Analysis of the melt spreading and MCCI during the ex-vessel phase of a severe accident in WWER-1000

Abstract

AbstractThe paper presents analysis results of melt spreading and core-concrete interactions in the containment of a WWER-1000 plant during the ex-vessel phase of a severe accident. The failure of the vessel takes place 8 h 35 min after the initiation of the accident. It has been assumed that the whole area of the containment floor is available for spreading, i.e. the door between the reactor cavity and the main part of the containment is not locked. The melt flow rate from the reactor pressure vessel was used as a boundary condition. The simulation of the melt spreading was performed with the LAVA code. The calculated spreading area varies from 60 to 100 m2 depending on the assumed values of the melt properties. The results from the LAVA calculations were used in parallel for COCOSYS and MELCOR calculations to study the core-concrete interactions. From the analyses it turned out: a larger spreading area leads to a faster cooling of the melt in the initial period of the accident, but in the long term the temperatures are the same. 60 h after start of the ex-vessel phase, the melt is not stabilised.