Appendix A, General Design Criteria (GDC) 4, of 10 CFR Part 50 states, in part, that the dynamic effects associated with postulated reactor coolant system pipe ruptures may be excluded from the design basis when analyses reviewed and approved by NRC demonstrate that the probability of fluid system piping rupture is extremely low under conditions consistent with the design basis. Licensees have typically demonstrated compliance with this probabilistic criterion through deterministic analyses that do not accurately simulate typical piping degradation. Given recent advances in probabilistic methodologies, the NRC staff and industry believe that performing a probabilistic analysis of primary system piping that fully addresses and quantifies uncertainties and directly demonstrates compliance with GDC 4 is more appropriate. NRC and industry expect that a robust probabilistic software tool, developed cooperatively, will facilitate meeting this goal; will improve licensing, regulatory decision making, and design; and will be mutually beneficial. Based on the terminology of GDC 4, this project is titled Extremely Low Probability of Rupture (xLPR). Development of the xLPR methodology and the corresponding software tool will involve many challenging technical decisions, modeling judgments, and sensitivity analyses. The purpose of the xLPR project is to develop a probabilistic assessment tool that can be used to demonstrate direct compliance with the requirements of 10 CFR 50 Appendix A GDC 4. The xLPR software tool will model active degradation mechanisms, such as primary water stress corrosion cracking, and the associated mitigation activities. The tool will be comprehensive with respect to known challenges, vetted with respect to scientific adequacy of models and inputs, flexible enough to permit analysis of a variety of in-service situations, and adaptable to accommodate evolving and improving knowledge. Successful execution of the xLPR project will involve a complex and diverse array of technical specialties and will require a well-organized and structured team of experts. This paper summarizes the objectives, organizational structure, and program plan of the collaborative NRC / Industry xLPR Project to develop a robust, flexible, probabilistic piping rupture assessment code. While the listed authors are individually responsible within their respective organizations for this project, the project organization includes a management team representing the xLPR Project task group structure. Individual members of this team will be listed in the paper along with their individual roles and contributions within the project. Companion papers from three of the four technical Task Groups of the xLPR Program (Computational, Modeling, and Inputs) provide substantial additional information detailing the technical approach and direction of this project.

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