Quantification of process variables for carbothermic synthesis of UC_1-xN_x fuel microspheres

This report details the continued investigation of process variables involved in converting sol-gel-derived, urania-carbon microspheres to ∼820-μm-dia. UC_1-xN_x fuel kernels in flow-through, vertical Mo and W crucibles at temperatures up to 2123 K. Experiments included calcining of air-dried UO₃-H₂O-C microspheres in Ar and H₂-containing gases, conversion of the resulting UO₂-C kernels to dense UO₂:2UC in the same gases and vacuum, and its conversion in N₂ to UC_1-xN_x (x = ∼0.85). The thermodynamics of the relevant reactions were applied extensively to interpret and control the process variables. Producing the precursor UO₂:2UC kernel of ∼96% theoretical density was required, but its subsequent conversion to UC_1-xN_x at 2123 K was not accompanied by sintering and resulted in ∼83–86% of theoretical density. Increasing the UC_1-xN_x kernel nitride component to ∼0.98 in flowing N₂-H₂ mixtures to evolve HCN was shown to be quantitatively consistent with present and past experiments and the only useful application of H₂ in the entire process. - Highlights: • Sol-gel feedstock conversion to UN through carbothermic reduction. • Investigation of process gas effect on final kernel quality and density. • Recommended process for consistent kernel production.