fixed a bug in MeshMaterialFilter.from_volumes (#3520)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: GuySten

openmc/filter.py

@@ -1082,7 +1082,12 @@ def from_volumes(cls, mesh: openmc.MeshBase, volumes: openmc.MeshMaterialVolumes
             A new MeshMaterialFilter instance
 
         """
-        bins = list(zip(*np.where(volumes._materials > -1)))
+        # Get flat arrays of material IDs and element indices
+        mat_ids = volumes._materials[volumes._materials > -1]
+        elems, _ = np.where(volumes._materials > -1)
+
+        # Stack them into a 2D array of (element, material) pairs
+        bins = np.column_stack((elems, mat_ids))
         return cls(mesh, bins)
 
     def __hash__(self):

tests/unit_tests/test_filter_meshmaterial.py

@@ -9,6 +9,7 @@ def test_filter_mesh_material(run_in_tmpdir):
     materials = []
     for i in range(4):
         mat = openmc.Material()
+        mat.id = 10*(i+1)
         mat.add_nuclide('Fe56', 1.0)
         materials.append(mat)
 
@@ -35,7 +36,7 @@ def test_filter_mesh_material(run_in_tmpdir):
     # MeshMaterialFilter with corresponding bins
     vols = mesh.material_volumes(model)
     mmf = openmc.MeshMaterialFilter.from_volumes(mesh, vols)
-    expected_bins = [(0, 1), (1, 1), (1, 2), (2, 2), (2, 3), (3, 3), (3, 4), (4, 4)]
+    expected_bins = [(0, 10), (1, 10), (1, 20), (2, 20), (2, 30), (3, 40), (3, 30), (4, 40)]
     np.testing.assert_equal(mmf.bins, expected_bins)
 
     # Create two tallies, one with a mesh filter and one with mesh-material