Postdoc, Stanford University
use the redox behavior to track the chemical signature used for fuel pallet process
The oxidation and corrosion behavior of the binary uranium oxide compounds, have been investigated extensively in nuclear community for decades due the concerns of spent fuel storage, structural stability of fuel pallet, and the chemical signature used for processing. Understanding the oxidation behavior under a certain temperature and humidity environment is crucial for chemical signatures correlated with uranium oxide processing, especially for uranium material oxidized and hydrated over certain period. However, the complicated outer-shell electron occupation close to Fermi level enable uranium to adopt several valence states in the uranium oxide system and therefore lead to a complex binary compound system.
The oxidation sequence in the uranium oxide system was previously studied. As the oxygen content increases, the excess oxygen will trigger the formation of oxygen clusters in the initial UO2 fluorute structure, eventually lead to the structural change from UO2 to U3O8.
In our work, we choose a different approach to elucidate the structural change and check the redox behavior, form the end product -- U3O8.
The characterization of the microstructure of high purity U3O8 powder sample, which was used as isotope standard sample, was performed using transmission electron microscope (TEM) and X-ray diffraction (XRD).
A redox behavior under humid environment was verified. With selected area electron diffraction (SAED), UO2 (cubic structure) precipitations with domain size of tens nanometer was observed at the edge while the U3O8 (orthorhombic structure) remained in the inner section. This redox mechanism, which is correlated to the exposure to high humidity conditions, is further discussed from the aspect of nano-structure analysis. This finding has revealed that an unexpected redox behavior may occur and the structural stability of U3O8 (end product) is not as stable as predicted from computational simulation works.
Abstract: The synthesis, crystal structure, and spectroscopic characterization of [U(V)(H2O)2(U(VI)O2)2O4(OH)](H2O)4 (1), a mixed-valent U(V)/U(VI) oxide material, are reported. The hydrothermal reaction of UO2(2+) with Zn and hydrazine at 120 degrees C for three days yields 1 in the form of a dark red crystalline solid. Compound 1 has been characterized by a combination of single-crystal and powder X-ray diffraction (XRD), elemental analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The structure consists of an extended sheet of edge and corner shared U(VI) pentagonal bipyramids that are further connected by edge sharing to square bipyramidal U(V) units. The overall topology is similar to the mineral ianthinite. The uranium L(III)-edge XAS revealed features consistent with those observed by single-crystal X-ray diffraction. High resolution XPS data analysis of the U4f region confirmed the oxidation states of U as originally assigned from XRD analysis and bond valence summations.
Pub.: 10 Oct '08, Pinned: 13 Jul '17
Abstract: Low-temperature oxidation of fine UO2 powders (<200 nm) is remarkably different compared to coarse-grained materials. The formation of an amorphous phase seems to be favored instead of U3O8, and no discrete surface oxide layer is observed. Oxidation proceeds via formation of higher uranium oxides (U4O9 and U3O7−z) in nanodomains. The development of long-period modulation in these domains occurs with increasing degree of oxidation.The nanostructure and phase evolution in low-temperature oxidized (40–250 °C), fine UO2 powders (<200 nm) have been investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The extent of oxidation was also measured via in situ thermogravimetric analysis. The oxidation of fine powders was found to proceed differently as compared to oxidation of coarse-grained UO2. No discrete surface oxide layer was observed and no U3O8 was formed, despite the high degree of oxidation (up to O/U = 2.45). Instead, nanosized (5–15 nm) amorphous nuclei (interpreted as amorphous UO3), unmodulated and modulated U4O9, and a continuous range of U3O7–z phases with varying tetragonal distortion (c/a > 1) were observed. Oxidation involves formation of higher uranium oxides in nanodomains near the grain surface which, initially, have a disordered defect structure (“disordered U4O9”). As oxidation progresses, domain growth increases and the long-period modulated structure of U4O9 develops (“ordered U4O9”). A similar mechanism is understood to happen also in U3O7–z.
Pub.: 25 Mar '16, Pinned: 13 Jul '17
Abstract: Uranium redox states and speciation in magnetite nanoparticles co-precipitated with U(VI) for uranium loadings varying from 1000 to 10000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M4 high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U(IV), U(V) and U(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L3 XAS and transmission electron microscopy (TEM) reveal that initially sorbed U(VI) species recrystallize to non-stoichiometric UO2+x nanoparticles within 147 days when stored under anoxic conditions. These U(IV) species oxidize again when exposed to air. U M4 HR-XANES data demonstrate strong contribution of U(V) at day 10 and that U(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L3 XAS indicates that this U(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS).
Pub.: 18 Jan '17, Pinned: 13 Jul '17
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