Modelling of liquid phase segregation in the Uranium-Oxygen binary system

In the uranium-oxygen (U-O) binary system, the existence of a miscibility gap induces the segregation between two immiscible liquid phases at thermodynamic equilibrium: an oxidic phase and a metallic one. Within the framework of severe accidents in a Light Water Reactor (LWR), the knowledge of in-vessel corium (U-O-Zr-steel system) behaviour and the associated vessel failure risk are of prime interest. The corium pool configuration plays a key role on the heat flux distribution and therefore on the vessel integrity. A coupled thermochemistry-thermalhydraulics model is needed for investigating the stratification kinetics of a corium pool. The work presented here is a first step in the development of such a model, and is focused on the modelling of the kinetics of liquid phase segregation in the U-O binary system. With this aim, we have developed a Cahn-Hilliard model, derived from a free energy functional, for investigating the liquid phase segregation. The present paper discusses the coupling of the transient Cahn-Hilliard solver with a U-O CALPHAD thermodynamic database and the parameterization of such a kinetic model.