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Title: Oxygen Isotope Fractionation in U3O8 during Thermal Processing in Humid Atmospheres

Journal Article · · ACS Omega

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.

Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Organization:
US Department of Homeland Security (DHS); USDOD Defense Threat Reduction Agency (DTRA); USDOE National Nuclear Security Administration (NNSA)
Grant/Contract Number:
AC52-07NA27344
OSTI ID:
1872280
Report Number(s):
LLNL-JRNL-827854; 1042962
Journal Information:
ACS Omega, Journal Name: ACS Omega Journal Issue: 4 Vol. 7; ISSN 2470-1343
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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