Densification behaviour of UO2 in six different atmospheres

The shrinkage behaviour of UO2 has been studied using a dilatometer in various atmospheres of Ar, Ar-8%H2, vacuum, CO2, commercial N2 and N2+1000 ppm of O2. The onset of shrinkage occurs at around 300-400 deg. C lower in oxidizing atmospheres such as CO2, commercial N2 and N2+1000 ppm O2 compared to that in reducing or inert atmospheres. Shrinkage behaviour of UO2 is almost identical in Ar, Ar-8%H2 and vacuum. The shrinkage in N2+1000 ppm O2 begins at a lower temperature than that in the commercial N2. The mechanism of sintering in the reducing, inert and vacuum atmospheres is explained by diffusion of uranium vacancies and that in the oxidizing atmospheres by cluster formation