UN Sintering Additives for Improved Control Over Advanced Fuel Forms

Uranium mononitride (UN) has been proposed as substitute for uranium dioxide (UO₂) monoliths or uranium carbide and uranium oxide (UCO) kernels in next generation nuclear fuels. The switch to UN would have three major benefits over UO₂ or UCO: higher uranium density (both), better thermal conductivity (UO₂), and higher melting point (UCO). However, the fabrication of high-density UN has historically proven challenging, especially after transitioning to the industrial scale. Sintering additives provide a straightforward approach to alleviate difficulties fabricating high-density UN while still achieving a microstructure capable of accommodating the needs of a wide variety of reactor applications. In this study, kernels were used as a model system to explore the effects of various sintering additives, because the additives can be easily and uniformly introduced into the precursor material. Kernels are fabricated in two major steps: microsphere precursor synthesis and conversion to the final material. Microsphere synthesis utilizes well-established sol-gel chemistry to produce kernel precursors with sintering additives controllably metered into the starting solution. The resulting microspheres were converted to their final ceramic form utilizing a rapid temperature profile designed to test the efficacy of each additive. The converted kernels were characterized using x-ray diffraction, scanning electron microscopy, and other typical kernel characterization techniques to observe how the additives affect composition, microstructure, and bulk properties. Density functional theory modeling was also used to provide insights into the atomic features underlying the improved densification caused by the additives, thereby creating a basis for material design selection. Overall, the improved understanding can be used to readily control the microstructure of the material without significantly affecting the bulk properties, opening avenues for the development of new fuel concepts.