Surface and electrochemical controls on UO2 dissolution under anoxic conditions

An Atomic-Scale Understanding of UO2 Surface Evolution during Anoxic Dissolution

“…The dissolution experiment was performed in a glovebox under argon atmosphere, with oxygen level ≤0.1 ppm of O 2 , in deaerated deionized water for 140 days, and the thin film sample was annealed in Ar/5% H 2 at 800 °C immediately before the leaching experiment. Further details for the leached sample on the sample production, the dissolution experiment, and characterization by SEM, electron backscatter diffraction (EBSD), and XRD are given in refs and .…”

“…The key to understanding the long-term rate of release of fission products and actinides from spent nuclear fuel is the rate of dissolution of the UO 2 matrix. , This dissolution will be surface-controlled and involve the evolution of the surface and potential new (secondary) phases forming depending on the environment. In our previous work regarding the dissolution of CeO 2 (a nonradioactive analogue of UO 2 ) and UO 2 − (anoxic conditions) we observed nanoscale surface alteration features and secondary phases that were difficult to identify because of their size. , Many unanswered questions surrounding secondary phase formation during anoxic dissolution of UO 2 remain, such as their crystal structure, location, dimensions, and the process by which they form. In addition, it is unclear whether they inhibit the dissolution process and if they support the common assumption that the crystalline actinide dioxides must be covered with an amorphous surface layer as a result of the dissolution. − To address all of these concerns, a high-resolution, submicron structural and chemical analysis approach is needed.…”

“…In particular, we use a combination of high-angle annular dark field (STEM-HAADF) and medium-angle annular dark field (STEM-MAADF) imaging, which are sensitive to lattice composition and strain, respectively, as well as high-resolution energy-dispersive X-ray spectroscopy (STEM-EDS) and electron energy loss spectroscopy (STEM-EELS) to examine the local chemical evolution of the system. For this purpose, a thin film sample of UO 2 subjected to anoxic dissolution in deionized water described in refs and was studied to reveal surface alteration features and identify secondary phase formations. The increase in resolution provided by atomic-scale chemical imaging of the dissolution process allows us to examine the extent of the amorphous surface hydroxide phase that is often postulated to mediate dissolution. , We find evidence for an alternative passivation mechanism through light element atomic-scale mapping, shedding light on the complex nature of dissolution in this system.…”

X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(iv),(v) and(vi) exposed to H2O-plasma under UHV conditions

“…29 Surface chemistry studies were also conducted on the anoxic dissolution of a single-crystalline thin film of UO 2 . 30,31 The authors concluded that dissolution and precipitation of uranium occurs via the tetravalent form instead of the hexavalent one. Another study revealed that the dissolution is initiated at surface grain boundaries and film cracks which was passivated with an oxidized layer via oxygen substitution into the central octahedral interstitial site into the UO 2 lattice.…”

Time-dependent surface modification of uranium oxides exposed to water plasma

“…7,[11][12][13][14][15][16][17] Such studies include interactions between aqueous radiolysis products and UO 2 surfaces, conducted in chemical and electrochemical experiments. 1,[18][19][20][21][22] Some studies were made by continuously exposing uranium oxide to low steadystate concentrations of oxidants produced by irradiating suspensions of uranium oxide or pellets immersed in aqueous solution. 8 Other studies focused on the leaching of pellets ( pure or doped with non-radioactive isotopes of fission products) [23][24][25][26][27] and even of thin films.…”