Oxygen Isotope and Fluorine Impurity Signatures during the Conversion of Uranium Ore Concentrates to Nuclear Fuel.

Within the front end of the nuclear fuel cycle, many processes impart forensic signatures. Oxygen-stable isotopes (δO values) of uranium-bearing materials have been theorized to provide the processing and geolocational signatures of interdicted materials. However, this signature has been minimally utilized due to a limited understanding of how oxygen isotopes are influenced during uranium processing. This study explores oxygen isotope exchange and fractionation between magnesium diuranate (MDU), ammonium diuranate (ADU), and uranyl fluoride (UOF) with steam (water vapor) during their reduction to UO. The MDU was precipitated from two water sources, one enriched and one depleted in O. The UOF was precipitated from a single water source and either directly reduced or converted to ADU prior to reduction. All MDU, ADU, and UOF were reduced to UO in a 10% hydrogen/90% nitrogen atmosphere that was dry or included steam. Powder X-ray diffraction (p-XRD) was used to verify the composition of materials after reduction as mixtures of primarily UO, UO, and UO with trace magnesium and fluorine phases in UO from MDU and UOF, respectively. The bulk oxygen isotope composition of UO from MDU was analyzed using fluorination to remove the lattice-bound oxygen, and then O was subsequently analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope compositions of the ADU, UOF and the resulting UO were analyzed by large geometry secondary ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU, ADU, and UOF experienced significant oxygen isotope exchange, and the resulting δO values of UO approached the values of the steam. When reduced without steam, the δO values of converted ADU, UO, and UO products remained similar to those of the UOF starting material. LG-SIMS isotope mapping of F impurity abundances and distributions showed that direct steam-assisted reduction from UOF significantly removed F impurities while dry reduction from UOF led to the formation of UO that was enhanced in F impurities. In addition, when UOF was processed via precipitation to ADU and calcination to UO, F impurities were largely removed, and reductions to UO with and without steam each had low F impurities. Overall, these findings show promise for combining multiple signatures to predict the process history during the conversion of uranium ore concentrates to nuclear fuel.