Shared Secondary Particle Bank (#3863)

Co-authored-by: John Tramm <jtramm@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Paul Romano <paul.k.romano@gmail.com>
Co-authored-by: Copilot <copilot@github.com>

docs/source/io_formats/settings.rst

@@ -741,14 +741,16 @@ 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:
 
@@ -1156,23 +1158,6 @@ based on constraints.
 
    *Default*: 0.05
 
--------------------------
-``<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
-
 --------------------------
 ``<source_point>`` Element
 --------------------------
@@ -1222,6 +1207,32 @@ 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
 ------------------------------

include/openmc/bank.h

@@ -34,18 +34,24 @@ 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

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

include/openmc/particle.h

@@ -71,7 +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

include/openmc/particle_data.h

@@ -50,8 +50,11 @@ struct SourceSite {
 
   // Extra attributes that don't show up in source written to file
   int parent_nuclide {-1};
-  int64_t parent_id;
-  int64_t progeny_id;
+  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 {
@@ -533,14 +536,14 @@ private:
   uint64_t seeds_[N_STREAMS];
   int stream_;
 
-  vector<SourceSite> secondary_bank_;
+  vector<SourceSite> local_secondary_bank_;
 
   // 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_;
+  int64_t current_work_ {0};
 
   vector<double> flux_derivs_;
 
@@ -563,7 +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};
@@ -693,12 +698,14 @@ public:
   int& stream() { return stream_; }
 
   // secondary particle bank
-  SourceSite& secondary_bank(int i) { return secondary_bank_[i]; }
-  const SourceSite& secondary_bank(int i) const { return secondary_bank_[i]; }
-  decltype(secondary_bank_)& secondary_bank() { return secondary_bank_; }
-  decltype(secondary_bank_) const& secondary_bank() const
+  SourceSite& local_secondary_bank(int i) { return local_secondary_bank_[i]; }
+  const SourceSite& local_secondary_bank(int i) const
   {
-    return secondary_bank_;
+    return local_secondary_bank_[i];
+  }
+  decltype(local_secondary_bank_)& local_secondary_bank()
+  {
+    return local_secondary_bank_;
   }
 
   // Number of secondaries created in a collision
@@ -747,13 +754,16 @@ 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_; }
 

include/openmc/settings.h

@@ -98,7 +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

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 +97,26 @@ 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;
+  }
+
   //! 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

include/openmc/simulation.h

@@ -49,6 +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
 
 //==============================================================================
@@ -59,7 +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();
@@ -70,8 +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
 //!
@@ -92,16 +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

openmc/lib/core.py

@@ -31,7 +31,10 @@ class _SourceSite(Structure):
                 ('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

openmc/settings.py

@@ -67,6 +67,10 @@ class Settings:
                     (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
@@ -256,8 +260,15 @@ 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
@@ -277,6 +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:
 
@@ -373,10 +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
 
@@ -482,6 +491,7 @@ class Settings:
         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
@@ -1306,6 +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
@@ -1924,6 +1943,11 @@ class Settings:
             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
@@ -2430,6 +2454,11 @@ class Settings:
         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:
@@ -2597,6 +2626,7 @@ class Settings:
         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)
@@ -2714,6 +2744,7 @@ class Settings:
         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)

src/bank.cpp

@@ -7,6 +7,7 @@
 #include "openmc/vector.h"
 
 #include <cstdint>
+#include <limits>
 #include <numeric>
 
 namespace openmc {
@@ -43,6 +44,13 @@ vector<vector<double>> ifp_fission_lifetime_bank;
 // 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
 
 //==============================================================================
@@ -60,6 +68,8 @@ void free_memory_bank()
   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)
@@ -68,13 +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) {
@@ -96,44 +106,200 @@ void sort_fission_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();
   }
 
-  if (settings::ifp_on) {
+  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 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()) {
+  // 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) {
+    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());
-  if (settings::ifp_on) {
+  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
+}
+
 //==============================================================================
 // C API
 //==============================================================================

src/event.cpp

@@ -1,6 +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"
 
@@ -64,7 +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();
@@ -136,7 +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);
   }
@@ -155,7 +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);
   }
@@ -176,4 +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

src/finalize.cpp

@@ -149,6 +149,8 @@ int openmc_finalize()
   settings::uniform_source_sampling = false;
   settings::ufs_on = false;
   settings::urr_ptables_on = true;
+  settings::use_decay_photons = false;
+  settings::use_shared_secondary_bank = false;
   settings::verbosity = -1;
   settings::weight_cutoff = 0.25;
   settings::weight_survive = 1.0;
@@ -219,6 +221,7 @@ int openmc_reset()
   settings::cmfd_run = false;
 
   simulation::n_lost_particles = 0;
+  simulation::simulation_tracks_completed = 0;
 
   return 0;
 }

src/ifp.cpp

@@ -32,13 +32,13 @@ 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() - 1];
+      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() - 1];
+      simulation::ifp_source_lifetime_bank[p.current_work()];
     simulation::ifp_fission_lifetime_bank[idx] = _ifp(p.lifetime(), lifetimes);
   }
 }

src/initialize.cpp

@@ -161,7 +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]);
@@ -173,14 +173,35 @@ 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;
@@ -366,6 +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;
@@ -460,6 +503,8 @@ bool read_model_xml()
   if (check_for_node(root, "tallies"))
     read_tallies_xml(root.child("tallies"));
 
+  check_pulse_height_compatibility();
+
   // Initialize distribcell_filters
   prepare_distribcell();
 
@@ -505,6 +550,8 @@ void read_separate_xml_files()
 
   read_tallies_xml();
 
+  check_pulse_height_compatibility();
+
   // Initialize distribcell_filters
   prepare_distribcell();
 

src/output.cpp

@@ -501,6 +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);
+  }
 }
 
 //==============================================================================

src/particle.cpp

@@ -94,7 +94,7 @@ bool Particle::create_secondary(
   // Increment number of secondaries created (for ParticleProductionFilter)
   n_secondaries()++;
 
-  auto& bank = secondary_bank().emplace_back();
+  SourceSite bank;
   bank.particle = type;
   bank.wgt = wgt;
   bank.r = r();
@@ -102,12 +102,21 @@ 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();
@@ -122,6 +131,16 @@ void Particle::split(double wgt)
     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)
@@ -168,6 +187,10 @@ void Particle::from_source(const SourceSite* src)
     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()
@@ -449,61 +472,72 @@ void Particle::event_collide()
 #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().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);
+  // 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();
   }
 }
 
@@ -515,6 +549,7 @@ void Particle::event_death()
 
   // Finish particle track output.
   if (write_track()) {
+    write_particle_track(*this);
     finalize_particle_track(*this);
   }
 
@@ -538,11 +573,17 @@ 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();
   }
 }
 
@@ -863,28 +904,27 @@ void Particle::write_restart() const
     write_dataset(file_id, "id", id());
     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);

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"
@@ -106,20 +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

src/physics.cpp

@@ -44,7 +44,7 @@ void collision(Particle& p)
 {
   // Add to collision counter for particle
   ++(p.n_collision());
-  p.secondary_bank_index() = p.secondary_bank().size();
+  p.secondary_bank_index() = p.local_secondary_bank().size();
 
   // Sample reaction for the material the particle is in
   switch (p.type().pdg_number()) {
@@ -127,7 +127,8 @@ 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() >= settings::max_secondaries) {
+      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 "
@@ -228,7 +229,7 @@ void create_fission_sites(Particle& p, int i_nuclide, const Reaction& rx)
     }
 
     // Set parent and progeny IDs
-    site.parent_id = p.id();
+    site.parent_id = p.current_work();
     site.progeny_id = p.n_progeny()++;
 
     // Store fission site in bank
@@ -253,7 +254,10 @@ void create_fission_sites(Particle& p, int i_nuclide, const Reaction& rx)
         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()++;
     }
 
@@ -1225,7 +1229,7 @@ void sample_secondary_photons(Particle& p, int i_nuclide)
 
     // 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_;
     }
   }

src/physics_mg.cpp

@@ -27,7 +27,7 @@ void collision_mg(Particle& p)
 {
   // Add to the collision counter for the particle
   p.n_collision()++;
-  p.secondary_bank_index() = p.secondary_bank().size();
+  p.secondary_bank_index() = p.local_secondary_bank().size();
 
   // Sample the reaction type
   sample_reaction(p);
@@ -179,7 +179,7 @@ void create_fission_sites(Particle& p)
     }
 
     // Set parent and progeny ID
-    site.parent_id = p.id();
+    site.parent_id = p.current_work();
     site.progeny_id = p.n_progeny()++;
 
     // Store fission site in bank
@@ -200,7 +200,10 @@ 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()++;
     }
 

src/settings.cpp

@@ -83,6 +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};
@@ -1320,6 +1321,27 @@ void read_settings_xml(pugi::xml_node root)
     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()

src/simulation.cpp

@@ -86,7 +86,7 @@ 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();
@@ -214,6 +214,20 @@ 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)
@@ -254,9 +268,17 @@ 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
@@ -324,6 +346,8 @@ const RegularMesh* ufs_mesh {nullptr};
 vector<double> k_generation;
 vector<int64_t> work_index;
 
+int64_t simulation_tracks_completed {0};
+
 } // namespace simulation
 
 //==============================================================================
@@ -547,7 +571,7 @@ 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();
@@ -571,26 +595,35 @@ 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;
@@ -598,6 +631,9 @@ 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;
 
@@ -611,9 +647,7 @@ 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
@@ -627,17 +661,21 @@ void initialize_history(Particle& p, int64_t index_source)
   p.write_track() = check_track_criteria(p);
 
   // Set the particle's initial weight window value.
-  p.wgt_ww_born() = -1.0;
-  apply_weight_windows(p);
+  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) {
@@ -655,13 +693,34 @@ 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);
@@ -816,7 +875,7 @@ void transport_history_based_single_particle(Particle& p)
         p.event_collide();
       }
     }
-    p.event_revive_from_secondary();
+    p.event_check_limit_and_revive();
   }
   p.event_death();
 }
@@ -826,11 +885,137 @@ void transport_history_based()
 #pragma omp parallel for schedule(runtime)
   for (int64_t i_work = 1; i_work <= simulation::work_per_rank; ++i_work) {
     Particle p;
-    initialize_history(p, 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);
+
+  // Phase 1: Transport primary particles and deposit first generation of
+  // secondaries in the shared secondary bank
+#pragma omp parallel
+  {
+    vector<SourceSite> thread_bank;
+
+#pragma omp for schedule(runtime)
+    for (int64_t i = 1; i <= simulation::work_per_rank; i++) {
+      Particle p;
+      initialize_particle_track(p, i, false);
+      transport_history_based_single_particle(p);
+      for (auto& site : p.local_secondary_bank()) {
+        thread_bank.push_back(site);
+      }
+    }
+
+    // Drain thread-local bank into the shared secondary bank (once per thread)
+#pragma omp critical(SharedSecondaryBank)
+    {
+      for (auto& site : thread_bank) {
+        simulation::shared_secondary_bank_write.thread_unsafe_append(site);
+      }
+    }
+  }
+
+  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. 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);
+
+    // Transport all secondary tracks from the shared secondary bank
+#pragma omp parallel
+    {
+      vector<SourceSite> thread_bank;
+
+#pragma omp for schedule(runtime)
+      for (int64_t i = 1; i <= simulation::shared_secondary_bank_read.size();
+           i++) {
+        Particle p;
+        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);
+        }
+      }
+
+      // Drain thread-local bank into the shared secondary bank (once per
+      // thread)
+#pragma omp critical(SharedSecondaryBank)
+      {
+        for (auto& secondary_site : thread_bank) {
+          simulation::shared_secondary_bank_write.thread_unsafe_append(
+            secondary_site);
+        }
+      }
+    } // End of transport loop over tracks in shared secondary bank
+    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;
@@ -848,33 +1033,7 @@ 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
@@ -883,4 +1042,122 @@ 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 secondaries from all particle buffers into shared bank
+    for (int64_t i = 0; i < n_particles; i++) {
+      for (auto& site : simulation::particles[i].local_secondary_bank()) {
+        simulation::shared_secondary_bank_write.thread_unsafe_append(site);
+      }
+      simulation::particles[i].local_secondary_bank().clear();
+    }
+
+    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 secondaries from all particle buffers into shared bank
+      for (int64_t i = 0; i < n_particles; i++) {
+        for (auto& site : simulation::particles[i].local_secondary_bank()) {
+          simulation::shared_secondary_bank_write.thread_unsafe_append(site);
+        }
+        simulation::particles[i].local_secondary_bank().clear();
+      }
+
+      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

src/tallies/filter_particle_production.cpp

@@ -19,7 +19,7 @@ void ParticleProductionFilter::get_all_bins(
 
   // Loop over secondary bank entries
   for (int bank_idx = start_idx; bank_idx < end_idx; bank_idx++) {
-    const auto& site = p.secondary_bank(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());

src/tallies/tally_scoring.cpp

@@ -945,7 +945,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
         if (p.type().is_neutron() && p.fission()) {
           if (is_generation_time_or_both()) {
             const auto& lifetimes =
-              simulation::ifp_source_lifetime_bank[p.current_work() - 1];
+              simulation::ifp_source_lifetime_bank[p.current_work()];
             if (lifetimes.size() == settings::ifp_n_generation) {
               score = lifetimes[0] * p.wgt_last();
             }
@@ -959,7 +959,7 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
         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() - 1];
+              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();
@@ -985,12 +985,11 @@ void score_general_ce_nonanalog(Particle& p, int i_tally, int start_index,
           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() - 1]
+              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() - 1]
-                .size());
+              simulation::ifp_source_lifetime_bank[p.current_work()].size());
           }
           if (ifp_data_size == settings::ifp_n_generation) {
             score = p.wgt_last();

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>

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

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()

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>

tests/regression_tests/particle_production_fission/local/results_true.dat

@@ -0,0 +1,3 @@
+tally 1:
+4.570000E+00
+1.047890E+01

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>

tests/regression_tests/particle_production_fission/shared/results_true.dat

@@ -0,0 +1,3 @@
+tally 1:
+5.180000E+00
+1.355140E+01

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()

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>

tests/regression_tests/particle_restart_fixed_shared_secondary/materials.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0"?>
+<materials>
+
+  <material id="1">
+    <density value="1.4" units="g/cc" />
+    <nuclide name="B10" ao="1.0" />
+  </material>
+
+</materials>

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

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>

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()

tests/regression_tests/pulse_height/local/inputs_true.dat

@@ -18,14 +18,12 @@
     <particles>100</particles>
     <batches>5</batches>
     <source type="independent" strength="1.0" particle="photon">
-      <space type="point">
-        <parameters>0.0 0.0 0.0</parameters>
-      </space>
       <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">

tests/regression_tests/pulse_height/shared/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/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>
+    <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>

tests/regression_tests/pulse_height/shared/results_true.dat

@@ -0,0 +1,201 @@
+tally 1:
+4.140000E+00
+3.443000E+00
+1.000000E-02
+1.000000E-04
+1.000000E-02
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tests/regression_tests/pulse_height/test.py

@@ -1,53 +1,53 @@
 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,subdir", [
+    (False, "local"),
+    (True, "shared"),
+])
+def test_pulse_height(shared_secondary, subdir):
+    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/cc', 3.7)
+        NaI.add_element('Na', 1.0)
+        NaI.add_element('I', 1.0)
 
-    model.materials = openmc.Materials([NaI])
+        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.source = openmc.IndependentSource(
-        space=openmc.stats.Point(),
-        energy=openmc.stats.Discrete([1e6], [1]),
-        particle='photon'
-    )
+        # 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='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, 1_000_000, 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()

tests/regression_tests/weightwindows/local/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>
+      <energy_bounds>0.0 0.5 20000000.0</energy_bounds>
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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>
+    <mesh id="2">
+      <dimension>5 6 7</dimension>
+      <lower_left>-240 -240 -240</lower_left>
+      <upper_right>240 240 240</upper_right>
+    </mesh>
+    <weight_windows id="2">
+      <mesh>2</mesh>
+      <particle_type>photon</particle_type>
+      <energy_bounds>0.0 0.5 20000000.0</energy_bounds>
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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>

tests/regression_tests/weightwindows/local/results_true.dat

@@ -0,0 +1 @@
+a78972fe3c0dadfc256cfb139c5ca8c7b634738e1895ad2d6286ed5e2567c6dc518e499363908e9058432684148a706a179a39110f703d5322491184a1d0c3e4

tests/regression_tests/weightwindows/results_true.dat

@@ -1 +0,0 @@
-a4a3ccca43666e2ca1e71800201b152cca20c387b93d67522c5339807348dcee5cada9acbed3238f37e2e86e76b374b06988742f07d4ea1b413e4e75d0c180b1

tests/regression_tests/weightwindows/shared/inputs_true.dat

@@ -64,6 +64,7 @@
       <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>

tests/regression_tests/weightwindows/shared/results_true.dat

@@ -0,0 +1 @@
+27c2d218ecf46161088003fbb39ccd63495f80d6e0b27dbc3b3e845b49b282dabf0eaba4fb619be15c6116fc963ec5ce1fe4197efbe4084972ec761e02e70eb5

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>
+      <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>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>

tests/regression_tests/weightwindows/survival_biasing/shared/inputs_true.dat

@@ -51,6 +51,7 @@
       <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>

tests/regression_tests/weightwindows/survival_biasing/shared/results_true.dat

@@ -0,0 +1 @@
+d17a437262d3316985fba4b48e21a7fcd5f32ac2d96ef0e66849f567deaeadc1e0f18d5d0bf5e9cab4246dbf823e7f43f249a1f67e928b27ae8c70b89f3cefbf

tests/regression_tests/weightwindows/survival_biasing/test.py

@@ -1,14 +1,16 @@
+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()
+
     # Material
     w = openmc.Material(name='Tungsten')
     w.add_element('W', 1.0)
@@ -38,13 +40,14 @@ def model():
     angle = openmc.stats.Monodirectional((1.0, 0.0, 0.0))
     energy = openmc.stats.Discrete([14.1e6], [1.0])
 
-    source = openmc.Source(space=space, angle=angle, energy=energy)
+    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)
 
@@ -61,7 +64,8 @@ def model():
     tallies = openmc.Tallies([flux_tally])
     model.tallies = tallies
 
-    lower_ww_bounds = np.loadtxt('ww_n.txt')
+    parent_dir = Path(__file__).parent
+    lower_ww_bounds = np.loadtxt(parent_dir / 'ww_n.txt')
 
     weight_windows = openmc.WeightWindows(mesh,
                                           lower_ww_bounds,
@@ -76,6 +80,12 @@ def model():
     return model
 
 
-def test_weight_windows_with_survival_biasing(model):
-    harness = HashedPyAPITestHarness('statepoint.5.h5', model)
-    harness.main()
+@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()

tests/regression_tests/weightwindows/test.py

@@ -1,14 +1,17 @@
+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)
@@ -43,6 +46,7 @@ def model():
     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))
@@ -71,10 +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
@@ -104,9 +108,15 @@ 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_wwinp_cylindrical():

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>

tests/regression_tests/weightwindows_pulse_height/local/results_true.dat

@@ -0,0 +1,201 @@
+tally 1:
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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>

tests/regression_tests/weightwindows_pulse_height/shared/results_true.dat

@@ -0,0 +1,201 @@
+tally 1:
+4.140000E+00
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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()

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)