Thermodynamics of the O–U system: III – Critical assessment of phase diagram data in the U–UO2+x composition range

In order to help understand the behavior of the UO2 fuel in a severe nuclear accident, the phase diagram data of the O–U system in the U–UO2 composition range are reviewed critically, so that the most reliable set of original data can be selected. In the course of this assessment, a special effort is made to evaluate as realistically as possible the associated uncertainties, not only for reliable data selection, but also as weights in the procedure, which is used later to optimize phase diagram and thermodynamic data. Among the criteria for selection, the non-congruent vaporization of the UO2±x phase and the interactions between UO2 and the containment materials are discussed, together with a detailed analysis of the experimental techniques used. A melting temperature for UO2 of 3138±15K is proposed, which is compatible with the selected data for the liquidus and solidus temperatures in this composition range. The solubility limit of oxygen in liquid uranium is selected as the smallest value among different and largely non-consistent values, and this selection leads to the existence of a large miscibility gap in the liquid phase between one metal rich liquid and one oxide rich liquid.