A simplified model for the quaternary U-Zr-Fe-O system in the miscibility gap

Abstract

The quaternary system of U-Zr-Fe-O, is characterized by a miscibility gap at high temperature, when both metal and oxide phases are in liquid state. In a molten pool, when both liquid phases co-exist, their relative position depends on the density of the metal. Therefore, prediction of the liquid metal composition is important to determine its density, as it may become heavier than the oxide phase, when U content becomes substantially high. In a previous study made by Salay and Fichot (2004), a fit to thermochemical equilibrium data for quaternary system had been derived and tested. In this paper, that previous fit is improved to derive a new fitting function, which is capable of covering a wider range of reactor designs (characterized by Cuz in present study) and accident scenarios (characterized by Cox). In order to derive this new fit, equilibrium calculations were made with the thermochemical database NUCLEA, developed at IRSN. One of the characteristic features of the phases at equilibrium, studied in this work, is that Fe found to be always absent from the oxide phase. Another feature observed, is that the solubility of O in the metal phase is quite limited. Finally, the analysis performed shows that the atomic ratio of U/Zr, is almost invariant in the oxide phase, and as a consequence, is almost invariant in the metal phase (at least for a sufficient dilution with Fe). From these observations, it is possible to propose a simplified representation of the equilibrium between liquid oxide and liquid metal, as a simple partition of (U-Zr) between both phases, the other atoms (O and Fe) remaining in their respective phases (oxide and metal). The proposed new fit in this paper consists in evaluating the (U-Zr) partition. The fit is defined by two parameters, which depend on the global degree of oxidation of Zr and on the global U/Zr ratio. Finally, a simple pseudo-binary model is proposed in order to simulate the transient evolution of a two-liquids mixture in the quaternary U-Zr-Fe-O system, towards an equilibrium.