Investigation of sol-gel feedstock additions and process variables on the density and microstructure of UN microspheres

Abstract The kernel of fully ceramic microencapsulated (FCM) fuel requires a material with high fissile density. For that reason, among others, UN is the appropriate chemical state for low enriched U fuel. The UN kernels are spheres ∼800 μm in diameter made using a sol-gel process. The effect of additives on the chemistry and density of the UN microspheres are investigated in this work. Gadolinium nitrate hexahydrate, Gd2O3, B, SiO2 and SiC were incorporated into the sol-gel broth in varying concentrations. It was found that Gd can serve as both a sintering aid and burnable poison when added to the sol-gel broth as a Gd(NO3)36H2O. However, even with the increased theoretical density of the UN microspheres, the U density was still too low for the FCM design that replaces UO2 pellets in a commercial light water reactor. Silicon carbide and B were also successfully added but produced a lower density final product. Other sol-gel processing variables were investigated. The pour density of the sol-gel feedstock was found to influence the final converted UN kernel density.