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

Abstract

Within the front end of the nuclear fuel cycle, many processes impart forensic signatures. Oxygen-stable isotopes (δ¹⁸O 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 (UO₂F₂) with steam (water vapor) during their reduction to UO_x. The MDU was precipitated from two water sources, one enriched and one depleted in ¹⁸O. The UO₂F₂ was precipitated from a single water source and either directly reduced or converted to ADU prior to reduction. All MDU, ADU, and UO₂F₂ were reduced to UO_x 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 U₃O₈, U₄O₉, and UO₂ with trace magnesium and fluorine phases in UO_x from MDU and UO₂F₂, respectively. The bulk oxygen isotope composition of UO_x from MDU was analyzed using fluorination to remove the lattice-bound oxygen, and then O₂ was subsequently analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope compositions of the ADU, UO₂F_2, and the resulting UO_x were analyzed by large geometry secondary ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU, ADU, and UO₂F₂ experienced significant oxygen isotope exchange, and the resulting δ¹⁸O values of UO_x approached the values of the steam. When reduced without steam, the δ¹⁸O values of converted ADU, U₃O₈, and UO_x products remained similar to those of the UO₂F₂ starting material. LG-SIMS isotope mapping of F impurity abundances and distributions showed that direct steam-assisted reduction from UO₂F₂ significantly removed F impurities while dry reduction from UO₂F₂ led to the formation of UO_x that was enhanced in F impurities. In addition, when UO₂F₂ was processed via precipitation to ADU and calcination to U₃O₈, F impurities were largely removed, and reductions to UO_x 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.