COMPARISON OF WWER-1000 REACTOR CORES BEHAVIOUR WITH DIFFERENT FUEL ASSEMBLIES IN SEVERE ACCIDENT CONDITIONS

The behaviour of two different WWER 1000 nuclear reactor cores has been compared and analysed in this paper: one with recently installed fuel assemblies TVSA-12 and another with previously used TVSA fuel assemblies. The goal is to analyze Large Break Loss of Coolant Accident (LB LOCA) with a double ended guillotine type scenario to simulate severe accident conditions in WWER1000 reactor. To create conditions for severe accident simultaneously with LB LOCA a station blackout is assumed. For this reason, the available safety systems are only hydro accumulators, which can prevent initial high heat up in the first few minutes until diesel generators are activated and start to inject cold water in the primary circuit. The calculations have been performed with MELCOR 2.1 computer code world widely to model the progression of severe accidents in nuclear power plants. One of the main objectives of comparison between nuclear reactor cores with TVSA-12 and TVSA fuel assemblies has been to examine reactor core heat up, followed by fuel damaging with further melting and relocation of fuel elements and reactor internals until the late in vessel phase. The paper is focused on investigating the impact of some important key phenomena during severe accident conditions on the accident progression as well as on the nuclear power plant response. Fuel behaviour has been investigated starting from oxidation of fuel cladding and fusion product release in the primary circuit after rupture of the pipe. Fuel and reactor core melting has been analysed as well as the disposition on the molten material to the bottom of the reactor vessel. The presented analysis can be used in assessment of safety operation of WWER-1000 NPP Kozloduy reactors with new fuel assemblies.