Oxygen Isotope Fractionation in U₃O₈ during Thermal Processing in Humid Atmospheres

The incorporation of oxygen isotopes from water into uranium oxides during industrial processing presents a pathway for determining a material's geographical origin. This study is founded on the hypothesis that oxygen isotopes from atmospheric water vapor will exchange with isotopes of oxygen in solid uranium oxides during thermal processing or calcination. Using a commonly encountered oxide, U₃O₈, the exchange kinetics and equilibrium fractionation with water vapor (in a concentration range of 50-55% relative humidity) were investigated using processing temperatures of 400, 600, and 800 °C. In an atmosphere containing only water vapor diluted in N₂, oxygen isotope equilibration in U₃O₈ occurred within 12 h at 400 °C and within 2 h at 600 and 800 °C. Fractionation factors (1000lnα, U₃O₈-H₂O) between the water and oxide were -12.1, -11.0, and -8.0 at 400, 600, and 800 °C, respectively. With both humidity and O₂ present in the calcining atmosphere, isotopic equilibration is attained within 2 h at and above 400 °C. In this mixed atmosphere, which was designed to emulate Earth's troposphere, isotopes are incorporated preferentially from water vapor at 400 °C and from O₂ at 600 and 800 °C. Rapid and temperature/species-dependent isotope exchange also elucidated the impact of retrograde exchange in humid air, showing a shift from O₂-dependent to H₂O-dependent fractionation as U₃O₈ cooled from 800 °C. These results confirm that uranium oxides inherit oxygen isotopes from humidity during thermal processing, illuminating an important mechanism in the formation of this forensic signature.