Equilibrium Fractionation of Oxygen Isotopes in the U₃O₈-Atmospheric Oxygen System

As a major component in the nuclear fuel cycle, octoxide uranium is subjected to intensive nuclear forensics research. Scientific efforts have been mainly dedicated to determine signatures, allowing for clear and distinct attribution. The oxygen isotopic composition of octoxide uranium, acquired during the fabrication process of the nuclear fuel, might serve as a signature. Hence, understanding the factors governing the final oxygen isotopic composition and the chemical systems in which U₃O₈ was produced may develop a new fingerprint concerning the history of the material and/or the process to which it was subjected. This research determines the fractionation of oxygen isotopes at different temperatures relevant to the nuclear fuel cycle in the system of U₃O₈ and atmospheric O₂. We avoid the retrograde isotope effect at the cooling stage at the end of the fabrication process of U₃O₈. The system attains the isotope equilibrium at temperatures higher than 300 °C. The average δ¹⁸O values of U₃O₈ in equilibrium with atmospheric oxygen have been found to span over a wide range, from -9.90‰ at 300 °C up to 18.40‰ at 800 °C. The temperature dependency of the equilibrium fractionation (1000 ln α_U ) exhibits two distinct regions, around -33‰ between 300 °C and -500 °C and -5‰ between 700 °C and -800 °C. The sharp change coincides with the transition from a pseudo-hexagonal structure to a hexagonal structure. A depletion trend in δ¹⁸O is associated with the orthorhombic structure and may result from the uranium mass effect, which might also play a role in the depletion of 5‰ versus atmospheric oxygen at high temperatures.