Oxygen Isotope Alterations during the Reduction of UO to UO for Nuclear Forensics Applications.

The fabrication of UO from UO 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 UO to UO was determined in the temperature range of 500-700 °C and for a duration of 2 to 6 h under a high-purity H atmosphere. Three UO samples, possessing a different oxygen isotopic composition, were used to investigate key parameters involved with the fractionation during the reduction process. All UO products did not maintain the original isotope composition of the starting UO under all conditions. The results show that the system UO-HO attains isotope equilibrium at 600 °C, provided the reduction process lasts at least 4 h or more. At 600 °C, UO was isotopically depleted by 2.89 ± 0.82‰ compared to the UO from which it was produced. We find that the HO formed during the reduction plays a major role in determining the final δO of UO prepared from UO The isotope equilibrium of the system UO-HO at 600 °C was calculated, indicating that δO of the HO was enriched by about 11‰ relative to the UO 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 UO samples and tracing back their production process.