Use cached property for openmc.data.Decay.sources (#3535)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: GuySten

openmc/data/decay.py

@@ -1,4 +1,5 @@
 from collections.abc import Iterable
+from functools import cached_property
 from io import StringIO
 from math import log
 import re
@@ -339,7 +340,6 @@ def __init__(self, ev_or_filename):
         self.modes = []
         self.spectra = {}
         self.average_energies = {}
-        self._sources = None
 
         # Get head record
         items = get_head_record(file_obj)
@@ -506,14 +506,9 @@ def from_endf(cls, ev_or_filename):
         """
         return cls(ev_or_filename)
 
-    @property
+    @cached_property
     def sources(self):
         """Radioactive decay source distributions"""
-        # If property has been computed already, return it
-        # TODO: Replace with functools.cached_property when support is Python 3.9+
-        if self._sources is not None:
-            return self._sources
-
         sources = {}
         name = self.nuclide['name']
         decay_constant = self.decay_constant.n
@@ -571,8 +566,7 @@ def sources(self):
             merged_sources[particle_type] = combine_distributions(
                 dist_list, [1.0]*len(dist_list))
 
-        self._sources = merged_sources
-        return self._sources
+        return merged_sources
 
 
 _DECAY_PHOTON_ENERGY = {}