Add test for borrowing FPY, update documentation

docs/source/io_formats/depletion_chain.rst

@@ -82,7 +82,7 @@ element has the following attributes:
 ------------------------------------
 
 The ``<neutron_fission_yields>`` element provides yields of fission products for
-fissionable nuclides. It has the follow sub-elements:
+fissionable nuclides. Normally, it has the follow sub-elements:
 
   :energies:
     Energies in [eV] at which yields for products are tabulated
@@ -100,3 +100,11 @@ fissionable nuclides. It has the follow sub-elements:
 
       :data:
         Independent yields for each fission product
+
+In the event that a nuclide doesn't have any known fission product yields, it is
+possible to have that nuclide borrow yields from another nuclide by indicating
+the other nuclide in a single `parent` attribute. For example:
+
+.. code-block:: xml
+
+    <neutron_fission_yields parent="U235"/>

openmc/deplete/nuclide.py

@@ -155,15 +155,16 @@ class Nuclide:
         return self.yield_data.energies
 
     @classmethod
-    def from_xml(cls, element, root, fission_q=None):
+    def from_xml(cls, element, root=None, fission_q=None):
         """Read nuclide from an XML element.
 
         Parameters
         ----------
         element : xml.etree.ElementTree.Element
             XML element to read nuclide data from
-        root : xml.etree.ElementTree.Element
-            Root XML element for chain file
+        root : None or xml.etree.ElementTree.Element
+            Root XML element for chain file (only used when fission product
+            yields are borrowed from another parent)
         fission_q : None or float
             User-supplied fission Q value [eV].
             Will be read from the element if not given

tests/unit_tests/test_deplete_nuclide.py

@@ -2,7 +2,7 @@
 
 import xml.etree.ElementTree as ET
 
-import numpy
+import numpy as np
 import pytest
 from openmc.deplete import nuclide
 
@@ -81,6 +81,44 @@ def test_from_xml():
         u235.yield_energies = [0.0253, 5e5]
 
 
+def test_fpy_parent():
+    """Test reading nuclide data with FPY borrowed from another nuclide."""
+
+    data = """
+<depletion_chain>
+  <nuclide name="U235" reactions="1">
+    <reaction type="fission" Q="193405400.0"/>
+    <neutron_fission_yields>
+      <energies>0.0253</energies>
+      <fission_yields energy="0.0253">
+        <products>Te134 Zr100 Xe138</products>
+        <data>0.062155 0.0497641 0.0481413</data>
+      </fission_yields>
+    </neutron_fission_yields>
+  </nuclide>
+  <nuclide name="U238" reactions="1">
+    <reaction type="fission" Q="200.0e6"/>
+    <neutron_fission_yields parent="U235"/>
+  </nuclide>
+</depletion_chain>
+    """
+
+    root = ET.fromstring(data)
+    elems = root.findall('nuclide')
+    u235 = nuclide.Nuclide.from_xml(elems[0], root)
+    u238 = nuclide.Nuclide.from_xml(elems[1], root)
+
+    # Make sure U238 yield is same as U235
+    assert np.array_equal(u238.yield_data.energies, u235.yield_data.energies)
+    assert np.array_equal(u238.yield_data.yield_matrix, u235.yield_data.yield_matrix)
+
+    # Make sure XML element created has single attribute
+    elem = u238.to_xml_element()
+    fpy_elem = elem.find('neutron_fission_yields')
+    assert fpy_elem.get('parent') == 'U235'
+    assert len(fpy_elem) == 0
+
+
 def test_to_xml_element():
     """Test writing nuclide data to an XML element."""
 
@@ -134,11 +172,11 @@ def test_fission_yield_distribution():
         act_dist = yield_dict[exp_ene]
         for exp_prod, exp_yield in exp_dist.items():
             assert act_dist[exp_prod] == exp_yield
-    exp_yield = numpy.array([
+    exp_yield = np.array([
         [4.08e-12, 1.71e-12, 7.85e-4],
         [1.32e-12, 0.0, 1.12e-3],
         [5.83e-8, 2.69e-8, 4.54e-3]])
-    assert numpy.array_equal(yield_dist.yield_matrix, exp_yield)
+    assert np.array_equal(yield_dist.yield_matrix, exp_yield)
 
     # Test the operations / special methods for fission yield
     orig_yields = yield_dist[0.0253]
@@ -151,18 +189,18 @@ def test_fission_yield_distribution():
 
     # Scale and increment fission yields
     mod_yields = orig_yields * 2
-    assert numpy.array_equal(orig_yields.yields * 2, mod_yields.yields)
+    assert np.array_equal(orig_yields.yields * 2, mod_yields.yields)
     mod_yields += orig_yields
-    assert numpy.array_equal(orig_yields.yields * 3, mod_yields.yields)
+    assert np.array_equal(orig_yields.yields * 3, mod_yields.yields)
 
     mod_yields = 2.0 * orig_yields
-    assert numpy.array_equal(orig_yields.yields * 2, mod_yields.yields)
+    assert np.array_equal(orig_yields.yields * 2, mod_yields.yields)
 
-    mod_yields = numpy.float64(2.0) * orig_yields
-    assert numpy.array_equal(orig_yields.yields * 2, mod_yields.yields)
+    mod_yields = np.float64(2.0) * orig_yields
+    assert np.array_equal(orig_yields.yields * 2, mod_yields.yields)
 
     # Failure modes for adding, multiplying yields
-    similar = numpy.empty_like(orig_yields.yields)
+    similar = np.empty_like(orig_yields.yields)
     with pytest.raises(TypeError):
         orig_yields + similar
     with pytest.raises(TypeError):