The Thermal Expansion of Uranium Mononitride (UN)

Uranium mononitride (UN) is an advanced ceramic nuclear fuel which has generated considerable interest in recent years with the on-going development of generation IV nuclear reactors (Streit, M. Eur. Ceram. Soc. 2005). Despite this interest, crucial physical and chemical properties remain relatively understudied, including the coefficient of thermal expansion (CTE) which describes a material’s volumetric expansion in response to temperature. A robust understanding of high temperature structures, thermophysical properties, and thermal oxidation behaviors of nuclear fuels is critically needed to evaluate fuel performance under operating and off-normal conditions. Herein, the CTE of UN was experimentally determined using in-situ high temperature X-ray diffraction (XRD) to observe changes in unit cell parameters from 30 to 500 °C under oxidizing condition. Simultaneously, we investigated phase alteration of UN under oxidizing conditions and mapped different phases from UN thermal oxidation as a function of temperature. Upon heating in the open-air, UN oxidized slowly, forming UO₂, with oxidation beginning at 200 °C and rapidly accelerating at 300 °C until complete of UN was complete at approximately 410 °C. Formation of UO₃ was not observed, which suggests that past work using TGA may have observed a thermal oxidation product with mass equivalent to UO₃ (Goncharov, V.G. J. Nucl. Mater. 2022). Additionally, while minor phases could not be definitively identified via XRD, several small reflections were successfully modeled as a uranium oxidation product with a fluorite structure and a smaller unit cell (approx. 5.31 Å) than UO_2+x. Raman spectroscopy was used to conclusively identify these minor surface species as UN_2-x products. The thermal expansion of UN was tracked up until the complete disappearance of the phase at 410 °C. At ambient conditions, the unit cell parameter a₀ was determined to be 4.8961(1) Å, which expanded at high temperatures to 4.9220(19) Å at 410 °C. It was noted that the ambient temperature lattice parameter, a₀, in this study is slightly higher than the (limited) literature reports (4.890 Å) by 0.12 %. Previous studies have attributed similar expansions in the unit cell of UN to either carbon impurities or differences in porosity, both of which will be considered going forward (Hayes, S.L. J. Nucl. Mater, 1990).The CTE was calculated using the XRD data by plotting fractional expansion of the unit cell as a function of temperature change and fitting a first order, temperature dependent function to the data. The CTE determined by the thermal oxidation study is higher than the few existing literature values, across several techniques and samples. Continuing investigations are underway to determine if the thermal expansion of UN observed during a thermal oxidation process, where some amount of O may diffuse into the UN lattice, is fundamentally different to the thermal expansion of UN isolated from its environment.