Store fission heating IncidentNeutron.from_ace

IncidentNeutron.from_ace now computes the fission heating
and a fission-less heating coefficient. The fission heating
is the product of the heating number and the fission cross
section, and stored as MT318.

The fission-less heating is the heating from all reactions
except fission, computed as heating_number * (total_xs - fission_xs).
The MT number for this is taken to be 999 as a temporary value.

A test is added for Am244 in test_data_neutron.py to examine the
new heating values

openmc/data/neutron.py

@@ -633,6 +633,18 @@ class IncidentNeutron(EqualityMixin):
             rx = Reaction.from_ace(ace, i)
             data.reactions[rx.mt] = rx
 
+        # If present, use fission xs to compute "fission heating" coefficient
+        fission_reaction = data.reactions.get(18)
+        if fission_reaction is not None:
+            fission_xs = fission_reaction.xs[strT].y
+
+            # Compute "fission-less" heating coefficient
+            no_fission_heating = Reaction(999)
+            no_fission_heating.xs[strT] = Tabulated1D(
+                energy, heating_number * (total_xs - fission_xs))
+            no_fission_heating.redundant = True
+            data.reactions[999] = no_fission_heating
+
         # Some photon production reactions may be assigned to MTs that don't
         # exist, usually MT=4. In this case, we create a new reaction and add
         # them

tests/unit_tests/test_data_neutron.py

@@ -205,6 +205,18 @@ def test_derived_products(am244):
     assert total_neutron.yield_(6e6) == pytest.approx(4.2558)
 
 
+def test_heating(run_in_tmpdir, am244):
+    assert 999 in am244.reactions  # TBD
+    strT = min(am244.reactions[1].xs.keys())
+    total_xs = am244.reactions[1].xs[strT].y
+    fission_xs = am244.reactions[18].xs[strT].y
+    total_heating = am244.reactions[301].xs[strT].y
+    no_fission_heating = am244.reactions[999].xs[strT].y
+    heating_number = total_heating / total_xs
+    assert no_fission_heating == pytest.approx(
+        heating_number * (total_xs - fission_xs))
+
+
 def test_urr(pu239):
     for T, ptable in pu239.urr.items():
         assert T.endswith('K')