Uncertainties and probabilities in nuclear reactor regulation

Nuclear reactor regulations in the USA originally specified deterministic criteria to be met by thermal/hydraulic codes. A set of results from a single run of the code was compared with a set of criteria. Codes are now becoming “realistic” with uncertainties incorporated into the process, usually requiring many runs of the code. The result is a range of results with associated probabilities. Regulations do not currently specify how such results should be evaluated against acceptance criteria. A suggestion has been made to use a level of confidence (e.g., 0.95) that the criteria are satisfied with at least some specified probability (e.g., 0.95). Some valid and invalid ways of doing this and their consequences are presented. It is suggested that an improved process would relate the thermal/hydraulic outputs to measures of core damage or some other suitable measure of risk. Furthermore, the uncertainties modeled in the code should be combined with uncertainties and probabilities, e.g., about functioning of equipment and operator actions, that are now represented in the probabilistic risk assessment (PRA). The task of combining thermal/hydraulic codes with PRA techniques to produce an effective and feasible technology presents a challenge to the technical community.