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, Gd 2 O 3 , B, SiO 2 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(NO 3 ) 3 6H 2 O. However, even with the increased theoretical density of the UN microspheres, the U density was still too low for the FCM design that replaces UO 2 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.