Application of the Integrated Safety Assessment Methodology to Sequences with Loss of Component Cooling Water System

Abstract

The Integrated Safety Assessment (ISA) methodology, developed by the Consejo de Seguridad Nuclear (CSN), Spanish Nuclear Regulatory Body, has been applied to a thermo-hydraulic analysis of Zion NPP for sequences with loss of the Component Cooling Water System (CCWS) in the context of the NEA application exercise SM2A (Safety Margin Application and Assessment). The ISA methodology allows among others to obtain the damage domain (the region where the PCT limit is exceeded) for each sequence of the dynamic event tree as a function of the uncertain times of operator actuations (secondary side cooling and recovery of CCWS). Sequences with available and unavailable accumulators (ACC) have been analyzed in order to distinguish all the possibilities (ACC demanded and successful, ACC demanded and failed and ACC not demanded). In this work, this damage domain and some parameters of every sequence have been obtained from the results of the simulations performed with TRACE code, these data as well as the time-density probability distributions of the considered uncertain parameters (manual actions) are used to obtain the exceedance frequency of the particular safety limit or damage limit. The results show the feasibility of ISA methodology in order to obtain accurate enough regions of uncertain parameters (time delays or physical parameters) where the particular safety limit of interest is exceeded, as well as the frequency of exceeding this limit.