Investigation of post LOCA cooldown procedure for Kozloduy NPP

This paper provides a discussion of various RELAP5 parameters, calculated for the investigation of nuclear power reactor parameters behavior in case of SB LOCA transient. It has been chosen 31.85 mm cold leg 6 break.The main purpose of the calculation is to investigate effective means and check the criteria for primary depressurization during SB-LOCA in VVER440 design. The basic scenario represents a 31.85 mm cold leg break. The aim of the calculations is to investigate the most likely response of the real NPP Kozloduy and the adequacy of the main operator actions during the post-LOCA Cooldown Procedure. The main strategy of the operator actions was primary side cooling by the secondary bleed (by BRU-A opening) and feed, Pressurizer water level supporting by opening pressurizer safety relief valve and HPPs. It has been investigated strategy with switching off all HPPs step by step. Calculations were performed by thermal-hydraulic code RELAP5/MOD3.2 and the 6 loops model of Kozloduy NPP, Unit M (VVER440/V230). A computer model for VVER440 was developed and validated at the Institute for Nuclear Research and Nuclear Energy (INRNE) - Bulgarian Academy of Sciences for analyses of operational occurrences, abnormal events, and design basis scenarios.Validation of the model has been done based on comparison with the real experimental plant data obtained from Kozloduy NPP. (author)