Use defaultdict(dict) when weighting fission yields

The Chain.form_matrix method looks for information and
fission yields for all isotopes with a fission reaction.
However, the AveragedFissionYieldHelper and FissionYieldCutoffHelper
are not guaranteed to return yields to all isotopes with
fission reactions. Instead, the union on two sets of yields
are returned:
1) yields from isotopes with a single set of fission yields
2) weighted yields computed for isotopes with both multiple sets of
   fission yields **and** non-zero densities.

This later item can cause some isotopes that the Chain anticipates
having fission yields, since they have a fission reaction, to not
exist in the yields library. An isotope with multiple sets of
yields but that is not physically present in the problem will not
have reaction rates and also not have fission yields computed
by these helpers. This will cause a KeyError during Chain.form_matrix
when fission yields for these isotopes are requested.

This commit changes the return type from weighted_yields on
ConstantFissionYieldHelper, AveragedFissionYieldHelper, and
FissionYieldCutoffHelper to be defaultdict(dict). This resolves
the above issue by returning an "empty" set of yields for isotopes
that potentially *should* have fission yields, but have zero density
across the problem, and therefore do not have tallied reaction
rates nor fission yields.

Documentation for the changes to these classes have been updated

openmc/deplete/abc.py

@@ -5,6 +5,7 @@ to run a full depletion simulation.
 """
 
 from collections import namedtuple
+from collections import defaultdict
 from collections.abc import Iterable
 import os
 from pathlib import Path
@@ -376,14 +377,17 @@ class FissionYieldHelper(ABC):
 
     Attributes
     ----------
-    constant_yields : dict of str to :class:`openmc.deplete.FissionYield`
+    constant_yields : collections.defaultdict
         Fission yields for all nuclides that only have one set of
-        fission yield data. Can be accessed as ``{parent: {product: yield}}``
+        fission yield data. Dictionary of form ``{str: {str: float}}``
+        representing yields for ``{parent: {product: yield}}``. Default
+        return object is an empty dictionary
+
     """
 
     def __init__(self, chain_nuclides):
         self._chain_nuclides = {}
-        self._constant_yields = {}
+        self._constant_yields = defaultdict(dict)
 
         # Get all nuclides with fission yield data
         for nuc in chain_nuclides:
@@ -407,14 +411,16 @@ class FissionYieldHelper(ABC):
         Parameters
         ----------
         local_mat_index : int
-            Index for material tracked on this process that
-            exists in :attr:`local_mat_index` and fits within
-            the first axis in :attr:`results`
+            Index for the material with requested fission yields.
+            Should correspond to the material represented in
+            ``mat_indexes[local_mat_index]`` during
+            :meth:`generate_tallies`.
 
         Returns
         -------
-        library : dict
-            Dictionary of ``{parent: {product: fyield}}``
+        library : collections.abc.Mapping
+            Dictionary-like object mapping ``{str: {str: float}``.
+            This reflects fission yields for ``{parent: {product: fyield}}``.
         """
 
     @staticmethod

openmc/deplete/helpers.py

@@ -5,6 +5,7 @@ from copy import deepcopy
 from itertools import product
 from numbers import Real
 import bisect
+from collections import defaultdict
 
 from numpy import dot, zeros, newaxis
 
@@ -179,9 +180,11 @@ class ConstantFissionYieldHelper(FissionYieldHelper):
 
     Attributes
     ----------
-    constant_yields : dict of str to :class:`openmc.deplete.FissionYield`
+    constant_yields : collections.defaultdict
         Fission yields for all nuclides that only have one set of
-        fission yield data. Can be accessed as ``{parent: {product: yield}}``
+        fission yield data. Dictionary of form ``{str: {str: float}}``
+        representing yields for ``{parent: {product: yield}}``. Default
+        return object is an empty dictionary
     energy : float
         Energy of fission yield libraries.
     """
@@ -237,7 +240,7 @@ class ConstantFissionYieldHelper(FissionYieldHelper):
 
         Returns
         -------
-        library : dict
+        library : collections.defaultdict
             Dictionary of ``{parent: {product: fyield}}``
         """
         return self.constant_yields
@@ -282,6 +285,11 @@ class FissionYieldCutoffHelper(TalliedFissionYieldHelper):
     fast_yields : dict
         Dictionary of the form ``{parent: {product: yield}}``
         with fast yields
+    constant_yields : collections.defaultdict
+        Fission yields for all nuclides that only have one set of
+        fission yield data. Dictionary of form ``{str: {str: float}}``
+        representing yields for ``{parent: {product: yield}}``. Default
+        return object is an empty dictionary
     results : numpy.ndarray
         Array of fission rate fractions with shape
         ``(n_mats, 2, n_nucs)``. ``results[:, 0]``
@@ -417,7 +425,7 @@ class FissionYieldCutoffHelper(TalliedFissionYieldHelper):
 
         Returns
         -------
-        library : dict
+        library : collections.defaultdict
             Dictionary of ``{parent: {product: fyield}}``
         """
         yields = self.constant_yields
@@ -469,9 +477,11 @@ class AveragedFissionYieldHelper(TalliedFissionYieldHelper):
 
     Attributes
     ----------
-    constant_yields : dict of str to :class:`openmc.deplete.FissionYield`
+    constant_yields : collections.defaultdict
         Fission yields for all nuclides that only have one set of
-        fission yield data. Can be accessed as ``{parent: {product: yield}}``
+        fission yield data. Dictionary of form ``{str: {str: float}}``
+        representing yields for ``{parent: {product: yield}}``. Default
+        return object is an empty dictionary
     results : None or numpy.ndarray
         If tallies have been generated and unpacked, then the array will
         have shape ``(n_mats, n_tnucs)``, where ``n_mats`` is the number
@@ -569,12 +579,13 @@ class AveragedFissionYieldHelper(TalliedFissionYieldHelper):
 
         Returns
         -------
-        library : dict
-            Dictionary of ``{parent: {product: fyield}}``
+        library : collections.defaultdict
+            Dictionary of ``{parent: {product: fyield}}``. Default return
+            value is an empty dictionary
         """
         if not self._tally_nucs:
             return self.constant_yields
-        mat_yields = {}
+        mat_yields = defaultdict(dict)
         average_energies = self.results[local_mat_index]
         for avg_e, nuc in zip(average_energies, self._tally_nucs):
             nuc_energies = nuc.yield_energies