Coupled Thermal-Hydraulic Analyses of the Molten Pool and Pressure Vessel during a Severe Accident

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 stratification could have on the cooling transient. A homogeneous and two stratified configurations of the debris (with different thickness of the metal layer) 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 three configurations for the heat transfer 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 predicted.