Fractionation Of Oxygen Isotopes In Uranium Oxides During Peroxide Precipitation And Dry Air Calcination

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. 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 degrees C to 1000 degrees 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 degrees C were performed at different time intervals to evaluate the rate of isotopic equilibration. The delta O-18 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 parts per thousand. 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 delta O-18 = +1.4 +/- 0.9 parts per thousand at 300 degrees C to +17.3 +/- 0.9 parts per thousand at 1000 degrees C. This variation in delta O-18 values was a result of oxygen exchange with atmospheric O-2 and was found to be rapid at calcination temperatures greater than 600 degrees 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.