SIDEBAR 5.1 DOE’s Safety Case for Yucca Mountain
The committee uses the term “safety case” in the broad sense described in Sidebar 2.1, that is, a collection of arguments reassessed at given stages of repository development in support of the safety of the repository. The committee believes that the Department of Energy (DOE), as the repository implementer, should develop the broad safety case, as described in Section 2.1 , for guiding its safety work and explaining its safety arguments. To see how this conforms to the licensing requirements in the United States, it is necessary to look at the terminology and procedures defined by the Nuclear Regulatory Commission (USNRC) regulations.
The term safety case is not used by the USNRC for the analysis of post-closure safety, which is of greatest relevance here. The applicant (DOE) is required to submit a complete application and an environmental impact statement that are sufficient for the USNRC to make a determination of safety for the repository, including “reasonable expectation that the materials can be disposed of without unreasonable risk to the health and safety of the public” (10 CFR Part 63.31). A central element of the complete application is to be a performance assessment. Performance assessment (definition from 10 CFR 63.2) means an analysis that:
“(1) Identifies the features, events, processes (except human intrusion), and sequences of events and processes (except human intrusion) that might affect the Yucca Mountain disposal system and their probabilities of occurring during 10,000 years after disposal;
(2) Examines the effects of those features, events, processes, and sequences of events and processes upon the performance of the Yucca Mountain disposal system; and
(3) Estimates the dose incurred by the reasonably maximally exposed individual, including the associated uncertainties, as a result of releases caused by all significant features, events, processes, and sequences of events and processes, weighted by their probability of occurrence” (66 Federal Register 55794, November 2, 2001).
Specific requirements for the performance assessment are also described in the regulations (see 10 CFR 63.114) and these are broadly similar to the safety case concept described by the committee. The USNRC stipulates that the performance assessment must:
“[…] (b) Account for uncertainties and variabilities in parameter values and provide for the technical basis for parameter ranges, probability distributions, or bounding values used in the performance assessment.
(c) Consider alternative conceptual models of features and processes that are consistent with available data and current scientific understanding and evaluate the effects that alternative conceptual models have on the performance of the geologic repository. […]