The triple oxygen isotope signature of uranium oxides in the nuclear fuel cycle

Uranium oxides, in the form of U ₃O ₈ and UO ₂, are principal compounds in the uranium nuclear fuel cycle and attract significant interest in the general field of nuclear forensics, for the development of new and credible signatures to support attribution. This study presents, for the first time, the analytical method and high-precision measurements of the triple oxygen isotope composition of U ₃O ₈ and UO ₂. We show that Δ’ ¹⁷O can be measured at the accuracy and reproducibility needed for nuclear forensics. Δ’ ¹⁷O exhibits a wide range, from -450 to -120 ppm, for both commercial and synthetic uranium oxides. The presentation of the data in Δ’ ¹⁷O-δ’ ¹⁸O space provides a two-dimension identification of uranium oxides for nuclear forensic characterization of samples. The relation between Δ’ ¹⁷O and the process from which the oxide was formed or manipulated, has the potential to trace the chemical history of the material. We determined the change in Δ’ ¹⁷O of the system U ₃O ₈ – atmospheric O ₂ resulting from isotope exchange at the temperature range of 298 K to 973 K. The dependency of the triple isotope exponent, θ, on temperature was calculated for this exchange reaction, concluding that the phase transition from pseudo-hexagonal to hexagonal structure plays a major role in determining Δ ¹⁷O and θ. A set of reduction reactions of U ₃O ₈ to UO ₂ indicate that Δ’ ¹⁷O undergoes minimal change during this process.