Fix Random Ray Solver Bug: Detect Multiple Point Sources in Same Subdivided Source Region

src/random_ray/flat_source_domain.cpp

@@ -1092,6 +1092,13 @@ void FlatSourceDomain::convert_external_sources()
         // point_source_map to determine if there are any point source terms
         // that should be applied.
         SourceRegionKey key {sr, mesh_bin};
+        auto it = point_source_map_.find(key);
+        if (it != point_source_map_.end()) {
+          fatal_error(
+            "Multiple point sources detected in the same subdivided source "
+            "region."
+            "This is not currently supported in the random ray solver.");
+        }
         point_source_map_[key] = es;
       } else {
         // If we are not using mesh subdivision, we can apply the external

tests/unit_tests/test_random_ray_multiple_point_sources.py

@@ -0,0 +1,164 @@
+"""
+Test for the Random Ray Solver multiple point sources error detection fix.
+
+This test verifies the fix for issue #3470 where multiple point sources 
+placed in the same subdivided source region would silently overwrite each other.
+The fix adds proper error detection to prevent this condition.
+"""
+
+import os
+import tempfile
+import pytest
+import openmc
+from openmc.examples import random_ray_three_region_cube
+
+
+def test_multiple_point_sources_same_location_error(run_in_tmpdir):
+    """Test that multiple point sources at the same location trigger an error 
+    when mesh subdivision is enabled."""
+
+    # Create a basic random ray model
+    openmc.reset_auto_ids()
+    model = random_ray_three_region_cube()
+
+    # Enable mesh subdivision - this is required to trigger the error
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (4, 4, 4)
+    mesh.lower_left = (0.0, 0.0, 0.0)
+    mesh.upper_right = (30.0, 30.0, 30.0)
+
+    # Get the source universe to apply mesh subdivision
+    source_universe = model.geometry.get_universes_by_name('source universe')[0]
+    model.settings.random_ray['source_region_meshes'] = [
+        (mesh, [source_universe])
+    ]
+
+    # Create multiple point sources at the exact same location
+    # This should trigger the error when mesh subdivision is enabled
+    point_location = openmc.stats.Point(xyz=(5.0, 5.0, 5.0))
+    energy_dist = openmc.stats.Discrete([10.0], [1.0])
+
+    source1 = openmc.IndependentSource(
+        space=point_location, 
+        energy=energy_dist, 
+        strength=1.0
+    )
+    source2 = openmc.IndependentSource(
+        space=point_location,  # Same location 
+        energy=energy_dist, 
+        strength=1.0
+    )
+
+    # Set the multiple point sources
+    model.settings.source = [source1, source2]
+
+    # Very short simulation to just trigger initialization
+    model.settings.batches = 1
+    model.settings.inactive = 0 
+    model.settings.particles = 10
+
+    # The error should be triggered during initialization/conversion of external sources
+    with pytest.raises(RuntimeError, match=r"Multiple point sources detected.*same subdivided.*source region"):
+        model.run()
+
+
+def test_multiple_point_sources_different_locations_success(run_in_tmpdir):
+    """Test that multiple point sources at different locations work correctly 
+    with mesh subdivision enabled."""
+
+    # Create a basic random ray model
+    openmc.reset_auto_ids()
+    model = random_ray_three_region_cube()
+
+    # Enable mesh subdivision 
+    mesh = openmc.RegularMesh()
+    mesh.dimension = (4, 4, 4)
+    mesh.lower_left = (0.0, 0.0, 0.0)
+    mesh.upper_right = (30.0, 30.0, 30.0)
+
+    # Get the source universe to apply mesh subdivision
+    source_universe = model.geometry.get_universes_by_name('source universe')[0]
+    model.settings.random_ray['source_region_meshes'] = [
+        (mesh, [source_universe])
+    ]
+
+    # Create multiple point sources at different locations
+    # This should work fine
+    point_location1 = openmc.stats.Point(xyz=(2.0, 2.0, 2.0))
+    point_location2 = openmc.stats.Point(xyz=(8.0, 8.0, 8.0))
+    energy_dist = openmc.stats.Discrete([10.0], [1.0])
+
+    source1 = openmc.IndependentSource(
+        space=point_location1, 
+        energy=energy_dist, 
+        strength=1.0
+    )
+    source2 = openmc.IndependentSource(
+        space=point_location2,  # Different location
+        energy=energy_dist, 
+        strength=1.0
+    )
+
+    # Set the multiple point sources
+    model.settings.source = [source1, source2]
+
+    # Very short simulation to just verify it initializes correctly
+    model.settings.batches = 1
+    model.settings.inactive = 0
+    model.settings.particles = 10
+
+    # This should not raise an error
+    try:
+        model.run()
+    except RuntimeError as e:
+        # If we get a RuntimeError, it should NOT be about multiple point sources
+        assert "Multiple point sources detected" not in str(e)
+        # Re-raise any other errors (could be due to test environment)
+        raise
+
+
+def test_multiple_point_sources_no_mesh_subdivision_success(run_in_tmpdir):
+    """Test that multiple point sources at the same location work correctly 
+    when mesh subdivision is disabled."""
+
+    # Create a basic random ray model without mesh subdivision
+    openmc.reset_auto_ids()
+    model = random_ray_three_region_cube()
+
+    # Explicitly ensure no mesh subdivision is configured
+    # (default behavior, but being explicit for clarity)
+    if 'source_region_meshes' in model.settings.random_ray:
+        del model.settings.random_ray['source_region_meshes']
+
+    # Create multiple point sources at the same location
+    # This should work fine without mesh subdivision
+    point_location = openmc.stats.Point(xyz=(5.0, 5.0, 5.0))
+    energy_dist = openmc.stats.Discrete([10.0], [1.0])
+
+    source1 = openmc.IndependentSource(
+        space=point_location, 
+        energy=energy_dist, 
+        strength=1.0
+    )
+    source2 = openmc.IndependentSource(
+        space=point_location,  # Same location, but no mesh subdivision
+        energy=energy_dist, 
+        strength=1.0
+    )
+
+    # Set the multiple point sources
+    model.settings.source = [source1, source2]
+
+    # Very short simulation to just verify it initializes correctly  
+    model.settings.batches = 1
+    model.settings.inactive = 0
+    model.settings.particles = 10
+
+    # This should not raise an error about multiple point sources
+    try:
+        model.run()
+    except RuntimeError as e:
+        # If we get a RuntimeError, it should NOT be about multiple point sources
+        assert "Multiple point sources detected" not in str(e)
+        # Re-raise any other errors (could be due to test environment)
+        raise