US Nuclear Regulatory Commission (USNRC) Standard Review Plan (SRP) 3.6.3, describes the current methodology for leak-before-break (LBB) piping safety assessment. Specifically, it describes a deterministic assessment procedure that can be used to demonstrate compliance with the 10CFR50 Appendix-A, General Design Criterion 4 (GDC-4) requirement that the primary system pressure piping exhibit an extremely low probability of rupture. However, SRP 3.6.3 does not permit assessment of piping systems with active degradation mechanisms, even though it is known that primary water stress corrosion cracking (PWSCC) has occurred in systems that have been granted LBB exemptions to remove pipe-whip restraints. To address this need, a program is being conducted with the long-term goal of developing a probabilistic assessment tool that can be used to directly demonstrate compliance with 10CFR50 Appendix–A, GDC-4, a tool that would account for the effects of both active degradation mechanisms and the mitigation activities that are being undertaken to address this degradation. This program has been termed “xLPR” as its goal is to demonstrate an eXtremely Low Probability of Rupture in pressure boundary piping systems. This methodology augments current LBB assessment models (leak rate and crack stability models) through the addition of best estimate models describing the initiation and propagation of flaws due to the various degradation mechanisms (fatigue, PWSCC, intergranular stress corrosion cracking (IGSCC), etc.), inspection models, and mechanical and chemical mitigation/remediation models that describe changes in stress state, pipe material and environment caused by mitigation/remediation efforts. Models currently used in LBB assessment will be updated, or replaced, with best estimate, probabilistic models, including those for leak rate and crack stability assessment. All models should account for the full distribution of input variables (where known) in order to account for both epistemic and aleatory uncertainties in as detailed a manner as feasible. This paper summarizes the structure and current activities of the Modeling Task Group within the framework of the overall xLPR Project, and the methodology used to select and develop the models for the xLPR Pilot Study. Preliminary information on the various models chosen for the Pilot Study, and how they are linked within the structure of the overall xLPR probabilistic code, is also provided.

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