Fractionation of Oxygen Isotopes in Uranium Oxides during Peroxide Precipitation and Dry Air Calcination
- Univ. of Utah, Salt Lake City, UT (United States)
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
The interaction between uranium and oxygen-containing substrates is nearly ubiquitous within the nuclear fuel cycle. Given the well-known and predictable oxygen stable isotope compositions of atmospheric oxygen and meteoric waters around the world, the use of these isotopes as a potential geolocation or processing signature of uranium compounds has been of interest within the nuclear safeguards community. This study focuses on measuring the oxygen-stable isotope composition of synthetically produced uranium oxides to determine the mechanism and extent of fractionation during materials processing relevant to the nuclear fuel cycle. Here, metastudtite [UO2O2(H2O)2] samples were first produced using water and hydrogen peroxide of a known oxygen isotope composition. This starting material was calcined in dry air at temperatures ranging from 300 °C to 1000 °C for 20 h producing oxides ranging in composition from UO3+x (0 ≤ x ≤ 0.5) to U3O8. In addition, calcinations between 500 and 800 °C were performed at different time intervals to evaluate the rate of isotopic equilibration. The δ18O values of water bound to metastudtite were measured by thermogravimetric analysis coupled to isotope ratio infrared spectroscopy (TGA–IRIS). A redesigned fluorination manifold was constructed at the University of Utah and was used to extract the bulk oxygen from precipitated and calcined products using bromine pentafluoride (BrF5). Mineralization water on metastudtite (measured by TGA–IRIS) was closely associated with the aqueous environment of precipitation with a fractionation of +3.5 ± 0.6‰. In contrast, bulk oxygen from metastudtite (measured by fluorination and gas chromatography combined with isotope ratio mass spectrometry) was found to have an inconsistent fractionation with precipitation water. Samples calcined in dry air exhibited a wide range of oxygen isotope compositions which increased from δ18O = +1.4 ± 0.9‰ at 300 °C to +17.3 ± 0.9‰ at 1000 °C. This variation in δ18O values was a result of oxygen exchange with atmospheric O2 and was found to be rapid at calcination temperatures greater than 600°C—with equilibration occurring in less than 2 h. These results provide insights into the incorporation and exchange of oxygen isotopes during precipitation and calcination processes comparable to those employed within the nuclear fuel cycle and establish a foundation for future investigations to build upon.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); Defense Threat Reduction Agency (DTRA)
- Grant/Contract Number:
- AC52-07NA27344
- OSTI ID:
- 1828115
- Report Number(s):
- LLNL-JRNL--819153; 1029903
- Journal Information:
- ACS Earth and Space Chemistry, Journal Name: ACS Earth and Space Chemistry Journal Issue: 6 Vol. 5; ISSN 2472-3452
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English