Oxygen Isotopic Composition of U₃O₈ Synthesized From U Metal, Uranyl Nitrate Hydrate, and UO₃ as a Signature for Nuclear Forensics

Triuranium octoxide (U₃O₈) is one of the main compounds in the nuclear fuel cycle. As such, identifying its processing parameters that control the oxygen isotopic composition could be developed as a new signature for nuclear forensic investigation. This study investigated the effect of different synthesis conditions such as calcination time, temperature, and cooling rates on the final δ¹⁸O values of U₃O₈, produced from uranium metal, uranyl nitrate hydrate, and uranium trioxide as starting materials. The results showed that δ¹⁸O of U₃O₈ is independent of the above-listed starting materials. δ¹⁸O values of 10 synthetic U₃O₈ were similar (9.35 ± 0.46‰) and did not change as a function of calcination time or calcination temperature. We showed that the cooling rate of U₃O₈ at the end of the synthesis process determines the final oxygen isotope composition, yielding a significant isotope effect on the order of 30‰. Experiments with two isotopically spiked 10 M HNO₃, with a difference of δ¹⁸O ∼75‰, show that no memory of the starting solution oxygen isotope signature is expressed in the final U₃O₈ product. We suggest that the interaction with atmospheric oxygen is the main process parameter that controls the δ¹⁸O value in U₃O₈. The uranium mass effect, the tendency of uranium ions to preferentially incorporate ¹⁶O, is expressed during the solid-gas oxygen exchange, which occurs throughout cooling of the system.