diff --git a/openmc/data/neutron.py b/openmc/data/neutron.py
index e903b6a92..a53194aa7 100644
--- a/openmc/data/neutron.py
+++ b/openmc/data/neutron.py
@@ -453,7 +453,7 @@ class IncidentNeutron(EqualityMixin):
             if rx.redundant:
                 photon_rx = any(p.particle == 'photon' for p in rx.products)
                 keep_mts = (4, 16, 103, 104, 105, 106, 107,
-                            203, 204, 205, 206, 207, 301, 444)
+                            203, 204, 205, 206, 207, 301, 444, 999)
                 if not (photon_rx or rx.mt in keep_mts):
                     continue
 
diff --git a/tests/unit_tests/test_data_neutron.py b/tests/unit_tests/test_data_neutron.py
index 7fccb33d8..22aebe20a 100644
--- a/tests/unit_tests/test_data_neutron.py
+++ b/tests/unit_tests/test_data_neutron.py
@@ -206,7 +206,7 @@ def test_derived_products(am244):
 
 
 def test_heating(run_in_tmpdir, am244):
-    assert 999 in am244.reactions  # TBD
+    assert 999 in am244.reactions
     strT = min(am244.reactions[1].xs.keys())
     total_xs = am244.reactions[1].xs[strT].y
     fission_xs = am244.reactions[18].xs[strT].y
@@ -215,6 +215,12 @@ def test_heating(run_in_tmpdir, am244):
     heating_number = total_heating / total_xs
     assert no_fission_heating == pytest.approx(
         heating_number * (total_xs - fission_xs))
+    # Re-read to ensure data is written
+    am244.export_to_hdf5("Am244.h5")
+    new = openmc.data.IncidentNeutron.from_hdf5("Am244.h5")
+    assert 999 in new.reactions
+    new_rxn = new.reactions[999].xs[strT].y
+    assert (new_rxn == no_fission_heating).all()
 
 
 def test_urr(pu239):