Warn when a material spans multiple cells in material_wise MGXS

openmc/model/model.py

@@ -2593,6 +2593,32 @@ def _isothermal_materialwise_mgxs(
                     for mat in mgxs_lib.domains
             }
 
+    def _warn_material_wise_spectral_averaging(self) -> None:
+        """Warn when a material fills more than one cell.
+
+        The "material_wise" method produces one flux-weighted cross section per
+        material, so a material spanning regions with different spectra (e.g.
+        steel near the source and deep inside a shield) is collapsed to a single
+        spectrum-averaged cross section. :meth:`Model.differentiate_mats` gives
+        each region its own material, and so its own cross section.
+        """
+        counts = {}
+        for cell in self.geometry.get_all_material_cells().values():
+            # A distribmat fill is a list that may contain None for void.
+            fills = cell.fill if cell.fill_type == 'distribmat' else [cell.fill]
+            for mat in fills:
+                if mat is not None:
+                    counts[mat] = counts.get(mat, 0) + 1
+
+        if any(n > 1 for n in counts.values()):
+            warnings.warn(
+                "The 'material_wise' method produces one spectrum-averaged "
+                "cross section per material, but one or more materials fill "
+                "multiple cells and will be averaged over locations that may "
+                "have different spectra. Use Model.differentiate_mats() to "
+                "generate a separate cross section per location."
+            )
+
     def _generate_material_wise_mgxs(
         self,
         groups: openmc.mgxs.EnergyGroups,
@@ -2635,6 +2661,8 @@ def _generate_material_wise_mgxs(
             Valid entries for temperature_settings are the same as the valid
             entries in openmc.Settings.temperature_settings.
         """
+        self._warn_material_wise_spectral_averaging()
+
         temp_settings = {}
         if temperature_settings is None:
             temp_settings = self.settings.temperature

tests/unit_tests/test_model.py

@@ -999,6 +999,25 @@ def modify_radius(radius):
     assert result.total_batches > 0
 
 
+def test_convert_to_multigroup_material_wise_warns_for_shared_material():
+    """material_wise generation warns when one material fills multiple cells, as
+    its cross section is then averaged over locations with possibly different
+    spectra."""
+    steel = openmc.Material(name="steel")
+    steel.add_element("Fe", 1.0)
+    steel.set_density("g/cm3", 7.9)
+
+    s1 = openmc.Sphere(r=1.0)
+    s2 = openmc.Sphere(r=2.0, boundary_type="vacuum")
+    # The same material object fills two separate cells (two "locations").
+    c1 = openmc.Cell(fill=steel, region=-s1)
+    c2 = openmc.Cell(fill=steel, region=+s1 & -s2)
+    model = openmc.Model(openmc.Geometry([c1, c2]), openmc.Materials([steel]))
+
+    with pytest.warns(UserWarning, match="differentiate_mats"):
+        model._warn_material_wise_spectral_averaging()
+
+
 def test_id_map_to_rgb():
     """Test conversion of ID map to RGB image array."""
     # Create a simple model