Authors: Aaron M. Chalifoux | Cody A. Nizinski | Miguel Cisneros | T. J. Tenner | Benjamin E. Naes | Kimberly N. Wurth | Erik Oerter | M. J. Singleton | Gabriel J. Bowen | Luther W. McDonald
Source: ACS Omega (article, 2024-02-27)
Field: Chemistry — Inorganic Chemistry

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 (δ<sup>18</sup>O 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 (UO<sub>2</sub>F<sub>2</sub>) with steam (water vapor) during their reduction to UO<sub><i>x</i></sub>. The MDU was precipitated from two water sources, one enriched and one depleted in <sup>18</sup>O. The UO<sub>2</sub>F<sub>2</sub> was precipitated from a single water source and either directly reduced or converted to ADU prior to reduction. All MDU, ADU, and UO<sub>2</sub>F<sub>2</sub> were reduced to UO<sub><i>x</i></sub> 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 U<sub>3</sub>O<sub>8</sub>, U<sub>4</sub>O<sub>9</sub>, and UO<sub>2</sub> with trace magnesium and fluorine phases in UO<sub><i>x</i></sub> from MDU and UO<sub>2</sub>F<sub>2</sub>, respectively. The bulk oxygen isotope composition of UO<sub><i>x</i></sub> from MDU was analyzed using fluorination to remove the lattice-bound oxygen, and then O<sub>2</sub> was subsequently analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope compositions of the ADU, UO<sub>2</sub>F<sub>2,</sub> and the resulting UO<sub><i>x</i></sub> were analyzed by large geometry secondary ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU, ADU, and UO<sub>2</sub>F<sub>2</sub> experienced significant oxygen isotope exchange, and the resulting δ<sup>18</sup>O values of UO<sub><i>x</i></sub> approached the values of the steam. When reduced without steam, the δ<sup>18</sup>O values of converted ADU, U<sub>3</sub>O<sub>8</sub>, and UO<sub><i>x</i></sub> products remained similar to those of the UO<sub>2</sub>F<sub>2</sub> starting material. LG-SIMS isotope mapping of F impurity abundances and distributions showed that direct steam-assisted reduction from UO<sub>2</sub>F<sub>2</sub> significantly removed F impurities while dry reduction from UO<sub>2</sub>F<sub>2</sub> led to the formation of UO<sub><i>x</i></sub> that was enhanced in F impurities. In addition, when UO<sub>2</sub>F<sub>2</sub> was processed via precipitation to ADU and calcination to U<sub>3</sub>O<sub>8</sub>, F impurities were largely removed, and reductions to UO<sub><i>x</i></sub> 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. Nuclear fuel cycle ; Uranium ; Uranium ore ; Nuclear fuel ; Oxygen ; Radiochemistry ; Isotopes of oxygen ; Impurity ; Isotope ; Fluorine ; Enriched uranium ; Environmental science ; Chemistry ; Materials science ; Nuclear chemistry ; Metallurgy ; Nuclear physics ; Radioactive waste ; Physics Radioactive element chemistry and processing ; Radioactive contamination and transfer ; Hydrocarbon exploration and reservoir analysis