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

Abstract

Triuranium octoxide (U 3 O 8 ) 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 δ 18 O values of U 3 O 8 , produced from uranium metal, uranyl nitrate hydrate, and uranium trioxide as starting materials. The results showed that δ 18 O of U 3 O 8 is independent of the above-listed starting materials. δ 18 O values of 10 synthetic U 3 O 8 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 U 3 O 8 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 3 , with a difference of δ 18 O ∼75‰, show that no memory of the starting solution oxygen isotope signature is expressed in the final U 3 O 8 product. We suggest that the interaction with atmospheric oxygen is the main process parameter that controls the δ 18 O value in U 3 O 8 . The uranium mass effect, the tendency of uranium ions to preferentially incorporate 16 O, is expressed during the solid−gas oxygen exchange, which occurs throughout cooling of the system.

Equilibrium Fractionation of Oxygen Isotopes in the U3O8–Atmospheric Oxygen System

“…Oxygen isotopic analyses of the U 3 O 8 samples were conducted using an isotope ratio gas-chromatography-mass spectrometer (irmGCMS, Thermo Scientific Delta Plus Advantage) and an IR CO 2 laser (10.6 μm, New Wave Research 25 W). The method is described in detail elsewhere. , Briefly, U 3 O 8 samples (1005–1596 μg) and SiO 2 samples (NBS-28, standard material for quality check and calibration; 260–450 μg) were placed in nickel cups in a stainless steel chamber and heated overnight at 80 °C under high vacuum. Pre-fluorination was performed thrice for the entire cell with 80 Torr of BrF 5 .…”

“…Two were synthesized from uranyl nitrate hydrate (UNH); U 3 O 8 -I has a δ 18 O value of 10.02‰ ± 0.31‰, and U 3 O 8 -II has a δ 18 O value of 0.28‰ ± 0.13‰. These two materials were synthesized intentionally differently (at the cooling step) to achieve a difference in their δ 18 O values . The third sample, U 3 O 8 -III, has a δ 18 O value of 4.78‰ ± 0.50‰, is a natural commercial U 3 O 8 material purchased from CETAMA, France (commercially known as “CHANTERELLE”), and is used as a calibration standard for impurities.…”

“…The fractionation of oxygen isotopes ( 16 O, 17 O, and 18 O) among phases is an important measure applied in many scientific fields, such as geochemistry, climate reconstruction, , and high-temperature magmatic and metamorphic processes, − to trace chemical reactions and environmental conditions. In the last 2 decades, several studies have investigated the use of oxygen isotopic composition in various uranium oxides as a signature for nuclear forensics applications. − Triuranium octoxide (U 3 O 8 ), the most stable form of uranium oxide, is a key compound in the uranium nuclear fuel cycle. U 3 O 8 is an intermediate product in the front end of the production cycle of the uranium nuclear fuel and a major compound in nuclear waste due to its thermal and chemical stabilities. − Understanding the mechanism governing the final oxygen isotopic composition of U 3 O 8 is fundamental to deciphering the material history in view of nuclear forensics investigations.…”

“…A series of studies investigated the main parameters of the manufacturing process, which determine the final oxygen isotopic composition (δ 18 O) of U 3 O 8 . The effects of calcination temperature, calcination time, different starting materials, different initial solutions, air humidity, and the cooling rate of the products were studied. ,,,− , Plaue synthesized U 3 O 8 from the U metal and different UO 2 samples and showed increased δ 18 O values with increasing temperature. Klosterman et al synthesized U 3 O 8 from uranium peroxide at 300–1000 °C and reported a fractionation of about −22‰ up to about −5‰, respectively, between the oxide and atmospheric oxygen with a retrograde isotope effect.…”

Novel Heteroleptic Uranyl(VI) Complexes Incorporating Tetradentate and Bidentate Chelating Ligands: Deviation from the Oyl‐U‐Oyl Linearity

“…However, at higher temperature, the UO 3 undergoes decomposition to form U 3 O 8 . This has become evident from the powder XRD studies of thermally decomposed compounds of both the complexes (Figures S8 and S9) [34a,b] . The sharp and intense peaks observed in the XRD pattern indicate that the highly crystalline U 3 O 8 was formedwith pure orthorhombic phase.…”