The US Nuclear Regulatory Commission (NRC) in conjunction with the US nuclear power industry under the leadership of the Electric Power Research Institute (EPRI) is developing a new probabilistic fracture mechanics (PFM) code as a means of demonstrating compliance with the 10CFR50 Appendix A, General Design Criterion 4 (GDC-4) requirement that primary system piping exhibit an extremely low probability of rupture. This PFM code, called xLPR (eXtremely Low Probability of Rupture) will be comprehensive by addressing all aspects of the problem, i.e., crack initiation, growth, stability, surface crack detection and leakage detection.

Previously, the NRC, along with two of its contractors, Battelle Memorial Institute and Engineering Mechanics Corporation of Columbus (Emc2), developed a probabilistic fracture mechanics code called PRO-LOCA[1] which was to have been used as a tool for re-evaluating the break frequency versus break size curves developed as part of the technical basis for the transition break size as part of the redefinition of the emergency core cooling system (ECCS) requirements in 10CFR50.46. PRO-LOCA was subsequently developed as part of an international cooperative research program led by Battelle called MERIT (Maximizing Enhancements in Risk-Informed Technology). Today PRO-LOCA is being further developed as part of another international cooperative program called PARTRIDGE (Probabilistic Analysis as a Regulatory Tool for Risk-Informed Decision GuidancE). The focus of this paper is three-fold. First, the relationship between PRO-LOCA and xLPR will be described. Secondly, the enhancements being made to PRO-LOCA will be discussed and compared to xLPR development. Finally, the results of some comparative cases where PRO-LOCA (Version 3.0) was benchmarked against xLPR (Version 1.0) are provided.

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