Porous UO2 microspheres, or kernels, can be produced by reduction of fired U3O8 kernels to the lower oxidation state. The temperature at which the U3O8 kernel is fired is a major parameter in establishing the degree of porosity, and subsequent reduction from the U3O8 state must be carefully controlled to prevent formation of internal cracks. Restricting the furnace temperature to < 1000°C and providing small incremental increases in the quantity of reducing gas added to the furnace inert atmosphere produced well-distributed porosity in the ceramic body without creating cracks in the internal structure of the kernel.