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 delta O-18 values of U3O8, produced from uranium metal, uranyl nitrate hydrate, and uranium trioxide as starting materials. The results showed that delta O-18 of U3O8 is independent of the above-listed starting materials. delta O-18 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 delta O-18 similar to 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 delta O-18 value in U3O8. The uranium mass effect, the tendency of uranium ions to preferentially incorporate O-16, is expressed during the solid-gas oxygen exchange, which occurs throughout cooling of the system.