Structural effects in UO2_2 thin films irradiated with fission-energy Xe ions

Uranium dioxide thin films have been successfully grown on LSAT (Al$_{10}$La$_3$O$_{51}$Sr$_{14}$Ta$_7$) substrates by reactive magnetron sputtering. Irradiation by 92 MeV $^{129}$Xe$^{23+}$ ions to simulate fission damage that occurs within nuclear fuels caused microstructural and crystallographic changes. Initially flat and continuous thin films were produced by magnetron sputtering with a root mean square roughness of 0.35 nm determined by AFM. After irradiation, this roughness increased to 60-70 nm, with the films developing discrete microstructural features: small grains (~3 $\mu$m), along with larger circular (up to 40 $\mu$m) and linear formations with non-uniform composition according to the SEM, AFM and EDX results. The irradiation caused significant restructuring of the UO$_2$ films that was manifested in significant filmsubstrate mixing, observed through EDX analysis. Diffusion of Al from the substrate into the film in unirradiated samples was also observed.Engineering and Physical Sciences Research Council (Grant ID: EP/ I036400/1), Radioactive Waste Management Ltd (formerly the Radioactive Waste Management Directorate of the UK Nuclear Decommissioning Authority, contract NPO004411A-EPS02), Russian Foundation for Basic Research (projects 13-03-90916), CSAR, Grand Accelélérateur National d’Ions Lourds (GANIL) Caen France, French Network EMIR, CIMAP-CIRIL, M.V.Lomonosov Moscow State University Program of Development, CKP FMI IPCE RA

The effect of fission-energy Xe ion irradiation on dissolution of UO2_2 thin films

The aim of this work was to study the effect of fission fragment damage on the dissolution of UO$_2$ thin films in water. For this purpose, thin films of UO$_2$ on LSAT (Al$_{10}$La$_3$O$_{51}$Sr$_{14}$Ta$_7$) substrates were produced and irradiated by 92 MeV $^{129}$Xe$^{23+}$ ions to a fluence of 4.8 × 10$^{15}$ ions/cm$^2$ to simulate the fission damage and induce chemical mixing that occur within nuclear fuels. The dissolution experiment was conducted under a nitrogen atmosphere (200–900 O$_2$ ppm in N$_2$ ) to study the effect of the induced irradiation damage and mixing on the dissolution of the UO$_2$ matrix. The irradiated samples showed a decrease in the amount of dissolved uranium, as compared to the corresponding unirradiated samples. This was ascribed to the irradiation-induced chemical mixing of the UO$_2$ films with the substrate elements, which resulted in stabilisation of the UO$_2$ matrix and increased its aqueous durability. Secondary phases were also observed on the surface of the UO$_2$ films after the dissolution experiment.The irradiation experiment was performed at the Grand Accelelerateur National d’Ions Lourds (GANIL) Caen, France, and supported by the French Network EMIR. AFM, SEM and EDX experiments were performed with support of M.V.Lomonosov Moscow State University Program of Development. 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

Effect of reducing groundwater on the retardation of redox-sensitive radionuclides

Laboratory batch sorption experiments were used to investigate variations in the retardation behavior of redox-sensitive radionuclides. Water-rock compositions were designed to simulate subsurface conditions at the Nevada Test Site (NTS), where a suite of radionuclides were deposited as a result of underground nuclear testing. Experimental redox conditions were controlled by varying the oxygen content inside an enclosed glove box and by adding reductants into the testing solutions

Research data supporting the publication: 'The effect of fission-energy Xe ion irradiation on dissolution of UO2_2 thin films'

Project: PhD work by A.J. Popel: ‘The effect of radiation damage by fission fragments on the structural stability and dissolution of the UO2 fuel matrix’. The Excel file ‘U_ICP-MS data’ with output ICP-MS data and calculations for water and acid dilutions supporting Figures 2 and 3 and ICP-MS results in the publication: A.J. Popel, V.G. Petrov, V.A. Lebedev, J. Day, S.N. Kalmykov, R. Springell, T.B. Scott, I. Farnan, The effect of fission-energy Xe ion irradiation on dissolution of UO2 thin films, J. Alloys Compd. 721 (2017) 586-592, https://doi.org/10.1016/j.jallcom.2017.05.084. The ICP-MS analysis was performed to measure 238U concentration in the extracted solutions. The data were generated on the 13-14th of October 2014 at the Department of Earth Sciences, University of Cambridge, Cambridge, UK, on a Perkin Elmer SCIEX Elan DRC II quadrupole ICP-MS. The data can be accessed through the University of Cambridge Data Repository

The nature of the chemical bond in UO<inf>2</inf>

The nature of the chemical bond in UO2 was analyzed taking into account the XPS structure parameters of the valence and core electrons, as well as the relativistic discrete variation (RDV) electronic structure calculation results for this oxide. The ionic/covalent nature of the chemical bond was determined for the UO8 (D4h) cluster, reflecting uranium’s close environment in UO2, and the U13O56 and U63O216 clusters, reflecting the bulk of solid uranium dioxide. The bar graph of the theoretical valence band (from 0 to ~35 eV) of XPS spectrum was built such that it was in satisfactory agreement with the experimental spectrum of a UO2 single crystalline thin film. It was shown that unlike the crystal field theory results, the covalence effects in UO2 are significant due to the strong overlap of the U 6p and U 5f atomic orbitals with the ligand orbitals, in addition to the U 6d AO. A quantitative MO scheme for UO2 was built. The contribution of the MO electrons to the chemical bond covalence component was evaluated on the basis of the bond population values. It was found that the IVMO electrons weaken the chemical bond formed by the OVMO electrons by 32% in UO8 and by 25% in U63O216.The work was supported by the RFBR grant #17-03-00277a. M.V. Ryzhkov acknowledges the support of research provided by the state assignment for the Institute of Solid State Chemistry of the Ural Brunch of RAS No AAAA-A16-116122810214-9. The authors acknowledge support from Lomonosov Moscow State University Program of Development for providing access to the XPS facility

XPS study of the surface chemistry of UO <inf>2</inf> (111) single crystal film

A (111) air-exposed surface of UO2 thin film (150 nm) on (111) YSZ (yttria-stabilized zirconia) before and after the Ar+ etching and subsequent in situ annealing in the spectrometer analytic chamber was studied by XPS technique. The U 5f, U 4f and O 1s electron peak intensities were employed for determining the oxygen coefficient kO = 2 + x of a UO2+x oxide on the surface. It was found that initial surface (several nm) had kO = 2.20. A 20 second Ar+ etching led to formation of oxide UO2.12, whose composition does not depend significantly on the etching time (up to 180 seconds). Ar+ etching and subsequent annealing at temperatures 100–380 °C in vacuum was established to result in formation of stable well-organized structure UO2.12 reflected in the U 4f XPS spectra as high intensity (~28% of the basic peak) shake-up satellites 6.9 eV away from the basic peaks, and virtually did not change the oxygen coefficient of the sample surface. This agrees with the suggestion that a stable (self-assembling) phase with the oxygen coefficient kO ≈ 2.12 forms on the UO2 surface

Research data supporting the publication: 'Structural effects in UO2_2 thin films irradiated with fission-energy Xe ions'

Project: PhD work by A.J. Popel: ‘The effect of radiation damage by fission fragments on the structural stability and dissolution of the UO2 fuel matrix’. The Excel file ‘XRD_UO2_films_LSAT’ with raw XRD data supporting Figure 10 and XRD results in the publication: A.J. Popel, V.A. Lebedev, P.G. Martin, A.A. Shiryaev, G.I. Lampronti, R. Springell, S.N. Kalmykov, T.B. Scott, I. Monnet, C. Grygiel, I. Farnan, Structural effects in UO2 thin films irradiated with fission-energy Xe ions, J. Nucl. Mater. 482 (2016) 210-217, https://doi.org/10.1016/j.jnucmat.2016.10.024. The XRD analysis was performed to assess crystallographic structure of the as-produced and irradiated thin films of UO2 on LSAT substrates. The data were generated in June-July 2014 at the Department of Earth Sciences, University of Cambridge, Cambridge, UK. A D8 Bruker diffractometer equipped with a primary Ge monochromator for Cu Ka1 and a Sol-X solid state detector operating in standard Bragg-Brentano geometry was used for the analysis. The samples were spun during signal collection and a zero-background sample holder was used in all cases. The data can be accessed through the University of Cambridge Data Repository