Influence of plutonium on the thermodynamic properties and phase equilibria in corium

Abstract : During a severe accident in a pressurized water reactor, very high temperatures can be reached (T>2500 K). In these conditions, the UO$_2$ or mixed oxide (U,Pu)O$_2$ nuclear fuel shall react with the Zircaloy cladding and then with the steel vessel, forming a complex mixture of solid and liquid phases called in-vessel corium. At very high temperature, the corium can be constituted of two metallic and oxide liquid phases due to the existence of a miscibility gap in the liquid state. The relative fraction and composition of these phases that form in the corium as a function of temperature are key input data for severe accident codes to predict corium thermal-hydraulics properties.The TAF-ID thermodynamic database [1] coupled to software like Thermo-Calc or Open Calphad is a powerful computational tool to predict the thermodynamic data of nuclear fuel materials. The database is developed using the Calphad method on the basis of the available experimental data. In case of severe accident applications, experimental data are missing, especially on systems containing plutonium. New measurements of thermodynamic and phase diagram data on subsystems of the prototypic in-vessel corium U-Pu-Zr-Fe-O are required. Such experiments are particularly difficult to carry out due to radiotoxicity of the materials and to the very high temperature level. New measurements using advanced laser melting techniques will be presented. The models derived from these new experimental data will be described. Finally the influence of the presence of plutonium in the mixed oxide fuel on the thermodynamic properties and phase equilibria of the in-vessel corium will be investigated by thermodynamic calculations.