Models for the Safe Storage of Dry and Wet Fissile Oxides

A model has been developed for calculating critically safe storage arrays of cylindrical vessels containing water/fissile oxide mixtures of PuO₂ (96% ²³⁹Pu and 4% ²⁴⁰Pu) or UO₂ (93.4% ²³⁵U and 6.6% ²³⁸U). It was assumed that these arrays were in air surrounded by a 12-in.-thick concrete vault. This model uses the concepts of surface density and unit surface-to-volume ratio to define safe array parameters. The model handles fissile densities ranging from that of the theoretical crystal down to ∼1kg/liter and containers whose shapes range from 300-cm-high thin cylinders to flat slab-shaped containers. Correction factors for dry oxide storage at various densities have also been developed for the cases where water can be prevented from entering the oxide containers. A considerably greater weight of oxide can be stored when water can be precluded. The effect of several inches of water on the concrete floor lowers k_eff ∼3%. (The units are assumed to be at least 12 in. above the floor.)