Probabilistic External Criticality Evaluation

This analysis is prepared by the Mined Geologic Disposal System (MGDS) Waste Package Development (WPD) department to provide a probabilistic evaluation of the potential for criticality of fissile material which has been transported from a geologic repository containing breached waste packages of commercial spent nuclear fuel (SNF). This analysis is part of a continuing investigation of the probability of criticality resulting from the emplacement of spent nuclear fuel in a geologic repository. The principal objective of this evaluation are: (1) to identify the range of possible concentrations of uranium which could be precipitated in the rock downstream (groundwater flow) form the repository, (2) to calculate the minimum critical masses of such concentrations (assuming a fissile enrichment of the uranium corresponding to the characteristics of commercial spent nuclear fuel, and assuming a concentration of water moderator consistent with the known porosity of the rock), (3) to estimate conservative upper bounds for the probability of occurrence of such critical masses, and (4) to estimate the consequences of any credible criticality and calculate the resulting risk, which is product of the probability of occurrence multiplied by the consequences.