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.

US NRC staff, working cooperatively with the Electric Power Research Institute through a memorandum of understanding, conducted a multi-year project that focused on the development of a viable method and approach to address the effects of PWSCC in primary piping systems approved for LBB. This project, called eXtremely Low Probability of Rupture (xLPR), defined the requirements necessary for a modular-based probabilistic fracture mechanics assessment tool to directly assess compliance with the regulations.

As reported in previous technical papers, the first version of the xLPR 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. Using the lessons learned from the pilot study, the production version of this code, designated as Version 2.0, focuses on those primary piping systems previously approved for LBB. In this version, the appropriate fracture mechanics-based models are employed to model the physical cracking behavior and a variety of computational options are provided to characterize, categorize and propagate problem uncertainties.

This paper examines the xLPR Version 2.0 model by presenting a brief overview of the xLPR scope, the code structure, computational framework and fracture mechanics-based models. As a demonstration of the xLPR Version 2.0 capabilities, an example is presented that focuses on PWSCC in large-bore piping systems. This example exercises some functionalities of the xLPR code to demonstrate its application to assess compliance with 10CFR50 Appendix A, GDC-4. This paper concludes with a brief discussion on the path forward and plans for the control and maintenance of the xLPR code.

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