Risk, Uncertainty in Risk, and the EPA Release Limits for Radioactive Waste Disposal

A conceptual model for the organization and execution of a performance assessment of a radioactive waste disposal site, including uncertainty and sensitivity analysis, is described. This model is based on a formal definition of risk as a collection of ordered triples, where the first element in each triple is a set of similar occurrences, the second element is the probability or frequency of the first element, and the third element is a vector of consequences associated with the first element. This division of risk into its three constituent parts provides a useful model for the structure of a performance assessment for several reasons. First, it provides a clear distinction between the major parts of a performance assessment. Second, it provides a way to distinguish between different types of uncertainty, including completeness, aggregation, model selection, imprecisely known variables, and stochastic variation. Third, it leads naturally to the representation of stochastic variation with a complementary cumulative distribution function (CCDF) and the representation of state of knowledge uncertainty with a family or distribution of CCDFs. Fourth, it provides a context in which the U.S. Environmental Protection Agency limits for radioactive releases to the accessible environment can be represented and calculated. The preceding ideasmore » are illustrated with results obtained in a preliminary performance assessment for the Waste Isolation Pilot Plant in southeastern New Mexico.« less