Add (currently failing) test with URR (n,gamma) reaction rate check

tests/unit_tests/test_urr_capture.py

@@ -0,0 +1,42 @@
+import openmc
+import pytest
+
+
+@pytest.fixture
+def th232_model():
+    # URR boundaries for Th232
+    e_min, e_max = 4000.0, 100000.0
+
+    model = openmc.model.Model()
+    th232 = openmc.Material()
+    th232.add_nuclide('Th232', 1.0)
+
+    surf = openmc.Sphere(r=100.0, boundary_type='reflective')
+    cell = openmc.Cell(fill=th232, region=-surf)
+    model.geometry = openmc.Geometry([cell])
+
+    model.settings.particles = 100
+    model.settings.batches = 10
+    model.settings.run_mode = 'fixed source'
+    energies = openmc.stats.Uniform(e_min, e_max)
+    model.settings.source = openmc.Source(energy=energies)
+
+    tally = openmc.Tally(name='rates')
+    tally.filters = [openmc.EnergyFilter([e_min, e_max])]
+    tally.scores = ['(n,gamma)', 'absorption', 'fission']
+    model.tallies.append(tally)
+    return model
+
+
+def test_urr_capture(run_in_tmpdir, th232_model):
+    # Export and run model
+    th232_model.export_to_xml()
+    openmc.run()
+
+    # Get reaction rates from tally
+    with openmc.StatePoint('statepoint.10.h5') as sp:
+        t = sp.get_tally(name='rates')
+        ngamma, absorption, fission = t.mean.flatten()
+
+    # In URR, the (n,gamma) rate should be equal to absorption - fission
+    assert ngamma == pytest.approx(absorption - fission)