Influence of the Corium Configuration on the Lower Head Thermal Behaviour in Case of Severe Accident in a PWR

Accidental scenarios in a PWR with core melt-down and flooding of the reactor pit have been considered in order to investigate the influence that the debris configuration has on the cooling transient.A homogeneous and three stratified configurations of the debris (with different thickness of the metal layer and distributionof the residual power) have been analysed by means of a finite element method.The model accounts for natural convection in the molten debris, heat transfer in the vessel wall, thermal radiation from the pool surface and for phase changes. The heat transfer between the external vessel wall and the cooling water has been considered by means of suitable correlations accounting for water boiling.In all the considered configurations the computed heat fluxes at the external vessel surface are below the CHF and the debris are retained within the vessel. Nevertheless the analyses show noticeable differences between the four configurations for the vessel wall behaviour and for heat fluxes from the molten pool to the cooling water.In particular for stratified configurations a peak of the lateral heat flux in correspondence of the metal layer has been observed along with an important erosion of the vessel wall.