Two models are described for calculating critically safe storage configurations for uranium (93.4% 235U) and plutonium (96% 239Pu, 4% 240Pu) metal. The first model deals with conventional arrays in air of fissile units surrounded by concrete walls. The model uses the concepts of surface density and unit surface-to-volume ratio to define safe-array parameters. The second model makes use of thick internal moderators (i.e., water) enclosing each storage unit in an array. These arrays are also surrounded by concrete walls. The internal moderators partially isolate adjacent storage units from one another and thus, in some cases, permit extremely high surface densities and vault loading. Several factors that influence the storage model are shape and density of the individual storage units and the degree of reflection of arrays of units.