Fix Mgxs::get_xs CHI_DELAYED indexing the wrong tensor when gout is null

include/openmc/xsdata.h

@@ -87,7 +87,7 @@ class XsData {
   // [angle][incoming group][outgoing group]
   tensor::Tensor<double> chi_prompt;
   // chi_delayed has the following dimensions:
-  // [angle][incoming group][outgoing group][delayed group]
+  // [angle][delayed group][incoming group][outgoing group]
   tensor::Tensor<double> chi_delayed;
   // scatter has the following dimensions: [angle]
   vector<std::shared_ptr<ScattData>> scatter;

src/mgxs.cpp

@@ -490,16 +490,19 @@ double Mgxs::get_xs(MgxsType xstype, int gin, const int* gout, const double* mu,
           val = xs_t->chi_delayed(a, 0, gin, *gout);
         }
       } else {
+        // sum of chi_delayed over outgoing groups; chi_delayed has shape
+        // [angle][delayed group][in group][out group] (the previous code
+        // indexed the rank-3 delayed_nu_fission tensor with four indices)
         if (dg != nullptr) {
           val = 0.;
-          for (int g = 0; g < xs_t->delayed_nu_fission.shape(2); g++) {
-            val += xs_t->delayed_nu_fission(a, *dg, gin, g);
+          for (int g = 0; g < xs_t->chi_delayed.shape(3); g++) {
+            val += xs_t->chi_delayed(a, *dg, gin, g);
           }
         } else {
           val = 0.;
-          for (int g = 0; g < xs_t->delayed_nu_fission.shape(2); g++) {
-            for (int d = 0; d < xs_t->delayed_nu_fission.shape(3); d++) {
-              val += xs_t->delayed_nu_fission(a, d, gin, g);
+          for (int d = 0; d < xs_t->chi_delayed.shape(1); d++) {
+            for (int g = 0; g < xs_t->chi_delayed.shape(3); g++) {
+              val += xs_t->chi_delayed(a, d, gin, g);
             }
           }
         }