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European Commission

Short description of the work
We have previously shown that the tetraphenylimidophosphinate ligand (TPIP) stabilises neptunium and uranium exclusively in the +VI oxidation states including complete conversion from their +V states. (1,2) Moreover, TPIP is able to form oligomeric species assembled by actinyl Lewis acid base adducts (cation-cation interactions) for uranyl (VI) ions and preliminary evidence suggests such interactions occur in mixtures of neptunyl (VI) and uranyl (VI) (i.e. a mixed metal cation-cation aggregate). The study of such cation-cation interactions are extremely important in evaluating and developing new separation processes based on simplified versions of the PUREX process.


Short description of the work
During the weeks spent at the JRC-ITU facility, uranium-americium mixed dioxides with Am/(U+Am) ratio equal to 15%, 30% and 50% (at.%) were observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM) in order to have information both on the surface morphology and on the nanometric structure of the grains.
In the TEM, electron energy loss spectroscopy (EELS) measurements were performed. In particular, EELS-spectra were collected on the Oxygen-K edge. Two main intense peaks due to O2p-5f and O2p-6d hybridization bands are observed with different resonances up to 570 eV due to multi-scattering (MS) contributions of successive coordination shells around oxygen atoms within the fluorite structure (typical of actinide dioxides). Both peaks and MS features are very sensitive to symmetry change and can be used to obtain information on the anion sub-lattice.
These data will be completed with XANES and EXAFS measurements on the same samples which will provide complementary information on the uranium-americium sublattice.
Analysis of the acquired data will now be performed jointly by ITU and CEA.


Short description of the work
1. Two U-based bcc alloys, U0.85Mo0.15 and U0.80Zr0.20, in the form of splats provided by CUNI, were subjected to a surface science study using photoelectron spectroscopy (XPS and UPS). Although the Mo-alloy has its surface covered, as expected, by oxidized Uranium, in the case of Zr alloy the U metal is invisible, and the surface is covered by Zr oxide. Using in situ sputtering by Ar ions, we succeeded to remove the surface layer and reveal the bulk of the alloy. Subsequent study of isochronal annealing (up to 700 oC) and/or exposure to O2 mapped conditions under which the surface segregation and oxidation proceeds under well defined UHV conditions.
2. Using sputter deposition from elemental U and Mo targets we succeeded to synthesize clean (oxygen-free) films of U-Mo alloys with variable stoichiometry. Electronic structure study using U-4f and valence-band spectra did not show any dramatic development (e.g. localization, electronic transfer) of the U-5f states in the course of Mo dilution. This is consistent with the known insensitivity of magnetic susceptibility to Mo concentration, and points to a significant hybridization of the U-5f and Mo electronic states, which prevents to 5f localization happening due to increasing U-U spacing. The 4f core-level spectra are rather similar for pure U (assumed to be in the alpha-U state) and Mo alloyed presumed gamma-U films, fine differences will be subject of detailed analysis. The spectra of corresponding films and splat mentioned above are in a good agreement.


Short description of the work
The Talisman JRP C05-14 was dedicated to the investigation of structural properties of uranium and americium mixed dioxides by XAS (X-ray absorption spectroscopy) and EELS (electron energy loss spectroscopy). XAS measurements were performed at the ROBL beamline of the European Synchrotron Radiation Facility (ESRF). One UO2 and five (U,Am)O2 samples with an Am/(U+Am) ratio equal to 15%, 30%, 50%, 60% and 70% were studied. XANES (X-ray absorption near edge structure) and EXAFS (Extended X-ray Absorption Fine Structure) spectra were recorded at both U and Am edges (U LIII, U LII and Am LIII) at four different temperatures: 15 K, 100 K, 150 K and 300 K. The analysis of the data will now be performed at CEA. XANES spectra will allow identifying the oxidation states of uranium and americium in the different compounds, whereas the EXAFS data will provide information on the local cation structure, which will be coupled with the EELS data. Moreover, thanks to the acquisition of EXAFS spectra at 4 different temperatures, it will be possible to determine the ratio between structural and thermal disorder in the Debye-Waller factors.


Short description of the work
During this stay, the work done at HZDR-IRE was dedicated to the synthesis and the structural characterization of the giant poly-oxo cluster Np38. Such type of complexes were already mentioned in the literature for uranium and plutonium but was never observed with neptunium. Our synthesis approach was based on the synthesis of U38, using a combination of tetrachloride actinide salt, benzoic acid, tetrahydrofuran and a small amount of water. In the case of neptunium, the syntheses were adapted to NpCl4 (Neptunium tetrachloride). The first attempt of Np38 synthesis gave very small crystals similar to those observed with uranium (octahedron), but too small for a single crystal X-ray diffraction (SC-XRD) analysis. Therefore, we optimized the synthesis parameters (mainly reaction time, amount of water) to improve the size and the quality of the crystals. This approach leaded to the production of Np38 crystals, good enough for SC-XRD and crystal data were collected during this stay at HZDR-IRE. The structure of Np38 was successfully solved shortly after this stay. Based on the obtained data, a manuscript is currently being prepared for publication.

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