Safety Assessment for Development of Severe Accident Management Guidelines Using In-House Code ‘Corves 2.0’ for KKNPP VVER-1000 Reactors

Severe accident conditions in nuclear reactors involve extensive degradation of fuel rods and partial/complete core melting occurring due to multiple failures of safety systems. Such severe accident progression in VVER-1000 reactors is generally categorized into two phases: In-vessel phase and Ex-vessel phase. As part of analysis tool development for severe accident analysis, computer code ‘CORVES 2.0’ has been developed for modeling degradation of reactor core and other internal components during in-vessel phase of KKNPP VVER-1000 reactors. CORVES 2.0 simulates various physio-chemical phenomena during severe accident progression and is used to provide analytical support for development of Severe Accident Management Guidelines (SAMG). During early phase of accident progression, the code calculates the heat-up of fuel during core uncovered conditions, and captures various phenomena such as: formation of U-Zr-O mixture due to chemical interaction between clad and fuel, oxidation of fuel rods and control rods, candling phenomena observed during metallic meltdown of severe accident. The developed code models elliptical shaped RPV lower head (specific feature of VVER) and calculates heat transfer in stratified corium configuration. CORVES 2.0 predicts RPV failure and also captures composition of corium outflow which will be used as input for the Core Catcher phenomena calculations. It is useful for determining the severe accident management strategies and these studies are considered for the development of the severe accident management guidelines (SAMG). Computer code ‘CORVES 2.0’ has been thoroughly validated using the results of international severe accident analysis codes. In the present study, the representative scenario of LB-LOCA along with ESBO for KKNPP has been analysed using ‘CORVES 2.0’ code.