From bd76fc056651e33d138d18dbee061c0a7bc83823 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Mon, 3 Nov 2025 11:11:57 -0600
Subject: [PATCH] Fix bug in normalization of tally results with no_reduce
 (#3619)

---
 src/tallies/tally.cpp              |  7 +++++-
 tests/unit_tests/test_no_reduce.py | 40 ++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 tests/unit_tests/test_no_reduce.py

diff --git a/src/tallies/tally.cpp b/src/tallies/tally.cpp
index b9c615ecb5..12ee3427eb 100644
--- a/src/tallies/tally.cpp
+++ b/src/tallies/tally.cpp
@@ -825,9 +825,14 @@ void Tally::accumulate()
       total_source = 1.0;
     }
 
+    // Determine number of particles contributing to tally
+    double contributing_particles = settings::reduce_tallies
+                                      ? settings::n_particles
+                                      : simulation::work_per_rank;
+
     // Account for number of source particles in normalization
     double norm =
-      total_source / (settings::n_particles * settings::gen_per_batch);
+      total_source / (contributing_particles * settings::gen_per_batch);
 
     if (settings::solver_type == SolverType::RANDOM_RAY) {
       norm = 1.0;
diff --git a/tests/unit_tests/test_no_reduce.py b/tests/unit_tests/test_no_reduce.py
new file mode 100644
index 0000000000..00ddb5a959
--- /dev/null
+++ b/tests/unit_tests/test_no_reduce.py
@@ -0,0 +1,40 @@
+"""Test the settings.no_reduce feature to ensure tallies are correctly
+reduced across MPI processes."""
+
+import openmc
+import pytest
+
+from tests.testing_harness import config
+
+
+@pytest.mark.parametrize('no_reduce', [True, False])
+def test_no_reduce(no_reduce, run_in_tmpdir):
+    """Test that tally results are correct with and without no_reduce."""
+
+    # Create simple sphere model with vacuum
+    model = openmc.Model()
+    sphere = openmc.Sphere(r=1.0, boundary_type='vacuum')
+    cell = openmc.Cell(region=-sphere)
+    model.geometry = openmc.Geometry([cell])
+    model.settings.run_mode = 'fixed source'
+    model.settings.batches = 10
+    model.settings.particles = 100
+    model.settings.source = openmc.IndependentSource(space=openmc.stats.Point())
+    model.settings.no_reduce = no_reduce
+
+    # Tally: surface current on vacuum boundary
+    surf_filter = openmc.SurfaceFilter(sphere)
+    tally = openmc.Tally()
+    tally.filters = [surf_filter]
+    tally.scores = ['current']
+    model.tallies = [tally]
+
+    # Run OpenMC with proper MPI arguments if needed
+    kwargs = {'apply_tally_results': True, 'openmc_exec': config['exe']}
+    if config['mpi']:
+        kwargs['mpi_args'] = [config['mpiexec'], '-n', config['mpi_np']]
+    model.run(**kwargs)
+
+    # The tally should be ~1.0 (every particle crosses the surface once)
+    tally_mean = tally.mean.flatten()[0]
+    assert tally_mean == pytest.approx(1.0)