Nuclear fuel research and development - influence of various manufacturing parameters on some characteristics of UO2 powders and their sintering behaviour

This work deals with the effects of various parameters in the process of manufacturing uranium dioxide from ADU on the characteristics and sintering behaviour of the powders obtained. It has been found that it is possible to manufacture sintered UO2 pellets having controlled sintered densities in the 87-97% TD range, by a proper ADU precipitation-calcination-reduction route, leading to UO2 powders with the desirable sintering properties. Calcination temperatures of the ADU significantly affect the physical properties and the sinterability of the correspondingly produced UO2 powders. It has been noticed that the static-bed thermal decomposition reaction of ADU powders is inhomogeneous, resulting in mixtures of β-UO3 and UOsub(2.9) in irreproducible composition ratios. The grain structure of poorly sintered UO2 pellets consists of a matrix with very small grains (2-3μm) surrounding few ''islands'' of large grains (10-20μm). Two types of pores are observable in UO2 sintered pellets, namely large (2-50μm) irregularly shaped intergranular pores, and small (<1μm) intragranular pores. The shape of the particles of most ADU powders is preserved during their thermal decomposition, up to about 550degC. The presence of fluorine in UO2 powders has been found to have a deleterious effect on sintering. Adhesion properties of UO2 powders are inversely proportional to their primary particles' size. (B.G.)