xLPR Pilot Study Report (NUREG-2110)
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Manuscript Completed: May 2012
Date Published: May 2012
NRC Project Manager: David Rudland
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Under the auspices of an addendum to the memorandum of understanding between the Electric Power Research Institute and the U.S. Nuclear Regulatory Commission’s Office of Nuclear Regulatory Research for cooperative research, a pilot study has been completed to evaluate the feasibility of developing a fully probabilistic, fracture-mechanics-based computational tool to evaluate the rupture probability of reactor coolant piping. This project, known as xLPR for Extremely Low Probability of Rupture, is initially focused on evaluating pipe rupture probabilities within Alloy 82/182 dissimilar metal welds located in lines licensed for leak-before-break (LBB) as allowed under General Design Criterion 4, "Environmental and Dynamic Effects Design Bases," of Appendix A, "General Design Criteria for Nuclear Power Plants," to Title 10 of the Code of Federal Regulations Part 50, "Domestic Licensing of Production and Utilization Facilities." The current LBB regulatory basis does not allow for assessment of piping systems subject to active degradation mechanisms, such as primary water stress-corrosion cracking, which has been detected in some systems that have been granted LBB approval. Although the piping systems susceptible to this type of corrosion have been shown through deterministic arguments to comply with the regulations, no fully probabilistic tool currently exists to directly assess this compliance.
Development of such a complex probabilistic computational tool is a daunting technical challenge, and the project team determined that a pilot study was necessary. The three key goals addressed include establishing the fundamental feasibility of such an undertaking, assessing whether the proposed organizational approach could accomplish the task, and informing the decision on the most appropriate computational platform to employ.
This report summarizes the results of that pilot study and provides an overview of the complete project documentation contained in a number of more detailed reports. The xLPR Pilot Study team demonstrated that it is feasible to develop a modular-based computer code for the determination of probability of rupture for LBB-approved piping systems. Furthermore, while the organization established to manage the project and accomplish the technical work of the Pilot Study successfully met that challenge, it identified important improvement opportunities that will be addressed as the project moves forward. Finally, substantial knowledge and experience were gained through the development of two parallel Pilot Study computational codes using a commercially licensed simulation framework code in one case and an open source framework code in the other. This approach has provided a strong basis for computational platform selection for further xLPR development. The xLPR Pilot Study successfully met its three key objectives and has established a solid base of knowledge and experience supporting further development.