The Development of the Fire PRA Model for Shimane Unit 2 Nuclear Power Plant

Abstract This paper outlines the methodology and analysis of the at-power Fire Probabilistic Risk Assessment (FPRA) of the Shimane Unit 2 Nuclear Power Plant (BWR). For most BWRs in Japan, the Fire PRA model is in the development phase. The Shimane Unit 2 NPP is one such plant, for which the FPRA project began in 2018 as a joint effort by The Chugoku Electric Power Company (Chugoku electric) and Hitachi-GE Nuclear Energy. The PRA process followed the EPRI/NRC Fire PRA Methodology (NUREG/CR-6850) as main guidance, with various other NUREG documents as supplementary reference. The project began with the collection of plant information for the initial quantitative screening (Task 7 of the EPRI/NRC methodology). This includes the definition of compartment boundaries (Task 1), selection of equipment and cables (Tasks 2, 3), and identification of ignition sources (Task 6). The internal events PRA model described in ICONE30-1361 served as the base model for the Fire PRA, and fire-specific considerations, such as multiple spurious operations, human failure events affected by fire (Task 12), and circuit analysis (Task 10) were also incorporated into the model (Task 5). Compartment-level Core Damage Frequencies were then quantified assuming whole-compartment damage for initial quantification. Following initial quantification, high-risk compartments were identified and carried over for detailed analysis (Tasks 8 and 11A), for which compartments were further subdivided into combinations of ignition source and target sets. A severity factor and non-suppression probability, quantified for each combination, provided additional realism in the estimates for the scenario fire risk. In addition, the final quantification considered risk contributions from main control room fires (Task 11B), multi-compartment fire scenarios (Task 11C), and fire damage to exposed structural steel (Task 11D). Assessment of seismic-fire interactions (Task 13) was not included in the project scope. This paper provides an analysis of plant features from the perspective of fire PRA elements, focusing on the relative risk mitigation between initial and final quantification for specific compartments of interest. The analysis also includes key takeaways and lessons learned during the development of the Fire PRA model. Moving forward, refinement measures will be taken to remove over-conservatism for risk-significant fire scenarios, and the model will be updated to reflect latest design changes. The next update also plans to address the Findings and Observations raised during the peer review. The effort included periodic plant walkdowns to confirm various as-built plant characteristics relative to design information. The PRA team, with support from Chugoku Electric, visited the Shimane site a total of four times to collect data regarding plant partitioning, ignition sources, distance between source and target, and control room features. As a key milestone, the project scope also included an online, external review at the end of Quantitative Screening to assess conformance with the ASME/ANS PRA Standard.