Oxygen Isotope Fractionation in U₃O₈ during Thermal Processing in Humid Atmospheres

Abstract

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, U 3 O 8 , 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 N 2 , oxygen isotope equilibration in U 3 O 8 occurred within 12 h at 400 °C and within 2 h at 600 and 800 °C. Fractionation factors (1000lnα, U 3 O 8 −H 2 O) between the water and oxide were −12.1, −11.0, and −8.0 at 400, 600, and 800 °C, respectively. With both humidity and O 2 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 O 2 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 O 2 -dependent to H 2 O-dependent fractionation as U 3 O 8 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.

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

“…Thus, we attribute the first-order transition state, at 300–400 °C, to the fact that the δ 18 O values reach a minimum and the second-order transition state, at 500–600 °C, to the inverse trend of the oxygen isotopic composition. We consider the possible isotope exchange with humidity H 2 O at 400 °C (Klosterman et al 22 ) minor to the isotope effect of the observed phase transition.…”

“…Klosterman et al 16 synthesized U 3 O 8 from uranium peroxide at 300−1000 °C and reported a fractionation of about −22‰ up to about −5‰, respectively, between the oxide and atmospheric oxygen with a retrograde isotope effect. Klosterman et al 22 determined the fractionation of oxygen isotopes between U 3 O 8 and N 2 /O 2 atmosphere at 400, 600, and 800 °C, and different calcination times (between 1 and 72 h) reported an average of Δ 18 showed that the cooling profile at the end of the fabrication process of U 3 O 8 determines the final oxygen isotopic composition, yielding a significant isotope effect in the order of 30‰, and that the interaction with atmospheric oxygen is the primary process that controls the δ 18 O value of U 3 O 8 . The common conclusion of the above studies is that the reaction with atmospheric oxygen is a crucial factor in determining the final oxygen isotopic composition of U 3 O 8 .…”

“…A series of studies investigated the main parameters of the manufacturing process, which determine the final oxygen isotopic composition (δ 18 O) of U 3 O 8 . The effects of calcination temperature, calcination time, different starting materials, different initial solutions, air humidity, and the cooling rate of the products were studied. ,,,− , Plaue synthesized U 3 O 8 from the U metal and different UO 2 samples and showed increased δ 18 O values with increasing temperature. Klosterman et al synthesized U 3 O 8 from uranium peroxide at 300–1000 °C and reported a fractionation of about −22‰ up to about −5‰, respectively, between the oxide and atmospheric oxygen with a retrograde isotope effect.…”

“…Klosterman et al 16 synthesized U 3 O 8 from uranium peroxide at 300–1000 °C and reported a fractionation of about −22‰ up to about −5‰, respectively, between the oxide and atmospheric oxygen with a retrograde isotope effect. Klosterman et al 22 determined the fractionation of oxygen isotopes between U 3 O 8 and N 2 /O 2 atmosphere at 400, 600, and 800 °C, and different calcination times (between 1 and 72 h) reported an average of Δ 18 O UO x -O 2 of −55.1‰ ± 1.0‰, −17.4‰ ± 2.2‰, and −8.2‰ ± 0.7‰, respectively. Assulin et al 17 synthesized U 3 O 8 in temperatures ranging from 650 to 850 °C and calcination times ranging from 30 min up to 168 h, concluding that under atmospheric conditions, the δ 18 O of U 3 O 8 is independent of the starting materials and of the original oxygen isotopic composition of UO 3 from which it had been prepared.…”

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

“…6 The comparison of several oxygen extraction methods from UO 2 , U 3 O 8 , and UO 3 compounds has revealed that different amounts of water molecules present in the uranium oxides must be removed, as they interfere with the isotope measurement 11 and exchange reactions with humid atmospheres, affecting the oxygen fractionation. 15 Klosterman et al 15 synthesized U 3 O 8 from uranium peroxide at 300−1000 °C, reporting fractionation of about −22‰ up to about −5‰, respectively, between the oxide and atmospheric oxygen with a retrograde isotope effect. 16 The exposure of U 3 O 8 and UO 2 to humidity in an oxidizing atmosphere showed that hydrated uranium oxide grows as a secondary mineral (metaschoepite) on aged U 3 O 8 and UO 2.…”