Add a simple test for time filter

tests/unit_tests/test_time_filter.py

@@ -0,0 +1,80 @@
+from math import sqrt
+from random import uniform
+
+import numpy as np
+import openmc
+import pytest
+
+
+def test_time_filter_basics():
+    f = openmc.TimeFilter([1.0, 2.0, 5.0, 9.0])
+    np.testing.assert_allclose(f.bins[0], [1.0, 2.0])
+    np.testing.assert_allclose(f.bins[1], [2.0, 5.0])
+    np.testing.assert_allclose(f.bins[-1], [5.0, 9.0])
+    assert len(f.bins) == 3
+
+    # Make sure __repr__ works
+    repr(f)
+
+    # to_xml_element()
+    elem = f.to_xml_element()
+    assert elem.tag == 'filter'
+    assert elem.attrib['type'] == 'time'
+
+
+@pytest.fixture
+def model():
+    # Select random sphere radius, source position, and source energy
+    r = uniform(0., 2.)
+    x = uniform(2., 10.)
+    E = uniform(0., 20.0e6)
+
+    # Create model
+    model = openmc.Model()
+    mat = openmc.Material()
+    mat.add_nuclide('Zr90', 1.0)
+    mat.set_density('g/cm3', 1.0)
+    model.materials.append(mat)
+    inner_sphere = openmc.Sphere(r=r)
+    outer_sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
+    center_cell = openmc.Cell(fill=mat, region=-inner_sphere)
+    outer_void = openmc.Cell(region=+inner_sphere & -outer_sphere)
+    model.geometry = openmc.Geometry([center_cell, outer_void])
+    model.settings = openmc.Settings()
+    model.settings.run_mode = 'fixed source'
+    model.settings.particles = 1000
+    model.settings.batches = 20
+    model.settings.source = openmc.Source(
+        space=openmc.stats.Point((x, 0., 0.)),
+        angle=openmc.stats.Monodirectional([-1., 0., 0.]),
+        energy=openmc.stats.Discrete([E], [1.0])
+    )
+
+    # Source particles will take a time of t = x/v to reach the inner sphere,
+    # where x is the distance from the source to the sphere and and v =
+    # sqrt(2E/m). Before this time, there should be no reactions,
+    MASS_NEUTRON_EV = 939.56542052e6  # eV/c²
+    C_LIGHT = 2.99792458e10  # cm/s
+    distance = x - r
+    velocity = sqrt(2*E / MASS_NEUTRON_EV) * C_LIGHT
+    t0 = distance / velocity
+
+    # Create tally with time filter
+    tally = openmc.Tally()
+    tally.filters = [openmc.TimeFilter([0.0, t0, 2*t0])]
+    tally.scores = ['total']
+    model.tallies.append(tally)
+    return model
+
+
+def test_time_filter_transport(model, run_in_tmpdir):
+    sp_filename = model.run()
+    with openmc.StatePoint(sp_filename) as sp:
+        t = sp.tallies[model.tallies[0].id]
+        values = t.mean.ravel()
+
+        # Before t0, the reaction rate should be zero
+        assert values[0] == 0.0
+
+        # After t0, the reaction rate should be positive
+        assert values[1] > 0.0