Experimental and thermodynamic evaluation of the melting behavior of irradiated oxide fuels

Onset of melting is an important performance limit for irradiated UO₂ and UO₂-based nuclear reactor fuels. Melting (solidus) temperatures are reasonably well known for starting fuel materials such as UO₂ and (U,Pu)O₂, however the influence of burnup on oxide fuel melting behavior continues to represent an area of considerable uncertainty. In this paper we report the results of a variety of melting temperature measurements on pseudo-binary fuel-fissia mixtures such as UO₂-PuO₂, UO₂-CeO₂, UO₂-BaO, UO₂-SrO, UO₂-BaZrO₃, and UO₂-SrZrO₃. These measurements were performed using the thermal arrest technique on tungsten-encapsulated specimens. Several low melting eutectics, the existence of which had previously been inferred from post-irradiation examinations of high burnup mixed oxide fuels, were characterized in the course of the investigation. Also, an assessment of melting temperature changes in irradiated oxide fuels due to the production and incorporation of soluble oxidic fission products was performed by application of solution theory to the available pseudo-binary phase diagram data. The results of this assessment suggest that depression of oxide fuel solidus temperatures by dissolved fission products is substantially less than that indicated by earlier experimental studies.