Abstract
Changes of the properties of UO2 compacts such as density, porosity, weight, O/U ratio and average grain size, with increasing sintering temperature were determined for three different kinds of UO2 powder, varying the sintering atmosphere. Hydrogen, vacuum and several combinations of these were compared. Conditions of compacting, sintering time, both heating and cooling rates were fixed constant throughout the experiment.
In the intermediate stage of sintering, it was revealed that the temperature range where the pronounced progress of densification occured was 200 to 300°C lower and that more marked grain growth was encountered in vacuum, compared with in hydrogen. As the kinetics of such acceleration of sintering, the followings were suggested. In. hydrogen atmosphere, the finer the particle size of the powder, the higher the surface energy, and thus the easier the sintering. In vacuum, the relation was similar so far as the particle size of the powder was concerned. Furthermore, it was concluded that the retention of higher excess oxygen content to the temperature higher than 1, 000°C, the activation of the surface layer of the powder by the low pressure, and the cleaning of the powder surface by the desorption of adsorbed gas could be the main mechanisms of acceleration of sintering in vacuum.
In the last stage of sintering, however, more closed pores were inclined to appear in vacuum compared with in hydrogen, which inhibited the grain growth, and the weight loss by mass evaporation became remarkable.
