Abstract: A process for producing large-grain, low-density UO.sub.2 micro-fuel spheres, the micro-fuel spheres having an average grain diameter of at least 5 .mu.m, which comprises:(a) isothermally heating one or more UO.sub.2 micro-fuel spheres in a stream consisting essentially of carbon dioxide for a time and at a temperature sufficient to produce grains having an increased average grain diameter of greater than 5 .mu.m, without substantially increasing the density of said micro-fuel spheres; and then(b) sintering said produced grains, in the absence of a sintering additive, in a reducing gas stream for a time and at a temperature sufficient to regulate both the ratio of oxygen to uranium in the produced grains, and the density of the grains.