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 degrees C and for a duration of 2 to 6 h under a high-purity H-2 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 degrees C, provided the reduction process lasts at least 4 h or more. At 600 degrees 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 delta O-18 of UO2 prepared from U3O8. The isotope equilibrium of the system UO2-H2O at 600 degrees C was calculated, indicating that delta O-18 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.