Add a test for non-90-degree periodic BCs

tests/regression_tests/periodic_6fold/inputs_true.dat

@@ -0,0 +1,34 @@
+<?xml version='1.0' encoding='utf-8'?>
+<geometry>
+  <cell id="1" material="1" region="9 10 -11 12" universe="0" />
+  <cell id="2" material="2" region="9 10 -12" universe="0" />
+  <surface boundary="periodic" coeffs="0.4999999999999999 0.8660254037844387 0.0 0.0" id="9" type="plane" />
+  <surface boundary="periodic" coeffs="0.4999999999999999 -0.8660254037844387 0.0 0.0" id="10" type="plane" />
+  <surface boundary="reflective" coeffs="5.0" id="11" type="x-plane" />
+  <surface coeffs="2.598076211353316 1.4999999999999998 2.0" id="12" type="z-cylinder" />
+</geometry>
+<?xml version='1.0' encoding='utf-8'?>
+<materials>
+  <material id="1">
+    <density units="g/cc" value="1.0" />
+    <nuclide ao="2.0" name="H1" />
+    <nuclide ao="1.0" name="O16" />
+    <sab name="c_H_in_H2O" />
+  </material>
+  <material depletable="true" id="2">
+    <density units="g/cc" value="4.5" />
+    <nuclide ao="1.0" name="U235" />
+  </material>
+</materials>
+<?xml version='1.0' encoding='utf-8'?>
+<settings>
+  <run_mode>eigenvalue</run_mode>
+  <particles>1000</particles>
+  <batches>4</batches>
+  <inactive>0</inactive>
+  <source strength="1.0">
+    <space type="box">
+      <parameters>0 0 0 5 5 0</parameters>
+    </space>
+  </source>
+</settings>

tests/regression_tests/periodic_6fold/results_true.dat

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+k-combined:
+1.858773E+00 1.550950E-02

tests/regression_tests/periodic_6fold/test.py

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+import openmc
+import numpy as np
+
+from tests.testing_harness import PyAPITestHarness
+
+
+class Periodic6FoldTest(PyAPITestHarness):
+    def _build_inputs(self):
+        # Define materials
+        water = openmc.Material(1)
+        water.add_nuclide('H1', 2.0)
+        water.add_nuclide('O16', 1.0)
+        water.add_s_alpha_beta('c_H_in_H2O')
+        water.set_density('g/cc', 1.0)
+
+        fuel = openmc.Material(2)
+        fuel.add_nuclide('U235', 1.0)
+        fuel.set_density('g/cc', 4.5)
+
+        materials = openmc.Materials((water, fuel))
+        materials.default_temperature = '294K'
+        materials.export_to_xml()
+
+        # Define the geometry.  Note that this geometry is somewhat non-sensical
+        # (it essentially defines a circle of half-cylinders), but it is
+        # designed so that periodic and reflective BCs will give different
+        # answers.
+        theta1 = (-1/6 + 1/2) * np.pi
+        theta2 = (1/6 - 1/2) * np.pi
+        plane1 = openmc.Plane(a=np.cos(theta1), b=np.sin(theta1),
+                              boundary_type='periodic')
+        plane2 = openmc.Plane(a=np.cos(theta2), b=np.sin(theta2),
+                              boundary_type='periodic')
+
+        x_max = openmc.XPlane(x0=5., boundary_type='reflective')
+
+        z_cyl = openmc.ZCylinder(x0=3*np.cos(np.pi/6), y0=3*np.sin(np.pi/6),
+                                 r=2.0)
+
+        outside_cyl = openmc.Cell(1, fill=water, region=(
+            +plane1 & +plane2 & -x_max & +z_cyl))
+        inside_cyl = openmc.Cell(2, fill=fuel, region=(
+            +plane1 & +plane2 & -z_cyl))
+        root_universe = openmc.Universe(0, cells=(outside_cyl, inside_cyl))
+
+        geometry = openmc.Geometry()
+        geometry.root_universe = root_universe
+        geometry.export_to_xml()
+
+        # Define settings
+        settings = openmc.Settings()
+        settings.particles = 1000
+        settings.batches = 4
+        settings.inactive = 0
+        settings.source = openmc.Source(space=openmc.stats.Box(
+            (0, 0, 0), (5, 5, 0)))
+        settings.export_to_xml()
+
+
+def test_periodic():
+    harness = Periodic6FoldTest('statepoint.4.h5')
+    harness.main()