Lessons Learned and Insights From Implementation of EPRI Guidelines on Internal Flooding at Fort Calhoun Station

Abstract

An Internal Flooding Probabilistic Risk Assessment (IF-PRA) study is required to meet the requirements of the Internal Flooding portions of the ASME Internal Events PRA standard [1, 2, 3] and RG 1.200 R1 [4] for risk-informed applications at Nuclear Power Plants (NPPs). An internal flooding study was performed recently for a Nuclear Power Plant (NPP), Fort Calhoun Station (FCS), with a nuclear steam supply system designed by Combustion Engineering. The study was performed using guidance that is currently being developed for industry use by EPRI [5]. The intent of the EPRI draft guidance is to describe how to perform the various tasks to meet the requirements and expectations of the ASME PRA Standard in order to comply with RG 1.200. Performing this analysis provided an opportunity to gain numerous insights on how to best perform the analysis from a technical standpoint. This paper provides lessons learned and insights gained in the implementation of the EPRI draft guidance on internal flooding for PRA. Insights and lessons learned on the assignment of initiating event frequencies, treatment of dynamic flood volumes and time-based operator response timings, and developing associated peer review materials are provided. Discussions dealing with various sources of pipe failure frequencies highlight key differences between commonly used data. Characterizations of spray scenarios were evaluated to determine their impact on plant risk caused by internal flooding events. Maintenance-induced flooding scenarios that are often neglected were evaluated to gain insights on their impact on plant risk. Insights on performing a timely completion of the analysis are provided. Optimization techniques for plant walkdowns to capture relevant information and identify high priority flood areas are discussed. The impact of the flooding scenarios on PRA-related equipment was integrated into the internal events PRA model to quantify the associated risk. The rules used to perform the integration process and the lessons learned from the integration are also discussed.