Probability and Consequences of Nuclear Criticality at a Geologic Repository—I: Conceptual Overview for Screening

This paper, Part I of two companion papers, reviews concepts underlying the basis for evaluating the criticality scenario for an assessment of performance after closure of a geologic repository for radioactive waste. In the United States, either a low-probability or low-consequence rationale can be the basis of excluding criticality, using the usual assumptions that (a) the interplay between the probability and consequence is not significant and (b) the mean of the epistemic uncertainty of the probability and consequence provides a sufficient approximation. Furthermore, the rationale can be based on either qualitative or quantitative arguments. For those situations with quantitative arguments, this paper provides additional perspective on evaluating the criticality scenario by combining quantitative estimates of low probability and low consequence as a complementary cumulative distribution function. As a demonstration, the low probability and low consequence of the criticality scenario for the Waste Isolation Pilot Plant (a repository for defense transuranic element waste) is presented.