NUMERICAL SIMULATION OF DROPLET FORMATION BY RAYLEIGH-TAYLOR INSTABILITY IN MULTIPHASE CORIUM

During a severe accident in a nuclear reactor, the melting of the core may lead to the formation of a multiphase liquid pool (corium) in the vessel lower head. The heat transfer at the boundary with the vessel is affected by diffusive and convective mass fluxes. In particular, the development of Rayleigh-Taylor instabilities influence the thickness of the top metallic layer and therefore the "focusing effect" of the heat flux, which is the main risk for the vessel integrity. We use a Cahn-Hilliard pseudo-binary model to describe the uranium/oxygen/zirconium/iron mixture. The diffusion and the convection are governed by the Cahn-Hilliard equation and the Navier-Stokes equations under the Boussinesq approximation. In this first step, the model is isothermal and the buoyancy force is only due to the gradient of chemical composition. The model is solved in two dimensions with a pseudo-spectral code. The initial configuration of a lighter layer of iron-rich fluid above a heavier layer of uranium/oxygen/zirconium mixture is mechanically stable. However, as diffusion progresses, the heavier metallic phase form at the interface. Due to the Rayleigh- Taylor instability, droplets of metallic phase grow and fall into the underneath layer. The droplet formation observed in a former experiment of corium stratification transient is well captured. The interface thickness in the Cahn-Hilliard model is shown to have an influence on the onset of convection and on the flow regime.