Adding ability to query for adjacent volumes.

src/pydagmc/dagnav.py

@@ -700,6 +700,16 @@ def surfaces(self) -> list[Surface]:
     def surfaces_by_id(self) -> dict[int, Surface]:
         return {s.id: s for s in self.surfaces}
 
+    @property
+    def adjacent_volumes(self) -> list[Volume]:
+        """Returns a list of volumes adjacent to this volume."""
+        adjacent_vols = set()
+        for surface in self.surfaces:
+            for vol in surface.volumes:
+                adjacent_vols.add(vol)
+        adjacent_vols.discard(self)
+        return sorted(list(adjacent_vols), key=lambda v: v.id)
+
     @property
     def num_triangles(self) -> int:
         """Returns the number of triangles in this volume"""

test/test_dagnav.py

@@ -1033,3 +1033,30 @@ def test_geometryset_check_tags_errors(request):
     with pytest.raises(ValueError, match="has no category or geom_dimension"):
         _ = pydagmc.Surface(model, raw_handle_surf_missing_all)
 
+
+def test_adjacent_volumes(fuel_pin_model):
+
+    # innermost volume
+    model = fuel_pin_model
+    vol1 = model.volumes_by_id[1]
+    adj_vols = vol1.adjacent_volumes
+    assert len(adj_vols) == 1
+    assert model.volumes_by_id[2] in adj_vols
+
+    # middle volume
+    vol2 = model.volumes_by_id[2]
+    adj_vols = vol2.adjacent_volumes
+    assert len(adj_vols) == 2
+    assert model.volumes_by_id[1] in adj_vols
+    assert model.volumes_by_id[3] in adj_vols
+
+    # outermost pin volume
+    vol3 = model.volumes_by_id[3]
+    adj_vols = vol3.adjacent_volumes
+    assert len(adj_vols) == 1
+    assert model.volumes_by_id[2] in adj_vols
+
+    # graveyard volume -- doesn't touch any other explicit volumes
+    vol6 = model.volumes_by_id[6]
+    adj_vols = vol6.adjacent_volumes
+    assert len(adj_vols) == 0