Continuing Development of PRO-LOCA for the Prediction of Break Probabilities for Loss-of-Coolant Accidents

As part of a possible risk-informed revision of the design-basis break size requirements for operating commercial nuclear power plants as specified in the Code of Federal Regulations (CFR), the NRC began development of a probabilistic piping fracture mechanics code called PRO-LOCA. The initial development of this code and its background was published at a prior PVP conference. Since that time, the development of the PRO-LOCA code has continued through an international group program entitled Maximizing Enhancements in Risk Informed Technology (MERIT). The MERIT program includes participation from Canada, Korea, Sweden, UK, and the US (NRC and EPRI). The PRO-LOCA code, which aides in predicting piping break frequencies as a function of break size, incorporates many enhancements in technology since some of the earlier probabilistic codes (e.g., PRAISE) were developed. These enhancements include improved crack stability analyses, leak rate models, crack initiation and growth models, and material property data. In addition, degradation mechanisms such as primary water stress corrosion cracking (PWSCC) for dissimilar welds in pressurized water reactors (PWRs) are included in the PRO-LOCA code. This paper reviews the ongoing development of the PRO-LOCA code by giving a brief description of the recent updates made to the models embedded in the code. Some of these capabilities include improvements to crack initiation and growth models, welding residual stress distribution inputs, the addition of weld overlays, past and future inspections, the addition of importance sampling, and bootstrap methods for predicting confidence limits on output. The current version of the PRO-LOCA code was used for a sensitivity analyses in order to demonstrate the effects of welding residual stress uncertainty on the probability of leak and rupture. Plans for the continuing development of the PRO-LOCA code conclude this paper.