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

Our present understanding of surface dissolution of nuclear fuels such as uranium dioxide (UO2) is limited by the use of nonlocal characterization techniques. Here we discuss the use of state-of-the-art scanning transmission electron microscopy (STEM) to reveal atomic-scale changes occurring to a UO2 thin film subjected to anoxic dissolution in deionized water. No amorphization of the UO2 film surface during dissolution is observed, and dissolution occurs preferentially at surface reactive sites that present as surface pits which increase in size as the dissolution proceeds. Using a combination of STEM imaging modes, energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy loss spectroscopy (STEM-EELS), we investigate structural defects and oxygen passivation of the surface that originates from the filling of the octahedral interstitial site in the center of the unit cells and its associated lattice contraction. Taken together, our results reveal complex pathways for both the dissolution and infiltration of solutions into UO2 surfaces.

The electronic structure and the nature of the chemical bond in CeO2.

The X-ray photoelectron spectral structure of CeO2 valence electrons in the binding energy range of 0 to ∼50 eV was analyzed. The core-electron spectral structure parameters and the results of relativistic discrete-variational calculations of CeO8 and Ce63O216 clusters were taken into account. Comparison of the valence and the core-electron spectral structures showed that the formation of the inner (IVMO) and the outer (OVMO) valence molecular orbitals contributes to the spectral structure more than the many-body processes. The Ce 4f electrons were established to participate directly in chemical bond formation in CeO2 losing partially their f character. They were found to be localized mostly within the outer valence band. The Ce 5p atomic orbitals were shown to participate in the formation of both the inner and the outer valence molecular orbitals (MOs). A large part in the IVMO formation is taken by the filled Ce 5p1/2, 5p3/2 and O 2s atomic shells, while the Ce 5s electrons participate weakly in the chemical bond formation. The composition and the sequent order of the molecular orbitals in the binding energy range of 0 to ∼50 eV were established. A quantitative scheme for the molecular orbitals of CeO2 was built. This scheme is fundamental for understanding the nature of chemical bonding and also for the interpretation of other X-ray spectra of CeO2. Evaluations revealed that the IVMO electrons weaken the chemical bond formed by the OVMO electrons by 37%.

XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films.

XPS determination of the oxygen coefficient kO = 2 + x and ionic (U(4+), U(5+), and U(6+)) composition of oxides UO2+x formed on the surfaces of differently oriented (hkl) planes of thin UO2 films on LSAT (Al10La3O51Sr14Ta7) and YSZ (yttria-stabilized zirconia) substrates was performed. The U 4f and O 1s core-electron peak intensities as well as the U 5f relative intensity before and after the (129)Xe(23+) and (238)U(31+) irradiations were employed. It was found that the presence of uranium dioxide film in air results in formation of oxide UO2+x on the surface with mean oxygen coefficients kO in the range 2.07-2.11 on LSAT and 2.17-2.23 on YSZ substrates. These oxygen coefficients depend on the substrate and weakly on the crystallographic orientation. On the basis of the spectral parameters it was established that uranium dioxide films AP2,3 on the LSAT substrates have the smallest kO values, and from the XRD and EBSD results it follows that these samples have a regular monocrystalline structure. The XRD and EBSD results indicate that samples AP5-7 on the YSZ substrates have monocrystalline structure; however, they have the highest kO values. The observed difference in the kO values was probably caused by the different nature of the substrates: the YSZ substrates provide 6.4% compressive strain, whereas (001) LSAT substrates result only in 0.03% tensile strain in the UO2 films. (129)Xe(23+) irradiation (92 MeV, 4.8 × 10(15) ions/cm(2)) of uranium dioxide films on the LSAT substrates was shown to destroy both long-range ordering and uranium close environment, which results in an increase of uranium oxidation state and regrouping of oxygen ions in uranium close environment. (238)U(31+) (110 MeV, 5 × 10(10), 5 × 10(11), 5 × 10(12) ions/cm(2)) irradiations of uranium dioxide films on the YSZ substrates were shown to form the lattice damage only with partial destruction of the long-range ordering.