The ambient- and high-temperature oxidation behaviour of U$^{3+}$N and Ln$^{3+}$N compounds relevant to spent nuclear fuel

Actinide and lanthanide binary nitrides (AnN, LnN), are isostructural cubic compounds relevant to next-generation nuclear fuels which require detailed understanding regarding their oxidative degradation for safe disposal, yet a systematic comparative description remains. Herein, the room- and high-temperature oxidation behaviour of U$^{3+}$N and Ln$^{3+}$N (Ln = Pr, Nd, Gd, Tb, Dy, Ho, Tm and Lu) compounds is examined via a combination of X-ray diffraction, electron microscopy, thermogravimetric, and X-ray absorption spectroscopy analysis. At room temperature, UN was found to undergo an oxygen mediated oxidising mechanism, which contrasted chemically and microstructurally to the LnN’s, which followed a consistent hydrolysis mechanism. At high temperature, more congruent behaviour is determined with direct occurrence of oxide products, where the onset temperature of oxidation was found to correlate with the ionic radii of examined Ln/U cations. The results provide insight into the behaviour of these compounds in UN-based spent fuel, particularly phase separation variability and incongruent behaviour during inadvertent oxidation.