diff --git a/src/relaxng/settings.rnc b/src/relaxng/settings.rnc
index 19a2948c58..282c685db1 100644
--- a/src/relaxng/settings.rnc
+++ b/src/relaxng/settings.rnc
@@ -3,6 +3,8 @@ element settings {
 
   element confidence_intervals { xsd:boolean }? &
 
+  element create_fission_neutrons { xsd:boolean }? &
+
   element cutoff {
     (element weight { xsd:double } | attribute weight { xsd:double })? &
     (element weight_avg { xsd:double } | attribute weight_avg { xsd:double })?
diff --git a/src/relaxng/settings.rng b/src/relaxng/settings.rng
index 606b978290..0018cb16bd 100644
--- a/src/relaxng/settings.rng
+++ b/src/relaxng/settings.rng
@@ -11,6 +11,11 @@
         <data type="boolean"/>
       </element>
     </optional>
+    <optional>
+      <element name="create_fission_neutrons">
+        <data type="boolean"/>
+      </element>
+    </optional>
     <optional>
       <element name="cutoff">
         <interleave>