2018 | AFC ACCOMPLISHMENTS 62 Fabrication of High Density UN Kernels for Fully Ceramic Matrix Fuels Principal Investigator: J. McMurray Collaborator: K.Terrani Uranium nitride (UN) has a high U density that is an important property for the accident tolerant fully ceramic micro- encapsulated fuel design. Gadolinium is burnable poison important for flat- tening the reactivity of the fuel. Project Description: A technological challenge associated with use of UN as the kernel for fully ceramic microencapsulated fuel, a candidate accident tolerant fuel form, is the reactivity must be decreased at the beginning of life with a burnable poison such as Gd.The focus of this research was to explore the viability of Gd additions to the sol-gel feedstock used to produce UN microsphere. An important physical property is the density, therefore the final density after carbothermic reduction and nitriding (CTRN) was measured and compared to benchmark UN.The impact of the chemical form of Gd additions to the sol-gel feedstock was investigated. It was shown that Gd is viable as an additive to UN microspheres and that very high theoretical densities (>95%) can be attained when introduced as a nitrate hexahydrate to the sol-gel broth Accomplishments: The goal of the research was to investigate the viability of Gd additions to UN microspheres. A significant discovery was that samples with Gd added using gadolinium nitrate hexahydrate, Gd(NO3)36H2O showed clear increases in density over those without it prepared with the same carbon, dispersant and processing parameters. Gadolinium mononitride is isostructural with UN; it was anticipated for the added Gd to be incorporated into the UN matrix to form (U,Gd)N. One aim of this work was to determine if solid solution sintering densification mechanisms would be active. Since the X-Ray Diffraction (XRD) results for the sample with 0.125 Gd by metal fraction showed ~12.5 wt% Gd2O3, it is not known whether the densification mechanism results from the solid solution, precipitation, or some combination thereof. The density decreased significantly over the baseline for Gd added as an oxide in the form of Gd2O3. It is proposed that Gd is better mixed when added as Gd(NO3)36H2O which dissolves in the ADUN whereas Gd2O3 nanoparticles form a suspension. Further, the dispersant effect could be altered by Gd2O3 resulting in C agglomerations that are thought to be detrimental to achieving high density kernels. While the Gd added as Gd(NO3)36H2O has been shown to improve the overall density of UN, it also has the effect of displacing U. It is reported that, 12.8 g/cm3 is required for the FCM design. Using the highest density numbers from this study only 12.4 g/cm3 was attained. Other routes must be taken to achieve higher U densities, for example hot isostatic pressing which has been shown to produce up UN microspheres with up to13.1 g/cm3 . Gadolinium can be added as a burnable poison to UN derived from the sol-gel method while at the same time increasing theoretical density.