Effect of Diel Cycling Temperature, Relative Humidity, and Synthetic Route on the Surface Morphology and Hydrolysis of α-U3O8

Tailoring Triuranium Octoxide into Multidimensional Uranyl Fluoride Micromaterials

Uranium microstructured materials with controlled size and shape are relevant to the nuclear industry and have found applications as targets for medical isotope production, fuels for nuclear reactors, standards for nuclear forensics, and energy sources for space exploration. Until now, most studies at the microscale have focused on uranium microspheres (oxides, nitrides, carbides, and fluorides), while micromaterials of uranium halides, carbides, and pnictides with other morphologies are largely unknown. A promising method to shape the morphology of uranium micromaterials is the replacement of O by F atoms in oxide materials using a solid-gas reaction. Here, with the aim to elaborate unexplored uranium fluoride micromaterials, the fluorination of uranium oxide (U3O8 and UO2) microspheres (ms), microrods (mr), and microplates (mp) in an autoclave at 250 °C with HF(g) (produced from the thermal decomposition of silver bifluoride (SBF)) and with ammonium bifluoride (ABF) was evaluated. We show that the reactions between U3O8 mr and U3O8 mp and SBF provided the most efficient way to elaborate mr and mp UO2F2 micromaterials in a high yield (∼90%). The resulting UO2F2 mr (length: 3-20 μm) and UO2F2 mp (width: 1-7.5 μm) exhibited a well-defined geometry that was identical to that of the U3O8 precursors. Agglomerated (NH4)3UO2F5 and UO2F2 ms (2-3.5 μm) were prepared from the reaction of U3O8 ms with ABF. It is noted that the reaction of UO2 ms with SBF and ABF did not provide any uranium fluoride micromaterials. The successful preparation of uranium fluoride microstructures (ms, mr, and mp) developed here opens the way to novel actinide fluoride micromaterials.

In Situ Liquid Cell Transmission Electron Microscopy Study of Studtite Particle Formation and Growth via Electron Beam Radiolysis

This study presents in situ observations of studtite (UO2O2(H2O)2·2H2O) crystal growth utilizing liquid phase transmission electron microscopy (LP-TEM). Studtite was precipitated from a uranyl nitrate hexahydrate solution using hydrogen peroxide formed by the radiolysis of water in the TEM electron beam. The hydrogen peroxide (H2O2) concentration, directly controlled by the electron beam current, was varied to create local environments of low and high concentrations to compare the impact of the supersaturation ratio on the nucleation and growth mechanisms of studtite particles. The subsequent growth mechanisms were observed in real time by TEM and scanning TEM imaging. After the initial precipitation reaction, a post-mortem TEM analysis was performed on the samples to obtain high-resolution TEM images and selected area electron diffraction patterns to investigate crystallinity as well as energy-dispersive X-ray spectroscopy spectra to ensure that studtite was produced. The results reveal that studtite particles form through various mechanisms based on the concentration ratio of uranyl to H2O2 and that studtite is initially produced through an amorphous intermediary prior to formation of the crystalline material commonly reported in the literature.

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

As a major component in the nuclear fuel cycle, octoxide uranium is subjected to intensive nuclear forensics research. Scientific efforts have been mainly dedicated to determine signatures, allowing for clear and distinct attribution. The oxygen isotopic composition of octoxide uranium, acquired during the fabrication process of the nuclear fuel, might serve as a signature. Hence, understanding the factors governing the final oxygen isotopic composition and the chemical systems in which U₃O₈ was produced may develop a new fingerprint concerning the history of the material and/or the process to which it was subjected. This research determines the fractionation of oxygen isotopes at different temperatures relevant to the nuclear fuel cycle in the system of U₃O₈ and atmospheric O₂. We avoid the retrograde isotope effect at the cooling stage at the end of the fabrication process of U₃O₈. The system attains the isotope equilibrium at temperatures higher than 300 °C. The average δ¹⁸O values of U₃O₈ in equilibrium with atmospheric oxygen have been found to span over a wide range, from -9.90‰ at 300 °C up to 18.40‰ at 800 °C. The temperature dependency of the equilibrium fractionation (1000 ln α_{U3O8-atm. O2} ) exhibits two distinct regions, around -33‰ between 300 °C and -500 °C and -5‰ between 700 °C and -800 °C. The sharp change coincides with the transition from a pseudo-hexagonal structure to a hexagonal structure. A depletion trend in δ¹⁸O is associated with the orthorhombic structure and may result from the uranium mass effect, which might also play a role in the depletion of 5‰ versus atmospheric oxygen at high temperatures.