Applying ANSYS/Multiphysics to In-Vessel and Ex-Vessel Core Melt Phenomena

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 or of the core catcher 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.

Different melt configurations can be assumed. They can be distinguished by the melt masses released into the LH, the melt composition, the segregation behaviour of the oxidic and the metallic component, and the density and the distribution of the internal heat sources. One of the most dangerous accident scenarios for a pressurized water reactor (PWR) assumes the relocation of a melt mass in the range of 200 Mg or more which is segregated into some 150 Mg of the heavier oxidic component at the bottom and some 50 Mg of a metallic melt above. The oxidic melt is surrounded by an oxidic crust due to the high solidification temperature of the oxide. The internal heat sources are mainly in the oxidic component, but due to the geometric configuration and the different fluid properties the highest heat fluxes and the major thermal ablation is expected between the metallic layer and the vessel wall. This phenomenon is called focussing effect.

An in-vessel-scenario of a PWR has been assumed and modelled with ANSYS/ Multiphysics. Using this Finite Element (FE) code package it is possible to simulate the velocity and temperature field for a fluid region and the corresponding temperature field within the surrounding solid structures. This thermo-fluiddynamic calculation can be coupled with a mechanical FE-model of the solid structures to analyse the stress and possible creeping forced by the mechanical and thermal loads. In this presentation, the recent modelling approach and the results obtained with the code ANSYS/Multiphysics are discussed. An outlook on the further development is given.