Estimates of component rupture probabilities for pressure boundary systems in LWRs

The goals of a multiyear program entitled Nondestructive Evaluation Reliability for Inservice Inspection of Light Water Reactors, which is being conducted at Pacific Northwest Laboratory (PNL) for the US Nuclear Regulatory Commission, are to determine the reliability of current in-service inspections of pressure boundary systems and components and to develop recommendations that can ensure a suitably high inspection reliability. The long-term objective is to develop recommendations for improved inspection plans that address inspection samples, frequencies, and methods. In meeting the program objectives, PNL has developed risk-based methods as a guide for establishing inspection priorities. In PNL's pilot application of risk-based methods, the limitations of historical failure rate data became evident, and it was not possible to exercise probabilistic fracture mechanics models. Therefore, expert judgment was selected for estimating failure probabilities. The goal was to obtain numerical estimates for probabilities of catastrophic failures (i.e., ruptures) in pressure boundary systems and components of pressurized water reactors. The representative plant selected for the study was the Surry nuclear power station unit 1 (Surry-1). This paper summarizes the process and the results of the expert judgment elicitation for selected systems at Surry-1