Criticality safety evaluation of shutdown diffusion cascade coolers

A criticality safety study of diffusion cascade coolers in a shutdown state is presented. The coolers represent six typical cascade coolers at a gaseous diffusion plant with accumulated deposits of UO2F2. The study involves keff calculations for the coolers with various distributions of UO2F2, which are assumed as part of several hypothetical accident scenarios. The results show that at least two independent failures must occur in order to have a criticality. Additionally, the distributions chosen represent the upper bounds for keff. Individual results show that the keff values for the cascade coolers designed for 80 and 97% enriched UF6 with deposit amounts <2.409 and 2.185 kg, respectively, will not exceed 0.9 for the accident scenarios modeled. All other coolers require shell-side flooding with H2O in order to cause a criticality, which is possible only if two or more independent failures occur