Modeling of the corium crust of a stratified corium pool during severe accidents in light water reactors

During Severe Accidents (SAs) in Light Water Reactors (LWRs), the core heat up due to the lack of fuel cooling in the Reactor Pressure Vessel (RPV) may lead to the formation of a so called corium pool (molten core oxidic and metallic liquid materials) relocating in the lower head of the RPV. Within the framework of SA, one strategy to minimize risks of containment failure is the In Vessel Retention (IVR) strategy. To demonstrate the feasibility of such a strategy and to ensure the vessel integrity during the accident, one of the key points is to correctly evaluate the thermal load applied on the RPV wall by the lower head corium pool. Among all the complex coupled transient phenomena taking place in the RPV, the possible solidification of the corium pool, forming the so-called corium crust, has been shown by experimental and numerical results to be a key phenomenon for the IVR strategy. However, in most SA codes, the corium crust is only taken into account as a stationary model playing the role of a thermal resistance between the corium pool and the RPV wall. In this paper, a transient corium crust model is presented which explicitly takes into account the mass, temperature, composition and thermodynamic properties of the crust and pool material. Numerical results, in the context of the IVR strategy, highlight the impact of the transient corium crust modeling in comparison to a stationary modeling. The proposed model opens new perspective of modeling such as the dissolution of the crust and/or its thermo-mechanical behavior.