diff --git a/src/energy_grid.F90 b/src/energy_grid.F90
index fa868c0c38..66b95be0de 100644
--- a/src/energy_grid.F90
+++ b/src/energy_grid.F90
@@ -2,7 +2,7 @@ module energy_grid
 
   use constants,        only: MAX_LINE_LEN
   use global
-  use list_header,      only: ListElemReal
+  use list_header,      only: ListReal
   use output,           only: write_message
 
 contains
@@ -18,9 +18,8 @@ contains
   subroutine unionized_grid()
 
     integer :: i ! index in nuclides array
-    type(ListElemReal), pointer :: list => null()
-    type(ListElemReal), pointer :: current => null()
-    type(Nuclide),      pointer :: nuc => null()
+    type(ListReal), pointer :: list => null()
+    type(Nuclide),  pointer :: nuc => null()
 
     message = "Creating unionized energy grid..."
     call write_message(5)
@@ -31,24 +30,18 @@ contains
       call add_grid_points(list, nuc % energy)
     end do
 
-    ! determine size of list
-    n_grid = 0
-    current => list
-    do while (associated(current))
-      n_grid = n_grid + 1
-      current => current % next
-    end do
+    ! Set size of unionized energy grid 
+    n_grid = list % size() 
 
     ! create allocated array from linked list
     allocate(e_grid(n_grid))
-    current => list
     do i = 1, n_grid
-      e_grid(i) = current % data
-      current => current % next
+      e_grid(i) = list % get_item(i)
     end do
 
     ! delete linked list and dictionary
-    ! call list_delete(list)
+    call list % clear()
+    deallocate(list)
 
     ! Set pointers to unionized energy grid for each nuclide
     call grid_pointers()
@@ -62,89 +55,64 @@ contains
 
   subroutine add_grid_points(list, energy)
 
-    type(ListElemReal), pointer :: list
+    type(ListReal), pointer :: list
     real(8), intent(in) :: energy(:)
 
-    integer :: i  ! index in energy array
-    integer :: n  ! size of energy array
-    real(8) :: E  ! actual energy value
-    type(ListElemReal), pointer :: current => null()
-    type(ListElemReal), pointer :: previous => null()
-    type(ListElemReal), pointer :: head => null()
-    type(ListElemReal), pointer :: tmp => null()
+    integer :: i       ! index in energy array
+    integer :: n       ! size of energy array
+    integer :: current ! current index 
+    real(8) :: E       ! actual energy value
 
     i = 1
     n = size(energy)
 
-    ! if the original list is empty, we need to allocate the first element and
+    ! 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)
-      current => list
       do i = 1, n
-        current % data = energy(i)
-        if (i == n) then
-          current % next => null()
-          return
-        end if
-        allocate(current % next)
-        current => current % next
+        call list % append(energy(i))
       end do
+      return
     end if
 
-    current => list
-    head => list
+    ! Set current index to beginning of the list 
+    current = 1
 
     do while (i <= n)
       E = energy(i)
 
       ! If we've reached the end of the grid energy list, add the remaining
       ! energy points to the end
-      if (.not. associated(current)) then
-        ! finish remaining energies
+      if (current > list % size()) then
+        ! Finish remaining energies
         do while (i <= n)
-          allocate(previous % next)
-          current => previous % next
-          current % data = energy(i)
-          previous => current
+          call list % append(energy(i))
           i = i + 1
         end do
-        current%next => null()
         exit
       end if
 
-      if (E < current % data) then
-        ! create new element and insert it in energy grid list
-        allocate(tmp)
-        tmp % data = E
-        tmp % next => current
-        if (associated(previous)) then
-          previous % next => tmp
-          previous => tmp
-        else
-          previous => tmp
-          head => previous
-        end if
-        nullify(tmp)
+      if (E < list % get_item(current)) then
 
-        ! advance index
+        ! Insert new energy in this position
+        call list % insert(current, E)
+
+        ! Advance index in linked list and in new energy grid
         i = i + 1
+        current = current + 1
 
-      elseif (E == current % data) then
-        ! found the exact same energy, no need to store duplicates so just
+      elseif (E == list % get_item(current)) then
+        ! Found the exact same energy, no need to store duplicates so just
         ! skip and move to next index
         i = i + 1
+        current = current + 1
       else
-        previous => current
-        current => current % next
+        current = current + 1
       end if
 
     end do
 
-    ! It's possible that an element was inserted at the front of the list, so we
-    ! need to move the list pointer back to the start of the list
-    list => head
-
   end subroutine add_grid_points
 
 !===============================================================================
diff --git a/src/finalize.F90 b/src/finalize.F90
index ac64e90fba..1006403fb3 100644
--- a/src/finalize.F90
+++ b/src/finalize.F90
@@ -69,6 +69,10 @@ contains
 #endif
 
 #ifdef MPI
+    ! Free all MPI types
+    call MPI_TYPE_FREE(MPI_BANK, mpi_err)
+    call MPI_TYPE_FREE(MPI_TALLYRESULT, mpi_err)
+
     ! If MPI is in use and enabled, terminate it
     call MPI_FINALIZE(mpi_err)
 #endif
diff --git a/src/initialize.F90 b/src/initialize.F90
index 1a9cc021fb..033917dc11 100644
--- a/src/initialize.F90
+++ b/src/initialize.F90
@@ -242,6 +242,9 @@ contains
     ! Commit derived type for tally scores
     call MPI_TYPE_COMMIT(MPI_TALLYRESULT, mpi_err)
 
+    ! Free temporary MPI type
+    call MPI_TYPE_FREE(temp_type, mpi_err)
+
   end subroutine initialize_mpi
 #endif
 
@@ -432,22 +435,22 @@ contains
     pair_list => universe_dict % keys()
     current => pair_list
     do while (associated(current))
-      ! find index of universe in universes array
+      ! Find index of universe in universes array
       i_univ = current % value
       univ => universes(i_univ)
       univ % id = current % key
 
-      ! check for lowest level universe
+      ! Check for lowest level universe
       if (univ % id == 0) BASE_UNIVERSE = i_univ
 
-      ! find cell count for this universe
+      ! Find cell count for this universe
       n_cells_in_univ = cells_in_univ_dict % get_key(univ % id)
 
-      ! allocate cell list for universe
+      ! Allocate cell list for universe
       allocate(univ % cells(n_cells_in_univ))
       univ % n_cells = n_cells_in_univ
 
-      ! move to next universe
+      ! Move to next universe
       next => current % next
       deallocate(current)
       current => next
@@ -462,16 +465,19 @@ contains
     do i = 1, n_cells
       c => cells(i)
 
-      ! get pointer to corresponding universe
+      ! Get pointer to corresponding universe
       i_univ = universe_dict % get_key(c % universe)
       univ => universes(i_univ)
 
-      ! increment the index for the cells array within the Universe object and
+      ! Increment the index for the cells array within the Universe object and
       ! then store the index of the Cell object in that array
       index_cell_in_univ(i_univ) = index_cell_in_univ(i_univ) + 1
       univ % cells(index_cell_in_univ(i_univ)) = i
     end do
-    
+
+    ! Clear dictionary
+    call cells_in_univ_dict % clear()
+
   end subroutine prepare_universes
 
 !===============================================================================
diff --git a/src/list_header.F90 b/src/list_header.F90
index 2d832dd0f4..a9311ec64a 100644
--- a/src/list_header.F90
+++ b/src/list_header.F90
@@ -19,16 +19,19 @@ module list_header
   type :: ListElemInt
     integer :: data
     type(ListElemInt), pointer :: next => null()
+    type(ListElemInt), pointer :: prev => null()
   end type ListElemInt
 
   type :: ListElemReal
     real(8) :: data
     type(ListElemReal), pointer :: next => null()
+    type(ListElemReal), pointer :: prev => null()
   end type ListElemReal
 
   type :: ListElemChar
     character(MAX_WORD_LEN) :: data
     type(ListElemChar), pointer :: next => null()
+    type(ListElemChar), pointer :: prev => null()
   end type ListElemChar
 
 !===============================================================================
@@ -128,6 +131,7 @@ contains
     else
       ! Otherwise append element at end of list
       this % tail % next => elem
+      elem % prev => this % tail
       this % tail => this % tail % next
     end if
 
@@ -152,6 +156,7 @@ contains
     else
       ! Otherwise append element at end of list
       this % tail % next => elem
+      elem % prev => this % tail
       this % tail => this % tail % next
     end if
 
@@ -176,6 +181,7 @@ contains
     else
       ! Otherwise append element at end of list
       this % tail % next => elem
+      elem % prev => this % tail
       this % tail => this % tail % next
     end if
 
@@ -499,8 +505,13 @@ contains
 
     else
       ! Default case with new element somewhere in middle of list
-      i = 0
-      elem => this % head
+      if (i_list >= this % last_index) then
+        i = this % last_index
+        elem => this % last_elem
+      else
+        i = 0
+        elem => this % head
+      end if
       do while (associated(elem))
         i = i + 1
         if (i == i_list - 1) then
@@ -509,11 +520,17 @@ contains
           new_elem % data = data
           
           ! Put it before the i-th element
-          new_elem % next => elem % next
-          elem % next => new_elem
+          new_elem % prev => elem % prev
+          new_elem % next => elem
+          new_elem % prev % next => new_elem
+          new_elem % next % prev => new_elem
           this % count = this % count + 1
+          this % last_index = i_list
+          this % last_elem => new_elem
           exit
         end if
+        i = i + 1
+        elem => elem % next
       end do
     end if
 
@@ -544,21 +561,31 @@ contains
 
     else
       ! Default case with new element somewhere in middle of list
-      i = 0
-      elem => this % head
+      if (i_list >= this % last_index) then
+        i = this % last_index
+        elem => this % last_elem
+      else
+        i = 0
+        elem => this % head
+      end if
       do while (associated(elem))
-        i = i + 1
-        if (i == i_list - 1) then
+        if (i == i_list) then
           ! Allocate new element
           allocate(new_elem)
           new_elem % data = data
           
           ! Put it before the i-th element
-          new_elem % next => elem % next
-          elem % next => new_elem
+          new_elem % prev => elem % prev
+          new_elem % next => elem
+          new_elem % prev % next => new_elem
+          new_elem % next % prev => new_elem
           this % count = this % count + 1
+          this % last_index = i_list
+          this % last_elem => new_elem
           exit
         end if
+        i = i + 1
+        elem => elem % next
       end do
     end if
 
@@ -589,21 +616,31 @@ contains
 
     else
       ! Default case with new element somewhere in middle of list
-      i = 0
-      elem => this % head
+      if (i_list >= this % last_index) then
+        i = this % last_index
+        elem => this % last_elem
+      else
+        i = 0
+        elem => this % head
+      end if
       do while (associated(elem))
-        i = i + 1
-        if (i == i_list - 1) then
+        if (i == i_list) then
           ! Allocate new element
           allocate(new_elem)
           new_elem % data = data
           
           ! Put it before the i-th element
-          new_elem % next => elem % next
-          elem % next => new_elem
+          new_elem % prev => elem % prev
+          new_elem % next => elem
+          new_elem % prev % next => new_elem
+          new_elem % next % prev => new_elem
           this % count = this % count + 1
+          this % last_index = i_list
+          this % last_elem => new_elem
           exit
         end if
+        i = i + 1
+        elem => elem % next
       end do
     end if
 
@@ -620,7 +657,6 @@ contains
     integer        :: data
 
     type(ListElemInt), pointer :: elem => null()
-    type(ListElemInt), pointer :: prev => null()
 
     elem => this % head
     do while (associated(elem))
@@ -632,12 +668,14 @@ contains
         if (associated(elem, this % head)) then
           this % head => elem % next
           if (associated(elem, this % tail)) nullify(this % tail)
+          if (associated(this % head)) nullify(this % head % prev)
           deallocate(elem)
         else if (associated(elem, this % tail)) then
-          this % tail => prev
+          this % tail => elem % prev
           deallocate(this % tail % next)
         else
-          prev % next => elem % next
+          elem % prev % next => elem % next
+          elem % next % prev => elem % prev
           deallocate(elem)
         end if
 
@@ -647,7 +685,6 @@ contains
       end if
 
       ! Advance pointers
-      prev => elem
       elem => elem % next
     end do
 
@@ -659,7 +696,6 @@ contains
     real(8)         :: data
 
     type(ListElemReal), pointer :: elem => null()
-    type(ListElemReal), pointer :: prev => null()
 
     elem => this % head
     do while (associated(elem))
@@ -671,12 +707,14 @@ contains
         if (associated(elem, this % head)) then
           this % head => elem % next
           if (associated(elem, this % tail)) nullify(this % tail)
+          if (associated(this % head)) nullify(this % head % prev)
           deallocate(elem)
         else if (associated(elem, this % tail)) then
-          this % tail => prev
+          this % tail => elem % prev
           deallocate(this % tail % next)
         else
-          prev % next => elem % next
+          elem % prev % next => elem % next
+          elem % next % prev => elem % prev
           deallocate(elem)
         end if
 
@@ -686,7 +724,6 @@ contains
       end if
 
       ! Advance pointers
-      prev => elem
       elem => elem % next
     end do
 
@@ -698,7 +735,6 @@ contains
     character(*)    :: data
 
     type(ListElemChar), pointer :: elem => null()
-    type(ListElemChar), pointer :: prev => null()
 
     elem => this % head
     do while (associated(elem))
@@ -710,12 +746,14 @@ contains
         if (associated(elem, this % head)) then
           this % head => elem % next
           if (associated(elem, this % tail)) nullify(this % tail)
+          if (associated(this % head)) nullify(this % head % prev)
           deallocate(elem)
         else if (associated(elem, this % tail)) then
-          this % tail => prev
+          this % tail => elem % prev
           deallocate(this % tail % next)
         else
-          prev % next => elem % next
+          elem % prev % next => elem % next
+          elem % next % prev => elem % prev
           deallocate(elem)
         end if
 
@@ -725,7 +763,6 @@ contains
       end if
 
       ! Advance pointers
-      prev => elem
       elem => elem % next
     end do