XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films

XPS determination of the oxygen coefficient k O =2+x and ionic (U 4+ , U 5+ and U 6+ ) composition of oxides UO 2+x formed on the surfaces of differently oriented (hkl) planes of thin UO 2 films on LSAT (Al 10 La 3 O 51 Sr 14 Ta 7 ) 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 UO 2+x on the surface with mean oxygen coefficients k O 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 k O 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 k O values. The observed difference in the k O values, 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 UO 2 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 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

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%.The work was supported by the RFBR grant № 17-03-00277a. M.V. Ryzhkov acknowledges financial support of FASO of Russia ISSC of the Ural Branch of RAS № AAAA-A16-116122810214-9. A.J. Popel acknowledges funding from the UK EPSRC (grant EP/I036400/1) and Radioactive Waste Management Ltd (formerly the Radioactive Waste Management Directorate of the UK Nuclear Decommissioning Authority, contract NPO004411A-EPS02), a maintenance grant from the Russian Foundation for Basic Research (projects 13-03-90916) and CSAR bursary

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 non-local 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 deionised water. No amorphisation 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 centre 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

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.The irradiation experiment was performed at the Grand Accelé rateur National d ́ ’Ions Lourds (GANIL) Caen, France, and supported by the French Network EMIR. The support in planning and execution of the experiment by the CIMAPCIRIL and the GANIL staff, especially I. Monnet, C. Grygiel, T. Madi, and F. Durantel, is much appreciated. The work was supported by RFBR grant no. 16-03-00914-a and partially supported by M.V. Lomonosov Moscow State University Program of Development. A.J.P. acknowledges funding from the UK EPSRC (grant EP/I036400/1) and Radioactive Waste Management Ltd. (formerly the Radioactive Waste Management Directorate of the UK Nuclear Decommissioning Authority, contract NPO004411A-EPS02), a maintenance grant from the Russian Foundation for Basic Research (projects 13-03-90916) and CSAR bursary. Thanks are given to A.M. Adamska, G.I. Lampronti, V.A. Lebedev, P.G. Martin, L. Payne, and A.A. Shiryaev for their help in characterization of the samples

Surface and electrochemical controls on UO2 dissolution under anoxic conditions

The escape of radionuclides from underground spent nuclear fuel disposal facilities will likely result from anoxic dissolution of spent nuclear fuel by intruding groundwater. Anoxic dissolution of various forms of uranium dioxide (UO2), namely bulk pellet, powder and thin film, has been investigated. Long-duration static batch dissolution experiments were designed to investigate the release of uranium ions in deionized water and any surface chemistry that may occur on the UO2 surface. The dissolved uranium concentration for anoxic dissolution of nearly stoichiometric UO2 was found to be of the order of 10−9 mol/l for the three different sample types. Further, clusters (∼500 nm) of homogenous uranium-containing precipitates of ∼20–100 nm grains were observed in thin film dissolution experiments. Such a low solubility of UO2 across sample types and the observation of secondary phases in deionized water suggest that anoxic UO2 dissolution does not only occur through a U(IV)(solid) to U(VI)(aqueous) process. Thus, we propose that dissolution of uranium under anoxic repository conditions may also proceed via U(IV)(solid) to U(IV)(aqueous), with subsequent U(IV) (precipitates) in a less defective form. Quantitative analysis of surface-sensitive EBSD diffractograms was conducted to elucidate lattice-mismatch induced cracks observed in UO2 thin film studies. Variable temperature anoxic dissolution was conducted, and no increased uranium concentration was observed in elevated temperatures

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ∼10−9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (0 0 1) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (∼0.1 O2 ppm) at ambient temperature (∼25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form.JRC.G.I.5-Advanced Nuclear Knowledg

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

10.1016/j.apsusc.2018.09.094Applied Surface Science464376-37

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ∼10−9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (0 0 1) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (∼0.1 O2 ppm) at ambient temperature (∼25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form