Improvement of in-vessel corium transient stratification modelling regarding the uranium and zirconium phase partitioning

• An improved model for mass transfer in U-O-Zr-Steel system is proposed. • Transfers of U and Zr are separated. • Coherent thermodynamic closures were obtained. • The model was applied to transient stratification and compared to previous model. • The updated model presented showed improvement compared to the previous one. One of the main threats on In-Vessel Melt Retention (IVR) mitigation strategy is the transient thermochemical stratification of the liquid corium in the vessel lowerhead. This paper proposes to improve the modelling of the transient liquid phase stratification of the {U,O,Zr,Steel} system within its miscibility gap. This work is an increment on a previous model considering the metal droplet forming during the relocation, and the focus is here on improving the mass transfer modelling by treating Uranium and Zirconium separately and not as a pseudo component. This means that in the model presented here the molar ratio of Uranium over Zirconium is not equal in the oxide and in the metallic phase. The equations of the model are presented as well as the thermodynamic closures it requires. Dissociating the transfers of Uranium and Zirconium allows for the use of thermodynamic closures that are closer to the actual thermodynamic state calculated using a Gibbs Energy minimizer. The improved model is applied to a synthetic transient where metal is added on top of an oxide phase leading to an equilibrium state with heavy metal layer creation. This synthetic transient is further modified to reach stratification inversion. The results are compared to the previous model and show significant improvements in computing the transient stratification.