Identification of Plant Operating States and Quantification of Initiating Event Frequency for Shutdown Probabilistic Safety Assessment of KKNPP-1&2

This paper discusses about the identification of Plant Operating States (POS) and quantification of Initiating Event (IE) frequency under each POS for VVER design. The tasks are performed as a part of In-house assessment of shutdown state risk for Kudankulam Nuclear Power Plant (KKNPP-1&2). As a part of this study, plant refueling shutdown (RSD) plans for both full and partial core refueling have been reviewed and plant operating states (POS) such as reactor dismantled state, core unloading and fuel loading state, maintenance phase etc. have been assessed based on reactor process parameters. Total 38 numbers of POS has been screened in for modeling based on similar conditions, events and component states. Initiating events (IE) under each POS are then investigated to model accident progression (Event tree ET model) using software Risk Spectrum ver. 1.1.3 leading to core damage (CD). This paper illustrates the criteria to identify POS, apportionment of POS duration under total shutdown duration as each POS presents specific risk condition. IEs under each POS have been identified and IE frequency pertaining to specified duration of POS has been calculated. Both generic and plant specific data has been utilized to quantify IE frequency. These inputs have been used in accident progression model and sequences leading further to core damage under each POS have been analysed for final quantification of overall core damage frequency (CDF).