Formation of columnar and equiaxed grains by the reduction of U3O8 pellets to UO2+x

The reduction of U3O8 pellets to UO2+x has been investigated at 1300 °C in H2, Ar and CO2 gas atmospheres by TGA, SEM, and X-ray diffraction. The selected U3O8 pellet was prepared by sintering a U3O8 powder compact. The TGA results show that the reduction rate is fastest in H2 gas, and X-ray diffraction indicates that U3O8 reduces to UO2+x without any intermediate phase. The reduced pellet, UO2+x, has a special grain structure that consists of equiaxed grains at the surface, columnar grains in the middle, and equiaxed grains in the center. The equiaxed grains and columnar grains are much smaller in H2 gas than in Ar or CO2 gas. The reducing gases significantly influence the morphology of the grain structure. This difference can be explained in terms of a relation between oxygen potential and critical nucleus size during the reduction.