Ability to specify a void material. Closes #66 on github.

src/constants.F90

@@ -72,8 +72,10 @@ module constants
   integer, parameter ::  &
        CELL_NORMAL  = 1, & ! Cell with a specified material
        CELL_FILL    = 2, & ! Cell filled by a separate universe
-       CELL_LATTICE = 3, & ! Cell filled with a lattice
-       CELL_VOID    = -1
+       CELL_LATTICE = 3    ! Cell filled with a lattice
+
+  ! Void material
+  integer, parameter :: MATERIAL_VOID = -1
 
   ! Lattice types
   integer, parameter ::  &

src/cross_section.F90

@@ -34,6 +34,9 @@ contains
     material_xs % fission    = ZERO
     material_xs % nu_fission = ZERO
 
+    ! Exit subroutine if material is void
+    if (p % material == MATERIAL_VOID) return
+
     mat => materials(p % material)
 
     ! Find energy index on unionized grid

src/hdf5_interface.F90

@@ -166,9 +166,13 @@ contains
        select case (c % type)
        case (CELL_NORMAL)
           call h5ltmake_dataset_string_f(temp_group, "fill_type", "normal", &
-               hdf5_err) 
-          call hdf5_make_integer(temp_group, "material", &
-               materials(c % material) % id)
+               hdf5_err)
+          if (c % material == MATERIAL_VOID) then
+             call hdf5_make_integer(temp_group, "material", -1)
+          else
+             call hdf5_make_integer(temp_group, "material", &
+                  materials(c % material) % id)
+          end if
        case (CELL_FILL)
           call h5ltmake_dataset_string_f(temp_group, "fill_type", "universe", &
                hdf5_err) 

src/initialize.F90

@@ -445,7 +445,9 @@ contains
        ! ADJUST MATERIAL/FILL POINTERS FOR EACH CELL
 
        id = c % material
-       if (id /= 0) then
+       if (id == MATERIAL_VOID) then
+          c % type = CELL_NORMAL
+       elseif (id /= 0) then
           if (dict_has_key(material_dict, id)) then
              c % type = CELL_NORMAL
              c % material = dict_get_key(material_dict, id)

src/input_xml.F90

@@ -368,9 +368,29 @@ contains
        ! Copy data into cells
        c % id       = cell_(i) % id
        c % universe = cell_(i) % universe
-       c % material = cell_(i) % material
        c % fill     = cell_(i) % fill
 
+       ! Read material
+       word = cell_(i) % material
+       call lower_case(word)
+       select case(word)
+       case ('void')
+          c % material = MATERIAL_VOID
+
+       case ('')
+          ! This case is called if no material was specified
+          c % material = 0
+
+       case default
+          c % material = str_to_int(word)
+
+          ! Check for error
+          if (c % material == ERROR_INT) then
+             message = "Invalid material specified on cell " // to_str(c % id)
+             call fatal_error()
+          end if
+       end select
+
        ! Check to make sure that either material or fill was specified
        if (c % material == 0 .and. c % fill == 0) then
           message = "Neither material nor fill was specified for cell " // & 

src/output.F90

@@ -344,6 +344,8 @@ contains
     ! Write information on material
     if (c % material == 0) then
        write(unit_,*) '    Material = NONE'
+    elseif (c % material == MATERIAL_VOID) then
+       write(unit_,*) '    Material = Void'
     else
        m => materials(c % material)
        write(unit_,*) '    Material = ' // to_str(m % id)

src/xml-fortran/templates/geometry_t.xml

@@ -8,7 +8,7 @@
   <typedef name="cell_xml">
     <component name="id" type="integer" />
     <component name="universe" type="integer" default="0" />
-    <component name="material" type="integer" default="0" />
+    <component name="material" type="word" length="12" default="''" />
     <component name="fill" type="integer" default="0" />
     <component name="surfaces" type="integer-array" />
   </typedef>