Fix density for Fe in MCNP model for heu-comp-inter-003 case 6.
Fix HEU densities in MCNP model for heu-comp-inter-003 case 6.
Fix reflectors in MCNP models for heu-met-fast-003 cases 8,9,10,11,12.
Removed redundant C-Nat in OpenMC model for heu-met-fast-018 and 019.