Releasing ID after creating dummy universe for cell plotting (#2663)

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

openmc/cell.py

@@ -618,7 +618,10 @@ class Cell(IDManagerMixin):
             Axes containing resulting image
 
         """
-        return openmc.Universe(cells=[self]).plot(*args, **kwargs)
+        # Create dummy universe but preserve used_ids
+        u = openmc.Universe(cells=[self], universe_id=openmc.Universe.next_id + 1)
+        openmc.Universe.used_ids.remove(u.id)
+        return u.plot(*args, **kwargs)
 
     def create_xml_subelement(self, xml_element, memo=None):
         """Add the cell's xml representation to an incoming xml element

tests/unit_tests/test_cell.py

@@ -345,3 +345,21 @@ def test_rotation_from_xml(rotation):
         elem, {s.id: s}, {'void': None}, openmc.Universe
     )
     np.testing.assert_allclose(new_cell.rotation, cell.rotation)
+
+
+def test_plot(run_in_tmpdir):
+    zcyl = openmc.ZCylinder()
+    c = openmc.Cell(region=-zcyl)
+
+    # create a universe before the plot
+    u_before = openmc.Universe()
+
+    # create a plot of the cell
+    c.plot()
+
+    # create a universe after the plot
+    u_after = openmc.Universe()
+
+    # ensure that calling the plot method doesn't
+    # affect the universe ID space
+    assert u_before.id + 1 == u_after.id