diff --git a/src/nuclide_header.F90 b/src/nuclide_header.F90
index 1831f0a89..2c5de1b14 100644
--- a/src/nuclide_header.F90
+++ b/src/nuclide_header.F90
@@ -87,9 +87,11 @@ module nuclide_header
 
     ! Reactions
     type(Reaction), allocatable :: reactions(:)
-    !type(DictIntInt) :: reaction_index ! map MT values to index in reactions
-                                       ! array; used at tally-time
-    integer, allocatable         :: rxn_index_MT(:)
+
+    ! Array that maps MT values to index in reactions; used at tally-time. Note
+    ! that ENDF-102 does not assign any MT values above 891.
+    integer :: rxn_index_MT(891)
+
     ! Fission energy release
     class(Function1D), allocatable :: fission_q_prompt ! prompt neutrons, gammas
     class(Function1D), allocatable :: fission_q_recov  ! neutrons, gammas, betas
@@ -561,8 +563,7 @@ contains
 
     n_temperature = size(this % kTs)
     allocate(this % sum_xs(n_temperature))
-    allocate(this % rxn_index_MT(1000))
-    this % rxn_index_MT(:)=0
+    this % rxn_index_MT(:) = 0
     do i = 1, n_temperature
       ! Allocate and initialize derived cross sections
       n_grid = size(this % grid(i) % energy)