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

Triuranium octoxide (UO) 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 δO values of UO, produced from uranium metal, uranyl nitrate hydrate, and uranium trioxide as starting materials. The results showed that δO of UO is independent of the above-listed starting materials. δO values of 10 synthetic UO 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 UO 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 HNO, with a difference of δO ∼75‰, show that no memory of the starting solution oxygen isotope signature is expressed in the final UO product. We suggest that the interaction with atmospheric oxygen is the main process parameter that controls the δO value in UO. The uranium mass effect, the tendency of uranium ions to preferentially incorporate O, is expressed during the solid-gas oxygen exchange, which occurs throughout cooling of the system.