Modelling the RPV lower head under the load of a segregated metal-oxide corium pool

For future nuclear power plants it is demanded that there are no consequences for the environment and the population even in the closest vicinity of the plant during and after every possible accident scenario. This includes the hypothetical scenario of a severe accident with subsequent core meltdown, corium relocation and formation of a melt pool with internal heat sources in the reactor pressure vessel (RPV) lower head (LH). Some reactor concepts have the aim to arrest the melt in the lower head removing the decay heat by external water flooding. In other concepts a dry reactor pit is designed and after the vessel failure a core catcher shall assure the long term stabilization of the corium within the containment. For both strategies investigations on the transient behaviour of the RPV are necessary. Two kinds of vessel failure can be distinguished: thermal and structural failure. Thermal failure means that the heat flux through the vessel wall becomes so high that the steel solidus temperature at any position of the vessel outside is exceeded. Structural failure means that a combination of thermal and mechanical loads causes the failure, e.g. the wall thickness of the vessel is reduced by thermal ablation and at the same time the internal pressure and the gravitational forces induce creeping with subsequent creep failure. (orig.)