diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 89a4a60e1..d3785aa6a 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -116,7 +116,8 @@ elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "PGI")
 
 elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "XL")
   # IBM XL compiler options
-  set(f90flags  "-WF,-DNO_F2008 -O2")
+  set(f90flags  "-O2")
+  add_definitions(-DNO_F2008)
   if(debug)
     set(f90flags  "-g -C -qflag=i:i -u")
     set(ldflags   "-g")
diff --git a/src/cross_section.F90 b/src/cross_section.F90
index dce4d75f3..c8a567938 100644
--- a/src/cross_section.F90
+++ b/src/cross_section.F90
@@ -35,8 +35,7 @@ contains
     integer :: j             ! index in mat % i_sab_nuclides
     real(8) :: atom_density  ! atom density of a nuclide
     logical :: check_sab     ! should we check for S(a,b) table?
-    type(Material), pointer, save :: mat => null() ! current material
-!$omp threadprivate(mat)
+    type(Material), pointer :: mat ! current material
 
     ! Set all material macroscopic cross sections to zero
     material_xs % total          = ZERO
@@ -150,9 +149,8 @@ contains
     integer :: u      ! index into logarithmic mapping array
     real(8), intent(in) :: E ! energy
     real(8) :: f             ! interp factor on nuclide energy grid
-    type(Nuclide),  pointer, save :: nuc => null()
-    type(Material), pointer, save :: mat => null()
-!$omp threadprivate(nuc, mat)
+    type(Nuclide),  pointer :: nuc
+    type(Material), pointer :: mat
 
     ! Set pointer to nuclide and material
     nuc => nuclides(i_nuclide)
@@ -286,8 +284,7 @@ contains
     real(8) :: f         ! interp factor on S(a,b) energy grid
     real(8) :: inelastic ! S(a,b) inelastic cross section
     real(8) :: elastic   ! S(a,b) elastic cross section
-    type(SAlphaBeta), pointer, save :: sab => null()
-!$omp threadprivate(sab)
+    type(SAlphaBeta), pointer :: sab
 
     ! Set flag that S(a,b) treatment should be used for scattering
     micro_xs(i_nuclide) % index_sab = i_sab
@@ -379,10 +376,9 @@ contains
     real(8) :: fission      ! fission cross section
     real(8) :: inelastic    ! inelastic cross section
     logical :: same_nuc     ! do we know the xs for this nuclide at this energy?
-    type(UrrData),  pointer, save :: urr      => null()
-    type(Nuclide),  pointer, save :: nuc      => null()
-    type(Reaction), pointer, save :: rxn      => null()
-!$omp threadprivate(urr, nuc, rxn)
+    type(UrrData),  pointer :: urr
+    type(Nuclide),  pointer :: nuc
+    type(Reaction), pointer :: rxn
 
     micro_xs(i_nuclide) % use_ptable = .true.
 
@@ -528,12 +524,11 @@ contains
 
   subroutine find_energy_index(E, i_mat)
 
-    real(8), intent(in) :: E     ! energy of particle
-    integer, intent(in) :: i_mat ! material index
-    type(Material), pointer, save :: mat => null() ! pointer to current material
-!$omp threadprivate(mat)
+    real(8), intent(in) :: E       ! energy of particle
+    integer, intent(in) :: i_mat   ! material index
+    type(Material), pointer :: mat ! pointer to current material
 
-    mat => materials(i_mat)      
+    mat => materials(i_mat)
 
     ! if the energy is outside of energy grid range, set to first or last
     ! index. Otherwise, do a binary search through the union energy grid.
diff --git a/src/energy_grid.F90 b/src/energy_grid.F90
index 676875a5b..9bde48a5e 100644
--- a/src/energy_grid.F90
+++ b/src/energy_grid.F90
@@ -26,10 +26,9 @@ contains
 
     integer :: i ! index in nuclides array
     integer :: j ! index in materials array
-    type(ListReal), pointer, save :: list => null()
-    type(Nuclide),  pointer, save :: nuc  => null()
-    type(Material), pointer, save :: mat  => null()
-    !$omp threadprivate(list, nuc, mat)
+    type(ListReal) :: list
+    type(Nuclide),  pointer :: nuc
+    type(Material), pointer :: mat
 
     call write_message("Creating unionized energy grid...", 5)
 
@@ -52,7 +51,6 @@ contains
 
       ! delete linked list and dictionary
       call list % clear()
-      deallocate(list)
     end do
 
     ! Set pointers to unionized energy grid for each nuclide
@@ -72,7 +70,7 @@ contains
     real(8) :: E_max                 ! Maximum energy in MeV
     real(8) :: E_min                 ! Minimum energy in MeV
     real(8), allocatable :: umesh(:) ! Equally log-spaced energy grid
-    type(Nuclide), pointer :: nuc => null()
+    type(Nuclide), pointer :: nuc
 
     ! Set minimum/maximum energies
     E_max = 20.0_8
@@ -116,7 +114,7 @@ contains
 
   subroutine add_grid_points(list, energy)
 
-    type(ListReal), pointer :: list
+    type(ListReal) :: list
     real(8), intent(in) :: energy(:)
 
     integer :: i       ! index in energy array
@@ -127,16 +125,6 @@ contains
     i = 1
     n = size(energy)
 
-    ! If the original list is empty, we need to allocate the first element and
-    ! store first energy point
-    if (.not. associated(list)) then
-      allocate(list)
-      do i = 1, n
-        call list % append(energy(i))
-      end do
-      return
-    end if
-
     ! Set current index to beginning of the list
     current = 1
 
@@ -189,8 +177,8 @@ contains
     integer :: index_e      ! index on union energy grid
     real(8) :: union_energy ! energy on union grid
     real(8) :: energy       ! energy on nuclide grid
-    type(Nuclide),  pointer :: nuc => null()
-    type(Material), pointer :: mat => null()
+    type(Nuclide),  pointer :: nuc
+    type(Material), pointer :: mat
 
     do k = 1, n_materials
       mat => materials(k)
diff --git a/src/geometry.F90 b/src/geometry.F90
index c4837e105..499ae5aa5 100644
--- a/src/geometry.F90
+++ b/src/geometry.F90
@@ -30,8 +30,7 @@ contains
     integer :: i_surface       ! index in surfaces array (with sign)
     logical :: specified_sense ! specified sense of surface in list
     logical :: actual_sense    ! sense of particle wrt surface
-    type(Surface), pointer, save :: s => null()
-!$omp threadprivate(s)
+    type(Surface), pointer :: s
 
     SURFACE_LOOP: do i = 1, c % n_surfaces
       ! Lookup surface
@@ -79,10 +78,9 @@ contains
     integer :: i                       ! cell loop index on a level
     integer :: n                       ! number of cells to search on a level
     integer :: index_cell              ! index in cells array
-    type(Cell),       pointer, save :: c => null()     ! pointer to cell
-    type(Universe),   pointer, save :: univ => null()  ! universe to search in
-    type(LocalCoord), pointer, save :: coord => null() ! particle coordinate to search on
-!$omp threadprivate(c, univ, coord)
+    type(Cell),       pointer :: c     ! pointer to cell
+    type(Universe),   pointer :: univ  ! universe to search in
+    type(LocalCoord), pointer :: coord ! particle coordinate to search on
 
     coord => p % coord0
 
@@ -132,15 +130,14 @@ contains
     logical,        intent(inout) :: found
     integer,        optional      :: search_cells(:)
 
-    integer :: i                       ! index over cells
-    integer :: i_xyz(3)                ! indices in lattice
-    integer :: n                       ! number of cells to search
-    integer :: index_cell              ! index in cells array
-    logical :: use_search_cells        ! use cells provided as argument
-    type(Cell),     pointer, save :: c => null()    ! pointer to cell
-    class(Lattice), pointer, save :: lat => null()  ! pointer to lattice
-    type(Universe), pointer, save :: univ => null() ! universe to search in
-!$omp threadprivate(c, lat, univ)
+    integer :: i                    ! index over cells
+    integer :: i_xyz(3)             ! indices in lattice
+    integer :: n                    ! number of cells to search
+    integer :: index_cell           ! index in cells array
+    logical :: use_search_cells     ! use cells provided as argument
+    type(Cell),     pointer :: c    ! pointer to cell
+    class(Lattice), pointer :: lat  ! pointer to lattice
+    type(Universe), pointer :: univ ! universe to search in
 
     ! Remove coordinates for any lower levels
     call deallocate_coord(p % coord % next)
@@ -294,8 +291,7 @@ contains
     real(8) :: norm      ! "norm" of surface normal
     integer :: i_surface ! index in surfaces
     logical :: found     ! particle found in universe?
-    type(Surface), pointer, save :: surf => null()
-!$omp threadprivate(surf)
+    type(Surface), pointer :: surf
 
     i_surface = abs(p % surface)
     surf => surfaces(i_surface)
@@ -575,9 +571,8 @@ contains
 
     integer :: i_xyz(3)       ! indices in lattice
     logical :: found          ! particle found in cell?
-    class(Lattice),   pointer, save :: lat => null()
-    type(LocalCoord), pointer, save :: parent_coord => null()
-!$omp threadprivate(lat, parent_coord)
+    class(Lattice),   pointer :: lat
+    type(LocalCoord), pointer :: parent_coord
 
     lat => lattices(p % coord % lattice) % obj
 
@@ -598,9 +593,9 @@ contains
     p % coord % lattice_x = p % coord % lattice_x + lattice_translation(1)
     p % coord % lattice_y = p % coord % lattice_y + lattice_translation(2)
     p % coord % lattice_z = p % coord % lattice_z + lattice_translation(3)
-    i_xyz(1) = p % coord % lattice_x 
-    i_xyz(2) = p % coord % lattice_y 
-    i_xyz(3) = p % coord % lattice_z 
+    i_xyz(1) = p % coord % lattice_x
+    i_xyz(2) = p % coord % lattice_y
+    i_xyz(3) = p % coord % lattice_z
 
     ! Set the new coordinate position.
     p % coord % xyz = lat % get_local_xyz(parent_coord % xyz, i_xyz)
@@ -676,13 +671,12 @@ contains
     real(8) :: a,b,c,k            ! quadratic equation coefficients
     real(8) :: quad               ! discriminant of quadratic equation
     logical :: on_surface         ! is particle on surface?
-    type(Cell),       pointer, save :: cl => null()
-    type(Surface),    pointer, save :: surf => null()
-    class(Lattice),   pointer, save :: lat => null()
-    type(LocalCoord), pointer, save :: coord => null()
-    type(LocalCoord), pointer, save :: final_coord => null()
-    type(LocalCoord), pointer, save :: parent_coord => null()
-!$omp threadprivate(cl, surf, lat, coord, final_coord, parent_coord)
+    type(Cell),       pointer :: cl
+    type(Surface),    pointer :: surf
+    class(Lattice),   pointer :: lat
+    type(LocalCoord), pointer :: coord
+    type(LocalCoord), pointer :: final_coord
+    type(LocalCoord), pointer :: parent_coord
 
     ! inialize distance to infinity (huge)
     dist = INFINITY
diff --git a/src/physics.F90 b/src/physics.F90
index 69bbcff34..0b276c9fc 100644
--- a/src/physics.F90
+++ b/src/physics.F90
@@ -74,8 +74,7 @@ contains
 
     integer :: i_nuclide    ! index in nuclides array
     integer :: i_reaction   ! index in nuc % reactions array
-    type(Nuclide), pointer, save :: nuc => null()
-!$omp threadprivate(nuc)
+    type(Nuclide), pointer :: nuc
 
     i_nuclide = sample_nuclide(p, 'total  ')
 
@@ -133,8 +132,7 @@ contains
     real(8) :: cutoff
     real(8) :: atom_density ! atom density of nuclide in atom/b-cm
     real(8) :: sigma        ! microscopic total xs for nuclide
-    type(Material), pointer, save :: mat => null()
-!$omp threadprivate(mat)
+    type(Material), pointer :: mat
 
     ! Get pointer to current material
     mat => materials(p % material)
@@ -195,9 +193,8 @@ contains
     real(8) :: f
     real(8) :: prob
     real(8) :: cutoff
-    type(Nuclide),  pointer, save :: nuc => null()
-    type(Reaction), pointer, save :: rxn => null()
-!$omp threadprivate(nuc, rxn)
+    type(Nuclide),  pointer :: nuc
+    type(Reaction), pointer :: rxn
 
     ! Get pointer to nuclide
     nuc => nuclides(i_nuclide)
@@ -313,9 +310,8 @@ contains
     real(8) :: f
     real(8) :: prob
     real(8) :: cutoff
-    type(Nuclide),  pointer, save :: nuc => null()
-    type(Reaction), pointer, save :: rxn => null()
-!$omp threadprivate(nuc, rxn)
+    type(Nuclide),  pointer :: nuc
+    type(Reaction), pointer :: rxn
 
     ! Get pointer to nuclide and grid index/interpolation factor
     nuc    => nuclides(i_nuclide)
@@ -418,8 +414,7 @@ contains
     real(8) :: v_cm(3)   ! velocity of center-of-mass
     real(8) :: v_t(3)    ! velocity of target nucleus
     real(8) :: uvw_cm(3) ! directional cosines in center-of-mass
-    type(Nuclide), pointer, save :: nuc => null()
-!$omp threadprivate(nuc)
+    type(Nuclide), pointer :: nuc
 
     ! get pointer to nuclide
     nuc => nuclides(i_nuclide)
@@ -497,7 +492,7 @@ contains
     real(8) :: mu_ijk       ! outgoing cosine k for E_in(i) and E_out(j)
     real(8) :: mu_i1jk      ! outgoing cosine k for E_in(i+1) and E_out(j)
     real(8) :: prob         ! probability for sampling Bragg edge
-    type(SAlphaBeta), pointer, save :: sab => null()
+    type(SAlphaBeta), pointer :: sab
     ! Following are needed only for SAB_SECONDARY_CONT scattering
     integer :: l              ! sampled incoming E bin (is i or i + 1)
     real(8) :: E_i_1, E_i_J   ! endpoints on outgoing grid i
@@ -509,8 +504,6 @@ contains
     real(8) :: frac           ! interpolation factor on outgoing energy
     real(8) :: r1             ! RNG for outgoing energy
 
-!$omp threadprivate(sab)
-
     ! Get pointer to S(a,b) table
     sab => sab_tables(i_sab)
 
@@ -1067,9 +1060,8 @@ contains
     real(8) :: phi          ! fission neutron azimuthal angle
     real(8) :: weight       ! weight adjustment for ufs method
     logical :: in_mesh      ! source site in ufs mesh?
-    type(Nuclide),  pointer, save :: nuc => null()
-    type(Reaction), pointer, save :: rxn => null()
-!$omp threadprivate(nuc, rxn)
+    type(Nuclide),  pointer :: nuc
+    type(Reaction), pointer :: rxn
 
     ! Get pointers
     nuc => nuclides(i_nuclide)
@@ -1175,8 +1167,7 @@ contains
     real(8) :: xi           ! random number
     real(8) :: yield        ! delayed neutron precursor yield
     real(8) :: prob         ! cumulative probability
-    type(DistEnergy), pointer, save :: edist => null()
-!$omp threadprivate(edist)
+    type(DistEnergy), pointer :: edist
 
     ! Determine total nu
     nu_t = nu_total(nuc, E)
diff --git a/src/source.F90 b/src/source.F90
index 3a12a3f71..4f931dc90 100644
--- a/src/source.F90
+++ b/src/source.F90
@@ -256,8 +256,7 @@ contains
 
     integer(8) :: particle_seed  ! unique index for particle
     integer :: i
-    type(Bank), pointer, save :: src => null()
-!$omp threadprivate(src)
+    type(Bank), pointer :: src
 
     ! set defaults
     call p % initialize()
diff --git a/src/string.F90 b/src/string.F90
index 5071493f8..ebe2310f2 100644
--- a/src/string.F90
+++ b/src/string.F90
@@ -151,7 +151,7 @@ contains
 ! TO_LOWER converts a string to all lower case characters
 !===============================================================================
 
-  elemental function to_lower(word) result(word_lower)
+  function to_lower(word) result(word_lower)
 
     character(*), intent(in) :: word
     character(len=len(word)) :: word_lower
@@ -174,7 +174,7 @@ contains
 ! TO_UPPER converts a string to all upper case characters
 !===============================================================================
 
-  elemental function to_upper(word) result(word_upper)
+  function to_upper(word) result(word_upper)
 
     character(*), intent(in) :: word
     character(len=len(word)) :: word_upper
diff --git a/src/tally.F90 b/src/tally.F90
index ded646d4d..c0e5b5e6e 100644
--- a/src/tally.F90
+++ b/src/tally.F90
@@ -58,9 +58,8 @@ contains
     real(8) :: macro_total          ! material macro total xs
     real(8) :: macro_scatt          ! material macro scatt xs
     real(8) :: uvw(3)               ! particle direction
-    type(Material),    pointer, save :: mat => null()
-    type(Reaction),    pointer, save :: rxn => null()
-!$omp threadprivate(mat, rxn)
+    type(Material),    pointer :: mat
+    type(Reaction),    pointer :: rxn
 
     i = 0
     SCORE_LOOP: do q = 1, t % n_user_score_bins
@@ -530,8 +529,7 @@ contains
     real(8) :: wgt                  ! post-collision particle weight
     real(8) :: mu                   ! cosine of angle of collision
     logical :: found_bin            ! scoring bin found?
-    type(TallyObject), pointer, save :: t => null()
-!$omp threadprivate(t)
+    type(TallyObject), pointer :: t
 
     ! Copy particle's pre- and post-collision weight and angle
     last_wgt = p % last_wgt
@@ -701,9 +699,8 @@ contains
     real(8) :: flux                 ! tracklength estimate of flux
     real(8) :: atom_density         ! atom density of single nuclide in atom/b-cm
     logical :: found_bin            ! scoring bin found?
-    type(TallyObject), pointer, save :: t => null()
-    type(Material),    pointer, save :: mat => null()
-!$omp threadprivate(t, mat)
+    type(TallyObject), pointer :: t
+    type(Material),    pointer :: mat
 
     ! Determine track-length estimate of flux
     flux = p % wgt * distance
@@ -810,9 +807,8 @@ contains
     integer :: i             ! loop index for nuclides in material
     integer :: i_nuclide     ! index in nuclides array
     real(8) :: atom_density  ! atom density of single nuclide in atom/b-cm
-    type(TallyObject), pointer, save :: t => null()
-    type(Material),    pointer, save :: mat => null()
-!$omp threadprivate(t, mat)
+    type(TallyObject), pointer :: t
+    type(Material),    pointer :: mat
 
     ! Get pointer to tally
     t => tallies(i_tally)
@@ -883,11 +879,10 @@ contains
     logical :: found_bin            ! was a scoring bin found?
     logical :: start_in_mesh        ! starting coordinates inside mesh?
     logical :: end_in_mesh          ! ending coordinates inside mesh?
-    type(TallyObject),    pointer, save :: t => null()
-    type(StructuredMesh), pointer, save :: m => null()
-    type(Material),       pointer, save :: mat => null()
-    type(LocalCoord),     pointer, save :: coord => null()
-!$omp threadprivate(t, m, mat, coord)
+    type(TallyObject),    pointer :: t
+    type(StructuredMesh), pointer :: m
+    type(Material),       pointer :: mat
+    type(LocalCoord),     pointer :: coord
 
     t => tallies(i_tally)
     matching_bins(1:t%n_filters) = 1
@@ -1115,10 +1110,9 @@ contains
     integer :: i ! loop index for filters
     integer :: n ! number of bins for single filter
     real(8) :: E ! particle energy
-    type(TallyObject),    pointer, save :: t => null()
-    type(StructuredMesh), pointer, save :: m => null()
-    type(LocalCoord),     pointer, save :: coord => null()
-!$omp threadprivate(t, m, coord)
+    type(TallyObject),    pointer :: t
+    type(StructuredMesh), pointer :: m
+    type(LocalCoord),     pointer :: coord
 
     found_bin = .true.
     t => tallies(i_tally)
@@ -1246,9 +1240,8 @@ contains
     logical :: x_same               ! same starting/ending x index (i)
     logical :: y_same               ! same starting/ending y index (j)
     logical :: z_same               ! same starting/ending z index (k)
-    type(TallyObject),    pointer, save :: t => null()
-    type(StructuredMesh), pointer, save :: m => null()
-!$omp threadprivate(t, m)
+    type(TallyObject),    pointer :: t
+    type(StructuredMesh), pointer :: m
 
     TALLY_LOOP: do i = 1, active_current_tallies % size()
       ! Copy starting and ending location of particle
@@ -1647,7 +1640,7 @@ contains
     real(8), allocatable :: tally_temp(:,:) ! contiguous array of results
     real(8) :: global_temp(N_GLOBAL_TALLIES)
     real(8) :: dummy  ! temporary receive buffer for non-root reduces
-    type(TallyObject), pointer :: t => null()
+    type(TallyObject), pointer :: t
 
     do i = 1, active_tallies % size()
       t => tallies(active_tallies % get_item(i))
@@ -1741,7 +1734,7 @@ contains
   subroutine tally_statistics()
 
     integer :: i    ! index in tallies array
-    type(TallyObject), pointer :: t => null()
+    type(TallyObject), pointer :: t
 
     ! Calculate statistics for user-defined tallies
     do i = 1, n_tallies
diff --git a/src/tracking.F90 b/src/tracking.F90
index e0a32b65c..c14d85b5d 100644
--- a/src/tracking.F90
+++ b/src/tracking.F90
@@ -36,8 +36,7 @@ contains
     real(8) :: d_collision            ! sampled distance to collision
     real(8) :: distance               ! distance particle travels
     logical :: found_cell             ! found cell which particle is in?
-    type(LocalCoord), pointer, save :: coord => null()
-!$omp threadprivate(coord)
+    type(LocalCoord), pointer :: coord
 
     ! Display message if high verbosity or trace is on
     if (verbosity >= 9 .or. trace) then
diff --git a/src/utils/openmc/cmfd.py b/src/utils/openmc/cmfd.py
index 6e7eb162a..8e2222215 100644
--- a/src/utils/openmc/cmfd.py
+++ b/src/utils/openmc/cmfd.py
@@ -514,8 +514,8 @@ class CMFDFile(object):
         self._norm = norm
 
 
-    @snes_monitor.setter
-    def snum_flushes(self, num_flushes):
+    @num_flushes.setter
+    def num_flushes(self, num_flushes):
 
         if not is_integer(num_flushes):
             msg = 'Unable to set the CMFD number of flushes to {0} ' \
diff --git a/src/utils/openmc/material.py b/src/utils/openmc/material.py
index 7e066bfce..06f1bdc9f 100644
--- a/src/utils/openmc/material.py
+++ b/src/utils/openmc/material.py
@@ -189,23 +189,27 @@ class Material(object):
 
     def add_nuclide(self, nuclide, percent, percent_type='ao'):
 
-        if not isinstance(nuclide, openmc.Nuclide):
-            msg = 'Unable to add an Nuclide to Material ID={0} with a ' \
+        if not isinstance(nuclide, (openmc.Nuclide, str)):
+            msg = 'Unable to add a Nuclide to Material ID={0} with a ' \
                   'non-Nuclide value {1}'.format(self._id, nuclide)
             raise ValueError(msg)
 
         elif not is_float(percent):
-            msg = 'Unable to add an Nuclide to Material ID={0} with a ' \
+            msg = 'Unable to add a Nuclide to Material ID={0} with a ' \
                   'non-floating point value {1}'.format(self._id, percent)
             raise ValueError(msg)
 
         elif not percent_type in ['ao', 'wo', 'at/g-cm']:
-            msg = 'Unable to add an Nuclide to Material ID={0} with a ' \
+            msg = 'Unable to add a Nuclide to Material ID={0} with a ' \
                   'percent type {1}'.format(self._id, percent_type)
             raise ValueError(msg)
 
-        # Copy this Nuclide to separate it from the Nuclide in other Materials
-        nuclide = deepcopy(nuclide)
+        if isinstance(nuclide, openmc.Nuclide):
+            # Copy this Nuclide to separate it from the Nuclide in
+            # other Materials
+            nuclide = deepcopy(nuclide)
+        else:
+            nuclide = openmc.Nuclide(nuclide)
 
         self._nuclides[nuclide._name] = (nuclide, percent, percent_type)
 
diff --git a/src/utils/openmc/universe.py b/src/utils/openmc/universe.py
index 810a8b5e7..963510c38 100644
--- a/src/utils/openmc/universe.py
+++ b/src/utils/openmc/universe.py
@@ -114,23 +114,30 @@ class Cell(object):
     @fill.setter
     def fill(self, fill):
 
-        if not isinstance(fill, (openmc.Material, Universe, Lattice)) \
-            and fill != 'void':
+        if isinstance(fill, str):
+            if fill.strip().lower() == 'void':
+                self._type = 'void'
+            else:
+                msg = 'Unable to set Cell ID={0} to use a non-Material or ' \
+                       'Universe fill {1}'.format(self._id, fill)
+                raise ValueError(msg)
+
+        elif isinstance(fill, openmc.Material):
+            self._type = 'normal'
+
+        elif isinstance(fill, Universe):
+            self._type = 'fill'
+
+        elif isinstance(fill, Lattice):
+            self._type = 'lattice'
+
+        else:
             msg = 'Unable to set Cell ID={0} to use a non-Material or ' \
                    'Universe fill {1}'.format(self._id, fill)
             raise ValueError(msg)
 
         self._fill = fill
 
-        if isinstance(fill, Lattice):
-            self._type = 'lattice'
-        elif isinstance(fill, Universe):
-            self._type = 'fill'
-        elif fill == 'void':
-            self._type = 'normal'
-        else:
-            self._type = 'normal'
-
 
     @rotation.setter
     def rotation(self, rotation):
@@ -247,7 +254,7 @@ class Cell(object):
         path = path[1:]
 
         # If the Cell is filled by a Material
-        if self._type == 'normal':
+        if self._type == 'normal' or self._type == 'void':
             offset = 0
 
         # If the Cell is filled by a Universe
@@ -348,6 +355,9 @@ class Cell(object):
             element.set("fill", str(self._fill._id))
             self._fill.create_xml_subelement(xml_element)
 
+        elif self._fill.strip().lower() == "void":
+            element.set("material", "void")
+
         else:
             element.set("fill", str(self._fill))
             self._fill.create_xml_subelement(xml_element)