Low-Temperature Oxidation of Fine UO2 Powders: Thermochemistry and Kinetics

Uranium oxides structural transformation in human body liquids

Uranium oxide microparticles ingestion is one of the potential sources of internal radiation doses to the humans at accidental or undesirable releases of radioactive materials. It is important to predict the obtained dose and possible biological effect of these microparticles by studying uranium oxides transformations in case of their ingestion or inhalation. Using a combination of methods, a complex examination of structural changes of uranium oxides in the range from UO₂ to U₄O₉, U₃O₈ and UO₃ as well as before and after exposure of uranium oxides in simulated biological fluids: gastro-intestinal and lung-was carried out. Oxides were thoroughly characterized by Raman and XAFS spectroscopy. It was determined that the duration of expose has more influence on all oxides transformations. The greatest changes occurred in U₄O₉, that transformed into U₄O_9-y. UO_2.05 and U₃O₈ structures became more ordered and UO₃ did not undergo significant transformation.

Oxidation of Micro- and Nanograined UO2 Pellets by In Situ Synchrotron X-ray Diffraction

When in contact with oxidizing media, UO₂ pellets used as nuclear fuel may transform into U₄O₉, U₃O₇, and U₃O₈. The latter starts forming by stress-induced phase transformation only upon cracking of the pristine U₃O₇ and is associated with a 36% volumetric expansion with respect to the initial UO₂. This may pose a safety issue for spent nuclear fuel (SNF) management as it could imply a confinement failure and hence dispersion of radionuclides within the environment. In this work, UO₂ with different grain sizes (representative of the grain size in different radial positions in the SNF) was oxidized in air at 300 °C, and the oxidation mechanisms were investigated using in situ synchrotron X-ray diffraction. The formation of U₃O₈ was detected only in UO₂ pellets with larger grains (3.08 ± 0.06 μm and 478 ± 17 nm), while U₃O₈ did not develop in sintered UO₂ with a grain size of 163 ± 9 nm. This result shows that, in dense materials, a sufficiently fine microstructure inhibits both the cracking of U₃O₇ and the subsequent formation of U₃O₈. Hence, the nanostructure prevents the material from undergoing significant volumetric expansion. Considering that the peripheral region of SNF is constituted by the high burnup structure, characterized by 100–300 nm-sized grains and micrometric porosity, these findings are relevant for a better understanding of the spent nuclear fuel behavior and hence for the safety of the nuclear waste storage.

The present work reports on the oxidation behavior of UO₂ disks having equal density but different grain size (3.08 μm, 478 nm, and 163 nm). After 21 h at 300 °C under air, the first two samples transformed into U₃O₈, while only U₃O₇ could be detected in the third one. This finding is relevant for nuclear waste management, since the grain size of the high burnup structure is comparable to that of the third sample.

Charge Localization and Magnetic Correlations in the Refined Structure of U3O7

Atomic arrangements in the mixed-valence oxide U₃O₇ are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U₆₀O₁₄₀ primitive cell (space group P4₂/n) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin-orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U₃O₇ to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO₂. Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) "oxo-type" bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μ_B, respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

Local Structure in U(IV) and U(V) Environments: The Case of U3O7

A comprehensive analysis of X-ray absorption data obtained at the U L₃-edge for a systematic series of single-valence (UO₂, KUO₃, UO₃) and mixed-valence uranium compounds (U₄O₉, U₃O₇, U₃O₈) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) methods were applied to evaluate U(IV) and U(V) environments, and in particular, to investigate the U₃O₇ local structure. We find that the valence state distribution in mixed-valence uranium compounds cannot be confidently quantified from a principal component analysis of the U L₃-edge XANES data. The spectral line broadening, even when applying the HERFD-XANES method, is sensibly higher (∼3.9 eV) than the observed chemical shifts (∼2.4 eV). Additionally, the white line shape and position are affected not only by the chemical state, but also by crystal field effects, which appear well-resolved in KUO₃. The EXAFS of a phase-pure U₃O₇ sample was assessed based on an average representation of the expanded U₆₀O₁₄₀ structure. Interatomic U–O distances are found mainly to occur at 2.18 (2), 2.33 (1), and 3.33 (5) Å, and can be seen to correspond to the spatial arrangement of cuboctahedral oxygen clusters. The interatomic distances derived from the EXAFS investigation support a mixed U(IV)–U(V) valence character in U₃O₇.

X-ray absorption methods (HERFD-XANES and EXAFS) at the U L₃-edge are applied to evaluate U(IV) and U(V) environments, and in particular, the U₃O₇ local structure. Recommendations on the evaluation of valence states using different XANES methods are worked out. New insights on the character of uranium environments in U₃O₇ are presented.