496e-02j,
+            1.159741300380281e02 - 3.595650922556496e-02j,
+            1.546932165729394e02 + 8.728391144940301e-02j,
+            1.546932165729394e02 - 8.728391144940301e-02j,
+            2.280349190818197e02 + 2.814037559718684e-01j,
+            2.280349190818197e02 - 2.814037559718684e-01j,
+            2.313004772627853e02 + 3.004628477692201e-01j,
+            2.313004772627853e02 - 3.004628477692201e-01j,
+            2.787470098364861e02 + 3.414179169920170e-01j,
+            2.787470098364861e02 - 3.414179169920170e-01j,
+            3.570711338764254e02 + 4.485587371149193e-01j,
+            3.570711338764254e02 - 4.485587371149193e-01j,
+            4.701059001346060e02 + 6.598089307174224e-01j,
+            4.701059001346060e02 - 6.598089307174224e-01j,
+            7.275819506342254e02 + 1.189678974845038e03j,
+            7.275819506342254e02 - 1.189678974845038e03j,
+        ]
+    )
+
+
+@pytest.fixture
+def ref_residues():
+    """Reference residues for real-pole test."""
+    return np.array(
+        [
+            [
+                -3.269879776751686e07 + 0.0j,
+                1.131087935798761e09 + 0.0j,
+                1.634151281869857e04 + 2.251103589277891e05j,
+                1.634151281869857e04 - 2.251103589277891e05j,
+                3.281792303833561e03 - 1.756079516325274e04j,
+                3.281792303833561e03 + 1.756079516325274e04j,
+                1.110800880243503e04 - 4.324813594540043e04j,
+                1.110800880243503e04 + 4.324813594540043e04j,
+                8.812700704117636e04 - 2.256520243571103e05j,
+                8.812700704117636e04 + 2.256520243571103e05j,
+                5.842090495551535e04 - 1.442159380741478e05j,
+                5.842090495551535e04 + 1.442159380741478e05j,
+                1.339410514130921e05 - 2.640767909713812e05j,
+                1.339410514130921e05 + 2.640767909713812e05j,
+                2.211245633333130e05 - 3.222447758311512e05j,
+                2.211245633333130e05 + 3.222447758311512e05j,
+                4.124430059785149e05 - 4.076023108323907e05j,
+                4.124430059785149e05 + 4.076023108323907e05j,
+                1.607378314999252e09 - 1.401163320110452e08j,
+                1.607378314999252e09 + 1.401163320110452e08j,
+            ]
+        ]
+    )
+
+
+@pytest.fixture
+def vector_test_data():
+    """Simple 2-signal test with known poles and residues."""
+    Ns = 101
+    s = np.linspace(3.0, 7.0, Ns)
+    poles = [5.0 + 0.1j, 5.0 - 0.1j]
+    residues = [[0.5 - 11.0j, 0.5 + 11.0j], [1.5 - 20.0j, 1.5 + 20.0j]]
+    f = np.zeros((2, Ns))
+    for i in range(2):
+        f[i, :] = np.real(
+            residues[i][0] / (s - poles[0]) + residues[i][1] / (s - poles[1])
+        )
+    weight = 1.0 / f
+    init_poles = [3.5 + 0.035j, 3.5 - 0.035j]
+    return s, poles, residues, f, weight, init_poles
+
+
+@pytest.fixture
+def poly_test_data():
+    """Test data with rational function plus polynomial terms."""
+    Ns = 201
+    s = np.linspace(0.0, 5.0, Ns)
+    poles = [-20.0 + 30.0j, -20.0 - 30.0j]
+    residues = [[5.0 + 10.0j, 5.0 - 10.0j]]
+    polys = [[1.0, 2.0, 0.3]]
+    f = evaluate(s, poles, residues, polys)
+    weight = 1.0 / f
+    init_poles = [2.5 + 0.025j, 2.5 - 0.025j]
+    return s, poles, residues, polys, f, weight, init_poles
+
+
+@pytest.fixture
+def real_poles_data(ref_poles, ref_residues):
+    """Large-scale signal using complex and real poles."""
+    Ns = 2000
+    s = np.linspace(1.0e-2, 5.0e3, Ns)
+    f = np.zeros((1, Ns))
+    for p, r in zip(ref_poles, ref_residues[0]):
+        f[0] += (r / (s - p)).real
+    weight = 1.0 / f
+    poles = np.linspace(1.1e-2, 4.8e3, 10)
+    poles = poles + poles * 0.01j
+    poles = np.sort(np.append(poles, np.conj(poles)))
+    return s, f, weight, poles
+
+
+@pytest.fixture
+def large_test_data():
+    """Stress test data with thousands of poles and samples."""
+    Ns = 3000
+    N = 200
+    s = np.linspace(1.0e-2, 5.0e3, Ns)
+    poles = np.linspace(1.1e-2, 4.8e3, N // 2) + 0.01j * np.linspace(
+        1.1e-2, 4.8e3, N // 2
+    )
+    poles = np.sort(np.append(poles, np.conj(poles)))
+    residues = np.linspace(1e2, 1e6, N // 2) + 0.5j * np.linspace(1e2, 1e6, N // 2)
+    residues = np.sort(np.append(residues, np.conj(residues))).reshape((1, N))
+    f = np.zeros((1, Ns))
+    for p, r in zip(poles, residues[0]):
+        f[0] += (r / (s - p)).real
+    weight = 1.0 / f
+    init_poles = np.linspace(1.2e-2, 4.7e3, N // 2) + 0.01j * np.linspace(
+        1.2e-2, 4.7e3, N // 2
+    )
+    init_poles = np.sort(np.append(init_poles, np.conj(init_poles)))
+    return s, f, weight, init_poles
+
+
+@pytest.fixture
+def eval_test_data():
+    """Reference data for evaluating rational + polynomial models."""
+    Ns = 101
+    s = np.linspace(-5.0, 5.0, Ns)
+    poles = [-2.0 + 30.0j, -2.0 - 30.0j]
+    residues = [5.0 + 10.0j, 5.0 - 10.0j]
+    polys = [1.0, 2.0, 0.3]
+    return s, poles, residues, polys
+
+
+def test_vector(vector_test_data):
+    """Test vectfit with vector samples and simple poles.
+    It is expected to get exact results with one iteration.
+    """
+    s, expected_poles, expected_residues, f, weight, init_poles = vector_test_data
+    poles, residues, _, fit, _ = vectfit(f, s, init_poles, weight)
+    assert np.allclose(
+        np.sort_complex(poles), np.sort_complex(expected_poles), rtol=1e-7
+    )
+    assert np.allclose(f, evaluate(s, poles, residues), rtol=1e-7)
+    assert np.allclose(f, fit, rtol=1e-5)
+
+
+def test_poly(poly_test_data):
+    """Test vectfit with polynomials."""
+    s, expected_poles, expected_residues, expected_polys, f, weight, init_poles = (
+        poly_test_data
+    )
+    poles, residues, cf, fit, _ = vectfit(f, s, init_poles, weight, n_polys=3)
+    poles, residues, cf, fit, _ = vectfit(f, s, poles, weight, n_polys=3)
+    assert np.allclose(
+        np.sort_complex(poles), np.sort_complex(expected_poles), rtol=1e-5
+    )
+    assert np.allclose(f, evaluate(s, poles, residues, cf), rtol=1e-5)
+    assert np.allclose(cf, expected_polys, rtol=1e-5)
+    assert np.allclose(f, fit, rtol=1e-4)
+
+
+def test_real_poles(real_poles_data, ref_poles, ref_residues):
+    """Test vectfit with more poles including real poles"""
+    s, f, weight, poles = real_poles_data
+    for _ in range(6):
+        poles, residues, _, fit, _ = vectfit(f, s, poles, weight)
+    assert np.allclose(
+        np.sort_complex(poles), np.sort_complex(ref_poles), rtol=1e-5, atol=1e-8
+    )
+    assert np.allclose(f, evaluate(s, poles, residues), rtol=1e-4)
+    assert np.allclose(f, fit, rtol=1e-3)
+
+
+def test_large(large_test_data):
+    """Test vectfit with a large set of poles and samples"""
+    s, f, weight, init_poles = large_test_data
+    poles_fit, residues_fit, _, f_fit, _ = vectfit(f, s, init_poles, weight)
+    assert np.allclose(f, f_fit, rtol=1e-3)
+
+
+def test_evaluate(eval_test_data):
+    """Test evaluate function"""
+    s, poles, residues, polys = eval_test_data
+
+    # Single signal, no polynomial
+    f_ref = np.real(residues[0] / (s - poles[0]) + residues[1] / (s - poles[1]))
+    f = evaluate(s, poles, residues)
+    assert np.allclose(f[0], f_ref)
+
+    # Single signal, with polynomial
+    for n, c in enumerate(polys):
+        f_ref += c * np.power(s, n)
+    f = evaluate(s, poles, residues, polys)
+    assert np.allclose(f[0], f_ref)
+
+    # Multi-signal, multi-residue, multi-poly
+    poles = [5.0 + 0.1j, 5.0 - 0.1j]
+    residues = [[0.5 - 11.0j, 0.5 + 11.0j], [1.5 - 20.0j, 1.5 + 20.0j]]
+    polys = [[1.0, 2.0, 0.3], [4.0, -2.0, -10.0]]
+    f_ref = np.zeros((2, len(s)))
+    for i in range(2):
+        f_ref[i, :] = np.real(
+            residues[i][0] / (s - poles[0]) + residues[i][1] / (s - poles[1])
+        )
+        for n, c in enumerate(polys[i]):
+            f_ref[i, :] += c * np.power(s, n)
+    f = evaluate(s, poles, residues, polys)
+    assert np.allclose(f, f_ref)
diff --git a/tests/unit_tests/test_void.py b/tests/unit_tests/test_void.py
new file mode 100644
index 000000000..8713d4b0b
--- /dev/null
+++ b/tests/unit_tests/test_void.py
@@ -0,0 +1,42 @@
+import numpy as np
+import openmc
+import pytest
+
+
+@pytest.fixture
+def empty_sphere():
+    openmc.reset_auto_ids()
+    model = openmc.Model()
+    surf = openmc.Sphere(r=10, boundary_type='vacuum')
+    cell = openmc.Cell(region=-surf)
+    model.geometry = openmc.Geometry([cell])
+
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 3
+    model.settings.particles = 1000
+
+    tally = openmc.Tally()
+    tally.scores = ['total', 'elastic']
+    tally.nuclides = ['U235']
+    tally.multiply_density = False
+    model.tallies.append(tally)
+
+    return model
+
+
+def test_equivalent_microxs(empty_sphere, run_in_tmpdir):
+    sp_file = empty_sphere.run()
+    with openmc.StatePoint(sp_file) as sp:
+        tally1 = sp.tallies[1]
+
+    mat = openmc.Material()
+    mat.add_nuclide('H1', 1e-16)
+
+    empty_sphere.geometry.get_all_cells()[1].fill = mat
+
+    sp_file = empty_sphere.run()
+    with openmc.StatePoint(sp_file) as sp:
+        tally2 = sp.tallies[1]
+
+    assert np.isclose(tally1.mean.sum(), tally2.mean.sum(), rtol=1e-10, atol=0)
+    assert tally1.mean.sum() > 0
diff --git a/tests/unit_tests/test_waste_classification.py b/tests/unit_tests/test_waste_classification.py
new file mode 100644
index 000000000..072590df9
--- /dev/null
+++ b/tests/unit_tests/test_waste_classification.py
@@ -0,0 +1,102 @@
+import random
+
+import openmc
+import pytest
+
+
+@pytest.mark.parametrize("metal", [False, True])
+def test_waste_classification_long(metal):
+    """Test classification when determined by long-lived radionuclides"""
+    f = 10.0 if metal else 1.0
+    limit = 8.0*f
+    mat = openmc.Material()
+    mat.add_nuclide('C14', 1e-9*f)
+    assert mat.get_activity('Ci/m3') < 0.1 * limit
+    assert mat.waste_classification(metal=metal) == 'Class A'
+
+    mat = openmc.Material()
+    mat.add_nuclide('C14', 1e-8*f)
+    assert 0.1 * limit < mat.get_activity('Ci/m3') < limit
+    assert mat.waste_classification(metal=metal) == 'Class C'
+
+    mat = openmc.Material()
+    mat.add_nuclide('C14', 1e-7*f)
+    assert mat.get_activity('Ci/m3') > limit
+    assert mat.waste_classification(metal=metal) == 'GTCC'
+
+
+@pytest.mark.parametrize("metal", [False, True])
+def test_waste_classification_short(metal):
+    """Test classification when determined by short-lived radionuclides"""
+    f = 10.0 if metal else 1.0
+    col1, col2, col3 = 3.5*f, 70.0*f, 700.0*f
+
+    mat = openmc.Material()
+    mat.add_nuclide('Ni63', 1e-10*f)
+    assert mat.get_activity('Ci/m3') < col1
+    assert mat.waste_classification(metal=metal) == 'Class A'
+
+    mat = openmc.Material()
+    mat.add_nuclide('Ni63', 1e-10*10*f)
+    assert col1 < mat.get_activity('Ci/m3') < col2
+    assert mat.waste_classification(metal=metal) == 'Class B'
+
+    mat = openmc.Material()
+    mat.add_nuclide('Ni63', 1e-10*200*f)
+    assert col2 < mat.get_activity('Ci/m3') < col3
+    assert mat.waste_classification(metal=metal) == 'Class C'
+
+    mat = openmc.Material()
+    mat.add_nuclide('Ni63', 1e-10*2000*f)
+    assert mat.get_activity('Ci/m3') > col3
+    assert mat.waste_classification(metal=metal) == 'GTCC'
+
+
+def test_waste_classification_mix():
+    """Test classification when determined by a mix of radionuclides"""
+    # Check example from 10 CFR 61.55 with mix of Sr90 and Cs137
+    mat = openmc.Material()
+    mat.add_nuclide('Sr90', 2.425e-9)
+    mat.add_nuclide('Cs137', 1.115e-9)
+
+    # In example, activity of Sr90 is 50.0 Ci/m3 and Cs137 is 22.0 Ci/m3
+    activity = mat.get_activity(units='Ci/m3', by_nuclide=True)
+    assert activity['Sr90'] == pytest.approx(50.0, 0.01)
+    assert activity['Cs137'] == pytest.approx(22.0, 0.01)
+
+    # According to example, the waste should be class B
+    assert mat.waste_classification() == 'Class B'
+
+
+def test_waste_rating_fetter():
+    """Test waste classification using the Fetter limits"""
+    # For Tc99, Fetter has a more strict limit. Here, we create a material with
+    # Tc99 at 1 Ci/m3 which exceeds Fetter but not NRC
+    density = 3.5561e-7
+    mat = openmc.Material()
+    mat.add_nuclide('Tc99', density)
+    assert mat.get_activity('Ci/m3') == pytest.approx(1.0, 1e-3)
+    assert mat.waste_disposal_rating(limits='NRC_short_C') < 1.0
+    assert mat.waste_disposal_rating(limits='Fetter') > 1.0
+
+    # With a lower density, it should be Class C under Fetter limits and Class A
+    # under NRC limits
+    mat = openmc.Material()
+    mat.add_nuclide('Tc99', 5.0e-2*density)
+    assert mat.waste_disposal_rating(limits='NRC_short_A') < 1.0
+    assert mat.waste_disposal_rating(limits='Fetter') < 1.0
+
+
+def test_waste_disposal_rating():
+    """Test waste_disposal_rating method"""
+    mat = openmc.Material()
+    mat.add_nuclide('K40', random.random())
+
+    # Check for correct classification based on actual activity
+    ci_m3 = mat.get_activity('Ci/m3')
+    assert mat.waste_disposal_rating(limits={'K40': 2*ci_m3}) < 1.0
+    assert mat.waste_disposal_rating(limits={'K40': 0.5*ci_m3}) > 1.0
+
+    wdr = mat.waste_disposal_rating(limits={'K40': 4*ci_m3}, by_nuclide=True)
+    assert isinstance(wdr, dict)
+    assert wdr['K40'] == pytest.approx(1/4)
diff --git a/tests/unit_tests/weightwindows/dagmc/__init__.py b/tests/unit_tests/weightwindows/dagmc/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/unit_tests/weightwindows/dagmc/nested_shell_geometry.h5m b/tests/unit_tests/weightwindows/dagmc/nested_shell_geometry.h5m
new file mode 100644
index 000000000..af1d5563d
Binary files /dev/null and b/tests/unit_tests/weightwindows/dagmc/nested_shell_geometry.h5m differ
diff --git a/tests/unit_tests/weightwindows/dagmc/test.py b/tests/unit_tests/weightwindows/dagmc/test.py
new file mode 100644
index 000000000..ed01a93ed
--- /dev/null
+++ b/tests/unit_tests/weightwindows/dagmc/test.py
@@ -0,0 +1,98 @@
+import pytest
+
+import openmc
+import openmc.lib
+
+pytestmark = pytest.mark.skipif(
+    not openmc.lib._dagmc_enabled(),
+    reason="DAGMC CAD geometry is not enabled.",
+)
+
+
+def test_dagmc_weight_windows_near_boundary(run_in_tmpdir, request):
+    """Ensure splitting near a boundary doesn't lose particles due to
+    a stale DAGMC history on the particle object."""
+
+    # DAGMC model overview:
+    # * Three nested cubes; innermost cube contains a fusion neutron source.
+    # * Two outer cubes filled with tungsten. Weight windows defined on a mesh
+    # cause particles to split moving outward.
+    # Outer cubes are similar in size (outer slightly larger) so particles
+    # frequently cross the problem boundary immediately after splitting. No lost
+    # particles are allowed to the correct DAGMC history after splitting is used.
+    model = openmc.Model()
+
+    dagmc_file = request.path.parent / 'nested_shell_geometry.h5m'
+    dagmc_univ = openmc.DAGMCUniverse(dagmc_file)
+    model.geometry = openmc.Geometry(dagmc_univ)
+
+    tungsten = openmc.Material(name='shell')
+    tungsten.add_element('W', 1.0)
+    tungsten.set_density('g/cm3', 7.8)
+    materials = openmc.Materials([tungsten])
+    model.materials = materials
+
+    settings = openmc.Settings()
+    settings.output = {'tallies': False, 'summary': False}
+
+    source = openmc.IndependentSource()
+    source.space = openmc.stats.Point((0.0, 0.0, 0.0))
+    source.angle = openmc.stats.Isotropic()
+    source.energy = openmc.stats.Discrete([14.1e6], [1.0])
+    settings.source = source
+
+    settings.batches = 2
+    settings.particles = 500
+    settings.run_mode = 'fixed source'
+    settings.survival_biasing = False
+    settings.max_lost_particles = 1
+    settings.max_history_splits = 10_000_000
+
+    settings.weight_window_checkpoints = {
+        'surface': True,
+        'collision': True,
+    }
+
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = (-60.0, -60.0, -60.0)
+    mesh.upper_right = (60.0, 60.0, 60.0)
+    mesh.dimension = (24, 1, 1)
+
+    weight_windows_lower = [
+        0.030750733294361156,
+        0.056110505674355333,
+        0.08187875047968339,
+        0.1101743496347699,
+        0.13982370013053508,
+        0.17443799246829372,
+        0.21576286623367483,
+        0.26416659508033646,
+        0.318574932646899,
+        0.3804031702117963,
+        0.42899359749256355,
+        0.4954283294279403,
+        0.49999999999999994,
+        0.43432341070872266,
+        0.38302303850488206,
+        0.32148375935490886,
+        0.2637416945702018,
+        0.21498369367288853,
+        0.17163611765361744,
+        0.13832102142074995,
+        0.10717772257151495,
+        0.07986176041282561,
+        0.05499644859408233,
+        0.03058023506703803,
+    ]
+
+    weight_windows = openmc.WeightWindows(
+        mesh,
+        lower_ww_bounds=weight_windows_lower,
+        upper_bound_ratio=5.0,
+    )
+    weight_windows.max_lower_bound_ratio = 1.0
+    settings.weight_windows = weight_windows
+    settings.weight_windows_on = True
+    model.settings = settings
+
+    model.run()
\ No newline at end of file
diff --git a/tests/unit_tests/weightwindows/test.py b/tests/unit_tests/weightwindows/test.py
index d6e509522..efa203b51 100644
--- a/tests/unit_tests/weightwindows/test.py
+++ b/tests/unit_tests/weightwindows/test.py
@@ -122,7 +122,8 @@ def model():
     return model
 
 
-def test_weightwindows(model, wws):
+@pytest.mark.parametrize("shared_secondary", [False, True])
+def test_weightwindows(model, wws, shared_secondary):
 
     ww_files = ('ww_n.txt', 'ww_p.txt')
     cwd = Path(__file__).parent.absolute()
@@ -131,6 +132,7 @@ def test_weightwindows(model, wws):
     with cdtemp(filepaths):
         # run once with variance reduction off
         model.settings.weight_windows_on = False
+        model.settings.shared_secondary_bank = shared_secondary
         analog_sp = model.run()
         os.rename(analog_sp, 'statepoint.analog.h5')
 
@@ -223,7 +225,8 @@ def test_lower_ww_bounds_shape():
     assert ww.lower_ww_bounds.shape == (2, 3, 4, 1)
 
 
-def test_photon_heating(run_in_tmpdir):
+@pytest.mark.parametrize("shared_secondary", [False, True])
+def test_photon_heating(run_in_tmpdir, shared_secondary):
     water = openmc.Material()
     water.add_nuclide('H1', 1.0)
     water.add_nuclide('O16', 2.0)
@@ -246,7 +249,8 @@ def test_photon_heating(run_in_tmpdir):
 
     model.settings.run_mode = 'fixed source'
     model.settings.batches = 5
-    model.settings.particles = 100
+    model.settings.particles = 101
+    model.settings.shared_secondary_bank = shared_secondary
 
     tally = openmc.Tally()
     tally.scores = ['heating']
@@ -260,7 +264,11 @@ def test_photon_heating(run_in_tmpdir):
     with openmc.StatePoint(sp_file) as sp:
         tally_mean = sp.tallies[tally.id].mean
 
-    # these values should be nearly identical
+    # Note: Our current physics model actually does allow this tally to
+    # occasionally go slightly negative. However, larger bugs can
+    # make this more common. We have selected a particle count for
+    # this test that happens to produce no negative tallies for both
+    # the shared and non-shared secondary PRNG streams.
     assert np.all(tally_mean >= 0)
 
 
diff --git a/tests/unit_tests/weightwindows/test_ww_gen.py b/tests/unit_tests/weightwindows/test_ww_gen.py
index 555421461..a4456e680 100644
--- a/tests/unit_tests/weightwindows/test_ww_gen.py
+++ b/tests/unit_tests/weightwindows/test_ww_gen.py
@@ -1,3 +1,4 @@
+import copy
 from itertools import permutations
 from pathlib import Path
 
@@ -303,7 +304,7 @@ def test_python_hdf5_roundtrip(run_in_tmpdir, model):
 
     openmc.lib.finalize()
 
-    wws_hdf5 = openmc.hdf5_to_wws()[0]
+    wws_hdf5 = openmc.WeightWindowsList.from_hdf5()[0]
 
     # ensure
     assert all(wws.energy_bounds == wws_hdf5.energy_bounds)
@@ -332,3 +333,72 @@ def test_ww_bounds_set_in_memory(run_in_tmpdir, model):
     wws.bounds = (bounds, bounds)
 
     openmc.lib.finalize()
+
+
+@pytest.mark.skipif(not openmc.lib._dagmc_enabled(), reason="DAGMC CAD geometry is not enabled.")
+def test_ww_generation_with_dagmc(run_in_tmpdir):
+    mat1 = openmc.Material(name="1")
+    mat1.add_nuclide("H1", 1, percent_type="ao")
+    mat1.set_density("g/cm3", 0.001)
+
+    materials = openmc.Materials([mat1])
+    dag_univ = openmc.DAGMCUniverse(
+        Path(__file__).parent.parent / "dagmc" / "dagmc_tetrahedral_no_graveyard.h5m")
+    bound_dag_univ = dag_univ.bounded_universe(padding_distance=1)
+    geometry = openmc.Geometry(bound_dag_univ)
+
+    settings = openmc.Settings()
+    settings.batches = 6
+    settings.particles = 30
+    settings.run_mode = "fixed source"
+
+    # Create a point source which are supported by random ray mode
+    my_source = openmc.IndependentSource()
+    my_source.space = openmc.stats.Point((0.25, 0.25, 0.25))
+    my_source.energy = openmc.stats.delta_function(14e6)
+    settings.source = my_source
+
+    model = openmc.Model(geometry, materials, settings)
+
+    rr_model = copy.deepcopy(model)
+    rr_model.settings.inactive = 3
+
+    rr_model.convert_to_multigroup(
+        method="stochastic_slab",
+        overwrite_mgxs_library=True,
+        nparticles=10,
+        groups="CASMO-2"
+    )
+
+    rr_model.convert_to_random_ray()
+
+    mesh = openmc.RegularMesh.from_domain(rr_model, dimension=(4, 4, 4))
+
+    # avoid writing files we don't make use of
+    rr_model.settings.output = {"summary": False, "tallies": False}
+
+    # Subdivide random ray source regions
+    rr_model.settings.random_ray["source_region_meshes"] = [
+        (mesh, [rr_model.geometry.root_universe])
+    ]
+
+    # less likely to get negative values in the weight window
+    rr_model.settings.random_ray["volume_estimator"] = "naive"
+
+    # Add a weight window generator to the model
+    rr_model.settings.weight_window_generators = openmc.WeightWindowGenerator(
+        method="fw_cadis",
+        mesh=mesh,
+        max_realizations=42,
+        particle_type='neutron',
+        energy_bounds=[0.0, 100e6]
+    )
+
+    rr_model.run()
+
+    model.settings.weight_windows_on = True
+    model.settings.weight_window_checkpoints = {"collision": True, "surface": True}
+    model.settings.survival_biasing = False
+    model.settings.weight_windows = openmc.WeightWindowsList.from_hdf5()
+
+    model.run()
diff --git a/tests/unit_tests/weightwindows/test_ww_list.py b/tests/unit_tests/weightwindows/test_ww_list.py
new file mode 100644
index 000000000..d148f382a
--- /dev/null
+++ b/tests/unit_tests/weightwindows/test_ww_list.py
@@ -0,0 +1,23 @@
+import openmc
+
+
+def test_ww_roundtrip(request, run_in_tmpdir):
+    # Load weight windows from a wwinp file
+    wwinp_file = request.path.with_name('wwinp_n')
+    wws = openmc.WeightWindowsList.from_wwinp(wwinp_file)
+
+    # Roundtrip them, writing to HDF5 and reading back in
+    wws.export_to_hdf5('ww.h5')
+    wws_new = openmc.WeightWindowsList.from_hdf5('ww.h5')
+
+    # Check that the new weight windows are the same as the original
+    assert len(wws) == len(wws_new)
+    for ww, ww_new in zip(wws, wws_new):
+        assert ww.particle_type == ww_new.particle_type
+        assert (ww.lower_ww_bounds == ww_new.lower_ww_bounds).all()
+        assert (ww.upper_ww_bounds == ww_new.upper_ww_bounds).all()
+        assert ww.survival_ratio == ww_new.survival_ratio
+        assert ww.num_energy_bins == ww_new.num_energy_bins
+        assert ww.max_split == ww_new.max_split
+        assert ww.weight_cutoff == ww_new.weight_cutoff
+        assert ww.mesh.id == ww_new.mesh.id
diff --git a/tests/unit_tests/weightwindows/test_wwinp_reader.py b/tests/unit_tests/weightwindows/test_wwinp_reader.py
index 434a36753..601517d47 100644
--- a/tests/unit_tests/weightwindows/test_wwinp_reader.py
+++ b/tests/unit_tests/weightwindows/test_wwinp_reader.py
@@ -52,7 +52,7 @@ n_mesh.z_grid = np.array([-100.0,
 n_e_bounds = (np.array([0.0,
                          100000.0,
                          146780.0]),)
-n_particles = ('neutron',)
+n_particles = [openmc.ParticleType.NEUTRON]
 
 # expected results - neutron and photon data
 np_mesh = openmc.RectilinearMesh()
@@ -63,7 +63,7 @@ np_mesh.z_grid = n_mesh.z_grid
 
 np_e_bounds = (np.array([0.0, 100000.0, 146780.0, 215440.0]),
                np.array([0.0, 1.0E8]))
-np_particles = ('neutron', 'photon')
+np_particles = [openmc.ParticleType.NEUTRON, openmc.ParticleType.PHOTON]
 
 # expected results - photon data only
 p_mesh = openmc.RectilinearMesh()
@@ -74,7 +74,7 @@ p_mesh.y_grid = np_mesh.y_grid
 p_mesh.z_grid = np.array([-50.0, 50.0])
 
 p_e_bounds = (np.array([0.0, 100000.0, 146780.0, 215440.0, 316230.0]),)
-p_particles = ('photon',)
+p_particles = [openmc.ParticleType.PHOTON]
 
 expected_results = [('wwinp_n', n_mesh, n_particles, n_e_bounds),
                     ('wwinp_np', np_mesh, np_particles, np_e_bounds),
@@ -96,7 +96,7 @@ def id_fn(params):
 def test_wwinp_reader(wwinp_data, request):
     wwinp_file, mesh, particle_types, energy_bounds = wwinp_data
 
-    wws = openmc.wwinp_to_wws(request.node.path.parent / wwinp_file)
+    wws = openmc.WeightWindowsList.from_wwinp(request.node.path.parent / wwinp_file)
 
     for i, ww in enumerate(wws):
         e_bounds = energy_bounds[i]
@@ -140,4 +140,4 @@ def test_wwinp_reader_failures(wwinp_data, request):
     filename, expected_failure = wwinp_data
 
     with pytest.raises(expected_failure):
-        _ = openmc.wwinp_to_wws(request.node.path.parent / filename)
+        _ = openmc.WeightWindowsList.from_wwinp(request.node.path.parent / filename)
diff --git a/tools/ci/gha-install-dagmc.sh b/tools/ci/gha-install-dagmc.sh
index 82759c9bc..8d0648a50 100755
--- a/tools/ci/gha-install-dagmc.sh
+++ b/tools/ci/gha-install-dagmc.sh
@@ -3,7 +3,7 @@
 set -ex
 
 # MOAB Variables
-MOAB_BRANCH='Version5.1.0'
+MOAB_BRANCH='5.5.1'
 MOAB_REPO='https://bitbucket.org/fathomteam/moab/'
 MOAB_INSTALL_DIR=$HOME/MOAB/
 
@@ -19,7 +19,7 @@ cd $HOME
 mkdir MOAB && cd MOAB
 git clone -b $MOAB_BRANCH $MOAB_REPO
 mkdir build && cd build
-cmake ../moab -DENABLE_HDF5=ON -DENABLE_NETCDF=ON -DBUILD_SHARED_LIBS=ON -DCMAKE_INSTALL_PREFIX=$MOAB_INSTALL_DIR -DENABLE_BLASLAPACK=OFF
+cmake ../moab -DENABLE_HDF5=ON -DENABLE_NETCDF=ON -DBUILD_SHARED_LIBS=ON -DCMAKE_INSTALL_PREFIX=$MOAB_INSTALL_DIR
 make -j && make -j install
 rm -rf $HOME/MOAB/moab $HOME/MOAB/build
 
diff --git a/tools/ci/gha-install-mcpl.sh b/tools/ci/gha-install-mcpl.sh
deleted file mode 100755
index 9b8609398..000000000
--- a/tools/ci/gha-install-mcpl.sh
+++ /dev/null
@@ -1,7 +0,0 @@
-#!/bin/bash
-set -ex
-cd $HOME
-git clone https://github.com/mctools/mcpl
-cd mcpl
-mkdir build && cd build
-cmake .. && make 2>/dev/null && sudo make install
diff --git a/tools/ci/gha-install-njoy.sh b/tools/ci/gha-install-njoy.sh
index 8255ffea8..168fcd4a7 100755
--- a/tools/ci/gha-install-njoy.sh
+++ b/tools/ci/gha-install-njoy.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 set -ex
 cd $HOME
-git clone https://github.com/njoy/NJOY2016
+git clone -b 2016.78 https://github.com/njoy/NJOY2016
 cd NJOY2016
 mkdir build && cd build
 cmake -Dstatic=on .. && make 2>/dev/null && sudo make install
diff --git a/tools/ci/gha-install-vectfit.sh b/tools/ci/gha-install-vectfit.sh
deleted file mode 100755
index bd38e1ea8..000000000
--- a/tools/ci/gha-install-vectfit.sh
+++ /dev/null
@@ -1,46 +0,0 @@
-#!/bin/bash
-set -ex
-
-PYBIND_BRANCH='master'
-PYBIND_REPO='https://github.com/pybind/pybind11'
-
-XTL_BRANCH='0.6.13'
-XTL_REPO='https://github.com/xtensor-stack/xtl'
-
-XTENSOR_BRANCH='0.21.3'
-XTENSOR_REPO='https://github.com/xtensor-stack/xtensor'
-
-XTENSOR_PYTHON_BRANCH='0.24.1'
-XTENSOR_PYTHON_REPO='https://github.com/xtensor-stack/xtensor-python'
-
-XTENSOR_BLAS_BRANCH='0.17.1'
-XTENSOR_BLAS_REPO='https://github.com/xtensor-stack/xtensor-blas'
-
-cd $HOME
-git clone -b $PYBIND_BRANCH $PYBIND_REPO
-cd pybind11 && mkdir build && cd build && cmake .. && sudo make install
-pip install $HOME/pybind11
-
-cd $HOME
-git clone -b $XTL_BRANCH $XTL_REPO
-cd xtl && mkdir build && cd build && cmake .. && sudo make install
-
-cd $HOME
-git clone -b $XTENSOR_BRANCH $XTENSOR_REPO
-cd xtensor && mkdir build && cd build && cmake .. && sudo make install
-
-cd $HOME
-git clone -b $XTENSOR_PYTHON_BRANCH $XTENSOR_PYTHON_REPO
-cd xtensor-python && mkdir build && cd build && cmake .. && sudo make install
-
-cd $HOME
-git clone -b $XTENSOR_BLAS_BRANCH $XTENSOR_BLAS_REPO
-cd xtensor-blas && mkdir build && cd build && cmake .. && sudo make install
-
-# Install wheel (remove when vectfit supports installation with build isolation)
-pip install wheel
-
-# Install vectfit
-cd $HOME
-git clone https://github.com/liangjg/vectfit.git
-pip install --no-build-isolation ./vectfit
diff --git a/tools/ci/gha-install.py b/tools/ci/gha-install.py
index 1cc792f8d..5488b9547 100644
--- a/tools/ci/gha-install.py
+++ b/tools/ci/gha-install.py
@@ -9,8 +9,8 @@ def install(omp=False, mpi=False, phdf5=False, dagmc=False, libmesh=False):
     os.mkdir('build')
     os.chdir('build')
 
-    # Build in debug mode by default with support for MCPL
-    cmake_cmd = ['cmake', '-DCMAKE_BUILD_TYPE=Debug', '-DOPENMC_USE_MCPL=on']
+    # Build in RelWithDebInfo mode by default with support for MCPL
+    cmake_cmd = ['cmake', '-DCMAKE_BUILD_TYPE=RelWithDebInfo', '-DOPENMC_USE_MCPL=on']
 
     # Turn off OpenMP if specified
     if not omp:
@@ -43,6 +43,9 @@ def install(omp=False, mpi=False, phdf5=False, dagmc=False, libmesh=False):
     # Build in coverage mode for coverage testing
     cmake_cmd.append('-DOPENMC_ENABLE_COVERAGE=on')
 
+    # Enable strict FP for cross-platform reproducibility in CI
+    cmake_cmd.append('-DOPENMC_ENABLE_STRICT_FP=on')
+
     # Build and install
     cmake_cmd.append('..')
     print(' '.join(cmake_cmd))
diff --git a/tools/ci/gha-install.sh b/tools/ci/gha-install.sh
index d8a3a7600..4cf62afbb 100755
--- a/tools/ci/gha-install.sh
+++ b/tools/ci/gha-install.sh
@@ -18,18 +18,13 @@ fi
 pip install 'ncrystal>=4.1.0'
 nctool --test
 
-# Install vectfit for WMP generation if needed
-if [[ $VECTFIT = 'y' ]]; then
-    ./tools/ci/gha-install-vectfit.sh
-fi
-
 # Install libMesh if needed
 if [[ $LIBMESH = 'y' ]]; then
     ./tools/ci/gha-install-libmesh.sh
 fi
 
 # Install MCPL
-./tools/ci/gha-install-mcpl.sh
+pip install mcpl
 
 # For MPI configurations, make sure mpi4py and h5py are built against the
 # correct version of MPI
@@ -39,7 +34,9 @@ if [[ $MPI == 'y' ]]; then
     export CC=mpicc
     export HDF5_MPI=ON
     export HDF5_DIR=/usr/lib/x86_64-linux-gnu/hdf5/mpich
-    pip install --no-binary=h5py h5py
+    # Install h5py without build isolation to pick up already installed mpi4py
+    pip install setuptools Cython pkgconfig
+    pip install --no-build-isolation --no-binary=h5py h5py
 fi
 
 # Build and install OpenMC executable
diff --git a/tools/ci/gha-script.sh b/tools/ci/gha-script.sh
index c7c634ffa..b40238ffb 100755
--- a/tools/ci/gha-script.sh
+++ b/tools/ci/gha-script.sh
@@ -14,5 +14,8 @@ if [[ $EVENT == 'y' ]]; then
   args="${args} --event "
 fi
 
-# Run regression and unit tests
-pytest --cov=openmc -v $args tests
+# Run unit tests and then regression tests
+pytest -v $args \
+  tests/test_matplotlib_import.py \
+  tests/unit_tests \
+  tests/regression_tests
diff --git a/vendor/xtensor b/vendor/xtensor
deleted file mode 160000
index 3634f2ded..000000000
--- a/vendor/xtensor
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 3634f2ded19e0cf38208c8b86cea9e1d7c8e397d
diff --git a/vendor/xtl b/vendor/xtl
deleted file mode 160000
index a7c1c5444..000000000
--- a/vendor/xtl
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit a7c1c5444dfc57f76620391af4c94785ff82c8d6