Fix for plotting model with multi-group cross sections (#3748)

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

Author: GuySten

openmc/model/model.py

@@ -1160,6 +1160,16 @@ def plot(
         y_min = (origin[y] - 0.5*width[1]) * axis_scaling_factor[axis_units]
         y_max = (origin[y] + 0.5*width[1]) * axis_scaling_factor[axis_units]
 
+        # Determine whether any materials contains macroscopic data and if so,
+        # set energy mode accordingly and check that mg cross sections path is accessible
+        for mat in self.geometry.get_all_materials().values():
+            if mat._macroscopic is not None:
+                self.settings.energy_mode = 'multi-group'
+                if 'mg_cross_sections' not in openmc.config:
+                    raise RuntimeError("'mg_cross_sections' path must be set in "
+                                       "openmc.config before plotting.")
+                break
+
         # Get ID map from the C API
         id_map = self.id_map(
             origin=origin,

tests/unit_tests/test_universe.py

@@ -1,3 +1,5 @@
+from pathlib import Path
+
 import lxml.etree as ET
 import numpy as np
 import openmc
@@ -104,6 +106,43 @@ def test_plot(run_in_tmpdir, sphere_model):
     plt.close('all')
 
 
+def test_mg_plot(run_in_tmpdir):
+    # Create a simple universe with macroscopic data
+    h2o_data = openmc.Macroscopic('LWTR')
+    water = openmc.Material(name='Water')
+    water.set_density('macro', 1.0)
+    water.add_macroscopic(h2o_data)
+    sph = openmc.Sphere(r=10, boundary_type="vacuum")
+    cell = openmc.Cell(region=-sph, fill=water)
+    univ = openmc.Universe(cells=[cell])
+
+    # Create MGXS library and export to HDF5
+    groups = openmc.mgxs.EnergyGroups([1e-5, 20.0e6])
+    h2o_xsdata = openmc.XSdata('LWTR', groups)
+    h2o_xsdata.order = 0
+    h2o_xsdata.set_total([1.0])
+    h2o_xsdata.set_absorption([0.5])
+    scatter_matrix = np.array([[[0.5]]])
+    scatter_matrix = np.rollaxis(scatter_matrix, 0, 3)
+    h2o_xsdata.set_scatter_matrix(scatter_matrix)
+    mg_library = openmc.MGXSLibrary(groups)
+    mg_library.add_xsdatas([h2o_xsdata])
+    mgxs_path = Path.cwd() / 'mgxs.h5'
+    mg_library.export_to_hdf5(mgxs_path)
+
+    # Set MG cross sections in config and plot
+    with openmc.config.patch('mg_cross_sections', mgxs_path):
+        univ.plot(width=(200, 200), basis='yz', color_by='cell')
+        univ.plot(width=(200, 200), basis='yz', color_by='material')
+
+    with pytest.raises(RuntimeError):
+        univ.plot(width=(200, 200), basis='yz', color_by='cell')
+
+    # Close plots to avoid warning
+    import matplotlib.pyplot as plt
+    plt.close('all')
+
+
 def test_get_nuclides(uo2):
     c = openmc.Cell(fill=uo2)
     univ = openmc.Universe(cells=[c])