Guard against using FPY tally results if not needed

If there are no nuclides with multiple sets of yields, then
the tallies needed for fission yields may or or may not exist.
Regardless, this means that using constant_yields is absolutely
appropriate and there is no need for unpacking on
FissionYieldCutoffHelper nor AveragedFissionYieldHelper.
The weighted_yields methods for these two classes simply return
the set of constant yields if this is the case.

openmc/deplete/abc.py

@@ -556,8 +556,11 @@ class TalliedFissionYieldHelper(FissionYieldHelper):
         """
         overlap = set(self._chain_nuclides).intersection(set(nuclides))
         if len(overlap) == 0:
+            if self._fission_rate_tally is None:
+                # No tally to update
+                return tuple()
             # tally no nuclides, but keep the Tally alive
-            self._fission_rate_tally.nuclides = None
+            self._fission_rate_tally.nuclides = []
             self._tally_nucs = []
             return tuple()
         nuclides = tuple(sorted(overlap))

openmc/deplete/helpers.py

@@ -390,6 +390,9 @@ class FissionYieldCutoffHelper(TalliedFissionYieldHelper):
 
     def unpack(self):
         """Obtain fast and thermal fission fractions from tally"""
+        if not self._tally_nucs:
+            self.results = None
+            return
         fission_rates = self._fission_rate_tally.results[..., 1].reshape(
             self.n_bmats, 2, len(self._tally_nucs))
         self.results = fission_rates[self._local_indexes]
@@ -424,8 +427,10 @@ class FissionYieldCutoffHelper(TalliedFissionYieldHelper):
         library : dict
             Dictionary of ``{parent: {product: fyield}}``
         """
-        rates = self.results[local_mat_index]
         yields = self.constant_yields
+        if not self._tally_nucs:
+            return yields
+        rates = self.results[local_mat_index]
         # iterate over thermal then fast yields, prefer __mul__ to __rmul__
         for therm_frac, nuc in zip(rates[0], self._tally_nucs):
             yields[nuc.name] = self._thermal_yields[nuc.name] * therm_frac
@@ -528,6 +533,9 @@ class AveragedFissionYieldHelper(TalliedFissionYieldHelper):
 
     def unpack(self):
         """Unpack tallies and populate :attr:`results` with average energies"""
+        if not self._tally_nucs:
+            self.results = None
+            return
         fission_results = (
             self._fission_rate_tally.results[self._local_indexes, :, 1])
         self.results = (
@@ -556,6 +564,8 @@ class AveragedFissionYieldHelper(TalliedFissionYieldHelper):
         library : dict
             Dictionary of ``{parent: {product: fyield}}``
         """
+        if not self._tally_nucs:
+            return self.constant_yields
         mat_yields = {}
         average_energies = self.results[local_mat_index]
         for avg_e, nuc in zip(average_energies, self._tally_nucs):