CONSIDERATION OF MULTIPLE SPURIOUS OPERATIONS IN THE FIRE PROBABILISTIC RISK ASSESSMENT OF SHIMANE UNIT 2 NUCLEAR POWER PLANT

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This paper outlines the methodology taken in the preparation and execution of the fire-induced multiple spurious operation (MSO) for the Shimane Unit 2 Nuclear Power Plant, as well as introduce key takeaways. Following the near-catastrophic fire event in the Browns Ferry Nuclear Power Plant in 1975, the US Nuclear Regulatory Commission (NRC) has taken action to require the evaluation of fires that could lead to the simultaneous spurious operation of multiple electrical equipment. In the Fire PRA, the ASME/ANS PRA standard [1] requires the PRA analyst to consider spurious operations that could contribute to an initiating event, affect the functionality of systems credited in the PRA, and/or result in the loss of reactor coolant system integrity. The Shimane Fire PRA is being performed in accordance with the standard, as well as the EPRI/NRC Fire PRA Methodology[2], and thus the applicability of MSO scenarios has been discussed in an expert panel held in June 2019. In this panel, experts on system design, electrical engineering, fire safe shutdown analysis, and PRA, gathered in the office of Hitachi-GE Nuclear Energy in Hitachi-city, Ibaraki, reviewed the compiled list of potential MSO scenarios and discussed their applicability for Shimane Unit 2 over a span of one week. This MSO list consisted of generic potential MSOs listed in NEI 00-01[3], augmented by plant-specific scenarios identified in the pre-panel reviews of piping & instrumentation diagrams and electrical schematic diagrams. The output of this expert panel was a plant-specific list of equipment failures to be added in the FPRA. As a result, the expert panel, over seventy scenarios were judged to be applicable to Shimane Unit 2, as well as additional action items to be considered later. An example of the identified scenarios is the failure of an injection system due to flow diversions caused by fire-induced inadvertent opening of multiple valves. Equipment associated with each applicable MSO scenario were identified and added to the Task 2 equipment list, and their cables were selected as part of the Task 3 cable selection. The event trees/fault trees were also modified to capture the effects of each scenario as part of the Task 5 process.