NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals.

Along with a series of existing qualitative steps to assure safety in LBB-approved lines experiencing PWSCC, NRC staff, working cooperatively with the Electric Power Research Institute through a memorandum of understanding, is developing a new, modular based, comprehensive piping system assessment methodology to directly assess compliance with the regulations. This project, called eXtremely Low Probability of Rupture (xLPR), will model the effects and uncertainties of relevant active degradation mechanisms and the associated mitigation activities. The resulting analytical tool will be comprehensive with respect to known and significant materials challenges (PWSCC, etc.), vetted with respect to the technical bases of models and inputs, flexible enough to permit analysis of a variety of in-service situations and adaptable such as to accommodate evolving and improving knowledge.

A multi-year project has begun that has been focused on the development of a viable method and approach to address the effects of PWSCC as well as define the requirements necessary for such a modular-based assessment tool. As reported in a previous paper, the first version of this code was developed as part of a pilot study, which leveraged existing fracture mechanics based models and software coupled to both a commercial and open source code framework to determine the framework and architecture requirements appropriate for building a modular-based code with this complexity. The pilot study focused on PWSCC in pressurizer surge nozzles, and was meant to demonstrate the feasibility of this code and approach and not to determine the absolute values of the probability of rupture.

This paper examines the plans for the xLPR Version 2.0 model which will broaden the scope of xLPR to all LBB-approved primary piping in pressurized water reactors (PWR), using an incremental approach that incorporates the design requirements and lessons learned from previous iterations. After a review of the Version 1.0 final results, this paper will document the plans for Version 2.0 including the revised management structure, the technical scope, and the progress in the code development effort to date.

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