[en] A double-electron excitation in L₃-edge x-ray absorption spectra of actinides has been observed for the first time. The investigation of this effect is presented. Actinide species in valence states IV and III were investigated by using experimental data of Th⁴⁺, U⁴⁺, Np⁴⁺, Pu³⁺, and Am³⁺ hydrates. The double-electron excitation was identified as a L₃N_6,7 shake-up effect. Energy positions of the double-electron features were found in good agreement with the Z+1 approximation. The LN resonance intensity corresponds with the resonance of the L edge. This has been observed by comparing the L₁, L₂, and L₃ XANES spectra of Th⁴⁺, as well as the L₃ XANES spectra of Np in oxidation states IV, V, VI, and VII. Influences of the double-electron excitation on the EXAFS signal are discussed

[en] Ru speciation is being investigated systematically from models of high level waste solutions right through to the calcination process and the vitrified glass product. The characterisation of these species is complicated due to the fact that a wide range of ruthenium nitrosyl/nitrite/nitrate complexes can be present in nitric acid waste solutions. The general formula for these complexes is RuNO(NO₃)_x(NO₂)_y(OH)_z(H₂O)_5-x-y-z^+3-x-y-z. A range of different techniques has been used for the characterisation of these species in solution, including electron absorption spectroscopy, vibrational spectroscopy, multi-nuclear magnetic resonance spectroscopy and X-ray absorption spectroscopy. (authors)

[en] Mixed oxide (MOX) fuel is usually considered as a solid solution formed by uranium and plutonium dioxides. Nevertheless, some physico-chemical properties of (U_1-y, Pu_y)O₂ samples manufactured under industrial conditions showed anomalies in the domain of plutonium contents ranging between 3 and 15 at.%. Cerium is commonly used as an inactive analogue of plutonium in preliminary studies on MOX fuels. Extended X-ray Absorption Fine Structure (EXAFS) measurements performed at the European Synchrotron Radiation Facility (ESRF) at the cerium and uranium edges on (U_1-y, Ce_y)O₂ samples are presented and discussed. They confirmed on an atomic scale the formation of an ideal solid solution for cerium concentrations ranging between 0 and 50 at.%

[en] Different Ce concentrations were incorporated in zirconia via the co-precipitation route, with the goal to stabilize the cubic zirconia polymorph. Due to the extraordinary luminescent properties of Eu, a trace amount of this lanthanide was added together with Ce in the synthesis process. A series of zirconia samples doped with 14 to 70 mol.% Ce was prepared. Seven selected compositions are presented here to demonstrate a clear phase transformation. The phase composition was evaluated by powder X-Ray diffraction (PXRD), and Raman spectroscopy. In the PXRD diffractograms, the monoclinic phase, characterized by diffraction peaks at 28.2° and 31.3°, is dominant only in the composition with 14 mol.% Ce. Above this concentration, a peak around 29.9°, assigned to the tetragonal phase, increases in intensity as a function of increasing Ce concentration in the zirconia matrix up to a concentration of 42 mol.%. Beyond this, the cubic phase starts to dominate the phase composition, with the intensive characteristic peak around 29.3°. Additionally, the presence of a tetragonal metastable phase in samples with 30, 42, and 50 mol % Ce, and a trace of the tetragonal phase in the composition with 70 mol % Ce were identified. Generally the diffraction peaks are shifted to lower 2 angles as a result of the substitution of Zr⁴⁺ by the larger Ce⁴⁺ cation which increases the lattice parameters. The Raman results corroborate the assignment of the different phase compositions identified in the PXRD studies. However, a large band at 512 cm^-1 becomes visible in the samples with 30 mol % Ce and grows in intensity with increasing Ce doping. This band has been reported to arise from Frenkel type defects, typically found in samples with oxygen vacancies or/and due to partial reduction of Ce⁴⁺ to Ce³⁺ occurring in the samples, and causing the formation of oxygen vacancies in the ZrO₂ structure for charge compensation. This, however, has to be verified in future XANES experiments. No solid phase separation was detected in both characterization analyses, PXRD and Raman. Luminescence spectroscopic studies, probing the Eu ion which has been incorporated together with Ce in all ZrO₂ solid phases, will be conducted in future studies. Preliminary results of these investigations will be shown.

[en] There is currently a great deal of research interest in the development of molten salt technology, both classical high temperature melts and low temperature ionic liquids, for the electrochemical separation of the actinides from spent nuclear fuel. We are interested in gaining a better understanding of actinide and key fission product speciation and electrochemical properties in a range of melts. Our studies in high temperature alkali metal melts (including LiCl and LiCl-KCl and CsCl-NaCl eutectics) have focussed on in-situ species of U, Th, Tc and Ru using X-ray absorption spectroscopy (XAS, both EXAFS and XANES) and electronic absorption spectroscopy (EAS). We report unusual actinide speciation in high temperature melts and an evaluation of the likelihood of Ru or Tc volatilization during plant operation. Our studies in lower temperature melts (ionic liquids) have focussed on salts containing tertiary alkyl group 15 cations and the bis(tri-fluor-methyl)sulfonyl)imide anion, melts which we have shown to have exceptionally wide electrochemical windows. We report Ln, Th, U and Np speciation (XAS, EAS and vibrational spectroscopy) and electrochemistry in these melts and relate the solution studies to crystallographic characterised benchmark species. (authors)

[en] The singular Pu^IV hexanuclear cluster [Pu₆(OH)₄O₄]¹²⁺ stabilized by 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) ligands was structurally characterized for the first time both in the solid state and in water solution by using X-ray diffraction and X-ray absorption and UV/Vis spectroscopy. The stability of this cluster in water and its high solubility over a large pH range are of upmost importance for plutonium environmental speciation with potential applications in a related migration model. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] This contribution provides an overview of a current research network funded by the German Federal Ministry of Education and Research (BMBF), entitled 'Fundamental investigations of actinide immobilization by incorporation into solid phases relevant for final disposal' - AcE. The AcE project aims at understanding the incorporation and immobilization of actinides (An) in crystalline, repository relevant solid phases, such as zirconia (ZrO₂) and UO₂, but also in zircon (ZrSiO₄), pyrochlores (Ln₂Zr₂O₇) and orthophosphates of the monazite type (LnPO₄), which may find use as host matrices for the immobilization and safe disposal of high-level waste streams. The main objectives of the AcE project are (i) the development of synthesis strategies for An(IV)-doped solid phases, (ii) understanding their associated structural and physical properties using combined modelling and experimental approaches and (iii) determining their performance after irradiation with particular regard to an assessment of their long-term stability, dissolution behavior, and suitability for An matrix incorporation. Recent results obtained for ZrO₂, the main corrosion product of the Zircaloy cladding material surrounding nuclear fuel rods, will also be discussed. ZrO₂ is monoclinic phase (P2(1)/c) at ambient conditions, and transforms into tetragonal (P4(2)/nmc) and cubic phases (Fm3 ̅m) at high temperatures of around 1200°C and 2370°C, respectively. However, particle size effects, the incorporation of foreign ions such as the actinides, as well as high radiation fields are known to also influence the stability fields of the polymorphs. A short overview of experimental studies conducted by the AcE partners, addressing both the ZrO₂ bulk structure, irradiation-induced changes, as well as the An environment during and after such structural transformations, will be given.

[en] Three tetravalent actinide (An^IV) hexanuclear clusters with the octahedral core [An₆(OH)₄O₄]¹²⁺ (An^IV=U^IV, Np^IV, Pu^IV) were structurally characterized in the solid state and in aqueous solution by using single-crystal X-ray diffraction, X-ray absorption, IR, Raman, and UV/Vis spectroscopy. The observed structure, [An₆(OH)₄O₄(H₂O)₈(HDOTA)₄].HCl/HNO₃.n H₂O (An=U(I), Np(II), Pu(III)), consists of a An^IV hexanuclear pseudo-octahedral cluster stabilized by DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) ligands. The six actinide atoms are connected through alternate μ₃-O^2- and μ₃-OH^- groups. Extended X-ray absorption fine structure (EXAFS) investigations combined with UV/Vis spectroscopy provide evidence for the same local structure in moderate acidic and neutral aqueous solutions. The synthesis mechanism was partially elucidated and the main physicochemical properties (pH range stability, solubility, and protonation constant) of the cluster were determined. The results underline the importance of: (1) considering such polynuclear species in thermodynamic models, and (2) competing reactions between hydrolysis and complexation. It is interesting to note that the same synthesis route with thorium(IV) led to the formation of a dimer, Th₂(H₂O)₁₀(H₂DOTA)₂.4 NO₃.x H₂O (IV), which contrasts to the structure of the other An^IV hexamers. (copyright 2017 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] Technetium, uranium and neptunium may all occur in the environment in more than one oxidation state (IV or VII, IV or VI and IV or V respectively). The surface of mackinawite, the first-formed iron sulfide phase in anoxic conditions, can promote redox changes so a series of laboratory experiments were carried out to explore the interactions of Tc, U and Np with this mineral. The products of reaction were characterised using X-ray absorption spectroscopy. Technetium, added as TcO₄^-, is reduced to oxidation state IV and forms a TcS₂-like species. On oxidation of the mackinawite in air to form goethite, Tc remains in oxidation state IV but in an oxide, rather than a sulfide environment. At low concentrations, uranium forms uranyl surface complexes on oxidised regions of the mackinawite surface but at higher concentrations, the uranium promotes surface oxidation and forms a mixed oxidation state oxide phase. Neptunium is reduced to oxidation IV and forms a surface complex with surface sulfide ions. The remainder of the Np coordination sphere is filled with water molecules or hydroxide ions

[en] The Rietveld refinement of HR-PXRD data on the superconducting Ag${}_{0.92}$Mo${}_6$S${}_8$ confirmed the Chevrel-like structure: SG R$\stackrel{¯<!--\; ??\; -->}{3}$, a = 9.3065 Å and c = 10.8310 Å. Magnetization measurements revealed a superconducting transition at T${}_c$ = 7.9 K. The lower and upper critical magnetic fields B${}_{c1}$ = 12 mT and B${}_{c2}$ = 3.5 T were deduced from the hysteresis. In contrast, measurements of the electrical resistivity in different magnetic fields resulted in B${}_{c2}$ = 7.4 T as well as the GL-coherence length ${\mathrm{\xi <!--\; ??\; -->}}_{GL}$ = 67 Å, the GL-parameter $\mathrm{\kappa <!--\; ??\; -->}$ = 33 and the London penetration depth ${\mathrm{\lambda <!--\; ??\; -->}}_L$ = 2204 Å. The specific heat jump at the superconducting transition $\mathrm{\Delta <!--\; ??\; -->}$ c${}_p$/$\mathrm{\gamma <!--\; ??\; -->}$ T${}_c$ = 1.77 is well above the value predicted by BCS theory (1.43), whereas the energy gap ratio $\mathrm{\Delta <!--\; ??\; -->}$(0)/k${}_B$T${}_c$ = 1.55 obtained from an exponential fit of the electronic specific heat c${}_{el}$ from 0.35 K to 4 K is smaller than the theoretically predicted value of 1.76. In agreement with the isostructural SnMo${}_6$S${}_8$ and PbMo${}_6$S${}_8$ Chevrel phases Ag${}_{0.92}$Mo${}_6$S${}_8$ is a two-band superconductor with the gap ratios ${\mathrm{\Delta <!--\; ??\; -->}}_1$/k${}_B$T${}_c$ = 2.39 (95%) and ${\mathrm{\Delta <!--\; ??\; -->}}_2$/k${}_B$T${}_c$ = 1.03 (5%).