Oxygen Isotope Alterations during the Reduction of U₃O₈ to UO₂ for Nuclear Forensics Applications

The fabrication of UO₂ from U₃O₈ is an essential reaction in the nuclear fuel cycle. The oxygen isotope fractionation associated with this reaction has significant implications in the general field of nuclear forensics. Hence, the oxygen isotope fractionation during the reduction of U₃O₈ to UO₂ was determined in the temperature range of 500-700 °C and for a duration of 2 to 6 h under a high-purity H₂ atmosphere. Three U₃O₈ samples, possessing a different oxygen isotopic composition, were used to investigate key parameters involved with the fractionation during the reduction process. All UO₂ products did not maintain the original isotope composition of the starting U₃O₈ under all conditions. The results show that the system UO₂-H₂O attains isotope equilibrium at 600 °C, provided the reduction process lasts at least 4 h or more. At 600 °C, UO₂ was isotopically depleted by 2.89 ± 0.82‰ compared to the U₃O₈ from which it was produced. We find that the H₂O formed during the reduction plays a major role in determining the final δ¹⁸O of UO₂ prepared from U₃O_8. The isotope equilibrium of the system UO₂-H₂O at 600 °C was calculated, indicating that δ¹⁸O of the H₂O was enriched by about 11‰ relative to the UO₂ due to the uranium mass effect. These findings could potentially have important implications for nuclear forensics, as they provide a new method for determining the history of UO₂ samples and tracing back their production process.