Study of the VVER-1000 Behaviour during LBLOCA with ID 850 mm Combined with Station Blackout Using Computer Code MELCOR 2.1

The purpose of this study is to analyse the key phenomena and processes during the transient “Station blackout (SBO) with large break loss of coolant accident (LBLOCA) with ID=850mm”. It has been analysed the processes, which show the accident progression since the beginning of LBLOCA in the cold leg to a rupture of the reactor vessel, including the total core uncovery, core heating up, hydrogen generation and fusion products release in the primary circuit after rupture of the fuel cladding, melting of the fuel and reactor core internals, and its further relocation to the bottom of the reactor vessel. For the presented analysis, it has been developing a new model for the reactor VVER-1000 with the computer code MELCOR 2.1 for severe accident. The reference nuclear power plant in this study is Kozloduy NPP, units 5 and 6. It has been compared the older version on the computer code MELCOR 1.8.5 with the new version of the code as using the same initial and boundary conditions. The difference in the development of the accident is determined mainly by differences in the models that have been recently developed and added to the MELCOR 2.1 version.