diff --git a/src/cmfd_data.F90 b/src/cmfd_data.F90
index 6555e24d79..52c6bc945b 100644
--- a/src/cmfd_data.F90
+++ b/src/cmfd_data.F90
@@ -12,7 +12,7 @@ contains
 
   subroutine set_up_cmfd()
 
-    use global,       only: cmfd
+    use global,       only: cmfd,cmfd_coremap
     use cmfd_header,  only: allocate_cmfd
     use cmfd_output,  only: neutron_balance,write_cmfd_hdf5
 
@@ -26,7 +26,7 @@ contains
     call neutron_balance()
 
     ! check for core map
-    if (allocated(cmfd % coremap)) then
+    if (cmfd_coremap) then
       call set_coremap()
     end if
 
@@ -117,7 +117,7 @@ contains
 
             ! detect zero flux
             if ((flux - 0.0D0) < 1.0D-10) then
-              if (.not. allocated(cmfd%coremap)) then
+              if (.not. cmfd_coremap) then
                 write(*,*) 'Fatal: detected zero flux without coremap'
                 stop
               else
@@ -238,7 +238,7 @@ contains
 
   subroutine compute_dtilde()
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     ! local variables
     integer :: nx                 ! maximum number of cells in x direction
@@ -323,7 +323,7 @@ contains
                 neig_hxyz = cmfd%hxyz(:,neig_idx(1),neig_idx(2),neig_idx(3))
 
                 ! check for fuel-reflector interface
-                if (allocated(cmfd % coremap)) then
+                if (cmfd_coremap) then
 
                   if (cmfd % coremap(neig_idx(1),neig_idx(2),neig_idx(3)) ==   &
                  &    99999 .and. cmfd % coremap(i,j,k) /= 99999) then
@@ -374,7 +374,7 @@ contains
 
   subroutine compute_dhat()
 
-    use global, only:cmfd 
+    use global, only:cmfd,cmfd_coremap 
 
     ! local variables
     integer :: nx                 ! maximum number of cells in x direction
@@ -458,7 +458,7 @@ contains
                      product(cmfd%hxyz(:,neig_idx(1),neig_idx(2),neig_idx(3)))
 
                 ! check for fuel-reflector interface
-                if (allocated(cmfd % coremap)) then
+                if (cmfd_coremap) then
 
                   if (cmfd % coremap(neig_idx(1),neig_idx(2),neig_idx(3)) ==   &
                  &    99999 .and. cmfd % coremap(i,j,k) /= 99999) then
diff --git a/src/cmfd_input.F90 b/src/cmfd_input.F90
index f048134a1a..8d198fb5f6 100644
--- a/src/cmfd_input.F90
+++ b/src/cmfd_input.F90
@@ -51,10 +51,11 @@ contains
       allocate(cmfd % coremap(cmfd % indices(1), cmfd % indices(2),            &
      &         cmfd % indices(3)))
       cmfd % coremap = reshape(mesh_ % map,(cmfd % indices(1:3)))
+      cmfd_coremap = .TRUE.
    end if
 
     ! check for core map activation by printing note
-    if (allocated(cmfd % coremap)) print *,"Core Map Overlay Activated"
+    if (cmfd_coremap) print *,"Core Map Overlay Activated"
 
     ! create tally objects
     call create_cmfd_tally()
@@ -301,7 +302,7 @@ contains
   subroutine read_cmfd_hdf5()
 
     use cmfd_header, only: allocate_cmfd
-    use global,      only: cmfd
+    use global,      only: cmfd,cmfd_coremap
 
 #ifdef HDF5
     use global, only: hdf5_output_file,hdf5_err
@@ -323,6 +324,8 @@ contains
     integer :: nz ! number of mesh cells in z direction
     integer :: ng ! number of energy groups
 
+    integer :: core_map_int 
+
     ! open output file
     call hdf5_open_output()
 
@@ -407,17 +410,51 @@ contains
     call h5dread_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%hxyz,dim4,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
 
-    ! read coremap to cmfd object
-    call h5dopen_f(hdf5_output_file,"cmfd/coremap",dataset_id,hdf5_err)
-    dim3 = (/nx,ny,nz/)
-    call h5dread_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%coremap,dim3,hdf5_err)
+    ! read in core_map logical 
+    call h5dopen_f(hdf5_output_file,"cmfd/coremap_active",dataset_id,hdf5_err)
+    dim1 = (/1/)
+    call h5dread_f(dataset_id,H5T_NATIVE_INTEGER,core_map_int,dim1,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
 
-    ! read mat_dim to cmfd object
-    call h5dopen_f(hdf5_output_file,"cmfd/mat_dim",dataset_id,hdf5_err)
-    dim1 = (/1/)
-    call h5dread_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%mat_dim,dim1,hdf5_err)
-    call h5dclose_f(dataset_id,hdf5_err)
+    ! now set coremap depending on logical
+    select case(core_map_int)
+
+      ! core map is not active
+      case(0)
+
+        ! set logical to false
+        cmfd_coremap = .FALSE.
+
+      ! core map is active
+      case(1)
+
+        ! set logical to true
+        cmfd_coremap = .TRUE.
+
+        ! allocate coremap in cmfd obj
+        allocate(cmfd % coremap(cmfd % indices(1), cmfd % indices(2),          &
+       &         cmfd % indices(3)))
+
+
+        ! read coremap to cmfd object
+        call h5dopen_f(hdf5_output_file,"cmfd/coremap",dataset_id,hdf5_err)
+        dim3 = (/nx,ny,nz/)
+        call h5dread_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%coremap,dim3,hdf5_err)
+        call h5dclose_f(dataset_id,hdf5_err)
+
+        ! read mat_dim to cmfd object
+        call h5dopen_f(hdf5_output_file,"cmfd/mat_dim",dataset_id,hdf5_err)
+        dim1 = (/1/)
+        call h5dread_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%mat_dim,dim1,hdf5_err)
+        call h5dclose_f(dataset_id,hdf5_err)
+
+      ! something is wrong
+      case default
+
+        write(*,*) 'FATAL ==> Could not detect core map in hdf5 output file'
+        stop
+
+      end select
 
     ! close output file
     call hdf5_close_output()
diff --git a/src/cmfd_loss_operator.F90 b/src/cmfd_loss_operator.F90
index 774b19b251..e0518b0b99 100644
--- a/src/cmfd_loss_operator.F90
+++ b/src/cmfd_loss_operator.F90
@@ -48,7 +48,7 @@ contains
 
   subroutine get_M_indices(this)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     type(loss_operator) :: this
 
@@ -62,7 +62,7 @@ contains
     this%nnz = 7 + ng - 1
 
     ! calculate dimensions of matrix
-    if (allocated(cmfd % coremap)) then
+    if (cmfd_coremap) then
       this%n = cmfd % mat_dim * ng
     else
       this%n = nx*ny*nz*ng
@@ -76,7 +76,7 @@ contains
 
   subroutine build_loss_matrix(this)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     type(loss_operator) :: this
 
@@ -123,7 +123,7 @@ contains
         XLOOP: do i = 1,nx
 
           ! check if not including reflector
-          if (allocated(cmfd % coremap)) then
+          if (cmfd_coremap) then
 
             ! check if at a reflector
             if (cmfd % coremap(i,j,k) == 99999) then
@@ -170,7 +170,7 @@ contains
               if (bound(l) /= nxyz(xyz_idx,dir_idx)) then
 
                 ! check for core map
-                if (allocated(cmfd % coremap)) then
+                if (cmfd_coremap) then
 
                   ! check that neighbor is not reflector
                   if (cmfd % coremap(neig_idx(1),neig_idx(2),neig_idx(3)) /=   &
@@ -272,7 +272,7 @@ contains
 
   function get_matrix_idx(g,i,j,k)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     ! arguments
     integer :: get_matrix_idx  ! the index location in matrix
@@ -285,7 +285,7 @@ contains
     integer :: nidx            ! index in matrix
 
     ! check if coremap is used
-    if (allocated(cmfd % coremap)) then
+    if (cmfd_coremap) then
 
       ! get idx from core map
       nidx = ng*(cmfd % coremap(i,j,k)) - (ng - g)
diff --git a/src/cmfd_output.F90 b/src/cmfd_output.F90
index 77055c62ee..f1136ff849 100644
--- a/src/cmfd_output.F90
+++ b/src/cmfd_output.F90
@@ -112,7 +112,7 @@ contains
 
   subroutine write_cmfd_hdf5()
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
 #ifdef HDF5
     use hdf5
@@ -144,8 +144,8 @@ contains
     ! write indices from cmfd object
     dim1 = (/4/)
     call h5screate_simple_f(1,dim1,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/indices",H5T_NATIVE_INTEGER,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/indices",H5T_NATIVE_INTEGER,       &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%indices,dim1,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -153,8 +153,8 @@ contains
     ! write totalxs from cmfd object
     dim4 = (/ng,nx,ny,nz/)
     call h5screate_simple_f(4,dim4,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/totalxs",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/totalxs",H5T_NATIVE_DOUBLE,        &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%totalxs,dim4,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -162,8 +162,8 @@ contains
     ! write p1scattxs from cmfd object
     dim4 = (/ng,nx,ny,nz/)
     call h5screate_simple_f(4,dim4,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/p1scattxs",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/p1scattxs",H5T_NATIVE_DOUBLE,      &
+   &                dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%p1scattxs,dim4,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -171,8 +171,8 @@ contains
     ! write scattxs from cmfd object
     dim5 = (/ng,ng,nx,ny,nz/)
     call h5screate_simple_f(5,dim5,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/scattxs",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/scattxs",H5T_NATIVE_DOUBLE,        &
+   &                dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%scattxs,dim5,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -180,8 +180,8 @@ contains
     ! write nfissxs from cmfd object
     dim5 = (/ng,ng,nx,ny,nz/)
     call h5screate_simple_f(5,dim5,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/nfissxs",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/nfissxs",H5T_NATIVE_DOUBLE,        &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%nfissxs,dim5,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -189,8 +189,8 @@ contains
     ! write diffcof from cmfd object
     dim4 = (/ng,nx,ny,nz/)
     call h5screate_simple_f(4,dim4,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/diffcof",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/diffcof",H5T_NATIVE_DOUBLE,        &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%diffcof,dim4,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -198,8 +198,8 @@ contains
     ! write current from cmfd object
     dim5 = (/12,ng,nx,ny,nz/)
     call h5screate_simple_f(5,dim5,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/current",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/current",H5T_NATIVE_DOUBLE,        &
+   &                dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%current,dim5,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -207,8 +207,8 @@ contains
    ! write flux from cmfd object
     dim4 = (/ng,nx,ny,nz/)
     call h5screate_simple_f(4,dim4,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/flux",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/flux",H5T_NATIVE_DOUBLE,           &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%flux,dim4,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -216,8 +216,8 @@ contains
     ! write dtilde from cmfd object
     dim5 = (/6,ng,nx,ny,nz/)
     call h5screate_simple_f(5,dim5,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/dtilde",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/dtilde",H5T_NATIVE_DOUBLE,         &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%dtilde,dim5,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -225,8 +225,8 @@ contains
     ! write dhat from cmfd object
     dim5 = (/6,ng,nx,ny,nz/)
     call h5screate_simple_f(5,dim5,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/dhat",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/dhat",H5T_NATIVE_DOUBLE,           &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%dhat,dim5,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -234,8 +234,8 @@ contains
     ! write albedo from cmfd object
     dim1 = (/6/)
     call h5screate_simple_f(1,dim1,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/albedo",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/albedo",H5T_NATIVE_DOUBLE,         &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%albedo,dim1,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
@@ -243,29 +243,54 @@ contains
     ! write hxyz from cmfd object
     dim4 = (/3,nx,ny,nz/)
     call h5screate_simple_f(4,dim4,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/hxyz",H5T_NATIVE_DOUBLE,dataspace_id, &
-   & dataset_id,hdf5_err)
+    call h5dcreate_f(hdf5_output_file,"cmfd/hxyz",H5T_NATIVE_DOUBLE,           &
+   &                 dataspace_id,dataset_id,hdf5_err)
     call h5dwrite_f(dataset_id,H5T_NATIVE_DOUBLE,cmfd%hxyz,dim4,hdf5_err)
     call h5sclose_f(dataspace_id,hdf5_err)
     call h5dclose_f(dataset_id,hdf5_err)
 
-    ! write coremap from cmfd object
-    dim3 = (/nx,ny,nz/)
-    call h5screate_simple_f(3,dim3,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/coremap",H5T_NATIVE_INTEGER,dataspace_id, &
-   & dataset_id,hdf5_err)
-    call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%coremap,dim3,hdf5_err)
-    call h5sclose_f(dataspace_id,hdf5_err)
-    call h5dclose_f(dataset_id,hdf5_err)
+    ! only write the following if core map is active
+    if (cmfd_coremap) then
 
-    ! write mat_dim from cmfd object
-    dim1 = (/1/)
-    call h5screate_simple_f(1,dim1,dataspace_id,hdf5_err)
-    call h5dcreate_f(hdf5_output_file,"cmfd/mat_dim",H5T_NATIVE_INTEGER,dataspace_id, &
-   & dataset_id,hdf5_err)
-    call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%mat_dim,dim1,hdf5_err)
-    call h5sclose_f(dataspace_id,hdf5_err)
-    call h5dclose_f(dataset_id,hdf5_err)
+      ! write coremap from cmfd object
+      dim3 = (/nx,ny,nz/)
+      call h5screate_simple_f(3,dim3,dataspace_id,hdf5_err)
+      call h5dcreate_f(hdf5_output_file,"cmfd/coremap",H5T_NATIVE_INTEGER,     &
+     &                 dataspace_id,dataset_id,hdf5_err)
+      call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%coremap,dim3,hdf5_err)
+      call h5sclose_f(dataspace_id,hdf5_err)
+      call h5dclose_f(dataset_id,hdf5_err)
+
+      ! write mat_dim from cmfd object
+      dim1 = (/1/)
+      call h5screate_simple_f(1,dim1,dataspace_id,hdf5_err)
+      call h5dcreate_f(hdf5_output_file,"cmfd/mat_dim",H5T_NATIVE_INTEGER,     &
+     &                 dataspace_id,dataset_id,hdf5_err)
+      call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,cmfd%mat_dim,dim1,hdf5_err)
+      call h5sclose_f(dataspace_id,hdf5_err)
+      call h5dclose_f(dataset_id,hdf5_err)
+
+      ! write core map logical to true (1)
+      dim1 = (/1/)
+      call h5screate_simple_f(1,dim1,dataspace_id,hdf5_err)
+      call h5dcreate_f(hdf5_output_file,"cmfd/coremap_active",                 &
+     &                 H5T_NATIVE_INTEGER,dataspace_id,dataset_id,hdf5_err)
+      call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,1,dim1,hdf5_err)
+      call h5sclose_f(dataspace_id,hdf5_err)
+      call h5dclose_f(dataset_id,hdf5_err)
+
+    else
+
+      ! write core map logical to false (0)
+      dim1 = (/1/)
+      call h5screate_simple_f(1,dim1,dataspace_id,hdf5_err)
+      call h5dcreate_f(hdf5_output_file,"cmfd/coremap_active",                 &
+     &                 H5T_NATIVE_INTEGER,dataspace_id,dataset_id,hdf5_err)
+      call h5dwrite_f(dataset_id,H5T_NATIVE_INTEGER,0,dim1,hdf5_err)
+      call h5sclose_f(dataspace_id,hdf5_err)
+      call h5dclose_f(dataset_id,hdf5_err)
+
+    end if
 
     ! close the CMFD group
     call h5gclose_f(group_id, hdf5_err)
@@ -278,7 +303,7 @@ contains
 
   subroutine write_cmfd_vtk()
 
-    use global,      only: cmfd,meshes
+    use global,      only: cmfd,cmfd_coremap,meshes
     use mesh_header, only: StructuredMesh
     use vtk_writer
 
@@ -339,7 +364,7 @@ contains
         XLOOP: do i = 1,nx
 
           ! check for non accelerated region
-          if (allocated(cmfd%coremap)) then
+          if (cmfd_coremap) then
             if (cmfd%coremap(i,j,k) == 99999) then
               cycle
             end if
@@ -405,7 +430,7 @@ contains
 
   function get_matrix_idx(g,i,j,k)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     ! arguments
     integer :: get_matrix_idx  ! the index location in matrix
@@ -418,7 +443,7 @@ contains
     integer :: nidx            ! index in matrix
 
     ! check if coremap is used
-    if (allocated(cmfd % coremap)) then
+    if (cmfd_coremap) then
 
       ! get idx from core map
       nidx = ng*(cmfd % coremap(i,j,k)) - (ng - g)
diff --git a/src/cmfd_prod_operator.F90 b/src/cmfd_prod_operator.F90
index 77a594be9a..d75caa67d2 100644
--- a/src/cmfd_prod_operator.F90
+++ b/src/cmfd_prod_operator.F90
@@ -48,7 +48,7 @@ contains
 
   subroutine get_F_indices(this)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     type(prod_operator) :: this
 
@@ -62,7 +62,7 @@ contains
     this%nnz = 7 + ng - 1
 
     ! calculate dimensions of matrix
-    if (allocated(cmfd % coremap)) then
+    if (cmfd_coremap) then
       this%n = cmfd % mat_dim * ng
     else
       this%n = nx*ny*nz*ng
@@ -76,7 +76,7 @@ contains
 
   subroutine build_prod_matrix(this)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     type(prod_operator) :: this
 
@@ -103,7 +103,7 @@ contains
         XLOOP: do i = 1,nx
 
           ! check if not including reflector
-          if (allocated(cmfd % coremap)) then
+          if (cmfd_coremap) then
 
             ! check if at a reflector
             if (cmfd % coremap(i,j,k) == 99999) then
@@ -152,7 +152,7 @@ contains
 
   function get_matrix_idx(g,i,j,k)
 
-    use global, only: cmfd
+    use global, only: cmfd,cmfd_coremap
 
     ! arguments
     integer :: get_matrix_idx  ! the index location in matrix
@@ -165,7 +165,7 @@ contains
     integer :: nidx            ! index in matrix
 
     ! check if coremap is used
-    if (allocated(cmfd % coremap)) then
+    if (cmfd_coremap) then
 
       ! get idx from core map
       nidx = ng*(cmfd % coremap(i,j,k)) - (ng - g)
diff --git a/src/global.F90 b/src/global.F90
index 7224dfe428..0f0661eecc 100644
--- a/src/global.F90
+++ b/src/global.F90
@@ -211,6 +211,9 @@ module global
   ! Flag for CMFD only
   logical :: cmfd_only = .FALSE.
 
+  ! Flag for coremap accelerator
+  logical :: cmfd_coremap = .FALSE.
+
 contains
 
 !===============================================================================