Improving the Performance of Uranium Mononitride as an Advanced Technology Fuel (ATF) with Niobium Alloying

Abstract

Enhancing the safety and efficiency of nuclear fuels is critical for the future of nuclear energy, given its potential to deliver consistent, low-carbon power. Uranium mononitride (UN) offers superior thermal conductivity and higher uranium density compared to uranium dioxide (UO2), but its susceptibility to oxidation and limited understanding of its behaviour present significant challenges. This study utilised direct current magnetron sputtering to fabricate niobium-doped UN thin films as a model system, enabling precise control over deposition parameters such as pressure, temperature, and substrate type. Thin films were characterised using X-ray diffraction (XRD), X-ray reflectometry (XRR), wavelength dispersive spectroscopy (WDS), and X-ray photoelectron spectroscopy (XPS). Notably, accelerated lattice shrinking inconsistent with Vegard’s Law was observed in glass-substrate samples, attributed to electronegativity differences and strain effects, exacerbated by the amorphous nature of the substrate. This finding underscores the critical role of substrate choice in stabilising thin film growth. A passive oxidation study revealed that niobium slowed but did not prevent long-term oxidation of UN thin films, with continued degradation observed over 37 months. Additionally, corrosion experiments in H2O2 demonstrated that niobium doping unexpectedly reduced the corrosion resistance of UN, despite uniform surface roughness showing even corrosion and a NbN sample withstanding corrosion. These experiments also identified a uranium oxynitride layer forming at the interface during degradation, corroborating structural and chemical analyses. The chemical states and valence band of niobium-doped UN were probed via XPS, confirming structural similarities between UN and NbN while revealing limited chemical modification due to doping. These insights, coupled with findings from complementary techniques, provide a comprehensive understanding of dopant effects on UN stability and corrosion behaviour. This work advances the fundamental understanding of UN and highlights its potential as a next-generation nuclear fuel material.

Date of Award	9 Dec 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Ross S Springell (Supervisor) & Tomas L Martin (Supervisor)