Hybrid lumped/distributed parameter model for treating the vessel lower head ablation by corium during a LWR severe accident

Abstract : Within the context of Severe Accident for Light Water Reactors, the studies of in-vessel corium behavior and associated risk of vessel failure are matters of prime interest. The corium heat flux at the pool interface can lead to the ablation of the steel vessel wall. The ablation kinetics is of prime interest when considering the possible formation of a “thin” metal phase on the top of the pool and the potential vessel failure at that point due to heat flux concentration, in particular when evaluating the chances of success of in-vessel retention (IVR) strategy. In the framework of so-called severe accident codes where fast-running models are used, standard models based on a 2D meshing of the wall can become impractical in particular when a too thin metal phase appears. Thus, this paper is focused on a simplified yet accurate modelling of the wall heating and ablation. It is based on a 1D axial meshing of the wall. For each wall mesh, radial heat fluxes at the wall internal and external boundaries are calculated following a lumped parameter modelling approach while the axial heat fluxes are approximated by a first-order finite difference formula. This model is validated against reference 2D solutions of the heat equation on pure conduction cases starting from a typical in-vessel configuration and constructed from nondimensionalization considerations. Finally, this model is applied to in-vessel corium transient calculations under IVR conditions.