Simulation Methods of Site Operating States Distribution for Multi-unit PSA

In general, PSA(Probabilistic Safety Assessment) models have been developed based on a single unit. Such models usually address a specific operating state including full power or 15 POSs(Plant Operating States) for LPSD(Low Power and Shutdown). Recently, the MUPSA(Multi-unit PSA) projects are on-going worldwide to capture holistic insights from a viewpoint of site level. One of the technical issues for MUPSA is to arrange the POSs of each unit such that the combination of POSs can be evaluated in the framework of PSA. Since each unit can have one full power state and 15 POSs, there are 16 states total. For combinations of multi-unit states, the amount will increase exponentially. For example, the site which has n units will have a combination of 16. In an engineer point-of-view, considering every single combination of POSs is inefficient and even impossible, and selecting important POSs and/or screening combinations of low possibilities are important. This paper suggests a simulation model for the combinations of the operating states of each unit in Kori site, total 8 units including the permanently stopped unit 1, using the operation records during 1983-2019 opening in the website of Korea from KINS(Korea Institute of Nuclear Safety) and KHNP(Korea Hydro & Nuclear Power) [1,2]. By using the simulation model, sorting out combinations so called Site Operating States(SOSs) that are screened out will be possible, increasing efficiency for MUPSA.